Lead Isotopic Constraints on the Provenance of Antarctic Dust and Relevant Atmospheric Circulation Patterns Prior to the Mid-Brunhes Event (~430 Kyr Ago)

A lead (Pb) isotopic record, covering the two oldest glacial-interglacial cycles (~572 to 801 kyr ago), from the European Project for Ice Coring in Antarctica Dome C ice core provides isotopic evidence for the provenance of dust in deep Antarctic ice prior to the Mid-Brunhes Event (MBE), ~430 kyr ago, characterized by less warm interglacials. The isotopic signatures suggest Patagonia and central-western Argentina (CWA) as the primary sources of dust in central East Antarctica during both pre-MBE glacials and interglacials, in concert with an equatorward shift of the southern westerly winds (SWW). The contribution from extra-Antarctic volcanic emissions appears to be important for non-dust Pb in the pre-MBE interglacial and intermediate climates, most likely due to the reduction of wet removal eciency with a weakening of the hydrologic cycle. Our results show a close coupling of the Southern Hemisphere atmospheric circulation patterns to climatic conditions prior to the MBE.


Introduction
The EPICA (European Project for Ice Coring in Antarctica) ice core drilled at Dome C (hereafter EDC) on the central East Antarctic Plateau (EAP) (75°06′S, 123°21′E, altitude 3,233 m above sea level) has provided unique archives of past climate changes over the last successive eight glacial-interglacial cycles back to Marine Isotope Stage (MIS) 20.2, ~ 800 kyr before present (B.P.) 1,2 . These eight climate cycles are characterized by a larger amplitude of climate variability with warmer interglacials after the Mid-Brunhes Event (MBE), ~ 430 kyr ago, compared to the earlier smaller climate changes with relatively cooler interglacials 2,3 . Together with the Antarctic temperature record, dust records from deep Antarctic ice cores are of particular interest as indicators of the sensitivity of atmospheric and surface conditions in lower latitude dust source regions to glacial-interglacial climate change, affecting the dust cycle at high latitudes 4,5 . However, the dust ux data alone cannot be used to ascertain to what extent the southern westerly winds (SWW) shifted in response to climate cycles, which is considered the underlying mechanism regulating glacial-interglacial variability of atmospheric CO 2 6-8 . A fundamental understanding of these paleo-atmospheric dynamics (that is, the latitudinal shift of the SWW) can be gained from study of climate-related dust provenance changes using isotopic evidence [9][10][11][12][13][14][15][16] .
Strontium (Sr) and neodymium (Nd) isotopic compositions have been used for tracing the provenance of dust trapped in East Antarctic ice 9-13, 15,16 . Compared to Sr-Nd isotope provenance studies, however, determination of the provenance of Antarctic dust using lead (Pb) isotopes has been limited to very few studies [17][18][19] , although Pb isotopes can be used as ngerprints to constrain dust sources and their evolution through space and time 20 . This is mainly because a reliable Pb isotope measurement in Antarctic deep ice cores remains an analytical challenge due to extremely low Pb concentrations at or below the picogram per gram (10 − 12 g g − 1 ) level and contaminants being brought to the outside of the deep ice cores that are inevitably drilled in holes lled with wall-retaining uids 17,18,21 .
Here we present the rst Pb isotope ratios in the EDC ice, dated from ~ 572 to 801 kyr B.P., corresponding to the two oldest glacial-interglacial cycles prior to the MBE over the past ~ 800 kyr. These data allow us to compare variations in Pb isotopic composition between our new data and those previously obtained for the recent two climate cycles in the same ice core 18 , thereby contributing to evaluations of dust provenance changes potentially linked to latitudinal shifts of the SWW between different climatic conditions before and after the MBE.

