Paleoenvironmental reconstruction of glaciomarine sediments in the Drygalski Basin of the western Ross Sea since the Last Glacial Maximum

Abstract

The repeated growth and reduction of the ice sheet have controlled the striking variation of the continental shelf environment in the Ross Sea. Particularly, glaciomarine shelf sedimentation is closely related to the ice sheet dynamics and ice shelf development since the Last Glacial Maximum (LGM). Using the four gravity (and box) cores obtained from the northern Drygalski Basin in the western Ross Sea, multi-proxies including sediment properties, geochemical proxies, carbon isotopic values of organic matter, clay mineral compositions of fine-grained sediments, and diatom assemblage were analyzed with AMS 14C dating from bulk organic matters. Based on the core description, X-radiograph observation, and results of grain size distribution, the core sediments were classified into the seven sediment facies: clast-rich muddy diamicton, clast-rich sandy diamicton, massive mud, laminated mud, bioturbated mud, massive sandy mud, and bioturbated sandy mud. The association of these sediment facies defines the four lithologic units in terms of the depositional evolution. Lithologic unit 1 represents diamicton deposited proximal to the grounding line. When the ice sheet and ice shelf retreated since the LGM, the unsorted sediment mass was deposited proximal to the grounding line. Lithologic unit 2 consists of the massive mud deposited under the sub-ice shelf condition. During the ice sheet retreat, the fine-grained particles were transported by the meltwater plume to the subice shelf floor. The biogenic materials were also transported from the seasonal open marine area to the sub-ice shelf by the currents. Lithologic unit 3 was composed of sandy mud and mud with abundant dropstones that might have been deposited at the calving line. As the calving line passed the Drygalski Basin, a large amount of the coarse fractions was deposited due to ice shelf breakup. The terrestrial sediments from the Victoria Land coast were transported from the grounding line of the remained local ice sheet, when the calving line passed the north side of Drygalski Ice Tongue. Lithologic unit 4 comprises the bioturbated mud and sandy mud that were deposited under the seasonal open marine condition. The biological productivity increased significantly during the late Holocene, while the terrestrial sedimentation decreased under seasonal open marine conditions. The diatom assemblage and delta 13C values of sediment organic matter indicate the mid-Holocene warm climate with more sea ice melting. Since the LGM, the paleoenvironmental change and glaciomarine depositional evolution on the northern Drygalski Basin were closely linked with the ice sheet dynamics and ice shelf activity, in addition to the climate change during the Holocene.