Sack Lunch Seminar (SLS)

From Glassy Sponges to the Marine Silicon Cycle: Paleoceanographic Applications of Silicon Isotopes






The marine silicon (Si) cycle is linked to global climate through coupling with the carbon cycle and the influence of tectonics and silicate weathering. In the modern ocean, biological precipitation of glass-like silica (opal) by diatoms is the dominant process removing dissolved Si, effectively stripping Si from surface waters, and is an efficient conveyor of organic carbon to the seafloor. Quantifying changes in the composition of the intermediate and deep waters that supply surface nutrients is necessary in order to understand the production of diatom opal, which provides a key control on atmospheric carbon dioxide (pCO2) and global climate. Here, I’ll discuss the use of the silicon isotopic composition (δ30Si) of biogenic opal as a direct method to quantify ocean water Si budgets. Diatom opal δ30Si, over timescales shorter than the residence time of Si in the ocean (~15 ka), can record changes in ocean productivity, because of fractionation processes that occur during uptake of dissolved Si in surface waters. Biogenic opal produced by glass sponges, conversely, is a potential proxy to quantify changes in whole-ocean dissolved Si and δ30Si over longer timescales, as well as changes in intermediate and deep-water Si. I will present the first downcore records of sponge silicon isotopes over the past glacial cycle from the Southern Ocean and South Atlantic, and discuss their paleoceanographic significance. Together, records of surface and deep ocean Si budgets can be used to explore how marine productivity has influenced pCO2 in the past.