Glacier biogeochemical cycling and downstream impacts

Far from being frozen and sterile environments, glaciers are biogeochemical reactors and regulators. In this Review, we discuss the hydrology and biogeochemistry of glacierized environments and their impact on downstream ecosystems. Supraglacial meltwaters export labile organic carbon associated with active supraglacial microbial communities, as well as carbon and nutrients delivered via atmospheric deposition. Meltwaters funnelled to the glacier bed and exiting at the glacier snout transport large quantities of rock flour as well as supraglacial and subglacial-derived organic carbon and nutrients to downstream ecosystems. Subglacial water flow paths influence rock–water contact times and vary greatly, affecting weathering reactions. For instance, the hydrology of mountain glaciers and the Greenland Ice Sheet is typically dominated by seasonal melt with short (hours) to medium (weeks) water residence times, although extended biogeochemical isolation can exist in more isolated parts of the Greenland Ice Sheet. Conversely, the Antarctic Ice Sheet is dominated by basal ice melt and residence times that can exceed years and decades. As a result, the latter supports extended biogeochemical isolation and more advanced chemical weathering. Microbial processes and physical-chemical weathering can both sequester or emit greenhouse gases, but the net effect remains unknown. Meltwaters can potentially fuel biological processes in downstream ecosystems by priming glacier-fed streams, fjords, and oceans with rock flour and nutrients. The rapid reduction in glacier area projected for the next century mandates that future research provides a critical assessment of the effects of deglaciation on watershed biogeochemistry, ecology and global biogeochemical cycles.