Antarctic sea ice forms a critical part of the Southern
Ocean and global climate system. The behaviour of Antarctic sea ice
throughout the last glacial-interglacial (G-IG) cycle (12 000–130 000 years) allows us to investigate the interactions between sea ice and climate under
a large range of mean climate states. Understanding both temporal and
spatial variations in Antarctic sea ice across a G-IG cycle is crucial to a
better understanding of the G-IG regulation of atmospheric CO2, ocean
circulation, nutrient cycling and productivity. This study presents 28
published qualitative and quantitative estimates of G-IG sea
ice from 24 marine sediment cores and an Antarctic ice core. Sea
ice is reconstructed from the sediment core records using diatom assemblages and from the ice core record using sea-salt sodium flux. Whilst all regions of the Southern Ocean display the same overall pattern in G-IG sea-ice variations, the magnitudes and timings vary between regions. Sea-ice cover is most sensitive to changing climate in the regions of high sea-ice outflow
from the Weddell Sea and Ross Sea gyres, as indicated by the greatest
magnitude changes in sea ice in these areas. In contrast the Scotia Sea
sea-ice cover is much more resilient to moderate climatic warming, likely
due to the meltwater stratification from high iceberg flux through “iceberg alley” helping to sustain high sea-ice cover outside of full glacial intervals. The differing sensitivities of sea ice to climatic shifts between
different regions of the Southern Ocean has important implications for the
spatial pattern of nutrient supply and primary productivity, which
subsequently impact carbon uptake and atmospheric CO2 concentrations
changes across a G-IG cycle.
Introduction
Antarctic sea ice is a crucial component of the Southern Hemisphere and
global climate system, through a strong albedo feedback (Hall, 2004),
its influence on ocean-atmosphere gas exchange (Rysgaard et al.,
2011), its roles in deep and bottom water formation (Rintoul, 2018) and consequently oceanographic circulation (Maksym, 2019). Sea ice helps to stabilise Antarctic ice shelves and marine terminating ice streams by buffering against wave and ocean swell induced calving (Massom
et al., 2018). Sea ice is also a crucial habitat for many Antarctic
organisms (Arrigo, 2014). Over the last four decades
Antarctic sea-ice extent has shown a slight increasing trend, although
within the last 5 years there have been 2 years with the lowest annual sea-ice
extent in the observational records (Parkinson, 2019). The
causes and drivers of this observed sea-ice extent trend are not well
understood, with numerous potential mechanisms invoked (Bintanja et al.,
2013; Ferreira et al., 2015; Lecomte et al., 2017; Turner et al., 2016).
Model simulations struggle with the stochastic nature of the sea-ice system
and have been unable to replicate sea-ice changes during the observational
period without an unrealistically reduced warming trend (Rosenblum and
Eisenman, 2017). In order to better understand sea-ice changes and diagnose
the response of the ocean system to these changes at the multidecadal
time scale we must investigate the regional patterns, magnitudes and timing
of Antarctic sea-ice changes throughout a full glacial-interglacial (G-IG)
cycle. Investigating the last G-IG cycle will allow us to document the
interactions between sea ice and climate during both very different mean
climate states, such as the warmer than present last Interglacial, and
transient climate states, such as glacial terminations or inceptions.
Sea ice has been linked to changes in carbon sequestration, ocean
circulation and productivity across G-IG cycles
(Kohfeld et al., 2022). Over the last G-IG cycle (12–130 ka) atmospheric CO2 has been observed to decrease in a stepwise fashion, with at least 3 major intervals of CO2 drawdown at 115 000–100 000 years (115–100 ka), 72–65 and 40–18 ka (Jouzel et al.,
2007). These intervals correspond to the Marine Isotope Stage (MIS) or substage
boundaries (MIS 5e-5d, MIS 5a-4 and MIS 3-2, respectively). These changes in
atmospheric CO2 have been tied to several mechanisms operating in the
Southern Ocean, including (a) reductions in air-sea gas exchange, (b) enhanced stratification of surface waters, (c) reduced upwelling and
vertical diffusion, (d) deep ocean circulation changes and (e) changes in
marine biological productivity and nutrient cycling. None of these
mechanisms are mutually exclusive, and all can be linked in some way to
changes to Antarctic sea-ice cover. Specifically, sea ice regulates deep
ocean outgassing through both surface stratification and by acting as a
physical barrier (Rysgaard et al., 2011). Brine rejection during
sea-ice formation densifies Antarctic bottom waters, thus increasing water
column stratification and reducing vertical mixing (Bouttes et al., 2010;
Ferrari et al., 2014; Galbraith and de Lavergne, 2019). Both surface and
deep stratification of the water column enhance CO2 sequestration in
Southern Ocean abyssal waters. Finally, sea-ice cover in the Southern Ocean
influences marine primary productivity, and thus CO2 uptake, both by
acting as a barrier to light and through the release of nutrients,
especially iron, in meltwaters within the marginal ice zone (Arrigo
et al., 2008). Changes in Southern Ocean sea-ice extent across a G-IG cycle
have therefore the potential to play a key role in atmospheric CO2
variability (Kohfeld and Chase, 2017; Peacock et al., 2006). As
a result, understanding the timing and magnitude of sea-ice changes in the
Southern Ocean is a crucial question for understanding the G-IG regulation
of ocean circulation, productivity, nutrient cycling, and carbon uptake.
