283Journal of Paleolimnology 26: 283–292, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

Lacustrine organic matter and the Holocene paleoenvironmentalrecord of Lake Albano (central Italy)*

D. Ariztegui1,*, C. Chondrogianni1, A. Lami2, P. Guilizzoni2 & E. Lafargue3

1Geologisches Institut, ETH Zentrum, 8092 Zürich, Switzerland2CNR-Istituto Italiano de Idrobiologia, 28922 Verbania-Pallanza, Italy3Institut Français du Pétrol, Rueil-Mailmaison, France*Present address: Institut Forel, University of Geneva, Geneva, Switzerland(E-mail: daniel.ariztegui@terre.unige.ch)

Received 10 July 2000; accepted 16 October 2000

Key words: Holocene, Rock-Eval® Pyrolysis, pigments, trophic-state, paleolimnology, organic carbon

Abstract

A combined bulk and detailed geochemical study of the sedimentary organic matter in Lake Albano, central Italy,provides critical data to track the response of this aquatic system to the environmental changes of variable amplitudethat occurred during the Holocene. Rock-Eval pyrolysis of this predominantly laminated, organic carbon-rich sedi-mentary sequence shows changes in hydrogen and oxygen indices that are related to variations in the dominance ofthe primary producers. These variations are further confirmed by the pigments and the carbon isotopic compositionof bulk organic matter showing that cyanobacteria dominated the lake waters during the early and late Holocene whereasdiatoms have been the main producers during the middle Holocene. Sharp decreases in productivity, 2–3 centurieslong, are identified at ca. 8.2, 6.4 and 3.8 ka. B.P. Changes in temperature and/or effective moisture are suggested asthe most probable causes, although human impact cannot be ruled out for the latest part of the Holocene.

Introduction

Holocene climatic fluctuations have been recently iden-tified in high-resolution paleoenvironmental recordsfrom different areas of the world. Multiproxy data fromlake cores show comparable variability during the Hol-ocene to isotope records from polar ice cores (Stager& Mayewski, 1997; Alley et al., 1998; Willemse &Törnqvist, 1999) and to an increasing number of marinecores (e.g., Lamb et al., 1995; Sirocko et al., 1996).These environmental changes have been particularlywell studied in several terrestrial records in centralEurope using a wide range of approaches (e.g., Niessen& Kelts, 1989; Niessen et al., 1992; Leemann & Niessen,1994; Ariztegui et al., 1996; Ramrath et al., 1999; Wilkes

et al., 1999). These sedimentary records have in com-mon that they all provide the environmental sensitiv-ity and the high-temporal resolution necessary to revealthe extent and effect of changes in the environmentduring the Holocene.

Several indirect estimations of a given parameter, or‘proxies’, such as the sedimentary organic fraction,have been used to reconstruct past environmental con-ditions from lake sediment archives. The compositionand amount of organic matter delivered to the lakesediments change in response to the types and abun-dance of organisms in and around the lacustrine basin(Tyson, 1995 and references herein; Meyers & Lallies-Vergès, 1999). Thus, the sedimentary organic matterpreserved in the sediments yields valuable paleoen-vironmental information. A combined study of bothsedimentological and organic matter compositionalvariations of the organic carbon-rich sediments con-tained in sedimentary cores of Lake Albano, Italy, pro-vides new, continuous information on the development

*This is one of a series of papers to be published in Journal ofPaleolimnology that were contributed from the keynote speakers atthe 2nd International Congress of Limnogeology, held May, 1999,in Plouzane, France, and organized by Dr. Jean-Jacques Tiercelin.

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of the lake throughout the Holocene. These results canbe used to further constrain the nature and regionalextent of Holocene environmental changes in the Medi-terranean region.

Site location

Lake Albano (ca. 293 m a.s.l.; 6 km2 surface; 175 mmaximum water depth) is a closed crater basin locatedin the Albani Hills in Latium, central Italy, ca. 25 kmsoutheast of Rome (Figure 1). This region of the Med-iterranean is climatically sensitive to both the NorthAtlantic-driven climate patterns and the monsoonalclimate (Chondrogianni et al., 1996a). It is, therefore,a key region to investigate the response of ecosystemsto these different climate regimes and to anthropogen-

ically-induced environmental changes through time (e.g.,Hoek, 2000). The sedimentary record of Lake Albano hasbeen extensively studied in the framework of the EU-project PALICLAS (Palaeoenvironmental Analysis ofItalian Crater Lake and Adriatic Sediments). The presentpaper discusses two cores recovered from 120-m wa-ter depth (PALB94-3A and B) that contain a completeHolocene sedimentary record of paleoenvironmentalchanges (Figure 1). Both cores were retrieved at the samelocation and are lithologically almost identical contain-ing the same sedimentary sequence.

