Examining the Utility of Coral Ba/Ca as a Paleo-Proxy for Interannual River Discharge Variability Along the Pacific Coast of Panamá

Climate along the Pacific coast of Panamá is largely dictated by seasonal N/S shifts in the Intertropical Convergence zone (ITCZ) and the consequent oscillations in precipitation. During the Panamanian wet season (May-Nov.) river discharge (Q) reaches its maximum and serves as a potential source of trace elements, such as Ba, to reefs. Near shore corals can record the waterborne trace metal history in their aragonite skeletons, which can then be exploited as a paleo-proxy for river discharge. We present two high-resolution Ba/Ca records from nearby Porites corals in the Gulf of Chiriquí, Panamá in an effort to better understand the long-term discharge and precipitation history of the region. Both corals record similar annual average Ba/Ca values throughout the time series’ (R=0.55) suggesting that they are faithfully recording water column Ba levels at a large scale. A monthly composite average of both coral Ba/Ca records is positively correlated to an average of all available river discharge data (n= 5) (R=0.42). While instrumental data are relatively sparse and discontinuous, there is a significant relationship between the two variables producing a Ba/Ca-discharge relationship where Q (m³/s)= Ba/Ca(μmol/mol)×49.97(μmol/mol)(m³/s)^-1-190.85. The Ba/Ca peaks correspond to the annual minima in our paired near-monthly resolved coral δ¹⁸O measurements, further supporting that maximum Q in the Gulf is concurrent with the annual salinity minima and precipitation maximum. Coral Ba/Ca in the Gulf of Chiriquí indicates that annual average river Q into the Gulf has varied from 50 to 133 m³/s over from 1966 to 1983. As inferred from our Ba/Ca data, interannual variability of river Q accounts for ~25% of total variance (after removing the seasonal cycle) and a long-term secular trend of increasing river Q accounts for ~30%. Our Porites coral Ba/Ca records from the Pacific side of Panamá provide an opportunity to supplement the limited instrumental river discharge data with coral-based geochemical analyses and to begin the quantification of longer-term variability of river discharge in this sector of the ITCZ.