Greenland melt drives continuous export of methane from the ice-sheet bed.

Ice sheets are currently ignored in global methane budgets1,2. Although ice sheets have been proposed to contain large reserves of methane that may contribute to a rise in atmospheric methane concentration if released during periods of rapid ice retreat3,4, no data exist on the current methane footprint of ice sheets. Here we find that subglacially produced methane is rapidly driven to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland ice sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice-sheet bed during the melt season. Pulses of high CH4(aq) concentration coincide with supraglacially forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season are indicative of subglacial methane reserves that exceed methane export, with an estimated 6.3 tonnes (discharge-weighted mean; range from 2.4 to 11 tonnes) of CH4(aq) transported laterally from the ice-sheet bed. Stable-isotope analyses reveal a microbial origin for methane, probably from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation5 to about 17 per cent of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive fluxes (4.4 to 28 millimoles of CH4 per square metre per day) rivalling that of major world rivers6. Overall, our results indicate that ice sheets overlie extensive, biologically active methanogenic wetlands and that high rates of methane export to the atmosphere can occur via efficient subglacial drainage pathways. Our findings suggest that such environments have been previously underappreciated and should be considered in Earth's methane budget.

High-Resolution in Situ Measurement of Nitrate in Runoff from the Greenland Ice Sheet.

We report the first in situ high-resolution nitrate time series from two proglacial meltwater rivers draining the Greenland Ice Sheet, using a recently developed submersible analyzer based on lab-on-chip (LOC) technology. The low sample volume (320 µL) required by the LOC analyzer meant that low concentration (few micromolar to submicromolar), highly turbid subglacial meltwater could be filtered and colorimetrically analyzed in situ. Nitrate concentrations in rivers draining Leverett Glacier in southwest Greenland and Kiattuut Sermiat in southern Greenland exhibited a clear diurnal signal and a gradual decline at the commencement of the melt season, displaying trends that would not be discernible using traditional daily manual sampling. Nitrate concentrations varied by 4.4 µM (±0.2 µM) over a 10 day period at Kiattuut Sermiat and 3.0 µM (±0.2 µM) over a 14 day period at Leverett Glacier. Marked changes in nitrate concentrations were observed when discharge began to increase. High-resolution in situ measurements such as these have the potential to significantly advance the understanding of nutrient cycling in remote systems, where the dynamics of nutrient release are complex but are important for downstream biogeochemical cycles.

A Lab-on-Chip Analyzer for in Situ Measurement of Soluble Reactive Phosphate: Improved Phosphate Blue Assay and Application to Fluvial Monitoring.

Here, we present a new in situ microfluidic phosphate sensor that features an improved "phosphate blue" assay which includes polyvinylpyrrolidone in place of traditional surfactants-improving sensitivity and reducing temperature effects. The sensor features greater power economy and analytical performance relative to commercially available alternatives, with a mean power consumption of 1.8 W, a detection limit of 40 nM, a dynamic range of 0.14-10 µM, and an infield accuracy of 4 ± 4.5%. During field testing, the sensor was continuously deployed for 9 weeks in a chalk stream, revealing complex relations between flow rates and phosphate concentration that suggest changing dominance in phosphate sources. A distinct diel phosphorus signal was observed under low flow conditions, highlighting the ability of the sensor to decouple geochemical and biotic effects on phosphate dynamics in fluvial environments. This paper highlights the importance of high resolution in situ sensors in addressing the current gross under-sampling of aquatic environments.

Lab-on-chip measurement of nitrate and nitrite for in situ analysis of natural waters.

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.

Lab-on-Chip for In Situ Analysis of Nutrients in the Deep Sea.

