L. Keith Fifield

Timing of the Last Glacial Maximum from observed sea-level minima

During the Last Glacial Maximum, ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today. But few direct estimates exist of the volume of the ice sheets, or the timing and rates of change during their advance and retreat. Here we analyse four distinct sediment facies in the shallow, tectonically stable Bonaparte Gulf, Australia—each of which is characteristic of a distinct range in sea level—to estimate the maximum volume of land-based ice during the last glaciation and the timing of the initial melting phase. We use faunal assemblages and preservation status of the sediments to distinguish open marine, shallow marine, marginal marine and brackish conditions, and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio-hydro-isostatic modelling. Our results indicate that from at least 22,000 to 19,000 (calendar) years before present, land-based ice volume was at its maximum, exceeding todays grounded ice sheets by 52.5 × 106 km3. A rapid decrease in ice volume by about 10% within a few hundred years terminated the Last Glacial Maximum at 19,000 ± 250 years.

Millennial and orbital variations of El Nino/Southern Oscillation and high-latitude climate in the last glacial period

The El Niño/Southern Oscillation (ENSO) phenomenon is believed to have operated continuously over the last glacial–interglacial cycle. ENSO variability has been suggested to be linked to millennial-scale oscillations in North Atlantic climate during that time, but the proposals disagree on whether increased frequency of El Niño events, the warm phase of ENSO, was linked to North Atlantic warm or cold periods. Here we present a high-resolution record of surface moisture, based on the degree of peat humification and the ratio of sedges to grass, from northern Queensland, Australia, covering the past 45,000 yr. We observe millennial-scale dry periods, indicating periods of frequent El Niño events (summer precipitation declines in El Niño years in northeastern Australia). We find that these dry periods are correlated to the Dansgaard–Oeschger events—millennial-scale warm events in the North Atlantic climate record—although no direct atmospheric connection from the North Atlantic to our site can be invoked. Additionally, we find climatic cycles at a semiprecessional timescale (∼11,900 yr). We suggest that climate variations in the tropical Pacific Ocean on millennial as well as orbital timescales, which determined precipitation in northeastern Australia, also exerted an influence on North Atlantic climate through atmospheric and oceanic teleconnections.

Tectonic uplift, threshold hillslopes, and denudation rates in a developing mountain range

Studies across a broad range of drainage basins have established a positive correlation between mean slope gradient and denudation rates. It has been suggested, however, that this relationship breaks down for catchments where slopes are at their threshold angle of stability because, in such cases, denudation is controlled by the rate of tectonic uplift through the rate of channel incision and frequency of slope failure. This mechanism is evaluated for the San Bernardino Mountains, California, a nascent range that incorporates both threshold hillslopes and remnants of pre-uplift topography. Concentrations of in situ–produced cosmogenic 10 Be in alluvial sediments are used to quantify catchment-wide denudation rates and show a broadly linear relationship with mean slope gradient up to ∼30°: above this value denudation rates vary substantially for similar mean slope gradients. We propose that this decoupling in the slope gradient–denudation rate relationship marks the emergence of threshold topography and coincides with the transition from transport-limited to detachment-limited denudation. The survival in the San Bernardino Mountains of surfaces formed prior to uplift provides information on the topographic evolution of the range, in particular the transition from slope-gradient–dependent rates of denudation to a regime where denudation rates are controlled by rates of tectonic uplift. This type of transition may represent a general model for the denudational response to orogenic uplift and topographic evolution during the early stages of mountain building.

Exposure dating and validation of periglacial weathering limits, northwest Scotland

Periglacial weathering limits on two mountains in northwest Scotland separate zones of contrasting exposure history. Exposure dating of bedrock below the weathering limits gives ages consistent with late Devensian deglaciation, but six out of seven samples from above the weathering limits give minimum exposure ages older than late Devensian ice expansion. These results suggest that mountain summits stood as nunataks above the last ice-sheet surface and rule out formation of weathering limits by periglacial rock breakdown since deglaciation, trimming of frost debris during an ice-sheet readvance, or enhanced weathering prior to climatic warming during ice-sheet downwastage. The dating results do not preclude the possibility that weathering limits mark a former englacial boundary between passive cold-based ice on mountain summits and erosive, warm-based ice at lower elevations, although 26 Al/ 10 Be ratios for high-level bedrock surfaces provide no evidence of prolonged static ice cover.

Australian desert dune fields initiated with Pliocene–Pleistocene global climatic shift

Development of continental aridity has been linked to late Cenozoic global cooling, but the evidence is indirect, based on terrestrial loess deposits and eolian silt in marine sediments, whereas direct dating of the inception of arid landforms has been frustrated by a lack of suitable methods. Here we report the first age determination of a major arid-zone dune field, based on cosmogenic 10 Be and 26 Al measurements of drill cores from dunes in the Simpson Desert, central Australia. Results show that the dune field began to form ca. 1 Ma, whereas dating using quartz optically stimulated luminescence indicates episodic dune building during late Quaternary ice ages. Less intense desertification began earlier; the previous cosmogenic exposure dating showed that neighboring stony deserts began to form at the onset of Quaternary ice ages 2–4 Ma. Aridity deepened and the dune field formed when ice age cycles increased their amplitude and switched their periods from 40 k.y. to 100 k.y. ca. 1 Ma.

