Holocene diatom stratigraphy in the Archipelago Sea,northern Baltic Sea

Two sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study the Holocene palaeoenvironmental history of the area. The diatom record was divided into local diatom assemblage zones (LDAZ). An age model was constructed using independent palaeomagnetic and AMS-¹⁴C methods. The early history of the Archipelago Sea was freshwater. Initial brackish-water influence is observed at 7,950 ± 80 cal. BP (LDAZ4), but fully brackish conditions were established at 7,700 ± 80 cal. BP (LDAZ5). Diatom assemblages indicate increasing salinity, warming climate, and possible increasing trophic state during the transition from lacustrine to brackish-water conditions. The decreasing abundance of Pseudosolenia calcar-avis (Schultze) Sundström and the increasing abundance of the ice-cover indicator species Pauliella taeniata (Grunow) Round and Basson indicate reduced salinity and climatic cooling after ～5,000 cal. BP. LDAZ boundaries do not always correlate with changes in the sediment appearance, which underlines the importance of defining biostratigraphic boundaries independently to the sediment visual character, in contrast with the conventional practice for classifying the Baltic Sea sediments.     

The late-glacial and early-Holocene palaeoecology of cladoceran microfossil assemblages at Kråkenes, western Norway, with a quantitative reconstruction of temperature changes

Cladoceran microfossil remains were analysed from a sediment core taken from a lake basin at Kråkenes, western Norway. The sequence included immediate post-glacial conditions (ca. 12,300 ¹⁴C BP), the Allerod, Younger Dryas, and early Holocene to approximately 8,500 ¹⁴C BP. The interpretation of changes in the cladoceran assemblages is based on the known ecology of the taxa, the documented environmental history of the study sequence, the variations in the organic content of the sediment, the radiocarbon dates, and the results of analyses of other biotic groups, including diatoms, macrophytes, and chironomids. In addition, a quantitative reconstruction of changes in air temperature is presented for the study period. This reconstruction is based on transfer functions developed from a separate Swiss surface-sediment cladoceran data set.The cladoceran assemblages throughout the sequence are dominated by littoral chydorid taxa. Bosmina, Daphnia, and Simocephalus represent the open-water component of the zooplankton. Chydorus piger and Daphnia were the only immediate post-glacial pioneer taxa. A rapid proliferation of the open-water and littoral cladoceran taxa began with the onset of the Allerod and persisted for approximately 1,000 yrs. At the start of the Younger Dryas a local glacier formed and drained into the lake, causing a sudden decline in chydorid diversity, with only Chydorus sphaericus and Acroperus harpae persisting throughout this period. Chydorid diversity started to recover in the upper Younger Dryas and continued in the early Holocene. Progressive acidification and oligotrophication are also discernible from the cladoceran assemblages present in the Holocene.The reconstructed mean summer air temperature was from 8-21 °C, with prediction errors of 1.8-2.5 °C. The Allerod was only slightly warmer than the Younger Dryas period, but a progressive increase in temperature is apparent during the early Holocene. In conclusion, the results of this study provide a further demonstration of the value of cladocera as indicators of a variety of palaeoenvironmental parameters, including temperature.     

Paleolimnology of three Jackman Sound Lakes,Southern Baffin Island,based on down-core diatom analyses

Three lakes, informally named Lakes Inqua, Instaar and Mercer, from the Jackman Sound area in Southern Baffin Island were cored. The elevation of the three lakes are 36 m above high tide (aht), 10 m aht and 1 m aht. The cores from Lakes Inqua and Mercer penetrated the marine/lacustrine boundary; the core from Lake Instaar is estimated to have come within ca. 5 cm of this boundary. In addition, grab samples and water samples were retrieved from the lakes. The cores have been ¹⁴C dated (AMS). Emergence of the three lake basins from the sea occurred approximately 8650 BP (Lake Inqua), 8630 BP (Lake Instaar) and 7900 BP (Lake Mercer, averaged date). This suggests a very rapid rate (>5 m/100 yrs) of glacio-isostatic rebound for this area. Down-core analyses of diatom assemblages indicate that the paleolimnology in the three lakes was similar. The diatom analyses suggest a change in climate between approximately 8000 and 6000 BP, which is also reflected in the marine record from the same area, and the onset of the neoglacial period around 4500 to 3800 BP. A decrease in diatom-inferred pH is evident in the lakes' development and can probably be related to the general change of the watershed vegetation with time.     

