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