Terrestrial Signature in Coral Ba/Ca, δ18O, and δ13C Records From a Macrotide-Dominated Nearshore Reef Environment, Kimberley Region of Northwestern Australia

Abstract

The geochemistry preserved in massive scleractinian corals has long been used as proxies for river runoff, but its reliability in naturally extreme environment (i.e., strong hydrodynamics and intensive thermal stress) has not been tested yet. Using Porites coral collected from the macrotidal nearshore Kimberley region of northwestern Australia, we assess the impacts of river runoff and associated changes in this extreme environment using elemental (Ba/Ca) and isotopic (δ18O and δ13C) compositions at both near monthly and annual resolutions. On the monthly timescales, significant terrestrial signatures were noted in skeletal Ba/Ca, ∆δ18O, and to a lesser extent in δ13C time series, highlighting their linkage to runoff input of Fitzroy River. However, all the geochemical time series as well as the observational coastal sea surface salinity exhibited a consistent ~1- to 2-month lag with river discharge, possibly a manifestation of the sluggish water and sediments exchange in King Sound which are likely induced by strong tidal forcing. On the annual timescales, Ba/Ca follows the variation in river discharge, while freshwater supplied by both runoff and rainfall all contributed to δ18O variations. In contrast, annual δ13C is mainly dominated by the 13C Suess effect, showing a gradually downward trend. Importantly, we find that δ18O and Ba/Ca records exhibit consistent and significant long-term trends, with δ18O being decreasing and Ba/Ca being increasing, coupled with the increased Australian monsoon, indicating that strengthened monsoon precipitation has likely brought more freshwater and sediment loads to the nearshore Kimberley region.