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Antarctic deglacial pattern in a 30 kyr record of sea surface temperature offshore South Australia

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Comparison of ice cores from Greenland and Antarctica shows an asynchronous two-step warming at these high latitudes during the Last Termination. However, the question whether this asynchrony extends to lower latitudes is unclear mainly due to the scarcity of paleorecords from the Southern Hemisphere. New data from a marine core collected off South Australia (~36°S) allows a detailed reconstruction of sea-surface temperatures over the Last Termination. This confirms the existence of an Antarctic-type deglacial pattern and shows no indication of cooling associated with the Northern Hemisphere YD event. The SST record also provides a new comparison with the more extensive paleoclimatic data available from continental Australia. This shows a strong climatic link between onshore and offshore records for Australia and to Southern Hemisphere paleorecords. We also show a progressive SST drop over the last ~6.5 kyr not seen before for the Australian region.
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Antarctic deglacial pattern in a 30 kyr record of sea surface
temperature offshore South Australia
Eva Calvo,
1
Carles Pelejero,
2
Patrick De Deckker,
3
and Graham A. Logan
4
Received 8 March 2007; revised 5 June 2007; accepted 18 June 2007; published 14 July 2007.
[1] Comparison of ice cores from Greenland and
Antarctica shows an asynchronous two-step warming at
these high latitudes during the Last Termination. However,
the question whether this asynchrony extends to lower
latitudes is unclear mainly due to the scarcity of
paleorecords from the Southern Hemisphere. New data
from a marine core collected off South Australia (36°S)
allows a detailed reconstruction of sea-surface temperatures
over the Last Termination. This confirms the existence of an
Antarctic-type deglacial pattern and shows no indication of
cooling associated with the Northern Hemisphere YD event.
The SST record also provides a new comparison with the
more extensive paleoclimatic data available from continental
Australia. This shows a strong climatic link between onshore
and offshore records for Australia and to Southern
Hemisphere paleorecords. We also show a progressive SST
drop over the last 6.5 kyr not seen before for the
Australian region. Citation: Calvo, E., C. Pelejero, P. De
Deckker, and G. A. Logan (2007), Antarctic deglacial pattern in a
30 kyr record of sea surface temperature offshore South Australia,
Geophys. Res. Lett.,34, L13707, doi:10.1029/2007GL029937.
1. Introduction
[2] Comparison of climate records from Greenland and
Antarctic ice cores, with the exception of the coastal
Antarctic site of Taylor Dome, has provided evidence for
an asynchronous pattern in the evolution of air temperatures
during the last ice age and Termination I [Blunier and
Brook, 2001]. During the last deglaciation, the Antarctic
Cold Reversal (ACR), that interrupted the deglacial warm-
ing in Antarctica, coincided with the warm Bølling-Allerød
in Greenland and occurred 1 kyr before its northern
hemisphere cold counterpart, the Younger Dryas (YD) event
[Jouzel et al., 1995]. This north-south asymmetry is con-
sistent with the bipolar seesaw hypothesis which proposes a
response to changes in the thermohaline circulation and heat
transport, causing Antarctica to cool when Greenland
warms and vice versa [Broecker, 1998]. While it appears
that climate changes in Greenland ice cores have been
synchronously recorded in most Northern Hemisphere
latitudes, evidence from the Southern Hemisphere is sparse.
In a compilation of SST records from the Pacific Ocean
[Kiefer and Kienast, 2005], only one SST record presented a
clear Antarctic pattern during the deglaciation [Pahnke et
al., 2003]. The only other record from the Southern Hemi-
sphere with enough temporal resolution to show an ACR
was a d
18
O record from the South Atlantic [Charles et al.,
1996] and more recently a U
37
k
0
SST record from the SE
Pacific [Kaiser et al., 2005].
[3] Here we present a new alkenone-derived sea-surface
temperature (SST) record from offshore southern Australia,
which covers the last deglaciation and the Holocene in
detail. These new data provide support for the idea that
deglacial SSTs in the mid latitudes of the Southern Hemi-
sphere evolved in synchrony with Antarctica. They also
offer a detailed Holocene marine record, still very scarce in
this area, to compare with the more extensive climatological
information from continental Australia.
2. Materials and Methods
[4] Sediment gravity core MD03-2611 (36°44
0
S,
136°33
0
E, 11.97 m long) was recovered from the Murray
Canyons area, off Kangaroo Island, South Australia during
the AUSCAN 2003 cruise (Figure 1) [Hill and De Deckker,
2004]. The site is located at 2,420 m water depth on a small
plateau over a ridge, avoiding the processes of erosion and
sediment redistribution that occur within the main canyons.
It is more than 200 km away from the present mouth of the
Murray River and 50 km away from the present 100 m
depth contour, which roughly delimits the broad continental
shelf, and sedimentation is mostly pelagic. A description of
the core and its mineralogical content is available in the
work of Gingele et al. [2004].
[5] Alkenone analyses on selected core samples were
performed following published methods [Calvo et al.,
2003] and alkenone-derived SSTs were reconstructed using
the relationship U
37
k
0
= 0.033 SST + 0.044 [Mu¨ller et al.,
1998], which provides annually averaged SSTs. The
accepted error bar is of the order of 1°C. The core-top
estimate provides a SST of 17.6°C, slightly warmer than the
modern annual mean of 16.5°C at 0 m depth [Conkright et
al., 2002]. This difference is probably due to the absence of
the most recent sediments from the top of the gravity core as
confirmed by a
14
C AMS date of the core-top sediment,
which provides a calibrated age of 683 years. A preferential
alkenone production during summer/autumm, when
upwelling develops along the coast of of the southern Eyre
Peninsula [Ka¨mpf et al., 2004], could also slightly bias the
reconstructed SSTs towards warmer values (SSTsummer =
18.4°C; SSTautumm = 17.1°C).
GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L13707, doi:10.1029/2007GL029937, 2007
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A
rticl
e
1
Institut de Cie`ncies del Mar, Consejo Superior de Investigaciones
Cientificas, Barcelona, Spain.
2
Institucio´ Catalana de Recerca i Estudis Avanc¸ ats and Institut de
Cie`ncies del Mar, Consejo Superior de Investigaciones Cientificas,
Barcelona, Spain.
3
Department of Earth and Marine Sciences, Australian National
University, Canberra, ACT, Australia.
4
Petroleum and Marine Division, Geoscience Australia, Canberra, ACT,
Australia.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL029937$05.00
L13707 1of6
[6] Oxygen isotopes were analysed on the planktonic
foraminifera Globigerinoides bulloides on a Finnigan
MAT251 mass spectrometer at GEOMAR (Germany). This
species was also used to establish the age model by means
of 16 AMS
14
C dates. From 0 to 26 cal kyr BP, radiocarbon
ages were converted into calendar ages using the Calib 5.01
software and the marine calibration dataset MARINE04
[Hughen et al., 2004], which includes a marine reservoir
correction of 400 years. The oldest
14
C age (24860
14
Cyr)
was calibrated using the CALPAL program and the CalPal
2005 SFCP calibration curve (available at http:/www.
calpal.de). According to this age model, the 765 cm
analysed in this study cover the last 30 kyr and sedimen-
tation rate averaged 27.6 cm/kyr. Thus, the 10-cm
sampling interval provides a time resolution of 360 years
for the alkenone record. The d
18
O record also present the
same time resolution for the top 400 cm of the core.
For the bottom 350 cm (between 16.8 and 30 kyr),
we sampled every 25 cm which provides a time resolution
of 890 years.
3. Results
[7] The alkenone-derived SST record shows a large
deglacial warming, up to 8°C, from 11°Cat19 cal kyr
BP (the coldest SST recorded during the Last Glacial
Maximum (LGM: 21 ± 2 kyr [Mix et al., 2001]) to
19.3°C at the beginning of the Holocene (Figure 2). A
similar amplitude was found using amino-acid racemization
in emu eggshells from Central Australia with a deglacial
average air temperature increase of 9°C[Miller et al., 1997].
SST amplitudes of 7 10°C were also found along the
Chilean margin at 35°S[Romero et al., 2006] and in
subantarctic waters southeast of New Zealand [Barrows et
al., 2000]. However, this change of 8°C is still considerable
when compared with other mid-latitude SST records of the
Southern Hemisphere (Figure 3). It is likely that the present
influence of the Leeuwin Current, which brings warm and
low-salinity waters from the Indian Ocean to the southern
Australian coast as far east as 130°E[Rochford, 1986], was
much reduced or absent during the LGM. This, together
with a greater influence of cold Southern Ocean waters
during the LGM would cause a larger SST amplitude than
expected for these latitudes.
[8] The deglacial warming is accomplished in two steps,
with an interruption between 15 and 13.3 kyr BP, which is
also captured by the d
18
O record. The first warming (5°C)
occurred between 19 and 15.5 kyr BP and the second
(3.5°C) between 13.3 and 11 kyr BP. After a small cooling
(0.6°C) at 11 kyr BP, SSTs exhibited a broad maximum
between 5.5 and 8 kyr BP, reaching the highest values of the
whole record (19.8°C) at 7.5 kyr BP. The record shows
small SST variability during this Holocene broad thermal
maximum, a period followed by a gradual decrease of 2°C
towards modern SST values.
[9]Thed
18
O record closely follows the glacial and
deglacial pattern displayed by the alkenone record, with a
2.2%change between glacial and Holocene sections
(Figure 2). Considering that the ice volume contribution
to this change is about 1%[Schrag et al.,1996],the
remaining 1.2%should reflect changes in local SST and
salinity. Given the 8°C warming recorded by the U
37
k
0
index, seawater d
18
O is expected to change by 1.6%over
the deglaciation [Bemis et al., 2002]. Accordingly, the 0.4%
isotopic difference is ascribed to an increase in local salinity
of about 0.8 units since the last glacial period (assuming a
change of 0.5%per 1 salinity unit). The decrease in salinity
during the LGM may be related to a northward displace-
Figure 1. Map showing location of marine core MD03-2611 (36°44
0
S, 136°33
0
E). Also displayed other marine and ice
cores discussed in the text.
L13707 CALVO ET AL.: DEGLACIAL SSTS OFFSHORE SOUTH AUSTRALIA L13707
2of6
ment of the Subtropical Front (STF: present location
between 38 40°S) and its associated salinity gradient.
Pahnke et al. [2003] also found a consistent decrease in
salinity during glacial times east of New Zealand, which
was attributed to a major influence of fresher subantarctic
waters from the south of the STF. Alternatively, the expo-
sure of the shallow shelf during the sea level drop at the
LGM may have also prevented the formation of the high
salinity surface waters that are formed today in the Spencer
Gulf, north of Kangaroo Island, as a result of strong summer
evaporation [Lennon et al., 1987].
[10] During the late Holocene, however, the evolution of
the d
18
O and alkenone records differs significantly. The
consistent cooling observed in the alkenone record for
the last 6 kyr is not identified in the isotope record. Instead,
the d
18
O measurements reach Holocene values at about
12 kyr BP and then follow a progressive depletion towards
the lowest d
18
O values of the whole record at the top of the
core. The contrasting evolution of the two proxies suggests
a significant influence of salinity in the planktonic d
18
O
record at this time.
4. Discussion
[11] The deglacial warming at site MD03-2611 closely
resembles the atmospheric temperature evolution recon-
structed in Antarctica since the end of the LGM, both in
timing and structure (Figure 2). The clear reversal in SST
and d
18
O observed during the deglaciation is synchronous
with the ACR described in Antarctic ice cores around 14
12.5 kyr BP [Jouzel et al., 1995], suggesting a close
coupling between mid- and high-southern latitudes. After
this reversal, SSTs increased gradually to the Holocene,
providing no evidence of any cooling associated with the
YD cold event (12.9 11.5 kyr BP). Whether this event,
which is characteristic of the Northern Hemisphere, also
affected the Southern Hemisphere has been the subject of
considerable debate. Denton and Hendy [1994] reported an
advance of the Franz Josef Glacier in New Zealand during
the YD, pointing to inter-hemispheric deglacial symmetry.
