Environmental variability in East African coral records: Links to human activity and the global tropics
by
Julia Cole
Department of Geosciences, University of Arizona, Tucson AZ 85721 USA

Environmental variability in tropical Africa results from changes in both physical climate and human activities. One of the few sources of annually resolved paleoenvironmental information in tropical Africa is the record preserved in reef corals, which are abundant (but threatened) on the east coast of the continent. New and published records from East African corals spanning the past 2 centuries reveal how regional environmental variability has responded to changes in climate and land use. We find that these factors interact to increase stresses on the reef environment. Oxygen isotopic data from several coral cores show a clear pattern of warming temperatures through the 20th century, with variability related to ENSO superimposed on interannual-decadal time scales. The temperatures during the most recent El Niño of 1997-8 were exceptionally warm; massive bleaching and mortality occurred at this time across the Western Indian Ocean. The ratio of Ba/Ca in the coral skeleton reflects the amount of terrestrial material transported to the reef environment by local rivers, i.e. soil erosion. A 200-year Ba/Ca record suggests a sudden increase in soil erosion in the early 20th century, concurrent with historical records of land degradation associated with colonial agriculture. Prior to 1900, variability in Ba/Ca is small and the mean is low; after 1930, the mean increases by ~35% and variance increases nearly threefold. In the latter interval, interannual variability in Ba/Ca corresponds with ENSO; El Niño years bring intense rains in October-November that are effective erosional agents on denuded soil. In the earlier interval, there is no relationship between Ba/Ca and El Niño, presumably because greater vegetation cover protected the soil against erosion by heavy rains. Thus 20th-century changes in land use in East Africa have enhanced the impact of ENSO on both soil erosion and on sedimentation on the reef. The effect of ENSO on the reef now includes two potentially significant stressors: warmer temperatures and enhanced sedimentation. These are only a small subset of the individual stresses on east African reef environments that may act synergistically to worsen reef health in the coming decades.

Date received: June 8, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahr-31.