Microfluidic reagent-based nutrient sensors offer a promising technology to address the global undersampling of ocean chemistry but have so far not been shown to operate in the deep sea (>200 m). We report a new family of miniaturized lab-on-chip (LOC) colorimetric analyzers making in situ nitrate and phosphate measurements from the surface ocean to the deep sea (>4800 m). This new technology gives users a new low-cost, high-performance tool for measuring chemistry in hyperbaric environments. Using a combination of laboratory verification and field-based tests, we demonstrate that the analyzers are capable of in situ measurements during profiling that are comparable to laboratory-based analyses. The sensors feature a novel and efficient inertial-flow mixer that increases the mixing efficiency and reduces the back pressure and flushing time compared to a previously used serpentine mixing channel. Four separate replicate units of the nitrate and phosphate sensor were calibrated in the laboratory and showed an average limit of detection of 0.03 µM for nitrate and 0.016 µM for phosphate. Three on-chip optical absorption cell lengths provide a large linear range (to >750 µM (10.5 mg/L-N) for nitrate and >15 µM (0.47 mg/L-P) for phosphate), making the instruments suitable for typical concentrations in both ocean and freshwater aquatic environments. The LOC systems automatically collected a series of deep-sea nitrate and phosphate profiles in the northeast Atlantic while attached to a conductivity temperature depth (CTD) rosette, and the LOC nitrate sensor was attached to a PROVOR profiling float to conduct automated nitrate profiles in the Mediterranean Sea.

Lab-on-chip analyser for the in situ determination of dissolved manganese in seawater.

A spectrophotometric approach for quantification of dissolved manganese (DMn) with 1-(2-pyridylazo)-2-naphthol (PAN) has been adapted for in situ application in coastal and estuarine waters. The analyser uses a submersible microfluidic lab-on-chip device, with low power (~ 1.5 W) and reagent consumption (63 µL per sample). Laboratory characterization showed an absorption coefficient of 40,838 ± 1127 L⋅mol-1⋅cm-1 and a detection limit of 27 nM, determined for a 34.6 mm long optical detection cell. Laboratory tests showed that long-term stability of the PAN reagent was achieved by addition of 4% v/v of a non-ionic surfactant (Triton-X100). To suppress iron (Fe) interferences with the PAN reagent, the Fe(III) masking agents deferoxamine mesylate (DFO-B) or disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron) were added and their Fe masking efficiencies were investigated. The analyser was tested during a deployment over several weeks in Kiel Fjord (Germany), with successful acquisition of 215 in situ data points. The time series was in good agreement with DMn concentrations determined from discretely collected samples analysed via inductively coupled plasma mass spectrometry (ICP-MS), exhibiting a mean accuracy of 87% over the full deployment duration (with an accuracy of > 99% for certain periods) and clear correlations to key hydrographic parameters.

Nitrate and Nitrite Variability at the Seafloor of an Oxygen Minimum Zone Revealed by a Novel Microfluidic In-Situ Chemical Sensor.

Microfluidics, or lab-on-a-chip (LOC) is a promising technology that allows the development of miniaturized chemical sensors. In contrast to the surging interest in biomedical sciences, the utilization of LOC sensors in aquatic sciences is still in infancy but a wider use of such sensors could mitigate the undersampling problem of ocean biogeochemical processes. Here we describe the first underwater test of a novel LOC sensor to obtain in situ calibrated time-series (up to 40 h) of nitrate+nitrite (ΣNOx) and nitrite on the seafloor of the Mauritanian oxygen minimum zone, offshore Western Africa. Initial tests showed that the sensor successfully reproduced water column (160 m) nutrient profiles. Lander deployments at 50, 100 and 170 m depth indicated that the biogeochemical variability was high over the Mauritanian shelf: The 50 m site had the lowest ΣNOx concentration, with 15.2 to 23.4 µM (median=18.3 µM); while at the 100 site ΣNOx varied between 21.0 and 30.1 µM over 40 hours (median = 25.1 µM). The 170 m site had the highest median ΣNOx level (25.8 µM) with less variability (22.8 to 27.7 µM). At the 50 m site, nitrite concentration decreased fivefold from 1 to 0.2 µM in just 30 hours accompanied by decreasing oxygen and increasing nitrate concentrations. Taken together with the time series of oxygen, temperature, pressure and current velocities, we propose that the episodic intrusion of deeper waters via cross-shelf transport leads to intrusion of nitrate-rich, but oxygen-poor waters to shallower locations, with consequences for benthic nitrogen cycling. This first validation of an LOC sensor at elevated water depths revealed that when deployed for longer periods and as a part of a sensor network, LOC technology has the potential to contribute to the understanding of the benthic biogeochemical dynamics.