Stephen A. Short

Wetting and drying of Australia over the past 300 ka

Alternately dry and wet climatic episodes across central and eastern Australia during the past 300 ka have greatly affected Australia9s rivers, lakes, and dune fields. Evidence of widespread climate and flow-regime changes has been provided by 75 thermoluminescence (TL) dates and 18 U/Th dates from alluvial and eolian sediments. Fluvial conditions dominated part of the last two interglacials (stage 5 and 7), resulting in large sand loads in rivers in the present Simpson Desert and southeastern Australia. During the last interglacial, fluvial activity in central Australia peaked at ∼110 ka (stage 5 pluvial), probably ∼5-10 ka behind world temperature and sea-level maxima. Following the last late interglacial wet phase, aridity associated with dune building spread from central Australia toward its margins, achieving greatest intensity during the last glacial maximum. A less widespread wet phase, identified at about 55-35 ka (stage 3 subpluvial), is associated with high lake levels and paleochannel activity in southeastern Australia. This TL record of variable continental aridity in Australia correlates well with global changes, including the variable eolian dust flux into central China, the northern Pacific Ocean, and Antarctica.

Comparative uranium-thorium and thermoluminescence dating of weathered quaternary alluvium in the tropics of Northern Australia

Abstract Thermoluminescence (TL) age determinations of alluvial sediments in the tropics are evaluated by comparison with U/Th age determinations of pedogenic accumulations in the alluvium of the lower Gilbert River, a large fan delta in the wet-dry tropics of northern Queensland, Australia. This study extends U/Th dating by applying it not only to calcretes, but also to Fe/Mn oxyhydroxide/oxide accumulations. While a direct correlation cannot be made between U/Th dates from secondary minerals and TL dates from the host sediments, both sets of data show broad consistency. In addition to providing a minima for acceptable TL ages, U/Th dates are useful for determining the chronology of pedogenesis/diagenesis. They show that calcretes and ferricretes have formed under similar climatic conditions in the wet-dry tropics of northern Australia during the late pleistocene. Beneath about 5–12 m the Gilbert fan delta consists of an extensive sand body older than 85,000 yr and probably about 120,000 yr in age, representative of a period of major fluvial activity not repeated since this time. Above are muds and fine sandy muds that extend uninterrupted to the present surface except in the downstream fan where they are bisected by a thin unit of medium sand that TL dates at 40,000–50,000 yr B.P. A system of sandy distributary channels over the fan surface represents an early Holocene fluvial phase probably more active than at present.

234U238U and 230Th234U activity ratios in the colloidal phases of aquifers in lateritic weathered zones

A procedure was developed for comparing solute and colloid phases of groundwaters in contact with uranium ore bodies at Nabarlek and Koongarra in the Alligator Rivers region, Northern Territory, Australia. Single-pass ultrafiltration of large volumes with cut-offs of 18 nm and 1 {mu}m was used. Colloids were composed of Fe and Si species with sorbed U and U daughters. Uranium isotopes were mostly present as soluble species. Thorium was significantly associated with the colloids. The {sup 234}U/{sup 238}U activity ratios (ARs) were similar in solute and colloid phases close to the ore bodies but further down-gradient colloids were generally more depleted of {sup 234}U than the solute. The {sup 230}Th/{sup 234}U ARs rose from very low values for both solute and colloid phases close to the ore bodies through several orders of magnitude to much higher values further down-gradient. Colloid {sup 230}Th/{sup 234}U ARs were always significantly greater than solute ARs. Results were consistent with a systematic leaching of U from colloids going down-gradient and very little mobilization of ore body. {sup 230}Th relative to U. Ubiquitous complexes {sup 232}Th appeared to suppress the solubility of {sup 230}Th.

sup 234 U/ sup 238 U and sup 230 Th/ sup 234 U activity ratios in the collidal phases of aquifers in lateritic weathered zones

A procedure was developed for comparing solute and colloid phases of groundwaters in contact with uranium ore bodies at Nabarlek and Koongarra in the Alligator Rivers region, Northern Territory, Australia. Single-pass ultrafiltration of large volumes with cut-offs of 18 nm and 1 {mu}m was used. Colloids were composed of Fe and Si species with sorbed U and U daughters. Uranium isotopes were mostly present as soluble species. Thorium was significantly associated with the colloids. The {sup 234}U/{sup 238}U activity ratios (ARs) were similar in solute and colloid phases close to the ore bodies but further down-gradient colloids were generally more depleted of {sup 234}U than the solute. The {sup 230}Th/{sup 234}U ARs rose from very low values for both solute and colloid phases close to the ore bodies through several orders of magnitude to much higher values further down-gradient. Colloid {sup 230}Th/{sup 234}U ARs were always significantly greater than solute ARs. Results were consistent with a systematic leaching of U from colloids going down-gradient and very little mobilization of ore body. {sup 230}Th relative to U. Ubiquitous complexes {sup 232}Th appeared to suppress the solubility of {sup 230}Th.

