METEORIC SPHAEROSIDERITE LINES: A TOOL FOR THE STUDY OF PALEOHYDROLOGY AND PALEOCLIMATES IN HUMID CONTINENTAL ENVIRONMENTS
L.A. Gonzalez, G.A. Ludvigson, R.A. Metzger, B.J. Witzke, and R.L. Brenner
The Geological Society of America
1996 GSA Annual Meeting
Denver, Colorado, October 28-31, 1996
1996 Abstracts with Programs, v. 28, no. 7, p. A-36
Unique vertical linear trends are defined by highly variable 13C values and relatively invariant 18O values of sphaerosiderites from the 1) Cretaceous Dakota Fm of Iowa-Nebraska 18O of -4.69 + 0.18 and -2.86 + 0.47: this study); 2) Late Cretaceous Whitemud Fm of Alberta 18O of -7.29 + 0.26 : Fritz et al., 1971, JSP 41:282-288), and 3) Early Triassic Rewan Gp of Australia 18O of -11.92 + 0.99 : Baker et al., 1996, Sedimentology 43: 77-88). These trends are analogous to well documented Meteoric Calcite Lines, (MCLs) that we define as Meteoric Sphaerosiderite Lines, MSLs.
Sphaerosiderites, a mm-scale radial-spherulitic siderite, appears to form exclusively in modern wetland soils. Ancient sphaerosiderites are found mostly in poorly-drained paleosol horizons within coal-bearing stratigraphic intervals, and like their modern counterparts are interpreted as forming below water tables. Sphaerosiderites precipitate from sulftate-poor Fe-rich reducing pore-fluids within or immediately above CH4-producing zones. The relatively invariant 18O composition of sphaerosiderites reflects formation in long-lived groundwater system with stable 18O composition at relatively constant temperatures. The 13C composition of the sphaerosiderites reflects contributions of dissolved CO2 from oxidation of organic matter and modified by either 1) 13C enrichment of the dissolved CO2 during CH4 formation by CO2 reduction, or 2) 13C depletion through anaerobic microbial oxidation of biogenic CH4. The formation of sphaerosiderite in reducing wetland soils offers the opportunity for extending the environmental range of continental sampling for 18O of carbonates into humid areas that have been widely presumed to lack the suitable proxy environmental records. MSLs provide us with a unique means of evaluating paleohydrology and paleoclimate in these continental humid settings as well as evaluating the chemistry of groundwater during sphaerosiderite formation.