DOI: 10.25921/badw-gv62
title: NOAA/WDS Paleoclimatology - Fiji Coral Annual Average d18O and Sr/Ca Data

description: The South Pacific Convergence Zone (SPCZ) is the largest and most persistent spur of the Intertropical Convergence Zone. At the southeastern edge of the SPCZ near 170°W and 15°–20°S a surface ocean salinity frontal zone exists that separates fresher Western Pacific Warm Pool water from saltier and cooler waters in the east. This salinity front is known to shift east and west with the phase of the El Niño Southern Oscillation. We have generated subannually resolved and replicated coral oxygen isotopic time series from Fiji (17°S, 179°E) and Rarotonga (21.5°S, 160°W) that have recorded interannual displacements of the salinity front over the last 380 years and also indicate that at lower frequencies the decadal mean position of the salinity front, and eastern extent of the SPCZ, has shifted east-west through 10° to 20° of longitude three times during this interval. The most recent and largest shift began in the mid 1800s as the salinity front progressively moved eastward and salinity decreased at both sites. Our results suggest that sea surface salinity at these sites is now at the lowest levels recorded and is evidence for an unprecedented expansion of the SPCZ since the mid 1800s. The expansion of the SPCZ implies a gradual change in the South Pacific to more La Niña-like long-term mean conditions. This observation is consistent with the ocean thermostat mechanism for the Pacific coupled ocean-atmosphere system, whereby exogenous heating of the atmosphere would result in greater warming in the western Pacific and a greater east-west surface temperature gradient.
description: Cores Fiji 1F and Fiji AB were drilled from Porites lutea colonies within 200m of each other. Fiji core 1F was collected in April 1997 in Savusavu Bay, Fiji. A 2.3 m coral core was recovered from a P. lutea colony growing at a depth of 10 m in the middle of Savusavu Bay on the south side of the island of Vanua Levu, Fiji (16°49'S, 179°14'E). Fiji core AB was collected in December 2001. Fiji core AB is 3.89m long continuous coral core from a colony of Porites lutea growing at 10 m depth in Savusavu Bay, Vanua Levu, Fiji (16°49'S, 179°14'E) (within 200 m of core 1F collected in 1997). METHODS SUMMARY For both Fiji 1F and Fiji AB coral slabs 7 mm thick were cut from the cores and cleaned in deionized water in an ultrasonic bath and with a probe sonicator. Dry slabs were sampled with a low-speed micro-drill along tracks parallel to corallite traces as identified in X-ray positives with a 1 mm round diamond drill bit. The X-ray collage for Fiji 1F was published in Linsley et al. (2004). Sr/Ca was measured at 1 mm resolution (12-13 samples per year) and d18O was measured at 2 mm resolution on the same samples (analyzing every other sample or 6-7 samples per year) in both Fiji cores. We measured oxygen isotope ratios on a Micromass Optima gas source mass spectrometer at the University at Albany-State University of New York with an individual acid reaction vessel system. External precision was better than 0.05 per mil for d18O based on analyses of 657 (13.5%) replicate samples. We used an inductively coupled plasma atomic emission spectro-photometer (ICP-AES) at Harvard University to measure coral skeletal Sr/Ca in Fiji core 1F following a technique described in detail by Schrag (1999). The external precision is better than 0.15% (RSD) based on analyses of replicate samples (10% replicates). Fiji 1F Sr/Ca data was further corrected by -0.14 mmol/mol to adjust for lab offset with the Lamont-Doherty Earth Observatory ICP-OES where Sr/Ca was measured on other cores in Dr. Linsley's collection. To construct the Fiji 1F and AB chronologies we tied the annual minima in Sr/Ca and/or d18O to February (on average the warmest month) and maximum Sr/Ca and d18O to August/September (on average the coolest months). Thus there were 2 tie-points per year. To generate the annual averages we then averaged all data with each calendar year. All mm-scale data. See methods in Linsley et al. (2004) and Linsley et al. (2006).