Modern, Preindustrial, and Past (Last 25 ka) Carbon Isotopic (δ ¹³ C) Variability in the Surface Waters of the Southwest Pacific

Carbon stable isotopes (δ ¹³ C) in modern seawater samples and planktic foraminifera Globigerina bulloides from core top and downcore sediments are used to estimate the distribution of δ ¹³ C of dissolved inorganic carbon (DIC) in the surface waters of the southwest Pacific in the modern, preindustrial (PI), and over the last 25 kyr. The predicted δ ¹³ C distribution in the modern (δ ¹³ C _DIC ), PI (δ ¹³ C _PI ), and late Holocene (from planktic foraminifera Globigerina bulloides [temperature corrected δ ¹³ C _{G.bulloidesTC} ] from core tops) displays a broad peak at the subtropical front) and subantarctic surface waters due to the combination of high biological productivity and thermodynamic air-sea gas exchange of CO ₂ in this region. The estimated δ ¹³ C _PI values and measured δ ¹³ C _{G.bulloidesTC} values from the core tops are higher than the modern values due to the Suess Effect. However, there is poor agreement between the δ ¹³ C _PI values and core top δ ¹³ C _{G.bulloidesTC} values south of 40°S as the back-calculation approach using chlorofluorocarbon-11 (CFC-11) method for removing the anthropogenic δ ¹³ C is not effective at these higher southern latitudes. The δ ¹³ C _{G.bulloidesTC} from a latitudinal transect of cores in the southwest Pacific were compiled by region using a Monte Carlo approach to determine the long-term trends in δ ¹³ C over the last 25 kyr. Glacial subantarctic δ ¹³ C _{G.bulloidesTC} values are low, while subtropical δ ¹³ C _{G.bulloidesTC} are high. The peak in δ ¹³ C _{G.bulloidesTC} values shifts south in the early Holocene. These latitudinal variations in δ ¹³ C _{G.bulloidesTC} are linked to changes in ocean circulation, biological productivity (associated with the shifts in the subtropical front), and air-sea CO ₂ exchange, likely related to the structure and position of the Southern Hemisphere Westerly Wind in the South Pacific region.