Trace metals in Holocene coastal peats and their relation to pyrite formation (NW Germany)

Ž .Chemical Geology 182 2002 423–442www.elsevier.comrlocaterchemgeo

Trace metals in Holocene coastal peats and their relation to pyritež /formation NW Germany

Olaf Dellwig a, Michael E. Bottcher b, Marcus Lipinski a, Hans-Jurgen Brumsack a,)¨ ¨a ( )Institute of Chemistry and Biology of the Marine EnÕironment ICBM , Carl Õon Ossietzky UniÕersity of Oldenburg, P.O. Box 2503,

D-26111 Oldenburg, Germanyb Max Planck Institute for Marine Microbiology, Department of Biogeochemistry, Celsiusstr.1, D-28359 Bremen, Germany

Received 8 September 2000; accepted 7 May 2001

Abstract

Three drill cores from the marshlands of NW Germany, which cover the entire Holocene, were analyzed at high-resolu-tion for bulk composition, Al, Fe, selected trace metals, and stable sulfur isotopes. The drill cores contain two lithological

Ž . Ž .types of peat: i basal peats overlying Pleistocene sands and ii intercalated peats situated between clastic sediments ofpredominantly marine origin. The peat layers are characterized by distinct enrichments in pyrite due to microbial sulfatereduction under almost open system conditions with respect to seawater sulfate as shown by sulfur isotope partitioning. Themain Fe source seems to be the freshwater environment. The determination of dissolved and particulate Fe of channels andsmall rivers close to the study area revealed a 50-fold higher Fe content of the freshwater environment when compared with

Ž .North Sea water. Pyrite enrichments are explained by two scenarios: i pyrite formation coincides with fen reed peat growthŽ . Ž . Ž .basal and intercalated under the influence of a brackish water zone salinity app. 5–15 and ii pyrite was formed after

Ž .peat growth in the lowest limnic basal peat intervals. Maximum pyrite accumulation TS 28% occurs in latter peats thatcontain thin clastic layers as a result of tidal channel activities after peat formation. The occurrence of clastic layers mayhave favoured the inflow of saline groundwater. The peat layers are also characterized by enrichments in redox-sensitive

Ž .trace metals As, Mo, Re, U and Cd, whereas Co, Cr, Cu, Mn, Ni, Pb, Tl, and Zn reflect the geogenic background.Leaching experiments have shown that As, Co, Cu, Mo, Re, and Tl are predominantly fixed as sulfides andror incorporatedinto pyrite. The remaining trace metals show no distinct trends, only Cr reveals a strong relation to the lithogenic detritus.Seawater is the dominating source for As, Cd, Mo, Re, and U. The remaining trace elements seem to have a freshwatersource similar to Fe. In contrast to the distribution of pyrite, highest amounts of redox-sensitive trace metals are seen in fen

Ž .reed peats basal and intercalated that were formed under a direct influence of seawater and brackish water, respectively.Therefore, we suggest that saline groundwater entering the basal peats was probably depleted in redox-sensitive trace metals,e.g. owing to microbially induced reduction of trace metals and subsequent precipitation as sulfides or fixation by organicmatter. q 2002 Elsevier Science B.V. All rights reserved.

Keywords: Trace metals; Pyrite; Sulfur isotopes; Coastal peats; Holocene

) Corresponding author. Fax: q49-441-7983404.Ž .E-mail address: brumsack@icbm.de H.-J. Brumsack .

0009-2541r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 01 00335-7

( )O. Dellwig et al.rChemical Geology 182 2002 423–442424

1. Introduction

During the Holocene sea-level rise, a sedimentarywedge was formed in the coastal area of NW Ger-many which consists predominantly of sediments of

Ž .marine origin Hoselmann and Streif, 1997 . A char-acteristic of these coastal deposits, however, are peatlayers, which can be differentiated into basal and

Ž .intercalated peats e.g. Streif, 1990 . The basal peatsoverlay Pleistocene sands, and their formation isoften encouraged by decreasing drainage due to theapproaching North Sea during the Holocene sea-level

Ž .rise Behre, 1982 . In contrast, intercalated peats aresituated between clastic sediments and were pre-dominantly formed under a distinct seawater influ-ence during phases of a moderate or even stagnantsea-level rise.

The geochemical investigation of Holocene peatsfrom the study area has only recently been started.These studies mainly focussed on the palaeoenviron-mental reconstruction of the Holocene developmentŽ .e.g. Dellwig et al., 1998, 1999 . As the coastal peats

Ž .grew in a brackish water zone Dellwig et al., 2001in close vicinity to the North sea, they are suppo-sed to reveal a geochemical composition different

Žfrom peat-forming environments further inland e.g..Naucke, 1990 . On the other hand, as a result of the

continuous seawater influence on the coastal peats,Žsimilarities to TOC-rich marine sediments e.g. sa-

propels, black shales, or sediments from upwelling.areas , which often contain elevated amounts of pyrite

and strong enrichments in specific trace metals andŽlight sulfur isotopes e.g. Brumsack, 1980, 1989;

Calvert, 1990; Warning and Brumsack, 2000; Passier.et al., 1999 , seem to be evident.

Therefore, several peat layers, originating fromthree drill cores from the marshlands of NW Ger-many, were analyzed at high-resolution for severaltrace metals in order to provide information abouttrace metal distribution and fixation within the peat.As Holocene coastal peats are often characterized by

Ž .a high abundance of pyrite Dellwig et al., 1999 ,which is an important carrier of several trace metalsŽe.g. Raiswell and Plant, 1980; Huerta-Diaz and

.Morse, 1992 , we also considered the relationshipbetween pyrite formation and trace metal partition-ing. For this reason, the first part of this contributionfocuses on pyrite formation, i.e. the geochemistry of

iron and sulfur, while in the second part, attention isgiven to the trace metal and sulfur isotope distribu-tion.

2. Regional setting, lithology, and stratigraphy

The investigated drill cores W2, W3, and W5Ž .archive No. KB5156, KB5750, KB5950 originatefrom the marshlands of the Wangerland about 20 kmNW of Wilhelmshaven close to Jade Bay, NW Ger-

Ž . Ž .many Fig. 1 . The cores diameter 10 cm weredrilled in summer 1996 with a drilling system pro-vided by the Geological Survey of the Federal State

Ž .of Lower Saxony, Germany Merkt and Streif, 1970 .Ž .The cores form part of a transect five cores W1–W5

of about 3 km in length which is located in aŽformerly sheltered so-called Crildumer Bay Petzel-

.berger, 1997 . In this communication, we reportdetailed results only for cores W2, W3, and W5,because they contain both basal and intercalated peatlayers.

The lithologies of cores W1–W5, based on resultsfrom inorganic-geochemical and microfacies analysisŽ .Dellwig, 1999 , are presented in Fig. 2. The botani-cal peat composition was investigated by visual coredescriptions and microscopic examination of plantfragments from several peat samples according to the

Ž .methods described by Grosse-Brauckmann 1962 .An overview about peat classification and character-istic plant species is presented in Table 1 in whichfive major peat types are listed. In the following

Ždiscussion, these peat types occur as basal peats on. ŽPleistocene material or intercalated peats between

.Holocene material . The intercalated peats consistmostly of fen reed peat. The only exception is theintercalated peat of core W3, which contains a transi-tion bog peat section in the upper part. On the otherhand, the basal peats are characterized by a morevariable composition. Except for core W4, theHolocene basis in all cores is formed by a fenwoodland peat layer which develops into fen sedgeand fen reed peat towards the top. The basal peat ofcore W5 even contains raised bog peat and transitionbog peat within its central part.

Several peat samples have been chosen for 14C-agedeterminations. The resulting average calibrated ageswere calculated using the 14C age calibration pro-

Ž .gram CALIB 3.0 Stuiver and Reimer, 1993 and are

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 425

Fig. 1. Map of the study area showing the location of the Wangerland core transect. The bold arrow in the left figure marks an assumed tidalŽ . Žchannel direction. Freshwater sample locations sluices around the Jade Bay and small rivers close to Oldenburg are marked by asterisks 1:

Ems-Jade-Channel, 2: Dangaster Sieltief, 3: River Jade, 4: Schweiburger Sieltief, 5: Eckwarder Sieltief, 6: River Hausbake, 7 and 8: River¨. Ž .Hunte . NLs the Netherlands, FRGsGermany .

14 w xFig. 2. Lithologies of the transect cores W1–W5 as well as C-age determinations of peat samples average calibrated years BP . The grayŽ .line indicates the German zero datum NN and the black lines within the basal peats of cores W1, W3, and W5 mark thin clastic layers.

Dashed lines indicate lithological transitions.

( )O. Dellwig et al.rChemical Geology 182 2002 423–442426

Table 1Classification and characteristic vegetation of peat layers of theinvestigated drill coresDrill core W4 is not included because it contains no peat layers.

Core Depth interval Peat classification Characteristic plantsw xm

W1 10.75–11.20 fen reed peat Phragmites australis11.20–11.40 fen woodland peat Alnus glutinosa

W2 4.90–5.01 fen reed peat P. australis8.20–8.45 fen reed peat P. australis10.58–10.70 fen woodland peat Betula pubescens

A. glutinosaW3 5.25–5.32 transition bog peat Sphagnum spp.

P. australis5.32–5.40 fen reed peat P. australis5.72–5.78 fen reed peat P. australis5.78–6.00 fen sedge peat Carex spp.6.00–6.04 fen woodland peat B. pubescens

W5 2.73–3.02 fen reed peat P. australis5.78–5.85 fen reed peat P. australis5.83–6.32 transition bog peatr Eriopherum

raised bog peat ÕaginatumCalluna ÕulgarisSphagnum spp.

6.32–6.42 fen woodland peat B. pubescens

also presented in Fig. 2. The age determinationsreveal that the onset of the formation of basal peatsoccurred approximately between 7300 and 6200 yearsBP due to the approaching North Sea which pre-vented drainage and led to a rising groundwater level

Ž .in the study area Behre, 1982 . Later, sea levelfluctuations led to changing conditions from fullymarine to supralittoral as seen by the occurrence oftidal flat sediments, brackish water sediments, and

Ž .intercalated peats Fig. 2 . Especially in cores W2and W5, silting up processes culminated in the for-mation of three intercalated peat layers.

Core W4 contains no basal peat but tidal channeldeposits which most likely eroded a formerly exist-

Ž .ing peat layer Fig. 2 . This tidal channel, whoseassumed direction is shown by the bold arrow in Fig.1, had a decisive influence on the geochemical com-position of the basal peats of cores W1, W3, andW5. Thus, especially during storm events, the intrud-ing water masses may have led to a partial buoyancyof the basal peats at transition zones, e.g. the transi-tion of fen woodland and raised bog peat in core W5.Therefore, the input of seawater and suspended par-ticulate matter was possible which led to the forma-

tion of thin clastic layers within the basal peats ofcores W1, W3, and W5 that are indicated by blacklines in Fig. 2. These clastic layers are 1–3 mm inthickness but may reach a few cm at other locations

Ž .in the study area Behre, 1982; Dellwig, 1999 . Amore detailed description of this phenomenon is

Ž .given by Dellwig et al. 2001 .Since one aim of this work is to provide informa-

tion about possible sources of metals in the peat, weanalyzed dissolved and particulate contents of Fe andselected trace metals in modern freshwater environ-ments close to the drill sites. Freshwater sampleswere taken from channels draining into the Jade Bayand from two rivers close to the city of OldenburgŽ .Fig. 1 .

3. Material and methods

Depending on lithology, high-resolution samplingŽ .847 samples was performed at 5 to 10 cm in clasticintervals and at 1 to 3 cm in the peat layers. The

Ž .samples were stored in polyethylene PE bags,sealed, and immediately frozen. Afterwards, thesamples were freeze-dried and homogenised in anagate mortar. The ground powder was used for allsubsequent geochemical analyses.

In order to provide an overview about the geo-chemical composition of the cores, all samples wereanalysed for the major elements Al, Fe and the tracemetals As, Co, Cr, Mo, Mn, Ni, Pb, V, Zn by XRFŽ .Philips PW 2400, equipped with a Rh-tube usingfused borate glass beads. Peat samples and sediment

Ž .samples with a total organic carbon TOC contentof more than 10% were heated to 500 8C to removeTOC prior to adding lithiumtetraborate and fusing inPt–Au-crucibles.

Ž . Ž .Total sulfur TS and total carbon TC wereanalysed in 601 samples after combustion using anIR-analyser Leco SC-444 while total inorganic car-

Ž .bon TIC was determined by a Coulometrics CM5012 CO2 coulometer coupled to a CM 5130 acidifi-

Žcation module Huffman, 1977; Engleman et al.,. Ž .1985 . The content of total organic carbon TOC

was calculated as the difference between TC andTIC.

Ž .An ICP-MS Finnigan MAT Element was usedŽto analyse total trace metals Cd, Co, Cr, Cu, Mo,

.Ni, Pb, Re, Tl, U in acid digestions of 64 peat

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 427

samples performed after Heinrichs and HerrmannŽ . Ž1990 in closed PTFE vessels PDS-6; Heinrichs et

.al., 1986 . A detailed description of sample prepara-Ž .tion is given by Dellwig et al. 1999 . The acid

digestions were also analysed for Al, Fe, As, Mn,ŽZn, and V by ICP-OES Perkin Elmer Optima

.3000XL .Five peat samples were subjected to leaching

experiments according to Huerta-Diaz and MorseŽ .1990 . The resulting HCl, HF, and HNO fractions3

were measured by ICP-OES for Fe, As, Mn, V, andZn, and other trace metals by ICP-MS. Averagerecovery, calculated from the sum of the three frac-tions and the bulk sediment measurements, was 94"20%.

Freshwater samples were collected from the sub-surface by using polyethylene bottles. After collec-tion, the samples were directly filtered through cellu-

Ž .lose acetate filters 0.45 mm; Nalgene , stored inpre-cleaned PE bottles, and acidified with HNO .3

Ž .Suspended particulate matter SPM was obtainedby filtering 250 ml through pre-weighed 0.45 mm

Ž .Millipore polycarbonate filters 47 mm diameter .Filters were rinsed with 18 MV water, dried at 608C, and reweighed. For element analysis, the filterswere treated with 0.5 ml HNO and 1.5 ml HClO in3 4

PTFE vessels at 150 8C overnight to decomposeorganic matter and the filter material. After that,SPM was digested in the same way as the peatsamples and appropriately diluted.

Freshwater and SPM samples were analysed forŽAl, As, Fe, Mn, V, and Zn by ICP-OES Perkin

.Elmer Optima 3000XL and for Cd, Co, Cr, Cu, Mo,ŽNi, Pb, Tl, and U by ICP-MS Finnigan MAT Ele-

.ment .Analytical precision and accuracy of XRF, ICP-

OES, and ICP-MS measurements were tested byŽreplicate analysis of geostandards GSD-3, -5, -6,.LKSD-1, PACS-1, SDO-1, SGR-1 and several in-

house standards. The precision of bulk parametermeasurements was checked in series of double runsand accuracy was determined by using in-house stan-

Ž .dards see Appendix A.1 .Selected peat samples were analyzed for the sul-

fur isotopic composition of total solid-bound sulfur.Prior to the isotope analysis, the samples were care-fully washed to remove sea-salts, and dried. Thesamples were directly analysed by means of combus-

Žtion isotope-ratio-monitoring mass spectrometry C-.irmMS using a Carlo Erba EA1108 elemental ana-

lyzer coupled to a Finnigan MAT 252 mass spec-trometer via a Finnigan MAT Conflo II interface as

Ž .described by Bottcher and Schnetger in press . Iso-¨tope ratios are given in the d-notation relative to theV-CDT standard. The reproducibility for the naturalsamples was better than "0.3‰.

