Introduction
Ice core records demonstrate that atmospheric CO2
concentrations (pCO2) were 80–100 ppm lower than
preindustrial levels during glacial intervals of the last ∼400 000 years (Petit et al., 1999). Of the ∼40–50 ppm
pCO2 decrease currently unaccounted for, the most plausible storage
location for this “missing” glacial CO2 is the deep ocean because
it is the largest easily exchangeable reservoir of carbon on the planet
(Sigman and Boyle, 2000). Paleoclimate studies have targeted the Southern
Ocean as a primary regulator for glacial–interglacial atmospheric
CO2 changes because the CO2 air–sea balance is set within
the Antarctic Zone by gas exchange with the deep ocean and by downward
CO2 transfer via the biological pump (Marinov et al., 2006). Proposed
physical mechanisms for drawing down atmospheric CO2 during glacial
intervals include increased Southern Ocean vertical stratification (Francois
et al., 1997) and (subsequently) decreased deep ocean ventilation rate
(Toggweiler, 1999). Because O2 is only added at the sea surface, if
glacial Southern Ocean ventilation was reduced, continuous organic matter
respiration as deep waters “aged” would have caused CO2 and
nutrients to accumulate as O2 was consumed. Hence bottom water
O2 concentrations provide an indirect assessment of whether the
biological pump and/or changes to ocean vertical water column structure
enhanced deep Southern Ocean CO2 storage during glacial intervals.
A recent compilation (Jaccard and Galbraith, 2012) of sedimentary
oxygenation proxies documents increased deep ocean ventilation between
the Last Glacial Maximum (LGM) and the early Holocene. Other studies
have sought to characterize global glacial–interglacial circulation
and deep ocean ventilation changes using benthic
13C/12C ratios (δ13C) (e.g.,
Curry and Oppo, 2005; Herguera et al., 2010; Hodell et al., 2003;
Ninnemann and Charles, 2002) or Δ14C (e.g., Keigwin,
2004; Robinson et al., 2005; Skinner et al., 2014) derived from
sedimentary CaCO3. The longer a water mass is isolated from
the surface and the more nutrients it accumulates, the lower the
δ13C of its pCO2 (Broecker and Peng,
1982). Similarly, surface waters dissolve CO2 with an
atmospheric 14C signature, but once a water mass leaves the
sea surface, its 14C content diminishes – increasing the
offset from the atmosphere – as the radiocarbon decays (Broecker and
Peng, 1982).
In the Atlantic Ocean, glacial δ13C and
Δ14C data imply different Southern Ocean chemistry
potentially resulting from changes to deep ocean vertical
stratification and mixing. South Atlantic benthic δ13C
values that are more than 1 ‰ lower than the modern ocean
support the existence of strong vertical stratification and poorer
ventilation (Ninnemann and Charles, 2002). However, interpretation of
δ13C is not entirely straightforward because it is
influenced by a combination of air–sea gas exchange, export
productivity, and water mass mixing (Kroopnick,
1985). δ13C also cannot uniquely identify whether
bottom water masses were oxygen-poor and/or export productivity was
high. Paired benthic-planktonic 14C ventilation measurements
from the northwest Atlantic Ocean (Keigwin, 2004; Keigwin and
Schlegel, 2002; Robinson et al., 2005) are also consistent with an
“old” Southern-sourced water mass below ∼2.5–3 km
water depth. However, Δ14C is also affected by water
mass mixing, and robust interpretation of this proxy requires accurate
determination of surface ocean reservoir age. As an added
complication, both δ13C and Δ14C
measurements require good carbonate preservation, but CaCO3 is
poorly preserved in the CO2-rich waters south of the Antarctic
Polar Front (Howard and Prell, 1994). Because every proxy is subject
to a unique set of biases, agreement among several proxies increases
confidence in our interpretations. Thus, it is prudent to seek
additional evidence for decreased bottom water O2/increased
CO2 concentrations in the glacial Southern Ocean from other
proxies.
