Climate change and human impact at Sacrower See (NE Germany) during the past 13,000 years: a geochemical record

Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS ¹⁴C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ¹³C_org and δ¹⁵N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ¹³C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.     

Sedimentological and geochemical records of past trophic state and hypolimnetic anoxia in large,hard-water Lake Bourget,French Alps

Sedimentological, geochemical and particle-size analyses were used to reconstruct the evolution of both trophic state and hypolimnetic anoxia in Lake Bourget (French Alps) during the last century. Radionuclide dating (²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am) confirmed the annual rhythm of laminations in the upper sediment profile. In Lake Bourget, biochemical varves are triplets composed of a diatom layer (spring lamina), a bio-precipitated calcite-rich layer (spring/summer lamina), and a layer rich in organic matter and detrital particles (winter lamina). The onset of eutrophication and the first appearance of an anoxic facies occurred simultaneously and were dated by laminae counting to AD 1943±1 year. Persistent anoxic conditions began in AD 1960. Eutrophication is characterised by drastic increases in the flux of biogenic silica (mostly diatoms), lacustrine organic matter, and larger calcite crystals (15–30 μm). The increase of organic matter also represents a marker of the onset of anoxic conditions in the hypolimnion. Our results show that eutrophication was the main factor controlling anoxia in the hypolimnion. This eutrophication was caused mostly by the inflow of untreated sewage effluents, and to a lesser extent, by input of fertilizer-derived phosphorus during floods of the Rhone River and run-off from the lake catchment. The Rhone River, however, can also be a source of re-oxygenation via underflows that originate during flood events. Oxygenation of the hypolimnion is also controlled by low winter temperatures, which enable turnover of the lake. Thus, global warming, associated with a forecasted reduction in precipitation, might reduce the efficiency of hypolimnetic re-oxygenation in Lake Bourget.     

Ecological responses to climate change in a bird-impacted High Arctic pond (Nordaustlandet,Svalbard)

A 28-cm sediment core from an Arctic pond (Nordaustlandet, Svalbard), which is currently subjected to the fertilizing effect of bird guano, was analysed for fossil invertebrates and the physical properties of the sediment. The objective was to examine aquatic community responses to climate warming. Our record reveals that faunal changes have occurred. Initially chironomid assemblages were dominated by a cold-indicating oligotrophic community but this was replaced by a community typical of more nutrient-enriched conditions and warmer water temperature at around AD 1,700–1,800. After AD 1,800, ostracods and Daphnia increase suggesting that a nutrient enrichment threshold was crossed, probably related to increased planktonic algal productivity. In the early twentieth century, organic content markedly increases and magnetic susceptibility values suddenly drop, indicating a further increase in nutrient input and lake productivity. Since the most likely source of nutrients in the lake is goose guano, this suggests that the size of the bird colony may also have increased over this period. These changes coincide with climate warming suggesting a positive feedback in which climate change is the primary driver of the increasing geese abundance and lake productivity. Our results further suggest that the predicted future warming in the Arctic will continue to have cascading effects on freshwater ecosystems in the region.     

Late holocene paleoclimatic and paleobiologic records from sediments of Devils Lake,North Dakota

