The Pliocene Expansion of the Indo-Pacific Warm Pool

The Indo-Pacific Warm Pool (IPWP) is the largest mass of very warm water (warmer than 28 °C) in the modern ocean, and a major reservoir and source of heat and moisture to the atmosphere. As such, the evolution of the IPWP has major consequences for reconstructions of tropical and global climate processes. Here, we produced a Mg/Ca record of surface-dwelling planktonic foraminifera (G. sacculifer, without the sac-like final chamber) to reconstruct sea surface temperature (SST) changes in the IPWP over the past 6 Myrs. We used three sites drilled during IODP Expedition 363 at either side of the Indonesian Throughflow: one Pacific Ocean site (U1488) north of Papua New Guinea, and two Indian Ocean sites (U1482 and U1483) -which together generate a single 6 Myr record (U1483/82)- on the NW Australian margin. These sites were sampled at high resolution (~3 kyrs) for intervals, between 40k and 60k years long, spaced over 6 Myrs in order to capture a range of glacial-interglacial temperature changes since the Miocene/Pliocene transition.

First, without incorporating possible changes in the Mg/Ca of seawater, we find no pronounced temperature difference at site U1488 throughout the record with average temperatures similar to today. During the Pliocene, both sites exhibit similarly stable temperature trends, staying within 1 °C of each other for most of the record until at least ~1.8 Ma. After ~1.8 Ma, site U1483 begins to slowly cool relative to U1488, and the youngest average temperatures are ~3 °C colder at U1483 than at U1488. This result, which is independent from changes in Mg/Ca of seawater, suggests that prior to the mid-Pliocene the IPWP was expanded relative to its modern extent, but not significantly warmer. Additionally, the oldest intervals (before 3.3 Ma) have narrower temperature ranges at both sites, while the younger intervals have colder cool temperatures, indicating weaker glacial periods in the Pliocene relative to the Pleistocene at both sites. To assess possible changes in Mg/Ca of seawater, we generated clumped-isotope temperature estimates for several intervals at both sites. There does not appear to be a temporal correlation in the difference between these two temperature estimates, suggesting that our Pliocene Mg/Ca SSTs are not systematically biased due to changes in Mg/Ca of seawater.