Graham Mortimer

A simple method for the precise determination of ≥ 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation

The combination of enriched isotopes and conventional elemental internal standards permits the precise determination of > 40 trace elements by ICPMS in a broad spectrum of geological matrixes. Enriched isotopes expand the suite of available reference isotopes spaced through the mass spectrum, so that the complex mass-dependent variations in sensitivity encountered during ICPMS analysis can be monitored and deconvolved. The method we have developed is straightforward, entailing simple sample preparation, instrument calibration, and data reduction procedures, as well as providing extended element coverage, improved precision, and both time and cost benefits compared to alternative analytical strategies. Analytical precision near or better than 1% RSD (relative standard deviation) is achieved for most elements with mass > 80 amu and between 1% and 4% (RSD) for elements with mass 80 amu and < 10 ng g−1 to 1 μg g−1 for elements with mass < 80 amu). The subtle geochemical differences which can be resolved using this method are demonstrated by analyses of Nb, Ta, Zr, and Hf in magmas from ocean islands and subduction zones. These data reveal significant departures from chondritic Zr/Hf and Nb/Ta values, and systematic trends which are consistent with greater incompatibility of Zr relative to Hf and also of Nb relative to Ta during melting of the upper mantle. The occurrence of significantly subchondritic Zr/Hf and Nb/Ta ratios in Nb-poor subduction zone magmas, supports the notion that the depletion of high-field strength elements in subduction magmas is due to their removal from the mantle wedge by prior melting events.

A high-resolution Sr/Ca and δ18O coral record from the Great Barrier Reef, Australia, and the 1982–1983 El Niño

Abstract A high-resolution (near weekly) Sr/Ca and oxygen isotopic record is presented for a coral from the Pandora Reef in the Great Barrier Reef (GBR) of Australia during the period 1978 to 1984. The records are well correlated except for periods of high rainfall when river runoff has significantly modified the δ18O value of seawater. Using the Sr/Ca temperature calibration of De Villiers et al. (1994), the Sr/Ca records exhibit seasonally controlled cyclical SST (sea surface temperature) variations of from ~21 to ~28δC. During the very strong El Nino of 1982–1983, the Sr/Ca systematics indicate a sharp drop in the winter SST to ~ 18.5δC. This represents a temperature anomaly of −3δC which is approximately twice that given by the δ18O variations, suggesting an ~×2 amplification of the anomaly by the Sr/Ca system, possibly due to the increasing dominance of inorganically controlled aragoniteseawater fractionation. The oxygen isotopic systematics show the combined effects of both temperature and changing seawater δ18O values, the latter reflecting the influx of 18O-depleted runoff during periods of high rainfall. Due to the extremely low (~10−3) Sr and Ca contents of river run off relative to seawater, it is possible to use the Sr/Ca thermometer to calculate temperatures independent of major floods and hence deconvolve the combined effects in the oxygen isotopic record of variable temperature and the δ18O value of seawater. Using this approach it is possible to quantitatively reproduce the volume of runoff from the Burdekin River during the periods of major flooding that occurred in early 1979 and 1981. The results of this study demonstrate that the combined use of high-resolution Sr/Ca and δ18O systematics in scleractinian corals is a powerful tool for providing quantitative constraints on past climate.

500 ka precipitation record from southeastern Australia: Evidence for interglacial relative aridity

230 Th/ 234 U dating of speleothems from southeastern Australia documents changes in effective precipitation over the past 500 k.y. at a temporal resolution not previously achieved. Results show that the highest effective precipitation for the southeastern interior of Australia occurred during stadials and cool interstadials of the past four glacial cycles. Interglacials and warm interstadials, as well as glacial maxima, are comparatively arid. We suggest that lower regional temperatures over the continent and changes in atmospheric circulation (stronger Walker circulation) produced the observed periods of increased effective precipitation.

High resolution windows into early Holocene climate: SrCa coral records from the Huon Peninsula

High-precision measurements of SrCa ratios are reported for Porites corals from the uplifted Holocene coral terraces at Huon Peninsula, Papua New Guinea. The early Holocene Porites have UTh mass spectrometric ages of 8920 ± 60 yr and 7370 ± 50 yr, and δ234U(t) values of 145 ± 2, similar to modern seawater. The SrCa coral records provide 5–6 year high resolution (near weekly) time windows into early Holocene sea surface temperatures. Seasonal temperature fluctuations are generally in the range of ± 1°C, with occasional excursions of ± 2°C, which may indicate the more frequent recurrence of very strong ENSO (El Nino-Southern Oscillation) events. Mean annual SrCa temperatures of 24.2 ± 1.1°C and 22.9 ± 0.8°C have been obtained, which are ∼ 2–3°C cooler than that exhibited by a modern Porites. These results indicate that, during the early Holocene, the equatorial western Pacific ocean was at least several degrees cooler than present-day temperatures. This is consistent with late glacial coral records from the Caribbean that indicate lower (∼ 6°C) sea surface temperatures for the equatorial oceans. The Huon Peninsula corals also indicate that SSTs were several degrees cooler than those in the Caribbean during the early Holocene. Thus, although the northern hemisphere summer radiation maximum occurred at ∼ 10 ka, there appears to have been a significant lag in the response of the equatorial western Pacific ocean to this warming. Cooler early Holocene sea surface temperatures in the western Pacific may have been due to changing patterns of ocean-atmosphere circulation, resulting from the exposure of large areas of continental shelf in the southeast Asia region, a consequence of lower glacial sea levels. It is likely that ocean temperatures in the Huon Peninsula were influenced by the opening at ∼ 7 ka of the Torres Strait, that now separates New Guinea from the Australian mainland.

Applications of the 238U_230Th decay series to dating of fossil and modem corals using MC-ICPMS

Methods for U-series dating of corals are described using a multi-collector (MC) ICPMS. The utility of this approach is illustrated with two examples: first, dating of Last Interglacial (∼105 years) corals from southwestern Western Australian; and second, the dating of near-modern (100 to 102 years) corals from the Great Barrier Reef. A previously unreported occurrence of Last Interglacial corals from the southernmost tip (Cape Leeuwin) of Western Australia is shown to have U-series ages ranging from 129 to 125 ka, indicating that relatively prolific coral growth occurred during the first half of the Last Interglacial period. This represents an extension of the southernmost limit of coral growth by almost ∼300–400 km compared with present-day limits, requiring significantly warmer (∼2°C) sea-surface temperatures, consistent with intensification of the Leeuwin Current. Furthermore, at this southernmost site, corals are preserved in growth position at heights of 2–2.5 m above present-day sea-level, confirming previous observations from along the stable coastal margin of Western Australia that sea-levels were ∼3 m higher during the first part of the Last Interglacial. The substantially improved sensitivity of MC-ICPMS is illustrated by 238U–230Th dating of modern coral samples extracted from a living Porites colony from the Great Barrier Reef. The 238U–230Th ages from annual samples are in excellent agreement with an independent chronology derived by counting of annual density bands and show that the coral grew continuously from 400 years ago to the present day. The typical precision achievable for corals ranging in age from tens to hundreds of years is shown to be 2–5 years, with the main limitation being uncertainties in the correction for initial non-radiogenic 230Th.