4. Results and discussion

4.1. Pyrite formation in coastal peats

4.1.1. Sulfur and ironFig. 3 shows depth profiles of total sulfur of the

drill cores W2, W3, and W5 on an organic matter-freeŽ .basis TS . This normalisation eliminates dilu-OMF

tion effects caused by organic matter of the seden-tary peat layers which can be easily distinguishedfrom the clastic sediments by their high TOC con-

Žtents TOC peat 16–52.6%; TOC clastic sediments.1.3–6.3% . The peat layers are highly enriched in

sulfur with highest amounts seen in the basal peat ofŽ .core W5. The non-normalized TS contents Table 2

of the intercalated and basal fen reed peats averageŽ .5.9% range 2.5–13.1% and are comparable to TS

values of other marine influenced peat-forming envi-ronments, like the Florida Everglades where values

Fig. 3. Depth profiles of total sulfur on an organic matter-freeŽ .basis TS of the drill cores W2, W3, and W5.OM F

( )O. Dellwig et al.rChemical Geology 182 2002 423–442428

Table 2Ž . Ž . Ž .Average TOC, TS, Fe and trace metal contents of fen reed peats cores W2, 3, 5 , fen woodland peats cores W2, 3, 5 , bog peat core W5 ,

Ž .and pyrite-rich sections of the basal peats containing clastic layers cores W3, 5Concentration ranges are presented in parenthesis.

Element Fen reed peat Fen woodland peat Transition bogr Basal peat intervalsy1w x Ž .mg kg intercalatedrbasal raised bog peat containing clastic layers

w x Ž . Ž . Ž . Ž .TOC % 33.3 19.3–48.6 34.8 33.0–37.6 45.2 35.4–52.6 17.2 16.0–18.9w x Ž . Ž . Ž . Ž .TS % 5.9 2.5–13.1 8.4 5.5–16.0 5.8 3.3–9.6 24.1 16.0–28.2w x Ž . Ž . Ž . Ž .Fe % 3.7 0.9–9.8 5.4 2.5–13.2 3.1 1.0–6.1 21.6 12.3–27.3

Ž . Ž . Ž . Ž .As 16 5–28 16 7–22 6 2–14 37 14–98Ž . Ž . Ž . Ž .Cd 0.1 0.02–0.3 0.1 0.05–0.3 0.05 0.02–0.1 0.2 0.1–0.3Ž . Ž . Ž . Ž .Co 5 0.7–15 5 1–14 1 0.7–3 9 4–23Ž . Ž . Ž . Ž .Cr 39 5–84 28 12–75 12 3–26 26 7–51Ž . Ž . Ž . Ž .Cu 9 4–24 9 5–18 6 4–19 6 4–9Ž . Ž . Ž . Ž .Mn 226 50–498 162 115–211 142 104–217 379 283–496Ž . Ž . Ž . Ž .Mo 13 4–34 11 2–24 2 1–7 18 13–23Ž . Ž . Ž . Ž .Ni 17 3–38 10 2–28 5 2–9 16 9–33Ž . Ž . Ž . Ž .Pb 9 1–22 6 2–16 2 1–5 6 2–14

y1w x Ž . Ž . Ž . Ž .Re mg kg 6 2–23 4 1–10 2 1–3 4 2–6Ž . Ž . Ž . Ž .Tl 0.3 0.01–0.7 0.2 0.06–0.6 0.06 0.01–0.1 0.2 0.05–0.5Ž . Ž . Ž . Ž .U 3 1–6 4 1–9 1 0.2–2 3 3–4Ž . Ž . Ž . Ž .V 56 11–106 20 12–35 15 2–29 32 11–60Ž . Ž . Ž . Ž .Zn 31 8–125 26 13–45 13 8–25 54 24–125

Žbetween 1.1% and 6.0% are observed e.g.Casagrande et al., 1977; Given and Miller, 1985;

.Price and Casagrande, 1991 .In contrast, the remaining basal peat intervals on

Ž .average contain 9.3% TS range 3.3–28.2% , thoughit should be noted that the lower values originatefrom raised bog peat samples of core W5. Althoughthe raised bogrtransition bog peat section of coreW5 is supposed to reflect a limnic and thereforesulfur-limited system, its average sulfur content of5.8% is extremely high when compared with otherfreshwater influenced peat-forming environments. TSvalues of sediments from the North American fresh-

Ž .water basin Everglades, Okefenokee Swamp , forŽinstance, are below 1.1% Casagrande et al., 1977;

.Altschuler et al., 1983; Bates et al., 1998 . Thehighest TS values are seen at transitions between fen

Žwoodland peat and fen sedge peat in core W3 TS.max. 16% and at the transition of fen woodland peat

Ž .and raised bog peat in core W5 TS max. 28.2% .These intervals are characterized by higher amountsin lithogenic elements like Al, Si, and Zr whichindicate the occurrence of clastic layers within the

Ž .basal peats of cores W3 and W5 Dellwig, 1999 .This finding corresponds to lower TOC contents of

these sections in comparison to the pure peat sam-Ž .ples Table 2 . The clastic layers most likely resulted

from tidal channel activities which led to a partialbuoyancy of the basal peats at transition zones and,therefore, allowing the intrusion of seawater and

Ž .suspended particulate matter see chapter 2 . Thinsection analyses showed that in these samples dis-crete pyrite layers occur between clastic material and

Ž .peat Dellwig et al., 2001 .Fig. 4 shows the correlation of TS versus Feavail.

for the intercalated and basal peats of drill cores W2,W3, and W5. Fe represents a calculated Feavail.

fraction which contains the Fe available for pyriteformation before the onset of early diagenesis, i.e.the formation of Fe-sulfides. Fe is calculatedavail.

Žusing the following empirical formula Brumsack,.1988 :

Fe sFey0.2PAl.avail .

The calculation of Fe is based on the assumptionavail.

that silicate-bound Fe amounts to about 20% of theAl content. As most Fe from sheet silicates is charac-terized by extremely high half-lives towards the reac-tion with dissolved sulphide, this Fe is not available

Žin the early stages of pyrite formation Canfield and

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 429

Ž .Fig. 4. Scatter plot of total sulfur TS versus Fe of the interca-lated and basal peats of the drill cores W2, W3, and W5. Thecontinuous line indicates the regression line and the dashed lineshows the theoretical pyrite ratio.

.Raiswell, 1991 . The remaining Fe consists mainlyof Fe-oxyhydroxides which react with dissolved sul-

Žphide during hours to several days Canfield et al.,.1992 . The distinct relation between TS and Feavail.

gives evidence for the formation of pyrite and showsthat almost all Fe is converted into pyrite. Thisavail.

finding is consistent with low concentrations of reac-Ž .tive Fe Fe : ave. 0.92%, range 0.32–1.58% andx

Ž .high DOP values ave. 0.92, range 0.87–0.95 of thepeat samples determined in the HCl fraction of the

Ž .leaching experiments Fig. 5 . In contrast to Leven-Ž .thal and Taylor 1990 , DOP was calculated accord-

ing to the following formula, where Fe is usedavail.

instead of TS to exclude diagenetically formed or-ganic sulfur:

DOPsFe r Fe qFe .Ž .avail . avail . x

The occurrence of organic sulfur compounds is indi-cated as most samples are situated above the theoret-

Ž .ical pyrite ratio in Fig. 4. Bates et al. 1995 , forinstance, reported organic sulfur contents of up to60% of the total sulfur inventory in peats fromFlorida. Formation of organo-sulfur compoundscompetes with iron sulfide formation and is oftenassumed to be related to a limitation in the amount

Žof reactive iron e.g. Bein et al., 1990; Hartgers et.al., 1997 .