However, other terrestrial reconstructions from New
Zealand provided contrasting results, more indicative of
asymmetry between hemispheres [i.e., Turn ey et al. ,
2003]. In the mid to high latitudes of South America,
studies on the existence of cooling related to the YD are
also contradictory [i.e., Glasser et al., 2004].
[12] An Antarctic-type deglacial pattern has been
reported in high resolution marine sediment cores using
alkenone- and Mg/Ca-SST methods from the southeast
Pacific and east of New Zealand [Kaiser et al., 2005; Lamy
et al., 2004; Pahnke et al., 2003]. In Figure 3, we compare
our MD03-2611 SST record with the three high resolution
Figure 2. (top) G. bulloides d
18
O record and (bottom) alkenone-derived SST record from core MD03-2611. Also plotted
Antarctica air temperatures, dD (shaded area [EPICA Community Members, 2004]). The vertical bar indicates the Antarctic
Cold Reversal (ACR).
L13707 CALVO ET AL.: DEGLACIAL SSTS OFFSHORE SOUTH AUSTRALIA L13707
3of6
and radiocarbon-dated alkenone-derived SST records avail-
able from these mid-latitudes of the Southern Hemisphere.
All four SST records show a synchronous onset of deglacial
warming at 19 kyr, comparable with the start of deglaci-
ation in Antarctica and SST records from tropical and
subtropical latitudes of the Pacific Ocean [Kiefer and
Kienast, 2005]. With the exception of core MD97-2121,
from north of Chatman Rise, all records display cooling
associated to the ACR. This cooling is clear in cores MD03-
2611 and ODP1233, off Chile, but only represents an
interruption of the warming in core MD97-2120, east of
New Zealand. However, Mg/Ca data from the same core
show a 1.5°C cooling during the ACR [Pahnke et al., 2003].
There is no evidence of cooling associated to the YD cold
event in any of the alkenone-derived SST deglacial records.
This contrasts with recent results from planktonic forami-
nifera d
18
O in two studies, which have been interpreted as
evidences of cooling associated to the YD in the mid to high
latitudes of the Southern Hemisphere. These studies
reported d
18
O enrichments at the time of the YD for a core
also retrieved South of Australia [Andres et al., 2003] and
for two cores from South of New Zealand, close to the Ross
Sea [Morigi et al., 2003]. Given the compilation of alke-
none-derived SST data described above (Figure 3) and that
foraminifera d
18
O is influenced by other parameters in
addition to SST, we suggest that the isotopic enrichment
may not solely be related to sea surface cooling.
[13] Considering that higher SST promotes evaporation,
formation of clouds and precipitation, alkenone-derived
SSTs in core MD03-2611 agree with the present knowledge
of the history of rainfall and moisture in continental
Southern Australia during the Holocene. The cold temper-
atures of the last 4 kyr coincide with a period of increased
aridity in southeastern Australia [Cupper, 2005; Harrison,
1993]. Drier conditions have also been inferred from the
mineralogical and isotopic study of the terrestrial compo-
nent of MD03-2611 [Gingele et al., 2007]. The d
18
O record,
however, show the lightest values of the whole record
during the late Holocene, indicating the existence of low
salinity water masses. Salinity changes can be driven by
Figure 3. Comparison of high resolution alkenone-derived SST records available from the mid-latitudes of the Southern
Hemisphere [Kaiser et al., 2005; Pahnke and Sachs, 2006]. See Figure 1 for location of the marine cores. ACR is the
Antarctic Cold Reversal. Light shaded bar indicates the onset of deglacial warming at 19 ± 1 kyr.
L13707 CALVO ET AL.: DEGLACIAL SSTS OFFSHORE SOUTH AUSTRALIA L13707
4of6
changes in the evaporation-precipitation balance and also by
the vertical/horizontal mixing of different water masses. The
arid conditions that prevailed in the area at that time cannot
account for these low salinity waters. Thus, changes in the
water masses bathing our core site is the most likely
explanation. A plausible mechanism to explain the cold
SSTs shown by the U
37
k
0
and the low salinities recorded by
the oxygen isotopes is a greater influence of Southern
Ocean waters versus the tropical waters of the Leeuwin
Current. Interestingly, marine cores from the western trop-
ical Pacific also show a similar trend during the Holocene,
with d
18
O decreasing from 10 ka to the present and Mg/Ca
ratios recording the lowest SSTs during the late Holocene
[Stott et al., 2004]. It may well be that the decrease in
salinity of the latter part of the Holocene is then a wider
feature across the Pacific Ocean, as already suggested by
Stott et al. [2004].
[14] Warmer SSTs coincide with higher precipitation
during the mid-Holocene, between 8 and 6 kyr BP, as
recorded in southeastern Australia lake levels [Bowler,
1981; Stanley and De Deckker, 2002] and in the history
of a swamp from the Fleurieu Peninsula [ Bickford and Gell ,
2005]. On the other hand, the progressive decrease in SST
from 6.5 kyr BP to modern times parallels other alkenone-
derived SST records from the mid latitudes of the Southern
Hemisphere, as recently reviewed by Lorenz et al. [2006].
Thus, our data gives support to the general finding that
Holocene SSTs experienced a warming in the tropics but a
cooling in the extratropics after 7 kyr BP.
5. Conclusions
[15] The reconstructed SSTs from core MD03-2611 rep-
resent the first continuous record of high temporal resolu-
tion for the Australian region over the last deglaciation. The
SST record shows significant colder conditions during the
Last Glacial Maximum, with temperatures 8°C lower than
during the early Holocene. This study confirms the exis-
tence of a cold reversal during the last deglaciation that
coincides with the ACR described in Antarctic ice cores. It
also provides no evidence of any cooling associated
with the northern hemisphere YD event. Thus, this new
alkenone-derived SST record corroborates the close link
between mid- and high-southern latitudes. Finally, the
SST evolution during the Holocene compares well with
information on hydrological and vegetation changes from
southeastern Australia showing a warm early Holocene at a
time of generally wetter conditions and a subsequent
decrease in temperature at a time of increasing aridity after
6.5 ka.
[16]Acknowledgments. We thank K. Pahnke for providing the
alkenone-SST data from core MD97-2120 and MD97-2121. E.C. and
C.P. acknowledge funding from the Spanish MEC and Generalitat de
Catalunya. The Australian National Oceans Office provided all the contract
funds for RV Marion Dufresne. Stable isotopes analyses were funded by an
ARC DP grant awarded to PDD. AMS dates were funded through AINSE
grant 05/036. We wish to also thank J. Shelley and A. Sturm for preparing
and running the foraminifera samples for isotopic analyses. G.A.L.
published with permission of the CEO Geoscience Australia.
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E. Calvo, Institut de Cie`ncies del Mar, Consejo Superior de Investiga-
ciones Cientificas, Pg. Marı´tim de la Barceloneta, 37-49, E-08003
Barcelona, Spain. (ecalvo@icm.cat)
P. De Deckker, Department of Earth and Marine Sciences, Australian
National University, Canberra, ACT 0200, Australia.
G. A. Logan, Petroleum and Marine Division, Geoscience Australia,
GPO Box 378, Canberra, ACT 2601, Australia.
C. Pelejero, Institucio´ Catalana de Recerca i Estudis Avanc¸ats and Institut
de Cie`ncies del Mar, Consejo Superior de Investigaciones Cientificas, Pg.
Marı´tim de la Barceloneta, 37-49, E-08003 Barcelona, Spain.
L13707 CALVO ET AL.: DEGLACIAL SSTS OFFSHORE SOUTH AUSTRALIA L13707
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... Despite the many efforts to understand the paleoceanographic setting south of Australia (e.g., Wells and Wells, 1994;Findlay and Flores, 2000;Barrows and Juggins, 2005;Nürnberg and Groneveld, 2006;Calvo et al., 2007;Moros et al., 2009;Spooner et al., 2011;De Deckker et al., 2012;Lopes dos Santos, 2012;Perner et al., 2018), no proxy studies and only a few modeling studies have concentrated on the subsurface development (e.g., Schodlok and Tomczak, 1997;Middleton and Cirano, 2002;Middleton and Platov, 2003;Cirano and Middleton, 2004;Middleton and Bye, 2007;Pattiaratchi and Woo, 2009). The aim of our study is to fill this important gap and to reveal changes in the Leeuwin Current over the last 60 kyr. ...
... ruber, O. universa, G. truncatulinoides) starting at ∼ 18 ka BP (Fig. 3a-c). It is further characterized by sea surface warming closely related to the Southern Hemisphere climate signal (WAIS Divide Project Members, 2013;EPICA Community Members, 2006) with SST Mg/Ca being overall warmer in the western core region and rather congruent to other deglacial SST proxy records from the region ( Fig. 4c; Lopes dos Santos et al., 2013;Calvo et al., 2007). ...
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The Leeuwin Current, flowing southward along the western coast of Australia, is an important conduit for the poleward heat transport and inter-ocean water exchange between the tropical and the subantarctic ocean areas. Its past development and its relationship to Southern Ocean change and Australian ecosystem response is, however, largely unknown. Here we reconstruct sea surface and thermocline temperatures and salinities from foraminiferal-based Mg/Ca and stable oxygen isotopes from areas offshore of southwestern and southeastern Australia, reflecting the Leeuwin Current dynamics over the last 60 kyr. Their variability resembles the biomass burning development in Australasia from ∼60–20 ka BP, implying that climate-modulated changes related to the Leeuwin Current most likely affected Australian vegetational and fire regimes. Particularly during ∼60–43 ka BP, the warmest thermocline temperatures point to a strongly developed Leeuwin Current during Antarctic cool periods when the Antarctic Circumpolar Current (ACC) weakened. The pronounced centennial-scale variations in Leeuwin Current strength appear to be in line with the migrations of the Southern Hemisphere frontal system and are captured by prominent changes in the Australian megafauna biomass. We argue that the concerted action of a rapidly changing Leeuwin Current, the ecosystem response in Australia, and human interference since ∼50 BP enhanced the ecological stress on the Australian megafauna until its extinction at ∼43 ka BP. While being weakest during the Last Glacial Maximum (LGM), the deglacial Leeuwin Current intensified at times of poleward migrations of the Subtropical Front (STF). During the Holocene, the thermocline off southern Australia was considerably shallower compared to the short-term glacial and deglacial periods of Leeuwin Current intensification.
... We compiled all available existing δ 18 O records for near-surface dwelling planktic foraminifera species (Globigerinoides ruber, G. bulloides, N. pachyderma; Supporting Information S1) spanning the last deglaciation (10-20 ka) from across the Southern Ocean (Bostock et al., 2004(Bostock et al., , 2015Caley et al., 2011;Calvo et al., 2007;Caniupán et al., 2011;Carlson et al., 2008;Charles et al., 1991;Chiessi et al., 2014;Crosta et al., 2004;Dyez et al., 2014;Fischer & Wefer, 1999;Gersonde et al., 2003;Gottschalk et al., 2015Gottschalk et al., , 2018Govin et al., 2009;Grobe & Mackensen, 1992;Haddam et al., 2018;Hasenfratz et al., 2019;Hodell et al., 2000Hodell et al., , 2003Labeyrie et al., 1996; LGM-Holocene δ 18 O ivc-gtc (δ 18 O calcite corrected for ice volume and global-mean SST changes; Section 2) at the core sites. The modern climatological meridional δ 18 O gradient is represented by the background shading ( Figure S1 in Supporting Information S1; darkest shade represents 0.25‰/°Lat, equivalent to ∼1°C/°Lat). ...
