A CHEMICAL INDICATOR OF TRADE-WIND REVERSAL IN CORALS FROM THE WESTERN TROPICAL PACIFIC

Historical levels of manganese have been measured in the annual rings of a reef coral from Tarawa atoll (1-degree-30'N,173-degrees-E) in order to assess the temporal variability of this trace element in surface waters of the western tropical Pacific. Seasonal variability associated with vertical mixing is not apparent as is the case in the eastern Pacific; however. Mn/Ca ratios are highly perturbed during three El Nino-Southern Oscillation (ENSO) events (1965, 1972, 1976) that occurred within the growth period 1960-1977. For approximately 6 months during each of these events, Mn/Ca ratios increase by 50-80% from a background of 27 nmol Mn/mol Ca. These features co-occur with negative delta-O-18 pulses recorded in the same coral which result from anomalously abundant rainfall. Wet deposition of Mn, however, does not appear to be an adequate source to account for the inferred surface ocean enrichments. The most plausible explanation has to do with the transient appearance of westerly winds along the equator during ENSO periods. Particularly vigorous westerly wind "bursts" (sometimes averaging 10 m/sec or mom over 16 hours) during the aforementioned years may have generated wind waves of sufficient strength to create an aura of particulate and diagenetically remobilized Mn from within the shallow westward facing lagoon of Tarawa. This interpretation is supported by relatively high Mn concentrations measured in pore waters of Tarawa lagoon sediments. The results suggest a useful new chemical tracer of historical ENSO conditions in the western Pacific basin.