Redox-sensitive trace metals are reliable proxies for sedimentary pore water
redox conditions (especially in the high latitude Southern Ocean where
sediments contain primarily biogenic silica and little to no CaCO3;
Diekmann, 2007) and provide an alternative approach to reconstructing deep
water ventilation. Additionally, by examining a suite of trace metals it is
possible to disentangle the relationship between low-oxygen bottom waters and
intensive oxidation of organic matter in sediments that can produce reducing
sedimentary pore waters. In this study we measured the redox-sensitive trace
metals Ag, Cd, Re, and Mo to reconstruct deep Southern Ocean oxygenation
history over the last ∼30 kyr for two piston cores from the
deep South Atlantic Ocean: RC13-254 (48.57∘ S, 5.127∘ E,
3636 m water depth), from the Cape Basin; and TN057-13-4PC
(53.1728∘ S, 5.1275∘ E, 2848 m water depth), from
the open ocean Antarctic Zone (Fig. 1). Additionally, we present new organic
carbon (Corg) concentrations for both cores.
Background
Trace metal geochemistry
Because trace metals are supplied to sediments via a number of
different delivery mechanisms, a suite of trace metals is required to
differentiate the processes that produce reducing conditions in pore
waters. Here we follow the definition of Berner (1981), and classify
sediments whose pore waters contain no measurable dissolved
oxygen or dissolved sulfide as suboxic, and those whose pore waters
contain measurable dissolved sulfide as anoxic. Onset of suboxic
conditions occurs between the reduction potentials of Mn and
NO3- reduction (Froelich et al., 1979). Silver and Cd are
thought to be delivered by sinking biogenic particles (Hendy and
Pedersen, 2005; Wagner et al., 2013) and preserved as their respective
sulfides where trace quantities of dissolved sulfide are present
(Crusius and Thomson, 2003; Rosenthal et al., 1995b). Hence, enhanced
preservation of these metals indicates suboxic conditions caused by
a high flux of organic matter (Hendy and Pedersen, 2005). Cadmium also
displays a strong tendency toward authigenic precipitation in
sediments where reducing conditions occur close to the sediment–water
interface (Calvert and Pedersen, 1993; Pedersen et al., 1989;
Rosenthal et al., 1995b). In contrast, authigenic Ag2S
precipitation appears to be negligible except in severely anoxic
sediments where sulfate reduction is occurring (McKay and Pedersen,
2008). Elevated Re concentrations also indicate pore water
suboxia. Rhenium exists in seawater as the conservative perrhenate
anion (ReO4-) (Koide et al., 1986). It diffuses into sediments
along concentration gradients and precipitates directly from seawater
under reducing conditions, possibly as ReO2 (Crusius et al.,
1996), although the probability for this reaction to proceed
spontaneously in typical seawater has recently been questioned (Helz
and Dolor, 2012; and references therein). Due to its low crustal
concentrations, nearly all sedimentary Re is authigenically
precipitated (Crusius et al., 1996). Hence, increases in Re
accumulation without corresponding increases in Ag and/or Cd indicate
that low bottom water O2 concentrations were likely present,
and this deficit was primarily responsible for suboxia in underlying
sediments.
Molybdenum enrichment occurs under anoxic conditions and requires
measurable sulfide levels to precipitate. In seawater Mo is present as
the conservative molybdate anion (MoO42-). Under oxic
conditions, Mo associates with Mn-oxyhydroxides (Calvert and Pedersen,
1993; Zheng et al., 2000) which produces low-level enrichments
unrelated to reductive processes. When redox conditions become
favorable to Mn-oxyhydroxide reduction, adsorbed Mo can be released to
pore waters (Crusius et al., 1996), decreasing preserved solid-phase
Mo. Significant enrichments of Mo (up to tens of ppm in non-euxinic
settings (Scott and Lyons, 2012)) above lithogenic background are
controlled by pore water H2S concentrations. At sulfide
concentrations below ∼11 µM, progressive
substitution of O for S atoms occurs in molybdate, leading to the
formation of multiple thiomolybdate species (MoOxS4-x2-)
that co-precipitate as a labile Mo-Fe-S phase (Erickson and Helz,
2000; Helz et al., 1996). The substitution reaction is kinetically
slow, but above the threshold value of 11 µM, a
“geochemical switch” (Helz et al., 1996) is flipped and complete
conversion to MoS42- (tetrathiomolybdate) occurs. Downcore
measurements of Mo and H2S in Santa Barbara Basin sediments
provide empirical support for this theoretical “geochemical switch”
(Zheng et al., 2000). Tetrathiomolybdate is highly particle-reactive
and adsorbs to pyrite, after which it is retained in sediments as
a Mo-Fe-S cubane compound (Bostick et al., 2003). Above sulfide
concentrations of ∼100 µM, direct precipitation of
a discrete Mo-sulfide phase is hypothesized to occur (Zheng et al.,
2000). Similar to Re, elevated Mo concentrations not coupled with
increased Ag and/or Cd concentrations imply that low bottom water
O2 concentrations caused anoxic conditions in pore waters.
