Radiocarbon-depleted water masses are typically found in the deep ocean ( 38 , 40 ) and hence we consider the possibility of mixing radiogenic Nd and radiocarbon-depleted waters into UCDW from below. A record from LCDW depths in the South Pacific shows invariable Nd isotopic compositions during the Holocene (ε Nd of ∼−8) ( 30 ) ( Fig. 2 ), in excellent agreement with the Nd isotopic compositions recorded by our Drake Passage corals from 1,701 and 1,750-m water depth at Sars Seamount, and inconsistent with an LCDW origin for the radiogenic signal in the UCDW layer. We therefore conclude that the Holocene Nd isotope evolution of UCDW in the Drake Passage was primarily controlled by the lateral admixture of radiogenic Nd at UCDW depths, and that this signal was propagated along isopycnal surfaces into shallower depths during UCDW upwelling and subsequently incorporated during AAIW formation.

Drake Passage cold-water coral data in radiocarbon–Nd isotope space. The benthic-atmosphere (B-atm) radiocarbon age offsets were calculated using 14 C atmosphere age of 0 y (1950 AD) for the modern seawater values ( 40 , 41 ). For past B-atm ages, we used coral 14 C ages ( 13 ) and IntCal13 atmospheric 14 C ages at the calendar age of each coral ( 41 ). (Inset) The dashed line shows a hypothetical conservative mixing calculation between modern NADW and PDW, with white dots indicating 10% intervals. Endmembers in the mixing calculation are modern NADW (ε Nd = −13.2, [Nd] = 17.6 pmol/kg, B-atm = 500 y, DIC [dissolved inorganic carbon] = 2,160 µmol/kg) ( 40 , 42 ) and PDW (ε Nd = −3.5, [Nd] = 44.4 pmol/kg, B-atm = 2,100 y, DIC = 2,350 µmol/kg) ( 40 , 43 ). The dashed gray rectangle indicates the modern Drake Passage seawater properties ( 24 , 40 ).

We emphasize that any local imprint of radiogenic terrestrial surface input could only have been preserved if it was exported directly to the Drake Passage (i.e., a scenario in which the inputs were sufficient to alter the mass balance of UCDW in the entire Southern Ocean is unlikely). Therefore, we may consider AAIW, which is formed from Southern Ocean surface waters, as a potential candidate for delivering a local radiogenic meltwater-sourced signal to middepths of the Drake Passage. Indeed, the Nd isotopic compositions of UCDW and AAIW are similar during the Middle Holocene ( Fig. 2 ). However, interpreting such values as a shuttle transferring radiogenic surface waters to middepth (i.e., both AAIW and UCDW layers) would also require 1) a significant deepening of the mixed layer down to the coral collection depth near 900-m water depth at Sars Seamount, or 2) a dramatic steepening of the isopycnals, or 3) a southward oceanic frontal shift of ∼3‒5° latitude in the Drake Passage to align AAIW isopycnals with the coral sampling locations within modern-day UCDW ( 3 , 4 ) ( Fig. 1 ). Critically, all of the above scenarios are difficult to reconcile with radiocarbon evidence from the same Drake Passage corals ( 13 ). Radiocarbon in the ocean is a function of surface ocean exchange with the atmosphere and water mass mixing and aging at depth ( 38 ), thereby providing an independent and complementary tracer to Nd isotopes ( 22 , 39 ). The Middle Holocene Nd isotope excursion is associated with poorly ventilated water masses at UCDW depths, expressed as a relatively high radiocarbon age offset between the coral and the contemporaneous atmosphere (B-atm) of ∼1,200 y ( Fig. 3 ). Therefore, the UCDW coral data are inconsistent with enhanced admixture of well-ventilated AAIW to UCDW depths. Instead, the Middle Holocene tracer distribution at these depth levels is consistent with the general pattern of the modern Southern Ocean overturning circulation ( Fig. 1B ), explaining both the similar Nd isotopic compositions of UCDW and AAIW and the offset in ventilation between these two waters masses ( Fig. 3 ). As such, the Middle Holocene radiogenic Nd isotope excursion was recorded in 2 independent settings (Sars Seamount and Burdwood Bank) and its origin must lie with a radiocarbon-depleted water mass source.

The skeletons of aragonitic cold-water coral specimens in the Drake Passage were shown to record an ambient Nd isotope seawater signal ( 24 ). Nevertheless, the observed Holocene Nd isotope variability in the Drake Passage could potentially reflect a number of different processes, since seawater Nd isotopic compositions can be altered through lithogenic input from dust ( 34 ), rivers ( 35 ), boundary exchange ( 36 ), or glacial erosion ( 37 ). The effect of dust input is typically restricted to the uppermost levels of the water column ( 34 ) and is negligible in this region of the Southern Ocean ( 37 ), while modern seawater Nd isotope data indicate that there is no influence of boundary exchange on CDW within the fast-flowing ACC in the Drake Passage ( 24 , 25 ). In particular, there is no observable release of Nd at the sampling locations in the modern day ( 24 ), and there is no reason to envisage more pronounced exchange during the Middle Holocene. Furthermore, a Sars Seamount coral from 1,701-m water depth dating to 5.69 ± 0.27 ka BP shows ε Nd of −8.2 ± 0.3 compared to ε Nd = −6.3 ± 0.2 in a coral from 869-m water depth at the same time (5.76 ± 0.06 ka BP; SI Appendix, Table S1 ). This 2-ε Nd offset is difficult to reconcile with a local benthic source of radiogenic Nd from the seamount. Although regional input fluxes could have differed in the past, the timing of major ice-sheet retreat in Antarctica ( Fig. 2 ) ( 32 , 33 ) and changes in terrestrial input from nearby potential source areas show no correspondence to the Middle Holocene Nd isotope maximum (see also SI Appendix, section 3 and Fig. S2 ).

Expansion of Pacific-Derived Waters into the Southern Ocean.

Compared to the modern day, the Middle Holocene UCDW properties show greater geochemical similarity to waters found in the middepth Pacific Ocean, for both Nd isotopes and radiocarbon (Fig. 3). Simple endmember changes in NADW or PDW are unable to explain the Nd isotope shift in UCDW, since the Middle Holocene Nd isotopic composition of PDW was largely invariable (44), while the Nd isotope signature of NADW would need to have changed to ∼−8, which is not supported by North Atlantic Nd isotope data (45). Consequently, we propose that the Middle Holocene Nd isotope shift to −5.8 in the Drake Passage records a significantly increased fraction of Nd from radiogenic PDW at UCDW depths.

Given the homogeneity of Nd isotopes in the modern Drake Passage water column (Fig. 4A; see also SI Appendix, section 4), one possibility to explain our data is to invoke wholesale changes of the entire Drake Passage water column during the Middle Holocene. Although the radiogenic Nd isotope signal recorded in our corals from UCDW and AAIW depths is not seen in any LCDW records from this time (Fig. 2), the lack of high-resolution LCDW data (and the age uncertainty of our Middle Holocene LCDW coral sample; SI Appendix, Table S1) means that we cannot rule out that short-lived changes may have occurred in LCDW, too. However, this scenario would require an abrupt invasion of Pacific-derived water masses at all depths and/or a substantial reduction of NADW input on centennial timescales in order to explain the large difference in ε Nd values recorded almost concurrently at ∼5.7 ka BP in UCDW (−6.3) and LCDW (−8.2) depths (Fig. 2 and SI Appendix, Table S1). Such large hydrographic changes are inconsistent with strong NADW export and the resulting unradiogenic Nd isotopic composition of LCDW in the Holocene Southern Ocean (28⇓–30) (Fig. 2). Moreover, considering that skeletal subsamples for Nd isotope analyses typically integrate the seawater signal of several decades (24), our observation of radiogenic values in 4 coral specimens retrieved from 2 different locations during the Middle Holocene suggests that this signal may have been a relatively persistent feature rather than a transient short-lived excursion. We therefore suggest that our data reflect changes restricted to UCDW and AAIW depths and indicate a vertical Nd isotope gradient in the Drake Passage during the Middle Holocene (Fig. 4B), which is remarkably different from the homogeneous Nd isotope distribution in the modern water column (24, 25).

Fig. 4. Vertical distribution of Nd isotopes in the modern and past Drake Passage. (A) Late Holocene coral data for AAIW (blue triangles) and UCDW (red circle) (<1.1 ka BP) (24) and foraminifera-derived data for South Pacific LCDW (green circle) (30) compared to modern seawater profiles from Sars Seamount (gray-filled circles) and Burdwood Bank (gray-filled triangles) framed by light-gray bar (ε Nd = −8.1 ± 0.5, 2 SD, n = 18, see also Fig. 2) (24). (B) Older Holocene coral data (filled colored symbols) compared to light-gray bar integrating modern seawater and coral data from A (ε Nd = −8.0 ± 0.6, 2 SD, n = 23) (24). Dark-gray polygon frames the Middle Holocene coral results (thick black outline; 6.8–5.8 ka BP) and concurrent LCDW data from the South Pacific (30).

The increased presence of PDW in the Drake Passage seems to have been closely preceded by large-scale equatorward SWW intensification during the Early to Middle Holocene (15⇓⇓⇓⇓–20, 46⇓–48), as reflected in increased sea spray on Macquarie Island north of the Polar Front in the southwest Pacific (54°S/158°E) (47) (Fig. 5E), enhanced moisture supply to southeast Australia (48) (Fig. 5F), and decreasing SST off New Zealand (15). Concurrent poleward weakening of the SWW is evident from decreasing fluvial runoff in southern Patagonia (53°S) (16) (Fig. 5D), reduced upwelling of CDW on the West Antarctic shelves (19, 20), and decreasing West Antarctic air temperatures (17) (Fig. 5C). Together, these changes suggest a more northerly position of the SWW in the South Pacific during the Middle Holocene (Fig. 5G). The northward SWW migration is part of large-scale climate reorganizations expressed in a concurrent positive Northern Hemispheric extratropical temperature anomaly (Fig. 5A), which has been ascribed to orbital forcing modulated by snow, ice, vegetation, and ocean circulation feedbacks (49). Here we postulate that SWW forcing played an important role in setting the Drake Passage water column structure, and hence the Nd isotope fingerprint, during the Holocene.

