Time-series rates of dissolved organic carbon production in the subtropical North Pacific Ocean

Over a 3-year period (April 2010-April 2013), we measured 14C-DOC production from vertical profiles used for determination of 14C-particle production, utilizing 0.2 um filtrates. Seawater for these experiments was collected from predawn CTD hydrocasts into acid-cleaned 500-ml polycarbonate bottles. A total of four replicate 500 ml bottles were subsampled per depth and each bottle was spiked with ~1.85 MBq 14C-bicarbonate. One hundred milliliters from one replicate per depth was vacuum filtered through a 0.2 mm polycarbonate filter and the filtrate served as a time zero blank. The remaining three bottles were hung on a free-drifting array, deployed before dawn, and incubated at their initial collection depths throughout the photoperiod (typically 11-13 hours). After sunset the array was recovered, and 100 ml subsamples of all bottles were filtered under gentle vacuum (&lt;50 mm Hg) onto 0.2 mm polycarbonate filters. These 0.2 mm filtrates were stored frozen (-20oC) until subsequent processing for determination of 14C-DOC productivity. Samples were processed as follows: 100 ml of the 14C-PC filtrates were thawed, poured into 500 ml polyethylene separatory funnels, and acidified by the addition of 500 µl of 2 M sulfuric acid (H2SO4). Samples were vigorously bubbled with air in a fume hood to remove 14CO2. A 70 ml subsample was removed from each separatory funnel and poured into a 100 ml glass serum bottle containing 1 ml of 2 M sodium hydroxide (NaOH) and 10 ml of 0.37 M potassium persulfate (K2S2O8) in 1 M NaOH. Bottles were sealed with rubber stoppers, crimp sealed with an aluminum cap, and autoclaved at 126°C for 200 minutes; oxidizing 14C-DOC to 14C-DIC in an alkaline solution. Once cooled to room temperature, samples were uncapped and resealed using rubber sleeve stoppers holding plastic center wells containing ~2 x 2 cm pieces of fluted chromatographic filter paper (Whatman 2) soaked with 0.2 ml of β-phenylethylamine. A syringe was used to inject 4 ml of 9 N H2SO4 into the solution, converting the 14C-labeled dissolved inorganic carbon (hereafter 14C-DIC) to 14CO2. Samples were stored undisturbed at room temperature, passively trapping the 14CO2 on the β-phenylethylamine soaked wick. After at least 100 hours, rubber sleeve stoppers were removed and center wells and wicks were placed in scintillation vials, followed by the addition of 10 ml of Ultima Gold LLT scintillation cocktail. Samples were subsequently counted on a Perkin Elmer Tri-Carb 2800TR liquid scintillation counter. Rates of 14C-DOC production were computed for each cruise as the mean of the triplicate bottles from each depth minus the average 14C-activity of the time zero (blank) samples.本研究于2010年4月至2013年4月的三年周期内，针对用于测定碳14标记颗粒（14C-particle）生成量的垂直剖面水样，采用0.2 μm滤液开展碳14标记溶解有机碳（14C-DOC）生成量的测定工作。本实验所用海水通过黎明前的温盐深剖面仪（CTD）采水系统采集，装入经酸清洗的500 mL聚碳酸酯（polycarbonate）瓶中。每个水深层级共设置4个500 mL平行采样瓶，向每个瓶中添加约1.85兆贝可（MBq）的碳14标记碳酸氢盐。从每个水深层级的1个平行瓶中取100 mL水样，经0.2 μm聚碳酸酯滤膜真空过滤，所得滤液作为零时刻空白对照样。剩余3个平行瓶被挂载于黎明前布设的自由漂移浮标阵列中，在初始采集水深处进行整个光照周期（通常为11~13小时）的原位培养。日落之后回收该浮标阵列，将所有采样瓶中的100 mL分样以弱真空（<50毫米汞柱）条件过滤至0.2 μm聚碳酸酯滤膜上。将这些0.2 μm滤液保存于-20℃冷冻环境中，直至后续开展14C-DOC生产力的测定工作。样品处理流程如下：取100 mL 14C标记聚碳酸酯滤液进行解冻，转移至500 mL聚乙烯分液漏斗中，加入500 μL 2 mol/L硫酸（H₂SO₄）进行酸化；在通风橱中向酸化后的水样中通入空气进行剧烈鼓泡，以去除生成的14C标记二氧化碳（¹⁴CO₂）。从每个分液漏斗中取70 mL分样，转移至100 mL玻璃血清瓶中，该血清瓶内预先装有1 mL 2 mol/L氢氧化钠（NaOH）以及10 mL 1 mol/L氢氧化钠溶液中溶解的0.37 mol/L过硫酸钾（K₂S₂O₈）。用橡胶塞密封血清瓶后，以铝盖进行压封，随后在126℃下高压灭菌200分钟，在碱性溶液中将14C-DOC氧化为14C标记溶解无机碳（14C-DIC）。待样品冷却至室温后，开盖并使用橡胶套塞重新密封，橡胶套塞内置塑料中心井，井中放置约2 cm×2 cm的槽纹层析滤纸（沃特曼2号滤纸，Whatman 2），该滤纸预先经0.2 mL β-苯乙胺（β-phenylethylamine）浸泡。使用注射器向瓶内溶液中注入4 mL 9 mol/L硫酸，将14C标记溶解无机碳（下文简称14C-DIC）转化为¹⁴CO₂。将样品静置存放于室温环境中，使生成的¹⁴CO₂被动吸附于经β-苯乙胺浸泡的滤纸上。静置至少100小时后，取下橡胶套塞，将塑料中心井及吸附滤纸转移至闪烁瓶中，随后加入10 mL Ultima Gold LLT闪烁液。随后使用珀金埃尔默（Perkin Elmer）Tri-Carb 2800TR型液体闪烁计数器对样品进行放射性计数。每个航次的14C-DOC生成速率按如下方式计算：取每个水深层级三份平行瓶的放射性活度均值，减去零时刻空白对照样的平均14C放射性活度。