Detrital fractions

The first uses of radiocarbon in deep-sea core dating were based on few data points and depended on extrapolation assuming the constant rate of titanium deposition (Arrhenius et al., 1951) or interpolation (Suess, 1956) for determination of rates of accumulation and chronology. The first systematic study of radiocarbon incorporating possible changes in accumulation rates with depth in a core was performed by Broecker et al. (1958). They showed that accumulation rates of both the carbonate fraction and the detrital fraction varied with time in the equatorial Atlantic and those variations were linked to paleoclimatic indicators inferred from paleontologic data (Figure 3). 

However, inorganic detrital components may be very difficult to identify and remove. Smear-slide examination is helpful, and SEM analysis can reveal the presence or absence of good crystal forms. Oxygen and carbon isotopic values, Mg/Ca and Sr/Ca ratios, or even Sr/ Sr ratios of any carbonates in the surrounding regions can be measured and compared with the measured values of carbonate sediment. While this does not prove conclusively that there is no detrital fraction, it can tell whether the detrital fraction occurs in enough abundance to affect the values of the non-detrital component. 

As an example, imagine a carbonate-rich sediment that contains a small but significant fraction of detrital silicate. The sediment might be a lake sediment, a surface coral, a deep-sea coral, a carbonate-rich bank sediment, or a speleothem (see for example, Richards and Dorale 2003). The carbonate has a very high U/ Th ratio (on the order of 10" by atom) and the detrital material has a lower %/ Th ratio of about 10°. The carbonate and detrital materials each have specific values that differ from each... 

Thorium is a highly insoluble element, mainly carried in the particulate form in river waters. This is well shown by Th data for the MacKenzie river (Vigier et al. 2001) and for the Kalix river (Andersson et al. 1995 Porcelli et al. 2001) in both cases, more than 95% of Th is carried by >0.45 pm particles. An important part of this Th is included within detrital material. This is illustrated by sequential extractions performed on sediments from the Witham river (Plater et al. 1992), which show the very low amount of Th in ion-exchangeable and organic-bound fractions compared to Th in Fe-Mn oxides... 

Preformed Refers to the fraction of a solute in a water mass whose origins are not related to remineralization of detrital particles in situ. 

The distribution of Jarosite in till samples is even more compelling than that for gold. Except for three samples with trace amounts, the most abundant Jarosite (from 1 to 25% of the grains in the heavy mineral fraction) occurs in samples adjacent to and within 7 km down-ice from the deposit (Fig. 4). Most Jarosite grains have a detrital morphology (variably... 

Analcime occurs only in the upper lagoonal complex in beds 150-300 m thick. It is most widespread in the cement of sandstones and fills the pores of many chemogenic rocks associated with them. The widespread occurrence of analcime in the upper complex and its absence in the. lower complex (despite the similar composition of detrital matter and the identical conditions of formation) would have been unaccountable were it not for one peculiar feature of the heavy mineral fraction. The heavy mineral content of rocks of the upper complex varies from fractions of a percent to 2 or 2.5%. Up ot 50% of the heavy mineral fraction consists of fresh, monoclinic pyroxene and amphibole. 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

Most of the epilimnetic particulate P was found in the 1.0-8.2- xm size range, which comprises small plankton and detrital material. During the mixing and early stratification periods, a major fraction of total particulate P was associated with diatoms. Two diatom-dominated size fractions, 63-19 and 211-114 xm, accounted for 20 and 12%, respectively, of the integrated total particulate P burden at peak levels in spring. Total diatom-P contri-... 

At the ecosystem scale, the significance of microbial community organization is that it determines the magnitude and efficiency of carbon transfer to other portions of the food web. High density communities like biofilms and floes can be directly grazed by metazoans, which divert microbial and detrital carbon out of the microbial loop (Wotton, 1994). In dilute planktonic systems, much of the carbon reaches metazoans through a microbial food-web after two to three trophic transfers, only a small fraction of microbial production remains. 

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