Extending the aridity record of the Southwest Kalahari: current problems and future perspectives

An extensive luminescence-based chronological framework has allowed the reconstruction of expansions and contractions of the Kalahari Desert over the last 50 ka. However, this chronology is largely based on near-surface pits and sediment exposures. These are the points on the landscape most prone to reactivation and resetting of the luminescence dating ‘clock’. This is proving to be a limiting feature for extending palaeoenvironmental reconstructions further back in time. One way to obviate this is to sample desert marginal areas that only become active during significant arid phases. An alternative is to find and sample deep stratigraphic exposures. The Mamatwan manganese mine at Hotazel in the SW Kalahari meets both these criteria. Luminescence dating of this site shows the upper sedimentary unit to span at least the last 60 ka with tentative age estimates from underlying cemented aeolian units dating back to the last interglacial and beyond. Results from Mamatwan are comparable to new and previously published data from linear dunes in the SW Kalahari but extend back much further. Analysis of the entire data set of luminescence ages for the SW Kalahari brings out important inferences that suggest that different aeolian forms (1) have been active over different time scales in the past, (2) have different sensitivities to environmental changes and (3) have different time scales over which they record and preserve the palaeoenvironmental record. This implies that future optically stimulated luminescence work and palaeoenvironmental reconstructions must consider both site location and its relationship to desert margins and sediment depositional styles, so that the resolution and duration of the aridity record can be optimally understood

Role of marine biology in glacial-interglacial CO2 cycles

It has been hypothesized that changes in the marine biological pump caused a major portion of the glacial reduction of atmospheric carbon dioxide by 80 to 100 parts per million through increased iron fertilization of marine plankton, increased ocean nutrient content or utilization, or shifts in dominant plankton types. We analyze sedimentary records of marine productivity at the peak and the middle of the last glacial cycle and show that neither changes in nutrient utilization in the Southern Ocean nor shifts in plankton dominance explain the CO2 drawdown. Iron fertilization and associated mechanisms can be responsible for no more than half the observed drawdown

Ecophysiological and bioclimatic foundations for a global plant functional classification

Question: What plant properties might define plant functional types (PFTs) for the analysis of global vegetation responses to climate change, and what aspects of the physical environment might be expected to predict the distributions of PFTs? Methods: We review principles to explain the distribution of key plant traits as a function of bioclimatic variables. We focus on those whole-plant and leaf traits that are commonly used to define biomes and PFTs in global maps and models. Results: Raunkiær's plant life forms (underlying most later classifications) describe different adaptive strategies for surviving low temperature or drought, while satisfying requirements for reproduction and growth. Simple conceptual models and published observations are used to quantify the adaptive significance of leaf size for temperature regulation, leaf consistency for maintaining transpiration under drought, and phenology for the optimization of annual carbon balance. A new compilation of experimental data supports the functional definition of tropical, warm-temperate, temperate and boreal phanerophytes based on mechanisms for withstanding low temperature extremes. Chilling requirements are less well quantified, but are a necessary adjunct to cold tolerance. Functional traits generally confer both advantages and restrictions; the existence of trade-offs contributes to the diversity of plants along bioclimatic gradients. Conclusions: Quantitative analysis of plant trait distributions against bioclimatic variables is becoming possible; this opens up new opportunities for PFT classification. A PFT classification based on bioclimatic responses will need to be enhanced by information on traits related to competition, successional dynamics and disturbance

A Tortonian (Late Miocene, 11.61-7.25 Ma) global vegetation reconstruction

For the Tortonian age of the Miocene Epoch (11.6–7.25 Ma) we present a global palaeobotanical and palaeoecologically-based vegetation dataset, combined with a best-fit Late Miocene climate-vegetation model experiment to create an advanced global data–model hybrid biome reconstruction. This new palaeoecological database and global vegetation reconstruction can be used both for the purposes of validating future palaeoclimate model simulations, as well as a land cover dataset to initialise palaeoclimate modelling experiments. Our Tortonian reconstruction shows significant changes in the distribution of vegetation compared to modern natural vegetation. For example in contrast to the modern scenario in the Northern Hemisphere, boreal forests reached 80°N and temperate forests were present above 60°N. Warm–temperate forests covered much of Europe, coastal North America and South-East Asia. Our reconstruction shows a spread of temperate savanna in central USA, the Middle East and on the Tibetan Plateau. Evidence for arid deserts is sparse, with the exception of the Atacama region (South America). Areas that exhibit arid desert today in the Tortonian were instead covered by shrublands, grasslands, savannas and woodlands. The extent of tropical forests in South America was likely reduced but expanded in the Indian sub-continent and East Africa. This pattern of global vegetation in the Late Miocene suggests a warmer and wetter world, which is supported by the pattern of climate anomalies predicted by our best-fit palaeoclimate-vegetation model experiment. Global mean annual temperature may have been as much as 4.5 °C higher than present day with many regions experiencing higher than modern amounts of precipitation over the annual cycle. The pattern of temperature and precipitation change reconstructed palaeobotanically, and predicted within our climate model experiment, infers a global forcing agent on Tortonian climate (e.g. such as elevated concentrations of greenhouse gases) to explain the observed and modelled climate anomalies. This is in contrast to current proxy records of Tortonian atmospheric CO2 which range from Last Glacial Maximum to mid-20th Century levels

Global vegetation dynamics and latitudinal temperature gradients during the Mid to Late Miocene (15.97–5.33 Ma)

A 617 site palaeobotanical dataset for the Mid to Late Miocene is presented. This dataset is internally consistent and provides a comprehensive overview of vegetational change from 15.97 to 5.33 Ma. The palaeobotanical dataset has been translated into the BIOME4 classification scheme to enable direct comparison with climate model outputs. The vegetation change throughout the Langhian, Seravallian, Tortonian and Messinian is discussed. The data shows that the Langhian, which includes the end of the Mid-Miocene Climatic Optimum, represents a world significantly warmer than today. The high northern latitudes were characterised by cool-temperate forests, the mid-latitudes by warm-temperate mixed forests, the tropics by tropical evergreen broadleaf forests and Antarctica by tundra shrub vegetation. Cool-temperate mixed forest existed during the Seravallian in the high northern latitudes, a reduction in warm-temperate mixed forests in the mid latitudes and a loss of tundra on Antarctica was noted. Tortonian vegetation distribution indicates that further cooling had occurred since the Seravallian. The major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm-temperate mixed forests. By the Messinian, this cooling trend had eliminated warm-temperate mixed forests from the western USA and Australia and had formed mid-latitude deserts. Despite the cooling trend throughout the Mid to Late Miocene, the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre-industrial conditions. The latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the Langhian would have been significantly shallower than at present and has gradually, but asymmetrically, become more modern towards the end of the Miocene. First the southern hemisphere distribution of bioclimatic zones became more modern, probably due to the climatic effects of a fully glaciated Antarctica. The northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the Miocene and slowly became more modern by the Messinian