Glendonite occurrences in the Tremadocian of Baltica first Early Palaeozoic evidence of massive ikaite precipitation at temperate latitude

• Published in: Scientific reports Vol. 9
• Main Authors: Popov, Leonid E, Alvaro, Javier J, Holmer, Lars E., Bauert, Heikki, Ghobadi Pour, Mansoureh, Dronov, Andrei V, Lehnert, Oliver, Hints, Olle, Männik, Peep, Zhang, Zhifei, Zhang, Zhiliang
• Format: Journal Article
• Language: English
• Published: 2019
• Subjects: Earth Science with specialization in Historical Geology and Palaeontology; Geovetenskap med inriktning mot historisk geologi och paleontologi
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-019-43707-4

The Tremadocian (Early Ordovician) is currently considered a time span of greenhouse conditions with tropical water surface temperature estimates, interpolated from oxygen isotopes, approaching 40 °C. In the mid-latitude Baltoscandian Basin, conodonts displaying low δ18O values, which suggest high temperatures (>40 °C) in the water column, are in contrast with the discovery of contemporaneous glendonite clusters, a pseudomorph of ikaite (CaCO3·6H2O) traditionally considered as indicator of near-freezing bottom-water conditions. The massive precipitation of this temperature sensitive mineral is associated with transgressive conditions and high organic productivity. As a result, the lower Tremadocian sediments of Baltoscandia apparently contain both "greenhouse" pelagic signals and near-freezing substrate indicators. This paradox points to other primary controlling mechanisms for ikaite precipitation in kerogenous substrates, such as carbonate alkalinity, pH and Mg/Ca ratios, as recently constrained by laboratory experiments. Preservation of "hot" conodonts embedded in kerogenous shales rich in δ18O-depleted glendonites suggests both the onset of sharp thermal stratification patterns in a semi-closed basin and the assumed influence of isotopically depleted freshwater yielded by fluvial systems.