Diatom assemblages preserved in marine sediments currently provide the most
robust proxy for investigating past changes in Antarctic sea ice over long
time intervals (Crosta et al., 2022; Thomas et al., 2019). Sea ice can be
reconstructed from past diatom assemblages both qualitatively, using the
relative abundance of sea-ice-related diatoms, and quantitatively, using
statistical transfer functions. In this study we have compiled both
qualitative and quantitative published sea-ice proxy records from 24 Southern Ocean marine sediment cores, alongside 1 Antarctic ice core
sea-ice proxy record (Fig. 1), for the last 150 ka, with particular focus
on the 12–130 ka interval, to answer the following questions:
Did different regions of the Southern Ocean display the same patterns of sea-ice advance and retreat across the last G-IG cycle?
Was the timing and magnitude of sea-ice changes during the last G-IG cycle consistent between different Southern Ocean regions?
Was sea ice more resilient/sensitive to climatic shifts in the different regions of the Southern Ocean?
Materials and methodsCore sites
We have compiled 28 sea-ice proxy records, whereby 27 records
were reconstructed from 24 marine sediment cores (some cores have
multiple sea-ice reconstructions utilising different approaches). One record
comes from the EPICA Dome C (EDC) ice core (Table 1). Data are presented for
the 0–150 ka age range (Fig. 1) but the focus of this study is the 12–130 ka range, as this interval covers a single G-IG cycle from the midpoint of
Termination II to the midpoint of Termination I. The data from 0–12 and
130–150 ka are included to give additional insights into Southern Ocean
sea-ice behaviour during other G-IG periods. Of the
27 sea-ice proxy records from marine sediment cores, 14 cover the full 12–130 ka interval and the other 13 only cover
part of the interval (Table 1). The cores presented in this study are
located between 43 and 62∘ S (Fig. 1), recovered from an average
water depth of 3343 m (with a range from 1652 to 5214 m), and the majority
are located in the oceanographic region between the modern winter sea-ice
extent (WSIE) and the modern Antarctic Polar Front (APF). Almost all
diatom-based sea-ice records covering the last G-IG cycle are restricted to
the north of the mean WSIE. Retrieving adequate cores south of the mean WSIE
is difficult for two reasons. Firstly, heavy sea-ice cover limits biogenic
silica production. Secondly, most of this region overlies abyssal plains
with depths greater than 4000 m. As a result of both low production and a
long settling time, dissolution of the more lightly silicified diatom
species (generally sea-ice-related species) increases (Leventer,
1998), which biases the preserved diatom assemblage to reflect warmer and
lower sea-ice conditions (Warnock et al., 2015).
Details for the 24 sediment cores and 1
Antarctic ice core which have sea-ice proxy records presented in this study,
including location, temporal coverage and sample resolution of the data,
what form the reconstructed sea-ice data comes in and references for both
the data and chronology for each record. Cores are ordered by latitude from
north to south. FCC: combined relative abundance of the diatom species
Fragilariopsis curta and F. cylindrus, MS: magnetic susceptibility, Nass: sea-salt sodium, SID: sea-ice duration, SST: sea-surface temperature, WSIC: winter sea-ice concentration.
LatitudeLongitudeTemporalSampleSea-iceDataChronology(∘ S)(∘ E)coverageresolutionreconstructionreferences(ka): meantype(min–max)SK200/22a43.7045.072–950.8 (0.1–2.1)SIDNair et al. (2019)Radiocarbon ages combined with correlation of the planktonic and benthic δ18O to Antarctic ice cores (Byrd & EDML) and the LR04 δ18O stack (Manoj and Thamban, 2015).PS2499-546.51-15.330–1301.4 (0.1–4.1)FCCGersonde and Zielinski (2000)Radiocarbon ages and 230Th excess dating combined with biofluctuation stratigraphies for Eucampia antarctica and Cycladophora davisiana and planktonic δ18O compared to SPECMAP (Gersonde and Zielinski, 2000).SK200/2749.0045.221–730.3 (0.1–1.3)SIDNair et al. (2019)Radiocarbon ages combined with correlation of the planktonic and benthic δ18O to Antarctic ice cores (Byrd & EDML) and the LR04 δ18O stack (Manoj and Thamban, 2015).PS1778-549.01-12.7012–1342.3 (0.3–14)FCCGersonde and Zielinski (2000)Radiocarbon ages and 230Th excess dating combined with biofluctuation stratigraphies for Eucampia antarctica and Cycladophora davisiana and planktonic δ18O compared to SPECMAP (Gersonde and Zielinski, 2000).ODP 109349.985.8711–1502.7 (1.1–6.3)FCCSchneider Mor et al. (2012)Correlation of transfer function summer SSTs, Neogloboquadrina pachydermaδ18O and MS to δD and dust records from the EDC ice core (Schneider Mor et al., 2012).PS1768-852.594.483–1461.6 (0.4–2.6)WSICEsper and Gersonde (2014)Radiocarbon ages and regional correlation of diatom species composition, estimated summer SSTs and MS between proximal cores (Xiao et al., 2016a) combined with 230Th excess dating (Frank et al., 1996).PS2102-253.07-4.990–35 72–1331 (0.3–4)WSIC FCCXiao et al. (2016a) Bianchi and Gersonde (2002)Radiocarbon ages combined with regional correlation of diatom species composition, estimated summer SSTs and MS between proximal cores (Xiao et al., 2016a). N. pachydermaδ18O changes compared to SPECMAP combined with diatom biofluctuation zones (Bianchi and Gersonde, 2002).ODP 109453.185.130–1502.4 (0.3–4.8)FCCSchneider Mor et al. (2012)Correlation of transfer function summer SSTs, N. pachydermaδ18O and MS to δD and dust records from the EDC ice core (Schneider Mor et al., 2012).TN057-13-PC453.205.100–460.9 (0.5–1.9)SIDStuut et al. (2004)Radiocarbon ages (Shemesh et al., 2002).
