Linda K. Ayliffe

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.

DIETS OF SOUTHERN AFRICAN BOVIDAE: STABLE ISOTOPE EVIDENCE

Abstract Although bovids have been studied for decades, debate still exists about their diets. To address this problem, we examined bovid dietary ecology through analysis of stable carbon isotopes. We analyzed tooth enamel, bone collagen, and hair from 312 individual bovids, representing 27 species from southern Africa. Although dietary information from the literature is usually supported by this technique, our results and the literature are sometimes highly divergent. For instance, our results indicate that Taurotragus oryx and Raphicerus campestris eat less grass than is widely believed. Furthermore, contrary to most theoretical expectations, our data indicate no relationship between body size and percentage of monocots consumed by southern African Bovidae. Although many researchers have abandoned the idea that bovid soft-tissue anatomy is strongly indicative of diet, we demonstrate a strong relationship between the percentage of grass in a bovid’s diet and several hard-tissue craniodental indices.

Oxygen isotope composition of the bone phosphate of Australian kangaroos: Potential as a palaeoenvironmental recorder

δ18O determinations of bone phosphate (δ18Op) have been performed on a suite of modern kangaroos and wallabies (known collectively as Macropods) that come from a wide variety of climatic zones within Australia. Macropod δ18Op values range from 16 to over 30%. (V-SMOW) and are found to correlate with environmental relative humidities. As Macropods are herbivorous mammals with low drinking water requirements that consume large amounts of plant leaf water, it is likely that the δ18Op signature of Macropods is reflecting leaf water fractionation processes that are controlled by relative humidity. These results indicate that δ18Op from fossil bones of Macropods and similar drought-tolerant herbivorous mammals may yield palaeohumidity estimates.

The retention of primary oxygen isotope compositions of fossil elephant skeletal phosphate

Fossil elephant remains from sediments of Plio-Pleistocene age in the vicinity of Lake Turkana, Kenya, provide insights into the ability of the phosphate-oxygen system to preserve original δ18O values (δ18Op) in the depositional environment. Individual fossil specimens exhibit systematic variations in the δ18Op values of their various tooth and bone components. Differences in δ18Op values between these skeletal components can be as high as 3.6%., despite the fact that equivalent phases for individual modern elephants have δ18Op values which vary by less than 0.6%.. XRD traces of the different skeletal phases of modern and fossil samples, also infer differential preservation of primary apatite crystallinities among the various fossil phases. Fossil samples of dentine, cementum and bone have X-ray spectra with sharper and more intense peaks compared to their modern counterparts, but little change in crystallinity is observed for fossil enamel samples. A model based on the premise that the enamel phase preserves primary isotope signatures and that the accompanying skeletal phases are partially re-equilibrated with the fluids associated with the microbial decomposition of the elephants carcass is consistent with the features of the 18Op dataset. This isotopic evidence, in conjunction with the physical and chemical attributes of enamel, has led us to postulate that enamel could be preserving primary isotope signatures while other skeletal components are affected to varying degrees by alteration processes in the burial environment.

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.

Determining biological tissue turnover using stable isotopes: the reaction progress variable.

The reaction progress variable is applied to stable isotope turnover of biological tissues. This approach has the advantage of readily determining whether more than one isotope turnover pool is present; in addition, the normalization process inherent to the model means that multiple experiments can be considered together although the initial and final isotope compositions are different. Consideration of multiple isotope turnover pools allows calculation of diet histories of animals using a time sequence of isotope measurements along with isotope turnover pools. The delayed release of blood cells from bone marrow during a diet turnover experiment can be quantified using this approach. Turnover pools can also be corrected for increasing mass during an experiment, such as when the animals are actively growing. Previous growth models have been for exponential growth; the approach here can be used for several different growth models.

An experimental study of nitrogen flux in llamas: is 14N preferentially excreted?

Nitrogen isotope analysis is now commonly used to investigate the diets, and to a lesser extent, the environments of ancient populations. These studies assume that mammals are predictably enriched in 15 N over their food, and concomitantly, that 15 N becomes increasingly concentrated as one moves up the food chain. The literature commonly states that this 15 N-enrichment of mammalian tissues is due to preferential excretion of light nitrogen ( 14 N), but there are few data to support this assertion. To address the gap, we conducted two nitrogen flux trials in which four llamas (Lama glama) were fed high- and low-protein diets. The ratios of fecal nitrogen loss to urinary nitrogen loss were 0.30 and 0.88 on the high- and low-protein diets respectively. Feces were enriched in 15 N by approximately +3‰ on both diets, whereas urinary nitrogen was depleted in 15 N( 2.1‰) on the low-protein diet, but not significantly different from intake 15 N on the high-protein diet. Most importantly, there was no statistically significant difference between dietary and total excreta 15 N on either diet. Given these data and theoretical considerations, we argue that the nitrogen influx and efflux of adult mammals at steady state should be isotopically commensurate. However, during growth, diet change, thermal or nutritional stress, animals may not be at steady state and fractionation between intake and excreta 15 Nm ay occur.

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.

Digestion and passage rates of grass hays by llamas, alpacas, goats, rabbits, and horses.

Many studies have suggested that South American camelids (SAC) have greater digestive efficiency than pecoran ruminants, but others have found no difference. In an effort to provide new data on this issue, we investigated the ability of SAC (alpacas and llamas), goats, horses, and rabbits to digest C 3 (Bromus inermis) and C4 (Cynodon dactylon) grass hay with nearly equal nitrogen and cell wall concentrations. Dry matter digestibility (DMD) of the C3 grass hay was not significantly different between SAC and goats. Foregut fermenters (SAC and goats) digested C3 and C4 grasses more efficiently than hindgut fermenters (horses and rabbits). SAC digested C4 grass hays more efficiently than goats, possibly due to their relatively longer particulate matter mean retention times (71 and 54 h, respectively). Apparent nitrogen digestibility was 9% higher for all species with the C3 grass in comparison to C4 grass. This suggests that the highly-vascularized bundle sheath cells of C 4 plants prevent efficient utilization of dietary nitrogen.

Extinction implications of a chenopod browse diet for a giant Pleistocene kangaroo

Kangaroos are the worlds most diverse group of herbivorous marsupials. Following late-Miocene intensification of aridity and seasonality, they radiated across Australia, becoming the continents ecological equivalents of the artiodactyl ungulates elsewhere. Their diversity peaked during the Pleistocene, but by approximately 45,000 years ago, 90% of larger kangaroos were extinct, along with a range of other giant species. Resolving whether climate change or human arrival was the principal extinction cause remains highly contentious. Here we combine craniodental morphology, stable-isotopic, and dental microwear data to reveal that the largest-ever kangaroo, Procoptodon goliah, was a chenopod browse specialist, which may have had a preference for Atriplex (saltbushes), one of a few dicots using the C4 photosynthetic pathway. Furthermore, oxygen isotope signatures of P. goliah tooth enamel show that it drank more in low-rainfall areas than its grazing contemporaries, similar to modern saltbush feeders. Saltbushes and chenopod shrublands in general are poorly flammable, so landscape burning by humans is unlikely to have caused a reduction in fodder driving the species to extinction. Aridity is discounted as a primary cause because P. goliah evolved in response to increased aridity and disappeared during an interval wetter than many it survived earlier. Hunting by humans, who were also bound to water, may have been a more decisive factor in the extinction of this giant marsupial.