Thorium-uranium disequilibrium dating of Late Quaternary ferruginous concretions and rinds

Radioactive ferruginous nodules from the Alligator Rivers Uranium Province, Northern Territory, Australia, were systematically examined by optical microscopy, XRD, alpha- and fission-track autoradiography, and analysed for U, Th, Ra and major stable elements. Correlation of autoradiographs with microscopic structure and analyses of selective extractions confirmed that U and Th were strongly associated with Fe oxides. Fission-track autoradiography showed no concentration gradient evidence for postdepositional leaching of uranium. Strong acid leaching studies showed that irreversibly adsorbed U, and authigenic 234U and 230Th, but not 226Ra, are quantitatively retained by accumulated oxide/oxyhydroxide. Correlation of the groundwater activity ratio with oxidic 234U/238U activity ratios indicated the latter was radiogenically consistent with oxidic 230Th/234U activity ratios < 1. Dense, accumulated Fe/Mn oxide matrices are apparently capable of forming radiogenic closed systems in respect of the 230U → 234U → 230Th decay set. 234U/238U and 230Th/234U activity ratios were used to derive similar mean ages of rinds for four nodules in the same horizon. Th/U ages were also determined, using a Th-index detrital correction method previously used for impure carbonates, of pedogenic Fe/Mn accumulations having background concentrations of U and Th from other locations in Australia. These ages were compared with TL and 14C ages of the-host sediments. The comparisions suggest that Th/U dating could be used to age indurated layers of Fe/Mn oxides in soil horizons < 350 ka old.

Evidence for pleistocene and holocene raised marine deposits, Sandon Point, New South Wales

In New South Wales there has been an elusive search for coastal deposits that might substantiate an elevated Holocene sea‐level. Chronostratigraphic evidence is presented for estuarine and beach deposits raised more than 1 and 2 m respectively above Australian Height Datum around Sandon Point, New South Wales, between 6900 and 1520 bp. The chronology is based upon 14C dating of shell and in situ mangrove stumps, and upon thermoluminescence dating of quartz sand. These elevations concur with other results determined along the east coast of Australia and in the south Pacific. Moreover the Holocene beach sediments lie above Pleistocene aeolian sand dating between 25 300 and 32 700 bp and estuarine mud which must be at least Last Interglacial in age. The latter units also lie more than 2 m above Australian Height Datum. Fossil coral found along the adjacent coast plus the elevation and orientation of the raised marine deposits imply that ocean temperature was warmer around 2800 bp by up to 2°C, that sea‐level...

Uranium infiltration from a river to shallow groundwater

The infiltration of uranium from the polluted River Glatt (Switzerland) into a hydraulically connected saturated aquifer was investigated during a period of almost five years. The uranium concentrations and the 234U238U activity ratios (ARs) were analysed monthly in the water of the river and of four groundwater sampling wells. The speciation of uranium and its relation to other aqueous and solid components were investigated experimentally and in model calculations. The uranium concentration and the ARs in the river varied seasonally with a maximum and a minimum, respectively, in summer. Processes, such as photo-reductive dissolution of iron-oxyhydroxides or the enhanced decomposition of organic matter and calcite in the upstream, eutrophic Lake Greifen may have caused these variations. Laboratory experiments confirmed a possible release of uranium by photochemical reactions. The concentration maximum of uranium was also detected in the shallow groundwater, but with a delay of about six months to the maximum in the river. Under the assumption that the maxima in the river and the groundwater were related, an in situ distribution coefficient (KD) for uranium of ≈7 mL g−1 can be calculated for this Quaternary gravel aquifer. Uranium was present predominantly as UO2-carbonate complexes, whereas the formation of phosphate and organic complexes (e.g., UO2-humates/EDTA/NTA) was negligible. Particles and colloids played a minor role for the transport of uranium in this aquifer. In the course of this study, we observed an ongoing general decrease of the uranium concentrations and a disappearance of its summerly maxima in the River Glatt. We relate this unexpected effect to a better control of the phosphate concentration in surface waters, which diminished the growth of aquatic biota. This, in turn, led to changes in the redox conditions of lake and river sediments, and herewith to a reduced release of heavy metals, including uranium. The results of this case study can be applied to estimate the migration behavior of other actinide oxyions (e.g., PuO22+NpO2+) in shallow heterogeneous aquifers, which abound in alpine and pre-alpine locations.