Continued.
LatitudeLongitudeTemporalSampleSea-iceDataChronology(∘ S)(∘ E)coverageresolutionreconstructionreferences(ka): meantype(min–max)PS2606-653.2340.801–360.4 (0.1–0.8)WSICXiao et al. (2016a)Radiocarbon ages combined with regional correlation of diatom species composition, estimated summer SSTs and MS between proximal cores (Xiao et al., 2016a).PS1652-253.665.101–331.8 (0.7–3.7)WSICXiao et al. (2016a)Radiocarbon ages combined with regional correlation of diatom species composition, estimated summer SSTs and MS between proximal cores (Xiao et al., 2016a).TPC06353.92-48.040–460.4 (0.1–4)FCCCollins et al. (2013)Correlation of relative paleointensity to the SAPIS RPI stack combined with Eucampia antarctica biofluctuation stratigraphy (Collins et al., 2012).PS2276-454.64-23.570–1301.8 (0.5–3.5)FCCGersonde and Zielinski (2000)Radiocarbon ages and 230Th excess dating combined with biofluctuation stratigraphies for Eucampia antarctica and Cycladophora davisiana and planktonic δ18O compared to SPECMAP (Gersonde and Zielinski, 2000).SK200/3355.0245.156–1501.5 (0.1–5)WSIC & SIDGhadi et al. (2020)Radiocarbon ages combined with the correlation of SST and SID to the temperature record from the EDC ice core (Ghadi et al., 2020).PS67/197-155.14-44.114–30 30–860.4 (0.2–0.7)WSIC FCCXiao et al. (2016a) Xiao (2011)Radiocarbon ages and regional correlation of Scotia Sea ash layers combined with MS correlation to the EDML ice core dust record, diatom abundance fluctuation patterns and recorded geomagnetic excursions (Xiao et al., 2016b).TPC07855.55-45.021–311.4 (0.7–2.5)FCCCollins et al. (2013)Radiocarbon ages combined with MS correlation to EDC ice core dust (Collins et al., 2012).SO136-11156.67160.233–1501.1 (0.6–2.2)SIDCrosta et al. (2004)Radiocarbon ages combined with the correlation of N. pachydermaδ18O to the SPECMAP reference stack (Crosta et al., 2004).PS67/219-157.22-42.475–30 30–1501 (0.2–10.1)WSIC FCCXiao et al. (2016a) Xiao (2011)Radiocarbon ages and regional correlation of Scotia Sea ash layers combined with MS correlation to the EDML ice core dust record, diatom abundance fluctuation patterns, recorded geomagnetic excursions and the last common occurrence of Rouxia leventerae (Xiao et al., 2016b).PS75/072-457.56-151.221–1501.1 (0.4–13.7)FCCStuder et al. (2015)Radiocarbon ages and the correlation of δ18O to the LR04 stack combined with regional correlation of elemental composition and MS between proximal cores (Benz et al., 2016).
Continued.
LatitudeLongitudeTemporalSampleSea-iceDataChronology(∘ S)(∘ E)coverageresolutionreconstructionreferences(ka): meantype(min–max)TAN1302-9659.09157.052–1401.8 (0.4–6.5)WSIC & SIDJones et al. (2022)Radiocarbon ages combined with the correlation of N. pachydermaδ18O to the LR04 stack (Jones et al., 2022).ELT27-2359.62155.242–521 (0.1–4.9)WSICFerry et al. (2015a)Radiocarbon ages combined with the correlation of N. pachydermaδ18O to the LR04 stack (Ferry et al., 2015a).TPC03459.79-39.600–621.8 (0.7–3.3)FCCAllen et al. (2011)Eucampia antarctica and Cycladophora Davisiana biofluctuation stratigraphies combined with correlation of the MS to EDC ice core dust (Allen et al., 2011).PS58/271-161.24-116.056–1480.6 (0.2–1.6)WSICEsper and Gersonde (2014)Correlation of physical parameters, elemental composition, diatom assemblage composition and derived WSIC and SSTs to the EDC ice core record combined with diatom biostratigraphic data (Esper and Gersonde, 2014).TPC28661.79-40.140–952 (0.2–21.9)FCCCollins et al. (2013)Correlation of relative paleointensity to the SAPIS RPI stack combined with Eucampia antarctica biofluctuation stratigraphy (Collins et al., 2012).EDC75.10123.351–1502 (2–2)Nass fluxWolff et al. (2006)Glaciological ice flow model constrained by dated volcanic horizons and matching of isotopic variations to other ice core records (Parrenin et al., 2007).
FCC (combined relative abundance of F. curta and F. cylindrus), WSIC (winter sea-ice concentration) and SID (sea-ice duration) records from Southern Ocean marine sediment cores and the Nass flux record from the EDC ice core shown
alongside a map of Antarctica with core locations, modern February (blue
line) and September (black line) sea-ice extents (data from
Fetterer et al., 2017) and the modern Antarctic Polar Front
position (grey line; Trathan et al., 2000). Dashed lines mark
the modern extents of the Weddell Sea (WS) and Ross Sea (RS) gyres. Downcore
records are arranged together in Southern Ocean regions, with distance from
the modern winter sea-ice (WSI) edge given in degrees latitude (plus sign
indicates locations north of the mean WSI edge, minus sign indicates locations south
of the mean WSI edge).
We selected records that stretch back beyond 30 ka, to ensure that they
cover more than just MIS 2, and also excluded short-term or “snapshot”
reconstructions (e.g. the Last Glacial Maximum synthesis by
Gersonde et al., 2005, or the MIS 5e synthesis by
Chadwick et al., 2020). Any records with a mean sample
resolution of > 3 ka were also excluded from this synthesis. To
avoid additional chronological uncertainties from converting between
multiple different age models (e.g. Capron et al., 2014), the
chronologies for all the records in this study are kept as originally
published. Therefore, the records presented in this study have a
chronological uncertainty of ∼ 2–4 ka (Bazin et al., 2013b;
Lisiecki and Raymo, 2005; Parrenin et al., 2007; Veres et al., 2013), which
is not significant compared to the time scale we focus on.
Sea-ice reconstructions
In this study, we present both quantitative and qualitative reconstructions
of sea ice (Fig. 1). Quantitative reconstructions are either the winter
sea-ice concentration (WSIC) or the sea-ice duration (SID), with two cores
(SK200/33 and TAN1302-96) that have both a WSIC and SID record. All but one
(core ELT27-23) of the quantitative reconstructions use a modern analog
technique (MAT) diatom transfer function, with cores PS1768-8, PS2102-2,
PS2606-6, PS1652-2, PS67/197-1, PS67/219-1 and PS58/271-1 run using the MAT
transfer function detailed in Esper and Gersonde (2014) and cores
SK200/22a, SK200/27, TN057-13-PC4, SK200/33, SO136-111 and TAN1302-96 run
using the MAT transfer function detailed in Crosta et al. (1998)
and their evolutions. Core ELT27-23 uses the generalised additive model
transfer function detailed in Ferry et al. (2015b). A comparison
of reconstructed sea-ice values using both the MAT and generalised additive
model transfer functions shows that both techniques yield robust and broadly
similar results (Ferry et al., 2015b) and supports our inclusion
in this study of records produced with either technique.
For cores where no quantitative sea-ice reconstructions were available we
have used a qualitative reconstruction in the form of the combined relative
abundance of the diatom species Fragilariopsis curta and F. cylindrus (FCC). The FCC proxy is a qualitative
indicator of winter sea-ice (WSI) presence, with abundances > 3 % associated with locations south of the mean WSI edge (Gersonde and
Zielinski, 2000). The sea-salt sodium (Nass) flux in the EDC ice core is also a qualitative indicator of WSIE, with a higher Nass flux associated with a greater WSIE (Wolff et al., 2010).
Normalised FCC (combined relative abundance of F. curta and
F. cylindrus), WSIC (winter sea-ice concentration) and SID (sea-ice duration) records from Southern Ocean marine sediment cores and the Nass flux record from the EDC ice core shown alongside a map of Antarctica with core locations, modern February (blue line) and September (black line) sea-ice extents (data from Fetterer et al., 2017) and the modern Antarctic Polar Front position (grey line; Trathan et al., 2000). Downcore records are arranged together in Southern Ocean regions, with distance from the modern winter sea-ice (WSI) edge given in degrees latitude (plus sign indicates
locations north of the mean WSI edge, minus sign indicates locations south of the
mean WSI edge). Each region also has a stacked record with the mean (black
line) and interquartile range (grey shading) shown.
Alongside the raw sea-ice data, we have also normalised all 28
records (Fig. 2) to allow a comparison of the timing of major changes
in sea ice between records. We used the following normalisation formula:
(Xsample-Xmean)/(Xmax-Xmin), where the mean, max and min are calculated using all available data within the 0–150 ka interval for
each record. Alongside the individual normalised sea-ice records we also
present a stack of normalised records for five different regions of the
Southern Ocean (Figs. 2 and 4). In order to stack the normalised records
for each region, the individual normalised records were first resampled at 2 ka resolution. This age resolution was chosen as a compromise between
maintaining the variation in each record and minimising the amount of
interpolation required.
Standardised WSIC (winter sea-ice concentration) and SID (sea-ice
duration) records from Southern Ocean marine sediment cores shown alongside
a map of Antarctica with core locations, modern February (blue line) and
September (black line) sea-ice extents (data from Fetterer et al.,
2017) and the modern Antarctic Polar Front position (grey line;
Trathan et al., 2000). Downcore records are arranged together
in Southern Ocean regions, with distance from the modern winter sea-ice
(WSI) edge given in degrees latitude (plus sign indicates locations north of the
mean WSI edge, minus sign indicates locations south of the mean WSI edge). Unlike in
Figs. 1 and 2, records from the East and Central Atlantic sectors have
been combined into a single Atlantic sector region.
Comparison of the stacks of normalised sea-ice records from the
five Southern Ocean sectors (black curves mark the means and grey shading
marks the interquartile ranges) alongside the stack of normalised Southern
Ocean sea-surface temperature (SST) records (the blue curve marks the mean
and the blue shading marks the interquartile range), the normalised LR04
benthic δ18O stack (purple curve; Lisiecki and Raymo, 2005), the normalised atmospheric CO2 record from the EDC ice core
(red curve; Bazin et al., 2013a) and PC1 (green
curve). The selection of the SST data is detailed in Sect. S1. Vertical
dashed lines mark MIS boundaries, with each MIS numbered in the centre of
the figure.
