Since the pioneering bottom camera observations of the
deep sea in the early 1980s, it has been understood that
pelagic sediments are formed by rapid settling from seasonal productivity events that occur in the surface ocean.
Rapid sedimentation to the sea floor, within days to weeks,
is mediated by aggregates of phytoplankton, often
enhanced by sticky, transparent extrapolymer particles
and by zooplankton fecal pellets. Normally this seasonal
input is rapidly reprocessed on the sea floor by the feeding
activity of benthic surface dwellers and burrowers that
effectively homogenize the sediment to depths of several
centimeters. When the activity of benthic organisms is
suppressed or eliminated, laminated sediments that record
the seasonal flux cycle are preserved. The benthos is most
commonly suppressed by low levels of bottom water oxygenation. This oxygen depletion generally occurs in
basins with sills where the bottom waters are not in direct
contact with the open ocean and so are not renewed.
Silled basins in which laminated sediments are preserved range in size from coastal fjords such as the Saanich Inlet in British Columbia to the California borderland
basins including the Santa Barbara Basin. On even larger
scales preservation of laminated sediments also occurs in
marginal seas with shallow sills such as the Baltic Sea or
the Black Sea. Bottom water oxygenation may also be sufficiently reduced to preserve laminae beneath highly productive upwelling regimes over open shelves, such as
the Gulf of California or the Peru Shelf, where the intense
flux of organic matter uses all available water column
dissolved oxygen. In many of these marginal settings,
there is an alternation of biogenic algal remains, often
formed by the spring bloom or “fall dump” with laminae
of terrigenous sediment commonly supplied by river runoff from seasonal rains. It is common to observe an annual
couplet of laminae (biogenic and terrigenous), but up to
22 individual laminae per year have been identified in
the Saanich Inlet which has rates of deposition exceeding
1 cm per year.
A further mechanism for the preservation of laminae is
the physical suppression of benthic activity by the rapid
deposition of tough mats of diatoms in “laminated diatom
mat deposits.” These are generated by the rapid flux of diatoms beneath oceanic frontal systems and occur extensively in ancient sediments of the equatorial Pacific and
around the Southern Ocean.
Annually laminated marine sediments are also referred
to as “marine varves” and may be used to develop marine
varve chronologies to form a method of establishing the
absolute and relative timing of climatic events. Time series
of interannual variability are also derived from lamina
sequences and are used to investigate past variation in
the frequencies of climatic phenomena such as El Niño.
Bibliography
Hughen, K. A., Overpeck, J. T., Peterson, L. C., and Trumbore, S.,
1996. Rapid changes in the tropical Atlantic region during the
last deglaciation. Nature, 380, 51–54.
Kemp, A. E. S. (ed.), 1996. Palaeoclimatology and palaeoceanography from laminated sediments. Geological Society of London, Special Publication, 116, 258pp.
Kemp, A. E. S., Pike, J., Pearce, R. B., and Lange, C. B., 2000. The
“fall dump”: a new perspective on the role of a shade flora in the
annual cycle of diatom production and export flux. Deep-Sea
Research II, 47(2129), 2154.
Kemp, A. E. S., Pearce, R. B., Grigorov, I., Rance, J., Lange, C.,
Quilty, P., and Salter, I., 2006. The production of giant marine
diatoms and their export at oceanic frontal zones: implications
for Si and C flux from stratified oceans. Global Biogeochemical
Cycles, 20, GB4S04, doi:10.1029/2006GB002698.
LAYERING OF OCEANIC CRUST
Juan Pablo Canales
Woods Hole Oceanographic Institution, Woods Hole,
MA, USA
Magma generated by decompression melting of the
upwelling mantle beneath mid-ocean ridges (MORs) rises
buoyantly and accumulates in crustal magma chambers
(e.g., Forsyth, 1992). The long-held view is that oceanic
crust is built from in situ crystallization of melts in these
reservoirs, as well as from melts extracted from the
magma chamber(s) in the form of dikes that eventually
break the seafloor during submarine eruptions (e.g.,
Sinton and Detrick, 1992). These processes result in oceanic crust that is magmatically layered, with a lower section typically 4–5 km thick composed of mafic plutons
(gabbros), a 1–2 km thick layer of sheeted basaltic dikes
above it, and a carapace of a few to several hundred meters
thick of extrusive basalts (pillow lavas and sheet flows),
known as the Penrose model of oceanic crust (Penrose
Conference participants, 1972). Although these units all
have common parental magmas, they exhibit distinct elastic properties because of differences in mineralogical textures produced by crystallization processes and different
porosity structure (i.e., the shallower extrusive section is
highly porous, with porosity decreasing with depth
throughout the crust). As a result, the speed of seismic
waves propagating through the crust increases with depth
while at the same time the rate at which seismic velocities
increase with depth (velocity gradient) diminishes with
depth. These seismic layers have been historically known
as seismic Layer 2 (upper and mid-crust, subdivided in 2A
and 2B based on changes in velocity gradient) and Layer
3 (lower crust) (e.g., White et al., 1992). To a large degree
they correspond to the lithological layering of the oceanic
crust, although deviations from this direct correlation are
known to exist (Christeson et al., 2007; Detrick et al.,
1994; Wilson et al., 2006).
The lithological and seismic layering described above
has successfully explained many observations made at
ophiolites (sections of oceanic crust exposed on land)
as well as observations at crust formed at MORs with
392
LAYERING OF OCEANIC CRUST
deep sea in the early 1980s, it has been understood that
pelagic sediments are formed by rapid settling from seasonal productivity events that occur in the surface ocean.
Rapid sedimentation to the sea floor, within days to weeks,
is mediated by aggregates of phytoplankton, often
enhanced by sticky, transparent extrapolymer particles
and by zooplankton fecal pellets. Normally this seasonal
input is rapidly reprocessed on the sea floor by the feeding
activity of benthic surface dwellers and burrowers that
effectively homogenize the sediment to depths of several
centimeters. When the activity of benthic organisms is
suppressed or eliminated, laminated sediments that record
the seasonal flux cycle are preserved. The benthos is most
commonly suppressed by low levels of bottom water oxygenation. This oxygen depletion generally occurs in
basins with sills where the bottom waters are not in direct
contact with the open ocean and so are not renewed.
Silled basins in which laminated sediments are preserved range in size from coastal fjords such as the Saanich Inlet in British Columbia to the California borderland
basins including the Santa Barbara Basin. On even larger
scales preservation of laminated sediments also occurs in
marginal seas with shallow sills such as the Baltic Sea or
the Black Sea. Bottom water oxygenation may also be sufficiently reduced to preserve laminae beneath highly productive upwelling regimes over open shelves, such as
the Gulf of California or the Peru Shelf, where the intense
flux of organic matter uses all available water column
dissolved oxygen. In many of these marginal settings,
there is an alternation of biogenic algal remains, often
formed by the spring bloom or “fall dump” with laminae
of terrigenous sediment commonly supplied by river runoff from seasonal rains. It is common to observe an annual
couplet of laminae (biogenic and terrigenous), but up to
22 individual laminae per year have been identified in
the Saanich Inlet which has rates of deposition exceeding
1 cm per year.
A further mechanism for the preservation of laminae is
the physical suppression of benthic activity by the rapid
deposition of tough mats of diatoms in “laminated diatom
mat deposits.” These are generated by the rapid flux of diatoms beneath oceanic frontal systems and occur extensively in ancient sediments of the equatorial Pacific and
around the Southern Ocean.
Annually laminated marine sediments are also referred
to as “marine varves” and may be used to develop marine
varve chronologies to form a method of establishing the
absolute and relative timing of climatic events. Time series
of interannual variability are also derived from lamina
sequences and are used to investigate past variation in
the frequencies of climatic phenomena such as El Niño.
Bibliography
Hughen, K. A., Overpeck, J. T., Peterson, L. C., and Trumbore, S.,
1996. Rapid changes in the tropical Atlantic region during the
last deglaciation. Nature, 380, 51–54.
Kemp, A. E. S. (ed.), 1996. Palaeoclimatology and palaeoceanography from laminated sediments. Geological Society of London, Special Publication, 116, 258pp.
Kemp, A. E. S., Pike, J., Pearce, R. B., and Lange, C. B., 2000. The
“fall dump”: a new perspective on the role of a shade flora in the
annual cycle of diatom production and export flux. Deep-Sea
Research II, 47(2129), 2154.
Kemp, A. E. S., Pearce, R. B., Grigorov, I., Rance, J., Lange, C.,
Quilty, P., and Salter, I., 2006. The production of giant marine
diatoms and their export at oceanic frontal zones: implications
for Si and C flux from stratified oceans. Global Biogeochemical
Cycles, 20, GB4S04, doi:10.1029/2006GB002698.
LAYERING OF OCEANIC CRUST
Juan Pablo Canales
Woods Hole Oceanographic Institution, Woods Hole,
MA, USA
Magma generated by decompression melting of the
upwelling mantle beneath mid-ocean ridges (MORs) rises
buoyantly and accumulates in crustal magma chambers
(e.g., Forsyth, 1992). The long-held view is that oceanic
crust is built from in situ crystallization of melts in these
reservoirs, as well as from melts extracted from the
magma chamber(s) in the form of dikes that eventually
break the seafloor during submarine eruptions (e.g.,
Sinton and Detrick, 1992). These processes result in oceanic crust that is magmatically layered, with a lower section typically 4–5 km thick composed of mafic plutons
(gabbros), a 1–2 km thick layer of sheeted basaltic dikes
above it, and a carapace of a few to several hundred meters
thick of extrusive basalts (pillow lavas and sheet flows),
known as the Penrose model of oceanic crust (Penrose
Conference participants, 1972). Although these units all
have common parental magmas, they exhibit distinct elastic properties because of differences in mineralogical textures produced by crystallization processes and different
porosity structure (i.e., the shallower extrusive section is
highly porous, with porosity decreasing with depth
throughout the crust). As a result, the speed of seismic
waves propagating through the crust increases with depth
while at the same time the rate at which seismic velocities
increase with depth (velocity gradient) diminishes with
depth. These seismic layers have been historically known
as seismic Layer 2 (upper and mid-crust, subdivided in 2A
and 2B based on changes in velocity gradient) and Layer
3 (lower crust) (e.g., White et al., 1992). To a large degree
they correspond to the lithological layering of the oceanic
crust, although deviations from this direct correlation are
known to exist (Christeson et al., 2007; Detrick et al.,
1994; Wilson et al., 2006).
The lithological and seismic layering described above
has successfully explained many observations made at
ophiolites (sections of oceanic crust exposed on land)
as well as observations at crust formed at MORs with
392
LAYERING OF OCEANIC CRUST