Figure 4. (a, b) Hydrologic balance reconstructed for two sites in tropical Africa using late Quaternary sediments (from Gasse and Van Campo, 1994), and (c) North Atlantic sea-surface density changes reconstructed using marine faunal and isotopic changes (from Duplessy et al., 1992, unit for density defined as sigma = [density -1] x 1,000). The observed paleohydrological changes suggest that dramatic abrupt shifts in hydrologic balance can occur on a spectrum of societal-relevant timescales. Although the deglacial (i.e., 13 to 7ka) shifts have been linked with meltwater discharge events into the North Atlantic, the cause(s) of the Holocene aridity and sea surface density events remain poorly understood.