Deglacial patterns of South Pacific overturning inferred from 231Pa and 230Th

The millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) is well documented for the last glacial termination and beyond. Despite its importance for the climate system, the evolution of the South Pacific overturning circulation (SPOC) is by far less well understood. A recently published study highlights the potential applicability of the 231Pa/230Th-proxy in the Pacific. Here, we present five sedimentary down-core profiles of 231Pa/230Th-ratios measured on a depth transect from the Pacific sector of the Southern Ocean to test this hypothesis using downcore records. Our data are consistent with an increase in SPOC as early as 20 ka that peaked during Heinrich Stadial 1. The timing indicates that the SPOC did not simply react to AMOC changes via the bipolar seesaw but were triggered via Southern Hemisphere processes.

www.nature.com/scientificreports/ Generally, Pacific seawater 231 Pa/ 230 Th-ratios are modulated by the longer residence time of deep-waters and thus reflect higher boundary scavenging intensity at the continental margins when compared to the Atlantic basin [16][17][18] . Accordingly, 231 Pa/ 230 Th has yet mainly been applied to reconstruct spatio-temporal changes in the MOC regimes of Atlantic and not in the Pacific basins. An active meridional overturning cell induces a general decrease in sedimentary 231 Pa/ 230 Th with water-depth as observed in the Atlantic Ocean [19][20][21] . The reported depth-dependent decrease is a consequence of the difference in particle reactivity between the two radionuclides with the relatively less particle-reactive 231 Pa, which is more prone to be advected, while 230 Th is preferentially exported and deposited into the underlying sediments, by reversible particle scavenging. Recently Luo et al. 15 examined to which extent the manifestation of ocean circulation can be recorded in core top sediments based on a compilation of > 250 231 Pa/ 230 Th measurements, covering large swaths of the Pacific Ocean (Fig. 1). The basin wide data-distribution underlines the anticipated predominant influence of particle fluxes and boundary scavenging sedimentary 231 Pa/ 230 Th. Yet, regional subdivisions of the data set reveal that in the central gyres and the Southwest Pacific region a discernable influence of SPOC is recorded by sedimentary 231 Pa/ 230 Th values. The vertical attenuation in 231 Pa/ 230 Th values along water depth in these regions, similar to the Atlantic, is interpreted as indicative of the influence of the overturning circulation 15 . Building on these findings, we measured 231 Pa/ 230 Th downcore profiles based on five sediment cores retrieved from the SW Pacific along a depth transect ranging between 835 and 4339 m back to ~ 30 ka to provide evidence for glacial and deglacial variations in PMOC dynamics.  15 , the SW-Pacific (orange triangles 15 ; red squares, this study), and the W Atlantic (blue diamonds; Table S1). Decreasing 231 Pa/ 230 Th with water depth in the W Atlantic (blue) is interpreted as an imprint of AMOC, while in the Pacific (grey) a similar correlation is observed only for the SW region (orang, red).

Results
Here we reconstruct changes in the SPOC from a depth transect of five sediment cores from the New Zealand Margin and the East Pacific Rise (Fig. 2). These sediment cores are bathed by the major Southwest Pacific deepwater masses, including Antarctic Intermediate Water (AAIW) the Upper and Lower Circumpolar Deep Water (UCDW and LCDW) as well as the Antarctic Bottom Water (AABW).
To assess temporal variations in the dynamic of SPOC, we make use of the sedimentary ratio of 231 Pa and 230 Th, which based on the recent results by Luo et al. 15 (Fig. 1), suggest that the Southwest Pacific is an area sensitive to circulation driven changes in 231 Pa/ 230 Th, as well as published ΔΔ 14 C-records 10 (Figs. 3,4,5). In the open ocean, the residence time of 231 Pa is about 10-times higher than of 230 Th Ref. 22 . Oceanic circulation thus results in the enhanced advection of 231 Pa and hence 231 Pa/ 230 Th values below the production ratio (0.093) 23 . Consequently, if deep-water circulation weakens, sedimentary 231 Pa/ 230 Th increases toward the production ratio 21,24 . As biogenic opal is widely known to decrease the residence-time of 231 Pa by preferentially removing it from the water column 25 , we analyzed the opal contents along with 231 Pa/ 230 Th (PANGAEA). The amount of biogenic opal in most of our samples is very low (< 3 wt%) and-more importantly-does not correlate with the pattern of 231 Pa/ 230 Th, implying that a significant impact on the scavenging behavior of 231 Pa at our core locations remains improbable 26 . However, we consider the contribution of biogenic opal export production poleward (i.e. upstream) of the core locations on the local 231 Pa/ 230 Th signal (supplementary information).
