Assessing the influence of glacial weathering on iron inputs to the ocean
Iron (Fe) is a growth-limiting nutrient and plays a critical role in ocean biogeochemical cycles. The solubility of Fe in seawater is extremely low and limits productivity in a number of ocean regions, which may play a role in regulating changes in atmospheric CO2 and climate. For example, increased dust transport during the last glaciation may have increased primary production and CO2 uptake. However the nature of potential far-field sources and the forms and solubility (and hence bioavailabilty – organisms generally incorporate soluble Fe in their cells) of the Fe delivered to the oceans remain poorly constrained.
Two major far-field sources of iron to the open ocean that have been identified are i) dust from arid regions, and ii) glacial weathering products.
While aeolian deposition is widely considered to be the major source of Fe to the remote oceans, the solubility of Fe from dust is extremely low. Glacial weathering products are an alternative source of iron: recent studies have demonstrated that glacial meltwaters as well as sediments in basal ice from Arctic, Antarctic and Alpine locations all contain nanoparticulate Fe-oxyhydroxides, which are highly soluble in seawater and which are readily taken up by marine organisms. In the coming decades, as the glaciers of the world recede, icecaps melt and arid regions expand, it is likely that the fluxes of Fe to the oceans will change. The primary research objective of this project is therefore to evaluate the relative flux and solubility of Fe derived from subglacial weathering products to the remote oceans.
Iron stable isotopes are a new geochemical tool and display large variations in low temperature environments and can be used to fingerprint specific sources of Fe to the oceans. Pilot data obtained for this project suggests that nanoparticulate Fe oxyhydroxides produced by glacial weathering processes are characterised by a distinct isotopic fingerprint. The goal of this project will be to develop Fe stable isotopes as a tracer of subglacial weathering processes and products, in particular nanoparticulate Fe-oxyhydroxides. This goal will be achieved by means of Fe- isotope studies of subglacial weathering processes carried out in well-characterised field sites in Iceland (the Vatnajokull ice sheet and Jokulsarlon localities, where ice is calving directly into seawater) and the French Alps (Bossons glacier).