Paradoxical High Productivity in the Eastern Equatorial Pacific Ocean During the Miocene “El Padre”, ODP Site 849, and IODP Sites U1337 and U1338

Abstract:

Prior to 4.6 Ma, SST proxy data has identified a 2° SST gradient from east to west across the equatorial Pacific, much smaller than the Holocene E-W gradient near 6°. The reduced Miocene gradient should have significantly reduced atmospheric Walker circulation and reduced the forcing for eastern Pacific upwelling. However, abundant data on biogenic deposition has found that, if anything, eastern equatorial Pacific productivity was higher in the Miocene and was still centered at the equator. The eastern equatorial Pacific is also marked by abrupt order-of-magnitude changes in production that last for 50-200 kyr throughout the middle and late Miocene. The latest (but not largest) of these productivity intervals began abruptly (<5 kyr onset) at 4745 ka at ODP Site 849, then at 0.6°S, and ended more gradually around 4555 ka. The high production interval can be traced as far north as Site U1337 at a paleolatitude of 2.5°N. Much higher relative diatom production occurs near the equator than at sites further north; instead they have high carbonate production, presumably by coccolithophorids. The opal and carbonate distributions will ultimately provide information on plankton response to changes in nutrient delivery.

We explore why the equatorial Pacific was more productive in the Miocene than in the late Pliocene and Pleistocene. Since the thermocline was warmer and the SST gradient with depth was more gradual in the Miocene, less wind stress is needed to lift the same nutrient load. Also, under warmer climates the ocean was probably more heterotrophic. Changes in organic carbon burial suggest that plankton decomposed higher in the water column in the Miocene than in the Pleistocene, and there was less cycling of particulate organic matter deep into the ocean interior. Nevertheless, abrupt but long lasting changes in production are a challenge to understand without abrupt dynamic changes in linkages between nutrient reservoirs. In the eastern Pacific the longer term evolution of biogenic sedimentation is associated with the closure of the Central American Seaway between 10 and 4 Ma.