Patrick De Deckker

Timing of the Last Glacial Maximum from observed sea-level minima

During the Last Glacial Maximum, ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today. But few direct estimates exist of the volume of the ice sheets, or the timing and rates of change during their advance and retreat. Here we analyse four distinct sediment facies in the shallow, tectonically stable Bonaparte Gulf, Australia—each of which is characteristic of a distinct range in sea level—to estimate the maximum volume of land-based ice during the last glaciation and the timing of the initial melting phase. We use faunal assemblages and preservation status of the sediments to distinguish open marine, shallow marine, marginal marine and brackish conditions, and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio-hydro-isostatic modelling. Our results indicate that from at least 22,000 to 19,000 (calendar) years before present, land-based ice volume was at its maximum, exceeding todays grounded ice sheets by 52.5 × 106 km3. A rapid decrease in ice volume by about 10% within a few hundred years terminated the Last Glacial Maximum at 19,000 ± 250 years.

Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration

The nature of compositional variability within the tiny calcitic shells (tests) that are precipitated by planktonic foraminifera has been investigated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Systematic large and correlated variation of Mg/Ca, Mn/Ca, Ba/Ca and Zn/Ca but relatively uniform Sr/Ca are observed through the test walls of analysed species (Globigerinoides sacculifer, Globigerinoides ruber, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei). Distinct chamber and chamber-wall layer compositions can be resolved within individual tests, and Mg/Ca compositional differences observed in sequentially precipitated test components of the different species analysed are consistent with seawater temperature changes occurring with habitat migration during their adult life-cycle stages. Estimated test calcification temperatures are in keeping with available seawater temperature constraints, indicating the potential for accurate seawater temperature reconstruction using LA-ICP-MS. Mg-rich (<1–6 mol% Mg) surface veneers that are also enriched in Mn, Ba, and Zn have been found on all species and all fossil tests, as well as on live-sampled tests of G. ruber, with the latter suggesting a possible biogenic origin. These Mg-rich surfaces bias bulk test compositions toward higher Mg/Ca values by between 5 and 35%.

Sea-level at the Last Glacial Maximum: evidence from northwestern Australia to constrain ice volumes for oxygen isotope stage 2

New sea-level information from the Bonaparte Gulf in northwestern Australia is used to constrain the magnitude and rates of change of ice volumes during the Last Glacial Maximum (LGM ). The region is tectonically stable and far from the former ice-covered regions. The glacio-hydro-isostatic adjustment of the coast is therefore relatively small, and the corrections for this eVect are not sensitive to details of the rebound model. Microfossil analysis and AMS radiocarbon dating of 11 gravity cores taken across the shelf and Bonaparte Gulf demonstrate that: (1) the LGM sea-levels were locally at ’125±4 m; (2) the LGM terminated abruptly at 19 000 cal yr BP with a rapid rise in sea-level of about 15 m over the next 500 years; and (3) the onset of the minimum sea-levels occurred before 22 000 cal yr BP. When corrected for the glacio-hydro-isostatic eVects, the increase of LGM ice volumes over the present-day ice volume is 52.5◊106 km3. The termination of the LGM is marked by a rapid ice discharge of 5.2◊106 km3.

Clay mineral distribution in surface sediments between Indonesia and NW Australia - source and transport by ocean currents

Abstract The clay mineral distribution in sediments between Indonesia and NW Australia has been assessed on the basis of 166 core-top samples. Clay mineral assemblages are closely related to the geology and weathering regime of the adjacent hinterland and allow the distinction of four clay mineral provinces. Three provinces, Western, Central and Eastern Province are situated along the Indonesian Islands Arc, from Sumatra in the west to Timor in the east. Illite is the major clay mineral of the Western and eastern Province, whereas the Central Province abounds with smectite. The fourth province comprises the NW and West Australian shelf and slope, as well as offshore plateaus and is dominated by kaolinite. Transport of clays by surface and subsurface ocean currents can be observed within the provinces, e.g. with the Leeuwin and West Australian Current in the NW Australian Province and with the outflow of low-salinity water through the Sunda and Lombok straits in the Central Province. Transport of clays across province boundaries is inhibited by strong salinity fronts, with the exception of the boundary between the Central and Eastern Province.

