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Coastal Hypoxia

Mewes et al. (2016)

Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion–Clipperton Fracture Zone

Abstract. The Clarion–Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal seawater circulation through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of dissolved constituent exchange between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m toward the basaltic basement. Nitrate (View the MathML source) concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. These profiles suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments and a downward diffusion of View the MathML source from sediment pore water into the basaltic crust. At one site, we determined that the 87Sr/86Sr ratios of the bottom water and of the deep sediment near the basaltic crust are similar, further supporting diffusive exchange between basaltic crust fluids and sediment pore water.

Transport-reaction modeling performed at two of the study sites revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the upper sediment and the overlying bottom water. These results further show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount.