A multi-proxy record of past climate from a stalagmite from Bone Cave in northwestern Botswana suggests relatively wet conditions late in the Little Ice Age and progressively drier conditions in the twentieth century. The proxies include surfaces of dissolution, variation in layer-specific width, changing mineralogy, varying abundance of detrital material, varying growth rate, and oxygen and carbon isotope data. They combine to suggest a period of exceptional wetness that can be assigned with greatest certainty to ~1810–1820 CE and may have been wettest in 1817. The same proxy data conversely suggest relatively dry conditions after about 1835, and increasing dryness after about 1930. Bone Cave lies in the summer rainfall zone, at the southern margin of the tropical rain belt associated with the Inter-Tropical Convergence Zone (ITCZ). Previously published data from other records combine with the Bone Cave record to support wetter conditions in the 1700s and until about 1830, and drier conditions thereafter. The period of exceptional wetness in ~1810–1820 recorded in BC97-14 coincides with a decade of exceptional cold in the Northern Hemisphere, and its acme may be coincident with the Tambora eruption and its resultant “year without a summer” in Europe and North America. These results are thus compatible with movement of the austral-summer ITCZ southward (rather than an increase in rain its customary position) when the Northern Hemisphere cooled relative to the Southern Hemisphere. Conversely, they suggest that the ITCZ has not come as far south as frequently during Earth's warming in the past century.
Bibliographical noteFunding Information:
Financial support for the fieldwork and laboratory analysis was provided by National Geographic Society grants 431-0520 to Callum Ross and Blythe Williams and 7201–02 to Brook and Eugene Marais, and by National Science Foundation grant 0725090 to Brook. We are grateful to the authorities and staff of the National Museum, Monuments and Art Gallery of Botswana for facilitating this research. Permission to work in Botswana was granted by the Office of the President, Republic of Botswana. We thank Callum Ross, Blythe Williams, Diane Waddle Stock, and Mohutsiwa Gabadirwe for their considerable help in the field.
- Bone Cave
- Global warming