A multiproxy perspective on Holocene hydroclimate dynamics: Diatom and biogeochemical insights from a Siberian lake system

In recent decades, accelerating climate warming has driven rapid hydroclimatic changes across northern latitudes. Lake ecosystems are under increasing pressure from rising air temperatures and the massive human activities such as the combustion of fossil fuels since the industrial period. These factors cause profound ecological and physical alterations of lake systems, even in remote areas. Throughout the entire Holocene interglacial, which began ~11650 years ago, hydroclimatic conditions in the Northern Hemisphere (NH) have varied on both decadal and millennial time scales. The NH experienced a general warming trend in the early Holocene, reaching a peak at the Holocene Thermal Maximum (HTM) during the mid-Holocene. Thereafter, palaeoclimate records from the NH indicate a general cooling trend in the late Holocene, until the recent climate warming on short time scales. However, the Holocene hydroclimate conditions are variable spatially and temporally in the northern latitudes. Especially palaeoclimate records from eastern Siberia are not only sparse but generally lack high-resolution data, particularly in remote areas. This restricts comprehensive hydroclimate reconstructions in this region. In addition, most meteorological records cover only the period since the end of the 19th century. Therefore, primarily high-resolution proxy data from natural archives, such as lake sediments, are required. Fossil diatoms in sediments and their stable isotopes (δ¹⁸Odiatom, δ³⁰Sidiatom) are valuable proxies for hydroclimatic reconstructions due to their wide distribution in northern lakes and their sensitivity to environmental changes. The thesis aims to deepen the understanding of Holocene hydroclimate variability over decadal to millennial time scales and its influence on lake ecosystems in the boreal region of Yakutia, eastern Siberia. It specifically focuses on sediment cores from Lake Khamra (59.99° N, 112.98° E), a remote lake in southwest Yakutia. This cumulative thesis includes three research articles. The first article presents an extraordinary ~220-year (ca. 1790–2015 CE) continuous oxygen isotope record of diatoms (δ¹⁸Odiatom). This provides a rare high-resolution, decadal-scale reconstruction of recent hydroclimatic variability. Despite rising air temperatures, winter precipitation was identified as the primary driver of δ¹⁸Odiatom variability, entering the lake as isotopically depleted snowmelt. A sharp rise in mercury levels in recent decades indicates the impact of human-induced air pollution and reflects that even remote lakes such as Lake Khamra are influenced by anthropogenic activities. The second article is based on the same samples as the first, providing a comprehensive decadal-scale multiproxy data set of the recent ~220 years. It focuses on diatom assemblages, carbon and nitrogen concentrations (TOC, TN), their stable isotopes (δ¹³C, δ¹⁵N), and diatom silicon isotopes (δ³⁰Sidiatom). The analysis displays a major shift in diatom assemblages around the 1970s. Presumably, climate warming has altered limnological conditions, featuring an earlier ice-out and enhanced summer stratification. In contrast, biogeochemical data show an increase in carbon and nitrogen concentrations since the 1950s, likely in response to hydroclimatic and fire-related changes in the catchment. The observed depletion of δ¹³C in recent sediment samples could be related to the burning of fossil fuels indicating human influence. The third article presents a millennial-scale δ¹⁸Odiatom record covering the entire Holocene. In addition, the study includes another outstanding high-resolution ~200-year decadal-scale multiproxy record within the HTM. This dataset offers key insights into both long-term and short-term hydroclimatic variability. A lake-internal comparison of multiproxy records from the recent warm phase and during the HTM highlights distinct differences in both the absolute values and the variability. In general, the thesis is an important contribution to the understanding of Holocene hydroclimate variability and lake ecosystem dynamics in eastern Siberia by combining multiproxy records on different time scales. The research provides a deeper insight into δ¹⁸Odiatom variability over different time scales and site-specific forcing factors. Multiproxy data from the same sediment samples minimise age-related uncertainties, resulting in robust datasets and improving the interpretation of individual proxies. Recent changes at Lake Khamra, such as diatom assemblages, biogeochemistry and δ¹⁸Odiatom variability contrast to the stable conditions of the HTM. These changes are consistent with findings from other lake studies in northern latitudes. This demonstrates the increasing influence of recent climate warming and human activities even on remote boreal lakes in eastern Siberia.