Stable carbon isotopes of dissolved inorganic carbon in the Western North Pacific Ocean: Proxy for water mixing and dynamics

The uptake of atmospheric CO2 and the cycle of dissolved inorganic carbon (DIC) in the ocean are the major mechanisms and pathways controlling global climate change and carbon cycling. The stable carbon isotope (delta C-13) of DIC, therefore, provides an important tracer for processes such as air-sea exchange, photosynthesis, and water dynamics in the ocean. Here, we present new delta C-13-DIC data on water samples collected from a north-south transect (13 degrees N-40 degrees N, 150 degrees E) in the western North Pacific (NP) Ocean in November 2019 and compare the results with those previously reported for similar transects (149.3 degrees E) during WOCE and CLIVAR projects over the past three decades. The values of delta C-13-DIC, ranging from -0.83 parts per thousand to 0.86 parts per thousand, were higher in the surface waters and decreased with depth. The high delta C-13-DIC values in the surface waters were influenced primarily by isotopic fractionation during air-sea exchange and photosynthesis. With depth, the movement of different water masses and mixing, as well as bathypelagic respiration in the dark water of the ocean, all play important roles in influencing the distribution and isotopic signatures of delta C-13-DIC in the western NP Ocean. The delta C-13-DIC values of the 0-200 m water layer varied from -0.17 parts per thousand to 0.86 parts per thousand, with lower values at high latitudes, affected by the low delta C-13-DIC values carried by the Oyashio Current to the Kuroshio Extension (KE) region. A downward trend was present in the delta C-13-DIC signature from north to south in the North Pacific Intermediate Water (NPIW) and Pacific Deep Water (PDW) in the western NP, which reflected the remineralization of organic matter with a horizontal transport of NPIW and PDW. We found a strong C-13 Suess Effect in the upper 2,000 m in the western NP Ocean, and delta C-13-DIC at the surface (<50 m) has decreased by 0.60 parts per thousand-0.85 parts per thousand since 1993. The mean delta C-13-DIC change in the surface ocean was estimated at 0.28 parts per thousand per decade between 1993 and 2019. The air-sea exchange and water mixing in the study area may have accelerated the absorption of anthropogenic CO2 in recent years, which likely caused a slightly faster rate of decrease in the delta C-13-DIC from 2005-2019 than that observed from 1993-2005.

Automated summary

The uptake and cycling of atmospheric CO2 in the ocean play a major role in global climate change and carbon cycling. The stable carbon isotope (delta C-13-DIC) of dissolved inorganic carbon (DIC) serves as a valuable tracer for various oceanic processes. This study presents new data on delta C-13-DIC from water samples collected in the western North Pacific Ocean and compares them with previous findings. The results show variations in the delta C-13-DIC values, influenced by air-sea exchange, photosynthesis, water dynamics, and isotopic fractionation. The study also reveals a decline in delta C-13-DIC values over time, indicating the impact of anthropogenic CO2 absorption and water mixing in accelerating this decrease.