PALEOENVIRONMENTAL INSIGHTS FROM THE 187OS/188OSI PARAMETER OF THE RE-OS GEOCHRONOMETER WITH AN EXAMPLE FROM THE LATE JURASSIC AGARDHFJELLET FORMATION, SVALBARD, NORWAY

Over the past decade, the rhenium-osmium (Re-Os) geochronometer has been proven to yield accurate and precise ages for black shales. In addition, the initial ¹⁸⁷Os/¹⁸⁸Os ratio (Os_i) parameter of the isochron can yield information about paleoenvironmental conditions at the time of deposition, especially when tied to additional chemical data.

Organic matter in marine sediment provides a reducing environment that draws down Re and Os from the overlying water column. Os sequestered in black shales records short-term changes in the proportions of various Os inputs due to its geologically brief ocean residence time of ~28 ka (Stein and Hannah, 2015, Springer). The ¹⁸⁷Os/¹⁸⁸Os ratio in seawater represents a balance of sources. Chemically primitive sources, such as the mantle, hydrothermal, and cosmogenic input, lower the ¹⁸⁷Os/¹⁸⁸Os ratio of seawater (e.g. present day cosmogenic ¹⁸⁷Os/¹⁸⁸Os ≈ 0.127). Weathering of chemically mature sources such as evolved crustal material will raise the ¹⁸⁷Os/¹⁸⁸Os ratio of seawater (e.g. average continental crust ¹⁸⁷Os/¹⁸⁸Os ≈ 1.4). As long as the Re-Os system is not affected by diagenesis, the Os_i in black shales are representative of the ¹⁸⁷Os/¹⁸⁸Os ratio in seawater at the time of deposition.

Drill core from the Late Jurassic Agardhfjellet Formation (AF) was collected near Longyearbyen, Svalbard for Re-Os geochronometry. The AF consists of black shales and siltstones with intermittent siderite/glauconite horizons deposited in a tectonically-quiet, shallow marine shelf. The Os_i from the Re-Os isochrons increase systematically over ~12 Ma from 0.402 ± 0.007 in the Oxfordian to 0.67 ± .05 in the Lower Volgian, also seen in the correlative Hekkingen Formation of the Barents Sea (Markey et al, submitted) indicating a regional trend. This reflects possible increases in continental weathering. This is corroborated by a decrease in δ¹³C_org from -25.25 ‰ to -26.95 ‰ of the same interval, a trend that has been identified in δ¹³C_org and δ¹³C_carb from correlative Boreal and Tethyan intervals. Other workers have interpreted this as increasing precipitation and chemical weathering in an increasingly hot and humid climate (Weissert and Mohr, 1996; Koevoets et al, 2016). The AF shows how the Os_i can be used in conjunction with additional chemical data to infer paleoenvironmental changes over time.