Tracing the source of sediment and phosphorus into the Great Barrier Reef lagoon

Neodymium and strontium isotopic systematics show that terrestrial phosphorus (P) entering the inner Great Barrier Reef (GBR) is dominated by the transport and dispersal of fine-grained basaltic soils. Soils derived from alkali basalts have high total P (3000-4000 mg/kg) and distinctive ¹⁴³Nd/¹⁴⁴Nd isotopic signatures (ε _Nd ∼+3 to +5), while the more common Palaeozoic granitic/metamorphic soils have much lower total P (300-600 mg/kg) and ¹⁴³Nd isotopic signatures ( ε _Nd ∼-8). The nearshore environment (<5 km from the coast) is dominated by coarse-grained, granitic-derived fluvial detritus, while >20 km from the coast, carbonate-rich sediments with increasing contributions from basaltic components become more important. In the offshore sites adjacent to coral reefs, it is shown that basalt-derived sediments can account for >90% of the terrestrial P, although making up less than half of the total terrigenous detritus. Equilibrium phosphorus concentration measurements on the marine sediments indicate that P enters the GBR lagoon via a two-stage process. Firstly, during episodic flood events, P is transported into the GBR lagoon on P-retentive fine-grained suspended sediments, with only minor desorption of P occurring in the low-salinity flood plumes. Desorption of P mainly occurs over longer timescales, predominantly in regions of sediment anoxia, with release of PO₄^3- directly into marine pore waters probably via reduction of ferric phosphates, and subsequent release into the water column by re-suspension. This process causes P depletion of the re-deposited sediments.