Premium
Methane in the southern North Sea: Low‐salinity inputs, estuarine removal, and atmospheric flux
Author(s) -
UpstillGoddard Robert C.,
Barnes Jonathan,
Frost Thomas,
Punshon Steven,
Owens Nicholas J. P.
Publication year - 2000
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/1999gb001236
Subject(s) - estuary , turbidity , salinity , hydrology (agriculture) , transect , oceanography , turbidite , environmental science , saturation (graph theory) , drainage basin , seawater , discharge , geology , structural basin , geomorphology , geography , geotechnical engineering , mathematics , cartography , combinatorics
Dissolved CH 4 was 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 CH 4 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 CH 4 concentrations in river waters world wide. In sea waters from the outer Wash estuary (U.K. coast) and adjacent to the Dutch coast, CH 4 was highly but nonlinearly correlated with salinity, consistent with strong CH 4 removal from river and/or estuarine CH 4 sources influencing these locations. In transects along the Humber and Tyne estuaries, CH 4 was highly negatively nonconservative, confirming the estuarine removal hypothesis. For both estuaries, highest CH 4 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 CH 4 supply associated with high turbidity. Time series measurements at two locations in the upper Tyne subsequently confirmed the strong correspondence of dissolved CH 4 and turbidity in the vicinity of the turbidity maximum. CH 4 removal estimated for the Humber, Tyne, Wash, and Rhine‐Scheldt estuaries was ∼ 90% of the low‐salinity CH 4 input. On the basis of this and river discharge data, −7.I×10 8 mol CH 4 may be removed annually in estuaries bordering the southern North Sea. Of this, ∼6.6×10 8 mol may be lost by air‐sea exchange. This represents an additional atmospheric CH 4 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 CH 4 concentrations implies a previously unaccounted for ∼6.3–24×10 9 mol (i.e., ∼ 0.1–0.4 × 10 12 g) CH 4 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 CH 4 levels at estuarine turbidity maxima, even this revision is likely to be conservative. Detailed studies of CH 4 distributions in major world estuaries would now be required in order to successfully reevaluate the CH 4 budget of the coastal marine atmosphere.