Strontium isotopes (87Sr/86Sr) are useful to trace processes in the Earth sciences as well as in forensic, archaeological, palaeontological, and ecological sciences. As very few large-scale Sr isoscapes exist in
Australia, we have identified an opportunity to determine
87Sr/86Sr ratios on archived fluvial sediment samples from the
low-density National Geochemical Survey of Australia. The present study
targeted the northern parts of Western Australia, the Northern Territory, and
Queensland, north of 21.5∘ S. The samples were taken mostly from
a depth of ∼60–80 cm in floodplain deposits at or near the
outlet of large catchments (drainage basins). A coarse (<2 mm)
grain-size fraction was air-dried, sieved, milled, and digested
(hydrofluoric acid + nitric acid followed by aqua regia) to release
total Sr. The Sr was then separated by chromatography, and the 87Sr/86Sr
ratio was determined by multicollector inductively coupled plasma mass
spectrometry. The results demonstrate a wide range of Sr isotopic values (0.7048
to 1.0330) over the survey area, reflecting a large diversity of source rock
lithologies, geological processes, and bedrock ages. The spatial distribution of
87Sr/86Sr shows coherent (multi-point anomalies and smooth
gradients), large-scale (>100 km) patterns that appear to be
broadly consistent with surface geology, regolith/soil type, and/or nearby
outcropping bedrock. For instance, the extensive black clay soils of the
Barkly Tableland define a >500 km long
northwest–southeast-trending unradiogenic anomaly (87Sr/86Sr<0.7182). Where sedimentary carbonate or mafic/ultramafic igneous
rocks dominate, low to moderate 87Sr/86Sr values are generally
recorded (medians of 0.7387 and 0.7422, respectively). Conversely, In proximity to the
outcropping Proterozoic metamorphic basement of the Tennant, McArthur,
Murphy, and Mount Isa geological regions, radiogenic
87Sr/86Sr values (>0.7655) are observed. A potential
correlation between mineralization and elevated 87Sr/86Sr values
in these regions needs to be investigated in greater detail. Our results
to date indicate that incorporating soil/regolith Sr isotopes in regional,
exploratory geoscience investigations can help identify basement rock types
under (shallow) cover, constrain surface processes (e.g. weathering and
dispersion), and, potentially, recognize components of mineral systems.
Furthermore, the resulting Sr isoscape and future models derived therefrom
can also be utilized in forensic, archaeological, palaeontological, and
ecological studies that aim to investigate, for example, past and modern animal
(including humans) dietary habits and migrations. The new spatial Sr isotope
dataset for the northern Australia region is publicly available (de Caritat
et al., 2022a; 10.26186/147473).
Australian GovernmentExploring for the Future 2020-2024Introduction
Strontium isotope ratios (87Sr/86Sr) can be measured in many
geological materials, as the trace element strontium (Sr) is relatively
abundant and readily substitutes for calcium (Ca) in minerals and organic
tissues. The 87Sr/86Sr of a mineral or rock is a function of (1) its initial and unchanging 86Sr content, (2) its initial rubidium (Rb)
content (thus Rb/Sr ratio), and (3) time (e.g. McNutt, 2000). Rubidium
substitutes readily for potassium (K) in minerals and is, thus, also relatively
common. As one of the two naturally occurring Rb isotopes, 87Rb,
which accounts for 27.8 % of Rb, decays over time (by emitting a negative
beta particle) to stable 87Sr (t1/2=49.6×109 years), and
the 87Sr/86Sr ratio of that material slowly increases with time
(e.g. Rotenberg et al., 2012; Nebel and Stammeier, 2018). During geological
processes, such as mineral dissolution or precipitation, or biological
processes, such as bone and tooth growth, the 87Sr/86Sr remains
constant, as there is no fractionation (e.g. Gosz et al., 1983; Nebel and
Stammeier, 2018). These characteristics make the Sr isotopic system very
useful in the geosciences, where it has been used for decades, for instance,
in the study of the following:
sediment diagenesis and low-grade metamorphism (e.g. Swart et
al., 1987; Schultz et al., 1989; Mountjoy et al., 1992; Schaltegger et al.,
1994);
sediment, dust, and soil provenance (e.g. Douglas et al., 1995;
Revel-Rolland et al., 2006; Bathgate et al., 2011; Bataille and Bowen, 2012;
De Deckker et al., 2014; Jomori et al., 2017; De Deckker, 2020; de Caritat
et al., 2022b);
stratigraphy and reservoir/basin analysis (e.g. Rundberg and
Smalley, 1989; Smalley et al., 1989, 1992; McArthur et al., 2012);
fluid flow (e.g. McNutt et al., 1987; Stueber et al., 1987;
Sullivan et al., 1990; Bagheri et al., 2014);
weathering and pedogenesis (e.g. Åberg et al., 1989; Faure
and Felder, 1981; Wickman and Jacks, 1992; Blum et al., 1994; Bullen et al.,
1994, 1997; Quade et al., 1995; Probst et al., 2000; Harrington and Herczeg,
2003; Oliver et al., 2003; Green et al., 2004);
terrestrial ecosystems and catchment processes (e.g. Graustein,
1989; Jacks et al., 1989; Lyons et al., 1995; Négrel and Grosbois, 1999;
Négrel and Pauwels, 2004; Gosselin et al., 2004; Chadwick et al., 2009;
Hagedorn et al., 2011);
hydrology and hydrogeology (e.g. Collerson et al., 1988;
Andersson et al., 1992, 1994; Douglas et al., 1995; Yang et al., 1996; Grobe
et al., 2000; Dogramaci and Herczeg, 2002; Ojiambo et al., 2003; Palmer et
al., 2004; Cartwright et al., 2007; Christensen et al., 2018; Shin et al.,
2021);
environment and environmental change (e.g. Andersson et al.,
1990; Åberg, 1995; Åberg et al., 1995; Oishi, 2021);
(palaeo)climatology and palaeogeographic reconstructions (e.g. Blum and Erel, 1995; Wei et al., 2018; Flecker et al., 2002);
determining the 87Sr/86Sr of seawater through
geological time (e.g. Veizer, 1989; Gruszczynski et al., 1992; Denison et
al., 1994a, b; Shields, 2007); and
ore genesis and mineral exploration (e.g. Le Bas et al., 1997;
de Caritat et al., 2005; Daneshvar et al., 2020; Wei et al., 2020; Zhao et
al., 2021).
