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自工业革命以来,随着化石燃料使用、土地利用方式的改变及森林的大量砍伐等人为活动加剧,大气CO2浓度明显升高,由工业革命前的280 μL·L−1快速升高至现在的410 μL·L−1 [1]. 与此同时,碳稳定同位素比值也发生了明显变化,主要原因是,地质历史时期形成的化石燃料相对当前大气CO2富集12C,其燃烧产生的CO2大量进入大气时,会明显降低大气CO2的δ13C值[2]. 有研究显示,大气CO2的δ13C值已由工业革命前的−6.5‰降至现在−8.0‰ [3-4]. 伴随大气CO2的δ13C值的降低,海水溶解无机碳(DIC)因与大气直接发生气体交换,其δ13C也会发生明显改变. 例如,太平洋表层海水的δ13CDIC在1970—1990年间降低了0.4‰[5].
鉴于δ13CDIC在示踪碳的来源和循环过程[6-8]及相关通量估算[5, 9-10]等方面的应用,迄今国际上已在多个海域开展海水δ13CDIC的分布特征及变化规律的研究. 例如,南印度洋海表δ13CDIC呈现夏季高于冬季的分布特征,且最大变化幅度约为0.3‰[11]. 阿拉伯海附近不同季风期海表δ13CDIC值分布范围在0.7‰—1.0‰之间,且主要受生物活动的影响[12]. 我国迄今关于海水δ13CDIC的相关研究开展较少,仅在南海北部和东北部、胶州湾,以及长江口外、黄河口外、珠江口外等海域有数据报道[7, 13-17].
当前,δ13CDIC正在成为研究海洋缺氧酸化形成机制的有效工具. 基于有机碳矿化产生CO2的δ13C值能继承其来源有机物δ13C特征的原理,在长江口、珠江口、墨西哥湾北部等区域,成功解析出缺氧酸化发生过程中耗氧有机物的来源组成,这将在很大程度上指导实施旨在改善水体缺氧酸化现象的管控措施[15, 17-18]. 目前黄渤海同样受到季节性缺氧酸化现象的影响. 据调查,黄海冷水团区域因群落呼吸作用积累起大量CO2,并且在夏、秋季水文条件下无法排放,造成显著的季节性酸化现象[19]. 另外,2011年至2017年间渤海夏季的底层水溶解氧(DO)呈现波动下降趋势,个别海域已经接近2 mg·L−1这一缺氧阈值[20].
黄渤海作为与西太平洋直接和间接连接的陆架边缘海,其生物地球化学特征的变化集中体现了人类活动和全球变化对边缘海的影响. 另外,黄渤海有着丰富的渔业资源和大量的水产养殖基地,是中国重要的海洋经济区. 然而季节性缺氧酸化现象的发生,会对黄渤海底栖生物的群落结构[21]和海水养殖产业产生严重影响,因此对黄渤海海域季节性缺氧酸化现象的产生机理及环境效应的研究势在必行. 开展δ13CDIC的调查研究,有助于深入认识该海域缺氧酸化的形成机制. 迄今在黄渤海区域除了在胶州湾和黄河口曾开展过δ13CDIC示踪水体碳化学过程外[7, 16],尚未见在黄渤海开阔海域报道关于δ13CDIC的结果,更无从探讨δ13CDIC的变化与该海域季节性缺氧酸化的关系.
水体δ13CDIC分析的前处理有沉淀法[22]和气体法[23]两种. 其中沉淀法不适用于海水等含有高浓度硫酸盐的水体[24],所以海水δ13CDIC的分析多基于气体法. 其原理是将水样加磷酸酸化后收集产生的CO2进行测定[25]. δ13CDIC的测定通常采用稳定同位素比质谱仪(IRMS),该法在国内外碳的稳定同位素测定方面应用很广[26-30]. 近年来,随着光腔衰荡光谱技术(CRDS)的发展,也有研究者将其用于CO2的δ13C测定[25, 31-32]. 相比于IRMS来说,CRDS分析技术具有更高的仪器稳定性、便携性,而且成本更低. 已有的研究显示,将基于CRDS技术的CO2同位素分析仪应用于海水δ13CDIC测定,可得到与IRMS相当的准确度和精密度[25].