Because Ag and Cd delivery to sediments is associated with biogenic
particles, and Cd precipitates in sediments where oxygen penetrates
0.5 cm or less (Morford and Emerson, 1999), enrichment of Ag
and Cd is assumed to be approximately contemporaneous with the
modelled sediment age. Under reducing conditions, Re and Mo
precipitation begin within ∼1 cm of the sediment–water
interface (Crusius et al., 1996; Morford et al., 2009), minimizing any
offset with the sediment age. A quantitative relationship has yet to
be shown between sedimentary trace metal accumulation and export
production or bottom water ventilation due to the combined effects of
biogenic particle remineralization, sedimentation rate, dilution by
other phases, and post-depositional re-oxidation of trace metals. For
this reason, trace metals can provide a qualitative determination of
the relative importance of export production vs. bottom water
ventilation by comparing Ag and Cd (metals associated with
biodetritus) enrichments with Re and Mo (authigenic metals)
enrichments. For this study, when Ag and/or Cd concentrations increase
with increasing Re and/or Mo concentrations, we interpret this pattern
as high export production that depletes sedimentary oxidants. When Ag
and/or Cd concentrations decrease with increasing Re and/or Mo
concentrations, we interpret this pattern as a relative decrease in
export production and concomitant decrease in bottom water
oxygenation. However it is important to note that export production
and bottom water oxygenation may operate synergistically to produce
contemporaneous trace metal enrichments.
Oceanographic setting
Core TN057-13-4PC was recovered from a small sedimentary basin
northeast of Bouvet Island, at the same location as ODP Site 1094
(Kanfoush et al., 2002), so the two cores can be directly
compared. Presently, TN057-13-4PC is located slightly to the south of
the Southern ACC Front (Orsi et al., 1995) and ∼2∘ north
of the average winter sea ice edge (Kanfoush et al., 2002). Primary
productivity in this region is high (Moore and Abbott, 2000) and
dominated by the heavily-silicified diatom Fragilariopsis kerguelensis (Abelmann et al., 2006). Sediments are composed
primarily of diatomaceous ooze and diatom mud (Shipboard Scientific
Party, 1999b). Site RC13-254 is located southwest of Meteor Rise in
the Cape Basin, on the northern edge of the Antarctic Polar Frontal
Zone (APFZ), and within the Permanently Open Ocean Zone. Productivity
within the APFZ is still high, but transitions from diatom-dominated
in the south to coccolithophorid-dominated in the north (Honjo et al.,
2000). Consequently, sediments in the upper portion of the core are
composed primarily of CaCO3 and transition to diatomaceous
ooze during deglaciation (Charles et al., 1991).
Today both cores are overlain by Lower Circumpolar Deep Water (LCDW)
(Fig. 2), a cold, saline, recirculating water mass of the Antarctic
Circumpolar Current (ACC). Modern LCDW is a mixture of North Atlantic
Deep Water (NADW), Antarctic Shelf Waters, and recirculated Pacific
and Indian Deep Waters (PDW and IDW) that is exported to the three
major ocean basins through Deep Western Boundary Currents (Talley,
2008). LCDW is ventilated from the north by penetration of NADW into
the ACC, and from the south by upward mixing of Antarctic Bottom Water
(Orsi et al., 1999). (Note, however, that bottom waters overlying
TN057-13-4PC receive less influence from NADW than sites further to
the north (Shipboard Scientific Party, 1999a), such as RC13-254/ODP
1091.) Once LCDW enters the Indian and Pacific basins, it upwells
diapycnally to form the major volume of PDW and IDW. These
subsequently re-enter the Atlantic basin through Drake Passage, mix
with ACC waters, and upwell isopycnally to form modern Upper
Circumpolar Deep Water (UCDW) (Talley, 2008), a nutrient-rich and
slightly lower-oxygen water mass. Thus in the modern ACC, waters are
well ventilated but the vertical water column structure can be
characterized as having a lower-oxygen water mass overlying
a higher-oxygen water mass (Fig. 2b).
