hydrography, lagoons can be divided into three classes,
normal, brackish, and hypersaline, which combined with
circulation, can exert a major influence on the sedimentological characteristics of the lagoon (Milliman, 1974).
Normal lagoons display temperatures and salinities similar to those of the adjacent ocean. In brackish lagoons,
freshwater influx from run-off and rainfall exceeds evaporation: the opposite is true of hypersaline lagoons.
Away from the land masses, lagoon sediments are
totally calcareous (apart from a few diatoms and sponge
spicules). Corals are common throughout apart from that
in deeper lagoons below the level of coral and algal
growth where black muds may dominate. Calcareous
algae and shell fragments from gastropods, echinoderms,
crustaceans, etc. may also be found in all parts of the
lagoon but foraminifera tend to dominate shallow waters
and Halimeda deeper zones beyond 50 m (Guilcher,
1988).
Studies of lagoon infill (e.g., Davies and Hopley, 1983;
Purdy and Gischler, 2005) show that once reefs reached
sea level at the end of the Holocene transgression, lateral
movement of sediment becomes a dominant process, with
infill rates of up to 8 m/ka. Occasional high energy storms
produce rates up to 18 m/ka. Lagoons in smaller platform
reefs may have totally infilled during the Holocene
(Hopley et al., 2007). However, as Purdy and Gischler
(2005) have shown for large atoll scale lagoons, the extent
of Holocene high stand lagoon filling may be minimal.
Lateral infilling also comes from patch reef growth, the
modal rate of which may be 8 m/ka which once the reef
is at sea level also reverts into lagoon infilling in the form
of a sediment halo around the patch reef. Although, current speeds, within lagoons are normally low, some sediment movement may take place with the help of
bioturbation (Frith, 1983).
Bibliography
Davies, P. J., and Hopley, D., 1983. Growth facies and growth rates
of Holocene reefs in the Great Barrier Reef. Bureau Mineral
Resources Journal, Australian Geology and Geophysics, 8,
237–251.
Frith, C. A., 1983. Some aspects of lagoon sedimentation and circulation at One Tree Reef, Southern Great Barrier Reef, Australia.
Bureau Mineral Resources Journal, Australian Geology and
Geophysics, 8, 211–221.
Guilcher, A., 1988. Coral Reef Geomorphology. New York: Wiley.
Hopley, D., 1982. Geomorphology of the Great Barrier Reef: Quaternary Development of Coral Reefs. New York: John Wiley -
Interscience, p. 453.
Hopley, D., 2006. Coral reef growth on the shelf margin of the Great
Barrier Reef with special reference to the Pompey Complex.
Journal of Coastal Research, 22, 150–158.
Hopley, D., Smithers, S. G., and Parnell, K. E., 2007. The Geomorphology of the Great Barrier Reef: Development, Diversity and
Change. Cambridge, UK: Cambridge University Press, p. 532.
Marshall, J. F., and Davies, P. J., 1982. Internal structure and Holocene evolution of One Tree Reef, southern Great Barrier Reef,
Australia. Coral Reefs, 1, 21–28.
Milliman, J. D., 1974. Marine Carbonates. Berlin, Heidelberg and
New York: Springer.
Purdy, E. G., 1974. Reef configurations, cause and effect. In
Laporte, L. F. (ed.), Reefs in Time and Space: Selected Examples
from the Recent and Ancient. Society of Economic Paleontologists and Mineralogists, Special Publication, 18, 9–76.
Purdy, E. G., and Gischler, E., 2005. The transient nature of the
empty bucket model of reef sedimentation. Sedimentary Geology, 175, 35–47.
Purdy, E. G., and Winterer, E. L., 2001. Origin of atoll lagoons.
Geological Society of America Bulletin, 113, 837–854.
Wyrwoll, K.-H., Zhong, R. Z., Collins, L. B., and Hatcher, B. G.,
2006. Origin of blue hole structures in coral reefs: Houtman
Abrolhos, Western Australia. Journal of Coastal Research, 22,
202–208.
Cross-references
Atolls
Blue Hole
Boat Channel
Faroes Reefs
Lagoon Circulation
Moating
Moats
Reef Classification by Hopley (1982)
Residence Time
Reticulated Reefs
LAST GLACIAL INTERSTADIALS
Yusuke Yokoyama
The University of Tokyo, Chiba, Japan
Definition
Relatively warm periods recorded in Greenland ice cores
during the last glacial.
Last glacial interstadials are strongly related to ocean circulation changes resulting from partial collapsing of continental ice. Sea-level changes also occurred. Ice cores
obtained from deep drilling in Greenland by international
collaborative efforts have revealed a detailed history of
paleoclimates. One of the important discoveries from the
ice cores was the identification of abrupt shifts in climate,
in particular during the last glacial. Shifts in Greenland
surface-air temperature were abrupt changing from cold
status to warm status by as much as 10–15
C in decadal
time scales or less. This had been previously found in
the interval between 20,000 and 75,000 years ago (e.g.,
Dansgaard et al., 1982; Johnsen et al., 1972, 1992). In
the meantime, deep-sea cores were also analyzed and indicated similar millennial scale oscillations. There are two
types of rapid climate change according to detailed examinations of the ice and marine cores, namely Dansgaard–
Oeschger (D–O) events and Heinrich events (HE). The
former has approximately 1,500 year cycles (e.g., NGRIP
Members, 2004) whereas the latter appeared every ca.
7,000 years (Yokoyama et al., 2001a; Clark et al., 2007).
HE were discovered from sedimentological analyses in
the North Atlantic as was noted from the synchronous
618
LAST GLACIAL INTERSTADIALS
normal, brackish, and hypersaline, which combined with
circulation, can exert a major influence on the sedimentological characteristics of the lagoon (Milliman, 1974).
Normal lagoons display temperatures and salinities similar to those of the adjacent ocean. In brackish lagoons,
freshwater influx from run-off and rainfall exceeds evaporation: the opposite is true of hypersaline lagoons.
Away from the land masses, lagoon sediments are
totally calcareous (apart from a few diatoms and sponge
spicules). Corals are common throughout apart from that
in deeper lagoons below the level of coral and algal
growth where black muds may dominate. Calcareous
algae and shell fragments from gastropods, echinoderms,
crustaceans, etc. may also be found in all parts of the
lagoon but foraminifera tend to dominate shallow waters
and Halimeda deeper zones beyond 50 m (Guilcher,
1988).
Studies of lagoon infill (e.g., Davies and Hopley, 1983;
Purdy and Gischler, 2005) show that once reefs reached
sea level at the end of the Holocene transgression, lateral
movement of sediment becomes a dominant process, with
infill rates of up to 8 m/ka. Occasional high energy storms
produce rates up to 18 m/ka. Lagoons in smaller platform
reefs may have totally infilled during the Holocene
(Hopley et al., 2007). However, as Purdy and Gischler
(2005) have shown for large atoll scale lagoons, the extent
of Holocene high stand lagoon filling may be minimal.
Lateral infilling also comes from patch reef growth, the
modal rate of which may be 8 m/ka which once the reef
is at sea level also reverts into lagoon infilling in the form
of a sediment halo around the patch reef. Although, current speeds, within lagoons are normally low, some sediment movement may take place with the help of
bioturbation (Frith, 1983).
Bibliography
Davies, P. J., and Hopley, D., 1983. Growth facies and growth rates
of Holocene reefs in the Great Barrier Reef. Bureau Mineral
Resources Journal, Australian Geology and Geophysics, 8,
237–251.
Frith, C. A., 1983. Some aspects of lagoon sedimentation and circulation at One Tree Reef, Southern Great Barrier Reef, Australia.
Bureau Mineral Resources Journal, Australian Geology and
Geophysics, 8, 211–221.
Guilcher, A., 1988. Coral Reef Geomorphology. New York: Wiley.
Hopley, D., 1982. Geomorphology of the Great Barrier Reef: Quaternary Development of Coral Reefs. New York: John Wiley -
Interscience, p. 453.
Hopley, D., 2006. Coral reef growth on the shelf margin of the Great
Barrier Reef with special reference to the Pompey Complex.
Journal of Coastal Research, 22, 150–158.
Hopley, D., Smithers, S. G., and Parnell, K. E., 2007. The Geomorphology of the Great Barrier Reef: Development, Diversity and
Change. Cambridge, UK: Cambridge University Press, p. 532.
Marshall, J. F., and Davies, P. J., 1982. Internal structure and Holocene evolution of One Tree Reef, southern Great Barrier Reef,
Australia. Coral Reefs, 1, 21–28.
Milliman, J. D., 1974. Marine Carbonates. Berlin, Heidelberg and
New York: Springer.
Purdy, E. G., 1974. Reef configurations, cause and effect. In
Laporte, L. F. (ed.), Reefs in Time and Space: Selected Examples
from the Recent and Ancient. Society of Economic Paleontologists and Mineralogists, Special Publication, 18, 9–76.
Purdy, E. G., and Gischler, E., 2005. The transient nature of the
empty bucket model of reef sedimentation. Sedimentary Geology, 175, 35–47.
Purdy, E. G., and Winterer, E. L., 2001. Origin of atoll lagoons.
Geological Society of America Bulletin, 113, 837–854.
Wyrwoll, K.-H., Zhong, R. Z., Collins, L. B., and Hatcher, B. G.,
2006. Origin of blue hole structures in coral reefs: Houtman
Abrolhos, Western Australia. Journal of Coastal Research, 22,
202–208.
Cross-references
Atolls
Blue Hole
Boat Channel
Faroes Reefs
Lagoon Circulation
Moating
Moats
Reef Classification by Hopley (1982)
Residence Time
Reticulated Reefs
LAST GLACIAL INTERSTADIALS
Yusuke Yokoyama
The University of Tokyo, Chiba, Japan
Definition
Relatively warm periods recorded in Greenland ice cores
during the last glacial.
Last glacial interstadials are strongly related to ocean circulation changes resulting from partial collapsing of continental ice. Sea-level changes also occurred. Ice cores
obtained from deep drilling in Greenland by international
collaborative efforts have revealed a detailed history of
paleoclimates. One of the important discoveries from the
ice cores was the identification of abrupt shifts in climate,
in particular during the last glacial. Shifts in Greenland
surface-air temperature were abrupt changing from cold
status to warm status by as much as 10–15
C in decadal
time scales or less. This had been previously found in
the interval between 20,000 and 75,000 years ago (e.g.,
Dansgaard et al., 1982; Johnsen et al., 1972, 1992). In
the meantime, deep-sea cores were also analyzed and indicated similar millennial scale oscillations. There are two
types of rapid climate change according to detailed examinations of the ice and marine cores, namely Dansgaard–
Oeschger (D–O) events and Heinrich events (HE). The
former has approximately 1,500 year cycles (e.g., NGRIP
Members, 2004) whereas the latter appeared every ca.
7,000 years (Yokoyama et al., 2001a; Clark et al., 2007).
HE were discovered from sedimentological analyses in
the North Atlantic as was noted from the synchronous
618
LAST GLACIAL INTERSTADIALS