Search

Explaining the Late Pleistocene demise of many of the world's larger terrestrial vertebrates is arguably the most enduring and debated topic in Quaternary science. Australia lost >90% of its larger species by around 40 thousand years (ka) ago, but the relative importance of human impacts and increased aridity remains unclear. Resolving the debate has been hampered by a lack of sites spanning the last glacial cycle. Here we report on an exceptional faunal succession from Tight Entrance Cave, southwestern Australia, which shows persistence of a diverse mammal community for at least 100 ka leading up to the earliest regional evidence of humans at 49 ka. Within 10 millennia, all larger mammals except the gray kangaroo and thylacine are lost from the regional record. Stable-isotope, charcoal, and small-mammal records reveal evidence of environmental change from 70 ka, but the extinctions occurred well in advance of the most extreme climatic phase. We conclude that the arrival of humans was probably decisive in the southwestern Australian extinctions, but that changes in climate and fire activity may have played facilitating roles. One-factor explanations for the Pleistocene extinctions in Australia are likely oversimplistic.

Abstract Maritime Continent (MC) convection drives zonal and meridional climate modes, including the Australasian monsoon system, but its role in past global climate change remains uncertain. Here we use speleothem oxygen isotope (δ 18 O) records from a latitudinal transect across the MC, and a new critically located record for southwest Sulawesi, Indonesia, to reconstruct spatial hydroclimate variability in the Australasian monsoon domain over the last 40,000 yr. Our results show that atmospheric convection in the core MC region was reduced from ∼40,000 to 12,000 yr ago and strengthened rapidly only after the inundation of shallow continental shelves in the region, well after the onset of deglaciation. Interestingly, the millennial-scale climatic impacts of North Atlantic Heinrich events are not evident in Sulawesi. Together with climate model simulations, we demonstrate that changes in deep atmospheric convection dominate glacial–interglacial hydroclimate over the MC, whereas latitudinal shifts in the mean location of the InterTropical Convergence Zone control rainfall patterns during abrupt climate changes. Highlights • We present new speleothem results from Sulawesi, Indonesia for the past 40,000 yr. • Atmospheric convection over the Maritime Continent was reduced from ∼40 to 12 kyr. • Convection strengthened following the inundation of shallow continental shelves. • Convection dominates glacial–interglacial hydroclimate over the Maritime Continent. • Latitudinal shifts in the ITCZ control rainfall patterns during abrupt climate events. [ABSTRACT FROM AUTHOR]

Abstract: Long, continuous records of Late Quaternary environmental change are rare in Southeast Asia, yet they are crucial for understanding the nature of early human dispersal and occupation in the Australasian region. We present a new record of palaeomonsoon activity extending back to 35,000 BP (years before the present), based on the analysis of oxygen isotope ratios (δ18O) in the freshwater bivalve Margaritanopsis laosensis excavated from the Tham Lod and Ban Rai rockshelters in Mae Hong Son Province, northwest Thailand. Long-term changes in the M. laosensis δ18O record reflect changes in the δ18O of the river water in which these organisms grew, and correlate well with changes in speleothem δ18O records of east Asian monsoon rainfall from Hulu Cave and Dongge Cave in China. The new northwest Thailand δ18O sequence indicates wetter and relatively unstable climatic conditions from 35,000 to 20,000 BP, followed by drier conditions from 20,000 to 11,500 BP. A period of peak aridity occurred around 15,600 BP during Heinrich Event 1, suggesting that the intertropical convergence zone shifted southward when the North Atlantic region cooled. However, there is little evidence for the Younger Dryas event at ∼12,800–11,500 BP. After 9,800 BP, precipitation increased substantially and climatic variability declined. Our findings provide an improved baseline against which to gauge interactions between early humans and climate change in Southeast Asia. For example, there was no significant change in the prehistoric flake stone technology used at Tham Lod and Ban Rai despite the bivalve δ18O evidence for substantial climate change in the region. Also, the climatic impact of the Younger Dryas event appears to have been less intense in northwest Thailand compared to the cooling and drying observed in China, and may explain why agriculture made a relatively late appearance in Thailand, possibly involving migrants from China. [Copyright &y& Elsevier]

Abstract: Uranium-series dating of oxygen and carbon isotope records for stalagmite SJ3 collected in Songjia Cave, central China, shows significant variation in past climate and environment during the period 20–10 ka. Stalagmite SJ3 is located more than 1000 km inland of the coastal Hulu Cave in East China and more than 700 km north of the Dongge Cave in Southwest China and, despite minor differences, displays a clear first-order similarity with the Hulu and Dongge records. The coldest climatic phase since the Last Glacial Maximum, which is associated with the Heinrich Event 1 in the North Atlantic region, was clearly recorded in SJ3 between 17.6 and 14.5 ka, in good agreement in timing, duration and extent with the records from Hulu and Dongge caves and the Greenland ice core. The results indicate that there have been synchronous and significant climatic changes across monsoonal China and strong teleconnections between the North Atlantic and East Asia regions during the period 20–10 ka. This is much different from the Holocene Optimum which shows a time shift of more than several thousands years from southeast coastal to inland China. It is likely that temperature change at northern high latitudes during glacial periods exerts stronger influence on the Asian summer monsoon relative to insolation and appears to be capable of perturbing large-scale atmospheric/oceanic circulation patterns in the Northern Hemisphere and thus monsoonal rainfall and paleovegetation in East Asia. Climatic signals in the North Atlantic region propagate rapidly to East Asia during glacial periods by influencing the winter land–sea temperature contrast in the East Asian monsoon region. [Copyright &y& Elsevier]

Recent research has revealed new insights into the temperature, size, and variability of the Indo-Pacific Warm Pool (IPWP) and the nature of the El Nin˜o-Southern Oscillation (ENSO) since the Last Glacial Maximum (LGM). Sea surface temperature (SST) reconstructions from foraminiferal Mg/Ca, alkenone, and revised coral Sr/Ca palaeothermometry agree that SSTs in the IPWP during the LGM were ∼3°C cooler than at present. In the central portion of the IPWP, the rapid post-glacial rise in SST led the deglaciation by ∼3000 years to produce near-modern SSTs by the early Holocene. In contrast, further west and north, post-glacial shifts in SSTs in the South China and Sulu Seas are synchronous with abrupt climate changes in the North Atlantic. New evidence for the nature of the Little Ice Age in the tropics has been obtained from a 420-year record of coral Sr/Ca and δ18O from the Great Barrier Reef, Australia. This indicates that SSTs and salinity were higher in the 18th century than in the 20th century. The results suggest that the tropical Pacific played a role as a source region of water vapour during the global expansion of Little Ice Age glaciers. The onset of modern ENSO periodicities is identified by palaeo-ENSO records throughout the tropical Pacific region ∼5000 years ago, with an abrupt increase in ENSO magnitude ∼3000 years ago. Individual ENSO events recorded by corals reveal that the precipitation response to El Nin˜o temperature anomalies was subdued in the mid-Holocene. The apparent non-linear onset of ENSO in the late Holocene appears to reflect abruptly enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone. Comparisons of precipitation variability recorded by Great Barrier Reef corals with ENSO indices for the last 350 years confirms that non-stationarity of ENSO teleconnections is a natural characteristic of modern climate. [Copyright &y& Elsevier]

During the Younger Dryas event, about 12,000 years ago, the Northern Hemisphere cooled by between 2 and 10 °C (refs 1, 2) whereas East Antarctica experienced warming. But the spatial signature of the event in the southern mid-latitudes and tropics is less well known, as records are sparse and inconclusive. Here we present high-resolution analyses of skeletal Sr/Ca and 18O/16O ratios for a giant fossil Diploastrea heliopora coral that was preserved in growth position on the raised reef terraces of Espiritu Santo Island, Vanuatu, in the southwestern tropical Pacific Ocean. Our data indicate that sea surface temperatures in Vanuatu were on average 4.5 ± 1.3 °C cooler during the Younger Dryas event than today, with a significant interdecadal modulation. The amplified annual cycle of sea surface temperatures, relative to today, indicates that cooling was caused by the compression of tropical waters towards the Equator. The positive correlation in our record between the oxygen isotope ratios of sea water and sea surface temperatures suggests that the South Pacific convergence zone, which brings O-depleted precipitation to the area today, was not active during the Younger Dryas period. [ABSTRACT FROM AUTHOR]

Abstract: The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between ∼5500 and 4300 years BP (∼1.2°C±0.3°C) and were similarly cool before ∼6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between ∼6600 and 6300 years BP (∼1.3°C±0.3°C), while similarly warm conditions may have also existed after ∼4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesting that the Asian monsoon–IOD interaction that exists at interannual time scales also persists over centennial to millennial scales. Associated mean changes in the Pacific ENSO modes may have also occurred during the mid-Holocene. The dynamic and inter-connected behaviour of the IPWP with tropical climate systems during the mid-Holocene highlights the fundamental importance of the warm pool region for understanding climate change throughout the tropics and beyond. [Copyright &y& Elsevier]

