changes in antarctic bottom water formation during

Changes in Antarctic Bottom Water Formation DuringInterglacial PeriodsS. K. Glasscock1, C. T. Hayes1 , N. Redmond1 , and E. Rohde1

1School of Ocean Science and Engineering, Division of Marine Science, The University of Southern Mississippi, StennisSpace Center, MS, USA

Abstract In themodern Southern Ocean and during the last interglacial period, Marine Isotope Stage 5e,there are observations that point to reduced Antarctic Bottom Water (AABW) formation. These reductionsare believed to be driven by an increase in the strength of the Southern Ocean density stratification due toAntarctic ice melt‐induced surface water freshening. Any reduction in AABW formation has importantimplications for global climate as AABW plays a vital role in the cycling of carbon in the world's ocean. Theprimary question this study seeks to answer is do these AABW reductions occur during any of the otherinterglacials of the past 470,000 years? To study AABW changes in the paleoceanographic record, we look atchanges in the redox record. Newly formed AABW is oxygen‐rich, so any reduction should lead to a decreasein oxygen concentrations in the deep Southern Ocean. The trace element uranium is useful for studyingthese redox changes as it is enriched in marine sediments under low‐oxygen conditions. When accountingfor other factors, such as paleoproductivity, that can also decrease the oxygen concentrations in sedimentaryporewater, it is possible to identify changes in AABW using authigenic uranium. The surveyconducted by this study found a possible AABW reduction during late Marine Isotope Stage 11 (~397 ka).The cause of this event is less clear than others studied, and we explore the possibilities of ice melt‐inducedfreshening or a change in the position or strength of the Southern Hemisphere westerly winds.

1. Introduction

As the Earth continues to experience warm climate conditions, it is becoming more important that weunderstand how the Earth will respond to ever‐increasing sea levels and global temperatures. In order tounderstand what the future may hold, we turn to past warm climate, or interglacial, periods for study. Ofparticular interest is Marine Isotope Stage 11 (MIS 11), one of the longest interglacial periods (374–424 ka) with orbital parameters similar to the present interglacial, the Holocene (~11.7 ka to present)(Droxler et al., 2003). Additionally, MIS 11 exhibited atmospheric CO2 levels that were similar to preindus-trial values (Lüthi et al., 2008), and Antarctic temperatures that were ~2.6°C warmer than Holocene condi-tions (Jouzel et al., 2007). These CO2 and temperature conditions led to sea levels that were, at a minimum,close to present values, with some estimates of MIS 11 sea level being 6 to 13 m or as much as 20 m higherthan present (Dutton et al., 2015; Raymo&Mitrovica, 2012). These high values would suggest significant col-lapse of both the Greenland and the West Antarctic Ice Sheets.

The purpose of this study is to determine how deep water formation in the Southern Ocean fared duringinterglacial periods of the past 470,000 years. The formation of deep water masses in the Southern Oceanis of critical importance for global climate because this is the primary location where CO2 sequestered indeep water is returned to the atmosphere via upwelling. The cause of this sea‐to‐air CO2 release is incom-plete nutrient utilization in Southern Ocean surface waters. In this location, deep water is upwelled and,subsequently, downwelled before all of the nutrients can be used by phytoplankton resulting in a net fluxof CO2 from the ocean to the atmosphere. However, over the past 2.5 million years, the time during whichthe Earth has experienced glacial‐interglacial cycling, this has not always been the case. Changes in nutrientutilization and the interaction between upwelling water and the atmosphere in the Southern Ocean are twoof the primary causes of the large amplitude atmospheric CO2 variations between glacial and interglacialperiods (Sigman et al., 2010).

In general, the deep glacial ocean is believed to have been more poorly ventilated than the interglacialocean (Sigman et al., 2010). We have less understanding as to how Antarctic Bottom Water (AABW)

©2020. American Geophysical Union.All Rights Reserved.

RESEARCH ARTICLE10.1029/2020PA003867

Key Points:• Southern Ocean record from

ODP1094 is analyzed for authigenicuranium for past 470 kyr

• Marine Isotope Stages 7 and 9display uranium peaks likely relatedto increased carbon export

• Marine Isotope Stage 11c displays alate peak in uranium likely relatedto a slowdown of AntarcticBottom Water

Correspondence to:C. T. Hayes,[email protected]

Citation:Glasscock, S. K., Hayes, C. T.,Redmond, N., & Rohde, E. (2020).Changes in Antarctic Bottom Waterformation during interglacial periods.Paleoceanography andPaleoclimatology, 35, e2020PA003867.https://doi.org/10.1029/2020PA003867

Received 28 JAN 2020Accepted 25 JUN 2020Accepted article online 2 JUL 2020

GLASSCOCK ET AL. 1 of 13

formation varied within interglacial periods. Today, AABW is formed from cold, salty waters forming inlocalized areas under Antarctic ice shelves (Thompson et al., 2018), as well as in localized areas of deepconvection below sea ice polynyas (Gordon, 2014). One of the most important sites of deep convection isthe Weddell Sea, which produces one of the largest and better ventilated components of AABW, WeddellSea Deep Water (Gordon et al., 2001). The Weddell contribution to AABW has been found to vary withthe strength of the Southern Hemisphere westerly winds, in particular by setting the baroclinicity andwind‐stress curl of the Weddell Gyre (Jullion et al., 2010; Meijers et al., 2016). Thus, with its shelf anddeep convention modes of production, AABW is likely to be sensitive to both freshwater forcing fromthe Antarctic ice sheet and the strength and/or position of the Southern Hemisphere westerly winds.Understanding AABW is becoming ever more important due to new observations and modelingstudies suggesting that anthropogenic global warming is already leading to reductions in deep waterformation around Antarctica and will continue to do so in the future (de Lavergne et al., 2014;Williams et al., 2016).

In addition to the modern reduction in AABW formation, there is strong evidence for a reduction in AABWduring the last interglacial period, Marine Isotope Stage 5 (MIS 5). Using authigenic uranium (aU), a portionof the sedimentary U inventory sensitive to redox conditions, Hayes et al. (2014) recorded low porewater oxy-gen conditions in the sediment core ODP 1094 at ~127 ka. Low oxygen in porewaters could be caused byreduced circulation supplying the oxygen or by increased removal by respiration of organic matter in thesediments. The 127 ka event occurred during a period of relatively low export and delivery of organic carbonto the sediments. In addition, these low oxygen conditions occurred during a period of sea level rise (Hayeset al., 2014). The prevailing hypothesis for the peak in aU at this time is a decrease in the bottom water oxy-gen concentrations due to a decrease in AABW formation caused by surface freshening induced stratifica-tion possibly related to Antarctic ice melt (Hayes et al., 2014). This hypothesis has since been supportedby identification of an Antarctic‐derived ice volume contribution centered on the timing of the aU peak(~127 ka) (Rohling et al., 2019).

The present study uses aU at ODP 1094 as a proxy for AABW formation over the past 470,000 years. This ispossible because the oxygen is delivered to this location primarily by recently ventilated AABW (Figure 1). Itis therefore possible to trace the influence of AABW at this location by looking at the redox record andaccounting for additional factors such as export production.

Figure 1. Location of sediment cores used in this study or mentioned in the text. ODP Sites 1094, 1093, 1091, and 1089 are shown as black, gray, gray, and bluedots, respectively. The two green dots (GeoB3603‐2 and MD96‐2081, comprising a spliced record of Agulhas Leakage Fauna; Peeters et al., 2004) are from nearlythe same location at slightly different water depths. The red dot is IODP Site U1305. The section on the right is dissolved oxygen from GLODAPv2.2019(Olsen et al., 2019) and is indicated on the map on the left by the red polygon. Major water masses Antarctic Bottom Water (AABW), Circumpolar Deep Water(CDW), and North Atlantic Deep Water (NADW) are indicated in the section.

10.1029/2020PA003867Paleoceanography and Paleoclimatology


2. Materials and Methods

The core used to identify the MIS 5 AABW reduction, ODP 1094 (53.18°S, 5.13°E, 2,807 m water depth;Figure 1), was used for this study. The sediment record from this location has demonstrated sensitivity tochanges in ocean circulation, with elevated aU concentrations during the last glacial period and the notedMIS 5 aU peak (Hayes et al., 2014; Jaccard et al., 2016). ODP 1094 is thus in a good position to record changesin AABW. Well within the core of Circumpolar Deep Water (CDW) and proximal to the Weddell basin andassociated locations of AABW formation, ODP 1094 sits in the ice‐free zone of the Antarctic Zone of theSouthern Ocean (Shipboard Scientific Party, 1999). Predominantly composed of siliceous sediments, ODP1094 provides a high‐resolution deep‐sea core, with an average sedimentation rate of ~17 cm/kyr. We ana-lyzed sediment samples within the composite depth range of 28.44 to 76.29 m corresponding to ages of ~156to ~504 ka. Analyzing material from all marine isotope stages between MIS 12 and MIS 5 helps put into con-text how unique events during MIS 11might have been. Samples were taken at ~10 cm intervals, allowing usto construct a record with an age resolution on the scale of hundreds of years. The U concentrations of thesediment samples were processed by published methods using strong acid digestion and column chromato-graphy (Hayes et al., 2017) and analyzed on a Thermo Scientific Element XR inductively‐coupled plasmamass spectrometer (ICP‐MS).

