Earth and Planetary Science Letters 184 (2001) 635^644
www.elsevier.com/locate/epsl
Timing and duration of the Last Interglacial inferred from
high resolution U-series chronology of stalagmite growth in
Southern Hemisphere
Jian-xin Zhao *, Qikai Xia, Kenneth D. Collerson
Department of Earth Sciences, University of Queensland, Steele Building, Brisbane, QLD 4072, Australia
Received 15 September 2000; received in revised form 14 November 2000; accepted 15 November 2000
Abstract
High-precision 230 Th^238 U ages for a stalagmite from Newdegate Cave in southern Tasmania, Australia define a rare
record of precipitation between 100 and 155 ka before the present. The fastest stalagmite growth occurred between
129.2 þ 1.6 and 122.1 þ 2.0 ka (V61.5 mm/ka), coinciding with a time of prolific coral growth from Western Australia
(128^122 ka). This is the first high-resolution continental record in the Southern Hemisphere that can be compared and
correlated with the marine record. Such correlation shows that in southern Australia the onset of full interglacial sea
level and the initiation of highest precipitation on land were synchronous. The stalagmite growth rate between 129.2
and 142.2 ka (V5.9 mm/ka) was lower than that between 142.2 and 154.5 ka (V18.7 mm/ka), implying drier conditions
during the Penultimate Deglaciation, despite rising temperature and sea level. This asymmetrical precipitation pattern is
caused by latitudinal movement of subtropical highs and an associated Westerly circulation, in response to a changing
Equator-to-Pole temperature gradient. Both marine and continental records in Australia strongly suggest that the
insolation maximum between 126 and 128 ka at 65³N was directly responsible for the maintenance of full Last
Interglacial conditions, although the triggers that initiated Penultimate Deglaciation (at V142 ka) remain
unsolved. ß 2001 Elsevier Science B.V. All rights reserved.
Keywords: thermal ionisation mass spectroscopy; absolute age; speleothems; interglacial environment; Australia; precipitation
1. Introduction
The precise timing and duration of the last and
previous interglacials are important for understanding mechanisms that triggered deglaciation,
and for obtaining insights into the course of the
* Corresponding author. Fax: +61-7-33651277;
E-mail: [email protected]
Holocene as we potentially approach the next
ice age. Despite accumulation of high-precision
U^Th dates for corals, calcite veins and marine
sediments, the timing and duration of the Last
Interglacial remain controversial [1^13]. For example, `strictly screened' coral age spectra from
coastal Western Australia (WA) [1,2] suggest
that the Last Interglacial high sea level started
at ca. 128 þ 1 ka, and terminated at ca. 116 þ
1 ka, with the main episode of coral growth con¢ned to a short interval between 128 and 122 ka.
In this record, the start of the Last Interglacial
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 3 5 3 - 8
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J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
high sea level appears to coincide with the onset
of the Milankovitch summer insolation peak at
65³N at 126^128 ka [14]. However, this observation appears to be in con£ict with a well-dated
calcite vein in Devil's Hole, Nevada [3^5], which
suggests that the initial sign of Penultimate Deglaciation warming started as early as V150 ka, and
full interglacial conditions were attained at V134
ka, some 5^6 ka earlier than indicated by the
coral record from WA. Oldest coral ages from
WA also appear to be younger than those elsewhere, e.g. Bahamas [6], Hawaii [7], Barbados [8],
and Huon Peninsula [9], which are as old as, or
older than, 130 ka. Recently, Esat et al. [9] reported evidence for rapid £uctuations of sea level
at Huon Peninsula during the Penultimate Deglaciation period (140^128 ka), which they suggested
may account for the discrepancy observed between WA and other coral records.
