Inka Meyer

Increase in African dust flux at the onset of commercial agriculture in the Sahel region

The Sahara Desert is the largest source of mineral dust in the world. Emissions of African dust increased sharply in the early 1970s (ref. 2), a change that has been attributed mainly to drought in the Sahara/Sahel region caused by changes in the global distribution of sea surface temperature. The human contribution to land degradation and dust mobilization in this region remains poorly understood, owing to the paucity of data that would allow the identification of long-term trends in desertification. Direct measurements of airborne African dust concentrations only became available in the mid-1960s from a station on Barbados and subsequently from satellite imagery since the late 1970s: they do not cover the onset of commercial agriculture in the Sahel region ∼170 years ago. Here we construct a 3,200-year record of dust deposition off northwest Africa by investigating the chemistry and grain-size distribution of terrigenous sediments deposited at a marine site located directly under the West African dust plume. With the help of our dust record and a proxy record for West African precipitation we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the seventeenth century. At the beginning of the nineteenth century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region. Our findings suggest that human-induced dust emissions from the Sahel region have contributed to the atmospheric dust load for about 200 years.

Grain size control on Sr-Nd isotope provenance studies and impact on paleoclimate reconstructions: An example from deep-sea sediments offshore NW Africa

Five sediment cores from offshore NW Africa were analyzed for strontium and neodymium isotope ratios to reconstruct temporal variations in continental weathering regimes. Sediments were taken from three time slices with well-known and distinctive environmental conditions: present-day (dry and warm), similar to 6 ka (wet and warm), and similar to 12 ka (dry and cold). Terrigenous sediment samples were split into two size fractions to distinguish between the two dominant transport mechanisms offshore NW Africa: fluvial (0-10 mu m) and aeolian (10-40 mu m). Sr isotope data record evidence of marked grain size control with higher isotopic ratios in the fine fraction. In contrast, epsilon(Nd) values are largely unaffected by grain size. Minor variability in Nd isotope data at each sampling site indicates near constant sources of terrigenous matter over the last similar to 12 ka. Variations in Sr isotope ratios are interpreted to reflect major changes in the evaporation-precipitation balance. We suggest that the Sr-Nd isotope data record a latitudinal shift of the northern limit of the African rainbelt and associated wind systems causing changes in the humidity and rate of chemical weathering over NW Africa. While hyperarid conditions prevailed similar to 12 ka, more humid conditions and intensified monsoonal rainfall at similar to 6 ka resulted in greater breakdown of easily weathered K-bearing phases and increased Sr-87/Sr-86 in the detritus. In late Holocene times the monsoonal circulation diminished resulting in a return to arid conditions. Our results clearly show that it is of vital importance in paleoenvironmental studies to carry out isotopic analyses on individual sediment fractions that were carried to the studied deposition site by distinct sediment transport mechanisms. If isotopic analyses are carried out on bulk sediments, the observed variability in isotopic values most likely represents changes in the particle size and mixing proportions of the sediment subpopulations.

The Little Ice Age in Southern South America: Proxy and Model Based Evidence

In this chapter climatic changes over southern South America during the last millennium with focus on the period of the Little Ice Age (LIA 1550–1800 AD) are investigated. Results presented are based on proxy and modelling evidence. Proxy studies include a variety of different sites, ranging from geomorphological, lacustrine, pollen to tree ring reconstructions. These different sources of evidence are combined into a multi-proxy network. Based on this network, spatio-temporal climatic changes in southern South America are assessed for the last millennium. A climate model simulation of the last millennium is used to investigate the influence of external forcing parameters, such as solar, volcanic and greenhouse gases on the local climate in southern South America. To compare proxy-based results and the output of the global climate model on a common basis, conceptual and quantitative downscaling and upscaling models are established. Based on these methodological approaches both reconstruction methods indicate a period of wetter conditions in south-eastern South America during the period of the LIA. Investigating the driving mechanisms for hydrological changes during the LIA, large-scale atmospheric circulation changes of the Southern Hemispheric Westerlies (SHWs) over southern South America are indicated in modelling results. Changes of the SHWs during the LIA also fit into the spatial pattern indicated by different proxies with wetter conditions and cooler temperatures in south-eastern South America accompanied with drier and warmer conditions to the north.