Paleo-seawater REE compositions and microbial signatures preserved in laminae of Lower Triassic ooids
Date
2017
Authors
Li, Fei
Yan, Jiaxin
Burne, Robert
Chen, Zhongqiang
Algeo, Thomas
Zhang, Wen
Tian, Li
Gan, Yuanlu
Liu, Ke
Xie, Shucheng
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Elsevier
Abstract
Signals indicative of paleo-seawater rare earth element (REE) composition and microbial activity were identified in marine ooids from the Lower Triassic of South China using in situ laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) together with petrographic and isotope geochemical techniques. Quantitative trace element profiles were generated across successive growth laminae in the cortices of selected ooids. The shale-normalized REE and yttrium (Y) data of these layers show 1) light rare earth element depletion (mean LaSN/YbSN =0.36±0.14), 2) positive La anomalies (mean (La/La*)SN =2.02±1.25), and 3) high ratios of Y/Ho (mean 53±12), which are similar to modern seawater compositions and Bahamian ooids. These findings imply that ooids in the study samples faithfully record Early Triassic paleo-seawater chemical signals such as REE+Y distributions. Petrographic observations revealed dark-colored layers with intense fluorescence containing structures that resemble microbial filaments or extracellular polymeric substances. These layers also possess more enriched REE compositions and higher concentrations of nutrient-like elements (e.g., Zn and Ba) than adjacent recrystallized light-colored laminae. Most ooid laminae contain negative Ce anomalies indicative of a well-oxygenated environment of formation, but some laminae have minimal negative anomalies suggesting the influence of suboxic porewater conditions. The present study provides a new perspective on the role of microbes in ooid genesis and on the utility of ooids as proxies for paleo-seawater chemistry
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Palaeogeography, Palaeoclimatology, Palaeoecology
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Journal article
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2037-12-31
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