On the use of benthic foraminiferal δ13C in palaeoceanography: constraints from primary proxy relationships
Andreas Mackensen, 2008. "On the use of benthic foraminiferal δ13C in palaeoceanography: constraints from primary proxy relationships", Biogeochemical Controls on Palaeoceanographic Environmental Proxies, W. E. N. Austin, R. H. James
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Recent findings are reviewed from observations in the field on the generation of the δ13C signal in shells of live (Rose Bengal stained) benthic foraminifera, and end up with implications for the interpretation of fossil signatures. The δ13C values of calcite tests of preferentially epifaunal foraminifera principally reflect the δ13C of dissolved inorganic carbon (DIC) of ambient seawater, whereas infaunal species record a porewater signal, both with an offset from equilibrium calcite. Species occupying the deepest average living depth in the sediment usually exhibit lowest δ13C test values, but δ13C values of conspecific specimens at a single site do not decrease with increasing subbottom depth and decreasing porewater δ13CDIC. Organic carbon fluxes to the sediment surface are generally reflected by infaunal species such that lowered δ13C values coincide with high fluxes, but even strictly epifaunal species may reflect seasonally pulsed phytodetritus supply by depleted test δ13C. In high-productivity environments, however, where dissolved oxygen and sedimentary carbonate contents are low, benthic foraminiferal tests show 13C enrichment probably due to carbonate-ion undersaturation. Ontogenetic increase in δ13C values of certain infaunal species suggests a slow-down of metabolic rates during test growth and decreasing fractionation with age. At sites of active methane discharge δ13C values of infaunal species reflect low pore water δ13CDIC values, documenting active methane release in the sediment, whereas lowered δ13C values of strictly epifaunal species are most probably the result of incorporation of 13C depleted methanotrophic biomass.
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Most of our information about the evolution of Earth’s ocean-climate system comes from the analysis of sediments laid down in the past. For example, the microfossil assemblage reflects the temperature, salinity and nutrient abundance of the water in which the organisms lived, while the chemical and isotopic composition of biogenic carbonates may be used to reconstruct past variations in the operation of the carbon cycle, as well as changes in ocean circulation.
Nevertheless, understanding the link between these sediment variables (or ‘proxies’) and environmental conditions is not straightforward. This volume adopts a novel approach by bringing together palaeontologists, geochemists and palaeoceanographers, who contribute evidence that is required to better constrain these proxies. Topics include: (i) processes of biomineralization, and their effect on the chemical and isotopic composition of different organisms; (ii) proxy validation, including field, laboratory and theoretical studies; (iii) the links between modern and fossil organisms.