Lower Circumpolar Deep Water Flow Through the SW Pacific Gateway for the Last 190 Ky: Evidence from Antarctic Diatoms

Abstract

Endemic Antarctic diatoms are incorporated into downwelling Antarctic Bottom Water (AABW) during its formation in the Weddell and Ross Seas and at the sea-ice interface on the shelves around Antarctica. We infer their subsequent entrainment in Lower Circumpolar Deep Water (LCDW) during upwelling of AABW into Lower LCDW around Antarctica. Antarctic diatoms are very effective tracers of Lower LCDW in the world's oceans. Significant quantities of displaced diatoms are recovered in core-tops along the known flow-path of Lower LCDW in the SW Pacific Gateway, east of New Zealand. In contrast, core-tops bathed in overlying water masses such as core North Atlantic Deep Water (NADW) are devoid of such diatoms. The interface between Lower LCDW and core NADW, at 3735-4097 m depth, is clearly defined in this region by the presence/absence of tracer diatoms, which closely matches the hydrographical observed interface at 3800 m. Tracer diatoms in cores located within the palaeo-LCDW flow-path may provide information on the flow of LCDW through the gateway for the last 190 ky. However, is it difficult to eliminate the effects of diatom productivity and preservation. AABW production rate and its mixing rate with LCDW will also have an effect. However, reworked diatoms suggest that Lower LCDW flow was greater during colder times. Separation of total tracers into open-ocean and sea-ice related taxa reveal variations in AABW source for the last 190 ky. AABW formed in the open-ocean appears to be relatively enhanced during colder times, while that formed over the shelf is relatively enhanced in warmer times.