Aubrey, 1991, p. 165). It may be caused by mass exchange
between land and sea (built up or melting of continental
ice and snow, changes in hydrological conditions on land)
or by a density change (temperature, salinity) of the ocean
water. The latter one is also called steric effect.
If the contribution of land ice is considered only, the
resulting effect is called glacial eustasy. However, the built
up or melting of ice masses will never result in a uniform
sea-level change. Here, the changes of the gravity field
due to the mass changes (“geoid effect”) and the response
of the solid earth due to changing mass loads (“glacioisostatic adjustment”) have to be considered as well.
For the time period 1993–2010, a value of 2.8 Æ
0.5 mm/year for the eustatic effect has been determined
(IPCC, 2013). The main contributions come from thermal
expansion (1.1 mm/year), glaciers and ice caps
(0.76 mm/year), and ice sheets (0.8 mm/year).
Bibliography
Emery, K. O., and Aubrey, D. G., 1991. Sea Levels, Land Levels,
and Tide Gauges. New York/Berlin/Heidelberg/London/Paris/
Tokyo/Hong Kong/Barcelona: Springer.
IPCC, 2013. Summary for policymakers. In Stocker, T. F., Qin, D.,
Plattner, G.-K., Tignor, M., Allen, S. K., Boschung, J., Nauels,
A., Xia, Y., Bex, V., and Midgley, P. M. (eds.), Climate Change
2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change. Cambridge, UK/New York: Cambridge University Press.
Pugh, D. T., 1987. Tides, Surges and Mean-Sea-Level. Chichester/
New York/Brisbane: Wiley.
Cross-references
Glacio(hydro)-isostatic Adjustment
Relative Sea-level (RSL) Cycle
Sea-Level
EVENTS
William W. Hay
Department of Geological Sciences, University of
Colorado at Boulder, Estes Park, CO, USA
Definition
Events are relatively short-lived phenomena recorded in
the geologic record by depositional, erosional, or geochemical features. They may be of local significance
(e.g., a storm deposit (tempestite), debris flow, submarine
landslide, tsunami deposit) or more extensive (e.g., a volcanic ash deposit, changes in the distribution of fossil species, changes in ocean water properties such as anoxia
reflected by sapropels) or even global (eustatic sea-level
changes, variations in isotopic ratios, changes in seawater
composition).
Major events in ocean history
Knowledge of the history of the oceans depends in part on
how much of the ocean floor is still available for an examination. As shown in Figure 1a, only half of the ocean
crust with its sediment that existed 180 million years ago
is still present. The oldest fragment of the ocean crust in
the ocean basins is between 180 and 200 million years
old, but other bits of the older ocean crust are preserved
as ophiolites in mountain ranges. Subduction and consequent loss of the geologic record primarily affect the
Pacific and Indian Oceans. Subduction in the Atlantic
Ocean basins is limited to the eastern peripheries of the
Caribbean and Scotia Plates.
The salinity of the ocean has generally declined during
the Phanerozoic as salt has been removed to form evaporite deposits. Erosion of older evaporites on the continental blocks slowly returns salt to the ocean, but overall, the
trend has been toward freshening of the ocean. Salinities
in the Precambrian are thought to have been generally
above 50 %. During the Paleozoic, they were of the order
of 48 % to 42 %. Figure 1b shows major salt extractions
and changes in the average ocean salinity since 180 Ma.
The breakup of Pangaea produced several isolated deep
basins on ocean crust, particularly the opening Gulf of
Mexico and southern South Atlantic, that accumulated
large amounts of salt, and each significantly reduced
ocean salinity. The deposition of salt in the isolated Mediterranean and Red Sea basins and the Persian Gulf
lowered salinity to its modern value of about 34.72 %.
The evaporites in these deposits on ocean crust will not
return to the ocean until continental collisions or other
plate tectonic processes bring them back above sea-level.
The buildup of ice on land during the Pleistocene glacials
increased ocean salinity to as high as 35.90 %, indicated
by the asterisk at the top of the figure.
Ocean anoxic events (OAEs), shown in Figure 1c, were
episodes of extensive anoxia in the intermediate and/or
deep ocean. Most of these are known only from the Tethys
and North Atlantic, as indicated by the width of the gray
bars in the figure. Durations of these events, as estimated
by Erba (2004) but here expressed in millions of years,
were as follows: the Toarcian OAE – ?1? myr, Weissert
OAE – 2 myr, OAE 1a – 1.250 myr, OAE 1b 0.046
myr, OAE 1c – 1 myr, OAE 1 day – 0.3 myr, and OAE
2 (the Bonarelli event) – 0.3–0.4 myr. Each of the OAEs
occurred during very warm greenhouse conditions.
During the Cretaceous and Paleocene, the ocean’s deep
waters were warm, containing only about half as much
dissolved oxygen as cold deep waters. This was undoubtedly one of the factors contributing to the development
of OAEs.
Also indicated in Figure 1c is the Cretaceous-Tertiary
boundary event, probably caused by the impact of an
asteroid in Yucatan. The global effect of this event is
marked in the ocean by the extinction of most calcareous
nannoplankton and planktonic foraminifera. The recovery
of the diversity in the plankton took several million years.
EVENTS
239
between land and sea (built up or melting of continental
ice and snow, changes in hydrological conditions on land)
or by a density change (temperature, salinity) of the ocean
water. The latter one is also called steric effect.
If the contribution of land ice is considered only, the
resulting effect is called glacial eustasy. However, the built
up or melting of ice masses will never result in a uniform
sea-level change. Here, the changes of the gravity field
due to the mass changes (“geoid effect”) and the response
of the solid earth due to changing mass loads (“glacioisostatic adjustment”) have to be considered as well.
For the time period 1993–2010, a value of 2.8 Æ
0.5 mm/year for the eustatic effect has been determined
(IPCC, 2013). The main contributions come from thermal
expansion (1.1 mm/year), glaciers and ice caps
(0.76 mm/year), and ice sheets (0.8 mm/year).
Bibliography
Emery, K. O., and Aubrey, D. G., 1991. Sea Levels, Land Levels,
and Tide Gauges. New York/Berlin/Heidelberg/London/Paris/
Tokyo/Hong Kong/Barcelona: Springer.
IPCC, 2013. Summary for policymakers. In Stocker, T. F., Qin, D.,
Plattner, G.-K., Tignor, M., Allen, S. K., Boschung, J., Nauels,
A., Xia, Y., Bex, V., and Midgley, P. M. (eds.), Climate Change
2013: The Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change. Cambridge, UK/New York: Cambridge University Press.
Pugh, D. T., 1987. Tides, Surges and Mean-Sea-Level. Chichester/
New York/Brisbane: Wiley.
Cross-references
Glacio(hydro)-isostatic Adjustment
Relative Sea-level (RSL) Cycle
Sea-Level
EVENTS
William W. Hay
Department of Geological Sciences, University of
Colorado at Boulder, Estes Park, CO, USA
Definition
Events are relatively short-lived phenomena recorded in
the geologic record by depositional, erosional, or geochemical features. They may be of local significance
(e.g., a storm deposit (tempestite), debris flow, submarine
landslide, tsunami deposit) or more extensive (e.g., a volcanic ash deposit, changes in the distribution of fossil species, changes in ocean water properties such as anoxia
reflected by sapropels) or even global (eustatic sea-level
changes, variations in isotopic ratios, changes in seawater
composition).
Major events in ocean history
Knowledge of the history of the oceans depends in part on
how much of the ocean floor is still available for an examination. As shown in Figure 1a, only half of the ocean
crust with its sediment that existed 180 million years ago
is still present. The oldest fragment of the ocean crust in
the ocean basins is between 180 and 200 million years
old, but other bits of the older ocean crust are preserved
as ophiolites in mountain ranges. Subduction and consequent loss of the geologic record primarily affect the
Pacific and Indian Oceans. Subduction in the Atlantic
Ocean basins is limited to the eastern peripheries of the
Caribbean and Scotia Plates.
The salinity of the ocean has generally declined during
the Phanerozoic as salt has been removed to form evaporite deposits. Erosion of older evaporites on the continental blocks slowly returns salt to the ocean, but overall, the
trend has been toward freshening of the ocean. Salinities
in the Precambrian are thought to have been generally
above 50 %. During the Paleozoic, they were of the order
of 48 % to 42 %. Figure 1b shows major salt extractions
and changes in the average ocean salinity since 180 Ma.
The breakup of Pangaea produced several isolated deep
basins on ocean crust, particularly the opening Gulf of
Mexico and southern South Atlantic, that accumulated
large amounts of salt, and each significantly reduced
ocean salinity. The deposition of salt in the isolated Mediterranean and Red Sea basins and the Persian Gulf
lowered salinity to its modern value of about 34.72 %.
The evaporites in these deposits on ocean crust will not
return to the ocean until continental collisions or other
plate tectonic processes bring them back above sea-level.
The buildup of ice on land during the Pleistocene glacials
increased ocean salinity to as high as 35.90 %, indicated
by the asterisk at the top of the figure.
Ocean anoxic events (OAEs), shown in Figure 1c, were
episodes of extensive anoxia in the intermediate and/or
deep ocean. Most of these are known only from the Tethys
and North Atlantic, as indicated by the width of the gray
bars in the figure. Durations of these events, as estimated
by Erba (2004) but here expressed in millions of years,
were as follows: the Toarcian OAE – ?1? myr, Weissert
OAE – 2 myr, OAE 1a – 1.250 myr, OAE 1b 0.046
myr, OAE 1c – 1 myr, OAE 1 day – 0.3 myr, and OAE
2 (the Bonarelli event) – 0.3–0.4 myr. Each of the OAEs
occurred during very warm greenhouse conditions.
During the Cretaceous and Paleocene, the ocean’s deep
waters were warm, containing only about half as much
dissolved oxygen as cold deep waters. This was undoubtedly one of the factors contributing to the development
of OAEs.
Also indicated in Figure 1c is the Cretaceous-Tertiary
boundary event, probably caused by the impact of an
asteroid in Yucatan. The global effect of this event is
marked in the ocean by the extinction of most calcareous
nannoplankton and planktonic foraminifera. The recovery
of the diversity in the plankton took several million years.
EVENTS
239