Corals are ideal time markers of sea level
change [15] and past high sea stands [2,7,8] because their 230 Th and 231 Pa ages can be precisely
determined by thermal ionisation mass spectrometric (TIMS) techniques [10,16]. Despite this,
their use may be complicated by a number of
factors such as poor preservation, diagenesis, uncertainty in the tectonic uplift history and hydroisostatic/glacio-isostatic processes [1,6,17]. These
factors may be partly responsible for the observed
age dispersion and discrepancy among di¡erent
Last Interglacial coral terraces worldwide. Deepsea sediment cores are ideal archives of global ice
volume change through time [18,19]. Unfortunately their direct dating is di¤cult, with only a
few corrected U^Th ages for marine sediments
deposited during the Last Interglacial and Penultimate Deglaciation periods being reported [11].
Similar dating problems also exist for ice cores
[20]. Speleothems are cave deposits composed
mainly of crystalline calcite (e.g. stalagmite, stalactite and £owstone) that are ideal for precise
and accurate dating by TIMS U-series chronology
[21]. They form across a wide range of climatic
zones, also during periods of severe climatic
changes, and are therefore widely available for
sampling. Speleothems, like Devil's Hole calcite
veins, contain valuable climatic information in
terms of variations in their physical, chemical
and isotopic compositions. Thus, despite some
complicating factors, they have greater potential
than many other terrestrial proxies such as lake
sediments and pollens to characterise global climatic events and processes [22^28].
The growth of speleothems is a complex function of dripwater Ca concentration, water supply
and soil CO2 concentration, all of which are climatically related [29^31]. Water supply (drip rate)
is directly correlated with e¡ective precipitation
over cave catchments. Soil CO2 concentration is
a function of bioproductivity, which is related to
both temperature and e¡ective precipitation. Ca
concentration in dripwater is controlled by the
ground water £ow path, which may also be related to palaeoclimatic changes albeit in a more
complicated fashion. Both experimental and observational evidence indicates that growth layers
in some stalagmites are annual [31,32] and growth
rate (or annual growth layer thickness) of a stalagmite is strongly and positively correlated with
e¡ective precipitation, and with palaeo-temperature in areas where water supply is abundant
[27,29,32,33]. Considering the sharply contrasting
mean annual temperature, precipitation and bioproductivity between glacial and interglacial periods, growth rates of speleothems are also expected
to signi¢cantly di¡er [24,26,27]. Growth rate
change of a stalagmite with a growth record
across glacial^interglacial cycles, particularly
from areas directly in£uenced by major global
circulation systems, may contain valuable information on the timing of global climatic events
and the duration of global climatic processes.
Whereas corals and foraminifera directly re£ect
sea levels and global ice volumes, respectively,
speleothems, like groundwater derived calcite
veins from Devil's Hole, Nevada, are important
proxies for palaeo-temperature and palaeo-precipitation on land [3^5,22^24,26^28]. The study of
speleothems may thus provide an insight into
the interplay between changes in di¡erent climatic
factors, such as global ice volume, sea level, palaeo-temperature and palaeo-precipitation. This is
potentially most useful in situations where
changes in palaeo-temperature and palaeo-precipitation are decoupled or out-of-phase as in Australia [34,35].
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J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
637
2. Results and discussion
To better understand the above questions, we
have dated growth layers for a 840 mm long stalagmite (NEW-B) from Newdegate Cave, southern Tasmania, Australia (Fig. 1) using the TIMS
U-series method. Newdegate Cave (146³56PE,
43³23PS) is located about 65 km southwest of Hobart. It was chosen for this study because Tasmania is surrounded by the Paci¢c and Southern
Oceans and the climate in southern Tasmania is
directly controlled by a prevailing, moisture-laden
Westerly circulation. Thus, global climatic signals
would likely be immediately recorded in terms of
e¡ective precipitation through tele-connections
(combined ocean^atmosphere circulations). The
prevailing Westerly rainfall over the area provides
an ideal condition for speleothems to grow continually across glacial^interglacial boundaries.
Southern Tasmania also has advantages over continental inland localities in recording general global climatic signals, as proxy records from the inland region may be complicated or masked by
local climate.
