HR: 0800h
AN: PP51A-0583    [Abstracts]
TI: A SIMS Calibration of Benthic Foraminiferal Mg/Ca
AU: * Curry, W B
EM: wcurry@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Marchitto, T M
EM: tom.marchitto@colorado.edu
AF: INSTAAR and Department of Geological Sciences, University of Colorado, Boulder, CO 80309 United States
AB: Using a suite of multi-core tops, we have produced a calibration of C. pachyderma Mg/Ca versus temperature spanning the temperature range of 5 to 18 °C. The core tops are located along the Florida margin south of Dry Tortugas (KNR166), along the Bahamas west of Andros Island and Great Bahama Bank (KNR166), and along the southeastern margin of Brazil (KNR159). Water depths range from about 200 to 800 m for the Florida Straits multi-cores and 400 to 800 m for the Brazil margin multi-cores. Five of the KNR166 core tops contain post-1950 bomb radiocarbon with F_modern> 1; several others have bomb radiocarbon mixed in with pre-bomb sediments to give ages less than 0 BP. Core top ages are generally older for the KNR159 multi-cores, but each is from a location with a well documented Holocene section. Sedimentation rates for KNR166 multi-cores vary from 10 to 100 cm kyr^-1; for KNR159 multi-cores, sedimentation rates vary from 5 to 10 cm kyr^-1. Elemental ratios were determined by Secondary Ionization Mass Spectrometry (SIMS) using a Cameca IMS 3f ion probe calibrated for Mg/Ca and Sr/Ca using two standards which were independently measured using ICP-MS. Using SIMS, the external precision of the calibration standards averages ±3.5% (1σ RSD) for Mg/Ca and ± 1.7% (1σ RSD) for Sr/Ca. SIMS elemental measurements were performed on one to three individual C. pachyderma tests in each core top; more than 30 tests have been measured from 18 multi-core tops. Mg/Ca variability within C. pachyderma tests averages ± 20% (1σ RSD) with a small but significant trend toward higher variability at higher Mg/Ca. Higher Mg/Ca is observed in warmer waters, but the Mg/Ca values are generally lower (at comparable warm temperatures) than observed in previous calibration studies. At temperatures below 8 °C, C. pachyderma Mg/Ca values are less than 2 mmole/mole. At temperatures warmer than 15 °C, C. pachyderma Mg/Ca values exceed 3 mmole/mole. The slope of Mg/Ca versus temperature (~0.14 mmole/mole °C^-1) is shallower than in previously published relationships because the highest values of Mg/Ca in these core tops are 3 to 6 mmole/mole lower than Mg/Ca values observed in the warm-water core tops of previous calibrations (OCE205, Little Bahama Bank). The apparent discrepancy in Mg/Ca is not the result of differences in analytical technique since Mg/Ca values produced by SIMS have been reproduced independently by ICP-MS. Thus there is a real difference in the measured Mg/Ca values in warm waters that needs to be explained by other biological or diagenetic factors. Sr/Ca varies far less within and among the C. pachyderma tests and for the SIMS Sr/Ca data there is no systematic relationship with temperature.
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4924 Geochemical tracers
DE: 4994 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005