The contribution of ice sheets to the global biogeochemical cycle of phosphorus is largely unknown, due to the lack of field data. Here we present the first comprehensive study of phosphorus export from two Greenland Ice Sheet glaciers. Our results indicate that the ice sheet is a hot spot of phosphorus export in the Arctic. Soluble reactive phosphorus (SRP) concentrations, up to 0.35 μM, are similar to those observed in Arctic rivers. Yields of SRP are among the highest in the literature, with denudation rates of 17-27 kg P km À2 yr À1. Particulate phases, as with nonglaciated catchments, dominate phosphorus export (>97% of total phosphorus flux). The labile particulate fraction differs between the two glaciers studied, with significantly higher yields found at the larger glacier (57.3 versus 8.3 kg P km À2 yr À1). Total phosphorus yields are an order of magnitude higher than riverine values reported in the literature. We estimate that the ice sheet contributes~15% of total bioavailable phosphorus input to the Arctic oceans (~11 Gg yr À1) and dominates total phosphorus input (408 Gg yr À1 ), which is more than 3 times that estimated from Arctic rivers (126 Gg yr À1 ). We predict that these fluxes will rise with increasing ice sheet freshwater discharge in the future.
Microfluidic technology permits the miniaturization of chemical analytical methods that are traditionally undertaken using benchtop equipment in the laboratory environment. When applied to environmental monitoring, these "lab-on-chip" systems could allow high-performance chemical analysis methods to be performed in situ over distributed sensor networks with large numbers of measurement nodes. Here we present the first of a new generation of microfluidic chemical analysis systems with sufficient analytical performance and robustness for deployment in natural waters. The system detects nitrate and nitrite (up to 350 μM, 21.7 mg/L as NO(3)(-)) with a limit of detection (LOD) of 0.025 μM for nitrate (0.0016 mg/L as NO(3)(-)) and 0.02 μM for nitrite (0.00092 mg/L as NO(2)(-)). This performance is suitable for almost all natural waters (apart from the oligotrophic open ocean), and the device was deployed in an estuarine environment (Southampton Water) to monitor nitrate+nitrite concentrations in waters of varying salinity. The system was able to track changes in the nitrate-salinity relationship of estuarine waters due to increased river flow after a period of high rainfall. Laboratory characterization and deployment data are presented, demonstrating the ability of the system to acquire data with high temporal resolution.
Subglacial chemical weathering plays a key role in global silicate weathering budgets, contributing to the cycling of silicon (Si) in terrestrial and marine systems and the potential drawdown of carbon dioxide from the atmosphere. Here, we use data from two Greenland Ice Sheet (GrIS) catchments to demonstrate how Si isotopes from dissolved and amorphous particulate fractions (δ 30 DSi and δ 30 ASi respectively) can be used together with major ion data to assess the degree of secondary silicate weathering product formation and redissolution in subglacial environments. We compare a time-series of summer melt seasons from the two study sites, which differ in catchment size (~600 km 2 for Leverett Glacier (LG) and ~36 km 2 for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na + and K + ions in relation to *Manuscript Ca 2+ and Mg 2+ ions, indicating a predominance of silicate weathering, whilst meltwaters from KS are characterised by carbonate weathering (hydrolysis and carbonation) throughout the melt season. Both catchments have mean δ 30 DSi values substantially lower than average riverine values (KS 0.41‰, LG-0.25‰, versus a global riverine mean of 1.25‰) and display a seasonal decline, which is more pronounced at LG. The δ 30 ASi values (discharge weighted mean values KS-0.44‰, LG-0.22‰) are lighter than the bedrock (mean values KS-0.18±0.12‰, LG 0.00±0.07‰) in both catchments, indicating a secondary weathering product origin or leaching of lighter isotopes during initial weathering of crushed rock. When used in combination, the major ion and silicon isotope data reveal that the extent of silicate weathering and secondary phase redissolution are more pronounced at LG compared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the δ 30 Si composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter δ 30 Si. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is likely to be very light in isotopic composition, and the flux is likely to increase with ice melt as the climate warms.
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