Cahyarini, S. Y.

Dullo, Wolf-Christian

Pfeiffer, Miriam

Zinke, Jens

English

OceanRep

Geophysical Research Abstracts,Vol. 11, EGU2009-6174, 2009EGU General Assembly 2009© Author(s) 2009Evaluating 20th century warming trends with modern Porites corals fromthe western Indian OceanM. Pfeiffer (1), J. Zinke (2), W.-C. Dullo (3), and S.Y. Cahyarini (4)(1) Universität zu Köln, Institut für Geologie und Mineralogie, Köln, Germany (Miriam.Pfeiffer@uni-koeln.de), (2) VrijeUniversiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands , (3) IFM-GEOMAR, Research Centerfor Marine Geosciences, Wischhofstrasse 1-3, 24148 Kiel, GERMANY , (4) Research Centre for Geotechnology, IndonesianInstitute of Sciences (LIPI), Bandung, IndonesiaRecent studies suggest that instrumental records of sea surface temperature (SST) are unreliable prior to 1965 dueto changes in the measurement procedures. Thompson et al. (2008) identified an artificial cool bias of up to 0.3˚Cin global mean SSTs between 1945 and the mid-1960s.Geochemical parameters in skeletons of massive corals can be used to infer past changes in climate on seasonal tocentennial time scales. The Sr/Ca ratio of coral aragonite is a widely used tool for deriving high-resolution proxyrecords of past sea surface temperatures. Application of the Sr/Ca paleothermometer relies on the assumption thatcoral Sr/Ca varies predictably with temperature and that seawater Sr/Ca is invariant on millennial timescales due tothe long residence time of Sr and Ca in the ocean. In contrast, the oxygen isotope ratios (δ18O) of coral aragonitevary in response to temperature and changes in the δ18O of seawater, the latter depending on the freshwater balance.Thus, coral δ18O may be used as a record of past sea surface temperatures only at sites were δ18O seawatervariations are negligible.SST in the western tropical Indian Ocean closely follows global mean temperature trends (Funk et al., 2008). Herewe present a set of Porites coral Sr/Ca and/or oxygen isotope records from the tropical Indian Ocean covering thepast 120-336 years (Seychelles, Chagos Archipelago). We computed a composite sea surface temperature recordfor the Western Indian Ocean using δ18O (Seychelles) and Sr/Ca (Chagos). This record clearly follows instrumentalSST trends in the Western tropical Indian Ocean, except in the 1945-1965 interval, were instrumental SST datashow a pronounced cooling not evident in the coral proxy index (the linear correlation coefficient between thecoral index and instrumental SST is r=0.86 if we omit the most problematic time period from 1945 to 1960).However, the coral index follows the global land surface air temperature trend, which is free of systematic biasesdue to changes in the measurement technique. We therefore conclude that the cooling of the tropical Indian Oceanindicated by instrumental SST is an artifact that may result from changes in the SST measurement techniques, assuggested by Thompson et al., 2008). Our results will have important implications for the investigation of large-scale teleconnections in the oceans (for example, the apparent shift in the Indian Ocean SSTs around 1945 hasbeen related to a regime change in the North Pacific Ocean).Thompson et al. (2008): A large discontinuity in the mid-twentieth century in observed global-mean surface tem-perature, Nature, 453: 646-649, DOI:10.1038/nature06982.Funk, C. et al. (2008): Warming of the Indian Ocean threatens eastern and southern African food security but couldbe mitigated by agricultural development, PNAS, 105: 11081–11086, DOI: 10.1073_pnas.0708196105.

Evaluating 20th century warming trends with modern Porites corals from the western Indian Ocean

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