Technical Note: Silica stable isotopes and silicification in a carnivorous sponge Asbestopluma sp.

[摘要] The stable isotope composition of benthic sponge spicule silica isa potential source of palaeoceanographic information about past deep seawaterchemistry. The silicon isotope composition of spicules has been shown torelate to the silicic acid concentration of ambient water, although existingcalibrations do exhibit a degree of scatter in the relationship. Less isknown about how the oxygen isotope composition of sponge spicule silicarelates to environmental conditions during growth. Here, we investigate thevital effects on silica, silicon and oxygen isotope composition ina carnivorous sponge, Asbestopluma sp., from the Southern Ocean. Wefind significant variations in silicon and oxygen isotopic composition withinthe specimen that are related to unusual spicule silicification. The largestvariation in both isotope systems was associated with the differentialdistribution of an unconventional, hypersilicified spicule type (desma) alongthe sponge body. The absence an internal canal in the desmas suggests anunconventional silicification pattern leading to an unusually heavy isotopesignature. Additional internal variability derives from a systematic offsetbetween the peripheral skeleton of the body having systematically a higherisotopic composition than the internal skeleton. A simplified silicon isotopefractionation model, in which desmas were excluded, suggests that the lack ofa system for seawater pumping in carnivorous sponges favours a lowreplenishment of dissolved silicon within the internal tissues, causingkinetic fractionation during silicification that impacts the isotopesignature of the internal skeleton. Analysis of multiple spicules should becarried out to "average out" any artefacts in order to produce more robustdowncore measurements.

[学科分类] 地球化学与岩石