Using the ICP‐MS, the 235U content of the samples was measured by isotope dilution using a spike of 236U(Eckert & Ziegler). Because themost abundant U isotope 238U was not measured, U concentrations were cal-culated using the seawater 238U/235U ratio of 137.824 (Weyer et al., 2008) according to Equation 1:

U concentrationμgg

� �¼

235USample −235UBlank

g sample×

238U.

235USW

1000: (1)

For this study, 83 duplicate and 3 triplicate samples were analyzed. The average standard deviation of the Uconcentrations of these replicates was ~5%. To assess external reproducibility, the gravimetrically calibratedsolution CRM 112a was used, with a concentration of 1 ppb. Measurements of this solution gave us aprecision of ~3%.

In order to determine the aU content of a sediment sample, a correction for the detrital portion of the Uwas made. Typically, this is done by measuring the amount of 232Th, a Th isotope only present in detritalmaterial, and making a correction based on the 238U/232Th ratio of the detritus (Henderson &Anderson, 2003). Concentrations of 232Th were determined by processing samples according to publishedmethods using strong acid digestion and column chromatography (Hayes et al., 2017) for a select group ofsamples. With a known concentration of 232Th, the detrital U concentration can be calculated usingEquation 2:

aUμgg

� �¼ UBulk −

232Th × 0:2429 ×UTh

� �AR

× 1:3297� �

; (2)

where 0.2429 and 1.3297 are constants to convert 232Th from concentration in μg/g to activity in dpm/gand U from activity in dpm/g to concentration in μg/g, respectively, and (U/Th)AR is an estimation ofthe U/Th activity ratio of detrital material. A range of estimations is available for (U/Th)AR between 0.4and 0.8; a value of 0.5 was used here, which is consistent with most other prior U/Th studies in theAtlantic sector of the Southern Ocean (Costa et al., 2020). The mean ± standard deviation of all detritaluranium corrections for the whole core was 9.5 ± 8.3% of the measured uranium. In general, glacial iso-tope stages had a higher detrital correction (13.8 ± 8.0%) than interglacial periods (6.7 ± 7.2%). Thus, theauthigenic uranium estimates for interglacial periods especially are relatively insensitive to the choice ofdetrital U/Th ratio. The 232Th concentrations were only measured for the portions of the core covering0–150 kyr (Hayes et al., 2014) and MIS 11. For the rest of ODP 1094, a detrital correction was estimatedbased on a linear correlation of the existing 232Th data and the amount of ice rafted detritus record forODP 1094 (Kanfoush et al., 2002). The regression equation was 232Th (dpm/g) = (IRD [grains/g] * 0.0033) + 0.0593 (r2 = 0.71, n = 132).



For this study, all the data analyzed from ODP 1094 were aligned with its latest age model based on benthicoxygen isotopes (Hasenfratz et al., 2019) which were tuned to the LR04 benthic stack (Lisiecki &Raymo, 2005).

Additional paleoceanographic proxies were used to determine possible causes in the changes in the redoxconditions of the sediment. Accounting for changes in productivity was particularly important, as increasedbenthic respiration can also drive down porewater oxygen concentrations. A high‐resolution paleoproduc-tivity proxy exists for ODP 1094, the Ba/Fe ratio (Jaccard et al., 2013). Barium and organic carbon fluxes havebeen shown to have a strong correlation (Dymond et al., 1992). By normalizing the Ba to Fe (which is mostlyof detrital origin) to account for detrital Ba, this allows for a qualitative assessment of the export productionfor ODP 1094 (Jaccard et al., 2013). Comparing the aU record to this paleoproductivity proxy allows for theidentification of peaks in aU due primarily to AABW circulation reductions rather than productivitychanges. For instance, periods of high aU concentrations with relatively low Ba/Fe values would mean thatreductive conditions in the sediment could not have been caused by high organic carbon fluxes, thereforemaking them likely candidates for AABW reductions.

Possible AABW reductions require a mechanism to trigger them. In the case of the MIS 5 AABW reduction,it appears that surface freshening due to Antarctic ice melt may have been a possible trigger (Hayeset al., 2014). This event occurred during a period of sea level rise, and glacial meltwater would have strength-ened the density stratification in the Southern Ocean leading to reduced AABW formation. Surface freshen-ing has also been hypothesized to be the cause of modern reductions in AABW formation (de Lavergneet al., 2014). To determine if AABW reductions during previous interglacial periods were related to increasedsurface freshening driven by glacial meltwater inputs, the aU reconstruction was compared to a globalreconstruction of sea level relative to 5 ka (Spratt & Lisiecki, 2016). This sea level reconstruction has millen-nial resolution (1 point per 1,000 years) and is based on multiple proxies, including the benthic δ18O of sea-water, which represents a global signal. For a more local perspective, we also investigated the δ18O of surfaceseawater at ODP 1094 derived from Mg/Ca and δ18O of Neogloboquadrina pachyderma (Hasenfratzet al., 2019).