An 84 cm sediment core collected from the center of Devils Lake, North Dakota, was analyzed at 1-cm intervals for,²¹⁰Pb,¹³⁷Cs, sediment conductivity, the concentrations of, biogenic silica, total organic carbon, carbon to nitrogen ratio, and the carbon and nitrogen isotopic composition of the organic fraction. Variations in²¹⁰Pb activities in the upper 20 centimeters indicate that sediment accumulations rates in Devils Lake are not constant, and that accumulation rates were highest during periods of high lake level. The mean sedimentation accumulation rate was calculated as 0.24 cm^–1 yr. The¹³⁷Cs profile is characterized by near-surface maximum concentrations, possibly the result of redistribution of¹³⁷Cs during salinity excursions.Biogenic silica is strongly correlated to lake level in Devils Lake. Periods of low lake level (characterized by high sediment conductivity) correspond to low biogenic silica concentrations. The trends in biogenic silica are attributed to variations in diatom productivity in the lake and to variations in sediment accumulation rates. Based on biogenic silica content and the composition of organic matter in the sediment (total organic carbon, carbon:nitrogen ratio and the ¹³C and ¹⁵N composition of total organic matter), paleobiologic conditions of Devils Lake during low lake stands were characterized by, (1) decreased primary productivity, (2) decreased input of detrital organic matter, and (3) increased nitrogen availability.During the 350 years of sediment accumulation represented by the 84-cm sediment core Devils Lake has experienced two periods of sustained high lake level; one between about 130 and 170 years ago (1820 to 1860 A.D.) and the second between 270 and 310 years ago (1680 to 1720 A.D.). Devils Lake experienced a period of intense drying about 260 years ago (1720 A.D.).     

A test of the integrity of metal records in sediment cores based on the documented history of metal contamination in Lac Dufault (Québec, Canada)

There is an important volume of published information on Lac Dufault (Québec) which describes the history of metal inputs over 70 years and the changes that occurred in the lake as a result of this contamination. We used this abundant source of chronological markers to test the hypothesis that lake sediments can provide true historical records of trace metal loading from metal mining. Sediment cores were obtained from the deepest zone of the lake (19 m). The sediments were dated using ²¹⁰Pb and ¹³⁷Cs and they were analyzed for total elemental concentrations (Ca, Cd, Cu, Fe, P, Pb, S, Zn). Metal profiles in the sediment core preserved the distinct signatures of different mine exploitations documented in the lake watershed. In particular, the core recorded: the beginning of industrialization in 1926; increasing sedimentation rates associated with perturbations in the lake watershed; the maximum of sediment [Cu] and the contemporary exploitation of ore bodies rich in Cu; the maximum of sediment [Fe] and the contemporary production of pyrite by a mining operation; the low sediment [Cu] and [Zn] in the 1950s and the low contemporary production of these metals by mining operations; the maximum of sediment [Cd] and the contemporary production of Cd by a subsequent mining operation. Anomalies in the distribution of ²¹⁴Pb activities in sediments reflected the intensity of acid mine drainage (AMD). There is good evidence that the lake resisted acidification from this AMD for the last thirty years. Overall, our results support the thesis that profiles of sedimentary Cd, Cu, Zn, Pb and Fe levels reflect the past history of metal input to Lac Dufault.     

A geochemical record of recent anthropogenic nutrient loading and enhanced productivity in Lake Nansihu, China

Total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ¹³C, δ¹⁵N), total phosphorus (TP) and organic phosphorus (OP) were measured in surface sediments and two short cores (DU-3 and WS-4) from Lake Nansihu, China to infer historical changes in anthropogenic nutrient inputs and corresponding shifts in lake primary productivity. Results indicate that organic matter preserved in the sediments is mainly autochthonous and that analyzed sediment variables were affected little by post-burial diagenesis. Increasing TOC, TN, OP and TP concentrations since the 1940s reflect increased P loading and elevated lake productivity. The δ¹³C values varied from ?21.5 to ?26.6‰ in the two sediment cores. Values were relatively more negative before the 1940s, but thereafter increased until the mid-1980s, reflecting elevated lake productivity. Since the mid-1980s, δ¹³C values remained relatively constant in core WS-4 and decreased in core DU-3, perhaps reflecting a change in the phytoplankton community. The δ¹⁵N values ranged from ?0.5 to 1.3‰ in core DU-3 and from 1.2 to 2.5‰ in core WS-4 before the mid-1980s, and increased to between 2.1 and 8.0‰ and 5.2 and 7.8‰, respectively, thereafter. Topmost sediments in the two cores display δ¹⁵N values similar to those recorded in the surface sediments (5.5–7.5‰). Higher δ¹⁵N values in recent deposits correspond to greater nitrogen concentration in water, and likely indicate anthropogenic nitrogen input, mainly from human and animal wastes.     

