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<p>In this paper we describe the implementation of the carbon isotopes <span class="inline-formula">¹³</span>C and <span class="inline-formula">¹⁴</span>C (radiocarbon) into the marine biogeochemistry model REcoM3. The implementation is tested in long-term equilibrium simulations where REcoM3 is coupled with the ocean general circulation model FESOM2.1, applying a low-resolution configuration and idealized climate forcing. Focusing on the carbon-isotopic composition of dissolved inorganic carbon (<span class="inline-formula"><i>δ</i>¹³</span>C<span class="inline-formula">_DIC</span> and <span class="inline-formula">Δ¹⁴</span>C<span class="inline-formula">_DIC</span>), our model results are largely consistent with reconstructions for the pre-anthropogenic period. Our simulations also exhibit discrepancies, e.g. in upwelling regions and the interior of the North Pacific. Some of these differences are due to the limitations of our ocean circulation model setup, which results in a rather shallow meridional overturning circulation. We additionally study the accuracy of two simplified modelling approaches for dissolved inorganic <span class="inline-formula">¹⁴</span>C, which are faster (15 % and about a factor of five, respectively) than the complete consideration of the marine radiocarbon cycle. The accuracy of both simplified approaches is better than 5 %, which should be sufficient for most studies of <span class="inline-formula">Δ¹⁴</span>C<span class="inline-formula">_DIC</span>.</p>