Marine Geology 280 (2011) 150–167

Contents lists available at ScienceDirect

Marine Geology

j ourna l homepage: www.e lsev ie r.com/ locate /margeo

Qualitative and quantitative approaches in palaeohydrography: A case study fromcore-top parameters in the Gulf of Cadiz

M. Rogerson a,⁎, J. Schönfeld b, M.J. Leng c

a Department of Geography, University of Hull, Cottingham Road, Hull, HU6 7RX, UKb IFM-GEOMAR, Wischhofstraße 1-3, 24148 Kiel, Germanyc NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK

⁎ Corresponding author. Tel.: +44 0 1482 466051.E-mail address: m.rogerson@hull.ac.uk (M. Rogerson

0025-3227/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.margeo.2010.12.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 January 2010Received in revised form 7 December 2010Accepted 11 December 2010Available online 23 December 2010

Communicated by G.J. de Lange

Keywords:palaeoceanographyAtlanticMediterraneanMediterranean Outflowstable isotopesgrainsizeforaminifera

Palaeoceanography relies on the assumption that parameters measureable in sediment cores correlatereliably to hydrographic parameters. Areas of dynamic watermass mixing, which generally have steephydrographic gradients, therefore provide both a tempting target (large spatial and temporal differences intemperature and salinity) and a significant challenge (high flow velocities and high “noise”) forpalaeoceanographers. Here, we investigate the ability of parameters measured on core tops to be used asthe basis for simulating regional hydrography within one globally important area of watermass mixing, theGulf of Cadiz. Using grainsize, sediment composition, benthic foraminiferal assemblage and benthicforaminiferal stable isotope analysis it is possible to build an effective framework for qualitative constraintof the position of the modern Mediterranean Outflow Water (MW) Plume. We propose that the Gulf of Cadizslope can be sub-divided into 8 hydrographic zones reflecting these parameters; Proximal MW, Core MW1(steep slopes), Core MW1 (gentle slopes), Distal MW1, Distal MW2, Lower Limit of MW plume, North AtlanticDeep Water and Ambient Atlantic Water. We anticipate that compilation of time-slice data will revealhydrographic sub-divisions of the slope for the past in a similar manner, improving our understanding of pastchanges in the size and position of the MW plume, but more work needs to be done before a secure protocolfor quantitative reconstruction can be created. Stable isotope analysis alone is insufficient for the task, withδ13C behaving in a non-conservative manner and δ18Ocalcite acting in an ambiguous way due to the competinginfluences of δ18Owater and temperature. It is likely that if a single parameter that unambiguously determinesthe difference between Mediterranean Water and Atlantic Water can be identified, combination withδ18Ocalcite will allow extension of our qualitative analysis into a quantitative means of palaeohydrographicreconstruction for this region. Similar complexity to that found in this study would be expected in any mixingregion, and in particular on the majority of sediment drifts. We recommend that core top surveys similar tothis study are performed on these sediment drift systems before any form of quantitative palaeohydrology isattempted.

).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Most palaeoceanography research is carried out on core materialrecovered from open ocean sites where bottom velocities aregenerally insufficient to entrain sand or silt-sized sediment particles(McCave and Hall, 2006). These sites not only represent a highproportion of the oceanic sea floor, and therefore a significantproportion of the surface of Earth, but they insulate interpretationof the marine proxy record from processes such as reworking andincorporation of allochthonous debris (McCave, 2002). The disadvan-tage of these sites is that regional horizontal property gradients in thewater overlying the sampled location tend not to be steep, and the

current systems that are typically the focus of palaeoceanographystudies are rather diffuse. Drift sediments from sites with strongbottom current systems that interact with the sediment surface, suchas marginal basin overflow systems like the Denmark Strait OverfloworMediterranean Outflow, can be viewed as the inverse of open oceansites. On these drifts, horizontal environmental gradients are high,making changes in the local current system very likely to influencepalaeoceanographic proxies. Their proximity to sites of significantwatermass transformation also means that local processes can havebasinwide implications for watermass properties and circulationpatterns (Hall et al., 2006; Lynch-Stieglitz et al., 1999; McManus et al.,2004; Robinson and McCave, 1994; Rogerson et al., 2005; Schönfeldand Zahn, 2000b; Stanford et al., 2006; Voelker et al., 2006). However,it is also true that the probability of the sedimentary record in thesedrift settings being polluted by allochthonous material is relativelyhigh (McCave, 2002).

151M. Rogerson et al. / Marine Geology 280 (2011) 150–167

One notably high-energy sedimentary drift-forming currentsystem is the Mediterranean Outflow plume on the southwest Iberianmargin (Fig. 1). On the continental slope of the Gulf of Cadiz, bottomwater potential temperature varies between +12.8 and +2 °C andsalinity between 38.5 and 34.9 psu (Alvarez et al., 2005). These rangesare higher than the total range of bottom water properties along ameridional section at 30 °W in the Atlantic by factors of 2–3 and ~8respectively. Furthermore, the Mediterranean Water current systemsignificantly alters the circulation of the mid-latitude eastern NorthAtlantic (Johnson et al., 2002) affecting distributions of heat and salt atmid-depth (Iorga and Lozier, 1999) and driving the formation of theAzores Front current system (Ozgokmen et al., 2001). Mediterranean-derived salt may also contribute to preconditioning of the Atlantic formeridional overturning (Bigg et al., 2003), making constraints of theGulf of Cadiz current system of potentially critical importance for theunderstanding of thermohaline collapse and, particularly, subsequentrecovery (Rogerson et al., 2006b). The Mediterranean Water reachesvelocities in excess of 2 m s−1 on the Gulf of Cadiz slope (Johnson et al.,1994), and is demonstrably capable of sediment entrainment, includingerosion of Tertiary sedimentary rocks that outcrop within its flow path(Hernandez-Molina et al., 2003; Kenyon and Belderson, 1973). Theprobability of biasing any sediment core record that has been under theinfluence of the Mediterranean Outflow with either reworked and orallochthonous material is therefore disconcertingly high.

Fig. 1. a) Location of the Gulf of Cadiz and position of the Mediterranean Outflow plume ob) Position of sedimentary sectors on the Gulf of Cadiz slope (after Hernandez-Molina et al

This study aims to address the issues of: 1) how reliable thesedimentary and palaeoceanographic record of Mediterranean Wateractivity in the Gulf of Cadiz can be expected to be, and 2) how theinfluence of allochthonous particles potentially may bias palaeohy-drological information from such records. Our approach uses a coretop dataset to demonstrate how accurately sedimentary parameterscan resolve the distribution of watermasses in the Gulf of Cadiz. Inaddition, this test is significant for any location where a mapping ofpast changes in the composition of local watermasses is performed.

1.1. The Gulf of Cadiz sediment drift

Onset of drift deposition in the Gulf of Cadiz is believed to haveoccurred at the end of the Messinian (~5.4 Ma), subsequent to thebreaching of the Strait of Gibraltar (Rodero et al., 1999). Since thistime the upper slope of the Gulf of Cadiz has been bathed by theMediterranean Water (MW), which is the product of the mixing ofdense Mediterranean Water, moving westward from the Strait ofGibraltar, with ambient Atlantic Central Water. Strong and perennialalong-slope flow dominates the sedimentary processes on thenorthern and eastern Gulf of Cadiz continental slope, causing theformation of a series of large sedimentary drifts between 500 and1200 m water depth (Faugères et al., 1984, 1986; Habgood et al.,2003; Hernandez-Molina et al., 2003, 2006; Kenyon and Belderson,

n the continental slope of western Iberia. Major river debouchments are also shown.., 2003).

152 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

1973; Stow et al., 1986). Surface sedimentary facies are highlyvariable in the Gulf of Cadiz (Habgood et al., 2003), but can be spatiallydivided into 5 sectors representing the impact of different flow regimes(Fig. 1b); (Hernandez-Molina et al., 2003). Sector 1 is positionedproximal to the Strait of Gibraltar and occupies the region of highestbottom shear stress (Kenyon and Belderson, 1973). The surface isdominated by abrasion surfaces, rock-outcrops and sand-dune fields(Hernandez-Molina et al., 2006; Kenyon and Belderson, 1973). To thenorthwest anddownslopeof Sector 1 is the regionwhere themajority ofMediterranean Water flow moves over the continental slope, causingthe formation of several scarp-bounded channels and both active andrelict sheeted andmounded drifts (Hernandez-Molina et al., 2003). Thisarea, Sector 3, is the most morphologically diverse part of the drift(Hernandez-Molina et al., 2006). Sector 4 is positioned to the north andupslope of Sector 3 and lies at the same depth as Sector 1. Sector 4represents an area where the shallowest part of the MediterraneanOutflow has already achieved neutral buoyancy (O'Neill-Baringer andPrice, 1999). This sector is where the majority of classic “contourite”deposition occurs (Stow et al., 1986). Silts and clayey silts with layers offine sands were encountered here (Kudrass, 1973; Schönfeld, 1997).Sector 5 is a distal, reduced energy extension of Sector 4, located to itswest on the southern Portuguese continental slope. The sediments aremainly clayey silts with a few layers richer in fine sand. Sector 2 isdescribed by Hernandez-Molina et al. (2003, 2006) as an “overflow-sedimentary lobe”. This sector is positioned west and downslope ofSector 1, in theflowpathof the lower limbof theMediterraneanOutflow(“MW2”) after it overflows over a sedimentary levee on the downslopemargin of Sector 1 (O'Neill-Baringer and Price, 1999). The predominantsediments are clayey silts and medium sands. However, on the basis ofdatingevidence for subsurface sandsRogersonet al. (2005,2006b) arguethat this is a relict drift reflecting the position of the major limb of theMediterranean Outflow during the Last Glacial Maximum.

