Abstract
AbstractEmissions of carbon dioxide from fossil fuel combustion are reducing the ratio 13C/12C, δ13C, in atmospheric CO2 and in the carbon in the ocean and terrestrial biosphere that exchanges with the atmosphere on timescales of decades to centuries. Future changes to fossil fuel emissions vary across different scenarios and may cause decreases of more than 6% in atmospheric δ13CO2 between 1850 and 2100. The effects of these potential changes on the three‐dimensional distribution of δ13C in the ocean have not yet been investigated. Here, we use an ocean biogeochemical‐circulation model forced with a range of Shared Socioeconomic Pathway (SSP)‐based scenarios to simulate δ13C in ocean dissolved inorganic carbon from 1850 to 2100. In the future, vertical and horizontal δ13C gradients characteristic of the biological pump are reduced or reversed, relative to the preindustrial period, with the reversal occurring in higher emission scenarios. For the highest emission scenario, SSP5‐8.5, surface δ13C in the center of Pacific subtropical gyres falls from 2.2% in 1850 to −3.5% by 2100. In lower emission scenarios, δ13C in the surface ocean decreases but then rebounds. The relationship between anthropogenic carbon (Cant) and δ13C in the ocean shows a larger scatter in all scenarios, suggesting that uncertainties in δ13C‐based estimates of Cant may increase in the future. These simulations were run with fixed physical forcing and ocean circulation, providing a baseline of predicted δ13C. Further work is needed to investigate the impact of climate‐carbon cycle feedbacks on ocean δ13C changes.
Highlights
The isotopic ratio 13C/12C in oceanic dissolved inorganic carbon (DIC) is influenced by physical, biological, and anthropogenic processes
We compare δ13C simulated by Model of Ocean Biogeochemistry and Isotopes (MOBI)-Transport Matrix Method (TMM) with reconstructions of δ13C in DIC based on observations from ocean cruise surveys, a δ13C climatology for the modern ocean (1994) and an estimate for the preindustrial period (1850), taken from Eide, Olsen, Ninnemann, Eldevik, and Johannessen (2017)
The potential future changes in atmospheric δ13C over this century are likely to change the three-dimensional distribution of δ13C of ocean DIC from its preindustrial and present state
Summary
The isotopic ratio 13C/12C in oceanic dissolved inorganic carbon (DIC) is influenced by physical, biological, and anthropogenic processes. The observed δ13C patterns are largely dominated by this biological effect, but ocean circulation and air-sea gas exchange act to oppose and partially counteract the patterns driven by the biological pump (Eide, Olsen, Ninnemann, & Johannessen, 2017; Schmittner et al, 2013). The temperature dependence of fractionation during air-sea gas exchange (Broecker & Maier-Reimer, 1992; Zhang et al, 1995) and the degree of isotopic equilibration, which relates to the surface residence time of seawater, are important in determining the ocean δ13C distribution (Khatiwala et al, 2019; Schmittner et al, 2013)
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