Zinc/silicon ratios of sponges: A proxy for carbon export to the seafloor

Reconstruction of past carbon export events is difficult. Most proxies for carbon export suffer from problems related to remineralisation and diagenetic effects above, at and below the sediment-water interface. Here we demonstrate that deepsea hexactinellid sponges incorporate zinc (Zn) into their silica skeletons in a direct relationship with the amount of particulate organic carbon (POC) accumulating in sediments. Such a relationship suggests that Zn incorporation into siliceous sponge spicules is from Zn associated with particulate organic matter (POM) 'raining down' from overhead waters. The dissolved Zn pool does not appear to be a major Zn source to deep-ocean sponges. Likewise, water temperature and pressure do not appear to strongly influence Zn incorporation into sponge silica. These results suggest that the Zn to silicon (Si) ratio of deep ocean sponges could serve as a useful proxy for reconstructing past POC burial events.