Stalagmite NEW-B shows a rather continuous
growth pro¢le, with the exception of a thin clayrich layer, possibly a growth hiatus, at the height
of 280 mm. A total of 16 samples, each covering
about a 4^8 mm band of growth layers, were taken along the growth axis and measured for U^Th
disequilibrium ages by TIMS (Table 1). The data
display a continual growth history from 100 to
155 ka, straddling the Last Interglacial. Based
on growth rate and initial 234 U/238 U activity ratio
variations, we have subdivided the growth history
into ¢ve stages, each characterised by a distinctive
average growth rate and discrete initial 234 U/238 U
activity ratios (Fig. 2).
Stage I (154.5^142.2 ka), within the glacial
maximum in age, has a moderate growth rate of
18.7 mm/ka, with intermediate initial 234 U/238 U
activity ratios of 1.69^1.79. Stage II (142.2^129.2
ka), correlated with the Penultimate Deglaciation
period, is characterised by a rather low growth
rate of 5.9 mm/ka, and much higher initial 234 U/
238
U activity ratios (2.01^2.19). Stage III (129.2^
122.1 ka), at the height of the Last Interglacial,
displays the highest growth rate of 61.5 mm/ka,
Fig. 1. Map showing sample locality (Newdegate Cave) and
winter atmospheric circulation pattern (July) in Australia.
Shaded belt indicates the mean location of the sub-tropical
Anti-Cyclones (STA). The study area Tasmania is under the
direct in£uence of the prevailing Westerly circulation.
more than 10 times higher than in Stage II, and
the lowest initial 234 U/238 U activity ratios (1.63^
1.69). Stage IV (122.1^116.7 ka), corresponding to
the later part of the Last Interglacial, shows a
signi¢cantly reduced growth rate of 16.1 mm/ka
with little change in initial 234 U/238 U activity ratios (1.62^1.69). During the last stage, Stage V
(116.7^100.3 ka), the growth rate drops to the
lowest value (0.3 mm/ka), and initial 234 U/238 U
activity ratios increase to a maximum of 2.70.
The most striking observation of our study is
that the period of most rapid stalagmite growth
(129.2 þ 1.6 to 122.1 þ 2.0 ka) coincides with episodes of proli¢c coral growth in WA (128.9 þ 0.6
to 121.7 þ 0.5 ka) (Fig. 2). During this period, the
coral record suggests that the sea level was about
30.7 to +5 m relative to the present Mean Low
Water Springs in WA [1,2]. Stalagmite growth
slowed down signi¢cantly during the period of
121.8^116.5 ka, a time of formation of a retrogressive coral terrace at Mangrove Bay, WA
(119.2 þ 0.6 to 116.1 þ 0.3 ka). Growth of both
the stalagmite and corals virtually stopped after
116 ka due to rapid sea level fall since then [1,2].
The excellent correlation between the stalagmite
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J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
record in Tasmania and the coral record from the
WA coast has signi¢cant climatic implications. It
suggests that changes in sea level and sea surface
temperature (SST) in the marine environment,
and palaeo-temperature and precipitation on
land were synchronous, at least for the southern
Australian region. The correlation rules out the
possibility of sampling or preservation bias as a
potential cause for the lack of older ages in the
WA coral population, and reinforces the argument for a striking climatic boundary at 128 þ 1
ka. This possibly marks phase convergence of major climatic factors such as sea level, global SSTs,
global continental temperature and global precipitation, in response to the onset of the summer
insolation peak at 65³N [14]. In fact, both Stirling
et al. [2] and Esat et al. [9] argued that coral age
spectra from many other localities, when critically
assessed, are in agreement with results from WA.