We also investigated if the strength of the Agulhas leakage circulation could provide any insight as to themechanism of AABW changes. Agulhas leakage is believed to play a large role in the Atlantic meridionaloverturning circulation (AMOC), as it supplies salty water that is advected to the North Atlantic, leadingto denser surface water that can be more readily incorporated into North Atlantic Deep Water (NADW)(Knorr & Lohmann, 2003; Weijer et al., 2001, 2002). The strength of the Agulhas leakage is influenced byoceanic and atmospheric factors. These include the position of the Southern Subtropical Front and thestrength and position of the Southern Hemisphere westerly winds. More southerly positions of the subtropi-cal front and the westerlies are believed to lead to more Agulhas leakage (Beal et al., 2011; Biastochet al., 2009; Caley et al., 2012; Peeters et al., 2004). However, this interpretation is based largely on observa-tions from the paleorecord. In comparison, ocean modeling studies of the Agulhas leakage suggest a morecomplex picture and some contrasting views. For instance, it was suggested that the strength of the wester-lies could be a more important factor than latitudinal position (Durgadoo et al., 2013). Furthermore, it wasfound that actually an equatorward shift in the westerlies could increase Agulhas leakage (Durgadooet al., 2013). Further still, it has been found that a stronger Agulhas Current could lead to less Agulhas leak-age (Van Sebille et al., 2009). These uncertainties aside, we use the Agulhas leakage record as some indica-tion of changing in the strength and/or position of the Southern Hemisphere westerly winds as this relates tothe strength of the AMOC.

We compared our data from ODP 1094 to a record of Agulhas Leakage Fauna (ALF), defined as an assem-blage of planktic foraminiferal species that are typically found in tropical‐subtropical regions but that aretransported through eddies with the Agulhas Current from the Indian Ocean into the subpolar CapeBasin of the South Atlantic (Peeters et al., 2004). This ALF record is a spliced record from GeoB‐3603‐2(35.13°S, 17.54°E, 2,840 m water depth) and MD96‐2081 (35.58°S, 17.68°E, 3,164 m water depth)(Figure 1). The splice was made during Marine Isotope Stage 7e or 235 ka, with MD96‐2081 representingthe older portion of the record back to 560 ka. This record had an older age model (Peeters et al., 2004)and thus it was necessary to update it for detailed comparisons in the sections around MIS 11 (containedentirely within the MD96‐2081 portion) by aligning its benthic oxygen isotope record with the LR04



benthic stack. The new tie points for MD96‐2081 used in this studyare reported in Table 1. A linear interpolation was used to derive agesbetween tie points.

Other factors that could possibly affect AABW formation includechanges in NADW formation. An antiphase relationship has beenobserved in the temperature records of the Northern and SouthernHemispheres during the last deglaciation, suggesting that increasesin deep ocean ventilation in one region are accompanied by decreasesin the other region (Broecker, 1998). To assess the influence ofNADW and AABW throughout the Atlantic, we consulted thebenthic record of δ13C from ODP Site 1089 (40.93°S, 9.88°E,

4,621 m water depth), one of the more highly resolved records in the South Atlantic (Hodell, Charles,et al., 2003) and IODP Site U1305 (57.48°N, 48.53°W, 3,459 m water depth), a very highly resolved drift sitein the far North Atlantic (Galaasen et al., 2020) (see Figure 1 for locations). The existing age models for bothof these cores were aligned well with the LR04 benthic stack. Benthic δ13C can be useful in assessing theinfluence of AABW versus NADW as the two have distinct isotopic values, with NADW being higher dueto greater nutrient utilization at its formation sites (Hodell, Venz, et al., 2003).

3. Data Availability Statement

New data presented here are archived at the NOAA Paleoclimatology database (https://www.ncdc.noaa.gov/paleo/study/28691).

