Calcium isotopes in planktonic foraminifera - core tops and sediment trap samples

For paleoceanographic studies, it is important to understand the processes that influence the calcium (Ca) isotopic composition of foraminiferal calcite tests preserved in the sediment record. Seven species of planktonic foraminifera from coretop sediments collectively exhibited a Ca temperature dependent fractionation of 0.013 per mil per °C. This is in agreement with previously published estimates for most species of planktonic foraminifera as well as biogenic and inorganic calcite and aragonite. Four species of planktonic foraminifera collected from a sediment trap showed a considerable amount of scatter and no consistent temperature dependent fractionation. Analyzed size fractions of coretop samples show no significant relationship with d44/40Ca. However, preliminary results suggest that the symbiotic and spinose foraminifera G. sacculifer might exhibit a relationship between test size and d44/40Ca. A one-box model in which Ca isotopes are allowed to fractionate by Rayleigh distillation from a biomineralization reservoir (internal pool) was used to constrain the isotopic composition of the original biomineralization Ca reservoir, assuming around 85% of the Ca reservoir is precipitated and the fractionation factor during precipitation is 0.9985 + 0.00002(T ºC). To explain the foraminiferal Ca isotope data, this model indicates that the Ca isotopic composition of the biomineralization reservoir is offset from seawater (approximately -0.8per mil).

Ba/Ca ratios in benthic foraminifera from recent core tops

The carbon isotope ratio (delta13C) and cadmium content (Cd/Ca) of benthic foraminifera shells have been used to reconstruct deep-water circulation patterns of the glacial oceans. These tracers co-vary with phosphorus in the modern ocean because they are nearly quantitatively regenerated from sinking biological debris in the upper water column. Hence they can be used to reconstruct the distribution of labile nutrients in glacial water masses. Independent constraints on glacial deep-ocean circulation patterns could be provided by a tracer of the distribution of silica and alkalinity, the deeply regenerated constituents of planktonic hard parts. Barium shares key aspects of its behaviour with these refractory nutrients because it is removed from solution in surface waters and incorporated into sinking particles which slowly dissolve deep in the water column and in the sediments. The fractionation of Ba between deep-water masses of the major ocean basins is largely controlled by thermohaline circulation patterns, so Ba conforms to different boundary conditions from Cd and 13C. As Ba substitutes into trigonal carbonates, it is a potential palaeoceano-graphic tracer if the Ba content of foraminifera shells reflects ambient dissolved Ba concentrations. Here we present data from Recent core-top benthic foraminifera which indicate that the Ba content of some recent calcitic benthic foraminifera does co-vary with bottom-water Ba.