Diffusive Transfer of Oxygen From Seamount Basaltic Crust Into Overlying Sediments: an Example From the Clarion-Clipperton Fracture Zone, Equatorial Pacific Ocean

Abstract:

Within the Clarion-Clipperton Fracture Zone (CCFZ) located in the equatorial Pacific Ocean numerous seamounts, with diameters ranging from 3 to 30 km and varying heights above the surrounding seafloor of up to 2500 m, occur throughout the deep-sea plain. There is evidence that these may serve as conduits for low-temperature hydrothermal circulation of seawater through the oceanic crust. During RV SONNE cruise SO205 in April/May 2010 and BIONOD cruise with RV L´ATALANTE in spring 2012 we took piston and gravity cores for geochemical analyses, as well as for high-resolution pore-water oxygen and nutrient measurements. Specifically, we took cores along a transect at three sites, located 400, 700 and 1000 m away from the foot of a 240 m high seamount, called ‘Teddy Bare’. At all 3 sites oxygen penetrates the entire sediment column of the organic carbon-poor sediment. More importantly, oxygen concentrations initially decrease with sediment depth but increase again at depths of 3 m and 7 m above the basaltic basement, suggesting an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments. This is the first time this has been shown for the deep subsurface in the Pacific Ocean. Mirroring the oxygen concentrations nitrate concentrations accumulate with sediment depth but decrease towards the basement. Transport-reaction modeling revealed that (1) the diffusive flux of oxygen from the basaltic basement exceeds 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 sediment surface and the overlying bottom water. We furthermore show that the upward diffusion of oxygen from the basaltic basement affects the preservation of organic compounds within the oxic sediment column at all 3 sites. Our investigations indicate that an upward diffusion of oxygen from the basalt into the overlying sediment may be a widespread phenomenon in this area of the Pacific Ocean that is characterized by numerous seamounts.