Methodology

The Rock-Eval® Pyrolysis method was initially devel-oped to measure both the free hydrocarbon content and

Figure 1. Crater Lake Albano is a hydrologically closed basin that receives water mainly from atmospheric precipitation and underwater springs.Core PALB94-3, located at 120 m water depth, comprises the entire Holocene sequence as shown in seismic line Q-Z. Assuming a p-wavevelocity of 1460 m/s in the sediments, 10 ms of two-way travel time in the 3.5 kHz. seismic profile corresponds to 7.3 m in the sediment cores.Black lines outline conspicuous seismo-stratigraphic units that correspond to the lithological units indicated by roman numerals (afterChondrogianni et al., 1996b).

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the hydrocarbons released by thermal conversion ofkerogen in rock and sediment samples (Espitalie et al.,1985). The method consists of progressive heating ofsediment samples and measurements of the amounts ofhydrocarbons that escape from the sediments at dif-ferent temperatures. Three main signals are generatedduring the heating from ca. 200–600 °C: gaseous hydro-carbons (S

0), volatile hydrocarbons (S

1) and hydrocar-

bon components produced by the thermal degradationof humic substances (S

2). Two parameters are obtained

from the pyrolysis: The Hydrogen Index (HI), ex-pressed in mg Hc × g–1 C

org, which gives an estimation

of the amount of hydrocarbon contained in the sedi-mentary organic matter (i.e., chemical quality of theorganic matter); and the Oxygen Index (OI), whichrepresents the amount of oxygen in mg CO

2 × g–1 C

org.

These parameters are indicative of the H/C and O/Cratios of organic matter, respectively, and can be relatedto the origin of the organic matter. Talbot & Livingstone(1989) have summarised the application of these indi-ces to the study of lacustrine sediments.

The carbon stable isotope composition of the bulkorganic matter, δ13C

(OM), was measured from CO

2 gas,

generated by combustion of decarbonated samples, onan Optima mass spectrometer at the ETH-Zürich. Thereproducibility of the measurements is ±0.2 ‰ and theresults reported are per mil (‰) deviation with respectto the international standard V-PDB.

Total pigments, chlorophyll plus chlorophyll deriva-tives (CD); and carotenoids (TC) were extracted with90% acetone. For comparison with previous studies,chlorophyll derivatives are calculated as absorbanceunits × g–1 organic matter (Wetzel, 1970; Lami et al.,1994). Total carotenoids are expressed as mg g–1 or-ganic matter (Züllig, 1982). Specific algal pigmentswere determined by ion pairing, reverse phase HPLC(Beckman), and are expressed as nMoles × g–1 organicmatter (Lami et al., 1994).

We defined the degree of bioturbation of the coresediments based on the Bioturbation Index (BI) schemeproposed by Behl & Kennett (1996). A value of 1 onthe index represents distinct, continuous lamination; 2represents diffuse, discontinuous or irregular lamina-tion; 3 is bioturbated sediments with few patches of dif-fuse lamination; and 4 describes completely bioturbatedsediments. The quality of the lamination was furtherevaluated using radiographs of 2 × 4 × 1 cm sub-sam-ples at selected depth intervals.

The cores were dated using a combination of AMSC-14 dating of terrestrial macrofossils, pollen stratig-raphy and tephrochronology.

The Holocene record: Results and discussion

Sedimentology

The seismic section in Figure 1 shows that core PALB94-3 contains a complete Holocene sequence with the Ple-istocene/Holocene transition securely dated at 11.48 ka.B.P. The identified sedimentological units could be as-signed to equivalent seismic reflections. Chondrogianniet al. (1996b) distinguished six different lithologicalunits in the Lake Albano sequence (Figure 2): (I) well-laminated coloured muds including whitish and reddishlaminae; the white laminae representing authigenic car-bonates; (II) massive dark olive brown muds inter-bedded with sections of coloured laminated mudsand diatom laminae towards the bottom. This unit con-tains the distinctive ‘Avellino’ tephra dated at 4.1 cal.ka. B.P. (Calanchi et al., 1996); (III) diatom beds andlaminae intercalated with coloured laminated muds;(IV) laminated coloured muds interbedded with diatomlayers; (V) massive dark olive gray muds intercalatedwith vaguely laminated sections; and (VI) massive lightolive gray spotted silts.

