Productivity Versus Preservation

The total annual primary productionby photosynthetic planktonic organisms in the modem world oceans has been estimated to be in the order of 30-50T0 tons of carbon (Berger et al. 1989 Hedges and Keil 1995). Oceanic carbon fixation is not evenly distributed, but displays zones of higher activity on continental margins 

Although the fraction of organic matter burned in sediments is small relative to the amount produced by photosynthesis in oceanic surface waters, empirical relationships were derived from the analysis of sedimentary organic matter to estimate oceanic paleoproductivity (PaP [g C m yr ]) in the geological past  

In this equation of Muller and Suess (1979), the Pleistocene paleoproductivity of the (oxic) ocean is related to the organic carbon content of a sediment in percent dry weight (C), the density of the dry sediment (p g cm ), its porosity ((() %) and the linear bulk 

The paleoprodnctivity eqnations above describe the relationship between snrface-water productivity and organic carbon accnmnlation, specifically under conditions of an oxic water colnmn. A different relationship for anoxic depositional settings, suggested by Bralower and Thierstein (1987), implies that at least 2 % of the organic carbon in the gross photosynthetic prodnction is preserved in the sediments  

3 Transport of Organic Matter Through the Water Column 

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An interesting proposal to differentiate the importance of productivity versus preservation... 

From the beginnings of ecology as a discipline, the mining industry has been at the center of the battle over preservation versus exploitation. As discussed in the introduction to this chapter, human activities such as mining, power production from fossil fuels and discharges of industrial and municipal wastes not only increase the rate at which metals enter the biosphere but may also drastically alter the speciation of metals from what it would be in the undisturbed geologic cycle. 

Reaction of dihalocarbenes with alkenes, [1 +2] cycloaddition, is the method of choice for the preparation of 1,1-dihalocyclopropanes. The reaction proceeds stereospecifically preserving the configuration of the alkene in the products. These observations allow the conclusion to be made that the dihalocarbene reacts in the singlet state with alkenes. Experimental data (relative activities of alkenes, selectivity indices as well as theoretical calculations indicate that dihalocarbenes are electrophilic species. This means that they react readily with electron-rich (nucleophilic) alkenes. Dihalocarbenes may also react with electron-poor alkenes, but at a much slower rate. In the case of alkenes with a fairly unreactive double bond, dihalocarbene may also attack other sites of the alkene molecule, e.g. insert into a C-H bond. The selectivity (reactivity) of dihalocarbenes depends on the temperature at 20 C typical dihalocarbenes can be arranged in the order given, with respect to their selectivities versus reactivities (Houben-Weyl, Vol. El9b, p 1598). 

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