Coupled evolution of stable carbon isotopes between the Southern Ocean and the atmosphere over the last 260 ka

Abstract

The oceanic thermocline circulation provides a route of communication between the surface and deep ocean and could have played an important role in the global carbon cycle, but studies on reconstructing past thermocline water properties are limited. Here we explore the potential use of left-coiling Globorotalia truncatulinoides as a recorder of thermocline conditions by measuring the stable oxygen and carbon isotopic compositions of this species from 28 surface sediments in the southwest Pacific near New Zealand. Our data show that G. truncatulinoides (sinistral) calcify mainly in the range of subsurface/thermocline depths in this study region between 100 and 850 m with their carbon isotopes largely corresponding to the surrounding seawater values. To understand the controlling factors of the thermocline δ13C evolution in the South Pacific, a 260 ka downcore δ13C record on this species (δ13CG.trunc) from core site ODP1123 is presented and compared with other δ13C records. The convergence of δ13C from thermocline, upper and lower circumpolar deep waters (UCDW/LCDW) during glacial terminations indicates that the deep ocean is the predominant source of increased atmospheric pCO2 and the δ13C anomalies in the upper ocean and atmosphere during the deglacials. This is evident in both the South Pacific and South Atlantic. A quantitative calculation of predicted surface ocean δ13C based on thermodynamic air-sea equilibrium implies this process has a significant control on the temporal thermocline water δ13C variation over the last glacial-interglacial (G-I) cycle. The lower deglacial δ13CG.trunc values in the South Atlantic compared to the Pacific further suggest a stronger upwelling in the Atlantic sector of the Southern Ocean, indicating that this was a major ventilation route with an important stock of light δ13C from the deep waters of this region. This study demonstrates the deep water influence (via upwelling) and atmospheric carbon isotope imprint (via air-sea exchange) on thermocline water δ13C evolution. It also provides important evidence for the rapid exchange of carbon between the Southern Ocean and atmosphere over multiple G-I cycles.