Constraining the conditions of phosphogenesis: Stable isotope and trace element systematics of Recent Namibian phosphatic sediments
Constraining the conditions of phosphogenesis: Stable isotope and trace element systematics of Recent Namibian phosphatic sediments
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DOI：
10.1016/j.gca.2021.03.022
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发表时间：
2021-03
影响因子：
5
通讯作者：
Kaarel Lumiste;Kaarel Mänd;J. Bailey;E. Stüeken;K. Paiste;Li-yuan Lang;Holar Sepp;A. Lepland;K. Kirsimäe
中科院分区：
文献类型：
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作者：
Kaarel Lumiste;Kaarel Mänd;J. Bailey;E. Stüeken;K. Paiste;Li-yuan Lang;Holar Sepp;A. Lepland;K. Kirsimäe
现代的磷酸化作用发生在受上升流和高初级生产力影响的大陆边缘。磷质沉积物的形成与全球气候波动、磷的生物循环和局部氧化还原条件有关。虽然在磷的过程中所涉及的描述，高分辨率的数据氧化还原和稳定同位素系统学的Recentin-situophosphorites是稀缺的。在这方面的贡献，我们调查了微量元素和硫，氮和有机碳稳定同位素组成的Recentin-situphosphatic沉积物的纳米比亚海岸。此外，我们检查了不同的广泛使用的地球化学指标在磷酸盐沉积物的可靠性。我们的研究结果表明，从硫化物的亚氧条件下，与固体磷酸钙矿物浓度的最大值相一致的转变。这种转变是伴随着钼和稀土富集和TOC丰度的单向变化。磷质沉积物中相对较低的黄铁矿δ 34 S值（约-20 ‰）表明在磷质生成过程中存在开放体系分馏作用。成磷作用的启动也伴随着沉积物δ 13 Corg和δ 15 N值的负移。磷酸盐结合硫酸盐（PAS）δ 34 S值低于现代海水硫酸盐值，表明参与化学无机营养硫氧化。我们的研究结果表明，在氧化还原条件从硫化物（亚）好氧，再加上活跃的硫循环的先决条件是磷酸化。磷质沉积物显示出大量的富集在U和V突出使用这些元素的复杂性，以及V/（V + Ni）和V/Cr，作为氧化还原代理，特别是在磷块岩和磷质沉积物。
Modern phosphogenesis occurs on continental margins influenced by upwelling and high primary productivity. The formation of phosphatic sediments is coupled to global climate fluctuations, biological cycling of phosphorus and local redox conditions. Although the processes involved in phosphogenesis are well described, high-resolution data on the redox and stable isotope systematics in Recentin-situphosphorites are scarce. In this contribution, we investigate the trace element and sulfur, nitrogen and organic carbon stable isotope composition of Recentin-situphosphatic sediments off the coast of Namibia. Also, we examine the reliability of different widely used geochemical proxies in phosphatic sediments. Our results suggest a shift from sulfidic to suboxic conditions, coinciding with the maximum in solid calcium phosphate mineral concentration. This shift is accompanied by unidirectional changes in Mo and Re enrichments and TOC abundance. Relatively low pyrite δ34S values (ca −20‰) of phosphatic sediments indicate open system fractionation during phosphogenesis. The initiation of phosphogenesis is also accompanied by negative shifts in sedimentary δ13Corgand δ15N values. Phosphate associated sulfate (PAS) δ34S values are lower than modern seawater sulfate values, suggesting the involvement of chemolithotrophic sulfur oxidation. Our results show a shift in redox conditions from sulfidic to (sub)oxic, coupled with active sulfur cycling are prerequisites for phosphogenesis. Phosphatic sediments show substantial enrichments in U and V highlighting the complexity of using these elements, as well as V/(V + Ni) and V/Cr, as redox proxies particularly in phosphorites and phosphatic sediments.