It can be seen from Fig. 4 that the highest amountof pyrite occurs within the basal peats. As seawaterrepresents the only source which provides sufficientamounts of sulfate, an enrichment of pyrite withinthe lower parts of the basal peats was unexpected

Ž .because of their botanical composition Table 1which excludes a direct seawater influence. On theother hand, the intercalated peats as well as the peat

Ž .layers of the uppermost basal parts max. 10 cmconsist predominantly of Phragmites reed which

Ž .grew under a mean salinity of about 10 range 5–15Žas indicated by diatom analyses Dellwig et al.,

.2001; Watermann et al., submitted for publication .Therefore, sufficient sulfate should be present for

Ž .sulfate-reducing bacteria SRB to produce H S for2

the formation of pyrite. As a direct seawater influ-ence on the lower parts of the basal peats is unlikely,

Ž .Dellwig et al. 2001 suggested that the occurrenceof clastic layers favoured the inflow of salinegroundwater, e.g. by tidal pumping. Hence, pyriteformation occurred subsequent to peat growth in thelower parts of the basal sections. However, the ques-

Ž . 34Fig. 5. Depth profile of total sulfur TS and d S values for theintercalated and basal peat of core W5 as well as DOP values offive peat samples which have been chosen for leaching experi-

Ž .ments black arrows .

( )O. Dellwig et al.rChemical Geology 182 2002 423–442430

tion remains: Why do the lowest parts of the basalpeats contain such high amounts of pyrite whencompared with the fen reed peats?

4.1.2. Stable sulfur isotopesTo provide information about sulfate availability

during pyrite formation, d34S values of reduced sedi-mentary sulfur species were determined for the inter-calated and basal peat of core W5. Fig. 5 shows thatall peat samples are characterized by a wide range of

34 Žnegative d S values y2.6‰ to y26.7‰ versus.V-CDT corresponding to a significant enrichment of

32 S compared to dissolved sulfate in coastal watersŽ 34of the modern North Sea d Ssq20.6‰; Bottcher¨

.et al., 1998 . The measured isotope signatures, there-fore, indicate that sulfate reduction in the peats wasassociated with sulfur isotope fractionation up toabout y47‰. This covers the range observed inexperiments with pure cultures of sulfate-reducing

Ž .bacteria Chambers and Trudinger, 1979 , and indi-cates that bacterial sulfate reduction in some peatsections took place under essentially open conditions

Žwith respect to seawater sulfate Hartmann and.Nielsen, 1969 .

While the pyrite-rich sections of the intercalatedfen reed peat show the most negative d

34S values,the basal raised bog and transition bog peat revealsdistinctly less negative values indicating conditionsduring pyrite formation more limited with respect to

Ž .dissolved sulfate Hartmann and Nielsen, 1969 , orassociated with higher cellular sulfate reduction ratesŽ .Chambers and Trudinger, 1979 . In contrast to theintercalated fen reed peat, which grew under a con-stant seawater influence, the raised bog peat receivedits sulfate more likely via diffusion. Considering the

34 Ž .correlation between TS and d S values Fig. 6 , twotrends for the intercalated and basal peat of core W5are obvious. The intercalated peat shows only littlevariation in TS when compared to the sulfur isotoperatios. On the other hand, the basal peat is character-ized by distinctly higher TS values but the d

34Sratios are slightly less negative. This emphasizes thatsulfate availability was not the limiting factor duringpyrite formation in the investigated peat-forming en-vironments.

As organic-rich coastal environments are typicallyŽcharacterized by high sulfate reduction rates e.g.

.Howarth and Teal, 1979; Howarth and Giblin, 1983 ,

Ž . 34Fig. 6. Scatter plot of total sulfur TS versus d S values of theintercalated and basal peat of core W5.

we suggest that the preservation of pyrite plays animportant role for elevated amounts of pyrite seen insuch environments. Studies on pyrite formation insalt marshes and coastal sediments have shown thatnet pyrite accumulation is small compared to the

Žhigh sulfate reduction rates Jørgensen, 1978;.Howarth and Teal, 1979 . The latter authors, for

instance, concluded that most pyrite was oxidized onan annual basis due to the metabolism of plant roots.Additionally, tidal flushing has to be considered,which leads to an input of oxygen and the removal

Ž .of soluble Fe Giblin and Howarth, 1984 . As plantactivity and tidal flushing both influence pyrite for-mation only in the reed peats, a better preservation ofpyrite can be assumed for the lowest parts of thebasal peats. Thus, most of the pyrite observed in thelower basal parts was formed after peat formationowing to the reaction of sulfate-rich waters andpeatland waters, of which the latter contain normallyhigh dissolved Fe due to its complexation by humic

Ž .substances Shotyk, 1988 .Sulfur isotope fractionation during bacterial sul-

fate reduction under experimental and natural low-sulfate conditions has been found to be smaller than

Žunder an excess of sulfate Harrison and Thode,.1958; Fry et al., 1995 . The high sulfur contents in

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 431

Ž .both peat types Fig. 6 indicate, however, that aneffect of sulfate availability on the isotope effectduring bacterial metabolism can be ruled out. An-other factor, however, which may have caused lighterisotope signatures in the intercalated peat may havebeen an enhanced influence of the reoxidation ofreduced sulfur species and bacterial disproportiona-tion reactions of sulfur intermediates on the sedimen-tary sulfur cycle, which are known to result in the

Žproduction of isotopically light hydrogen sulfide e.g..Strauss, 1999 . Finally, the diffusion and further

reaction of dissolved biogenic hydrogen sulfide fromsediment sections which are limited with respect toiron into deeper parts of the sediment profile, asshown for organic-rich sediments of the Mediter-

Žranean and the Baltic Sea Passier et al., 1999;.Bottcher and Lepland, 2000 , may have influenced¨

the parts below the strongest TS enrichments foundŽ .in the profile of core W5 Fig. 5 . This process

should lead to the formation of pyrite which isenriched in 34S compared to the sediment sectionwhere the H S was produced. The dominant factor2

determining sulfur isotope partitioning, however, wasthe availability of dissolved sulfate.

4.2. Trace metals in the coastal peats

4.2.1. Trace metal distributionTable 2 provides an overview about the trace

Žmetal contents of the investigated peats fen reedpeat, fen woodland peat, transition bogrraised bog

.peat as well as of the pyrite-rich basal peat intervalswhich contain clastic layers. The element contentsreveal a large variability which is probably causedby changing palaeoenvironmental conditions associ-ated with the Holocene sea level fluctuations. Acomparison of the data presented here with otherpeat-forming environments is difficult. In addition tothe aforementioned climatic changes, the geochem-

Ž .istry of peat especially fen peat strongly dependsŽ .on hydrogeological conditions weathering , i.e. the

Ž .source rock composition. Christanis et al. 1998determined trace elements in Holocene fen peat sam-ples from Greece. They found, for instance, a signifi-cant enrichment in the uranium contents which couldbe related to the weathering of granitic rocks. Never-theless, fen sedge peats from Finland analyzed by

Ž .Sillanpaa 1972 showed trace metal contents com-¨¨

parable to the fen reed peats presented in this study.The only exception is Mo, which is 15-fold higher inthe samples of the present study. Mo and Re concen-trations in the investigated fen reed peats are highand are comparable to values reported for anoxiccoastal sediments of the Saanich Inlet and the Peru

Žmargin, and for sapropels of the Black Sea Gold-.berg, 1987; Crusius et al., 1996 . Anthropogenic

influences on trace metal composition can be ruledout for the investigated Holocene peats as seen from206r207Pb ratios determined for several Holocene peatsamples from the study area which reflect a local

Ž .geogenic background of 1.20 Dellwig, 1999 .Fig. 7 shows the Mo profiles of cores W2,OMF

ŽW3, and W5. Similar to the TS distribution Fig.OMF.3 the Mo profiles show enrichments in the peatOMF

layers. However, in contrast to TS , almost all fenOMFŽ .reed peats intercalated and basal show higher

Mo values when compared with the lowest partsOMF

of the basal peats where the highest pyrite enrich-ments are seen. Similar enrichments are also seen forthe redox-sensitive trace elements As, Re, and U,

Ž .while other trace metals e.g. Co, Cr, Zn show onlyslight or no enrichments.

