Variation in the eastern North Pacific subtropical gyre (California Current system) during the Middle to Late Miocene as inferred from radiolarian assemblages

Abstract

Abstract The California Current has influenced the primary productivity along the California margin, which plays a significant role in global carbon cycles. Although previous studies have explored the response of the California Current to climate change since the Pliocene, comparatively little is known about that during the Middle and Late Miocene when the paleoclimate changed significantly. Here, we present variations of the California Current based on new data of radiolarian fossil assemblages integrated with previously published data from the Middle to Late Miocene, and show that the California Current was strengthened from 12.3 to 10.5 Ma, 9.0 to 8.2 Ma, and 7.2 to 6.9 Ma at ODP Site 1021. The biogenic opal declined during periods of increased strength of the California Current from the Middle to Late Miocene at Site 1021 in offshore area. These data suggest that the biogenic opal decreased at Site 1021 in offshore area due to the deep nutricline depth by the strengthened California Current. R-mode cluster analyses divided into three major assemblages; surface/warm, cool/surface to thermocline, and intermediate-deep water taxa. The pronounced faunal changes of radiolarian assemblages to the dominance of intermediate-deep water taxa might reflect an initiation of the modern-type California Current system since ca. 10.5 Ma in the early Late Miocene due to the tectonic changes (e.g. partial closing of tropical gateways and uplift of the Himalaya and the Rocky Mountains) and/or significant high latitude cooling.