CLIMATIC IMPLICATIONS OF SURFACE DOMAINS IN ARRAYS OF δD AND δ18O FROM HYDROXYL MINERALS: GOETHITE AS AN EXAMPLE

Abstract

Paired D/H and 18O/16O analyses of natural goethites range from -224%% to -93%% for δD and from -15.5%% to +2.8%% for δ18O. The isotopic systematics of these goethites seem to be most closely mimicked by the goethite-water D/H and 18O/16O fractionation curves of Yapp (1987, 1990). Most of these goethites are of unknown age, but the large range of their isotopic compositions illustrates the possible paleoenvironmental significance of ''surface domains'' calculated for hydroxyl minerals. A modern surface domain (MSD) for goethite in coordinates of δD and δ18O was calculated from the following information: (a) scatter in the modern relationship between surface temperatures and δ18O of meteoric water (Rozanski et al., 1993); (b) average annual surface air temperatures from 0°C to 30°C for the meteoric water sample sites; (c) the meteoric water line (MWL) of Craig (1961); and (d) the goethite-water isotopic fractionation factors of Yapp (1987, 1990). Goethites that had formed in continental environments in approximate thermal equilibrium with the Earth's modern atmosphere would be expected to lie within the MSD, and 16 of 31 goethites do so. The remaining 15 are ''outliers.'' Of the 16 MSD samples, 12 (including the three samples from active soil or bog environments) are ''concordant.'' The calculated isotopic temperatures of the concordant goethites range from 6°C to 27°C and are the same (within analytical uncertainty) as the modern average surface air temperatures at the respective sites. Calculated temperatures of the four ''discordant'' MSD samples range from 7°C to 25°C and are higher than corresponding modern surface temperatures.A ''warm Earth surface domain'' (WESD) was calculated with an assumed temperature of 35°C for the tropics. Eight of the 15 outlier samples plot within the portion of the WESD that does not overlap the MSD. Thus, these eight goethites may have formed at times when the Earth's global climate was warmer than at present. Temperatures calculated from the eight WESD samples range from 20°C to 33°C and suggest lower latitudinal temperature gradients as predicted by climate models of a warmer Earth. Calculated temperatures of the seven samples that lie outside both the MSD and the WESD (non-sd samples) range from 19°C to 69°C. The non-sd status of these seven samples suggests that they formed in environments dominated by subsurface heat sources.Surface domains can be calculated for any hydroxyl mineral for which both the hydrogen and oxygen isotope mineral-water fractionation factors are known. The concept of surface domains provides a criterion for assessing the likelihood that a particular sample records information on ancient climatic temperature.