Maki Morimoto

Interannual sea surface salinity changes in the western Pacific from 1954 to 2000 based on coral isotope analysis

[i] Changes in sea surface salinity (SSS) in the western Pacific warm pool (WPWP) provide information on the variability of the El Nino Southern Oscillation (ENSO) because SSS reflects the balance between evaporation and precipitation. We reconstructed the long-term variability of SSS at the northwestern edge of the WPWP from oxygen isotope analysis of annual coral bands. The estimated SSS corresponded well with in situ observations, demonstrating that isotopic records from coral can be used as a paleosalinometer. Estimated SSS between 1954 and 2000 peaked during the El Ninos of 1972-73, 1982-83, and 1997-98. These SSS peaks were caused by anomalously low precipitation accompanying the eastward shift in the WPWP. Citation: Iijima, H., H. Kayanne, M. Morimoto, and O. Abe (2005), Interannual sea surface salinity changes in the western Pacific from 1954 to 2000 based on coral isotope analysis.

A 6.5-year continuous record of sea surface salinity and seawater isotopic composition at Harbour of Ishigaki Island, southwest Japan

We produced continuous records of sea surface salinity and isotopic composition from 1998 to 2004 at Ishigaki Island, southwest Japan, and found clear seasonal variations in salinity and oxygen isotopic composition and increasing trends of them after 1999. These increasing trends could be principally due to the decreasing difference between local precipitation (P) and evaporation (E), as a result of the reduction of horizontal vapour transport from adjacent oceans. When samples collected in heavy rainfall events were excluded, the average Δδ18O/Δ salinity slope was obtained as 0.31, 0.35 in summer and 0.28 in winter. Estimated E/P ratios based on the isotopic box model are in good agreement with the ratios of independently estimated E to observed P. †Updated paper: originally presented on the IAEA International Symposium “Quality Assurance for Analytical Methods in Isotope Hydrology” (August 2004, Vienna).

Precision and long-term stability of clumped-isotope analysis of CO2 using a small-sector isotope ratio mass spectrometer.

RATIONALE The ratio of the measured abundance of (13)C-(18)O bonding CO(2) to its stochastic abundance, prescribed by the δ(13)C and δ(18)O values from a carbonate mineral, is sensitive to its growth temperature. Recently, clumped-isotope thermometry, which uses this ratio, has been adopted as a new tool to elucidate paleotemperatures quantitatively. METHODS Clumped isotopes in CO(2) were measured with a small-sector isotope ratio mass spectrometer. CO(2) samples digested from several kinds of calcium carbonates by phosphoric acid at 25 °C were purified using both cryogenic and gas-chromatographic separations, and their isotopic composition (δ(13)C, δ(18)O, Δ(47), Δ(48) and Δ(49) values) were then determined using a dual-inlet Delta XP mass spectrometer. RESULTS The internal precisions of the single gas Δ(47) measurements were 0.005 and 0.02‰ (1 SE) for the optimum and the routine analytical conditions, respectively, which are comparable with those obtained using a MAT 253 mass spectrometer. The long-term variations in the Δ(47) values for the in-house working standard and the heated CO(2) gases since 2007 were close to the routine, single gas uncertainty while showing seasonal-like periodicities with a decreasing trend. Unlike the MAT 253, the Delta XP did not show any significant relationship between the Δ(47) and δ(47) values. CONCLUSIONS The Delta XP gave results that were approximately as precise as those of the MAT 253 for clumped-isotope analysis. The temporal stability of the Delta XP seemed to be lower, although an advantage of the Delta XP was that no dependency of δ(47) on Δ(47) was found.

Seasonal Radiocarbon Variation of Surface Seawater Recorded in a Coral from Kikai Island, Subtropical Northwestern Pacific

A coral radiocarbon (Delta (super 14) C) investigation with a high time-resolution is crucial for reconstructing secular and seasonal Delta (super 14) C changes in the surface seawater which potentially reflect ocean circulations and dynamic ocean-atmosphere interactions. The Delta (super 14) C values of a modern coral (Porites sp.) from Kikai Island, southern Japan, in the subtropical northwestern Pacific, were determined for the period of 1991-1998 at a monthly resolution. A coral Delta (super 14) C time series for the 8 yr indicated seasonal cycles superimposed on a secular decreasing trend of 3.8 per mil per yr. The seasonal amplitude of the coral Delta (super 14) C was about 18 per mil on the average and the minimum Delta (super 14) C was observed in late spring and summer. The Delta (super 14) C changes were tentatively explained by horizontal oceanic advections around Kikai Island or over the wide range of the equatorial and subequatorial Pacific.