In order to compare the trace metal inventory ofthe peat samples with the geogenic background, wecalculated enrichment factors versus average shaleŽ .Wedepohl, 1971, 1991; Re: Colodner et al., 1993

Ž .on an organic matter-free basis EFS . Fig. 8OMF

Fig. 7. Depth profiles of Mo on an organic matter-free basisŽ .Mo of the drill cores W2, W3, and W5.OM F

( )O. Dellwig et al.rChemical Geology 182 2002 423–442432

Ž .Fig. 8. Enrichment factors for Fe and selected trace metals versus average shale on an organic matter-free basis EFS for reed peatsOM FŽ . Ž . Ž .cores W2, 3, 5 and pyrite-rich basal peat samples containing clastic layers cores W3, 5 . Average shale data from Wedepohl 1971, 1991 .

Ž .Re value from crustal abundance after Colodner et al. 1993 .

shows the average EFS values of 38 peat sam-OMFŽples including the fen reed peats intercalated and

.basal fen reed peats of cores W2, W3, W5 and thepyrite-rich basal peat intervals of cores W3 and W5which contain clastic layers. Highest enrichments areseen for Mo and Re accompanied by slight enrich-ments in As, Cd, and U. In comparison to the basalpeats, higher enrichments in As, Mo, Re, and U areseen for the fen reed peats. In contrast to these tracemetals, Fe shows a higher abundance in the basalpeats which reflects the pyrite distribution seen from

Ž .the TS profiles Fig. 3 . The remaining traceOMF

metals plot close to the shale level indicating nosignificant enrichments. These results are in contrastto findings on organic-rich marine sediments whichtypically show a wide spectrum in trace metal en-

Ž .richments e.g. Warning and Brumsack, 2000 .

4.2.2. Trace metal sourcesTable 3 shows the seawater concentrations, aver-

age shale contents, and seawaterrshale ratios of theinvestigated trace metals. With respect to the seawa-terrshale ratios, similarities for three groups of trace

Ž .metals become evident: a The redox-sensitive tracemetals Re and Mo show the highest seawaterrshaleratio followed by U and As, because these elementsoccur as oxoanions in relative high concentrations inoxygenated seawater while their geogenic back-

Žground is comparatively low Bruland, 1983; Martinand Whitfield, 1983; Goldberg, 1987; Wedepohl,

.1991; Colodner et al., 1993 . The same relation

holds for Cd even though it does not occur as an0 Žoxoanion, but as CdCl complex in seawater Bru-2

.land, 1983 , its seawaterrshale ratio is high as well.Ž .b Cr and V also occur as oxoanions in seawater.

Ž .Their geogenic background, however, is high. cFinally, Co, Cu, Mn, Ni, Pb, Tl, and Zn formcations, carbonato, chloro, andror hydroxy com-

Table 3Seawater concentrations and average shale contents as well asseawaterrshale ratios of the investigated trace metals

a bElement Seawater Ave. shale Seawaterrave.y1 y1 y3w x w xmg l mg kg shale=10

As 1.7 10 0.17Cd 0.08 0.13 0.62Co 0.001 19 0.00005Cr 0.21 95 0.002Cu 0.25 39 0.006Mn 0.27 850 0.0003Mo 10.6 1.3 8.2Ni 0.47 68 0.007Pb 0.002 22 0.0001

c dRe 0.008 0.0005 16.6Tl 0.01 0.68 0.02

eU 3.2 3 1.1V 1.8 130 0.014Zn 0.39 115 0.003

a Ž .Bruland 1983 .b Ž .Wedepohl 1971, 1991 .c Ž .Goldberg 1987 .d Ž .Crustal abundance, Colodner et al. 1993 .e Ž .Martin and Whitfield 1983 .

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 433

plexes and are characterized by low seawater con-centrations in comparison to their geogenic back-

Ž .ground Wedepohl, 1971; Bruland, 1983 .The seawater element contents from the two latter

groups, therefore, should not significantly contributeto the composition of the peat. Hence, it can beconcluded that similar to sulfur, seawater representsthe main source for As, Cd, Mo, Re, and U. Incontrast, the contents of the remaining trace metalscan essentially be explained by the input of detritalmaterial, e.g. suspended particulate matter con-tributed by flooding which is characterized by ageochemical composition almost similar to average

Ž .shale Dellwig et al., 2000 . Thus, processes ofelement mobilisation from the surrounding clasticsediments and fixation within the peats seem to playonly a minor role. However, regarding the Fe enrich-ment in Fig. 8, a further element source is necessarywhich introduces sufficient Fe to the peats becausethe concentration of dissolved and particulate Fe is

Ž .very low in seawater and North Sea water Table 4 ,respectively.

Ž .Dellwig et al. 2001 suggested a scenario duringmoderate sea-level rise for the Holocene coastalpeat-forming environment of NW Germany which is

characterized by the occurrence of a brackish waterzone of comparatively low tidal activity. According

Žto diatom analyses Watermann et al., submitted for.publication , the mean salinity of the brackish water

zone ranged between 5 and 15 depending on theexposition to the open sea. While the seawater pro-vides sufficient sulfate, the investigation of the geo-chemical environment of the study area shows that

Žfreshwater may contribute large amounts of Fe Ta-.ble 4 . We used a simple balance calculation to

estimate the brackish water volume needed for theFe and trace metal inventory of 1 kg of the interca-lated fen reed peat of core W5 considering a salinity

Ž .of 5 and 15 Table 4 . The endmember salinitieschosen for freshwater and seawater are Ss0 andSs30, respectively. Additionally, Al was chosen torepresent the detrital fraction. We limit the followingdiscussion on data from core W5 because it providesthe most complete database, including age determi-nations which enable estimating compaction of the

Ž .intercalated peat see below .The balance is based on the assumption that the

actual element concentrations of the freshwater envi-ronment and of the North Sea are comparable to thesituation in the past. Eolian input and processes

Table 4Ž .Dissolved and particulate concentrations of Al, Fe and selected trace metals in freshwater small riversrcreeks close to the study area ,

Ž .average seawater, and Southern North Sea water Salinity)30The particulate phase concentrations are given per unit volume of water. Element contents of the intercalated peat layer of core W5Ž . Ž .corrected for water content and compaction and calculated volumes of brackish water including dissolved and particulate phase at a

Ž .salinity S of 5 and 15 necessary to explain the element inventory of 1 kg of the intercalated peat layer.

Element Freshwater Freshwater Seawater North Sea Intercalated Volume at Volume ata a b c a 3 3w x w xdissolved particulate dissolved particulate peat W5 Ss5 m 1Ss15 mcorr.

y1 y1 y1 y1 y1w x w x w x w x w xmg l mg l mg l mg l mg kg

Al 300 729 0.54 125 5500 6.3 9.6Fe 1678 2973 0.06 87 4844 1.2 2.0

dCo 0.8 0.3 0.001 0.04 1 1.1 1.8Cr 0.9 0.9 0.21 0.65 8 5.0 6.2Mn 290 56 0.27 5.4 38 0.1 0.2

dMo 0.4 0.02 10.6 0.03 0.9 0.4 0.2e dU 0.1 0.03 3.2 0.005 0.3 0.5 0.2

V 3.3 1.7 1.8 0.23 12 2.7 3.5y1w xSPM mg kg 28 7

a This work.b Ž .Bruland 1983 .c Ž .Dellwig et al. 2000 .d Ž .J. Hinrichs, ICBM, submitted for publication .e Ž .Martin and Whitfield 1983 .

