Terrestrial data spanning the Last Glacial Maximum (LGM) and deglaciation
from the southern Australian region are sparse and limited to discontinuous
sedimentological and geomorphological records with relatively large
chronological uncertainties. This dearth of records has hindered a critical
assessment of the role of the Southern Hemisphere mid-latitude westerly
winds on the region's climate during this time period. In this study, two
precisely dated speleothem records for Mairs Cave, Flinders Ranges, are
presented, providing for the first time a detailed terrestrial hydroclimatic
record for the southern Australian drylands during 23–15 ka. Recharge to
Mairs Cave is interpreted from the speleothem record by the activation of
growth, physical flood layering, and
The Mairs Cave record indicates that the Flinders Ranges were relatively wet during the LGM and early deglaciation, particularly over the interval 18.9–15.8 ka. This wetter phase ended abruptly with a shift to drier conditions at 15.8 ka. These findings are in agreement with the geomorphic archives for this region, as well as the timing of events in records from the broader Australasian region. The recharge phases identified in the Mairs Cave record are correlated with, but antiphase to, the position of the westerly winds interpreted from marine core MD03-2611, located 550 km south of Mairs Cave in the Murray Canyons region. The implication is that the mid-latitude westerlies are located further south during the period of enhanced recharge in the Mairs Cave record (18.9–16 ka) and conversely are located further north when greater aridity is interpreted in the speleothem record. A further comparison with speleothem records from the northern Australasian region reveals that the availability of tropical moisture is the most likely explanation driving enhanced recharge, with further amplification of recharge occurring during the early half of Heinrich Stadial 1 (HS1), possibly influenced by a more southerly displaced Intertropical Convergence Zone (ITCZ). A rapid transition to aridity at 15.8 ka is consistent with a retraction of this tropical moisture source.
Evidence in the paleoclimate record for the nature and timing of changes in the Australian environment at the time of HS1 can assist in the critical assessment of how the climate system reacts to complex feedbacks in the ocean and atmosphere system (Clark et al., 2012). However, little is currently known about the impact of Heinrich stadials on Australian climate, and the Southern Hemisphere mid-latitudes in general, as reliable terrestrial paleoclimate records from this region are sparse (Broecker and Putnam, 2012). The latitudinal position of the westerly winds at the Last Glacial Maximum (LGM) is also an important research question which still remains debated despite long-running attention, both specifically in the southern Australian sector (Bowler, 1978; Bowler and Wasson, 1984; Wywroll et al., 2000; Shulmeister et al., 2004, 2016; Williams et al., 2009; Hesse et al., 2004; Turney et al., 2006; Haberlah et al., 2010; Cohen et al., 2011; De Deckker et al., 2012) and elsewhere in the Southern Hemisphere (Gasse et al., 2008; Kohfeld et al., 2013; Sime et al., 2013).