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The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesised to play a key role in regulating atmospheric CO2 over glacial-interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ18O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9-7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (LGM; 20 ka) relative to the mid-Holocene (6.5 ka). Climate models from the Palaeoclimate Modelling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO2 (R2=0.98). Experiments with a 0.25° resolution ocean-sea-ice-carbon model suggest that shifting the westerlies equatorward reduces the overturning rate of the ocean below 2 km depth, leading to a suppression of CO2 outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO2 rise, and suggest outgassing of natural CO2 from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.
... For example, high latitudes in the Northern Hemisphere were generally cold during this period (Cacho et al., 2001); paleohydrological records in mid-low latitudes revealed the existence of two-phase southward movements of the Intertropical Convergence Zone (ITCZ) (Broecker and Putnam, 2012;Asmerom et al., 2017;McGee et al., 2018;Huang et al., 2019;B. Zhao et al., 2021); paleotemperature records from the Antarctic and the southern Indian Ocean showed a gradual warming trend (EPICA Community Members, 2006;Calvo et al., 2007). More investigations on paleoclimate during the MI are required to provide insights into the dynamics of the Earth's climate system and improve climate models and projections. ...
Article
Exploring the seasonal atmospheric processes involved with hydroclimate changes of the East Asian Summer Monsoon (EASM) during the Mystery Interval (MI, 17.5–14.5 ka BP) will enhance understanding of the EASM climate system. Here we report on the δ²H values of leaf wax n-alkanes (δ²Halk) in a wetland core retrieved from southern China. These values are relatively negative during the MI, contrasting to positive excursion recorded by stalagmite δ¹⁸O in nearby caves during the same period. Considering the overlap of the primary synthesis time of leaf waxes in deciduous woody plants and periods of precipitation δ²H alteration in the EASM (late spring to early summer), we interpret the relatively negative signal of δ²Halk in the wetland core as the early onset of summer rainfall during the MI. The early onset of summer rainfall was likely due to the early northeastward shifting of the Western Pacific subtropical high due to the low land-sea thermogradient between the Qinghai-Tibet Plateau and the West Pacific Ocean.
... Other records in the western Pacific region referred to in the text are also indicated: yellow squares, records of terrestrial climate cooling or drying during the period of LGM climax (LGM-b); white circles, records of decreased sea-surface temperature (SST) during LGM-b. 1, Lake Mikata (Gotanda et al., 2002); 2, MS 2008E (Peterse et al., 2011); 3, MD98-2195 (Ijiri et al., 2005); 4, MD97-2141 (Rosenthal et al., 2003); 5, MD01-2390 (Steinke et al., 2008); 6, MD98-2181 (Stott et al., 2007); 7, MD9821-62 (Visser et al., 2003); 8, MD98-2176 (Stott et al., 2007); 9, Gempa Bumi Cave (Krause et al., 2019); 10, Liang Luar Cave (Ayliffe et al., 2013); 11, GeoB10069-3 (Dubois et al., 2014); 12, MD98-2170 (Stott et al., 2007); 13, Ball Gown Cave (Denniston et al., 2013); 14, Lynch's Crater (Muller et al., 2008); 15, MD03-2611 (Calvo et al., 2007); 16, MD97-2121 (Pahnke and Sachs, 2006); 17, Okarito (Barrell et al., 2013); 18, Galway tarn (Barrell et al., 2013); 19, MD97-2120 (Pahnke and Sachs, 2006). (B) Monthly temperature and precipitation from 1981 to 2010 AD at nearby Guangchang meteorology station (116 12 0 E, 26 30.6 0 N, 142 m a.s.l.),~15 km southeast of the LJF. ...
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Recent global sea-level data indicate that a final growth of ice volume occurred towards the end of the Last Glacial Maximum (LGM). The LGM culmination, LGM-b, lasted from about 21 to 17 ka and was followed by rapid deglaciation. This finding suggests a millennial-scale state with extreme glacial conditions (at least regionally) that immediately preceded the onset of full deglaciation marked by Heinrich event 1. In order to check how regional climate and vegetation in subtropical East Asia responded to the global-scale LGM culmination and the deglaciation onset, a new high-resolution pollen record from the Lijiafang peatland (LJF: 26°36.849′N, 116°08.463′E, 270 m) in Southeast China was generated. This record demonstrates a massive expansion of herbs during LGM-b. The dominance of herbaceous pollen suggests sparse forest cover in the lowlands of the subtropical East Asian Summer Monsoon (EASM) region and indicates an approximately 3300-year-long interval of the driest climate conditions of the entire last glacial-interglacial cycle. The LJF pollen-based reconstruction is consistent with the other vegetation records from the larger region, suggesting the climate change during LGM-b significantly impacted terrestrial ecosystems. The environmental change in the EASM region is likely linked to the Sea Surface Temperatures (SSTs) decrease that weakened the Walker cell convection over the western Pacific Ocean and reduced convection and moisture content in the Intertropical Convergence Zone. The strengthened decrease in moisture supply during LGM-b seems to have resulted in extreme and widespread megadroughts in the EASM and Australian regions. Our data indicate that the cold SSTs may have also decreased air temperatures in the neighboring parts of the continents. We suggest that the widespread SSTs decline in the western Pacific may have been linked to the cooling of the Southern Ocean and the expansion of Antarctic sea ice during LGM-b.
... However, the Tibetan Plateau experienced cooling events due to the slowdown of AMOC during the MI (Opitz et al., 2015). In contrast, the SST of the South Hemisphere increased rapidly during the MI and reached its highest values after 16.0 ka BP ( Fig. 6k; EPICA Community Members, 2006;Calvo et al., 2007). Therefore, the greater temperature gradient might have resulted in a slower process of pressure transition between the Tibetan Plateau and the ocean of the Southern Hemisphere and consequently delayed the early formation of the ITCZ in the Northern Hemisphere. ...
