A 70 m long continental sediment record was recovered at Darwin
Crater in western Tasmania, Australia. The sediment succession includes a
pre-lake silty sand deposit overlain by lacustrine silts that have
accumulated in the ∼816 ka meteorite impact crater. A total of 160 m
of overlapping sediment cores were drilled from three closely spaced holes.
Here we report on the drilling operations at Darwin Crater and present the
first results from petrophysical whole core logging, lithological core
description, and multi-proxy pilot analysis of core end samples. The
multi-proxy dataset includes spectrophotometry, grain size, natural gamma
rays, paleo- and rock magnetism, loss on ignition, and pollen analyses. The
results provide clear signatures of alternating, distinctly different
lithologies likely representing glacial and interglacial sediment facies.
Initial paleomagnetic analysis indicate normal magnetic polarity in the
deepest core at Hole B. If acquired at the time of deposition, this result
indicates that the sediment 1 m below commencement of lacustrine deposition
post-date the Matuyama–Brunhes geomagnetic reversal ∼773 ka.
Introduction
Pleistocene glacial and interglacial cycles involved repeated massive
reorganisations of the Earth system. Proxies in ocean, ice-core, and
continental records capture a range of these changes at global to regional
and local scales. Continental records show how terrestrial environments
responded to the large-scale shifts between glacial and interglacial states,
providing valuable insights into the interactions between the atmosphere,
oceans, cryosphere and biosphere, and the long-term context from which to
interpret current and future trends. This is particularly important in the
mid- to high latitudes of the Southern Hemisphere, where Antarctic climate and
circumpolar atmosphere–ocean dynamics exert a dominant influence not
only on southern terrestrial climates but also on the global carbon cycle
(Saunders et al., 2018; Skinner et al., 2010; Toggweiler, 2009; Toggweiler et
al., 2006). These studies highlight the role of the Southern Hemisphere
westerly winds (SWWs) in regulating the Southern Ocean carbon sink and the
global carbon cycle. Empirical data from long paleoclimate records are
therefore needed to test different theories of how the Earth system operates
and if they can be simulated in Earth system models.
Crater lake sediment records often have great scientific value and can
provide local to global climate histories (e.g. Wilke et al., 2016). Examples
of impact craters with lacustrine sediment records include Bosumtwi in Ghana
(6∘30′ N; 1 Ma; Scholz et al., 2007), Colônia
in Brazil (23∘52′ S; 5 Ma; Ledru et al.,
2015), El'Gygytgyn in Russia (67∘30′ N; 2.8 Ma; Melles et al.,
2012), and Pingualuit in Canada (61∘16′ N; 1.4 Ma; Guyard et al.,
2011). There is, to date, no sediment record from an impact crater in the
mid-latitudes of the Southern Hemisphere, however there are volcanic crater
sediment records, such as Potrok Aike in south-eastern Patagonia
(51∘58′ S; 107 m; 51 ka; Zolitschka et al., 2013), Lake Pupuke
in New Zealand (37∘47′ S; 48 ka; Stephens et al., 2012), and a
series of volcanic craters in the Newer Volcanics Province in south-eastern
Australia (∼38∘; Matchan et al., 2017 and references therein).
Long Pleistocene lake sediment records are extremely rare in Australia
because of the aridity and the general absence of glaciers and recent
tectonic activity able to form deep freshwater basins. Many mainland
Australian lakes are ephemeral, and very few originate from before the last
glacial period (Falster et al., 2018). Exceptions include discontinuous
paleo-lakes in mainland Australia, such as Stony Creek basin
(37∘21′ S; 1.85–1.55 Ma; Sniderman et al., 2007) and mega-lake
Bungunnia (∼35∘ S; 2.5–0.8 Ma; McLaren and Wallace, 2010),
and glacial lakes extending back to the last glacial in Tasmania (Beck et
al., 2017; Colhoun, 2000; Colhoun et al., 1999). Our knowledge of Pleistocene
climate in Australia is therefore incomplete.
Here we report on the drilling operations at Darwin Crater, a ∼816 ka
old meteorite impact crater in western Tasmania, Australia
(42∘18′ S, 145∘39′ E). Drilling at Darwin Crater
40 years ago recovered 60 m of lacustrine sediments overlying 160 m of
coarser crater-fill deposits, including polymictic and sandy unconsolidated
breccia overlying deformed slates (Howard and Haines, 2007). While
low-resolution pollen data were compiled on the upper 20 m of that core,
revealing excellent pollen preservation and clear shifts between glacial and
interglacial vegetation and climate (Colhoun and van der Geer, 1998), little
further analyses were completed or published from this important archive. A
note on paleomagnetic polarity of the original core by Barton (1987) in
congress proceedings stated that the base of the Darwin lake sediments was of
normal polarity, however no paleomagnetic data were published. Here, we
present the first results from a new drilling campaign at Darwin Crater in
2018. The aim of this drilling was to recover the lacustrine sediment
sequence (the uppermost 60 m of sediments in the crater) to bridge a time
gap in the Australian paleoclimate record and provide a long continental
record of Pleistocene climate in the mid-latitudes of the Southern
Hemisphere. The results include data from non-destructive whole core logging
(natural gamma ray, magnetic susceptibility, and resistivity) and multi-proxy
pilot analysis of core end samples (spectrophotometry, grain-size, magnetic,
loss-on-ignition, and pollen analyses). We interpret this data and outline the ongoing and
potential future research directions.
