DETERMINING MOLYBDENUM SPECIATION TO UNDERSTAND THE DEPOSITIONAL HISTORY OF BLACK SHALES

Unaltered black shales are used to interpret past depositional redox conditions by examining rock geochemistry. Measuring redox sensitive trace element concentrations and isotopic signatures, can successfully identify ancient oxygen depleted episodes. For example, molybdenum (Mo) isotopic measurements have allowed researchers to make breakthroughs reconstructing the timing of early Earth’s oxygenation. Additionally, higher Mo concentration are usually associated with severe euxinia (e.g., free sulphide in the water column). However, the interpretation of Mo systemics is too qualitative which limits the full potential of the Mo proxy. Our research uses X-ray Absorption Fine Structure (XAFS) as an innovative approach to determine the molecular geochemistry (speciation) of Mo in rocks.

The early Triassic (~250-240 Ma) Montney formation is in the Alberta Sedimentary Basin along the Canadian Rockies. It is composed of black shales and interbedded siltstone/sandstone packages in transgressive-regressive cycles. Field and well observations the Lower Montney have interpreted these cycles to indicate distal offshore provenance with an anoxic or possibly euxinic water column due to the amount of organic matter preserved. Yet, there is some debate on the degree of reducing conditions during deposition of the Lower Montney. Recent research characterizing organic matter (O.M.) fractions found that O.M. migrated into the formation post-depositionally, complicating the distal offshore interpretation. To untangle the depositional history of black shales within this formation, we measured the Mo concentration and speciation using inductively coupled plasma mass spectrometry (ICP-MS) and XAFS, respectively. Our preliminary results reveal that low temperature diagenesis altered Mo speciation indicating depositional euxinia is not the main mechanism concentrating Mo.