Earth Science Student Research Grants Recipients 2006

Investigating Deglacial Climate Records: The Carbon Isotope Minimum Event and High-Resolution Radiocarbon Ages

Kathryn Rose

Many studies have focused on the transition from the Last Glacial Maximum (LGM) into the deglaciation as a foundation for understanding the mechanisms controlling global climate change. One pervasive characteristic of glacial terminations over the last 400,000 years is reorganizations in the ocean-atmosphere system. Associated with these reorganizations are perturbations in the carbon isotope record ( 13C, the ratio of 13C to 12C) in fossil foraminifera, used to trace changes in deep ocean circulation. A characteristic feature of the early deglacial record at each transition is an abrupt decrease in 13C of seawater as recorded by fossil foraminifera. These low 13C/12C ratios have been documented by studies throughout the Southern Hemisphere as the carbon isotope minimum event (CIME; Spero & Lea, 2002). The reoccurrence of the CIME on the last few glacial terminations suggests a strong linkage to processes driving the abrupt transition from glacial to interglacial climate modes. However, conclusive evidence for the mechanisms of this event remains elusive. One hypothesis for the origin of the CIME suggests that low 13C signals derived from poorly ventilated waters in the deep ocean contain 13C-depleted "old" carbon that was transmitted through the thermocline via Antarctic Intermediate Water (AAIW). However, new data obtained recently (Spero et al., in prep) suggest the CIME is associated with a young, excess radiocarbon (14C) anomaly that is not consistent with an 'old' carbon mechanism. The timing of the 14C anomaly and the CIME are not fully understood but have the potential to reveal important clues concerning the source of carbon cycle changes at the start of southern hemisphere deglaciation that will provide insight into the forcing mechanisms and responses of the ocean at glacial terminations. Therefore, I will use geochemical analyses on two cores from chemically distinct water masses to determine whether the structure and timing of the CIME and associated 14C anomaly is consistent in sub-polar and sub-tropical waters of the Southern Hemisphere.