Results and discussion
Oxidative burndown
Oxidative burndown occurs when previously deposited sediments become
re-exposed to oxygen, remobilizing reduced trace metal phases. It is
more likely to occur in slowly accumulating sediments and during times
of rapid environmental change, such as during a glacial–interglacial
transition (Tribovillard et al., 2006). Once remobilized, trace metals
can either diffuse downward into the sediment column if concentration
gradients decrease in that direction, or diffuse upward out of
sediments into the water column. In the former case, evidence of
oxidative burndown usually presents as a sharp peak below a redox
front. The effect has been observed downcore for Re in organic
carbon-rich turbidites (Crusius and Thomson, 2003) and for Cd and U in
Subantarctic sediments (Rosenthal et al., 1995a) directly below the
glacial–interglacial transition.
Burndown is likely to have affected the upper ∼200 cm of
TN057-13-4PC and the upper ∼60–70 cm of RC13-254. Evidence for
oxidative burndown is clearest for RC13-254. An abrupt transition from very
low to glacial-level trace metal concentrations occurs between ∼60 and
70 cm core depth (∼13–13.5 ka), coinciding with decreases in
pore water NO3- and increases in pore water Mn concentrations at
a nearby core (King et al., 2000). No evidence for oxidative burndown is
apparent further downcore as at no other point do all redox-sensitive trace
metals decrease abruptly and coherently to crustal concentrations. Muted
trace metal enrichments in TN057-13-4PC from the core top to ∼200 cm (∼5.5 ka) suggest the onset of oxidative burndown
during the late Holocene. However, NO3- and Mn pore water profiles
(King et al., 2000) indicate that the present oxic-suboxic transition occurs
∼20–30 cm core depth (∼0.7–1 ka), implying that
a slowdown in trace metal delivery rather than re-oxidation may exert primary
control over trace metal enrichments in the upper portion of the core.
However, available proxy data fail to give an unambiguous determination of
the existence and timing of burndown in TN057-13-4PC near the
glacial–interglacial transition. The sharp Ag and Cd peaks observed in
TN057-13-4PC between ∼820 and 880 cm core depth (∼21.6–26.9 ka) indicate possible burndown. If true, co-occurring Re peaks
appear to have migrated further downcore. However, U fluxes increase slightly
from ∼832 to 884 cm (∼22.1–26.5 ka) (Anderson et al.,
2009), suggesting that burndown was minimal or affected U in a similar –
though less severe – manner to Ag and Cd. Changes in Corg
flux can control the onset of burndown (Crusius and Thomson, 2000) and may
help to diagnose when burndown began. Sedimentary Corg
concentrations in TN057-13-4PC decline from a maximum of about 0.5 % to
about 0.3 % from 30 ka to the end of MIS 3 (∼22 ka), coincident
with the large Ag, Cd, and Re peaks. Subsequently, agreement among Ag, Cd,
Re, and Corg peaks between ∼760 and 820 cm
(∼16.2–21.6 ka) suggests a true increase in export productivity.
Sedimentary Corg concentrations in the Antarctic Zone
therefore increase going into the LGM and a decrease in the
intensity of organic matter respiration cannot be responsible for
re-oxygenation of sediments. Rather, burndown appears to have occurred
sometime during late MIS 3. Therefore, with the evidence at hand, we will
assume that oxidative burndown has had a minimal effect on trace metal
concentrations near the glacial–interglacial transition.
Glacial–interglacial changes in Southern Ocean redox conditions
Trace metal concentrations in Holocene-age sediments reported here for
the Atlantic sector Southern Ocean are low, similar to those measured
for Pacific sector core top sediments both within and outside the opal
belt (Wagner et al., 2013). These data suggest that marine chemistry
unfavorable to trace metal accumulation exists throughout the modern
Southern Ocean. In contrast, trace metal concentrations at both sites
are significantly higher downcore (Figs. 3a–d, 4a–d). Although both
RC13-254 and TN057-13-4PC are situated in the open ocean, trace metal
concentrations in deglacial- and glacial-age sediments are similar to
concentrations found at modern continental margin sites (e.g., McKay
and Pedersen, 2008; Morford and Emerson, 1999). These continental
margin sites are often located in oxygen minimum zones such as the
eastern Pacific Ocean where dissolved oxygen concentrations are much
lower than the ∼200–225 µM typical of modern LCDW
(Fig. 2b).