Reaching back ~ 35,000 years, our dataset displays coherent variations in the sedimentary 231 Pa/ 230 Th values. The general trends of the independent 231 Pa/ 230 Th-data and ΔΔ 14 C-records 10 , with bulges in the records in the glacial and deglacial sections imply a certain level of consistency throughout time and space (Fig. 4). Glacial 231 Pa/ 230 Th-ratios are generally higher and more variable than during the Holocene (Fig. 3), as are the ΔΔ 14 Crecords of this region 10 . The most noticeable feature is a transient increase in 231 Pa/ 230 Th values between ~ 20 and 18 ka in cores PS75/104-1, SO213-76-2, and SO213-82-1. With a maximum of 0.12, the signal recorded by SO213-82-1 is particularly salient, as it significantly exceeds the production ratio. After ~ 17 ka, we report a constant decrease in 231 Pa/ 230 Th values gradually declining toward Holocene values in all sediment records (Fig. 3).

Discussion
The bathymetric transect of sediment cores presented here comprise records from different water depths and thus, different water masses and circulation regimes. The shallowest record PS75/104-1, is bathed by AAIW recently formed in the Antarctic Polar Zone, an area of high opal production (Fig. 1). This implies that changes in opal export production, upstream of our core location might have a significant impact on the 231 Pa/ 230 Th pattern locally 27 . Thus, we interpret this record as well as the older part of PS75/059-2 (> 18 ka), when the main area of high opal production migrated northwards, reflecting a combination of opal productivity, and (to a lesser extent) ocean circulation (see supplementary information for more details, Fig. S1). In a similar way to PS75/104-1, AABW record SO213-76-2 might have been influenced by waters recently formed in an area of high opal production (Fig. S1). However, as this core location is also under the influence of an admixture of LCDW, we assume that this effect is less severe than in our AAIW record. Mid-depth records SO213-82-1 and PS75/100-4 are bathed by southbound CDW/PDW (Circumpolar/Pacific Deep Water). It is expected that the concentration of 231 Pa builds up relatively to 230 Th along increasing travel time as a function of PMOC strength. Thus, changes in the upstream export of 231 Pa are able to exert a dominant influence on the SW-Pacific 231 Pa/ 230 Th ratios as shown by Luo et al. 15 . Holocene ratios of all sediment cores (Fig. 3) fall within the modern budget of the SW-Pacific 17 , ranging between ~ 0.06 and 0.08.  Th-values close to or even higher than the production ratio. In general, we observe generally higher 231 Pa/ 230 Th-ratios during the glacial, compared to the Holocene, indicative of either weaker SPOC, higher glacial particle fluxes or most likely a combination of both ( Fig. 1) 15 .