Downslope of the sediment drifts, normal gravity flow processesdominate, with 2 additional small gravity flow lobes positionedwithin Sector 2 (Habgood et al., 2003). In addition, sedimentincorporated into the Gulf of Cadiz drift systems at depth is believedto be derived from mass wasting from the continental shelf(Hernandez-Molina et al., 2006). This would imply that downslopeprocesses also dominate above the MW plume, and this contention issupported by the presence of the empty tests of Shelf-Dwellingbenthic foraminifera in surface sediment across much of the sedimentdrifts (Rogerson, 2003). The fine-grained sediment incorporated intothe sediment drifts is ultimately sourced from the large Iberian riverswhich debouch into the Gulf of Cadiz (Lopez-Galindo et al., 1999).

1.2. Approach

The physical oceanography of the mid-latitude eastern NorthAtlantic at intermediate depths is primarily controlled by thedistribution of Mediterranean Water (Iorga and Lozier, 1999;O'Neill-Baringer and Price, 1999; Ozgokmen et al., 2001; Reid,1979). However, pure Mediterranean Water does not extend morethan a few tens of kilometres westward from the Strait of Gibraltar(Price et al., 1993). Mapping this plume therefore depends upon theability to estimate the proportional admixture of the MediterraneanWater into “ambient” water masses on the continental slope of theGulf of Cadiz and out into the open Atlantic (Alvarez et al., 2005). Inorder to trace the palaeoceanography of the Mediterranean Outflow,spatial distribution data for a given proxy parameter, from whichhydrography can be inferred, have to be generated. However, whenconsidering this approach a number of significant limitationsbecome apparent. Whatever method is employed, palaeoceanogra-phy is limited to sites where reasonably continuous sedimentaryrecords can be recovered. We therefore cannot incorporate intocalibration datasets samples from any location where no netdeposition occurs or where coarse sand obstructs the retrieval of

conventional gravity or piston cores. This is very unfortunate, as theregion that is ‘invisible’ to palaeoceanography therefore encom-passes Sector 1 and parts of Sector 3 where the majority of mixing ofMediterranean Outflow Water (O'Neill-Baringer and Price, 1997;Price and O'Neill-Baringer, 1994; Price et al., 1993), flow splitting(Borenas et al., 2002; Jungclaus and Mellor, 2000; O'Neill-Baringerand Price, 1997; Serra et al., 2005) and spreading (Ambar andHowe, 1979; Davies et al., 2002; Jungclaus and Mellor, 2000;O'Neill-Baringer and Price, 1997) occur. This immediately limitsthe ability of palaeoceanographic methods to reveal changes in thepast MW.

Our resulting concept is to investigate the ability of a sedimentsurface geochemical and sedimentary data set to predict theproportional distribution of Mediterranean Water within thebottom water in the Gulf of Cadiz today. To ensure that the sitesselected for analysis could, in principle, be incorporated into apalaeoceanographic dataset, we restrict our samples to the tops ofsediment cores at least 1 m long. Combining the collections withinBOSCORF (British Ocean Sediment COre Repository Facility), South-ampton, and the Lithotek at IFM-GEOMAR, Kiel, a total of 62 coretops were used for analysis (Table 1). Core top samples comprise theuppermost 1 cm of the sediment cores. Due to the elevatedproportion of silt or fine sand in near-surface sediments, theuppermost part of the cores were physically stable and wellpreserved. In particular, the surface oxidized layer and a dark-brown horizon stained by Mn oxides was usually recognized a fewcentimetres below the sediment surface indicating an undisturbedsedimentary record (Schönfeld, 2001).

2. Methods

2.1. Hydrographic data

Core top datasets have a lower spatial resolution than hydro-graphic survey data, and consequently investigating the performanceof palaeoceanographic parameters compared to hydrographic data ishampered by different numbers of observations. Consequently, wepresent two hydrographic datasets of potential temperature (θ) andsalinity (S) taken from MEDATLAS (MEDATLAS, 2002). The first,named “All Hydro”, comprises all bottom data (i.e. deepest datapointwithin each completed CTD cast) within the Gulf of Cadiz, and thesecond, named “Core-Site Hydro”, comprises data interpolated fromthe All Hydro dataset onto the sites from which core top samples areused for our study. Interpolation was done using a simple squareddistance interpolation using up to 7 and at least 3 nearest neighbours.

To calculate the proportional presence of MW from the basichydrographic data we use a simple three end-member mixingapproach for θ and S. At least in concept, a palaeoceanographicapproach utilising any binary geochemical dataset might effectivelybe used in the same manner. It is important to recognise, however,that this is a significant simplification of the region, in which at least 9end member watermasses can be discerned (Alvarez et al., 2005).Consequently, the selection of our three end-members requires somejustification.

One surface water component in the Alvarez et al. (2005) analysis,the Maderia ModeWater, does not seem to have a strong influence onthe subsurface system, and consequently is ignored. The Labrador SeaWater, Eastern North Atlantic Deep Water and Iceland Sea OverflowWater can reasonably be approximated to a single North AtlanticDeep Water (NADW) composition (θ=2 °C; S=35 psu). PureMediterranean Water represents a second unique end-member(MW; θ=12.8 °C; S=38.5 psu). Following Price and O'Neill-Baringer(1994), we represent the water into which Mediterranean Waterpasses as it leaves the Strait of Gibraltar with a single end-membercomposition (θ=12 °C; S=35.5 psu) and term this watermass“Ambient Atlantic Water” (AAW). This composition is similar to the

Table 1Location of core tops used in this study.

Core Latitude (°N) Longitude (°N) Water depth (m)

D13679 35.86 7.25 952D13680 35.86 7.22 1010D13682 35.77 7.61 1458D13683 35.86 7.50 1163D13684 35.86 7.35 1012D13685 35.76 6.92 952D13686 35.85 7.18 956D13687 35.72 7.20 1232D13688 35.72 7.09 1237D13689 35.77 7.06 1128D13690 35.76 6.92 861D13691 35.86 7.06 1072D13692 35.89 7.03 984D13693 35.76 6.75 620D13694 35.90 7.04 851D13695 35.96 7.01 853D13696 36.02 7.11 885D13697 35.98 7.04 891D13698 36.55 8.20 1055D13700 36.32 7.92 1191D13701 36.22 7.81 1061D13702 36.11 7.77 1123D13703 36.06 7.76 1161D13706 36.10 8.02 1946D13707 36.20 8.02 1533D13884 36.09 8.14 1949D13885 36.04 8.23 1979D13887 36.13 7.97 1543D13889 35.84 7.48 1580D13891 36.08 8.19 1971D13892 35.78 7.72 1497D13893 35.71 7.72 1442D13896 36.04 7.19 817D13897 35.97 7.30 957D13898 35.89 7.42 1111D13899 35.84 7.48 1179D13900 35.81 7.52 1297M39002-2 36.03 7.78 1209M39003 36.11 7.22 802M39004 36.24 7.73 966M39005-3 36.54 6.74 118M39005-3-7 36.54 6.74 118M39006-1 36.51 6.77 214M39008-4 36.38 7.07 577M39009-1 36.35 7.14 681M39016-1 36.78 7.70 581M39017-3 36.65 7.41 533M39017-5 36.65 7.41 533M39018-1 36.75 7.25 496M39019-2 36.75 8.10 730M39020-1 36.74 8.11 726M39021-5 36.61 8.26 901M39022-1 36.71 8.26 668M39023-3 36.74 8.26 730MSM01/3-136 35.46 9.00 3066MSM01/3-147 35.56 9.51 3859MSM01/3-158 35.79 8.42 2211MSM01/3-181 35.66 7.33 1323MSM01/3-183 35.77 8.26 2094MSM01/3-240 35.29 6.62 401MSM01/3-260 35.37 6.87 574MSM01/3-266 35.34 6.74 569

153M. Rogerson et al. / Marine Geology 280 (2011) 150–167

“H-mode” water of Alvarez et al. (2005) or the North Atlantic CentralWater (NACW) of other authors.

Proportional contributions of these three end-members arecalculated from:

fMOW =θi−θAAWð Þ SNADW−SAAWð Þ− θNADW−θAAWð Þ Si−SAAWð Þ

θMW−θAAWð Þ SNADW−SAAWð Þ− θNADW−θAAWð Þ SMW−SAAWð Þ ð1Þ

fNADW =θi−θAAW

θNADW−θAAW− θMW−θAAW

θNADW−θAAWfMOW ð2Þ

fAAW = 1−fNADW−fMW ð3Þ

Where f represents proportion, θ represents potential tempera-ture, S represents salinity and subscripts indicate that values are forend-member watermasses (MW, AAW and NADW) and sampledwater (i). Calculated values for fNADW are occasionally b0, presumablydue to minor admixture of an end-member watermass warmer than12.8 °C (e.g. Madeira Mode Water). In these cases, a value of 0 isenforced for fNADW and carried forward into Eq. (3).

