Resolving sea ice dynamics in the north-western Ross Sea during the last 2.6 ka: From seasonal to millennial timescales

Time-series analyses of satellite images reveal that sea ice extent in the Ross Sea has experienced significant
changes over the last 40 years, likely triggered by large-scale atmospheric anomalies. However, resolving
how sea ice in the Ross Sea has changed over longer timeframes has until now remained more elusive. Here
we used a laminated sediment piston core (14.6 m) collected from the Edisto inlet (Western Ross Sea) to
reconstruct fast ice dynamics over the last 2.6 ka. Our goal was to first understand the climate expression of
selected well-defined sediment laminae and then use these characteristics for reconstructing past sea ice
behaviour across the whole sedimentary sequence. We used the recently established sea ice diatom
biomarker proxy IPSO25 in combination with diatom census counts and bulk analyses. Analyses performed
on a suite of discrete laminae revealed statistically significant differences between dark and light laminae
reflecting different depositional conditions. Based on their respective biogeochemical fingerprints, we infer
that dark laminae accumulated during sea ice thaws in early summer. Under these conditions, laminae
contain relatively high concentrations of IPSO25 and display an enriched d13C composition for the bulk
organic matter (OM). While diatom assemblages in dark laminae are relatively homogenous, as the thaw
continues later in the summer, Corethron pennatum becomes the dominant diatom species, resulting in the
formation of light laminae characterized by low IPSO25 concentrations. Since C. pennatum can migrate
vertically through the water column to uptake nutrients and avoid competition in oligotrophic waters, its
high concentration likely reflects stratified and ice-free surface waters typical of late summer.
Down-core trends show that the correlation between sediment brightness and geochemical fingerprint
(i.e., IPSO25 and d13C) holds throughout the record. Based on the knowledge gained at lamina level,
our down-core high-resolution reconstruction shows that the summer fast ice coverage changed
dramatically during the late Holocene. Specifically, we conclude that the Edisto inlet experienced regular
early summer opening between 2.6 ka, and ca. 0.7 ka, after which, coastal fast ice persisted during
summer months and ice-free conditions became less frequent. Comparison with previous regional ice
core data suggests that the sudden cooling recorded over the Victoria Land Coast region since 0.7 ka
might potentially explain our observation of persistent summer fast ice in the Western Ross Sea. Our
study has shown that multi-proxy data derived from laminated sediments can provide hitherto unknown
detail regarding past summer sea ice dynamics in coastal Antarctic regions.