HR: 14:45h
AN: PP52B-05    [PDF]
TI: The use of alkenone $\delta$$^{13}$C as a paleo-growth rate proxy
AU: * Pagani, M
EM: mark.pagani@yale.edu
AF: Yale University, 210Whitney Ave, New Haven, CT 06520 United States
AU: Hall, J
EM: jenney.hall@yale.edu
AF: Yale University, 210Whitney Ave, New Haven, CT 06520 United States
AB: The carbon isotopic fractionation that occurs during marine photosynthetic carbon fixation ($\epsilon$$_{p}$) is primarily a function of the concentration of CO$_{2aq}$, cellular growth rate, and cell geometry. Field data from natural haptophyte populations provide evidence for a robust relationship between the $\epsilon$$_{p}$ values derived from diunsaturated alkenones ($\epsilon$$_{p37:2}$) and the concentration of reactive soluble phosphate. Given our understanding of the factors controlling $\epsilon$$_{p37:2}$, it is likely that differences in algal growth rates are responsible for this relationship. Records of $\epsilon$$_{p}$ reconstructed from the stable carbon isotopic of diunsaturated alkenones and coeval planktonic foraminifera have been used to reconstruct the long-term evolution of atmospheric carbon dioxide during the Neogene by restricting analysis to relatively stable oceanographic environments and thus minimizing the effects of variable nutrient availability and cellular growth rates. However, $\epsilon$$_{p37:2}$ trends from different ocean sites do not only reflect global pCO2 levels, because there are significant differences in isotopic trends between basins. These differences likely reflect regional differences in haptophyte growth rates. For example, records of $\epsilon$$_{p37:2}$ from site 516 in the southwest Atlantic records a large, and permanent decrease in $\epsilon$$_{p37:2}$ between 20.3 to 19.5 Ma, coincident with a ~4 degree decline in surface water temperature inferred from the $\delta$$^{18}$O of planktonic foraminifera. This decrease in $\epsilon$$_{p37:2}$ is interpreted as reflecting both higher growth rates and nutrient concentrations in the upper-water column, caused by a reduction in upper water column stratification. Disappearance of measurable alkenones by ~17 Ma occurs prior to a reduction in surface-to-thermocline $\Delta$$\delta$$^{18}$O and $\Delta $$\delta$$^{13}$C gradients, and is attributed to low nutrient concentrations and reduced influence of proto-Antarctic Intermediate Water. We continue to investigate a potential nutrient control on the character of $\epsilon$$_{p37:2}$ at site 516 by evaluating the Cd/Ca and Ba/Ca character of depth stratified foraminifera coeval with the decrease in $\epsilon$$_{p37:2}$ between 20.3 to 19.5 Ma. Initial results suggest that the nutrient character of the upper water column experienced substantial change coincident with the $\epsilon$$_{p37:2}$ shift.
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1065 Trace elements (3670)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting