Changes in the possible behavior of El Niño–Southern Oscillation (ENSO) with global warming have provoked interest in records of ENSO from past “greenhouse” climate states. The latest Cretaceous laminated Marca Shale of California permits a seasonal-scale reconstruction of water column flux events and hence interannual paleoclimate variability. The annual flux cycle resembles that of the modern Gulf of California with diatoms characteristic of spring upwelling blooms followed by silt and clay, and is consistent with the existence of a paleo–North American Monsoon that brought input of terrigenous sediment during summer storms and precipitation runoff. Variation is also indicated in the extent of water column oxygenation by differences in lamina preservation. Time series analysis of interannual variability in terrigenous sediment and diatom flux and in the degree of bioturbation indicates strong periodicities in the quasi-biennial (2.1–2.8 yr) and low-frequency (4.1–6.3 yr) bands both characteristic of ENSO forcing, as well as decadal frequencies. This evidence for robust Late Cretaceous ENSO variability does not support the theory of a “permanent El Niño,” in the sense of a continual El Niño–like state, in periods of warmer climate.

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