Whilst the normalised records allow us to investigate changes in the timing
of major sea-ice changes between records they do not allow a comparison of
the magnitude of sea-ice changes. For this comparison we have standardised
the quantitative sea-ice records (Fig. 3) using the following formulas:
(XWSIC-20)/100 and (XSID-1)/10. The 20 % and 1 month yr-1
values in the two formulas represent the mean WSIE, north of which sea ice
occurs only episodically. This means x-axis values in the Fig. 3 graphs
can indicate the relative position of the mean WSI edge, with positive
x-axis values indicating periods when the mean WSI edge is located north
(equatorward) of the core site and negative values indicating when the mean
WSI edge is to the south (poleward) of the core site. The 100 % and 10 months yr-1 in the two formulas are scaling factors that show how the WSIC or SID compares to a theoretical maximum. In the modern day, SIDs < 8 months yr-1 have a good linear correlation to WSICs (Fig. S1 in the Supplement). Given all the SID values in our records are < 5 months yr-1 we have used a theoretical maximum of 10 months yr-1 SID to keep this scaling against WSICs,
rather than the actual maximum of 12 months yr-1 SID. The FCC and Nass
flux proxies are only qualitative indicators of sea ice and cannot be scaled
in the same way as the WSIC and SID records. Records with these qualitative
proxies are thus excluded from the map of standardised records (Fig. 3).
To allow the comparison of sea-ice patterns and trends between different
regions of the Southern Ocean the records in Figs. 1 and 2 are grouped
into: the Scotia Sea (60–30∘ W), the Central Atlantic (30–0∘ W), the East Atlantic (0–30∘ E), the West Indian (30–60∘ E) and the Pacific (150∘ E–105∘ W) sectors. These sectors
were chosen to allow us to investigate longitudinal variations in sea ice,
whilst ensuring there are at least four core records in each sector. In
Fig. 3 the Central and East Atlantic sectors are combined into a single
Atlantic sector (20∘ W–30∘ E) due to the smaller number of
standardised records.
Principal component analysis
A principal component analysis (PCA) was applied to the resampled and
normalised data using PAST v3.15 software (Hammer et al., 2001) in
order to identify the main trends present in the 27 marine-based
sea-ice records (from 24 sites) and the EDC Nass flux record. We applied the PCA to the normalised data to avoid over-representation of a given proxy or record (e.g. FCC varies between 0 % and 20 % in our records whereas Nass flux varies between 200 and 1000 µg m-2 yr-1). The
PCA was performed over the entire 0–150 ka period. Many of the records do
not cover the whole period (Fig. 1) and missing values were coped with by
using the mean value imputation option.
Results and discussionPatterns and trends in sea ice
The PCA of all the Southern Ocean sea-ice records in this study reveals that
the dominant trend, principal component (PC) 1, in sea ice during the last
130 ka is the G-IG cyclicity, with high sea-ice cover during MIS 2 and low
sea ice during MIS 5 (Figs. 4 and S2). PC1 explains 40 % of the total
variance, while PC2 accounts for only ∼ 10 % (Fig. S3)
and if the PCA is applied solely to the long records, which cover over half
of the 0–150 ka interval, PC1 accounts for > 50 % of the
variance and PC2 is reduced to 8 %. All PCs except PC1 fall at or below
the broken stick curve (Frontier, 1976; Fig. S3) and therefore, only
PC1 appears to bear enough signal to be interpreted. The G-IG trend in PC1
is also evident in the normalised stacks for the different sectors of the
Southern Ocean, with the exception of the Scotia Sea sector where there is
high sea ice throughout the 0–100 ka period (Figs. 2 and 4). The PCA does
not produce any grouping based on the different methods of sea-ice
reconstruction used (i.e. FCC, WSIC, SID) (Fig. S4), which suggests that
all three estimates of sea ice display similar patterns and support our
decision to include all three in this study. There is also no apparent PCA
grouping by Southern Ocean sector, although the two most southerly Scotia
Sea sector cores (TPC034 and TPC286) do have a very different trend to all
the other records (Fig. S4).
All Southern Ocean sectors in this study have the same pattern in sea-ice
retreat, with a prominent drop during the glacial termination at 140–130 ka,
but the pattern in sea-ice advance varies between the sectors (Figs. 1, 2
and 4). The West Indian and Pacific sectors have low sea ice from the
glacial termination through until ∼ 80–70 ka, after which the
normalised sea-ice values display a gradual increase to maxima at 25–20 ka
(Figs. 2 and 4). The East Atlantic sector also has low sea ice until
∼ 70 ka but this is followed by a rapid increase to high
normalised sea-ice values during MIS 4 and then a retreat at the beginning
of MIS 3, although not as pronounced as during MIS 5 (Fig. 4). Normalised
sea-ice values in the East Atlantic then increase gradually from
∼ 55 ka until a maximum concurrent with the other Southern
Ocean sectors (Fig. 4). Both the Central Atlantic and Scotia Sea sectors
have sea-ice expansion to glacial or near-glacial levels in the normalised
values by 100 ka. Normalised sea-ice values in the East Atlantic reach a
maximum during the 30–20 ka period in phase with other sectors, except in
the Scotia Sea where normalised sea-ice values do not present much variation
over the last 100 ka. As such, the pattern in the Central Atlantic sector sea ice indicates a transition between the normalised sea-ice trends in the East Atlantic and Scotia Sea sectors (Figs. 2 and 4). The EDC Nass flux record has an almost bimodal pattern in values, with both rapid decreases and increases between the interglacial and glacial “states” (Figs. 1 and 2). Between 70 and 20 ka there is almost no variation in the EDC Nass
flux whereas between 130 and 110 ka there is almost a quadrupling of the EDC
Nass flux (Fig. 1). This is in contrast to the trends seen in the
sediment core sea-ice records (Figs. 1, 2 and 4) and was suggested by
Röthlisberger et al. (2010) to indicate that the Nass
flux in Antarctic ice cores is more sensitive to sea-ice changes during
interglacial intervals than during glacial intervals. However, it should be
noted that the EPICA Dronning Maud Land (EDML) ice core (75.00∘ S, 0.07∘ E) Nass flux record does show more variation during the 70–20 ka
interval (Fischer et al., 2007), which suggests that sea-ice cover
may be more dynamic in the Weddell Sea than around the Wilkes Land margin,
in agreement with the results in Ghadi et al. (2020).
