Glendonite occurrences in the Tremadocian of Baltica: first Early Palaeozoic evidence of massive ikaite precipitation at temperate latitude (original) (raw)

Glendonite occurrences in the Tremadocian of Baltica: first Early palaeozoic evidence of massive ikaite precipitation at temperate latitudes

Scientific Reports , 2019

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 δ 18 O 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 (CaCO 3 ·6H 2 O) 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 δ 18 o-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. Except one rather controversial note 1 , the record of glendonites displays an apparent gap from Neopoterozoic 2 to Permian 3 times. However, similar calcareous nodular aggregates embedded in Tremadocian black shales of the East Baltic (Fig. 1a), the so-called "antraconites", have been known for more than 150 years. These aggregates are documented from 24 geographical localities in the Türisalu and Koporiye formations (Cordylodus angulatus-Paltodus deltifer pristinus zones) and sporadically in the Orasoja Member (upper part of the Kallavere Formation; Cordylodus angulatus-Paltodus deltifer pristinus zones), exposed along 600 km of the Baltic-Ladoga Glint 4 , a transect linking North Estonia to the eastern St Petersburg area (Fig. 1b). All these units were accumulated in the