HR: 0800h
AN: PP21C-1580    [Abstracts]
TI: Seasonal Variation in Upwelling Recorded in Peruvian Mollusks: Implications for Reservoir Correction Estimates.
AU: * Andrus, C T
EM: fandrus@geo.ua.edu
AF: Department of Geological Sciences, 202 Bevill Building, University of Alabama, Tuscaloosa, AL 35487 United States
AU: Hodgins, G W
EM: ghodgins@physics.arizona.edu
AF: National Science Foundation-Arizona Accelerator Mass Spectometry Facility, Department of Physics, University of Arizona, Tuscon, AZ 85721 United States
AU: Sandweiss, D H
EM: Dan_Sandweiss@umit.maine.edu
AF: Department of Anthropology and Institute for Quaternary and Climate Studies, 120 Alumni Hall, University of Maine, Orono, ME 04469 United States
AB: A Protothaca asperima clam from Paracas, Peru (13° 50' S) was analyzed via accelerator mass spectrometry (AMS) for Δ¹⁴C through ontogeny. AMS microsampling was conducted at high temporal resolution (19 samples over approximately two years of growth). The clam was collected in 1948, so no nuclear bomb-derived radiocarbon is present and thus the Δ¹⁴C profile is a function of the combined effects of vertical mixing and ventilation age of the local waters in which the clam grew. Δ¹⁴C varies through ontogeny following a generally sinusoidal pattern in tandem with growth increments. The ¹⁴C age range in this sample spans 479±39 to 627±31 yrs BP. This is equivalent to ΔR (local variation in reservoir effect) oscillating through ontogeny by approximately 150 years between 1947 and 1948. The lifespan of this clam is generally considered to fall within one of the most stable periods with respect to El Niño/Southern Oscillation (ENSO) during the last century, the source of the greatest variation in upwelling of deep water on the coats of Peru. Therefore the variability measured here was unexpected. Only some portions of the shell we describe fall within the range of ΔR estimated by others, while the rest of the shell displays significantly lower values. This high seasonal variation in ΔR suggests caution in the application reservoir corrections derived from whole-shell analyses, even in the absence of El Niño events occurring during their growth. All of the 7 currently used ΔR estimates from coastal waters from Ecuador, Peru, and Chile are from whole shell analyses of uncertain lifespan that may or may not have experienced El Niño. High variability in ΔR induced by ENSO may be present but undetected in each shell. Additionally, our data presented here suggest that even in the absence of El Niño, ΔR may vary significantly through ontogeny. Without more detailed information of the season and duration of maximal shell growth it is difficult to assess if these whole-shell ΔR measurements are biased. Therefore it may be advisable for future ΔR estimates to be derived from ontogenetic Δ¹⁴C profiles as opposed to whole shell analyses. Further, these data may provide a useful comparative baseline for studies of paleo-upwelling in this region.
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1105 Quaternary geochronology
DE: 4922 El Nino (4522)
DE: 4964 Upwelling (4279)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005