MIS 5 substages can be seen in both the EDC record and several of the
Pacific and Atlantic sector core records (PS1778-5, PS1768-8 and
PS75/072-4), whereas the Indian sector records maintain low sea ice from
Termination II through until MIS 4 (Fig. 1). The appearance of these
substages in the records for cores PS75/072-4 and PS1768-8 (Fig. 1) is
likely due to these cores being located near the output regions of the Ross
Sea and Weddell Sea gyres, respectively. Both the raw and normalised FCC
records from core PS67/219-1 also appear to show MIS 5 substages but with an
offset of ∼ 5 ka to younger ages compared to the other records
(Figs. 1 and 2).
For the standardised records in Fig. 3, positive values indicate intervals
when the core site is located south of the mean WSI edge. The standardised
sea-ice records indicate that the majority of cores with WSIC or SID
reconstructions were located within the mean WSIE during MIS 2, with the
exception of the most northerly West Indian sector cores (Fig. 3). Our
compilation shows that most regions experienced a first significant sea-ice
expansion during MIS 4, even though some records indicate that sea ice may
have expanded during MIS 5 stadial periods (Figs. 1, 2 and 4). However, a
robust assessment of the position of the mean WSI edge prior to MIS 4 is
complicated by the scarcity of standardised records for this period (Fig. 3). In the West Indian sector the mean WSI edge was located south of all the
core sites in this study until MIS 2 (Fig. 3). In the Pacific sector the
SO136-111 and TAN1302-96 core sites were located north of the mean WSI edge
during the 130–70 ka interval (MIS 5), with the standardised record for
TAN1302-96 indicating it was north of the mean WSI edge during the 130–25 ka
interval (MIS 5 through MIS 3, inclusive). In contrast, the standardised
sea-ice record for core PS58/271-1 shows high frequency, but low amplitude,
variability around the y-axis for the majority of the interval from 110 to
30 ka (Fig. 3), indicating that the mean WSI edge was located at or near
this core site throughout this period.
Magnitude of sea-ice changes
The standardised records show that the largest amplitude changes in sea ice
are found in the core records located at the output of the Weddell Sea gyre
(Atlantic sector, Fig. 3), indicating that the sea ice in this region is
the most sensitive to climate variations. The higher sensitivity of sea ice
at the Weddell gyre output than other regions of the Southern Ocean supports
the greater sea-ice advance seen in this region in studies of the Last
Glacial Maximum (Gersonde et al., 2005) as well as the substantial
retreat estimated by model runs of the Last Interglacial
(Holloway et al., 2017). The output regions of Southern
Ocean gyres are areas of high sea-ice drift in the present day, with changes
in Southern Hemisphere winds strongly influencing this drift
(Holland and Kwok, 2012; Kwok et al., 2017). As discussed in
Sect. 3.1, cores located at the output of the Southern Ocean gyres also
show larger normalised sea-ice variations during the colder MIS 5 substages
(5b and 5d) than other cores (Fig. 2), further indicating the sensitivity
of sea ice in these regions to lower amplitude climatic shifts than full
G-IG transitions. Many of the cores in this study, especially those in the
West Indian and western Pacific sectors, show relatively little sea-ice
signal outside of the higher values during MIS 2-4 (Fig. 1), suggesting
that these cores are located too far north to pick up small magnitude
changes in sea ice.
In the East and Central Atlantic sectors, the normalised stacks indicate a
substantial reduction in sea ice during MIS 3 (Fig. 4). For the Central
Atlantic sector stack this trend is largely driven by the normalised sea-ice
values in core PS1778-5 (Fig. 2), which is the most easterly Central
Atlantic sector core site for which an MIS 3 record is presented (Fig. 2).
This behaviour in the two Atlantic sectors contrasts to the normalised
stacks for the other Southern Ocean sectors, where high sea ice is
maintained throughout the entire MIS 2-4 interval (Fig. 4). In the Pacific
sector the only records to show low sea ice during MIS 3 are those from core
TAN1302-96, in clear contrast with the nearby SO136-111 and ELT27-23 core
records (Fig. 1). However, the lower sea ice during MIS 3 in core
TAN1302-96 compared to SO136-111 is attributed to a lower sample resolution
in the former (Jones et al., 2022). The reduction in sea ice during MIS 3 in the Central and East Atlantic but not in the other Southern Ocean
regions, raises intriguing questions regarding the spatial differences in
Antarctic sea-ice sensitivity. Additional records in these other Southern
Ocean sectors, at more southerly latitudes, are necessary to assess the
robustness of the different sea-ice patterns during MIS 4-3 and the
underlying drivers.
