Carbon sources and sinks within the Oman-UAE ophiolite: implications for natural atmospheric CO2 sequestration rates

Hyperalkaline (pH >11) spring waters across the Oman-UAE ophiolite have precipitated large-scale surface and subsurface freshwater carbonate deposits within and on mantle-sequence peridotites. Ten carbonate lithofacies have been identified that display a common stratigraphy across the region, with some on-going deposition. Alternating wet and arid climate periods, consistent with previous Quaternary climate data for Oman and the UAE, can be inferred from the morphologies and stratigraphy of carbonate lithofacies. Carbon and oxygen stable isotope ratios for individual lithofacies are consistent across the ophiolite, indicating uniform formation processes throughout the region. Modern carbonates actively precipitate via the drawdown of atmospheric CO2 into hyperalkaline groundwaters, indicated by very negative δ13C values (≈ –25‰) and δ18O (≈ –18‰) caused by kinetic fractionation. In contrast, ancient travertines display a much wider range in δ13C and δ18O, reflecting mixing of several C and O sources including atmospheric CO2, dissolved limestone CO32– and soil DIC. Radiocarbon data for ancient travertines show a series of stratigraphically impossible 14C “ages” which are interpreted to be due to the incorporation of a proportion of 14C-dead carbon. The sources of 14C-dead carbon are a mixture of dissolved limestone CO32–, soil DIC and the re-working of older carbonate deposits. The proportion of carbon from different sources must be taken into account when calculating sequestration rates of atmospheric CO2 into carbonates. The incorporation of 14C-dead carbon into carbonate lithofacies leads to a systematic offset in conventional radiocarbon ages towards older ages. Use of offset, older ages will cause underestimation of the rate of carbon sequestration, whilst the presence of 14C-dead carbon will cause overestimation of the volume of atmospheric carbon being stored. Age offsets in carbonates mean that sequestration rates taking into account all carbon in the system, e.g. from atmospheric or limestone CO32–, will be higher than previously thought.