4. Results

Figure 2 displays the aU record developed by this study (~156–470 ka) and Hayes et al. (2014) (~1.22–158 ka)along with paleoproductivity (Ba/Fe), global sea level, and atmospheric CO2 records. This allows forlarge‐scale observations of how the aU record at ODP 1094 is related to ocean circulation, global climate,and Southern Ocean productivity. The ODP 1094 aU record displays elevated values during glacial periods(yellow highlighted areas), when sea level is low and deep ocean circulation is sluggish leading to increaseddeep ocean carbon storage, and depleted deep ocean oxygen concentrations. The other major features of theaU record are the aU peaks associated with periods of high productivity typically during deglaciations(orange highlighted areas, or Terminations II, III, IV, and V). Increased organic matter delivery to the sedi-ment results in increased respiration in the sediment and an associated reduction in porewater oxygen con-centrations, thus leading to the precipitation of aU.

For MIS 7 and 9 (shown in greater detail in Figure 3), large aU peaks are associated with periods character-ized by rising sea levels and maximum atmospheric CO2 values. The aU peaks centered on the glacial termi-nations are highly correlated with the Ba/Fe record, making it likely that these particulate aU enrichmentsare produced by excess organic matter delivery to the sediment and subsequent drawdown of pore water oxy-gen. There is not a simple linear scaling between the magnitude of the Ba/Fe and aU peaks, and more quan-titative information about porewater or bottomwater oxygen concentrations cannot be drawn. Interestingly,during the glacial‐like period of MIS 7d (220–224 ka), there is another aU peak with low Ba/Fe values, indi-cating a likely reduction in bottom water oxygen concentrations driven by the sluggish glacial circulation ofthe Southern Ocean, similar to that seen in glacial stages MIS 3/4, 6, 8, 10, and 12 (Figure 2).

For MIS 5, though it is difficult to visualize at the scale of Figure 2, the aU record also shows an initial pro-ductivity peak, along with a second, immediately following peak associated with an AABW reduction. Thisis discussed and plotted in greater detail by Hayes et al. (2014).

The MIS 11 record displays some features not typical of the other interglacials analyzed for this study. Themajority of our discussion of this record will focus on the first 30,000 years of MIS 11, characterized bythe broad minimum in ice volume, sometimes referred to as MIS 11c (Candy et al., 2014). First, the produc-tivity history does not display the same trend as MIS 7 or 9, with large increases leading into the interglacial(Figure 2). Instead, MIS 11 displays elevated Ba/Fe values for a longer period of time, indicating moreextended and potentially pulsed productivity events in comparison to the other interglacials. We note a

Table 1New Tie Points to Update Existing Age Model for MD96‐2081 (Peeters et al., 2004)During MIS 11 for Comparison With ODP‐1094 Records

Age (ka) Depth in MD96‐2081 (cm)

341.2 1,373385 1,508391 1,548402 1,593405 1,618410 1,633427 1,703



caveat for MIS 11 in that the X‐ray fluorescence (XRF) record of Ba/Fe is known to contain an importanthiatus due to the presence of opal CT concretions at approximately 401–403 ka, and this could obscurepeak Ba/Fe values (Figure 4). The extended period of high productivity contains three peaks of aUcentered around 422, 411, and 405 ka (orange highlighted areas in Figure 4) that are likely influenced byorganic matter oxidation, and we cannot decisively ascribe AABW circulation changes here. There is,however, a later peak in aU after the period of peak productivity (purple highlighted area ~397 ka) thatwe hypothesize may have been related to AABW reduction.

5. Discussion

The ~397 ka aU/low oxygen event requires an explanation other than enhanced productivity. Even thoughthe relationship between aU and productivity is nonlinear, it is difficult to reconcile increasing aU concen-trations with decreasing productivity (Figure 4). Therefore, the oxygen concentration in the deep water

Figure 2. Overview of the ODP 1094 aU record (Hayes et al., 2014, 0–158 ka, red line and this study, 156–470 ka, black line) for Marine Isotope Stages (MIS) 1 thru12 in relation to (from bottom up) paleoproductivity (Ba/Fe) from ODP 1094 (Jaccard et al., 2013, dark green line), Agulhas Leakage Fauna (ALF) (light greenline) from GeoB‐3603‐2/MD95‐2081 (Peeters et al., 2004), atmospheric CO2 (Lüthi et al., 2008, gray line) and global sea level relative to 5 ka (Spratt &Lisiecki, 2016, blue line). The orange highlighted areas indicate likely productivity‐induced aU enrichments, the yellow highlighted areas show glacial aUenrichments, and the thin purple highlighted areas in MIS 5 and 11 show possible circulation induced aU enrichments.



surrounding ODP 1094 must have been reduced during this time, and this event is a likely candidate for anAABW reduction.

