HR: 1330h
AN: B12B-0782 [PDF]
TI: Ground-Truthing the Boron Isotope - Paleo {\it p}H Proxy in Planktonic Foraminifera Shells: Partial
Dissolution and Shell Size Effects
AU: * H\"{o}nisch, B
EM: hoenisch@ldeo.columbia.edu
AF: School of Earth and Environmental Sciences, Queens College, 65-30 Kissena Blvd., Flushing, NY
11367-1597 United States
AU: * H\"{o}nisch, B
EM: hoenisch@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-8000 United States
AU: Hemming, N G
EM: hemming@qc.edu
AF: School of Earth and Environmental Sciences, Queens College, 65-30 Kissena Blvd., Flushing, NY
11367-1597 United States
AU: Hemming, N G
EM: hemming@qc.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-8000 United States
AB:
Empirical calibration studies have shown that seawater {\it p}H controls the boron isotope composition ($\delta^{11}$B) of
planktonic foraminifera shells. First applications of this new tool revealed promising parallels between reconstructed
surface seawater {\it p}H and past {\it p}CO$_{2}$ levels as measured in ice cores. However, little attention has been paid
to secondary parameters that might affect the $\delta^{11}$B-{\it p}H record of marine calcium carbonate. Here we investigate
$\delta^{11}$B of different size fractions of the symbiont-bearing planktonic foraminifer {\it Globigerinoides sacculifer},
ranging from 250 to 865 $\mu$m in shell diameter. Sediment samples from the Ontong Java Plateau in the Pacific and the
$90\deg$ East Ridge in the Indian Ocean reveal a systematic size pattern with up to +2.4$\permil$ heavier $\delta^{11}$B
values in larger individuals. This pattern is most likely due to differences in symbiont photosynthetic activity and its
integrated effect on the {\it p}H of the foraminiferal microenvironment. Smaller individuals must either live deeper in the
water column where light intensities are lower or the symbiont density in smaller individuals is lower as compared to large
specimens. Sediment samples from different water depths were studied to investigate dissolution effects on foraminiferal
$\delta^{11}$B by looking at shell weight. Because {\it in situ} dissolution studies have shown that smaller individuals of a
certain foraminifera species are more susceptible to dissolution than larger ones, we expected partial shell dissolution to
have a more significant affect on the chemical composition of smaller foraminifera shells relative to larger shells. Between
sediment cores of various depths we observe decreases in $\delta^{11}$B on the order of -0.7 to -1.6$\permil$ that are
associated with lower shell weights. Analyses of Mg/Ca ratios and shell surface microstructure will test our interpretation
of the shell size and depth transect $\delta^{11}$B data.
DE: 1040 Isotopic composition/chemistry
DE: 3030 Micropaleontology
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 4806 Carbon cycling
SC: Biogeosciences [B]
MN: 2003 Fall Meeting