SEA LEVEL CHANGE AND DEFORESTATION ON THE EWA PLAIN OF OAHU DURING POLYNESIAN SETTLEMENT: A CASE FOR COINCIDENCE

Ordy Pond is a shallow closed basin on the leeward coastal plain of Oahu, Hawaii, containing a 9500-year record of rapid and continuous sediment deposition. The sediments forming this beautifully laminated record are almost entirely authigenic and thus primarily a function of local and regional climate history. A time series analysis of the water chemistry has shown the modern pond to be highly eutrophic and nearly entirely anoxic. The sediment laminae appear to be varves resulting from seasonal changes in the water chemistry. Carbonate laminae formed during hot dry seasons when respiration of organic matter is prevalent, while organic laminae formed in the wet seasons during increased productivity.

Previous palynological study found the local area became deforested approximately 930 YR BP, coincident with the arrival of Polynesian settlers. Attempting to link Polynesians to the cause, the authors cited the Polynesian rat, Rattus exulans, as the most likely cause of forest decline. However, our investigation indicates the environment was undergoing significant change 500+ years prior to Polynesian settlement. A 1‰ drying trend in the d¹⁸O_carbonate began as early as 1540 YR BP, perhaps as the climate warmed or the forest began to open allowing increased evaporation. This was followed at 1300 YR BP by a shift in carbonate mineralogy from calcite to aragonite, a clear signal of changing water chemistry. We believe that not only did forest decline begin before the Polynesians arrived, but we believe the decline was due to a ~2 m drop in sea level from the mid-Holocene sea level highstand. The drop in sea level would have lowered the water table, thus stressing the arboreal flora and favoring arid grass species. A steepening of the groundwater slope and a seaward shift in the meteoric/marine groundwater interface could explain the 3‰ d¹⁸O_carbonate freshening, and the increasing return to calcite precipitation from 1030 to 670 YR BP.