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Methane stable isotopic ratios and concentrations as indicators of methane dynamics in estuaries
Author(s) -
Sansone Francis J.,
Holmes M. Elizabeth,
Popp Brian N.
Publication year - 1999
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 - 10.1029/1999gb900012
Subject(s) - estuary , bay , seawater , oceanography , environmental chemistry , water column , δ13c , fractionation , environmental science , geology , stable isotope ratio , chemistry , organic chemistry , quantum mechanics , physics
Mixing diagrams of methane (CH 4 ) concentration and stable isotopic ratio (δ 13 C‐CH 4 ) were used to examine the fate of river‐borne CH 4 as it crosses a variety of estuaries: Columbia River (Oregon/Washington), Parker River (Massachusetts), Great Bay (New Hampshire), Kaneohe Bay (Hawaii), and Elkhorn Slough (California). Unlike the surface of the open ocean, these systems are not in near atmospheric equilibrium with respect to concentration or δ 13 C‐CH 4 value. The range of observed CH 4 concentrations and δ 13 C‐CH 4 values were 33–440 n M and −36 to −58 per mil, respectively, for the freshwater end‐members for these systems, 12–330 n M and −48 to −60 per mil for water at the mouths of the estuaries, and 1.6–6 n M and −45 to −60 per mil for the seawater end‐members. In the Kaneohe Bay estuary, CH 4 concentration and δ 13 C‐CH 4 displayed near‐conservative behavior. In the Columbia River estuary, there was loss of riverine CH 4 coupled with shifts to heavier isotopic values, apparently the result of in situ CH 4 oxidation; this oxidation exhibited an apparent kinetic isotopic fractionation factor of 1.0042−1.012. In contrast, the other estuaries showed elevated concentrations and more negative δ 13 C‐CH 4 values apparently resulting from inputs of biogenic CH 4 from midestuary marshes and sediments. The upper reaches of all these systems were well out of equilibrium with the atmosphere on a concentration basis, indicating that they are atmospheric CH 4 sources. However, these first δ 13 C‐CH 4 measurements show that there is a wide range of isotopic variation in these waters, which indicates that it will be difficult to estimate the collective isotopic contribution of estuaries to the global methane budget.