Jan-Hendrik May

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.

Early and Middle Holocene Hunter-Gatherer Occupations in Western Amazonia: The Hidden Shell Middens

We report on previously unknown early archaeological sites in the Bolivian lowlands, demonstrating for the first time early and middle Holocene human presence in western Amazonia. Multidisciplinary research in forest islands situated in seasonally-inundated savannahs has revealed stratified shell middens produced by human foragers as early as 10,000 years ago, making them the oldest archaeological sites in the region. The absence of stone resources and partial burial by recent alluvial sediments has meant that these kinds of deposits have, until now, remained unidentified. We conducted core sampling, archaeological excavations and an interdisciplinary study of the stratigraphy and recovered materials from three shell midden mounds. Based on multiple lines of evidence, including radiocarbon dating, sedimentary proxies (elements, steroids and black carbon), micromorphology and faunal analysis, we demonstrate the anthropogenic origin and antiquity of these sites. In a tropical and geomorphologically active landscape often considered challenging both for early human occupation and for the preservation of hunter-gatherer sites, the newly discovered shell middens provide evidence for early to middle Holocene occupation and illustrate the potential for identifying and interpreting early open-air archaeological sites in western Amazonia. The existence of early hunter-gatherer sites in the Bolivian lowlands sheds new light on the region’s past and offers a new context within which the late Holocene “Earthmovers” of the Llanos de Moxos could have emerged.

Mid- to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin

The scale, spatial variability and implications of pre-Columbian human-induced changes in the Amazon basin are controversial. While some scholars believe that large settlements and complex societies were limited to areas with favourable environmental conditions and human disturbance was localized, others propose that social complexity developed regardless of environmental constraints and opportunities and that human disturbance was widespread. In order to understand the extent to which environmental preconditions influenced the development of pre-Columbian societies, research is needed that integrates both environmental reconstructions and archaeological data. The present study explores past human–environment interactions in the Llanos de Moxos (LM) in the Bolivian Amazon. Combining extensive fieldwork and remote sensing image analysis, we reconstruct mid- to late-Holocene fluvial activity in the southeastern LM and the formation of a sedimentary lobe left by the Grande River. The lobe deposition created the conditions for the development of fertile, drained soils. We also show how pre-Columbian inhabitants adapted to the sedimentary lobe and managed to maximize the area of land suitable for agriculture by building a drainage/irrigation infrastructure. Our results provide an interpretative framework for the diversity of archaeological remains in the LM and suggest that people reached high levels of social complexity as a result of two necessary factors: favourable environmental conditions and human ingenuity.

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.

A geomorphological map of the Quebrada de Purmamarca, Jujuy, NW Argentina

Abstract Please click here to download the map associated with this article. Geomorphological maps summarize a huge quantity of geomorphological information within a spatial context and are thereby a valuable framework for the interpretation of Quaternary landscape evolution at any location. Geomorphological mapping in the Quebrada de Purmamarca (Eastern Cordillera, NW Argentina) integrates information gained from topographical maps, remote sensing imagery and field work. The resulting map provides insights into a dynamic landscape evolution, and documents the interplay between tectonic and climatic controls at various temporal and spatial scales. Several remnants of planation surfaces are interpreted as the products of the long-term uplift and segmentation due to thrusting since the Tertiary. Three terrace levels were mapped and reect complex regional scale cycles of erosion and deposition, possibly in relation to climatically intensi-ed periglacial activity during most of the Quaternary. Finally, the association of smaller scale landforms such as landslides, alluvial fans and colluvial slopes in the study area points to rapid incision followed by several minor climatic fluctuations in Late Quaternary and Holocene times. In conclusion, the presented map serves as a valuable tool for Quaternary research, and provides the base for further analyses of the mapped forms and processes.

Infrared stimulated luminescence dating of 19th century fluvial deposits from the upper Rhine River

Abstract Knowledge of the age of fluvial deposits is an important aspect in the understanding of river dynamics, which is pre-requisite for sustainable river management and restoration back to more natural conditions and processes. Presented here is a case study on using feldspar Infrared Stimulated Luminescence (IRSL) to date low-energy fluvial sediments that formed after correction of the Upper Rhine River in the first half of the 19th century. A rigorous testing programme is carried out to characterise the IRSL properties of the samples, including thermal transfer, dose recovery and fading. All samples reveal complex distributions of equivalent dose, implying the presence of differential bleach-ing in the samples. It is shown that multi-grain aliquots overestimate the known-age by up-to 200 years, i.e. apparent IRSL ages are twice as old as the true age of the sediment. The use of single grains results in ages that are in excellent agreement with the expected age, therefore the age overestimation in multi-grain aliquot measurements is likely explained by signal averaging effects. While the application of single grains appears mandatory for dating young low-energy fluvial deposits, the small absolute offset associated with the multi-grain approach might be acceptable when dating sediments of such type that are older than a few 1000 years.

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.