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CHAPTER 4 • Plate Tectonics and Long-Term Climate 71

Winter hemisphere model simulation of highly seasonal changes in mois-
L ture between wet summer monsoons and dry winter
monsoons.
H

Tectonic Control of CO Input: BLAG
2
Spreading Rate Hypothesis
H Our examination of both the polar position hypothesis
L
and the climate of Pangaea suggests that changes in
Earth’s geography alone cannot explain the climatic
variations between warm greenhouse climates and cold
Summer hemisphere
A Seasonal pressure and winds icehouse climates during the last 450 Myr. Another
likely factor in these climatic changes is variations in the
Winter hemisphere
CO concentration of the atmosphere. In the remainder
2
4 of this chapter we examine two hypotheses that attempt
to explain why CO has changed through time. One
2
2 hypothesis emphasizes changes in CO input by volca-
2
4 4 noes; the other focuses on changes in CO removal by
2
6 6
8 weathering.
4-6 Control of CO Input by Seafloor Spreading
2
A hypothesis published in 1983 proposed that climate
changes during the last several hundred million years
Summer hemisphere
B Seasonal precipitation (mm/day) > 4 2–4 < 2 have been driven mainly by changes in the rate of CO 2
input to the atmosphere and ocean by plate tectonic
FIGURE 4-15 “Supermonsoons” on Pangaea Climate processes. This hypothesis is called the BLAG hypothe-
models simulate (A) very large seasonal changes in surface sis, based on the initials of its authors, the geochemists
pressure and winds and (B) monsoonal precipitation on Robert Berner, Antonio Lasaga, and Robert Garrels.
Pangaea. Summer heating creates a low-pressure region (L) We will refer to it as the spreading rate hypothesis.
and draws in moist oceanic winds, which drop heavy In a world of active plate tectonic processes, carbon
precipitation along the subtropical east coast. Winter cooling cycles constantly between Earth’s interior and its sur-
creates a high-pressure cell (H) that sends dry air out from face (Figure 4-16). Most CO is expelled to the atmos-
2
land to sea and reduces precipitation. (Adapted from J. E. phere by volcanic activity along two kinds of locations:
Kutzbach, “Idealized Pangean Climates: Sensitivity to Orbital (1) margins of converging plates, where parts of the
Change,” Geological Society of America Special Paper 288 [1994]: subducting plates melt and form molten magmas that
41–55.)
rise to the surface in mountain belt and island arc volca-
noes, delivering CO and other gases from Earth’s inte-
2
rior; and (2) margins of divergent plates (ocean ridges),
blew from the land out to sea. The subtropical margins where hot magma carrying CO erupts directly into
2
of Pangaea were places of enormous contrast in sea- ocean water.
sonal precipitation, alternating between very wet sum- Some volcanoes also emit CO at sites distant from
2
mers and dry winters. plate boundaries where thin plumes of molten material
Geologic evidence of seasonal moisture contrasts on rise from deep within the interior and reach the surface
Pangaea comes from the common occurrence of red at volcanic hot spots (see Figure 4-16 bottom). Addi-
beds, sandy or silty sedimentary rocks stained various tional CO is released to the atmosphere by the slow
2
shades of red by oxidation of iron minerals. Red- oxidation of old organic carbon in sedimentary rocks
colored soils accumulate today in regions where the eroded at Earth’s surface (Chapter 3).
contrast in seasonal moisture is strong. The process of The centerpiece of the BLAG hypothesis is the con-
oxidation is analogous to rust that forms on metal tools cept that changes in the rate of seafloor spreading over
left out in the rain. In a geologic context, the wet season millions of years have controlled the rate of delivery of
provides the necessary moisture, and the rust forms CO to the atmosphere from the large rock reservoir of
2
during the dry season or shorter dry intervals. Red beds carbon, and that the resulting changes in atmospheric
were more widespread on Pangaea than during other CO concentrations have had a major impact on Earth’s
2
geologic intervals, a finding that is consistent with the climate.