Article
The inconsistency of paleoclimate records between the mid-low and high latitudes during the Mystery Interval (MI, 17.5–14.5 ka BP) is not well resolved. To provide a mechanism analysis, this study conducts spatial comparisons of hydroclimate changes in the Asian Summer Monsoon (ASM) region during the MI by reconstructing the paleohydrology of the south of East Asian Summer Monsoon (EASM) region using the molecular distributions and carbon isotope ratio of n-alkanes, as well as elemental and isotopic composition of bulk organic matter, in a sedimentary core retrieved from Dingnan (DN) wetland. The multiple indices of the DN core confirmed the occurrence of dry-wet and wet-dry transitions at ca. 17.5 ka BP and 16.0 ka BP, respectively. Synchronous with the dry-wet transition in the DN records, the Indochina Peninsula at a slightly lower latitude became wetter. In contrast, the middle Yangtze region at a more northern latitude became slightly drier, and North China did not show a hydrological variation. Moreover, the wet-dry transition at the mid-MI was strong in the Indochina Peninsula (10–20°N, at about 16.5 ka BP) and the southern EASM region (23–27°N, at about 16.0 ka BP) but was nearly absent in the middle Yangtze region (28–32°N) and North China (35–40°N). The first hydrological transition possibly connects with the synchronous southward movement of the Intertropical Convergence Zone (ITCZ) and the westerly jet caused by the cooling of the Northern Hemisphere. The absence of the second hydrological transition is interpreted as a result of the decoupling of the ITCZ and the westerly jet caused by the delayed formation of the ITCZ due to the warming of the Southern Hemisphere. This study reveals spatial differences in the hydroclimate variations during the MI and highlights the potential influence of coupling between the westerly jet and the ITCZ on rainfall in the ASM region.
Chapter
Megafans are partial cones of river sediment that reach unexpectedly large dimensions, with the largest on Earth being 700 km long. Due to recent developments in space-based observations, global mapping efforts have shown that modern megafan features cover vast landscapes on most continents. This book provides a new inventory of nearly 300 megafans across five continents. Chapters focus on regional studies of megafans from all continents barring North America and Antarctica. The major morphological attributes of megafans and multi-megafan landscapes are discussed, and the principal controls on megafan development are examined. The book also compares megafans with alluvial fans, deltas, floodplains and the recently recognised 'major avulsive fluvial system' (MAFS). The final part of the book discusses the application of megafan research to economic geology, aquifers and planetary geology including layered deposits on Mars. This is an invaluable reference for researchers in geomorphology, sedimentology and physical geography.
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Antarctic ice cores have revealed the interplay between dust and climate in the Southern Hemisphere. Yet, so far, no continuous record of dust provenance has been established through the last deglaciation. Here, using a new database of 207 Rare Earth Element (REE) patterns measured in dust and sediments/soils from well-known potential source areas (PSA) of the Southern Hemisphere, we developed a statistical model combining those inputs to provide the best fit to the REE patterns measured in EPICA Dronning Maud Land (EDML) ice core (E. Antarctica). Out of 398 samples measured in the EDML core, 386 samples have been un-mixed with statistical significance. Combined with the total atmospheric deposition, we quantified the dust flux from each PSA to EDML between 7-27 kyr BP. Our results reveal that the dust composition was relatively uniform up until 14.5 kyr BP despite a large drop in atmospheric deposition at ~18 kyr with a large contribution from Patagonia yielding ~68% of total dust deposition. The remaining dust was supplied from Australia (14-15%), Southern Africa (~9%), New Zealand (~3-4%) and Puna-Altiplano (~2-3%). The most striking change occurred ~14.5 kyr BP when Patagonia dropped below 50% on average while low-latitude PSA increased their contributions to 21-23% for Southern Africa, 13-21% for Australia and ~4-10% for Puna-Altiplano. We argue that this shift is linked to long-lasting changes in the hydrology of Patagonian rivers and to sudden acceleration of the submersion of Patagonian shelf at 14.5 kyr BP, highlighting a relationship between dust composition and eustatic sea level. Early Holocene dust composition is highly variable, with Patagonian contribution being still prevalent, at ~50% on average. Provided a good coverage of local and distal PSA, our statistical model based on REE pattern offers a straightforward and cost-effective method to trace dust source in ice cores.
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The morphology and development of several submarine canyons offshore southeast South Australia and western Victoria are described. The existence of three of those canyons had been foreshadowed in 1963 by N. Boutakoff, who thought them to be linked to ancient courses of the Glenelg River. These canyons occur on the outer continental shelf where their heads are situated in depths shallower than 1000 m. Sinuous channels are visible within two of the canyon heads, indicating that water and sediment may still travel downslope and cause erosion, and thus are geologically recent. Several other canyons are also documented and named. Two canyons are confined to depths below 3000 m; they may be much older and seem not to be linked to ancient river courses. They are also oblique to the upper canyons. The other characteristic feature of the area is the presence of numerous undersea slides. These occur at three specific depths (∼1200, ∼1500 and ∼1800 m) and are coincident with stratigraphic horizons in which continental groundwater flows have been identified in adjacent exploratory oil and gas wells drilled into the continental shelf. Sapping of groundwater may likely have occurred during very wet periods inland. We suggest that these undersea slides could be the first step in the formation of deep-sea canyons that are not necessarily linked to ancient river courses. We also postulate that the ‘sliding’ of large piles of sediment down the continental slope has tsunamigenic potential and may have occurred during significant wet climate on land. • KEY POINTS • Several deep-sea canyons and numerous submarine slides are documented along the continental shelf offshore the coastline of western Victoria and southeast South Australia in the Otway Basin. • Some of these canyons are linked to previous courses of the Glenelg River, which changed over time in response to regional tectonic changes such as faulting and significant upwarping. • The orientation of most of these canyons tends to align with ancient terrestrial lineaments that likely extend offshore. • Sinuous channels within the upper canyon reaches are clearly visible and not infilled with sediment; they are likely active and linked to ongoing groundwater seepage. • It is proposed that the underwater slides are generated by the submarine discharge of continental groundwater on the continental slope, at three specific water depths with discharge associated with particular lithologies. These slides may still be active. • Superimposed slides may engender the formation of new canyons. • The volume of some of these undersea slides is of tsunamigenic scale.
