Millimeter-scale scanning of redox-sensitive elements in Tremadocian Alum Shale for revealing subtle redox variations and cyclicity, northwestern Estonia, Baltic Palaeobasin

Alum Shale is a finely laminated organic-rich sedimentary rock which has recorded an anoxic-euxinic period which lasted for more than 20 My during tthe Cambrian and early Ordovician in the current-day Scandinavian region. A younger section (Tremadocian) of Alum Shale formation is located in Estonia, eastern part of the Baltic Paleobasin.

A 10-meter section of Alum Shale (black shale) and associated sediments (glauconitic sandstone, grey shale, sandstone) were scanned in core PED-403 with a Geotek XRF analyser, attached to automated Geotek-MSCL workstation. The concentrations of Mo, U, Ti, Al, S, and Si were analysed and corrected by measuring in-house reference samples.

Mo and U mostly co-vary in the sediments, but certain sections indicate a preferential uptake of Mo instead of U, indicating periods where particulate shuttle was active during slightly more oxic periods. Fine laminae also contain very low Mo and U enrichments, pointing to short-lived oxygenated conditions instead of prevailing perennial oxygen minium zone conditions on the shelf. Suble redox changes do not correlate with sedimentary textures described in the core. Systematic cyclicity was detected in the case of Ti, Al and Si. Sørensen et. Al. (2020) have demonstrated that astronomically forced climate cycles have been recorded in Cambrian-age Scandinavian Alum Shale cores. With follow-up analyses we wish to reveal whether cycles detected in the Estonian core have similar forcings.

This study was supported by EGT-TWINN project (GA no 101079459).