At RC13-254 (Cape Basin, modern APFZ), the obvious difference in trace
metal enrichments between late MIS 2 and the late Holocene
(Fig. 3a–d) indicates a significant change in deep Southern Ocean
chemistry ∼3.6 km water depth. Prior to ∼13.5 ka,
the record presents a strong case for pore water suboxia near the
sediment–water interface. During MIS 3, mostly elevated Ag
(79–273 ppb) and Cd (0.22–1.26 ppm) concentrations
coupled with a trend of relatively stable Re concentrations (average
16.3 ppb) that do not correspond to changes in Ag and Cd
concentrations suggest that low bottom water O2 concentrations
acted synergistically with variations in primary productivity to
produce suboxic pore waters. Transitioning into the Last Glacial
Maximum (∼19.1–21.1 ka; ∼181–206 cm),
decreasing Ag and Cd and increasing Re concentrations (maximum
38.5 ppb) suggest that low bottom water O2
concentrations became more important in controlling sedimentary redox
conditions. Subsequent deglacial increases in Ag and Cd concentrations
that correspond to Re concentrations from ∼13.5–19.1 ka (∼70–181 cm) imply that suboxic pore waters resulted primarily
from high organic matter flux to sediments. After ∼13.5 ka
(above ∼70 cm), Ag, Cd, and Re concentrations greatly
decrease to near or below crustal concentrations, probably due to
oxidative burndown (see Sect. 4.1). The Mo record is dissimilar to the
other trace metals and suggests little, if any, authigenic enrichment
(Fig. 3d). A possible exception occurs ∼25.4 ka (∼309 cm) when a corresponding increase in Cd concentrations is
observed.
Glacial- and deglacial-age Ag, Cd, and Re enrichments (Fig. 3a–c) do
not likely result from lithogenic input. Estimates of terrigenous
content do not exceed 35 % (Latimer and Filippelli, 2001, Fig. 3g)
and Ag, Cd, and Re concentrations are nearly always greater than bulk
continental crust, a purely lithogenic phase
(Fig. 3a–c). Furthermore, Corg concentrations
(Fig. 3e) correspond well to Ag and Cd concentrations, substantiating
their interpretation as a record of export production.
The pattern of trace metal enrichments at TN057-13-4PC (modern Antarctic
Zone) is dissimilar to that at RC13-254. The overall record (Fig. 4a–d)
suggests that bottom waters at the site have been well ventilated for
approximately the last 25 kyr, with organic matter respiration mostly
responsible for periods of sedimentary suboxia. Silver, Cd, and Re show large
enrichments prior to ∼16 ka (below ∼760 cm)
(Fig. 4a–c). Rhenium concentrations as high as 65 ppb (Fig. 4c)
between ∼25.2 and 30.6 ka (∼860–921 cm) suggest that
low bottom water O2 concentrations caused suboxic pore waters during
this time. Rhenium concentrations then decrease as Ag and Cd concentrations
increase to maxima of 589 ppb and 1.13 ppm, respectively,
between ∼23.4 and 25.2 ka (∼840–860 cm). These trends
suggest increasing export production and bottom water O2
concentrations. From ∼16.2–23.4 ka (∼760–840 cm) Re
peaks show reasonable agreement with Ag and Cd, indicating a change to
suboxia driven by high export production. From ∼11–16.2 ka (∼560–760 cm), relatively low Ag, Cd, and Re concentrations suggest
improved bottom water ventilation at the site. Long-term increases in Ag, Cd,
Re, and Corg concentrations between ∼5.5 and 11 ka
(∼200–560 cm) suggest that trace metal enrichments responded
to pore water suboxia driven by high export production, but coeval opal
fluxes decline. Well-oxygenated sediments similar to modern conditions
appeared ∼5.5 ka (∼200 cm), coincident with the onset of
the Neoglacial (Hodell et al., 2001) and increased sea ice presence at the
site (Stuut et al., 2004, Fig. 4h). Burndown and/or a slowdown in trace metal
delivery appears to be responsible for low trace metal concentrations after
∼5.5 ka (see Sect. 4.1). Similar to site RC13-254, Mo concentrations
are low throughout the core (Fig. 4d), thus indicating no authigenic
enrichment.
