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This addendum to the International Ocean Discovery Program (IODP) Expedition 378 Scientific Prospectus details changes to the expedition schedule including additional drilling depth approvals and updates to the operations plans that were presented in the original prospectus. In September 2018, Expedition 378 was rescheduled for 2020 to accommodate the construction and installation of two new propellers for the vessel with minimal disruption to the slate of expeditions scheduled in the Southern Ocean. The change in expedition timing resulted in a change of the port of origination from Lyttelton, New Zealand, to Lautoka, Fiji, and a change in the number of transit days. Consequently, the operations schedule has been revised. The other development prompting an addendum to the Expedition 378 Scientific Prospectus was the approval by the Environmental Protection and Safety Panel (EPSP) to drill deeper at Deep Sea Drilling Project (DSDP) Site 277 than the previous approved maximum depth of 480 meters below seafloor (mbsf) (now approved to 670 mbsf). 

International Ocean Discovery Program (IODP) Expedition 378 is designed to recover the first comprehensive set of Paleogene sedimentary sections from a transect of sites strategically positioned in the South Pacific to reconstruct key changes in oceanic and atmospheric circulation. These high southern–latitude sites will provide an unparalleled opportunity to add crucial new data and geographic coverage to existing reconstructions of Paleogene climate. As the world’s largest ocean, the Pacific Ocean is intricately linked to major changes in the global climate system. Previous drilling in the low-latitude Pacific Ocean during Ocean Drilling Program (ODP) Legs 138 and 199 and Integrated Ocean Drilling Program Expeditions 320 and 321 provided new insights into the mechanisms of the climate and carbon system, productivity changes across the zone of divergence, time-dependent calcium carbonate dissolution, bio- and magnetostratigraphy, the location of the Intertropical Convergence Zone, and evolutionary patterns for times of climatic change and upheaval. Expedition 378 in the South Pacific Ocean uniquely complements this work because appropriate high-latitude records are unobtainable in the Northern Hemisphere of the Pacific Ocean. To optimize the recovery of Paleogene carbonates buried under red clay sequences at present latitudes of 40°–52°S and enable a full range of paleoceanographic proxy-based investigations, Expedition 378 will drill a transect of sites primarily situated along magnetic Anomaly 25n on ~56 Ma crust. Additional sites are located on 40 Ma crust (Anomaly 18). The drilling strategy will also redrill the sedimentary record at Deep Sea Drilling Project (DSDP) Site 277 to obtain a continuous record of a previously spot-cored, classic Paleogene high-latitude site and provide a crucial, continuous record of the shallow Subantarctic South Pacific Ocean from the Paleocene to late Oligocene. These new cores and data will significantly contribute to the challenges of the “Climate and Ocean Change: Reading the Past, Informing the Future” theme of the IODP Science Plan (How does Earth’s climate system respond to elevated levels of atmospheric CO2? How resilient is the ocean to chemical perturbations?). Furthermore, Expedition 378 will provide material from the far South Pacific Ocean in an area with no previous scientific drilling as part of a major regional slate of expeditions in the Southern Ocean to fill a critical need for high-latitude climate reconstructions. The operational plan is to occupy seven primary sites (with two proposed alternate sites) along an east–west transect to recover the most complete sedimentary succession possible, which includes coring three holes at each site with wireline logging operations at the two deepest penetration sites. Basement will be tagged in at least one of the holes at each site.