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Ocean circulation, but simulations using general circulation models provide a
range of differences in circulation for open and closed seaways.
Steph et al. (2010) wrote a lengthy paper dealing with changes in the Pacific
between about 4.8 and 3.6 mybp but its relevance to the onset of glaciation after
that time is somewhat nebulous to this writer.
In an interesting related study, Cane and Molnar (2001) pointed out that
global climate change around 3-4 mybp is thought to have influenced the evolution
of hominids, via the aridification of Africa, and may have been the precursor to
Pleistocene glaciation about 2.75 mybp . Whereas most explanations of these
climatic events involve changes in circulation of the North Atlantic Ocean due to
the closing of the Isthmus of Panama, they suggested that closure of the Indone-
sian seaway 3-4 mybp ago could be responsible for the aridification of Africa and
other climate changes. Their model indicated that northward displacement of New
Guinea, about 5 mybp , might have switched the source of flow through Indonesia
from warm South Pacific waters to relatively cold North Pacific waters. This
would have decreased sea surface temperatures in the Indian Ocean, leading to
reduced rainfall over eastern Africa. They further suggested that changes in the
equatorial Pacific might have reduced atmospheric heat transport from the tropics
to higher latitudes, stimulating global cooling and the eventual growth of ice
sheets.
Uriarte (2009) provided a revealing discussion of the relation between the
formation of the Isthmus of Panama and the freezing of the Arctic. He described
''a somewhat paradoxical theory'' in which the closure of the isthmus reduced
leakage of warm Caribbean water to the Pacific, and thus increased the flow of
warm water transported by the Gulf Stream to the North Atlantic. This increased
evaporation, and ''as a result, rainfall levels rose in Siberia, thus increasing the
amount of water discharged by the Siberian rivers into the Arctic, facilitating
its winter freezing, since fresh water freezes at a higher temperature than salty
water. In a positive feedback process, the freezing of the surface waters in the
Arctic increased the albedo over a vast region and furthermore isolated the
ocean from the atmosphere, thus decreasing the transfer of heat from the
water to the air.'' Uriarte illustrated this theory with a figure as shown in Figure
2.38a .
Uriarte also described a second theory that
postulates that before the formation of the isthmus, the Gulf Stream
penetrated much further into the Arctic than afterwards. Thus, before the change
in ocean currents, the Arctic remained unfrozen, at least in summer. When the
Panama isthmus was still open, part of the flow of very salty water from the
Atlantic equatorial current escaped out into the Pacific instead of turning north.
This meant that the salinity and density of the surface current of both the Gulf
Stream and the North Atlantic Drift was lower, which facilitated its penetration
into the Arctic Ocean. However, when the isthmus finally closed off the passage
between the Atlantic and the Pacific, the proportion of tropical waters in the
upward flow increased, as did its salinity. The water mass transported by the
''
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