Currently, only a single proxy record exists from within the Australian
sector for the position and strength of the westerly winds during the LGM
and deglaciation (De Deckker et al., 2012). Marine core MD03-2611 (33–10 ka)
(Fig. 1a) is located east of the Great Australian Bight in the Murray
Canyons region at the present position of the subtropical front (STF). The
paleo-position of the STF is interpreted from the foraminifera micro-fossil
record (
Location of the Flinders Ranges and other sites described in the
text
Southern Australia is presently dominated by semi-arid drylands that skirt
the southern margin of Australia's arid interior. The proximity to the arid
interior means that it is hydrologically sensitive to episodes of climatic
change (Fitzsimmons et al., 2013). Terrestrial data that span the LGM and
the subsequent deglaciation are particularly sparse in this region and
limited to discontinuous sedimentological and geomorphological archives
(e.g. salt lakes, dune systems, ephemeral fluvial systems). Such records
typically carry large chronological uncertainties due to preservation issues
(e.g. lack of organic material, reworking of sediments, erosion) as well as
the limitations of applicable age measurement techniques (Fitzsimmons et
al., 2013). Additionally, the interpretation of geomorphic records may not
be straightforward for other reasons. For example, dune activation may
respond non-linearly to precipitation and be a function of sediment supply
as well as aridity (Fitzsimmons et al., 2013), whilst the generally large
catchments of Australia's arid interior contribute uncertainty over whether
fluvial systems are recording local or distal conditions. For example, Lake
Frome receives water from a catchment of nearly 63 000 km
Speleothems offer a number of advantages over the geomorphic archives described above, including being recorders of local recharge due to a relatively small catchment, and typically being well preserved in stable cave environments. Speleothems also yield precise and accurate chronologies based on U-series age measurements that typically result in age uncertainties of < 1 %
(2
Cave monitoring studies have shown dripwater
Cave monitoring studies and speleothem records in general are primarily
focused on temperate or tropical environments. Far less monitoring has been
undertaken within semi-arid/arid climates (with the exception of Ayalon et
al., 1998; Pape et al., 2010; Cuthbert et al., 2014a, b; Markowska et al.,
2016). In these drier landscapes, where there is less frequent recharge, and
temperatures conducive to evaporation occur, one would expect evaporative
processes to increasingly affect dripwater
There has been relatively little use of speleothem records to reconstruct southern Australia's dryland history. To date, just three speleothem stable isotope records exist for this region (Desmarchelier et al., 2000; Bestland and Rennie, 2006; Quigley et al., 2010) from marine isotope stages 6 and 5d, and the Holocene, but all are of particularly low-resolution and contain lengthy millennial-duration hiatuses. Previous research has produced age measurements of speleothems from caves in this region resulting in age–frequency histograms for Naracoorte Caves, the Flinders Ranges, and Kangaroo Island (Fig. 1a) (Ayliffe et al., 1998; St Pierre et al., 2012; Cohen et al., 2011, 2012). These studies highlight that speleothem growth in the southern margin of Australia's drylands is primarily a function of water balance, with enhanced speleothem growth coinciding with intervals of potentially greater moisture availability (Ayliffe et al., 1998; St Pierre et al., 2012; Cohen et al., 2011; Fitzsimmons et al., 2013). For example, the first such study by Ayliffe et al. (1998) demonstrated that speleothem growth at Naracoorte Caves is more frequent in the stadial periods of the last glacial cycle. This was attributed to increased effective precipitation as a result of reduced evaporation (Ayliffe et al., 1998).
A more recent study highlighted the climatic relationship between speleothem growth and pluvial intervals in this region via the overlap of two stalagmite records from Mairs Cave in the Flinders Ranges with lake-full conditions at Lake Frome. Lake Frome is 200 km NE of the cave site and is fed by run-off from the eastern slopes of the Flinders Ranges (Cohen et al., 2011). This relationship between enhanced speleothem growth during periods of increased effective precipitation is also observed outside Australia (e.g. Wang et al., 2004; Vaks et al., 2006; Stoll et al., 2013) and highlights stalagmite growth as a useful on/off indicator of recharge in semi-arid to arid environments.
In this study, we present the geochemical records of these two stalagmites from Mairs Cave. As it was not practical to monitor Mairs Cave owing to its remote location, lack of active dripwater, and infrequent recharge, we compare our data with the modern analogue study at Wellington Caves, in a temperate semi-arid environment 1000 km NE of Mairs Cave (Fig. 1a). We also draw on a comparison of our data with independent evidence for the location of the westerly winds, inferred from the foraminifer-derived proxy record of the STF from marine core MD03-2611 (De Deckker et al., 2012) and tropical speleothems from northern Australasia (Ayliffe et al., 2013; Denniston et al., 2013a, b) as well as other archives of climatic change relevant to the southern Australian region.
Today, the Flinders Ranges and surrounding region are classified as semi-arid to arid, and speleothem growth in the isolated pockets of Cambrian-age limestone karst is very sparse. The Flinders Ranges are a rugged 400 km long topographic divide intercepting the path of the westerly winds from the Southern Ocean, providing orographically enhanced rainfall in a region otherwise characterised by flat, arid dunefields and large salt lakes (Fig. 1a, b).