Outside of the geosciences, food tracing and provenancing have also been
underpinned by the use of Sr isotopes, although, in this case, generally relying
on the bioavailable Sr rather than the total Sr (e.g. Voerkelius et al., 2010; Di
Paola-Naranjo et al., 2011; Vinciguerra et al., 2015; Hoogewerff et al.,
2019; Moffat et al., 2020). Anthropological studies have relied on
87Sr/86Sr isotope ratios to locate archaeological artefacts or
reconstruct ancient human behaviours (e.g. Frei and Frei, 2013; Willmes et
al., 2014, 2018; Adams et al., 2019; Pacheco-Forés et al., 2020;
Washburn et al., 2021). Animal migration studies have also relied on Sr
isotope data (e.g. Price et al., 2017). More recently, large-scale
compilations and machine-learning-based predictions of the 87Sr/86Sr
variations up to the continental and even global scale have been proposed
(e.g. Bataille et al., 2014, 2018, 2020).
Strontium isotope landscape maps (“isoscapes”) provide the fundamental
context required for the interpretation of more detailed scientific research
about processes or provenance. Despite the plethora of research using Sr
isotopes to address various scientific questions, very few Sr isoscapes
exist in the Southern Hemisphere, particularly for soils or covering large
swathes of the Earth's surface (see Bataille et al., 2020). Two exceptions
to this are (1) the work by Adams et al. (2019), which reported
87Sr/86Sr in plant, soil, and biota over ∼300000 km2 on the Cape York Peninsula in Australia, and (2) the
∼500000 km2 Sr isoscape of inland southeastern
Australia recently published by our team (de Caritat et al., 2022b). The
present study affords an opportunity to further redress this deficiency and
will reduce the Northern Hemisphere bias in future global
87Sr/86Sr models. It also pertains to a land surface that has not
been rejuvenated by recent glaciation, consisting of over 85 % regolith
or weathered material (Wilford, 2012), and, as a result, is abundant in
minerals such as kaolinite, illite–smectite, goethite, and hematite. The
choice of total rather than bioavailable Sr as the focus of this work was
driven by an emphasis on geological sources and processes.
Setting
The study area in northern Australia stretches across Western Australia, the
Northern Territory, and Queensland (hereafter abbreviated to WA, NT, and Qld,
respectively), north of 21.5∘ S (Fig. 1). It is surrounded by
the Indian Ocean to the west, the Timor and Arafura seas and the Gulf of
Carpentaria to the north, and the Coral Sea (Pacific Ocean) to the east. The
main climate zones in the area are described as “Hot humid summer” in the
north and along the coast and “Warm humid summer” further south and inland
(Bureau of Meteorology, 2022a). The vegetation zones are dominated by “Tropical savanna” and
“Hot grassland with winter drought” (Bureau of Meteorology, 2022b). The 10-year (1996–2005)
average minimum and maximum temperatures mostly range from 15 to 24 ∘C
and from 27 to 36 ∘C, respectively (Bureau of Meteorology, 2022c). Average annual rainfall
over the 4-year period to November 2009 (when the bulk of the sampling
was completed) mostly ranges from 400 to 1500 mm yr-1 and is strongly seasonal (summer
rain) (Bureau of Meteorology, 2022d). Physiographically, the study area includes, from west to
east, the Kimberley, North Australian Plateaus, Barkly–Tanami Plains,
Carpentaria Fall, Carpentaria Lowlands, Peninsular Uplands, and Burdekin
Uplands provinces (Pain et al., 2011). Topographic altitude ranges from sea
level to 1622 m a.s.l. (above sea level) at Mount Bartle Frere (Qld's highest
point, located just south of Cairns), and the mean altitude is
∼260 m a.s.l. (Geoscience Australia, 2008).
The soil types most commonly encountered in the study area are, according to the
Australian Soil Classification scheme (Isbell and National Committee on Soil and Terrain, 2021; Australian Soil Resource Information System, 2022a), Vertosol, Kandosol, or Tenosol (∼24 % of the
sample sites each), followed by Rudosol, Hydrosol, or Sodosol (7 %–9 %) and
the rarer Calcarosol, Chromosol, Dermosol, or Podosol (<2 %). The
major river basins that divide the area belong, from west to east, to the
Pilbara, Sandy Desert–Mackay, Fitzroy, North Kimberley, Ord, Victoria,
Tennant, Daly, Darwin, Melville, Alligator, Roper, Arnhem, McArthur, Channel
Country, Burketown, Leichhardt, Flinders, Gilbert, Weipa, Mitchell, Princess
Charlotte Bay, Burdekin, Barron, Cooper Creek, and Whitsunday water regions
(Geoscience Australia, 1997). Land use over the area is overwhelmingly “Grazing natural
vegetation”, followed by “Other protected areas (inc. indigenous uses)”
and “Nature conservation” (Australian Soil Resource Information System, 2022b).
The northern Australia Sr isotope study area (see the inset for
location: WA – Western Australia, NT – Northern Territory, Qld –
Queensland, NSW – New South Wales, ACT – Australian Capital Territory, Vic – Victoria, Tas – Tasmania, and SA – South Australia) and the National
Geochemical Survey of Australia (NGSA) and Northern Australia Geochemical
Survey (NAGS) 87Sr/86Sr sample locations (small black squares)
shown with towns (large orange squares) and places (medium orange squares), main roads (solid green lines in panel a), secondary roads (dashed grey
lines in panel a), and the NGSA catchment boundaries (medium blue lines in panel b). Map projection:
Albers equal area.
The main geological region groups (Blake and Kilgour, 1998) over the study
area are, from west to east, the Pilbara, Canning, Kimberley, Bonaparte,
Ord, Victoria, Pine Creek, Victoria, Wiso, Arunta–Tanami, Tennant, Money,
McArthur, Arafura, Georgina, Isa, Carpentaria, Eromanga, Georgetown–Coen,
Quinkan, and Paleozoic-Qld groups (see Fig S1 in the Supplement). The higher-level
geological region domains (Blake and Kilgour, 1998) over the study area are,
from west to east, the Pilbara, Paleozoic, North Australian Craton,
Proterozoic, and Meso-Cenozoic domains (see Fig S2 in the Supplement). Thus, the study
area displays a complex and protracted geological history spanning over 3.6 Gyr (Ollier, 1988; Braun et al., 1998; Betts et al., 2002; Blewett, 2012;
Withnall et al., 2013; Ahmad and Scrimgeour, 2013), with two cratonic nuclei
in the west and centre (the Archean Pilbara Craton and Palaeoproterozoic–Mesoproterozoic North Australian Craton) flanked by younger Proterozoic and
Palaeozoic orogenic belts and basins, including the Phanerozoic Tasmanides to
the east. Mesozoic and Cenozoic sedimentary sequences of the Eromanga and
Carpentaria basins conceal much of the basement terrain over the eastern
third of the study area. The whole region has experienced extensive
weathering, resulting in a ubiquitous and locally thick regolith mantle.
Figure 2a shows that the decidedly dominant bedrock unit intersected by the
sample sites is “Sedimentary and low-grade metamorphics” (89 % of sites),
followed by the much less frequent “Felsic igneous intrusive” (4 %) and
“Mafic to ultramafic igneous volcanic” (3 %) lithologies. Representing 2 % or less each are the “Medium-grade metamorphics”, “Felsic
to mafic igneous volcanic”, “High-grade metamorphics”, “Mafic igneous
intrusive”, and “Felsic to intermediate igneous volcanic” lithologies.
Further investigation of the dominant bedrock unit “Sedimentary and low-grade metamorphics”
reveals that it is comprised of “Regolith” (79 %), “Sedimentary
siliciclastic” (4 %), “Feldspar- or lithic-rich arenite to rudite” (2 %), “Sedimentary carbonate” (2 %), and <1 % each of
“Argillaceous detrital sediment”, “Metasedimentary siliciclastic”, and
“Quartz-rich arenite to rudite”. Further, the “Regolith” class is reported
to mainly consist of “Alluvium” (51 %), “Sand plain” (9 %), “Estuarine
and delta deposit” (4 %), “Colluvium” (4 %), “Calcrete” (3 %),
“Undifferentiated sediment” (3 %), “Lake deposit” (2 %), and <2 % each of “Black soil plain”, “Dune”, and “Ferruginous duricrust”.
The bedrock ages (periods) intersected at the sample sites are
overwhelmingly Tertiary–Quaternary (58 %), followed by much less frequent
Mesoproterozoic (8 %), Cenozoic (6 %), Cambrian (5 %),
Palaeoproterozoic (5 %), Palaeoproterozoic–Mesoproterozoic (3 %),
Carboniferous–Permian (3 %), Permian (3 %), Devonian–Carboniferous (2 %), Archaean (2 %), and Cambrian–Ordovician (2 %) ages. Representing
<2 % are the Devonian, Jurassic–Cretaceous, Ordovician,
Silurian–Devonian, Jurassic, Neoproterozoic–Cambrian, and Triassic periods.
Numerous mineral occurrences are found in northern Australia, and Fig. 2b
shows the most important ones classified as “Mineral Deposits” (i.e. those
with an inferred resource), “Operating Mines” (currently producing), or
“Developing Projects” (approved but not yet producing). As mineralization is
the end point of geologically “unusual” processes taking place regionally in
“mineral systems” (Wyborn et al., 1994), it is useful to investigate if
mineral system processes leave a recognizable 87Sr/86Sr
fingerprint in the country rock and sediment derived therefrom. The study
area is particularly rich in “Base Metals” deposits (both Pb–Zn- and
Cu-dominated deposits, e.g. the Mount Isa, McArthur River, and George Fisher deposits) and is host to
the “North Australian Zinc Belt”, the world's largest Zn–Pb province (Huston
et al., 2022). In addition, notable occurrences of “Battery or Alloy Metals”
(Ni, Co, Mn, V, Mo, and Mg, e.g. the Spinifex Ridge, Ripon Hills, Julia Creek, and
Richmond deposits) and “Precious Metals” (Au and Ag, e.g. the Mount Isa and George Fisher deposits) are
also found here. Further, “Other Metals” (Sn, Sb, W, Ta, and Nb, e.g. the Mount
Carbine deposit), “Rare Earth Elements” (e.g. Thunderbird and Nolans Bore deposits), “Platinum
Group Elements” (Pt, Pd, and Rh, e.g. the Munni Munni deposit), and “Heavy Mineral Sands”
(e.g. Thunderbird deposit) occurrences are also catalogued. Thus, the area is significant
for underpinning Australia's critical mineral resources supply now and into
the future (e.g. Department of Industry, Science, Energy and Resources, 2022), without which a global transition to a
lower-carbon economy will be challenging.
The northern Australia Sr isotope study area and National
Geochemical Survey of Australia (NGSA) and Northern Australia Geochemical
Survey (NAGS) 87Sr/86Sr sample locations (small black squares)
shown with (a) surface geology from Raymond et al. (2012) and (b) NGSA
catchment boundaries (medium blue) and mineral occurrences (Geoscience Australia, 2022a). Map
projection: Albers equal area.
Material and methodsMaterial
This study makes use of archive “catchment outlet sediment” samples
collected during the National Geochemical Survey of Australia (NGSA), which
covered ∼80 % of Australia (de Caritat and Cooper, 2011a,
2016; de Caritat, 2022). The sampling philosophy of the NGSA was to collect
naturally mixed and fine-grained fluvial/alluvial sediments from large
catchments, thereby obtaining a representative average of the main soil and
rock types contributing sediment through weathering. This allowed an
ultralow sampling density (approximately one sample per 5200 km2) that was still
representative of large-scale natural variations (de Caritat and
Cooper, 2011b). Catchment outlet sediments are similar to floodplain
sediments in the sense that they are deposited during receding floodwaters
outside the riverbanks; however there is an added complexity in that, in Australia,
many areas can also experience the addition (or loss) of aeolian material. The
sampled floodplain geomorphological entities are typically vegetated and
biologically active (e.g. plants, worms, and ants), thereby making the collected
materials true soils, albeit soils developed on transported alluvium parent
material.
