Global Radiation and Onset of Stratification as Forcing Factors of Seasonal Carbonate and Organic Matter Flux Dynamics in a Hypertrophic Hardwater Lake (Sacrower See,Northeastern Germany)

A 2-year (October 2003–October 2005) high-resolution sediment trap study was conducted in Sacrower See, a dimictic hardwater lake in northeastern Germany. Geochemical and diatom data from sediment trap samples were compared with a broad range of limnological and meteorological parameters to quantify the impact of single parameters on biochemical calcite precipitation and organic matter production. Our goals were to disentangle how carbonaceous varves and their sublaminae form during the annual cycle to better understand the palaeorecords and to detect influences of dissolution, resuspension as well as of global radiation and stratification on lake internal particle formation. Total particle fluxes in both investigated years were highest during spring and summer. Sedimentation was dominated by autochthonous organic matter and biochemically precipitated calcite. Main calcite precipitation occurred between April and July and was preceded and followed by smaller flux peaks caused by resuspension during winter and blooms of the calcified green algae Phacotus lenticularis during summer. In some of the trap intervals during summer up to 100% of the precipitated calcite was dissolved in the hypolimnion. High primary production due to stable insolation conditions in epilimnic waters began with stratification of the water column. Start and development of stratification is closely related to air and water surface temperatures. It is assumed that global radiation influences the onset and stability of water column stratification and thereby determining the intensity of primary production and consequently of timing and amount of calcite precipitation which is triggered by phytoplanktonic CO₂ consumption. Sediment fluxes of organic matter and calcite are also related to the winter NAO-Index. Therefore these fluxes will be used as a proxy for ongoing reconstruction of Holocene climate conditions.     

New insights into Late Pleistocene glacial and postglacial history of northernmost Ungava (Canada) from Pingualuit Crater Lake sediments

The Pingualuit Crater was formed by a meteoritic impact ca. 1.4 million years ago in northernmost Ungava (Canada). Due to its geographical position near the center of successive North American ice sheets and its favorable morphometry, the Pingualuit Crater Lake (water depth = 246 m) promises to yield a unique continuous sedimentary sequence covering several glacial/interglacial cycles in the terrestrial Canadian Arctic. In this paper, we suggest the existence of a subglacial lake at least during the Last Glacial Maximum (LGM) by hydraulic potential modeling using LGM ice-surface elevation and bed topography derived from a digital elevation model. These results support the hypothesis that the bottom sediments of the Crater Lake escaped glacial erosion and may contain a long-term continental sedimentary sequence. We also present the stratigraphy of a 9 m-long core retrieved from the deep basin of the lake as well as a multiproxy reconstruction of its deglacial and postglacial history. The base of the core is formed by very dense diamicton reflecting basal melt-out environments marking the end of subglacial conditions at the coring site. The overlying finely laminated silt are related to the onset of proglacial conditions characterized by extremely low lacustrine productivity. Infra Red Stimulated Luminescence and AMS ¹⁴C dating, as well as biostratigraphic data indicate sediment mixing between recent (e.g. Holocene) and much older (pre- to mid-Wisconsinan) material reworked by glacier activity. This process prevents the precise dating of these sediments that we interpret as being deposited just before the final deglaciation of the lake. Two finer grained and organic-rich intervals reflect the inception of lacustrine productivity resulting from the cessation of glacial meltwater inputs and ice-free periods. The lower organic interval corresponds to the early postglacial period (6850–5750 cal BP) and marks the transition between proglacial and postglacial conditions during the Holocene Thermal Maximum, while the uppermost organic-rich core section represents late Holocene sediments (～4200–600 cal BP). The organic intervals are separated by a basin-scale erosive slide occurring around 4200 cal BP and likely related to 1) a seismic event due to the glacio-isostatic rebound following the last deglaciation or 2) slope instabilities associated with rapid discharge events of the lake.     

Laminated lake sediments in northeast Poland: distribution, preconditions for formation and potential for paleoenvironmental investigation

Glacial landscapes of the Land of Great Masurian Lakes and Suwa?ki Lakelands in northeast Poland are characterized by very high abundance of lakes. These two areas were surveyed for lakes containing laminated sediments. Using bathymetry as a criterion, 60 small, deep lakes, representing preferred conditions for formation and preservation of lacustrine non-glacial varves, were selected for gravity coring. We found laminated sediments in 24 of the lakes, 15 in the Land of Great Masurian Lakes and 9 in the Suwa?ki Lakeland. Seven of these 24 sediment records were laminated in the topmost part only. Analysis of lake morphometric variables showed that the relation between surface area and maximum water depth can be used to identify lakes with laminated sediments. Most of the newly discovered lakes with laminated deposits have surface areas ≤0.3 km² and maximum depths of 15–35 m. Multivariate statistical analysis (Linear Discriminant Analysis) of the lake dataset identified the morphological features of lake basins and their catchments that largely control preservation of laminated sediments. Microscopic and geochemical analyses revealed a biogenic (carbonaceous) type of lamination typical for lakes in northeast Poland. Such lakes are characterized by a spring-summer lamina that is rich in calcium carbonate and an autumn-winter lamina composed of organic and minerogenic detritus. This pattern may be modified by multiple periods of calcite deposition during a single year or substantial contribution of clastic material. Laminations and high sedimentation rates offer the possibility of high-resolution investigation of past climate and environmental changes through application of myriad biological, isotopic and geochemical proxies.     

Conditions for deposition of annually laminated sediments in small meromictic lakes: a case study of Lake Suminko (northern Poland)

A three-year field study was conducted in Lake Suminko, Poland, to gain an understanding of the limnological variables that influence the formation and spatial extent of annually laminated sediments in the lake. The water body is divided into three depth strata, the mixolimnion, chemocline and monimolimnion, each defined by distinct values of temperature, electrical conductivity and oxygen concentration. Typical for meromictic lakes, the monimolimnion remains perennially anoxic and is rich in dissolved solids and nutrients. The annual pattern of particle flux in Lake Suminko is closely related to biochemical processes in the euphotic zone. During winter months we observed very low accumulation rates of non-carbonaceous matter, while during the rest of the year, three periods of calcite deposition were recorded (April, July–August, and October–November). The periods of high calcite deposition corresponded with algal blooms and oxygen concentration maxima. Two principal factors contribute to meromixis in Lake Suminko: (1) the basin is sheltered, preventing wind mixing, and (2) biochemical processes in the lake associated with high lacustrine productivity. Meromictic conditions must control the formation and preservation of laminated sediments in the lake because preserved laminations occur only in areas where the lake floor lies below monimolimnetic waters.     

SOLAR ACTIVITY VARIATIONS RECORDED IN VARVED SEDIMENTS FROM THE CRATER LAKE OF HOLZMAAR – A MAAR LAKE IN THE WESTEIFEL VOLCANIC FIELD, GERMANY

Annually laminated sediments from Lake Holzmaar (Germany) provide high resolution sedimentological and palaeoenvironmental data of the last 22,500 years. Weichselian periglacial varves and Holocene organic varves indicate different depositional environments. For the best preserved sections from both parts, spectral analyses were performed in order to detect cyclic fluctuations in varve thickness. Weichselian varves are dominated by an 88 year periodicity. Linear spectral analysis of Holocene varves provides no significant cyclicity. But, supposing nonlinear transformations of the solar signal through the Lake Holzmaar ecosystem during the Holocene, an adequate nonlinear spectral analysis method was able to detect periodicities of 11, 88, and 208 years. The existence of these cyclicities, comparable to the solar activity fluctuations, give further evidence for the existence of a sun-climate relationship, based on a not yet completely understood mechanism.     

Late-glacial pollen and diatom changes in response to two different environmental perturbations: volcanic eruption and Younger Dryas cooling

A high-resolution pollen and diatom stratigraphy has been studied from late-glacial annually laminated sediments of Holzmaar (425 m a.s.l., Germany). The sediment sequence studied comprises 475 varves and includes two environmental perturbations of different type and duration: the short, abrupt deposition of the late Allerød tephra layer of the Laacher See volcano (LST, 11 000 yr B.P.), and the more gradual onset of the 'Younger Dryas climatic cooling.Numerical analyses involving (partial) redundancy analyses in connection with Monte Carlo permutation tests suggest that the deposition of 78 mm of Laacher See Tephra had a statistically significant effect on the pollen stratigraphy (percentage and accumulation rates), most probably because of the proximity of the site to the volcano. The diatom accumulation rates also show a statistically significant change, whereas the diatom percentage data do not change significantly. The between-sample rates-of-change in both biostratigraphies are higher at and just after the LST event than at the transition to the Younger Dryas biozone. Sequence splitting of pollen and diatom accumulation rate data also shows a clustering of significant splits at the LST event. A close correlation between changes in the pollen and diatom percentage data for the investigated time-interval suggests a common underlying climatic signal, whereas the accumulation rates of both biostratigraphies behave more individualistically and show more short-term variability due, in part, to the inherent noise in the two data sets. Variance partitioning shows that the local pollen and diatom assemblage zones explain much of the variance in the data-sets. Statistical modelling using redundancy analysis shows that the changes in the diatom assemblages are best predicted by the Younger Dryas biozone and the main changes in the pollen stratigraphy (as represented by the first PCA axis of the pollen data).The results suggest that the biostratigraphies studied at Holzmaar reflect generally stable systems which were disturbed by the deposition of the Laacher See Tephra. After a phase of recovery both systems again reached a new phase of stability prior to the long-term Younger Dryas climatic deterioration that perturbed the assemblages again. The very close and statistically significant parallelism between the major stratigraphical patterns in the pollen and diatom percentage data highlights the responses of the two biological systems to environmental perturbations at different temporal scales.     

