HR: 17:00h
AN: OS24B-01 INVITED    [Abstracts]
TI: Understanding the Distribution and Behavior of Si Isotopes in the Ocean
AU: * De La Rocha, C L
EM: Christina.De.La.Rocha@awi.de
AF: Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, Bremerhaven, 27515, Germany
AU: Moritz, S
EM: Stefan.Moritz@awi.de
AF: Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, Bremerhaven, 27515, Germany
AU: Lohmann, G
EM: Gerrit.Lohmann@awi.de
AF: Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, Bremerhaven, 27515, Germany
AU: Wolf-Gladrow, D
EM: Dieter.Wolf-Gladrow@awi.de
AF: Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, Bremerhaven, 27515, Germany
AB: The Si isotopic composition (δ³⁰Si) of sedimentary opal is a proxy for dissolved silicon (DSi) use by diatoms. Some scenarios explaining lower atmospheric carbon dioxide during glacials have been evaluated against δ³⁰Si records, requiring that we understand them well. Our general understanding of δ³⁰Si as being controlled by isotopic fractionation during biological silicification, distillation of isotope ratios in surface waters, and pumping of Si to deep waters during the dissolution of sinking opal paints a reasonable but imperfect picture of the data collected so far. Closer look at the processes which control the distribution of Si isotopes in the ocean and sediments is needed. This could be done best by combining new measurements with modeling. Diatoms produce opal with a δ³⁰Si value about 1.5 ‰ lower than that of DSi. Such fractionation raises the δ³⁰Si of surface waters but the virtual depletion of DSi in the surface ocean means that the average δ³⁰Si of both deep waters and opal sediments should equal that of the inputs (i.e. about +1.2 ‰). Average riverine δ³⁰Si should then be +1.4 ‰. Some big questions remain. For example, how much of the roughly 1 ‰ variability in the δ³⁰Si of deep waters is real versus noise in the measurements? Real variability could contribute to downcore variations in δ³⁰Si. Some of the scatter in the surface water and sedimentary data, at least, is real, caused by regional and temporal differences in the ratio of nutrient replenishment via upwelling and depletion via biological uptake, complicating interpretation of sedimentary δ³⁰Si values. Regarding our understanding of Si isotope cycling, Southern Ocean surface water data and predictions from a GCM show a marked but imperfect correspondence and need considering. Lastly, C and N isotope values of opal are linked to the diatom species composition of the sediments and the same may be true for δ³⁰Si. Modeling sedimentary variations in δ³⁰Si may help to identify the extent to which the downcore signal reflects factors other than nutrient utilization.
DE: 4805 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912)
DE: 4855 Phytoplankton
DE: 4870 Stable isotopes (0454, 1041)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting