NOAA/WDS Paleoclimatology - Padang Bai, Indonesia 20th Century Coral 14C Data

STUDY NOTES: Coral radiocarbon (Delta-14C) on untreated, low-speed drilled samples. Precision is ~±3.5 per mil as documented by an in-house homogenized coral standard, and presented as AGE CORRECTED DELTA-14C [see Stuiver and Polach, 1977]. Coral cores were drilled in January 1990 from an exceptionally large Porites colony growing at a depth of 5 m at Padang Bai, Bali on the southern edge of Lombok Strait. The recovered coral cores span nearly 300 yr of growth and a long (1 mm resolved) stable isotope record can be found in the work of Charles et al. [2003]. We have focused our radiocarbon work on the uppermost ~70 cm of the coral which mainly covers the pre to postbomb period. The cores were cut into 9 mm slabs, ultrasonically cleaned in distilled water, and air dried. After identifying the major vertical growth axis, the coral was sequentially sampled at 2 mm increments with a low-speed drill. Where necessary, we overlapped parallel sample tracts in order to adequately splice sections together. 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 (8-9 mg) were placed in individual reaction chambers, evacuated, heated, and acidified with orthophosphoric acid at 90C. The evolved CO2 was purified, trapped, and converted to graphite in the presence of iron catalyst and a stoichiometric excess of hydrogen [Vogel et al., 1987]. Graphite targets were measured at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory. Radiocarbon results are reported in conventional per mil (%) notation as age corrected DELTA(14C) as defined by Stuiver and Polach [1977] and include a background correction using 14C-free calcite, and the d13C correction obtained from the stable isotope results. Analytical precision and accuracy of the radiocarbon measurements is ~±3.5% (1s sd). We created a preliminary age model based upon the seasonal structure within the d13C record and sclerochronology. The final, and reported age model is based on tuning preliminary age model relating the coral d18O to the seasonal cycle of (d18Ow) salinity and temperature. The seasonal extremes were anchored to March/April and September of each year consistent with instrumental observations of temperature, salinity, and cloudiness. In keeping with the definition of "high-precision radiocarbon" [Stuiver and Polach, 1977] the data have been rounded to zero significant figures.
ABSTRACT SUPPLIED BY ORIGINATOR: We have explored surface water mixing in the Lombok Strait through a bimonthly resolved surface water D14C time series reconstructed from a coral in the Lombok Strait that spans 1937 through 1990. The prebomb surface water D14C average is -60.5%, and individual samples range from -72% to 134%. The annual average postbomb maximum occurs in 1973 at 122%. The timing of the postbomb maximum is consistent with a primary subtropical source for the surface waters in the Indonesian seas. During the postbomb period, the coral records regular seasonal cycles of 5% to 20%. Seasonal high D14C occur during March-May (warm, low salinity), and low D14C occur in September (cool, higher salinity). The D14C seasonality is coherent and in phase with the seasonal D14C cycle observed in Makassar Strait. We estimate the influence of high D14C Makassar Strait (North Pacific) water flowing through the Lombok Strait using a two end-member mixing model and the seasonal extremes observed at the two sites. The percentage of Makassar Strait water varies between 16% and 70%, and between 1955 and 1990, it averages at 40%. The rich D14C variability has a biennial component reflecting remote equatorial Indian Ocean forcing and a component in the ENSO band, which is interpreted to reflect Pacific forcing on the D14C signature in Lombok Strait.