Climatic and oceanographic change from high-resolution records in large fossil Porites, Magnetic Island, Queensland (C4.1S)

Task leader: Mr Stephen Lewis, CRC Reef and James Cook University.

Task associate: Dr Janice Lough, Australian Institute of Marine Science.

Recovering corals from Nelly Bay

The present concern of climatic changes due to 'greenhouse' warming has highlighted the importance of understanding past climates and to aid predicting the effect climate change will have on coastal environments. Observations of steady global sea level rise and warming accompanied with predictions of enhanced intensities and frequencies of drought and storm/flood events have received extensive media coverage. Significant challenges have surfaced for climatologists who have the task of collating and interpreting the massive amount of data currently being generated from weather stations around the world in order to understand long term climatic trends.

Scientists are restricted in producing long-term climatic records as detailed instrumental records are limited to the last century. Fortunately, organisms such as trees and corals faithfully record daily weather in their growth structures which can be physically and geochemically measured to assess past climatic trends where instrumental records are not available.

A valuable data resource has recently come to light with the recovery of eight large 2-3 m fossil colonies of Porites coral in Nelly Bay, Magnetic Island. These particular corals died around 5630 and 5620 ± 90 years BP and contain a record spanning about a hundred years. An additional coral colony which died in the year 2000 has been cored will provide a means of calibrating sea surface temperature with the elemental ratios.

The primary aim of the study is to construct a climate and evolutionary history of Magnetic Island over the Holocene. Geotechnical data from the harbour development accompanied with additional sediment cores and topographical surveying, will allow a detailed stratigrahic model of Nelly Bay to be constructed over the Holocene. This model will particularly focus on the fossil corals providing clues into the cause of the coral’s mortality as well as changes that have occurred in the coastal environment. Carbon dating of the sediment cores and beach ridges will allow age control and the position of sea level to be determined over the Holocene.

Sr/Ca, U/Ca, and Mg/Ca ratios measured in the modern coral core will be compared with instrumental records over an equivalent timeframe accessing their use as a possible sea surface temperature palaeothermometer. There is also an opportunity to evaluate and compare physical growth and element ratios as proxies for producing sea surface temperatures.

Oxygen isotopes will be examined for its potential for estimations of sea surface salinity comparing it with luminescence data and rainfall measurements.

The Ba/Ca ratio will be correlated with measured nutrients and suspended sediment concentrations obtained from a previous study (Muslim, 1995) evaluating its use as a nutrient and turbidity indicator. The ratio will also be measured over periods of dredging activity to determine if the reefs were affected by elevated turbidity during this time.

The geochemical proxies will then be collated to produce a climatic record for a period in the mid Holocene. This record can then be compared to previous mid Holocene records for north Queensland, as well as evaluating any potential anthropogenic impacts on Magnetic Island and on the Great Barrier Reef.