Terrestrial and marine POC export fluxes estimated by 234Th–238U disequilibrium and δ13C measurements in the East China Sea shelf

IF 3.9 3区 环境科学与生态学 Q2 ENVIRONMENTAL SCIENCES Biogeochemistry Pub Date : 2024-04-29 DOI:10.1007/s10533-024-01136-0
Qiangqiang Zhong, Dekun Huang, Qiugui Wang, Jinzhou Du, Fule Zhang, Jing Lin, Tao Yu
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Abstract

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 (τTh–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.

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通过 234Th-238U 失衡和 δ13C 测量估算东海大陆架陆地和海洋 POC 出口通量
利用234Th-238U不平衡来估算上层海洋颗粒有机碳(POC)的下沉通量已被广泛采用。本文测定了东海大陆架水体中234Th相对于238U的缺失和POC的碳同位素特征(δ13C),并以此区分海洋和陆地POC输出通量的比例。在东海大陆架的整个水体中,观察到 234Th 相对于 238U 的极度缺乏,234Th/238U 放射性活度比在 0.158 ± 0.045 到 0.904 ± 0.068 之间(平均值为 0.426 ± 0.159)。颗粒活性放射性核素234Th(τTh-T)在大陆架水体中的停留时间为9-44天,明显短于大陆坡区和海盆区。这一现象表明,与大陆坡和盆地上层水相比,ECS 陆架水的颗粒物清除过程更为迅速。通过应用基于 δ13C 的两端成员混合模型,估计陆地 POC 的比例为 0-74%(平均值:30±22%),海洋 POC 的比例为 25%-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存在显著的空间差异,分别为 11 至 129 毫摩尔碳/平方米/天和 2.6 至 38 毫摩尔碳/平方米/天。南部和北部大陆架的平均陆地 POC 通量相似(约 21 至 24 毫摩尔碳/平方米/天),而北部的海洋 POC 通量(86 ± 37 毫摩尔碳/平方米/天)约为南部的四倍(26 ± 20 毫摩尔碳/平方米/天)。有趣的是,海洋和陆地 POC 的估计出口通量比以前报告的 POC 在底层沉积物中的埋藏通量(从 1.1 ± 0.1 到 11.4 ± 1.1 毫摩尔碳/平方米/天)高出约一个数量级,这表明从上层水体出口的陆地和海洋 POC 大部分(90%)在大陆架沉积物中降解。这种 "碳缺失 "现象在很大程度上可归因于其他过程(包括微生物再加工、跨大陆架迁移以及底栖生物可能的消耗)的快速分解。我们的研究结果突显了大陆架碳循环的动态性质,以及进一步研究了解这些过程和改进碳预算评估的必要性。
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来源期刊
Biogeochemistry
Biogeochemistry 环境科学-地球科学综合
CiteScore
7.10
自引率
5.00%
发文量
112
审稿时长
3.2 months
期刊介绍: Biogeochemistry publishes original and synthetic papers dealing with biotic controls on the chemistry of the environment, or with the geochemical control of the structure and function of ecosystems. Cycles are considered, either of individual elements or of specific classes of natural or anthropogenic compounds in ecosystems. Particular emphasis is given to coupled interactions of element cycles. The journal spans from the molecular to global scales to elucidate the mechanisms driving patterns in biogeochemical cycles through space and time. Studies on both natural and artificial ecosystems are published when they contribute to a general understanding of biogeochemistry.
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