Southwest Pacific Ocean surface reservoir ages since the last glaciation: Circulation insights from multiple-core studies

Radiocarbon (C) in dissolved inorganic carbon in the ocean can trace the age of ocean water relative to the atmosphere and provide insight into climate-driven changes in ocean circulation since the last glaciation. Here we estimate surface radiocarbon ages from the last glaciation through the deglaciation into the Holocene in the southwestern Pacific by using tephras, both as stratigraphic tie points and for the availability of existing radiocarbon dates from terrestrialbased analyses of the organic carbon associated with them, as markers of past atmospheric ΔC. The glacial surface reservoir age of subtropical waters was ~700 C years older than the coeval atmosphere at ~25,000 cal yr B.P. This was significantly older (more C depleted) by ~ 300 C years, than modern reservoir ages. At the same time, subantarctic surface water reservoir age was ~3200 C years, almost 5 times the modern reservoir age, making the difference in age between subtropical and subantarctic surface water masses treble the modern difference. This pattern is attributed to the upwelling and exchange of very old deepwaters from the glacial abyss in the Southern Ocean. In the early deglaciation, surface reservoir ages were ~600 to 700 C years. Recent atmospheric ΔC calibrations project that these surface reservoir ages were older than modern by 1.2-fold to 2-fold. This increased reservoir effect can be attributed to shallow circulation that differed from modern, delivering waters with lower C content to the region. Early Holocene surface reservoir ages of ~300 to 500 C years, similar to recent, suggest modern circulation patterns were in place by that time.