Rainfall at Mairs Cave is predominantly derived during the austral winter
months (Fig. 1c) but recharge in typical years is small to negligible,
explaining the lack of modern speleothem growth. The region experiences
limited connectivity with tropical moisture sources, owing to its position
on the southern limb of the Hadley cell, although the development of
continental troughs during summer months can favour advection of moisture
from the eastern Indian Ocean or Coral Sea and generate heavy summer rains
(Schwerdtfeger and Curran, 1996; Pook et al., 2014). In a study of the
synoptic climatology of heavy rainfall (> 25 mm day
The 1974 CE filling of Lake Frome and Lake Callabonna (Fig. 1b) is one of the most significant hydrological episodes in the historical record and happened in a year when a La Niña also coincided with a negative Indian Ocean Dipole (IOD) event. Negative IOD events occur when sea surface temperatures in the eastern tropical Indian Ocean are warmer than average, feeding moisture into Australia's interior (Risbey et al., 2009). The lakes were fed both by direct runoff from the Flinders Ranges and from rain further north, via Strzelecki Creek (Cohen et al., 2011, 2012). During a single week-long event, rainfall records were broken across much of the arid interior. Persistent rainfall was generated by sustained moist tropical airflow. Eighty percent of the resulting rainfall came from the eastern Indian Ocean during the first 4 days before switching to the western Pacific Ocean, as shown by trajectory analysis (Fig. 1d).
Mairs Cave stalagmites MC-S1
Mairs Cave (138
The two stalagmites used in this study were collected from Mairs Cave
(Fig. 1b) in 1998 CE. MC-S1 was collected from the main chamber
The two stalagmites are considerably different in appearance (Fig. 2a, b). MC-S1 is darker “coffee-coloured” (Fig. 2a) and contains well-defined parallel laminae appearing throughout the specimen (Fig. 2c), consisting of columnar and open-columnar fabric (Frisia et al., 2000). The growth surface of MC-S1 was wide and flat-topped, with gour features along the stalagmite flanks indicative of high dripwater Ca concentrations and high drip rate. The coffee colour is also indicative of organics, which would suggest greater connectivity to the soil than MC-S2. MC-S1 was sawn from a flowstone-covered boulder. Approximately 10–15 mm of its base was not recoverable owing to the saw cut.
Stalagmite MC-S2 is pale, semi-translucent, and forms a narrow candle-stick
stalagmite in its upper half. It contains thin lenses of calcite alternating
with layers of calcified sediment in the lower half (Fig. 2b). This lower
section captures the earliest growth phase of MC-S2, when calcite deposition
competed with aggradation of sediment on the floor of the cave where MC-S2
grew (Fig. 2b). These sediment layers, assumed to be deposited by
floodwaters, are more prominent and thicker in the lowest half of MC-S2, but
are still visible on the stalagmite flanks between
Two 5 mm slabs were sectioned longitudinally from each stalagmite. One slab
was further sectioned along the longitudinal axis for stable isotopes and
trace element analyses, while the
Powders were obtained for stable isotope analysis via continuous
micro-milling at 300 and 100
Trace elements were analysed by laser ablation inductively coupled plasma
mass spectrometry (LA-ICPMS) at RSES using a 193 nm excimer laser masked by
a slit that resulted in a rectangular area ablated from the sample surface
that was 120
Petrographic observations were carried out on uncoated, polished thin
sections under plane (PPL) and cross-polarised light (XPL) using a Zeiss
Axioskop optical microscope and a Leica MZ16A stereomicroscope at the
University of Newcastle. Fabrics were coded following a conceptual framework
proposed in Frisia (2015), which is based on models of fabric development as
a response to changes in cave environmental parameters. The changes of
fabrics through time were obtained by assigning numbers to each fabric
recognised in the stalagmites and plotting their variability along the
vertical axis (with respect to distance from top). In this conceptual
framework (see Frisia et al., 2000; Frisia, 2015), number 1 is assigned to
clean, columnar calcite. The highest numbers are commonly given to fabrics
reflecting high driving force (high supersaturation), which result in
spherulitic growth. These are absent in Mairs stalagmites, which are
dominated by columnar calcite. Thus, the progression of numbers in the fabric
stratigraphy (log) reflect an increasing amount of impurities “polluting”
clean columnar calcite (1), which result in the development of faint laminae (2),
parallel and visible laminae (3), laminae with triangular-shaped
(rhombohedra) tips (4) up to the point when from compact the fabric becomes
open (5). Then, the criteria follow a similar
U and Th isotope data and age determinations (in depth order) for stalagmites MC-S1 and MC-S2, Mairs Cave, Flinders Ranges, South Australia. Square brackets indicate activity ratios. MCS2-UM10 was omitted from the age–depth model as it is out of stratigraphic order.