Estimates of terrigenous content from Ti-normalization (Fig. 4g)
suggest that increased lithogenic input probably contributed to high
total Ag, Cd, and Re concentrations during early MIS 2 and MIS
3. However, similar to site RC13-254, estimates of terrigenous content
are no more than 55 % and therefore cannot be the only factor
contributing to high trace metal concentrations. Sedimentary
Corg concentrations (Fig. 4e) also correspond
reasonably well to Ag and Cd concentrations in TN057-13-4PC. However,
as noted in Sect. 4.1, it is impossible to unequivocally state that
oxidative burndown near the glacial–interglacial transition has not
affected the record. Additionally, increasing terrigenous input might
dilute biogenic phases.
In summary, Ag, Cd, and Re enrichments in sediments from the modern
APFZ (RC13-254) and the Antarctic Zone (TN057-13-4PC) reveal
millennial-scale changes in both productivity and bottom water
ventilation. These changes indicate a significant shift in Southern
Ocean water chemistry from the last glacial period and deglaciation to
the late Holocene, when sedimentary trace metal accumulation is nearly
absent. Additionally, the two cores are separated by only ∼5∘ of latitude, and so the different trace metal records
demonstrate glacial/deglacial spatial heterogeneity in the Southern
Ocean.
Bottom water ventilation changes
Constraining the timing, location, and intensity of bottom water ventilation
changes can help to unravel the mechanism of glacial–interglacial
atmospheric CO2 variations. Rhenium is a valuable tool for assessing
bottom water ventilation changes because it precipitates directly from
seawater under suboxic conditions and is not cycled with biogenic particles
(Colodner et al., 1993; Crusius et al., 1996). The radiocarbon
(14C) content of deep ocean waters is an alternate, more common
measure of water mass ventilation that has been applied to the last glacial
period and deglaciation (e.g., Broecker and Barker, 2007; Marchitto et al.,
2007; Robinson et al., 2005). By comparing these two measures of bottom water
ventilation, a consistent picture emerges in the Atlantic. Rhenium
concentrations from RC13-254, Δ14C apparent ventilation ages
from Subantarctic core MD07-3076 (44.074∘ S, 14.208∘ W,
3770 m) (Skinner et al., 2010), and Δ14C apparent
ventilation ages from northeast Atlantic core MD99-2334K (37.80∘ N,
10.17∘ W, 3146 m) (Skinner et al., 2014) were binned into
500 year intervals to establish a common age scale. Both Δ14C
apparent ventilation age records are positively correlated with Re
concentrations at this temporal resolution (Fig. 5). Most prominently,
elevated Re concentrations from ∼19 to 20 ka correspond to the highest
measured benthic-atmospheric Δ14C offsets of nearly
4 kyr (Fig. 6), thus supporting the conclusion of Skinner
et al. (2010) that the carbon sequestration capacity of the Southern Ocean
was enhanced at this time due to slower ventilation rates at the depth of
modern LCDW. Overall, oxygen depletion appears to have intensified in the
deep Atlantic as global climate moved into the full glacial conditions of the
LGM, became less intense during deglaciation, and then Southern Ocean oxygen
concentrations reset to more modern conditions sometime after ∼13.5 ka.
A lower-resolution Re record from PS2489-2 (42.873∘ S,
8.973∘ E, 3700 m) (Martínez-Garcia et al., 2009,
Fig. 6b) corroborates the pattern from RC13-254 of lower Re
concentrations/lower ventilation ages during MIS 3 and higher Re
concentrations/higher ventilation ages during MIS 2. Rhenium data
(this study) also support the prediction (Sarnthein et al., 2013) that
oxygen-depleted conditions similar to the modern Eastern Pacific
oxygen minimum zone (see Sect. 4.2) occurred in the Atlantic sector of
the Southern Ocean at the LGM. Furthermore, recent deep-sea coral
ΔΔ14C measurements from Drake Passage (Burke and
Robinson, 2012) tentatively support better ventilation at depths
corresponding to modern UCDW compared to LCDW during the LGM.