Establishing a chronology for lacustrine sediments using a multiple dating approach—A case study from the Frickenhauser See, central Germany

In order to establish a reliable chronology for lacustrine sediments of the Frickenhauser See (central Germany) different dating methods have been applied. A total of 17 AMS ¹⁴C dates, all within the last 2000 years, were supplemented with ¹³⁷Cs/²¹⁰Pb dating and varve counting of the uppermost sediments (131 years). The age–depth model for the Frickenhauser See has to cope with highly variable sedimentation rates and overlapping probability distributions of calibrated ¹⁴C dates. The uncertainty of calibrated ¹⁴C dates could be considerably reduced by including the stratigraphic relationship of the dated samples, the age information derived from short-lived isotopes and varve counting as well as an upper and lower limit of realistic sedimentation rates as ‘a priori’ information in the calibration procedure. Sets of possible age combinations obtained by repeated sampling from the modified probability distributions were used to calculate continuous age–depth relationships based on monotonic smoothing splines. The obtained age–depth model for the sediment record of the Frickenhauser See represents the average of over 16,000 such model runs and suggests a drastic increase in sedimentation rates from around 1–2 mm a⁻¹ (200–1000 AD) to over 25 mm a⁻¹ for the period between 1100 and 1300 AD. From then on, sedimentation rates exhibit relatively stable values around 3–9 mm a⁻¹. ‘Conventional’ age–depth models such as general polynomial regression or cubic splines either do not include the obtained age-information in a satisfying manner (the model being too “stiff”) or exhibit “swings” causing age-reversals in the model. Although the age–depth relationships obtained for monotonic smoothing splines and mixed-effect regression are generally very similar, they differ in their respective sedimentation rates as well as in their uncertainties. Mixed-effect regression resulted in much higher sedimentation rates of more than 37 mm a⁻¹. These results suggest that monotonic smoothing splines give better control of the age–depth model characteristics and are well suited in situations, where the integrity of ¹⁴C dates is high, i.e. the dated material represents the age of the respective layer.     

Transient simulations,empirical reconstructions and forcing mechanisms for the Mid-holocene hydrological climate in southern Patagonia

This study investigates the atmospheric circulation in transient climate simulations with a coupled atmosphere–ocean general circulation model (GCM) for the mid-Holocene (MH) period 7–4.5 ka BP driven with combinations of orbital, solar and greenhouse gas forcings. The focus is on southern South America. Statistical downscaling models are derived from observational data and applied to the simulations to estimate precipitation in south-eastern Patagonia during the MH. These estimates are compared with lake level estimates for Laguna Potrok Aike (LPA) from sediments. Relative to pre-industrial conditions (i.e. 1550–1850), which show extraordinarily high lake levels, the proxy-based reconstructed lake levels during the MH are lower. The downscaled simulated circulation differences indicate higher LPA precipitation during the MH from March to August, higher annual means, and reduced precipitation from September to February. Thus the reconstructed lower LPA lake levels can not be explained solely by the simulated precipitation changes. Possible reasons for this discrepancy are discussed. Based on proxy data from southern South America hypotheses have also been proposed on the latitudinal position of the southern hemispheric westerlies (SHWs). In agreement with some of these hypotheses our simulations show an increased seasonal cycle of the latitudinal position of the SHWs during the MH, which can be explained by the orbital forcing. The simulations also show stronger SHWs over southern Patagonia during austral summer and weaker SHWs during winter. The downscaling model associates weaker SHWs with increased precipitation in the LPA region. However, this relationship is only moderate, and therefore the downscaling model does not support the assumption of a strong link between mean SHWs and precipitation over south-eastern Patagonia, which is the basis of many proxy-based hypotheses about the SHWs.     

Paleosecular variation and paleointensity records for the last millennium from southern South America (Laguna Potrok Aike,Santa Cruz,Argentina)

High-resolution paleo- and rock magnetic studies were performed on a group of four sediment cores from Laguna Potrok Aike (Santa Cruz, Argentina) representing the time period AD 1300–2000. The rock magnetic analyses show that the main magnetic mineral is (titano)magnetite with a concentration between 0.01 and 0.08%, and a grain size of 4–15 μm. This study is helpful in order to complete the paleosecular variation (PSV) and paleointensity type curves for South America which do not have a detailed record for the last millennium. The comparison with the study carried out for Lake El Trébol shows a very good agreement, supporting that PSV records of south-western Argentina can be developed into a stratigraphic correlation tool on a regional scale.     

Responses of testate amoebae assemblages (Amoebozoa: Arcellinida) to recent volcanic eruptions,inferred from the sediment record in Laguna Verde,southern Patagonia,Argentina

Journal of Paleolimnology - There is little knowledge about testate amoebae in lakes and their responses to volcanic eruptions. To address this knowledge gap, we studied the paleoecology of these...