PP53B-2323
Comparative Study of δ18O Compositions Determined for Fossil Holocene Planktic Foraminifera by In Situ SIMS Measurements and Standard Gas-Source IRMS Bulk Shell Analyses
Friday, 18 December 2015
Poster Hall (Moscone South)
Jody Wycech1, Daniel Clay Kelly1, Reinhard Kozdon2, Kouki Kitajima3, Howard J Spero4, Ian J Orland5, Noriko Kita6 and John W Valley7, (1)University of Wisconsin Madison, Madison, WI, United States, (2)Lamont Doherty Earth Observatory, Palisades, NY, United States, (3)University of Wisconsin, Madison, WI, United States, (4)University of California Davis, Department of Earth and Planetary Sciences, Davis, CA, United States, (5)University of Minnesota Twin Cities, Dept. of Earth Sciences, Minneapolis, MN, United States, (6)UW-Madison, Madison, WI, United States, (7)University of Wisconsin Madison, WiscSIMS, Dept. of Geoscience, Madison, WI, United States
Abstract:
The use of SIMS to perform in situ δ18O analyses on micron-scale domains within individual foraminiferal shells is an emerging technique that holds much promise for elucidating new paleoclimate information from deep-sea records. High precision and accuracy are possible for inorganic calcite, but continued testing is essential to establish the accuracy of this novel technique for biocarbonates; hence, a comparative study was conducted using both in situ SIMS (IMS 1280) and standard gas-source IRMS analyses to measure the δ18O in the same chamber of a foraminifera shell. An initial experiment was performed on 18 Orbulina universa shells (~600 μm) exhibiting varying states of preservation (3 well-preserved glassy, 1 intermediate, 14 frosty) handpicked from the uppermost 2 cm (Holocene) of a piston core (PC9) retrieved from the northwestern Atlantic. The spherical chamber of each shell was manually broken into smaller fragments with the majority of the fragments pooled for δ18O analysis by IRMS, and a remaining fragment cast in epoxy for SIMS. Comparison of average SIMS δ18O values to corresponding IRMS δ18O values for the same chamber reveals a linear relationship across a 3‰ range in glassy and frosty shells with the SIMS values being 0.9 ± 0.7‰ (2SD) lower. This 0.9‰ offset is also seen in O. universa shells (1 glassy, 3 intermediate, 10 frosty) that have undergone hydrogen peroxide cleaning and sonication. The δ18O offset is being further investigated using manually fragmented O. universa shells from the core-top of PC9 that have been vacuum roasted as well as cultured specimens grown under controlled conditions. Through these additional experiments we are exploring possible offset mechanisms including: errors in SIMS and gas-source analysis; differences between the coarsely crystalline homogeneous SIMS standards and complex biocarbonate samples; and zoning in shells due to growth, diagenesis or other components not targeted by SIMS.
