Lethaia, Vol. 36(3), pp. 227-254, September 2003
Abstract
The continuity through the past 300 million years of key tropical
sediment types, namely coals, evaporites, reefs and carbonates, is
examined. Physical controls for their geographical distributions are
related to the Hadley cell circulation, and its effects on rainfall
and ocean circulation. Climate modelling studies are reviewed in this
context, as are biogeographical studies of key fossil
groups. Low-latitude peats and coals represent everwet climates
related to the Intertropical Convergence Zone near the Equator, as
well as coastal diurnal rainfall systems elsewhere in the tropics and
subtropics. The incidence of tropical coals and rainforests through
time is variable, being least common during the interval of Pangean
monsoonal climates. Evaporites represent the descending limbs of the
Hadley cells and are centred at 10? to 40? north and south in
latitudes that today show an excess of evaporation over
precipitation. These deposits coincide with the deserts as well as
seasonally rainy climates, and their latitudinal ranges seem to have
been relatively constant through time. Reefs also can be related to
the Hadley circulation. They thrive within the regions of clear water
associated with broad areas of downwelling which are displaced toward
the western portions of tropical oceans. These dynamic features are
ultimately driven by the subtropical high-pressure cells which are the
surface signature of the subsiding branches of the Hadley
circulation. Carbonates occupy the same areas, but extend into higher
latitudes in regions where terrestrial surface gradients are low and
clastic runoff from the land is minimal. We argue that the
palaeo-latitudinal record of all these climate-sensitive sediment
types is broadly similar to their environments and latitudes of
formation today, implying that dynamic effects of atmospheric and
oceanic circulation control their distribution, rather than
temperature gradients that would expand or contract through time.