Sponge spicules in marine sediments: a significant silicon sink

Abstract

Siliceous sponges produce a skeleton of silica spicules. Upon sponge death, spicules are released to the sediment, but little is known about the fate of those free spicules, their spatial patterns of distribution, and their role in the silica cycle. In this study, we examined sediments obtained from piston and gravity cores collected from a variety of sedimentary marine environments, including continental shelves, slopes and abyssal basins from various seas and oceans (Antarctic, Western and Eastern Atlantic, Western Mediterranean, Western and Eastern Central Pacific, and Arctic). In each core, the sediment sequence extended from Recent Time to about Mid and Late Holocene (i.e., 4,000 to 10,000 years). Through light microscopy inspection and alkaline digestions of sediment subsamples, we estimated the amount of sponge biogenic silica (mg) relative to the total biogenic silica per gram of sediment at various burial depths within each core. We discovered that sponge silica mass may vary over an order of magnitude, with much variation depending on the particular deposition environment but little between burial depths within the core, what suggests no major changes in local sponge abundance during the Holocene. The relative importance (%) of sponge biogenic silica versus total biogenic silica substantially increased with burial depth within the cores, indicating higher preservation of spicules relative to diatoms. Scanning electron microscopy confirmed spicules buried longer than 4,000 years to have only very incipient dissolution scars. These findings revealed that preservation of sponge-produced silica in continental-margin sediments is notable, being responsible for much silicon export from the marine biological cycle to the geological cycle. To date, most silicon burial in the oceans has been attributed to planktonic diatoms and directly connected to levels of planktonic primary production in the overlying photic waters. Our findings strongly disrupt this traditional view and introduce the role of benthic sponges as significant exporters of silicon, through burial of biogenic silica that has been produced following processes decoupled from primary production. These findings have important implications to improve the current understanding of processes controlling silicon turnovers in several ocean compartments used to model the global silicon cycle