Results And Discussion
Pb and Ba concentrations and Pb isotopes Elemental concentrations and Pb isotopic compositions measured in the innermost parts of individual samples are illustrated in Figure 1, together with the pro les of dust ux or deuterium (δD) as a function of the age of the ice, and all data are listed in Table S1. Figure 1 Figure S2). This re ects that dust was the main source of Pb in the EDC ice during cold climatic conditions 21,24 .
Furthermore, the Pb concentrations show a sharp decrease when the δD values increase during colder glacials and remain very low when the δD values are above -420‰ ( Figure S3). indicates an increased non-dust (that is, volcanic) contribution.
In Figure (Table S2), exhibiting a tendency (except for sample no. 27, ~712 kyr B.P., Figure S4) to converge toward the partially overlapping eld between Patagonia, Tierra del Fuego (TdF), and the southern and middle CWA (S-CWA and M-CWA, respectively) ( Figure 2). Considering the Sr-Nd isotopic evidence for a dominant dust contribution from northern Patagonia relative to southernmost Patagonia (including TdF) in the post-MBE cold climates 16 , the pre-MBE glacial dust Pb isotopic compositions emerging within the overlapping eld between Patagonia and TdF may be signals of northern Patagonia. As a result, the ratios distributed in a relatively narrow range between Patagonia, S-CWA, and M-CWA are thought to be a consequence of equally signi cant contributions from these potential dust sources to the EAP glacial dust during the pre-MBE glacials. The isotopic signature of sample no. 27 moves toward the most radiogenic McMurdo-Erebus volcanic eld ( Figure S4), likely associated with the effects of volcanic eruptions, probably Antarctic, as mentioned before ( Figure S3). Interestingly, our new dust isotopic data show no dust transport from the PAP to the EAP during glacials prior to the MBE, which contrasts with previous observations for the post-MBE glacial dust as described above.
The pre-MBE interglacial dust-dominant Pb isotopes distribute within the Patagonian eld (sample no. 11) or the overlapping eld between Patagonia and TdF (sample nos.  A large difference in the isotopic compositions before and after the MBE is also observed for non-dust dominant Pb. In Figure 2, both the post-MBE glacial and interglacial isotopes of non-dust dominant Pb vary over a wide range of the mixing line between the non-radiogenic Patagonian dust eld and the very radiogenic McMurdo-Erebus volcanic eld. In contrast, the pre-MBE non-dust dominant Pb isotopes remained less radiogenic, except for four samples (sample nos. 20, 28, 29 and 40, Figure S4), relative to the post-MBE values ( Figure 2 and Table S2). The distinctive non-radiogenic compositions of the pre-MBE volcanic Pb are likely due to extra-Antarctic volcanic contributions in association with the reduction of wet removal e ciency, coupled with an equatorward shift of the SWW belt (see next sections).
Dust provenance and its relevance to a shift of the SWW before the MBE New isotopic evidence for Patagonian dust in the EDC ice core during the pre-MBE glacials indicates that Patagonia was the major dust source throughout the glacial periods over the past 800 kyr, consistent with previous Sr-Nd isotopic ngerprints of Patagonian glacial dust in the EAP ice prior to the MBE 13,28 . This suggests that the SWW remained over Patagonia (particularly northern Patagonia) under both pre-MBE and post-MBE Antarctic cold climates.
The dominance of Patagonian dust during glacial periods over the past 800 kyr would be linked to the glacial advances of the Northern and Southern Patagonian Ice Sheet (PIS), stretched from 37°S to 56°S, and the associated increase of uvioglacial outwash deposits, allowing the enhanced dust entrainment associated with an increase in the vigour of atmospheric circulation 12,32,33 . Despite the lack of detailed records of glacier uctuations in the PIS far back to MIS 20, the PIS glaciers likely reached its greater extent during glacial periods since the Great Patagonian Glaciation (~1 Ma ago) 34  Another prominent feature of pre-MBE glacial isotopic compositions is the absence of a PAP dust signature, which contrasts with the hypothesis of its potential contribution during the post-MBE glacials, attributed to an equatorward movement of the subtropical westerly jet stream (SJT) over the PAP, a high elevation basin (~4,000 m a.s.l.) 15,16,19,28 . We attribute the non-contribution of PAP to the pre-MBE glacial dust in the EAP to environmental conditions that reduced either dust production in the PAP or e cient transfer of PAP dust to the EAP under cold climates of Antarctica prior to the MBE, probably linked to shorter pre-MBE glacials than younger ones, which consequently reduced the dust productivity in the major dust sources at lower latitudes in the Southern Hemisphere 41 . Although hypothetical, a less pronounced increase in the EDC glacial dust uxes prior to the MBE (notably MIS 16, 18 and 20) relative to the post-MBE glacials (e.g., MIS 8 and 10), as noted previously 36 , would be partly due to the absence of any glacial dust supply from PAP (Figure 1), assuming the similarity of a strengthening of SSA PSAs and the transport e ciency of dust from SSA PSAs to the EAP between the pre-MBE and post-MBE glacial conditions 5 . A tentative assessment of the relative dust contribution in the EDC glacial ice, using the Nd isotopic composition between a Patagonian and a PAP end-member, suggested that when glacial dust input to the EAP increased, the contribution from Patagonia decreased, while the contribution from a secondary source, PAP, increased, and vice versa, during Pleistocene glacial times 28 , supporting the above hypothesis.
During the pre-MBE interglacials, our Pb isotopic signatures characterize a dominance of dust from Patagonia with the existence of additional input of dust from CWA, suggesting favorable atmospheric circulation for the persistence of dust production and transfer of dust from these source regions.

