Life in the Deep Sediments
Location

           As sediment covers most of the deep ocean and ocean margins and 70% of the earth's surface is covered by ocean, the extent of these communities is quite vast.  However, the depth to which these communties prevail has been increasing as deeper and deeper cores are logged.  Bacterial counts show a logarythmic decrease of bacterial numbers with depth (eg. Parkes et al. 1994).  Significant populations are predeicted to exist below 1000m by a logarythmic regression, but in ever decreasing numbers.


(Parkes et al. 2000).

            Assuming all sulfate reduced at the methane-sulfate transition zone is used for methane oxidation, the diffusive flux of methane upward can be estimated (D'Hondt et al. 2002).  If the diffusive flux of methane is in steady-state with the rate of methane production below, then the rate of bacterial activity in the methanogenic zone can be estimated and the respiration of the entire sediment column can be estimated by the downward flux of sulfate into the sediment column.  D’Hondt calculated this for six ODP cores, showing: 1. Total rates of subsurface SO42- reduction are at least two to three orders of magnitude higher at the methane-rich ocean-margin sites than at the sulfate-rich open-ocean sites. 2.  At the open-ocean sites, total rates of subseafloor SO42- reduction are so low as to be nearly indistinguishable from zero, showing either greatly reduced bacteria populations, or extremely low bacterial activity.  D’Hondt et al. 2002 divided the oceans into two provinces: a sulfate-rich open-ocean and an ocean-margin province where sulfate was limited to shallow sediments.  Methane is produced in both regions but is abundant only in sulfate-depleted sediments.  Metabolic activity is greatest in narrow zones of sulfate-reducing methane oxidation along ocean margins.  (D’Hondt et al. 2002)
(A). Global SO42- map. Symbol color indicates the depth of the shallowest sample where subsurface SO42- concentrations either stabilize at a nonzero value (circles) or reach zero (diamonds) (white, <=5 mbsf). Circle size indicates the SO42- concentration at which each subsurface SO42- profile stabilizes. (B) Global CH4 map. Circle color indicates the depth of the shallowest sample that exhibits a CH4 concentration above the laboratory background level (white, <=5 mbsf). Circle size indicates the peak subsurface concentration. Crosses mark sites where CH4 never rises above laboratory background. Cross color marks the depth of the deepest sample analyzed.  (D'Hondt et al. 2002).


Deep Sediment: Introduction

Deep Sediment: Counting Methods

Deep Sediment: Energy Source

Deep Sediment: Signal in Sediment

Deep Sediment: Total Carbon

Deep Sediment:  Sustainability

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