Evaluating the potential of early Pliocene mollusc shells from Peru as El Nino records - permanent "El-Nino-like" state or interannual El Nino variability
Zusammenfassung der Projektergebnisse
It was proposed that the tropical Pacific in the early Pliocene (3-5 Ma) might have been characterized by a stable permanent “El Nino-like” state without interannual El Niño variability. Due to the lack of seasonally resolved proxy data from low latitude sites this hypothesis remains controversial. To establish the potential of early Pliocene bivalve shells from southern Peru as climate archive, a screening procedure was developed to evaluate possible diagenetic alterations. Oxygen isotope records of two well preserved bivalve shells (Dosinia ponderosa) were generated to provide time-windows of past seasonal and interannual climate variability from the early Pliocene warm period (3.5-4.5 Ma). The seasonal cycle of sea surface temperatures for these two short time windows (3-4 years each) were compared to Holocene datasets. This comparison shows that the amplitudes in the early Pliocene shells were reduced by half compared to the seasonal cycle recorded in the shells of modern and mid to early Holocene bivalves of the species Mesodesma donacium. Orbital parameters and hence the magnitude of insolation varied considerably during the Holocene. The consistent difference between the observed seasonal cycle in the early Pliocene compared to the late, mid and early Holocene hence can not be easily explained by differences in the orbital parameters alone. Instead, the reduced seasonal cycle could be a direct effect of a deeper thermocline and/or a reduced Walker circulation in the eastern subtropical Pacific, which both would lead to the suppression of upwelling of cooler subsurface waters. Such a scenario is consistent with early Pliocene proxy data suggesting that the meridional sea surface temperature (SST) gradient between the eastern and western tropical to subtropical Pacific was much reduced compared to today, resembling an El Niño-like state. However, the amplitude of one seasonal cycle recorded in the oxygen isotope record of Dosinia ponderosa exceeds the mean seasonal cycle in the early Pliocene by far. This high amplitude isotope event could represent an ENSO event during the early Pliocene and hence would indicate that the El-Nino like state was not permanent but was characterized by ENSO variability.
Projektbezogene Publikationen (Auswahl)
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(2007). Early Pliocene mollusc shells from Peru al El Niño records - permanent "El Nino-like" state or interannual El Nino variability? In: 13th Bathurst Meeting of Carbonate Sedimentologists, Norwich, UK
Reuning, L. and DeVries, T. J.
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(2008) A reduced early Pliocene seasonal temperature cycle recorded in a bivalve shell from southern Peru. Geo2008, Jahrestagung der DGG/GV, Aachen, Schriftenreihe der DGG, Issue 60, ISBN 978-3-510-49 07-7, p. 347
Reuning, L., and DeVries Th.
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(2008) Seasonal and interannual climate variability recorded in early Pliocene mollusc shells from coastal Peru, Geophysical Research Abstracts, Vol. 10, EGU2008-A-02142, 2008; EGU General Assembly, Vienna
Reuning, L. and DeVries, T. J.
