Conference Agenda

Harmful cyanobacterial blooms (cyanoHABs) represent a major threat to lake ecosystems and water quality worldwide. These blooms produce potent neurotoxins and hepatotoxins as well as secondary metabolites (such as geosmin and 2-methylisoborneol), which impair water quality and threaten aquatic ecosystems, wildlife, and human health. As rising global temperatures are expected to increase the frequency and intensity of cyanoHABs, understanding their long-term occurrence has become increasingly important.

In this study, we investigated cyanoHAB formation over an annual cycle in Lake Bordesholm, northern Germany, to identify lipid signatures capable of tracing harmful cyanobacterial blooms in the geological record. Bloom formation occurred between June and September and was dominated by unicellular Microcystis and heterocytous genera including Aphanizomenon and Anabaena. Bloom development coincided with an increase in heterocyte glycolipids (HGs), specific biomarkers for diazotrophic cyanobacteria. Moreover, the relative distribution of HGs shifted throughout summer in response to changes in cyanobacterial community composition. Periods of intense bloom formation were further characterized by nitrogen isotope values close to 0‰, consistent with enhanced nitrogen fixation.

Elevated HG abundances combined with low nitrogen isotope values produced a distinct signal in the underlying sediment record. Our results demonstrate that the combined application of HG biomarkers and nitrogen isotope analyses provides a promising new approach for reconstructing cyanoHAB occurrence and dynamics in the geological past.

Understanding Earth´s past climate variability is crucial for predicting future changes. However, robust paleotemperature proxies are sparse for continental settings. Proxies based on lipids of cyanobacteria, which are widespread in many lakes worldwide, can potentially close this gap. Heterocyte glycolipids (HGs) are exclusive to heterocytes, specialized cells of members of the Nostocales, and are dedicated to nitrogen fixation. The ratio of fully hydroxylated versus keto-functionalized aglycone moieties of HGs are affected by growth temperature and hence correlate with lake water surface temperatures. Our investigation into the varying hydroxy/keto ratios across cultured isolates revealed a species effect, resulting in substantial differences between species grown at the same temperature. Our newly proposed multivariate heterocyte temperature proxy (HTP) averages multiple HG proxies, diminishing the species effect on the proxy response. Averaging of related proxies may be more widely applicable and should be explored for other proxy systems in the future. We show that sedimentary averaging of lipid signatures likely explains the strong correlations between lake water temperature and proxy signals in natural systems despite substantial variations in cyanobacterial communities, which is also expected for other paleotemperature proxies. Sedimentary averaging may thus be a key element that allows paleotemperature proxies to deliver meaningful reconstructions. Despite the observed significant species effect, correlation coefficient and temperature residuals of the HTP are comparable to established organic paleotemperature proxies such as U^K₃₇ or TEX₈₆, accordingly allowing temperature reconstructions in most lacustrine sediment archives. HG hydroxy/keto ratios thus represent some of the most promising paleotemperature proxies for continental settings.

“Lab experiments show that a constant water flow along a flat bottom produces stationary vortices which are visible if small particles with larger densities than water. If particles are moved by a stable vortex, these particles are deposited at quiet positions between two vortices. It is shown that in lakes particles are deposited along sand-strips perpendicular to the flow, obviously during a phase of large and ±constant flow. This happens in a lake after a sub-lacustrine slide generating a seiche, a phase of ±constant occurs if the seiche flow is a maximum. The depositional result are series of sand-strips in a mud environment and connected by monogranular sand layers. Example are deposits in the huge lake at the Cretaceous/Paleogene boundary from the Gulf of Mexico up to 1000 km northward and near the Chicxulub impact, and an actual 4x6 km² lake connected eastward and westward to the Lake Lucerne-system where 1601 a sub-lacustrine slide occurred. It is generally assumed that both events generated seiches; the depositional results in both examples include mud inclusions in the sand deposits, indicating that the flow stability during the sand deposits is fragile. It is just this general instability during the deposits of sand which is the general feature of seiche deposits.”

