HR: 0830h
AN: PP21C-1190    [PDF]
TI: What Lies Beneath: 5 Million Year Record of Subsurface and Surface Ocean Conditions From the Eastern Equatorial Pacific Ocean
AU: * Wara, M W
EM: mwara@es.ucsc.edu
AF: Ocean Sciences Department, UC Santa Cruz, 1156 High Street, A316 Earth and Marine Sciences, Santa Cruz, CA 95064
AU: Flower, M
AF: Ocean Sciences Department, UC Santa Cruz, 1156 High Street, A316 Earth and Marine Sciences, Santa Cruz, CA 95064
AU: Ravelo, C
EM: acr@es.ucsc.edu
AF: Ocean Sciences Department, UC Santa Cruz, 1156 High Street, A316 Earth and Marine Sciences, Santa Cruz, CA 95064
AB: Our results show that the Plio-Pleistocene (5.3 - 0.0 Ma) has been a period of dramatic change in the subsurface temperature and vertical structure of the Eastern Equatorial Pacific Ocean (EEP). We present records of subsurface and surface ocean temperature derived from magnesium calcium ratios (Mg/Ca) in foraminiferal calcite. Our estimates of absolute ocean temperature at the base of the photic zone ($\sim$100m) indicate a total cooling over the last 5 m.y. of approximately $10\deg$C, occurring in two major steps. The first subsurface cooling, from $\sim$$19\deg$C to $\sim$$14\deg$C, occurred between 4.6 and 4.0 Ma and is not associated with a major change in EEP sea surface temperature (SST). The second subsurface cooling, from $\sim$$14\deg$C to $\sim$$10\deg$C, occurred between 2.5 and 0.0 Ma and is associated with an equivalent reduction in SST. We interpret a gradual trend in the carbon isotope composition of the subsurface dwelling foraminifera over the entire 5.3 Ma interval to indicate a change in the relative contributions of northern and southern sources to the thermocline in the EEP. Our estimates of subsurface ocean temperature are derived from Mg/Ca ratios in {\it Globorotalia tumida} (355-425 $o$m) at Ocean Drilling Program site 847 ($0\deg$N, $95\deg$W, 3347m water depth). Our SST estimates are derived from {\it Globogerinoides sacculifer} (w/o sac; 355-425 $o$m) in the same samples. {\it G. tumida} has been shown in plankton tow and core top studies to occupy a relatively stable depth at the base of the photic zone ($\sim$100m in the EEP) while {\it G. sacculifer} has been shown to calcify predominantly in the mixed layer. Thus Mg/Ca ratios in {\it G. tumida} and {\it G. sacculifer} are a monitor of ocean temperature at these respective depths. We present a core-top calibration verifying the Mg/Ca-temperature relationship for {\it G. tumida} in the modern tropical Atlantic and Pacific oceans. The shift from a warmer ($\sim$$19\deg$C) to a cooler ($\sim$$10\deg$C) regime at 100m in the EEP has two possible causes. The first is an overall cooling of the subsurface driven by temperature changes in the subtropical northeast and southeast Pacific ocean, the regions that supply water to the EEP thermocline. The second is a shift in the depth of the thermocline from below to above 100m driven by changes in wind driven upwelling.
DE: 3309 Climatology (1620)
DE: 3670 Minor and trace element composition
DE: 4231 Equatorial oceanography
DE: 4267 Paleoceanography
DE: 4835 Inorganic marine chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting