We present downcore records of redox-sensitive authigenic uranium (U) and manganese (Mn) concentrations based on five marine sediment cores spanning a meridional transect encompassing the Subantarctic and Antarctic zones in
the southwestern Indian Ocean covering the last glacial cycle. These records
signal lower bottom water oxygenation during glacial climate intervals and
generally higher oxygenation during warm periods, consistent with
climate-related changes in deep-ocean remineralized carbon storage. Regional
changes in the export of siliceous phytoplankton to the deep sea may have
entailed a secondary influence on oxygen levels at the water–sediment
interface, especially in the Subantarctic Zone. The rapid reoxygenation
during the deglaciation is in line with increased ventilation and enhanced
upwelling after the Last Glacial Maximum (LGM), which in combination
conspired to transfer previously sequestered remineralized carbon to the
surface ocean and the atmosphere, contributing to propel the Earth's climate out of the last ice age. These records highlight the still insufficiently documented role that the Southern Indian Ocean played in the air–sea partitioning
of CO2 on glacial–interglacial timescales.
Introduction
On glacial–interglacial timescales, the ocean plays a dominant role in
regulating changes in the global carbon cycle (e.g., Sigman and Boyle, 2000),
as the deep ocean has a sufficiently voluminous and dynamic carbon reservoir
to modulate the air–sea partitioning of CO2 and by inference climate.
In particular, the Southern Ocean acts as a major conduit connecting the
vast ocean interior and the atmosphere, as deep CO2-rich water masses
outcrop along tilted density surfaces (isopycnals) promoting exchange with
the atmosphere (Marshall and Speer, 2012; Talley, 2013).
Accordingly, a number of distinct and often synergistic mechanisms, focusing on
changes in Southern Ocean circulation, nutrient biogeochemistry, and sea ice
dynamics have been proposed to have contributed to lower atmospheric
CO2 during past ice ages (e.g., Adkins, 2013; Ferrari et al., 2014; Hain et al., 2010; Sigman et al., 2010, 2021). However, the mechanisms accounting for the generally reduced glacial atmospheric CO2 inventory are still debated and not yet fully resolved. Radiocarbon (14C) data suggest that the deep (> 1000–1500 m) ocean was generally more poorly
ventilated during the last ice age than during the Holocene (Sarnthein et
al., 2013; Skinner et al., 2017) (although a portion of this signal could be
related to decreased air–sea gas exchange; Galbraith et al., 2015). The
formation of saltier (less buoyant) bottom waters around Antarctica due to
more dynamic sea ice cycling would have strengthened the vertical
stratification and isolation of deeper waters during the Last Glacial
Maximum (LGM) (Adkins, 2013; Adkins et al., 2002; Bouttes et al., 2010;
Ferrari et al., 2014; Stein et al., 2020). Furthermore, a northward shift of
the upwelling region might have led to the exposure of shallower waters,
resulting in reduced CO2 outgassing and enhanced carbon sequestration
in the ocean interior (Sigman and Boyle, 2000; Toggweiler, 1999; Toggweiler
et al., 2006; Watson et al., 2015).
In addition to these physical mechanisms affecting ocean circulation,
changes in marine biology and nutrient biogeochemistry further contributed
to sequester carbon away from the atmosphere (Francois et al., 1997;
Galbraith and Jaccard, 2015; Sigman and Boyle, 2000; Sigman et al., 2010,
2021). A generally more efficient biological carbon pump during glacial
periods, sustained by generally more complete nutrient utilization and/or
increased Fe-bearing dust supply would have contributed to curb CO2
outgassing from the Southern Ocean (e.g., Ai et al., 2020; Galbraith and
Skinner, 2020; Gottschalk et al., 2016; Jaccard et al., 2013; Kohfeld et
al., 2005; Kumar et al., 1995; Martínez-García et al., 2014;
Sigman et al., 2010; Studer et al., 2015).
At the onset of the last glacial termination (TERM I) approximately 17.5 ka
ago, Southern Ocean ventilation resumed as the Earth emerged from the last
ice age, and previously sequestered, radiocarbon-depleted CO2 was
released to the atmosphere (e.g., Basak et al., 2018; Bauska et al., 2016;
Burke and Robinson, 2012; Gottschalk et al., 2016, 2020b; Jaccard et al.,
2016; Rae et al., 2018; Skinner et al., 2010). Coupled with enhanced
upwelling, nutrient- and CO2-rich subsurface waters were transported to the sunlit surface ocean, supporting high levels of biological production
with less complete nutrient consumption south of the Polar Front (e.g.,
Anderson et al., 2009; Frank et al., 2000; Jaccard et al., 2013; Kohfeld et
al., 2005; Thöle et al., 2019). At the same time, the Fe-bearing dust supply
started to dwindle, causing biogenic export production to decline in the
Subantarctic Zone of the Southern Ocean (Anderson et al., 2014; Jaccard et
al., 2016, 2013; Martínez-García et al., 2014; Thöle et al.,
2019), further decreasing marine carbon storage. In combination, these
processes would thus have contributed to increase atmospheric CO2
concentrations, providing the necessary impetus for Earth's climate to
transition out of the last ice age.
While this narrative represents the prevailing regionally integrated
understanding of the leverage the Southern Ocean bears on the air–sea
partitioning of CO2 across the last deglacial termination, observations
are largely based on records from the southern Atlantic. Records from the
Pacific and Indian sectors of the Southern Ocean are consistent with the
first-order paleoceanographic evolution described above, but regional
specificities exist. In particular, it remains unclear how export production
patterns vary regionally in the Indian sector of the Southern Ocean
characterized by a complex frontal structure (e.g., Durgadoo et al., 2008).
Additionally, the source of bioavailable Fe, which is crucial to sustain
phytoplankton growth, may vary regionally. Indeed, observations suggest that
a large fraction of the Fe supplied to the open Indian Ocean may arise from
deep winter mixing (Tagliabue et al., 2014).
Reconstructing past changes in bottom water oxygenation has the potential to
further disentangle and offer a clearer understanding of some of these
processes. Variations in the storage of respiratory carbon are accompanied
by large changes in dissolved oxygen concentration associated with organic
matter remineralization (e.g., Anderson et al., 2019; Gottschalk et al.,
2016, 2020b; Hoogakker et al., 2015; Jaccard et al., 2016; Jacobel et al.,
2017). The temporal evolution of bottom water oxygenation can thus be
reconstructed qualitatively using the distribution of redox-sensitive metals
in the marine sedimentary record (e.g., Calvert and Pedersen, 1996; Francois
et al., 1997; Frank et al., 2000; Nameroff et al., 2002). Here, we focus on
authigenic uranium (U) and manganese (Mn), which are both sensitive to
dissolved oxygen concentrations typically encountered in open-ocean
conditions. The analyses were carried out on a set of five marine sediment
cores spanning a meridional transect in the still underrepresented Indian
sector of the Southern Ocean. Combining these observations with preserved
opal flux reconstructions allows for deciphering the different processes
affecting bottom water oxygenation and inferring their relative
contributions in sequestering CO2 in the ocean interior, away from the atmosphere. The core site locations were characterized by temporally
contrasted export production patterns (Manoj and Thamban, 2015; Nair et al.,
2019) and thus provide an interesting case study to critically test some of
the prevailing assumptions underlying the sequence of events, which lead the
Earth's climate to transition into the last ice age.
Study site, materials, and methodsCore locations and material
The five marine sediment cores were retrieved along a meridional transect
including Del Caño Rise and Conrad Rise and reaching as far south as
Enderby Abyssal Plain in the SW Indian Ocean (Fig. 1). Cores DCR-1PC
(46∘01.34′ S, 44∘15.24′ E, 2632 m b.s.l.) and COR-1bPC (54∘16.04′ S, 39∘45.98′ E, 2828 m b.s.l.) were collected during expedition KH-10-7 on R/V Hakuho-maru in 2010–2011. The sediment cores
were retrieved from the southern flank of Del Caño Rise (DCR-1PC) and
Conrad Rise (COR-1bPC), respectively. Cores PS2609-1 (51∘29.9′ S, 41∘35.8′ E, 3113 m b.s.l.), PS2606-6 (53∘13.9′ S,
40∘48.1′ E, 2545 m b.s.l.), and PS2603-3 (58∘59.2′4 S,
37∘37.7′ E, 5289 m b.s.l.) were retrieved during ANT-XI/4
expedition on R/V Polarstern in 1994. Cores PS2609-1 and PS2606-6 were retrieved
from Conrad Rise as well: the former on its northern flank and the
latter on the rise itself. Core PS2603-3 is located furthest to the south
in the Enderby Abyssal Plain and at the greatest water depth of the five
cores. All cores are bathed by Circumpolar Deep Water (CDW) with the
exception of core PS2603-3, which is influenced by Antarctic Bottom Water
(AAWB) (Fig. 1). CDW comprises variable contributions of North Atlantic
Deep Water (NADW), as well as recirculated deep water originating from the
Pacific and Indian Ocean basins. CDW is typically depleted in dissolved
oxygen as the water mass is not ventilated at the ocean surface but instead
forms as a blend of pre-aged subsurface water masses.
(a) Core locations in the SW Indian Ocean across the Southern Ocean frontal system. The fronts from north to south are the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF), and the Southern Antarctic Circumpolar Current Front (SACCF). The zones between them are defined as the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), Antarctic Zone (AZ), and Continental Zone (CZ) (Orsi et al., 1995). (b) Cross section of core locations with modern oxygen concentrations (plotted with the ODV software, Schlitzer, 2018).
DCR-1PC is the northernmost core and lies in the Subantarctic Zone (SAZ)
of the Southern Ocean; it is composed of nannofossil and diatom ooze with
variable amounts of clay. All other cores predominantly consist of diatom
ooze (Kuhn, 2003a, b, c; Oiwane et al., 2014) with variable amounts
of lithogenic material and lie south of today's position of the Polar Front
(PF) in the Antarctic Zone (AZ).
Age models
For DCR-1PC, seven radiocarbon (14C) ages were obtained using
mono-specific samples of planktic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma (sinistral) using the accelerator
mass spectrometry (AMS) facilities at the University of Tokyo (Crosta et
al., 2020). Calibration of the 14C measurements was performed using the
CALIB7.02 software and the Marine13 calibration curve (Reimer et al., 2013)
applying a regionally informed marine reservoir age correction of
890 ± 100 years (Butzin et al., 2005). The Marine13 calibration curve
was given preference because the Marine20 curve is not adequate for
application in polar regions characterized by variable sea ice extent
(Heaton et al., 2020). In addition to the 14C dates, the diatom-based
sea surface temperature (SST) record was graphically aligned to the EPICA
Dome C deuterium (δD) record, assuming both records are synchronous
(Crosta et al., 2020). It is worth noting that the depth interval 33–41 cm is
characterized by a substantial reduction in sedimentation accumulation
rates, possibly reminiscent of a sedimentary hiatus (Fig. 2).
Age versus depth of each core.
The stratigraphy for core COR-1bPC is based on 23 calibrated
14C measurements using mono-specific samples of planktic foraminifera
Neogloboquadrina pachyderma (sinistral) (Oiwane et al., 2014). All dates were corrected for the regional reservoir age (890 years) (Bard, 1988) and converted to calibrated years before present (cal yr BP) using
the CALIB 6.1.0 software package (Stuiver and Reimer, 1993).
Tie points of cores PS2609-1, PS2606-6, and PS2603-3.
MagSus stands for magnetic susceptibility, BSiO2 stands for biogenic silica, LR04 is the
global benthic δ18O LR04 stack (Lisiecki and Raymo, 2005), and EDC
is Epica Dome C (Lambert et al., 2012).
Regarding the PS cores, preliminary age pointers were based on available
radiocarbon dates (Xiao et al., 2016) and biostratigraphic constraints
(Table 1). The 14C measurements were obtained via AMS using the sedimentary
humic acid fraction in the absence of sufficiently well-preserved
foraminiferal carbonate. Radiocarbon ages were calibrated using
the CALIB4.2 (Stuiver et al., 1998) software package after applying a
reservoir age correction of 810 years (Bard, 1988). The preliminary age models were first refined by graphically aligning biogenic opal (BSiO2)
measurements to the LR04 δ18O benthic stack, assuming an
in-phase relationship. This approach inherently assumes that sedimentary
BSiO2 concentrations are modulated by climate variability in the
Southern Ocean (e.g., Hasenfratz et al., 2019) and more specifically in the
Indian sector of the Southern Ocean (Kaiser et al., 2021; Manoj and Thamban,
2015). Similarly, the sedimentary magnetic susceptibility (MagSus) signal
contains a coherent climate-related component and was thus used for initial
age model tuning of cores PS2609-1 and PS2606-6 (e.g., Weber et
al., 2012, 2014). These age solutions were then further refined by
graphically aligning the X-ray fluorescence (XRF) Ca / Ti and Ti records to the EPICA Dome C (EDC) dust record (Lambert et al., 2012) for cores PS2609-1 and PS2606-6
(Table 1) and assuming an in-phase relationship between both proxies and
archives (e.g., Martínez-Garcia et al., 2014; Lamy et al., 2014). For
the age model for core PS2603-3, the extinctions of three diatom species
served as additional biostratigraphic markers (Table 1): Rouxia leventerae at 130 ka,
Hemidiscus karstenii at 191 ka, and Rouxia constricta at 300 ka (Zielinski and Gersonde, 2002). There is evidence
for a 30 kyr sedimentary hiatus associated with a sediment disturbance
at the marine isotope stage (MIS) 5–MIS 4 boundary. Independent absolute age constraints with the
constant rate supply (CRS) model (Geibert et al., 2019) yielded similar ages
for PS2603-3, except for the interval around the hiatus. Finally, the age
models have been tested by comparing the solutions to independently defined
age models. Specifically, our age model for PS2606-6 is very similar to
the stratigraphic framework published by Ronge et al. (2020). The age model
for core PS2603-3, which arguably contains the fewest tie points, was
critically assessed using an independent approach based on CRS (Geibert et
al., 2019). Both approaches provided very similar ages, with age offsets of
< 1.5 kyr for the last 20 kyr. In summary, the presented age models
may certainly be perfectible, but given the constraints and limitations they are
probably realistic and permit meaningful regional comparisons on
multi-millennial timescales.
Bottom water oxygenation proxies
In oxygenated seawater, uranium (U) is present as soluble U(VI). In
oxygen-depleted environments, however, U is reduced and precipitated as
insoluble U(IV) in the form of uraninite (Langmuir, 1978; Morford and
Emerson, 1999). Uranium concentrations in sediment porewaters decreases
under reducing conditions, creating a concentration gradient between bottom
waters and the uppermost sediment layers. This gradient leads to the
diffusion of dissolved U into the sediment and to the precipitation of
authigenic U (aU) phases (Klinkhammer and Palmer, 1991; Langmuir, 1978).
The authigenic fraction of U can be determined by calculating the excess
238U relative to detrital thorium (232Th), which is done by assuming a constant,
regionally informed detrital 238U /232Th ratio. This ratio can vary locally, with lower values generally associated with crustal lithogenic
sources. We considered a ratio of 0.5 for cores DCR-1PC, COR-1bPC,
PS2909-1, and PS2909-6 (Francois et al., 2004; Henderson and Anderson,
2003). For PS2603-3, the core farthest to the south and potentially
influenced by lithogenic material originating from the Antarctic continental
crust, the minimum 238U /232Th activity ratio observed is
0.27 ± 0.01. We therefore set the lithogenic end-member
conservatively to 0.27.
aU=AU238total-ATh232total×AU238ATh232det
The U and Th isotope compositions were quantified by isotope dilution
following Anderson and Fleer (1982) and later modified by Choi et al. (2001),
Pichat et al. (2004), and Lippold et al. (2009). Briefly, freeze-dried
marine sediments (150–200 mg) were spiked (229Th, 236U) and
completely digested in a pressure-assisted microwave (Tmax= 180 ∘C) using concentrated HNO3, HCl, and HF. Both elements
were separated and purified by anion exchange column chromatography using
AG1-X8 resin. Measurements were conducted using a Thermo Fisher Scientific
Neptune Plus multi-collector inductively coupled plasma mass spectrometer
(MC-ICP-MS) at the University of Bern, and for the PS cores a
single-collector ICP-MS (Thermo Fisher Scientific Element 2) at the Alfred
Wegener Institute (AWI) in Bremerhaven was used. Internal standards were used in each
batch to assess precision and reproducibility for the sample preparation
process as well as for the measurements. Approximately half of the samples
of cores PS2609-1, PS2606-6, and PS2603-3 had been prepared earlier
following a similar procedure, albeit with slightly less sediment material
(50 mg) and higher temperatures during digestion (Tmax= 210 ∘C). The chromatographic separation and subsequent purification of the U and Th fractions was performed using UTEVA resin (Eichrom). The isotope measurements were corrected with a calibrated
standard (UREM-11 Sarm 31) and yielded a relative standard deviation of less
than 3.8 % and 3.5 % for 238U and 234U, respectively, and less than
5.7 % and 4.9 % for 230Th and 232Th, respectively. The
measurement differences between the two mass spectrometers remain within
these error ranges.
In marine sediments, manganese (Mn) precipitates under well-oxygenated
conditions as oxyhydroxides (Mn(III) and (IV)) (Calvert and Pedersen, 1996).
Manganese enrichments in sediments can be observed where the accumulation of
organic matter is low and oxic conditions generally prevail (e.g., Calvert
and Pedersen, 1993). Under more reducing conditions, the sedimentary
distribution of Mn is controlled by the input of insoluble detrital
fraction. Any Mn present in the sediment that is in excess relative to the
concentration expected from the detrital fraction is assumed to have
accumulated authigenically under oxic conditions. Here, we use the XRF core
scanner peak intensity count ratios between Mn and Ti to constrain excess
Mn, assuming a constant detrital ratio between both elements. Ti is assumed
to be associated exclusively with lithogenic sources.
For Mn and Ti analyses in cores PS2609-1 and PS2606-6, the samples
were fully digested, evaporated, and redissolved in 20 mL 1M HNO3. An
aliquot was then diluted 1 : 100, and rhodium was added as an internal standard. The Mn and Ti concentrations were measured on the single-collector ICP-MS (Thermo Fisher Scientific Element 2) at AWI in Bremerhaven. Reference
material NIST 2702 was digested with each batch and measured with the
samples. For cores COR-1bPC and DCR-1PC, the Mn and Ti measurements
were acquired by XRF core logging with a Tatscan-F2 at the Kochi Core
Center, Japan (Sakamoto et al., 2006).
Preserved opal export
The sedimentary biogenic opal fraction is predominantly composed of diatom
frustules and minor amounts of radiolarians and sponge spicules. Diatoms
dominate carbon export around and (mostly) south of the
PF in the Southern Ocean (Cortese et al., 2004; Ragueneau et al., 2000). Sedimentary biogenic silica
(BSiO2), together with other proxies, has been widely used to reconstruct past
changes in marine export production (e.g., Anderson et al., 2009; Bradtmiller
et al., 2007; Chase et al., 2003). The accumulation of biogenic opal is
influenced not only by opal production in the sunlit surface ocean but also
by dissolution in the water column and at the seabed (Dezileau et al., 2003;
Pondaven et al., 2000; Ragueneau et al., 2000). Empirical studies of modern
opal export patterns suggest that the spatial distribution of opal burial
predominantly reflects diatom productivity and opal export (e.g., Chase et
al., 2003; Nelson et al., 2002; Pondaven et al., 2000; Sayles et al., 2001).
However, the link between opal and carbon export is not straightforward, as
other factors such as Fe availability can affect C and N uptake in diatoms
relative to Si (Boutorh et al., 2016; Meyerink et al., 2017; Pichevin et
al., 2014). A multi-proxy approach would provide a more unambiguous
reconstruction of paleo-productivity in the region. However, based on the similar
glacial–interglacial patterns of different paleo-productivity proxies in the
region (Thöle et al., 2019), we assume that changes in biogenic opal
fluxes provide a robust, first-order approximation of past changes in
organic carbon delivery to the sediment.
Sedimentary biogenic opal concentrations were determined using Fourier
transform infrared spectroscopy (FTIRS) at the University of Bern for cores
DCR-1PC and COR-1bPC (Vogel et al., 2016). A laboratory-specific internal reference
material was measured with each batch of samples to assess precision and
reproducibility. For the PS cores, the sedimentary opal content was
determined by alkaline extraction of silica according to Müller and
Schneider (1993) at AWI, Bremerhaven. Both methods provide comparable
results.
The 230Th normalization approach was used to reconstruct vertical
fluxes of BSiO2. The method allows for accounting and correcting for potentially
obfuscating effects, such as lateral sediment redistribution by bottom
currents (Bourne et al., 2012; Costa et al., 2020; Francois et al., 2004;
Henderson and Anderson, 2003). The flux of scavenged 230Th
(F230Th) settling to the seafloor at a specific water depth z is assumed to be equal to its known production rate (β230) from 234U decay within the water column. The resultant inverse relationship between the scavenged 230Th and the total vertical flux of particulate matter can be used to calculate preserved vertical fluxes (prFv) from the activity of initial scavenged 230Th in the sediment ATh230,(0)scav.
prFv=β230×zATh230,(0)scav
The equation to derive the initial scavenged sedimentary 230Th
ATh230,(0)scav takes into account (i) 230Th produced in situ from the decay of aU (238U), (ii) 230Th produced in situ from the decay of lithogenic U (238U), and (iii) radiogenic decay of 230Th after deposition. Normalizing the concentration of a specific sedimentary component (j) with the 230Th approach provides a quantitative estimate of its preserved vertical flux prFj through time:
prFj=prFv×fj,
where fj is the weight fraction of constituent j in the sediment.
ResultsSubantarctic Zone of the SW Indian OceanRedox-sensitive metal records
The sedimentary aU concentrations in core DCR-1PC show the highest values
of all cores (Fig. 3). The general pattern is consistent with a
climate-related signal, with typically higher concentrations during cold
periods and relatively low concentrations during warmer intervals. More
specifically, the highest values occur during MIS 6 and MIS 3–MIS 2. During
MIS 6 the values decrease gradually, show a peak around 130 ka, and then
decrease steeply to levels well below 1 ppm at the start of MIS 5. The
sedimentary aU concentrations are characterized by transient oscillations
during MIS 5 (106–100, 88–85 ka), and concentrations then increase at
the end of MIS 5 and stabilize during MIS 4. At the onset of MIS 3, aU
levels increase steeply, with values remaining high throughout MIS 3,
typically ranging between 5 and almost 8 ppm. Authigenic U concentrations
start declining after 27 ka and reach the lowest levels of the entire
glacial cycle at around 17.5 ka, remaining low throughout the Holocene. Mn
is typically enriched during the two major warm climate intervals of MIS 5
and the Holocene, during which values are higher and show high-frequency
variability, while Mn / Ti peak intensity count ratios hover around lower
values during cold climate intervals, including a period between
115 and 100 ka.
(a) Authigenic uranium concentrations (in blue), opal fluxes (in orange), and Mn / Ti ratios (in purple) from XRF scanning of sediment core DCR-1PC in the Subantarctic Zone. (b) Atmospheric CO2 concentrations from EPICA Dome C ice core, the composite record (Bereiter et al., 2015, and references therein), and the δD record from EPICA Dome C ice core, reflecting Antarctic air temperatures (Jouzel et al., 2007). Light blue bars show cold periods MIS 2, 4, and 6 (Lisiecki and Raymo, 2005).
Preserved opal export
Preserved biogenic opal fluxes vary between 0 and 0.7 g cm-2 kyr-1 (Fig. 3). Opal flux reconstructions show a pattern generally consistent
with glacial–interglacial cyclicity, with typically higher opal fluxes
during cold periods and lower fluxes during warmer climate intervals,
consistent with paleoceanographic reconstructions from the region (Manoj and
Thamban, 2015; Nair et al., 2019). Biogenic opal fluxes decrease about 5 ka
before the onset of the glacial terminations during both MIS 6 and MIS 2.
There are distinct differences between the downcore aU and BSiO2 flux records.
During warm intervals and the transition into MIS 4, both aU and BSiO2 flux
records appear to be coupled, but at the start of MIS 3 both parameters
start decoupling, suggesting a more nuanced relation between the two
proxies.
Antarctic Zone of the SW Indian OceanRedox-sensitive metal records
In the following sections, the four cores retrieved from the Antarctic Zone
of the Southern Ocean will be described from north to south wherever
possible. The downcore sedimentary aU records vary similarly in all four
cores (Fig. 4). Sedimentary aU concentrations vary between 0 and 3.5 ppm,
with the southernmost core, PS2603-3, showing the lowest values and
generally more subdued temporal variability. The lowest values in all cores
are consistently found during MIS 5 and the Holocene. In both PS2609-1
and PS2606-6, sedimentary aU concentrations increase during MIS 4, a
feature more salient in PS2609-1. Another increase mostly discernible in
PS2609-1 is observed at the end of MIS 5 (85–80 ka). The low values
during MIS 3 hover around 1 ppm in PS2609-1 and around 0.5 ppm in
PS2606-6, respectively. In COR-1bPC, the core with the shortest
sediment record, aU levels start increasing gently from the middle of MIS 3, before
reaching highest values at the end of the last ice age just before the start
of the deglacial transition.
(a–d) Authigenic uranium concentrations (in blue), opal fluxes (in orange), and Mn / Ti ratios (in purple) in the Antarctic Zone south of the Polar Front. (e) Atmospheric CO2 from the EPICA Dome C ice core, the composite record (Bereiter et al., 2015, and references therein), and the δD record from EPICA Dome C ice core, reflecting Antarctic air temperatures (Jouzel et al., 2007). Light blue bars show cold periods MIS 2 and 4 (Lisiecki and Raymo, 2005).
The highest aU concentrations in cores PS2609-1, PS2606-6, and
COR-1bPC occur during MIS 2 with a gradual increase from about 30 ka,
peaking during the LGM. Thereafter, aU levels decline sharply to values well
below 1 ppm at 2–4 ka, concomitant with the onset of the last glacial
termination. After TERM I, a small increase around 11–8 ka is apparent.
Throughout the Holocene, aU levels stay below 1 ppm. In PS2603-3 the
highest values are also found within MIS 2, but the downcore variability is
more muted. The determination of sedimentary aU concentrations inherently
assumes that the 238U /232Th ratio of the lithogenic material
supplied to the core site remained constant in space and time. Although the
detrital 238U /232Th ratio may have fluctuated in response to changing
detrital sources (Manoj et al., 2012, 2013), the authigenic fraction typically amounts to > 60 % of the total U. Such a decrease in the 238U /232Th ratio during the last ice age would likely only marginally affect the absolute sedimentary aU concentrations and would not affect
the first-order downcore patterns. Authigenic Mn levels remain low
throughout the entire ice age and increase at the onset of the last glacial
termination. The Mn / Ti records show similar trends, with low values during MIS 5 and MIS 4 and elevated or more variable values from about 15 ka.
Preserved opal export
The cores south of the PF show a consistent pattern, with relatively low
preserved opal fluxes throughout most of MIS 2, 3, and 4; slight
increases and higher variability during MIS 5; and a prominent peak after the
LGM, all of which is consistent with available paleoceanographic records from the SW Indian
Ocean (Manoj and Thamban, 2015; Nair et al., 2019). The values reached
during this peak are much higher than in the SAZ, with values up to
4 g cm-2 kyr-1. PS2603-3 shows the overall lowest values,
reaching 1.2 g cm-2 kyr-1 after both TERM I and II.
The downcore variations in BSiO2 flux show a very different pattern than the
aU records, especially during the LGM and TERM I. A rapid increase in BSiO2 fluxes at the onset of the last
deglaciation is observed at the same time as the aU
decrease. After that point, BSiO2 fluxes slowly decrease over the
course of the Holocene. During MIS 5 there is a small increase in opal
fluxes around 85–75 ka. The Mn / Ti values remain low in all cores and rise after the LGM in parallel with the BSiO2 flux.
Interpretation and discussionDynamics of bottom water oxygenation in the SAZ
The downcore aU and Mn / Ti patterns are generally coherent (Fig. 3), and
together the records provide a consistent picture reflecting past changes in
bottom water oxygenation. Overall, the data indicate generally more reducing
conditions during cold periods, whereas sediments were more oxidizing during
warmer climate intervals, consistent with previous observations spanning the
last deglacial transition in the region (Sruthi et al., 2012). The two
redox-sensitive elements, however, are not perfectly anti-correlated,
consistent with their inherent sensitivities to changing redox conditions
(Tribovillard et al., 2006). When comparing both redox-sensitive metal
records to opal fluxes, the proxies broadly allude to similar oxygenation
conditions, in particular during glacial inception periods. However, towards peak
glacial conditions the records show some degree of divergence.
The first drop in atmospheric pCO2 at around 115 ka, marking the last glacial inception, coincides with a reduction in Mn / Ti values in core DCR-1PC, which could be attributed to a transition towards more reducing
conditions (Fig. 3). At the transition from MIS 5 to MIS 4, both redox
proxies show a clear shift towards more reducing conditions, concomitant
with a rise in biogenic opal export and coinciding with a substantial increase
in dust and lithogenic and iron deposition rates recorded in Antarctic ice
cores (e.g., Lambert et al., 2012) and subantarctic marine sediments (e.g.,
Anderson et al., 2014; Lamy et al., 2014; Manoj and Thamban, 2015;
Martínez-García et al., 2014; Thöle et al., 2019). Kohfeld and
Chase (2017) suggest a major change in ocean circulation, contemporaneous
with this second drop in CO2 centered at 72–65 ka. In particular,
sedimentary neodymium (Nd) isotope records and 13C data suggest a major
reorganization of deep-ocean circulation at that time (e.g., Oliver et al.,
2010; Wilson et al., 2015). Indeed, the shoaling of the Atlantic overturning
cell may have left the abyssal ocean dominated by dense southern-sourced
water (e.g., Lynch-Stieglitz et al., 2016; Matsumoto et al., 2002), which may have
increased the vertical density gradient and contributed to isolate the deep
ocean, making it more prone to sequester remineralized carbon (Hain et al.,
2010; Skinner, 2009; Yu et al., 2016). We therefore suggest that both
increased export production and by inference organic matter respiration, as
well as a decrease in deep-ocean ventilation, preconditioned the sediments,
maintaining sufficiently reducing conditions to allow for recording the more
subtle ventilation changes of the last ice age (Gottschalk et al., 2020a).
The decrease in sedimentary aU concentrations and associated reoxygenation
of bottom waters started within MIS 2 and not, as expected, towards the end
of the last ice age, when ocean circulation and upwelling began to intensify
as the Southern Hemisphere warmed (Basak et al., 2018; Gottschalk et al.,
2016, 2020b; Jaccard et al., 2016; Rae et al., 2018; Ronge et al., 2020;
Skinner et al., 2010). Rather than a regionally disparate initiation of
upwelling, the decrease in aU accumulation in core DCR-1PC may be related
to diagenetic “burn-down”. Diagenetic redeposition of sedimentary aU is indeed
observed when oxygenated conditions in the uppermost layers of the sediment
recur (e.g., Thomson et al., 1990), for example as a result of reinvigorating
deep-water ventilation. As oxygen diffuses into pore waters, previously
precipitated U will dissolve and will be either lost to the overlying water
or diffuse deeper into the sediment, where conditions are still reducing and
where it will be reprecipitated (Colley et al., 1989; Jacobel et al., 2017;
Mangini et al., 2001). A high sediment accumulation rate would limit the
depth interval over which burn-down would affect the downcore aU record.
Core DCR-1PC has the lowest sedimentation rates of the five cores
investigated and thus is potentially more prone to be affected by oxidative
burn-down (Korff et al., 2016). With the lowest sedimentation rate within
the core being reported between 17 and 25 ka (< 1 cm kyr-1,
Fig. 5), re-dissolution provides a plausible explanation accounting for
the early aU decrease. Alternatively, the observed decrease in aU
concentrations during the LGM may reveal a transient sedimentary hiatus,
which may have affected the sedimentary record in this core. During the
penultimate glacial termination (TERM II), aU appears to have only been
marginally affected by remobilization processes. The sedimentation rates
during this interval are indeed higher than at the end of the last ice age
(Fig. 5).
(a–e) Sedimentation rates (in black) and authigenic uranium concentrations (in light blue) in all cores from north to south. Light blue bars show cold periods MIS 2, 4, and 6 (Lisiecki and Raymo, 2005).
The early decrease in bSiO2 export towards the end of both glacial periods
could be related to increasingly complete Si(OH)4 consumption south of the Polar Front (Dumont et al., 2020). Upwelling and thus nutrient supply to the surface water south of the PF were substantially reduced during glacial times (Anderson et al., 2009; Francois et al., 1997). Biological
productivity was reduced, but at the same time nitrate consumption was more
complete (Ai et al., 2020; Studer et al., 2015), stemming CO2
outgassing from the ocean interior. Part of these surface waters are
transported towards the north, and when they reach the location of
DCR-1PC they are largely depleted of Si(OH)4. Transitioning towards
peak glacial conditions, there would have been gradually less Si(OH)4
available for opal production in the SAZ, as reflected by the downcore opal
flux records. Relaxed Fe limitation as a result of enhanced bioavailable
Fe input with dust would have lowered the Si / N uptake ratio in diatoms
(Brzezinski et al., 2002). Nitrogen isotope studies show that nitrate
utilization was more complete in glacial periods when compared to
interglacials (Ai et al., 2020; Horn et al., 2011; Martínez-García
et al., 2014; Studer et al., 2015), whereas silicon isotopes show an
opposite pattern (Brzezinski et al., 2002; Dumont et al., 2020). Despite
preferred nitrate uptake and therefore potentially more silicate leakage
towards the north, the overall amount of Si(OH)4 upwelled from depth
might have been declining, thus limiting diatom growth (Brzezinski et al.,
2002; Dumont et al., 2020). Moreover, as the age model and the low
sedimentation rate may indicate, a possible hiatus during this critical
interval may provide a complementary explanation to account for the early
decrease in opal fluxes.
Dynamics of bottom water oxygenation in the AZ
South of the PF, all cores, with the exception of the slowly accumulating
core retrieved from Enderby Basin, record similar (paleo)oceanographic
evolutions. The sedimentary aU and Mn / Ti levels show a consistent downcore
pattern with generally more reducing conditions during peak glacial times
and rapid (re)oxygenation during glacial terminations. These observations
are consistent with recent radiocarbon-based evidence suggesting that the
deep Indian Ocean was generally more poorly ventilated during the last ice
age (Bharti et al., 2022; Gottschalk et al., 2020b; Ronge et al., 2020).
Preserved opal fluxes are persistently low during cold periods, while aU and
Mn / Ti values are consistent with increasingly reducing conditions. The rapid increase in preserved opal fluxes coinciding with more oxygenated conditions during TERM I imply that export production and thus the respiratory demand for organic matter remineralization only played a secondary role, whereas ventilation imposed a primary control on sedimentary oxygenation levels. The rapid increase in biogenic opal fluxes at the onset of both glacial terminations is likely a result of enhanced upwelling of subsurface waters, rich in both (micro)nutrients and CO2 (Anderson et al., 2009; Gottschalk et al., 2020b; Jaccard et al., 2016; Skinner et al., 2010).
With the available data of redox-sensitive elements U and Mn and the
preserved opal fluxes in these cores, an alternative interpretation needs to
be considered. As Fe scarcity reduces the C and N uptake ratio relative to
Si (Boutorh et al., 2016; Meyerink et al., 2017; Pichevin et al., 2014), the
opal peak during the deglaciation can at least partly be attributed to
Fe limitation, which is possibly induced by a reduction of Fe-bearing dust input. To
further explore this alternative possibility, downcore changes in Si / Fe
ratios in diatom shells could be analyzed. However, considering multi-proxy
studies of export production in the AZ (e.g., Chase et al., 2003; Thöle
et al., 2019) and enhanced upwelling intensities during the deglacial (e.g.,
Basak et al., 2018; Gottschalk et al., 2020b; Ronge et al., 2020; Skinner et
al., 2010), we suggest the observed opal peak to be the result of increased
production due to enhanced nutrient availability from upwelled deep waters,
in line with a meridional shift of the Southern Ocean frontal system.
Reinvigorating ventilation in the Southern Ocean associated with the glacial
termination would leave a spatially coherent oxygenation pattern in all
cores, as it was related to an overall reorganization in ocean circulation.
The associated opal signal might slightly differ in time given that the
upwelling region shifts along with the frontal system (Anderson et al.,
2009). However, as the timing of aU peak emplacement cannot robustly be
defined and the sampling resolution may be insufficient, the potential time
lag between the onset of the aU decrease and the sharp rise in opal
production and deposition cannot reliably be assessed.
In the southernmost core PS2603-3, aU and preserved opal fluxes suggest
similar oceanographic dynamics in the Enderby Abyssal Plain. The opal peak that is concomitant with the onset of TERM I suggests that the core is similarly
recording enhanced upwelling, but the maximum values of
1.2 g cm-2 kyr-1 are consistent with the core being located further away from the most vigorous upwelling region. The slight increase in the aU record during the glacial maxima suggests that bottom water oxygenation was generally reduced during ice ages.
Ventilation and circulation changes on glacial–interglacial timescales and their impact on atmospheric pCO2
Across the transect, the core site locations are bathed by different water
masses (Fig. 1). With the exception of the southernmost core, which is
bathed by Antarctic Bottom Water (AABW), the other four sediment cores are
located in Circumpolar Deep Water (CDW), which is formed partly from North
Atlantic Deep Water (NADW) and mixed with AABW and other deep waters that
originate from the Indian and Pacific oceans (Talley, 2013). CDW is upwelled
in the Southern Ocean along tilted isopycnals. The lower CDW, which stems
mainly from dense, high-salinity NADW, then moves towards the south as a
precursor for AABW. The less dense upper CDW, which has a more
oxygen-depleted signature from older Indian and Pacific deep waters, is
transported to the north by Ekman flow (Talley, 2013). The lower and upper
CDW thus have distinct oxygen concentrations, but the range in dissolved
oxygen concentration changes is smaller than the ranges reported for the
last glacial cycle due to increased carbon sequestration (Anderson et al.,
2019; Galbraith and Skinner, 2020). Therefore, any variations in aU and
Mn / Ti values are unlikely to be primarily driven by local variations in
oxygenation, but rather record a coherent regional picture. The main factor
controlling changes in oxygenation at the sediment–water interface and in
pore waters is thus more likely related to bottom water ventilation changes.
This can be argued for by the decoupling of aU and opal fluxes in DCR-1PC
(Fig. 3) during the later phase of the glacial periods and by the more
pronounced anti-phasing in the records south of the PF. This anti-phasing at
TERM I with invigorated circulation and thus rejuvenation of the deep ocean
is linked to enhanced supply of nutrient-rich waters to the surface, which
in turn fueled biological production. However, this nutrient-fueled phytoplankton
growth was not efficient to quantitatively fix dissolved carbon, thus
allowing CO2 to escape to the atmosphere (Ai et al., 2020; Sigman et
al., 2010; Studer et al., 2015). The increase in opal fluxes in connection
with more ventilated bottom waters at the end of the glacial periods fits
well with higher atmospheric CO2 inventories associated with the
release of previously sequestered carbon from subsurface waters (Burke and
Robinson, 2012; Jaccard et al., 2016; Ronge et al., 2020; Skinner et al.,
2010).
The major slowdown in deep-ocean circulation occurred mostly after the
MIS 5–MIS 4 transition. Before this point, at the onset of the glacial period, other
factors may have contributed to the drawdown of atmospheric carbon. During
the first CO2 drop within MIS 5, cooling sea surface temperatures at
high latitudes in both hemispheres led to CO2 reduction through barrier
mechanisms. Sea ice formation and seasonally induced meltwater would lead
to stronger surface water stratification and affect air–sea gas exchange and
impede the ability of lower waters to rise to the productive surface (Watson and Naveira
Garabato, 2006; Wolff et al., 2010). This barrier mechanism is consistent
with the observed first increase in nitrate consumption associated with
enhanced stratification of the surface ocean (Ai et al., 2020; Studer et
al., 2015). Only at the second CO2 drop at the MIS 5–MIS 4 transition does deep-ocean circulation slow down, as indicated by increased aU and decreased Mn / Ti, in good agreement with other proxy data (Jimenez-Espejo et al.,
2020; Kohfeld and Chase, 2017; Oliver et al., 2010; Wilson et al., 2015).
This second drop was accompanied by enhanced biological export production
north of the PF fueled by enhanced dust input (Lambert et al., 2012). Our
results show a regionally coherent increase in sedimentary aU
concentrations, especially after the MIS 5–MIS 4 transition, and indicate
the gradually more sluggish overturning circulation that contributed to
partitioning carbon into the ocean interior (Gottschalk et al., 2020a;
Jaccard et al., 2016, 2013; Kohfeld and Chase, 2017).
South of the PF (in core PS2609-1), the aU record indicates reoxygenation
during MIS 3, while in the SAZ reducing conditions intensify. This
contrasting behavior could be explained by a reorganization of deep-water
masses. NADW that forms CDW in which our cores are located could have
retracted northwards during MIS 3, while AAWB, which is generally more oxygenated
than NADW, bathed the cores south of the PF (Sigman et al., 2010).
Progressing towards the glacial maximum, the upwelled water masses would
successively be more isolated from atmospheric forcing by extended sea ice,
leading to more oxygen-depleted and carbon-rich AABW (Ferrari et al., 2014),
and thus resulting in the increased aU levels in the southern cores during
MIS 2.
Conclusions
Five marine sediment cores from the Indian sector of the Southern Ocean were
considered in this study. They were retrieved across a transect spanning a
latitudinal band of about 15∘ from Del Caño Rise (north of the
SAF), Conrad Rise, and as far south as the Enderby Abyssal Plain, which is close to the
Southern ACC Front (SACCF). Redox-sensitive aU and Mn / Ti ratios were studied in detail and compared to 230Th-normalized preserved opal export fluxes to better constrain bottom water oxygenation in the context of carbon
sequestration since the last glacial inception. Our results suggest that
more sluggish circulation dynamics and thus ventilation changes are the
major contributor accounting for enhanced carbon sequestration in deep-water
masses during glacial periods. Our paleoceanographic records covering a
meridional transect increase the spatial resolution of deep-water
oxygenation records in the Southern Ocean and are overall in agreement with
previous reconstructions from other deep-water sites (Chase et al., 2001;
Dezileau et al., 2002; Francois et al., 1997; Frank et al., 2000; Gottschalk
et al., 2020b; Jaccard et al., 2016; Thöle et al., 2019).
The influence of locally enhanced biological export production on
oxygenation states due to increased Fe fertilization by dust input cannot be
ruled out completely, especially in the SAZ and at the transition from MIS 5
to 4 (Jaccard et al., 2016, 2013; Martínez-García et al., 2014).
In the AZ, however, export production likely played a minor role in
partitioning carbon from the atmosphere. More importantly, decreased
ventilation and the associated slowdown of overall ocean circulation during
cold periods (Wu et al., 2021) led to more carbon being sequestered in the
ocean interior. Our results indicate a major drawdown of atmospheric
CO2 by a more sluggish overturning circulation. The most substantial
circulation changes are suggested to have occurred at the transition of
MIS 5–MIS 4, as has been proposed by Kohfeld and Chase (2017). South of the PF,
reducing conditions recede during MIS 3, most likely due to a reorganization
of deep-water circulation with larger expansion of AABW, reaching the
southern core locations (Ferrari et al., 2014). Later during MIS 2,
these deep waters that upwell to newly form AABW were also increasingly
isolated from atmospheric forcing due to expanded sea ice cover. Generally,
our reconstructions support the hypothesis that ventilation dynamics are the
main driver of oxygenation changes in the Southern Ocean and thus exert a
major control on the air–sea partitioning of CO2 over the last glacial cycle.
Data availability
The data set of this study is accessible on BORIS portal: 10.48620/69 (Amsler, 2022).
Author contributions
HEA and SLJ devised the study. HEA, LMT, IS, and WG carried out U / Th
measurements. HEA and GK carried out biogenic opal measurements. IS and WG
conducted the absolute Mn and Ti measurements. OE contributed largely to the age models for the PS cores. MI and GK planned the cruises
and provided access to the core material and XRF measurements. HEA wrote the
initial version of the manuscript, and all co-authors contributed to the
manuscript.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank the captain, crew members, and scientists of the R/V Hakuho-maru and R/V Polarstern for the recovery of the sediment cores during the KH-10-7 (lead scientist: Minoru Ikehara) and ANT-XI/4 cruises (lead scientist: Gerhard Kuhn) in the Southern
Ocean. We thank Martin Wille, Igor Villa, Jörg Rickli, David Janssen,
Edel O'Sullivan, Alessandro Maltese, and Jörg Lippold for their help
with the MC-ICP-MS measurements; Julijana Krbanjevic and Hendrik Vogel for
helping with the opal measurements; and Takuya Matsuzaki for the Tatscan
measurements. Funding for this study was provided by the Swiss National
Science Foundation (grant nos. PP00P2_144811 and 200021_163003). Oliver Esper, Walter Geibert, Gerhard Kuhn, and Ingrid Stimac
were funded by the Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-
und Meeresforschung PACES II research program. Minoru Ikehara was funded by the Japan Society for the Promotion of Science KAKENHI (Grants-in-Aid for
Scientific Research P23244102 and JP17H06318). The isotope data were obtained
on a Neptune MC-ICP-MS acquired with funds from NCCR
PlanetS, supported by the Swiss National Science Foundation (grant no. 51NF40-141881).
Financial support
This research has been supported by the Swiss National Science Foundation (grant nos. PP00P2_144811 and 200021_163003).
Review statement
This paper was edited by Luc Beaufort and reviewed by two anonymous referees.
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