Page 8 - GSA Today December 2022

Figure 4. Idealized range of soil gas distributions on Earth over the past 4 billion years. Gas concentrations varied within the envelope shown
depending on seasonal productivity and waterlogging, and atmospheric levels of gases inferred from paleosol consumption are shown at
the surface. PAL—preindustrial atmospheric level (280 ppm).

soils show more variable concentrations with Neoproterozoic (Fig. 3), perhaps from newly of weathering derived from compilation of
seasons, within profiles, and geographically evolved fungal-lichen microbial earths experimental studies (Schwartzmann, 2017)
(Breecker and Retallack, 2014). Thus, biotic (Retallack, 2013; Kump, 2014). The advent for three reasons. First, experimental stud-
enhancement of weathering was not just a of land plants did draw down atmospheric ies reveal enhancement factors of major
matter of changing the atmosphere (Kasting, CO (Berner, 1997) but did not appreciably steps in terrestrial productivity, such as the
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2010), because soil gases at the site of sili- alter rates of CO consumption by either sili- evolution of trees (Retallack, 2022b), and
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cate and apatite weathering were critical cate or apatite weathering at the coarse 500- does not consider the origin of microbial
(Kump, 2014). Neoproterozoic consumption m.y. scale of this investigation (Fig. 3). life in soils and prokaryotic evolutionary
of CO was less by increased silicate weath- During the past 16 million years, range advances in microbiome weathering. Second,
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ering than by increased apatite weathering expansion and contraction of carbon-hungry global carbon sequestration has been aided
(Fig. 3), suggesting a role for ligands from soils such as Mollisols and Oxisols, with by the growth of land area through time.
life on land (Neaman et al., 2005). With later reciprocal adjustment of carbon-lean soils The estimates of land-area increase used
evolution of land plants, soil CO rose orders such as Gelisols and Aridisols have acted as here are based on estimates of continental
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of magnitude higher than in the atmosphere, a planetary thermostat. Mollisol-Oxisol area and paleohypsometry (Cawood and
supplying carbonic acid for both silicate and expansion curbs greenhouse CO spikes, Hawkesworth, 2019), which are relatively
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apatite weathering (Berner, 1997; Retallack, but Gelisol-Aridisol expansion cannot over- conservative, but show a factor of three,
2022a, 2022b). ride continued volcanic degassing of CO rather than a factor of 100 increase through
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Estimates of CO consumption by Paleo- (Retallack, 2022a). Too few Archean paleo- time (Fig. 3C). Third, nutrient depletion
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proterozoic and Archean soils do not show sols are now known to demonstrate such fuels biomass carbon sequestration increases
expected (Kasting, 2010) high amounts of counterbalancing carbon sequestration, but of about the same magnitude (Retallack,
soil or atmospheric CO (Sheldon, 2006; biotic enhancement of weathering is sus- 2022a). Geographic spread and temporal
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Retallack, 2018; Retallack et al., 2016, 2021). pected then as well. The record of paleosols fluctuation in areas of various kinds of
Common sulfates formed in Archean paleo- reveals that atmospheric and soil CO show paleosols will be needed for a full account-
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sols despite low atmospheric O suggest that considerable temporal and presumably also ing of planetary temperature regulation by
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strong sulfuric acid produced by anaerobic geographic variation (Fig. 3) but not a mono- soils, as has been possible for the Neogene
sulfur oxidizing bacteria, creating more tonic increase (Fig. 4). Nevertheless, carbon fossil record of soils (Retallack, 2022a).
amorphous colloids such as imogolite than sequestration by silicate weathering and
clay, may have been more important than phosphorus depletion did rise (Fig. 4), as CONCLUSIONS
weak carbonic acid in Archean silicate predicted in theory (Schwartzmann, 2017). Paleosols are now evidence for progres-
weathering (Retallack, 2018; Retallack et al., sive CO and CH greenhouse reduction by
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2016). Paleoproterozoic atmospheric oxida- COMPARISON WITH EXPERIMENTS biologically enhanced weathering to offset
tion raised rates of atmospheric CO con- Increases of three orders of magnitude in increased stellar luminosity and continued
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sumption by both oxidative silicate and apa- nutrient depletion of individual paleosols volcanic greenhouse gas emission. Biological
tite weathering from aerobic cyanobacteria (Fig. 3A) and global carbon sequestration regulation of soil and atmospheric gases may
and actinobacteria, but increases in apatite, (Fig. 3C) is greater than an estimate of two have maintained habitable surface condi-
not silicate, weathering rates are seen in the orders of magnitude of biotic enhancement tions on Earth for the past 3.7 Ga.

8 GSA TODAY | December 2022