Jonathan Barnes

Methane in the southern North Sea: Low‐salinity inputs, estuarine removal, and atmospheric flux

Dissolved CH4was measured in coastal waters of the southern North Sea, in two adjacent U.K. estuaries with well-defined turbidity maxima (Humber and Tyne) and in their associated river catchments, during a series of campaigns covering the period 1993–1999. In general, samples from all three environments were significantly to highly CH4 enriched relative to atmospheric air. Observed river water concentrations, ∼ 33–152 nmol L−1 (940–4305% saturation) for the Humber river catchment and ∼ 3–62 nmol L−1 (86–1754% saturation) in the river Tyne, were within but toward the low end of the range of CH4 concentrations in river waters world wide. In sea waters from the outer Wash estuary (U.K. coast) and adjacent to the Dutch coast, CH4 was highly but nonlinearly correlated with salinity, consistent with strong CH4 removal from river and/or estuarine CH4 sources influencing these locations. In transects along the Humber and Tyne estuaries, CH4 was highly negatively nonconservative, confirming the estuarine removal hypothesis. For both estuaries, highest CH4 concentrations, ∼190–670 nmol L−1 (6000–21,000% saturation) in the Humber and ∼650 nmol L−1(21,800% saturation) in the Tyne, were observed at very low salinity in the vicinity of the turbidity maximum. Importantly, these concentrations greatly exceeded measured river water values, implying for both situations the existence of a large in situ CH4 supply associated with high turbidity. Time series measurements at two locations in the upper Tyne subsequently confirmed the strong correspondence of dissolved CH4 and turbidity in the vicinity of the turbidity maximum. CH4removal estimated for the Humber, Tyne, Wash, and Rhine-Scheldt estuaries was ∼ 90% of the low-salinity CH4 input. On the basis of this and river discharge data, −7.I×108 mol CH4 may be removed annually in estuaries bordering the southern North Sea. Of this, ∼6.6×108mol may be lost by air-sea exchange. This represents an additional atmospheric CH4 flux from the North Sea unaccounted for in previous work, which may have, consequently, underestimated this source by ∼50%. Upward scaling of this estimate based on the mean of reported river water CH4 concentrations implies a previously unaccounted for ∼6.3–24×109 mol (i.e., ∼ 0.1–0.4 × 1012 g) CH4 yr−1 which may be lost globally to gas exchange in estuaries, increasing previous such estimates by ∼8–50%. However, as it is based on data that exclude the possibility of elevated CH4 levels at estuarine turbidity maxima, even this revision is likely to be conservative. Detailed studies of CH4 distributions in major world estuaries would now be required in order to successfully reevaluate the CH4 budget of the coastal marine atmosphere.

Methane and nitrous oxide fluxes in the polluted Adyar River and estuary, SE India.

We measured dissolved N(2)O, CH(4), O(2), NH(4)(+), NO(3)(-) and NO(2)(-) on 7 transects along the polluted Adyar River-estuary, SE India and estimated N(2)O and CH(4) emissions using a gas exchange relation and a floating chamber. High NO(2)(-) implied some nitrification of a large anthropogenic NH(4)(+) pool. In the lower catchment CH(4) was maximal (6.3+/-4.3 x 10(4)nM), exceeding the ebullition threshold, whereas strong undersaturation of N(2)O and O(2) implied intense denitrification. Emissions fluxes for the whole Adyar system approximately 2.5 x 10(8) g CH(4)yr(-1) and approximately 2.4 x 10(6)gN(2)O yr(-1) estimated with a gas exchange relation and approximately 2 x 10(9) g CH(4)yr(-1) derived with a floating chamber illustrate the importance of CH(4) ebullition. An equivalent CO(2) flux approximately 1-10 x 10(10)gy r(-1) derived using global warming potentials is equivalent to total Chennai motor vehicle CO(2) emissions in one month. Studies such as this may inform more effective waste management and future compliance with international emissions agreements.

