DOI: 10.25921/mbgc-jc86
title: NOAA/WDS Paleoclimatology - Nauru Island - Radiocarbon Data

description: We have generated a high resolution coral Δ14C record spanning the last 50 years to document the seasonal and interannual redistribution of surface waters in the western tropical Pacific. Prebomb (1947–1956) Δ14C values average −63‰ and have a total range of 30‰. Values begin to increase in 1957, reaching a maximum of 137‰ in mid-1983. Large interannual variability of up to 80‰ closely follows the El Niño-Southern Oscillation (ENSO). During each ENSO warm phase, Δ14C values begin to increase, reflecting the reduction of low-14C water upwelling in the east and the invasion of subtropical water into the western equatorial tropical Pacific. Maximum Δ14C values are in phase or lag the corresponding sea surface temperature maxima in the eastern tropical Pacific, whereas the rapid return to more negative Δ14C is in phase with eastern Pacific ENSO indices. The highest-amplitude excursions occur during the 1965/1966 and 1972/1973 events, when the 14C contrast is highest between the eastern Pacific and subtropics. The 1982/1983 El Niño, although a larger ENSO event, has a lower Δ14C amplitude, reflecting the penetration of bomb radiocarbon into the equatorial undercurrent and the reduced contrast in Δ14C between thermocline and subtropical surface waters at that time. This coral record demonstrates the potential for using similar radiocarbon time series for documenting variability in Pacific shallow circulation over interannual and decadal timescales.
description: Coral radiocarbon (Delta-14C) on untreated, low-speed drilled samples. Precision is ±4 per mil as documented by an in-house homogenized coral standard, and presented as AGE CORRECTED DELTA-14C [see Stuiver & Polach, 1977]. Cores were taken from a 1.6 meter Porties spp coral on the north side of Nauru Island (166E 0.5S), 14m bottom depth, in July of 1995 by D. Schrag & M. Moore. Cores were split and cleaned with distilled water. After identifying the major vertical growth axis, the coral was sequentially sampled at 2 mm increments with a low-speed drill. Splits (~1 mg) were reacted in vacuo in a modified autocarbonate device at 90C and the purified CO2 analyzed on a gas source stable isotope ratio mass spectrometer. The remaining sample splits (~10 mg) were placed in individual reaction chambers, evacuated, heated, and then acidified with orthophosphoric acid at 90C. The evolved CO2 was purified, trapped, and converted to graphite in the presence of cobalt catalyst in individual reactors [Vogel et al., 1987]. Graphite targets were measured at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory [Davis et al., 1990]. Radiocarbon results are reported as Delta-14C as defined by Stuiver and Polach [1977] and include the d13C correction obtained from the stable isotope results. Analytical precision and accuracy of the radiocarbon measurements is ±4 per mil (1s) based on repeated analyses of an in-house homogenized near-modern coral and acceptedsecondary and tertiary radiocarbon standards. In keeping with the definition of "high-precision" (<3 per mil, 1s) data have been rounded to zero significant figures. The age model for these data was derived from a preliminary delta-13C age model which was then optimized by mapping the delta-18O data to a local instrumental precipitation record. Age model errors are estimated (brute force approach) at ±2-3 months. Precision is ±4 per mil, based on analyses of an in-house homogenized, near modern coral sample. These data are age corrected (Stuiver & Polach, 1977).