The  . In order to investigate if this general glacial to Holocene trend reflects a glacial reduction in upstream removal of 231 Pa, we turned to several sediment records along the Equatorial East Pacific Rise. These records show a consistent trend from lower glacial, to increased Holocene values that approach and exceed the production ratio between ~ 17 ka and 11 ka (Fig. S2) 35 . It is thus plausible that the decrease in 231 Pa/ 230 Th as observed in our records (Fig. 3), represents diminished export of 231 Pa from the north after ~ 17 ka. It is plausible to assume that the equatorial records 35 were themselves also influenced by upstream processes, which is in good agreement with the processes outlined by Luo et al. 15 . Nevertheless, it is important to note, that these records are probably not exactly upstream of our core locations but provide the only available approximation of an upstream signal from 231 Pa/ 230 Th downcore profiles from the literature to date. Despite a sufficient temporal resolution, we did not observe any changes during HS2 as manifested in Atlantic sediment cores (Fig. 3). Depending on the boundary conditions, HS2 might not have been associated with any sizeable variation in the SPOC 13 . An additional feature of our records is the inverse evolution of 231 Pa/ 230 Th and ΔΔ 14 C throughout the end of the last glacial (Figs. 3,4,5). At ~ 20 ka, our data show increased 231 Pa/ 230 Thratios, accompanied by enhanced ventilation (ΔΔ 14 C) 10 (Figs. 4,5). Appearing contradictory at first, we argue that both patterns are likely the result of increasing water mass advection. Upon SPOC increase, excess 231 Pa gets transported via PDW/CDW downstream to our core locations 15 , increasing the 231 Pa/ 230 Th-ratio far above the production ratio (Figs. 4, 6B).
Hence, at this water-depth, increased 231 Pa/ 230 Th-values may not reflect a SPOC slow-down 15 but rather imply its reinvigoration. This is in good agreement with the ΔΔ 14 C-records that indicate more active overturning and ventilation during this time period 10 . Thus, the evolution of both proxies ( 231 Pa/ 230 Th and ΔΔ 14 C) can be explained with the same mechanism, an increase in SPOC that transported excess 231 Pa to our core location, while also leading to an increase in ΔΔ 14 C Ref. 10 . From ~ 19 ka on, PS75/104-1 (AAIW) recorded lower 231 Pa/ 230 Th-ratios, while the mid-depth cores still experienced elevated values (Fig. 3). This short interval was likely caused by increasing opal production in the Antarctic Zone of the Southern Ocean 7 that stripped 231 Pa from the water before reaching the downstream location. Subsequent to this peak, 231 Pa/ 230 Th-values rapidly increased and paralleled the pattern observed in both mid-depth cores (Fig. 3 While the AMOC plummeted, South Pacific 231 Pa/ 230 Th-and ΔΔ 14 C-data (Fig. 4) indicate a progressive evolution of the SPOC toward modern values, starting as early as ~ 20 ka, which culminated during HS1 (Fig. 4). Other records from the Southwest Pacific also corroborate this timing, featuring a significant decrease of εNd as a result of deep-water destratification and enhanced mixing 11 .  The declining ventilation and circulation of mid-depth SO waters 8,10,29 paralleled peak glacial SH climate such as low temperatures, changes in the density structure of intermediate-and bottom-waters, expanded sea ice, and displaced or weakened Southern Westerly Winds (SWW) [36][37][38][39] . According to the age models of our sediment cores, the mid-depth SPOC recovered faster from the HS1 disturbance than the AMOC (Fig. 4). Hence, the initial impulse that triggered the increase in deglacial SPOC and upwelling 7 must have arisen from the SO or the SH and not via NADW. During this period, glacial climate conditions reversed and gradually exposed the upwelling area of CDW to the surface. The combination of different parameters such as reduced northward www.nature.com/scientificreports/ heat advection and shifts in the Intertropical Convergence Zone and wind belts increased Southern Ocean upwelling and CO 2 -release 40,41 . In addition, local changes in orbital forcing, are considered to be an important factor driving early deglacial changes in the West Antarctic and Pacific sector 40 . Hence, we argue that the SPOC increase that preceded the end of the LGM (Fig. 6A) was triggered by processes centered in the SH and not by changes in NADW dynamics. In this respect, independent records of Antarctic Ice Sheet retreat 3 and the early West Antarctic warming phase 40,42 are consistent with the timing observed in our records (Fig. 4). Deep-to bottom-water sediment core SO213-76-2 (4339 m) however, differs from the deglacial pattern of the other records, as it marks an interval of decreasing 231 Pa/ 230 Th from ~ 19 on (Figs. 4B, 5). This interval is in very good agreement to the increasing ΔΔ 14 C-values measured on the same sediment core 10 that also show a similar shift at ~ 20 ka (Fig. 5). Today, SO213-76-2 is influenced by LCDW and AABW 29 . During the LGM however, the core location of SO213-76-2 was probably only exposed to LCDW. We argue that during peak glacial times, AABW was too dense to be exported to the north of the Pacific Antarctic Ridge, in a similar manner as it was observed in the Atlantic sector of the SO 43 . In the Pacific sector, processes associated with the early Southern Hemisphere warming 42 began to erode the deep stratification at ~ 20ka 11 . This erosion allowed AABW to reach the core location and thus reduce the influence of excess CDW 231 Pa on SO212-76-2.
With our new data, we might also be able to add to a debate of Pacific studies that argued for or against a glacial reduction in Pacific overturning 11,[44][45][46][47] . Our results are consistent with the notion of enhanced glacial carbon storage within mid-depth South Pacific deep-waters 10,45 . In combination with more complete surface nutrient consumption 36,48 the reported slowdown in glacial SPOC might account for the sequestration of carbon in the deep sea along with a progressive decay of its 14 C-content. However, to confine the sequestered carbon into the deeper ocean, climatic conditions must have changed in parallel. The stratification of the Southern Ocean's water column was intensified by an increased glacial density gradient 11,37,39 , Antarctic sea ice noticeably expanded to the north 49 , while the SWW were displaced toward the north 38 . Ultimately, all factors significantly hampered the exchange of deep-waters and the atmosphere. In combination, stratification, expanding sea ice, and displaced winds reduced the upwelling of Circumpolar Deep Waters (CDW) during the glacial, thus are likely the driving parameters behind the observed slowdown of the SPOC. This glacial slowdown extended the residence time of deep-waters within the ocean's interior by several thousand years 8,10,33,34 , increased 231 Pa/ 230 Thratios due to boundary scavenging 15 , and ultimately allowed for the progressive accumulation of carbon (CO 2 ) in the glacial ocean.
At the final phase of the LGM, we argue that South Pacific overturning progressively increased (Fig. 6). Possibly supported by atmospheric teleconnections, the increase in SPOC sparked the re-ventilation of Southwest Pacific deep-waters, leading North Atlantic processes by almost 2000 years (Fig. 4). This mechanism transported excess 231 Pa toward the South Pacific, resulting in the observed transient increase in 231 Pa/ 230 Th. Ultimately, this process is probably linked to the upwelling of carbon-rich deep-water and culminated in the transfer of old, 14 C-depleted CO 2 to the atmosphere (Fig. 6C). In addition, the re-invigoration of the SPOC is also a likely process, which carried warm CDW onto the shelf regions, fostered the early retreat of Antarctic ice shelves 2,3 (Figs. 4B,F, 6C). Hence, our reconstructions of South Pacific overturning present a physical link between increasing deepwater ΔΔ 14 C Refs.8,10,33,34 , declining atmospheric Δ 14 C Ref. 50 , rising atmospheric CO 2 -levels 4 , retreating Antarctic ice shelves 3 , and rising global sea level. However, as mentioned above, several reconstructions of benthic δ 13 C values and sortable silt 44 and foraminiferal εNd Ref. 47 argue against a pronounced decrease in glacial deep Pacific overturning. To a large extent, these studies cover water masses deeper than 3000 m. As our main signal was observed in a water depth of 2066 m, that is also in the range of the so-called floating glacial carbon pool 10,51 , this discrepancy might be explained by the different hydrographic sections sampled. However, to obtain a more comprehensive overview in past SPOC changes and 231 Pa/ 230 Th budgets, we stress the need for more Pacific downcore records.

