A magnetic and geochemical approach to the changing sedimentation accumulation on the upper slope of the great barrier reef, northeastern Australian margin

Abstract

The Great Barrier Reef (GBR) located along the northeastern margin of Australia is the largest coral reef system in the world. Modern climatic alterations are quickly changing the GBR ecosystem. To understand the implications of these changes it is important to reconstruct the geological history of GBR. Here we use geochemical and magnetic proxies to evaluate past climatic fluctuations and their consequences on sediment deposition along the GBR margin. IODP Expedition 325 – Hole M0058A, drilled on the uppermost slope at ca. 170 m water depth, reveals the depositional history of the GBR margin during the interval of MIS 7 to 5 and MIS 1. Magnetic and geochemical variations along the core section reveal detailed information on sediment accumulation and on the variations in terrigenous input in relation to sea-level fluctuations and climate change. Sea-level variations influenced margin deposition between MIS 7 and 6 impacting shoreline progradation/retrogradation and siliciclastic redistribution, resulting in a mixture of finer to coarser magnetic assemblages with no significant changes in terrigenous input. At the end of MIS 6 a decline in the deposition of carbonate sediments concomitant with the deposition of fine-grained magnetite-rich terrigenous sediments suggests an intensification of the monsoon in response to global warming trends. Arid periods over NE Australia were established after the glacial/interglacial transition (the MIS 6-5e) and at the middle Holocene (after the MIS 2–1 transition at ca. 7 ka), which favored dust deposition over the region. Enhanced dust fertilization subsequently promoted primary productivity at these intervals resulting in the presence of biogenic magnetite at Hole M0058A produced by magnetotactic bacteria.