Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in 1 Paleozoic black shales 2 3 Michael A. Kipp a,b,*,† , Thomas J. Algeo c,d,e , Eva E. Stüeken b,f , Roger Buick a,b 4 5 a Department of Earth & Space Sciences and Astrobiology Program, University of Washington, 6 Seattle, WA, 98195-1310, USA 7 b Virtual Planetary Laboratory, NASA Nexus for Exoplanet System Science, Seattle, WA, 98195- 8 1310, USA 9 c Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA 10 d State Key Laboratory of Biogeology and Environmental Geology, China University of 11 Geosciences, Wuhan 430074, China 12 e State Key Laboratory of Geological Processes and Mineral Resources, China University of 13 Geosciences, Wuhan 430074, China 14 f School of Earth & Environmental Sciences, University of St. Andrews, St. Andrews, Fife, KY16 15 9AL Scotland, United Kingdom 16 *correspondence to mkipp@caltech.edu 17 † Present address: Division of Geological & Planetary Sciences, California Institute of Technology, 18 Pasadena, CA, 91125, USA 19 20 Abstract 21 Mass-dependent variations in selenium stable isotope ratios have recently been developed as a 22 paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox 23 potential, this system holds promise for probing conditions relevant to the evolution and 24 diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. 25 Although several studies have identified selenium isotopic variability during oxygenation events 26 in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling 27 this isotopic variability. This currently limits the robust interpretation of selenium isotope 28 variability to first-order mechanisms driving large-magnitude changes. Here, we explore 29 selenium isotope variability within and among Paleozoic black shales deposited on the North 30 American craton that have been well-studied using a variety of other paleo-environmental 31 proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance 32 and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late 33 Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous 34 selenium recycling, leading to very high concentrations in sediments and enrichment of heavy 35 isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian 36 units, differences in local basinal hydrography led to a gradient in selenium abundance and 37 isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and 38 causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case 39 studies, the additional context provided by complementary paleo-environmental proxies was 40 critical for distinguishing between possible drivers of selenium isotopic variability. When 41 extending such studies to other paleo-environmental settings, we suggest that the continued use 42 of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. 43 Moreover, further development of techniques for high-precision and phase-specific selenium 44