Conclusions
Our analysis of downcore 231 Pa/ 230 Th-records based on SW-Pacific sediment cores allowed us to investigate the applicability of this proxy with respect to Pacific core top 231 Pa/ 230 Th-data 15 . In addition, when interpreted as a circulation signal, our new 231 Pa/ 230 Th data set sheds light on the impact changes in the SPOC had on the glacial Pacific carbon pool, and it's deglacial erosion, and the release of CO 2 . In particular, we conclude that: a. As shown in the study by Luo et al. 15 , SW-Pacific 231 Pa/ 230 Th is potentially sensitive to changes in Pacific overturning circulation. b. Based on this sensitivity, we argue that the deglacial onset of the SPOC presumably transported excess 231 Pa from the north Pacific to our core locations via PDW/CDW, resulting in values exceeding the production ratio. www.nature.com/scientificreports/ 6. After ~ 17 ka, upstream records along the Equatorial EPR show a progressive removal of 231 Pa. This upstream removal likely put an end to the transport of excess 231 Pa to our cores, causing the progressive decrease in 231 Pa/ 230 Th as observed by us following this time interval. 7. The transient rise in 231 Pa/ 230 Th, during HS, ending earlier in the Pacific than in Atlantic records, is likely indicative of the transport of 14 C-depleted and CO 2 -rich waters from the floating Pacific carbon pool 10 toward the surface. 8. Ultimately, this process is a likely common driver between the rise of atmospheric CO 2 Ref. 4 , it's drop in Δ 14 C Ref. 50 , and the observed collapse of Antarctic glaciers 3 . 9. Changes in the SPOC did not simply react to AMOC changes via the bipolar seesaw, but were probably triggered by Southern Hemisphere changes in orbital forcing, shifting atmospheric systems, and CO 2 -release 40,41 . 10. Our findings highlight the need for additional downcore records from the South Pacific to obtain a more reliable 231 Pa/ 230 Th as well as better insight into past changes in SPOC.

Methods
Sediments and sample treatment. The sediment cores analyzed in our study were recovered during R/V Polarstern expedition ANT-XXVI/2 (PS75) in 2010 and R/V Sonne expedition SO213/2 in 2011 (Fig. 2). The sediments predominantly consist of foraminifer-and nannofossil-bearing muds with negligible quantities of biogenic opal 52,53 . Sedimentation rates vary between 3. For the geochemical analyses, we used three to four cubic centimeters of bulk sediment from the working halves, except for core PS75/100-4, where we had to sample the archive halves, as no material was left in the working halves. Unfortunately, PS75/100-4 was so intensively sampled in the deglacial interval that literally no material was left for our analyses of this time period.

Radiocarbon dating and age models.
To better constrain the distinct HS1 pattern of SO213-82-1, we expanded the radiocarbon record of Ronge et al. 10 . Briefly, monospecific planktic foraminifera Globigerina bulloides and mixed benthic species (Cibicidoides spp. and Uvigerina spp), were separated and analyzed at the Alfred-Wegener-Institute's MICADAS facility. Following the procedure previously outlined in Ronge et al. 10 , we added six data-points into the stratigraphy of core SO213-82-1 and updated the stratigraphy of PS75/104-1 according to Küssner et al. 56 . However, as the stratigraphy of SO213-82-1 in Ronge et al. 10 is based on a 14 C-independent method, via the correlation to the EDC ice core record, we updated these age models using a 14 C-related approach. To convert 14 C-ages into calendar ages, we used the Calib 7.1 solution 57 along with the IntCal13 calibration curve 50 as well as the New Zealand Margin surface reservoir ages of Skinner et al. 45 . However, as the surface reservoir age reconstruction of Skinner et al. 45 is insufficient to calibrate all our data points, we also used modelled 14 C-ages 58 . A direct comparison of modelled 58 and reconstructed 45 surface reservoir ages reveals an offset between both methods, ranging from only 20 to about 300 years. To account for the ages of data points without a direct 14 Cage, we applied two independent Bayesian approaches (see below). Using both methods, we are confident that our age models provide the necessary resolution to discern and characterize millennial-scale changes in deep ocean circulation.
Two samples with prominent planktic age reversals (46-47 cm and 86-87 cm) were ignored for our age models.
