Demonstrating the Utility of Clumped Isotopes in Benthic Foraminifera to Estimate Subsurface Ocean Temperatures

Delia Oppo, Geology & Geophysics
Weifu Guo , Geology & Geophysics

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2015 OCCI Funded Project

Abstract

Measurements in pore water fluids from just a few ocean locations suggest that during the Last Glacial Maximum (LGM), the salinity of the deep Southern Ocean was higher than that of the North Atlantic, opposite to the situation in the modern ocean (Adkins et al., 2002).  It is therefore assumed that as deep waters must have been near the freezing point, deep density contrasts were driven by salinity.  Even North Atlantic Deep Water (NADW), which was presumably shallower than 2000 m during the LGM (e.g.  Curry and Oppo, 2005) is hypothesized to have been near the freezing point, so that when it upwelled near Antarctica, it did not provide significant heat to melt sea ice and ice shelves as observed today (Miller et al., 2012).  This cold NADW plays a key role in the more recent of a multitude of hypotheses related to ocean storage of carbon during the LGM (e.g.  Ferrari et al., 2014).  Regional heat accumulation in the subsurface North Atlantic during the LGM is hypothesized to have triggered ice sheet instability and an Atlantic Meridional Overturning Circulation (AMOC) shutdown (Marcott et al., 2011).  The same AMOC shutdown is hypothesized to have been critical to the subsequent release of ocean carbon to the atmosphere (e.  g.  Sigman et al., 2007).  Others have postulated that geothermal heating during this same, long interval of significantly reduced AMOC ultimately destabilized the water column and caused AMOC resumption and an observed, abrupt North Atlantic warming (the transition to the Bolling-Allerod warm period ~14,500 years ago) (Thiagarajan et al., 2014).  These (and other) hypotheses can only be tested with accurate paleo-temperature estimates.
 
The most commonly used proxy for Bottom Water Temperature (BWT) is Mg/Ca in benthic foraminifera (e.g.  Skinner and Shackleton, 2005) but many lines of evidence suggest that Mg/Ca in benthic foraminifera is controlled by factors other then BWT.  Here, we propose to make preliminary clumped isotopes measurements in LGM and late Holocene benthic foraminifera from Atlantic sediment cores to (1) demonstrate, for a future NSF proposal, that we can make meaningful estimates of subsurface cooling using this method and (2) to provide the first, preliminary estimates of glacial cooling in Atlantic deep waters using clumped isotopes in benthic foraminifera.  Measurements would be made in 3 cores from different depths, with the expectation that glacial cooling would be greatest at the shallowest (<1000 m) site.