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doi:10.2204/iodp.proc.346.203.2017

Introduction

Anaerobic oxidation of methane (AOM) may be a dominant process in shallow sediment across continental slopes (D’Hondt et al., 2002; Dickens, 2001). During AOM, downward diffusing sulfate reacts with upward migrating methane as follows (Reeburgh, 1976, Barnes and Goldberg, 1976):

The microbially mediated reaction (Boetius et al., 2000) typically occurs across a relatively thin and conspicuous geochemical horizon (Devol and Ahmed, 1981), generally now referred to as the sulfate–methane transition (SMT). Other than producing bicarbonate and hydrogen sulfide ions, AOM indirectly affects the chemistry of other elements, notably Ba, Ca, Fe, Mg, and Sr. This is because of precipitation and dissolution of various minerals (e.g., barite, calcite, greigite) within sediment at or near the SMT (Torres et al., 1996; Dickens, 2001; Luff and Wallmann, 2003; Snyder et al., 2007.

Although both AOM and associated SMTs are now widely discussed in the scientific literature, a full understanding of chemical impacts across the SMT remains uncertain. This is because many sites where AOM has been suggested lack high-resolution interstitial water (IW) sampling over an extended sedimentary depth, a full suite of pertinent analyses, or both. Previous work in the marginal sea between the Eurasian continent, the Korean Peninsula, and the Japanese Islands (hereafter simply called the “marginal sea”; Fig. F1) highlights this issue. For example, at Ocean Drilling Program (ODP) Site 798, which presumably has a SMT, measurements of dissolved SO₄^2– and Ba were made approximately every 5.5 m and 20 m, respectively (Shipboard Scientific Party, 1990). Along Umitaka Spur, Snyder et al. (2007) determined alkalinity and dissolved S, Ba, Ca, Mg, and Sr at high sample resolution, but without measurements of CH₄ or dissolved HS^–, and the piston cores only penetrated 4 m of sediment.

A shallow SMT probably occurs in sediment across much of the southern marginal sea (Fig. F1). In 2013, International Ocean Drilling Program (IODP) drilled seven locations in the marginal sea. Site U1426 is located at 37°2.00′N, 134°48.00′E, and 902 m water depth at the same location as ODP Site 798. Four holes at Site U1426 penetrated 396, 34, 204, and 99 meters below seafloor (mbsf) with recovery <100% (see the “Site U1426” chapter [Tada et al., 2015b]). Site U1427 is located at 35°57.92′N, 134°26.06′E, and 330 m water depth. Three holes at Site U1427 cored 548, 405, and 351 mbsf with a recovery <98.9% (see the “Site U1427” chapter [Tada et al., 2015c]). While principally drilled for paleoceanographic objectives, an additional focus at these sites in the southern marginal sea was generation of high-resolution IW profiles for a wide array of dissolved species. This mission was accomplished, but as noted in the “Site U1426” and “Site U1427” chapters (Tada et al., 2015b, 2015c), there were some data irregularities with shipboard SO₄^2– and Ba measurements (Fig. F2). The SO₄^2– concentrations did not reach micromolar concentrations below the SMT as expected from abundant work at locations around the world, including piston core KH-77-3-L4 located 54 km northeast of Site U1426 (Fig. F1; Masuzawa and Kitano, 1983). The dissolved Ba concentrations were approximately one order of magnitude higher than those measured at ODP Site 798. The dissolved Fe concentrations were only measured on a few samples.

Here, we reexamine most of the pore water splits by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) at Rice University approximately 4 months after the expedition (Fig. F3). The analyses include S, which we use a proxy for SO₄^2–, Ba, and Fe. Smooth high-resolution and internally consistent concentration profiles were determined for most solutes on board ship, including Ca and Mg, so we purposely tuned analyses to these three elements and did not measure any other analytes.

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