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Summary for Field Activity Number 2007-030-FA

Area of Operations: Gulf of Alaska, United States, North America, North Pacific

Dates: August 15, 2007 to September 21, 2007

Objectives: Determining concentrations, sources and fluxes of dissolved and particulate iron and solid-phase speciation to the North Pacific ocean.

Type of Activity: Sampling

Information to be derived: Samples and Chemical Analysis
specifically: 1) speciation and bioavailability of particulate iron in the Gulf
2) iron speciation in suspended sediment from various tributaries
3) degree of solubility of source iron in seawater
and 4) rate of transport of iron-rich nearshore waters toward iron-poor offshore regions of the Gulf

Summary: Iron is a nutrient that is believed to limit primary productivity in about 30 to 40 percent of the ocean s surface waters, including much of the northern North Pacific, where iron addition has been shown to stimulate plankton growth. By facilitating phytoplankton blooms, iron supply to surface waters may lead to a transfer of carbon to the deep sea and thus decrease the concentration of atmospheric CO2. Iron supply could also impact the fish yield of ecosystems controlled by nutrient supply. Before connections can be made between iron supply and these broader topics, however, some fundamental questions must be addressed, including (1) how does naturally occurring iron move from the continent to the open ocean? and (2) what fraction of that iron is bioavailable —in a form that is accessible by such organisms as phytoplankton? Continental sources of iron to the marine environment are numerous and include airborne dust, riverine input, continental-shelf sediment resuspension, submarine groundwater discharge, and remobilization during sediment diagenesis. However, the supply and bioavailability of iron from these sources is poorly constrained and likely to vary in both time and space. Improving our understanding of the processes governing iron transport and bioavailability in marine waters could prove critical in predicting the response of marine ecosystems to environmental change. Some of the specific research objectives of the cruise were to (1) measure the iron content in waters of the northwestern Gulf of Alaska, an area for which few data existed; (2) examine how mixing of iron-rich coastal waters with high-nutrient, low-chlorophyll waters leads to enhanced phytoplankton biomass in the northwestern Gulf of Alaska; and (3) assess what fraction of the particulate iron is reactive or bioavailable. The cruise involved extensive water sampling in both the coastal and offshore marine waters of the Gulf of Alaska. Once in the gulf, the Thompson tracked in and out of the sediment-laden Alaska Coastal Current and the offshore waters of the Alaska Gyre, while the scientists collected both surface samples and depth profiles and made various shipboard measurements. The terrestrial sediment supply to the Gulf of Alaska is large and primarily glacially derived, a product of extensive mechanical weathering of bedrock by glaciers within the interior mountain ranges of Alaska and by tidal glaciers flowing into the Gulf of Alaska. The scientists observed suspended glacial flour kilometers offshore in the Alaska Coastal Current. This glacial sediment, transported offshore by various processes, could be an important source of iron to the waters of the Alaska Gyre in this region. Schroth s research focuses on determining the speciation and bioavailability of particulate iron in the Gulf of Alaska and on assessing the variations in such parameters as a function of particulate source. Schroth collected suspended-sediment samples through filtration of both surface waters (collected from Bruland s fish ultraclean surface-water-sampling system) and water collected at depth (with an assembly consisting of a Niskin bottle rosette and a conductivity-temperature-depth [CTD] sensor). These samples were collected in various areas—the sediment-laden Alaska Coastal Current, sediment-rich near-bottom (nepheloid) layers near the continental shelf of the Gulf of Alaska, fiords immediately adjacent to tidal glaciers draining areas with multiple bedrock types, and offshore waters of the Alaska Gyre—to encompass an array of water masses that could contain unique iron particulate phases and distributions. Schroth will determine the solid-phase speciation of iron in these samples by using synchrotron-based X-ray-absorption spectroscopy. In addition, Schroth will determine iron speciation in suspended sediment from tributaries with different catchment bedrock geology and degree of glaciation in the Copper River drainage system—a primary source of sediment to the Gulf of Alaska. Exposed glacial sediment was also sampled at the toe of glaciers that differ in catchment geology; Schroth will examine the solubility of iron in this sediment when reacted with seawater from the gulf. The terrestrial component of this project will allow USGS scientists to assess how iron speciation in glacial sediment and bedrock influences the reactivity of iron that is transported to the Gulf of Alaska. Specifically, Schroth s research seeks to answer the following questions: What is the solid-phase speciation of iron across different water masses and biogeochemical gradients in the Gulf of Alaska, and how does iron speciation relate to iron bioavailability in these marine waters? Does the solid-phase speciation of iron vary by terrestrial source, and what role do glaciers and their catchment geology play in iron reactivity in riverine and dust loads delivered to the ocean near glaciated coasts? Crusius work involved examining the dynamics of mixing of nearshore iron-rich waters with offshore iron-poor waters, using radium isotopes as tracers. Radium is enriched in nearshore surface waters, both by desorption from sediment surfaces and by discharge of saline ground water. The goal of Crusius work is to understand the rates at which nearshore waters, rich in iron, are transported toward offshore iron-poor regions of the Gulf of Alaska using the radium isotopes as tracers.

USGS Project Number: 2921-C4C70 - Project Description

Contact: John Crusius (

InfoBank: uses the identifier T-1-07-GA. Visit this site for more information.

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