Carbon recycling in ophiolite-hosted carbonates, Oman-UAE

Large-scale surface and subsurface freshwater carbonate deposits of probable Quaternary age have formed on the Oman-UAE ophiolite. Here, serpentinisation reactions in ultramafic rocks have produced calcite and magnesite. These carbonates are frequently cited as examples of natural atmospheric CO₂ sequestration, but the possibility of carbon recycling has not been addressed. The aim of this study is to assess the degree of atmospheric CO₂ being incorporated into carbonates versus that which has been recycled from alternative sources such as soil CO₂, or limestones that underlie the ophiolite. This has been determined through δ¹³C/δ¹⁸O, ⁸⁷Sr/⁸⁶Sr and ¹⁴C analysis of all major carbonate lithofacies identified. Our analyses of modern carbonate crusts forming on the surface of stagnant hyperalkaline (pH >11) waters show highly depleted δ¹³C and δ¹⁸O values (-25.5‰ ×0.5 PDB and -16.8‰ ×0.5 PDB respectively). This depletion has been attributed to a kinetic isotope effect occurring during atmospheric CO₂ exchange with Ca(OH)₂ hyperalkaline waters [1]. By comparison, inactive travertine deposits show a large range in δ¹³C (-10.5 to -21.8‰ PDB) which lies on a trajectory from the composition of modern crusts towards bicarbonate fluids in equilibrium with soil CO₂. We interpret this trend as being produced by the mixing of different carbon sources, either at the time of formation or during later alteration. Modern carbonates and inactive travertines also have ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations similar to Cretaceous and Tertiary limestones which surround the ophiolite, whilst subsurface veins also display ⁸⁷Sr/⁸⁶Sr ratios similar to these Cretaceous limestones. Carbon recycling can also be determined with ¹⁴C. Modern atmospheric CO₂ has a global average of 105-106% modern ¹⁴C (pMC), therefore freshwater carbonates forming solely from atmospheric CO₂ would be expected to contain >100 pMC. However, modern carbonates display varied results from 94.5-101.4 pMC. Low values could be caused by meteoric waters incorporating ¹⁴C 'dead' carbon through the dissolution of limestones and/or uptake of soil CO₂. This 'dead' carbon would then be assimilated into veins and surface deposits, offsetting pMC values. Inactive travertines show significant fluctuations in ¹⁴C values within a single hand sample, where stratigraphically younger samples give older radiocarbon 'ages' outside of error. These fluctuations may have been caused by the presence of limestone sourced 'dead' carbon in waters at time of formation, surface runoff containing soil CO₂ or by later recrystallisation. Isotopic evidence indicates that mixing of contemporary atmospheric carbon and recycled older carbon has taken place during the on-going carbonation of the Oman-UAE ophiolite sequence. Failure to account for this recycled carbon could lead to inaccurate estimates of natural CO₂ sequestration rates. References [1] Clark, I.D. and Fontes, J. (1990) Palaeoclimatic reconstruction in Northern Oman based on carbonates from hyperalkaline groundwaters. Quaternary Res, 33, 320-336