[M-IS27_2PO1] MarineMarine manganese deposits: Origin, growth processes, and environment

[MIS27-P02] Uranium isotope composition in ferromanganese crusts: Implications for the paleoredox proxy

*Kosuke T. GOTO¹, Ariel D. ANBAR², Gwyneth W. GORDON², Stephen J. ROMANIELLO², Gen SHIMODA¹, Yutaro TAKAYA³, Ayaka TOKUMARU³, Tatsuo NOZAKI⁴, Katsuhiko SUZUKI⁴, Shiki MACHIDA⁵, Takeshi HANYU⁴, Akira USUI⁶ (1.GSJ, AIST, 2.Arizona state University, 3.The University of Tokyo, 4.JAMSTEC, 5.Waseda University, 6.Kochi University)

Keywords:Uranium, 238U/235U, 234U/238U, paleoredox, ferromanganese crust, isotope geochemistry

Variations of the 238U/235U ratio (d238U) in sedimentary rocks have been proposed as a possible proxy for decoding the paleo-oceanic redox conditions, although the marine U isotope system is not fully understood (Stirling et al., 2007 GCA; Weyer et al., 2008 EPSL). Here we investigate the spatial variation of d238U in modern ferromanganese crusts by analyzing U isotopes in the surface layer (0-3 mm depth) of 19 samples collected from 6 seamounts in the Pacific Ocean. The d238U values in the surface layers show little variation and range from -0.59 to -0.69 permil. The uniformity of d238U values is consistent with the long residence time of U in modern seawater (Dunk et al., 2002 Chem. Geol.), although the d238U values are lighter than that of present-day seawater by ~0.24 permil (Stirling et al., 2007 GCA; Weyer et al., 2008 EPSL). The light d238U is consistent with the isotope offset found during the adsorption experiment of U to birnessite (Brennecka et al., 2011 ES&T). Our results suggest that removal of lighter U from seawater to ferromanganese crusts is responsible for the second largest uranium isotopic fractionation in the modern marine system and could provide a source of heavy U to seawater.Depth profiles of U isotopes (d234U and d238U) in two ferromanganese crusts were investigated to reconstruct the evolution of oceanic redox state during the Cenozoic. The depth profiles of d238U show very limited ranges, and have similar values with those of the surface layer samples. The absence of any resolvable variations in the d238U depth profiles suggests that the relative proportions of oxic and reducing uranium sinks have not varied significantly over the past 40 Myr. However, the d234U depth profiles in the same samples suggest the possible U redistribution after deposition. Therefore, the d238U values may have been overprinted by secondary mobilization with pore-water or seawater. These results suggest that careful evaluation of secondary disturbance is required before applying chemical and isotope depth profiles of ferromanganese crusts to understand paleocean environmental changes.To assess the potential effect of U removal by Mn oxides on seawater d238U, we calculated the seawater d238U under different fractions of U removal by Mn oxides using a simple isotope balance model. This calculation suggests that seawater d238U could have varied significantly throughout the Earth's history along with the changes of the Mn oxides accumulation rate.