The sedimentary record of Quaternary glacial to interglacial sea‐level change on a subtropical carbonate ramp: Southwest Shelf of Australia. Issue 2 (24th September 2020) - Deik, Hanaa
Abstract: In the last decades, the understanding of temperate carbonate systems has improved considerably, but their development over glacial–interglacial timescales is still understudied in comparison to their tropical counterparts. A key question is how do temperate carbonate platforms respond to high‐amplitude, glacial–interglacial sea‐level changes? Integrated Ocean Drilling Program Site U1460 was drilled at the uppermost slope of the Southwest Shelf of Australia at the transition between the subtropical Carnarvon Ramp and the warm–temperate Rottnest Shelf. The origin and composition of the sediments in the upper 25 m below seafloor at Site U1460 were investigated using X‐ray diffraction, scanning electron, and light microscopy. The Middle Pleistocene to Holocene sequence at Integrated Ocean Drilling Program Site U1460 contains a record of sea‐level controlled sedimentary cycles. Carbonate sediments deposited during interglacial sea‐level highstands (Marine Isotope Stages 1, 5, most of 7, 9 and 11) are mainly fine‐grained (<63 µm) and dominated by low‐Mg calcite from pelagic bioclasts such as planktic foraminifera. The glacial lowstand intervals (Marine Isotope Stages 2 to 4, 6, 8, 7d, 10 and 12), instead are coarser‐grained and relatively rich in aragonite and high‐Mg calcite from neritic bioclasts, such as bryozoans. These changes in texture, mineralogy and composition are best explained by the deposition of neritic bioclasts closer to the shelf edge during glacial sea‐level lowstands. During early transgression, reworking of bioclast‐rich coastal dune deposits likely leads to transport and redeposition of neritic clasts on the upper slope. In contrast, dominantly pelagic sediments characterize deposition at the platform edge during interglacial highstands. These results highlight regional differences in the response of temperate carbonate systems to sea‐level change: A previously published model developed for early Pleistocene temperate carbonates from the Great Australian Bight indicates that shelfal material was exported to the upper slope during sea‐level highstands. It is argued that this difference is related to the change in duration and amplitude of glacial–interglacial sea‐level cycles before and after the Mid‐Pleistocene transition.