Abundant microspherules from the Upper Ordovician of northern Tarim Basin, Northwest China: Origin and palaeoenvironmental implications

A continuous record of abundant microspherules, pyrite framboids, and bacterial fossils is obtained from the Upper Ordovician in the Dawangou section of northern Tarim Basin, Northwest China. Chemical and physical characteristics of these microspherules were determined using a scanning electron microscope and an energy dispersive spectrometer. Microspherules were formed as a single spherule, spherule aggregates, and drop‐like particles. Physical morphology of the spherules has no relation to its formational process. Energy dispersive spectrometry results indicate that microspherules are mainly composed of Ca, C, O, Si, Al, S, Fe, and Ti. Thus, these microspherules are further categorized into 5 major types: iron, titaniferous‐iron, siliceous, calcareous, and organic microspherules. The first 3 types of microspherules are likely of volcanic origin rather than extraterrestrial origins (i.e., impact event or cosmic dusts) or modern industrial pollutions due to their morphologies and chemical compositions. Both calcimicrospherules and organic microspherules are biogenic, and they are distributed in the lower part of the studied section (Beds 4–6), in which cephalopods, brachiopods, and bacterial fossils are also commonly present and pyrite framboids are absent. These features generally point to an oxic condition. In contrast, other microspherules are abundant in the upper part of the section (Beds 9–13), in which tiny pyrite framboids (mean diameters 4.6–7.3 μm) are rather abundant and typical of euxinic to dysoxic conditions. Both microspherule and pyrite framboid evidences indicate that the northern Tarim region became oxygen‐poor conditions, which were coupled with intensive volcanic eruptions during the Late Ordovician.