The Arctic is warming nearly four times faster than the global average, with profound consequences for the cryosphere, ocean circulation, and marine ecosystems. One emerging consequence of this climatic change is “Atlantification”, where warmer waters of Atlantic origin penetrate further into the Arctic Ocean. This process can weaken stratification, reduce sea-ice cover, and drive a poleward expansion of Atlantic taxa. To better understand what will happen in a changing future, we must turn to the past, commonly studied using proxies. Biological proxies are widely used, but are generally restricted to organisms that preserve as fossils, emphasizing the need to complement the biological proxy toolbox.

A promising complement involves dormant stages of heat-loving bacteria, known as thermospores, which have paradoxically been found in permanently cold sediments worldwide. Because thermophilic growth requires temperatures of at least 40°C, their presence in these cold environments implies a previous dispersal from a warm source environment where they were last active as vegetative cells. Their ability for long-distance transport, combined with being largely unaffected by genetic modification, makes thermospores promising as proxies for past microbial ocean current dispersal. Yet, their use as such remains insufficiently explored, especially in the Arctic Ocean, where little is known about their spatial distribution, dispersal pathways, and source environments. To address this, sediments from three locations on the outer Laptev Sea Shelf were incubated with radioactive sulfate to measure the activity of sulfate-reducing, endospore-forming bacteria. Together with 16S rDNA sequencing, thermospores were detected at one of the three investigated sites. While some sequences matched taxa from distant locations, including Svalbard fjord sediments and sediments from Aarhus Bay offshore Denmark, indicating long-distance transport, others were associated with the deep biosphere and environments such as hydrocarbon reservoirs and hydrothermal systems, pointing to possible Arctic sources. However, their occurrence at only one site suggests a limited dispersal and the presence of a dispersal barrier, challenging the view of unlimited dispersal and highlighting the need for methodological improvements before thermospores can be reliably used as proxies. 

 The cosmopolitan coccolithophore Gephyrocapsa huxleyi is a calcifying phytoplankton with a key role in the biogeochemical cycling of carbon and sulfur. Over the past decades of Atlantification, it has become more prevalent in the marginal seas of the Arctic Ocean. However, the preservation of carbonate nannofossils is generally poor in Arctic marine sediments, making it difficult to determine their spatiotemporal distribution based on mineral remains alone. Molecular genetic methods, such as sedimentary ancient DNA (sedaDNA), may offer a promising, yet understudied, complement to micropaleontological approaches. In this thesis, the occurrence of G. huxleyi was explored in both surface sediments and longer sediment cores from the Lomonosov Ridge and Arctic marginal seas using shotgun-sequenced DNA. The aim was to test whether a molecular genetic approach can reliably determine the presence or absence of G. huxleyi, despite poor carbonate preservation. The results were compared with nannofossil data and showed overall good agreement but also discrepancies, indicating methodological limitations with both methods. For shotgun-sequenced DNA, these mismatches were attributed to low read counts, short and degraded fragments, and incomplete reference databases. Nevertheless, the detection of sedaDNA from G. huxleyi, or closely related taxa, in sediments lacking preserved nannofossils and going back to at least MIS 5, demonstrates the method´s potential to complement micropaleontological approaches. Thus, once the methodological challenges identified herein are overcome, sedaDNA may have the capacity to refine nannofossil-based age estimates of Quaternary Arctic marine sediments.

 Together, these studies comprise a multidisciplinary framework for investigating Arctic biogeography and paleoceanography. By combining genomic, geochemical, and paleontological approaches, this thesis advances our understanding of microbial dispersal and biostratigraphic patterns across space and time in the context of a warming Arctic Ocean.