Modern, Preindustrial, and Past (Last 25 ka) Carbon Isotopic (δ¹³C) Variability in the Surface Waters of the Southwest Pacific

Abstract

Carbon stable isotopes (δ13C) in modern seawater samples and planktic foraminifera Globigerina bulloides from core top and downcore sediments are used to estimate the distribution of δ13C 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 δ13C distribution in the modern (δ13CDIC), PI (δ13CPI), and late Holocene (from planktic foraminifera Globigerina bulloides [temperature corrected δ13CG.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 CO2 in this region. The estimated δ13CPI values and measured δ13CG.bulloidesTC values from the core tops are higher than the modern values due to the Suess Effect. However, there is poor agreement between the δ13CPI values and core top δ13CG.bulloidesTC values south of 40°S as the back‐calculation approach using chlorofluorocarbon‐11 (CFC‐11) method for removing the anthropogenic δ13C is not effective at these higher southern latitudes. The δ13CG.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 δ13C over the last 25 kyr. Glacial subantarctic δ13CG.bulloidesTC values are low, while subtropical δ13C G.bulloidesTC are high. The peak in δ13CG.bulloidesTC values shifts south in the early Holocene. These latitudinal variations in δ13C G.bulloidesTC are linked to changes in ocean circulation, biological productivity (associated with the shifts in the subtropical front), and air‐sea CO2 exchange, likely related to the structure and position of the Southern Hemisphere Westerly Wind in the South Pacific region.

Early deglacial CO2 release from the Sub-Antarctic Atlantic and Pacific oceans

“…A two-point calibration using international standards NBS 18 and NBS 19 was used to correct to Vienna PeeDee Belemnite (VPDB), the standard deviation based on in-house carbonate standard GS-1 is 0.02 ‰ (1σ). Oxygen and carbon stable isotopes on G. bulloides were previously published for TAN1106-28 (Maxson et al, 2019).…”

“…a) 230 Th normalised alkenone abundance (this study). b) TAN1106-28  13 CG.bulloides fromMaxson et al (2019). c)  11 BG.bulloides derived pCO2 with 68% and 95% uncertainty envelopes.…”