Geology Reference
In-Depth Information
buoyancy forces acting on a density contrast between
the sediment-water mixture in the current and that in
the surrounding water. Contour currents consist of
fl ows that normally follow bathymetric contours, are
produced by differences in the density of water masses,
and can be greatly infl uenced by the geostrophic gyre
(Heezen et al. 1966 ), although they are also modifi ed
and infl uenced by wind-driven currents, boundary cur-
rents, and tidal currents. Wind-driven currents only
occur when strong winds blow in the same direction
for an extended period of time, and generally do not
affect really deep waters, although some have been
known to persist down to at least 1,500 m depth (Vidal
et al. 1992 ) with velocities up to 25 cm/s at 500 m
depth (Shanmugam 2008 ), a velocity high enough to
form sinuous coarse-sand ripples and to move pea-
sized gravel (Fig. 14.2 ). Because they only last while
the wind blows (usually seasonally), and the bottom
current portion commonly joins the geostrophic fl ow
(Vidal et al. 1992 ), their effects are often hard to dif-
ferentiate from contour currents, especially when deal-
ing with sediments or rocks. Internal waves are waves
that occur along density boundaries or gradients within
water bodies. These only contact the seafl oor when the
density boundary or gradient is at the seafl oor, such as
against a seamount or continental slope. They probably
have many different formation and modifi cation mech-
anisms including wind, gravity settling of dense fl uid,
and convection. Internal waves include deep-water
tidal currents (baroclinic tides), which are internal
waves with tidal periodicities. In deep-water settings,
tidal currents consist of internal waves that form along
density boundaries in the ocean by conversion of sur-
face tides (barotropic tides - really the 'whole ocean'
tide, as they affect more than just the water surface
where they can most easily be measured) to internal
waves along topographically rough surfaces such as
continental slopes, continental shelves, submarine
landslides, seamounts, or any other rough bit of seafl oor
(Garrett and Kunze 2007 ) .
None of these processes operate in isolation, how-
ever. A contour current fl owing through a constriction,
for example, may resuspend enough sediment to
generate a turbidity current (Mulder et al. 2006 ; Özsoy
et al. 2001 ). Because the contour current will continue
along at more or less the same depth, and the turbidity
current will fl ow down-slope, the two currents should
split up relatively soon after generation of the turbidity
current. Likewise it has long been suspected that tidal
currents may be forcing mechanisms for the generation
of turbidity currents, especially near the head of sub-
marine canyons, and a coincidence of strong down-
canyon fl ows with high surface tides reinforces this
contention (Shepard et al. 1979 ) . In some settings
internal tidal currents and contour currents may inter-
fere such that an internal tidal current can augment the
velocity of a contour current part of the time, and
negate it part of the time (McCave et al. 1980 ) . The
largest unknown is what the relative contribution of
each process is on both the modern seafl oor, and to
preserved sediments in the rock record. An under-
standing of the former can only come from modern
observations, while an understanding of the latter
must come from detailed outcrop studies, keeping
in mind the recognition of turbidites, tidalites, and
contourites is sometimes enigmatic and certainly
problematic.
14.2.1 Internal Waves and Internal Tides
Density boundaries in water bodies along which inter-
nal waves can form can be due to the variable salinity
of the water, particulate matter (including organic mat-
ter, sediment from river plumes, and eolian dust), and
temperature stratifi cation. Internal waves vary greatly
in amplitude, from a few centimeters to hundreds of
meters. Likewise their wavelengths can vary from a
few centimeters or meters to thousands of kilometers
(Zhenzhong et al. 1998 ). The period of internal waves
can vary from a few minutes to days or possibly longer
(seasonal). LaFond ( 1962 ) showed that some internal
waves impart oscillatory shear at the fl uid boundary,
indicating that they may sometimes behave like sur-
face waves; this type of wave may therefore also
generate symmetrical ripples or hummocky cross-
strata (Fig. 14.3 ) (Heezen and Rawson 1977 ; Kneller
et al. 1997 ) .
Internal tides are internal waves that approximate
the diurnal or semi-diurnal tidal cycle in period,
although they may be signifi cantly out of phase with
the surface tides. Internal tides are thought to be a sig-
nifi cant factor in helping to mix the deep oceans (Legg
2004 ; Munk and Wunsch 1998 ; St. Laurent and Garrett
2002 ), and in providing energy to deep marine areas
that otherwise might not see much energy fl ux from
currents, such as mid-ocean ridges or abyssal plains
(Egbert and Ray 2000 ). Internal tides are probably
 
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