The relationship between silicon isotope fractionation in sponges and silicic acid concentration: Modern and core-top studies of biogenic opal

Recent work has shown the silicon isotope composition, denoted by delta Si-30, of deep-sea sponges reflects the concentration of ambient silicic acid (Si(OH)(4)) in seawater. However, existing calibrations are based predominantly on living sponges collected from the Southern Ocean. These data cannot, however, be used to determine whether other parameters that correlate with silicic acid in the Southern Ocean, such as temperature and salinity, influence delta Si-30 of sponges. Furthermore, the published data do not demonstrate whether disaggregated core-top sedimentary spicules preserve the primary delta Si-30 signal recorded in living sponges. Here, we address both of these issues. We refine and widen the existing calibration by including a global distribution of modern sponges. In addition, we provide the first systematic calibration from spicules picked from core-top sediments that covers sites from different ocean basins. The relationship between Si(OH)(4) and delta Si-30 in sponge spicules is the same in different ocean basins, between specimens that grew in different temperature and salinity conditions. Our core-top data agree well with the modern sponge calibration indicating there are no significant post-depositional effects or early diagenetic overprints. These two new datasets support the assertion that sponge delta Si-30 can be used as a proxy for silicic acid concentrations in the past. (C) 2011 Elsevier Ltd. All rights reserved.