Evidence from chlorin nitrogen isotopes for alternating nutrient regimes in the Eastern Mediterranean Sea
Evidence from chlorin nitrogen isotopes for alternating nutrient regimes in the Eastern Mediterranean Sea
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DOI：
10.1016/j.epsl.2009.12.009
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发表时间：
2010-02-15
影响因子：
5.3
通讯作者：
Pearson, Ann
中科院分区：
文献类型：
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作者：
Higgins, Meytal B.;Robinson, Rebecca S.;Pearson, Ann
叶绿素降解产物氯的氮同位素反映了海洋浮游植物群落利用的营养氮的同位素组成。在东地中海更新世和全新世的沉积物中，氯和总氮的δ N-15值一致变化，其偏移量近似于5‰，反映了叶绿素生物合成过程中所传递的分异。来自综合海洋钻探计划站点964和969的样品以类似于4-10 cm的采样分辨率进行了分析，这些样品聚集在类似于100-170 ka的推动事件2、3、4和5周围。在低有机含量沉积物中，氯值接近于0 / 1000，而总氮值接近于+5 / 1000，这表明后者反映了原始生物量，而不是成岩同位素富集的结果。在浅层层，氯和总氮值为负的千分之五（分别类似于千分之五和千分之零），类似于先前报道的现代水柱颗粒（类似于千分之零）。我们认为，东地中海的营养条件对应于三种情况，而推进体与现代大块三角洲N-15之间的相似性是巧合。缺乏有机物的泥灰岩沉积物形成于低营养条件下，其地表生产力是由大西洋源硝酸盐上涌造成的。腐殖体的特征是重氮化作用增强，这可能是由于河流向地表水的磷通量增加所致。目前的条件受人为氮源的支配。这些情景都与土壤冲积物形成的模型相一致，在该模型中，由于P:N营养失衡，淡水输入增加引起的分层导致了氮固定。产量的提高与分层作用相结合，促进和维持了缺氧的深水，从而增加了有机质的保存。这样的模型可能与解释过去海洋中其他强烈的有机物沉积事件有关。(C) 2010 Elsevier B.V.版权所有
Nitrogen isotopes of chlorins, degradation products of chlorophyll, reflect the isotopic composition of nutrient N utilized by marine phytoplankton communities. Here we show that in sediments of the eastern Mediterranean Pleistocene and Holocene, values of delta N-15 for chlorins and total nitrogen vary in concert, with a consistent offset of similar to 5 parts per thousand reflecting the fractionation imparted during chlorophyll biosynthesis. Samples from the Integrated Ocean Drilling Program Sites 964 and 969 were analyzed at a sampling resolution of similar to 4-10 cm, clustered around sapropel events 2, 3, 4 and 5 (similar to 100-170 ka). In low organic content sediments, chlorin values of similar to 0 parts per thousand coincident with total nitrogen values of similar to+5 parts per thousand indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment. In sapropel horizons, the chlorin and total nitrogen values are 5 parts per thousand more negative (similar to-5 parts per thousand and similar to 0 parts per thousand, respectively), resembling previously-reported, modern-day water-column particulates (similar to 0 parts per thousand). We suggest that nutrient conditions in the Eastern Mediterranean correspond to three scenarios and that the similarity between sapropel and modern-day bulk delta N-15 is coincidental. Organic-poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of Atlantic-sourced nitrate. Sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine P fluxes to surface waters. Present-day conditions are dominated by anthropogenic N sources. These scenarios agree with a model of sapropel formation in which stratification caused by increased fresh-water inputs led to N fixation due to P:N nutrient imbalance. Enhanced production combined with stratification promoted and maintained anoxic deep waters, consequently increasing organic matter preservation. Such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans. (C) 2010 Elsevier B.V. All rights reserved.