Can corals reveal land-use change? Trace element and isotopic records suggest patterns of land-use change in the Republic of Palau.

Trace element and isotopic analyses of coral cores are combined with land-use analysis to test corals as recorders of land-use change in the Republic of Palau. Previous studies have shown increased sedimentation in the Ngeremeduu Bay in short-term periods, which may be correlated to earth-moving projects. This project reconstructs a multi-decadal record of sedimentation to inform land-use management decisions in Palau. Coral cores (20-41cm in length) were sampled along a high-to-low sedimentation gradient: near major rivers (high-impact) and ocean (low-impact). The samples were measured for isotopic indicators of environmental conditions: salinity and temperature (δ18O) and light (δ13C). The δ18O and δ13C values (% VPDB) ranged from -5.0 to -6.0 (δ18O) & -2.5 to -6.0 (δ13C) in high-impact sites and -5.5 to -6.5 & -2.5 to -3.5 in low-impact sites. The results indicate an isotopic transition along the high-to-low sedimentation gradient suggesting increased freshwater in Ngeremeduu Bay. The periods of lowest carbon fixation (correlated with low δ13C values) occur in high-impact sites, suggesting light-depleted conditions. These spatial and temporal variations may correlate with land-use change patterns calculated from a twenty-year record of earth-moving permits. Trace element data, including Barium to Calcium ratio, indicate a peak in sedimentation during 2006 and 2007, with a gradual increase in sediment leaving the bay since 2004. These results correlate to large-scale road construction and corroborate previous findings that Ngeremeduu Bay has reached a tipping point of retaining sediment.