Developing Replicated Coral Geochemical Reconstructions to Understand South Pacific Convergence Zone Dynamics in a Changing Climate

The South Pacific Convergence Zone (SPCZ) is a high rainfall region of the atmosphere. It is the largest and most constant spur of the global Intertropical Convergence Zone (ITCZ) covering a large area of the tropical and subtropical South Pacific. The SPCZ and ITCZ are important parts of the global water cycle. These regions contribute around 30% of total global precipitation and serve as source regions for atmospheric moisture transfer to higher latitudes. In addition, the SPCZ is the main feature controlling Southern Hemisphere hydroclimate and South Pacific tropical cyclones. Despite the importance of the SPCZ, there are questions about whether the SPCZ is intensifying and the factors that control its position. The researchers have previously generated oxygen isotope data from coral cores collected from four sites in the SPCZ (American Samoa, Fiji, Tonga, and Rarotonga). These data record ocean-atmosphere conditions from the present back several centuries. A key question is if the observed coral oxygen isotope trend in the SPCZ is due to surface ocean warming, greater rainfall, or both. This project will analyze temperature sensitive elements preserved in these same coral cores to reconstruct multi-year and long-term trends in ocean temperature and rainfall. This work will allow for a better understanding of the influence global warming, El Niño events, and volcanic eruptions have on the SPCZ. The results may also help in predicting future SPCZ behavior as the climate changes. Further, this project will support a PhD student, undergraduate students, and high school students.The overarching goal of this research is to better understand variability in the South Pacific Convergence Zone (SPCZ) size and position over the last several hundred years. The project will address the question of how did the SPCZ respond to global climate change and western Pacific warming in the 1700s and 1800s compared to the late 20th century? Important climate uncertainties include what ocean-atmosphere parameters control the long-term position of the SPCZ and whether the SPCZ is currently intensifying. Coral oxygen isotope (d18O) time series extending back to the 1500s and 1600s CE from American Samoa, Fiji, Tonga and Rarotonga in the SPCZ region all contain a long-term secular trend towards lower (more negative) d18O, but the ocean-atmosphere significance is unclear. Given the importance of the SPCZ to southern Hemisphere hydroclimate and the combined ITCZ+SPCZ to global hydroclimate, understanding the significance of these coral d18O trends is a high priority paleoclimate objective with implications for future SPCZ changes. The amplitude of the d18O trend at American Samoa, Fiji, and Rarotonga is ~ 0.6‰ which is ~ 4X greater than expected if the forcing is the 0.5°C sea surface temperature (SST) warming since the 1850s suggested by instrumental sea surface temperature. Recent work interpreted the trends towards lower d18O in Fiji and Rarotonga corals to be evidence of an expansion and/or intensification of the SPCZ to the southeast since the 1800s. But gridded instrumental SST products are known to be based on sparse data before the mid-1900s, and nothing is known about SST annual means prior to the 1850s. To address the past SST uncertainty in the SPCZ region the researchers have demonstrated the potential of using coral skeletal Li/Mg and Sr/Ca measured by inductively coupled plasma mass spectrometry (ICP-MS) as SST proxies in this region. The goals of this project are to generate replicated d18O, Li/Mg and Sr/Ca time-series on six Porites lutea coral cores from American Samoa, Fiji and Tonga. Work on this project will allow both the reconstruction of SST, but also seawater salinity and rainfall. Replication will provide chronology checks and also additional error estimation. Analysis of this unique collection of long coral cores and replication approach will lead to a better understanding of SPCZ long-term dynamics and will facilitate modeling efforts to predict future changes as the Earth warms.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.