2.2. Sedimentary parameters

There is some potential for sedimentary carbonate and organiccarbon content or the grain size characteristics of the siliciclasticcomponents to record the distribution of strong bottom currents, andtypically palaeoceanographic assessments of MW have relied uponsuch sedimentary data (Llave et al., 2006; Mulder et al., 2002; Nelsonet al., 1993, 1999; Rogerson et al., 2005, 2006b,c; Schönfeld and Zahn,2000b; Sierro et al., 1999; Toucanne et al., 2007; Voelker et al., 2006).A few cubic centimetres from the core top samples were washedthrough a 63 μm screen. The residue was dried at 60 °C. Grainsizeanalysis on fractions N63 μm were done by hand-sieving duringmicropalaeontological preparation at grainsize intervals 63, 90, 125,150 and 250 μm. The fraction b63 μm was collected in jars,decarbonatised with 20% acetic acid, washed and centrifuged withdeionised water and sodium polyphosphate was added to aconcentration of 0.05%. The samples were left overnight in an orbitalshaker for homogenisation. The grain size distribution of the fractions1.1 to 63.1 μm was measured with a Micrometrics Sedigraph 5100 atIFM-GEOMAR, Kiel, Germany. The precision of the instrument is 1mass percent over the whole grainsize spectrum (Webb and Orr,1997). The analytical reproducibility is up to 9 weight percent for allfractions, and 2% in the 10 to 63 μm (Sortable Silt) fraction (Bianchi etal., 1999). Data for the b63 μm fraction is converted to the MeanSortable Silt (McCave and Hall, 2006; McCave et al., 1995) and UP10(Frigola et al., 2007) parameters for further analysis, which representthe mean grainsize in the 10–63 μm range and the proportion of massN10 μm respectively.

A split of the b63 μm samples was dried and analysed for sedimentcomposition. This approach loses some compositional informationfrom the coarser fraction, particularly from the % carbonate data, butwas necessary to ensure sufficient benthic foraminifera could beextracted from core top samples, as these can be quite sparse withinmaterial from the Gulf of Cadiz. Percentage carbonate is calculatedfrom the mass lost during immersion of the disaggregated sample inexcess 20% acetic acid. Percentage Total Organic Carbon (%TOC) iscalculated from the mass lost from the residue left after aciddissolution during combustion at 550 °C and the mass of theb63 μm fraction before acidification.

2.3. Benthic foraminiferal assemblage data

To allow comparison with previous benthic foraminiferal datafrom the region (Schonfeld, 2002; Schönfeld, 1997, 2001, 2002b), andto reduce the impact of post-mortem reworking (Rogerson et al.,2006a), we report assemblages from the N250 μm fraction. Allindividuals within the sample were identified, or where tests wereabundant at least 300 individuals from an aliquot taken from theoriginal sample using a Green Geological microsplitter. The forami-niferal assemblages comprise the fauna living at the time of samplingand the dead fauna, and these two component assemblages areusually very different (Murray, 2006). The average concentration ofempty foraminiferal tests in Gulf of Cadiz sediments is 4096 speci-mens and the average standing stock is 24 individuals per 10 cm3

(Schönfeld, 2002b). These data imply that only 2 among 300 countedbenthic foraminifera may have been alive at the time of core retrieval.

Table 2Offsets used in compilation of δ18Oepifauna and δ18Oinfauna datasets. Values are takenfrom (Fontanier et al., 2006).

InfaunaTaxon Δδ18O (δ18OU. peregrina - δ18Otaxon)Uvigerina mediterranea +0.26‰Globobulimina affinis −0.15‰

EpifaunaTaxon Δδ18O (δ18OC. kullenbergi - δ18Otaxon)Cibicides lobatulus −0.34‰

154 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

The faunal census data from this study therefore resemble the deadassemblages that would be preserved within the core record.

2.4. Stable isotope analysis

Foraminiferal δ18O and δ13C were measured on aliquots of thecrushed and homogenised tests of 3–10 Cibicidoides kullenbergi,Cibicides lobatulus, Planulina ariminensis, Uvigerina mediterranea,

Fig. 2. a) All Hydro and Core Top Hydro sample locations. Underlain interpolation is fMW fromline contours showing Core Top Hydro dataset. The critical region of no data in the core site d

Uvigerina peregrina parva or Globobulimina spp. Approximately30–100 microgrammes of carbonate were used for isotope analysisusing a GV IsoPrimemass spectrometer plusMultiprep device. Isotopevalues (δ13C, δ18O) are reported as per mille (‰) deviations of theisotopic ratios (13C/12C, 18O/16O) calculated to the VPDB scale using awithin-run laboratory standard calibrated against NBS standards.Analytical reproducibility of the standard calcite (KCM) is b0.1‰ forδ13C and δ18O. Single-species stable isotope datasets cannot begenerated due to the restricted regional distributions of the speciescaused by strong ecological gradients induced by the activity of theMW plume (Schonfeld, 2002; Schönfeld, 1997, 2002b). To maximisethe range of fMW encompassed by single δ18O datasets, data arecombined into compilations of infauna (Uvigerina spp. and Globobu-limina I spp.) and epifauna (Cibicidoides spp., Cibicides spp.) usinginter-species offsets taken from the literature (Table 2).

2.5. GIS

The datasets described above were compiled into a GeographicInformation System within the ArcGIS software. All contour maps

All Hydro dataset; b) fMWwith filled contours showing All Hydro dataset and overlainataset, roughly corresponding to sedimentary Sector 1, is shown as the grey shaded area.

155M. Rogerson et al. / Marine Geology 280 (2011) 150–167

used in this paper were compiled using a squared inverse distanceweighted regression. At least 3 and up to 7 neighbours are included.

2.6. Statistical analysis

Correlation analysis was performed in the SPSS software (V. 16),using Kendall's τb and Spearman's ρ analyses. Both of these analysesare non-dimensional and make no assumption as to the nature ofthe correlation (e.g. linear, exponential). Two-tailed variances aregiven.

3. Results

3.1. Hydrography

The hydrographic data for the Gulf of Cadiz are summarised inFig. 2a, in which sample points for the All Hydro dataset are shown asvertical crosses (+) and sample points for the Core-Site dataset areshown as diagonal crosses (×). Underlying these datasets is a map of

Fig. 3. a) UP10 index; d) Sortable Silt index. Line contour

fMW inferred from the All Hydro dataset. Note that the area where fMW

in the All Hydro exceeds 30% is almost free from locations that areincludedwithin the Core-Site Hydro dataset. This reflects the inherentsampling bias in any core-based assessment of the MW plume, asdescribed in Section 1.2. Potential temperature (Online supportingFig. S1a) and salinity (Fig. S1b) show significant deviation betweenthe two datasets. The distribution of fMW in the Core-Top Hydrodataset (Fig. 2b) also yields a significantly different pattern than theAll Hydro dataset, in that it more definitely resolves the upper core ofthe plume (MW1); (O'Neill-Baringer and Price, 1997, 1999) than thelower core (MW2) giving the impression that the MW core flows atshallower depth than is the case in All Hydro. Despite not resolvingMW2 well, the Core-Site Hydro dataset also indicates extension ofdilute MW significantly further downslope in the region between35.75 and 36 °N, 8–8.5 °W than the All Hydro dataset. However, theCore Top Hydro dataset does resolve fNADW reasonably well, probablyreflecting a higher density of samples in the area where fNADWgradients are steepest (Fig. S1c). The sensitivity of these hydrographicreconstructions both to sampling density and distribution of sampled

s overlain are fMW from the Core Top Hydro dataset.

Fig. 4. a) % Calcium Carbonate; b) Total Organic Carbon. Line contours overlain are fMW from the Core Top Hydro dataset.

156 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

locations is a very significant complication to any attempt at core-based reconstruction, and will need to be taken into account for anyassessment of palaeohydrography in this area.

3.2. Sedimentary parameters

Sand only dominates on the upper slope, with silt and claygenerally increasing with depth (Fig. S2a). Coarse sand (N150 μm;Fig. S2b) is concentrated in two areas, which coincide with theboundary of sedimentary Sector 1 and the deeper channels in Sector 3.The UP10 index reflects the same trends (Fig. 3a), but more closelyreflects the main pathways of MW through zones 1, 2 and 3. Sortablesilt (Fig. 3b) shows peak values close to 35.8 °N, 7.5 °W and 36.6 °N,6.7 °W and 36.7 °N, 7.7 °W and minima close to 36 °N, 7 °W and36.1 °N, 8.3 °W; a distribution that shows little apparent relationship

Fig. 5. Benthic foraminiferal data; a) % of assemblage Elevated Epifauna; b) % of assemblagoverlain are fMW from the Core Top Hydro dataset.

with fMW. Similarly, the percentage mass of carbonate in the b63 μmfraction shows a distribution little like that of fMW (Fig. 4a).

