Tim J Cohen

Continental aridification and the vanishing of Australia's megalakes

The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The final disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridification that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water filling these lakes at different times during the last full glacial cycle.

Mind the gap: an absence of valley-fill deposits identifying the Holocene hypsithermal period of enhanced flow regime in southeastern Australia

The Holocene sedimentary record in southeastern Australia is present in a range of landscape settings, such as upland swamps (dells), internally drained lake basins, alluvial fans, and mid-catchment and lowland floodplains. An assessment of the best-constrained basal radiocarbon dates in valley-fill locations between 30 and 42°S and upstream of last-glacial eustatic influences yields an intriguing pattern. The record for fluvial sites with catchment areas > 30 km2 exhibits a distinct gap in the alluvial record between 8 and 4 ka BP (10—4.5 ka). In contrast, data for eleven upland-swamp sites with catchment areas < 50 km2 exhibit a broader spectrum of basal ages, albeit with some reduction of activity during the alluvial gap. We suggest that the period 8—4 ka BP in the sedimentary record at the fluvial sites reflects the early to mid-Holocene climatic optimum independently recognized in proxy climate data in the region. It was a period of enhanced water discharges, stable well-vegetated catchments and low sediment yields, and therefore greatly limited sediment sequestration, and it has been termed the Nambucca Phase. In upland swamps, however, threshold-driven processes produce an episodic landscape responses during much of the Holocene. Contrasting results in upland compared with middle and lower basin locations demonstrate the nonuniform landscape response to climatic changes during the Holocene in southeastern Australia.

A reconstruction of extratropical Indo-Pacific sea-level pressure patterns during the Medieval Climate Anomaly

AbstractSubtropical and extratropical proxy records of wind field, sea level pressure (SLP), temperature and hydrological anomalies from South Africa, Australia/New Zealand, Patagonian South America and Antarctica were used to reconstruct the Indo-Pacific extratropical southern hemisphere sea-level pressure anomaly (SLPa) fields for the Medieval Climate Anomaly (MCA ~700–1350 CE) and transition to the Little Ice Age (LIA 1350–1450 CE). The multivariate array of proxy data were simultaneously evaluated against global climate model output in order to identify climate state analogues that are most consistent with the majority of proxy data. The mean SLP and SLP anomaly patterns derived from these analogues illustrate the evolution of low frequency changes in the extratropics. The Indo-Pacific extratropical mean climate state was dominated by a strong tropical interaction with Antarctica emanating from: (1) the eastern Indian and south-west Pacific regions prior to 1100 CE, then, (2) the eastern Pacific evolving to the central Pacific La Niña-like pattern interacting with a +ve SAM to 1300 CE. A relatively abrupt shift to –ve SAM and the central Pacific El Niño-like pattern occurred at ~1300. A poleward (equatorward) shift in the subtropical ridge occurred during the MCA (MCA–LIA transition). The Hadley Cell expansion in the Australian and Southwest Pacific, region together with the poleward shift of the zonal westerlies is contemporaneous with previously reported Hadley Cell expansion in the North Pacific and Atlantic regions, and suggests that bipolar climate symmetry was a feature of the MCA.

Late-Holocene climatic variability indicated by three natural archives in arid southern Australia:

Three terrestrial climate proxies are used to investigate the evolution of Holocene palaeoenvironments in southern central Australia, all of which present a coherent record of palaeohydrology. Single-grain optically stimulated luminescence from sediments supplemented by 14C from charcoal and lacustrine shells was obtained to date shoreline deposits (Lake Callabonna) and the adjacent Mt Chambers Creek alluvial fan. Our findings are complemented by a U/Th-based record of speleothem growth in the Mt Chambers Creek catchment, which we interpret to reflect increased precipitation. Together, these archives shed light on the timing of, and possible sources of water for, Holocene pluvial intervals. We identified several phases of elevated lake levels dated at ~5.8–5.2, 4.5, 3.5–2.7 and 1 kyr, most of which correspond to fluvial activity resulting from increased precipitation in the adjacent ranges. The enhanced hydrology during phases of the late Holocene likely increased the reliability of resources for regional human populations during a time of reduced winter rainfall. When considered within the framework of the current understanding of Holocene palaeoclimate in central Australia, our data suggest that the pattern of landscape response was broadly synchronous with larger scale climatic variability and punctuated by pluvial periods greater than today.