Radiographs from different depths of the record (Fig-ure 2) illustrate variable development of millimetre-scalelamination throughout the sequence with the exceptionof the middle Holocene which is characterised by thickdiatom layers (up to 3.0 cm) and higher sedimentationrates. These fluctuations in the development of lami-nation have been semi-quantified using the BI (Figure3). The absence of lamination characterises Pleistocenesedimentation in the lowermost part of the core (UnitVI). An incipient development of laminae can be ob-served during the early Holocene (Unit V) that is soonfollowed (Unit IV onwards) by increasing lamination oc-casionally masked by the presence of microlaminationwithin the thick diatom mats (Units IV to II) duringintervals of high primary productivity (Ryves et al.,1996).

A robust chronology has been established showinga distinct change in sediment accumulation in Unit IV(Figure 2) separating the older part of the sequence(Units V and VI) with low rates (0.8 mm/a) from theyounger sediments (Unit III onwards) with relativelyhigher rates (1.1 mm/a).

Sedimentary organic matter

Figure 3 shows the variations in total organic carboncontent (TOC), hydrogen and oxygen indices (HI, OI)and in the bioturbation index (BI). Early Holocene sedi-

286

mentation is marked by a two-step increase in TOC atthe onset of units V and IV reaching highest values of6% (dry mass). A distinctly continuous decrease fol-lows through units III and II up to the tephra layer t1(4.1 ka. B.P.). Late Holocene sedimentation (above t1)displays average values of 5% with increasing contentsduring the last century. A similar picture is shown bythe HI variation. A gradual increase at the onset of theHolocene (Unit V) reaches a maximum (460 mg Hc ×g–1 C

org) in Unit IV. The generally continuous decrease

through Units III and the lowermost part of Unit II isterminated with a minimum at tephra t1. A secondgradual increase during late Holocene peaks in a maxi-

mum (500 mg Hc × g–1 Corg

) by the end of Unit II anddisplays averages values of 420 mg Hc × g–1 C

org up to

the top of the core.OI ranges between 120 and ca. 200 mg CO

2 × g–1.

Unit III and parts of Units II and IV show higher OIand comparatively lower HI values at the very well-laminated, diatom-rich intervals. Furthermore, the dia-tom-rich part of the core is characterized by a mirroreffect of HI to OI, whereas a general covariance isobserved in the remainder of the core. Figure 4a showsa HI-OI plot with the entire set of data, which approxi-mates the van Krevelen type plot of elemental H/C andO/C ratios. Although all samples plot within the Type

Figure 2. Log of core PALB94-3 including lithological units and X-radiographs at selected sediment depths. Shaded sections within the li-thology indicate diatom beds. Chronology is based upon combined AMS 14C age determinations; pollen data (well-dated rise of Olea and apeak in Castanea); and tephrochronology. Shaded intervals in the age/depth plot indicate lithological units. Notice that the most substantialchange in accumulation rate throughout the Holocene occurred at ca. 880 cm.

287

II organic matter there is a distinct differentiation be-tween the unit specific values.

The mineral matrix can produce artifacts on the cal-culation of both HI and OI (Espitale et al., 1985; Pe-ters, 1986). In particular, clays, due to their highlyreactive surface, can retain the hydrocarbon producedfrom the cracking of the organic matter through absorp-tion leading to lower S

2 values. This, in turn, will af-

fect the HI (HI = S2 normalized to TOC content) which

will show anomalously low values. This problem canbe eliminated by classifying the organic matter usingS

2 vs. TOC diagrams (Langford & Blanc-Valleron,

1990; Ariztegui, 1993). Figure 4b shows a S2 vs. TOC

diagram for the entire Holocene sequence where thedifferent units are represented by distinct symbols.Boundaries between organic matter types (dashed linesin Figure 4b) have been drawn following Langford andBlanc-Valleron (1990). As in the van Krevelen type dia-gram (Figure 4a) all samples fall within Type II organicmatter. A regression line can be fitted to the entire se-quence and to each unit separately as shown in Table1. A matrix effect is indicated by a positive ×-interceptof the regression line on the S