The Scotia Sea records indicate an enlarged sea-ice extent, relative to the
modern, almost throughout the last 150 ka, with the only exception being
during MIS 5e (Fig. 1). The high raw and standardised WSIC values in cores
PS67/197-1 and PS67/219-1 indicate that these cores were located south of
the mean WSI edge in the Late Holocene at ∼ 4 ka (Figs. 1
and 3). This suggests that the Late Holocene WSI edge in the Scotia Sea was
located ∼ 5∘ further north than its present location. The
location of these cores in the modern day “iceberg alley” (Weber et al.,
2014) could account for the pervasive sea-ice presence and low sensitivity
to climatic variations. A maintained flux of icebergs through this region
would release cold and fresh meltwaters, stratifying the surface ocean and
promoting sustained sea-ice presence, even during warmer periods. The
notable reduction during MIS 5e in the Scotia Sea could therefore either
indicate a change in the pathway or flux of Weddell Sea icebergs, or
sufficiently warm sea-surface temperatures, maybe as a result of a more
southerly position of the Antarctic Circumpolar Current in the Drake Passage
and Scotia Sea region (Wu et al., 2021), to outweigh the influence of the
iceberg meltwaters. The large amplitude changes in sea ice at the Weddell
Sea gyre output, as discussed above, are also likely related to “iceberg
alley”, with an easterly expansion of the modern iceberg field (Weber et
al., 2014) during colder glacial periods providing greater iceberg meltwater
stratification in the East Atlantic sector, which promoted and maintained an
increased WSIE.
Timing of sea-ice changes
The different chronologies applied to the records in this study (Table 1)
prevent us from being able to identify any small (∼ 2–4 ka)
differences in the timing of sea-ice changes between the different Southern
Ocean regions. However, during both Termination I and II, we observe up to 5 ka offsets in the retreat time of sea ice between the different regions
(Fig. 4). For both the Pacific and West Indian sectors, the normalised
stack shows that sea-ice retreat during Termination II started at
∼ 140 ka and was finished by ∼ 130 ka, largely
coincident with the increases in both the Southern Ocean SSTs (cores sites
shown in Fig. S5) and the atmospheric CO2 record from the EDC ice
core (Figs. 4 and S6). In contrast, the normalised stacks for both the
Central and East Atlantic sectors display sea-ice retreat between
∼ 135 and ∼ 125 ka, consistent with the
timing of Termination II in the LR04 benthic δ18O stack (Fig. 4). In the Scotia Sea sector, the only record covering Termination II is
from core PS67/219-1 and indicates a longer period of sea-ice retreat
lasting from ∼ 140 until ∼ 125 ka (Figs. 1
and 4). The earlier retreat in the West Indian sector compared to the two
Atlantic sectors is likely due to the West Indian sector cores being located
further north of the modern winter sea-ice extent than the Atlantic sector
cores, with an average of 9.2∘ of latitude compared to 3.8∘,
respectively (Fig. 1). However, this does not explain the earlier sea-ice
retreat in the Pacific sector, where the cores are located an average of
only 3.7∘ of latitude north of the modern winter sea-ice extent
(Fig. 1).
During Termination I, the Pacific sector normalised stack again displays an
earlier sea-ice retreat than the other regions, with retreat starting at
∼ 21 ka compared to ∼ 18 ka in the East Atlantic
and West Indian sectors and ∼ 16 ka in the Central Atlantic
sector (Fig. 4). Whilst the timing of Termination I in the East Atlantic
and West Indian sectors occurs within chronological uncertainty of
Termination I in either the Central Atlantic or the Pacific sector, these
latter regions differ by > 4 ka in their timing for Termination I. The onset of Termination I sea-ice retreat in the East Atlantic, Central
Atlantic and West Indian sectors is concurrent with the onset of Southern
Hemisphere surface air temperature increases, which dominantly begin at
∼ 17 ka (Osman et al., 2021),
and increase in Southern Ocean SSTs (Fig. 4). The earlier sea-ice retreat
during Termination I in the East Atlantic sector normalised stack, compared
to the Central Atlantic sector stack, is likely due to the proximity of the
eastern core sites to the Weddell Sea gyre output. Gersonde et al. (2005) reconstructed a “tongue” of summer sea ice in this region during the
Last Glacial Maximum which suggests sea ice in this area is more dynamic and
may have been more susceptible to early melting and retreat during
Termination I than the sea ice located further west. The discrepancy between
the earlier retreat of sea ice in the East Atlantic sector, relative to the
Central Atlantic sector, during Termination I but not Termination II could
indicate that either the sea-ice “tongue” in the East Atlantic sector was
more resilient to melting and retreat during MIS 6 than during MIS 2, or
that this “tongue” did not exist during MIS 6. The high sensitivity of East
Atlantic sector sea ice during Termination I is also indicated by the rapid
retreat to interglacial levels, with a deglaciation duration of only
∼ 2–4 ka compared to the ∼ 5–8 ka deglaciation
duration in the other Southern Ocean sectors (Fig. 4). The normalised
stack for the Scotia Sea does not show any clear signal for Termination I
(Fig. 4), with only cores TPC063 and TPC078 displaying the sharp decrease
in sea ice between 20 and 12 ka that characterises Termination I in the
other records (Fig. 1).
Summary and outstanding questions
The present compilation of sea-ice records suggests that all regions of the
Southern Ocean display the same general pattern in sea ice during the last
G-IG cycle, with high sea ice during MIS 2 and 4 and low sea ice during MIS 5e. However, the different areas of the Southern Ocean do have varying
magnitudes, timing and short-term trends in sea ice during the 12–130 ka
interval, especially with respect to sea-ice advance between MIS 5e and 4.
Sea-ice cover seems to be most sensitive to changing climate at the outputs
of the Weddell Sea and Ross Sea gyres, with the greatest magnitude sea-ice
changes occurring in cores located in these regions. Records located near
the Weddell Sea gyre output have especially high sensitivity, with a more
rapid sea-ice advance and retreat than other areas of the Southern Ocean and
a more pronounced MIS 3 sea-ice reduction than the other regions. In
contrast, the sea-ice cover in the Scotia Sea region is seemingly much more
resilient to changing climate, with the only notable reduction during the
peak of the MIS 5e interglacial. This resilience to moderate warming could
be related to the large iceberg flux through the Scotia Sea, with
iceberg-meltwater stratification and cooling helping to sustain sea ice
outside of full glacial intervals.
