2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 135-20
Presentation Time: 9:00 AM-6:30 PM


CHILTON, Kristin D., ROMANS, Brian W. and FLYNN, Shauna, Geosciences, Virginia Polytechnic Institute and State University, 1405 Perry Street, Blacksburg, VA 24061, kchilton@vt.edu

Thermohaline circulation is critical in governing global heat transfer and supporting marine ecosystems via the circulation of nutrients throughout the ocean. Thus, understanding the response of ocean circulation to abrupt climate changes in Earth history is of particular importance and relevance considering contemporary global climate change. However, understanding the response of deep ocean circulation to past global change has been difficult to accomplish at high temporal resolution due to the commonly slow sedimentation rates (<1 cm/kyr) in the deep ocean. Contourite drift deposits are depositional systems with relatively high accumulation rates (>5 cm/kyr) and therefore contain expanded sections of the geologic record. In this study, grain-size data of the terrigenous fraction are obtained from the mud-dominated Newfoundland Ridge drift complex at IODP Site U1411, which is interpreted to have formed under the influence of the Deep Western Boundary Current (DWBC). To examine the response of the DWBC to abrupt global cooling at the Eocene-Oligocene Transition (EOT), we analyzed 160 samples that span 150 m of stratigraphy from 36-26 Ma.

The main objectives of the grain-size analysis are to obtain data on (1) the ‘sortable silt’ (SS) fraction (10-63 µm) to generate a record of relative change in current velocity and (2) the abundance of lithogenic sand (>63 µm), which might relate to the occurrence of ice-rafted detritus. Preliminary data reveal a change across the EOT, from highly variable mean SS in the Eocene to a more restricted mean SS in the Oligocene. The data also show a long-term increase in mean grain size over the studied interval, which is most apparent in the measured range (1-63 µm). These results suggest a change in the DWBC at the EOT from a temporally variable current intensity to a relatively stable one, which may support hypotheses of EOT invigoration of ocean circulation. The abundance of lithogenic sand increases from negligible to measurable weight percentages across the EOT with a long-term increase through the Oligocene. More detailed examination of the grain-size distributions of the fine fraction, as well as comparisons to other proxies, will be helpful in deciphering the nature of DWBC changes and in informing our understanding of the evolution of ocean circulation during the EOT.