The Al and U concentration data (not shown) were used as a broad guide to
select calcite with low-detrital and highest U content for age measurements.
Both stalagmites were examined in thin section. Fabric and possible
dislocations in growth were documented and also used to guide subsampling
for age measurements. U-Th age measurements were conducted at the University
of Melbourne following the methods of Hellstrom (2003; Table 1). Briefly,
samples of 20–120 mg were dissolved in concentrated HNO
The Mairs Cave speleothem record presented here is dated by a total of
19 high-precision U-series disequilibrium age measurements (Table 1;
Fig. 2a, b; age–depth plots are given in Fig. S1 of the Supplement)
and collectively spans 24 to 15 ka. A lens of calcite formed at
the base of MC-S2 at 24.2 ka that was subsequently covered by sediment,
before calcite recommenced growing at 23 ka. The exact growth interval of
this lower lens could not be constrained with further age measurements owing
to the amount of detrital material in this section. Age modelling shows that
MC-S2 grew relatively slowly at 10
Mairs Cave stalagmites MC-S1 and MC-S2: age
measurements
MC-S1 growth was initiated by 17.2 ka at a moderate rate (60–90
Both stalagmites are comprised predominately of columnar calcite. The compact columnar fabric of MC-S2 is, in its lower part (pre-20.6 ka), punctuated by several (> 20) thin detrital layers, most evident on the vertical flank of the stalagmite (Fig. 3c). Calcite re-nucleation occurs above the detritus rich layer and is marked by geometric selection whereby only crystals best aligned for mass-transfer processes within the environment of deposition continue to grow at the expense of neighbouring crystals (see Gonzalez et al., 1992; Self and Hill, 2003). However, growth of the dominant forms mostly occurred in optical continuity with the substrate. In the upper 40 mm (post-20.6 ka) the sediment layers cease to drape over the growth axis although sediment lenses are still evident on the flank of the stalagmite up until ca. 18.9 ka (Fig. 2b). In this portion of the stalagmite, the fabric is compact columnar calcite and lamination is not visible or extremely faint (Fig. 3c).
