HR: 17:15h
AN: PP44A-06    [Abstracts]
TI: Boron Isotopes in Benthic Foraminifers: Monospecific Coretop Calibration and Paleo-Reconstruction Through Two Glacial Cycles
AU: * Hoenisch, B
EM: hoenisch@uni-bremen.de
AF: University of Bremen, Marum Leobener Strasse, Bremen, 28359 Germany
AU: * Hoenisch, B
EM: hoenisch@uni-bremen.de
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Bickert, T
EM: g05e@uni-bremen.de
AF: University of Bremen, Marum Leobener Strasse, Bremen, 28359 Germany
AU: Hemming, N G
EM: hemming@qc.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Hemming, N G
EM: hemming@qc.edu
AF: Queens College School of Earth and Environmental Sciences, 65-30 Kissena Boulevard, Flushing, NY 11367 United States
AB: One hypothesis for the observed ~30% lower glacial atmospheric pCO₂ suggests an increased rain ratio of organic to inorganic carbon during glacial times (Archer and Maier-Reimer, Nature, 1994). In this scenario degradation of the excess organic carbon would cause porewater dissolution of calcite in sediments above the glacial lysocline, thus increasing seawater alkalinity and promoting the uptake of CO₂ from the atmosphere into the ocean. This hypothesis was supported by a boron isotope study on mixed benthic foraminifers, which suggested a +0.3 unit higher deep ocean pH during glacial times (Sanyal et al., Nature, 1995). However, the suggested increase in deep ocean carbonate ion concentration ~+100 mumol kg^-1 is inconsistent with sedimentary records of carbonate preservation and other geochemical proxy records such as benthic Zn/Ca. Although several studies have shown that the boron isotopic composition of marine biogenic carbonates is primarily controlled by seawater pH, a much higher glacial deep water pH has long been questioned. A likely source of error in the previous deep ocean pH reconstructions may have been the use of mixed benthic foraminifera species. We present the first boron isotope validation study on the single epibenthic foraminifer Cibicidoides wuellerstorfi, which includes coretop and glacial data from a depth profile (1000-4500 m water depth) at the outer Walvis Ridge. In addition we show C. wuellerstorfi data from core ODP 668B in the eastern equatorial Atlantic, the same samples recently used to quantitatively reconstruct surface ocean pH and atmospheric pCO₂ through two full glacial cycles (Hoenisch and Hemming, EPSL, 2005). Our monospecific benthic boron isotope data match the shape of the modern pH profile at Walvis Ridge. Importantly, the pH difference between glacial and interglacial deep ocean is much smaller than previously estimated.
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
DE: 4924 Geochemical tracers
DE: 4926 Glacial
DE: 4930 Greenhouse gases
DE: 4936 Interglacial
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005