The percentage mass of organic carbon in the b63 μm fraction(Fig. 4b) shows some correspondence to the distribution of fMW, withmaximum TOC occurring at the lower limit of MW2, probably reflectingthe behaviour of refractory organic matter as a sedimentary particle(very low density and consequent low settling velocity). Unlike othersedimentary particles, organic carbon remains entrained within theMediterranean Outflow plume beyond the point at which it achievesneutral buoyancy in the westernmost Gulf of Cadiz, reflecting its hightransportationpotential (Ambar et al., 2002; Freitas andAbrantes, 2002).

3.3. Benthic foraminiferal assemblages

Over 100 taxa were identified within the core top samplesassemblages, which are summarised in Supporting Table 1. For spatial

e Shelf-Dwelling fauna; c) % of assemblage Anoxic Infauna. In all cases, line contours

157M. Rogerson et al. / Marine Geology 280 (2011) 150–167

158 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

mapping we combined individual taxa into conceptual groups. The“Elevated Epifauna” group (Cibicides spp, Discannomalina spp, P.ariminensis and Valvulina pennulata) was defined by Schönfeld (1997,2002b) and reflects filter feeders specialised for environments withstrong bottom currents, such as the core of the MediterraneanOutflow plume. The proportional presence of this group in the deadassemblage has previously been proposed as a proxy for palaeo-velocity (Schönfeld, 2002a). The “shelf dwelling taxa” group (Ammo-nia spp., Cancris spp., Discorbis spp., Elphidium spp., Hanzawaiaboueana (H. rhodiensis of authors), Hyalinea balthica, Planorbulinamediterranensis and Rosalina spp.) are organisms known to live only atshallower depths than represented by the sampled sites (Murray,2006; Villanueva-Guimerans, 2000; Villanueva-Guimerans andCervera-Currado, 1999; Villanueva-Guimerans et al., 1999), andwhich are therefore expected to have been reworked from the shelf.The “Anoxic Infauna” group (Chilostomella ovata, Bulimina costata andGlobobulimina spp.) consists of known facultative anaerobes (Jorissenet al., 1995), and will reflect areas with low oxygenation of near-surface pore waters. This group has also previously been shown toreflect the distribution of transported refractory organic matter in thedistal parts of submarine canyons (Fontanier et al., 2005; Rogerson etal., 2006a).

The “Elevated Epifauna” group (Fig. 5a) shows a distributionsimilar to fMW (Fig. 2b), but like UP10 reflects the pathways of MWthrough Zones 2, 3 and 4, where flow velocity is enhanced by localtopographic effects, rather than exclusively the distribution ofMediterranean Water. A single taxon (C. lobatulus) dominates thisgroup, contributing more than half (mean=56%) of the total numberof individuals, with the only other major contributor beingP. ariminensis. Shelf-Dwelling taxa occur in two peaks, one abovethe MW1 core and one within the sandy parts of sedimentary Sectors2 and 3. The shallow sample reflects peak abundance in Ammonia spp.and Cancris spp. and probably reflects unaltered shelf assemblages,whereas the deeper peak reflects a significant flux of tests downthe slope with distribution linked to the highest velocity parts of theMW (Fig. 5b), and dominantly comprises Elphidium spp. andP. mediterranensis. The major channel in Zone 2 (the Gil EanesChannel) is particularly well resolved by mapping transportedindividuals, but these taxa occur as a belt crosscutting the pathwayof MW1 in the northeast, probably reflecting a transitional regionbeyond which the flow is no longer able to maintain these tests insuspension. The “Anoxic Infauna” group show a prominent maximumin the lower parts of the MW flow core in the same region as whereTOC reaches maximum values (Fig. 5c), though analysis of correlationindicates no significant relationship between these parameters.

3.3.1. Distribution of individual taxaIn addition to these groups, certain individual taxa show distribu-

tions of potential significance for constraining the past distribution ofMediterraneanWater. Planulina ariminensis (Fig. 6a), though includedas part of the Elevated Epifauna group, shows a clear preference forupper slope depths (b800 m) regardless of whether these locationsare within high fMW (northern area of Fig. 6a) or low fMW regions(south-eastern area of Fig. 6a), though it should be noted that thelatter is represented by only a single datapoint. In combination withother parameters this taxon is nonetheless potentially useful as anindicator for the geostrophic (MW1) limb of the MW. Martinotiellacommune (Fig. 6b), though not a member of the Elevated Epifaunagroup, shows a highly analogous pattern. Pullenia bulloides (Fig. 6c)rarely contributes N2% of the assemblage, but shows a distinctpreference for low fMW areas regardless of depth.

There is also significant spatial variability within the ShelfDwelling taxa group. Fig. 7 shows the percentage abundance of themost common of these taxa (Elphidium crispum, Elphidium macellum,P. mediterranensis, Ammonia spp. and Cancris spp.) within samplesordered according to their percentage of Elevated Epifauna; this

effectively places them with bottom velocity increasing to the right(Schonfeld, 2002). A significant difference in the peak abundances ofthese taxa is evident within this dataset. Ammonia spp. and Cancrisspp. both well reflect the bimodal distribution of the Shelf-Dwellingfauna group, with local maxima in both low- and high-velocity areas.Maximum percentage of Elphidium spp. occurs where ElevatedEpifauna are most common, with E. crispum abruptly tailing off to~0% outside of the highest velocity sites. Elphidium macellum extendsfurther down the velocity gradient, being present in low numbers atabout half of the sites. Planorbulina mediterranensis shows a distinctmaximum in the zone where E. macellum is present but E. crispum isnot, and reaches peak values N40% of all tests N250 μm in these areas.For completeness Globobulimina spp. are also shown, indicating peakvalues immediately abutting the P. mediterranensis zone, againreflecting a relationship with transported organic matter, and anotherpeak in the ~0 ms−1 parts of the system.

3.4. Stable isotope analyses

For δ18Ocalcite both in epifauna and infauna theMediterraneanWaterresolves as an isotopically heavy band of water (Fig. 8a and b) in thenorthern Gulf of Cadiz. Though the P. ariminensis dataset (Fig. 8c) isspatially restricted to the northernmost part of the Gulf of Cadiz, MW isalso resolved as an isotopically heavy watermass in the north of theregion studied. However, for the epifauna and infauna (Fig. 8a and b),this picture is complicated by isotopically similar water lying in thecentral and southern Gulf of Cadiz. This watermass is NADW which isisotopically light (δ18Owater ~+0.16‰ (Schmidt et al., 1999)) but is alsocold (~2 °C). Thus it exhibits a relatively high equilibrium value forcalcite (δ18OEpifauna=+2.41‰). This is consistent with the expectedequilibrium behaviour of calcite (Fig. 9), as the balance of δ18Owater andtemperature influences result in calcites growingwithin theMWhavingan intermediate value (δ18Oequilibrium calcite=+2.35‰) betweenNACW (=+1.4‰) and NADW (=+3.26‰). The spatial consequenceof this is that the MW:NACW mixture that dominates within theMediterranean Outflow plume outside of the MW1 core presents as aprominent isotopicminimumwithin the epifauna and infauna δ18Ocalcite

maps.δ13Ccalcite in all species (Figs. S3a–f) show a spatial association with

TOC, with low values occurring on the downslope margin of the plumewhere TOC is high. This is consistentwith the view that remineralisationof this organic matter may be reflected in this parameter. However, thismay indirectly result in this parameter having someassociationwith thedistribution of MW via the particle hydraulic behaviour of organicmatter, with the MW plume presenting as a local isotopic maximum(see northwestern area in Figs. S3a–f).

3.5. Analysis of correlation

Non-parameteric correlations between selected core top data andθi, Si and fMW are shown in Table 3, with correlations significant at the95% confidence level shown in italics and at the 99% confidence levelin bold. At both confidence levels, more core top parameters correlatewith temperature (respectively for τb and ρ, 8 and 7 at 95–99%; 9 and9 at N99%) and salinity (6 and 5 at 95–99%; 10 and 11 at N99%)separately than do with fMW (6 and 4 at 95–99%; 1 and 3 at N99%),indicating significant complexity in the relationship between the coretop parameters and watermass distribution. This complexity probablyarises from the fact that although temperature may have amechanistic relationship with certain parameters (e.g. specificforaminiferal distributions or δ18Ocalcite), the relationship with fMW

can only be indirect and is subject to potentially competingcomponents (e.g. temperature and velocity). Only the percentageoccurrence of Spiroplectinella wrightii is correlated to fMW at 99%confidence using both methods. Of the sedimentary parameters,warm and salty bottomwater correlates reliably with highmass in the

Fig. 6. a) % of assemblage P. ariminensis; b) % of assemblageM. commune; c) % of assemblage P. bulloides. In all cases, line contours overlain are fMW from the Core Top Hydro dataset.

159M. Rogerson et al. / Marine Geology 280 (2011) 150–167

Fig. 7. Distribution of individual benthic foraminiferal taxa along the velocity gradientindicated by % Elevated Epifauna.

160 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

150–250 μm range and low mass in the b63 μm range. The isotopicmeasurements perform notably poorly, with only δ18Ocalcite inepifauna and δ13C in the spatially restricted P. ariminensis dataset(N=13) showing N95% correlation to fMW.

4. Discussion

4.1. Qualitative assessment of fMW using core data

As with previous assessments of the Gulf of Cadiz slope (Faugèresand Stow, 1993; Faugères et al., 1984; Gonthier et al., 1984; Habgood etal., 2003; Hernandez-Molina et al., 2003, 2006; Lopez-Galindo et al.,1999; Marches et al., 2007; Nelson et al., 1993, 1999; Schonfeld, 2002;Schönfeld, 2002b; Stow et al., 1986), the data presented in the previoussection show that the presence of the Mediterranean Outflowdominates the distribution of all the core top parameters investigated.However, no single parameter is completely successful in recording theconcentration of MW. An attempted reconstruction of changes in theflowpath of the MW in the path will therefore require a multi-proxyapproach. Qualitatively, UP10 index, the percentage mass in the 150–250 μm range, TOC, δ18Oepibenthic, percentage of benthic foraminifera inthe Elevated Epifauna and Anoxic Infauna groups, the distribution ofindividual shelf taxa (especially Elphidium spp. and P. mediterranensis),percentage P. ariminensis and also taxa significantly correlating to highfMW (B. elongata, S. wrightii andM. commune) and low fMW (C. bradyi andP. bulloides) are the most useful parameters for consideration.

For the purposes of palaeoceanography, it is therefore useful toconsider a qualitative zonation of the Gulf of Cadiz slope fromwhich adescriptive analysis of the past distribution of watermasses can beinferred. As a spatial pattern, this can be used to constrain relativechange in past systems compared to the present and to provide aqualitative target for circulation modelling efforts. On the basis of ourdata, we propose an 8-fold zonation with the focus on resolving thehydraulic behaviour of the Mediterranean water. The distribution ofthese zones for core top data are shown in Fig. 10.

4.1.1. Zone 1; proximal MW (fMW N0.35)Key characteristics; erosion or hiatus. As with canyon and fan

systems (Rogerson et al., 2006a), areas presenting no data (i.e.absence of deposits or micropalaeontology) represent an end-member of the system under consideration and should not beexcluded from analysis. In the modern Gulf of Cadiz, this zone occurswithin the channel leading from the Gibraltar Strait, in the vicinity ofthe termination of that channel and patchily throughout Sectors 1 and3 of Hernandez-Molina et al. (2003, 2006). For erosion to take place,maximum sheers imparted by the flow of MW must exceed theentrainment sheer of coarse sand, indicating bottom velocities inexcess of 0.5 ms−1 (Pidwirny, 2006).

4.1.2. Zone 2; core MW1 (0.25b fMWb0.35)Key characteristics; sediment≥7.5% by mass≥150 μm. Percentage

abundance P. ariminensis≥5%; δ18OP. ariminensis≥+1‰, δ18OCibicidoides spp.

+1 to +2‰. MW1 is the geostrophic limb of the MW and represents amixture of MW with AAW, without significant admixture of NADW.Today, the core of MW1 traverses the Gulf of Cadiz slope between 500and 700 m, encompassing sedimentary Sector 4 and the upper part ofSector 3 (Hernandez-Molina et al., 2003, 2006). Distinction of this corefrom MW2 is significant to the behaviour of the system of glacial/interglacial times, due to the recognised switching between flows beingdominantly shallow during periods of high sea level and dominantlydeep during periods of low sea level (Nelson et al., 1999; Rogerson et al.,2005; Schönfeld and Zahn, 2000a).

4.1.3. Zone 3; core MW on steep slope (0.2b fMWb0.3)Key characteristics; ≥5% 150–250 μm by mass, UP10≥40% total

mass, abundant Elevated Epifauna (≥15%), abundant reworkedshelf fauna (≥15%), adult E. crispum present, intermediate values inδ18Oepifauna(+1.3 to +2‰) and high values in δ18Oinfauna (+2.5 to+2.7‰). Local velocity within the MW plume is controlled by:

ui = g ρi−ρambientð Þρrð Þαi = f

where ui is local velocity, ρi is local density of MW product water,ρambient is the density of ambient water at the same depth (i.e. eitherthe density of AAW or NADW, depending on location), ρr is areference density, g is the acceleration due to gravity, αi is the localslope of the sediment surface and f is the Coriolis accelerationterm (Price and O'Neill-Baringer, 1994). It is therefore not surprisingthat Sector 3, where slope inclination is ~19×10−3 compared to~7.8×10−3 for Sector 4, velocity indicators (UP10, Elevated Epifauna)show maximum values. Topographic effects mean that the bottomvelocity fields provided by these proxies should be interpreted withcaution because they may be invariable, even under variablehydrography. However, it is useful to map and constrain thesezones of high flow speed in the past as a means of monitoring theinternal behaviour of the MW.

4.1.4. Zone 4; distal MW1 (0.2b fMWb0.3)Key characteristics; P. ariminensis≥5% benthic fauna, ≥2% M.

commune benthic fauna, equilibrium δ18O low (δ18Oinfauna≤+ 2.15‰,δ18Oepifauna≤+1.5‰). In the northern Gulf of Cadiz, the MW1 plumereaches neutral buoyancy and decelerates as a result of continued

Fig. 8. a) δ18Oepifauna; b) δ18Oinfauna; c) δ18OP. ariminensis. In all cases, line contours overlain are fMW from the Core Top Hydro dataset.

161M. Rogerson et al. / Marine Geology 280 (2011) 150–167

fNADW

fAA

W

Fig. 9. Calculated equilibrium values for δ18Ocalcite in the MW-AAW-NADW mixingsystem investigated in this study.

162 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

erosion of it salinity (due to mixing with AAW) and momentum (dueto friction). This is well reflected both in sedimentological datasets(within sedimentary Sectors 4 and 5) and in the coretop dataset. Thecontinued presence of M. commune and P. ariminensis, but lowabundance of Elevated Epifauna, low UP10 and δ18O steadily reducingto the west provides a distinct signature of this part of the plumewithin core records.

4.1.5. Zone 5; distal MW2 (0.1b fMWb0.2)Key characteristics E. crispum absent but P. mediterranensis≥2% of

total fauna, δ18O low (δ18Oinfauna≤+2.15‰, δ18Oepifauna≤+1.5‰).The spatial separation of E. crispum and P. mediterranensis (Fig. 5),which is probably driven by their different particle hydraulicbehaviour, provides a means of discrimination of the upper andlower parts of MW2 in the region it descends the slope (SedimentarySectors 2 and 3). The absence of P. ariminensis and abundance of lightshelf dwelling taxa also provides a clear means of distinguishing distalMW2 from distal MW1, and thus the descending limb of the plumefrom the geostrophic limb.

4.1.6. Zone 6; lower limit of MW (fMW~0.1)Key characteristics; high proportion of Anoxic Infauna (≥15%),

low δ13Cepifauna (≤+1‰) and high TOC (≥10% mass≤63 μm). δ18Olow (δ18Oinfauna≤+ 2.15‰, δ18Oepifauna≤+1.5‰). Measurements ofsuspended sediment within the water column reveal that terrigenousand biogenic sediment is suspended within the MW where it is incontact with the slope, but that it is virtually indistinguishable fromambient Atlantic watermasses once it has reached neutral buoyancy(Ambar et al., 2002). However, MW does retain a significant level ofsuspended particulate organic matter (Freitas and Abrantes, 2002).The sporadic occurrence of faunas dominated by deep infaunal taxa atthe lower limit of the sedimentary drift reflects the deposition of thissuspended organic matter, providing a powerful means of identifyingthe lower limit of MW penetration. This is a particularly importantcharacteristic of the plume to identify, as it constrains how deep theMediterranean water penetrates into the Atlantic watermass.

4.1.7. Zone 7; sub-MW (NADW)Key characteristics; local maximum in δ18O, low Elevated Epifauna

(≤1%), low transported Shelf-Dwelling taxa (≤0.5%), low AnoxicInfauna (≤0.5%). Today, MW is underlain by NADWbut this may havechanged in the past (Shackleton et al., 2000; Skinner et al., 2003).Nevertheless, the most effective means of establishing the character-

istics of this water mass will be from abyssal depths (N3000 m) on theIberian and Moroccan margins.

4.1.8. Zone 8; ambient Atlantic water (fMWb0.1)Key characteristics; local minimum in δ18Oepifauna, low Elevated

Epifauna (≤1%), low Anoxic Infauna (≤0.5%). The watermassoverlying MW is complex, reflecting both recirculated Mediterraneanwater (“MW0”); (Serra et al., 2005) and the Atlantic Inflow current(O'Neill-Baringer and Price, 1999). As with the sub-MW layer, this islikely to have varied through time, with penetration of the AzoresFront during glacial periods (Rogerson et al., 2004) and incursion offreshened surface water during Heinrich Stadials (Rogerson et al.,2010). Consequently, as with the sub-MW layer, the best approach tocharacterising this layer is likely to be via shallow (100–400 m) coresfrom the Iberian margin.

