HR: 17:10h
AN: H34A-04 INVITED [Abstracts]
TI: Overview and Brief History of the Boron Isotope Proxy for Past Seawater pH
AU: * Hoenisch, B
EM: hoenisch@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory
of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Hemming, G
EM: hemming@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory
of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Hemming, G
EM: hemming@ldeo.columbia.edu
AF: School of Earth and Environmental Sciences
Queens College, 65-30 Kissena Blvd., Flushing, NY 11367, United States
AB:
In 1992 Hemming and Hanson (GCA, vol. 56, p. 537-543) showed that a variety of modern marine carbonates
revealed a boron isotopic composition close to the isotopic composition of dissolved borate at modern seawater
pH, suggesting this was the boron species preferentially adsorbed and incorporated into marine carbonates.
With a constant offset between the trigonal and tetrahedrally coordinated boron species and a pH-dependent
variation in their fractions, it appeared that this system would be sensitive to pH changes in the natural range of
seawater. Accordingly, it was suggested that the boron isotope composition of marine carbonates is a proxy for
past seawater pH. Subsequent culture studies with living planktic foraminifers and corals, as well as synthetic
precipitation experiments confirmed that the boron isotopic composition follows the isotopic composition of
borate across a wide range of seawater pH. In order to use the proxy with confidence, however, all other controls
apart from pH need to be thoroughly understood. Recent laboratory and sediment experiments have
demonstrated that vital effects and partial shell dissolution have the potential to modify the primary seawater pH
signal recorded in the boron isotopic composition of planktic foraminifers. However it has also been shown that
careful sample selection allows for avoiding these potential complications. A record of reconstructed surface
seawater pH and estimated aqueous PCO2 shows a remarkable match between boron isotope based
atmospheric pCO2 estimates and the Vostok ice core CO2 record. This convincingly demonstrates that boron
isotopes in planktic foraminifers allow quantitative estimates of atmospheric pCO2 in the past, and confirms that
glacial surface ocean pH was ~0.2 units higher compared to interglacial periods.
We are going to review and discuss the achievements generated in Gil Hanson's lab over the past 15 years in the
light of recent empirical measurements of the boron isotope fractionation between boric acid and borate in
seawater.
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 4930 Greenhouse gases
SC: Hydrology [H]
MN: 2007 Joint Assembly