Middle Eocene to early Oligocene paleoceanography of the Antarctic Ocean (Maud Rise, ODP Leg 113, Site 689): change from a low to a high productivity ocean

Abstract

The sediment record from the middle Eocene (about 45 Ma) to the early Oligocene (about 33 Ma) on Maud Rise Site 689, Weddell Sea (paleodepth 1400–1650 m), has been studied by means of a coarse fraction analysis in order to reconstruct variations in surface water productivity and bottom water masses. Productivity has been estimated using carbonate dissolution and accumulation rates of radiolaria, diatoms, benthonic and planktonic foraminifers, sponge debris, ostracods and echinoids. Accumulation rates of Bolboforma spp. vary parallel to these proxies. Productivity increased with time. Three episodes were found in which productivity increased sharply, and productivity varied cyclically with a period of 430 ky. The middle Eocene, ca. 45.44-37.26 Ma, was a period of generally low productivity. No opal was deposited and carbonate preservation was excellent. During periods of a slight increase in productivity accumulation rates of benthonic and planktonic foraminifers, ostracods and echinoids increased in spite of an increase in carbonate dissolution. In the period ca. 41.62–43.07 Ma productivity rose in three distinct pulses. Clinoptilolite, formed from biogenous opal, is a common mineral in these sediments. These three productivity maxima coincide with a mica-rich unit. Mica probably originated from erosion on Antarctica, and was transported by an intermediate water depth current. In the late Eocene, ca. 36.9–34.95 Ma, productivity increased considerably and opal skeletons were deposited. Carbonate dissolution increased but planktonic foraminifer accumulation rates were not markedly decreased in periods of higher productivity, because productivity increase exceeded or equalled rises in dissolution rates. Increasing opal accumulation rates coincided with increases in the rates for benthonic foraminifers, sponge debris, ostracods and echinoids. In the late Eocene-early Oligocene period, 34.95-33.05 Ma, a further increase in productivity led to a strong dissolution of planktonic foraminifers in high productivity periods. The productivity increase is probably due to the increasing pole-equator temperature gradient which led to stronger atmospheric and oceanic turnover and mixing in the oceans. In the uppermost late Eocene two unusual layers suggest changes in bottom water masses affecting the seafloor at Site 689. A sediment interval with especially well-preserved carbonate microfossils underlies an interval that has poor carbonate preservation despite evidence of low productivity. These intervals suggest a change in bottom water conditions from Warm Saline Deep Water (WSDW), which favors carbonate preservation, to Antarctic Bottom water (AABW), which promotes carbonate dissolution. Cooling of surface waters over a period of 300 ky led to a thickening of the deep AABW layer, and an upward shift of the overlying WSDW, until the AABW finally reached the top of the Maud Rise in the area of Site 689. This period of AABW influence on top of the Maud Rise lasted 200 ky.