Stirling et al. [2] also suggested that ages from
slowly uplifting reefs are expected to be slightly
older than the oldest Last Interglacial ages from
WA, as these corals probably started growing
during episodes of sea level rise leading up to
the Last Interglacial. In faster uplifting localities
like the Huon Peninsula, major reef-building
events may have occurred even earlier (V134
ka) [9], when the rate of sea level rise is similar
to the crustal uplift rate (V2 m/ka). It is also
possible that the sudden onset of proli¢c reef
growth in WA and abrupt increase in stalagmite
growth rate in Tasmania at 129^128 ka is due to a
sudden change in regional climate over Australia
at that time. However, it must be emphasised that
coastal WA represents the only tectonically stable
region, far from the glacio-isostatic in£uence of
Table 1
U^Th isotopic and age data for stalagmite NEW-B
Sample
NEW-BT-1
NEW-BT-2
NEW-B1
NEW-B2
NEW-B3
NEW-B4
NEW-BM
NEW-B5
NEW-B5-2
NEW-B6
NEW-B7
NEW-B7-2
NEW-B7-3
NEW-B8
NEW-B9
NEW-BB
Height
U
(mm)
(ppm)
840
835
748
665
586
506
411
337
310
289
274
251
234
177
135
52
0.2585
0.1225
0.1097
0.1018
0.1084
0.1154
0.1224
0.1223
0.1070
0.0995
0.1399
0.0550
0.0720
0.1080
0.1230
0.0777
(230 Th/232 Th)
(234 U/238 U)
(230 Th/238 U)
230
Th/238 U
age
(ka)
Initial
(234 U/238 U)
3250
502
537
384
995
1490
894
96
477
241
258
92
145
693
1802
1068
2.2758 þ 113
1.4475 þ 55
1.4839 þ 38
1.4210 þ 53
1.4792 þ 46
1.4613 þ 48
1.4434 þ 50
1.4318 þ 40
1.4617 þ 27
1.8090 þ 27
1.6875 þ 56
1.7003 þ 62
1.4951 þ 33
1.4554 þ 46
1.5121 þ 34
1.5138 þ 81
1.4726 þ 88
0.9978 þ 48
1.0527 þ 90
1.0088 þ 117
1.0610 þ 112
1.0529 þ 98
1.0457 þ 80
1.0486 þ 85
1.0697 þ 65
1.3763 þ 98
1.2877 þ 113
1.3012 þ 63
1.1575 þ 67
1.1319 þ 55
1.1922 þ 65
1.2123 þ 68
100.3 þ 1.1
116.7 þ 1.3
122.1 þ 2.0
123.2 þ 2.7
124.5 þ 2.4
125.8 þ 2.2
127.3 þ 2.0
129.4 þ 2.6
129.2 þ 1.6
133.3 þ 2.1
135.5 þ 2.7
135.6 þ 2.7
142.2 þ 2.2
144.7 þ 1.8
147.3 þ 1.8
151.8 þ 2.5
2.6959 þ 98
1.6240 þ 70
1.6850 þ 60
1.6842 þ 85
1.6826 þ 70
1.6592 þ 68
1.6365 þ 67
1.6287 þ 86
1.6670 þ 46
2.1858 þ 85
2.0136 þ 103
2.0407 þ 133
1.7458 þ 73
1.6871 þ 61
1.7778 þ 52
1.7906 þ 97
Ratios in parentheses are activity ratios calculated from measured atomic ratios. Errors are at the 2c level for the least signi¢cant
digits. The ages are calculated using half-lives from 230 Th and 234 U of 75 380 and 244 600 years, respectively. The calculated ages
and initial (234 U/238 U) ratios include a negligible to small correction for initial/detrital U and Th using average crustal 232 Th/238 U
atomic ratio of 3.8 þ 1.9 (230 Th, 234 U and 238 U are assumed to be in secular equilibrium). Analytical procedures are similar to
those in [1,4,16]. Sample chips of V1 g were dissolved in HNO3 , and spiked with a 229 Th^233 U mixed tracer, co-precipitated
with FeOH3 , and puri¢ed with 0.8 ml Dowex AG-1 X8 200^400# anion columns held in universal pipette tips. U and Th fractions were loaded separately onto zone-re¢ned rhenium single ¢laments and sandwiched into two graphite layers. A Fisons VG
Sector 54-300 TIMS equipped with a WARP ¢lter and an ion counting Daly detector was used to determine U and Th concentrations and isotopic compositions. Both Th and U are measured on the Daly detector in peak-jumping mode. Total procedural
blanks are negligible at 6 10 pg for both Th and U. Repeated analysis of the international HU-1 uraninite standard yields an
average 234 U/238 U activity ratio of 1.0015 þ 0.0009 (2cm , N = 16), which is within error of the value reported in [4].
EPSL 5704 4-1-01
J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
639
Fig. 2. Heights and initial 230 U/238 U activity ratios versus TIMS U^Th ages for sub-samples of stalagmite NEW-B from Newdegate Cave. Open squares with error bars and black dots represent heights and initial 230 U/238 U activity ratios, respectively. Best¢t line through the open squares shows a ¢ve-stage growth history for NEW-B, with the time range and growth rate for each
stage also marked in parentheses. See text for discussion.
the major Northern Hemisphere ice centres, for
which a large quantity of high-precision `strictly
screened' coral ages are available. Data for this
region are expected to be more coherent than
those for locations susceptible to active vertical
tectonic in£uences and/or glacio^hydro^isostatic
e¡ects. This, coupled with the fact that other localities show the same clustering of ages as WA
when critically assessed, is strongly suggestive of
an abrupt shift in climate, not just locally in Australia, but also globally, over the Earth as a whole
at 129^128 ka.
It is also important to note that the change at
122^121 ka shown in both stalagmite and coral
records in Australia appears to re£ect the e¡ect of
a `short-lived' globally signi¢cant cooling event,
termed `intra-Eemian cold event' in Europe,
which was also recorded by major change in global reef growth patterns and features in some deepsea cores [2]. However, the abrupt reduction in
stalagmite growth rate, apart from re£ecting a
possible global climate event, may have also
been further in£uenced by cave hydrological
changes, e.g. lateral migration of the ceiling drip
point above the stalagmite tip or Ca concentration change in the dripwater.
The second major observation of our study is
that stalagmite growth rate during the Penultimate Deglaciation transitional to full Last Interglacial, i.e. during Stage II (129.2^142.2 ka), was
three times slower than that during the preceding
full-glacial period, Stage I (142.2^154.5 ka) (Figs.
2 and 3). This suggests that, during the Penultimate Deglaciation when temperature and sea level
were generally rising, the climate in Tasmania was
the driest, with the lowest e¡ective precipitation.
This observation is consistent with other continental records (pollens, lake levels, sand dunes,
charcoals, dusts, speleothem growth frequency
distribution) [26,34^36] for the Last Deglaciation
period in Australia, which suggest that southern
Australia and Tasmania experienced the driest
conditions, with lowest e¡ective precipitation during the Last Glacial Maximum (LGM) to Holocene transition (18^10 ka), signi¢cantly lower
than before the LGM (see Fig. 4).
Interpretation of the above observations is controversial [34^36]. According to Harrison [35],
such an asymmetrical precipitation pattern was
caused by the combined e¡ects of the latitudinal
shifts in the main circulation belts (i.e. the Subtropical Anti-Cyclones (STA) and the associated
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J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
Fig. 3. (A) Comparison between growth rate variations of stalagmite NEW-B and histogram of coral ages from WA coast [2].
The period of highest growth rate of stalagmite NEW-B is perfectly correlated with the episode of proli¢c coral growth in coastal
WA. (B) The SPECMAP [19] and Devil's Hole [3] oxygen isotope records versus time. Also shown for comparison is the 65³N
summer insolation curve. Note that the onset of highest stalagmite and coral growth in Australia is broadly consistent with the
isolation peak at 126^128 ka.
Westerlies) and the changes in the strength of the
Walker Circulation. Kershaw [36], by contrast,
attributed the lowest e¡ective precipitation during
the deglaciation period to the interplay between
cool oceans, high degrees of continentality, and
rapidly rising continental temperature and evaporation rates. We believe the asymmetrical precipitation pattern is best explained by a polarward
shift of the STA and the associated Westerlies
(Fig. 1) during deglaciation. The important supporting evidence is that the rapid air temperature
rise over Antarctic ice sheets started as early as
19 ka [20], whereas a low SST (6³C lower than
present) in the SW Paci¢c persisted until as late as
10 ka [37]. Such a substantially reduced Equatorto-Pole temperature gradient between 19 and
10 ka would have caused a polarward shift of
the STA and the associated Westerly belt [35].