Nitrous oxide and methane during the 1994 SW monsoon in the Arabian Sea/northwestern Indian Ocean

Partial pressures of dissolved and atmospheric nitrous oxide, N2O, and methane, CH4, were measured during the latter stages of the southwest (SW) monsoon and subsequent intermonsoon transition of 1994 in the Gulf of Oman and the northern and central Arabian Sea, during Discovery cruises D210 and D212 of the United Kingdom Joint Global Ocean Flux Study of the northwestern Indian Ocean (NWIO). Mean observed atmospheric mixing ratios, 310±3 ppbv N2O, 1706±17 ppbv CH4 (SW monsoon), and 311±3 ppbv N2O, 1784±20 ppbv CH4 (intermonsoon), were analytically indistinguishable from contemporary global baseline data. Mean surface mixed layer saturations were spatially and temporally heterogeneous. Largest variation was observed for N2O in an upwelling region adjacent to the Oman coast; mean N2O saturations were 140±40% (SW monsoon) and 119±17% (intermonsoon), with corresponding CH4 saturations of 170±55% and 179±15%. These apparent differences were largely a consequence of less detailed station coverage during D212, reflecting large variability on a relatively small spatial-scale rather than true seasonal variation; for individual stations in the coastal upwelling, temporal changes in mean mixed layer saturations were not significant. This suggests that within this region the processes of gas exchange, net production, and supply/removal by advection and vertical mixing were more or less in balance during the period studied. Open ocean saturations were lower and less variable: 106±7% N2O, 130±5% CH4 (SW monsoon) and 104±6% N2O, 115±2% CH4 (intermonsoon). Large supersaturation maxima for N2O (saturations ∼400–800%) and CH4 (saturations ∼200–400%) just below the base of the mixed layer were ubiquitous and followed a trend of progressive deepening toward the south. All deep N2O profiles were characterized by a second, more vertically extensive maximum (saturations ∼400–600%) between 500 and 1000 m. For these, plots of ΔN2O versus AOU were consistent with their formation by coupled nitrification-denitrification, with denitrification becoming progressively more important with distance toward the core of the oxygen depleted zone. Sea-to-air fluxes for a 6-month period represented by the study were determined from measured air-sea partial pressure differences and gas transfer velocities derived from in situ wind speeds. Estimated semiannual emissions, ∼0.41–0.75 × 1012 g N2O, ∼0.1–0.18 × 1012 g CH4, were within most previously reported ranges for the NWIO. The data indicate that seasonal changes in wind speed rather than seasonal changes in air-sea partial pressure differences due to monsoon-driven mixing and upwelling are the dominant control on air-sea gas exchange in the NWIO.

Spatial and temporal distribution of methane in an extensive shallow estuary, south India

Sedimentary methane (CH4) fluxes and oxidation rates were determined over the wet and dry seasons (four measurement campaigns) in Pulicat lake, an extensive shallow estuary in south India. Dissolved CH4 concentrations were measured at 52 locations in December 2000. The annual mean net CH4 flux from Pulicat lake sediments was 3.7 × 109 g yr-1 based on static chamber measurements. A further 1.7 × 109g yr-1 was estimated to be oxidized at the sediment-water interface. The mean dissolved concentration of CH4 was 242nmol ¦-1 (ranging between 94 and 501 nmol ¦-1) and the spatial distribution could be explained by tidal dynamics and freshwater input. Sea-air exchange estimates using models, account only for ∼13% (0.5 × 109 g yr-1) of the total CH4 produced in sediments, whereas ebullition appeared to be the major route for loss to the atmosphere (∼ 63% of the net sediment flux). We estimated the total atmospheric source of CH4 from Pulicat lake to be 0.5 to 4.0 × 109g yr-1.

A multiple regression model of normal central and peripheral motor conduction times

The effects of age, height, and gender on magnetic central and peripheral motor conduction times (CMCT, PMCT) were analyzed using a multiple regression model.