Global climate during the Pliocene (5.33-2.58 Ma) was similar or even warmer than today’s climate providing a potential analog on how future global climate change may look like. Although the Pliocene has been studied extensively, major gaps in our knowledge of the east Pacific are still existing. During the early Pliocene the east Pacific was likely warmer than today, the equatorial cold tongue did not exist yet and the Oxygen Minimum Zones (OMZ) may have been less intense or absent. This overview will look at the history of water column stratification in the east Pacific both on a long-term timescale, i.e. from the early Pliocene to after the onset of Northern Hemisphere Glaciation, and on a shorter, glacial-interglacial timescale. We reconstruct variability potentially related to the OMZ from several (I)ODP sites (Site U1338, Site 1236, Site 1241) using different foraminifera-based proxies and assemblages. Changes in the stratification of the water column, related to upwelling intensity are reconstructed by temperature (Mg/Ca) and relative salinity (d¹⁸O_sw) analysed on different species of foraminifera representing different waterdepths (Trilobatus sacculifer, Neogloboquadrina dutertrei, Globorotalia crassaformis). Nutrient utilization in the water column related to upwelling is reconstructed by nitrogen isotopes determined on the tests of T. sacculifer and may be related to changes in the OMZ. Specific changes in the OMZ are reconstructed by the abundance and test porosity of the planktic foraminifera Globorotaloides hexagonus, which shows a strong affinity with the OMZ.
 

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