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Close examination of key and well-dated Holocene sites, both on land and at sea in the Australian region indicate that at the very beginning of the Holocene, as a result of strong westerlies, there must have been a continuous positive Southern Annular Mode (SAM). Following from that, the entire region switched to a negative SAM scenario and, during that time, the westerlies must have retreated further south. Afterwards, a period of time spaning ∼8200 to ∼5500 years ago temperatures were higher than today. We refer to it as the Holocene Hyspithermal. Coincident to this period, lake levels and postulated rainfall were extraordinarily high and vegetation spectra in places very different compared to today. The extent of this period varies by a few centuries between sites, but this may result from the level of resolution and also appears to be controlled by latitude. There is also clear indication that the influence of the westerlies was reduced over Australia during those two and a half millennia. Nevertheless, air temperatures recognised in Antarctic ice cores are at the opposite to those recognised in Australia. In addition, during the Australian Holocene Hypsithermal, CO2 levels were at their lowest in Antarctic ice cores. Climatic conditions then progressively deteriorated everywhere a bit after ∼6000 years BP until recent times as ENSO signals with alternating El Niño and La Niña conditions across the entire Pacific region as already described by Perner et al. (2018) based on the same cores studied here. Brief mention is also made to the presence of humans in SE Australia during the Holocene. It seems that human activities changed well after the period of high temperatures and rainfall, with more sedentary activities along the major rivers, with an enhancement of food production in organized settings suggestive of villages.
Thesis
A l’échelle des cycles glaciaires-interglaciaires (G/IG) du Quaternaire, la concentration atmosphérique en CO₂ (p CO₂) est associée à d’importantes variations, notamment pendant les transitions entre périodes glaciaires et interglaciaires, aussi appelées terminaisons, qui enregistrent des augmentations de pCO₂ de l’ordre de 100 ppm en quelques milliers d’années. Alors que les rôles de la circulation océanique et de l’étendue de la couverture de glace sur le pCO₂ sont étudiés depuis plusieurs années, peu de travaux s’intéressent à l’impact de la productivité biologique. L’objectif de cette thèse est de reconstruire les changements de productivité biologique et d’estimer leur contribution sur les variations de pCO₂ atmosphérique au cours des derniers 800 000 ans (ka), en portant une attention particulière à la Terminaison V (~425 ka) et au stade isotopique (MIS) 11, l’interglaciaire enregistré vers 400 ka. Pour cela, deux approches ont été combinées pour remonter aux variations globales et locales de la productivité biologique. Des mesures de Δ¹⁷O de O₂ effectuées dans les bulles d’air piégé dans la carotte de glace EPICA Dome C (Antarctique) entre 400 et 800 ka, ont permis de compléter les mesures préexistantes et remonter aux variations de la productivité biosphérique globale exprimée en flux d’oxygène au cours des derniers 800 ka. Des analyses micropaléontologiques (coccolithes, foraminifères) et géochimiques (COT, CaCO₃, XRF) ont été effectuées sur une carotte sédimentaire du secteur Indien de l’Océan Austral (MD04-2718) pour remonter aux variations d’efficacité de la pompe biologique au cours des derniers 800 ka. Les résultats obtenus durant cette thèse ont permis de montrer qu’à l’échelle des cycles G/IG, la productivité biosphérique globale et la productivité carbonatée dans l’Océan Austral sont plus importantes pendant les interglaciaires comparées aux glaciaires, ce qui minimise l’impact de la productivité organique marine sur la pCO₂ atmosphérique. La Terminaison V et MIS 11 enregistrent la plus forte productivité biosphérique des derniers 800 ka et la plus importante production carbonatée marine des 9 derniers interglaciaires dans l’Océan Austral. Alors que la forte productivité carbonatée est une source de CO₂ pour l’atmosphère, l’augmentation de la productivité de la biosphère permettrait de contrebalancer l’augmentation de pCO₂ via la photosynthèse.
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Climatic fluctuations recorded in Antarctica and Greenland during the last deglaciation (18 10 ka) differ markedly in their timing. It remains controversial whether local climate fluctuations recorded in southern mid-latitudes relate primarily to northern or southern polar records. We present multiproxy results from New Zealand that show strong evidence for a minor cooling or slowdown in the rate of warming at the time of the North Atlantic late glacial interstadial. The Younger Dryas chronozone in New Zealand was a period of resumed warming and increased westerly airflow. Differences between the hemispheres at this time were probably due to a reorganization of the thermohaline circulation system and associated changes in the meridional temperature gradient.
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Knowledge of the state of the earth at the Last Glacial Maximum (LGM, an interval around 21,000 years ago) is an important benchmark for understanding the sensitivity of global environmental systems to change. Much progress in understanding climates of the LGM has occurred in the ∼20 years since the end of the CLIMAP project of the 1970s (Climate Long-range Investigation, Mapping and Prediction). Here we review this progress, based on presentations and discussion at a first open science meeting of the EPILOG project (Environmental Processes of the Ice age: Land, Oceans, Glaciers) held in Delmenhorst, Germany, May 1999. We outline key controversies and document protocols for EPILOG contributions, so that a new synthesis of the LGM Earth can emerge as an open project of the world's community of scientists.
