Southern Ocean control of silicon stable isotope distribution in the deep Atlantic Ocean

Abstract

The fractionation of silicon (Si) stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon ( δ 30 Si) dissolved in seawater a sensitive tracer of the oceanic biogeochemical Si cycle. We present a high‐precision dataset that characterizes the δ 30 Si distribution in the deep Atlantic Ocean from Denmark Strait to Drake Passage, documenting strong meridional and smaller, but resolvable, vertical δ 30 Si gradients. We show that these gradients are related to the two sources of deep and bottom waters in the Atlantic Ocean: waters of North Atlantic and Nordic origin carry a high δ 30 Si signature of ≥+1.7‰ into the deep Atlantic, while Antarctic Bottom Water transports Si with a low δ 30 Si value of around +1.2‰. The deep Atlantic δ 30 Si distribution is thus governed by the quasi‐conservative mixing of Si from these two isotopically distinct sources. This disparity in Si isotope composition between the North Atlantic and Southern Ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate ( δ 15 N‐NO 3 ). We infer that the meridional δ 30 Si gradient derives from the transport of the high δ 30 Si signature of Southern Ocean intermediate/mode waters into the North Atlantic by the upper return path of the meridional overturning circulation (MOC). The basin‐scale deep Atlantic δ 30 Si gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes, which fractionates Si isotopes between the abyssal and intermediate/mode waters formed in the Southern Ocean.