Further evidence for this hypothesis comes from the ice core CO2 record and the Ca/Fe record of ODP 1094(Figure 4). This aU peak occurs during a period of decreasing atmospheric CO2 concentrations, reflectingincreased carbon storage in the deep ocean (as observed during MIS 5e by Hayes et al., 2014).Additionally, immediately after the aU peak, there is a slight increase in atmospheric CO2, perhaps due toincreased ocean–atmosphere exchange. However, these features occur during a more general decrease inatmospheric CO2. With regard to the Ca/Fe record, Jaccard et al. (2013) showed from the million year recordat ODP 1094 that sediments at this site are relatively low in CaCO3 for the majority of the Pleistocene andlarge deglacial peaks of enhanced carbonate preservation occurred during deglacial releases of CO2 fromthe deep Southern Ocean. This CO2 release must have been accompanied by relatively vigorous overturningin the Southern Ocean. The precipitous drop in Ca/Fe during the 397 ka aU peak is consistent with reductionin Southern Ocean ventilation causing reduced carbonate preservation (Figure 4).

An important process to consider in interpreting authigenic uranium is postdepositional remobilization ofuranium within the core. For instance, we know that authigenic uranium forms not at thesediment‐water interface but at some subsurface depth that is mainly a function of the core's accumulationrate. This has the potential to make aU peaks appear at an older age than concurrent processes recorded bythe surface sediment (such as organic matter deposition). We assess this effect by calculating the potentialdepth offset between the sediment water interface and depth of authigenic uranium enrichment usingEquation 3 (François et al., 1993):

zu¼ D × Usw

S × ρ × aU; (3)

where D is the diffusitivity for UO22+, Usw is the U concentration of seawater, S is the sedimentation rate,

ρ is the dry bulk density, and aU is the concentration of authigenic uranium in the sediment. This depth

Figure 3. Detailed view of Marine Isotope Stages (MIS) 7 (left) and 9 (right) of (bottom to top) authigenic uranium (aU) and the Ba/Fe productivity proxy fromODP 1094, atmospheric CO2 (Lüthi et al., 2008), and global sea level relative to 5 ka (Spratt & Lisiecki, 2016). The orange bars indicate productivity‐drivenenrichments in aU, and the yellow bar indicates the glacial‐like aU enrichment likely due to sluggish Southern Ocean circulation during MIS 7d.



offset, zu, represents the distance over which dissolved U is diffusing between the sediment‐water interfaceand the maximum depth at which authigenic uranium is precipitated in the sediments. As this study alsodeals with changes in the bottom water oxygen concentration which would affect this depth, it isimportant to note that suboxic bottom water conditions should only reduce zu if they could also beaccounted for in this equation. Our assumption is that we cannot resolve uranium enrichment eventsthat occur over a distance shorter than zu. The closest significant uranium feature to the 397 ka event isthe preceding aU peak around 401 ka. These two aU peaks are 1.86 m apart (Figure 5), whereas thecalculated zu values for this section of the core are ~15–25 cm. This is another way of showing thethese two aU peaks, separated by 1.86 m in distance and 4 kyr in time, are likely not significantly

Figure 4. Relationship between (from bottom to top): authigenic uranium (aU), Ba/Fe and Ca/Fe ratios measured byX‐ray fluorescence (XRF) scanning (Jaccard et al., 2013), the δ18O of surface seawater (ssw) (Hasenfratz et al., 2019),atmospheric CO2 (Lüthi et al., 2008), and global sea level relative to 5 ka (Spratt & Lisiecki, 2016) during MIS 11. Notethere is a break in the XRF data between 401 and 403 ka indicated by a white box. The purple highlighted area shows apossible AABW reduction, while the orange highlighted areas show likely productivity‐induced uranium enrichments.



influenced by remobilization. During this section of the core, the sedimentation rate is 44 cm/kyr,meaning the zu corresponds to a potential age offset of 0.3–0.6 kyr between the uranium and the Ba/Ferecords, which is relatively minor considering the resolution of our record (Figure 4). We do findpotential depth offsets of up to 1 m during glacial periods due to lower sedimentation rates, and thisshould be taken into account when interpreting millennial‐scale events during glacial periods in this core.

Taking for granted the observations in ODP1094, if the MIS 11 aU peak is the result of a reduction in AABW,what could have been the trigger? During MIS 5, the trigger appears to have been increased density stratifi-cation due to surface freshening from Antarctic ice melt (Hayes et al., 2014; Rohling et al., 2019). The evi-dence supporting this cause during the MIS 11 event is less clear (purple highlighted area in Figure 4).Global sea level was falling across the 397 ka aU event, arguing against a substantial contribution ofAntarctic ice melt at that time. However, the effect of global averaging may make it difficult to pinpointan Antarctic event, so the surface freshening hypothesis cannot be fully excluded. In fact, the δ18O of surfaceseawater (δ18Ossw, largely a function of salinity) from ODP 1094 (estimated by combining Mg/Ca tempera-ture and δ 18O of N. pachyderma; Hasenfratz et al., 2019) suggests a multimillennial reduction in sea surfacesalinity across the 397 ka (Figure 4). Propagated analytical and calibration uncertainties in the estimate ofδ18Ossw are 0.25‰ (or about 25% of the observed reduction), and this uncertainty is likely an underestimateconsidering some difficulty in defining species‐specific offsets discussed by Hasenfratz et al. (2019).Additionally, there is no δ18Ossw data available between 396 and 358 kyr, likely due to the poor foraminiferapreservation during that time, meaning that the putative reduction in sea surface salinity is not fullyresolved.

