| Summary: | <p>To inform cost-effective monitoring of offshore geological storage of carbon dioxide (CO<sub>2</sub>), a unique field experiment, designed to simulate leakage of CO<sub>2</sub> from a sub-seafloor storage reservoir, was carried out in the central North Sea. A total of 675 kg of CO<sub>2</sub> were released into the shallow sediments (∼3 m below seafloor) for 11 days at flow rates between 6 and 143 kg d<sup>-1</sup>. A set of natural, inherent tracers (<sup>13</sup>C, <sup>18</sup>O) of injected CO<sub>2</sub> and added, non-toxic tracer gases (octafluoropropane, sulfur hexafluoride, krypton, methane) were used to test their applicability for CO<sub>2</sub> leakage attribution and quantification in the marine environment. All tracers except <sup>18</sup>O were capable of attributing the CO<sub>2</sub> source. Tracer analyses indicate that CO<sub>2</sub> dissolution in sediment pore waters ranged from 35 % at the lowest injection rate to 41% at the highest injection rate. Direct measurements of gas released from the sediment into the water column suggest that 22 % to 48 % of the injected CO<sub>2</sub> exited the seafloor at, respectively, the lowest and the highest injection rate. The remainder of injected CO<sub>2</sub> accumulated in gas pockets in the sediment. The methodologies can be used to rapidly confirm the source of leaking CO<sub>2</sub> once seabed samples are retrieved.</p>
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