Bachelor of Science (Honours)
ANZSRC / FoR Code
040305 Marine Geoscience, 040310 Sedimentology
School of Earth & Environmental Sciences
Schofield, Harrison John, Holocene changes in mean sea-surface temperatures from Central Pacific fossil microatolls, Bachelor of Science (Honours), School of Earth & Environmental Sciences, University of Wollongong, 2011.
The equatorial Pacific east-west sea surface temperature (SST) gradient (SST gradient) is a dynamic component of the El Niño Southern Oscillation (ENSO) and is a key determinant of global climate patterns. The SST gradient, which is averaged over decades to millennia, is a major component of the Pacific mean state, and changes in its strength are related to ENSO variability. Understanding the interaction between Pacific SST gradient and ENSO is important because changes in mean SSTs are projected to alter the Pacific mean SST gradient, but it is not clear what impact this will have on ENSO. By investigating the historical relationship between these two parameters, a better understanding of the interaction between the SST gradient and ENSO can be gained. Studies of climate of the past 10 thousand years (kyr) have documented changes in ENSO, yet little is known about the major components of the Pacific mean state, including the mean SST gradient.
In this thesis, 112 Porites sp. fossil corals collected from Kiritimati Island (157°25‘W, 1°50‘N) were available to reconstruct central equatorial Pacific Ocean (CP) SSTs. Thorough screening of every coral sample for diagenetic alteration by XRD and thin section techniques eliminated 64 coral samples, leaving 52 different corals from which SSTs could be accurately reconstructed. Radiocarbon and uranium-thorium dating of 45 of these corals revealed their ages, and the ages of the individual corals were distributed unevenly between 6 kyr BP and the present day. The average SST for each coral was estimated based on the corals‘ Sr/Ca ratio, which was measured using Inductively Coupled Plasma Atomic Emission Spectrometery. All of these reconstructed SST obtained from the corals, form a time series of central equatorial Pacific SSTs for the mid- to late Holocene. This time series is then compared with other Holocene Pacific SST reconstructions and ENSO records to better understand the Pacific SST gradient and ENSO interaction.
Weighted mean coral Sr/Ca SSTs for every 500-year interval from 6 to 1 kyr before present (BP) indicated that mean SSTs in the CP show centennial to millennial scale change in SSTs, with the largest deviations from modern SSTs occurring at 4.5-4 kyr BP where SSTs mean were ~2.9 °C ± 1.1°C cooler than today. Results also indicated SSTs were ~1.4°C ± 0.6°C cooler from 5.5-4.3 kyr BP, which combined with data from the western equatorial Pacific Ocean (WP) and eastern equatorial Pacific Ocean (EP) and modelling results provides evidence to suggest that a Pacific La Niña-like or La Niña Modoki-like mean state was active during the mid-Holocene.
For the full mid- to late Holocene period, mean SSTs in the CP and the WP appear to have undergone similar centennial to millennial scale variations, whereas EP SSTs show a consistent warming trend. Although CP mean SSTs indicate that mean zonal SSTs have been more variable throughout the mid and late-Holocene than previously thought, mean SST gradients (EP to CP and CP to WP) correlated well with previously documented Holocene SST gradient shifts.
Finally, changes in two mean SST gradients (CP minus EP and EP minus WP) were compared with ENSO evolution from 6 kyr to 1 kyr BP. Results revealed that since the mid-Holocene, both CP – EP and EP – WP gradients have weakened, and El Niño events have increased in frequency. Additionally, the proposed strengthening of the Pacific cold tongue during the 4.5-4 kyr BP correlated with very low El Niño activity. This result suggests that the centennial to millennial trends in mean SSTs have influenced ENSO behaviour since the mid-Holocene.