The Carbon Isotope Minimum Event and the History of Radiocarbon Ages Across the Deglaciation

The most recent transition from the Last Glacial Maximum (LGM) into the deglaciation was associated with an abrupt decrease in the carbon isotope composition of planktonic foraminifera. These low 13C/12C ratios, referred to as the carbon isotope minimum event (CIME), have been linked to changes in Southern Ocean circulation at the end of the LGM (Spero & Lea, 2002). One hypothesis for the origin of the CIME suggests the low δ13C waters are derived from an `old' carbon, deepwater source. However, recent data suggest the initiation of the CIME is associated with an excess 14C age anomaly that is not consistent with an `old' carbon mechanism (Spero, Miller and Guilderson, in prep). We present new radiocarbon data from high-sedimentation rate core MD97-2120 (43.32°S, 174.55°E; water depth 1210 m, mean sed. rate ~16 cm/kyr) from the South Chatham Rise, in the Southwest Pacific, which expands on previously published isotope and radiocarbon data by Pahnke et al. (2003). Published radiocarbon and carbon isotope analyses indicate the CIME occurs between ~16-20 cal kyr. The timing of the age anomaly and CIME are not fully understood but have the potential to reveal important clues concerning the reduction of deep-water production or a change in the source water of AAIW. New radiocarbon analyses from the planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma (s.), at the initiation of the CIME, yield 14C ages between 18.0 and 18.3 kyr. These data resolve an age discrepancy originally published by Pahnke et al. (2003). We will present a continuous high-resolution radiocarbon dataset across the CIME in order to document its surface reservoir age history and characterize the extent of the age anomaly during the CIME. The age anomaly at the initiation of the CIME suggests the mechanism of the event is related to a `young' carbon source advecting into AAIW or the subsurface thermocline at the start of southern hemisphere deglaciation. The reoccurrence of a CIME at the start of the last few glacial terminations suggests the event reveals a fundamental component of the initial processes leading to the transition from glacial to interglacial climate modes. A greater understanding of the CIME will provide insight into the forcing mechanisms and responses of ocean circulation to climate change.