Corals record low mobile barium concentrations in the Burdekin River during the 1974 flood: evidence for limited Ba supply to rivers?

Porites corals collected from two coastal locations in the Great Barrier Reef (GBR) of Australia, have been analysed for trace elements by laser-ablation ICP-MS, and fluorescence by photographic techniques. The sites were selected because they experience freshwater discharge from nearby river systems (Pandora Reef from the Burdekin River, King Reef from several rivers including the Tully River). The analyses spanned approximately 6 years from the late 1960s to mid 1970s and included the year 1974, which is the largest recorded flood of the Burdekin River. Both corals display large enrichments in Ba/Ca (by up to 4×), which coincide with the timing of river flood plumes as indicated by the luminescent flood bands. From these observations, it is inferred that peaks in coralline Ba are caused by Ba desorbing from suspended sediment in the low-salinity region of the flood plume and being taken up by the coral as a dissolved component. The effective river end-member Ba concentration (Ba EREM) for floods from both the Burdekin and Tully Rivers are calculated from Ba/Ca anomalies in the corals and salinity models generated by the Australian Institute of Marine Sciences (AIMS). The Ba EREMs in the Pandora Reef coral are negatively correlated with river discharge. Especially notable is the very low EREM for the 1974 Burdekin River flood (226±13 nM). Ba EREMs are more variable in the King Reef coral, reflecting a complex mix of freshwaters from several rivers. The 1974 event, however, is dominated by freshwater from the Burdekin River, and the EREM determined from Ba in the King Reef coral (196±20 nM) is identical to that calculated from the Pandora coral. The low EREM for the large 1974 flood, and the negative trend with discharge, suggest that the supply of mobile (dissolved and labile particulate) Ba to the Burdekin River may be limited.