Year

2019

Degree Name

Bachelor of Science (Advanced) (Honours)

Department

SEALS

Advisor(s)

Helen McGregor

Abstract

The 6-3 kyr BP time period is characterised by enhanced precessional forcing and an abrupt global climate event known as the ‘4.2ka event’ megadrought and is a key study period for characterising the sensitivity of El Nino-Southern Oscillation (ENSO) ˜ to external forcing. Recent studies have suggested ENSO as a possible driver of the ‘4.2ka event’ megadrought. This thesis aims to develop a deeper understanding of ENSO in the mid-Holocene and investigate possible links between ENSO and the ‘4.2ka event’. A Sr/Ca-SST record was generated from XM35, a fossil coral from Kiritimati Island in the equatorial central Pacific that lived between 4.4-4.2 kyr BP and from which a δ 18O record has previously been generated (McGregor et al. 2013). With paired measurements of Sr/Ca and δ 18O, sea surface temperature (SST) and δ 18Osw (interpreted as a hydrological cycle proxy) can be reconstructed. The Sr/Ca-SST results from XM35 show a 53% reduction in ENSO variance, supporting previous studies showing that mid-Holocene ENSO was weaker relative to present. The Sr/Ca-SST record also shows a cooling trend of approximately 1.5°C over the 175-year length of the record. Analysis of the temporal structure of El Nino events at 4.4-4.2 kyr BP shows ˜ peaking of El Nino events in September, slightly earlier than in the present-day climate. ˜ Temporal differences in El Nino events and the SST annual cycle are consistent with ˜ the mid-Holocene precessional forcing model of Clement et al. (2000), which posits that mid-Holocene ENSO weakening was caused by insolation seasonality arising from precessional forcing. A co-timing of abrupt oceanic cooling shown in the XM35 Sr/Ca SST record with severe drought in southern Asia from the ‘4.2ka event’ megadrought evidenced in a speleothem δ 18O record (Berkelhammer et al. 2013) supports expression of the ‘4.2ka event’ in the equatorial central Pacific. Although ENSO has previously been hypothesised as an ultimate driver of the ‘4.2ka event’ (Toth & Aronson 2019; Walker et al. 2012), this thesis instead proposes that an abrupt tropical climate shift linked to ENSO and Pacific Decadal Oscillation-like variability may have played a role in or driven the ‘4.2ka event’. The results from XM35 indicate that further investigation of a ‘4.2ka event’ driver originating in the Pacific Ocean or elsewhere in the low-latitudes is warranted.

FoR codes (2008)

040605 Palaeoclimatology

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.