Terrestrial sediments, carbon isotope

Schoeninger MJ, DeNiro MJ (1984) Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals, Geochim Cosmochim Acta 48 625-639 Scholten SO (1991) The distribution of nitrogen isotopes in sediments, PhD Thesis University of Utrecht... 

Schubert, C. J., and Calvert, S. E. (2001). Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments Implications for nutrient utilization and organic matter composition. Deep-Sea Res. 148, 789-810. 

Marine DOC has stable carbon isotope values between —21%o and —22%o (Dmtfel et al., 1992) consistent with a largely marine source. While these data seem to exclude a significant contribution from C3 terrestrial plants, there is increasing evidence for an important contribution from C4 plants to persistent POM in marine sediments on the continental shelf and slope (see below). Desorption of C4 plant carbon and incorporation into oceanic DOC would be difficult to detect by isotopic or molecular biomarker analyses. 

Figure 3 Loss of terrestrial OC in deltaic systems, (a) Organic carbon to mineral surface area ratio (OC SA) plotted against bulk stable carbon isotopic compositions for riverine suspended sediments (closed symbols) and deltaic surface sediments (open symbols). A shift to lower OC SA values indicates net loss of organic matter, and a shift to heavier (i.e., C-enriched) isotopic compositions indicates increasing contributions from marine organic matter, (b) The average ( 1 SD) total amount of terrestrial OC persisting in deltaic sediments, based on the changes in OC SA and composition between river suspended sediments and deltaic sediments for four river systems...

Prahl et al. (1994) also concluded that terrestrial OC contributes significantly to Washington Margin sediments. These authors determined bulk elemental and stable carbon isotopic compositions and concentrations of a range of vascular plant biomarkers (epicuticular wax-derived n-alkanes, lignin-derived phenols and cutin-derived hydroxy-alkanoic acids) for sediments from the Columbia... 

I. 0). The isotopic composition of alkanes and of other classes of compounds can also be used as a general source input indicator. Isotopic fractionation resulting from the metabolic pathways involved in the synthesis of biologically produced compounds, when preserved in a dlagenetlc product, is frequently used to differentiate between terrestrial and aquatic sources. Hydrogen and carbon isotopic compositions of biogenic methanes from shallow aquatic environments is discussed in a later chapter of this volume (R. A. Burke and W. M. Sackett). The applicability of carbon isotopic data to tracing the source of deep-sea Mesozoic sediments is discussed by R.M. Joyce and E. S. Van Vleet. 

Carbon isotope ratios are principally useful to distinguish between marine and terrestrial organic matter sources in sediments and to identify organic matter from different types of land plants. The stable carbon isotopic composition of organic matter reflects the isotopic composition of the carbon source as well as the discrimination (fractionation) between and... 

Figure 3. Idealized carbon isotope cycle in a small stratified lake. The isotopic composition of organic matter buried in sediments is determined by the proportions of different terrestrial and lacustrine organic mailer. Ihccartron isotopic composition of dissolved inorganic carbon (DlC), and the rates of primary production and respiration within the water column. Isotope enrichment factors (e), listed here as the difference between the product and the substrate, vary with the form of DlC that lake algae assimilate (e.g., C02(aq) or HCOy ). Inorganic carbonate (CaC03> typically forms in isotopic equilibrium with the dissolved inorganic carbon pool and. as such, is indirectly affected by organic matter sources and primary production and respiration rales.

Stable carbon isotopes data—The data from stable isotope and radiocarbon measurements are summarized in Table 1. Analytical precision is 0.l%o for C. Stable carbon isotope ratios of the different fractions span from — 20.14 to — 27.82%o with the unextracted and extracted sediments exhibiting values, as expected, in the midrange around — 22%o (Fig. 1). The only exception is the fatty acid fraction which is anomalously depleted in Stable carbon isotope numbers of the different compound classes from the SMB samples, in general, suggest a mixing of terrestrial and marine components with humic and fidvic acids implying major contribution from marine sources (Sweeney and Kaplan, 1980 Stuermer et al., 1978). 

Biomarker Carbon-Isotope Analyses. While a forest/non-forest carbon-isotope signature is readily identifiable in terrestrial environments, it is very difiScult to obtain complete, well dated fluvial sequences. Complete sedimentary sequences are readily available fi-om the submarine fan deposits which lie off the mouth of the Amazon River 82 but the analysis of bulk sediments fi om the marine environment is not appropriate because terrestrially-derived riverbome carbon in the sample rapidly becomes mbced with marine-derived carbon offshore fi om the river mouth. 

Gas-chromatography-combustion-mass-spectrometry (GC-C-MS) enables the carbon-isotope analysis of n-alkanes of each carbon number, and therefore, the separate analysis of alkanes derived from alga] acterial and vascular plant sources. This ability has previously been utilized to determine the origin of waxy n-alkanes in a variety of sediments 85,86). The carbon-isotope composition of n-alkanes derived fi-om vascular plants can, therefore, be used to determine the relative contributions of C3- and C4-derived carbon to organic matter in marine, terrestrial or composite sediment samples. This approach has been applied in a preliminary manner to representative samples fi om the Amazon Basin (Bird and Summons, unpublished data). 

The results discussed above should be conridered preliminary, but they do suggest that the carbon-isotope composition of terrestrially derived n-all es accurately reflect the isotopic composition of the bulk vegetation from which they were derived. They also suggest that this terrestrial isotopic signature can be resolved in marine sediments, where the bulk terrestrial carbon-isotope signature is obscured by admixture with marine-derived carbon. 

The calcium carbonate shells of marine microfauna are a large repository of terrestrial calcium and constitute a potential record of changes in the cycling of calcium at and near the earth s surface (Zhu and MacDougall 1998 De La Rocha and DePaolo 2000 Schmitt et al. 2003a,b). To understand the record held in deep sea carbonate sediments, it is necessary to document any Ca isotopic fractionation that occurs between dissolved seawater Ca and carbonate shell material. 

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