OS24B-01 INVITED
Understanding the Distribution and Behavior of Si Isotopes in the Ocean
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.
OS24B-02
Estimates of glacial Si and N dynamics in the Glacial Southern Ocean using Isotopic Mass balance
Evaluating the role of the Southern Ocean in regulating glacial-interglacial atmospheric CO2 cycles remains a major issue in paleoceanography. The Silicic Acid Leakage Hypothesis purports that increases in the export of silicic acid from the Southern Ocean to low latitudes via Subantarctic Mode Water (SAMW) during glacial times enhanced diatom productivity over that of calcifying forms thus contributing to lower atmospheric pCO2 through changes in ocean alkalinity. Recent analyses indicate that the influence of Southern Ocean nutrients on low- latitude alkalinity changes is driven more by changes in the silicic acid : nitrate ratio in SAMW than by the absolute amount of silicic acid exported in this water mass. We present an analysis of the nitrate and silicic acid content of SAMW during the Holocene and the last glacial maximum (LGM) in the Indian sector of the Southern Ocean using records of Si isotopes and diatom-bound N isotopes from the Antarctic and Subantarctic zones. For the LGM diatom-bound N isotopic values combined with Th-corrected opal fluxes suggest a reduction in upwelling in the Antarctic zone to the South of the Polar Front. Si and N isotopes suggest the Subantarctic received considerable silicic acid from the Antarctic, but no nitrate requiring an increase in nutrients supplied by vertical mixing in the Subantarctic to produce the known opal deposit. Depending on the magnitude of the reduction in Antarctic upwelling and the magnitude of the Subantarctic vertical source the ratio of silicic acid:nitrate in SAMW ranged from 0.5 to 6.7 during the LGM. The influence of such changes on atmospheric pCO2 is estimated using numerical modeling .
OS24B-03
The Silicon Isotope Composition of Silicic Acid in the Equatorial Pacific
Natural variations in silicon isotopes within silicic acid were investigated between 4° N and 3° S latitude along 110° W longitude in the equatorial Pacific. Variations in silicon isotopes map onto discrete water masses between 0 and 300 m. Silicic acid in equatorial surface waters is isotopically light compared to the subtropical waters to the north and south. The Equatorial Undercurrent (EUC) is revealed as a local maximum in δ 30Si of +1.8 to +2.4 ‰ compared to values of +1.6 to +1.8 ‰ in waters with similar silicic acid concentrations to the north and to the south. The anomalously positive isotopic composition of silicic acid within the undercurrent may reflect the biological consumption of silicic acid in the Southern Ocean and North Pacific water masses that supply silicic acid to the EUC. Estimates of the fractionation factor ε calculated using an open system isotope model range between -0.6 and -1.0 ‰ with most of the variability in ε being due to uncertainties in the characteristics of source waters supplying silicic acid to surface waters. Deep waters beneath the equator show relatively higher δ 30Si values, +1.4 ‰, compared to previous measurements from the north Pacific of ca. +1.0 ‰. Deep waters flow from the north to the south at 110°W suggesting that the deep waters beneath the equator at 110°W may be influenced by the North Pacific silicic acid plume that lies to the north of our study site. Samples of silicic acid from the core of the Si plume reveal δ 30Si values all >+1.4 ‰ consistent with this hypothesis.
OS24B-04
An Updated Method for the Measurement of Silicon Isotopes From Opal, Examples From Cariaco Basin Sediment Traps
We modified a new method for the complete digestion of silicate minerals (van den Boorn et al., 2006, JAAS, 21, 734-742), replacing a Parr bomb NaOH digestion with a hot plate digestion using Teflon beakers. The method is ideal for amorphous silicate minerals such as opal, and has been tested using diatoms recovered from the >125 μm fraction of sediment trap samples from the Cariaco Basin. A complete digestion of these samples was achieved using this revised method. The method has been used to prepare samples for silicon (Si) isotope analyses on a Neptune MC-ICPMS. The MC-ICPMS method was tested using the Si isotope reference material ‘big batch', correcting for mass bias by standard-sample bracketing. The δ29Si and δ30Si values of big batch, normalized to NIST RM-8546 (formerly NBS-28), are in agreement with published values (van den Boorn et al., 2006). This method will be evaluated with other Si reference materials (e.g., diatomite). Ultimately the new digestion method and MC-ICPMS method will be used to measure Si isotope variability in diatoms from the Cariaco Basin sediment trap samples. Furthermore, replicates will be used in an interlaboratory calibration to test the reproducibility between different techniques (e.g., MC-ICPMS vs. gas source).