Potential ZnS fossilization of gastropods (Middle Jurassic claystones from Central Poland)

Abstract

The rich fossil fauna in the Middle Jurassic claystones that crop out in the Krakow-Czestochowa Upland is extensively replaced by sulfide minerals, mainly pyrite. Sphalerite (ZnS) is rare and restricted to the internal casts of gastropods, often together with framboidal and euhedral pyrite and calcite. Scanning electron microscopy-energy dispersive spectrometer study was undertaken to explain this curious association. The results show that although direct infilling of the carbonate shell, similar to processes occurring during pyritization, is probable, it does not explain all textures observed. We propose that the carbonate shells were initially infilled by calcite and iron sulfides and sphalerite subsequently replaced the calcite casts. Preferential occurrence of ZnS in gastropods could result from accumulation of higher concentration of zinc during the organisms' life. After death, this Zn was introduced into the carbonate making gastropods more prone to ZnS replacement. Formation of ZnS casts was probably a late diagenetic event as zinc content of the surrounding sediment does not appear to influence sphalerite formation.

Large-Field Electron Imaging and X-ray Elemental Mapping Unveil the Morphology, Structure, and Fractal Features of a Cretaceous Fossil at the Centimeter Scale

“…The fossilization process of some fossils from the Ipubi Formation occurs through pyritization in several stages, as previously shown by our group . In this way, the hypothesis of formation of ZnS through different stages is another possibility, similarly to the process occurring for gastropods . First, the animal is infilled by iron sulfides and calcite, and finally sphalerite or wurtzite substitutes for the calcite casts.…”

“…The presence of ZnS in fossils, appearing as sphalerite or wurtzite, was observed in biogenic remains in the form of internal casts, as infillings of voids in skeletons, and also as replacement of skeleton materials. , More specifically, this compound has been observed in fossils from deep-sea hot springs at hydrothermal vent fields present along crests of volcanic spreading ridges. − Fossils in these areas were formed from fauna commonly comprising worms, crabs, and a variety of fish. , The mineralization of ZnS in these fossils is related to the acidification of the solution and the presence of hydrogen sulfide, which consequently induce the precipitation of zinc sulfide. , …”

“…In the case of gastropods, the organisms accumulate a large quantity of Zn during their lives, allowing this chemical element to be available in the environment after their deaths to form compounds such as ZnS. Regarding the shrimp A1, the presence of Zn can also be attributed to an accumulation process during life, although it is known through modern toxicological studies that shrimps are not an efficient trap for Zn , as are gastropods, ,− and are able to regulate their total body levels of Zn . Obviously, we cannot exclude a different origin for Zn, such as its introduction by water with a high concentration of zinc.…”

“…The process of ZnS formation itself can be very complex, as pointed out in the case of gastropod fossils. In fact, at least two possibilities were found: a direct infilling of the carbonate shells by ZnS, or the infilling of the carbonate shells by calcite and/or pyrite, with the replacement of these materials by sphalerite or wurtzite . Related to the direct infilling by ZnS, it was shown that the presence of sulfate-reducing bacteria cultured in a ZnSO 4 /FeSO 4 medium leads to ZnS precipitation, but iron sulfides such as pyrite are not precipitated .…”

The significance of mudstone fabric combined with palaeoecological evidence in determining sedimentary processes – an example from the Middle Jurassic of southern Poland

“…1), sit u ated in the south east ern tip of the out crop belt. They have been doc u mented as re gards their stra tig ra phy, palae ontol ogy, geo chem is try and palaeo ec ol ogy (e.g., Ró¿ycki, 1953;Pazdrowa, 1967;Szczepanik et al, 2005;Marynowski et al, 2007;Salamon and Zatoñ, 2007;Szczepanik and Saw³owicz, 2008;Zatoñ and Tay lor, 2009;Zatoñ, 2011;Zatoñ et al, 2011Zatoñ et al, , 2012Barski, 2012). The Blanowice sec tion ex poses ~12 m of dark grey and black, lam i nated mudstones with sev eral ho rizons of, and loosely scat tered, sid er ite and cal car e ous con cretions (Fig.…”