By compiling long-term sea-ice records of the last G-IG cycle from
throughout the Southern Ocean this study has identified how changes in sea
ice vary between different regions of the Southern Ocean. The more dynamic
state of sea ice in the East Atlantic over long time scales is likely
associated with the expansion and contraction of “iceberg alley”. Weddell
Sea icebergs have been identified as an important source of iron to the
Atlantic sector of the Southern Ocean (Shaw et al.,
2011) and large amplitude changes in the spatial extent of iceberg outflow
over G-IG cycles therefore helps explain the high magnitude variations in
G-IG iron flux in this region of the Southern Ocean
(Martínez-Garcia et al., 2009). Large amplitude changes in both
the East Atlantic sector WSIE and the spatial extent of “iceberg alley” over a G-IG cycle have important implications for the supply of nutrients,
especially iron, and subsequently primary productivity in this region. This,
in turn, helps regulate oceanic CO2 sequestration across a G-IG cycle,
in an area of the Southern Ocean with high modern primary productivity
(Vernet et al., 2019).
Our compilation also allows us to identify where further research would most
help to develop the results and findings of this study. It is clear from the
discussion in Sect. 3.3 that establishing a harmonised chronology across
all core records could help reduce the ∼ 2–4 ka chronological
uncertainties presented in this study and allow further analysis of any
short duration leads and lags between the sea-ice changes in the different
Southern Ocean sectors. Alongside an improved chronology, N-S transects of
sea-ice records, especially in the regions with the highest amplitude of
G-IG sea-ice variability (e.g. East Atlantic sector), would allow the rates
of sea-ice advance and retreat to be estimated. A refined chronology would
also allow sea-ice dynamics to be compared between hemispheres and provide
insights into how sea ice influences global ocean circulation dynamics
across a G-IG cycle.
Model-proxy comparisons of sea-ice extent over the last 130 ka display mixed
results, with good agreement between model and proxy estimates of WSIE
during the Last Glacial Maximum (Green et al., 2022) but
poor model-data consistency in estimates of summer sea-ice extent during the
Last Glacial Maximum (Green et al., 2022) and WSIE during the
Last Interglacial (Chadwick et al., 2022a; Otto-Bliesner et al., 2021).
Antarctic sea-ice changes during the observational period have also not been
replicated in model simulations, without an unrealistically reduced warming
trend (Rosenblum and Eisenman, 2017). Better parameterisation of sea
ice (and assimilation of proxy records) is required to improve our
understanding of the drivers and feedbacks active over both short and long
time scales.
Finally, whilst this study presents a good spatial coverage in some areas of
the Southern Ocean (e.g. the Atlantic sector), there are parts of the
Southern Ocean for which G-IG sea-ice changes are minimally constrained
(Figs. 1–3). The central and eastern Pacific sector (90–180∘ W)
currently has only two long G-IG sea-ice records and the eastern Indian
sector (60–150∘ E) has none. Similarly, the majority of the G-IG
sea-ice records presented here are located north of the modern mean WSIE
(Figs. 1–3), largely due to the difficulties associated with
reconstructing robust sea-ice estimates from diatom records located beneath
persistent or long duration sea-ice cover (as discussed in Sect. 2.1). The
development of new sea-ice proxies from beneath this persistent sea ice, e.g. highly branched isoprenoids (Belt, 2018; Lamping et al., 2021;
Vorrath et al., 2019) or DNA from sea-ice associated organisms (De
Schepper et al., 2019), could allow the reconstruction of sea ice from more
southerly core sites, without which any small amplitude sea-ice variations
during warmer interglacial periods (e.g. MIS 5a, 5c and 5e) cannot be fully
investigated.
Data availability
The sea-ice proxy data for all 27 marine sediment core records are available from PANGAEA (10.1594/PANGAEA.943275,
Chadwick et al., 2022b).
The supplement related to this article is available online at: https://doi.org/10.5194/cp-18-1815-2022-supplement.
Author contributions
MC contributed to the conceptualisation, data curation, formal analysis, investigation, methodology, visualisation and writing the original draft. XC contributed to the conceptualisation, formal analysis, methodology, resources, supervision, visualisation and reviewing and editing the writing. OE contributed to the conceptualisation, resources and reviewing and editing the writing. LT contributed to the data curation, investigation and resources. KEK contributed to the conceptualisation, funding acquisition, investigation, project administration, supervision and reviewing and editing the writing.
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.
Special issue statement
This article is part of the special issue “Reconstructing Southern Ocean sea-ice dynamics on glacial to historical time scales”. It is not associated with a conference.
Acknowledgements
This work was conducted as part of Phase 1 of the Cycles of Sea-Ice Dynamics
in the Earth system (C-SIDE) PAGES scientific working group. We thank both Claire S. Allen and Jacob Jones for directly providing us with their sea-ice proxy records from cores TPC034 and TAN1302-96, respectively.
Financial support
This research has been supported by a Past Global Changes Data Stewardship Scholarship (grant no. DSS_105) for Matthew Chadwick and a National Sciences and Engineering Research Council of Canada Discovery Grant (grant no. RGPIN342251) for Karen E. Kohfeld.
Review statement
This paper was edited by Marit-Solveig Seidenkrantz and reviewed by two anonymous referees.
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