Updating the age models resulted only in minor changes in the radiocarbon records provided by Ronge et al. 10 . A comparison of these records to our updated age models is shown in Figure S2. The inclusion of new ΔΔ 14 C data points furthermore improves the agreement of SO213-82-1 with other CDW records from the Pacific (MD97-2121) 45 and the Atlantic (MD07-3076) 8 .
Bayesian age modelling. We applied the Bayesian age-depth model hummingage, which is developed at AWI and freely available at https:// github. com/ hummi ngbird-dev/ hummi ngage. The GitHub repository provides source code for the R programming language and a Jupyter Notebook containing detailed explanations. Additionally, an easy-to-use web service is provided at https:// hummi ngage. awi. de/ for applying hummingage online in the browser.
The age-depth model hummingage can be an alternative to the widely used Bacon 59 method. In addition to hummingage we also applied Bacon to our data and show the comparison of both approaches in the supplements (supplementary information; Fig. S4).
Geochemistry. 231 Pa/ 230 Th measurements. The concentrations of sedimentary 231 Pa, 238 U, 230 Th, and 232 Th were jointly measured by isotope dilution in a co-operation between AWI and Heidelberg University with contributions from the GEOMAR Kiel and using mass spectrometers at the AWI (Element 2), Heidelberg (Element 2, Neptune, iCap) and Kiel (Neptune).
The chemical separation and cleaning followed standard procedures 60  www.nature.com/scientificreports/ calculated from the measured bulk concentrations 22 . Lithogenic contributions were corrected by applying a detrital 238 U/ 232 Th activity ratio of 0.5 based on a basin-wide average lithogenic background as suggested by Henderson and Anderson 22 or Bourne et al. 62 . As well, a disequilibrium of 4% for 234 U/ 238 U in the lithogenic fraction has been considered to account for preferential 234 U loss via the recoil-effect 62 . We excluded samples SO213-76-2 200 cm and 314 cm from our interpretations. These samples contain high concentrations of volcanic glass that may have interfered with our measurements. Nevertheless, all measurements, including SO213-76-2 200 cm and 314 cm, are included in the PANGAEA datafiles (https:// doi. panga ea. de/ 10. 1594/ PANGA EA. 889934).
Opal measurements. As the presence of biogenic opal may affect the ratio of 231 Pa and 230 Th, by preferentially scavenging 231 Pa Ref. 25 , we determined the sedimentary biogenic opal content of our samples following the method of Müller and Schneider 63 . For the analysis, we leached 20-100 mg of bulk sediment, using 100 ml of 1.0 M NaOH at 85 °C. The alkaline solution was then injected into the analyzer. In the analyzer, the leachate was treated with 0.088 M H 2 SO 4 , a molybdate reagent, oxalic acid and an ascorbic acid in order to form molybdateblue complexes, which were analyzed using a photometer for dissolved silica. For most sediment cores, the average SiO 2 content was well below 2%. Since these low biogenic opal concentrations would only marginally affect the sedimentary 231 Pa/ 230 Th-ratios 26,64,65 , and since the signals of both metrics did not covary, we dismiss any significant impact of biogenic opal in driving downcore changes in 231 Pa/ 230 Th. Rather, we interpret changes in sedimentary 231 Pa/ 230 Th as primarily reflecting temporal changes in overturning circulation.
Biogenic barium. Biogenic barium (Ba Bio ) is considered to be a reliable proxy for paleoproductivity 66 . Ba Bio was calculated based on the difference of total Ba and lithogenic Ba (Ba lith ), measured on our bulk samples via ICP-MS. To calculate Ba lith from 232 Th (measured by ICP-MS via isotope dilution), we followed the method of Costa et al. 67 : Ba lith = 51.4 * 232 Th.
To assess the role, vertical mass fluxes and paleoproductivity might have played on the 231 Pa/ 230 Th-ratios in the Southwest Pacific, we compared both, Th xs0 (Fig. S5A) and Ba Bio (Fig. S5B) to our 231 Pa/ 230 Th-records. Due to the weak correlation of these proxies, we are confident to interpret 231 Pa/ 230 Th in terms of paleocirculation ( Figure S6). www.nature.com/scientificreports/