Collaborative Research: Was there an anomalous Indian Ocean carbon reservoir? Searching for old radiocarbon and quantifying Ice Age changes in deep Indian Ocean ventilation

Carbon cycles between the earth, ocean, and atmosphere on geological timescales. The relative amount of carbon stored in each of these respective locations is one important control on Earth’s average temperature. During the last ice age 18,000 years ago, carbon moved out of the atmosphere causing decreased global temperatures. However, it remains unknown where that carbon was stored. This project uses radiocarbon dating to test whether the deep Indian Ocean stored substantial amounts of carbon across the last ice age. The researchers will study microscopic fossils in four sediment cores from across the Indian Ocean basin. These tiny fossil shells made by single-celled organisms record the radiocarbon content of the seawater they lived in at the time of their growth. By comparing the radiocarbon age recorded in these microfossils to the age of the sediment, researchers will determine if deep Indian Ocean carbon was abnormally old during the last ice age. The size of the sediment grains will reveal how quickly currents at the bottom of the ocean flowed and how quickly this old, stored carbon could be transported out of the deep Indian Ocean and released back to the atmosphere. Combined, these data will tell researchers whether changes in deep Indian Ocean carbon were—or were not—important for past changes in Earth’s temperature. The work will support early career scientists and provide research opportunities for graduate students. Seawater radiocarbon content is a powerful tracer of air-sea carbon dioxide exchange and ocean carbon storage. Existing observations suggest that Indian Ocean Bottom Water radiocarbon was significantly lower (i.e., having a much older radiocarbon ventilation age) than all other ocean basins during the Last Glacial Maximum, which could reflect a much slower overturning of these waters and enhanced carbon storage via the biological carbon pump. If these existing measurements accurately reflect the entire Indian Ocean, they suggest that the carbon sequestration capacity of the glacial Indian Ocean has been greatly underestimated. In this project, researchers will create four new glacial- interglacial records of Indian Ocean Bottom Water radiocarbon to answer the following question: Do the existing (very old) Indian Ocean Bottom Water records accurately represent the region? In addition to answering this primary question, collaborators will analyze sortable silt content to estimate ocean current speeds and build this data into an inverse model to improve understanding of the processes driving the observed changes. This project will fund a graduate student and will continue the “First Gen BEES” (Becoming an Earth & Environmental Scientist) program initially designed and produced by first generation college students. 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.