TY - JOUR
T1 - Distal and proximal controls on the silicon stable isotope signature of North Atlantic Deep Water
AU - de Souza, Gregory F.
AU - Slater, Richard D.
AU - Hain, Mathis P.
AU - Brzezinski, Mark A.
AU - Sarmiento, Jorge Louis
N1 - Funding Information:
The authors gratefully acknowledge helpful comments on an earlier version of this manuscript by Timothy Conway and Beatrice Radden Keefe, and the constructive reviews of two anonymous reviewers. This work was supported by Swiss National Science Foundation post-doctoral fellowships PBEZP2-140169 and P300P2-147747 granted to GFDS, NOAA grant NA11OAR4310066 to JLS, and UK NERC grant NE/K00901X/1 to MPH.
Publisher Copyright:
© 2015 Elsevier B.V.
PY - 2015/12/15
Y1 - 2015/12/15
N2 - 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.
AB - 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.
KW - Biogeochemical cycles
KW - Meridional overturning circulation
KW - Silicon isotopes
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U2 - 10.1016/j.epsl.2015.10.025
DO - 10.1016/j.epsl.2015.10.025
M3 - Article
AN - SCOPUS:84946210337
SN - 0012-821X
VL - 432
SP - 342
EP - 353
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -