Terrestrial Signature in Coral Ba/Ca, δ<sup>18</sup>O, and δ<sup>13</sup>C Records From a Macrotide‐Dominated Nearshore Reef Environment, Kimberley Region of Northwestern Australia
Xuefei Chen, Wenfeng Deng, Gangjian Wei, Malcolm T. McCulloch
Abstract
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 (δ 18 O and δ 13 C) compositions at both near monthly and annual resolutions. On the monthly timescales, significant terrestrial signatures were noted in skeletal Ba/Ca, ∆δ 18 O, and to a lesser extent in δ 13 C 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 δ 18 O variations. In contrast, annual δ 13 C is mainly dominated by the 13 C Suess effect, showing a gradually downward trend. Importantly, we find that δ 18 O and Ba/Ca records exhibit consistent and significant long‐term trends, with δ 18 O 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.