Honours Thesis Abstract
Rachel Skelly Vazey
Newcastle University, NSW
2007 AIG Honours Bursary Winner
This thesis is a preliminary investigation into the reliability of modern and fossil coral reef-flat microatolls as proxy indicators of sub-tropical sea surface temperature (SST) and sea surface salinity (SSS) in the Southern Cook Islands. Microatolls are discoid colonies of corals characterized by lateral rather than upward growth due to exposure at the air/water interface during lowest astronomical tides. These corals have been chosen as they provide the opportunity to couple sea-level reconstructions from microatoll topographic surveys, with palaeo-SST and SSS time series derived from geochemical analysis. Microatoll samples of the coral species Porites lutea were collected from
Rarotonga and Aitutaki Islands, and investigations centred on the use of stable isotope ratios of oxygen (δ 18
O) and elemental strontium and calcium (Sr/Ca) ratios were completed. Results from the δ 18
O
(coral)
analysis exhibited a strong relationship (R
2
=
0.73) with regional instrumental SST datasets, however negligible relationships were indicated for SST and Sr/Ca
(molar)
. The annualized palaeo-SSS reconstructions showed a weak relationship with local station precipitation records. Reproducibility of the spatial and temporal variation of the δ 18 O
(coral)
datasets was demonstrated between modern samples growing in close proximity. A δ 18
O-SST transfer function (SST (°C) =
(δ 18
O
(coral)
‰ + 0.43) / – 0.18) derived from least squares regression analysis between the modern samples and the instrumental dataset was applied to a suite of fossil microatolls and a discontinuous time series was generated. The Cook Island δ 18
O-SST time series were compared with other Pacific coral δ 18
O-SST time series and found to be consistent, with the exception of an anomalously cool δ 18
O-SST estimate for the period
950 AD. Low average coral growth rates for this coral, MaI, are half that observed in the modern record and are implicit with cooler SST. However, screening for diagenesis reveals that the magnitude of the cooling observed in MaI is enhanced by postdepositional acicular aragonite cement causing a further cooling of the δ 18 O-SST estimate. In summary the results of this research indicate that δ
18
O
(coral)
values from modern and fossil reef-flat microatolls have excellent potential as palaeo-SST proxies provided they are first screened for diagenetic alteration.