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Late Quaternary climate fluctuated between extremes of glaciations, lasting ∼90 kyr on average, and interglacial episodes, lasting ∼10 kyr. Still largely unknown are the spatial and temporal evolution of these global climate states, with vigorous debate still underway on the mechanisms responsible for glacial inceptions and terminations. Though it is widely believed that the Southern Hemisphere oceans play a central role in global climate changes, few paleoclimate records exist from there. Here we present three new alkenone-derived SST records from the midlatitude Southern Hemisphere spanning the last 160 kyr, a full glacial-interglacial cycle. Our subtropical SST records from the last glacial period are characterized by (1) warming 47–23 kyr B.P., when high latitudes in both hemispheres cooled, and (2) milder temperatures during the penultimate glacial period than during the last glacial interval. These SST features are found to be of tropical- to subtropical-wide extent, implying increased thermal gradients at times of high-latitude ice sheet growth. This has implications for the vigor of atmospheric and upper ocean circulation and the transport of heat and moisture to the poles that may have been instrumental in the growth and maintenance of polar ice sheets during glacial periods.
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We report observational existence of a large seasonal coastal upwelling system that establishes in austral summer (December-April) along Australian southern shelves. Wind-driven upwelling events occur simultaneously in three upwelling centres spanning a distance of ~800 km. During each summer period there are ~2-3 major upwelling events, each lasting ~1 week. The simultaneous, rapid response of SST to wind forcing in the upwelling centres, which display vastly different shelf widths, points to the existence of a larger-scale process that carries cold water onto the shelf prior to the upwelling season. Exploration of a major upwelling event in March 1998 shows the evolution of peak surface chlorophyll-a concentrations of >4 mug/L lagging the onset of upwelling by ~1 week. The associated (exponential) growth rate can be estimated at 0.4 d-1. Another week later we found a distinct sub-surface chlorophyll-a maximum at a depth of 50 m centred along the upwelling front. Reasons for the formation of this maximum are not fully understood.
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We have analysed alkenones in 149 surface sediments from the eastern South Atlantic in order to establish a sediment-based calibration of the U37K′ paleotemperature index. Our study covers the major tropical to subpolar production systems and sea-surface temperatures (SST’s) between 0° and 27°C. In order to define the most suitable calibration for this region, the U37K′ values were correlated to seasonal, annual, and production-weighted annual mean atlas temperatures and compared to previously published culture and core-top calibrations. The best linear correlation between U37K′ and SST was obtained using annual mean SST from 0 to 10 m water depth (U37K′ = 0.033 T + 0.069, r2 = 0.981). Data scattering increased significantly using temperatures of waters deeper than 20 m, suggesting that U37K′ reflects mixed-layer SST and that alkenone production at thermocline depths was not high enough to significantly bias the mixed-layer signal. Regressions based on both production-weighted and on actual annual mean atlas SST were virtually identical, indicating that regional variations in the seasonality of primary production have no discernible effect on the U37K′ vs. SST relationship. Comparison with published core-top calibrations from other oceanic regions revealed a high degree of accordance. We, therefore, established a global core-top calibration using U37K′ data from 370 sites between 60°S and 60°N in the Atlantic, Indian, and Pacific Oceans and annual mean atlas SST (0–29°C) from 0 m water depth. The resulting relationship (U37K′ = 0.033 T + 0.044, r2 = 958) is identical within error limits to the widely used E. huxleyi calibrations of Prahl and Wakeham (1987) and Prahl et al. (1988) attesting their general applicability. The observation that core-top calibrations extending over various biogeographical coccolithophorid zones are strongly linear and in better accordance than culture calibrations suggests that U37K′ is less species-dependent than is indicated by culture experiments. The results also suggest that variations in growth rate of algae and nutrient availability do not significantly affect the sedimentary record of U37K′ in open ocean environments.
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The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Boelling (B) around 14.5 ky BP, spanned through the Alleroed (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus. the coldest part of the ACR. which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record. 87 refs., 5 figs.
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A fossil pollen and charcoal record from a cyperaceous swamp on the Fleurieu Peninsula, South Australia, was investigated to provide a history of pre and post-European settlement vegetation change. It was found that the swamp initiated sometime before 8220 calibrated years BP. High rates of peat development and the expansion of swamp species between 7000 BP and 4500 cal. BP indicated wet conditions at that time. The swamp became drier in the late Holocene and some peat may have been lost through deflation. Macroscopic charcoal and Typha pollen suggested that Aborigines deliberately burned the upland wetlands during the mid to late Holocene. Prior to European settlement climate changes generated community shifts in the terrestrial vegetation. The record revealed a transition from an early Holocene Eucalyptus woodland to an Allocasuarina wet-heath in the humid mid-Holocene, a community type with no modern analogue in the region. In the drier late Holocene, a Eucalyptus-dominated woodland returned. The impacts of European settlement were clearly seen in changes in sedimentation rates and in both terrestrial and wetland flora. Allocasuarina verticillata declined early in the European period and fire tolerant species were promoted, before the almost complete removal of native vegetation through broad scale land clearance and its replacement with nonnative pasture species. Compositional changes to the swamp flora were marked through the European phase with Acacia expanding early in settlement and later being replaced by Leptospermum, in response to changed fire and regional hydrological regimes. The impact of European land use is discussed in relation to Holocene climate-driven vegetation changes and aspects of Aboriginal land use.
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Three major water masses occur for all or part of the year within the shelf and slope region off southern Australia. A Leeuwin Current carries the warmest water mass of relatively low salinity into the region, principally along the shelf break as far east as 130ºE. This water mass first enters the western end of the region in May, disappears from the eastern end after July and from the western end by September-October. A warm and very high salinity water mass is present in the central and eastern half of the Great Australian Bight for most of the year. This central Bight water mass drifts to the south-east and occupies much of the shelf and slope region east of 135ºE., particularly in winter. A West Wind Drift cold water mass of lowest salinity is found throughout the year off the slope region of southern Australia and periodically intrudes into the shelf break, especially when the Leeuwin Current is weakly developed. The central Bight waters, which provide a second source of warm waters in the eastern half of the region, greatly complicate the interpretation from satellite imagery of warm waters in that region as being derived solely from the Leeuwin Current. Adequate salinity data and sea surface temperatures derived from satellite imagery are required to determine more accurately the eastward extent of the Leeuwin Current.