Robbins Island: The Index Site for Regional Last Interglacial Sea Level, Wave Climate and the Subtropical Ridge Around Bass Strait, Australia
A unique index-record of Last Interglacial (Marine Isotope Stage 5e MIS5e) relative sea level (RSL) and wave climate history in South-east Australia is presented from Robbins Island, in western Bass Strait. This is applied to interpret the wider MIS5e coastal evidence around Bass Strait. At Robbins Island, the combination of low wave and wind energy, a tide-modified regime and a sand supply resulted in the shoreline progradation throughout MIS5e. This preserved a time-series of paleo-sea level across a 7 km wide strandplain (Remarkable Banks). After a highstand, MIS5e RSL attained a stillstand of +5.75 ± 0.5 m above modern mean sea level during 126 to ∼119 ka BP. The MIS5e RSL interpretation is underpinned by modern analogues and hydrodynamic modelling of waves, tides and currents. A high resolution LiDAR Digital Elevation Model (DEM) supported by morpho-sedimentary studies, ground-penetration radar (GPR) surveys and a geochronology based upon Optically Stimulated Luminescence (OSL) methods were used to define the proxy RSL record. The observed RSL history was compared to modelled RSL history that accounted for the theoretical fall in RSL (regression) throughout MIS5e, due to the Glacio-Isostatic Adjustment (GIA) forcing.
Three stages of RSL change occurred during MIS 5e: (i) RSL fall during phase 1 from ∼129 to 126 ka BP, and during phase 3 between ∼118 and 114 ka BP.; and, (ii) a multi-millennial stillstand during the intervening phase 2 from 126 to ∼119 ka BP. The stillstand departure from GIA theory, points unambiguously to persistent polar meltwater contributions to sea level of ∼2 m from 126 to 119 ka BP, where the component of RSL fall due to GIA was balanced by the RSL rise from meltwater. The potential contributions of paleo wave climate (direction) and boundary current histories were reconstructed from across all Bass Strait sites to determine an RSL budget. In addition, the paleo wave climate history allowed the triangulation of directional ocean wave synoptic sources and identified a 5° poleward shift in the Subtropical Ridge during MIS5e.