Exploring the use of sedaDNA to provide a palaeogenomic-based biostratigraphy in central Arctic Ocean sediments

The Arctic is currently undergoing rapid warming, which in the near future is expected to result in summers with an ice-free Arctic Ocean interior, and winters having thinner, and more mobile ice. Changes in sea-ice cover will have profound impacts on Arctic oceanography, its marine ecosystem, and ultimately on our climate from regional to global scales. To better understand what will happen in a changing future, we must look into the past. Arctic marine sediments provide a range of proxies that provide valuable palaeoceanographic, and palaeoclimatic information, documenting changes to the cryosphere. Yet, a confident interpretation of palaeoceanographic, and palaeoclimatic changes across glacial cycles of the Pleistocene is still hampered by our inability to accurately date Arctic marine sediments.

Studies conducted by Jakobsson et al. in the early 2000s transformed age-model interpretations in the Arctic following the identification of the coccolithophore Gephyrocapsa huxleyi (formerly known as Emiliania huxleyi) in a sediment core from the Lomonosov Ridge. Although G. huxleyi evolved globally ca 290 ka during MIS 8 (300-243 ka), it is generally believed that this species first appeared in the Arctic Ocean during the last interglacial period (MIS 5, 71-130 ka). The biostratigraphic datum provided by the first appearance of this species has therefore been central to much palaeoceanographic research conducted in the Arctic. However, identifying nannofossils in Arctic Ocean sediments is non-trivial, as their mineral remains are often poorly preserved or entirely absent due to unfavorable taphonomic conditions. This has led to ambiguous age estimates, as revealed by a recent study by Razmjooei et al. (2023) revising the calcareous nannofossil biostratigraphy in the Arctic, indicating that previously inferred sub-stages of MIS 5 may actually represent full interglacial periods rather than interstadials.

Recent advances using sedimentary ancient DNA (sedaDNA) now permit genome-based approaches to identify calcareous photosynthetic algae in marine sediments. We hypothesize that the sedaDNA approach may provide a “palaeogenomic biostratigraphic” age control when the conventional fossil-based approach is not usable due to poorly preserved or absent fossil remains. Given its importance in Arctic biostratigraphy, and since this is the only coccolithophore with an existing reference genome, we focus on G. huxleyi. An initial pilot study analyzing 5 samples from one sediment core from the central Arctic Ocean has previously showed positive reads for G. huxleyi in sediment layers argued to be from the last interglacial (MIS 5). Expanding on this pilot-study, we conducted a high-resolution sampling, totaling 93 sedaDNA samples, of two additional cores from the central Arctic Ocean. By integrating metagenomics with fossil, and climate proxy data, we aim to more confidently place the first appearance of G. huxleyi in the biostratigraphic framework of Quaternary Arctic marine sediments.