A 12.8-ka-long palaeoenvironmental record revealed by subfossil ostracod data from lacustrine freshwater tufa in Lake Sinijärv,northern Estonia

High-resolution quantitative analysis of ostracod assemblages from 4.3-m-thick freshwater tufa-rich sediments, deposited during the last 12.8 ka in Lake Sinijärv, northern Estonia, yielded information on water level, trophic state conditions, and temperature changes since the late glacial. AMS ¹⁴C dates from aquatic mosses provided time constraints on the palaeoenvironmental development of the region. In the ostracod assemblage structure, four faunal zones (OFZ) were determined. The most significant change in the ostracod fauna occurred at 10,590 cal. y BP, when a typical littoral, polythermophilic fauna was replaced by a mostly sublittoral, species-rich meso- to stenothermophilic fauna. The ostracod data indicate two major low-water-level periods in the lake at 12,800–10,590 and 7,600–3,700 cal. y BP. Sediment analysis indicates the most intensive tufa precipitation occurred during these low stand periods, rather than during the warmest climate in Estonia between 8,000 and 4,500 cal. y BP. The late glacial low water level in the groundwater-fed Lake Sinijärv at 12,800–10,590 cal. y BP coincides partly with the regression in the Lake Peipsi basin (14,000–12,100 cal. y BP) and with the last drainage event of the Baltic Ice Lake at 11,600 cal. y BP. The low-water-level period in Lake Sinijärv occurred earlier than in lakes in the SE sector of Scandinavian glaciation. The change from low to high water level in Lake Sinijärv at 10,590 cal. y BP preceded the first post-glacial transgression events in the small lowland lakes of Estonia, southern Sweden, Poles`ye in Belarus, and Valday in NW Russia. In general, the mid-Holocene low-water-level period in Lake Sinijärv between 7,600 and 3,700 cal. y BP is concurrent with the regressions in the lakes of the SE sector of Scandinavian glaciation.     

Paleoenvironmental reconstruction of Holocene coastal depositsfrom the Southeastern Buenos Aires Province, Argentina

An estuarine sequence outcropping at La Ballenera Creek (BuenosAires Province), dated between 6,800 and 4,100 ¹⁴C years BP, wasinvestigated for diatoms and molluscs. The sea level history along the BuenosAires coastline has been reconstructed from discrete beach ridge sequencesalong low-lying plains. The La Ballenera profile is located on a cliffcoast where a sequence recorded environmental changes. Fifty-eightdiatom species were grouped based on their salinity and life formcharacteristics. Cluster analysis allowed the division of the sequence intothree diatom zones. Mollusc tolerances were also used to discern theenvironmental changes induced by sea-level fluctuations. The base of thesequence recorded the initiation of the marine influence about6,790–6,200 years ago. Diatom assemblages consist ofbrackish-freshwater tychoplankton accompanied by brackish benthicdiatoms. Among the molluscs, the freshwater-brackish species Heleobiaparchappii dominates. Between 6,200 and ca. 4,800 ¹⁴C years BP, anestuarine lagoon environment is indicated by benthic and epiphyticmarine-brackish diatoms, as well as by the estuarine snail Heleobiaaustralis. After 4,800 ¹⁴C years BP, the diatom assemblages and therelative abundance of freshwater molluscs indicate a marshy environment withlower salinity content. The La Ballenera records the salinity changes thataffect an estuary that infilled during the 2,700 years after the maximum sealevel reached in mid-latitudes of South America.     

Late-Glacial and Holocene Record of Lake Levels of Mathews Pond and Whitehead Lake,Northern Maine,USA

Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 ¹⁴C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 ¹⁴C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 ¹⁴C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 ¹⁴C yrs BP and remained low until 3000 ¹⁴C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes. Electronic supplementary material to this article is available at and accessible for authorized users.     

