Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea
Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea
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DOI：
10.1016/j.gca.2018.05.023
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发表时间：
2018-08
影响因子：
5
通讯作者：
S. Smart;H. Ren;S. Fawcett;R. Schiebel;M. Conte;P. Rafter;K. Ellis;M. A. Weigand;S. Oleynik;G. Haug;D. Sigman
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文献类型：
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作者：
S. Smart;H. Ren;S. Fawcett;R. Schiebel;M. Conte;P. Rafter;K. Ellis;M. A. Weigand;S. Oleynik;G. Haug;D. Sigman
本文报道了2009 - 2013年在马尾藻海北部百慕大时间序列站点采集的浮游有孔虫的氮(N)同位素比值（δ15N），这些有孔虫分别来自上层海洋网拖（表层至200 m）、系泊沉积物圈闭和岩心顶部沉积物。与以往低纬度岩心-顶部沉积物的测量结果一致，在净流中收集到的浮游带、鞭毛共生有孔虫壳内结合的有机质年平均δ15N（平均2.3‰）接近浅层温跃层（~ 200 m）硝酸盐（2.6‰），后者是马尾藻海表层水新氮的主要来源。无鞭毛共生生物的深栖有孔虫具有较高的δ15N（平均3.6‰）。我们观察到在净捕有孔虫中，体组织和壳结合的δ15N没有系统差异。从净水系（6.8 nmol/mg）到沉积物捕集器（5.4 nmol/mg）和表层沉积物（3.0 nmol/mg），壳氮含量呈下降趋势。平均而言，壳界δ15N从净拖带（3.1‰）上升到沉积物圈闭（3.7‰），但进入沉积物后没有进一步变化（3.7‰）。综上所述，这些观测结果表明，在下沉至500 m以上的过程中，具有较低δ15N和较高N含量的壳或壳部分优先损失，然后在下沉和掩埋之间出现非同位素分异的N含量下降。沉积物圈闭（以及较小程度的地表净拖带）的时间序列数据显示出季节性的δ15N变化，在早春最小，在晚春最大，从夏季到秋季逐渐下降。这些变化似乎是由上层海洋总生物量δ15N的季节性变化引起的，而这种变化又是由早春硝酸盐供应、随后的硝酸盐减少以及在夏末秋初铵循环的相对重要性增加所驱动的。上层海洋总生物量与有孔虫之间的δ15N关系表明，有孔虫结合的δ15N记录了低营养（如亚热带）环境中年硝酸盐供应的δ15N，但也对高营养区域的硝酸盐消耗程度敏感，在某些条件下可能对上层海洋铵循环的变化敏感。
We report the nitrogen (N) isotope ratios (δ15N) of planktic foraminifera collected from upper-ocean net tows (surface to 200 m), moored sediment traps, and core-top sediments at the Bermuda Time-series Site in the northern Sargasso Sea between 2009 and 2013. Consistent with previous measurements from low-latitude core-top sediments, the annually-averaged δ15N of organic matter bound within the shells of euphotic zone-dwelling, dinoflagellate symbiont-bearing foraminifera collected in net tows (2.3‰ on average) approximates that of shallow thermocline (∼200 m) nitrate (2.6‰), the dominant source of new N to Sargasso Sea surface waters. Deeper-dwelling foraminifer species without dinoflagellate symbionts tend to have a higher δ15N (3.6‰ on average). We observe no systematic difference between the bulk tissue and shell-bound δ15N in net tow-collected foraminifera. A decline in shell N content is observed from net tows (6.8 nmol/mg) to sediment traps (5.4 nmol/mg) and surface sediment (3.0 nmol/mg). On average, shell-bound δ15N rises from net tows (3.1‰) to sediment traps (3.7‰) but does not change further upon incorporation into the sediments (3.7‰). Together, these observations are consistent with preferential loss of shells or shell portions with lower δ15N and higher N content during sinking through the upper 500 m, followed by a non-isotope fractionating decrease in N content between sinking and burial. Time-series data from sediment traps (and to a lesser extent, surface net tows) exhibit seasonal δ15N variations, with a minimum in early spring, a maximum in late spring and a decline from summer to fall. These variations appear to arise from seasonal changes in the δ15N of total upper-ocean biomass, which are, in turn, driven by early springtime nitrate supply, subsequent nitrate drawdown, and an increase in the relative importance of ammonium recycling into the late summer and early fall. The δ15N connection between total upper ocean biomass and foraminifera indicates that foraminifer-bound δ15N records the δ15N of the annual nitrate supply in oligotrophic (e.g., subtropical) environments but will also be sensitive to the degree of nitrate consumption in high-nutrient regions and possibly to changes in upper-ocean ammonium recycling under some conditions.