M. Elliot, M.T. McCulloch, M.K. Gagan, G. Cabioch* and J.
*Institut de Recherche pour le Developpement, Nouméa,
We measured the trace
element composition (Sr/Ca) and oxygen isotope ratio (18O)
of a modern Tridacna maxima shell collected live in December
2001 from New Caledonia. Bivalves secrete daily layers of carbonate
which can be sub-sampled to obtain information with seasonal
resolution. T. maxima are found in tropical reef environments
and can live for over one-hundred years. In contrast with corals,
they precipitate their shell in isotopic equilibrium with the
water in which they grow and thus have the potential to be valuable
tools in paleo-environmental studies. Sr/Ca and Mg/Ca of marine
carbonates are commonly used to reconstruct past temperatures.
However previous studies have not succeeded in establishing a
clear relationship between water temperature and the Sr/Ca of
T. maxima bivalves. Our approach was to measure skeletal Sr/Ca
using high resolution by laser ICP-MS and the 18O
of a modern specimen which has grown in a controlled environment.
A section of the giant T. maxima shell reveals
3 distinct zones. The hinge and the outer zones present well-defined
successive dark and light yearly increments, whereas the inner
layer is homogenous and reveals no obvious annual banding. The
Sr/Ca of different zones was analysed in order to assess any
differences in the distribution throughout the shell. We obtained
high-resolution profiles of Sr/Ca from the hinge and inner zones,
which were unaltered relative to the outer zone. Two parallel
scans perpendicular to the growth layers were performed in order
to check the reproducibility of the laser ICP-MS Sr/Ca measurements
which were obtained every 50 microns. 18O
was measured on samples of carbonate powder extracted from the
hinge area with an average resolution of 300-400 microns using
a high precision microdrill.
The profiles of Sr/Ca in each zone all exhibit
seasonal variations but with different amplitude (Figure 12).
The 18O profile along the hinge area
also shows seasonal variations which have been used to develop
an annual chronology. High Sr/Ca and 18O
values are associated with the dark-winter increments in the
hinge zone which is similar to what has been observed in corals.
We have thus been able to obtain seasonal variability of trace
elements which covary with the 18O
record. This observation supports the possibility of a temperature-controlled
uptake of trace elements. However, our results show that the
distribution of the trace elements throughout the shell varies
significantly. The biological and physical processes controlling
the uptake of trace elements from the surrounding water and their
distribution within the shell of T. maxima will have to be better
understood before obtaining a reliable palaeothermometer.
Figure 12: Sr/Ca variations measured by
laser ICP-MS along the inner layer (top) and along the hinge
area (bottom) of a modern T. maxima collected live in December
2002, in New Caledonia. The estimated age of this specimen is
approximately 49 years. The transect of the inner layer covers
the last ~33 years, whereas the transect along the hinge area
covers the period 1943-1985.