Cross-shelf transport of particulate organic carbon (POC) is an essential but poorly quantified process regulating the POC flux and the biological pump efficiency in marginal seas. Here, we estimated the cross-shelf export flux of ²³⁴Th and POC from the northern South China Sea (NSCS) shelf based on a dataset of ²³⁴Th in the water column and the underlying sediments over the shelf (bottom depth ≤ 200 m) and the adjacent slope (bottom depth > 200 m). Based on a full mass balance of ²³⁴Th throughout the water column and the underlying sediments, we found that besides radioactive production and decay, sedimentary accumulation and cross-shelf export associated with drifting particles were the most important processes in the ²³⁴Th budget. The input via horizontal shelf-slope water exchange and along-shelf water transport was a ²³⁴Th source to the shelf water, and neglecting this component would underestimate the actual vertical ²³⁴Th and POC fluxes by ~10–33%. The POC flux via vertical export from the euphotic zone was 26 ± 1 mmol m⁻² d⁻¹ while that via cross-shelf transport was 9.9 ± 3.4 mmol m⁻² d⁻¹. The cross-shelf exported POC presumably inject into the intermediate/deep waters and could be trapped for several decades in the SCS. Our results highlight the importance of cross-shelf POC transport in regulating carbon storage in the SCS on seasonal to longer time scales. A compilation of the case studies regarding cross-shelf transport suggests its role in the carbon cycle might be more important than previously recognized and needs to be re-evaluated.

颗粒有机碳（POC）的跨架运输是调节边缘海中的POC通量和生物泵效率的必不可少但量化程度不高的过程。在这里，我们根据水柱中的²³⁴ Th数据集和该架子上的底层沉积物（底部深度≤200 m）估算了南海北部陆架²³⁴ Th和POC的跨架出口通量。以及相邻的斜坡（底部深度> 200 m）。基于一个完整的质量平衡²³⁴整个水柱Th和底层沉积物中，我们发现，除了放射性产生和衰变，沉积的积累和与漂流颗粒相关的跨陆架出口是最重要的工序在²³⁴预算。通过水平架子坡水交换和沿架子水输送的输入是架子水的²³⁴ Th源，而忽略此分量将使实际的垂直²³⁴ Th和POC通量低估约10–33％。从富营养区垂直出口的POC通量为26±1 mmol m ^-2 d ^-1，而跨架运输的POC通量为9.9±3.4 mmol m ^-2 d ^-1。跨架出口的POC可能注入了中/深水域，并可能被困在SCS中数十年。我们的研究结果突出表明，跨季节POC传输对于调节SCS中季节性到较长时间范围内的碳存储非常重要。有关跨架运输的案例研究汇编表明，其在碳循环中的作用可能比以前认识的更为重要，需要重新评估。