Re-Os geochronology and geochemical evolution of late Cambrian to Middle Ordovician Alum and Tøyen shales, Sweden

Abstract

The limited number of accurate and precise radiometric ages through the ∼100 Myr span of the Cambrian and Ordovician impedes reliable age determinations for stage boundaries in these periods. Here, we fill significant gaps in the early Paleozoic chronostratigraphy by providing precise Re-Os time-pins. Sample selection is linked to a firm biostratigraphic framework built on the appearance and distribution of trilobites, graptolites, and conodonts. A Furongian (upper Cambrian) Alum Shale section (Andrarum-3 drill core, Scania, Sweden) at the onset of the Steptoean Positive Carbon Isotopic Excursion (SPICE) yields highly non-isochronous Re-Os isotopic data from a section with wildly fluctuating δ¹³C_org; however, selected data from a narrow sediment band with steady carbon isotope stratigraphy provides an imprecise Re-Os age of 497 ± 28 Ma (2σ; Model 3; n = 3), with an initial ¹⁸⁷Os/¹⁸⁸Os ratio (Os_i) of 0.74 ± 0.05. Organic-rich Alum Shale (Tomten-1 drill core, Västergötland, Sweden) from ∼120 cm below the Cambrian-Ordovician boundary yields a Model 1 age of 488.6 ± 5.1 Ma (2σ; MSWD = 1.5; n = 25) and an Os_i of 0.82 ± 0.04 for Stage 10, uppermost Cambrian. Biostratigraphic data indicate the dated Alum Shale is from an interval slightly below the Top Of Cambrian Excursion (TOCE) and slightly above the First Appearance Datum (FAD) of the agnostoid Lotagnostus americanus. Organic-rich Tøyen Shale (Lerhamn drill core, Scania, Sweden) yields a precise Model 1 Re-Os age of 469.7 ± 1.4 Ma (2σ; MSWD = 1.0; n = 10) and Os_i of 0.802 ± 0.002 for the maximum age of the Floian–Dapingian stage boundary (Lower–Middle Ordovician boundary). The Os isotopic composition of seawater from the latest Ediacaran through the Cambrian to Early-Middle Ordovician hovers around 0.8 but falls to 0.54 by early Silurian. This significant decrease in seawater ¹⁸⁷Os/¹⁸⁸Os is consistent with reduced chemical weathering and cooler seawater temperatures through the Middle–Late Ordovician. Overall, Redox Sensitive Element (RSE; Re, Os, Mo, U) abundances correlate positively with Total Organic Carbon (TOC), suggesting efficient removal of these elements from an anoxic water column by organic matter. However, these relationships break down for high TOC (>10%) shales depositing under euxinic conditions. The RSE-TOC relationship breakdown supports enhanced metal drawdown from the water column with local pyrite accumulation. Geochemical data suggest the deposition of Alum and Tøyen shales under hydrographically restricted settings with increased primary productivity along the Baltica's margin during the latest Cambrian to Early-Middle Ordovician.