Thermoluminescenge and Uranium‐Thorium chronologies of pleistocene coastal landforms of the Illawarra region, New South Wales

SUMMARY TL determinations of the ages of aeolian sands and U/Th determinations of the ages of crusts on rock platforms in the Illawarra region, especially at Red Point, indicate that these techniques give considerable promise of a major advance in deciphering the Pleistocene coastal record in Australia. The methods of dating are outlined, and problems of laboratory and field interpretation are considered. The longstanding debate as to whether the +2 m level of platforms is modern or is a Pleistocene relict has been resolved for the Illawarra coast, where these platforms can be shown to have been formed during the Last Interglacial high stand of the sea. By inference, the +4 m level must be of at least the same age. The +6 to 7 m level was cut by about 300Ka and may be as old as Pliocene. Four aeolian units at Red Point reworked from nearby barrier beach complexes date from c. 25Ka, 45Ka, 125Ka, and 300 to >400Ka. Reconnaissance surveys and dating indicate that aeolian sands of similar ages occur elsewhere...

Stratigraphy and chronology of late pleistocene reefs in the Southern Cook Islands, south Pacific

The Makatea Islands in the southern Cook Islands consist of a degraded volcanic core surrounded by emergent “makatea” limestones. While much of this reefal limestone is of Tertiary age, the seawardmost reefs are late Pleistocene. Last-interglacial reefs (oxygen isotope substage 5e) identified by U-series dating reach 12.2 m on Atiu (mean age of three samples 126,000 ± 5000 yr B.P.), 9.8 m on Mitiaro (mean age of five samples 119,000 ± 3000 yr B.P.), 10.0–12.7 m on Mauke (mean age of four samples 128,000 + 5000, −4,000 yr B.P.), and 14.5–20.0 m on Mangaia (mean age of four samples 115,000 ± 7000 yr B.P.). On Atiu, Mauke, and Mitiaro a lower reef unit has been identified, and is separated from the overlying substage 5e reef by a sharp discontinuity, often with a crust of coralline algae. U-series dating from Atiu and Mauke indicates that this is a stage 7 reef formed in the penultimate interglaciation. The Makatea Islands have emerged as a result of flexure of the lithosphere in response to loading by the Pleistocene volcanic island of Rarotonga. Isolated outcrops of makatea limestone on Rarotonga reach 3.5 m and are probably of substage 5e age. Elastic and viscoelastic models of lithospheric flexure predict that much of the compensatory movement should occur shortly after the emplacement of the load. However, the age and elevation of late Quaternary reefs on the southern Cook Islands, where a Pleistocene volcanic island has loaded relatively old ocean floor (>80 myr), indicate that differential uplift has been continuing over the last 250,000 yr.

DISTRIBUTION OF URANIUM- AND THORIUM SERIES RADIONUCLIDES IN MINERAL PHASES OF A WEATHERED LATERITIC TRANSECT OF A URANIUM ORE BODY

Abstract A surficial, weathered 400 m long profile of the Ranger One ore body #3 (Northern Territory, Australia) has been characterised to a depth of 10 m in terms of the total uranium, 238 U , 234 U , 232 Th , 230 Th and 228 Th distributions, and measurements of other geochemically important elements. The characterisation was limited to the 234 U , 230 Th and 228 Th , precipitation due to evaporation of ground- and interstitial water, and the adsorption of uranium and thorium within a time frame less than that required for 234 U / 238 U and 230 Th / 234 U to achieve secular radioactive equilibrium. The mineralogical composition of the soil is to some extent responsible for local variations in the measured properties. The differences in chemical speciation and behaviour of uranium and thorium enhance the variability in the 230 Th / 234 U activity ratios. The monsoonal climatic and hydrological variations intensify some of the observed effect.