John Hellstrom

Iolite: Freeware for the visualisation and processing of mass spectrometric data

Iolite is a non-commercial software package developed to aid in the processing of inorganic mass spectrometric data, with a strong emphasis on visualisation versus time of acquisition. The goal of the software is to provide a powerful framework for data processing and interpretation, while giving users the ability to implement their own data reduction protocols. It is intended to be highly interactive, providing the user with a complete overview of the data at all stages of processing, and allowing the freedom to change parameters and reprocess data at any point. The program presents a variety of windows for the selection and viewing of data versus time, as well as features for the generation of X-Y plots, summary reports and export of data. In addition, it is capable of generating X-Y images from laser ablation rasters, and combining information from up to four separate elemental concentrations (intensities of red, green and blue, and the z-axis) in a false-colour three-dimensional image. By virtue of its underlying computing environment—Igor Pro—Iolite is capable of processing very large datasets (i.e., millions of timeslices) rapidly, and is thus ideal for the interrogation of multi-hour sessions of laser ablation data that can not be easily manipulated in conventional spreadsheet applications, for example. It is also well suited to multi-day sessions of solution-mode inductively-coupled plasma mass spectrometer (ICPMS) or thermal ionisation mass spectrometer (TIMS) data. A strong emphasis is placed on the interpolation of parameters that vary with time by a variety of user selectable methods including smoothed cubic splines. Data are processed on a timeslice-by-timeslice basis, allowing outlier rejection and calculation of statistics to be employed directly on calculated results. This approach can reduce the risk of processing biases associated with the manipulation of integrated datasets, while also allowing the implementation of more complex data reduction methods.

Increasing Australian–Indonesian monsoon rainfall linked to early Holocene sea-level rise

The Australian–Indonesian summer monsoon affects rainfall variability across the Indo–Pacific region. Reconstructions of monsoon strength from stalagmites show that precipitation increased from 11,000 to 7,000 years ago, as rising global sea level caused the flooding of the Indonesian continental shelf. The Australian–Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo–Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation1,2,3, but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak4. Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise5,6,7. We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone8. However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago.

Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone

A severe drought in parts of low-latitude northeastern Africa and southwestern Asia ∼4200 yr ago caused major disruption to ancient civilizations. Stable isotope, trace element, and organic fluorescence data from a calcite flowstone collected from the well-watered Alpi Apuane karst of central-western Italy indicate that the climatic event responsible for this drought was also recorded in mid-latitude Europe. Although the timing of this event coincides with an episode of increased ice-rafted debris to the subpolar North Atlantic, the regional ocean-atmosphere response seems atypical of similar Holocene ice-rafting events. Furthermore, comparison of the flowstone data with other regional proxies suggests that the most extreme part of the dry spell occurred toward the end of a longer-term climate anomaly.

Evidence for Obliquity Forcing of Glacial Termination II

Oblique Reasoning In Milankovich theory, the canonical theory of glaciation and deglaciation, ice sheets wax and wane in response to the amount of summer insolation at a latitude of 65°N, which is consistent with the observed timing of the last deglaciation. The penultimate glaciation behaved quite differently, however. Now, Drysdale et al. (p. 1527, published online 13 August) offer firmer constraints on the timing of the penultimate deglaciation, by correlating a difficult-to-date marine record of ocean volume to a precisely datable nearby speleothem (terrestrial stalagmite). Ocean volume began to increase about 141,000 years ago, thousands of years before the rise in 65°N summer insolation. Thus, instead of the forcing mechanism proposed by Milankovich, variations in Earths obliquity may be mostly responsible for the disappearance of ice sheets. Marine records suggest that the early onset of the penultimate deglaciation was due to changes in Earth’s obliquity. Variations in the intensity of high-latitude Northern Hemisphere summer insolation, driven largely by precession of the equinoxes, are widely thought to control the timing of Late Pleistocene glacial terminations. However, recently it has been suggested that changes in Earth’s obliquity may be a more important mechanism. We present a new speleothem-based North Atlantic marine chronology that shows that the penultimate glacial termination (Termination II) commenced 141,000 ± 2500 years before the present, too early to be explained by Northern Hemisphere summer insolation but consistent with changes in Earth’s obliquity. Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.

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.

