EFFECTS OF MINERAL WEATHERING ON IRON, SULFUR AND PHOSPHORUS GEOCHEMISTRY ALONG A DEGLACIATED CHRONOSEQUENCE IN SOUTHWEST GREENLAND

Continental ice sheet retreat exposes deglaciated landscapes underlain by glacially comminuted sediments, but chemical weathering of minerals differs across the glacial foreland as sediments are exposed to the atmosphere and colonized by terrestrial ecosystems following ice retreat. We evaluate how postglacial landscape evolution affects iron (Fe), phosphorus (P) and sulfur (S) dynamics between glaciated and deglaciated watersheds along a chronosequence in southwest Greenland, and how these changes impact magnitudes of P export by combining P concentrations with stream discharge measurements. Our evaluations are based on mass balance models of measured solute concentrations fitted to mineral weathering reaction stoichiometry. Mass balance models indicate that sulfuric acid mineral weathering is greatest in coastal deglaciated watersheds with the longest exposure ages and highest precipitation, suggesting more intense sulfur cycling and iron sulfide mineral oxidation facilitated by soil microbiota. Iron sulfide oxidation promotes the formation of iron oxide compounds, which have an affinity to sorb P. Sequential chemical extractions of sedimentary P reservoirs in stream sediments show a 10-fold increase in the quantity of bioavailable inorganic P (ion exchangeable + Fe-associated P) in deglacial compared to glacial stream sediments, the majority of which is retained by associations with Fe oxide minerals. The increase in bioavailable inorganic P storage in deglaciated watersheds is accompanied by significantly (p < 0.01) lower P exports from these streams to the coastal ocean, likely reflecting the preferential retention of P on Fe oxide-rich soils and sediments. These results suggest that weathering of Fe sulfide minerals and formation of Fe oxide minerals increases potential storage of P with ice sheet retreat, and may decrease P export to coastal phytoplankton communities. Changes in the bioavailability and export of Fe and P may have implications for primary productivity and atmospheric CO₂ exchange in both terrestrial and coastal ecosystems with continued ice sheet retreat.