Efficient extraction of past seawater Pb and Nd isotope signatures from
Southern Ocean sediments
Abstract
Radiogenic lead (Pb) and neodymium (Nd) isotope compositions extracted
from authigenic phases in marine sediments are sensitive tracers to
reconstruct past ocean circulation and water mass mixing. Chemical
reductive leaching of hydrogenetic ferromanganese oxyhydroxides from
bulk sediments is the most practical way to recover past seawater Pb and
Nd isotope signatures in the Southern Ocean, due to the scarcity of
alternative archives. However, the leached signal could be compromised
if substantial quantities of Pb and Nd were released from
non-hydrogenetic sediment fractions during chemical extraction. Here we
developed a very short 10-seconds leaching method to extract reliable
seawater Pb and Nd isotope signals from sediments in the Atlantic sector
of Southern Ocean. The effect of a previously recommended MgCl prewash,
the role of chelate ligands in the leaching solution and length of
leaching time were investigated. The results show that 10 seconds
exposure time of sediments to reductive leaching extracted sufficient
and more reliable hydrogenetic Pb and Nd compared with the commonly used
30-minute leaching approaches. The robustness of our improved leaching
method was validated via direct comparison of Pb and Nd isotope
signatures with actual seawater, porewater and corresponding sediment
leachates from three stations in front of the Antarctic Filchner-Rønne
Ice Shelf. Our findings suggest that in contrast previously studied
sites on the West Antarctic continental shelf, the southern Weddell Sea
shelf is not a location of pronounced benthic Nd fluxes to the water
column.