Rapid and accurate U/Th dating using parallel ion-counting multi-collector ICP-MS

Using the parallel ion-counting capability of an appropriately configured multi-collector ICP-MS it is possible to simultaneously measure the activity ratios 230Th/234U and 234U/238U in a single solution. A method has been developed which fully internalises the measurement of elemental fractionation, instrument mass bias and ion counter gain, allowing on-line calculation of U/Th ages on a cycle-by-cycle basis during data acquisition. Because of the high efficiency of this technique very small sample sizes can be used, with permil-level precision possible on both 230Th/238U and 234U/238U from analyses of a few tens of nanograms of uranium. As U and Th fractions do not need to be collected and analysed separately sample throughput is very high, using a simple TRU-Spec ion exchange resin procedure. Repeat analyses of Harwell Uraninite, NIST SRM 960 and two carbonate age standards show excellent agreement with results from other laboratories.

Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo

Further U-series dating and the magnetic stratigraphy of the hosting cave deposits show that Australopithecus sediba lived just under 2 million years ago, near or just before the emergence of Homo. Newly exposed cave sediments at the Malapa site include a flowstone layer capping the sedimentary unit containing the Australopithecus sediba fossils. Uranium-lead dating of the flowstone, combined with paleomagnetic and stratigraphic analysis of the flowstone and underlying sediments, provides a tightly constrained date of 1.977 ± 0.002 million years ago (Ma) for these fossils. This refined dating suggests that Au. sediba from Malapa predates the earliest uncontested evidence for Homo in Africa.

Multi-proxy constraints on the climatic significance of trace element records from a New Zealand speleothem

Trace element concentrations and uranium isotope ratios are reported for a speleothem from the South Island of New Zealand. TIMS uranium-series dating of the speleothem indicates it to have grown from 31 000 years ago to the present, providing a continuous record of the last deglaciation. Trace element abundances measured in this speleothem using ICP-MS are found to exhibit strong temporal relationships with δ13C and other measured variables, and with the environmental changes inferred from them. Strontium and barium concentrations are positively correlated with inferred changes in the productivity and extent of vegetation cover above the cave. Magnesium concentration appears to have responded to changes in groundwater residence time, assumed to have an inverse relationship with effective meteoric precipitation above the cave. The degree of 234U/238U disequilibrium also appears to have varied in response to hydrological changes, and together with the magnesium data implies a post-glacial increase in regional effective precipitation to have culminated ca. 13 000 cal yr B.P., some 2000 years after a dramatic post-glacial increase in forest extent previously inferred for the region. This increase in forest extent is thus unlikely to have been caused by a significant increase in precipitation, and is assumed to have been driven predominantly by a rapid increase in regional temperature, centred on 15 000 calendar years before present.

Stalagmite evidence for the precise timing of North Atlantic cold events during the early last glacial

Evidence of millennial-scale cold events following the last interglacial are well preserved in North Atlantic marine cores, Greenland ice, and pollen records from Europe. However, their timing was previously undetermined by radiometric dating. We report the first precise radiometric ages for two such events, C23 (105.1 ± 0.9 ka to 102.6 ± 0.8 ka) and C24 (112.0 ± 0.8 ka and 108.8 ± 1.0 ka), based on stable carbon and oxygen isotope measurements on a stalagmite from Italy (CC28). In addition to providing new information on the duration of these events in southern Europe, the age data provide invaluable tuning points for the Melisey I (C24) and Montaigu (C23) pollen zones identified in western Europe. The former event is of particular significance because it represents the end of the Eemian interglacial forest phase in western Europe. The new age data will also allow fine tuning of the timing and duration of Greenland stadial 24 (equivalent to C23) in the North Greenland Ice Core Project ice core and, via a common gasage chronology, tuning of the Vostok and EPICA (European Project for Ice Coring in Antarctica) ice cores.

An arid-adapted middle Pleistocene vertebrate fauna from south-central Australia

How well the ecology, zoogeography and evolution of modern biotas is understood depends substantially on knowledge of the Pleistocene. Australia has one of the most distinctive, but least understood, Pleistocene faunas. Records from the western half of the continent are especially rare. Here we report on a diverse and exceptionally well preserved middle Pleistocene vertebrate assemblage from caves beneath the arid, treeless Nullarbor plain of south-central Australia. Many taxa are represented by whole skeletons, which together serve as a template for identifying fragmentary, hitherto indeterminate, remains collected previously from Pleistocene sites across southern Australia. A remarkable eight of the 23 Nullarbor kangaroos are new, including two tree-kangaroos. The diverse herbivore assemblage implies substantially greater floristic diversity than that of the modern shrub steppe, but all other faunal and stable-isotope data indicate that the climate was very similar to today. Because the 21 Nullarbor species that did not survive the Pleistocene were well adapted to dry conditions, climate change (specifically, increased aridity) is unlikely to have been significant in their extinction.