Fig. 5. Drake Passage water mass mixing compared to Holocene climate parameters. All site numbers refer to locations shown in SI Appendix, Fig. S1. (A) Interhemispheric extratropical temperature anomaly, where positive values represent Northern Hemisphere positive anomalies and vice versa (49). (B) Excess rhenium ( XS Re) from 1,015-m water depth off Chile, located at hinge depth between high O 2 (AAIW) and low O 2 (PDW) (site 5) (50). (C) Antarctic Peninsula deuterium isotope-based temperature record from James Ross Island (note reversed axis) (site 10) (17). (D) Accumulation rate of terrestrial organic carbon in a Patagonian fjord, recording SWW-induced fluvial input (site 4) (16). (E) Diatom-inferred conductivity as a tracer for SWW-controlled sea spray on Macquarie Island north of the PF (site 2) (47). (F) Depth of southeast Australian Lake Gnotuk, indicative of SWW-driven precipitation–evaporation (P-E) balance (site 1) (48). (G) Summary panel of the latitudinal SWW trends, with peak northward intensity between ∼7.5 and 5.5 ka BP highlighted by the yellow shading (15⇓⇓⇓⇓–20, 46⇓–48). (H) EPICA Dome C (EDC) ice core CO 2 record (11-point running mean) (site 13) (55, 56). (I) Drake Passage Nd isotope data from UCDW (red), LCDW (green), and AAIW depths (blue) (this study and <0.5 ka BP coral data from ref. 5). Gray bar at the y axis represents the range of modern local seawater Nd isotopic compositions (see legend of Fig. 2 for details) (24). Thick colored lines in C, E, and F are 3-point running means of the respective datasets.

Poleward SWW weakening can reduce the wind forcing over the ACC, in particular where the zonal ACC flow is constrained by topography (1, 3, 51). Such a reduction is dynamically coupled to diapycnal mixing in the Southern Ocean water column, including between UCDW and LCDW (8). Recent model results highlight the role of Southern Ocean diapycnal mixing for the structure of the Atlantic overturning circulation (52), while other simulations indicate a pronounced reorganization of the overturning circulation in the Pacific in connection with SWW changes (51, 53). Reduced SWW forcing was shown to decrease upwelling of NADW in the Southern Ocean and enhance upwelling of NADW-influenced deep waters (via diapycnal mixing) in the Indo-Pacific (51). Deep-water upwelling rates in the Pacific are slow, but these waters are funneled via the PDW outflow into the Southern Ocean, where they join the circumpolar flow predominantly at UCDW depth levels (1, 2, 27) (Fig. 1B). We therefore propose that poleward SWW weakening caused a decrease of diapycnal mixing in the deep Southern Ocean, which was offset by increased upwelling in the Pacific Basin, thereby fueling enhanced PDW export to the Drake Passage. The combination of these processes resulted in the observed vertical Nd isotope gradient in the Drake Passage water column between ∼7 and 6 ka BP (Fig. 4B).

This hypothesis is supported by reduced oxygenation at intermediate water depths off Chile (50) (Fig. 5B) and the presence of depleted radiocarbon within PDW off New Zealand (B-atm ∼ 2,100 y at 7 ka BP) (54), which are both consistent with a southward expansion of poorly ventilated PDW during the Middle Holocene and an enhanced influence of PDW in the Drake Passage. While there are few well-resolved Nd isotope records from intermediate waters during the Holocene, a reconstruction from AAIW depths in the southwest Atlantic shows a small Middle Holocene excursion toward more radiogenic Nd isotope values (31) (Fig. 2), consistent with our observations from the Drake Passage. Given the more pronounced changes recorded by the UCDW corals in the Drake Passage, and their more Pacific-like Nd isotopic compositions, we suggest that changes in the Nd isotopic composition of UCDW originated through increased PDW influence in the Pacific sector of the Southern Ocean and were advected downstream to exert a control on the Holocene evolution of Nd isotopes in South Atlantic AAIW (Fig. 1B).

Interestingly, many proxy data including the interhemispheric temperature distribution (49) (Fig. 5A), expansion of PDW in the southeast Pacific (50) (Fig. 5B), and the poleward SWW weakening in Patagonia (16) (Fig. 5D) show rather gradual changes over the Early to Middle Holocene, whereas the change in Nd isotopic composition of UCDW seems relatively abrupt and delayed in comparison (Fig. 5I). The abrupt nature of the Nd isotope pattern is more similar to SWW strength in the southwest Pacific (47) (Fig. 5E) and precipitation changes in southeast Australia (48) (Fig. 5F). Rapid Nd isotope shifts in the Drake Passage suggest that diapycnal mixing in the Southern Ocean and PDW export into UCDW can change on relatively short timescales, possibly in response to latitudinal SWW changes across a critical threshold. Furthermore, they also relate to the unique ability of the cold-water coral archive to record abrupt oceanographic changes (39).