Weathering of quartz as an Archean climatic indicator
Introduction
Climate during the Archean Era (∼3.8 to 2.5 Ga) is relevant to the history of life on Earth. However, only limited geological analysis bears on the topic. Oxygen-isotope studies of cherts provide oceanic temperature estimates for 3.5–3.2 Ga of 55–85 °C [1]. Evidence of thorough weathering in the Archean, despite the absence of vascular plants, has prompted temperature estimates as high as 85 °C [2], [3]. In addition, the limited rock record shows that glaciation was uncommon in the Archean. See the work of Young et al. [4] for a possible exception at 2.9 Ga.
Theoretical studies of Archean temperature model a greenhouse atmosphere composed of carbon dioxide and methane. The commonly cited upper limit, 85 °C, is model dependent [5], [6], [7], [8]. Conversely, reasonable amounts of these gases do not necessarily imply temperatures much higher than those of the modern tropics or the Cretaceous Earth [8]. Limited direct estimates of atmospheric CO2 come from observations that siderite is present in 3.2-Ga pebble weathering rinds [9] but is absent from a 2.75-Ga paleosol [10] (Fig. 1).
In this paper, we suggest that the weathering behavior of quartz in monomineralic quartzite and chert and as grains in granitic rocks is a viable Archean paleothermometer. The tendency on the modern Earth for quartz to weather mechanically, unlike the dissolution weathering of carbonates, is notorious. “As unrelenting flint to drops of rain.” (Shakespeare, Titus Andronicus, Act II, Scene III). “He plies her hard; and much rain wears the marble.” (Shakespeare, The Third Part of King Henry the Sixth, Act III, Scene II). There is an urge to regard evidence of this difference on the ancient Earth as unremarkable. Rather, quartz does dissolve some in modern environments [11]. Its solubility and dissolution rate of quartz are strong functions of temperature. Quartz will dissolve and chemically disaggregate readily in a sufficiently hot climate, as has been suggested for the Archean. Instead, field observations indicate that quartz in the Archean mostly resisted chemical weathering, like it does under modern clement conditions. Here, we quantify the effect using solubility and kinetic data to extrapolate from the chemical behavior of quartz observed in modern climates.
Because the Barberton sequence (like most Mesoarchean sequences) is predominately syntectonic, we restrict our attention to sediments derived during periods of relatively slow exhumation, such that chemical weathering could proceed. We eschew stratigraphic levels dominated by highly immature sediments, like the mechanically derived conglomerates or poorly fractionated volcaniclastic deposits underlying the Moodies Group.
Section snippets
Archean geological observations
Hessler studied well-preserved 3.2-Ga clastic sediments from Barberton Mountain Land for her thesis [12]. We concentrate on Moodies Group rocks north of the Inyoka Fault, which have been the subject of extensive geological work [13], [14], [15], [16], [17]. A striking feature of these rocks is that the degree of weathering prior to fluvial deposition is within the range of that observed at the warm, humid end of the climate spectrum on the modern Earth. This observation is elaborated below for
Modern geological analogs
The weathering of modern granitic rocks to quartz-rich sand and clay-rich shale is well known. So is the occurrence of some dissolution features in quartz [11]. Modern weathering occurs when reaction rates are fastest, during the wet part of the year in seasonally humid climates and during the warm part of the year in temperate climates [26]. Modern weathering thus occurs at ∼25 °C except at very high latitudes.
Although lower soil levels will be tapped during intensive erosion events, it is
Inferences about Archean climate
We wish to compare the above modern analogs with the Archean record, particularly with the mid-Archean sediments preserved in the Moodies Group. To a first order, the Archean sediments we have discussed are similar to modern ones. If anything, quartz dissolution features are less common in the ancient rocks [11]. A tempting inference is that the Archean climate was like the modern. Yet we need to identify observations that are particularly temperature sensitive, because there were many features
Conclusions
We have applied rather banal observations on Archean sediments and modern environments to obtain a paleothermometer for surface conditions on the early Earth. We concentrated on quartz because it is a simple common mineral for which chemical data exist. Its weathering is not strongly affected by atmospheric CO2. The observations are quite low-tech involving point counts of sediments, visual observation of pebbles, and microscopic observations of grains in sediments and weathered rock. In the
Acknowledgements
We thank Don Lowe for help with field and laboratory aspects of this study and Dave DesMarais for discussion of chemical cycles and Archean climate. Jim Kasting, S. Doerr, Rob Rye, Susan Brantley, Ray Pierrehumbert, Lee Kump, Marjorie Schulz, Patricia Dove, Don Rimstidt, Tsuyoshi Hattanji, and Roger Buick provided prompt responses to our questions. Kevin Zahnle provided helpful comments. Wouter Bleeker and Lee Kump provided helpful comments. Cores were provided by the Sheba Royal Gold Mine in
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