Silicon isotopes in sponge spicules: the story of a proxy

Abstract:

The silicon isotope composition of deep-sea sponges has been shown to reflect the concentration of dissolved silicon, silicic acid, in seawater. This discovery has lead to the development of a novel geochemical proxy for past deep water nutrient status, which has already been applied to a wide range of palaeoceanographic questions ranging from Southern Ocean silicic acid leakage on glacial-interglacial and millennial timescales, to the proliferation of diatoms at the Eocene-Oligocene boundary. The initial calibrations based on modern sponge samples showed some scatter in the relationship between sponge silicon isotopes and silicic acid concentration, but without any apparent systematic influence from other environmental factors (temperature, pH, or other nutrients), morphology or species. However, a silicon isotope calibration of core top spicules, based on measurements made on a large number of spicules extracted from sediments, shows a tighter relationship with silicic acid concentrations, indicating that there are variations between and within individuals that are "averaged out" during palaeoceanographic studies. As is the case for all novel geochemical proxies, there is a need to test the proxy rigorously to ensure robust interpretation of any downcore signal. Here, we will present new studies of modern sponge specimens that have been carried out to shed light on the processes that could result in differences in silicon isotopic fractionation between and within individual sponges. Our findings highlight where caution is required in order to produce robust downcore records of past ocean silicic acid concentrations.