Evidence of past climate change in the Little Ice Age and of the Pacific Decadal Oscillation periodicity in Africa: comparison between two neighbor stalagmites from Namibia and Botswana (preliminary results)

Stalagmites are potential sources of paleoenvironmental records because their characteristics respond to climate change. In this preliminary study we compare isotopic values resolvable at annual to bi-annual scale from two stalagmites from caves in Proterozoic dolomitic marbles, as a proxy for the past climate variability in Southern Africa. Stalagmite DP1 is from Dante Cave in the Otjozondjupa region of Namibia, and Stalagmite BC97-14 is from Bone Cave, in the Koanaka Hills of Northwest Botswana, ~125km west of the Okavango Delta. Dante and Bone Cave receive respectively ~532 and ~475mm annual rainfall with an annual average temperature of 21.0°C and 22.6°C. Both caves are in semi-arid areas with highly seasonal rainfall. Vegetation is distinctive for each location: deciduous woodland savanna with a relatively high diversity of scattered tall trees on the hill in which Dante is located and a sparser tree cover on the hillsite of Bone Cave area. The data from the two stalagmites are very different, reflecting the distinctive hydrological and vegetation settings of the caves where they formed. δ18O in BC97-14 is highly variable, ranging from -11.6 to -5.8 ‰, while δ13C is nearly constant (-10.5 to -8.5 ‰). In contrast, δ13C and δ18O are covariant in DP1, with much higher δ13C values (-10.8 to -5.1‰) and much lower δ18O (-11.7 to -6.5 ‰) compared to that of Bone Cave. The distance from the water vapor source (Indian Ocean) might explain the generally lower δ18O values from the Namibian stalagmite compared to that of Botswana; while the difference in vegetation cover reflects the difference between δ13C for the two areas. Regardless of this isotopic difference, the stalagmite data for Bone Cave show a marked 15- to 22-year periodicity that is much more subdued in the Namibia speleothem. This cycle matches with the 17- or 18-year periodicity in the Botswana historical rainfall record. The periodicity is very well pronounced between AD 1825 to 1925, when abrupt wet seasons follow drier ones every 30 to 40 years. We propose this cycle would match with the Pacific Decadal Oscillation, a pattern of change (warm or cool surface waters) in the Pacific Ocean's climate that occurs every 20 to 30 years. Furthermore, a major climatic shift around the 1600s is also apparent, when both δ18O and δ13C in the Dante Cave stalagmite decrease by ~ 3-4 ‰, indicating a dramatic change from dry to wet conditions. This change is followed by a shift to a drier climate from 1650 to 1950 for both DP1 and BC97-94. The abrupt change in isotopic data is coeval with the transition from the Medieval Climate Anomaly (prior to AD 1600) to the Little Ice Age (~AD 1350-1850), and possibly the onset of the Maunder Sunspot Minimum (1645-1715). The stalagmite data show that major Late Holocene climate events recorded in the Northern Hemisphere also impacted the Southern Hemisphere, including Southern Africa. The rapid changes in isotopic values are also mirrored by changes in stalagmite petrography. Crystals of calcite with a distinctly brownish/yellowish color in DP1 samples reflect the wet period in Namibia, and a Type E (erosional) layer-bounding surface in BC97-14 marks the onset of the very wet period in Botswana.