NOAA/WDS Paleoclimatology - Southwest Africa Rock Hyrax Midden Isotope Data

The discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central Namib Desert, Africa, is providing unprecedented insight into the region's environmental history. High-resolution stable carbon and nitrogen isotope records spanning 0-11,700 cal (calibrated) yr B.P. indicate phases of relatively humid conditions from 8700-7500, 6900-6700, 5600-4900, and 4200-3500 cal yr B.P., with a period of marked aridity occurring from 3500 until ca. 300 cal yr B.P. Transitions between these phases appear to have occurred very rapidly, often within <200 years. Of particular importance are: (1) the observed relationship between regional aridification and the decline in Northern Hemisphere insolation across the Holocene, and (2) the significance of suborbital scale variations in climate that covary strongly with fluctuations in solar forcing. Together, these elements call for a fundamental reexamination of the role of orbital forcing on tropical African systems, and a reconsideration of what factors drive climate change in the region. The quality and resolution of these data far surpass any other evidence available from the region, and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern Africa. Please contact principle investigator Brian Chase to obtain these data. Stable isotope data (d13C and d15N) from three Rock Hyrax middens on the south-facing flank of Klein Spitzkoppe, western-central Namibia, Southern Africa (21.8317°S, 15.1955°E). The d13C values vary between -23.4 per mil and -17.0 per mil (Figure 2). As a representation of hyrax diet, these variations primarily reflect the proportion of C3 plants (in this environment, generally trees and shrubs) versus C4 plants (generally grasses) with average d13C value of ~-26 per mil and ~-12per mil respectively (Codron et al., 2007; Smith and Epstein, 1971). Observations indicate that hyraxes consume an exceptionally wide range of plants (as many as 79 species of grass, shrubs, and trees), but prefer grasses to the leaves and bark of trees and shrubs (Hoeck, 1975). In xeric savannah regions such as at Spitzkoppe, however, the latter become an important part of their diet during dry periods when grass is scarce (Hoeck, 1975). Considering this, it is expected that during times of less or less regular rains d13C values would become relatively lower as hyraxes come to depend increasingly on the arboreal taxa that can access reserves of groundwater (Walker et al., 1981). Overall, the d13C records obtained from these middens suggest that marked seasonality has been a part of the region's rainfall regime throughout the Holocene, with C3 plants consistently constituting a significant part of the animals' diet. The d15N values vary from 13.4 to 7.4 per mil. The potential of nitrogen stable isotope compositions as a climatic indicator has long been recognised, but studies of modern ecosystems have produced complex data sets, that have proved difficult to interpret (e.g. Heaton, 1987; Heaton et al., 1986). Among herbivores, 15N abundance in animal tissues is strongly influenced by climate, diet and/or physiology (Ambrose and DeNiro, 1986; Heaton et al., 1986). While early studies focused on the possible effects that animal metabolism would have on the signal (Ambrose and DeNiro, 1986), subsequent studies of d15N values in plants across aridity gradients have indicated that it may not be necessary to look towards herbivore metabolism for the link between d15N values and rainfall, as clear correlations are identified between higher d15N values and decreased rainfall in both C3 and C4 plants (Heaton, 1987; Murphy and Bowman, 2006; Swap et al., 2004) and soils (Aranibar et al., 2004). Murphy and Bowman's (2006) spatially-extensive studies of d15N values in both grass and kangaroo bone from across Australia reveal a remarkably consistent relationship between plant and bone d15N signals, suggesting that water availability, through its influence on the isotopic signature of plants/diet, is the primary control on animal d15N with metabolism having no clear effect