Coral records of central tropical Pacific radiocarbon variability during the last millennium

Abstract

PALEOCEANOGRAPHY, VOL. 25, PA4212, doi:10.1029/2009PA001788, 2010 Coral records of central tropical Pacific radiocarbon variability during the last millennium Laura K. Zaunbrecher, 1,2 Kim M. Cobb, 1 J. Warren Beck, 3 Christopher D. Charles, 4 Ellen R. M. Druffel, 5 Richard G. Fairbanks, 6 Sheila Griffin, 5 and Hussein R. Sayani 1 Received 12 May 2009; revised 18 May 2010; accepted 7 June 2010; published 10 November 2010. [ 1 ] The relationship between decadal to centennial changes in ocean circulation and climate is difficult to discern using the sparse and discontinuous instrumental record of climate and, as such, represents a large uncertainty in coupled ocean‐atmosphere general circulation models. We present new modern and fossil coral radiocarbon (D 14 C) records from Palmyra (6°N, 162°W) and Christmas (2°N, 157°W) islands to constrain central tropical Pacific ocean circulation changes during the last millennium. Seasonally to annually resolved coral D 14 C measurements from the 10th, 12th–17th, and 20th centuries do not contain significant interannual to decadal‐scale variations, despite large changes in coral d 18 O on these timescales. A centennial‐scale increase in coral radiocarbon from the Medieval Climate Anomaly (∼900–1200 AD) to the Little Ice Age (∼1500–1800) can be largely explained by changes in the atmospheric D 14 C, as determined with a box model of Palmyra mixed layer D 14 C. However, large 12th century depletions in Palmyra coral D 14 C may reflect as much as a 100% increase in upwelling rates and/or a significant decrease in the D 14 C of higher‐ latitude source waters reaching the equatorial Pacific during this time. SEM photos reveal evidence for minor dissolution and addition of secondary aragonite in the fossil corals, but our results suggest that coral D 14 C is only compromised after moderate to severe diagenesis for these relatively young fossil corals. Citation: Zaunbrecher, L. K., K. M. Cobb, J. W. Beck, C. D. Charles, E. R. M. Druffel, R. G. Fairbanks, S. Griffin, and H. R. Sayani (2010), Coral records of central tropical Pacific radiocarbon variability during the last millennium, Paleoceanography, 25, PA4212, doi:10.1029/2009PA001788. 1. Introduction [ 2 ] Ocean circulation changes in the tropical Pacific strongly influence global climate, as demonstrated during El Nino‐Southern Oscillation (ENSO) extremes. During strong El Nino events, a relaxation of the trade winds results in a large reduction of the equatorial upwelling of cooler subsurface waters and reshapes the wind‐driven surface currents in the tropical Pacific [Taft and Kessler, 1991]. This reorganization of equatorial currents causes anomalously warm waters in the eastern and central tropical Pacific, ulti- mately driving a reorganization of the large‐scale global atmospheric circulation. While instrumental data resolve seasonal to interannual variability in tropical Pacific circu- lation [Picaut and Tournier, 1991; Donguy and Meyers, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA. Now at Department of Geosciences, Georgia State University, Atlanta, Georgia, USA. Physics and Geosciences Department, University of Arizona, Tucson, Arizona, USA. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA. Earth System Science Department, University of California, Irvine, California, USA. Earth and Planetary Science Department, Rutgers, State University of New Jersey, New Brunswick, New Jersey, USA. Copyright 2010 by the American Geophysical Union. 0883‐8305/10/2009PA001788 1996], the nature of decadal to centennial‐scale changes in tropical Pacific circulation remain unknown. Resolving such low‐frequency ocean circulation variability and its rela- tionship to low‐frequency regional and global climate changes is critical to the improvement of ocean models used for climate prediction. [ 3 ] Radiocarbon ( 14 C) is a useful tracer of water mass mixing, as deep waters that have been isolated from the atmosphere are depleted in 14 C due to radioactive decay, whereas surface waters are relatively enriched. Large sur- face water 14 C gradients arise from horizontal and vertical mixing – upwelling brings relatively depleted 14 C waters to the ocean surface whereas prolonged air‐sea gas exchange in the mid‐ocean gyres drives 14 C enrichment in these areas. Thus, regional water masses are ‘tagged’ with a distinct 14 C signature depending on the regional oceanographic setting. Changes to regional seawater 14 C values through time imply changes in either horizontal or vertical mixing. [ 4 ] Corals are useful tools for the reconstruction of sea- water 14 C signatures as they incorporate the 14 C of the dissolved inorganic carbon of the seawater in which they grow into their skeletal matrix, and can live for decades to centuries [Druffel and Linick, 1978; Dunbar and Cole, 1999; Druffel et al., 2007]. Annual band counting in mod- ern corals and/or U/Th dating in fossil corals ensure accurate, C‐independent absolute chronologies for the construction of coral‐based records of seawater radiocarbon variability through time. Indeed, the incorporation of “bomb 14 C” PA4212 1 of 15