Provenance of intermediate waters in the western North Pacific deduced from thermodynamic imprint on δ13C of DIC

Abstract

Stable carbon isotopes of dissolved inorganic carbon (δ13CDIC), together with phosphate concentrations (PO3-4) of the western North Pacific marginal area are presented. Although δ13CDIC controlled not only air-sea exchange but also biological activities, by subtracting PO3-4 as a biological factor from the δ13CDIC, the thermodynamic imprint on the carbon isotopic composition (δ13Cas) is used as a conservative tracer. The linear relationship between δ13Cas and temperature with the δ13Cas/temperature slope of -0.104‰°C is observed in the North Pacific subtropical region. This relationship can be extended to the Weddell Sea deep waters. Therefore oceanic DIC is apparently in temperature-dependent isotopic equilibrium with atmospheric CO2. To characterize each water mass, the "isotopically labeled temperature" (Tiso), which is derived from δ13Cas-temperature relationship obtained for the subtropical gyre water and Weddell Sea Water, is introduced. In contrast, the δ13Cas-temperature relationship is not necessarily linear for the northern Japan Sea Water, the Kuroshio-Oyashio Mixed Water and the Okhotsk Sea Water. This nonlinearity implies apparent air-sea carbon isotopic disequilibria resulting from water cooling without accompanying air-sea carbon isotope equilibrium. By using the potential temperature (Tpot) and the Tiso, the source waters constituting intermediate waters (26.8σθ) have been estimated. The results clearly indicate that (1) the intermediate water in the southern Okhotsk Sea contains up to 20% Japan Sea Water, and that (2) "new" North Pacific Intermediate Water (NPIW) consists of Okhotsk Sea Water, subarctic gyre water, and "old" subtropical NPIW. Copyright 2003 by the American Geophysical Union.