Paleo-seawater REE compositions and microbial signatures preserved in laminae of Lower Triassic ooids

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 La_SN/Yb_SN = 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.