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

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 (delta O-18 and delta C-13) compositions at both near monthly and annual resolutions. On the monthly timescales, significant terrestrial signatures were noted in skeletal Ba/Ca, increment delta O-18, and to a lesser extent in delta C-13 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 delta O-18 variations. In contrast, annual delta C-13 is mainly dominated by the C-13 Suess effect, showing a gradually downward trend. Importantly, we find that delta O-18 and Ba/Ca records exhibit consistent and significant long-term trends, with delta O-18 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. Plain Language Summary The geochemistry preserved in massive corals living in the coastal and nearshore region can provide reliable records for terrestrial input of freshwater and sediment. This capacity is not compromised even in highly dynamic, naturally extreme reef environment as revealed in this study. Using elemental (Ba/Ca) and isotopic (delta O-18 and delta C-13) compositions of a long-lived Porites coral from the nearshore Kimberley region of northwestern Australia, we evaluated the runoff-related signals in both monthly and annual coral geochemical records, to explore their possibility being freshwater and sediments tracers. The seasonal Ba/Ca peaks are found to follow the large discharge events, and monthly.d18O variation is associated with the changes of coastal salinity driving by the influx of freshwater supplied by Fitzroy River. Terrestrial signal is less manifested in skeletal delta C-13 records. Additionally, a consistent 1- to 2-month temporal lag was found in monthly records, a manifestation of sluggish exchange of water and sediment in this embayment which is likely induced by tidal forcing. The long-term increase in Ba/Ca baseline ratios and decrease in delta O-18 from the 1920s onwar, pointed to an enhanced freshwater and sediment loads into the embayment, possibly linking to the lasting strengthening of Australian monsoon. These findings highlight that corals living in the naturally extreme environment can provide reliable records for freshwater and terrestrial sediments transport to the nearshore region by using skeletal delta O-18 and Ba/Ca.