HOLOCENE RECORD OF SEASONALITY IN A TROPICAL POND, OAHU, HAWAII

Ordy Pond is a highly eutrophic, brackish, coastal pond in Oahu, Hawaii. Sediments record long and short term changes in the pond environment over the past ~10,000 years. Major long-term influences on the pond environment include sea level fluctuations, climate and vegetation changes, and introduction of humans to the region 750-1000 years ago. Shorter-term influences include fluctuations in rainfall on seasonal and annual to decadal (ENSO- and PDO-related) time scales, and changes in land use over the past century. Ordy Pond formed when rising sea level flooded Oahu’s Pleistocene carbonate platform roughly 10,000 years ago. Aquatic sediments (the upper 13.5 m of core) have three main compositional components: authigenic carbonate, organic matter, and diatom tests. The lowermost 7m of aquatic sediment are finely laminated couplets of light carbonate-rich layers alternating with dark layers rich in organic matter and diatoms. At a sub-bottom depth of ~ 6.5 m (about 1000 yr BP), laminations give way to thinly banded sediments, and the upper 5 m of core (roughly 0-150 yr BP) are faintly banded, organic-rich, and gelatinous to spongy. Diatoms are dispersed throughout the aquatic sediments. Studies of modern water column processes, and time-series analysis of color spectra in core photographs, indicate the laminae couplets represent seasonal cycles of sedimentation, forced by differences in temperature and rainfall. Inorganic carbonate precipitation is dominant during the hot, dry summer and organic-rich diatomaceous sediments are deposited in response to winter rain events. Several sediment parameters have been calculated as indicators of the degree of seasonality. The difference between % carbonate in light and dark layers should be maximized during time periods of greater seasonal climate differences. Similarly, the difference in silica content of dark vs light layers should reflect the degree of seasonality. Analysis of these parameters indicate that seasonal differences were greater during deposition of the laminated sequence, and several long-term cycles of seasonality are apparent in the sediments dated from 1000 to 10,000 yrs BP. At roughly 1000 yr BP, both indicators of seasonal differences decline, and the past 150 yrs of pond history are characterized by cyclical fluctuations of decreased seasonality.