HR: 08:45h
AN: PP41D-04    [Abstracts]
TI: Molybdenum Isotope Variations in the Toarcian (Early Jurassic): Constraining Paleoredox Changes During an Ocean Anoxic Event
AU: * Pearce, C R
EM: c.r.pearce@open.ac.uk
AF: Department of Earth Sciences, CEPSAR, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Cohen, A S
EM: a.s.cohen@open.ac.uk
AF: Department of Earth Sciences, CEPSAR, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Coe, A L
EM: a.l.coe@open.ac.uk
AF: Department of Earth Sciences, CEPSAR, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Burton, K W
EM: k.w.burton@open.ac.uk
AF: Department of Earth Sciences, CEPSAR, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AB: Molybdenum is a redox sensitive element that is sequestered by marine sediments and chemical precipitates, and is capable of recording changes in oceanic conditions over time. Whilst Mo concentrations have been used in conjunction with other geochemical information as an indicator of the local water conditions (e.g. Re/Mo ratios; Crusius et al., 1996), the natural isotopic variations in Mo also offer considerable potential as a global paleoredox proxy. Previous Mo isotope studies have looked at long-term changes in the oxygenation state of the oceans (e.g. Siebert et al., 2003; Arnold et al., 2005), as the long residence time of the dominant molybdate ion ensures that oceanic Mo is isotopically uniform and consequently provides a truly global record of marine conditions over intervals >1 Ma. To date little attention has been paid to the possibility of Mo isotope variations that may have occurred over timescales shorter than this. However, there is increasing evidence that sudden and substantial changes have occurred in the marine environment in the past. The geochemical behaviour of Mo is such that relatively rapid isotopic changes may now be resolvable using the latest high precision stratigraphical and analytical techniques. We have produced Mo isotope data from a suite of organic rich sedimentary rocks deposited over an interval of ~1.5 Ma spanning the Toarcian Ocean Anoxic Event (OAE) that occurred ~183 Ma ago. As with other Mesozoic OAEs, the Toarcian event is associated with a dramatic increase in the organic carbon content of the sedimentary deposits and with a mass extinction. Recent geochemical studies have also revealed a stepped δ¹³C_org negative excursion of ~ -7‰, marked excursions in the seawater ¹⁸⁷Os/¹⁸⁸Os and ⁸⁷Sr/⁸⁶Sr ratios, and evidence for a sudden temperature increase of ~10°C, all of which indicate a substantial environmental change at that time (Cohen et al., 2004; Kemp et al., 2005). Our Mo isotope data were obtained using a Nu Instruments MC-ICP-MS and are presented as δ^98/95Mo relative to an in-house standard. The natural and instrumental fractionation factors are determined using a ¹⁰⁰Mo-⁹⁷Mo double spike and an offline Newton-Raphson deconvolution procedure, resulting in an external reproducibility of <0.15‰ (2 s.d.). Preliminary results reveal at least two abrupt δ^98/95Mo excursions of 0.6 and 0.8‰, both of which appear to be correlated with the rapid negative shifts in δ¹³C_org. In addition, longer term changes in δ^98/95Mo can be resolved across the OAE. These findings have important implications for our understanding of the changes in the marine environment that occurred during the Toarcian OAE. Our results also show that variations in the Mo isotope composition of seawater have occurred relatively suddenly at certain times in the past, and indicate that this isotope system has the ability, under certain circumstances, to record abrupt changes in global oceanic conditions. Crusius, J. et al. (1996), EPSL 145, 65-78; Siebert, C. et al. (2003), EPSL 211, 159-171; Arnold, G.L. et al. (2005), Science 304, 87-90; Cohen, A.S. et al. (2004), Geology 32, 157-160; Kemp, D.B. et al. (2005), Nature In Press.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1094 Instruments and techniques
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005