Channel instability in a forested catchment: a case study from Jones Creek, East Gippsland, Australia

Abstract Jones Creek, a forested sub-catchment of the Genoa River, Victoria, Australia has experienced channel metamorphosis induced by a series of floods since 1971. In the flood of record, in 1971, the Genoa River widened by up to two times at the confluence with Jones Creek. This effectively shortened the course of Jones Creek, resulting in a lagged tributary response. Incision and bed steepening during floods in 1975 and 1978 triggered significant changes in channel form along Jones Creek. Channel changes between 1972–1997 resulted in a four-fold increase in cross-sectional area. Channel depth increased by up to 1.5 m, but has subsequently refilled by around 0.6 m. Initially, incision resulted in increased stream power as a result of increased mean depth, while sinuosity was maintained. This was followed by channel widening, a reduction in sinuosity and a continued increase in slope. Estimated stream power remains high as the channel continues to laterally adjust. This study highlights the dynamic nature of tributary–trunk stream relationships in a cut-and-fill landscape, demonstrating how trunk stream adjustments can induce profound tributary instability in a forested subcatchment. Channel widening of the trunk stream primed the tributary for change. Bedlevel incision and increases in bedslope breached the threshold of landscape stability in this steep alluvial tributary. Three periods of channel change can be identified, reflecting the complex response of the system to channel incision. Rather than developing simple cutoffs, Jones Creek now exhibits a range of lateral adjustment and realignment features. Responses to external disturbance provide insights into ‘natural’ recovery mechanisms in this forested setting. Lateral adjustment and associated sediment deposition have been accompanied by rapid rates of vegetation colonisation and stabilisation of realignment features, effectively reducing the volume of sediment that is available to be reworked through the channel network.

A synoptic climatology of heavy rain events in the Lake Eyre and Lake Frome catchments

The rare occasions when Lake Eyre in central, southern Australia fills with water excite great interest and produce major ecological responses. The filling of other smaller lakes such as Lake Frome, have less impact but can contribute important information about the current and past climates of these arid regions. Here, the dominant synoptic systems responsible for heavy rainfall over the catchments of Lake Eyre and Lake Frome since 1950 are identified and compared. Heavy rain events are defined as those where the mean catchment rainfall for 24 hours reaches a prescribed threshold. There were 25 such daily events at Lake Eyre and 28 in the Lake Frome catchment. The combination of a monsoon trough at mean sea level and a geopotential trough in the mid-troposphere was found to be the synoptic system responsible for the majority of the heavy rain events affecting Lake Eyre and one in five of the events at Lake Frome. Complex fronts where subtropical interactions occurred with Southern Ocean fronts also contributed over 20% of the heavy rainfall events in the Frome catchment. Surface troughs without upper air support were found to be associated with 10% or fewer of events in each catchment, indicating that mean sea level pressure analyses alone do not adequately capture the complexity of the heavy rainfall events. At least 80% of the heavy rain events across both catchments occurred when the Southern Oscillation Index (SOI) was in its positive phase, and for Lake Frome, the SOI exceeded +10 on 60% of occasions, suggesting that the background atmospheric state in the Pacific Ocean was tilted towards La Nina. Hydrological modeling of the catchments suggests that the 12-month running mean of the soil moisture in a sub-surface layer provides a low frequency filter of the precipitation and matches measured lake levels relatively well.

Identifying extreme pluvials in the last millennia using optical dating of single grains of quartz from shorelines on Australia's largest lake

The filling of Kati Thanda-Lake Eyre (KT-LE), Australia’s ‘inland sea’ has captured scientific and cultural interest for over a century and a half. However, despite the presence of multiple shorelines around the modern playa at or near the modern maximum lake-filling levels, no quantitative estimates of major late-Holocene filling events have ever been documented. We develop a preliminary chronological data set using single-grain optically stimulated luminescence (OSL) on lake shoreline samples in order to determine the timing of large lake-filling events (equivalent to 1974 Common Era (CE) as Australia’s wettest year on record) for KT-LE, Australia’s largest lake basin. Despite quartz grains with very low natural dose luminescence (Ln) signal, we derive palaeodoses from geologically recent deposits (decades to centuries) using standard rejection criteria and highlight no signs of partial bleaching but occasional bioturbation in modern deposits. Major modern filling episodes, such as the1974 and 1949/1950 filling events, are successfully captured in the geochronological record, as are two major lake-filling episodes in 1854 ± 21 CE years and 1598–1654 CE. Two additional periods of potential lake-filling events have been identified at 1.2 ± 0.09 and 1.9 ± 0.14 ka, but stratigraphic control on these events is less robust. These chronostratigraphic records, while discontinuous, provide important hydrological evidence for extreme pluvial events akin to 1974 or 1949/1950, and the approach holds promise for identifying climate extremes and landscape response over the late Holocene.