Abstract:
Lord Howe Island and Balls Pyramid are subaerial landmasses in the centre of the Tasman Sea, remnants of eroded Miocene volcanoes. Lord Howe has a rhomboidal shaped shelf 24 km wide and 36 km long with an average water depth of 50 m, whereas Balls Pyramid has a slightly smaller shelf, located 20 km to the south. A piston core was collected from 780 m water depth in the trough which separates the two shelves, recovering 2.8 m of carbonate sediment composed of fine grained sandy-mud. Several periods of coral reef growth have occurred on the shelf of Lord Howe Island during the Late Quaternary, maintained by the tropical East Australian Current. The southern limit of this current, termed the Tasman Front, presently sweeps the island annually and during the Last Glacial Maximum, it is thought to have moved northward, allowing cooler waters to encircle Lord Howe Island. The aim of this study was to assess how these changes are reflected in palaeo-temperature records of foraminifera preserved within the sediment core.
Shell fragments are the major constituent of the sediment, followed by foraminifera, miscellaneous carbonates and bryozoans. The planktonic foraminifera species assemblage is consistent with the transitional-subtropical water masses typical of the region. An absence of coral and coralline algae within the core also indicates little transport from the mid and inner shelf regions, with sediment primarily sourced from the shelf slopes. Oxygen and carbon isotope analyses were conducted using the planktonic foraminifera G. ruber and G. bulloides as well as the benthic Uvigerina Sp. These analyses combined with radiocarbon dating indicate a sedimentation record back to Oxygen Isotope Stage (OIS) 4. The upper part of the core to 800 mm depth is composed of material slumped off the island flanks during OIS 2 - 3. The middle section of the core (800 - 2000 mm) was deposited during OIS 3, with the lower section of the core (2000 - 2800 mm) being of OIS 4 age. Sea surface temperatures range from 10 - 12°C during OIS 4 (2000 - 2800 mm) to a mean of ~10°C in OIS 3 (800 - 2000 mm) and warming to a mean of ~11°C in the upper slumped section. Temperature fluctuations of over 1.5°C occur at the top of the core due to the mixed sediment source related to slumping. For benthic foraminifera a mean of ~9°C is calculated for all stages. This study indicates that despite being adjacent to an island sound palaeo-temperatures can be extracted and indicate a northward movement of the Tasman Front during OIS 2 -4.
