HR: 09:45h
AN: PP21A-08    [Abstracts]
TI: Systematic temporal thallium isotope variations in a 72 Ma old Fe-Mn crust: A proxy for changes in ocean chemistry?
AU: * Nielsen, S G
EM: sunen@earth.ox.ac.uk
AF: University of Oxford Department of Earth Science, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Klemm, V
EM: klemm@erdw.ethz.ch
AF: ETH Zurich Department of Earth Science, Clausiusstrasse 25, Zurich, 8092, Switzerland
AU: LaRowe, D
EM: d.larowe@geo.uu.nl
AF: Utrecht University Department of Earth Science, P.O. Box 80.021, Utrecht, 3508 TA, Netherlands
AU: Halliday, A N
EM: Alex.Halliday@earth.ox.ac.uk
AF: University of Oxford Department of Earth Science, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Hein, J R
EM: jhein@usgs.gov
AF: USGS, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: A large equilibrium isotope fractionation of about 2 per mil occurs between thallium (Tl) in seawater and modern Fe-Mn crusts [1].
We measured the Tl isotope composition through the entire thickness of the Pacific Fe-Mn crust CD29-2, which has recently been dated using Os isotope stratigraphy [2]. The samples have a spatial resolution of between 0.2 and 1 mm which equates to about 0.1 to 0.5 Myrs.
The most prominent variations occur in the time interval between 72 Ma (the base of the crust) and 50 Ma. The Tl isotope composition decreases from ε²⁰⁵Tl = +7.5 (ε²⁰⁵Tl represents the deviation of the ²⁰⁵Tl/²⁰³Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 10⁴) at 72 Ma to a minimum of ε²⁰⁵Tl = +5.0 at 67 Ma and immediately rebounds to ε²⁰⁵Tl = +6.0. No change is thereafter recorded until 59 Ma, where ε²⁰⁵Tl increases smoothly from +6.0 at 59 Ma to +10.5 at 50 Ma. The remainder of the crust displays only limited Tl isotope variations. Five previous analyses at various depths in CD29-2 [3] are fully consistent with our more detailed investigation.
The variations most likely reflect either changes in the Tl isotope composition of seawater over time [3] or could represent a change in the Tl isotope fractionation factor between Fe-Mn crusts and seawater.
Changing the Tl isotope composition of seawater requires that the fluxes of ocean inputs or outputs change over time. Based on previous studies, it appears that only a change in the relative outputs of Tl associated with Fe-Mn oxyhydroxides adsorbed onto pelagic sediments and Tl uptake into oceanic crust altered at low temperatures has the necessary isotopic leverage to drive the observed variations. Recent theoretical calculations of the mechanism controlling Tl isotope fractionation have predicted that there is a large equilibrium isotope fractionation between Tl¹⁺ and Tl³⁺ [4]. Published Tl isotope compositions of modern Fe-Mn crusts [1] and altered mid-ocean ridge basalts [5] are consistent with these two reservoirs exclusively incorporating Tl³⁺ and Tl¹⁺, respectively. Thus, changes in the speciation of Tl in seawater may also be responsible for the observed Tl isotope variation recorded in Fe-Mn crusts. However, under normal aqueous conditions only Tl¹⁺ should be thermodynamically stable. We are therefore conducting further investigations to establish if the oxidized form of Tl is stable in seawater.
[1] Rehk\¨{a}mper et al., EPSL, 2002, [2] Klemm et al., EPSL, 2005, [3] Rehk\¨{a}mper et al., EPSL, 2004, [4] Schauble, GCA, 2007, [5] Nielsen et al., EPSL, 2006
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
DE: 1635 Oceans (1616, 3305, 4215, 4513)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting