Late Miocene Early Pliocene

The Late Miocene witnessed further high latitude cooling, summarized in Hodell and Kennett (1986). A cooling of Southern Ocean upper circumpolar and intermediate water masses is documented in benthic foraminiferal oxygen isotope records from ODP Site 1088 in the sub-Antarctic South

Atlantic, which display a two-step increase between ~7.4 and 6.9 Ma (Billups, 2002). Associated with this late Neogene cooling are further changes in Southern Ocean oceanography.

A 10-5 Ma hiatus in the sediment package underlying the Bounty Fan at ODP Site 1122 suggests an intensification of the ACC, associated with the Late Miocene climate cooling and expansion of the WAIS (Carter et al.,

2004). The strengthening of North Atlantic Deep Water (NADW) in the Late Miocene led to global oceanic circulation patterns approaching the modern configuration (e.g. Wright et al., 1991). Late Miocene strengthening of NADW is supported by benthic foraminiferal carbon isotope records from ODP Site 1088 in the sub-Antarctic South Atlantic, which indicate an influx of a nutrient-depleted water mass into the Southern Ocean at ~ 6.6 Ma (Billups, 2002). The relative flux of NADW into the Southern Ocean reached modern proportions by ~6Ma, and subsequently intensified yet further, supporting speculation that the climatic warmth of the Early Pliocene resulted from enhanced thermohaline overturn (Kwiek and Ravelo, 1999; Billups, 2002).

The warm period in the Early Pliocene coincides with an apparent increase in biological productivity as indicated by increased opal deposition rates at ODP Sites 1165, 1095, 1096 and several gravity core sites in the Southern Ocean, which has been interpreted as reflecting a reduction in the extent of sea ice (Hillenbrand and Futterer, 2002; Griitzner et al., 2005; Hillenbrand and Ehrmann, 2005; Hillenbrand and Cortese, 2006). Surprisingly, this warm interval appears to coincide with glacial advance (indicated by increased terrigenous input from Antarctica), perhaps supporting the ''snowgun hypothesis'' of Prentice and Matthews (1991) (Grutzner et al., 2005). Large oscillations in the Al/Ti ratio, the lead isotopic composition and the neodymium isotopic composition of sediments at DSDP Site 266 in the Australian-Antarctic basin have been interpreted to reflect either variations in the intensity of the ACC or variations in weathering fluxes resulting from periods of instability of the Antarctic Ice Sheet around 1.9 and 3.3 Ma (Vlastelic et al., 2005).

Diatom assemblages at ODP Sites 1165 and 1166 suggest that sea-ice cover remained lower than today through much of the Pliocene (Whitehead et al.,

2005), and silicoflagellate assemblages have been used to pinpoint three intervals (3.7, 4.3-4.4 and 4.6-4.8 Ma) within the Pliocene when sea surface temperatures in the Southern Ocean were roughly 5°C warmer than today (Whitehead and Bohaty, 2003). Similar events are recorded in ODP Leg 178 Sites, pointing to circum-Antarctic warming that may have been responsible for ice-sheet loss (Escutia et al., 2007).

0 0

Post a comment