Interactions of Biogenic opal, Sediments and Seawater in the Southern Ocean Tracked by Stable Silicon Isotope in Pore Waters

Ivia Closset, University of California Santa Barbara, Santa Barbara, CA, United States, Mark A Brzezinski, University of California, Marine Science Institute, Santa Barbara, CA, United States, Damien Cardinal, Universite Pierre et Marie Curie, LOCEAN, Paris, France, Arnaud Dapoigny, LSCE, Gif sur Yvette, France, Janice Jones, University of California Santa Barbara, Marine Science Institute, Santa Barbara, CA, United States and Rebecca S Robinson, University of Rhode Island, Graduate School of Oceanography, Narragansett, United States
Diatoms are known to fractionate silicon isotopes during the formation of their siliceous frustules. The silicon isotopic composition of opal (δ30Si) is then closely linked to the degree of silicic acid consumption in surface waters. In this context, studies use diatom δ30Si preserved in sediment archives to reconstruct past changes in silicic acid utilization. However, evaluation of how diagenesis of diatom in marine sediments affects the δ30Si of opal preserved in oceanic archives has received little attention.

Here we report the first measurements of silicon isotopic composition of pore water (δ30Sipw) from the Southern Ocean using 8 sediment cores collected along a transect south of New Zealand. South of the Polar Front, δ30Sipw values can be extremely low with a range of variability from +0.20‰ to +1.14‰, the heaviest isotopic compositions occurring in the uppermost part of the sediment cores. North of the Polar Front, δ30Sipw values are more homogeneous and display a gradient from South (high δ30Sipw values) to North (low δ30Sipw values) which is inversely correlated with the decreasing gradient of pore water silicon concentration.

This high-resolution dataset allowed us to estimate variations of the diffusion flux from the sediments (from 49 to 171 mmol m-2 yr-1) and the burial of opal (from net dissolution at -35 to high burial rate up to +1238 mmol m-2 yr-1) in the different zones of the Southern Ocean.

Preliminary results of δ30Si of opal from those samples show variations both within and among cores with lighter values south. These variations along with those observed within the pore waters will be interpreted relative to opal and clay content, as well as selected dissolved cation concentrations to evaluate the interactions between opal, sediment and seawater in the upper layers of the ocean seafloor. This study aims challenging our knowledge of the mechanisms responsible for the formation of authigenic alumino-silicate phases and how they affect the silicon isotope fractionation of opal during early diagenesis in marine sediments.