Conclusions
New Pb isotope data for the oldest part of the EDC ice core provide the rst evidence for Patagonia and CWA as the persistent sources of East Antarctic dust during both cold glacial and lukewarm interglacial climates over the two oldest glacial-interglacial cycles prior to the MBE. Combined with implications of the isotopic signatures for an increased advection of extra-Antarctic volcanic Pb to the EAP during pre-MBE interglacial and intermediate climates, our isotopic data suggest the emergence of CWA as an important potential dust supplier to the EAP during the pre-MBE interglacials, which is most likely related to a northward shift and/or extension of the SWW belt in combination with a weakened hydrologic cycle as a result of cooler climatic conditions during the corresponding periods relative to the post-MBE interglacials. Our ndings highlight a sensitive feature for changes in the position of the SWW belt and hydrologic cycle intensity in response to pre-MBE climate cycles. Further work is necessary to better understand the modes of variability of the southern climatic system on different time-scales of climate changes.

Methods
Ice core samples and decontamination procedure We have analyzed 40 samples obtained from 30 core sections of the 3,260-m EDC ice core, with depths from 2,973.91 (572,800 yr B.P., MIS 15.1) to 3,189.45 m (801,590 yr B.P., MIS 20.2) 58 (Figure 1). The depth and estimated age of each sample are given in Table S1 in the supporting information. Each of the 30 ice core sections (55 cm in length and 5 cm in radius) from the 3,260-m EPICA Dome C ice core, drilled in a uid-lled hole 1 , was mechanically decontaminated using an acid-cleaned polyethylene lathe and ultraclean working procedures at the Korea Polar Research Institute (KOPRI) 21,59 . These involve the chiseling of successive layers of ice in progression from the contaminated outside toward the center of the section using acid-cleaned ultraclean stainless steel chisels. An external ~2 mm thick layer of the most highly contaminated ice was scraped away before decontaminating. All the equipment used during the entire operation was extensively acid-cleaned prior to use and the chiseling was performed inside a laminar ow class 100 clean bench located in a cold room at -15°C. After the chiseling was completed, the inner core was then cut into two consecutive 20 cm long parts when the whole inner core was available. Altogether, 40 samples were analyzed for this study. Each sample was melted at room temperature in ultra-clean wide mouth low-density polyethylene (LDPE) 1 L bottles within a class 100 clean bench inside a class 10,000 clean room at KOPRI. About 10 mL aliquots were taken into acidcleaned ultraclean 15 mL LDPE bottles and were then transported frozen to Curtin University in Perth, Australia, for Pb and Ba concentrations and Pb isotope analysis.

Mass spectrometry
The procedures of sample processing and analysis by thermal ionization mass spectrometry (TIMS) have (Thermo Scienti c) TIMS 60 . All ion beams were measured with a secondary electron multiplier (SEM), collecting ~300 isotope ratios per sample. The accuracy of the Pb and Ba concentrations is estimated to be ±10% (95% con dence interval), attributed mainly to the accuracy of dispensing the spike into the sample 60 . Two procedural blanks and two or more reference material samples containing a ~100 pg of NIST 981 SRM Pb isotopic standard were analyzed together with each batch of up to ~21 samples for quality control and monitoring of instrumental mass fractionation 60 . A correction for isotopic fractionation of 0.11 ± 0.08% per mass unit was applied to the measured ratios. Pb isotope ratio uncertainties at 95% con dence interval (Table S1, supporting information) are associated with the sample analysis, the isotopic composition of the Pb blank and the instrumental mass fractionation correction.

Validation methodology for decontamination procedures
Although careful elimination of the signi cant contamination from the outside of the core sections were performed by mechanical chiseling as describe above, changes in the measured Pb and Ba concentrations and Pb isotope ratios as a function of radius from the outside to the inside of the core were investigated for the selected core sections to check the e ciency of the decontamination. Examples of such concentrations and isotope pro les are shown in Figure S7      Comparison of 206Pb/207Pb ratios between the pre-MBE and post-MBE intervals as a function of the dust fraction of Pb in the EDC ice core samples. All uncertainties are 95% con dence intervals. The endmembers of volcanic 206Pb/207Pb ratios for the potential volcanic sources come from published literature (see Figure S6). The locations of individual volcanoes are shown in Figure S7.  Comparison of 206Pb/207Pb ratios between the pre-MBE and post-MBE intervals as a function of the dust fraction of Pb in the EDC ice core samples. All uncertainties are 95% con dence intervals. The endmembers of volcanic 206Pb/207Pb ratios for the potential volcanic sources come from published literature (see Figure S6). The locations of individual volcanoes are shown in Figure S7.

Supplementary Files
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