2 vs. TOC diagram, and

the position of the intercept is a measure of the amountof adsorption. The ×-intercept in Figure 4b is 1% TOC

which indicates the amount of organic material (witha given HI) that must be present before hydrocarbonsare liberated from the sediments by pyrolysis. Theoverall high degree of correlation for all the units in-dicates a common origin for each individual group ofsamples. According to Langford et al. (1990) ten timesthe slope of the regression line gives a good estimateof the percent of pyrolizable hydrocarbons in the totalorganic carbon. When the entire sedimentary sequenceis considered, this fraction is 45.1% (Table 1). The es-timated percent of pyrolizable hydrocarbons increasesat times of relatively high productivity, as during thedeposition of Units I to IV, whereas times of decreas-ing productivity (Units V and VI) display substantiallylower values.

The ratio of chlorophyll derivatives (CD) to totalcarotenoids (TC) provides a good estimation of thebalance between allochthonous and autochthonoussources of organic matter (Guilizzoni et al., 1982;Sanger, 1988). Figure 5a shows that the average valueof this ratio for most of Lake Albano sedimentary or-ganic matter has been produced in the lake, althoughhigher than average values are observed in the upper-most part of Unit II. A few peaks reaching ~ 120, ob-served in Unit III, are associated with turbidites (i.e.,higher contribution of allochthonous material). A dia-tom study of the Lake Albano sequence showed thatUnit III, and to a lesser extend Unit IV and the lowerpart of Unit II (up to t1), contain mostly diatom-pro-duced organic matter (Ryves et al., 1996). Previousstudies have shown that diadinoxanthin is a characteris-tic carotenoid of diatoms, whereas echinenone char-acterizes cyanobacteria. Thus, the diadinoxanthin/echinenone ratio, shown in Figure 5a, can be used asan indicator of the relative contribution of diatomsand cyanobacteria, respectively. While most of thesequence displays diadinoxanthin/echinenone ratio≤ 1, the ratio in Unit III increases to values of 3 andhigher (i.e., mostly diatom-produced organic matter)confirming previous observations.

It has been shown, in both marine and lacustrineenvironments, that cyanobacteria have H/C ratios by12% higher than diatoms (1.87 and 1.66, respectively;Pelet, 1981). Therefore, relative changes in the contri-bution of these organisms to the sedimentary organicmatter should also imply variations in the HI values.

Isorenieratene is a diagnostic pigment for a groupof phototrophic bacteria (e.g., Phaeobium) that growsunder strongly anoxic conditions (Züllig, 1985). Con-centrations of isorenieratene in the Lake Albano coredisplay the lowest values in Unit III and into Unit II

Figure 3. Percent TOC, HI (solid line) and OI (dashed line) wereanalysed in core PALB94-3A whereas BI was described in corePALB94-3B. Notice the different scales for HI and OI. Interpretedlithological units are shaded and labelled with roman numerals. Theindicated unit time-boundaries are based on the age model.

288

(Figure 5a). This implies that the oxygen state of bot-tom waters was substantially different to that observedin the remaining of the Holocene sequence. High levelsof productivity have been suggested as an alternativeexplanation for the low concentration of isorenier-atene in the sediments due to limiting light availabil-ity (Ariztegui et al., 1996). Although distribution ofother bacterial pigments in the sediments and the BIindex indicates that anoxia during deposition of unit IIIwas not as intense, the poor correlation observed be-tween HI and isorenieratene concentrations (Figure 5b)suggests that changes in the oxygen-status of the wa-ter column have not been the most important factor forthe preservation of the sedimentary organic matter. Fur-

thermore, a careful analysis of the data indicates thatvariations in HI and OI in the Holocene sedimentarysequence of Lake Albano are most probably related tochanges in the dominant primary producers within thelake. Figure 5c displays a HI vs. diadinoxanthin/ ech-inenone plot discriminating the different lithologicalunits. Unit III displays relatively low HI (average 300)and the highest values of this pigment ratio in the wholesequence. The lowermost HI values within Units II andIII are due to the higher contribution of terrestrial ma-terial during the deposition of the previously mentionedturbidites.

The δ13C(OM)

of selected samples shows that the dia-tom-rich units with comparatively low HI display morepositive carbon isotopic compositions (Figure 5d). Lab-oratory experiments and phytoplankton studies in bothmarine and lacustrine systems have shown that bloom-forming diatoms can be 6‰ more positive on averagethan other associated plankton and the bulk particulateorganic matter (Fry & Wainwright, 1991; Tyson, 1995and references herein).

Paleoenvironmental reconstruction

The collective sedimentary and organic matter evidenceimplies that most of the variations in HI values ob-served in Lake Albano Holocene sediments can be at-

Figure 4. (a) Rock-Eval® van Krevelen-type diagram for the entire Lake Albano sedimentary organic matter. Symbols indicate the interpretedunits and dashes lines trace maturity paths for organic matter types. (b) S2 vs. TOC diagram. Dashed lines define boundaries of organic mattertypes (see text), whereas the solid line indicates the regression line for the entire set of data.

Table 1. The average HI for each sedimentary unit is given by theslope of the regression line × 10 representing the percent ofpyrolizable hydrocarbons

Unit no. Fitted regression line % pyrolizable Hc

Total y = 4.7562 + 4.5106× R2 = 0.766 45.1I y = 7.9535 + 5.1257× R2 = 0.890 51.2II y = 5.9455 + 5.1382× R2 = 0.627 51.4III y = 12.718 + 5.1356× R2 = 0.837 51.3IV y = 5.9794 + 4.8077× R2 = 0.759 48.1V y = 3.2069 + 4.0849× R2 = 0.990 40.8VI y = 5.6803 + 1.4516× R2 = 0.644 14.5

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tributed to changes of algal source between cyano-bacteria or diatom dominance and, to a lesser extent,the fluctuating degree of water-column anoxia. Varia-tions in these conditions are in turn related with envi-ronmental changes through time.

High productivity levels characterise the entire Hol-ocene sequence, as also estimated by reconstructedtrophic status (Ryves et al., 1996). The early Holocenedeposits are particularly organic-rich, as shown by hightotal organic content (TOC) as well as by relatively highHI values (Figure 3). During this interval (between ca.9.8 and 6.6 cal. ka. B.P.) TOC values averaged 6%, andthe laminated nature of the lake deposits as shown bylow BI suggests that hypolimnetic anoxia developed inrelatively deep waters. Cyanobacteria dominance in-dicates an increasing stabilisation of the water columnin warming climates (Züllig, 1989) as well as low N toP ratios (Shapiro, 1990). In fact, stratification, reducedfree CO

2 concentrations during summer photosyn-

thesis; and phosphorous accumulation are common

conditions triggering natural cyanobacteria blooms(McGowan et al., 1999). A similar scenario probablyoccurred in the early Holocene.

Between 8.1 and 7.6 cal. ka. B.P., however, there isa decrease in TOC fluxes and HI values, a substantialdecrease in pigment concentrations, a marked decreasein isorenieratene ratios and dominance of diatoms overother types of algae (Ryves et al., 1996), all of whichsuggests a short-lived episode of reduced anoxia con-temporaneous with a decline in primary productivityand/or preservation. In addition, Manca et al. (1996)reported a drop in concentration and diversity of chy-dorids (type of Cladocera) during this short interval tovalues characteristic of the Würm late-glacial period,suggesting that a short-lived cold event affected theregion during the mid-Holocene. Comparable evidencehas been found in other terrestrial and marine recordsof the Mediterranean region and equated with the glo-bally distributed Early-Middle Holocene transition(EHMT) cooling event (Ariztegui et al., 2000).

Figure 5. (a) Diagnostic pigment curves for biological sources and degree of anoxia in Lake Albano sediments (core PALB94-3A). CD/TC= Chlorophyll derivatives/total carotenoids. Pigment data are expressed in nM gLOI–1. (b) The low regression line for the HI vs. isorenieratenedistribution (dashed line) implies that the degree of anoxia in the water column did not play a major role increasing HI values. (c) Adiadinoxanthin/echinenone vs. HI diagram for the different units reveals that comparatively low HI can be associated with the increasingcontribution of diatoms (particularly Unit III). (d) A δ13C(OM) vs. HI plot of selected samples shows that higher contributions of diatoms-pro-duced organic matter is reflected in comparatively lower HI values (see text).

290

From 7.6 to ca. 4.0 cal. ka. B.P. the progressive toalmost complete dominance of diatoms indicated bycomparatively low HI, enriched δ13C

(OM), high dis-

tinct pigment concentrations as well as the presenceof microlamination imply substantial changes in theenvironmental conditions. These microlaminae oftenmask the macrolamination giving an unexpected mas-sive aspect to the sediments (i.e., high BI values). Theplankton cycle of modern seasonally stratifying lakewaters demonstrate the interplay of turbulent mixing,light and nutrient distributions (Tyson, 1995). Cyano-bacteria usually develop during stable, well lit but lownitrogen conditions – as in Lake Albano – whereasdiatoms blooms in many temperate lakes occur pref-erentially during the season with comparatively moreturbulent waters and higher silica concentrations. Pol-len and magnetic evidence indicate increasing catchmentdisturbance and erosive input towards the younger sed-iments (Rolph et al., 1996). The latter would havetriggered the observed change in the dominant plan-kton population and an overall reduction in aquaticproductivity (Ryves et al., 1996). Whether the com-positional changes in the organic matter are related toclimate or human activities in the lake catchment can-not be answer conclusively from the existing evidence.Simulations with a synchronously coupled atmos-phere-ocean-vegetation model as well as numerouspaleodata in Europe and North Africa, however, im-ply substantial changes in the climate system around6.0 ka. B.P. (Ganopolski et al., 1998 and referencesherein).

At ca. 3.8 ka. B.P, a distinctive environmental altera-tion can be interpreted from both sedimentary organicmatter and lithological features. Most often these cha-nges have been interpreted as associated to deforestationand other human-induced disturbance of the environ-ment for early-populated areas such as the Lazio region(Birks, 1986; Roberts, 1989). Pollen profiles and themagnetic record of lakes Albano and Nemi support thishypothesis (Rolph et al., 1996). However, well-con-strained environmental changes, such as lake level re-gressions and glacial advances, have been documentedin remote regions of Africa (Talbot et al., 1984, Halfmanet al., 1994; Lamb et al., 1995) whereas increasingeffective moisture have been described for the At-acama Altiplano in Northern Chile (Valero-Garcés etal., 1996). Moreover, results of paleolimnological stud-ies in the Sahara area provide evidence of a major de-cline in precipitation at around 4.5 ka. B.P. (Ritchie etal., 1985). Thus, there is global evidence of substan-tial rapid moisture balance shifts during this last part

of the Holocene (Kelts, 1997) that might have influ-enced human migration and activity, rather than viceversa.

Conclusions

Combined sedimentological, bulk and detailed analy-sis of the organic matter in Lake Albano indicateshigh levels of primary productivity during the entireHolocene. Changes in the dominant producers are rec-ognised at different intervals using HI, OI and the iso-topic composition of the bulk organic matter. Thesechanges are further confirmed by more detailed analy-sis of the sedimentary organic fraction and can be usedto infer former limnological conditions. During both theearly Holocene and the last 4.0 ka. B.P. cyanobacteria-produced organic matter was higher than the diatomcontribution. Comparatively low HI and high OI val-ues during the middle Holocene, in contrast, indicateintervals where diatoms are the predominant primaryproducers. Following the diatom-dominated productiv-ity interval a sharp decrease in organic matter contentis observed at around 4.0 ka. B.P. Approximately 300years later, this drop in primary productivity is followedby the accumulation of organic-rich, cyanobacteria-dominated sediments that display the highest HI val-ues of the sequence. These variations are consistent withother records from the region (Magri, 1997; Ramrath etal., 1999). Although human activity in the catchmentis evident, the global signal indicates that changes inclimatic variables such as wind intensity, precipitationand temperature are the most probable factors affect-ing these environmental changes.

Acknowledgements

We are indebted to all the participants of the EU-Envi-ronment Project PALICLAS (contract EV 5V CT93-0267) for stimulating discussions on the differentaspects of the multiproxy data set as well as their openand helpful collaboration. We thank W. T. Andersonand C. Vasconcelos for their help in the Stable Iso-topes Laboratory of the ETH-Zürich and I. Hajdas forthe radiocarbon dating. The authors acknowledge theconstructive and careful comments of J. Teranes andan anonymous reviewer. The Swiss participation inthe EU-Project was supported by the Bundesamt fürBildung und Wissenschaft (BBW No. 93.0276).

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