Sedimentary organic matter preservation; a test for selective degradation under oxic conditions

Abstract

We report here a test of the hypothesis that the extent of organic matter preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water/sediment interface. Oxygen penetration depths, organic element compositions, and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the Washington continental shelf, slope, and adjacent Cascadia Basin. Individual amino acid, sugar, and pollen distributions were analyzed for a 11 to 12 cm horizon from each core, and 14 C-based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites. Sediment accumulation rates decreased, and dissolved O 2 penetration depths increased offshore along the sampling transect. As a result, oxygen exposure times (OET) increased seaward from decades (mid-shelf and upper slope) to more than a thousand years (outer Cascadia Basin). Organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location. In particular, organic carbon/surface area ratios decreased progressively offshore and with increasing OET. Three independent compositional parameters demonstrated that the remnant organic matter in farther offshore sediments is more degraded. Both concentration and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible ''oxic effect.'' OET helps integrate and explain organic matter preservation in accumulating continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere.

Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope

“…Cowie and Hedges, 1994), the clearest signal shown by all three parameters is that in both the Yokosuka and Goa transects, which extend below the OMZ, there is a consistent trend towards a greater degree of alteration that parallels the increase in DO levels from ∼ 1000 m near the base of the OMZ to the maximum sampling depth of 2056 m. Thus, the lower %C org values observed below the OMZ appear to be linked to increasing extent of OM decay, as previously observed on the Pakistan margin (Vandewiele et al, 2009), which can be linked to a progressive rise in O 2 exposure time (e.g. Hartnett et al, 1998;Hedges et al, 1999).…”

“…Rather, these trends are due to increasing decay with longer O 2 exposure, as concluded previously for several other margins, such as off Washington, Mexico and Pakistan (e.g. Hartnett et al, 1998;Hedges et al, 1999;Vandewiele et al, 2009).…”

Community Structure of Denitrifiers, Bacteria , and Archaea along Redox Gradients in Pacific Northwest Marine Sediments by Terminal Restriction Fragment Length Polymorphism Analysis of Amplified Nitrite Reductase ( nirS ) and 16S rRNA Genes

“…Besides large distances on the microbial scale, one major factor that might influence the selection of different communities is the differing amounts of carbon resources at our sites. Puget Sound sediment has more carbon, which is also less degraded than is Washington margin sediment carbon (18). Furthermore, among the Washington margin sites, the amount of carbon decreases with greater distance from shore (26).…”

Biogeochemistry of fatty acids in a river-dominated Mediterranean ecosystem (Rhône River prodelta, Gulf of Lions, France): Origins and diagenesis

“…Hedges et al (1999) stated that oxic degradation should be more appropriately viewed as a set of as yet unknown processes which characteristically prevail under sedimentary conditions where O 2 is present, as opposed to a specific mechanism that directly involves molecular oxygen.…”