Distal and proximal controls on the silicon stable isotope signature of North Atlantic Deep Water

It has been suggested that the uniquely high δ³⁰Si signature of North Atlantic Deep Water (NADW) results from the contribution of isotopically fractionated silicic acid by mode and intermediate waters that are formed in the Southern Ocean and transported to the North Atlantic within the upper limb of the meridional overturning circulation (MOC). Here, we test this hypothesis in a suite of ocean general circulation models (OGCMs) with widely varying MOCs and related pathways of nutrient supply to the upper ocean. Despite their differing MOC pathways, all models reproduce the observation of a high δ³⁰Si signature in NADW, as well showing a major or dominant (46-62%) contribution from Southern Ocean mode/intermediate waters to its Si inventory. These models thus confirm that the δ³⁰Si signature of NADW does indeed owe its existence primarily to the large-scale transport of a distal fractionation signal created in the surface Southern Ocean. However, we also find that more proximal fractionation of Si upwelled to the surface within the Atlantic Ocean must also play some role, contributing 20-46% of the deep Atlantic δ³⁰Si gradient. Finally, the model suite reveals compensatory effects in the mechanisms contributing to the high δ³⁰Si signature of NADW, whereby less export of high-δ³⁰Si mode/intermediate waters to the North Atlantic is compensated by production of a high-δ³⁰Si signal during transport to the NADW formation region. This trade-off decouples the δ³⁰Si signature of NADW from the pathways of deep water upwelling associated with the MOC. Thus, whilst our study affirms the importance of cross-equatorial transport of Southern Ocean-sourced Si in producing the unique δ³⁰Si signature of NADW, it also shows that the presence of a deep Atlantic δ³⁰Si gradient does not uniquely constrain the pathways by which deep waters are returned to the upper ocean.