Radiocarbon and Stable Carbon Isotope Constraints on the Propagation of Vent CO2 to Fluid in the Acidic Kueishantao Shallow Water Hydrothermal System. (13th October 2022)

• Record Type: Journal Article
• Authors: Wang, Shing‐Lin; Lin, Yu‐Shih; Burr, George S.; Wang, Pei‐Ling; Lin, Li‐Hung
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 23:Number 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-13
• Subjects: radiocarbon -- shallow water hydrothermal system -- CO2‐rich gas -- high temperature low pH fluid -- DIC -- POC; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2022GC010508
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: We report radiocarbon and stable carbon isotope measurements from vent gas (CO2(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 CO2 samples to facilitate Accelerator Mass Spectrometry (AMS) analysis, and evaluate radiocarbon background levels during processing to characterize hydrothermal‐sourced CO2 that contains negligible radiocarbon. The result shows that both CO2(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 CO2(g) and DIC. A combination of equilibrium and kinetic isotopic fractionation can adequately explain relatively high CO2(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 CO2(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. Plain Language Summary: Kueishantao (KST) shallow water hydrothermal vents constantly discharge CO2 ‐dominated gases and acidic fluid into ambient seawater, of which all chemical properties are significantly affected. To track the footprint of CO2 emitted from the hydrothermal vents into the adjacent carbon reservoirs, we first measure radiocarbon in the vent CO2 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 CO2 and the acidic fluids beneath vent orifices. We also used simulations to show how hydrothermal vents take up the magmatic‐sourced carbon beneath vent orifices and then diffuse to the seawater above. How organisms living near KST hydrothermal vents take in the magmatic‐sourced carbon will be discussed in a future work. Key Points: A precipitation method removed sulfur compounds in hydrothermal samples and resulted in graphite targets suitable for radiocarbon analysis More than 90% of the dead‐carbon component in vent CO2 and acidic fluid indicates gas and fluid were well exchanged under a high flow rate Aqueous CO2 dominates dissolved inorganic carbon (DIC) species in acidic fluid and is responsible for radiocarbon and stable carbon isotope values in Kueishantao fluid DIC