Modeling the evolution of pulse-like perturbations in atmospheric carbon and carbon isotopes: the role of weathering–sedimentation imbalances

• Main Authors: A. Jeltsch-Thömmes, F. Joos
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-03-01
• Series: Climate of the Past
• Online Access: https://www.clim-past.net/16/423/2020/cp-16-423-2020.pdf

<p>Measurements of carbon isotope variations in climate archives and isotope-enabled climate modeling advance the understanding of the carbon cycle. Perturbations in atmospheric <span class="inline-formula">CO₂</span> and in its isotopic ratios (<span class="inline-formula"><i>δ</i>¹³C</span>, <span class="inline-formula">Δ¹⁴C</span>) are removed by different processes acting on different timescales. We investigate these differences on timescales of up to 100&thinsp;000 years in pulse-release experiments with the Bern3D-LPX Earth system model of intermediate complexity and by analytical solutions from a box model. On timescales from years to many centuries, the atmospheric perturbations in <span class="inline-formula">CO₂</span> and <span class="inline-formula"><i>δ</i>¹³CO₂</span> are reduced by air–sea gas exchange, physical transport from the surface to the deep ocean, and by the land biosphere. Isotopic perturbations are initially removed much faster from the atmosphere than perturbations in <span class="inline-formula">CO₂</span> as explained by aquatic carbonate chemistry. On multimillennial timescales, the <span class="inline-formula">CO₂</span> perturbation is removed by carbonate compensation and silicate rock weathering. In contrast, the <span class="inline-formula"><i>δ</i>¹³C</span> perturbation is removed by the relentless flux of organic and calcium carbonate particles buried in sediments. The associated removal rate is significantly modified by spatial <span class="inline-formula"><i>δ</i>¹³C</span> gradients within the ocean, influencing the isotopic perturbation of the burial flux. Space-time variations in ocean <span class="inline-formula"><i>δ</i>¹³C</span> perturbations are captured by principal components and empirical orthogonal functions. Analytical impulse response functions for atmospheric <span class="inline-formula">CO₂</span> and <span class="inline-formula"><i>δ</i>¹³CO₂</span> are provided.</p> <p>Results suggest that changes in terrestrial carbon storage were not the sole cause for the abrupt, centennial-scale <span class="inline-formula">CO₂</span> and <span class="inline-formula"><i>δ</i>¹³CO₂</span> variations recorded in ice during Heinrich stadials HS1 and HS4, though model and data uncertainties prevent a firm conclusion. The <span class="inline-formula"><i>δ</i>¹³C</span> offset between the Penultimate Glacial Maximum and Last Glacial Maximum reconstructed for the ocean and atmosphere is most likely caused by imbalances between weathering, volcanism, and burial fluxes. Our study highlights the importance of isotopic fluxes connected to weathering–sedimentation imbalances, which so far have been often neglected on glacial–interglacial timescales.</p>