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

• Published in: Earth and planetary science letters 2003-05, Vol.210 (1-2), p.249-258
• Main Authors: McCulloch, Malcolm, Pailles, Christine, Moody, Philip, Martin, Candace E.
• Format: Article
• Language: English
• Subjects: Environmental Sciences; Global Changes; Marine
• ISSN: 0012-821X
• DOI: 10.1016/S0012-821X(03)00145-6

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 super(143)Nd/ super(144)Nd isotopic signatures ( epsilon sub(Nd)~+3 to +5), while the more common Palaeozoic granitic/metamorphic soils have much lower total P (300-600 mg/kg) and super(143)Nd isotopic signatures ( epsilon sub(Nd)~-8). The nearshore environment (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 PO sub(4) super(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.