The tropical eastern Indian Ocean is a key area for the Indian Ocean Dipole (IOD), Asian monsoon and El Niņo-Southern Oscillation (ENSO) systems, which have major impacts on climate throughout the tropics and beyond. Characterising Holocene climate variability in this region has the potential to improve our understanding of modern climate variability and the predictability of climate change during 21st century greenhouse warming. To this end, this thesis uses geochemical tracers in the skeletons of modern and fossil Porites corals from the Mentawai Islands, Indonesia, to reconstruct environmental conditions in the tropical eastern Indian Ocean over the past 7000 years. These palaeoclimate reconstructions provide the first insights into the evolution of the IOD system over the Holocene and extend our knowledge of the interactions between the IOD, Asian monsoon and ENSO systems. The coral records are also used to examine the recent death of the Mentawai Islands reef so that potential threats to coral reef ecosystems may be better understood and managed in the future.

Below are abstracts for two of the published papers based on this thesis:

Coral Reef Death During the 1997 Indian Ocean Dipole Linked to Indonesian Wildfires
Abram et al. (2003) Science 301, 952-955
Geochemical anomalies and growth discontinuities in Porites corals from western Sumatra, Indonesia, record unanticipated reef mortality during anomalous Indian Ocean Dipole upwelling and a giant red tide in 1997. Sea surface temperature reconstructions show that although some past upwelling events have been stronger, there were no analogous episodes of coral mortality during the past 7000 years, indicating that the 1997 red tide was highly unusual. We show that iron fertilization by the 1997 Indonesian wildfires was sufficient to produce the extraordinary red tide, leading to reef death by asphyxiation. These findings highlight tropical wildfires as an escalating threat to coastal marine ecosystems.

Seasonal characteristics of the Indian Ocean Dipole during the Holocene epoch
Abram et al. (2007) Nature 445, 299-302
The Indian Ocean Dipole (IOD) causes climatic extremes and socio-economic hardship throughout the tropical Indian Ocean region. There is much debate about how the IOD interacts with the El Nino/Southern Oscillation (ENSO) and the Asian monsoon, and recent changes in the historic ENSO-monsoon relationship raise the possibility that the properties of the IOD may also be evolving. Improving our understanding of IOD events and their climatic impacts thus requires the development of records defining IOD activity in different climatic settings, including prehistoric times when ENSO and the Asian monsoon behaved differently from the present day. Here we use coral geochemical records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events over the past 6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of strong surface ocean cooling, together with droughts that peaked later than those expected by El Nino forcing alone. Climate model simulations suggest that this enhanced cooling and drying was the result of strong cross-equatorial winds driven by the strengthened Asian monsoon of the middle Holocene. These IOD-monsoon connections imply that the socioeconomic impacts of projected future changes in Asian monsoon strength may extend throughout Australasia.

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