Biogenic Si cycling along the Cocos Ridge: differences between in situ and ex situ extracted pore waters imply rapid rates of dissolved Si uptake upon core recovery

The cycling of silicon (Si) in marine sediments can have major impacts on global ocean nutrient dynamics and productivity. Si cycling in the sediment of the Cocos Ridge and overlying waters of the eastern equatorial Pacific was investigated using unique in situ and traditional ex situ pore water collection techniques as well as solid phase analysis, core incubation, and sediment trap collection. Si remineralization and burial fluxes show little variation between the four sites investigated, regardless of a ∼1600 m difference in water depth among stations. Dissolved Si concentrations in pore water from sediment depths >10 cm collected in situ were significantly and consistently higher than those collected ex situ implying a previously unrecognized sampling artifact associated with sediment core recovery and processing. The loss of dissolved Si in the ex situ pore waters is also associated with the fractionation of Si isotopes. In situ pore water δ³⁰Si is lighter than ex situ pore water δ³⁰Si at three of the four stations, though only significantly lighter at one. The preferential loss of light Si in ex situ pore waters is attributed to authigenic clay formation during core collection and transport from the cold, high pressure benthos to the ocean's surface. However, consistency between in situ and ex situ pore water Si gradients within 10 cm of the sediment water interface indicates remineralization fluxes previously determined using sediment core-derived pore waters remain accurate.