Do the corals off Molokai,Hawaii preserve a long-term groundwater discharge record?

Understanding long-term trends in coastal groundwater discharge on the island of Molokai, Hawaii, may provide important clues to better understand the nature of exchange across the land/sea interface and the impact of climate change. Human pressure also affects such exchange through changes in withdrawals rates. In response to increased urbanization, demand for coastal groundwater has also risen, as has the potential for coastal groundwater contamination. Coral cores were collected from several shallow sites along the south shore of Molokai and analyzed for a suite of trace elements, including select groundwater tracers. Long-term (1913-2002) stream discharge records from Molokai reveal a downward trend in base flow that imply a decrease in rainfall and coastal groundwater flow. In the Molokai corals, there was a statistically significant downward trend in monthly resolved yttrium and rare earth to calcium ratios over the last several decades. Thus the coral geochemical records appear to respond to changes in groundwater discharge associated with a decrease in base flow since 1913. These findings are further explored by testing naturally occurring radium isotopes as a groundwater tracer and oxygen isotopes as a freshwater tracer in the coral record.