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On the use of benthic foraminiferal ¹³C in palaeoceanography: constraints from primary proxy relationships

Alfred Wegener Institute for Polar and Marine Research, 27568 Bremerhaven, Germany (e-mail: Andreas.Mackensen{at}awi.de)

Recent findings are reviewed from observations in the field on the generation of the ¹³C signal in shells of live (Rose Bengal stained) benthic foraminifera, and end up with implications for the interpretation of fossil signatures. The ¹³C values of calcite tests of preferentially epifaunal foraminifera principally reflect the ¹³C 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 ¹³C test values, but ¹³C values of conspecific specimens at a single site do not decrease with increasing subbottom depth and decreasing porewater ¹³C_DIC. Organic carbon fluxes to the sediment surface are generally reflected by infaunal species such that lowered ¹³C values coincide with high fluxes, but even strictly epifaunal species may reflect seasonally pulsed phytodetritus supply by depleted test ¹³C. 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 ¹³C 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 ¹³C values of infaunal species reflect low pore water ¹³C_DIC values, documenting active methane release in the sediment, whereas lowered ¹³C values of strictly epifaunal species are most probably the result of incorporation of ¹³C depleted methanotrophic biomass.

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