Person: Helen Evans

The depth of oxygen penetration into marine sediments differs considerably from one region to another. In areas with high rates of microbial respiration, O₂ penetrates only millimetres to centimetres into the sediments, but active anaerobic microbial communities are present in sediments hundreds of metres or more below the sea floor. In areas with low sedimentary respiration, O₂ penetrates much deeper but the depth to which microbial communities persist was previously unknown. The sediments underlying the South Pacific Gyre exhibit extremely low areal rates of respiration. Here we show that, in this region, microbial cells and aerobic respiration persist through the entire sediment sequence to depths of at least 75 metres below sea floor. Based on the Redfield stoichiometry of dissolved O₂ and nitrate, we suggest that net aerobic respiration in these sediments is coupled to oxidation of marine organic matter. We identify a relationship of O₂ penetration depth to sedimentation rate and sediment thickness. Extrapolating this relationship, we suggest that oxygen and aerobic communities may occur throughout the entire sediment sequence in 15–44% of the Pacific and 9–37% of the global sea floor. Subduction of the sediment and basalt from these regions is a source of oxidized material to the mantle.

Source: http://dx.doi.org/10.1038/ngeo2387

Publications > Journal Article

Scientific Drilling

Assessment and use of NGR instrumentation on the JOIDES Resolution to quantify U, Th, and K concentrations in marine sediment

Authors: Ann G. Dunlea, Richard W. Murray, Robert N. Harris, Maxim A. Vasiliev, Helen Evans, Arthur J. Spivack, Steven L. D'Hondt

Published: March 29, 2013

C-DEBI Contribution Number: 162

Abstract

Concentrations of uranium (U), thorium (Th), and potassium (K) in geological materials provide insight into many important lithological characteristics and geologic processes. In marine sediment, they can aid in identifying clay compositions, depositional environments, and diagenetic processes. They can also yield information about the alteration and heat production of rocks (Ketcham 1996; Barr et al., 2002; Revillon et al., 2002; Brady et al., 2006; Bartetzko, 2008). Measurements of the concentrations of these elements in geological materials are relatively straightforward in shore-based laboratories. Rapidly determining their abundance within cores of sedimentary and igneous rock sequences onboard a research vessel is a more challenging but potentially very useful method to non-destructively and quickly provide important geochemical information about the concentrations of U, Th, and K within the sequences being cored.

Source: http://dx.doi.org/10.2204/iodp.sd.15.05.2013

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