269 Geochemistry II

Wednesday, 28 September 2016
8:00 AM - 12:00 PM
Room 403

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

Paper number: 269-5
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

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 (Osi) 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 Osi 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 Osi 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 δ13Corg from -25.25 ‰ to -26.95 ‰ of the same interval, a trend that has been identified in δ13Corg and δ13Ccarb 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 Osi can be used in conjunction with additional chemical data to infer paleoenvironmental changes over time.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287096

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