During this period, the bulk of the Australian
continent would have been under the direct in£uence of the dry, expanded STA, resulting in re-
duced precipitation and increased aridity. This
model satisfactorily explains a higher precipitation during the pre-LGM glacial period, as the
pre-LGM Equator-to-Pole temperature gradient
may not be signi¢cantly di¡erent from that of
the interglacial/Holocene periods, allowing part
of southern Australia to receive rainfall from the
Westerly circulation. In addition, a cooler continental temperature during the glacial period may
also have led to a reduced evaporation rate and a
higher e¡ective precipitation. Apart from the reduced Equator-to-Pole temperature gradient, a
positive SST gradient between Eastern and Western Paci¢c regions (cf. coral SSTs between Barbados and Vanuatu in [37]) may also have been
established during the later part of the Last Deglaciation (14^10 ka). This may have facilitated a
weakened Walk Circulation, causing the observed
maximum aridity in Australia for this period [34^
36].
The continental records in southern Australia
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J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
Fig. 4. Comparison between growth rate variations of stalagmite NEW-B in Tasmania (top) and frequency distribution
of high lake levels in southern Australia (bottom, after [35]).
The lowest e¡ective precipitation is inferred for the deglaciation periods (PD and LD) from both records, suggestive of a
common mechanism responsible for such a climatic pattern.
Abbreviations are: LI, Last Interglacial; PD, Penultimate
Deglaciation; PG, Penultimate glacial period; LD, Last Deglaciation; LG, Last glacial period.
are not extensive enough to allow comparison
with our stalagmite record from Tasmania. However, other limited evidence does suggest that the
climatic system may have behaved similarly. The
Antarctic ice core records imply an earlier start of
deglaciation, as re£ected by the ¢rst isotopic sign
of warming at 150^140 ka [20], despite uncertainty of þ 10 ka in the model ages. Earlier warming of Antarctica has been attributed to large local changes in precession [38]. On the other hand,
Sr/Ca records in a V130 ka old coral from Alladin's Cave at Huon Peninsula [39] suggest that
SST in the SW Paci¢c is 6³C lower than that of
the Last Interglacial. This situation is similar to
that for the Last Deglaciation, when Equator-toPole temperature gradient has dropped, leading to
a polarward shift and expansion of the STA and
resulting in the lowest e¡ective precipitation in
641
Tasmania. If this was the case, the rapid increase
in stalagmite growth and sudden onset of proli¢c
coral growth in Australia at 129^128 ka may have
been related to an abrupt SST rise at around this
time [39].
Speleothem growth records in this study are in
contrast with those reported for caves from the
Naracoorte region in South Australia (140³45PE,
36³58PS, see Fig. 1), where speleothems grew
mainly during stadials and cool interstadials,
rather than warm interglacials or Holocene [26].
Our model can also explain such contrasting precipitation patterns. In southern Tasmania, precipitation has been mainly supplied by the moistureladen Westerly circulation, which was only brie£y
interrupted by a polarward migration of the STA
during deglaciation periods. Because precipitation
in this area is often larger than evaporation, longterm variations in the e¡ective precipitation (or
net precipitation), as re£ected by the speleothem
growth rate, were usually correlated with changes
in moisture levels in the Westerly circulation, and
were therefore broadly in phase with global climatic cycles. However, climate in the Naracoorte
region of South Australia has been dominantly
in£uenced by the dry STA, characterised by
much lower rainfall. In this area, long-term variations in e¡ective precipitation were likely to be
inversely correlated with changes in the evaporation rate, or air temperature. In this case, a lower
evaporation rate, coupled with a possibly enhanced Walker Circulation [26], would have resulted in an increased e¡ective precipitation during the stadials and cool interstadials in the
Naracoorte region.
