Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 90

''The supercontinent of Gondwana broke apart, separating into subsections
that became Africa, India, Australia, South America, and Antarctica, and
passageways opened between these new continents, allowing oceans to flow
between them. When Antarctica was finally severed from the southern tip of
South America to create the Drake Passage, Antarctica became completely
surrounded by the Southern Ocean. The powerful Antarctic Circumpolar
Current began to sweep all the way around the continent, effectively isolating
Antarctica from most of the warmth from the global oceans and provoking
large-scale cooling.''
However, the explanation for the delay in glaciation of the Northern
Hemisphere is more problematic. Haug (2004) emphasized the ''huge gap'' in
Central America that allowed tropical water to flow between the Atlantic and
Pacific Oceans. When the Isthmus of Panama formed about 3 mybp , it partitioned
the Atlantic and Pacific Oceans and may have fundamentally changed global
ocean circulation. According to Haug (2004), before the Isthmus of Panama
formed, Pacific surface waters mixed with Atlantic waters, roughly balancing the
two oceans' salinity. However, the North American, South American, and Carib-
bean Plates began to converge about 5 mybp slowly forming the Isthmus of
Panama. This gradually restricted the exchange of water between the Pacific and
Atlantic, and their salinities diverged. Evaporation in the tropical Atlantic and
Caribbean left those ocean waters saltier and put fresh water vapor into the
atmosphere. The Trade Winds carried the water vapor from east to west across
the low-lying Isthmus of Panama and deposited fresh water in the Pacific through
rainfall. As a result, the Pacific became relatively fresher, while salinity steadily
increased in the Atlantic. As this salinity increased after closing of the Pacific-
Atlantic connection—also known as the ''Central American Seaway'' (CAS)—the
water transported northward in the Atlantic became warmer and saltier. As Haug
(2004) explained:
''As this water reaches high North Atlantic latitudes, it transfers heat and
moisture to the atmosphere, leaving behind cold, salty, dense water that sinks
toward the ocean floor. This water flows at depths, southward and beneath the
Gulf Stream, to the Southern Ocean, then through the Indian and Pacific Oceans.
Eventually, the water mixes with warmer water and returns to the Atlantic to
complete the circulation. The principal engine of this global circulation, often
called the Ocean Conveyor, is the difference in salt content between the Atlantic
and Pacific Oceans.''
According to Haug (2004), the Gulf Stream intensified as a result of the
closure of the Pacific-Atlantic connection, and this transported more warm salty
watermasses to high northern latitudes, amplifying the Ocean Conveyor. In 1968,
Peter Weyl hypothesized that this would have brought a critical ingredient for ice
sheet growth to the Northern Hemisphere—moisture—leading to a buildup of ice
sheets in the north (Keigwin, 1982). However, evidence later developed that the