By comparison, MC-S1 fabric is characterised by both compact, translucent
and open, milky columnar subtypes with well-defined laminae. The laminae are
particularly well defined by the presence of brown organic-rich calcite from
the base to 6 mm below the top (i.e. 17.2–15.6 ka; Fig. 3c). The fabrics
were further defined by the shape of the crystal tips, which were flat or
acute (Turgeon and Lundberg, 2001; Frisia, 2015). Such distinction allows an
immediate recognition of the original thickness of the film of fluid bathing
the stalagmite tip (Turgeon and Lundberg, 2001). Mairs cave columnar fabrics
are characterised by two common types of crystal terminations as seen in
thin section: rhombohedra (which appear as a triangle) and flat (which
appear as a line). The rhombohedra tips reflect the emergence of either
cleavage {10.1}, steep {01.2}, or acute {40.1} rhombohedra
at the speleothem surface at the time of formation. Of these, the
{10.1} and {40.1}
are the most commonly observed forms in speleothems, as the steep
rhombohedron is more typical of marine waters, at higher supersaturation
than the typical cave waters. The steep form also yields length-fast
individuals (see Fairchild and Baker, 2012), which is not the case of MC-S1
and MC-S2 columnar crystals. The height of the rhombohedra “tips” in both
stalagmites suggest that they grew in a film of fluid of up to 100
Scatter plot of MC-S1 and MC-S2
MC-S1 and MC-S2
The shorter duration but faster growing MC-S1 record is approximately
0.5 ‰ lower compared with MC-S2 during the overlapping
growth period (17.2–15.6 ka) during which MC-S1 is dominated by decadal
isotopic variability of 1–1.5 ‰ (Fig. 3d). To compare the
records more closely, a smoothing spline was applied to MC-S1
Mean MC-S2
With respect to millennial trends, MC-S1
Scatter plots (Fig. 4a) show that MC-S2
A notable characteristic present in each of the speleothem isotopic records
is the coincident troughs in both isotopes defining a saw-tooth pattern,
particularly in regard to
Spectral analysis of MC-S1 and MC-S2
Speleothem mean Mg
MC-S2 Mg
With regard to Sr
The relationship between speleothem Mg
Further detailed comparison with the isotopic record was attempted to tease
out potential drivers such as soil processes, dilution, and autochthonous
versus allochthonous sources. For example, lowered Mg
There are several characteristics that suggest that isotopic disequilibrium
is impacting the Mairs Cave speleothem record and that this impact varies
through time and spatially between stalagmites: (i) MC-S2 is isotopically
enriched compared with MC-S1, (ii) relatively slow growing MC-S2 has
particularly high mean
Typically, isotopic disequilibrium is considered to be caused by either
(i) fractionation during the degassing process enhanced by high dripwater
supersaturation and slow drip rates (Fantidis and Ehhalt, 1970; Day and
Henderson, 2011) or (ii) fractionation driven by within-cave evaporation,
from either low relative humidity or high ventilation (Deininger et al.,
2012). We can expect several of these to be more common in semi-arid karst
settings (e.g. low drip rates, low cave air relative humidity; Cuthbert et
al., 2014a). However, it appears that the calcite has precipitated closer to
isotopic equilibrium across the top of stalagmite MC-S2 (Sect. 4.3.2). This
suggests isotopic disequilibrium could be occurring in the parent
dripwaters, perhaps by incomplete equilibration or evaporation in the
soil/epikarst karst stores (Bar-Matthews et al., 1996; Cuthbert et al.,
2014a). The fact that variability in
We argue that the impact of recharge is also evident in the multi-decadal to
centennial variability, which appear as saw-tooth type features displaying
rapid 1–2.5 ‰ decreases in
The persistence of sediment bands representing cave floor flooding, the erratic thickness of MC-S1 laminae argued to be drip-rate-controlled, and the occasional dissolution feature identified via thin section further support a hydrological driver, i.e. that the cave is affected by intermittent recharge. Dissolution features in this time interval implicate occasional rapid infiltration of high intensity events resulting in soil zone bypass or inundation by floodwaters.
The abrupt shift in the isotopic records at 18.9 ka, coinciding with the
peak of the LGM, is characterised by (i) a 2 ‰ abrupt
decrease in both
The absence of sediment bands in MC-S2 after 18.9 ka (Fig. 2b) could also suggest a hydrological change, although we cannot exclude that this is simply a function of the stalagmite growth outpacing streamwater levels or a reduction in sediment supply. MC-S1 began growing by 17.2 ka during this proposed period of enhanced recharge. Initiation of a new stalagmite suggests activation of a new flow path, further supporting more effective recharge. The occurrence of bundles of laminae showing parallel versus rhombohedra-tipped layers suggests that the increase in effective recharge varied, periodically, from 17.2 to 16.2 ka (Fig. 3c), with episodes of dissolution (highest recharge of undersaturated waters) between 16.7 and 16.2 ka. The presence of laminae indicates input of colloidal particles during infiltration when water was at its lowest supersaturation state (Frisia et al., 2003). The occurrence itself of the visible organic colloids would suggest that maximum infiltration occurred in a cooler context (Frisia et al., 2003), which prevented efficient organic matter degradation.