( )O. Dellwig et al.rChemical Geology 182 2002 423–442434

Ž .during estuarine mixing Chester, 1990 are disre-garded. As today the contents of several heavy

Ž .metals e.g. As, Cd, Cu, Ni, Pb, Tl, Zn may beinfluenced by current anthropogenic activity, we re-stricted the mass balance calculation to Co, Cr, Mn,Mo, U, and V which most likely reflect natural

Ž .concentrations Lipinski, 1999 . We used dissolvedand particulate element concentrations of channelsaround the Jade Bay and two small rivers close to

Ž .Oldenburg with a salinity below 0.5 Fig. 1 , as wellas dissolved element concentrations of average sea-

Ž .water Bruland, 1983; Martin and Whitfield, 1983and particulate matter of the Southern North SeaŽDellwig et al., 2000; J. Hinrichs, ICBM, submitted

.for publication . It should be noted that the particu-late element concentrations in Table 4 are given inunits of volume of water and that the element con-tents of the intercalated fen reed peat layer of coreW5 are corrected for water content and compactionŽ .74%, 46%; Dellwig et al., 2001 .

Regarding the calculated water volumes, whichare necessary to explain the composition of 1 kg ofthe intercalated fen reed peat layer of core W5, three

Ž .groups of elements can be differentiated Table 4 .The highest volumes at a salinity of 5 are calculatedfor Al, Cr, and V which are more abundant infreshwater compared to seawater. As Al is associatedwith the clay fraction this value reflects the volumeneeded for supplying the lithogenic component withinthe peat. However, it has to be considered that, incontrast to Cr or V, Al is not involved in redox-processes, which may lead to element fixation andenrichment. Therefore, the dissolved phase should bedisregarded, as a result of which the required vol-umes at salinities of 5 and 10 increase to 8.8 and10.5 m3, respectively.

The second group consists of Fe and Co, whichare represented by an intermediate volume due totheir elevated abundance in freshwater. The lowestvolumes are seen for Mn, Mo, and U. Although thefreshwater concentrations of Mo and U are compara-tively low, their high dissolved abundance in seawa-ter contributes sufficient amounts even at a salinityof 5. In the contrary, freshwater introduces highamounts of Mn to the system which explains its lowvolume. However, significant enrichments in com-parison to average shale are not seen for Mn, whichmay be related to its high mobility under anoxic

Ž . Ž .conditions. In contrast to Fe II , fixation of Mn II ismore difficult. Precipitation of MnCO or MnS is3

described, for instance, for organic-rich sediments ofŽthe Baltic Sea Suess, 1979; Bottcher and Huckriede,¨

.1997 . Nevertheless, enhanced alkalinities or ex-tremely high concentrations in dissolved sulfide arerequired for the formation of authigenic Mn carbon-

Žate and sulfide minerals, respectively Bottcher and¨.Huckriede, 1997 . Regarding the water volumes at a

salinity of 15, increasing volumes are seen for Al,Fe, Co, Cr, Mn, and V, while Mo and U show adecrease by a factor of about 2, which reflects theimportance of the interaction between freshwater andseawater within the investigated peat-forming envi-ronment.

We suggest that the high concentrations of Co,Fe, and Mn in the freshwater environment of thestudy area are derived from the catchment area of therivers, which is characterized by the occurrence ofbogs and swamps. Thus, metals that are mobilisedfrom soils under reducing and acidic conditionsŽ .Viers et al., 1997 can be complexed by dissolvedorganic matter leading to an export from the bog and

Ž .swamp areas Pettersson et al., 1997 . This sugges-tion is supported by enhanced amounts of organicmatter seen in the investigated freshwater environ-

Ž y1ment average DOC 29 mg l , average POC 3.6 mgy1 .l , which are correlated to metals like Co, Cr, or

Ž .Fe Lipinski, 1999 .

4.2.3. Trace metal fixationMost of the trace metals determined in this study

form stable sulfides. Therefore, leaching experi-ments, according to the method described by

Ž .Huerta-Diaz and Morse 1990 , were performed onfive pyrite-rich peat samples in order to provideinformation about the importance of pyrite as a sinkfor trace metals in coastal peats. The chosen samplesare indicated by black arrows in Fig. 5. From theseleaching experiments, three acid fractions were ob-tained. While the HCl fraction contains the reactivecompounds, HF and HNO react with clay minerals3

and pyrite, respectively. The relative trace metalabundance in each fraction is shown in Fig. 9. Thestandard deviations for each element in each fractionare on average 9% with a maximum value of 19.5%.

The trace elements which are enriched due to theseawater influence in comparison to the geogenic

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 435

Fig. 9. Relative abundance of selected trace metals in three acid fractions resulting from leaching experiments according to the methodŽ .described by Huerta-Diaz and Morse 1990 .

background are As, Cd, Mo, Re, and U. From theseelements As, Mo, and Re show a high abundance inthe pyrite fraction because these elements may beincorporated into pyrite andror form stable sulfidesŽ .e.g. Belzile and Lebel, 1986 . However, the low Moabundance of 65% in the pyrite fraction is quiteunexpected because Mo is one of the trace metals

Žtypically enriched in pyrite Raiswell and Plant, 1980;.Huerta-Diaz and Morse, 1992 . Although the HF-

fraction normally represents the detrital bound met-als, the high concentration of on average 6.6 mgkgy1 Mo in the HF fraction excludes a sole detritalfixation, when considering the Mo content of 1.3 mg

y1 Ž .kg in average shale Wedepohl, 1991 . For thatreason, we assume that the affinity of Mo to organicmatter has caused this unexpected behavior, as sug-

Ž .gested previously by Brumsack and Gieskes 1983Ž .and Christanis et al. 1998 . Limited information is

available on the fixation mechanism of Re. However,recent studies have shown that Re can be highly

Ženriched in pyrite-rich anoxic marine sediments e.g..Crusius et al., 1996; Warning and Brumsack, 2000

which suggests its accumulation as a sulfide.Although Cd is also commonly present as sulfide

Ž . ŽCdS in anoxic sulphidic sediments Framson andLeckie, 1978; Elderfield et al., 1979; Brumsack,

.1980 pyrite is generally an unimportant sink for CdŽ .Huerta-Diaz and Morse, 1992 . According to Morse

Ž .and Luther 1999 , this behavior is related to thefaster water exchange reaction kinetics of Cd com-pared to Fe2q, leading to metal sulfide precipitation

prior to Fe-sulfide formation, respectively. With re-spect to the relatively high solubility product of CdS

Žwhen compared, e.g. with CuS Dyrssen and Krem-.ling, 1990 , its high abundance in the HCl fraction is

likely related to the leaching procedure.U forms no sulfides under sedimentary conditions

but it shows a strong relation to organic matter underŽanoxic conditions Szalay and Szilagyi, 1967;´

.Cheshire et al., 1977; Brumsack and Gieskes, 1983 .However, a correlation between U and TOC does notexist because the organic matter originated from thesedentary peat-forming process. A further reason forthe observed U enrichment in Fig. 8 is the increasing

4q Žparticle reactivity of reduced U Anderson et al.,.1989 . The U content of the HCl fraction represents

the excess U bound to organic matter and particles.In contrast, the HF fraction may reflect the detritalfraction, which is consistent with the slight enrich-

Ž .ment of U in the peats Fig. 8 when compared withMo or Re.