Last ice age millennial scale climate changes recorded in Huon Peninsula corals

Uranium series and radiocarbon ages were measured in corals from the uplifted coral terraces of Huon Peninsula (HP), Papua New Guinea, to provide a calibration for the (super 14) C time scale beyond 30 ka (kilo annum). Improved analytical procedures, and quantitative criteria for sample selection, helped discriminate diagenetically altered samples. The base-line of the calibration curve follows the trend of increasing divergence from calendar ages, as established by previous studies. Superimposed on this trend, four well-defined peaks of excess atmospheric radiocarbon were found ranging in magnitude from 100% to 700%, relative to current levels. They are related to episodes of sea-level rise and reef growth at HP. These peaks appear to be synchronous with Heinrich Events and concentrations of ice-rafted debris found in North Atlantic deep-sea cores. Relative timing of sea-level rise and atmospheric (super 14) C excess imply the following sequence of events: An initial sea-level high is followed by a large increase in atmospheric (super 14) C as the sea-level subsides. Over about 1800 years, the atmospheric radiocarbon drops to below present ambient levels. This cycle bears a close resemblance to ice-calving episodes of Dansgaard-Oeschger and Bond cycles and the slow-down or complete interruption of the North Atlantic thermohaline circulation. The increases in the atmospheric (super 14) C levels are attributed to the cessation of the North Atlantic circulation.

Terrestrial vegetation change inferred from n-alkane σ13C analysis in the marine environment

Abstract We report gas chromatography-isotope ratio monitoring-mass spectrometry (GC-IRM-MS) measurements of the δ13C values of individual biomarker compounds (n-alkanes) extracted from a 3 m marine sediment core taken near the mouth of the Johnstone River, North Queensland, Australia. The technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine-derived carbon. The results indicate that there has been a 2% increase in the δ13C values of terrestrially derived n-alkanes (C29−C33) since clearing of the forested Johnstone River drainage basin for sugarcane and pasture began in the late 19th century. A much slower ∼1% increase in δ13C values after 6,000 years BP and prior to European settlement may be related to a decrease in rainfall in the basin, or to an increase in the abundance of C4 plants as a result of increased aboriginal burning. The results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin, and demonstrate that the δ13C values of terrestrially derived n-alkanes in the marine environment can be used to assess basin-wide vegetation changes in adjacent river catchments on geological timescales.

Punctuated eustatic sea-level rise in the early mid-Holocene

Whether eustatic sea-level rise through the Holocene has been punctuated or continuous has remained controversial for almost two decades. Resolving this debate has implications for predicting future responses of remaining ice sheets to climate change and also for understanding the drivers of human settlement and dispersal patterns through prehistory. Here we present a sea-level curve for the past 8900 yr from Singapore, a tectonically stable location remote from ice-loading effects. We also present critical and unique sedimentation rate, organic δ13C, and foraminiferal δ13C proxy records of sea-level change derived from a shallow-marine sediment core from the same area over the same time interval. The sea-level curve, corroborated by the independent proxy records, suggests rapid rise at a rate of 1.8 m/100 yr until 8100 cal (calibrated) yr B.P., a near cessation in the rate of sea-level rise between 7800 and 7400 cal yr B.P., followed by a renewed rise of 4–5 m that was complete by 6500 cal yr B.P. We suggest that this period of relatively stable sea level during the early to mid-Holocene enabled modern deltas to advance, providing a highly productive environment for the establishment of coastal sedentary agriculture. Periods of rapid sea-level rise before and after may have catalyzed significant postglacial episodes of human dispersal in coastal regions.

Late-surviving megafauna in Tasmania, Australia, implicate human involvement in their extinction

Establishing the cause of past extinctions is critical if we are to understand better what might trigger future occurrences and how to prevent them. The mechanisms of continental late Pleistocene megafaunal extinction, however, are still fiercely contested. Potential factors contributing to their demise include climatic change, human impact, or some combination. On the Australian mainland, 90% of the megafauna became extinct by ≈46 thousand years (ka) ago, soon after the first archaeological evidence for human colonization of the continent. Yet, on the neighboring island of Tasmania (which was connected to the mainland when sea levels were lower), megafaunal extinction appears to have taken place before the initial human arrival between 43 and 40 ka, which would seem to exonerate people as a contributing factor in the extirpation of the island megafauna. Age estimates for the last megafauna, however, are poorly constrained. Here, we show, by direct dating of fossil remains and their associated sediments, that some Tasmanian megafauna survived until at least 41 ka (i.e., after their extinction on the Australian mainland) and thus overlapped with humans. Furthermore, a vegetation record for Tasmania spanning the last 130 ka shows that no significant regional climatic or environmental change occurred between 43 and 37 ka, when a land bridge existed between Tasmania and the mainland. Our results are consistent with a model of human-induced extinction for the Tasmanian megafauna, most probably driven by hunting, and they reaffirm the value of islands adjacent to continental landmasses as tests of competing hypotheses for late Quaternary megafaunal extinctions.

Cosmogenic Cl-36 dating of postglacial landsliding at The Storr, Isle of Skye, Scotland

Major postglacial rock slope failures are a common feature of the Scottish Highlands and other mountainous areas that were deglaciated at the end of the Pleistocene, but evaluation of the causes and triggers of failure has been hindered by a lack of reliable dating evidence. We report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at The Storr on the Isle of Skye, Scotland, using 36Cl surface exposure dating. Exposure ages of 6.3 ± 0.7 cal. ka BP and 6.6 ± 0.8 cal. ka BP were obtained for rock samples from two separate landslide blocks, giving an overall age estimate of 6.5 ± 0.5 cal. ka BP for rock slope failure at this site. This date is consistent with AMS radiocarbon dating of windblown sand derived from the failure scarp, and with previous inferences (based on relative dating evidence) concerning an early-Holocene age for most rock slope failures in the Scottish Highlands. The long time lag (. 7 ka) between deglaciation and failure suggests that progressive joint extension and shearing of rock bridges and asperities were of critical importance in conditioning failure, though a seismic trigger cannot be ruled out. The methodology of surface exposure dating in this context is described and its future potential assessed.