Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...

Speculative interpretations of Triassic climates have long
played a central role in Triassic studies. Pre-1970 work focused
primarily on classic arguments on the origin of red beds, as sum-
marized by Krynine (1950) and Van Houten (1973). More re-
cently, paleoclimatological research has focused on dynamic
climatological models, such as those presented by Robinson
(1973). This paradigm shift is in part due to the understanding
that the red color of sedimentary rocks probably reflects mineral
alteration through time of both detrital and authigenic ferric ox-
ides (Van Houten, 1973). Recent research has concentrated on
four interrelated climatic factors: (1) a general atmospheric circu-
lation model, (2) northerly drift of plates across lines of latitude,
(3) impact of rift topography, and (4) periodic and cyclic climate
changes interpreted according to the Milankovitch astronomical
theory of climate change.
Large continents profoundly modify their climates through
the way in which they respond to seasonal variations in solar
radiation. Landmasses typically have low heat capacity and thus
warm and cool rapidly, transferring these effects to air masses
above. Eurasia and North America today, for example, have
marked seasonal variations of temperature that cause concomi-
tant changes in pressure gradient forces and in wind direction. By
analogy, Pangaea probably experienced an effective monsoonal
circulation.
Figure 2 is an interpretive sketch of summer climatic condi-
W. Manspeizer and Others
tions over Laurasia and of winter conditions over Gondwanaland
near the beginning of the breakup of Pangaea (Robinson, 1973).
In this interpretation the rift basins were dominated by a Tethyan
high-pressure cell that pumped warm, moist air into the conti-
nent. Hot dry summer winds, blowing from the west across the
continent, probably had little impact on the rift basins, which lay
east of the Intertropical Convergence Zone. During the Laurasian
winter, however, the proto-Atlantic basins would have been dom-
inated by a subtropical high-pressure cell, while Gondwanaland
was overlain by the Intertropical Convergence Zone. Winds,
blowing clockwise about the high-pressure cell, typically brought
cold, dry air from aloft across the basins, but on occasion carried
cool, moist maritime air from Arctic Canada to the proto-
Atlantic region.
Rift basins, because of their special topographic characteris-
tics, create unusual climates that cannot be explained through a
general atmospheric circulation model alone (Manspeizer, 198 1,
1982). The detrital basins on the American plate had substantial
relief and extended for long distances across lines of latitude and
prevailing wind systems (Fig. 2). As moist air (from either Tethys
or Arctic Canada) was uplifted over the shoulders of the rift
system, it would have cooled adiabatically, yielding rainwater for
high-discharge streams and high-altitude lakes. Conversely, as the
air descended into the axis of the rift, it would have warmed
adiabatically, becoming hot and dry and thus increasing the rates