Our spectral analysis demonstrates that multi-decadal to centennial
variability in speleothem
However, we consider that, in a semi-arid environment, moisture source
variation cannot be reliably fingerprinted owing to the additional isotopic
impact of evaporation of water in karst stores between recharge events
(Cuthbert et al., 2014a, b), as well as the likely scenario that recharge
will be biased to intense
A final point to raise when considering mean speleothem
The point 15.8 ka marks a transition in the Mairs Cave record evidenced by an abrupt
The transition in the MC-S1 record at 15.8 ka also occurs approximately at
the time of overall isotopic enrichment and higher Mg
The Mairs Cave record overlaps chronologically with a nearby sedimentary
archive, the “Flinders silts” (Fig. 1b), a thick sequence (up to 18 m) of
slackwater laminae from the western side of the central Flinders Ranges
(Callen, 1984; Haberlah et al., 2010). These fluvial deposits are
approximately 100 km from Mairs Cave and date from
The Mairs Cave and Flinders silts records correlate remarkably well in terms of their timing of hydrological change with a “switching-on” of recharge at 24 ka and “switching-off” at 16 ka and a significant change in the hydrological characteristics at 19 ka. However, they differ somewhat in the interpretation of the hydrological change at 19 ka, i.e. reduced storm frequency/intensity in the silt record versus more effective recharge in the stalagmite record. Although speculative, combining this evidence may indicate something of the nature of this change in terms of the frequency/magnitude characteristics of the rainfall, i.e. a shift to more frequent, lower-magnitude events leading to more continuous recharge, or the speleothem isotopic record may just reflect reduced evapotranspiration over this interval. The records do agree in the 23–19 ka interval in terms of significant hydrological events of approximate centennial frequency, i.e. high-magnitude floods in the silts record are consistent with significant recharge occurring approximately every 130–180 years in the Mairs Cave record.
The Mairs Cave
We note here, also, that it was previously unresolved whether the termination of Flinders silts at 16 ka was due to a lack of floods or the exhaustion of silt supply (Haberlah et al., 2010). However, the match with the abrupt transition to aridity in our data supports a climatic driver for the termination of the floodwater lamina.
Increased hydrologically effective precipitation in the 19–16 ka period is
also supported by optically stimulated luminescence ages from beach ridges at Lake Frome
(Fig. 1a),
indicating that it was 15–20 times the modern volume between 18 and 16 ka (Cohen
et al., 2011, 2012), coincident with relatively high levels of charcoal and
woodland taxa pollen present in the sediments of the lake floor (Singh and
Luly, 1991; Luly, 2001). The shift to aridity at 15.8 ka in Mairs Cave is
supported by significant reductions in
Comparing the Mairs Cave record with the MD03-2611 marine record (De Deckker
et al., 2012) reveals that the transitions identified in the STF record also
coincide remarkably with those at Mairs Cave in terms of timing, i.e. 19 and
16 ka (Fig. 6b). However, we highlight the following inconsistency. The
MD03-2611 STF record is interpreted as a proxy of westerly winds, with the
westerlies interpreted to have shifted
These observations raise an interesting problem: that the Mairs Cave record,
which is sensitive to recharge, appears to be hydrologically out of tune
with the evidence in the marine record. We explore three possibilities for
the disagreement in the marine and terrestrial records:
Sea surface temperatures (SSTs) in the Southern Ocean were more important for moisture delivery rather
than mean latitudinal position of the westerlies; the water balance was sensitive to temperature (i.e. evaporation) rather
than simply rainfall (5.2.3); or rainfall to Mairs Cave was dependent on another moisture source other than
the westerlies (5.2.4).