4.2. Quantitative assessment of fMW using core data

For effective synthesis of empirical and simulation methods,qualitative constraint of the MW is insufficient and quantitativeassessment of past fMW distribution must be possible. The failure ofcore top parameters to correlate significantly with fMW (Table 3), theambiguous behaviour of δ18Oequilibrium within the mixing system, theimpact of remineralisation of sedimentary carbon on δ13Cforaminifera,the failure of the Sortable Silt approach to constrain high velocitiesand the clear topographic effect on the Elevated Epifauna and UP10indices are all significant blows to the potential of the parametersinvestigated to perform this function. Nevertheless, coretop δ18Ocalcite

parameters do present a distribution apparently consistent with asignificant level of hydrographic control. It is possible to determinewhether this control is potentially worth utilising for past timeslicesby substituting δ18Ocalcite into Eqs. (1)–(3) in the place of temperatureor salinity, with endmember values determined by calculation fromendmember temperature and δ18Owater values (δ18OMW=+1.50‰;δ18OAAW=+0.38‰; δ18ONADW=+0.16‰) taken from the GlobalSeawater δ18O database (Schmidt et al., 1999). Correlation ofhydrographic fMW and values for fMW derived from sequentialsubstitution of δ18Oepifauna or δ18Oinfauna for θi or Si are shown inFig. 11. Where δ18Ocalcite is substituted for temperature, r2 values are0.745 (n=52) and 0.599 (n=39) for infauna and epifaunarespectively, and t-tests indicate these correlations are significant atthe 0.01 confidence level. Further confirmation of the ability ofδ18Ocalcite and salinity to resolve the distribution of fMW can bedemonstrated via a spatial comparison of calculated fMWwith core tophydrographic data (Fig. 12). Note, however, a false indication offMW N0.2 in the southeastern region of the δ18Oepifauna dataset(Fig. 12b)., perhaps due to confusion caused by a small proportionaladmixture of NADW into AAW or from sporadic presence of meddiesshed from the Portimao canyon or Moroccan margin (Serra et al.,2005) However, where δ18Ocalcite is substituted for salinity, nosignificant correlation is found, reflecting the important ambiguitiesin both θ (MW≈AAW) and δ18Ocalcite (MW≈1:1 mix of AAW andNADW). This analysis indicates that δ18Ocalcite has been proven to bean effective substitute for temperature for analysis of fMW, however itsambiguous behaviour means that successful quantitative palaeocea-nography still relies upon identification of a second, unambiguousconservative tracer.

As the above correlationsmay be affected by theway hydrographicdata has been compiled, we performed an additional independentanalysis comparing data for the “M39” cores with CTD data taken onsite during core recovery (Schonfeld, 2002). This analysis (SupportingFig. S4)makes aminimal difference to the conclusions drawn from theabove, with compilations of δ18O and salinity still performing better(r2=0.36 and 0.65 for infauna and epifauna respectively) thancompilations of δ18O and temperature (r2=0.006 and 0.05 respectively).

Table 3Correlation coefficients of core top parameters with θ, S and fMW from the Core Top Hydro dataset.

Kendall'sτb

T-pot S fMW Spearman'sρ

T-pot S fMOW

Correlationcoefficient

Sig.(2-tailed)

Correlationcoefficient

Sig.(2-tailed)

Correlationcoefficient

Sig.(2-tailed)

Correlationcoefficient

Sig.(2-tailed)

Correlationcoefficient

Sig.(2-tailed)

Correlationcoefficient

Sig.(2-tailed)

N

SS .217 0.025 0.051 0.597 0.138 0.155 .340 0.015 0.095 0.509 0.220 0.120 51UP10 0.094 0.286 0.096 0.275 0.138 0.117 0.139 0.282 0.134 0.299 0.196 0.127 62% mass N250 .188 0.031 0.093 0.288 −0.050 0.564 0.247 0.053 0.141 0.274 −0.078 0.549 62% mass 150–250 .315 0.001 .265 0.002 0.143 0.102 .473 0.000 .426 0.001 0.240 0.061 62% mass 63–90 0.144 0.100 0.102 0.241 0.135 0.121 0.221 0.084 0.156 0.227 0.207 0.106 62% mass b63 −.242 0.005 −.174 0.046 −0.109 0.213 −.384 0.002 −.290 0.022 −0.172 0.181 62TOC content −0.02 0.816 0.161 0.084 −0.102 0.273 −0.025 0.854 0.209 0.126 −0.132 0.335 55Carbonate content 0.178 0.055 0.077 0.408 −0.003 0.971 0.261 0.054 0.117 0.393 0.004 0.979 55δ18OInfauna 0.014 0.884 0.118 0.214 0.084 0.378 0.026 0.856 0.154 0.272 0.125 0.374 53δ18Oepifauna −0.126 0.260 −0.150 0.179 −0.215 0.054 −0.208 0.203 −0.229 0.161 −.319 0.048 39δ13CU. peregrina −.279 0.034 −.435 0.001 −0.037 0.778 −.396 0.034 −.592 0.001 −0.096 0.622 29δ13CU. mediterranea 0.178 0.322 0.163 0.364 −0.074 0.680 0.195 0.453 0.178 0.495 −0.134 0.609 17δ13CCibicidoidids 0.060 0.642 0.053 0.681 0.182 0.159 0.045 0.813 0.053 0.779 0.216 0.252 30δ13CP. ariminensis −0.142 0.501 0.103 0.625 .477 0.024 0 1 0.187 0.541δ13CGlobigerinids 0.251 0.087 0.200 0.172 0.087 0.551 0.346 0.098 0.307 0.144 0.150 0.485 24δ13CC. lobatulus 0.017 0.910 0.052 0.735 0.135 0.381 0.066 0.770 0.055 0.807 0.170 0.451 22B. costata −.310 0.000 −.327 0.000 −0.030 0.729 −.452 0.000 −.487 0.000 −0.0601 0.643 62B. elongata .358 0.001 .358 0.001 .261 0.011 .437 0.000 .441 0.000 .327 0.009 62Cancris spp. .206 0.039 .204 0.040 .254 0.011 .270 0.034 .262 0.040 .331 0.009 62C. laevigata .269 0.007 .223 0.026 0.157 0.118 .340 0.007 .280 0.027 0.198 0.122 62C. lobatulus .257 0.004 .244 0.006 0.077 0.385 .389 0.002 .379 0.002 0.108 0.402 62C. bradyi 0.021 0.844 −0.098 0.350 −.256 0.015 0.025 0.845 −0.115 0.373 −.308* 0.015 62Discorbis spp. .202 0.035 .298 0.002 0.153 0.110 .276 0.030 .413 0.001 0.206 0.108 62K. bradiana −.225 0.022 −.252 0.010 −0.045 0.649 −.303 0.017 −.329 0.009 −0.060 0.642 62M. communis 0.172 0.056 .198 0.028 .186 0.039 0.224 0.080 .280 0.027 .261 0.041 62P. bulloides −0.071 0.475 −0.084 0.395 −.237 0.0164 −0.083 0.519 −0.105 0.416 −.302 0.017 62S. schlumbergeri .222 0.017 0.181 0.051 0.101 0.275 .300 0.018 .253 0.047 0.132 0.307 62S. bulloides 0.150 0.111 .244 0.009 0.086 0.359 0.191 0.137 .345 0.006 0.120 0.352 62S. carinata −0.153 0.121 −.288 0.003 0.018 0.858 −0.203 0.113 −.360 0.004 0.026 0.841 62S. wrightii 0.090 0.390 0.166 0.112 .327 0.002 0.110 0.395 0.203 0.004 .403 0.001 62U. peregrina −.195 0.026 −.238 0.007 −0.059 0.504 −.267 0.036 −.354 0.005 −0.085 0.512 62“Shelf dwellers” .260 0.00 .244 0.005 0.157 0.074 .375 0.003 .357 0.004 0.238 0.063 62“Elevated epifauna” .230 0.009 .175 0.046 0.067 0.447 .337 0.007 .257 0.044 0.106 0.412 62“Anoxic infauna” −.403 0.000 −0.280 0.001 −0.048 0.585 −.566 0.000 −0.416 0.001 −0.080 0.535 62

163M.Rogerson

etal./

Marine

Geology

280(2011)

150–167

Fig. 10. Qualitative Palaeo-hydrographic zones on the modern Gulf of Cadiz slope.

a b

c d

Fig. 11. fMW calculated from Core Top Hydro compared to fMW estimated from a) δ18Oepifauna and θ; b) δ18Oepifauna and Salinity; c) δ18Oinfauna and θ; d) δ18Oinfauna and Salinity.

164 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

Fig. 12. fMW estimated from Core Top Hydro salinity and a) δ18Oepifauna; b) δ18Oinfauna.

165M. Rogerson et al. / Marine Geology 280 (2011) 150–167

166 M. Rogerson et al. / Marine Geology 280 (2011) 150–167

4.3. Perspectives for palaeoceanographic analysis of other mixingsystems

It is clear that palaeoceanographic analysis of regions with strongbottom currents is not straightforward, even where hydrographicgradients are highly significant. Despite spatial patterns generallyreflecting in situ conditions rather than transportation, all the core-based parameters used in this study effectively integrate competinginfluences, only some of which reflect regional hydrography.Indicators of flow speed (grainsize, Elevated Epifauna, transportedforaminifera) reflect slope steepness as well as negative buoyancy inthe bottom water, ecological indicators such as the benthic forami-nifera assemblages indicate the pelagic flux of fresh organic particlesand the advective supply of sedimentary carbon by the MWcomplicating the relationship of individual taxa. The isotopic tracersare defective either because they are non-conservative (δ13C) orbecause they are ambiguous (δ18O). It would be expected that othersedimentary drift systems would present similar complexity, makingconstraint of past hydrography in these settings potentially extremelychallenging. Selection of the parameters from which palaeohydrologyis carried out is of critical importance, and it is only through exercisessuch as that presented here that effective and ineffective parameterscan be distinguished.