Throughout the Quaternary, the flora and fauna of Australia evolved and adapted to the high-amplitude, low- and high-frequency climate changes that characterized the ice-age cycles. However, during the last glacial cycle, between ∼120 and 15 ka, unprecedented irreversible changes in flora and fauna occurred, and in that same interval modern humans established their first firm presence in the landscape. Disentangling the impacts of the first-order trend toward a colder, drier planet through the Late Quaternary from the impacts of human colonization has been challenging, from both the chronological and paleoenvironmental perspectives. We utilize the stable isotopes of carbon and oxygen preserved in near-continuous time series of Dromaius (emu) eggshell from five regions across Australia to provide independent reconstructions of ecosystem status and climate over the past 100 ka. Carbon isotopes are determined by the diet consumed by the female bird, whereas oxygen isotopes record the status of local moisture balance in the months prior to breeding. Together, δ 13 C and δ 18 O provide ecosystem status and climate from the same dated sample, reducing correlation uncertainties between proxies. Combined with recent improvements in the chronologies of Late Quaternary shorelines fringing inland lake basins and deflation during arid times, these data collectively reaffirm that Australia generally became increasingly, albeit irregularly, drier from the last interglaciation through to the last glacial maximum. Dromaius eggshell δ 18 O documents peak aridity between 30 and 15 ka, but shows no evidence of exceptional climate change between 60 and 40 ka. In contrast, Dromaius δ 13 C diet documents an irreversible loss of the majority of palatable summer-rainfall-related C4 grasses across the Australian arid zone between 50 and 45 ka, about the same time that the giant megafaunal bird, Genyornis , became extinct, and coincident with human dispersal across the continent. Our data indicate that changes unique to Australia occurred between 50 and 45 ka that led to a new climate-vegetation relationship and an overall reduction in effective moisture across much of the continent. The large summer-rainfall-dominated lakes of interior Australia failed to re-fill subsequently, despite a wide range of global climate states. A full explanation for the mechanisms behind these changes remains elusive, but they are almost certainly related to human agency. Plausible explanations include a change in fire regime resulting from human-lit fires, a change in fire regime following extinction of megafaunal browsers, and/or a threshold response to increasing aridity. Of these, the climate change explanation is least likely, given the lack of evidence for unprecedented aridity between 60 and 40 ka, and the successful adaptation of Australian ecosystems to 2.5 Ma of similar changes. [ABSTRACT FROM AUTHOR]

Abstract: The tropics are the major source of heat and moisture for the Australasian region. Determining the tropics' response over time to changes in climate forcing mechanisms, such as summer insolation, and the effects of relative sea level on exposed continental shelves during the Last Glacial period, is an ongoing process of re-evaluation. We present a synthesis of climate proxy data from tropical Australasia spanning the last 30,000 years that incorporates deep sea core, coral, speleothem, pollen, charcoal and terrestrial sedimentary records. Today, seasonal variability is governed largely by the annual migration of the inter-tropical convergence zone (ITCZ), influencing this region most strongly during the austral summer. However, the position of the ITCZ has varied through time. Towards the end of Marine Isotope Stage (MIS) 3, conditions were far wetter throughout the region, becoming drier first in the south. Universally cooler land and sea-surface temperature (SST) were characteristic of the Last Glacial Maximum, with drier conditions than previously, although episodic wet periods are noted in the fluvial records of northern Australia. The deglacial period saw warming first in the Coral Sea and then the Indonesian seas, with a pause in this trend around the time of the Antarctic Cold Reversal (c. 14.5 ka), coincident with the flooding of the Sunda Shelf. Wetter conditions occurred first in Indonesia around 17 ka and northern Australia after 14 ka. The early Holocene saw a peak in marine SST to the northwest and northeast of Australia. Modern vegetation was first established on Indonesia, then progressively south and eastward to NE Australia. Flores and the Atherton Tablelands show a dry period around 11.6 ka, steadily becoming wetter through the early Holocene. The mid-late Holocene was punctuated by millennial-scale variability, associated with the El Niño-Southern Oscillation; this is evident in the marine, coral, speleothem and pollen records of the region. [Copyright &y& Elsevier]

Evidence for nearly synchronous climate oscillations during the last deglaciation has been found throughout the Northern Hemisphere but few records are based on independent time scales of calendar years. We present a rare uranium-series dated oxygen–carbon isotope record for a speleothem from Tangshan Cave, China, which demonstrates that abrupt deglacial climatic oscillations from 16 800 to 10 500 yr BP are semi-synchronous with those found in Greenland ice core records. Relatively rapid shifts in speleothem oxygen isotope ratios demonstrate that the intensity of the East Asian monsoon switched in parallel with the abrupt transitions separating the Bølling–Allerød, Younger Dryas, and pre-Boreal climatic reversals. However, the dated isotopic transitions appear to have lasted longer. Our results demonstrate the dominant role of atmospheric teleconnections in the rapid propagation of deglacial climatic signals on a hemispheric scale, and highlight the importance of U-series dated speleothems in the timing and characterization of abrupt climate change. [Copyright &y& Elsevier]