本研究设计并优化了一种海水中δ13CDIC分析样品的前处理方法,并且与基于CRDS技术的CO2同位素分析仪联用,测试了黄渤海夏季缺氧酸化发生海域典型断面的δ13CDIC,探讨其分布特征及可能的影响因素.
溶解无机碳稳定同位素分析样品的前处理方法及黄渤海夏季数据初步分析
An optimization of the acid-extraction pretreatment procedure for precisely detecting stable isotopic composition of seawater dissolved inorganic carbon: Its application in the Bohai and Yellow Seas
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摘要: 设计并优化了一套海水中溶解无机碳(DIC)稳定同位素(δ13CDIC)的预处理装置,并通过光腔衰荡光谱法对其进行分析测定. 在整个预处理及测定过程,通过向海水样品中添加磷酸并在高纯氮气的吹扫下使用气体采样袋收集其产生的CO2气体,随后将气体采样袋连接Picarro G2121-i型仪器进行δ13CDIC的测定. 通过对影响因素的研究确定了预处理条件为:海水样品体积30 mL, 85%(V/V)浓磷酸使用量1 mL,吹扫流速150 mL·min−1,吹扫时间4 min 20 s,仪器测定时间约10 min. 在此条件下DIC回收率超过99%,且δ13CDIC的测定标准偏差小于0.1‰,极差小于0.2‰. 基于此条件,对2019年夏季黄渤海典型断面不同水层的δ13CDIC进行了测定. 表层海水中δ13CDIC数据大部分集中在−0.5‰与0.5‰之间,其中黄海表层平均值为−0.2‰±0.5‰;底层海水中δ13CDIC在−2.0‰至−0.4‰的范围内,平均值为−1.3‰±0.5‰. 这种表层高、底层低的整体趋势主要受控于海气交换、光合作用、呼吸/矿化作用等生物地球化学过程,也与这两个海域经常发生夏季底层水耗氧酸化现象一致. 本研究为海水中δ13CDIC水样的前处理提供了一种经济、高效的手段.Abstract: We designed and optimized an acid-extraction pretreatment procedure for precisely detecting stable isotopic composition of seawater dissolved inorganic carbon (δ13CDIC). The CO2 was generated by adding phosphoric acid to the seawater samples, and collected in a gas sampling bag under the purge of nitrogen. Then, the gas sample was injected to a cavity ring-down spectroscopy analyzer (model: Picarro G2121-i) for δ13CDIC determination. The pretreatment conditions were optimized as follows: 30 mL seawater sample, 1 mL phosphoric acid (85%(V/V)), 150 mL·min−1 carrier gas flow rate, 4 min 20 s purge time, 10 min isotope determination. Under these conditions, the sample recovery of dissolved inorganic carbon (DIC) was more than 99%, the standard deviation of δ13CDIC determination was estimated to be less than 0.1‰, and the range was less than 0.2‰. Appling this procedure, we determined summertime distributions of δ13CDIC in the Bohai and Yellow Seas in 2019. Most surface δ13CDIC values ranged between −0.5‰ and 0.5‰, with an average of −0.2‰±0.5‰ in Yellow Sea surface waters. δ13CDIC in bottom waters ranged from −2.0‰ to −0.4‰, with an average of −1.3‰±0.5‰. The general δ13CDIC pattern could be explained by common biogeochemical processes such as air-sea gas exchange, photosynthesis and respiration/mineralization. It was also consistent with the earlier-reported summertime oxygen-depletion and seasonal acidification in the two coastal oceans. This study provides an efficient and reliable technique for the precision determination of δ13CDIC in seawater.
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图 7 黄海δ13CDIC (a)、表观耗氧量(b)和DIC (c)与水温的关系. 虚线椭圆指示黄海夏季底层低温水.
Figure 7. Relationships of stable isotopic compositions of dissolved inorganic carbon (DIC) versus seawater temperature (a), apparent oxygen utilization (AOU) versus seawater temperature (b), and DIC versus seawater temperature (c) in the Yellow Sea. Data obtained in the summertime cold water mass are enclosed within ellipses.
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