UC San Diego

The origin and influence of the “4.2 kyr event”– a climate excursion characterized by century-long drought hypothesized to have affected civilizations throughout the northern subtropics– is uncertain, despite its registration in multiple paleoclimatic archives. As the dominant source of global interannual climate variability, the El Niño-Southern Oscillation (ENSO) might be expected to have shaped the expression of global precipitation and temperature across the 4.2 kyr event. However, the role of ENSO has not been explored, owing to a general lack of temporally extended observations in the tropical Pacific. Here I analyze a number of tropical Pacific Porites coral fossils spanning this interval, between ~4600 and 4100 years B.P., building on the previously published record of ENSO activity as well as the prevailing mean state of the tropical Pacific climate. Monthly resolved oxygen isotopic records of three corals from Fanning Island and one from Christmas Island overlap with and extend existing records to produce a nearly-continuous sequence that spans several hundred years. Oxygen isotopic values of the overlapping segments of the fossil corals strengthen the reliability of the paleoclimate record and the radiometric dating. The new coral data generated in this study confirm the previous inference of low ENSO activity over an extended time interval encompassing the 4.2 kyr event. Additionally, the new coral data show that the interannual variability, though reduced in amplitude relative to the modern climate, was expressed as repeated episodes of prolonged and intense La Niña conditions. This characterization stands in contrast with a single multi-decadal excursion, as initially hypothesized for the 4.2 kyr event. Finally, the mean values of overlapping coral segments from Fanning and Christmas Islands are similar, suggesting that the modern temperature and salinity gradient was not present during the interval of the 4.2 kyr event. The simplest explanation for this lack of temperature/salinity gradient is that the southern boundary of the North Equatorial Counter Current was shifted north of Fanning Island.