Radiocarbon and Stable Carbon Isotope Constraints on the Propagation of Vent CO 2 to Fluid in the Acidic Kueishantao Shallow Water Hydrothermal System

We report radiocarbon and stable carbon isotope measurements from vent gas (CO 2(g) ), dissolved inorganic carbon (DIC), and particulate organic carbon (POC), from two vents of the Kueishantao (KST) shallow‐water hydrothermal system, offshore northeast Taiwan. The purpose of this research is to investigate how magmatic‐sourced carbon enters various carbon pools in the hydrothermal system. We utilize a precipitation method to eliminate sulfur compounds from CO 2 samples to facilitate Accelerator Mass Spectrometry (AMS) analysis, and evaluate radiocarbon background levels during processing to characterize hydrothermal‐sourced CO 2 that contains negligible radiocarbon. The result shows that both CO 2(g) and DIC in the fluids below two vent orifices fall within a narrow range of fraction of modern carbon (F 14 C) from 0.013 to 0.136 and δ 13 C from −8.3‰ to −5.1‰. The F 14 C values correspond to approximately 90% magmatic‐sourced carbon in CO 2(g) and DIC. A combination of equilibrium and kinetic isotopic fractionation can adequately explain relatively high CO 2(g) δ 13 C to DIC, beneath vent orifices. Above the vent orifices, DIC becomes enriched in both 14 C and 13 C as a result of physical mixing with ambient seawater. POC F 14 C values confirm a significant magmatic carbon contribution into the POC pool within the KST system, with rapid hydrothermal circulation. Our results identify the physiochemical processes responsible for magmatic carbon migration from CO 2(g) into the DIC pool, and demonstrate how a dual carbon isotope approach can serve as an effective tool in understanding carbon flow in high temperature‐low pH hydrothermal systems. Kueishantao (KST) shallow water hydrothermal vents constantly discharge CO 2 ‐dominated gases and acidic fluid into ambient seawater, of which all chemical properties are significantly affected. To track the footprint of CO 2 emitted from the hydrothermal vents into the adjacent carbon reservoirs, we first measure radiocarbon in the vent CO 2 and hydrothermal fluid. Radiocarbon is a good tracer for this purpose since magmatic‐sourced carbon is usually devoid of 14 C. However, sulfur content is typically high in hydrothermal samples and makes radiocarbon measurements difficult. We tested three methods to remove the sulfur and found that precipitation is the most efficient method for hydrothermal samples. We show that in the KST system, more than 90% of the magmatic‐sourced carbon was retained in the vent CO 2 and the aci