A silicon depleted North Atlantic since the Palaeogene: Evidence from sponge and radiolarian silicon isotopes

Despite being one of Earth’s major geochemical cycles, the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica (DSi) concentration through geologic time remain poorly constrained. Silicon isotope ratios (expressed as δ³⁰Si) in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling. In particular, the δ³⁰Si of siliceous sponge spicules has been demonstrated to be a useful proxy for past DSi concentrations. We analysed δ³⁰Si in radiolarian tests and sponge spicules from the Blake Nose Palaeoceanographic Transect (ODP Leg 171B) spanning the Palaeocene-Eocene (ca. 60 - 30 Ma). Our δ³⁰Si results range from +0.32 to +1.67‰ and -0.48 to +0.63‰ for the radiolarian and sponge records, respectively. Using an established relationship between ambient dissolved Si (DSi) concentrations and the magnitude of silicon isotope fractionation in siliceous sponges, we demonstrate that the Western North Atlantic was DSi deplete during the Palaeocene-Eocene throughout the water column, a conclusion that is robust to a range of assumptions and uncertainties. These data can constitute constraints on reconstructions of past-ocean circulation. Previous work has suggested ocean DSi concentrations were higher than modern ocean concentrations prior to the Cenozoic and has posited a drawdown during the Early Palaeogene due to the evolutionary expansion of diatoms. Our results challenge such an interpretation. We suggest here that if such a global decrease in oceanic DSi concentrations occurred, it must predate 60 Ma.