Addressing the first point, we note that the
Addressing the second point, it was shown previously (Williams et al., 2006)
that recharge to the Flinders Ranges at the LGM could be enhanced simply
because evaporation would be reduced in a cooler environment (Williams et
al., 2006). We demonstrate this also by using the Thornthwaite method to
estimate evaporation (Thornthwaite, 1948). Figure 1c shows calculations of
monthly hydrologically effective precipitation (HEP) for (i) present-day
monthly rainfall and temperature in the Flinders Ranges and (ii) LGM
temperatures, whereby monthly temperature was offset by
Thirdly, we consider whether effective precipitation may be higher if the region is being watered from systems other than the westerlies. There is evidence that northern Australia and southern Indonesia were wetter during parts of the Last Termination and this has been linked to changes in the Indo-Australian summer monsoon (IASM) activity/Western Pacific Warm Pool (WPWP) dynamics and/or a southward shift in the Intertropical Convergence Zone (ITCZ) (e.g. Nott and Price, 1994; English et al., 2001; Turney et al., 2004; Denniston et al., 2013a; Ayliffe et al., 2013). A more southerly displaced ITCZ could increase the availability of tropical moisture into northern Australia, as was demonstrated in a modelling study under HS1 boundary conditions (Mohtadi et al., 2014). This could have increased the availability of tropically sourced moisture to the mid-latitudes via continental troughs as described in Sect. 2.
Figure 6 shows the speleothem records from Ball Gown Cave in NW Western
Australia (Denniston et al., 2013a), Cape Range in Western Australia
(Denniston et al., 2013b) and Liang Luar in Flores, Indonesia (Ayliffe et
al., 2013), as well as marine record SO189-39KL of reconstructed tropical
eastern Indian Ocean salinity, from the northern Mentawai Basin, off the
coast of Sumatra (Mohtadi et al., 2014) (see Fig. 1a for locations). Both
the Ball Gown Cave and Liang Luar records are considered to be influenced by
the intensity/location of the IASM with a more southerly displaced IASM
inferred during HS1 (Denniston et al., 2013; Ayliffe et al., 2013). While
it is difficult to judge against the Ball Gown Cave record, given that the
uncertainty in its chronology is approximately
The above offers an explanation for the period of enhanced recharge seen in
the Mairs Cave record during HS1. However, in the Mairs Cave record, this
period of enhanced recharge occurred within the context of an earlier shift
to relatively wetter conditions at 18.9 ka, evidenced by the decrease in
speleothem
In the modern record, the delivery of moisture from the warm seas surrounding northern Australia to its interior is strongly governed by tropical ocean patterns associated with the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) (Ummenhofer et al., 2009). Variability in tropical Pacific and Indian Ocean SSTs, in particular, strongly influences southern Australian rainfall (Ummenhofer et al., 2009; Pook et al., 2014) and has been shown to display decadal variability (Ummenhofer et al., 2011). A more La Niña-like state in the glacial period has been previously invoked (e.g. Sarnthein et al., 2011; Muller et al., 2008). However, a reconstruction of tropical eastern Indian Ocean salinity, from marine sediments off the coast of Sumatra, suggests anomalously dry conditions prevailed from approximately 19–20 until 16 ka (Mohtadi et al., 2014; Fig. 6h). This site lies within the core of the IOD zone of upwelling, suggesting that it is unlikely that the IOD was responsible for enhanced recharge to Mairs Cave from 18.9 to 15.8 ka. In the study by Mohtadi et al. (2014), the strong anti-phase relationship between hydroclimate records from NW and SE Indonesia (reduced rainfall over Sumatra versus enhanced rainfall over Flores) was noted and reproduced in their model simulation. This was argued to further support the southward position of the ITCZ during HS1. Interestingly, in the Sumatran record, this drying trend appears to begin as early as some time between 19 and 20 ka (Fig. 6h).
An alternative explanation is that a weaker subtropical ridge across Australia may have permitted deeper penetration of troughs into interior Australia. This possibility is supported by a modelling study by Sime et al. (2013), who showed that the strength of the Hadley cell in the subsidence regions was reduced during the LGM. Thus recharge to Mairs Cave throughout the 23–15.8 ka period may have been favoured by a weakened subtropical ridge, with a further enhancement of moisture delivery by a southward displaced ITCZ during HS1.
