Page 7 - GSA Today December 2022

The Archean acid-sulfate weathering
style was geographically limited by late
Archean spread of carbonic acid weather-
ing, which dominated after the 2.45 Ga
GOE (Rye and Holland, 1998). The rise of
cyanobacteria as part of a largely freshwa-
ter and terrestrial clade of “Terrabacteria”
(Battistuzzi and Hedges, 2009) maintained
soil productivity, promoting perineutral
carbonic acid hydrolysis and free oxygen in
both soil and air (Fig. 4). Perineutral pH in
soils by 2.4 Ga is indicated by pedogenic
carbonate in paleosols of that age and in
aridland soils ever since (Pekkarinen, 1979).
Thus, hydrolytic weathering systems geo-
graphically displaced archaic acid-sulfate
weathering, now limited to areas of sulfide
ore weathering and anaerobic parts of
waterlogged soils and lakes (Benison and
Bowen, 2015).
Phosphorus depletion of paleosols rose
during the GOE, and again during the NOE
(Fig. 3B). The Neoproterozoic does not
signify a fundamental change in style of
weathering, but rather the evolution of more
effective biologically produced ligands,
which were mainly bacterial during the
GOE, but supplemented by more effective
ligands of fungi and lichens during the
Neoproterozoic (Neaman et al., 2005;
Retallack, 2013; Kump 2014). Both increases
in terrestrial productivity coincide in time
with Snowball Earth cooling events (Walker,
1982; Kasting, 2010).
IMPLICATIONS FOR SOIL GASES IN
DEEP TIME
Some of these same paleosols also have
been used to calculate CO consumption as a
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Figure 3. Base (A) and phosphorus depletion (B) and carbon consump- guide to atmospheric evolution (Sheldon,
tion (C) inferred from tau analysis of paleosols over the past 3.7 Ga. 2006; Retallack et al., 2021), but they are
(A–B) Closed symbols are individual paleosols, and large open sym-
bols are mean for 500 Ma intervals. Only a single paleosol is known for imperfect guides to the atmosphere. Today,
1000–500 Ma. (C) Annual rates of C consumption from base depletion soils may have up to three orders of magni-
and apatite weathering (see supplemental material [see text footnote
1]) and global land area increase calculated from continental area and tude more CO than the atmosphere because
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freeboard estimates (Cawood and Hawkesworth, 2019). Upper and of soil respiration, and three orders of magni-
lower box bounds and error bars are two standard deviations. GOE— tude less O due to waterlogging (Elberling
Great Oxidation Event; NOE—Neoproterozoic Oxidation Event. 2
et al., 2011). The differences in CO and O
2
2
from the atmosphere are less marked in well-
haze (Haqq-Misra et al., 2008). Modeling of mass of atmospheric N and a H 0.1 mixing drained soils with open-soil structure (Kyaw
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methane production rates from a P-limited ratio (Wordsworth and Pierrehumbert, Tha Paw et al., 2006). Calculations of gas
and SO -poor Archean ocean would not 2013). This seems unlikely because N in consumption from paleosols (Sheldon, 2006;
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have produced enough methane for a signif- the atmosphere was limited to 1.1–0.5 bars Retallack et al., 2021), combined with mod-
icant CH -greenhouse (Laakso and Schrag, judging from nitrogen and argon isotopic ern soil gas measurements (Elberling et al.,
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2019), but anaerobic methanogenesis would ratios in fluid inclusions dated to 3500 Ma 2011; Kyaw Tha Paw et al., 2006), allow
have been more widespread in well-drained (Marty et al., 2013), and total atmospheric idealized hypotheses for gas concentrations
Archean soils than its current geographic pressure at 2700 Ma may have been only within well-drained alluvial soils over the
limitation to waterlogged wetlands (Benison half modern judging from the size of lava past 3.7 billion years (Fig. 4). Both O and
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and Bowen, 2015). Another Archean warm- vesicles and raindrop impressions (Som et CO are higher in modern than in Pre-
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ing possibility is three times the current al., 2016). cambrian soils, and geologically younger
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