Historical changes in sediment and phosphorus loading to the upper Mississippi River: mass-balance reconstructions from the sediments of Lake Pepin

Long-term changes in sediment and phosphorus loading to the upper Mississippi River were quantified from an array of 25 sediment cores from Lake Pepin, a large natural impoundment downstream of the Minneapolis-St Paul metropolitan area. Cores were dated and stratigraphically correlated using ²¹⁰Pb, ¹³⁷Cs, ¹⁴C, magnetic susceptibility, pollen analysis, and loss-on-ignition. All cores show a dramatic increase in sediment accumulation beginning with European settlement in 1830. Accumulation rates are highest and show the greatest post-settlement increases in the upper end of the lake. Present-day sediment-phosphorus concentrations are roughly twice those of pre-settlement times, and the Fe/Al-bound fraction makes up a greater portion of the total. Diatom assemblages record a marked increase in nutrient availability over the last 200 years, changing from clear-water benthic forms and mesotrophic planktonic taxa in pre-settlement times to exclusively planktonic assemblages characteristic of highly eutrophic conditions today. Lake-water total-phosphorus concentrations, estimated by weighted averaging regression and calibration, increased from 50 to 200 μg l⁻¹ during this period. Sediment loading to Lake Pepin from the Mississippi River has increased by an order of magnitude since 1830. Modern fluxes are about 900,000 metric tons annually, and are more than 80% detrital mineral matter. About 17% of the lake’s volume in 1830 has been replaced by sediment, and at current accumulation rates the remainder will be filled in another 340 years. Phosphorus accumulation in Lake Pepin sediments has increased 15-fold since 1830, rising from 60 to 900 metric tons annually. This rise represents a sevenfold increase in phosphorus loading from the Mississippi River coupled with more efficient retention of phosphorus inflows by bottom sediments. More efficient trapping of phosphorus in Lake Pepin over the last century resulted from higher rates of sediment burial. The most dramatic changes in nutrient and sediment inputs to Lake Pepin have occurred since 1940, although gradual increases began shortly following European settlement. Sediment accumulation rates rose sharply between 1940 and 1970 and then leveled off, while phosphorus inflows record their largest increases after 1970. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.     

The transport and mass balance of fallout radionuclides in Blelham Tarn, Cumbria (UK)

Although lake sediment archives are widely used for reconstructing historical records of atmospherically delivered pollutants, the quantitative relationship between fallout levels and their record in the sediments is complex and not well known. The original input signal from the atmosphere can be significantly distorted by mediating transport processes in the catchment, through the water column and within the sediments themselves. Since these processes also affect the fallout radionuclide ²¹⁰Pb commonly used to date sediments, a better understanding of their impact is also important to improving the accuracy and reliability of sediment dating. Blelham Tarn has been the subject of a number of palaeolimnological investigations using radiometric dating techniques since the early 1970s. More recently it was the site of a major study carried out within the EU Transuranics project concerning the long-term fate of fallout radionuclides in catchment/lake systems. This paper reviews the radiometric data from this study and uses the results to determine mass balances for fallout ²¹⁰Pb, ¹³⁷Cs and ^239+240Pu in Blelham Tarn, and their spatial distribution over the bed of the lake. Atmospheric fluxes were determined by measuring concentrations in rainwater and cumulative inventories in soil cores from non-eroding sites. Sediment records in a grid of 16 cores were used to determine the spatial distribution over the bed of the lake, and net inputs from the catchment. Mass balance calculations indicate that c. 47% of ²¹⁰Pb in the sediments derives from erosive inputs from the catchment. For ^239+240Pu the figure rises to 61%. Reduced amounts of ¹³⁷Cs in the sediments are attributed to greater losses of this radionuclide from the water column via the outflow due to its greater solubility. Inputs of radionuclides from the catchment are concentrated near one of the major input streams. Away from this part of the lake the sediment record is dominated by direct atmospheric fallout, though the detailed pattern is influenced by sediment focussing. A one parameter catchment/lake transport model is developed that incorporates the assumption that transport rates will decline with time as fallout on the catchment diffuses into the soil and becomes less available for removal. Values of the transport parameter were calculated for ²¹⁰Pb and ^239+240Pu and found to be comparable. The results suggest that it will take c. 11000 years to remove 50% of ^239+240Pu from the catchment to the lake.     

