CHANGING PATTERNS IN HOLOCENE PRECIPITATION AS RECORDED BY DOMINICAN CORAL PROXIES

Holocene coral reefs exposed in the Enriquillo Valley, Dominican Republic may record changes in regional precipitation on seasonal, decadal and perhaps millennial scales from ~7 ka to ~5 ka. During the Mid-Holocene, the Enriquillo Valley served as an embayment of the Caribbean Sea. The unique geography of the paleo-Enriquillo Valley ensured the formation of a fringing reef in a marine environment particularly sensitive to changes in local salinity. The stable carbon and oxygen isotopic compositions of Siderastrea sp. and Montastraea sp. corals collected from the valley reveal dramatic variations in local salinity thought to reflect regional precipitation patterns.

High-resolution oxygen isotopic data from Enriquillo corals reflect a decrease in the seasonal amplitude of Holocene precipitation as governed by changes in orbital induced insolation from ~7.2 ka to ~5.3 ka. A decrease in summer precipitation may be attributed to a decrease in the latitudinal extent of the Atlantic Intertropical Convergence Zone (ITCZ). In addition to a strong seasonal mode of variability, oscillations in the geochemical composition of these corals occur with a consistent frequency of 15-20 years. The decadal to multi-decadal mode of variability appears to represent regional fluctuations in tropical storm-related precipitation. Sustained periods of isotopic depletion are attributed primarily to the multi-year influx of rivers during and after the passage of storms. The cyclic post-storm diversion of rivers occurred as alluvial shedding from the surrounding mountains forced the river toward more northerly drainage paths. A mechanism whereby tropical Atlantic sea surface temperature patterns and/or thermohaline circulation mediate Mid-Holocene tropical storm cyclicity is invoked. Enriquillo corals may also record millennial scale reversals in regional SST and/or precipitation during the Mid-Holocene. The oxygen isotopic compositions of Enriquillo corals are interpreted to indicate the onset and termination of a Mid-Holocene wet/warm period at ~7 ka and 5.9-5.3 ka respectively. These dates are in agreement with other proxy records of a Mid-Holocene moist/humid period from the tropical Atlantic and Africa. These millennial scale events are likely forced by complex changes in ocean/atmosphere dynamics.