The ostracode record from Harris Lake, southwestern Saskatchewan: 9200 years of local environmental change

Holocene paleoenvironments of Harris Lake, southwestern Saskatchewan, are reconstructed from the ostracode stratigraphy of a 10.4 m sediment core. Twenty three taxa, representing nine genera, were identified and counted from 113 samples. At each depth, a theoretical faunal assemblage was derived from the raw counts. The mean and variance of chemical, climatic and physical variables were inferred from modern analogues of the fossil assemblages, using existing autecological data from 6720 sites, mostly in western Canada. These data suggest four paleoenvironments: an early-Holocene (9240–6400 years BP) variable climate supporting aspen parkland vegetation; the warm dry hypsithermal (6400–4500 years BP); a short transitional period of ameliorating climate and expanding subboreal forest (4500–3600 years BP); and the present environment since 3600 years BP. A change in regional climate with the draining of Glacial Lake Agassiz (ca. 8500 years BP) and landsliding in the watershed (ca. 4000 years BP) caused relatively rapid environmental change. The ostracode record generally corroborates the interpretations of other proxy data previously published for Harris Lake. Most of the discrepancy involves the timing and severity of maximum Holocene warmth and aridity. Peak aridity interpreted from the pollen data is earlier than in the other proxy records. Both the diatoms and ostracodes indicate highest paleosalinity between ca. 6500 and 5000 years BP, but maximum salinity in the diatom record occurs between ca. 6000–5700 years BP, whereas the ostracode-inferred salinity is relatively low at this time and peaks later at ca. 5000 years. Neither of these reconstructions suggests the short episodes of hypersalinity interpreted from the mineralogy.     

Quantitative Holocene climatic reconstruction from Arctic Russia

Vegetation changes reflected in fossil pollen spectra are a primary source of information about climate fluctuations in the past. A statistical-information (transfer function) method based on the correlation of recent pollen spectra with modern climate conditions has been used to reconstruct Holocene climatic changes from fossil pollen. Climatic variables used for the reconstructions are the mean annual, January, July temperatures and annual precipitation. Peat sections with pollen and ¹⁴C data from the Arctic Russia were used for the reconstructions. The reconstructed climate fluctuations are similar to the climate changes obtained from many sites in the former USSR. A clear signal for Younger Dryas cooling, 11,000-10,000 yr BP and early Preboreal warming is apparent. The early Preboreal (10,000-9000 yr BP) was the warmest time for sites from modern coastal and island areas. The warm interval occurred in the Boreal period, about 8500 yr BP. According to the reconstructions the warmest time for non-coastal areas was the last half of Atlantic period, 6000-4500 yr BP. Other warm intervals were reconstructed about 3500 and 1000 yr BP. Reconstructions show that warming periods are primarily defined as times of increased summer temperatures, and cooling periods as time of decreased winter temperatures. The precipitation followed the temperatures: during the warming periods precipitation increased and during the cooling periods it decreased. Precipitation maximum, about 100 mm higher than present, are reconstructed for the warmest interval, 6000-4500 yr BP at all sites.     

Early Holocene Tree Growth at a High Elevation Site in the Northernmost Scandes of Sweden (Lapland): A Palaeobiogeographical Case Study Based on Megafossil Evidence

The paper focuses on early Holocene tree growth and alpine tree-limits in the northernmost Swedish Scandes (Lapland). Megafossil wood remnants in peats and lakes were searched for over a large area at elevations high above the modern tree-limits. Wood of Pinus sylvestris, Betula pubescens spp. tortuosa and Alnus incana was discovered near the shore of a small lake (999 m a.s.l.) c. 500 m higher than today's tree-limit of Pinus sylvestris in this region. Radiocarbon dating yielded values of unprecedented age, c. 8500–8100 ¹⁴C years BP for all three species and in addition 5400–4500 ¹⁴C years BP for Betula and Alnus. The highest position of the Pinus tree-limit occurred during the early Holocene, which compares well with the situation reconstructed by megafossils in the southern Scandes. It now appears that the long-term tree-limit and climate histories are broadly the same in entire mid- and northern Fennoscandia. Corrected for glacio-isostatic land uplift, the tree-limit record suggests that the summers were c. 2.4°C warmer than today at 8500 ¹⁴C years BP . A dry continental climate with substantial seasonal contrasts is likely to have prevailed during this period, which restricted the occurrence of glaciers and glacier activity. Most circumstances point to the overriding importance of the Milankovitch orbital theory for pacing or forcing the long-term postglacial climate change. The results are inconsistent with most inferences based on pollen, certain macrofossil records and general circulation simulations. These proxy environmental histories have frequently advocated a mid-Holocene thermal optimum and an oceanic and humid climate in northern and western Fennoscandia during the early Holocene. The uncovered discrepancy between the outcome of the objective and factual megafossil method and more subjective/inferential microfossil methods should be important for Quaternary plant ecology in general, stressing the usefulness of megafossil studies.     