A sediment record of recent nutrient loading and trophic state change in Lake Norrviken,Sweden

Human-induced perturbations in the Lake Norrviken catchment, Sweden, over the last 100+ years have left distinctive geochemical imprints in the sediments. Disposal of sewage, industrial, and agricultural run-off into the lake since the end of the nineteenth century changed the trophic status from eutrophic to hyper-eutrophic. The primary organic matter (OM) source in the lake is in situ algal material. Total organic carbon (TOC) concentrations increased near the mid-section of a short sediment core collected from the deepest part of the lake, reflecting elevated epilimnetic productivity and consequent hypolimnetic anoxia. Accompanying shifts to lighter stable organic C and total N isotopic compositions suggest that cyanobacterial productivity increased during this period. The distribution of pigments in the core indicates a shift in the phytoplankton community to a cyanobacteria-dominated system. Moreover, pigments confirm that N₂-fixing versus non-N₂-fixing phytoplankton varied depending upon the external inputs of N and P. Conditions in the lake improved after sewage input was diverted and the lake is currently mesotrophic.     

Assessment of nutrient sources and paleoproductivity during the past century in Longgan Lake,middle reaches of the Yangtze River,China

Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ¹³C_Org, δ¹⁵N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ¹³C_Org, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ¹⁵N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.     

Recently induced anoxia leading to the preservation of seasonal laminae in two NE-German lakes

The recent sediments of two lakes in the NE German lowland became seasonally laminated at different times. Anoxic bottom conditions resulted from a surplus of organic matter (OM), in the early stage indicated by irregularly laminated sediments comprising abundant iron-sulfide framboids. Their diagenetic formation predates the preservation of biochemical calcite varves. In the larger, deeper Lake Tiefer See near Klocksin, anoxia developed stepwise. A first anoxic pulse was contemporary with inflow narrowing by railway-dam construction and accumulation of OM. It was favored by a decrease of the intensity of lake circulation (turnover). Nutrients introduced from artificial fertilizer then increased the primary production (diatoms) to the point of OM surplus and seasonal laminae formation started 40 years later in 1924. In the smaller, shallower Lake Tiefer See in the Uckermark, a massive pulse of iron sulfide was centered around 1960, seven years after installation of piped field drainage into the lake. Anoxia developed rapidly with the nutrients drained from a fertilized groundwater catchment that is 10 times larger than the surface catchment, while surface erosion was reduced. Reducing bottom conditions became regular and the seasonal lamination was preserved after 1967. Morphological criteria to screen lakes for varved sediments should include reductions of natural lake inflow and catchment increase, such as by inflow of field drainage. Similar developments of increased nutrient input or intensity decrease of lake circulation may result from historical human activities but also from natural processes.     

Sediment-chemistry response to land-use change and pollutant loading in a hypertrophic lake, southern Sweden

