Coral-based climate reconstructions in the central and western Indian Ocean from the Holocene to the present-day : orbital forcing, internal variability and anthropogenic disturbances
Aachen (2019, 2020) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (XX, 177 Seiten) : Illustrationen, Diagramme, Karten
In order to better understand natural and anthropogenically-induced climate variability, long records of tropical climate parameters covering the past hundreds to thousand years are needed. Usually, instrumental records are too short or lack spatial homogeneity. Only a few records provide continuous measurements of tropical climate parameters prior to 1950. Proxy archives, indirect climate indicators, can help to fill the gaps in the instrumental database. Tropical corals can constitute an ideal proxy archive to reconstruct seasonal, interannual and decadal tocentennial climate variations. They can be used to reconstruct past changes of environmental parameters such as sea surface temperature (SST) or rainfall by measuring Sr/Ca and d18O. In the past decades, the network of coral paleoclimate reconstructions has been growing. The tropical Indian Ocean has been underrepresented in this network. However, the Indian Ocean is of enormous relevance in terms of global warming as it is the largest contributor to global mean SST rise and has been warming faster than any other ocean basin during the last century. The Chagos Archipelago is located in the central Indian Ocean. It lies at the eastern margin of the Seychelles-Chagos thermocline ridge and features open ocean upwelling. In situ SST have been recorded by temperature loggers since 2006, which record upwelling-related cooling and is closely tracked by high-resolution satellite SST. Two modern Porites coral cores were collected from different colonies from a patch reef in the lagoon of Peros Banhos, which experiences low temperature variability and high mean SST. An open ocean Porites coral core was collected at the outer reef slope of Diego Garcia, a site characterized by large temperature fluctuations related to open ocean upwelling. The Sr/Ca record of the open ocean coral shows clear seasonal cycles that closely track the satellite SST. Between 2007 and 2010, Sr/Ca of both lagoon coral cores also tracks satellite SST. However, between 2003 and 2006 the Sr/Ca curves are almost flat and annual coral growth rates are low. For this period, warm SST and coral bleaching have been reported for Chagos. This suggests that observed reduction in coral growth rates is a response to prolonged warming events. Corals from the open ocean at Chagos are adapted to large temperature variability and appear to be unaffected by recent warm events, because upwelled water relieves corals from thermal stress. Based on that, it can be concluded that the temperature variability at a reef site is important to determine the sensitivity of a coral to thermal stress, with corals from sites with small temperature variations being more susceptible. Further, this study is the first to show that these Sr/Ca signatures are reproducible as they affect coral cores from different colonies. On interannual timescales, the dominant modes of climate variability in the tropical Indian Ocean are the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Therefore, ENSO events also have a strong influence on the Chagos Archipelago. A stable ENSO-SST relationship between tropical Indian and Pacific Ocean is known for the period from the present back, but not prior, to 1950. Three monthly-resolved Sr/Ca records of sub-fossil coral samples from the Chagos Archipelago are used to reconstruct past SST covering the periods 1675-1716 (Maunder Minimum), 1836-1867 (late Little Ice Age) and 1870-1909 (early warming period). The records were combined with an additional record from the same site that was already published. All Sr/Ca records reveal typical ENSO periodicity between 3 and 8 years. Frequency and intensity of ENSO events vary between studied time periods. At Chagos, El Niño and La Niña events are more often recorded in modern corals (1965-1995) and satellite SST (1981-2018) reflecting the actual ENSO frequency compared to the coral covering the Maunder Minimum period. The results of this study suggest that the ENSO-SST teleconnection in the central Indian Ocean was stationary over the last centuries prior to 1950. The mid-Holocene (6 kyr BP) is a target period to study climate variability and its response to orbital forcing, because the latitudinal and seasonal distribution of insolation was different than today, whereas other climate parameters, such as ice volume and greenhouse gas concentrations, were similar to today. As rainfall in East Africa is coupled to changes in SST in the tropical Indian Ocean, it is a key region in which coral paleoclimatic reconstructions have to be conducted to better understand the complex interactions between atmosphere and ocean in the Indian Ocean. Three coral samples dated to the mid-Holocene and one modern coral from East Kenya were used to reconstruct past SST and d18O of the seawater. Both modern and mid-Holocene coral SST do not only follow solar radiation, but also reflect the monsoon circulation. The results further show a reduction of the mean annual SST cycle during the mid-Holocene compared to the present-day and a shift of the annual SST maxima between mid-Holocene and present-day, which can be related to orbital forcing. In addition to SST, coral d18O reflects rainfall, oceanic advection processes, or all three parameters. However, this signal is not as a clear as the signals in the Sr/Ca records. Overall, this study presents three modern and three sub-fossil coral Sr/Ca records from the central Indian Ocean (Chagos Archipelago) and three fossil coral Sr/Ca as well as one fossil and one modern coral d18O record from the western Indian Ocean / East Africa (Kenya), which are used to reconstruct SST and d18O of the seawater. Thus, this thesis contributes new records that help to further close the data gaps. Closing these gaps is needed to better understand the Indian Ocean’s climate system.