Open University Uranium-SeriesLaboratory    
Earth and Environmental Sciences, The Open University, Milton Keynes, UK    

Mechara Stalagmites in Ethiopia, high-resolution dating (Dr A Baker Newcastle/Birmingham IP/786/0902)
 

The study of calcareous speleothems as archives of climate change is a major area of Quaternary paleoclimate research (see reviews in: Lauritzen and Lundberg, 1999; McGarry and Baker, 2000; Lauritzen, 2003; McGarry and Caseldine, 2004; Harmon et al., 2004; McDermott, 2004; McDermott et al., 2005; Fairchild et al., 2005; Fairchild et al., 2006). All these reviews agree that many of the variables that can be measured in speleothem may serve as climate proxies. However, interpretation of reliable paleoclimate signals from speleothems (especially stalagmites) depends on many factors, including the type and diversity of climate proxies being investigated. In addition to investigating new proxies, multi-proxy study of stalagmites is considered to be crucial (Fairchild et al., 2005; McDermott et al., 2005).

 

The Mechara Karst area, in southeastern Ethiopia was chosen with the aim to improve our understanding of the relationships between the modern cave climatology, from actively growing stalagmites, straw stalactites, and modern drip waters, in order to help interpret older stalagmite records (Baker et al., submitted). A high resolution multi-proxy record from a Mid-Holocene stalagmite was obtained. This annually laminated stalagmite grew for 443 ± 8 years starting at 5023 y BP (± 160 y) as determined by lamina-tuned U-Th dating. The speleothem shows three growth phases: (1) an initial phase where the stalagmite has a candlestick shape, deposited by relatively slow-rate drip water (2) a middle growth phase where laminae grow on the flanks, suggesting faster drip rates and (3) a final growth phase where the speleothem becomes narrower due to a decrease in drip water supply towards the end of deposition (see Figure). Morphometry, annual growth rate, fluorescence index, and 234U/238U isotope ratio show marked differences between the growth phases, while δ13C and δ18O do not show significant variation, except within the third phase towards the top of the stalagmite, where they have higher values. The study indicates that a multi-proxy approach is crucial as the different proxies respond to single climate forcing in different manners. Our methods can be widely applied to speleothem studies to distinguish the relative importance of the various proxies in recording cave ‘external’ (climate, hydrology) and cave ‘internal’ (e.g., evaporation processes). The results complement Late Quaternary climate studies in the region mainly recorded from lake archives (Mohammed et al., 2004). Understanding of natural climate variability is of crucial interest in this region where the frequency of droughts has a strong influence on the sustainability of human society.

 

 

 
 
 

left: Ach-1 stalagmite image;

right: schematic view of the stalagmite where sample points for dating and isotope analysis are marked. Three growth layers and their boundaries along with some traces of visible lamina are also shown. Horizontal dashed lines mark the boundaries of growth phases, From Asrat et al., 2006

 
   

Asrat et al. 2005 Journal of Quaternary Science 21(0) 53-63
Baker et al. 2005 Speleology in press
Fairchild et al. 2006 Earth Science Reviews 75 105-153
Fairchild et al. 2007 in: Geochemical sediments and landscapes. Nash and McLaren eds. Blackwells in press
Harmon et al. 2004 in: Studies in cave sediments. Sasowsky et al. eds. Kluwer Academic 199–226
Lauritzen 2003 in: Global change in the Holocene. Mackay et al. eds. Hodder Arnold 242–263
Lauritzen and Lundberg 1999 The Holocene 9 643–647
McDermott 2004 Quaternary Science Reviews 23 901–918
McDermott et al. 2005 in: Isotopes in paleoenvironmental research. Leng ed. Springer Verlag 185-226
McGarry and Baker 2000 Quaternary Science Reviews 19 1087–1101
McGarry and Caseldine 2004 Quaternary Science Reviews 23 2389–2404
Mohammed et al. 2004 in: Past climate variability through Europe and Africa. Battarbee ed. Springer Verlag 159–180
 
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© Peter van Calsteren
Last updated: 23 December, 2011 10:57
 
December 23, 2011 10:57December 23, 2011 10:57