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 10th, 15th, and 17th 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, 15th and 17th 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 17th century, when ENSO was very prominent, averaged -51‰°, compared to -57‰° and -60‰° during the 15th and 10th 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.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1637 Regional climate change;
- 4916 Corals (4220);
- 4918 Cosmogenic isotopes (1150)