Drilling target and geological settings
Darwin Crater is a 1.2 km diameter unconfirmed meteorite impact crater in
western Tasmania, Australia (42∘18′13′′ S,
145∘39′36′′ E; Fig. 1). Gravity and magnetic surveys and
scientific drilling in 1975 and 1983 revealed that the circular depression is
filled with 60 m of lake sediments overlying 40 m of mixed muds, sands and
rock fragments (unconsolidated breccia) and 120 m of deformed, brecciated,
slumped, and fractured slate and quartzite laying over coherent local slate
(Richardson, 1984; Howard and Haines, 2007). The stratigraphy is consistent
with a small simple impact crater, however it is unconfirmed, because no
diagnostic shock indicators have been found (Howard and Haines, 2007). The
impactite “Darwin glass” is found within a 400 km2 strewn field
around Darwin Crater, where it most likely originates (Fudali and Ford, 1979;
Howard and Haines, 2007; Howard, 2009) and is dated at 816±7 ka by
40Ar/39Ar (Lo et al., 2002).
Darwin glass is frequently found in archaeological cave sites in the region
because this sharp and resistant material was used as a cutting tool by
Aboriginal Tasmanians during the Pleistocene (Allen et al., 2016).
(a) Zonal wind speed at 850 mbar in the mid-latitude of the
Southern Hemisphere, and average winter and summer core position of the
Southern Hemisphere westerly winds (SWWs). (b) The SWWs are a
dominant climate control in western Tasmania, where rainfall is strongly
correlated to wind intensity and orography. (c) Regional topographic
map showing the location of Darwin Crater, and (d) aerial oblique
drone image of Darwin Crater during the scientific drilling operations in
April 2018. The drill and camp site are visible in white. Wind data from the
NCEP and NCAR Reanalysis V1 (Kalnay et al., 1996).
Darwin Crater lies 170 m above sea level, and the lake that formerly
occupied the crater drained a small catchment area of 2.2 km2
(Fig. 1d). The regional geology is dominated by sedimentary rocks and
Quaternary deposits in the valley around the crater, volcanic and
volcaniclastic rocks to the north-west (e.g. Mount Darwin, Mount Sorell), and
quartzite to the east (e.g. Frenchman's Cap in the Engineer Range). The
modern climate is cool temperate, with mean annual temperatures at nearby
Queenstown (20 km distant) ranging from 5.6–16.4 ∘C and with
2405 mm rain per year (Australian Bureau of Meteorology). The climate is
dominated by the SWWs which, together with orography, control precipitation
in Tasmania and throughout the mid- to high latitudes of the Southern
Hemisphere (Fig. 1b) (Garreaud, 2007). The crater is vegetated with shrubs,
sedges, and grasses (including Melaleuca squamea,
Leptospermum spp., Monotoca glauca, Gymnoschoenus sphaerocephalus, and Restio australis) occupying the
basin floor, whilst the surrounding slopes are occupied by a cool temperate
rainforest assemblage including Lagarostrobos franklinii,
Nothofagus cunninghamii, and Phyllocladus aspleniifolius.
Drilling operations
The scientific drilling operations at Darwin Crater took place on
18–25 April 2018. The drill team was composed of six people, including two
drillers, two students, and two researchers. Darwin Crater is in the remote
Tasmanian Wilderness World Heritage Area. While a road was bulldozed to
access the crater for drilling in the 1970s, today the crater is inaccessible
by road. Therefore, the team, drill rig, camp, and equipment were transported
to the site by helicopter (Fig. 2a–b).
Photos of TAS1803 scientific drilling at Darwin Crater. (a) Helicopter-assisted drill rig assembly. (b) The
drill team and view of the
drilling and core handling set-up. (c) After recovery, the cores were
immediately wrapped in black plastic to protect from light to allow for OSL
dating. (d) The drill bit, (e) core end view from the top, (f) core end view
from the base, together with the core catcher, and (g) core end sample C8
includes a white mineral tentatively identified as vivianite.
The drill site TAS1803 (Fig. 1d; 42∘18′16.46′′ S,
145∘39′33.09′′ E) was selected in the approximate centre of the
paleolake and in open sedgeland vegetation to facilitate camp set-up. Three
parallel holes were drilled using a Boart Longyear 47 drill rig and HQ pipes
(96 mm outside and 63.5 mm inside diameter) with a diamond drill bit
(Fig. 3d). The same tools were used for all cores drilled. Core catchers were
only required in the first few runs of Hole A because sediment lower down the
sequence were sufficiently cohesive to be retained in the tube without loss.
The drill rig was pushed 1.1 m eastward from Hole A to Hole B and
1.3 m eastward from Hole B to Hole C. The sediment recovery was initially low in the
upper half of Hole A as we worked on establishing the best method for
sediment retention. The optimal method proved to involve minimal flushing
with water and rotation of the core barrel to allow efficient recovery of the
poorly consolidated lake sediments. Once the optimal method was identified,
operations went smoothly, and Hole B was successfully drilled the next day
with 95.6 % recovery (Table 1). Holes A and B were abandoned when the top
of the pre-lake crater infill described by Howard and Haines (2007) was
reached, which indicated that the full lake sediment sequence had been
successfully recovered. Hole C was drilled to ca. 44 m to obtain full
overlap of the uppermost part of the record, which had lower recovery than
the lower part of the record (Table 1).
Core recovery log for each core hole
and whole core MSCL-scanning data. Grey indicates full recovery, and white
indicates no recovery. Natural gamma ray (NGR) was measured at 40 cm
intervals, and volumetric magnetic susceptibility (kLF) and
resistivity were measured at 1 m intervals.
Cores were retrieved in standard 10 ft long (3.048 m) transparent PVC HQ
wire-line core liners. Each core was immediately wrapped in black plastic
foil to protect from sunlight and allow for future optically stimulated
luminescence (OSL) dating analyses (Fig. 2c). The core end sediments that
protruded from the core liners or those retained in the core catchers (up to
6.5 cm) were collected in labelled bags (Fig. 2e–g) for preliminary
analyses. Each 3 m core was cut into ca. 150 cm sections to facilitate
transport and storage (total 93 core sections). We use the nomenclature
“-TM” for the top to middle and “-MB” for the middle to base sections (e.g. core TAS1803-B10 is cut
into TAS1803-B10-TM and TAS1803-B10-MB). Cores are stored in cold rooms and
curated at the University of Melbourne within the School of Geography. All
data will be deposited upon publication in the National Oceanic and
Atmospheric Association (NOAA), NEOTOMA (https://www.neotomadb.org/,
last access: 11 January 2019), and PANGAEA (https://pangaea.de/, last access: 11 January 2019) public
data repositories.
Summary of holes drilled and core recovery at Darwin Crater site
TAS1803.
HoleStartEndDrilledRecoveredRecovery(m)(m)(m)(m)(%)A1.6670.7669.1055.3980.2B0.5669.2668.7065.6695.6C043.6643.6639.5890.7Total181.5160.6Core scanning and multi-proxy analyses of core end samples
Whole core scanning of non-destructive petrophysical properties was performed
using a Geotek Multi-Sensor Core Logger (MSCL) for all cores from Holes A, B,
and C, in the Petrophysics Laboratory at the University of Melbourne.
Volumetric magnetic susceptibility in low field (kLF; 0.565 kHz)
was measured using a Bartington MS2C loop sensor with a diameter of 8 cm, and the
non-contact resistivity was measured using a Geotek sensor at 1 cm
intervals. Natural gamma ray (NGR) emission integrated over 20 cm of core
was counted for 1 min using Geotek NaI(Tl) at 40 cm intervals on the
cores from Hole A.
The cores from Hole B were split lengthwise using a Geotek core splitter.
Core splitting was conducted under red light, and one core half was wrapped in
black plastic foil and stored at 4 ∘C for OSL and paleomagnetic
sampling. The other half was used for visual lithological core descriptions
and sent to Australia's Nuclear Science and Technology Organisation (ANSTO)
for micro-XRF (Itrax) core scanning and core photography. The scanned half
will be subsampled for multi-proxy analysis (including pollen, charcoal,
grain size, magnetic properties, and aDNA). These procedures are repeated for the
cores from each hole.
The core end samples from Holes A, B, and C (total 57 samples) were analysed
for their magnetic properties, spectrophotometry, grain size, and
loss on ignition, while only select core end samples were analysed for
pollen.
Room-temperature magnetic measurements were performed at The Australian
Archaeomagnetism Laboratory at La Trobe University to characterise the
magnetic mineral assemblage and evaluate the potential for changes in the
paleomagnetic field to provide a geochronological tool. Standard 8 cm3
paleomagnetic boxes (unoriented) were used for all the core end samples from
Holes A, B, and C. In addition, four box samples (A20, A21, B9, and B10) were
taken partially oriented from well-preserved cylinder-shaped core ends (known
vertical z axis), and four contiguous cubes were taken oriented (1, 2, 3,
and 4) in the deepest core of Hole B (section TAS1803-B-23MB). Magnetic
susceptibility was measured using a Bartington MS2B sensor, and the natural
and laboratory-induced (anhysteretic in peak alternating field (AF) 0.1 T
with 0.05 mT DC biasing field and isothermal in 0.3 T and 2 T DC field)
remanent magnetisations (NRM, ARM, and IRM) were
acquired using an AGICO AF Demagnetizer LDA5, an AGICO Magnetizer PAM1, a
Magnetic Measurements Pulse Magnetizer MMPM10, and an AGICO JR-6 Spinner
Magnetometer. Demagnetisation data were analysed using the Demagnetization
Analysis in Excel (DAIE) workbook (Sagnotti, 2013). Finally, first-order reversal curves (FORC) were acquired
for three samples using a LakeShore VSM8600 Vibrating Sample Magnetometer to
investigate the magnetic domain state. The FORCs were processed in Forcinel
3.0 (Harrisson and Feinberg, 2008) using VARIFORC smoothing (Egli, 2013).
Quantitative sediment colour (L*, a*, and b*) was acquired using a handheld Konica Minolta CM-700d
Spectrophotometer applied directly on the sediments of the paleomagnetic
box samples.
Grain-size analysis was carried out on all the core end samples from Holes A,
B, and C using a Beckman Coulter LP13320 particle sizer at the School of
Geography at the University of Melbourne. Samples were pre-treated with
hydrogen peroxide (10 %–30 %) for at least 4 weeks until organic-matter digestion was complete. Sediments were then treated with a dispersing
agent, sodium pyrophosphate, and sonicated for at least 5 min prior
to analysis. The statistical grain-size parameters were calculated with the
GRADISTAT software (Blott and Pye, 2001).
Loss-on-ignition (LOI) analysis was performed on all core end samples using
standard techniques, which involved sequential temperature treatments of
1 cc sediment to ascertain water content (60 ∘C overnight),
carbonate content (950 ∘C for 2 h), and organic content in two
parts (360 ∘C for 24 h for labile carbon; 550 ∘C for 4 h
for black carbon; Heiri et al., 2001). The residual sediment after
temperature treatments is an inorganic fraction that may include
siliciclastic material, diatoms, oxides and sulfides, and it is herein after
referred to as the siliciclastic fraction.
Pollen analysis was performed on the 20 core end samples of Hole B following
standard techniques (Faegri and Iversen, 1989), with a set volume (0.5 cc) of
sediment sieved with a 100 µm sieve, followed by sequential acid
and alkali treatments (KOH, HCl, HF, and acetolysis)
to isolate the pollen, spores, and other palynomorphs from the sediment
matrix. Pollen and spore counts were then tallied to a minimum count of
300 pollen grains of terrestrial origin under 400X and 630X magnification. An
exotic marker was added to each sample to allow calculation of pollen, spore,
and other palynomorph (e.g. Botryococcus, microscopic charcoal, and
pyrite spherules) concentrations.
Principal component analysis (PCA) of selected core end data (pollen, LOI,
grain size) was performed to identify relationships between these variables.
The data were first normalised to the standard deviation to meet the
requirements of data normality required by PCA.
(a) Superimposed plots of volumetric magnetic susceptibility
(kLF) and resistivity data from Holes A, B, and C
(smoothed over 50 data points), and simplified stratigraphy of site TAS1803
Hole B, which was split in half lengthwise, visually described, and
photographed. The position of the core end samples and the paleomagnetic
samples in Fig. 6 are indicated with square and asterisk (*),
respectively. (b) Core end cube samples from Hole B, and (c) representative
lithologies for black-brown silt (section 9-MB), olive-green silt (section
10-TM) and pre-lake deposits (section 23-MB). The blue mineral visible in
sections 9-MB and 10-TM, and cubes 8b and 10 is tentatively identified as
vivianite.
Initial resultsCore scanning
The drilled depths and recovery information (Table 1) were combined with the
MSCL data to derive a common depth scale (“hole corrected depth” used in
Figs. 3, 4, and 8). Recovery gaps were generally assumed to occur at the base
of core drives, except for the compaction of peat which resulted in sediment
gaps at the hole top (Hole A – unknown; Hole B – 56 cm; Hole C – 70 cm). In most
cases, the top depth of the MSCL data from each core section sequentially
follows the base depth of the previous core section. If the previous core
section had sediment gap, the top depth of MSCL core data was set at the
measured drilling depth. The Hole corrected depth did not include the core
end thickness (sediment in the core end), which ranged from 0 to 6.5 cm per
drilled core (3 m). The difference between the drilled depth and the hole-corrected depth and core end thickness can be attributed to sediment
expansion. Hole B depth was further corrected to account for sediment gaps
seen in core sections during splitting (nine sections had gaps ranging from
2–8 cm) and thus constitutes the most representative depth with respect to
the sediments accumulated in the crater.
