Expression of seasonal and ENSO forcing in climatic variability at lower than ENSO frequencies: evidence from Pleistocene marine varves off California

Abstract

Upper Pleistocene marine sediments along the upper continental slope off northern and central California contain alternations of varved and bioturbated sediments and associated changes in biota and sediment composition. These alternations can be related to conditions that accompany El Niño and anti-El Niño (ENSO) circulation. Anti-El Niño conditions are characterized by increased upwelling and productivity and by low concentrations of dissolved oxygen in the oxygen minimum zone that resulted in varve preservation. El Niño conditions are characterized by little or no upwelling, low productivity, and higher concentrations of dissolved oxygen that resulted in zones of bioturbation. Alternations of varves and zones of bioturbation, that range from decades to millennia, occur through the upper Pleistocene section. The inferred long-term alternations in El Niño and anti-El Niño conditions appear to be a re-expression of ENSO's primary 3–7 year cycle. Decadal to millennial cycles of productivity associated with El Niño and anti-El Niño conditions may have served as a “carbon pump” and transferred atmospheric CO 2 to the marine reservoir. Changes in sediment composition and organisms associated with El Niño or anti-El Niño conditions can be related to both seasonal and ENSO phenomena. Expression of these changes at lower-than-ENSO frequencies may be partly explained by adding the effects of seasonal variability to effects produced by a self-oscillating ENSO system. However, deterministic mechanisms, including solar modulation of ENSO, may also contribute to long-term alternations of El Niño and anti-El Niño conditions.