HR: 0800h
AN: GC51A-0167    [Abstracts]
TI: Radiocarbon Variability in the Tropical Pacific During the Last Millennium
AU: * Zaunbrecher, L K
EM: laura.zaunbrecher@eas.gatech.edu
AF: Georgia Institute of Technology, Department of Earth and Atmospheric Sciences 311 Ferst Drive NW, Atlanta, GA 30332, United States
AU: Cobb, K M
EM: kcobb@eas.gatech.edu
AF: Georgia Institute of Technology, Department of Earth and Atmospheric Sciences 311 Ferst Drive NW, Atlanta, GA 30332, United States
AU: Druffel, E R
EM: edruffel@uci.edu
AF: University of California, Irvine, Earth System Science Department, Irvine, CA 92697, United States
AU: Griffin, S M
EM: sgriffin@uci.edu
AF: University of California, Irvine, Earth System Science Department, Irvine, CA 92697, United States
AB: The strength of upwelling in the tropical Pacific strongly influences global climate, as demonstrated during El Niño Southern Oscillation (ENSO) extremes. Understanding the causes of past variability in tropical Pacific upwelling will help to predict how future climate may evolve under radiative forcing caused by the increase in greenhouse gases. We measured radiocarbon (¹⁴C) records in corals collected from Palmyra (6°°N, 162°°W) and Christmas (2°°N, 157°°W) Islands, located in the central tropical Pacific, to reconstruct high-resolution variations in tropical Pacific upwelling over the last millennium. Corals incorporate ¹⁴C/¹²C ratios equal to those in the dissolved inorganic carbon of surrounding seawater, and thus provide monthly-resolved records of surface ocean radiocarbon concentrations back through time. Surface ocean waters are more enriched with radiocarbon than deep waters, owing to the atmospheric source for radiocarbon. Upwelled waters are depleted in radiocarbon because significant radioactive decay has occurred during their isolation in the deeper ocean. Previous work with central tropical Pacific corals has determined that upwelled waters are approximately -55‰‰ to -65‰‰ in this region, while currents flowing from the west Pacific to the east Pacific carry waters with a higher ¹⁴C signature, roughly -38‰, and waters from the east have a value of -72‰‰ (Konishi et. al, 1981, Druffel, 1981). Therefore, variations in ¹⁴C in the Palmyra and Christmas corals reflect the mixing of the cool, radiocarbon-depleted, waters associated with equatorial upwelling and the warm, radiocarbon-enriched waters advected from the western tropical Pacific. Oxygen isotopic (δ¹⁸O) analyses of the Palmyra and Christmas fossil corals reveal a rich climatic history of interannual to centennial variability (Cobb et al., 2003). Our approach targets specific time intervals associated with strong interannual to centennial δ¹⁸O anomalies for high-resolution radiocarbon analysis. We compare seasonally-resolved radiocarbon measurements from the Palmyra and Christmas modern corals to their fossil counterparts. The 1941 El Niño event is not associated with a significant radiocarbon departure, despite significant changes in upwelling and horizontal advection inferred from sea-surface temperature records and coral δ¹⁸O data. Indeed, high-resolution radiocarbon sampling of the 10^th, 15^th, and 17^th century fossil corals reveals no significant radiocarbon variations associated with ENSO activity. Decadal to centennial-scale variations in radiocarbon variability are resolved by annually-averaged samples during the 10%^th, 15^th and 17^th centuries. These analyses indicate that during active ENSO periods, the corals are more enriched in ¹⁴C than during suppressed ENSO activity. Coral radiocarbon from the 17^th century, when ENSO was very prominent, averaged -51‰‰, compared to -57‰‰ and -60‰‰ during the 15^th and 10^th centuries, respectively. Discussion of the mechanisms that would allow low variability in ¹⁴C, and large changes in δ¹⁸O, including the variations in response times of the corals to incorporating temperature differences versus radiocarbon will be presented.
DE: 1637 Regional climate change
DE: 4916 Corals (4220)
DE: 4918 Cosmogenic isotopes (1150)
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting