The use of ²³⁴Th–²³⁸U disequilibrium has been widely employed to estimate the sinking flux of particulate organic carbon (POC) from the upper sea and ocean. Here, the deficits of ²³⁴Th relative to ²³⁸U in the water column and the carbon isotope signature (δ¹³C) of POC in the East China Sea (ECS) Shelf were measured, which was used to distinguish the fraction of marine and terrestrial POC export fluxes. In the ECS Shelf, very strong deficits of ²³⁴Th relative to ²³⁸U were observed throughout the water column, with ²³⁴Th/²³⁸U activity ratios ranging from 0.158 ± 0.045 to 0.904 ± 0.068 (averaging 0.426 ± 0.159). The residence times of particle reactive radionuclide ²³⁴Th (τ_Th–T) in the ECS shelf water varied between 9 and 44 days, which is significantly shorter than that in the continental slope area or the basin area. This phenomenon indicates that there is a more rapid particle scavenging process in the ECS shelf water compared to the continental slope and basin upper water. By applying a two-end-member mixing model based on the δ¹³C, the fraction of terrestrial POC was estimated to be 0 to 74% (mean: 30 ± 22%) and the fraction of marine POC was in the range of 25% to 100% (mean: 70 ± 22%). Fluxes of marine and terrestrial POC settling to the seafloor exhibited significant spatial differences among different stations, ranging from 11 to 129 mmol C/m²/day and from 2.6 to 38 mmol C/m²/day, respectively. The averaged terrestrial POC fluxes in the southern and northern ECS Shelf were similar (~ 21 to 24 mmol C/m²/day), while the marine POC fluxes in the north (86 ± 37 mmol C/m²/day) were approximately four times higher than those in the south (26 ± 20 mmol C/m²/day). Interestingly, the estimated export flux of both marine and terrestrial POC were approximately one order of magnitude higher than the previously reported burial fluxes of POC (ranging from 1.1 ± 0.1 to 11.4 ± 1.1 mmol C/m²/day) in the underlying bottom sediments, indicating that the majority (> 90%) of both terrestrial and marine POC exported from the upper water column are degraded in the sediments of the ECS Shelf. This “carbon missing” phenomenon can greatly be attributed to rapid decomposition by other processes (including microbial reworking, cross-shelf transport, and possible consumption by benthic organisms). Our findings highlight the dynamic nature of carbon cycling in the continental shelf and the need for further research to understand these processes and improve carbon budget assessments.

²³⁴ Th– ²³⁸ U不平衡已被广泛用于估算上层海洋和海洋的颗粒有机碳 (POC) 的下沉通量。在这里，测量了东海（ECS）陆架水体中²³⁴ Th 相对于²³⁸ U的赤字以及 POC 的碳同位素特征（δ ¹³ C），用于区分海洋和陆地的比例POC 出口通量。在 ECS 陆架中，在整个水体中观察到²³⁴ Th 相对于^{238 U 存在非常强烈的赤字， 234} Th/ ²³⁸ U 活性比范围为 0.158 ± 0.045 至 0.904 ± 0.068（平均 0.426 ± 0.159）。颗粒反应放射性核素²³⁴ Th (τ _Th–T ) 在东海陆架水中的停留时间在 9 ～ 44 d 之间，明显短于大陆坡区或盆地区。这一现象表明，与大陆坡和盆地上部水相比，东海陆架水存在更快速的颗粒清除过程。通过应用基于 δ ¹³ C的两端元混合模型，陆地 POC 的比例估计为 0 至 74%（平均值：30 ± 22%），海洋 POC 的比例在 25 范围内。 % 至 100%（平均值：70 ± 22%）。沉降到海底的海洋和陆地POC通量在不同站点之间表现出显着的空间差异，范围分别为11至129 mmol C/m ² /天和2.6至38 mmol C/m ² /天。 ECS 陆架南部和北部的平均陆地 POC 通量相似（约 21 至 24 mmol C/m ² /天），而北部的海洋 POC 通量（86 ± 37 mmol C/m ² /天）约为比南方地区高四倍（26 ± 20 mmol C/m ² /天）。有趣的是，海洋和陆地 POC 的估计输出通量比之前报道的底层沉积物中POC 埋藏通量（范围从 1.1 ± 0.1 到 11.4 ± 1.1 mmol C/m ² /天）大约高一个数量级，表明从上层水体输出的陆地和海洋 POC 的大部分（> 90%）在 ECS 陆架的沉积物中降解。这种“碳缺失”现象很大程度上归因于其他过程的快速分解（包括微生物改造、跨大陆架运输以及底栖生物可能的消耗）。我们的研究结果强调了大陆架碳循环的动态性质，以及需要进一步研究以了解这些过程并改进碳预算评估。