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

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Sea-ice reconstruction over the last 3ka in the Ross Sea (Antarctica)

The aim of the project Holoferne was to obtain a continuous and highly resolved record of sea ice dynamics and other environmental parameters during the late Holocene. With this goal in mind, we collected a laminated sediment core (14.6 m long) in the Edisto inlet (Ross Sea, Antarctica). The coring site was chosen in the inner bay, where the Holocene unit is particularly expanded (>60 m thick). By means of AMS 14C datings of the acid insoluble organic fraction, the expanded record covers the last ca. 2800 years indicating an average sedimentation rate of ca. 0.5 cm y-1. X-ray radiographs and visual inspections show well-preserved laminated sediments dominated by alternating dark- and light-brown diatom oozes. In this study, we present XRF core scanning data, diatom assemblages, high-resolution bulk organic matter geochemistry (stable isotopes and contents of organic carbon and total nitrogen), opal, grain size and lipid biomarker results with focus on the new IPSO25 proxy (di-unsaturated highly branched isoprenoid) of landfast sea ice. A sub-sample of well-defined dark and light laminae (n=33) exhibited a statistically different (t-test) composition in terms of stable carbon isotopes, IPSO25, relative percentage of same the diatoms such as Corethron pennatum, as well as porosity. Dark laminae are likely indicative of spring blooms when d13C signature and IPSO25 concentration are higher, whereas porosity is comparatively low. The heavy d13C signature and high IPSO25 values are consistent with deposition of sea-ice diatoms grown in reduced availability of dissolved inorganic carbon typical of sea-ice matrix and released in the water column during the sea ice melting season. By contrast, the IPSO25 in light laminae decreases by up to three-orders of magnitude together with a marked decrease of d13C. We interpret these trends to reflect a protracted opening of the bay later in summer and a greater availability of carbon for photosynthesis. Under these conditions, the greater Corethron pennatum concentration in the light-brown laminae might indicate a different environment likely associated with open sea conditions. In addition, the relatively higher abundance of Corethron pennatum resulted in a “sponge-like” matrix, which explains the marked difference in porosity between light and dark laminae. If confirmed, this interpretation allows some inferences on the Late Holocene temporal variability of the regional wind pattern, which in turn exerts first-order control on the landfast sea ice dynamics and diatom ecology in the Edisto Inlet, and in general along the western coast of the Ross Sea

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 δ13C 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 δ13C) 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