NOAA/WDS Paleoclimatology - Langkai, Makassar Strait 120 Year 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]. Cores were taken from a large Porites lutea coral in 4m water depth from the south coast of Langkai Island 35km west of Sulawesi by 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 hydrogen and an iron 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 age-corrected Delta-14C (per mil) 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 ~±3.5 per mil (1s) based on repeated analyses of an in-house homogenized near-modern coral and accepted secondary and tertiary radiocarbon standards. An initial age-model was constructed using the seasonal variability in coral d13C. To generate the primary time series we pinned the minimum d13C to February of each year, and linearly interpolated the data between marker points. Coral d18O is negatively correlated with water temperature, the summer d18O coral signal is heavily influenced by rainfall and runoff, so we matched the coral d18O maxima to the winter SST minimum from the GOSTA SST atlas. Age model uncertainties are estimated to be better than +/- 2 months. In keeping with the definition of "high-precision radiocarbon" data have been rounded to zero significant figures. Langkai Island, South Makassar Strait, Indonesia: 5°02'S, 119°04'E, 4m water depth, 2.3m core length
ABSTRACT SUPPLIED BY ORIGINATOR: To explore the seasonal to decadal variability in surface water masses that contribute to the Indonesian throughflow, we have generated a 115-year bimonthly coral-based radiocarbon time series from a coral in the Makassar Straits. In the pre-bomb (pre-1955) era from 1890 to 1954, the radiocarbon time series occasionally displays a small seasonal signal (10-15%). After 1954 the radiocarbon record increases rapidly, in response to the increased atmospheric 14C content caused by nuclear weapons testing. From 1957 to 1986 the record displays clear seasonal variability from 15 to 60% and the post-bomb peak (163 per mil) occurred in 1974. The seasonal cycle of radiocarbon can be attributed to variations of surface waters passing through the South Makassar Strait. Southern Makassar is under the influence of the Northwest Monsoon, which is responsible for the high austral summer radiocarbon (North Pacific waters) and the Southeast Monsoon that flushes back a mixture of low (South Pacific and upwelling altered) radiocarbon water from the Banda Sea. The coral record also shows a significant 14C peak in 1955 due to the bomb-14C water advected into this region from nuclear weapons tests in the Marshall Islands in 1954.