Proxies

 
 

Fossil pollen

Hyrax middens contain rich pollen records that have been proven to provide important information concerning vegetation dynamics during the last 50,000 years (Gil-Romera et al., 2006; Gil-Romera et al., 2007; Scott et al., 2004; Scott and Woodborne, 2007). Beyond the considerable and well-known palaeoecological value of these data, pollen records also provide palaeoclimatic information that may be quantified using a variety of statistical models (Ortega-Rosas et al., 2008; Roeland et al., 1988). As there are few modern analogues for the past vegetation in this region, we will use a probabilistic approach based on the relationship between the modern geographical distribution of plant taxa and spatial climate.

Stable isotopes

Our recent work has shown that hyrax middens also provide an excellent archive for the study of δ13C and δ15N as proxies for long-term environmental change, with the extreme variability seen in modern ecosystem studies being controlled for by the spatial and temporal averaging intrinsic in hyraxes’ wide dietary preferences (as many as 79 species of grass, shrubs, and trees (Hoeck, 1975)), restricted range (usually within 60 m of their shelters (Sale, 1965)), and the relatively long periods of time incorporated into each one mm sample (approximately 10 – 50 years).

With the exception of the Cold Air Cave speleothem record (Holmgren et al., 2003), these stable isotope records from hyrax midden records are now providing the most detailed, highly-resolved palaeoenvironmental records from southern Africa. The clarity of events such as the Younger Dryas (Figure 5) is unsurpassed, and even sub-millennial – decadal-scale events such ast the 2700 BP Event, the Medieval Warm Epoch and the Little Ice Age are clearly visible (Figure 6). Perhaps most interesting, is the dramatic post-Little Ice Age increase in humidity, which may signal regional responses to global warming. Ongoing work is focussing on obtaining higher resolution records for the last millennium, which can be compared to instrumental records of precipitation and temperature to calibrate the stable isotope signals to obtain long-term quantified records of climate change.

Figure 17: Comparison of δ15N (a) and δ13C (b) records from the De Rif hyrax midden from the Cederberg Mountains of southwestern Africa (Chase et al., 2011) with sea-surface temperature estimates from marine sediment core ODP1084B off the coast of Namibia (c; Farmer et al., 2005), the δ18O record from Greenland’s GRIP ice core (d; Dansgaard et al., 1993), Antarctic temperatures as reconstructed from δD variations at Dome C (e; Jouzel et al., 2007). Taken together, these records indicate a broadly uniform climatic signal across the southern tropics during the Holocene (cf. Chase et al., 2010). Shaded areas indicate the Younger Dryas (YD) Antarctic Cold Reversal (ACR) and Heinrich Stadial 1 (HS1).

 

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