Δ14C (MD07-3076) and trace metal (RC13-254) results
therefore suggest that ocean circulation changes may have inhibited
glacial LCDW from mixing with better ventilated waters and/or that
vertical stratification increased, similar to conclusions drawn from
δ18O and δ13C tracer data (Lund et al.,
2011). Other observations further support these conclusions: (1)
Glacial North Atlantic Intermediate Water (GNAIW) penetrated only to
∼30∘ S (Curry and Oppo, 2005), reducing ventilation
from the north. Lund et al. (2011) also suggest that shoaling of
modern North Atlantic Deep Water to GNAIW reduced mixing by bottom
topography. (2) Indian Ocean δ15N measurements
(Francois et al., 1997) suggest that increased Southern Ocean vertical
stratification was responsible for lowered bottom water O2
concentrations in glacial LCDW. One plausible mechanism to increase
Southern Ocean vertical stratification invokes increased sea ice
extent and cooling of the open ocean water column as a driver for
increased brine formation (Keeling and Stephens, 2001).
The sum of the data therefore points to glacial LCDW as the storage reservoir
of glacial CO2. Because modern LCDW forms the bulk of Indian and
Pacific Deep Waters (Talley, 2008), a low 14C signature during the
last glacial period is also expected for deep Pacific and Indian Ocean sites
downstream of glacial South Atlantic LCDW. In agreement with this prediction,
apparent ventilation ages ∼3600 years older than modern values (and
∼2500 years older than those reported by Skinner et al., 2010) were
measured at Chatham Rise (Sikes et al., 2000; Vandergoes et al., 2013), where
the Deep Western Boundary Current flows into the Southwest Pacific (Carter
and McCave, 1997). Additionally, increased LGM benthic-planktonic age
differences have been observed in the deep North Pacific (Galbraith et al.,
2007). Targeting glacial LCDW and its downstream deep water products (glacial
IDW and PDW, as well as deep Atlantic waters; Orsi et al., 1999) allows for
a radiocarbon-depleted Southern Ocean source to be diffused into a large
volume of deep ocean water, thereby circumventing the need for a smaller,
extremely depleted 14C reservoir (Broecker and Barker, 2007).
Based on glacial age benthic-planktonic age differences from the western
Pacific that are similar to modern values, it has been argued that an
isolated, 14C-depleted abyssal reservoir could only have existed
below 2.8 km water depth (Broecker et al., 2008). This extremely
“old” reservoir would then have dissipated by mixing into the global ocean
at the deglaciation (Broecker and Barker, 2007). However, Burke and Robinson
(2012) point out that the age of such a reservoir would need to be less
extreme if a portion of the trapped CO2 is allowed to vent directly
to the atmosphere, perhaps through increased Southern Ocean upwelling
(Anderson et al., 2009). Using the volume and average depth of each major
ocean basin, a simple calculation estimates that the total volume of the
Atlantic, Indian, and Pacific Oceans below 2.8 km water depth is
∼25 % of global ocean volume. Because modern LCDW feeds all three
basins, it is reasonable to suppose that glacial LCDW acted as the isolated
reservoir, dispersing radiocarbon-depleted water throughout the deep ocean.
Solid evidence for a globally distributed, “old” oceanic carbon reservoir
is so far lacking. However, benthic-planktonic age differences often assume
constant surface reservoir ages. If this assumption is not valid, and surface
water reservoir ages are high, the “true” age of deep waters may not be
apparent. Furthermore, CaCO3 is not well preserved in the deep
Pacific, and suitable samples from depths below 2.8 km are difficult
to obtain (Broecker et al., 2007).
Presently, LCDW bathes site TN057-13-4PC (Antarctic Zone); however, the lack
of strong Re enrichments (Fig. 4c; Table S1) suggests that a different,
well-ventilated water mass (perhaps a glacial equivalent of UCDW) existed at
2.8 km. The contrast with ventilation records from RC13-254 (Cape
Basin) showing older, more O2-depleted glacial LCDW suggests that the
modern water mass arrangement in which a lower-O2 component of CDW
overlies a higher-O2 component may have been reversed during the last
glacial period. The ventilation source at TN057-13-4PC is unclear, however,
and the paucity of cores from all sectors of the Antarctic Zone leaves the
glacial vertical water column structure an open question.
Comparison to other trace metal records
Most other trace metal records available from the Southern Ocean utilize Cd
or authigenic U as suboxic indicators, although interpreting bulk sediment U
is not straightforward because its accumulation is influenced by
a combination of bulk sedimentation rate/sediment focusing, organic carbon
rain rate, and bottom water O2 concentration (Frank et al., 2000).
Higher Cd and U accumulation rates/concentrations in Subantarctic sediments
during the last glacial period have been primarily interpreted to record
suboxic conditions caused by a northward shift of the ACC high productivity
zone (Anderson et al., 1998; Bareille et al., 1998; Chase et al., 2001; Frank
et al., 2000; Kumar et al., 1995; Rosenthal et al., 1995a). Elevated glacial
Ag, Cd, and Corg concentrations (Fig. 3a, b, e) concur with
previous work by demonstrating strong export production north of the modern
APF. In comparison, Antarctic Zone sediments from the Indian and Atlantic
sectors give inconsistent results. In the Atlantic sector, some studies show
minima in bulk sediment U concentrations and Corg and
Sibio (opal) fluxes at the LGM before increasing during
deglaciation (Anderson et al., 1998; Frank et al., 2000), when the high
productivity zone shifted southward again. Other studies from the Indian
sector demonstrate modest increases in bulk sediment Cd and U accumulation
rates during the LGM similar to deglacial accumulation rates (Rosenthal
et al., 1995a). Organic carbon accumulation rates also show a modest increase
during the LGM, but afterward increase rapidly during the early deglaciation,
suggesting that preservation issues could have influenced proxy accumulation.
South Atlantic trace metal concentrations (this study) follow the results of
Anderson et al. (1998) most closely by decreasing toward the LGM and
increasing through the deglacial and Holocene (Fig. 4a–d).
A simple explanation of trace metal enrichment patterns is complicated by Cd
and U accumulation trends that imply decreased CDW oxygen concentrations
(Bareille et al., 1998; Francois et al., 1997; Rosenthal et al., 1995a) in
addition to glacial–interglacial shifts of the high productivity zone.
Increasing Re concentrations coupled with decreasing Ag and Cd concentrations
during the LGM (Fig. 3a–c) support this conclusion, and suggest that export
production and bottom water O2 concentrations both influenced trace
metal accumulation north of the modern APF. However, available data do not
provide a clear indication of oxygenation levels in the glacial Antarctic
Zone, and how they may have changed over time. Further studies employing
a suite of redox proxies are needed from all three sectors of the Southern
Ocean to sufficiently characterize glacial deep water ventilation.
Conclusions
This study is the first to present a suite of trace metal (Ag,
Cd, Re, and Mo) concentrations in Atlantic sector Southern Ocean sediments
over the last ∼30 kyr, recording substantial changes in
regional ventilation and export production history. Higher concentrations of
all trace metals during deglaciation and the last glacial period, compared to
late Holocene sediments, reveal significantly different past Southern Ocean
water chemistry. Cape Basin sediments (RC13-254) were suboxic from at least
∼30 to ∼13.5 ka due to a combination of high export production
and reduced bottom water ventilation. After ∼13.5 ka, trace metal
concentrations fall to crustal levels as oxidative burndown erases the
authigenic record. The presence of burndown underscores the
glacial–interglacial redox shift in Southern Ocean water mass chemistry.
Furthermore, during the LGM (19–23 ka) elevated Re concentrations
unconnected to export production coincide with oldest Δ14C
apparent ventilation ages (Skinner et al., 2010, 2014).
Trace metal enrichments in the Antarctic Zone (TN057-13-4PC) are mostly
weaker than those from the Cape Basin and indicate suboxic sediments due to
enhanced export production from ∼16.2 to 25.2 ka, and during the early
Holocene from ∼5.5 to 11 ka. Increased Re concentrations indicate that
sedimentary suboxia due to poor bottom water ventilation may have existed
from ∼25.2 to 30.6 ka, although burndown during MIS 3 and/or at the
glacial–interglacial transition cannot be entirely ruled out.
Poorer ventilation of the glacial Southern Ocean requires changes in
global deep ocean circulation. We hypothesize that changes in water
mass mixing and stratification altered deep water chemistry such that
the model of a modified modern Southern Ocean vertical water column
structure may not be appropriate. Furthermore, our work suggests that
glacial CO2 was stored within glacial LCDW and its downstream
products, Indian and Pacific Deep Waters and the deep Atlantic.