5. Conclusions

Despite evidence of postmortem transportation of microfossils,grainsize, sediment composition and benthic foraminiferal assem-blage data provide a secure means by which to qualitatively subdividethe Gulf of Cadiz slope into zones reflecting different proportionalpresence, flow speed and settling behaviour of the MediterraneanOutflow. P. mediterranensis on the Gulf of Cadiz slope is considered tobe reworked from the shelf environment, and correlates well withother taxa of similar ecology (e.g. Elphidium spp.). However, its thinwall and flat morphology make it particularly transportable allowingit to be carried into the distal parts of the descending limb of theplume (MW 2). It is generally not abundant within the along-slopeflowing (i.e. geostrophic limb of the plume (MW1)). Planulinaariminensis, however, though part of the Elevated Epifauna groupseems to be limited inwater depth to b800 m,making it common onlywithin the geostrophic limb of the flow (MW1). Critically, therelatively abundances of these taxa do allow discrimination betweendynamically different parts of the plume.

Peaks in TOC, abundance of “anoxic” infauna and sporadically lowδ13Cinfauna at the lower limit of the sediment drift indicates thatrefractory organic matter accumulates here in close analogy to similarbehaviour in the outer parts of submarine canyons (Fontanier et al.,2005; Rogerson et al., 2006a). Identification of this dysoxic “halo”zone at the edge of the plume activity could prove vital inmapping thedistribution of Mediterranean water on the Iberian margin in the past,giving greater emphasis to the report of unique associations of benthictaxa at this transition in living assemblages on the Portuguese slope(Schönfeld, 1997).

Stable isotopes (δ13C and δ18O) prove to be inadequate to constrainthedistributionofMediterraneanwater on theGulf of Cadiz slope today,and, unless otherwise demonstrated, almost certainly are unreliable inthe past too. However, δ18Ocalcite can be combined with bottom salinityfor prediction of fMWwith asmuch success as a bottom temperature. Thestrong implication of this finding is that combined analysis of δ18Ocalcite

with a second, more explicit tracer of Mediterranean water may provesuccessful for palaeohydrography.

Sediment drifts are attractive locations for palaeoceanographersdue to their strong hydrographic gradients and the importance ofthese locations to basin scale ocean dynamics. However, this studyindicates that data must be approached with caution, especially whenonly single locations are studied. Selection of parameters appropriate

to the specific mixing system under study is vital, and realistically canonly be made on the basis of analysis of parameters in core-topdatasets relative to modern hydrography.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.margeo.2010.12.008.

References

Alvarez, M., Perez, F.F., Shoosmith, D.R., Bryden, H.L., 2005. Unaccounted role ofMediterranean Water in the drawdown of anthropogenic carbon. Journal ofGeophysical Research-Oceans 110 (C9).

Ambar, I., Howe, M.R., 1979. Observations of the Mediterranean Outflow II — the deepcirculation in the vacinity of the Gulf of Cadiz. Deep-Sea Research 26A, 555–568.

Ambar, I., et al., 2002. Physical, chemical and sedimentological aspects of theMediterranean outflow off Iberia. Deep-Sea Research II 49, 4163–4177.

Bianchi, G.G., Hall, I.R., McCave, I.N., Joseph, L., 1999. Measurement of the sortable siltcurrent speed proxy using the Sedigraph 5100 and Coulter Multisizer IIe: precisionand accuracy. Sedimentology 46 (6), 1001–1014.

Bigg, G.R., Jickells, T.D., Liss, P.S., Osborn, T.J., 2003. The role of the oceans in climate.International Journal of Climatology 23 (10), 1127–1159.

Borenas, K.M., Wahlin, A.K., Ambar, I., Serra, N., 2002. The Mediterranean outflowsplitting — a comparison between theoretical models and CANIGO data. Deep-SeaResearch II 49, 4195–4205.

Davies, P.A., Guo, Y., Rotenberg, E., 2002. Laboratory model studies of MediterraneanOutflow adjustments in the Gulf of Cadiz. Deep-Sea Research II 49, 4207–4223.

Faugères, J.C., Stow, D.A.V., 1993. Bottom-current-controlled sedimentation: a synthesisof the contourite problem. Sedimentary Geology 82, 287–297.

Faugères, J.C., Gonthier, E., Stow, D.A.V., 1984. Contourite drift remolded by deepMediterranean outflow. Geology 12, 296–300.

Faugères, J.C., Gonthier, E., Peypouquet, J.P., Pujol, C., Vergnaud-Grazzini, C., 1986.Distribution et variations des courants de fond sur la ride, de Faro (Golfe de Cadiz),temoins des modifications de échanges Mediterranee Atlantique au QuaternaireRecent. Bulletin Société Géologique de France 2 (3), 423–432.

Fontanier, C., et al., 2005. Live foraminiferal faunas from a 2800 m deep lower canyonstation from the Bay of Biscay: faunal response to focusing of refractory organicmatter. Deep-Sea Research I 52 (7), 1189–1227.

Fontanier, C., et al., 2006. Stable oxygen and carbon isotopes of live benthic foraminiferafrom the Bay of Biscay: microhabitat impact and seasonal variability. MarineMicropaleontology 58 (3), 159–183.

Freitas, P.S., Abrantes, F., 2002. Suspended particulate matter in the Mediterraneanwater at the Gulf of Cadiz and off the southwest coast of the Iberian Peninsula.Deep-Sea Research Part II-Topical Studies in Oceanography 49 (19), 4245–4261.

Frigola, J., et al., 2007. Holocene climate variability in the westernMediterranean regionfrom a deepwater sediment record. Paleoceanography 22 (2).

Gonthier, E., Faugères, J.C., Stow, D.A.V., 1984. Contourite Facies of the Faro Drift, Gulf ofCadiz. In: Stow, D.A.V., Piper, D.J. (Eds.), Fine-grained Sediments: Deep-waterProcesses and Facies. Geological Society Special Publication, London, pp. 275–292.

Habgood, E., et al., 2003. Deep-water sediment wave fields, bottom current sandchannels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic.Sedimentology 50, 1–27.

Hall, I.R., et al., 2006. Accelerated drawdown of meridional overturning in the late-glacialAtlantic triggered by transient pre-H event freshwater perturbation. GeophysicalResearch Letters 33 (16).

Hernandez-Molina, J., et al., 2003. Looking for clues to palaeoceanographic imprints: adiagnosis of the Gulf of Cadiz contourite depositional systems. Geology 31 (1), 19–22.

Hernandez-Molina, F.J., et al., 2006. The contourite depositional system of the Gulf ofCadiz: a sedimentary model related to the bottom current activity of theMediterranean outflow water and its interaction with the continental margin.Deep-Sea Research Part Ii-Topical Studies in Oceanography 53 (11–13), 1420–1463.

Iorga, M.C., Lozier, M.S., 1999. Signatures of the Mediterranean outflow from a NorthAtlantic climatology 1. Salinity and density fields. Journal of Geophysical Research104 (C11), 25,985–26,009.

Johnson, G.C., Lueck, R.G., Sanford, T.B., 1994. Stress on the Mediterranean OutflowPlume: part II. Turbulent dissipation and shear measurements. Journal of PhysicalOceanography 24, 2084–2092.

Johnson, J., Ambar, I., Serra,N., Stevens, I., 2002. Comparative studies of the spreadingof theMediterraneanwater through the Gulf of Cadiz. Deep-Sea Research II 49, 4179–4193.

Jorissen, F.J., deStigter, H.C., Widmark, J.G.V., 1995. A conceptual model explainingbenthic foraminiferal microhabitats. Marine Micropalaeontology 26, 3–15.

Jungclaus, J.H., Mellor, G.L., 2000. A three-dimensional model study of the Mediterraneanoutflow. Journal of Marine Systems 24, 41.

Kenyon, N.H., Belderson, R.H., 1973. Bed forms of the Mediterranean undercurrentobserved with side-scan sonar. Sedimentary Geology 9, 77–99.

Kudrass, H.R., 1973. Sedimentation am Kontinentalhang vor Portugal und Marokko imSpatpleistozan und Holozan. “Meteor” Forsch.-Ergebn., C(13): 1-63.

Llave, E., et al., 2006. High-resolution stratigraphy of the Mediterranean outflowcontourite system in the Gulf of Cadiz during the late Pleistocene: the impact ofHeinrich events. Marine Geology 227 (3–4), 241–262.

Lopez-Galindo, A., Rodero, J., Maldonado, A., 1999. Subsurface facies and sedimentdispersal patterns: southeastern Gulf of Cadiz, Spanish continental margin. MarineGeology 155, 83–98.

Lynch-Stieglitz, J., Curry, W.B., Slowey, N., 1999. Weaker gulf stream in the FloridaStraits during the Last Glacial Maximum. Nature 402.

167M. Rogerson et al. / Marine Geology 280 (2011) 150–167

Marches, E., et al., 2007. Contourite drift construction influenced by capture ofMediterranean Outflow Water deep-sea current by the Portim(a)over-tildeosubmarine canyon (Gulf of Cadiz, South Portugal). Marine Geology 242 (4), 247–260.

