Sinks marine sediments

Marine sediments can be both sources and sinks for U. Uranium has repeatedly been shown to be scavenged from solution into reducing sediments. 

The main mechanism for removal of organic carbon from the ocean is burial in sediments. This flux is equal to the average global sedimentation rate for marine sediments times their weight percent organic carbon. The total sink... 

If Vtii/Ptii = 1 then the accumulation of °Thxs in marine sediments would provide an assessment of their sedimentation rate. For instance, if Pm is N dpm m yr , and N dpm are found in the upper 1 cm of 1 m of seafloor, then the sedimentation rate must be lcmyr Sedimentation rate is an important variable in paleoceanographic reconstruction as it provides the timescale for the continuous record of environmental change recorded in marine sediments. Sedimentation rate is also a key geochemical variable as sediments are the major sink for most chemical species in the ocean. A tool allowing assessment of past sedimentation rates is therefore an appealing prospect. 

In terms of organic carbon generation, the coccolithophorids are a minor player, representing only 6 to 8% of global marine primary production. But their detrital remains contribute disproportionately to the burial of carbon in marine sediments. This is due to near complete loss of POC via remineralization as the detrital hard and soft parts settle to the seafloor. As estimated from Broecker s Box model in Chapter 9, only about 1% of the POM that sinks out of the surfece water is buried in marine sediments. In comparison, about 20% of the biogenic PIC survives to become buried in the sediments. 

There may be a cycling of S compounds of different oxidation state between anaerobic and aerobic zones in the soil, such as at the soil—floodwater interface. In reduced lake and marine sediments this leads to accumulation of insoluble sulfides as S04 carried into the sediment from the water above is immobilized. Such deposits function as sinks for heavy metals. Plants absorb S through their roots as S04 H2S is toxic to them. Therefore HS must be oxidized to S04 in the rhizosphere before it is absorbed. 

Lithium is a conservative element in the ocean with a residence time of abont one million year. Its isotope composition is maintained by inputs of dissolved Li from rivers (average 5 Li + 23%c, Huh et al. 1998) and high-temperature hydrothermal fluids at ocean ridges at one hand and low temperature removal of Li into oceanic basalts and marine sediments at the other. Any variance in these sources and sinks thus should cause secular variations in the isotope composition of oceanic Li. And indeed in a first attempt Hoefs and Sywall (1997) interpreted Li isotope variations in well preserved carbonate shells as indicating secular variations of the oceanic Li-cycle. 

Polycyclic aromatic hydrocarbons (PAH) are produced by the combustion, under fuel rich conditions, of almost any fuel. Although a few PAH with vinylic bridges (such as acenaphthylene) are lost, most PAH are quite stable in the atmosphere and eventually accumulate in environmental sinks such as marine sediments. Spatial and historical measurements of PAH in sediments Indicate that these compounds are stable, conservative markers of man s energy producing activities. 

The sediments represent the major sink for material in the oceans. The main pathway to the sediments is the deposition of suspended particles. Such particles may be only in transit through the ocean from a continental origin or be formed in situ by chemical and biological processes. Sinking particles can scavenge material from solution. Accordingly, this section introduces the components found in marine sediments, but emphasises processes that occur within the water column that lead to the formation and alteration of the deposited material. 

Though microbial biotransformation is an important removal process, it is not complete in the sense that the biphenyl backbone is not broken. Moreover, recent studies [116-118] indicate PCBs that flow to terrestrial, fresh water, and ocean surfaces are returned to the atmosphere. So it is important to identify permanent PCB sinks that is, sinks that result in the complete removal of the PCB molecule from atmospheric cycling. Burial of PCBs in fresh water or marine sediments below the resuspension layer and transformation processes initiated by OH attack in the troposphere are identified as major permanent sinks. 

Marine biota comprise only —3 Gt of OC, and sinking and suspended particulate OC account for a further 10-20 Gt. The majority of OC in the oceans is in the form of DOC (680 Gt) and organic matter sequestered in the upper meter of marine sediments (—100 Gt). Concentrations of marine DOC are highest in the upper ocean, and in the coastal zone. Typical open ocean DOC concentrations in surface seawater range from... 

The S04 concentration in sediments affects S04 reduction only when concentrations are quite low. The reduction of SOl in marine sediments appears to be zero-order with respect to S04 to concentrations of 2 mM (Boudreau and Westrich, 1984 Goldhaber and Kaplan, 1974). In freshwaters, SO concentrations must be much lower before they limit SO reduction (Bak and Pfennig, 1991 Lovley and Klug, 1983b Sinke et al., 1992). Because freshwater contains very little sot compared to seawater, the importance of SO4 reduction in sediments increases in an estuary as the salinity increases (Capone and Kiene, 1988). Therefore, the vertical extent of the SO4 reduction zone increases substantially as more SO4 becomes available, while the metha-nogenic zone is pushed deeper into the sediment and its contribution to carbon mineralization decreases in importance. 

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