Unlocking the power of lake multiproxy analyses by understanding subsurface biosphere processes

<p>Lake sediments bear valuable information allowing multidisciplinary research to address paleoclimatic and paleoenvironmental reconstructions at regional to global scales. Sedimentological, geochemical, paleontological and biological tools are commonly used to tackle these questions, which are generally driven by a set of intricated parameters. Among them, the importance of biogeochemical cycling is largely acknowledged in the lake (paleo-) water columns and has been at the heart of most paleolimnological studies. The way these signals are transferred to lake sediments has largely been studied. However, microbial communities - the principal actors in the biogeochemical cycling framework - keep being active in the sediment, and continue to influence the preservation and retention of organic and inorganic matter while buried. Gathered within the “early diagenesis” black box, these processes, once qualified, can help better interpret the proxies they may influence, and even constitute new ones. Within this work, we provide examples showing that the integration of studies of the subsurface biosphere within geo- and paleo-limnology investigations can help unlock or secure the potential of multiproxy analysis for reconstructing the paleoenvironments, paleoclimates and paleo-ecology of lake basins. The use of now well-developed OMICS methods, through the analysis of environmental and/or ancient DNA and lipids in particular has been coupled to mineralogical, isotopic and magnetic information in the Dead Sea (Levant) to demonstrate the differential preservation of mineralogic and sedimentologic signals along the last two glacial-interglacial cycles (Thomas et al., 2015, 2016; Ebert et al., 2018). Similar signals have been unlocked in Lake Towuti (Indonesia) and in Laguna Potrok Aike (Argentina) (Vuillemin et al., 2015, 2017). In Lake Ohrid (North Macedonia/Albania), environmental DNA has provided limited inputs on that perspective (Thomas et al., 2020), but has shown that ancient/fossil DNA could provide valuable information regarding the lake primary productivity and the status of its watershed land-cover. Integrating OMICS methods to tackle the identity and activity of the ancient and modern subsurface biosphere of lakes therefore holds an immense potential not only for microbiology investigations, but also for paleoclimatic and paleoenvironmental reconstructions.</p><p>Ebert et al. (2018) Overwriting of sedimentary magnetism by bacterially mediated mineral alteration. Geology <strong>46</strong>, 2–5.</p><p>Thomas et al. (2016) Microbial sedimentary imprint on the deep Dead Sea sediment. The Depositional Record 1–21.</p><p>Thomas et al. (2020) Weak influence of paleoenvironmental conditions on the subsurface biosphere of lake ohrid over the last 515 ka. Microorganisms <strong>8</strong>, 1–20.</p><p>Thomas et al. (2015) Impact of paleoclimate on the distribution of microbial communities in the subsurface sediment of the Dead Sea. Geobiology <strong>13</strong>, 546–561.</p><p>Vuillemin et al. (2015) Recording of climate and diagenesis through fossil pigments and sedimentary DNA at Laguna Potrok Aike, Argentina. Biogeosciences Discussions <strong>12</strong>, 18345–18388.</p><p>Vuillemin et al. (2017) Preservation and Significance of Extracellular DNA in Ferruginous Sediments from Lake Towuti , Indonesia. Frontiers in Microbiology <strong>8</strong>, 1–15.</p>

A contribution to adequate use of freshwater sponges as a proxy in paleoenvironmental studies

This paper provides a comprehensive review of the environments where freshwater sponges occur and evaluates the use of sponge spicules as a proxy in paleoenvironmental studies in the Neotropical region. The paper aims to: I) review the information about the ecology of inland sponges to facilitate the use of spicules as a paleoenvironmental tool; and II) identify possible incongruities in the use of this information in paleoenvironmental reconstructions that have been conducted in Neotropical regions. The study compiled data on 77 sponge species, specialist or generalist that occur under certain environmental conditions, such as: substrate type for growth, hydrodynamic types, as well as salinity and acidity concentrations. In addition, it provides a comparison of the paleoenvironmental conditions applied to reconstruction studies that have been carried out within this biogeographic region, highlighting incongruities regarding the current ecology of the sponges.

The δ13C, δ18O and Δ47 records in biogenic, pedogenic and geogenic carbonate types from paleosol-loess sequence and their paleoenvironmental meaning

Paleoenvironmental reconstructions are commonly based on isotopic signatures of a variety of carbonate types, including rhizoliths and land-snail shells, present in paleosol-loess sequences. However, various carbonate types are formed through distinct biotic and abiotic processes over various periods, and therefore may record diverging environmental information in the same sedimentological layer. Here, we investigate the effects of carbonate type on δ13C, δ18O, and clumped isotope-derived paleotemperature [T(Δ47)] from the Quaternary Nussloch paleosol-loess sequence (Rhine Valley, SW Germany). δ13C, δ18O, and T(Δ47) values of co-occurring rhizoliths (-8.2‰ to -5.8‰, -6.1‰ to -5.9‰, 12–32°C, respectively), loess dolls (-7.0‰, -5.6‰, 23°C), land-snail shells (-8.1‰ to -3.2‰, -4.0‰ to -2.2‰, 12–38°C), earthworm biospheroliths (-11‰, -4.7‰, 8°C), and “bulk” carbonates (-1.9‰ to -0.5‰, -5.6‰ to -5.3‰, 78–120°C) from three sediment layers depend systematically on the carbonate type, admixture from geogenic carbonate, and the duration of formation periods. Based on these findings, we provide a comprehensive summary for the application of the three isotopic proxies of δ13C, δ18O, and Δ47 in biogenic and pedogenic carbonates present in the same sediment layer to reconstruct paleoenvironments (e.g., local vegetation, evaporative conditions, and temperature). We conclude that bulk carbonates in Nussloch loess should be excluded from paleoenvironmental reconstructions. Instead, pedogenic and biogenic carbonates should be used to provide context for interpreting the isotopic signature for detailed site- and time-specific paleoenvironmental information.

Palynofacies analysis – principles and application for paleoenvironmental studies: implications from Bulgaria

The use of palynofacies analysis has been steadily increasing during the last decades. This stems from both commercial interests (i.e., hydrocarbon exploration) and academic research of past environments and studies of the global climate changes that took place during the Phanerozoic. In both commercial and academic contexts, there is a need for a better understanding of palaeoenvironmental changes in order to correctly date and interpret the depositional facies recorded in boreholes and outcrops. The present paper introduces this important analysis in the Bulgarian literature with characteristic examples of different palynofacies types documented by the author in Cretaceous sections in Bulgaria and their interpretation. These examples highlight the value of using palynofacies and palynomorph associations in an integrated approach for paleoenvironmental reconstructions in Bulgaria.