Today, the 'normal' mean state of the tropical Pacific is temporarily perturbed by short-lived El Nino events, which have global climate effects. However, there is growing evidence that the mean state has not been stable over long (geologic) timescales due to changes in sub-surface oceanic (thermocline) conditions, and that changes in the mean state have profound global climate effects that could explain the onset of the ice ages. The proposed study will focus on the last 5 Ma of earth history, which includes the early Pliocene warm period, the most recent period of sustained global warmth relative to today, and the global cooling that led to the ice ages. This project will test the hypotheses that 1) the early Pliocene warm period included El Nino-like mean conditions and the transition to cooler ice age climate was accompanied by a gradual shoaling of the thermocline and a switch from permanent 'El Nino-like' to modern 'normal' conditions and 2) higher frequency (41 kyr, glacial-interglacial) sea surface temperature (SST) variability in the tropical Pacific is related to changes in thermocline conditions. To address these hypotheses, we will analyze the elemental (Mg/Ca) and stable isotope composition of fossil foraminifera as well as organic biomarkers and sediment composition from deep sea sediments to reconstruct SST, precipitation, wind strength, and thermocline depth. This study will contribute to a fundamental understanding of the role of tropical Pacific processes in global climate change and will provide a data set for modelers to improve our interpretations and test their hierarchy of models. A graduate student and undergraduates will conduct research on this project. Research-based learning is encouraged for undergraduates. The PI's are involved with initiatives and/or established programs to increase student diversity and to expose high school students to cutting-edge science.
