Towards the use of oxygen isotopes for tracking North Atlantic waters in the Southern Ocean
Abstract
North Atlantic Deep Water (NADW), formed in the subpolar North Atlantic, propagates southward at 1–4 km depth before joining the Antarctic Circumpolar Current (ACC). These Atlantic-ventilated waters ultimately leave the ACC via three outlets: upwelling south of the ACC, entrainment into northward-flowing Antarctic Bottom Water, or northward propagation at mid-depths (1–3 km) in the Indian and Pacific. The relative importance of outlets has major implications for ocean ventilation and carbon storage. However, mixing in the ACC rapidly erodes NADW properties, making NADW pathways difficult to quantify. Here we trace NADW around the Southern Ocean using seawater oxygen isotope (d18O) observations. A persistent d18O maximum at mid-water densities marks the NADW core and weakens eastward from the Atlantic sector. Combining these observations with water-mass fractions from three ocean-transport models (TMI, OCIM, NEMO), we reconstruct deep-ocean d18O, estimating end-members for six surface source regions by least-squares optimization. Differences in bulk mixing among models produce contrasting NADW distributions and d18O maximum reconstructions around the ACC. The TMI reconstruction captures the high d18O signature only in parts of the ACC, indicating excessive mixing. The OCIM reconstruction compares well with previous estimates of deep-ocean d18O, showing improved fidelity in the deep South Atlantic. The main limitation for d18O water-mass fingerprinting stems from measurement offsets between laboratories. Targeted circumpolar sampling of salinity and d18O in the ACC, alongside improved characterization of offsets, would help to constrain NADW pathways.
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BibTeX
@misc{millet2026towards,
title = {{Towards the use of oxygen isotopes for tracking North Atlantic waters in the Southern Ocean}},
author = {Bruno Millet and William R Gray and Casimir de Lavergne and Gilles Reverdin and Claire Waelbroeck and Mark Holzer and Helen C. Bostock and Frerk Pöppelmeier and Didier M. Roche},
howpublished = {HAL},
year = {2026},
month = sep,
url = {https://hal.science/hal-05766693v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Towards the use of oxygen isotopes for tracking North Atlantic waters in the Southern Ocean
AU - Bruno Millet
AU - William R Gray
AU - Casimir de Lavergne
AU - Gilles Reverdin
AU - Claire Waelbroeck
AU - Mark Holzer
AU - Helen C. Bostock
AU - Frerk Pöppelmeier
AU - Didier M. Roche
PY - 2026
DA - 2026/09/28
PB - HAL
UR - https://hal.science/hal-05766693v1
AB - North Atlantic Deep Water (NADW), formed in the subpolar North Atlantic, propagates southward at 1–4 km depth before joining the Antarctic Circumpolar Current (ACC). These Atlantic-ventilated waters ultimately leave the ACC via three outlets: upwelling south of the ACC, entrainment into northward-flowing Antarctic Bottom Water, or northward propagation at mid-depths (1–3 km) in the Indian and Pacific. The relative importance of outlets has major implications for ocean ventilation and carbon storage. However, mixing in the ACC rapidly erodes NADW properties, making NADW pathways difficult to quantify. Here we trace NADW around the Southern Ocean using seawater oxygen isotope (d18O) observations. A persistent d18O maximum at mid-water densities marks the NADW core and weakens eastward from the Atlantic sector. Combining these observations with water-mass fractions from three ocean-transport models (TMI, OCIM, NEMO), we reconstruct deep-ocean d18O, estimating end-members for six surface source regions by least-squares optimization. Differences in bulk mixing among models produce contrasting NADW distributions and d18O maximum reconstructions around the ACC. The TMI reconstruction captures the high d18O signature only in parts of the ACC, indicating excessive mixing. The OCIM reconstruction compares well with previous estimates of deep-ocean d18O, showing improved fidelity in the deep South Atlantic. The main limitation for d18O water-mass fingerprinting stems from measurement offsets between laboratories. Targeted circumpolar sampling of salinity and d18O in the ACC, alongside improved characterization of offsets, would help to constrain NADW pathways.
ER -