LATE NEOGENE CHANGES IN DIATOM SEDIMENTATION IN THE NORTH PACIFIC

During the late Neogene, North Pacific diatom sedimentation underwent major changes in response to high-latitude cooling and changes in surf ace and deep water circulation. At 9 Ma diatom mass accumulation rates (MARs) increased in the NW Pacific and off northeast Japan, possibly due to shoaling of the Isthmus of Panama, which lead to an enrichment of nutrients in North Pacific deep waters. During the latest Miocene, diatom MARs increased progressively off southern California (6.5 Ma), at high latitudes of the North Pacific (6.2 Ma), and off northeastern Japan (5.5 Ma), presumably in response to high latitude cooling. At ab out 4.5 Ma diatom sedimentation abruptly increased in the NW Pacific b ut declined off Japan and California, coincident with the onset of a p rolonged period of high-latitude warmth. Enhanced upwelling of nutrien t-rich deep waters in the NW Pacific probably stimulated diatom produc tion there. A major step in high latitude cooling at 2.7 Ma caused a r eversal of these mid Pliocene diatom sedimentation patterns. Upwelling of deep, nutrient-rich waters waned at higher latitudes, leading to a decline in diatom productivity; while wind-driven, coastal upwelling increased off southern California and stimulated diatom growth. (C) 19 98 Elsevier Science Ltd. All rights reserved.