In an alternative to surface freshening due to ice melt, here we explore another possible mechanism to sup-port the AABW reduction hypothesis, relating to a millennial‐scale northward shift or weakening of theSouthern Hemisphere westerlies. In fact, the reduced salinity around 397 ka inferred from the δ18Ossw datacould also have been consistent with a northward shift in the Southern Hemisphere fronts, as in the modernSouthern Ocean there is an increase in salinity from the Antarctic to the Subantarctic. The primary evidencewe discuss, however, for the frontal change possibility comes from the ALF proxy data described in section 2.ALF tends to be highest at the onset of interglacial periods (Peeters et al., 2004) (Figure 2). While there is adrastic increase in ALF at the onset of MIS 11, peak ALF values are not seen until late in the interglacial

Figure 5. The potential depth offset (zu) between the sediment‐water interface and authigenic uranium (aU)enrichments, highlighting the distance between the late MIS 11 aU peaks in ODP 1094.



stage, around 401–395 ka, closely corresponding to the timing of the 397 ka aU event (Figure 6). Thisincrease in ALF is thought to have led to a maintenance of a strong AMOC late in the interglacial periodpotentially caused by stronger wind systems (Dickson et al., 2010). A strong AMOC, leading to upwellingof warm CDW that can infiltrate to the Antarctic shelves, could exacerbate basal melting of the AntarcticIce Sheet (Thompson et al., 2018), potentially providing a link to a freshwater‐driven disruption of AABWformation. However, since productivity at ODP 1094 is fueled by upwelling CDW (Anderson et al., 2009;Jaccard et al., 2013), the ODP 1094 Ba/Fe productivity record itself argues that upwelling peaked roughly4 kyr prior (~401 ka) and was declining during the 397 ka aU event. Perhaps a northward shift orweakening of the Southern Hemisphere westerlies moved the region of upwelling and peak productivitynorth of ODP 1094 during this time. This would in effect reduce AABW formation by reducing deepconvection contributions to AABW such as from the Weddell Sea, without requiring a large Antarcticmelt water event. This potential change in the winds goes against the original interpretations of Biastochet al. (2009) and Peeters et al. (2004) that ascribed a southward shift of the westerlies to increased Agulhasleakage. However, given the current uncertainty in modeling the connection between Agulhas leakageand the wind systems (section 2), we hypothesize that a northward shift or weakening of the SouthernHemisphere westerlies could be consistent with increased Agulhas Leakage and reduced AABW formation.

Figure 6. Marine Isotope Stage 11 behavior of (bottom to top) ODP 1094 authigenic uranium (aU, this study), AgulhasLeakage Fauna (ALF) from MD96–2081 (Peeters et al., 2004), δ13C of benthic foraminifer Cibicidoides wuellerstorfi fromODP 1089 (Hodell, Charles, et al., 2003), δ13C of C. wuellerstorfi from IODP Site U1305 (Galaasen et al., 2020), andbenthic δ18O from all cores compared with the LR04 stack (Lisiecki & Raymo, 2005) to demonstrate age modeluncertainty.



Reconstructions of the position of the SouthernHemisphere frontal systems duringMIS 11 are relatively rarein the paleoclimate record. In one significant example, Kemp et al. (2010) attempted this based on mappingthe presence of productivity‐driven laminated diatom mats at ODP Sites 1091, 1093, and 1094 (see Figure 1for locations). Kemp et al. suggested there was a large‐scale southward shift of the Antarctic Polar Front fromamore northerly position during 700–420 ka to more southerly position from 420 ka to present (i.e., the tran-sition is thought to have occurred right at the MIS 12/11 deglacial boundary). This observation would arguein favor of weakened Southern Hemisphere westerlies rather than northward shift during the 397 ka aUpeak, though the diatom‐mat mapping technique is qualitative and not done on a millennial timescale.Nonetheless, a slight reduction in summer sea surface temperature (based on diatom assemblages) atODP 1094 during the latter part of MIS 11c (Schneider‐Mor et al., 2005) also supports the possibility of amillennial‐scale northward frontal shift.

