A spatial and temporal study of the carbon cycle; the role of carbonates in buffering Earth’s climate

Canonically, changes in Earth’s atmospheric CO₂ concentrations have been attributed to an imbalance between volcanic degassing rates and silicate weathering. To force environmental changes in this way, such as the CO₂ decline observed during the Cenozoic Period (66 Myr - present), requires either a decrease in volcanic degassing, or an increase in silicate weathering and a change in the total amount of carbon in the combined ocean-atmosphere (OA) system, which is hard to square against proxy observations of invariant silicate weathering and volcanic degassing rates. Recent rethinking about carbon cycling suggests that rather than increasing sources or sinks of carbon, environmental change can be forced by redistributing carbon between the ocean and atmosphere, which satisfies the condition of maintaining a similar amount of carbon in the OA system. A proposed mechanism to achieve this redistribution is via increasing rates of carbonate weathering, which provides a renewed onus on investigating the impact carbonate rocks, and dissolution thereof, may have on buffering changes in Earth’s climate. The long term process of chemical weathering is of particular interest currently, as some suggest silicate mineral dissolution reactions provide a scalable escape route from anthropogenic greenhouse-gas emissions, and subsequent disruptive environmental perturbations. This seems to be at odds with decades of work prior, showing the timescales upon which silicate weathering operates are far too sluggish to be useful in the next 50-100 years. Indeed, the energy required to catalyse silicate weathering (i.e., mining, grinding) and transport minerals to field-sites has not been convincingly shown to outweigh potential CO₂ removal, yet. Furthermore, in watersheds it is difficult to prove additionality as a consequence of enhancing silicate mineral dissolution. In large, open systems it is hard to trace additional carbon removal from the atmosphere as a consequence of enhanced silicate dissolution, data are often very noisy and the fractal nature of river networks means signals are diluted very quickly. Seemingly, appreciation of the timescales upon which chemical weathering operates have become skewed. This thesis investigates the transport of carbon between the terrestrial and ocean realm, on both short and geological timescales. Carbonate rocks weather rapidly in comparison to silicates, and carbonate terrains can be very efficient at delivering alkalinity to the oceans. However, the efficiency of the carbonate weathering alkalinity pump is hampered by the solubility of CaCO₃, which commonly precipitates at Earth’s surface and in doing so out-gasses CO₂. These inputs of alkalinity to the global oceans are measurable, and dynamic enough to be observed on short timescales and are investigated. Given this, the chemistry of contemporary carbonate terrains is investigated using stable Mg isotopes to understand how their carbon transfer capacity can be improved. A global riverine carbonate chemistry model is presented to quantify the present day maximum global carbonate weathering flux of alkalinity to the oceans. A method for quantifying carbon removal rates using radiocarbon data from sites where the chemical weathering process has been expedited is also presented. The fate of alkalinity in the global oceans requires a much longer frame of reference, given the residence time of carbon and base cations in the ocean. Therefore, a Cenozoic palaeo-record of stable Mg isotopes in seawater is presented, and potential drivers of carbon redistribution between the OA-system are assessed.