International Ocean Discovery Program Expedition 378 Scientific Prospectus Addendum

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, bioand 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 highlatitude 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. Expedition schedule International Ocean Discovery Program (IODP) Expedition 378 is based on IODP drilling Proposal 567-Full4 (http://iodp.tamu.edu/scienceops/expeditions/south_pacific_ paleogene_climate.html). Following evaluation by the IODP Scientific Advisory Structure, the expedition was scheduled for the R/V JOIDES Resolution, operating under contract with the JOIDES Resolution Science Operator (JRSO). At the time of publication of this Scientific Prospectus, the expedition is scheduled to start in Lyttelton, New Zealand, on 14 October 2018 and end in Papeete, Tahiti, on 14 December. A total of 61 days will be available for the initial port call, transit, drilling, coring, and downhole measurements described in this report. For the current detailed schedule, see http://iodp.tamu. edu/scienceops/. Further details about the facilities on board the JOIDES Resolution can be found at http://iodp.tamu. edu/publicinfo/drillship.html. Introduction The South Pacific Paleogene Climate (SPLAT) science program is based on IODP Proposal 567-Full4. The goal is to investigate the record of Cenozoic climate and oceanography through a drilling transect in the high-latitude southern Pacific Ocean. In particular, we will target shallowly buried, carbonate-bearing sediments deposited during the very warm late Paleocene and early Eocene, including the Paleocene/Eocene boundary and Eocene–Oligocene transition, to investigate how the Eocene Earth maintained high global temperatures and high heat transport to the polar regions despite receiving near-modern levels of solar energy input. Investigation of the recovered sediments also will constrain the subpolar Pacific Ocean climate, oceanographic structure, and biogeochemical cycling of much of the Cenozoic. Recovered sediments will be used to characterize water masses, deep and shallow ocean temperature, latitudinal temperature gradients, the strength of upwelling, and the strength of the zonal winds to study both the atmospheric and oceanic climatic subsystems. Expedition 378 will investigate the system of convergences, divergences, and mixing zones in subpolar to polar latitudes. A zone of high biological productivity is associated with physical mixing at the polar front and the upwelling and divergence region poleward of the polar front. The array of seven primary sites and two alternate sites were chosen to encompass 33° of longitude (Deep Sea Drilling Project [DSDP] Site 277 at 166°E and proposed Site SP-14A at 133°W) and 12° of latitude (Site 277 at 52°S and proposed Site SP15A at 40°S), providing comprehensive regional coverage to achieve the expedition objectives (Figure F1). The sites planned for Expedition 378 are located across the early Paleogene transition from subtropical to subpolar surface water masses. This transect spans the region where intermediate waters are formed in the modern ocean. Although it is debated where and how these waters might have formed in the Eocene, their locus of formation can be monitored by the strength of oceanic fronts and convergences in the South Pacific, as recorded by numerous proxies. The width of the subpolar front measures the strength of convergence and the extent to which Antarctic Intermediate Water is formed. In the process of measuring these gradients, the drill sites along the transect will define the physical size of the subtropical gyre and the subpolar climate zone during the very warm climates of the early Paleogene. One of the primary needs for both atmospheric and oceanic circulation models is robust sea-surface temperature (SST) control. These data form a major boundary condition for constraining these models, the most easily observable signal of circulation patterns. Temperature gradient can be measured in a relative sense even if the absolute calibrations are off with respect to the Subantarctic or equatorial region. Redrilling Site 277 will allow for the direct sampling of intermediate water compositions in addition to making use of the latitudinal gradient. The Drake Passage (or Drake Isthmus) separated the abyssal South Atlantic from the abyssal South Pacific through much of the Paleogene (there was likely a shallow connection via the Transantarctic Seaway prior to the opening of the Drake Passage), and it is likely that South Pacific thermohaline structure was different from the South Atlantic. Expedition 378 will allow researchers