Tracking changes in silicon isotopic composition during diatom descent and dissolution in the Cariaco Basin

摘要

We measured the Si isotopic composition (i.e. delta30Si) of sinking diatom opal in the Cariaco Basin to evaluate primary controls on diatom delta30Si values. The Cariaco Basin time series consists of four sediment traps at progressively greater depths on a single mooring and monthly hydrographic and dissolved nutrient measurements within the water column. Monthly measurements of diatom delta30Si from sediment traps between 225-m and 1200-m were made over a full seasonal cycle (May 2005--Feb. 2006). Diatom delta30Si values remained largely unchanged from 225-m to 1200-m despite a host of changing physical parameters in the basin over the course of the year as well as during diatom decent and dissolution in the water column. Diatom delta30Si showed no statistical relationship with the seasonal upwelling cycle in the basin or the associated changes in opal flux. A dissolution driven change in delta30Si is absent in sediment trap material over the depth range 225-m to 1200-m. However, delta30Si of bSiO2 showed a ca. 0.35 +/- 0.2 / shift towards heavier values from 225-m to 410-m during seven of the nine months. However, this shift is largely erased as diatom delta 30Si values decrease by an average of -0.31 +/- 0.2 / below 410m, possibly associated with incongruent dissolution of diatom species. Further research on the dissolution dynamics of the warm water and anoxic microbial community, as well as diatom species composition should shed light on a dissolution driven delta30Si-bSiO2 signal. Although variation in diatom delta30Si within the water column is unexpected and unexplained with the current dataset, this study demonstrates that the diatom delta30Si value produced at the surface matches that received close to the seafloor, a desirable result for paleoceanographic reconstructions using diatom delta30Si.