DOI: 10.25921/jzj0-d148
title: NOAA/WDS Paleoclimatology - Namibia 5000 Year Rock Hyrax Midden Nitrogen Isotope Data

description: Presented here are stable nitrogen isotope data from a rock hyrax (Procavia capensis) middens from northwestern Namibia that record a series of rapid aridification events beginning at ca. 3800 cal yr BP, and which mark a progressive decrease in regional humidity across the Holocene. Strong correlations exist between this record and other terrestrial and marine archives from southern Africa, indicating that the observed pattern of climate change is regionally coherent. Combined, these data indicate hemispheric synchrony in tropical African climate change during the Holocene, with similar trends characterising the termination of the 'African Humid Period' (AHP) in both the northern and southern tropics. These findings run counter to the widely accepted model of direct low-latitude insolation forcing, which requires an antiphase relationship to exist between the hemispheres. The combined dataset highlights: 1) the importance of forcing mechanisms influencing the high northern latitudes in effecting low-latitude climate change in Africa, and 2) the potential importance of solar forcing and variations in the Earth's geomagnetic shield in determining both long-term and rapid centennial-scale climate changes, identifying a possible mechanism for the variations marking the AHP termination in both the southern and northern tropics.
description: Please contact principle investigator Brian Chase to obtain these data. 

This dataset comprises stable nitrogen isotope data from a rock hyrax (Procavia capensis) middens from northwestern 
Namibia that record a series of rapid aridification events beginning at c. 3800 cal yr BP, and which mark a progressive decrease in regional humidity across the Holocene. Strong 
correlations exist between this record and other terrestrial and marine archives from southern Africa, indicating that the observed pattern of climate change is regionally coherent. 
Combined, these data indicate hemispheric synchrony in tropical 
African climate change during the Holocene, with similar trends 
characterising the termination of the 'African Humid Period' (AHP) in both the northern and southern tropics. These findings run counter to the widely accepted model of direct low latitude 
insolation forcing, which requires an anti-phase relationship to exist between the hemispheres. The combined dataset highlights: 
1) the importance of forcing mechanisms influencing the high northern latitudes in effecting low latitude climate change 
in Africa, and 2) the potential importance of solar forcing 
and variations in the Earth's geomagnetic shield in determining 
both long-term and rapid centennial-scale climate changes, identifying a possible mechanism for the variations marking the AHP termination in both the southern and northern tropics. 

Stable isotope data (d15N) from a rock hyrax midden recovered from northwest-facing Austerlitz cave in northwestern Namibia (20°27.83'S, 14°26.54'E) on the southern margin of the limestone karstveld region, south of the Etendeka 
and Kamajab plateaus. 

AMS radiocarbon analyses of the Austerlitz midden indicate that it was deposited between 4538-4825 cal (calibrated) yr BP (UBA 9441) and 1385-1522 cal yr BP (UBA-9436). The distribution of ages as a function of depth shows continuous deposition, 
with no apparent hiatuses. Accumulation rates average ~30 µm yr-1, with slightly higher rates (~48 µm yr-1) at the base of the section. Each 1 mm isotope sample thus integrates approximately 20 – 35 
years of accumulation. 

The d15N values of the Austerlitz midden vary from 12.5 to -19.4‰. From the beginning of the record, there is a clear, progressive enrichment in d15N values, with sharp increases occurring between c. 3500 – 3400 cal yr BP and c. 3000 – 2800 cal yr BP. These trends 
show close similarities with the d15N record from the hyrax middens recovered from Spitzkoppe massif, 160 km to the southwest (Chase et al., 2009), and reveal a coherent, regional-scale midden 
15N signal at this time. 

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