The whole core MSCL-scanning results are presented in Fig. 3. The NGR
displays small amplitude variations, with higher values of naturally
occurring gamma radiation reflecting a higher content of mineral phases rich
in potassium (K), uranium (U), and thorium (Th), such as clay minerals and K
feldspar. The magnetic susceptibility (kLF) values display a
series of sharp and large amplitude changes and good reproducibility of
depth between smoothed records from the three
holes (Fig. 4). kLF varies by more than 2 orders of magnitude,
which indicates large changes in the concentrations of ferrimagnetic
minerals. Intervals with high kLF values generally correspond to
low NGR values, especially in the lower section of the core (ca. 65–45 m).
A series of peaks in kLF of up to ca. 1000×10-5 SI
were measured in the lowermost
portion of the core from Hole B (Fig. 3). Such high values are not observed
in the parallel core from Hole A and hint at possible differential
post-depositional diagenesis, chemical precipitation or differential
preservation of iron oxides in the different cores. The resistivity values
vary mostly between 5–500 Ωm, which
is a small range relative to the full range of variability found in
geological samples (0.01–10 000 Ωm; Gueguen and Palciauskas, 1994).
The resistivity over that range of values has two main controls: the porosity
of the material (clays having lower values than sands), and the water content
(Gueguen and Palciauskas, 1994). The intervals with consistent inter-hole
resistivity primarily reflect the sediment porosity. The intervals with
different resistivity behaviours (Fig. 4) reflect variable water content of
the sediment and are useful for identifying potential minor disturbances in
individual core sections from drilling operations, transport and handling of
the cores.
Core description
The simplified lithology for Hole B (Fig. 4a) is based on visual observations
of split core surfaces. The transition from the pre-lake sediment
(unconsolidated sand-dominant breccia) to lake sediment is at ∼65 m
and is marked by a shift to higher magnetic susceptibility and lower NGR
values (Fig. 3). The lake sediment unit is 61 m thick and is capped by a
∼3 m thick coarse-grained clastic layer followed by a ∼1 m
thick peaty soil layer. The lake sediment is primarily composed of alternating
black-brown and olive-green muds with gradual colour transitions. There are
occasional sharp contacts between these lithologies and grey muds, mottled
muds, and sandy layers. The sediment displays occasional vertical structures
interpreted as drilling disturbances or deformation from pressure release
between sediment layers of different densities.
Deposits of a white mineral, ranging from very small “specks” to structures
> 3 cm in size (Figs. 2g and 4b–c), occur throughout the cores,
in both black-brown and olive-green lake sediment units. These minerals were
observed changing colour from white to blue upon contact with air (oxidation)
and to bright yellow upon contact with a solution of ammonium molybdate and
nitric acid. This indicates that it is a phosphate mineral that we
tentatively identify as authigenic vivianite, which can form in organic and
iron-rich lake sediments under reducing conditions (Rothe et al.,
2016). Vivianite constitutes a promising target for uranium series dating
(Goetz and Hillaire-Marcel, 1992; Nuttin et al., 2013).
Mean grain-size distribution for the two main types of lake
sediments at Darwin Crater. (a) Olive-green silts and (b) black-brown silts.
Shifts to finer grain sizes in olive silts at 52 and 34 m define three
depth intervals.
Selected paleo- and rock-magnetic results. Orthogonal
projections and alternating field demagnetisation plot for (a) lake
sediment sample B9t, and (b) oriented pre-lake sediment sample B23-4
displaying normal polarity. The down-core position of the samples are
indicated by an asterisk in Fig. 5. The acquisition of gyroremanence (GRM) in
AF > 40 mT is typical of greigite (Roberts et al., 2011). A hard
coercivity component is present in sample B23-4, where about one third of the
NRM remains after AF 65 mT. First-order reversal curve (FORC) diagrams for
samples (c) B8 and (d) B23. The closed peak distribution indicates single
domain (SD) magnetic particles (Roberts et al., 2014), which are stable
remanence carriers.
Core end sample analysesPhysical properties (spectrophotometry and grain size)
The lake sediment deposit has distinctively finer grain sizes (48 samples;
fine to coarse silts with mean size of 20 µm) than the pre-lake
deposit (three samples; silty sands and sandy silts with mean size of
102 µm) and the post-lake deposit (three samples; silty sands and
sandy silts with mean size of 167 µm) and has higher L* values
(darker colour). Within the lake sediment unit, L* (black to white) and
b* (blue to yellow) have distinct values for the two principal lithotypes,
with relatively lower L* and b* values in the black-brown silts and
higher L* and b* values in the olive-green silts (Fig. 8d–e). The lake
sediment succession above 34 m is more fine grained (fine silts with mean
size of 12 µm; 26 samples) than the lowermost part (coarse silts
with mean size of 28 µm; 22 samples). Figure 5 illustrates that
this shift to finer grains is attributable to the olive-green silts. The mean
grain-size distribution of olive-green silts in the uppermost part of the
lacustrine sediment succession (4–34 m; solid line in Fig. 5a) is comparable to that of the black-brown silts
(Fig. 5b); however, distinctively coarser grain sizes are present in the
olive-green silts of the lowermost part (34–65 m; dashed lines in Fig. 5a).
Pollen
A total of 114 different pollen, spore, and palynomorphs were identified in
the core end samples from drill Hole B. Overall, the pollen spectra are
characterised by either (1) a high proportion of cold climate indicators
(Poaceae, Asteraceae, and alpine taxa), including
Tubuliforidites pleistocenicus, which is now palynologically extinct, and an Asteraceae pollen grain known
only from glacial stage pollen flora in south-eastern Australia (Macphail et
al., 1993) or (2) a high proportion of warm climate indicators (rainforest
plants such as Lagarostrobos franklinii, Nothofagus cunninghamii, and Phyllocladus aspleniifolius; Fletcher and Thomas, 2007). While the coarse resolution of the pollen
data currently precludes alignment of the record with glacial-to-interglacial
climate shifts, observed variability in pollen abundance clearly reflect
shifts between cooler and warmer periods.
PCA bi-plot of core end data (pollen, LOI, and grain size).
Correlations between data and axes > 0.4r2
are shown as red arrows (indicating the direction of correlation). A
preliminary interpretation of the environmental significance of the axes is
included.
Loss on ignition
LOI results reveal high detrital siliciclastic contents in the TAS1803
sediment succession, with values ranging from 38 % to 97 % and an average
value of 80 %. The total organic carbon content varies between 1 % and 61 %.
Carbonate content is consistently < 4 % for all core end samples.
Magnetic properties
The room-temperature magnetic properties indicate complex magnetic mineral
assemblages. The concentration-dependant parameters display large amplitude
changes (kLF, χ, NRM, ARM, IRM), with low χ values characteristic of greigite,
pyrrhotite, goethite, and hematite as well as high values
indicative of magnetite, titanomagnetite, and maghemite (Peters and Dekkers,
2003). The magnetic assemblage is dominated by low-coercivity minerals, as
indicated by mostly saturated samples in 0.3 T field (average
IRM0.3T/ IRM2T value of 0.96). The
magnetic grain-size indicators (SIRM /kLF,
kARM /kLF, and ARM / IRM)
display different behaviours at times that indicate non-uniform down-core
magnetic mineralogy. Stepwise alternating field (AF) demagnetisation of the
lake sediment samples (cubes B9t, B10t, A20t, A21t, B23-MB-1, and B23-MB-4; at 2444, 2743, 5896, 6201, 6523, and 6531 cm
hole-corrected depth) reveal a stable and well-defined component of NRM
(Fig. 6) and the presence of high coercivity minerals (10 %–40 % of
NRM remaining after AF 60 mT) and greigite (gyroremanent magnetisation;
Fig. 6a; Roberts et al., 2011).
Principal component analysis
The first two PCA axes accounted for 55 % of the variance within the
pollen, LOI, and grain-size data. PCA axis 1 (37 % explained variance)
separates samples with high siliciclastic content, coarser grain size, and
cold climate pollen taxa from samples with high organic content, finer grain
size, and warm climate pollen taxa (Fig. 7). This suggests that these
parameters will help to differentiate between cold and warm climate states.
PCA axis 2 (18 % explained variance) separates samples with cool climate
rainforest pollen taxa, high aquatic taxa (Isoetes,
Botryococcus, and Myriophyllum) medium grain size, and higher
carbonate values. This suggests that parameters on this axis will help to
differentiate between wetter and drier climates.
Discussion
The core logging data and pilot multi-proxy analysis of the TAS1803 cores
show a stratigraphy broadly in agreement with the first drillings at Darwin
Crater (Howard and Haines, 2007) and provide exciting new insights into the
nature and timing of lake sediment deposition. Two main lithotypes alternate
throughout the 61 m thick lacustrine sediment succession; dark
organic-matter-rich silts and olive-green siliciclastic silts. These
lithotypes are interpreted as being deposited under interglacial and glacial
climate conditions, based primarily on the NGR, spectrophotometry (L* and
b*), LOI, and pollen data (Fig. 8). The olive-green silts, with higher NGR
counts, relatively high L* and b* values, higher siliciclastic content,
coarser grain size, and more abundant cold climate pollen taxa correspond to
the sediments deposited during glacial periods. The brown-black silts with
low NGR counts, relatively low L* and b* values, low siliciclastic
content, finer grains, and more abundant warm climate pollen taxa possibly
correspond to sediments deposited during warm interglacial climate conditions
(horizontal grey bars, Fig. 8). This is supported by the PCA of core
end pollen and grain-size and LOI data,
which indicate a principal gradient that separates samples that have coarse
grains, low siliciclastic content, and pollen types
indicative of a colder climate from samples that have fine grains, high
organic content, and pollen types indicative of warmer climate conditions
(Figs. 7 and 8j). These results are consistent with previous pollen studies,
which indicate that glacial stages in western Tasmania are characterised by
sparse open vegetation and poor organic soil development, which allows
deposition of coarse siliciclastic sediment into lake basins (Beck et al.,
2017; Colhoun, 2000; Colhoun et al., 1999). In contrast, interglacial periods
in western Tasmania are characterised by maximum rainforest development,
well-developed organic soils, and deposition of fine-grained and highly
organic lake sediment (Beck et al., 2017; Colhoun, 2000; Colhoun et al.,
1999).
Selected parameters of the pilot multi-proxy
analyses. (a) Mass-normalised magnetic susceptibility (χ), (b) mean grain size, (c) Natural gamma ray
(NGR), (d) Color data L*
and (e)b*, (f) Magnetic grain-size indicator
kARM /kLF, (g) coarse
silt, (h) siliciclastic content, (i) cold climate pollen
taxa, and (j) PCA axis 1 from left (warm) to right (cold). The
starting and ending ages of the Darwin lacustrine sediment succession are
unknown and will be identified by ongoing dating work. (k) A benthic
δ18O stack (Lisiecki and Raymo, 2005) is shown as a
reference for Pleistocene glacial and interglacial climate cycles. A
simplified lithology log for Hole B is included. The
horizontal grey shaded bars tentatively indicate interglacial periods.
Different symbols are used for different sample types: square symbols for
measurements performed on the same cube samples, large circles for core
end samples, and small circles for whole core MSCL samples.
The preliminary results of the Darwin Crater record suggest that the
61 m-thick lake deposit apparently covers a minimum of the seven full
glacial cycles (interglacials tentatively highlighted in grey in Fig. 8). The
length of each apparent glacial cycle is variable, which may reflect
different sedimentation rates, lake levels, and climate regimes through time.
Similarly, the different grain-size distributions in the glacial olive silts
(Fig. 5) may reflect changes in lake levels, sediment sources, availability,
preservation, and/or climate regimes. High-resolution Itrax surface element
scanning, multi-proxy analyses, and dating of the sediment archive (OSL,
36Cl, U series, and magnetostratigraphy) are underway and will be
used to precisely define, date, and determine how many climate cycles are
present, especially in the lower part of the core where some proxies display
high-frequency changes.
The preliminary results provide evidence for reducing conditions in the lake
sediments. The visual observation of vivianite throughout the cores and the
magnetic evidence for greigite (see GRM in Fig. 6a) imply reducing
conditions. Moreover, framboidal pyrite was observed under the microscope in
the basal core end samples at > 60 m depth (samples B21, A22,
B22,
and B23) as well as C6 (16.7 m). The magnetic susceptibility values are not
null for these samples with pyrite, which suggests that iron oxides were not
limiting the pyritisation process and/or that ferrimagnetic minerals formed
sometime after pyrite formation. The kLF variability, including
intervals with near-zero values, does not seem correlated with the occurrence
of pyrite, greigite, and vivianite. This suggests that factors other than the
redox conditions may also control the kLF, such as detrital
input, biogenic iron oxides, their relative dilution in dia- and paramagnetic
material, and/or chemical precipitation of magnetic minerals.
The lake in Darwin Crater formed sometime after 816±7 ka
(40Ar/39Ar dating of Darwin glass; Lo et al., 2002) and
lake sedimentation terminated during a warm interglacial period which will be
identified by ongoing dating work (Sect. 7). Pilot paleomagnetic analysis of
the deepest core of Hole B (cube samples TAS1803-B23-MB-1, -2, -3, and -4)
indicate normal magnetic polarity 1 m under the lake deposit (Fig. 6b). If
this remanence was acquired at the time of deposition, this result constrains
the entire lacustrine deposition to an age younger than 773 ka
(Matuyama-Brunhes geomagnetic reversal; Ogg, 2012; Singer, 2014), which is
consistent with the inferred interpretation of seven glacial cycles discussed
above. Ongoing paleo- and environmental magnetic investigations aim to
characterise the complex magnetic mineral assemblage, investigate reducing
diagenesis, and build a full-vector paleomagnetic field
record and paleoclimate proxies.
