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

W.S Reeburgh,Jolynn Carroll,S.C Tyler

doi:10.1016/j.dsr2.2006.03.018

Abstract

We report measurements of d 13 C-CH4 and d 2 H-CH4 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-CH4 should permit partitioning Black Sea methane sources into fossil and recent

Highlights

Methane geochemistry was studied during the 1988 US–Turkey Black Sea Expedition (Reeburgh et al, 1991)
The depth distributions of d2H-CH4 and d13CCH4 at the central Black Sea station are shown in Figs. 2 and 3
Methane concentrations obtained from the Reeburgh et al (1991) study are plotted for the respective sample depths

Summary

Introduction

Two cores, one on the shelf (41135.50 N, 28146.920 E, 170 m, BS5-1), and the other at the deep station. Detailed measurements of water-column methane concentration were performed at sea using a gas chromatograph equipped with a stripping-trapping manifold. These measurements were precise to 0.8% over most of the concentrations encountered. W.S. Reeburgh et al / Deep-Sea Research II 53 (2006) 1893–1900 methane in the Black Sea water-column. Results from the two techniques were indistinguishable, with an overall average deep Black Sea methane oxidation rate for both techniques of $2.3 nM dayÀ1. Sediment methane concentration and oxidation rate measurements were made at the central station (BSK-2) only. These results are summarized in Reeburgh et al (1991). Samples for stable isotope analysis, the subject of this paper, were collected during the 1988 expedition at the central deep station

Methane budget

Methods

Methane traps

Oxidation and mass spectrometry

Results and discussion