Stable carbon and hydrogen isotope measurements on Black Sea water-column methane

Abstract

We report measurements of d 13 C-CH 4 and d 2 H-CH 4 at a central station in the Black Sea. We considered the Black Sea as a ''biogeochemical bucket'' and the single station as a basin-wide integrator of processes affecting methane. Considering the rapid (3.6-18 yr) turnover of methane and the similarity of these stable isotope distributions to the methane concentration and oxidation rate profiles [Reeburgh, Ward, Whalen, Sandbeck, Kilpatrick, Kerkhof, 1991. Black Sea methane geochemistry. Deep-Sea Research 38, S1189-S1210], it appears that methane is being added approximately as fast as it is being oxidized. Methane can be thought of as ''running in place'' in the Black Sea water column.Recent reports of extensive vents on the northern side of the Black Sea suggest that they might be a methane source capable of effectively balancing the Black Sea methane budget. Unfortunately, we have limited information on basin-wide seep fluxes and cannot identify them with stable isotope measurements. Methane oxidation (and accompanying isotope fractionation) is so extensive that the water-column stable isotope measurements provide little information on methane sources. Future measurements of 14 C-CH 4 should permit partitioning Black Sea methane sources into fossil and recent components. r

Basin-wide estimates of the input of methane from seeps and clathrates to the Black Sea

“…This increase in CH 4 inventory would cause the δ 13 C of CH 4 dissolved in the water column to decrease by 1.4 ± 0.7‰ over this 13 yr period [49]. While this difference is small, our measurements of δ 13 C-CH 4 are indistinguishable from those conducted 13 yr ago [50] (Fig. 2).…”

“…Subsequent measurements of CH 4 concentration profiles in sediment cores [5,6] indicate that the diffusive flux of CH 4 from sediments to the water [47]. By convention [47], all radiocarbon results are normalized to 13 C. Samples were collected in the (○) central Black Sea water column in July 1988 [1,50] and (▴) western Black Sea water column in May 2001. Precision of ±1σ for the: 1) CH 4 concentration measurements is 3-4% based on replicate analyses of samples, 2) δ 13 C-CH 4 measurements is 0.3‰ based on replicate analyses of standard samples, and 3) 14 C-CH 4 measurements is 0.1 pMC.…”

Methane oxidation coupled to oxygenic photosynthesis in anoxic waters

“…The decrease in methane concentrations was accompanied by a strong increase in methane carbon isotope values (d 13 C-CH 4 ; Figure 1c) from À 75 to À 65% in the anoxic layer to À 35% at 8.5 m depth, suggesting microbial methane oxidation at the chemocline. Despite their fundamentally different biochemistry, both aerobic and anaerobic oxidation of methane result in a narrow range of fractionation factors for methane carbon (a C ; 1.003-1.039 and 1.009-1.039, respectively; Holler et al, 2009;Reeburgh et al, 2006). The fractionation factor a for Lago di Cadagno calculated from in situ d…”

Oceanic Methane Biogeochemistry

“…Only 15% of the methane source needed to maintain the steady-state Black Sea methane distribution could be identified. Distributions of methane concentration, δ 13 C-CH 4 , and methane oxidation rate were uniform in waters below 600 m. 86,250 Reports of extensive seeps, hydrate deposits, and mud volcanoes along the northern margin appeared after 1991 and were suspected as the source of the "missing" 85% of the methane source. The methane emitted from these seeps was expected to be of hydrate or thermogenic origin and was expected to contain little or no radiocarbon, so measurements of natural 14 C-CH 4 were proposed.…”

“…176,251 The methane concentrations, methane oxidation rates, and δ 13 C-CH 4 distributions are uniform below 1000 meters, leading to the suggestion that methane is being added as rapidly as it is being consumed in this depth interval. 250 Kessler, Reeburgh, and Tyler 251 compared the stable isotope and methane concentration distributions in the Cariaco Basin and the Black Sea. Between-basin differences in the deep parts of the basins are large, 9‰ for δ 13 C-CH 4 and 83‰ for δ …”