McCave, I.N., 2002. A poisoned chalice? Science 298, 1186–1187.McCave, I.N., Hall, I.R., 2006. Size sorting in marine muds: processes, pitfalls, and

prospects for paleoflow-speed proxies. Geochemistry Geophysics Geosystems 7.McCave, I.N., Manighetti, B., Robinson, S.G., 1995. Sortable silt and fine sediment size/

composition slicing: parameters for palaeocurrent speed and palaeoceanography.Paleoceanography 10 (3), 593–610.

McManus, J.F., Francois, R., Gherardi, J.M., Keigwin, L.D., Brown-Leger, S., 2004. Collapseand rapid resumption of Atlantic meridional circulation linked to deglacial climatechanges. Nature 428 (6985), 834–837.

MEDATLAS, 2002. MEDATLAS/2002 database. Mediterranean and Black Sea database oftemperature salinity and bio-chemical parameters. Climatological Atlas: IFREMEREdition. MEDAR Group

Mulder, T., et al., 2002. Studying past deep-ocean circulation and the Palaeoclimaterecord in the Gulf of Cadiz. EOS 83, 481–488.

Murray, J., 2006. Ecology and Applications of Benthic Foraminifera. CambridgeUniversity Press, Cambridge.

Nelson, C.H., Baraza, J., Maldonado, A., 1993. Mediterranean undercurrent sandycontourites, Gulf of Cadiz, Spain. Sedimentary Geology 82, 103–131.

Nelson, C.H., et al., 1999. Influence of the Atlantic Inflow and Mediterranean outflowcurrents on Late Quaternary sedimentary facies of the Gulf of Cadiz continentalmargin. Marine Geology 155, 99–129.

O'Neill-Baringer, M., Price, J.F., 1997. Mixing and spreading of the Mediterraneanoutflow. Journal of Physical Oceanography 27, 1654–1677.

O'Neill-Baringer, M., Price, J.F., 1999. A review of the physical oceanography of theMediterranean outflow. Marine Geology 155, 63–82.

Ozgokmen, T.M., Chassignet, E.P., Rooth, C.G.H., 2001. On the connection between theMediterranean Outflow and the Azores current. Journal of Physical Oceanography31, 461–480.

Pidwirny, M., 2006. Erosion and Deposition, Fundamentals of Physical Geography2ndedition.

Price, J.F., O'Neill-Baringer, M., 1994. Outflows and deep water production by marginalseas. Progress in Oceanography 33, 161–200.

Price, J.F., et al., 1993. Mediterranean Outflow Mixing and Dynamics. Science 259,1277–1282.

Reid, J.L., 1979. On the contribution of the Mediterranean Sea outflow to theNorwegian–Greenland Sea. Deep-Sea Research 26A, 1199–1223.

Robinson, S.G., McCave, I.N., 1994. Orbital forcing of bottom-current enhanced sedimen-tation on Feni Drift, NE Atlantic, during the Mid-Pleistocene. Paleoceanography 9 (6),943–972.

Rodero, J., Pallares, L., Maldonado, A., 1999. Late Quaternary seismic facies of the Gulf ofCadiz Spanish margin: depositional processes influenced by sea-level change andtectonic controls. Marine Geology 155, 131–156.

Rogerson, M., 2003. Palaeoceanography and Sedimentology of the Gulf of Cadiz; 30–0 kaBP. PhD Thesis, University of Southampton, Southampton.

Rogerson, M., Rohling, E.J., Weaver, P.P.E., Murray, J.W., 2004. The Azores Front since theLast Glacial Maximum. Earth and Planetary Science Letters 222, 779–789.

Rogerson, M., Rohling, E.J., Weaver, P.P.E., Murray, J.W., 2005. Glacial to InterglacialChanges in the Settling Depth of the Mediterranean Outflow Plume. Paleoceano-graphy 20. doi:10.1029/2004PA001106.

Rogerson, M., et al., 2006a. Benthic Foraminifera of a Miocene Canyon and Fan. MarineMicropalaeontology 60, 295–318.

Rogerson, M., et al., 2006b. Colour Logging as a Tool in High-resolution Palaeoceano-graphy. In: Rothwell, R.G. (Ed.), New Ways of Looking at Sediment Cores and CoreData. Geological Society Special Publications, London, pp. 99–112.

Rogerson, M., Rohling, E.J., Weaver, P.P.E., 2006c. Promotion of meridional overturningby Mediterranean-derived salt during the last deglaciation. Paleoceanography 21.doi:10.1029/2006PA001306.

Rogerson, M., et al., 2010. Enhanced Mediterranean-Atlantic Exchange During AtlanticFreshening Phases. Geochemistry Geophysics Geosystems 11, 22.

Schmidt, G.A., Bigg, G.R., Rohling, E.J., 1999. Global Seawater Oxygen-18 Database.Schonfeld, J., 2002. A new benthic foraminiferal proxy for near-bottom current

velocities in the Gulf of Cadiz, northeastern Atlantic Ocean. Deep-Sea Research PartI-Oceanographic Research Papers 49 (10), 1853–1875.

Schönfeld, J., 1997. The impact of the mediterranean outflow water (MOW) on benthicforaminiferal assemblages and surface sediments at the southern Portugesecontinental margin. Marine Micropalaeontology 29, 211–236.

Schönfeld, J., 2001. Benthic foraminifera and pore water oxygen profiles: a re-assessment of species boundary conditions at the western Iberian margin. Journalof Foraminiferal Research 31, 86–107.

Schönfeld, J., 2002a. A new benthic foraminiferal proxy for near-bottom currentvelocities in the Gulf of Cadiz, northeastern Atlantic Ocean. Deep-Sea Research I 49,1853–1875.

Schönfeld, J., 2002b. Recent benthic foraminiferal assemblages in deep high-energyenvironments from the Gulf of Cadiz (Spain). Marine Micropalaeontology 853, 1–22.

Schönfeld, J., Zahn, R., 2000a. Late Glacial to Holocene history of the MediterraneanOutflow. Evidence from benthic foraminiferal assemblages and stable isotopes at thePortugese margin. Palaeogeogrpahy, Palaeoclimatology, Palaeoecology 159, 85–111.

Schönfeld, J., Zahn, R., 2000b. Late Glacial to Holocene history of the MediterraneanOutflow. Evidence from benthic foraminiferal assemblages and stable isotopes at thePortuguese margin. Palaeogeography Palaeoclimatology Palaeoecology 159 (1–2),85–111.

Serra, N., Ambar, I., Kase, R.H., 2005. Observations and numerical modelling of theMediterranean outflow splitting and eddy generation. Deep-Sea Research Part Ii-Topical Studies in Oceanography 52 (3–4), 383–408.

Shackleton, N., Hall, M.A., Vincent, E., 2000. Phase relationships between millennial-scale events 64,000–24,000 years ago. Paleoceanography 15 (6), 565–569.

Sierro, F.J., Flores, J.A., Baraza, J., 1999. Late glacial to recent palaeoenvironmentalchanges in the Gulf of Cadiz and formation of sandy contourite layers. MarineGeology 155, 157–172.

Skinner, L.C., Shackleton, N.J., Elderfield, H., 2003. Millennial-scale variability of deep-water temperature and δ18Odw indicating deep-water source variations in theNortheast Atlantic, 0–34 cal. ka BP. Geochem. Geophys. Geosyst. 4.

Stanford, J.D., et al., 2006. Timing of meltwater pulse 1a and climate responses tomeltwater injections. Paleoceanography 21 (4). doi:10.1029/2006PA001340.

Stow, D.A.V., Faugères, J.C., Gonthier, E., 1986. Facies distribution and textural variationin Faro Drift contourites: velocity fluctuation and drift growth. Marine Geology 72,71–100.

Toucanne, S., et al., 2007. Contourites of the Gulf of Cadiz: a high-resolution record of thepaleocirculation of the Mediterranean outflow water during the last 50,000 years.Palaeogeography Palaeoclimatology Palaeoecology 246 (2–4), 354–366.

Villanueva-Guimerans, P., 2000. Bolivinidae y Buliminidae (foraminíferos bentónicos)del margen septentrional del Golfo de Cádiz (España). Revista Espanola deMicropalaeontologia 32, 131–141.

Villanueva-Guimerans, P., Cervera-Currado, J.L., 1999. Distribution of Planorbulinacea(benthic foraminifera) assemblages in surface sediments on the northernmargin ofthe Gulf of Cadiz. Boletín Instituto Español de Oceanografía 15 (1–4), 181–190.

Villanueva-Guimerans, P., Canudo-Sanagustin, J.I., Cervera-Currado, J.L., 1999. Losforaminíferos bentónicos recientes de la superfamilia Miliolacea Ehrenberg, 1839en los sedimentos superficiales del margen septentrional del Golfo de Cádiz.Revista Espanola de Micropalaeontologia 31, 99–122.

Voelker, A.H.L., et al., 2006. Mediterranean outflow strengthening during northernhemisphere coolings: a salt source for the glacial Atlantic? Earth and PlanetaryScience Letters 245 (1–2), 39–55.

Webb, P.A., Orr, C., 1997. Analytical methods in fine particle technology. Micromeritics.301 pp.