Expedition 363 summary

Abstract

International Ocean Discovery Program Expedition 363 sought to document the regional expression and driving mechanisms of climate variability (e.g., temperature, precipitation, and productivity) in the Indo-Pacific Warm Pool (IPWP) as it relates to the evolution of Neogene climate on millennial, orbital, and geological timescales. To achieve our objectives, we selected sites with a wide geographical distribution and variable oceanographic and depositional settings. Nine sites were cored during Expedition 363, recovering a total of 6956 m of sediment in 875–3421 m water depth with an average recovery of 101.3% during 39.6 days of on-site operations. Two moderate sedimentation rate (~3–10 cm/ky) sites are located off northwestern Australia at the southwestern maximum extent of the IPWP and span the late Miocene to present. Seven of the nine sites are situated at the heart of the Western Pacific Warm Pool (WPWP), including two sites on the northern margin of Papua New Guinea with very high sedimentation rates (>60 cm/ky) spanning the past ~450 ky, two sites in the Manus Basin (north of Papua New Guinea) with moderate sedimentation rates (~4–14 cm/ky) recovering upper Pliocene to present sequences, and three sites with low sedimentation rates (~1–3 cm/ky) on the southern and northern Eauripik Rise spanning the early Miocene to present. The wide spatial distribution of the cores, variable accumulation rates, exceptional biostratigraphic and paleomagnetic age constraints, and mostly excellent or very good foraminifer preservation will allow us to trace the evolution of the IPWP through the Neogene at different temporal resolutions, meeting the primary objectives of Expedition 363. Specifically, the high–sedimentation rate cores off Papua New Guinea will allow us to better constrain mechanisms influencing millennial-scale variability in the WPWP, their links to high-latitude climate variability, and implications for temperature and precipitation in this region under variable mean-state climate conditions. Furthermore, the high accumulation rates offer the opportunity to study climate variability during previous warm periods at a resolution similar to that of existing studies of the Holocene. With excellent recovery, Expedition 363 sites are suitable for detailed paleoceanographic reconstructions at orbital and suborbital resolution from the middle Miocene to Pleistocene and thus will be used to refine the astronomical tuning, biostratigraphy, magnetostratigraphy, and isotope stratigraphy of hitherto poorly constrained intervals within the Neogene timescale (e.g., the late Miocene) and to reconstruct the history of the Asian-Australian monsoon and the Indonesian Throughflow on orbital and tectonic timescales. Results Y. Rosenthal et al. Expedition 363 summary from high-resolution interstitial water sampling at selected sites will be used to reconstruct density profiles of the western equatorial Pacific deep water during the Last Glacial Maximum. Additional geochemical analyses of interstitial water samples in this tectonically active region will be used to investigate volcanogenic mineral and carbonate weathering and their possible implications for the evolution of Neogene climate. Introduction The Western Pacific Warm Pool (WPWP), often defined by the 28°C isotherm, is the warmest part of the Indo-Pacific Warm Pool (IPWP), which spans the western waters of the equatorial Pacific and eastern Indian Ocean (Figure F1). The region is a major source of heat and moisture to the atmosphere and a location of deep atmospheric convection and heavy rainfall. Small perturbations in the sea-surface temperature (SST) of the WPWP influence the location and strength of convection in the rising limbs of the Hadley and Walker cells, affecting planetary-scale atmospheric circulation, atmospheric heating, and tropical hydrology (Neale and Slingo, 2003; Wang and Mehta, 2008). These perturbations may also influence heat uptake and storage in the WPWP thermocline, as well as transport to the Indian Ocean through the Indonesian Throughflow (ITF). These processes also constitute important feedbacks in the climate system due to their influence on the dynamic ocean-atmosphere coupling in the equatorial Pacific, thereby exerting a strong influence on global temperatures and atmospheric pCO2 (Oppo and Rosenthal, 2010). Detailed paleoceanographic records, such as those recovered during Expedition 363, offer the opportunity to study the behavior of the WPWP under different mean-state background conditions and its effects on both regional and global climate. Seasonal to interannual climate variations in the WPWP are dominated by fluctuations in precipitation associated with the seasonal march of the monsoons, migration of the Intertropical Convergence Zone (ITCZ), and interannual changes associated with variability of the El Niño Southern Oscillation (ENSO) (e.g., Ropelewski and Halpert, 1987; Halpert and Ropelewski, 1992; Rasmusson and Arkin, 1993) (Figure F2). At present, departures from expected weather patterns associated with the monsoon and ENSO systems impact the lives of many people in the tropics and many regions around the world. For example, El Niño events are associated with a nearly global fingerprint of temperature and precipitation anomalies (Ropelewski and Halpert, 1987; Rasmusson and Arkin, 1993; Cane and Clement, 1999). However, considerable uncertainty exists regarding the response of the tropical Pacific climate, primarily precipitation, to rising greenhouse gas concentrations because of our limited understanding of the past variability of the WPWP and conflicting results from data compared to models. For example, models simulating the response of the equatorial Pacific Ocean to greenhouse gas forcing disagree about whether the zonal temperature gradient will increase or decrease and what the implications will be for the Walker circulation and the hydrologic cycle in the tropics (Forster et al., 2007). These simulations typically use an ENSO analogy to predict future climate; in a similar way to interannual ENSO variability, long-term changes in the mean climate state of the equatorial Pacific are often evaluated primarily as changes in the east–west SST gradient. However, other simulations of global warming effects suggest that the tropical Pacific does not become more El Niñoor La Niña-like in response to increased greenhouse gases (DiNezio et al., 2009). Instead, these simulations suggest a different equilibrium state, whereby shoaling and increased tilt of the equatorial Pacific thermocline is associated with weakening of the trade winds without a concomitant change in the zonal SST pattern, which argues against using ENSO as an analog for long-term changes in tropical conditions (DiNezio et al., 2010). In turn, changes in the structure of the thermocline can have a major effect on the ocean heat content, and thus global climate, and also on the character of ENSO variability, which has been documented for the Last Glacial Maximum (LGM) (Ford et al., 2015). Changes in thermocline temperature/structure have also been suggested as a possible mechanism responsible for the slowdown in surface warming from ~2000 to 2014 (e.g., England et al., 2014). A primary goal of this expedition was to assess the regional expression of climate variability (e.g., precipitation, temperature, pCO2, and biological productivity) within the WPWP in the context Figure F1. Mean annual sea-surface temperature within the IPWP with locations of sites cored during Expedition 363 (yellow circles). Black arrows mark the path of the Indonesian Throughflow. Red arrow = Leeuwin Current. Blue arrow = South Equatorial Current. Data source: ODV World Ocean Atlas (https://odv.awi.de/en/data/ocean/world-ocean-atlas-2013).