NOAA/WDS Paleoclimatology - South Africa Deglacial Hyrax Midden Stable Isotope Data

STUDY NOTES: Please contact principle investigator Brian Chase to obtain these data. Here we present stable carbon and nitrogen isotope data obtained from a rock hyrax (Procavia capensis) midden recovered from the Cederberg Mountains of South Africa's Western Cape (32.446°S, 19.221°E). Located in the core of southern Africa's winter rainfall zone (sensu Chase and Meadows (2007)), the site presently receives c. 380 mm yr-1 of rainfall with >80% falling between April and October. This marked seasonality is a product of the annual expansions and migrations of westerly storm tracks and associated frontal systems. Each winter these systems bring rain to southwestern Africa and each summer, as they contract poleward, their influence is replaced by the southward displacement of the South Atlantic Anticyclone and the development of coastal upwelling cells. The behaviour and influence of these systems on terrestrial environments over centennial to multi-millennial timescales, however, remains largely unresolved. Accelerator mass spectrometry radiocarbon analyses of 10 samples from a 400 mm section of the De Rif midden indicate that it was deposited between ca. 19,500 and 7300 cal yr B.P., spanning the entire Last Glacial-Interglacial Transition (LGIT). Further, the distribution of ages as a function of depth shows that accumulation throughout this period was broadly continuous, with no hiatuses indicated. Upper (0-70 mm) and lower (235-400 mm) sections are composed primarily of urine and accumulated at a rate of ~20–30 um yr-1. Separating these sections is a 165-mm-thick layer that contains a greater admixture of fecal pellets, which increased the rate of deposition to ~94 um yr-1. Samples for stable isotope analysis were drilled by hand with a 1-mm-diameter drill bit with an average sample interval of 1.5 mm. The d15N values vary from 5.3‰ to -2.3‰. Among herbivores, such 15N abundance in animal tissues is influenced by climate, diet, and/or physiology (Ambrose and DeNiro, 1986; Heaton et al., 1986). While many studies have focused on the possible effects of animal metabolism on the signal (Ambrose and DeNiro, 1986), studies of d15N in plants across aridity gradients indicate clear correlations between enriched d15N and decreased rainfall (Murphy and Bowman, 2006; Schwarcz et al., 1999), suggesting that metabolism per se may have negligible or relatively minor influence on the signal. In particular, spatially extensive studies of d15N in both grass and kangaroo bone from across Australia reveal a strong, consistent relationship between plant and bone d15N values, suggesting that water availability, through its influence on the isotopic signature of consumed vegetation, is the primary control on animal d15N, with metabolism having no clear effect (Murphy and Bowman, 2006). These findings are supported by stable isotope records obtained from hyrax middens in Namibia, which show strong similarities between variations in d15N and a range of paleoenvironmental proxies reflecting changes in precipitation over multimillennial time scales (Chase et al., 2009). The d13C values from the midden vary between -28.3‰ and -26.9‰. As a reflection of hyrax diet, these values indicate that the site has hosted a predominantly to purely C3 vegetation throughout the recorded period. This ecosystem stability over time provides a unique opportunity for variations in d13C to be used as a proxy for climate rather than vegetation. Although variations in d13C in middens are often interpreted in terms of changing proportions of C3 versus C4 plants in the landscape (Scott and Vogel, 2000), in ecosystems supporting only C3 plants, variations in d13C are primarily a function of leaf-level changes in water-use efficiency (Ehleringer and Cooper, 1988). Thus, the d13C variations in the De Rif hyrax midden are interpreted as primarily reflecting changes in effective precipitation. This interpretation is supported by the strong similarities that are evident between the d13C and d15N records (and argues against a significant influence of long-term changes in atmospheric CO2; Arens et al., 2000), providing mutual validation that climate is the primary determinant of the observed signals
ABSTRACT SUPPLIED BY ORIGINATOR: Our ability to identify the timing and extent of past major climate fluctuations is central to understanding changes in the global climate system. Of the events that have occurred in recent geological time, the Younger Dryas (YD, 13-11.5 ka), an abrupt return to near-glacial conditions during the last glacial-interglacial transition (ca. 18-11.5 ka), is one of the most widely reported. While this event is apparent throughout the Northern Hemisphere (Peteet, 1995), evidence for its occurrence in the Southern Hemisphere remains equivocal due to a lack of well-dated terrestrial records. Here we report high-resolution stable carbon and nitrogen isotope records obtained from a rock hyrax midden, revealing the first unequivocal terrestrial manifestation of the YD from the southern African subtropics. These results provide key evidence for the relative influence of the YD, and suggest that a subtropical-temperate transition zone existed along the oceanic Subtropical Front (~41°S) across the Southern Hemisphere, with the Northern Hemisphere exerting a strong influence on all but the higher latitudes of the Southern Hemisphere after the Heinrich Stadial 1 (15 ka).