Cyanobacteria as a trigger for increased primary productivity during sapropel formation in the Baltic Sea—a study of the Ancylus/Litorina transition

A sediment sequence from the Gotland Basin, representing the Ancylus Lake/Litorina Sea transition, was analysed for pigments, stable isotopes (δ¹³C and δ¹⁵N), diatoms and carbon and nitrogen content. The negative correlation between the cyanobacterial specific pigment zeaxanthin and δ¹⁵N supports the hypothesis that cyanobacteria in the early Litorina Sea indeed were nitrogen fixers. Their incorporation of nitrogen could have acted as a trigger for eutrophication during the Litorina Sea stage of the Baltic Sea. As cyanobacteria normally flourish in eutrophicated waters, the increasing concentrations of zeaxanthin during early Litorina Sea also corroborate that high primary production was an important part of the formation of organic rich (sapropel) sediments in the Baltic Sea. The first occurrence pre-dates the formation of laminated sediments, but the peaks of both zeaxanthin and organic carbon are within laminated sequences. This implies that the oxygen conditions of the bottom water also play a major role in the formation of sapropel sediments.     

Preliminary evidence of environmental changes at Lake Bambili (Cameroon, West Africa) since 24,000 BP

Preliminary analyses of diatoms, phytoliths, and siliceous protozoan plate records in a 16 m sediment core from Lake Bambili (Cameroon, West Africa; 2264 m AMSL) provide evidence of pronounced climatic changes in the West Cameroon Highlands since ~24,000 ¹⁴ C yrs BP. Percentages of planktonic diatoms rose with increased precipitation:evaporation ratios around 24,000 BP, ~15,000-9500 BP, and ~2400-2000 BP. Since 15,000 BP, Bambili appears to have experienced climatic changes of comparable timing and magnitude, but with signs in opposition to those registered in the West African lowlands. Much of this pattern may be attributable to variability in montane stratiform cloud formation, which in turn is related to paleo-wind regimes and upwelling dynamics in the Gulf of Guinea.     

Late-glacial and early-Holocene Trichoptera (Insecta) from Kråkenes Lake, western Norway

Fossil Trichoptera (caddisfly) remains have been identified and quantitatively recorded in the late-glacial and early-Holocene sediments from Kråkenes Lake, western Norway. The sediment sequence was deposited between 12,300 and 8850 ¹⁴C BP, covering the Allerød, Younger Dryas, and early-Holocene periods. The first Trichoptera were recorded at 12,000 ¹⁴C BP, and during the Allerod a diverse assemblage of Limnephilidae taxa developed in the lake. By about 11,400 ¹⁴C BP the relatively thermophilous Polycentropus flavomaculatus and Limnephilus rhombicus were present, suggesting that the summer water temperature was at least 17 °C. This temperature fell by 5-8 °C at the start of the Younger Dryas, and the thermophilous taxa were replaced within 20-40 ¹⁴C yrs by Apatania spp., including the arctic-alpine A. zonella, suggesting a maximum summer water temperature of 10-12 °C. The Trichoptera assemblage was impoverished in numbers and in diversity over the next 200 yrs as the severe conditions of the Younger Dryas developed. As soon as temperatures rose and glacial meltwater and silt input ended about 700 ¹⁴C yrs later, the resident Apatania assemblage expanded immediately, within 10 yrs. About 130 yrs later, thermophilous taxa replaced Apatania, and a much more diverse assemblage than in the Allerod occupied the varied habitats made available by the development of the Holocene lake ecosystem. The 130 yr delay may have been caused by a gradual temperature increase crossing a critical threshold, or by the time taken for thermophilous taxa to migrate from their Younger Dryas refugia.     

Stratigraphy of the mid-Holocene black bands in Lakes Michigan and Huron: Evidence for possible basin-wide anoxia

The post-glacial history of the Great Lakes has involved several changes in lake levels throughout the latest Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, outlet incision and isostatic rebound. The final lowering of lake levels occurred at approximately 7600 ¹⁴C yr BP, after which lake levels began to rise again to the Nipissing highstand at approximately 4700 ¹⁴C yr BP. During this time of rising lake levels, black bands of iron sulfide were being formed in the sediments of all five of the Great Lakes. These bands signify suboxic to anoxic conditions, at least within the sediments and possibly at the sediment-water interface, during the middle Holocene warm interval. During this interval, the climate was warmer and drier than present, possibly resulting in the occasional absence of seasonal turnover in the lakes. We examined a series of piston cores from northern Lakes Michigan and Huron and found that the black bands are correlatable among cores taken from within the same basin. The observation that the banding can be correlated suggests a basin-wide cause, near-bottom or sub-bottom anoxia in the northern Michigan and northern Huron sediments during the mid-Holocene warm period. The sedimentary and geochemical processes in the Great Lakes during the middle Holocene warm interval are good indicators of possible future scenarios for the lakes as a result of global warming, as 21^st-century temperatures are predicted to reach similar levels due to increased concentrations of greenhouse gases.     

Diatom,pollen, and geochemical evidence for the palaeosalinity of Medicine Lake,S. Dakota,during the late-Wisconsin and early Holocene

Medicine Lake is a highly saline meromictic lake in eastern South Dakota. A lithostratigraphic and biostratigraphic study of the late-glacial and early post-glacial sediments shows that it was a relatively deep dilute freshwater lake during the period of the Picea pollen zone. When spruce forest changed to a Betula and then to a Quercus/Ulmus woodland in the early post-glacial period, the lake water became more concentrated but remained fresh. However, during the subsequent rapid transition to prairie in the early Holocene, when Gramineae, Ambrosia, and Artemisia dominated dry-land vegetation, the freshwater diatom flora was progressively replaced by a saline assemblage characterized by Cyclotella quillensis, Chaetoceros, and eventually Cyclotella caspia. The lake became permanently saline at about 9000 yr BP. A comparison of the fossil diatom assemblages with surface-sediment samples from a range of lakes in the Dakotas indicates that the change involved an increase in conductivity from about 500 S cm^–1 in the late-glacial period to > 10 000 S cm^–1 in the early Holocene. This rapid change is also marked by an abrupt increase in sulphate concentration in the sediment, the first appearance of bands of gypsum crystals, and the beginning of a well-laminated core sequence that continues through the remaining sediment record. Conditions of high salinity have prevailed to the present day.Contribution 299, Limnological Research Center.     

Luminescence-dating zeroing tests in Lake Hoare, Taylor Valley, Antarctica

In two of the perennially ice-covered lakes in the McMurdo Dry Valleys, lakes Hoare and Bonney in the Taylor Valley, bottom water has ¹⁴C ages of 2.7 ka and 10 ka (respectively), rendering ¹⁴C ages of bottom sediments highly problematic. Consequently, we tested the effectiveness of thermoluminescence (TL) zeroing in polymineral fine silt material from several depositional environments around and on the lake (stream suspensions, ice-surface sand dune, and silty sand from near the top of the more-than-3m-thick ice). We also conducted TL and infrared-stimulated-luminescence (IRSL) dating tests on material from three box cores recovered from the bottom of Lake Hoare, in a transect away from the abutting Canada Glacier. We observed effective zeroing of light-sensitive TL in suspended silt from one input stream and less effective zeroing from another stream. We observed effective zeroing of light-sensitive TL also in silt from a glacier-proximal eolian ice-surface dune and from sand from within the upper 5 cm of ice. In contrast, in box-core 1, the bottom sediment yielded minimum TL apparent ages of 1500-2600 yrs, with no discernable stratigraphic depth trend. IRSL dating applied to the same box-core samples produced significantly lower age estimates, ranging from ~600 ± 200 yrs to 1440 ± 270 yrs top-to-bottom, an improvement over the depth-constant ~2200 yrs TL ages. In two other cores closer to the Canada Glacier, IRSL ages from ~600 ± 200 yrs (top) to ~ 2900± 300 yrs (at depth) were measured. Not only are the IRSL ages a significant improvement over the TL results, but the near-core-top IRSL ages are also a dramatic improvement over the ¹⁴C results (~2.7 ka). IRSL dating has a demonstrated potential to supplant ¹⁴C dating for such antarctic lacustrine deposits.     

