Chemical, Biological and Hydrological Controls on the 14C Content of Cold Seep Carbonate Crusts: Numerical Modelling and Implications for Convection at Cold Seeps

Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of 14C (D14C) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on 14C incorporation in cold seep carbonates. Our simulations show that increased amounts of CH4 in the expelled fluids result in elevated crust D14C, while high Ca2+ and HCO3 concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust D14C. Advection, bioirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust D14C. In addition, the presence of old or modern carbon (MC) in host sediments prior to cementation and the 14C-decay associated to the time needed to form the crust contribute in defining the D14C of carbonate crusts. We then use the model to reproduce the 14C content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of 14C (-945.0

明晰冷泉（cold seep）环境的水文特征，对精准估算沉积楔处的流体与化学通量至关重要。浅部对流过程可能影响流体通量估算，还可能促进天然气水合物藏的失稳，进而提升沉积物-海洋甲烷通量。然而，冷泉环境中存在对流的证据仍较为有限。我们以东地中海冷泉处形成的碳酸盐岩结壳中的14C浓度（Δ14C）作为示踪剂，用以追踪海水源流体的对流循环过程。我们采用数值模型探究冷泉碳酸盐岩中14C掺入的控制因素。模拟结果显示，排出流体中CH₄含量升高会使结壳的Δ14C值上升，而高Ca²⁺与HCO₃⁻浓度则会产生相反效果。对流是唯一能显著提升结壳Δ14C值的输运过程；平流、生物灌溉、涡旋扩散与生物扰动则对结壳Δ14C值影响微弱，甚至会使其降低。此外，胶结作用前宿主沉积物中存在的古老或现代碳（MC），以及与结壳形成所需时间相关的¹⁴C衰变，共同决定了碳酸盐岩结壳的Δ14C值。随后，我们利用该模型重现东地中海冷泉结壳的¹⁴C含量，以约束其形成所需的化学与水文条件。部分结壳的¹⁴C含量相对较低（-945.0