Finally, we consider the relationship between the Mairs Cave record and
regional data for the transition to drier conditions at 15.8 ka in the Mairs
Cave record. As noted by Zhang et al. (2016), both the Flores and Ball Gown
Cave records have low
Two stalagmites from Mairs Cave, Flinders Ranges, are interpreted to record
shallow groundwater recharge to the over the LGM/early deglacial. Relative
recharge is primarily indicated by the on/off activation of speleothem
growth and degree of isotopic disequilibrium, supported by Mg 23–18.9 ka: MC-S2 activates but shows isotopic disequilibrium driven by
evaporation in the soil/epikarst water stores, punctuated with multi-decadal
periods of higher effective recharge and cave flooding. 18.9–15.8 ka: MC-S2 has reduced isotopic disequilibrium, indicating increased
infiltration and/or reduced evapotranspiration and MC-S1 activates due to
relatively more recharge. Speleothem 15.8 ka: MC-S1 records a shift to aridity, coinciding with the
termination of MC-S2 and, eventually, MC-S1 growth, indicating the end of
effective recharge.
These findings agree well with other regional geomorphic evidence for high-magnitude floods of approximately centennial frequency in the Flinders silts (coinciding with phase I), lake highstands (coinciding with phase II), and re-activation of dunefields (overlapping phase III) in the southern Australian drylands. In comparison, the Mairs Cave record is the most precisely dated and highest-resolution record of these archives to date, and the first able to confirm that recharge to this region is responding to key global events during the Last Termination (LGM and HS1).
The source of moisture responsible for enhanced recharge could not be
reliably isotopically fingerprinted for the Mairs Cave record, as it is
within the hydrological uncertainty of the speleothem
The spectral analysis reveals that episodes of recharge to Mairs Cave occurred in approximately 180-year cycles that persisted through the whole 23–16 ka interval. This suggests that the mechanism for multi-centennial variability in recharge was operating throughout this period. This may be due to a weakened subtropical ridge with other mechanisms amplifying recharge from 18.9 ka, particularly during HS1. For example, Southern Ocean SSTs, reduced evapotranspiration, a further increase in availability of tropical moisture from a more southerly displaced ITCZ during HS1, or some combination of these. A future modelling study investigating these factors is warranted, as are further data from this region. In particular, high-resolution terrestrial reconstructions for the LGM/deglaciation period from higher latitudes that are more sensitive to the westerly winds and less influenced by tropically sourced moisture, e.g. southwest Western Australia and western Tasmania, are warranted.
Stable isotope data from Fig. 3 are available from the
NOAA Paleoclimatology database
(
Pauline C. Treble performed the stable isotope analyses for MC-S1 and
the trace element analyses for both stalagmites as well as the majority of
the data interpretation and manuscript drafting; John C. Hellstrom performed the U
The authors declare that they have no conflict of interest.
We would like to thank Rod Wells for the use of stalagmite MC-S2 and discussions about Mairs Cave; James Shulmeister, Pandora Hope, John Chappell, Ed Rhodes, Martin Williams, Ian Houshold, and David Haberlah for discussions on the region's climate (both today and in the past) and geomorphology; and Monika Markowska for discussions on semi-arid speleothem records. We thank also Janece McDonald, Islay Laird, Krista Simon, and Joan Cowley for assistance with milling and isotope analysis; Stuart Hankin for drafting Fig. 1a, b; Patrick De Deckker, Matthias Moros, and Rhawn Denniston for supplying published data; and Dan Sinclair for discussions on quantifying PCP. We also thank the four anonymous reviewers, who provided valuable feedback that improved this manuscript. The stable isotope measurements on MC-S1 and MC-S2 were supported by Australian Research Council LIEF grants LE0989624 and LE0668400, and U-Th age measurements by Australian Research Council LIEF grant LE0989067. The majority of the data were collected whilst Pauline C. Treble was a research fellow at the Australian National University. This paper is a contribution to the SHAPE IFG. Edited by: Andrew Lorrey Reviewed by: four anonymous referees