A strong phreatomagmatic eruption in the Eifel Region ~13,000 years ago gave rise to the nowadays largest volcanic lake in Germany. Lake Laach, a system enclosed within a small catchment that is still affected by the underlying volcanic activity, provides a unique natural laboratory for investigating carbonate proxy formation and element cycles in environmental gradients. The lake is continuously affected by magmatic CO₂ input. Moffetes are known from the lake margins and subaquatically in the basins of the 50 meter deep lake, where they are often linked to pockmark occurence. We sampled the water column on a seasonal basis, as well as porewaters and sediments from several short gravity cores and a piston composite core up to 6 meter long from a reference site. These provide a basis for assessing the sediment as an active biogeochemical reactor, inducing the formation of (Fe, Mn, Ca, Mg) carbonate solid-solutions, as further detailed by sediment extractions and SEM-EDX.

Further examination of sediments and porewaters from different pockmark sites shows high dissolved inorganic carbon (DIC) fluxes and enhanced diagenesis. In particular, pockmark sediments show alternating white and brownish laminae, characterized as rhodochrosite and/or siderite via XRD. Micro-facies investigations identified carbonates as pore-filling phases, evidenced by the analysis of thin sections unsing confocal Raman spectroscopy. The C isotopic composition of DIC in the porewaters points towards a volcanic-CO₂ isotope endmember, while the isotope signature of (Fe-Mn) carbonate solid-solutions reflect the signature of DIC in the pockmarks.

Un-glaciated terrestrial sites in East Antarctica host numerous lakes and ponds that can contain sedimentary archives on regional deglaciation histories and relative sea level changes. These sediment records also provide the unique possibility to study limnic (paleo)ecosystems in these extreme and diverse environments. Here we present paleoenvironmental reconstructions from lake sediment records that were obtained from two small lakes (Informally named Eye Lake and Ear Lake) on Thomas Island (Bunger Hills, East Antarctica) during the EASI 3 "RV Polarstern" voyage PS141. We use a multiproxy approach, combining sedimentological, elemental (µXRF core scanning) and lipid biomarker data to trace the evolution of the lakes since their formation following local glacier/ice sheet retreat, taking into account changes in hydrology and paleoecology. Our findings show that the lake records differ significantly from one another, even though the lakes are geographically close to each other and are subjected to similar climatic forcings. Ear Lake (c. 15 m deep) is brackish at present day and authigenic carbonate laminae in the sedimentary record indicate frequently recurring changes in PH, likely triggered by increased in-lake productivity. Eye Lake is shallower (4 m water depth) and fresh. Here, water mosses on the modern-day lake bottom and in the sedimentary record indicate an oligotrophic, stable lake environment. By comparing the two lakes, it is possible to investigate the response of individual lake systems to local influences but also to the impact of changing climatic conditions.

This study investigates sources and sinks of different forms of carbon and sulfur in a coastal peatland at the Baltic Sea coastline. The area is currently rewetted, but was and is under temporal flooding by brackish seawater. Investigations were carried out using geochemical analyses of peat and sediment samples taken from cores up to several meters long. Geochemical measurements included total Hg, extractable metals and sulfur species, and the stable isotope signatures of oxidized and reduced carbon and sulfur phases to identify element sources and sinks, and biogeochemical transformations.

Element profiles reflect a consistent stratigraphy of peat over sand. The C and N isotope composition of peat is characteristic of C₃ vegetation on land. Extractable Na contents in the top parts reflect episodic brackish water influence. Hg is enriched in the peat layer, likely extracted from seawater and bound to organic matter. Based on the C and O isotope composition, and Sr and Mg contents, carbonates in deeper layers are essentially of marine origin. Sulfur in the peat layer is found both in organic and inorganic phases. The stable sulfur isotope composition (³⁴S/³²S) of reduced sulfur phases was generally more negative than sulfate dissolved in the Baltic Sea and showed the most positive values in the peat decreasing with depth. This confirms the importance of microbial sulfate-reduction for carbon mineralization and sulfur fixation, with varying impact of sulfate reservoir effects, possibly superimposed by transport processes. A transect of sediment cores allows for elucidating the role of distance to the coastline.