时间：2019年12 月30日 9：40-10：40
Cenozoic climate has transitioned from a warmhouse to an icehouse, manifesting as decreasing seawater temperature and atmospheric CO2. What caused the changes in temperature and CO2, however, has not been fully resolved. Since ocean chemistry is closely coupled with global carbon cycle on multi-million-year timescales, studies on ocean chemistry can provide potentials to understand long-term changes in carbonate system including atmospheric CO2. Previous studies show that the concentrations of major ions in seawater such as Mg and Ca ([Mg]sw and [Ca]sw) have changed over the Cenozoic, but existing proxies yielded inconsistent [Ca]sw records with large uncertainties. Therefore, new proxies are needed to constrain the secular change in [Ca]sw. Here I propose to use Na/Ca in foraminifera as a proxy for [Ca]sw. This proxy is testified by culture experiments and core-top calibrations, showing that foraminiferal Na/Ca increases as [Ca]sw decreases. The down core Na/Ca records of three species of foraminifera are generated from multiple locations in the Atlantic, Pacific, and Indian Ocean. A generally increasing trend in the Na/Ca records toward present suggests a monotonic decrease in [Ca]sw since Eocene. Combined with [Mg]sw record from fluid inclusions, it suggests an increase in (Mg/Ca)sw since then. Such a long-term change in [Ca]sw and (Mg/Ca)sw may be a result of coupled changes in continental weathering and hydrothermal flux. Coupled records of [Ca]sw and carbonate saturation state may help explain the atmospheric CO2 record over the Cenozoic.
Dr. Xiaoli Zhou uses geochemical tools to investigate paleoclimate and paleoceanography. She was trained in geochemistry as an undergraduate at University of Science and Technology of China. After graduating in 2011, she started working with Prof. Zunli Lu as a PhD student at Syracuse University. During her PhD study, she developed redox proxies with Prof. Lu by using carbonate- and organic- bound-iodine, and applied them to multiple hyperthermal events in Earth’s history. The studies suggest that I/Ca in carbonate and I/TOC in bulk sediment can indicate redox conditions of the ocean at different water depths at various temporal resolutions. The results were published in peer-reviewed journals such as Paleoceanography & Paleoclimatology and Chemical Geology with Dr. Zhou as the first author and also in other top journals such as Science and Nature collaborated with scientists around the world. After receiving her PhD degree in 2016, Dr. Zhou joined Prof. Yair Rosenthal’s group as a postdoctoral associate at Rutgers University to work on the development of foraminiferal Na/Ca as a proxy for seawater calcium concentration over the Cenozoic Era. The preliminary results were presented at the Goldschmidt conference in 2018 and received extensive attention and comments. This new proxy has the potential to reconstruct seawater calcium concentration throughout the Cenozoic at a high resolution, and shed light on global calcium cycle with connections to climate change and carbon cycle.