Future plans
The next stages of the project will focus on producing a chronologically
constrained paleoclimate record. Core splitting of Holes A and C, Itrax
XRF scanning, and subsampling for the planned multi-proxy analyses are
underway, and collaborators have been engaged for testing and development of
other sediment proxies, such as beryllium isotopes, diatoms, Cladocera, aDNA,
and stable isotopes.
Planned analysesDating
Our immediate focus is on building a robust chronostratigraphy including OSL
dating of quartz and feldspar grains in the sandy deposits, 36Cl in the
pre-lake deposits, uranium–thorium dating of vivianite, and radiocarbon
dating of the uppermost sediments. There is potential for a full-vector
paleomagnetic record combining remanent magnetisation and cosmogenic nuclide
beryllium-10 (10Be) for relative dating, using the global geomagnetic
dipole field at the millennial scale (eg., Channell et al., 2009; Ziegler et
al., 2011; Simon et al., 2016), and for independent comparison to
paleoclimate records, including other continental records, marine sediments,
and ice cores (via cosmogenic isotopes). The preliminary pollen data indicate
potential targets for a tuning approach to changes in orbital geometry and
associated insolation changes and global-scale climate records such as the
LR04 benthic oxygen isotope stack (Fig. 8k; Lisiecki and Raymo, 2005).
Paleoclimate
For paleoclimate reconstructions, we are planning to employ a multi-proxy and
high-resolution approach combining physical and biogenic indicators,
including pollen, charcoal, magnetic properties, grain size, stable isotopes,
and elemental composition. The long record of the Darwin Crater will be combined
with a long record (2–3 glacial cycles; unpublished data) from neighbouring
Lake Selina (location in Fig. 1) to help to establish a long and continuous
continental record in Australia and one of the oldest in the Southern
Hemisphere. The proxy dataset will then be used to test global climate model
simulations to help understand climate dynamics and interactions (e.g.
Menviel et al., 2014; Pedro et al., 2018).
Questions that will be addressed
The multi-proxy dataset will be collectively applied to address the main
question motivating this project: what is the role of the SWWs in the Pleistocene climate cycles? In
particular, we will investigate the following questions:
How did the SWWs respond during the transition from glacial to interglacial
climates (e.g. terminations), and what were the environmental impacts of
these changes in Western Tasmania?
Are inferred changes in SWW position or intensity related to changes in the
concentration of atmospheric CO2 over glacial cycles, and can these be
related to changes in the capacity of the Southern Ocean CO2 sink?
Do the SWWs shift equatorward during glacial phases (Toggweiler et al., 2006;
Toggweiler, 2009)?
Do the SWWs display the proposed pattern of poleward contraction and the coupling
with North Atlantic climate variability during terminations (Denton et al.,
2010)?
What is the environmental impact of the middle Pleistocene transition (if
recorded) on the terrestrial environment in the southern mid-latitudes?
It is anticipated that the new lake sediment paleoclimate record from the
mid-latitude Australian region will constitute an empirical test for
conceptual models of SWW dynamics and provide essential boundary conditions
for predictive climate models.
Data availability
The pilot data presented here form the basis of several
more detailed studies which are currently underway. Once these studies are
finished and published, sediments from the curated Darwin Crater cores will
be available to the scientific community, and the data will be made available
in publicly available repositories.
Author contributions
MF, ALP, MB, HH, DH, and JP collaborated on the project design and funding
acquisition. ALP, MF, TM, and RL were part of the drilling team. ALP, MF, TM,
MM, and PG performed analysis in MF, AH, and HH's research laboratories. ALP
wrote the original draft, and all co-authors provided contributions and
reviews.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
Thanks to Max Harvey and Adam Debresteli for drilling operations and Bruce
Maxwell for helicopter transport. Thanks to Malcolm Wallace and David Belton
for fruitful discussions, Brad Dodrill from Lake Shore for performing FORC
analysis, Rachael Fletcher for pollen sample preparation, and Chee Hoe Chuan
for LOI analysis. This project is funded by the Australian Research Council
(ARC) Discovery Indigenous project IN170100062 to Michael S. Fletcher and
Agathe Lisé-Pronovost. Agathe Lisé-Pronovost is supported by a
McKenzie Fellowship at the University of Melbourne and funding from La Trobe
University's Deputy Vice Chancellor Research (DVCR). Tom Mallett is supported by
a La Trobe University Postgraduate Research Scholarship. Joel B. Pedro
acknowledges support from the European Research Council under the European
Union's Seventh Framework Programme (FP7/2007-2013) and ERC grant agreement
no. 610055 (the ice2ice project). Edited by:
Thomas Wiersberg Reviewed by: Marie-Pierre Ledru, James M.
Russell, and Hendrik Vogel
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