Uptake of Mg and Sr in the euryhaline ostracod Cyprideis determined from in vitro experiments

This paper describes a series of in vitro experiments during which juveniles of the euryhaline ostracod Cyprideis australiensis were grown to adulthood in waters of ranging salinities (from 10‐70‰.) and Mg=Ca and Sr=Ca ratios, and under two different constant temperature regimes, 20o and 25oC. Results indicate the following thermodependence for Mg in the calcitic valves of adult Cyprideis australiensis: T.Mg/D 2:69C 5230 [Mg=Ca]ostracod=[Mg=Ca]water .A minor thermodependence was also found for this ostracod species with respect to the uptake of Sr. There is a very strong relationship between the Mg=Ca of the ostracods and the Mg=Ca of the waters in which they moulted. The same can be said between the Sr=Ca of the ostracods and the waters’ Sr=Ca. There is no direct relationship between water salinity and either the Mg=Ca ratio nor the Sr=Ca ratio of the ostracod valves. The 20o and 25oC experiments established the following partition coefficients which we believe can be applied to any Cyprideis taxa grown in waters within the 5 to 20 Mg=Ca range: KD[Mg]D 0:000514C 0:00019 Temperature oC , and KD[Sr]D 0:223C 0:0086 Temperature oC . This relationship for Sr, which has been mentioned for the first time for Cyprideis is tentative. The Mg=Ca of ostracods is strongly dependant on temperature, but this effect can easily be masked by variations in the Mg=Ca of the host water commonly found in natural lakes. Ostracod physiology is adapted to constructing valves from low-Mg calcite, so when growing in waters with high (>30), or very low (<1) Mg=Ca, Cyprideis species will not take up Mg in accordance with the calculated KD.

Magnesium and strontium compositions of Recent benthic foraminifera from the Coral Sea, Australia and Prydz Bay, Antarctica

Analyses of the Mg/Ca and Sr/Ca ratios of the modern benthic foraminifera, Cibicides wuellerstorfi (epifaunal) and Uvigerina species (infaunal) from the Coral Sea, and Cibicides refulgens (epifaunal) and Trifarina angulosa (infaunal) from Prydz Bay, Antarctica revealed relationships with temperature that have possible applications for reconstructions of bottom-water paleotemperatures. A positive relationship exists between the Mg/Ca and Sr/Ca ratios of Cibicides wuellerstorfi and Cibicides refulgens and ambient temperatures, at least within the range of -2 and 6°C. For the correlation between Mg/Ca compositions and temperature the r2 values range from 0.78 (C. wuellerstorfi alone) to 0.88 (C. wuellerstorfi and C. refulgens together). At present, the Mg/Ca-temperature relationship must be regarded as tentative because of significant overlap of standard error values. The relationship between the Sr/Ca compositions of C. wuellerstorfi and bottom-water temperature yields an r2 value of 0.95. These results indicate that Sr/Ca and possibly Mg/Ca ratios of Cibicides wuellerstorfi may provide useful information for the assessment of paleotemperature. Single-species data are presently insufficient to assess the influence of ambient temperature on trace-element compositions of Uvigerina species. Trifarina angulosa may have Mg/Ca compositions which are positively related to temperature, but Sr/Ca values seem unaffected by temperature. This may be due to pore-water influences on infaunal tests or to vital effects. Although more modern data are needed, our present results suggest that Sr/Ca ratios and possibly Mg/Ca ratios of some benthic foraminifera have the potential to be useful paleothermometers, at least within a temperature range of −2 to 6°C.

Uncertainties in seawater thermometry deriving from intratest and intertest Mg/Ca variability in Globigerinoides ruber

intertest Mg/Ca variability within these deep-sea core top samples is a source of significant uncertainty in Mg/Ca seawater temperature estimates and is notable for being site specific. Our results indicate that widely assumed uncertainties in Mg/Ca thermometry may be underestimated. We show that statistical power analysis can be used to evaluate the number of tests needed to achieve a target level of uncertainty on a sample by sample case. A varying bias also arises from the presence and varying mix of two morphotypes (G. ruber ruber and G. ruber pyramidalis), which have different mean Mg/Ca values. Estimated calcification temperature differences between these morphotypes range up to 5C and are notable for correlating with the seasonal range in seawater temperature at different sites.

Ostracod shell chemistry: A new palaeoenvironmental indicator applied to a regressive/transgressive record from the gulf of Carpentaria, Australia

Abstract We demonstrate the feasibility of determining a continuous record of chemical changes occurring during marine transgressive/regressive phases, using the Mg/Ca and Sr/Ca ratios measured in some 300 single valves of the euryhaline ostracod Cyprideis from two cores from the Gulf of Carpentaria, Australia. The molar Mg and Sr distribution coefficients Kd[Mg] and KD[Sr] for modern Australian Cyprideis specimens (Chivas et al. 1986b), K D [ Mg ] at 25°C=( Mg/Ca ) shell /( Mg/Ca ) water =0.00458±0.00072 K D [ Sr ]=( Sr/Ca ) water /( Sr/Ca ) water =0.475±0.057 are used here to reconstruct the palaeoenvironmental record of the gulf. The two cores studied show similar patterns during the past 40,000 yr for equivalent hydrologic phases recognized in the cores. The Mg/Ca ratios of the water inferred from ostracod shell chemistry indicate that Carpentaria was a fresh or slightly saline lake between ≈ 40,000 and ≈ 13,000 yr B.P., before being transgressed by the sea during sea level rise. Around 26,000 yr B.P., a phase of substantial precipitation of authigenic calcite caused a dramatic increase in the Sr/Ca and Mg/Ca ratios of the lake water, and this was registered by the ostracod shell chemistry. From ≈ 26,000 to ≈ 13,000 yr B.P., the lake remained fresh or slightly saline despite the Ca depletion of the water which caused the Sr/Ca and Mg/Ca of the ostracods to be unusually high.

A Late Quaternary pollen record from deep-sea core Fr10/95, GC17 offshore Cape Range Peninsula, northwestern Western Australia

Abstract Pollen and charcoal analysis on marine sediment core Fr10/95, GC-17 provides a record of vegetation, fire and climate change for the last 100 ka, with a hiatus from 64 to 46 ka, for the Cape Range Peninsula, Western Australia. Our results indicate significantly drier conditions and reduced summer rain after 46 ka compared with 100–64 ka. Periods of maximum summer rain occurred at 100, 80 and 70 ka. Vegetation changed from open Eucalyptus woodlands rich in grasses to open Eucalyptus and Gyrostemon shrublands rich in Chenopodiaceae/Amaranthaceae and Asteraceae Tubuliflorae, in the period from 46 to 40 ka. The charcoal record does not suggest human involvement in this vegetation change. The period from 14 to 3 ka was wetter with heavier summer rain compared to today, probably as a result of higher sea-surface temperatures. Increased strength of the Leeuwin Current during the last 5000 years is suggested by the presence of Pteridophyta spores derived from Indonesia.

Australian salt lakes: their history, chemistry, and biota — a review

A vast number of large lakes (≫ 100 km2) are typically very old features of the Australian landscape; they occupy areas which have changed little tectonically (e.g., they occupy ancient drainage systems in Western Australia or lie in deep depressions such as the Great Artesian Basin: Lake Eyre) and have not been transgressed by the sea since at least the Palaeogene. Other salt lakes, most of which are small (≪ 50 km2), have been affected morphologically during recurring glacial-interglacial cycles (e.g., lakes associated with gypsum or clay lunettes, sabkhas, pans, lakes near the coast behind barrier dunes as a result of sea-level changes) and their sedimentary records represent comparatively much shorter periods of time. There are also a number of unusually young (< 30 000 years) crater lakes, some of which are the best studied lakes in Australia.The major ions encountered today in Australian salt lakes consist of sodium and chloride although some lakes are also calcium sulphate rich. The origin of these ions is briefly discussed. Sodium carbonate lakes are rare in Australia today. Under past climatic/hydrological conditions the chemistry of a number of lakes was apparently different.The biota of Australian salt lakes is mostly endemic; it is highly diversified as witnessed by the crustacean fauna and is well adapted to the harsh conditions prevailing in saline water. This is the result of a long history of aridity in Australia. The characteristics of this biota are presented together with data on its distribution which is primarily related to climatic conditions.