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Intellectual Merit We propose to measure silicon isotope distributions along the US GEOTRACES section from Peru to Tahiti scheduled for the Fall of 2013. The silicon isotope proxy is increasingly being used to assess the role of diatoms and silicic acid in past shifts in ocean productivity and their implications for climate. It is becoming clear that successful application of the silicon isotope proxy for diatom productivity requires a mechanistic understanding of how the silicon isotopic composition of ventilating waters masses varies in time and space. Model results suggest control through the fractionation of isotopes of Si during silica production and during silica dissolution coupled to the movement of Si by biological pump and the meridional overturning circulation. Empirical support for model predictions is equivocal. The very few full ocean depth profiles of δ30Si(OH)4reveal systematic variations among ocean basins that can be understood in terms of thermohaline circulation patterns, but in the eastern Pacific the relationship between δ30Si(OH)4and [Si(OH)4] in deep waters are the opposite of what current models predict. The spatial resolution of the present data set is inadequate to resolve conflicting hypotheses regarding the mechanisms driving large-scale δ30Si(OH)4distributions in this region, but we hypothesize that the North Pacific Silicic Acid Plume plays a critical role.
We propose to test model predictions regarding δ30Si(OH)4distributions within key water masses in the Pacific. Among the water masses sampled will be Lower Circumpolar Deep Water flowing from the south that should each be isotopically light due to incomplete nutrient depletion in the Southern Ocean. North Pacific Deep Water that flows into the region from the North should carry the signature the North Pacific Silicic Acid Plume. We will also investigate the influence of hydrothermal inputs along the East Pacific Rise. The strong gradient in productivity within surface waters between the Peru Shelf and Tahiti is also of interest as surface waters off Peru become preferentially depleted in silicic acid relative to nitrate. This strong decoupling of N and Si use by phytoplankton should result in contrasting N and Si isotope distributions allowing an empirical test of the ability of Si and N isotopes to predict relative nutrient depletion patterns as is assumed in paleo-applications of these proxies. We also propose to expand our current modeling to construct the first numerical model that will incorporate the influence of the North Pacific Silicic Acid Plume that may account for the anomalous Si isotope patterns in the Pacific.
Broader Impacts - While we can test hypotheses regarding the controls on Si isotope distributions using the proposed ocean section it is clear from anomalies in the current global δ30Si(OH)4data set that a comprehensive test will require spatially resolved δ30Si(OH)4data from multiple oceans basins. International GEOTRACES sections completed or planned by Canada, Great Britain, France, Germany, Sweden and India include δ30Si(OH)4measurements that will compliment those proposed here. While Si isotopes are not a core GEOTRACES parameter all data collected as part of this project will be submitted to the GEOTRACES database to aid this larger global synthesis. The PI supplies both secondary standards as well as reference seawater samples to all PI’s measuring Si isotopes as part of International GEOTRACES.
The research will also involve undergraduates who will be trained to assist in verification of [Si(OH)4] of each sample using colorimetric methods, data entry and data management.