Project Title: The Role of Speciation and Provenance in the Bioavailability of Iron in the Gulf of Alaska
Mendenhall Fellow: Andrew W. Schroth (508) 457-2295, aschroth@usgs.gov
Duty Station: Woods Hole, MA
Start Date: October 1, 2007
Education: M.S. Geochemistry, Northern Arizona University (2001); Ph.D. Geochemistry, Dartmouth College (2007)
Research Advisors: John Crusius, (508) 457-2353, jcrusius@usgs.gov; Ken Bruland (UCSC), (831) 459-4587, bruland@ucsc.edu; Ed Sholkovitz (WHOI), (508) 289-2346, esholkovitz@whoi.edu
Project Description: Although ubiquitous and often enriched in the continental crust, iron (Fe) is only present at trace level dissolved concentrations in most marine waters and is thought to be the limiting nutrient to primary productivity in many areas of the ocean. In such areas, Fe limitation of plankton growth may influence the amount of carbon sequestration associated with algal export and the stock of fish populations in ‘bottom up’ ecosystems. While the distribution of dissolved iron throughout much of the world’s oceans is known or estimated at various spatial scales, a comprehensive understanding of the role of individual continental sources of Fe (aeolian dust, riverine input, continental shelf sediment resuspension, subterranean groundwater discharge, and remobilization during diagenesis of sediments) to the marine environment and the relative bioavailability of each Fe source remains elusive. A mechanistic model of the delivery of potentially bioavailable forms of Fe from the continent to marine waters is required to predict ecosystem response to the various forms of iron, its coupling to carbon cycling, and its response to current and future environmental change.
The objective of this study is to quantify and mechanistically describe the generation and delivery of bioavailable Fe from airborne and riverine sources to the Gulf of Alaska, with emphasis on the role of glaciers and variable bedrock mineralogy in this process.
Specifically, this work will:
- Examine the solid-phase speciation of iron across geochemical gradients in the Gulf of Alaska to identify the various forms of particulate iron and their distribution within subarctic marine waters. These samples were collected during August and September 2007 aboard the RV Thomas G. Thompson on a research cruise led by Ken Bruland. Iron speciation will be determined by synchrotron-based X-ray absorption spectroscopy (XAS).
- Quantify the speciation and flux of iron in glaciated and nonglaciated tributaries of the Copper River of differing bedrock Fe mineralogy to examine the role of glaciers and underlying bedrock in the fluvial delivery of reactive Fe to coastal waters. This will be accomplished by collecting suspended sediment and water samples within select tributaries and along the main stem of the Copper River during the summer of 2008. Analyses will be carried out by various techniques including XAS and wet chemical digestions.
- Examine the solubility of iron in glacial flour (sourced in catchments of different Fe mineralogy) in seawater to determine the role glacially-derived dust storms could play in the production of bioavailable Fe in arctic and subarctic iron-limited marine waters. In addition, these data can then be compared to the reactivity of Fe in desert-derived dust which has been previously studied to compare Fe reactivity by dust source. This will be accomplished by collecting samples of glacial flour within the Copper River watershed and then performing a variety of leaching experiments with Fe-limited water collected from the Gulf of Alaska on the research cruise.
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