Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes
Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes
复制标题
华南扬子地台南缘早寒武世海洋氧化还原变化与氮循环：来自氮和有机碳同位素的证据
DOI：
10.1016/j.precamres.2015.06.009
复制
发表时间：
2015-09
影响因子：
3.8
通讯作者：
Yao Su-Ping
中科院分区：
文献类型：
--
作者：
Wang Dan;Struck Ulrich;Ling Hong-Fei;Guo Qing-Jun;Shields-Zhou Graham A.;Zhu Mao-Yan;Yao Su-Ping
早寒武世是一个关键的时期，在此期间，海洋环境遇到了重大的扰动以及关键的生物创新。古沉积物的氮同位素组成具有记录海洋生态系统和环境条件之间的共变的潜力。在这里，我们报告散装氮和有机碳同位素组成伴随钼浓度数据（一个氧化还原条件的代用指标）。对于寒武纪Fortunian阶段，可变δ 15 N体积值（−4至+2‰）和低δ 13 Corg值元嘉剖面的平均含水量（-35 ‰ ~-33 ‰）表明缺氧沃茨向透光层变浅，在盆地区域氨同化与厌氧氨氧化竞争。晚寒武世第二阶段，δ 15 Nbulk和δ 13 Corg值较低，Mobulk浓度较高（δ 15 Nbulk = −5 ~ 0‰; δ 13 Corg = −33.5 ~ −31.0‰; Mobulkin三岔剖面>100 ppm，δ 15 Nbulk = 100 ‰; δ 13 Corg = δ −33‰ >100 ppm莫布尔金元甲剖面），反映了强烈的N2固定伴随着间歇性的氨同化在扩大的缺氧条件下与变浅的化学跃层可能进入透光层。早寒武世第3阶段，两个剖面相对较高的δ 15 N体积值，以及正的δ 13 Corg偏移，表明15 N富集的硝酸盐同化作用的贡献增加，因此更多的好氧条件。寒武系第3阶中晚期陆架边缘沉积物的δ 15 N值较低（<10 ‰），以盆地区最低（-3 ~0 ‰）。结合深盆沉积物中高Mobulk浓度（>100 ppm）和高Mo/TOC体积比（平均1.18），这些数据表明透光带再次变为缺氧，但硫化物条件仅发生在盆地区域。与第2阶段相比，这一时期的洋地黄分布范围较小，光合生物量占主导地位，化能自养生物量占主导地位。有可能重建沿着南海南缘各种海洋环境中氧化还原条件的空间变化和时间演变。扬子地台。盆地地区似乎已占主导地位的缺氧和间歇性euxinia在透光带在大部分的早寒武世（Fortunian阶段到第3阶段），在早期阶段3的更多的好氧条件除外。在该平台上，除寒武纪第二阶段缺氧事件外，含氧浅水占主导地位。早寒武世第二阶段至第三阶段，外陆架区经历了缺氧，尽管面积可能有限。高的MoBulk浓度和Mo/TOC体积比表明，华南海盆与富氧程度较高的开阔洋有着良好的联系，而低的δ 15 N体积比（δ 15 N +1.6 ‰）可能反映了早寒武世海水硝酸盐的δ 15 N组成普遍低于现代值。
The early Cambrian is a crucial interval, during which the marine environment encountered significant perturbations alongside key biological innovations. The nitrogen isotopic composition of ancient sediments has the potential to record co-variation between the marine ecosystem and environmental conditions. Here we report bulk nitrogen and organic carbon isotopic compositions accompanied by Mo concentration data (a proxy for redox conditions) for a shelf marginal drill core section (Sancha) and a deep basinal outcrop section (Yuanjia) in Hunan province, South China.For the Cambrian Fortunian Stage, variable δ15Nbulkvalues (−4 to +2‰) and low δ13Corgvalues (−35‰ to −33‰) from the Yuanjia section indicate that anoxic waters shoaled into the photic zone, with ammonia assimilation competing with anammox in the basinal area. In the late Cambrian Stage 2, low δ15Nbulkand δ13Corgvalues, together with high Mobulkconcentrations (δ15Nbulk= −5 to 0‰; δ13Corg= −33.5 to −31.0‰; >100 ppm Mobulkin Sancha section and δ15Nbulk= ∼0‰; δ13Corg= ∼−33‰ >100 ppm Mobulkin Yuanjia section), reflect intense N2-fixation accompanied by intermittent ammonia assimilation under expanded euxinic condition with shoaling chemocline possibly into the photic zone. In the early Cambrian Stage 3, relatively high δ15Nbulkvalues, alongside positive δ13Corgexcursions, at both sections indicate an increasing contribution from15N-enriched nitrate assimilation and hence more oxic conditions. Shelf margin sediments from the middle and late parts of Cambrian Stage 3 are characterized by low δ15Nbulkvalues (∼0‰), being lowest in the basinal area (−3 to 0‰). In combination with high Mobulkconcentration (>100 ppm) and high Mo/TOCbulkratios (averaging ∼18) in deep basinal sediments, these data suggest that the photic zone became anoxic again, but that sulfidic conditions occurred in the basinal area only. Compared with Stage 2, euxinia was less extensive during this interval, with photosynthetic biomass dominating over chemoautotrophic biomass at both localities.Based on a combination of δ15N data from this study and published data from studies on contemporaneous sections in Yunnan Province and the Three Gorges area, it is possible to reconstruct the spatial variability and temporal evolution of redox conditions in the various marine environments along the southern margin of the Yangtze platform. The basinal area appears to have been dominated by anoxia and intermittent euxinia in the photic zone during most of the early Cambrian (Fortunian Stage to Stage 3) with the exception of more oxic conditions during early Stage 3. On the platform, oxygenated shallow water prevailed with the exception of the notable Cambrian Stage 2 anoxic event. The outer shelf area experienced anoxia from Stage 2 to Stage 3 of the early Cambrian, although of possibly limited areal extent. The high Mobulkconcentrations and Mo/TOCbulkratios suggest that the South China basin was well connected with the considerably oxygenated open ocean and that the mean low δ15Nbulkvalues (∼+1.6‰) from the basin may reflect generally lower δ15N composition for seawater nitrate in the early Cambrian compared with modern value.
登录
查看更多内容
影响因子：
5
作者：
U. Struck;K. Emeis;M. Voss;M. Krom;G. Rau
通讯作者：
U. Struck;K. Emeis;M. Voss;M. Krom;G. Rau
影响因子：
8.8
作者：
S. Schutter
通讯作者：
S. Schutter
DOI：
--
发表时间：
2008-07
期刊：
--
影响因子：
--
作者：
S. Crowe;D. Fowle;S. Katsev;B. Sundby;A. Mucci;G. Haffner
通讯作者：
S. Crowe;D. Fowle;S. Katsev;B. Sundby;A. Mucci;G. Haffner
影响因子：
3.4
作者：
Goldberg, Tatiana;Gordon, Gwyneth;Rehkaemper, Mark
通讯作者：
Rehkaemper, Mark
影响因子：
--
作者：
P. Cappellen;E. Ingall
通讯作者：
P. Cappellen;E. Ingall