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.