The source of the trace metals Co, Cu, Cr, Mn,Ni, Tl, V, and Zn is the detrital material whichmainly originates from the freshwater environmentŽ .Table 4 . These metals show no enrichment inrelation to the geogenic background. Therefore, theirabundance in the HNO fraction probably reflects3

mobilisation of the reactive non-silicate bound phaseunder anoxic conditions followed by fixation in pyriteand formation of metal sulfides, respectively. This isespecially the case for Co, Cu, and Tl which show ahigh abundance in the HNO fraction and are known3

( )O. Dellwig et al.rChemical Geology 182 2002 423–442436

Žto form stable sulfides e.g. Duchesne et al., 1983;.Luther, 1991 . Such a process is in accordance with

the shale-like distribution of the detrital-derived met-als because they are only transferred between differ-ent mineral phases. Enrichments in these metals inthe investigated peats, therefore, is unlikely.

Except for Cr, which concentration is highest inŽthe HF fraction, the remaining metals Mn, Ni, Pb,

.V, Zn show no clear distribution in the acid frac-tions. Pb and Zn reveal a behavior similar to Cd dueto their faster water exchange reaction kinetics than

2q Ž .Fe Morse and Luther, 1999 , which prevents afixation in pyrite. On the other hand, the waterexchange kinetics of Ni are slower than for Fe2q

favouring its incorporation into pyrite. Extractionexperiments of pure metal sulfides carried out by

Ž .Cooper and Morse 1998 have shown that CdS,PbS, and ZnS extract completely in cold HCl whilethe dissolution of Ni-sulfides ranges between 1% and28%. This finding may explain the higher abundanceof Ni in the HNO fraction. Mn shows an almost3

equal distribution in the HCl and HNO fraction.3

Although a MnS phase is not easily formed Mn canbe incorporated into pyrite at high DOP levels. Vresembles U because it is also characterized by arelationship to organic matter under anoxic condi-

Ž .tions Szalay and Szilagyi, 1967 . Different to all´other trace metals, Cr shows the highest abundancein the HF fraction. This behavior reflects a strongrelation of Cr to alumosilicates which is consistent

Ž .with a correlation between Cr and Al rs0.99 . Inaddition, Cr sulfides are unstable in comparison to

Žrapidly formed insoluble Cr-hydroxides Smillie et.al., 1981; Eary and Rai, 1989 .

4.2.4. Processes influencing the trace metal distribu-tion

One final question remains: Which processescaused the different abundance of redox-sensitivetrace metals between the individual peat sections? Asseawater represents the most important source for Sand redox-sensitive trace elements, their higher en-

Žrichment in the fen reed peats intercalated peats and.uppermost parts of the basal peats in comparison to

the lower parts of the basal peats of cores W3 andW5 is rather unexpected, because in the basal peatsthe highest amounts of pyrite occur. The fen reed

peats consist mainly of P. australis, which grewunder a continuous influence of seawater and brack-ish water, respectively. In contrast, the botanical

Žcomposition of the lower parts of the basal peats fen.woodland and raised bog peat reflects exclusively

limnic conditions. Therefore, the difference in tracemetal composition seems to be related to the timingof pyrite formation, i.e. caused by an early direct orlater indirect influence of seawater.

Scatter plots of Mo and U versus d34S values in

Ž .core W5 Fig. 10a and b reveal a general trend ofincreasing trace metal contents with increasing sul-fate availability and open system conditions, respec-tively. This trend is clearly reflected by the samplesof both the intercalated reed and the basal peat. Thesamples with the highest Mo and U contents and themost negative sulfur isotope data originate from theupper part of the intercalated fen reed peat. Althoughthis peat presumably reflects a regressive develop-ment of the sea level, the upper part was influencedby the onset of a further transgression. Thus, anincreasing salinity up to app. 15 within the interca-lated peat could be evidenced by its diatom inven-

Ž .tory Watermann et al., submitted for publication .The samples of the basal fen reed peat show anindifferent behavior especially with respect to Mo.As the mean salinity during the formation of thebasal fen reed peat was lower in comparison to the

Ž .intercalated peat Dellwig et al., 2001 , we suggestthat the inventory of redox-sensitive trace metalsresulted to a certain degree from downward diffusingseawater from the brackish and tidal flat sediments,respectively.

As and Re show a behavior similar to Mo, how-ever, the Re contents are distinctly lower in basal fenreed peat samples compared to Mo. The average

Ž y3 .RerMo-ratio 0.8=10 of the intercalated fenreed peat reflects the seawater ratio, whereas thebasal fen reed peat shows a lower ratio of 0.2=10y3.

Ž .Crusius et al. 1996 reported that Re can be accumu-lated in suboxic sediments while Mo needs anoxicconditions. Therefore, it seems likely that downwarddiffusing seawater was depleted in Re within theoverlying suboxic clastic sediments before enteringthe basal fen reed peat.

The scatter plots of Mo and U versus TS of coreŽ .W5 Fig. 10c and d reflect the aforementioned

difference between the intercalated fen reed peat and

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 437

34 Ž .Fig. 10. Scatter plots of Mo and U versus d S values and total sulfur TS of the intercalated and basal peat of core W5.

the lower basal peat, which can be also seen for Reand less pronounced for As. Two trends are obviousfor the intercalated and basal peat with a transitionalbehavior of the basal fen reed peat samples. Theintercalated peat shows only little variation in TSwhen compared to the trace metal contents, whereasthe basal peat reveals the reverse behavior. It shouldbe noted that this relationship does not provide anyinformation about the fixation of the trace metals.Even if As, Mo, and Re are fixed as sulfides, this isnot the case for U, as was shown by the leachingexperiments.

Taking into account that S and redox-sensitivetrace metals originate from the same source, thehigher trace metal contents in relation to TS of theintercalated fen reed peat suggests a different com-position of the brackish water. Thus, the intercalated

fen reed peats are directly influenced by brackishwater while the lower parts of the basal peat, wherethe clastic layers occur, received saline groundwatervia tidal pumping. This water is most likely affectedby diagenetic processes during transport, which ledto a partial removal of trace metals. Such diageneticprocesses are for instance microbial reduction oftrace metals through enzymatical mechanisms as

Ž .shown by Lovley 1993 and fixation by organicmatter which therefore lead to decreasing trace metalcontents. Besides the direct reduction by sulfate-re-ducing bacteria, the reduction of U is also initiatedby dissolved sulfides produced by sulfate-reducing

Ž .bacteria Klinkhammer and Palmer, 1991 . Further-more, the conservative MoO2y anion shows a parti-4

cle reactive behavior after reaction with soft ligandslike humic-bound thiols and the resulting compounds

( )O. Dellwig et al.rChemical Geology 182 2002 423–442438

Ž .are more susceptible to reduction Helz et al., 1996 .Similar to the basal fen reed peat, the RerMo-ratio

Ž y3 .of this peat section 0.2=10 is distinctly lowerthan the seawater ratio of 0.8=10y3 which indi-cates a stronger depletion of Re in comparison toMo. This is also seen in the EFS values of theOMF

intercalated and basal peats in Fig. 8. On the otherhand, sulfate is supposed to be less affected by theseprocesses because its concentration is orders of mag-nitudes higher in comparison to the trace metals.

5. Summary and concluding remarks

This work has shown that the investigatedHolocene coastal peat-forming area acted as an im-portant sink for redox-sensitive trace metals and Federived from seawater and rivers, respectively. To-day, modern dike-building lead to an increase in tidalaction in the coastal areas of NW Germany, which

Žtherefore prevents the formation of peat Dellwig et.al., 2000 . Nevertheless, it can be assumed that the

processes of trace metal fixation described here arealso relevant for other present coastal peat-formingenvironments, e.g. the East Coast of the USA. Theresults can be summarized as follows.

The investigated drill cores contain basal peatsŽfen woodland peat, fen reed peat, and raised bog

.peat which form the Holocene basis and intercalatedŽ .peats mostly fen reed peat . The peat layers contain

elevated amounts of pyrite owing to microbial reduc-tion of seawater sulfate under almost open systemconditions. Highest contents of pyrite are seen inlimnic basal peat sections which contain thin clasticlayers as a result of tidal channel activities. Theoccurrence of clastic material within the basal peatsmay have favoured the intrusion of saline groundwa-ter after peat formation by tidal pumping. In contrast,pyrite formation in the fen reed peats coincides withpeat growth under phases of moderate sea-level rise.These scenarios are supported by the isotopic com-position of sedimentary sulfur.

