Carbon and nutrient export from intertidal sand systems elucidated by Ra-224/Th-228 disequilibria
Carbon and nutrient export from intertidal sand systems elucidated by Ra-224/Th-228 disequilibria
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Ra-224/Th-228 不平衡阐明潮间沙系统的碳和养分输出
DOI：
10.1016/j.gca.2020.02.007
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发表时间：
2020
影响因子：
5
通讯作者：
Shi Xiangming
中科院分区：
文献类型：
--
作者：
Cai Pinghe;Wei Lin;Geibert Walter;Koehler Dennis;Ye Ying;Liu Wei;Shi Xiangming
我们提出了一种替代方案，利用224Ra/228次不平衡来研究可渗透沙质海床的碳和养分输出。在福建围头湾附近的潮间带沙滩(中国)和德国库克斯黑文附近的瓦登海潮滩上，测定了两个不同的潮间带沙体系中溶解的224Ra，总计224Ra和228Th的沉积剖面。在沙子系统上方20或30 cm的沙层中，发现总计224Ra相对于228Th的显著亏损。我们构造了一个简单的二维平流循环模式来模拟周期性潮汐淹没和激流作用下沙子系统中的界面流体输运。基于沉积物中的224Ra/228次不平衡，模型估计了围头湾沙质海滩高潮、中潮和低潮位置的水交换通量分别为20.3、9.1和1.9 L m−2 h−1。与之相比，该模型估算的沃登海潮滩水交换通量为7.2 L m−2 h−1。孔隙水中溶解无机碳的生成是沙质滩涂向海输出的速率限制步骤，可合理地模拟为一级动力学反应。海滩上界面流体的输运模式促进了沉积物中氧化还原条件的水平分带，从涨潮位置的完全氧化状态逐渐演化到低潮位置的亚氧状态。有明显的证据表明，在亚缺氧状态下，氮素通过反硝化作用而损失，我们估计在这个位置的脱氮率为3.3mmolN m−2d−1。对于这两个潮间带沙系统，DIC输出通量在20.1~89.4mmolC m−2d−1之间，与在富含有机质的河口沉积物中测定的通量相当。同时，溶解无机氮(DIN)的输出通量从0.8mmolN m−2d−1变化到18.6mmolN m−2d−1。总体而言，这项研究表明，沙质沉积物在碳和养分的生物地球化学循环中的作用需要重新考虑。
We propose an alternative scheme for the use of224Ra/228Th disequilibria to investigate carbon and nutrient export from a permeable sandy seabed. Sediment profiles of dissolved224Ra, total224Ra and228Th were determined at two different intertidal sand systems - an intertidal sandy beach near Weitou Bay in Fujian (China), and a tidal sand flat in the Wadden Sea near Cuxhaven (Germany). Dramatic deficit of total224Ra relative to228Th was identified in the upper 20 or 30 cm sand layer over the sand systems. We construct a simple two-dimensional advective cycling model to simulate interfacial fluid transport in a sand system that is subject to periodic tidal inundation and swash actions. Based on the224Ra/228Th disequilibria in the sediment, the model gives estimates of 20.3, 9.1, and 1.9 L m−2h−1for water exchange flux at the high tide, mid-tide, and low tide position over the sandy beach at Weitou Bay, respectively. In comparison, the model provides an estimate of 7.2 L m−2h−1for water exchange flux at the tidal sand flat in the Wadden Sea.The production of dissolved inorganic carbon (DIC) in porewater is the rate-limiting step for DIC export from the sandy beach into the sea, and can be reasonably simulated as a first-order kinetic reaction. The pattern of interfacial fluid transport over the beach facilitates a horizontal zonation of redox condition in the sediment, which evolves progressively from a fully oxic state at the high tide position to a suboxic state at the low tide position. There is clear evidence of nitrogen loss via denitrification in the suboxic status, and we estimate a nitrogen removal rate of 3.3 mmolN m−2d−1at this site. For the two intertidal sand systems, DIC export fluxes range from 20.1 to 89.4 mmolC m−2d−1, comparable in magnitude to fluxes determined in organic rich estuarine sediments. In the meantime, export fluxes of dissolved inorganic nitrogen (DIN) change from 0.8 to 18.6 mmolN m−2d−1. Overall, this study suggests that the role of sandy sediments in the biogeochemical cycling of carbon and nutrients needs to be revisited.