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.

Saltwater Intrusion and Submarine Groundwater Discharge: Acceleration of Biogeochemical Reactions in Changing Coastal Aquifers

“…They observed an increase in the reconstructed SGD corresponding to the escalating trend of local rainfall and suggested that "coral paleohydrological records can contribute to model-data comparisons in SGD models under different pumping scenarios." Chen et al (2020) measured Ba/Ca, δ 18 O, and δ 13 C in coral skeletal material from the nearshore reef environment in the Kimberley Region of Northwestern Australia. They concluded the skeletal material contained significant terrestrial signatures of Ba/Ca, residual δ 18 O, and to a lesser extent δ 13 C on a monthly time scale.…”

Pacific‐Driven Salinity Variability in the Timor Passage Since 1777

“…Because the NCF δ 18 O record only extends to 1983, it does not capture the recent trends that are present in many instrumental records of the Australian monsoon and ITF (Chen et al., 2020; Gallego et al., 2017; He et al., 2022; Higgins et al., 2022; Ummenhofer et al., 2021). Instead, the long‐term variability of NCF δ 18 O gives insight into the internal variability prior to the late 20th century.…”

“…Freshwater fluxes are integral to both the monsoon and ITF, imprinting their variations on ocean surface salinity (Ummenhofer et al., 2021). Long‐lived corals track past changes in salinity through the oxygen isotopic composition (δ 18 O) of their skeletons, revealing long‐term changes that relate to ITF transports and regional freshwater balance (Cahyarini et al., 2014; Chen et al., 2020; Hennekam et al., 2018; Linsley et al., 2017; Murty et al., 2017, 2018). Such records are increasingly useful as networks of data emerge to place local variability into regional and basin‐scale context (Abram et al., 2020; Tierney et al., 2015; Ummenhofer et al., 2021).…”

“…The Australian summer monsoon system dominates annual hydroclimate variability across northern Australia (McBride & Nicholls, 1983). Instrumental and paleoclimate records (Chen et al., 2020; He et al., 2022; Higgins et al., 2022; Smith et al., 2008), along with related indicators of monsoon strength (Gallego et al., 2017), suggest that monsoon rainfall in northern Australia is increasing. However, the timescales and locations of these studies vary, and they include few sites from the remote Top End (the peninsula between the Gulf of Carpentaria and the Timor Sea, where Nightcliff Reef is located; Figure 1).…”

Environmental records from coral skeletons: A decade of novel insights and innovation

“…First, as reviewed by Saha et al (2016) there has been rapid expansion and refinement of proxies for upwelling (Grottoli et al, 2013;LaVigne et al, 2010LaVigne et al, , 2016Watanabe et al, 2017), runoff/river discharge (Brenner et al, 2017;Chen et al, 2020;Grove et al, 2012;Kaushal et al, 2020Kaushal et al, , 2021LaVigne et al, 2016;Maina et al, 2012;Saha et al, 2018), and/or sediment flux on to the reef (Carriquiry & Horta-Puga, 2010;Fleitmann et al, 2007;Ito et al, 2020;Mallela et al, 2013;Prouty et al, 2010;Tanzil et al, 2019) from elements with nutrient-like profiles in the water column (Ba/Ca, P/Ca, Y/Ca, Cd/Ca, δ 13 C) and from skeletal luminescence. For example, while Ba/Ca proxy has been widely applied for some time, the interpretation of these records has varied greatly among sites.…”