Responses to recent land-use changes and pollutant loading in the sediment of a hypertrophic lake in southern Sweden were studied by comparison of geochemical, pollen and magnetic records with historical land-use data. A chronology was constructed for the last two centuries by correlating changes in the pollen diagram to major events in the land-use history. Sediment accumulation was low (mean c. 0.2 g cm^-2 yr^-1) prior to 1800 AD, when less than 25% of the catchment was arable land. Reorganization of the agrarian system during the 19th century increased the annually tilled area by 300%, which accelerated soil erosion and substantially increased the accumulation of allochtonous matter in the lake. Since the turn of the century 90% of the catchment has been ploughed every year. The deposition of clastic matter in the lake has, however, decreased due to a gradual rerouting of the drainage system, which has reduced the effective catchment area by c. 85%.Authigenic vivianite (Fe3(PO4)2.8H2O) is a major P phase in the preindustrial non-sulphidic sediments, which suggests that the sediments at that time served as a fairly efficient sink for P. The arable expansion, increased manuring and, eventually, the introduction of artificial fertilizers during the 19th century led to a massive influx of nutrients, which elevated primary production in the lake. Subsequent development of bottom water anoxia around 1900, in combination with an additional pollutant burden of sulphate within the lake basin, led to major alterations of the biogeochemical cycles. The most critical change in the post-1900 sediments involved the cycling of Fe and P. The linkage between the lacustrine P and Fe cycles can explain that FeS formation was paralleled by a release of P from the sedimentary pool. This supply of P to the lake basin must have supplemented the nutrient supply by modern agriculture and contributed to recent hypertrophication. The bacterial sulphate reduction also affected the generation of alkalinity which supported a significant calcite precipitation in the post-1900 sediments.S is enriched 10-fold in the post-1900 sediments compared to preindustrial values. Along with the rise in S, soot particles derived from fossil fuel combustion appear in the sediments for the first time. Therefore, Bussj¨osj¨on is thought to be a good example of how a well-buffered, highly productive lake may respond to the pollution by sulphur from acid rain.     

Lake ecosystem responses to catchment disturbance and airborne pollution: an 800-year perspective in southern Sweden

Sediment sequences spanning the last 800 years from two small lakes in the south Swedish uplands were explored for assessment of effects of changing human population, local land-use practices and airborne pollution on lake-ecosystem functioning and resilience. Variations in nutrient cycling and deposition of lithogenic elements were studied, using a multi-proxy stratigraphic approach. Carbon and nitrogen elemental and isotopic analyses were applied in combination with records of hydrocarbons (n-alkanes) to investigate the sources and depositional conditions of sediment organic matter. Changes in fluvial and airborne delivery of inorganic matter were based on X-ray fluorescence measurements. The results reveal that population growth and related increases in land-use pressure had a major impact on catchment erosion and input of terrestrial organic matter to the lakes from the 1500s to the end of the 1800s. Evidence also exists of a brief period of catchment disturbance at ca. 1200–1300, followed by recovery, likely connected to the Black Death pandemic. At ca. 1900 synchronous shifts in most of the proxy records suggest a marked change in external forcing common to the two lakes related to a major decrease in population density and the introduction of modern forestry following the industrial revolution. Interestingly, the two sediment records exhibit generally coherent trends in C/N ratio, organic C content and δ¹³C, both before and after 1900, indicating broadly similar sensitivities of the lake ecosystems to human impact. In contrast, deviating trends in total N content, δ¹⁵N and lithogenic element concentrations (K, Ti, Rb and Zr) reflect site-specific responses to local disturbances during the last century due to different nutrient conditions and catchment properties. Our companion sediment records highlight the importance of understanding long-term human impact on watersheds and demonstrate how regional versus local forcing of lake ecosystems, as well as site-specific responses related to catchment characteristics can be reconstructed.     

Laghi di Monticchio (Southern Italy,Region Basilicata): genesis of sediments—a geochemical study

