No abstract

No abstract

Detrital zircon U-Pb ages of Lower Cambrian and Middle Devonian have been determined for sandstone in Estonia through LA-ICP-MS (Nu instruments, Wrexham, UK). Both sandstones have a similar zircon age spectrum with distinct age clusters that reflect the basement geology of Baltica, i.e. (Sveconorwegian). Noteworthy is a cluster at 750-550 Ma, because rocks of such age are absent within the core of Baltica. The present results suggest a possible link between Baltica and Avalonia/Cadomia during the Late Neoproterozoic.

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.