Preto, Nereo, Borsato, Andrea, Carampin, Raul, della Porta, Giovanna, Frisia, Silvia, Gattolin, Giovanni, Klügel, Andreas, Himmler, Tobias, Westphal, Hildegard ORCID: https://orcid.org/0000-0001-7324-6122 and Zorzi, Federico (2025) Microbialites as archives for palaeo-seawater trace element composition – two case studies from the Triassic of the Dolomites (northern Italy) compared. In: Nucleation and Growth of Sedimentary Minerals. , ed. by Meister, Patrick, Fischer, Cornelius and Preto, Nereo. Special Publication, 50 . International Association of Sedimentologists, Gent, pp. 267-301. ISBN 978-1-7398845-2-9

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Abstract

Marine authigenic carbonates in microbialites are widely used as archives of the trace element composition of seawater. This is because microbialites form in seawater, hence, their constituting carbonates are sedimentary minerals which are expected to preserve a faithful record of past seawater Rare Earth Elements and Y composition. Here, Triassic microbialites with a range of diagenetic alterations were investigated with the aim of testing the potential and limitations of microbialite carbonates as archives of past seawater chemical composition. Microbialites from the Triassic of the Dolomites (northern Italy) were analysed for their trace element composition by using both Laser Ablation Inductively Coupled Plasma Mass Spectrometry with a spot size of 100 μm and an electron microprobe with spot sizes of ca. 2 μm. Samples from the upper Anisian Latemar platform include thrombolites with clotted peloidal fabric. These are associated with fossils whose skeletons were made of aragonite, now completely replaced by calcite spar. Abundant marine radiaxial fibrous cements occlude all but the largest, centimetre-scale pores. These cements, along with microbialites, now consist of low-Mg calcite. Microbialites with clotted peloidal fabric were also sampled from the lower Carnian patch-reefs of the Cortina d’Ampezzo Basin. In this case, aragonitic shells and skeletons of sponges and corals retained their original mineralogy and ultrastructure and microbialites are still made of high-Mg calcite with 10-14 mole % MgCO3. Marine cements are scarce, while most pores are filled by shallow burial siderite or ferrous calcite cements. The original carbonate minerals in this lower Carnian case study are better preserved than in the upper Anisian Latemar platform and it could be expected that they retain the trace element signature of Carnian seawater. However, marine cements and most microbialites from the upper Anisian Latemar platform show seawater-like, shale-normalised Rare Earth Elements and Y patterns, whereas microbialites in the lower Carnian of the Cortina d’Ampezzo Basin mostly yield patterns unlike seawater, resembling, instead, those of the shallow burial siderites and Fe-calcites. Microbialites from both localities were initially highly porous, but, while the pores of upper Anisian microbialites were filled by marine cements soon after deposition, the pores of lower Carnian microbialites were filled during shallow burial by Fe-rich carbonate cements. The intermixing of microbialite carbonate and shallow burial, Fe-rich cement occurs at such a small scale that even the spatial resolution of electron microprobes is not high enough to ensure that microbialite carbonates analyses are not contaminated by pore-filling cements. We conclude that standard analytical techniques such as laser ablation inductively coupled plasma mass spectrometry are not suitable to isolate and analyse fossil microbialites, because they are lacking the necessary spatial resolution, and as a consequence, it is mostly the pore-filling cements that are detected. Hence, trace element analysis on microbialites only has the potential to faithfully record the chemical composition of past seawater if the pore-filling phase is a marine cement. In such instances, millimetre-scale rims or crusts of marine cements usually occur as well and should be preferred for trace element analyses.

Document Type:	Book chapter
Programme Area:	PA4
Research affiliation:	Biogeochemistry and Geology > Geoecology & Carbonate Sedimentology
Date Deposited:	03 Mar 2026 09:44
Last Modified:	03 Mar 2026 09:44
URI:	https://cris.leibniz-zmt.de/id/eprint/6133

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