The sedimentation record of Lago Grande di Monticchio (LGM) is one of the most prominent paleoclimatic archives in the non-glaciated areas of Europe. However, the modern lake system has never been the subject of intense limnological studies. On the basis of hydrochemical water profiles, detailed investigations of sediment short cores and in situ pore water profiles from the littoral to the profundal zone, we elucidate spatial variations of sediment genesis within the lake basin and the importance of various depth sections for the lake’s internal nutrient cycling. Sediments from the smaller meromictic Lago Piccolo di Montichio are discussed as a reference. Our study demonstrates: (i) distinctly higher sediment accumulation for the centre of the lake basin by focussing of the settling particle flux; (ii) decline of carbonate from the littoral to the profundal zones; (iii) non-synchronous change of calcite net-accumulation for various water depths; (iv) exceptionally high cation release from sediments covering the steeply inclining sector of the lake basin; (v) relatively constant dissolved silica concentrations in the pore waters (SiO₂ ∼42 mg/l) independent of water depth and sediment composition; (vi) influx of oxygen-bearing groundwater into the anoxic hypolimnion after heavy rainfall and the associated precipitation of Fe-oxihydroxides; (vii) higher release of NH₄ by anaerobic degradation of organic matter at a water depth of 23 m than for sediments at a maximum water depth of 32 m, whereby the latter reflects the importance of seasonal sediment re-oxidation for anaerobic degradation of organic debris; (viii) although seasonal re-oxidation of sediments from various water depths is quite different, Oxygen Index values of LGM sediments fall in a small range, which reflects rapid microbial consumption of seasonally re-generated easily bio-degradable organic molecules.     

Carbon cycling in Lake Erie during cultural eutrophication over the last century inferred from the stable carbon isotope composition of sediments

A ~106-cm sediment core from the eastern basin of Lake Erie was examined to investigate biogeochemical processes in this large lake during its cultural eutrophication over the last century. We measured stable carbon isotopes of total organic carbon and calcium carbonate (δ¹³C_TOC and d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} ) as well as the concentrations of total organic carbon (TOC) and calcium carbonate (CaCO₃). δ¹³C_TOC and TOC show a strong positive correlation throughout the core and record changes in phytoplankton productivity and nutrient loading. CaCO₃ and TOC concentrations display a negative correlation throughout the core, suggesting that CaCO₃ concentrations are controlled primarily by decomposition of TOC in the hypolimnion and the sediments, although temperature and invasive mussels are also potential controlling factors. d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values show a positive correlation with δ¹³C_TOC between 1909 and 1969, indicating phytoplankton productivity was the primary control for d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values during eutrophication. However, a negative correlation between d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} and δ¹³C_TOC from 1970 to 2002 suggests that these two proxies tracked different aspects of the carbon cycle in the lake in more recent times. The cause for the negative correlation is not yet known, but it is perhaps associated with temperature variations and seasonal differences in productivity.     

Holocene trophic state history of a subtropical blackwater lake,South Georgia,USA

Organic-rich sediment from Lake Louise, a dystrophic sinkhole lake in south Georgia, displays variations in C, N, P, C/N, δ¹³C, δ¹⁵N, biogenic silica (BSi) and diatom flora that document changes in trophic state over the past ~9,500 years. The lake initially was oligotrophic and moderately productive, but by the middle Holocene a rising regional water table, driven by eustatic sea level rise, caused expansion of wetlands around the lake and a shift to humic waters. Low rates of sediment accumulation, low C contents, rising C/N, and light δ¹³C and δ¹⁵N indicate this was a time of low productivity, more anoxic bottom waters and extensive recycling of littoral organic matter. These conditions persisted until ~1800 AD when a physical disturbance to the watershed, probably the Great Hurricane of 1780, resulted in a dramatic increase in productivity that has continued to the present day. We attribute this shift, recorded by a >tenfold increase in sediment accumulation rate, higher C, P, and δ¹⁵N, and lower BSi, to establishment of an inflow stream that increased nutrient delivery to the lake, raised water level, and expanded the wetland area around the lake. Since ~1930, logging, farming, and highway construction have impacted the lake, further accelerating biological productivity as well as the delivery of terrigenous sediment. Results of this study illustrate the potential of a single, catastrophic event to permanently alter the hydrology and chemistry of a lacustrine system and confirm that dystrophic lakes can be highly productive and therefore promising targets for paleolimnological study.     

