Using 224Ra/228Th disequilibrium to quantify benthic fluxes of dissolved inorganic carbon and nutrients into the Pearl River Estuary
Using 224Ra/228Th disequilibrium to quantify benthic fluxes of dissolved inorganic carbon and nutrients into the Pearl River Estuary
复制标题
利用 224Ra/228Th 不平衡量化进入珠江口的溶解无机碳和营养物的底栖通量
DOI：
10.1016/j.gca.2015.08.015
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发表时间：
2015-12-01
影响因子：
5
通讯作者：
Dai, Minhan
中科院分区：
文献类型：
--
作者：
Cai, Pinghe;Shi, Xiangming;Dai, Minhan
最近在沿海沉积物中观察到的Ra-224/Th-228不平衡已被证明是追踪调节溶质在沉积物-水界面转移的底栖生物过程的极好代理。为了更好地利用这一代用物，有必要了解沉积物中Ra-224的反应动力学。本研究采集了珠江口（PRE）样带块状沉积物中Ra-224和Th-228的深度剖面。与大量沉积物测量一起，还测定了孔隙水和上覆水中的溶解Ra-224、溶解无机碳（DIC）和营养物质（NO2- + NO3-、NH4+）。PRE沉积物中Ra-224相对于Th-228存在明显的亏缺，且空间变化较大。通过对沉积物中溶解和吸附的Ra-224分布的成岩模型，我们推断吸附从水相中去除Ra-224的速率为0.1 +/- 1.1-2000 +/- 400 d（-1）。此外，对Ra-224的吸附表现为缺氧淡水>缺氧淡水>缺氧咸淡水速率序列，可能反映了氧化还原条件和离子强度对Ra-224吸附-解吸动力学的影响。根据观测到的沉积物中Ra-224的亏缺，采用一维质量平衡交换模型估算了底底生物的Ra-224通量。研究表明，灌溉是控制溶质在沉积物-水界面转移的主要过程，而分子扩散和沉积物混合共同贡献了来自底部沉积物的Ra-224总通量的5%以下。然后，我们利用Ra-224/Th-228不平衡方法来量化DIC和营养物质的底栖通量。我们发现，在旱季，沉积物间隙水向PRE输送了大约42 +/- 6 × 10(9) mol的DIC和类似的16 +/- 1 × 10(9) mol的NH4+。相反，它从上覆水柱中去除约13 +/- 1 × 10(9) mol NO3-。DIC的底栖生物通量相当于该季节河流输入的18%。在营养物方面，我们的研究结果表明，海底沉积物是水体NO3-的主要汇，也是PRE中NH4+的主要来源。总体而言，本研究表明灌溉是河口DIC和养分质量平衡中必须考虑的重要过程。(C) 2015 Elsevier Ltd.版权所有。
The Ra-224/Th-228 disequilibrium that was recently observed in coastal sediments has been proven to be an excellent proxy for tracing the benthic processes that regulate solute transfer across the sediment-water interface. In order to better utilize this proxy, there is a need to understand the reaction kinetics of Ra-224 in sediments. In this study, depth profiles of Ra-224 and Th-228 in bulk sediments were collected along a transect in the Pearl River Estuary (PRE). Together with bulk sediment measurements, dissolved Ra-224, dissolved inorganic carbon (DIC), and nutrients (NO2- + NO3-, NH4+) in pore water and in the overlying waters were also determined. A marked deficit of Ra-224 with respect to Th-228 with large spatial variations was observed in the PRE sediments. By use of a diagenetic model for the distributions of dissolved and adsorbed Ra-224 in sediments, we infer that adsorption removes Ra-224 from aqueous phase at a rate of 0.1 +/- 1.1-2000 +/- 400 d(-1). In addition, adsorption of Ra-224 exhibits a rate sequence of oxic freshwater > anoxic freshwater > anoxic brackish water, probably reflecting the effect of the redox conditions and ionic strength on the adsorption-desorption kinetics of Ra-224.Benthic fluxes of Ra-224 were estimated from the observed deficit of Ra-224 in the sediments using a one-dimensional (1D) mass balance exchange model. We demonstrated that irrigation was the predominant process that controls solute transfer across the sediment-water interface, whereas molecular diffusion and sediment mixing together contributed < 5% of the total Ra-224 fluxes from bottom sediments. We then utilized the Ra-224/Th-228 disequilibrium approach to quantify the benthic fluxes of DIC and nutrients. We showed that sediment interstitial waters delivered approximately 42 +/- 6 x 10(9) mol of DIC and similar to 16 +/- 1 x 10(9) mol of NH4+ into the PRE in the dry season. In contrast, it removed about 13 +/- 1 x 10(9) mol of NO3- from the overlying water column. The benthic flux of DIC is equivalent to similar to 18% of the riverine input in this season. In terms of nutrients, our results suggest that bottom sediments are a major sink of water column NO3-, and are a predominant source of NH4+ in the PRE. Overall, this study indicates that irrigation is an important process and must be considered in the mass balance of DIC and nutrients in estuaries. (C) 2015 Elsevier Ltd. All rights reserved.