van Beek P.1,
R. François2, M. Conte2, J.-L. Reyss3, S. Honjo2,
M. Bacon2 and M. Charette2
1now at University of Edinburgh, Grant Institute, West Mains Rd, Edinburgh EH9 3JW, Scotland, UK (Tel. + 44 131 650 8511, Fax + 44 131 668 3184, pieter.vanbeek@glg.ed.ac.uk), 2Woods Hole Oceanographic Institution, Woods Hole MA02543, USA and 3
Laboratoire des Sciences du Climat et de l’Environnement, 91198 Gif-sur-Yvette, France
228Ra/226Ra activity ratio in suspended matter to track barite
formation and transport in the water column
In recent years, several geochemical proxies were developed
in order to better constrain past changes in the flux of organic matter
exported from the euphotic layer. Among these proxies, barite (BaSO4)
crystals appear to be very promising. These crystals are assumed to be
precipitated within microenvironments formed during the decay of organic matter
in the upper water column. However, a lack of understanding of the mechanisms
of barite formation in undersaturated seawater has hindered rigorous
application of the proxy.
Chemical analogues of barium, radium isotopes (i.e.,
228Ra, T1/2=5.75 y, and 226Ra, T1/2=1602
y) are incorporated in barite and can be used to trace these crystals in the
water column. In this work, we present 228Ra/226Ra
activity ratios measured in suspended matter (that reflect the ratios in
suspended barite) collected at the Bermuda time-series site using large volume in
situ pumps. The profiles thus obtained are compared to those from the
seawater and from the sinking matter collected using sediment traps. These
results provide information on both the depth of formation and transport of
barite within the water column, with implications for the use of barite as a
proxy for productivity.
Results of leaching experiments conducted in suspended matter collected in the North-West Pacific (JPAC programme) will be also shown. These investigations were designed to better understand the relationship between barite formation and the degradation of organic matter in the water column.