The deep Southern Ocean (SO) circulation plays a key role in the storage and release of CO2 in Earth’s climate system. Uptake and release of CO2 strongly depend on the redistribution of well and poorly ventilated water masses. Here, we present new neodymium isotope data (εNd) from three sediment cores from the Atlantic sector of the SO to assess the distribution of water masses during the past 150 ka. ODP 1094 (2807 m) reveals a tremendous Holocene εNd-variability (-0.25 to -6.07), which most likely reflects local influences. PS 1768-8 (3299 m) and ODP 1093 (3624 m), show far more unradiogenic interglacial εNd-signatures, which are similar to the present-day Antarctic Bottom Water (AABW) (εNd~-8). During peak glacial periods, radiogenic εNd-values of ~-2.5 to -3.5 are recorded. This confirms a predominance of glacial Pacific-sourced deep water at depths of 3.3-3.6 km in the South Atlantic, with proportions close to 100% and lacking AABW. We advocate for the presence of Pacific Deep Water even during interglacials and further hypothesize that a possible intrusion due to a warming climate could accelerate climate warming. The persistent occurrence of such a highly radiogenic water mass substantially changes the view regarding the selection of the Southern Hemisphere εNd-endmember for the Atlantic Ocean and reinforces the major importance of carbon storage in the Glacial SO.

<p>Palaeoceanographic studies of ocean circulations are crucial for understanding the ocean´s impact on the Earth´s climate system. Circulation patterns and the provenance of water masses can be detected from temporal variations of the neodymium isotopic composition (εNd) of authigenic neodymium, preserved in deep sea sediment.</p><p>Inductively coupled plasma source mass spectrometry allows for the precise and accurate determination of εNd-values of samples and reference material.</p><p>Here, we reevaluate the mass spectrometric measurement protocol and instrument setting with respect to precision and accuracy defined by neodymium standards.</p><p>The shape of the ion beam plays a crucial role, which is manifested in the result that an optimal adjustment of the beam shaping quadrupoles can increase precision by a factor of 4.</p><p>In addition, the optimal standard neodymium concentration level is roughly 50 ppb yielding uncertainties of the mean of repeated measurements as low as 0.07 ε units whereas 5 times lower concentrations yield 10 times higher uncertainties.</p><p>The statistical nature of precision is further demonstrated through an uncertainty inversely proportional to the square root of N measurements. As a consequence, with an increase from 30 to 80 consecutive measurements precision was improved by a factor of 1.22.</p><p>Taking all evaluated aspects into account, precision and accuracy of standards and thus sediment samples can be strongly improved, hence contributing to a better comprehension of past ocean circulation, where neodymium isotope gradients are small.</p>

<p>The deep Southern Ocean (SO) circulation is of major significance for the understanding of the ocean´s impact on Earth’s climate as uptake and release of CO<sub>­­­2</sub> depend strongly on the redistribution of well and poorly ventilated water masses.</p><p>Neodymium isotopes preserved in deep sea sediment<em> </em>have proven useful to study the deep ocean circulation and water mass provenance thanks to basin scale isotope gradients between the Pacific and the North Atlantic. Here we present novel neodymium isotope data (ε<sub>Nd</sub>) of three sediment cores in 2.8, 3.3 and 3.6 km depth in the Atlantic sector of the SO to assess the presence of old and poorly ventilated Pacific sourced Deep Water (PDW) during the past 150 ka.</p><p>The sediment cores indicate dramatic temporal changes of ε<sub>Nd</sub> spanning a range of 7.7 ε-units from -1.0 to -8.3. While the ε<sub>Nd</sub> variability of the two deeper cores is driven by changes in ocean circulation, the shallowest drilling site is likely influenced by a local source of radiogenic Nd, such as weathering of volcanic material.</p><p>During peak glacial periods with maximum ice extent and a shoaled AMOC we observe radiogenic ε<sub>Nd</sub> values of ~-2.5 to -3.5. This confirms a predominance of glacial PDW at depths of >3 km with proportions close to 100% and thus increasing the water volume portion with enhanced respired carbon. We further advocate for the persistent presence of PDW even during interglacials although with a much smaller proportion.</p><p>Hence, our results enforce the leading role of the SO in storing and reinjecting respired CO<sub>2</sub> into the deep Atlantic Ocean and the Atmosphere during glacial-interglacial terminations.</p>