Climate reconstruction using 44/40Ca isotopes in annually banded reef corals.

This report discusses the influence of temperature and salinity on calcium isotope

fractionation in coral skeleton. Currently, the fractionation and calcium pathway in

corals are not fully constrained. Biological (vital effects) or environmental parameters

have influenced the isotopic composition. To better constrain the processes, the

dataset is based on an accurate and high-resolution sampling coupled with a multiproxy

approach.

Samples were micro-drilled from fossil and modern tropical corals. The last deglacial

fossil coral samples were cored during the IODP expedition 310 in Tahiti (Camoin et

al., 2007). Ongoing analyses include modern corals cultured in monitored

environmental conditions at different salinities (36, 38 and 40 ‰, Centre Scientifique

de Monaco), following the method described in Reynaud-Vaganay et al. (1999). Ca

and O isotopic composition as well as the Sr/Ca ratio were measured on the same

sample. Ca isotope measurements are based on the method reported in Heuser et

al. (2002) using a TIMS Finnigan Triton T1.

Our preliminary results on fossil corals (Porites sp.) show that the fractionation is

almost constant within the error bars. No relationship between precipitation rate and

fossil coral δ44Ca is recognized. The weak temperature dependence reported in

Böhm et al. (2006) is possibly not the only parameter that is responsible for the

fractionation. Indeed, subseasonal temperature variations reconstructed by δ18O and

Sr/Ca ratio are not mirrored in coral δ44Ca. Removing sea-surface temperature from

the δ18O signal (δ18Osw) results in a coral δ44Ca curve that closely matches δ18Osw,

pointing to salinity as an important factor that affects coral δ44Ca. However, one

should note Cahyarini et al. (2008) were not able to reconstruct salinity variations

based on δ18O and Sr/Ca in modern coral from Tahiti.

To further constrain the influence of salinity on calcium isotope fractionation, current

investigation focuses on cultured corals. Furthermore, we examine inter-colonies and

inter-genus variability (Acropora sp., Porites sp., Montipora sp. and Stylophora sp.) to

improve the understanding of calcium isotope behavior.