A number of attempts have been made to explain the substantial phase shift in the well-documented, high-precision Devil's Hole record
[12,13,10,2,5]. A plausible interpretation is that
the Devil's Hole record picked up an `early warming event' in the North Atlantic region at 145^
150 ka, leading global ice-volume and sea level
records by thousands of years [13]. In this study,
we consider that the 5^6 ka phase o¡set between
the Tasmania and the Devil's Hole records in the
onset of full Last Interglacial conditions (Fig. 3) is
mainly due to the fact that the growth rate of the
Tasmania speleothem is largely controlled by var-
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J.-x. Zhao et al. / Earth and Planetary Science Letters 184 (2001) 635^644
iations in the Westerly related precipitation,
whereas the Devil's Hole records re£ect the continental palaeo-temperature regime in North
America. Despite such a phase shift, however,
the Tasmanian record, like the Devil's Hole archive, also implies an early start of deglaciation
at 142 ka (cf. 150 ka in Devil's Hole), re£ected by
the beginning of a reduced growth rate in Stage II
(Figs. 2 and 3). The 142 ka ¢gure agrees well with
those inferred from several other studies [9^11,20].
Backdating of the Penultimate Deglaciation does
point to the need for alternative, more complicated, driving mechanisms that triggered deglaciation [9^11], although both marine and continental records in Australia strongly suggest that the
high-latitude insolation maximum between 126
and 128 ka was directly responsible for the maintenance of full Last Interglacial conditions.
According to Esat et al. [9] and McCulloch et
al. [39], rapid Penultimate Deglaciation sea level
oscillations at Huon Peninsula were characterised
by a sea level peak at 314 m below present sea
level at 135 ka, followed by a severe sea level drop
by 60^80 m at 130 ka, and subsequent rapid rise
to above present sea levels after 130 ka. Such
severe climatic oscillation signals are not obvious
in the Tasmania stalagmite record, probably due
to the lack of su¤cient resolution in our data.
The only discernible sign in our record is re£ected
by the two analytically indistinguishable ages of
135.5 þ 2.7 and 135.6 þ 2.7 ka at the heights of 274
and 251 mm, respectively. Given the linear growth
rate for Stage II (Fig. 2), the expected age for the
sample at 251 mm height should be 139.4 ka, 3.8
ka older than its 135.6 ka age. The deviation is
slightly larger than the analytical error of 2.8 ka.
Thus, an alternative, though speculative, explanation would be that this section (274^251 mm)
represents a period of faster stalagmite growth
at around 135.5 ka, coincident with the high sea
level records at both Huon Peninsula and WA
[2,9].
It is also important to note that initial 234 U/
238
U activity ratios in stalagmite NEW-B vary
systematically with growth rates (Fig. 2). A similar 234 U/238 U variation observed in the Soreq
cave, Israel was attributed to the e¡ects of rainfall
and soil weathering conditions on drip-water
composition, with low rainfall accompanied by
high initial 234 U/238 U activity ratio [28]. Our study
clearly con¢rms that initial 234 U/238 U in speleothems, like growth rate, may provide an excellent
and robust proxy for palaeo-precipitation.
Acknowledgements
Stalagmite samples from Newdegate Cave were
collected with the permission of Parks and Wildlife Service Tasmania (Permit No. ES96277). We
especially wish to thank Dr. Ian Houshold and
Dr. Kelvin Kiernan for their hospitality and logistic support in the ¢eld and for their friendship.
The manuscript bene¢ts greatly from constructive
reviews by Tezer Esat and Claudine Stirling and
incisive discussions with Balz Kamber. This study
was supported by an Australian Research Council
(ARC) research fellowship to J.-x.Z. (No. ARF/
F39802775), and OPRS and University of
Queensland (UQ) post-graduate research scholarships to Q.X. We also wish to acknowledge funding support to our Isotope Geochemistry Laboratory by ARC and UQ. J.-x.Z. owes his research
progress to the help and support of Faye Liu,
Liqing Cheng and Guisong Liu.[AH]
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