Testing a novel genomic-based approach for Arctic Ocean biostratigraphy

Biostratigraphy is a fundamental tool for age-calibrating marine sediments and enabling palaeoceanographic reconstructions. However, establishing age control in marine sediments of the Arctic Ocean is challenging, due to low micro- and nannofossil abundances, discontinuous occurrences across glacial and interglacial periods, as well as spatial and temporal variations in carbonate preservation. Gephyrocapsa huxleyi is a globally distributed coccolithophore whose first occurrence at ca 290 ka is widely used to date Quaternary marine sediments. Yet, its initial appearance and stratigraphic range in the Arctic Ocean are debated. Here, we present the first combined seda DNA-nannofossil approach to trace the occurrence of G. huxleyi throughout three sediment cores from the Lomonosov Ridge recovered during the Arctic Ocean 2016 expedition on icebreaker Oden. Sed aDNA was extracted from 87 samples spanning key lithological boundaries. Following shotgun sequencing, the presence of G. huxleyi was assessed using three independent bioinformatic tools Kraken2, BWA and BLAST. These results were integrated with lithological data and Gephyrocapsa nannofossil assemblages to evaluate the potential of palaeogenomics as a biostratigraphic tool in the Arctic that could potentially overcome the limitations of conventional nannofossil-based approaches. Although we found broadly overlapping seda DNA and nannofossil results, key discrepancies and methodological limitations do not allow us to confidently identify the first occurrence of G. huxleyi in the studied sequences. We discuss both the potential and the challenges of seda DNA as a complementary biostratigraphic tool to improve age control of Arctic marine sediments. • We present a novel combined seda DNA-nannofossil approach to trace the occurrence of the key biostratigraphic marker Gephyrocapsa huxleyi in Arctic Ocean sediments. • Seda DNA from 87 samples, spanning and extending below the established biostratigraphic range of G. huxleyi, is analyzed together with nannofossil assemblages. • Broadly overlapping seda DNA and nannofossil results are revealed, but discrepancies and methodological limitations prevent confident placement of the first occurrence of G. huxleyi.