The sampling medium and density were both strategically chosen in the NGSA
project to prioritize coverage over resolution. This was justified by the
fact that the NGSA was Australia's first and, to date, only fully
integrated, internally consistent geochemical survey with a truly national
scope. In terms of the present study area, it is clear that these choices
have implications with respect to the granularity of the patterns revealed by the Sr
isoscape; as the collection of Sr isotope data in Australia using NGSA
samples grows in the future (e.g. de Caritat et al., 2022b, and this
contribution), it is hoped that the value of coverage will prevail over a
relative low resolution of detailed features.
The NGSA collected samples at two depths: a “top outlet sediment” (TOS) from
a shallow (0.1 m) soil pit approximately 0.8 m × 0.8 m in area and a
“bottom outlet sediment” (BOS) from a minimum of three auger holes that were generally
drilled within ∼10 m of the TOS pit. The auger holes were
drilled as deep as possible (to refusal or to a maximum depth of 1 m), and the
BOS sample was collected from an average depth of 0.6–0.8 m from all
drilled holes. A field manual was compiled to record all sample collection
method details, including site selection (Lech et al., 2007). Sampling for
the NGSA took place between July 2007 and November 2009, and the field data
were recorded in Cooper et al. (2010). In the laboratory, the samples were
air-dried at 40 ∘C for a minimum of 48 h (or to constant mass)
before being further prepared (see de Caritat et al., 2009); for the
comprehensive geochemical analysis programme of the NGSA, the reader is referred to de Caritat et
al. (2010). An aliquot of a minimum of ∼1 g of sample was milled
to a fine powder using a carbon steel ring mill or, for a few samples only,
an agate micromill. The main sample type selected for the present Sr isotope
study was NGSA BOS <2 mm in order to be as representative as
possible of the geogenic background unaffected by modern land use practices
and inputs (e.g. fertilizers). A few NGSA TOS <2 mm samples,
prepared in an identical fashion, were also analysed.
Several additional samples from the Northern Australia Geochemical Survey
(NAGS; Bastrakov and Main, 2020) were included in this project, as they
provide a higher-density coverage over part of the study area. The NAGS
project used the same sampling philosophy and sample collection, preparation,
and analysis methods as the NGSA, with a higher sampling density of one sample
per ∼500 km2. The NAGS project collected only TOS (0–0.1 m depth) samples, and the NAGS TOS <2 mm aliquots were prepared and
analysed as per the NGSA samples. Sampling for the NAGS took place in May
and June 2017.
Overall, 326 NGSA BOS <2 mm, 18 NGSA TOS <2 mm (including
15 with BOS also analysed), and 28 NAGS TOS samples, resulting in a total of 372
analyses from 357 samples, were analysed for Sr isotopes as detailed in
Sect. 3.2 below. Given that there are ∼10 % field
duplicates in the NGSA and the smaller NAGS catchments are nested within
NGSA catchments, all of those samples originate from within 307 NGSA
catchments, which together cover 1.536×106 km2 of northern
Australia (see Fig. 1).
Methods
Samples were prepared and analysed for Sr isotopes (87Sr/86Sr) at
the Wollongong Isotope Geochronology Laboratory (WIGL). Approximately 50 mg
of sample was weighed and digested in a 2:1 mixture of hydrofluoric and
nitric acids. All reagents used were SEASTAR™ BASELINE® grade,
with Sr concentrations typically <10 ng kg-1. Following
digestion, samples were redissolved in aqua regia (twice if needed) in
order to eliminate any fluorides, followed by nitric acid twice. Finally,
samples were redissolved in 2 M nitric acid prior to ion exchange
chromatography. Strontium was isolated from the sample matrix using
an automated, low-pressure chromatographic system (Elemental Scientific
prepFAST MC™) and a 1 mL Sr–Ca column (Eichrom™)
(Romaniello et al., 2015). The Sr elutions were redissolved in 0.3 M nitric
acid. Strontium isotope analysis was performed on a
Neptune Plus™ (ThermoFisher Scientific) multicollector inductively coupled plasma mass spectrometer
(MC-ICP-MS) at WIGL. The sample introduction system consists of
Apex-ST PFA MicroFlow nebulizer (Elemental Scientific, Inc.) with an uptake rate of ∼0.1 mL min-1, a quartz SSI dual cyclonic spray chamber, jet sample, and
X-skimmer cones. Measurements were performed in low-resolution mode. The
instrument was tuned at the start of each session with a 20 µgkg-1 Sr solution, and sensitivity for 88Sr was typically around 4 V.
Masses 88, 87, 86, 85, 84, and 83 were collected on Faraday cups.
Instrumental mass bias was internally corrected using measured
87Sr/86Sr. Masses 85 and 83 were used to correct for the isobaric
interference of 87Rb and 86Kr, respectively. Maps were prepared
using the open software QGIS® (version 3.16.14 Hannover) and applying an
Albers equal area projection. Symbology for displaying 87Sr/86Sr
data here either classified point data in eight equal quantile classes
(12.5 % of the data each, using a gradient from green denoting low to red denoting high) at the sampling
site or attributed the same value and colour to the whole catchment from
which the outlet sediment comes, reflecting the sampling medium, catchment
outlet sediment, being a representative sample of the average materials in
the catchment (see Sect. 3.1).
Quality assessment
National Institute of Standards and Technology (NIST) strontium carbonate
isotope Standard Reference Material SRM987 was used as a secondary standard
and measured after every five samples to assess accuracy during analysis.
The accuracy of the whole procedure was assessed by processing United States
Geological Survey (USGS) reference material basalt from the Columbia River
standard BCR-2 (Plumlee, 1998). The mean ±2 standard errors 87Sr/86Sr value for
BCR-2 in this study is 0.704961±35 (n=13), which is within the error of the
value in Jweda et al. (2016) (0.704500±11). Total procedure blanks
ranged between 0.025 and 0.245 ng Sr (n=12). A total of 20 field duplicate
sample pairs (collected at a median distance of ∼80 m from
one another on the same landscape unit; see Lech et al., 2007) were analysed
for 87Sr/86Sr in the BOS <2 mm sample, and they returned a
median relative standard deviation of 0.17 %. The relative standard
deviation from field duplicates includes natural variability
(mineralogical/chemical heterogeneity of the alluvial deposit) as well as
sample collection, preparation, and analysis uncertainties.
Overall, we feel that the quality of the 87Sr/86Sr data presented
herein is adequate for the purpose of regional mapping and that reporting
87Sr/86Sr data to the third decimal place with an indicative
fourth decimal place is appropriate for this work. This relatively low
precision obtained for field duplicates is attributed to the heterogeneity of
the alluvial deposits, as precision relating to sample preparation and
analysis for Sr isotopes is at the fifth decimal place (see the results for
BCR-2 above).
Data analysis
Data management was performed using Microsoft Excel®,
graphing and visualization was carried out using IMDEX ioGas®, and spatial
analysis and mapping was undertaken with QGIS® (an open-source geographical
information system). For the purposes of generating the Sr isoscape from
combined BOS and TOS samples, calculated “BOS-equivalent” values of TOS were
derived from regression analysis (see Sect. 5.2). Catchment-based Sr
isoscapes were constructed by assigning the
87Sr/86Sr value of a catchment's outlet sediment sample to each corresponding NGSA catchment or, if more than
one sample was analysed per catchment (e.g. field duplicates or
higher-resolution NAGS samples), by assigning the mean of those multiple samples. All
maps are shown in Albers equal area projection.
Results
The soil 87Sr/86Sr values reported herein range from 0.7048 to
1.0330 (range = 0.3282). The median is 0.7405 and the mean is 0.7532
(standard deviation = 0.0480; kurtosis = 8.1602; skewness = 2.4640).
Figure 3 illustrates the univariate structure of the new data. Spatially,
the 87Sr/86Sr values define large-scale, coherent patterns with
multi-point low and high regions (Fig. 4). The main high-value
(radiogenic) regions are found in the central and northern parts of the NT,
most of Cape York, and along some of the coast (including the WA
part of the study). Prominent low-87Sr/86Sr-value (unradiogenic)
regions include a large, central, northwest–southeast-trending elongated
area in the NT, a smaller and similarly trending area in central Qld, and the northernmost and easternmost parts of the Qld study area.
Univariate distribution of the 87Sr/86Sr data (n=357)
from the northern Australia Sr isotope study area: (a) histogram (20 bins,
0.016 wide); (b) cumulative frequency plot; and (c) Tukey box plot (the solid white dot shows the mean, circles denote outliers, and triangles represent extreme outliers) (Tukey,
1977). The sample medium is the <2 mm fraction of NGSA bottom outlet
sediment (BOS) or equivalent (see the text for further information).
Strontium isoscape for the northern Australia study area with (a) data points classed by quantiles at the sampling sites and (b) as whole
catchments coloured by same colour ramp. The sample medium is the <2 mm
fraction of NGSA bottom outlet sediment (BOS) or equivalent (see the text for
further information). Map projection: Albers equal area.
DiscussionComparison with other datasets
A comparison of the present results with selected soil 87Sr/86Sr
datasets from around the world is offered in Table 1. All discoverable data
from Australia and the Southern Hemisphere comprising more than a handful of
sites were included, whereas only selected Northern Hemisphere datasets
focusing on recent and large-scale datasets were included. Table 1
combines both bioavailable/exchangeable and bulk/total 87Sr/86Sr
data, representing variable soil grain-size fractions (where reported),
parent materials, and land uses. Despite this variability, one can make
several observations. Firstly, the northern Australia Sr isotope dataset has
the highest maximum (1.0330) and largest range (0.3282) of
87Sr/86Sr values amongst those compiled. Secondly, there is no
observed standard protocol for collecting or preparing soils for either
bioavailable or total 87Sr/86Sr determination. Thirdly, total
87Sr/86Sr datasets tend to have a wider range and greater variance
than bioavailable ones, at least partly accounting for the higher values
encountered here. These issues complicate data compilation and integration
across projects/countries but do not preclude them. Indeed, through
contemporaneous data analytics, including artificial intelligence and
machine learning, it is likely that the relationships between bioavailable
and total Sr isotope values and a host of other environmental variables
(including from climatic, topographic, biotic, and geoscientific categories)
can be teased out and that high-spatial-resolution models/predictions of
bioavailable or total Sr isotope distribution can be derived (e.g. Bataille
et al., 2020). This is indeed a future research direction that we propose/support
for isoscape science in general as well as in an Australian Sr isoscape in
particular.
Comparison of 87Sr/86Sr data from this study
with selected datasets from around the world, with an emphasis on Australia and
the Southern Hemisphere (count, minimum, median, mean, standard deviation,
maximum, and range). The regions are abbreviated as follows: Aus – Australia, Qld – Queensland, SA – South
Australia, SE – southeastern, Vic – Victoria, and WA – Western Australia.
The digestions are abbreviated as follows: AcAc – acetic acid (CH3COOH), AmAc – ammonium acetate
(CH3COONH4), AmNt – ammonium nitrate (NH4NO3), and AR – aqua
regia.
RegionSample TypeDigestionnMinMedMeanSDMaxRangeSourceSouthern Hemisphere: Aus Northern AusSoil (on alluvium), <2 mmMilled, HF + HNO3+ AR3570.70480.73900.75270.04841.03300.3282This studySE AusSoil (on alluvium), <2 mmMilled, HF + HNO3+ AR1120.70890.71990.72200.07360.75110.0422de Caritat et al. (2022b)Mostly SA, Vic (19 sites)Soil (calcrete)AcAc260.70610.71020.71150.00510.73290.0267Quade et al. (1995)SE Aus; Murray–Darling BasinAlluvium, <2µmHF + HNO3260.70800.71630.72670.01880.77510.0672Gingele and De Deckker (2005)Southern Aus (eight sites)Soil (calcrete)AcAc460.70940.71510.71680.00580.73740.0280Dart et al. (2007)Soil (calcrete)HNO3+ HF + HCl380.71250.73820.75380.05880.99850.2860Dart et al. (2007)Canning Coast, WADust, <2µmAcAc140.71410.73610.73850.01630.77270.0586De Deckker (2019)Cape York Peninsula, QldSoil, <2 mmAcAc930.70750.72270.72520.01540.79110.0835Adams et al. (2019)Southern Hemisphere: Other Brazil (three regions)Soil (agricultural), <2 mmAmAc30.71220.71320.71290.00060.71330.0011de Almeida (2021)Southern ChileSoil (forest)AmAc220.70910.70980.71010.00070.71190.0029Kennedy et al. (2002)Southern PeruSoil (agricultural)Ashed, AmAc1140.70200.70760.70770.00170.71890.0169Knudson et al. (2014)South Africa (four regions)Soil (agricultural), <180µmHNO3+ H2O2670.70810.71300.71260.00200.71590.0078Vorster et al. (2010)Southern New ZealandSoil, <2 mmAmNt830.70400.70790.70780.00130.71120.0072Adam Martin, personal communication (2021)Northern Hemisphere BritainSoilH2O260.70740.70930.70920.00100.71150.0041Evans et al. (2010)FranceSoil, <2 mmAmNt5240.70330.71130.71190.00420.73080.0275Willmes et al. (2014)EuropeSoil (grazing), <2 mmAmNt5650.70380.70900.71030.00520.75230.0485Hoogewerff et al. (2019)Soil (agricultural), <2 mmAmNt6220.70380.70930.71120.00630.75960.0558Hoogewerff et al. (2019)IsraelSoil, <2 mmAmNt600.70580.70860.70850.00080.71020.0044Moffat et al. (2020)CambodiaSoilAshed, HF600.70340.71150.71300.00600.74240.0389Shewan et al. (2020)SoilAshed, AmNt460.70370.71100.71170.00390.72040.0167Shewan et al. (2020)JapanSoil, <2 mmH2O2130.70670.70850.70880.00160.71290.0062Oishi (2021)ItalySoilExchangeable and bulk2730.70530.70910.70990.00270.72380.0185Lugli et al. (2022)Top–bottom relationship
Based on the 15 sites where both TOS and BOS samples were analysed, a strong
correlation (Fig. 5) between the aforementioned two depths is found:
(87Sr/86Sr)BOS=0.9913×(87Sr/86Sr)TOS+0.0069(r2=0.97;p<0.001;n=15).
Equation (1) was used to infer BOS 87Sr/86Sr from those sites where
only TOS samples could be obtained (i.e. 3 NGSA and 28 NAGS sites),
effectively deriving a BOS-equivalent 87Sr/86Sr value for the
purposes of performing internally consistent statistical and spatial
analysis.
As a result of the robust correlation between TOS and BOS, the BOS (or
subsurface) Sr isoscapes of Fig. 4 can be recalculated to TOS (or surface)
Sr isoscapes by replacing the legend values for the eight quantile class
boundaries with the values 0.7050–0.7156–0.7215–0.7293–0.7401–0.7522–0.7674–0.7910–1.0276. A TOS map and values would be useful when
studying surface processes, such as plant and animal uptake or the effect of
land use on sediment composition and dynamics.
Scatterplot of the BOS (deep) versus TOS (surface)
87Sr/86Sr data (n=15) from the northern Australia Sr isotope
study area with total uncertainty derived from field duplicates (0.17 %;
grey plus symbols) and linear square regression line (dashed line).
Relationship to bedrock
When grouped by age of their respective geological region, the new
87Sr/86Sr data show a general trend of increasing
87Sr/86Sr with increasing bedrock age, as illustrated by the box
and scatter plots in Fig. 6. In Fig. 6,
average numerical ages (mid-points between beginning and end ages) are
attributed to each geological period group. The observed trend is consistent
with that of the known controls on mineral (and rock) 87Sr/86Sr
values, namely their age.
Distribution of the 87Sr/86Sr data from the northern
Australia Sr isotope study area by geological region age: (a) notched Tukey
box plots (Tukey, 1977) of, left to right, the whole dataset first (grey; n=357) and then sorted by average age (early Tertiary, n=6; Middle
Jurassic to Holocene, n=80; Jurassic to Tertiary, n=5; Cretaceous, n=4; Palaeozoic to Mesozoic, n=28; Silurian to Triassic, n=6;
Palaeozoic, n=61; Neoproterozoic to early Permian, n=7; Neoproterozoic
to Devonian, n=49; Mesoproterozoic to Permian, n=2; Mesoproterozoic to
Neoproterozoic, n=2; Mesoproterozoic, n=4; Palaeoproterozoic to
Permian, n=7; Palaeoproterozoic to Neoproterozoic, n=13;
Palaeoproterozoic to Mesoproterozoic, n=59; Palaeoproterozoic, n=7;
Archaean to Palaeoproterozoic, n=12; and Archaean, n=5); (b) a scatterplot against the average age of the geological region with the group averages
(crosses) and linear square regression line (dashed line).
Another known control on Sr isotopic values is the Rb content, which, over
time, contributes 87Sr by radiogenic decay. Closer examination of the
geology in the catchments with the 10 highest 87Sr/86Sr values
indicates that they contain lithologies likely to be enriched in Rb relative
to Sr (e.g. felsic, felsic-to-intermediate igneous rocks, and medium- to
high-grade metamorphic rocks), as shown using detailed maps in the Supplement.
Of the 5 top 10 87Sr/86Sr values in the northern NT, 3 are
from the Pine Creek geological region. The two northernmost of these
(sample nos. 2007190099 and 2007191390) with 87Sr/86Sr values of 0.9413 and 0.9900, respectively, are in catchments downstream of 1780–1790 Ma I-type granite intrusions that have been reported to have
87Sr/86Sr values from 0.71 to 8.13, 87Rb/86Sr values from ∼1 to 290, Rb concentrations of up to 956 mg kg-1, and Sr concentrations of up to 459 mg kg-1 (Riley,
1980, their Table 1).
The elevated samples from the WA coast are also generally proximal to, or
downstream of, felsic igneous lithologies (including the granitoid and
gneiss underlying the Mallina Basin; Van Kranendonk et al., 2002) and are
consistent with 87Sr/86Sr values reported for clay fraction
sediments from the region by De Deckker (2019), as shown in the Supplement.
The northernmost samples along the coast of WA overlap with the King Leopold
Orogen, abutting the western flank of the Kimberley geological region. Few
87Sr/86Sr data are available for this region, but those that exist
within the Hooper Complex (Griffin et al., 2000) indicate radiogenic
87Sr/86Sr values: 0.7708–0.9620 for the Whitewater Volcanics,
0.7336–0.8524 for the Kongorow Granite, and 0.7353–0.7602 for the Lennard
Granite (Bennett and Gellatly, 1970, their Table 2). Thus, observed elevated
87Sr/86Sr values of 0.8080, 0.7709, and 0.7923 in three sediment
samples (sample nos. 200719103, 2007190317, and 2007190966, respectively)
near Derby, WA, are entirely compatible with these bedrock source rocks.
Page et al. (1980, their Table 5) published Rb and Sr data from Alligator
Rivers uranium field rocks, including from the Nimbuwah Complex granitoid
close to Cooper Creek, <20 km north of the Nabarlek uranium deposit
(their sample no. 7212.4063). For this sample, they reported
87Rb/86Sr values of 1.25–1.47, 87Sr/86Sr values of 0.7387–0.7442, Rb concentrations of
146–164 mg kg-1, and Sr concentrations of 320–336 mg kg-1. Our sample (sample no. 2007190710) from this
same catchment has a 87Sr/86Sr value of 0.8019 and is located on a
felsic granite polygon.
Black et al. (1983, their Tables 3 and 5) reported calculated initial rock
87Sr/86Sr ratios in the range from 0.76 to 0.92 from granites,
including the Wangala and the Haverson granites, in the northern Arunta
geological region (southern NT). These intrusions intersect three sampled
NGSA catchments for which we obtained elevated 87Sr/86Sr values,
namely 0.8140, 0.7763, and 0.8651 (sample nos. 2007190727, 2007190562,
and 2007190123, respectively). These catchments are also just
upstream of some of our top 10 87Sr/86Sr values (between 0.8935
and 1.0330).
In the Georgetown geological region of northwestern Qld, broad consistency
between whole-rock 87Sr/86Sr and catchment sediment
87Sr/86Sr is also observed. For instance, our sample
no. 2007190858, from a catchment that drains mostly the Esmeralda Granite
(average whole-rock initial 87Sr/86Sr of 0.7314±0.0116;
Black, 1973, their Table 2) on the western edge of the Georgetown Inlier, has a
relatively elevated 87Sr/86Sr value of 0.7467. Conversely, the
three catchments that mostly directly drain the less radiogenic Newcastle
Range Volcanics in the centre of the Georgetown Inlier, which have a lower average
whole-rock initial 87Sr/86Sr of 0.7152±0.0011 (Black,
1973, Table 2), have correspondingly lower sediment 87Sr/86Sr
values of 0.7318 (sample no. 2007190572), 0.7268 (sample no. 2007190348), and a field
duplicate pair of 0.7258 (sample no. 2007190548) and 0.7282 (sample no. 2007190548).
The floodplain sediment Sr isotopic values recorded in areas dominated by
sedimentary carbonate and mafic/ultramafic igneous rocks are usually within
an intermediate range between the more radiogenic and unradiogenic
end-members discussed above. Indeed, samples sited within 0.1∘
(∼10 km) of lithologies recorded as sedimentary carbonate and
mafic/ultramafic igneous rocks have median values of 0.7387 (n=96) and
0.7422 (n=42), respectively.
Distribution of the 87Sr/86Sr data from the northern
Australia Sr isotope study area by mineral occurrence hosted in same
catchment. The categories shown, from left to right, are as follows: all (n=97); Base Metals – Cu (Zn, Pb,
Ag, Au) (n=5); Base Metals – Zn, Pb (Cu, Ag) (n=19); Battery/Alloy Metals
– Ni, Co, Mn, V, Mo, Mg (n=17); Bauxite (n=3); Coal (n=2); Diamond (n=3);
Fertilizer Elements – P, K (n=12); Graphite (n=2); Heavy Mineral Sands
(n=1); Iron Ore (n=5); Other Metals – Sn, Sb, W, Ta, Nb (n=6); Platinum
Group Elements (n=2); Precious Metals – Au, Ag (n=13); Rare Earth Elements
(n=6); and Uranium (n=1).
Relationship to mineralization
Figure 7 illustrates the range of 87Sr/86Sr values of catchments
that contain known mineralization of various types. A total of 44 NGSA
catchments host 97 mineral occurrences as catalogued by Geoscience Australia (2022a). These
resources include 19 “Base Metals – Zn, Pb (Cu, Ag)”; 17
“Battery/Alloy Metals – Ni, Co, Mn, V, Mo, Mg”; 13 “Precious Metals – Au,
Ag”; 12 “Fertilizer Elements – P, K”; 6 “Rare Earth Elements”; 6 “Other
Metals – Sn, Sb, W, Ta, Nb”; 5 “Iron Ore”; and 5 “Base Metals –
Cu (Zn, Pb, Ag, Au)” occurrences. Whilst the vast majority of mineral
occurrences are found in catchments with a sediment outlet
87Sr/86Sr signature ≪0.84, six outlier
occurrences are associated with higher catchment outlet sediment
87Sr/86Sr values: 0.8434 (the Coronation Hill Pt–Pd deposit in the
catchment containing sample no. 2007191552); 0.9373 (Nolans Bore hard-rock rare earth element (REE) and Mount Peake vanadium deposits in the catchment containing sample
no. 2007191112); 0.9748 (Batman Au deposit in the catchment containing
sample no. 2007191329); and, highest of all, 0.9900 (the Browns Pb–Zn and the
Browns Co deposits in the catchment containing sample no. 2007191387). Some
mineral occurrence types are not found in catchments with
87Sr/86Sr signature less radiogenic than 0.75 in this study: the
minima reported for the Platinum Group Elements, Rare Earth Elements/Heavy Mineral Sands, and Diamond commodity groups are 0.7529, 0.7606, 0.7606, and 0.7567, respectively.
Whilst it is unlikely that mineral deposits themselves impart a significant
control on the 87Sr/86Sr of sediment collected down-catchment
given their usually limited size, the hosting country rock may, however,
potentially be more widely affected by mineral system processes, and, upon further investigation, appears
to record some Sr isotopic effect that may be useful for mineral vectoring. One of the most radiogenic 87Sr/86Sr
signatures (0.9373) is found for the catchment hosting both the Mount Peake
vanadium deposit (Simandl and Paradis, 2022) and the Nolans Bore hydrothermal REE
deposit (Schoneveld et al., 2015). Hydrothermal or magmatic REE deposits
are not usually characterized by highly radiogenic Sr signatures (e.g.
0.7029–0.7262 at Bayan Obo – Le Bas et al., 1997; 0.701–0.708 in global
review of carbonatite deposits – Bolonin, 2019; and 0.7037–0.7062 at Caotan – Wei
et al., 2020) and nor is Nolans Bore specifically (0.7054–0.7079;
Huston et al., 2016). Thus, the elevated 87Sr/86Sr reported here
from this catchment is attributed to the radiogenic bedrock occurring in the
study area and perhaps in upstream catchments immediately to the south of
it.
Data availability
The new spatial Sr isotope dataset for the northern Australia region is
publicly available from 10.26186/147473 (de
Caritat et al., 2022a) and via the Geoscience Australia portal
https://portal.ga.gov.au/restore/cd686f2d-c87b-41b8-8c4b-ca8af531ae7e
(last access: 27 February 2023; Geoscience Australia, 2023). Metadata are also available via the
Geoscience Australia portal (https://portal.ga.gov.au/metadata/geochronology-and-isotopes/isotopes/rbsr-isotope-points/4cacd9e8-3340-4c27-99fe-48d404e67ca8,
last access: 27 February 2023) (Geoscience Australia, 2022b).
Conclusions
A total of 372 new strontium (Sr) isotopic compositions
(87Sr/86Sr) are reported from 357 catchment outlet sediment
samples from northern Australia (north of 21.5∘ S). The analysed
material originates from the sample archives of the National Geochemical
Survey of Australia (NGSA; n=344) and Northern Australia Geochemical
Survey (NAGS; n=28) projects, both of which targeted overbank or floodplain
landforms near the outlet of large catchments. The sampled catchments
together cover 1.536×106 km2 of northern Australia. For the most
part, bottom outlet sediment (BOS) samples, retrieved mostly by drilling (with an auger) to,
on average, 0.6 to 0.8 m depth, were analysed. However, a few top outlet sediment
(TOS) samples, collected from the top 0.1 m of soil, were also
included, notably all of the NAGS samples and 18 NGSA samples. Total digestion of
milled <2 mm grain-size fractions from these sediments yielded a
wide range of 87Sr/86Sr values from a minimum of 0.7048 to a
maximum of 1.0330.
To date, the present study represents the largest Sr isoscape in the Southern
Hemisphere, a region that is critically under-represented in terms
of isotopic data worldwide. We found a very strong correlation between the
87Sr/86Sr values in the BOS and TOS samples, allowing us to
confidently infer BOS-equivalent values where only TOS samples were
analysed. A map of the 87Sr/86Sr distribution (isoscape) across
northern Australia reveals spatial patterns reflecting the ages and
lithologies of the source material for the sediment, which is principally
carried down catchment by fluvial processes. There is an overall increase in
87Sr/86Sr observed with increasing age of the geological region
intersected at the sampling points. In areas of outcropping or sub-cropping
felsic igneous rocks, relatively radiogenic Sr signatures are observed, whereas the inverse is noted for areas of mafic igneous rocks or marine sediments. The new Sr
isotopic data are also examined in terms of the mineral occurrences
found in their catchment. Whilst most mineral occurrences in the region are
found in a catchment with an 87Sr/86Sr signature ≪0.84, six outlier occurrences are associated with higher values. Some
mineral occurrence types (Platinum Group Elements, Rare Earth
Elements, Heavy Mineral Sands, and Diamond) are not found in catchments
with 87Sr/86Sr signature less radiogenic than 0.75 in this study.
Whether mineral system or regional petrogenetic processes are responsible
for the association of relatively elevated 87Sr/86Sr with mineral
deposits is a subject for further investigation.
Although we have focused the discussion of the new 87Sr/86Sr data
on sediment sources in terms of rock ages and types, potential applications
of the present isoscape and modelling derived therefrom could be extended to
studies of mineralization, hydrology, food tracing, dust
provenancing or sourcing, and historic migrations of people and animals.
The supplement related to this article is available online at: https://doi.org/10.5194/essd-15-1655-2023-supplement.
Author contributions
PdC provided the concept, samples, and funding,
curated and analyzed the data, created the figures, and wrote and edited the manuscript. AD provided technical guidance, resources, and supervision, curated the data, and edited the manuscript. FD carried out the analyses, provided technical support, and curated the data.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
This paper is published with the permission of the Chief Executive Officer, Geoscience Australia.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The National Geochemical Survey of Australia (NGSA)
project would not have been possible without Commonwealth funding through
the “Onshore Energy Security Program” (http://www.ga.gov.au/ngsa,
last access: 27 February 2023), and Geoscience Australia appropriation. The
Northern Australia Geochemical Survey (NAGS) project was funded under the
“Exploring for the Future” programme (https://eftf-production.ga.gov.au/northern-australia-geochemical-survey,
last access: 27 February 2023) and Geoscience Australia appropriation.
Collaboration with the geoscience agencies of all states and the Northern
Territory is gratefully recognized. We acknowledge all land owners and custodians, whether
private, corporate, and/or traditional, for granting access to the field
sites for the purposes of sampling. We are also grateful to Geoscience Australia laboratory
staff for assistance with preparing and analysing the samples. We thank
Geoscience Australia reviewers, Kathryn Waltenberg and David Huston; journal
reviewers, Ian Moffat, Malte Willmes, and Jodie Pritchard; and the journal
editor, Attila Demény, for their detailed and constructive critique of our work.
Financial support
This research has been supported by the Australian Government (Exploring for
the Future, https://www.eftf.ga.gov.au/about, last access: 27 February 2023). Geoscience Australia's Exploring for the Future programme
provides pre-competitive information to inform decision-making by government,
community, and industry on the sustainable development of Australia's
mineral, energy, and groundwater resources. By gathering, analysing, and
interpreting new and existing pre-competitive geoscience data and knowledge,
we are building a national picture of Australia's geology and resource
potential. This leads to a strong economy, resilient society, and sustainable
environment for the benefit of all Australians. This includes supporting
Australia's transition to net-zero emissions; strong, sustainable resources
and agriculture sectors; and economic opportunities and social benefits for
Australia's regional and remote communities. The Exploring for the Future
programme, which commenced in 2016, is an 8-year, AUD 225 million investment by
the Australian Government.
Review statement
This paper was edited by Attila Demény and reviewed by Jodie Pritchard, Ian Moffat, and Malte Willmes.
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