Investigating Deglacial Climate Records: The Carbon Isotope Minimum Event and High-Resolution Radiocarbon Ages
Kathryn Rose1, Elisabeth Sikes2, Thomas P. Guilderson3, Rainer Zahn 4, Tessa M. Hill1, Howard Spero1
1Department of Geology, University of California, Davis
4 Institució Catalana de Recerca i Estudis Avançats, Universitat Autònoma de Barcelona, Institut de Ciencia i Tecnologia Ambientals, E-08193 Bellaterra (Cerdanyola), Spain
Institute of Marine and Coastal Sciences, Rutgers University
Center for Accelerator Mass Spectrometry, University of California/ Lawrence Livermore National Laboratory, USA
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 that the low d13C signal is derived from poorly ventilated waters in the deep ocean. These waters would have contained radiogenically “old” 14C-depleted carbon that was transmitted through the thermocline via Antarctic Intermediate Water (AAIW) and/ or Subantarctic Mode Water (SAMW).
In this study, we reconstruct the reservoir age history of the CIME by comparing stable isotope (C and O) and radiocarbon (14C) data from two high resolution sediment cores from chemically distinct water masses in the Southwest Pacific: one core located in the Southern Ocean; south of the Subtropical Frontal Zone (on the Chatham Rise; MD97-2120 (43°32.06’S, 174°55.85’E; 1210 m)) and a second core that is located in subtropical water, north of the Chatham Rise, RR0503-64 JPC (37°25.34’S, 174°55.85’E; 650 m). Radiocarbon and stable isotope data from the LGM (~20 kybp) to ~14 Ka, were obtained from planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma (s) (sub-polar core MD97-2120), and Globorotalia inflata (sub-tropical core RR0503-64). Our data indicate that the CIME is present in both sub-tropical and sub-polar waters. Radiocarbon data show that a large decrease in 14C reservoir age occurred within sub-polar waters in G. bulloides and N. pachyderma (s). immediately prior to the initiation of the CIME (20 Ka), thereby yielding significantly younger 14C ages of 12.9 to14.0 kyr, respectively. The reservoir age discrepancy is absent from 14C analyses of G. inflata, which inhabits sub-tropical thermocline waters. However, mixed benthic ages centered in Subtropical Mode Waters (STMW) suggest that the signal advected below thermocline waters but above STMW in the South Pacific sub-tropical gyre. These new results are consistent with previously documented 14C ages from the Eastern Equatorial Pacific, which also indicate excess radiocarbon at the initiaion of the CIME (Spero et al in prep). We hypothesize that the 14C signal is related to waters with a “young” carbon source contributing to upper SAMW and advecting into the subpolar thermocline at the start of southern hemisphere deglaciation.