We turn now to discussion of Atlantic benthic δ13C data for further investigation between the possible con-nections between AMOC and the 397 ka aU event. NADW carries with it a high δ13C signature, due to its lowpreformed nutrient content. Therefore, we also might expect to see increased NADW input into theSouthern Ocean during an AABW slow down as was observed during MIS 5e (Hayes et al., 2014).

The δ13C record of the Cape Basin record from ODP 1089 (Hodell, Charles, et al., 2003) demonstrates a per-iod where benthic δ13C is high roughly coinciding with the late MIS 11 period of elevated ALF and aU(~397–403 ka) (Figure 6). While this does not necessarily confirm the connection, it does fit with the predic-tion that increased Agulhas leakage would lead to greater NADW formation during this time (Hodell,Charles, et al., 2003; Peeters et al., 2004). There is a significant decrease in δ13C following the ~397 ka event,possibly indicating a resumption in AABW formation. Furthermore, the benthic δ13C record from the NorthAtlantic IODP Site U1305 (highly‐resolved, with the three‐point running mean representing roughly 250 yrspacing) displays several centennial‐scale shifts demonstrating some interglacial instability in NADW for-mation (Galaasen et al., 2020). There is a shift to higher, North Atlantic‐like δ13C values at around 401 ka,consistent with the ALF data indicating a strong AMOC at that time. However, Site U1305 indicates a strongNADW until the end of the measured values in MIS 11c, not reflecting a strong resumption in AABW andconcomitant NADW slowdown as was seen around 125 ka after the 127 ka aU event in ODP 1094 duringMIS 5e (Galaasen et al., 2014). There is likely age model uncertainty of at least 1 kyr (see the comparisonof the benthic δ18O records in Figure 6) in comparing the U1305 record with the less highly resolved ODP1094 and 1089 records. Nonetheless, the benthic δ13C records of the deep Atlantic at least are not inconsis-tent with a possible increase in AMOC during the 397 ka event that could have interrupted AABW formationeither through accelerated Antarctic ice melting or a northward and/or weakened Southern Hemispherewesterly wind field.

6. Conclusions

Large‐scale observations of glacial‐interglacial AABW variability show reduced Southern Ocean ventilationduring all glacial periods of the past 470,000 years due to their elevated aU concentrations. The interglacialstages in general show a better ventilated Southern Ocean with relatively little aU enrichments. Deglacialpeaks in Southern Ocean productivity caused aU enrichments due to increased organic matter delivery ateach of the past five glacial terminations.

During the later stages of the interglacial period MIS 11c, an abrupt low bottomwater oxygen event occurredthat suggests a reduction in AABW around ~397 ka. This is the second such interglacial AABW reductionfound in the aU record of ODP 1094, with the other reduction (~127 ka) occurring during MIS 5e (Hayeset al., 2014). While both events are characterized by low bottom water oxygen conditions, they may have dif-ferent driving mechanisms. The MIS 5e event was likely caused by a decrease in Southern Ocean surfacedensity due to Antarctic ice melt. Antarctic ice melt may have also impacted the late MIS 11c AABW produc-tion, though current sea level records cannot currently pinpoint an Antarctic melt event. Alternatively, theMIS 11c event could possibly have been caused by a change in the intensity or location of the SouthernHemisphere westerly winds, reducing the Weddell Sea contribution to AABW.

This MIS 11 event may not be exactly analogous to the present interglacial, as the westerlies are predicted tomove poleward and intensify, associated with the current recovery of stratospheric ozone loss (Russell



et al., 2006; Thompson & Solomon, 2002). In fact, this effect is hypothesized to overcome the increase in den-sity stratification occurring in the modern Southern Ocean (Russell et al., 2006). However, this still raisessome perplexing questions as to what happened during late MIS 11 that caused a different response inAABW compared to the other interglacials of the past 500 ka. The following questions will be fruitful targetsof future research.

Is theMIS 11 AABW reduction truly driven by some change in the Southern Hemisphere westerlies, and if sowhat kind?

What could cause the westerlies to move or weaken so abruptly?

Could future sea level records reveal an Antarctic ice melt event around 397 ka?

Was this same mechanism at work during MIS 5, working in concert with Southern Ocean surfacefreshening?

It is important that we understand the mechanisms at work during these past interglacial periods, and par-ticularly MIS 11, as it has orbital parameters similar to the Holocene and higher‐than‐present sea levels andit may give some insight into future climatic conditions.

Data Availability Statement

Data are archived at theNOAAPaleoclimatology database (https://www.ncdc.noaa.gov/paleo/study/28691).

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changes in antarctic bottom water formation during

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