Influence of marine biochemical cycles on seasonal variation of Ba/Ca in the near-shore coral Cyphastrea, Rat Island, southern Great Barrier Reef

Abstract

Coral reef ecosystems maintain high biodiversity and have great economic value. Nonetheless, inshore coral reefs of the Great Barrier Reef (GBR) are facing threats from poor water quality due to extensive modification of coastal catchments following European settlement. Ba/Ca ratios in coral skeletons have been used to infer past water quality, but in order to use a proxy to infer past environments, it is critical to develop a firm understanding of the parameters that control the proxy. This study investigated various environmental, climatic and oceanographic factors to understand a long-term (1976 to 2016) record of biogeochemical cycling of Ba/Ca preserved in a Cyphastrea sp. coral colony recovered from near-shore Rat Island (near Port Curtis), southern GBR. Our Cyphastrea was found to have simple skeletal microstructure, similar to that of Porites, with unaltered skeletal preservation, clear annual density banding, and an extended growth history with seasonally resolved Sr/Ca signals. This combined with a wide geographic distribution make Cyphastrea coral a potential new palaeoenvironmental archive. Although Rat Island is located near the mainland and is influenced directly by discharge from the Calliope and Boyne rivers, temporal oscillations of Ba/Ca did not show peaks associated with high runoff during summers. Rather, Cyphastrea's high-resolution Ba/Ca data shows, for the first time, consistent seasonal cycles with a gradual decrease from spring through summer and gradual rise from autumn through winter. Biogeochemical cycling of Ba in the semi-confined turbid water of Port Curtis may be controlled by active or passive planktic uptake during the spring bloom stage and removal of barite minerals formed in decaying phytoplankton during post-bloom stage. The subsequent increase of Ba/Ca from autumn through winter may be attributed to reduction of phytoplankton abundance. The semi-enclosed embayment of Port Curtis has restricted water turnover with a large terrigenous runoff load that may have significant influence on phytoplankton dynamics. Our work suggests that Ba cycling, and hence, the Ba/Ca coral

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

“…Nevertheless, we cannot rule out other possible factors that may produce such seasonal Ba/Ca signals like sediment resuspension, seepage of submarine groundwater, upwelling, or biological activity (e.g., coral spawning and phytoplankton blooms) (Alibert et al, ; Esslemont et al, ; Lea et al, ; Saha, Webb, et al, ). Given the sparse report and the unfavorable ocean conditions for upwelling along the western Australia coast (Varela et al, ), bottom seawater Ba transport is unlikely to be an important factor in the nearshore Kimberley.…”

“…This is commonly achieved by subtracting the temperature contribution from the δ 18 O coral utilizing either satellite record or coral Sr/Ca temperature proxy (Gagan et al, ; McCulloch et al, ). As for coral Ba/Ca, the scenario is more complicated due to the complexity of biogeochemistry of Ba in coastal coral skeletons (Chen et al, ; Lewis et al, ; Saha, Webb, et al, ; Sinclair, ; Tanzil et al, ). Monthly variability of coral Ba/Ca is not only always corresponded to river discharge peaks during wet season but would also show seasonal cycles or anomalously episodic spikes possibly linked to upwelling (Alibert & Kinsley, ; Lea et al, ), submarine groundwater (Alibert et al, ; Jiang et al, ), biological activity (Gillikin et al, ; Lewis et al, ; Saha, Webb, et al, ; Sinclair, ; Wyndham et al, ), anomalously lower temperature (Chen et al, ), or sediment resuspension (Alibert et al, ; Esslemont et al, ).…”

“…As for coral Ba/Ca, the scenario is more complicated due to the complexity of biogeochemistry of Ba in coastal coral skeletons (Chen et al, ; Lewis et al, ; Saha, Webb, et al, ; Sinclair, ; Tanzil et al, ). Monthly variability of coral Ba/Ca is not only always corresponded to river discharge peaks during wet season but would also show seasonal cycles or anomalously episodic spikes possibly linked to upwelling (Alibert & Kinsley, ; Lea et al, ), submarine groundwater (Alibert et al, ; Jiang et al, ), biological activity (Gillikin et al, ; Lewis et al, ; Saha, Webb, et al, ; Sinclair, ; Wyndham et al, ), anomalously lower temperature (Chen et al, ), or sediment resuspension (Alibert et al, ; Esslemont et al, ). Similarly, coral δ 13 C also has limitations as an environmental proxy as drivers such as internal carbon source, coral‐symbionts metabolism, kinetic effects, and other environmental variables may vary case by case (e.g., Allison & Finch, ; Deng et al, ; Grottoli, ; McConnaughey et al, ; McConnaughey, ; Swart, Healy, et al, ; Swart, Leder, et al, ).…”

“…Skeletal Ba/Ca ratios are the most commonly used proxy to constrain historical changes in terrestrial inputs (e.g., Alibert et al, ; Fallon et al, ; Grove et al, 2013; Jupiter et al, ; Lewis et al, , ; McCulloch et al, ; Prouty et al, ; Saha et al, , Saha, Rodriguez‐Ramirez, et al, ; Sinclair & McCulloch, ; Walter et al, 2013), despite that uncertainty associated with this proxy remains enigmatic in that seasonal cycles or anomalous peaks of coral Ba/Ca were occasionally reported to decouple from terrestrial runoff (Chen et al, ; Lewis et al, , ; Saha, Webb, et al, ; Tanzil et al, ; Sinclair, ; Tudhope et al, ). In the open ocean, barium is a nutrient‐like element with its abundance depleted in the surface ocean but enriched in the deep waters (Chow & Goldberg, ; Wolgemuth & Broecker, ).…”

Coral-based high-resolution rare earth element proxy for terrestrial sediment discharge affecting coastal seawater quality, Great Barrier Reef

“…Anomalous peaks in the proxies that are decoupled from river discharge and other known events have been reported in some studies, suggesting that these proxies are not solely controlled by terrigenous sediment runoff. Other factors, including regional hydrodynamics, upwelling, barite inclusions, phytoplankton blooms, dilution of proxy signals during wet seasons, submarine groundwater seeps, and episodic intrusion of trace metals to coastal water when local mangrove sediment carrying capacities are breached, can potentially have significant influences on the temporal variability of single element proxies (Alibert et al, 2003;Carriquiry and Horta-Puga, 2010;Chen et al, 2011;Lewis et al, 2012;Saha et al, 2018b;Sinclair, 2005;Wyndham et al, 2004). Hence, the reliability of these proxies for environmental reconstruction is complex necessitating the development of additional proxies.…”