Award: OCE-2048998

Our participation in the GEOTRACES program has focused on using ocean sections to understand mechanisms controlling the distribution of silicon isotopes within dissolved silicic across the global ocean. This particular study sampled the GP17_OCE section for the silicon isotopic composition of silicic acid in the water column. This section was ideal for testing the prediction from numerical models that processes in the Southern Ocean control the distribution of Si isotopes throughout the global ocean. The mechanisms underlying Southern Ocean dominance involve the unique physics, chemistry and biology of the region. Briefly, the Southern Ocean meridional counter current circulation combines with the biological pump to create feedback that enhances silicic acid concentrations in the Southern Ocean often described as the Southern Ocean Silicon Trap. Using modelling and our measurements we found  that the exceptionally light isotopic character of the deep southern ocean arises from this circulation pattern that exposes deep water to repeated biological fractional by diatoms in surface waters. Some of these light deep waters flow northward at deep ocean depths while the residual heavy isotopes of Si are exported from the Southern Ocean in more shallow northward flowing mode waters redistributing Si isotopes of much of the low latitude ocean.

 A portion of the GP17_OCE section followed a longitudinal section in the Southern Ocean above the hydrothermal plume emitted from the Pacific Antarctic Ridge. Here we made a discovery that silicic acid may be absorbing onto iron oxyhydroxides in the water column fractionation silicon isotopes. Such interactions have been explored within pore waters of deep-sea sediments, but the phenomenon has not been previously been observed in the water column. We re-evaluated data from other hydrothermal sites samples by GEOTRACES and found similar patterns. Prior to this fractionation of silicon isotopes in the sea was assumed to be driven mainly by biology where diatoms fractionate isotopes of Si when they construct their siliceous shells in ocean surface waters. This new discovery, if it is upheld by further study, suggests a strong abiotic process affecting Si isotopes distributions in the sea and a new link between the marine silicon and iron cycles.

Last Modified: 08/20/2026
Modified by: Mark A Brzezinski