Atlantic Trade Wind Biome
189
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The entire coastline has a significant shelf width, from the West Florida Shelf (250 km)
to the equally wide Campeche Bank off Yucatan. The northern coast is dominated by the
bird’s-foot delta of the Mississippi River, so that riverborne silt nourishes the sedimentary
regime of the northern and western coasts of the Gulf of Mexico. The northern coast
is also characterized by the existence of an almost continuous series of coastal lagoons
situated behind bar beaches; the existence of these extensive features has consequences
for this entire shelf. The DeSoto canyon separates the sedimentary regime of the northern
coast from the coralline sediments that dominate the West Florida Shelf; these sediments
may serve as a model for depth zonation on similar coastlines elsewhere. Pure quartz
sand lies outside the beaches with foraminiferan silt prominent in shallow water, and
quartz-shell sand mixtures dominate a little deeper. At midshelf depths, there is a band
of pure shell sand while, deeper yet, a zone of sand formed of particles of calcareous
algae gives way at about midshelf to sand formed almost entirely of oolite particles. At
the break of shelf, almost everywhere in this province, there is a “mud line” at the limit
of a highly organic, sedimentary regime, the origin of extensive slumping of sediments
into deep water.
The low-lying coastline from western Florida to southern Yucatan is largely free of coral
formations because of limiting winter temperatures and excessive runoff of freshwater
and deposition of riverine material. Fringing reefs are abundant only in the Caribbean,
whereas an offshore barrier reef occurs along the eastern coast of Yucatan and extends
south onto the Mesquite Bank. Several carbonate banks of continental shelf dimension
and depth lie in deep water: the Campeche and Bahamas Banks and the Mesquite Bank on
the eastern coast of Honduras. The terraced outer platforms of these bear calcareous and
pellet muds, with oolites and coral-algal fragments in very shallow water. These white,
calcareous sediments are forced into waves and ripples by tidal streams. The Antilles
islands are typically endowed with leeward reefs on their western coasts.
The northern coasts are close to the critical latitude for the formation of diurnal tides
so that tidal streams and fronts are relatively weak through the province. Storm surges
driven by the hurricanes typical of the West Indies region are, on the other hand relatively
common and devastating to coastal ecosystems.
Defining Characteristics of Regional Oceanography
The major source of energy for water motion in this province is wind; the seasonal
migration of the ITCZ over South America is (relative to that over Africa) rather small,
so easterly Trades blow almost constantly over the Caribbean, although more strongly
during boreal spring and summer. At this season, coastal upwelling is induced along the
northeast coasts of the Yucatan peninsula by the conjunction of high current speed (of
order 25 m sec
−1 ) and strong winds from the southeast.
A very important component of wind forcing in this province are the “weekly easterly
waves” that may develop into full-blown hurricanes, propagating generally northward
across the province; in such events, surface wind stress is twice that associated with
normal trade winds. This stress creates transient circulation, open ocean upwelling, and
storm surges and may, in exceptional circumstances, reduce surface temperature by as
much as 3–4
C and cause a 30- to 40-m deepening of the mixed layer (Mooers and Maul,
1998). The wind regime of the Gulf of Mexico is characterized by winter westerlies along
the northern shelf, although their effects are less than those of the quasiweekly “northers”
that cool shelf waters seasonally: in January, the 30-m temperature over the shelf here
may be taken down to about 20
C by these winds. Nevertheless, mixed-layer depths are
189
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The entire coastline has a significant shelf width, from the West Florida Shelf (250 km)
to the equally wide Campeche Bank off Yucatan. The northern coast is dominated by the
bird’s-foot delta of the Mississippi River, so that riverborne silt nourishes the sedimentary
regime of the northern and western coasts of the Gulf of Mexico. The northern coast
is also characterized by the existence of an almost continuous series of coastal lagoons
situated behind bar beaches; the existence of these extensive features has consequences
for this entire shelf. The DeSoto canyon separates the sedimentary regime of the northern
coast from the coralline sediments that dominate the West Florida Shelf; these sediments
may serve as a model for depth zonation on similar coastlines elsewhere. Pure quartz
sand lies outside the beaches with foraminiferan silt prominent in shallow water, and
quartz-shell sand mixtures dominate a little deeper. At midshelf depths, there is a band
of pure shell sand while, deeper yet, a zone of sand formed of particles of calcareous
algae gives way at about midshelf to sand formed almost entirely of oolite particles. At
the break of shelf, almost everywhere in this province, there is a “mud line” at the limit
of a highly organic, sedimentary regime, the origin of extensive slumping of sediments
into deep water.
The low-lying coastline from western Florida to southern Yucatan is largely free of coral
formations because of limiting winter temperatures and excessive runoff of freshwater
and deposition of riverine material. Fringing reefs are abundant only in the Caribbean,
whereas an offshore barrier reef occurs along the eastern coast of Yucatan and extends
south onto the Mesquite Bank. Several carbonate banks of continental shelf dimension
and depth lie in deep water: the Campeche and Bahamas Banks and the Mesquite Bank on
the eastern coast of Honduras. The terraced outer platforms of these bear calcareous and
pellet muds, with oolites and coral-algal fragments in very shallow water. These white,
calcareous sediments are forced into waves and ripples by tidal streams. The Antilles
islands are typically endowed with leeward reefs on their western coasts.
The northern coasts are close to the critical latitude for the formation of diurnal tides
so that tidal streams and fronts are relatively weak through the province. Storm surges
driven by the hurricanes typical of the West Indies region are, on the other hand relatively
common and devastating to coastal ecosystems.
Defining Characteristics of Regional Oceanography
The major source of energy for water motion in this province is wind; the seasonal
migration of the ITCZ over South America is (relative to that over Africa) rather small,
so easterly Trades blow almost constantly over the Caribbean, although more strongly
during boreal spring and summer. At this season, coastal upwelling is induced along the
northeast coasts of the Yucatan peninsula by the conjunction of high current speed (of
order 25 m sec
−1 ) and strong winds from the southeast.
A very important component of wind forcing in this province are the “weekly easterly
waves” that may develop into full-blown hurricanes, propagating generally northward
across the province; in such events, surface wind stress is twice that associated with
normal trade winds. This stress creates transient circulation, open ocean upwelling, and
storm surges and may, in exceptional circumstances, reduce surface temperature by as
much as 3–4
C and cause a 30- to 40-m deepening of the mixed layer (Mooers and Maul,
1998). The wind regime of the Gulf of Mexico is characterized by winter westerlies along
the northern shelf, although their effects are less than those of the quasiweekly “northers”
that cool shelf waters seasonally: in January, the 30-m temperature over the shelf here
may be taken down to about 20
C by these winds. Nevertheless, mixed-layer depths are