DOI: 10.25921/c417-zb83
title: NOAA/WDS Paleoclimatology - Sabine Bank, Vanuatu stable isotope salinity reconstruction from 1842-2007 CE

description: Climate variability associated with the El Niño-Southern Oscillation (ENSO) results in large sea-surface temperature (SST) and sea-surface salinity (SSS) anomalies in many regions of the tropical Pacific Ocean. We investigate interannual changes in SSS driven by ENSO in the southwestern Pacific at Sabine Bank, Vanuatu (SBV, 166.04°E, 15.94°S) using monthly variations in coral d18O from 1842 to 2007 CE. We develop and apply a coral d18O -SSS transfer function, which is assessed using a calibration-verification exercise (1970-2007 CE). The 165-year reconstructed SSS record contains a prominent trend toward freshening from 1842 to 2007 CE; mean SSS for 1842-1872 CE is 35.46 ± 0.28 psu, which contrasts with a mean value of 34.85 ± 0.31 psu for 1977-2007 CE, with a freshening trend during the latter part of the 20th century that is not unprecedented with respect to the overall record. Variance in the record is concentrated in the interannual (42%) and interdecadal (29%) bands. The SBV-SSS record matches well with a similarly reconstructed SSS time series at Malo Channel, Vanuatu, which is located ~120 km to the east of SBV. This regional signal is likely driven by ENSO-related changes in the SPCZ and interdecadal changes in surface water advection. The pattern of interdecadal variability at SBV agrees reasonably well with coral records of interdecadal variability from Fiji and Tonga, especially in the pre-1940 portions of the records, further evidence for the regional extent of the salinity signal at Sabine Bank, Vanuatu.
description: Stable isotopic analyses (d18O and d13C) from a modern Porites lutea coral cored at Sabine Bank, Vanuatu in December 2006 and October 2007. The d18O anomaly data is used to investigate salinity variability at this location from 1842-2007 CE (span of 165 years). A 2-m long core (06SB-A1) was extracted from a Porites lutea coral head in 8 m of water at Sabine Bank, Vanuatu (166.07°E, 15.94°S), in December 2006. A 2.9-m long core (07SB-A2) was extracted from the same coral head in October 2007, reaching the base of the coral. We prepared the 8-cm diameter cores by cutting them into 6-mm thick slabs; X-radiographs of these slabs helped to determine the maximum growth axis of the coral, which guided the selection of the most appropriate sampling paths for micromilling powders. Using a computer assisted micromill drilling system, we collected coral sample powders at 1 mm intervals, which equates to approximate monthly sampling, given an estimate of an annual extension rate of 1.2 cm/yr. Core 06SB-A1 was sampled in its entirety and 07SB-A2 was sampled from 0 to 40 cm (to confirm overlap of the two cores and to gain an extra year in the time series) and from 143 to 290 cm, which overlaps with the lower end of 06SB-A1 and extends the time series back to 1842 CE. Results are only reported to a depth of 200 cm as a result of diagenetic alteration at the bottom of the core, which is visible as dark patches in the X- radiographs and large excursions, as well as the lack of an annual cycle in the geochemical time series. Stable isotope determinations were made on aliquots of sample powder using a Thermo-Finnigan MAT253 Isotope Ratio Mass Spectrometer (IRMS), with a Kiel IV Carbonate Device, and on a Thermo-Finnigan Delta V Plus IRMS with Gasbench II (GB II) connected to a Conflo IV, both at the Analytical Laboratory for Paleoclimate Studies (ALPS) at the Jackson School of Geosciences, University of Texas at Austin. The precision of the Kiel IV/MAT253 IRMS for samples in this study is 0.05‰ for d18O and 0.03‰ for d13C (1-sigma), as estimated via multiple analyses of a carbonate standard (n = 224), which is consistent with long- term precision for this instrument of 0.06‰ for d18O and 0.03‰ for d13C. The precision of the GB II/Delta V Plus IRMS is 0.07‰ for d18O and 0.03‰ for d13C (1-sigma), as estimated via multiple analyses of the same carbonate standard (n = 25), which is consistent with long-term precision for this instrument of 0.07‰ for d18O and 0.03‰ for d13C. Replicate coral samples analyzed on both mass spectrometers yield similar results (mean difference = 0.03‰ for d18O and 0.12‰ for d13C; n = 35). All stable isotope values are reported relative to Vienna Pee Dee Belemnite (VPDB), in standard delta notation. Reconstructed SSS = 34.861 + 1.772*d18Oanomaly. See the paper for a full description of the methods used.