|Home||Archived February 20, 2019||(i)|
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. As a result, private companies have begun to express interest in iron "fertilization" of the ocean because of the value of atmospheric CO2 reduction in the carbon-offset market. Understandably, this concept is both intriguing and highly controversial (see "Fertilizing the Ocean with Iron"). 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 ground-water discharge, and remobilization during sediment diagenesis (the changes that take place in sediment after burial). 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.
During August and September 2007, U.S. Geological Survey (USGS) scientists Andrew Schroth (Mendenhall Postdoctoral Research Fellow) and John Crusius (research geochemist) of the Coastal and Marine Geology Team in Woods Hole, Massachusetts, participated in a research cruise focused on iron and funded by the National Science Foundation. Ken Bruland, professor of ocean sciences at the University of California, Santa Cruz, led the expedition on the research vessel Thomas G. Thompson. In addition to Bruland, Crusius, and Schroth, 25 other scientists associated with various universities participated in the cruise, supported by the 22-member crew of the vessel. 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 6-week cruise, which began in Seattle, Washington, and ended in Dutch Harbor, Alaska, 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 USGS 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. Though relying on different methodologies, Schroth's and Crusius' research projects both seek to answer fundamental questions about the transport and bioavailability of terrestrially derived iron in the marine environment.
Schroth's research focuses on determining the speciation and bioavailability of "particulate" irondistinct particles of iron, as opposed to dissolved ironin the Gulf of Alaska and on assessing the variations in such parameters as a function of particulate source. "Speciation" refers to the form, or "phase," in which iron exists, including its oxidation state and bonding environment. Along with dissolved iron (present at low concentrations in seawater), particulate iron is believed to be an important source of iron for biota if it is present in a bioavailable form. 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 areasthe 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 Gyreto 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 at the Stanford Synchrotron Radiation Laboratory in Stanford, California (the same facility where USGS scientist Jim Hein is studying deep-sea iron-manganese crusts; see Sound Waves story, "Beam Time at the Stanford Linear Accelerator Awarded to USGS Scientist").
In addition, Schroth will determine iron speciation in suspended sediment from tributaries with different catchment (drainage basin) bedrock geology and degree of glaciation in the Copper River drainage systema 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:
Crusius' work on the Gulf of Alaska cruise 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. Radium exists as four naturally occurring isotopes with half-lives of 3.6 d, 11.4 d, 5.8 y, and 1,600 y; each of these isotopes is continuously supplied from a long-lived parent isotope. If the radium sources maintain near-constant ratios of these isotopes, the water-column ratios of short-lived to longer lived isotopes will change as nearshore waters are advected or mixed offshore, reflecting the aging of this coastally derived signal (see paper by Willard S. Moore, 2000, in Continental Shelf Research, URL http://www.sciencedirect.com/science/journal/02784343, v. 20, no. 15)
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. Together, the work of Schroth and Crusius should help improve our understanding of the processes that transport iron from coastal sources to the open Gulf of Alaska, one of the largest iron-limited regions of the world's oceans.
in this issue:
|Home||Archived February 20, 2019|