History of a presently slightly acidic lake in northeastern Alberta,Canada as determined through analysis of the diatom record

The Holocene sedimentary diatom record from Otasan Lake, Alberta, has been analyzed to determine the development of this presently slightly acidic lake. The changes in the lake have been linked to the development of the Sphagnum-dominated catchment. Analysis of the stratigraphic data revealed four distinct zones. The lake record began ca. 8200 yrs BP with a benthic and alkaline diatom assemblage dominated by Ellerbeckia arenaria (Moore) Crawford. At ca. 7300 yrs BP planktonic species began to increase and dominate indicating increased water levels, decreased turbidity, and increased nutrient levels. Throughout the Holocene the peatland in the catchment encroached toward the modern lake margin and by ca. 5000 yrs BP lake acidity had changed sufficiently such that acidic diatom species dominated. Tabellaria flocculosa (Roth) Kütz.v. flocculosa Strain IIIp sensu Koppen dominated the record from ca. 5000 to ca. 3100 yrs BP. The lowest lake water pH was inferred for this zone. From ca. 3100 yrs BP to the present Fragilaria species, primarily F. construens v. venter (Ehr.) Hustedt, dominated the diatom assemblage. Diatom productivity and inferred pH were interpreted as stable. From correspondence analysis of the fossil samples, and from species assemblages, underlying gradients of pH, nutrient level, and water depth were inferred. The change from alkaline to slightly acidic conditions took place between ca. 8200 and ca. 5000 yrs BP. From ca. 3000 yrs BP to the present, lake water pH has remained fairly constant. Nutrient levels and water depth were inferred to have altered together. After ca. 8200 yrs BP, nutrients and water level began to increase until ca. 6000 yrs BP. Then, there was a gradual decline in these variables over the most acidic zone until ca. 3000 yrs BP, after which they, too, have remained fairly constant. Dominant Boreal Upland Vegetation was established by ca. 7200 yrs BP, and it was inferred that dominant climate patterns had been established at that time, but small changes in climate have occurred and the landscape in northeastern Alberta has only been stable for the last 3000 years.     

Diatom responses to late-glacial and early-Holocene environmental changes at Kråkenes, western Norway

A stratigraphic diatom sequence is presented for the period from 13,870-9,170 cal BP from Kråkenes Lake, western Norway. Changes in species assemblages are discussed with reference to the changing environmental conditions in the Allerød, Younger Dryas, and the early Holocene and to the development of the aquatic ecosystem. The site is sensitive to acidification, and diatom-based transfer functions are applied to estimate the past pH status. The combination of rapid sediment accumulation together with an excellent calibrated radiocarbon chronology means that the rate of inferred pH change and associated increase in [H⁺] can be assessed and compared with recent, anthropogenically acidified situations.The Allerød diatom assemblages are dominated by benthic taxa particularly Fragilaria species, indicating an unproductive, alkaline, turbid, and immature system. Diatoms are absent in the early part of the Younger Dryas, but subsequently a sparse planktonic flora develops reflecting decreased turbidity and/or increased nutrient supply. A clear sequence of diatom assemblages is seen in the early Holocene. A short-lived peak of Stephanodiscus species indicating a period of increased nutrient availability occurred at ca. 11,500 cal BP. Throughout the early Holocene, acid-tolerant species increasingly replaced less acidophilous, circumneutral taxa.The lake became slightly more acid during the Allerød, but this was statistically insignificant in a trend test involving regression of pH or [H⁺] in relation to age. Diatom-inferred pH declined rapidly during the early Holocene period investigated (9,175-11,525 cal BP) with a statistically significant overall rate of 0.024 pH units per 100 yrs. This consisted of an older (ca. 11,525-10,255 cal BP) phase, where pH fell more rapidly at up to 0.095 pH units per 100 yrs; and a younger phase after ca. 10,500 cal BP where the pH fall was extremely slow (0.008 pH units per 100 yrs) and was not statistically significant.In the Allerød a combination of low catchment productivity together with disturbance, weathering, and minerogenic inwash ensured that the base-cation status remained relatively high. In the Holocene the catchment soils stabilised and base cations were sequestered by terrestrial vegetation and soil. This resulted in reduced base-cation leaching and this, together with the production of organic acids caused the lake to acidify, reaching an equilibrium by ca. 10,000 cal BP.     