Anthropogenic impacts recorded in recent sediments from Otisco Lake,New York,USA

Geochemical analysis of ²¹⁰Pb-dated sediment cores from Otisco Lake (New York) combined with analysis of recent land cover change in the watershed revealed the history of land use change, lake management, and industrial pollution since European settlement in the northeastern US. Clearance of forestland for agriculture characterized the early settlement era that was marked in the Otisco Lake sediments by a decline in organic carbon (OC) and OC:N ratios, which indicate a change in the sources of organic matter to the lake. Agricultural land use reached its greatest areal extent in the Otisco watershed around 1900, followed by field abandonment and reforestation over the last century. In the 1920s sediment accumulation rates began to increase coincident with residential development along the lake shore. Nitrogen and organic carbon, which are transported from the watershed, show increased fluxes to the lake in response to this change. Deposition of inorganic carbon increased markedly over a short period from the 1940s to the 1980s, which is consistent with enhanced mobilization of watershed calcium by increased acidic deposition, followed by alkalinization of the lake waters and calcite precipitation. An increase in copper content in the sediments reflects application, since 1942, of the algicide copper sulfate to the lake waters to control algal blooms. This CuSO₄ marker confirmed the accuracy of the ²¹⁰Pb chronology. Atmospheric mercury (Hg) fluxes to the lake were affected by increased sediment transport from the watershed. The maximum Hg peak in the sediment record, however, was dated to the early 1970s and coincides with maximum Hg emissions to the atmosphere in the Great Lakes region.     

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.     

Quantitative climate reconstruction linking meteorological,limnological and XRF core scanner datasets: the Lake Sanabria case study,NW Spain

Monthly limnological monitoring in Lake Sanabria (Spain) since 1986 provided a unique opportunity to test relationships among climate, hydrology and lake dynamics and how they are recorded in the lake sediments. Four datasets were employed: (1) meteorological (monthly maximum and minimum air temperature and total precipitation), (2) limnological (Secchi disk, water temperature, conductivity, pH, dissolved oxygen, nitrate, silicon, total and reactive phosphorus, and total chlorophylls and chlorophyll a), (3) hydrological (Tera River water input and output), and (4) XRF core scanner measurements carried out in short cores. Linear models between the different dataset variables allowed us to characterize the climate signal transmission from one to the other and cross-correlation analyses permitted us to identify the different response times (if any) between them. Principal Component Analyses (PCA) of the limnological and geochemical datasets allowed us to identify the main processes that link lake dynamics, primarily nutrient supply and organic productivity, with some sedimentological processes, e.g. organic matter and phosphorus accumulation. Sediment chronology was established by gamma spectrometry (²¹⁰Pb). Water input to Lake Sanabria is controlled mostly by the Tera River input and is linked directly to precipitation. Response of the Lake Sanabria water budget to climate oscillations is immediate, as the strongest correlation between these two datasets occurs with no lag time. PCA of the limnological dataset indicated that most of the variance is related to nutrient input, and comparison with the Tera River water discharge shows that nutrient input was controlled mainly by oscillations in the hydrological balance. The lag time between the hydrological and limnological datasets is 1 month. The PCA of the XRF core scanner dataset showed that the principal process that controls the chemical composition of the Lake Sanabria sediments is related to sediment and nutrient delivery from the Tera River and organic productivity. Comparison of the nutrient input reconstructed using the limnological dataset and the XRF core scanner data indicated that the sediments act as a low-pass filter, smoothing the climate signal. It was, however, possible to establish the link between these datasets, and obtain a quantitative reconstruction of precipitation for the 1959–2005 AD period that captures the regional variability. This quantitative precipitation reconstruction suggests it is possible to obtain accurate climate reconstructions using non-laminated sediments.     

How reliable is the210Pb dating method? Old and new results from Switzerland

We present a historical overview of applications of²¹⁰Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of²¹⁰Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several²¹⁰Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by¹³⁷Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm^–2 y^–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the²¹⁰Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm^–2 y^–1. In other parts of the lake a significant deficit in the inventory of²¹⁰Pb was found in the sediments. This could be due to a chemical redissolution of²¹⁰Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of²¹⁰Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of²¹⁰Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved²¹⁰Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and²¹⁰Pb in sediments of Lake Zurich was found.This is the first of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.