The trace metals Mo and Re show highest enrich-ments in the peat layers followed by As, Cd, and Uwhile Co, Cr, Cu, Mn, Ni, Pb, Tl, V, and Zn reflectthe geogenic background. The trace metal composi-tion of the peats can be explained by the presence of

a brackish water zone with sulfate-rich seawater andFe-rich freshwater as endmembers. Seawater is themain source for As, Cd, Mo, Re, and U, whereas thecontents of the remaining elements are most likelycontributed by freshwater.

Leaching experiments have shown that As, Co,Cu, Mo, Re, and Tl are predominantly fixed assulfides andror incorporated into pyrite. The remain-ing trace metals show no distinct trend, only Crreveals a strong relation to the lithogenic detritus.

In contrast to pyrite, which shows highest accu-mulation in the lowest basal peats, the redox-sensi-tive trace metals As, Mo, Re, and U are moreenriched in the intercalated fen reed peats. As thesource of sulfate and metals is the same, this differ-ence is likely related to the different scenarios ofpyrite formation in the lower basal and intercalatedpeats. The intercalated peats were formed under adirect influence of seawater and brackish water, re-spectively. Hence, pyrite formation and trace metalfixation occurred during peat growth. On the otherhand, the saline groundwater which enters the basalpeats after their formation is most likely affected bydiagenetic processes and is, therefore, partly depletedin redox-sensitive trace metals.

Acknowledgements

The authors wish to thank B.M. Scholz-Bottcher¨Ž .ICBM for stimulating discussions, and J. Barck-

Žhausen Geological Survey of the Federal State of.Lower Saxony, Hannover for drilling support and

lithological core descriptions. Furthermore, we wouldlike to thank M.A. Geyh for 14C determinationsŽ .Geol. Surv. of Lower Saxony, Germany and W.

Ž .Bartels LUFA, Soil-physical Laboratory, Germanyfor the botanical macro-residual analyses.

The study was funded by German Science Foun-Ž .dation DFG through grant No. Scho 561r3-1, 4-1

during priority research program DFG-SPP ABiogeo-chemical changes over the last 15,000 years—con-tinental sediments as an expression of changing envi-ronmental conditionsB. The authors appreciate usefulcomments on the manuscript by two anonymousreviewers.

( )O. Dellwig et al.rChemical Geology 182 2002 423–442 439

Appendix A

A.1. Precision and accuracy of analysed elements

w x w xElement Method Precision SD 2s % Accuracy %

TS IR-analyser 2.3 6.2TC IR-analyser 4.0 1.3TIC Coulometry 1.1 0.2Al XRFrICP-OES 1.3r1.6 1.1r0.9Fe XRFrICP-OES 1.1r4.8 0.9r2.5As XRFrICP-OES 4.6r13.7 6.7r15.6Cd ICP-MS 11.7 10.7Co XRFrICP-MS 7.3r8.5 7.7r5.2Cr XRFrICP-MS 3.6r9.7 5.2r6.5Cu ICP-MS 10.2 2.4Mo XRFrICP-MS 15.3r11.2 15.2r6.2Mn XRFrICP-OES 1.6r4.2 4.1r2.2Ni XRFrICP-MS 3.9r10.8 8.7r4.0Pb XRFrICP-MS 5.6r12.4 2.7r5.1Re ICP-MS 18.6 –Tl ICP-MS 10.9 9.3U ICP-MS 10.4 7.1V XRFrICP-OES 3.0r7.8 3.1r5.1Zn XRFrICP-OES 3.4r4.7 4.9r7.7

[ ] [ y 1 y 1]A.2. TOC, TS, Al, Fe % and trace metal data mg Kg , Re mg Kg of selected samples of major peattypes

Peat type Core Depth TOC TS Al O Fe O As Cd Co Cr Cu Mn Mo Ni Pb Re Tl U V Zn2 3 2 3w xm

raised bog W5 6.00 45.0 5.8 1.1 4.5 6 0.04 2 13 4 145 2 6 3 2 0.05 0.8 14 12raised bog W5 6.02 44.7 7.8 0.8 7.0 6 0.05 1 9 19 147 3 4 3 3 0.07 1.0 12 13raised bog W5 6.12 46.6 4.7 1.7 2.7 4 0.05 1 16 5 115 3 7 2 1 0.06 1.8 15 9raised bog W5 6.16 47.0 5.6 2.5 1.9 3 0.04 1 20 4 104 2 5 3 2 0.11 1.5 17 8raised bog W5 6.18 50.2 3.9 0.9 1.9 3 0.04 1 9 5 136 2 3 2 1 0.05 0.9 8 9raised bog W5 6.24 50.4 3.3 0.2 1.5 2 0.05 1 3 4 120 1 2 1 1 0.01 0.2 2 8raised bog W5 6.26 52.6 4.0 0.1 2.0 2 0.07 1 3 8 132 1 2 1 1 0.01 0.2 2 8raised bog W5 6.29 50.6 5.0 0.1 3.6 2 0.11 1 7 5 142 2 4 1 1 0.01 0.2 7 12fen woodland W2 10.57 14.7 4.0 5.0 5.1 16 0.10 5 39 7 280 10 13 10 3 0.3 4.8 50 42fen woodland W2 10.59 33.0 6.6 2.3 5.0 21 0.06 3 21 5 211 16 7 3 3 0.1 6.3 35 25fen woodland W2 10.62 34.5 8.2 1.5 6.8 22 0.06 2 15 5 189 10 5 3 3 0.1 3.0 16 21fen woodland W2 10.65 35.7 5.5 1.1 3.6 13 0.05 1 12 5 136 2 2 2 2 0.1 0.8 11 13fen woodland W5 6.36 18.9 28.2 2.5 38.5 16 0.10 4 20 5 496 19 10 4 2 0.1 3.4 27 33fen woodland W5 6.38 16.7 28.0 3.2 39.1 18 0.10 6 26 5 304 17 13 5 3 0.2 3.0 31 36fen woodland W5 6.40 33.0 16.0 2.0 18.9 7 0.11 6 14 10 159 2 9 4 1 0.2 1.5 15 23fen woodland W5 6.42 21.4 1.2 1.3 0.9 1 0.07 1 13 5 120 1.0 4 3 1 0.1 0.7 5 5

( )O. Dellwig et al.rChemical Geology 182 2002 423–442440

fen reed W2 4.96 35.4 6.0 5.0 4.7 24 0.13 6 55 8 155 13 27 10 6 0.26 3.2 62 54fen reed W2 8.26 39.8 7.6 1.9 6.8 28 0.11 3 15 5 227 17 13 4 7 0.16 4.8 26 23fen reed W2 8.35 47.2 5.4 0.4 2.4 10 0.02 1 5 4 171 5 2 1 2 0.01 1.3 11 8fen reed W3 5.26 25.8 3.2 4.8 3.8 20 0.18 7 48 13 247 16 20 9 29 0.29 8.8 79 38fen reed W3 5.34 46.6 3.8 2.0 1.3 5 0.05 1 21 8 50 9 6 3 4 0.11 2.4 56 10fen reed W3 5.37 37.8 4.1 4.0 2.5 7 0.04 3 33 8 60 11 10 8 3 0.22 2.7 54 19fen reed W5 2.80 34.1 6.6 4.2 6.6 26 0.21 11 38 10 498 28 31 8 23 0.29 5.8 65 14fen reed W5 2.86 33.8 5.2 5.0 4.0 12 0.10 2 41 8 342 11 18 9 9 0.28 2.3 66 13fen reed W5 2.91 20.4 3.0 9.5 3.5 9 0.10 7 71 14 225 4 20 14 5 0.67 2.1 106 32

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