Holocene environmental dynamics of south-eastern Brazil recorded in laminated sediments of Lago Aleixo

Environmental changes of the last 9,300 years were reconstructed by geochemical and pollen analyses of a 14-m-long, laminated sediment core from Lago Aleixo, south-eastern Brazil. Fossil pollen assemblages indicate open savannah vegetation (campo cerrado) and gallery forests until approximately 6,900 cal. BP. During that time, siderite laminae were deposited under anoxic conditions at the lake bottom. Then, increased rainfall and a shorter annual dry period allowed gallery forests and semi-deciduous forests to expand, leading to more closed cerrado vegetation. High-intensity rainfall events during this period are recorded as peaks in K and Ti concentrations. The sediment facies during this period consists of alternating layers of diatoms and minerogenic matter. C/N ratios imply that algae and perhaps soils, too, were the main contributors to sediment organic matter. Biogenic silica and δ¹³C_org variations indicate increasing primary productivity, which was related to higher nutrient flux from intensified leaching of soils, as shown by rising K/Al ratios. Around 800 cal. BP, a closed, semi-deciduous forest developed under present-day climate conditions. Slope stabilization diminished erosion processes in the catchment and caused reduced input of minerogenic matter into the basin. Human impact is evident in the topmost homogeneous sediments, as removal of the stabilizing forest cover amplified soil erosion. The continuous trend to more humid conditions during the Holocene probably reflects increased influence of the Amazon Basin as a moisture source. We conclude that the Lago Aleixo sediment archive was a sensitive recorder of environmental dynamics in tropical South America, which were mainly controlled by changes in precipitation patterns.     

Mid- to Late Holocene pollen and alluvial record of the arid Andean piedmont between 33° and 34°S,Mendoza, Argentina: Inferences about floodplain evolution

In arid and semiarid regions of Central West Argentina, paleoenvironmental conditions and history of the Holocene vegetation is still scanty. This paper presents the analysis of palynological records of two Mid- to Late Holocene alluvial sequences (La Escala -LES- and Brazo Abandonado -BA-) in the fluvial basin of the Arroyo La Estacada (33°27′S and 69°03′W), placed in the Cordillera Frontal central piedmont of Mendoza province. Variations in pollen assemblages of the two sequences related to the environments developed in the floodplains of the arroyo allowed us to infer the evolution of local plant communities. Between the ca. 4000 and 3000 ¹⁴C years BP and between ca. 500 and 400 ¹⁴C years BP, a larger representation of hydrophytic communities related to environments with local water availability was inferred; and between ca. 3000 and 500 ¹⁴C years BP an increase in shrubby xerophytic communities related to environments with lower water availability was detected. These results led us to propose that the Holocene pollen spectra fluctuations observed in the alluvial sedimentary sequences prove mainly changes in plant communities of the floodplain environment in response to the fluvial dynamics of the arroyo.     

A late Holocene palaeolimnological record from central Mexico, based on faunal and stable-isotope analysis of ostracod shells

A late Holocene palaeolimnological record for central Mexico has been obtained from Lake Pátzcuaro, using recent and fossil ostracods. Lake Pátzcuaro, Michoacán, is a closed-basin lake which responds rapidly to changes in the ratio of precipitation/evaporation in the region. The record from a single lake-sediment core, dated by AMS radiocarbon method, covers the last ~3,530 yrs, and is based on ostracod faunal palaeoecology coupled with analysis of the stable-isotope (¹⁸O/¹⁶O and ¹³C/¹²C) composition of ostracod valves. The faunal distribution is determined by the presence or absence of aquatic vegetation and, to a lesser extent, salinity. The ¹⁸O/¹⁶O and ¹³C/¹²C ratios in ostracod calcite show good agreement with palaeolimnological inferences from the faunal assemblages, principally recording changing precipitation/evaporation and primary-productivity levels, respectively. Wetter conditions existed in central Mexico between approximately ~3,600 and ~2,390 yr BP, between ~1,330 to ~1,120 yr BP, and from ~220 yr BP to present, characterised by fluctuating lake levels. A dilution of the sediment load in the lake reduced turbidity levels allowing for a marked increase in productivity. During these phases, the combination of a deeper lake and increased macrophyte cover reduced the degree of mixing of the waterbody. In the earliest of these phases there was sufficient stratification of the waterbody for methanogenesis to occur in the sediment interstices. The wet phases were separated by prolonged dry periods, during which time the climatic conditions were relatively stable. Good agreement was found between the findings of this study and others from the central Mexican/Caribbean region suggesting that abrupt climate changes occurred at least at a regional scale.