Past and present coral distribution at the latitudinal limit of reef development, southwest Pacific Ocean

Coral reefs are particularly vulnerable to climate-change impacts such as warming sea-surface temperatures, ocean acidification and increased storm activity. In response to these changes, corals may alter their geographical distributions and expand their ranges into higher latitudes. Coral reef range expansions have occurred during past periods of warming and coral populations have survived in regions protected from adverse conditions, termed ‘refugia’, until conditions improved and reefs replenished. Modern-day climate refugia have been hypothesised in higher latitudes as well as deeper, mesophotic waters (30-150 m depth). Few studies have investigated the role of higher latitude, mesophotic environments in supporting modern corals and their potential as habitat for coral refugia and expansion. This thesis investigates past and present coral distribution around the subtropical, mesophotic Balls Pyramid shelf and draws comparisons to the adjacent Lord Howe Island shelf. Balls Pyramid is a steep, 552 m high volcanic pinnacle in the southwest Pacific Ocean. The pinnacle occurs 24 km south of Lord Howe Island, which was considered to be the southernmost limit of modern and Late Quaternary reef development in the Pacific Ocean. This thesis aims to: 1) determine the extent to which the Balls Pyramid shelf may have supported past coral reef development; 2) establish the extent to which modern coral populations colonise the shelf; 3) predict suitable areas of coral habitat; and 4) assess whether an understanding of past and present reef development can inform on the future potential of the shelves as substrates for coral refugia and expansion