Controls on the seasonal and interannual dynamics of organic matter carbon isotopes in mesotrophic Lake Holzmaar, Germany

We investigated the particulate organic matter (POM) flux and its stable carbon isotope composition in response to changes in phytoplankton primary production and abiotic environmental affects in mesotrophic Lake Holzmaar. POM fluxes were determined by sediment traps emptied biweekly from 1995 to 2004. Water chemical measurements and meteorological observations were done concurrently. POM fluxes within and directly below the lake's epilimnion were relatively low during winter (<0.5 g m(-2) d(-1)) and increased to 1.0-1.5 g m(-2) d(-1) during summer months because of enlarged phytoplankton production. In contrast, in the lake's hypolimnion a much higher POM flux was observed during winter as result of resuspension of particulate material from the lake's bottom. C:N (weight, total organic carbon : total nitrogen) ratios of settling organic matter vary around 8 at all three depths, indicating a predominant phytoplankton origin of POM. The annual epilimnic POM flux correlates with the epilimnic phosphate concentration early in the seasonal cycle, indicating that the supply of phosphate is the major control on the amount of algal biomass. A recurrent annual oscillation of the stable carbon isotope composition of POM with low delta C-13 values during winter and more positive values during summer is interpreted as the result of phytoplankton primary production. A close delta C-13(POM)-POM flux relationship was observed, testifying to the primary dependence of delta C-13(POM) on lacustrine primary production. Contrary to POM flux, the epilimnic delta C-13(POM) signal is transferred to the lakes bottom notwithstanding sediment resuspension and microbial alteration. Our results demonstrate that sedimentary stable carbon isotopes are a valuable proxy for lacustrine paleo-productivity in Lake Holzmaar

Transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake (Lake Holzmaar, Germany)

We have investigated the transfer of oxygen isotope signals of diatomaccous silica (delta P-18(diatom)) from the epilimnion (0-7 m) through the hypolimnion to the lake bottom (similar to 20 m) in freshwater Lake Holzmaar, Germany. Sediment-traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days. The 7 m trap collected diatoms from the epilimnion being the main zone of primary production, while the 16 m trap collected material already settled through the hypolimnion. Also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years. The delta O-18(diatom) values of the 7 m trap varied from 29.4 parts per thousand in spring/ autumn to 26.2 parts per thousand in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study. Remarkably, despite the short settling distance delta(18)Odimom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from April to September delta O-18(diatom) values of the 16 m trap were roughly similar to 1 parts per thousand enriched in O-18 compared to those of the 7 m trap. Isotopic exchange with the isotopically lighter water of the hypolimnion would shift the delta O-18(diatom) value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation. Dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral pH of the water should only cause a minor enrichment of the 16 m values. Nevertheless, opal from the bottom sediment was found to be 2.5%. enriched in O-18 compared to the weighted average of the opal from the 7 m trap. Thus, resuspension of bottom material must have contributed to the intermediate delta O-18(diatom) signal of the 16 m trap during summer. Dissolution experiments allowed further investigation of the cause for the remarkably enriched delta O-18(diatom), value of the bottom sediment. Experiments with different fresh diatomaceous materials show an increase of opaline O-18 at high pH values which is remarkably reduced when organic coatings of the cells still exist or at near neutral pH. In contrast, high-pH conditions do not affect the delta O-18(diatom) values of sub-fossil and even fossil opal. IR analyses show that the O-18 enrichment of the sedimentary silica is associated with a decrease in Si-OH groups and the formation of Si-O-Si linkages. This indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment. Silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration. The dynamics of this process is of particular importance for the quantitative interpretation of sedimentary delta O-18(diatom) values in terms of palaeothermometry. (c) 2006 Elsevier Inc. All rights reserved

Late glacial environment and climate development in northeastern China derived from geochemical and isotopic investigations of the varved sediment record from Lake Sihailongwan (Jilin Province)

Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (delta(15)) and stable organic carbon isotope (delta C-13(org)) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM. Phases of high TOC content, high TN content, depleted delta C-13(org) values and high delta N-15 values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point towards a reduced phytoplankton discrimination against N-15 due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic (delta C-13(org), delta N-15) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450 to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial