oa Botswana Notes & Records - Annual growth layers in a stalagmite from Drotsky's Cave, Ngamiland: relationships between layer thickness and precipitation

Volume 24, Issue 1
  • ISSN : 0525-5090



A half metre high stalagmite from Drotsky's Cave in Botswana has more than 1 200 annual growth layers. Most of the layers consist of calcite overlain by a thin deposit of aragonite. It seems likely that calcite is deposited during the summer rainy season when there is an abundant supply of water to the stalagmite and that the overlying aragonite is laid down during the early part of the following dry season as the supply of dripwater to the stalagmite slows and possibly stops. Preliminary studies indicate that layer thickness in the most recent 100 layers varies with the amount of precipitation at Shakawe, 150km from Drotsky's Cave. In fact, three-year mean precipitation at Shakawe for the period 1936/37 to 1983/84 explained 30% of the variation in three-year mean layer thickness, the regression being significant at the 0,002 confidence level. Peaks and troughs in the record of stalagmite layer thickness are each separated by an average of 21-22 years, a period almost identical to the 22-year sunspot cycle and close to the 18-year cycle identified in southern African precipitation. When the stalagmite has been analyzed fully, it could provide the longest proxy record of precipitation in southern Africa, giving information to at least 800 AD.

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