Download Marine Sediments and Climate History

EAS/BIOEE 154
Lecture 6
Introduction to Oceanography
Marine Sediments and Climate
History
Principal of Superposition
Sediments get older downward.
Unconformity
 Gap in time in the sedimentary sequence.
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Types of Marine Sediment
Detrital
 particles which arise from weathering on the continents
Chemical
 Precipitated from seawater
 Ions transported in solution from site of erosion
 Precipitation often biologically mediated
Distribution of Sediments
Neritic sediments
 Continent-derived sands and muds – mostly detrital sediments
 Confined to continental margins
Pelagic Sediments: deep ocean
 Biogeneous: Calcareous and siliceous oozes
 Abyssal Clays: Very fine clays confined to the deep ocean basins
 Hydrogenous sediments: Abiologic chemical precipitates (rare)
Pelagic Biogenous Sediments: Oozes
Oozes are sediment composed predominantely of shells of micro-organisms
Calcareous
 Confined to shallower regions (<4500 meters depth)
 Foramifera (protozoans)
 Pteropods (floating snails)
 Coccolithophorids (algae)
Siliceous
 Shells of silica (same as quartz or glass)
 Radiolarians (protozoans)
 Diatoms (algae)
 Most common beneath areas of high biologic productivity
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Hydrogenous Sediments
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Direct precipitates from seawater (without biological action).
Manganese Nodules the most common example.
Occur only where sedimentation rates are very low.
Grow at rates of 1 mm/1000 years and less
Reading the History Book: Climate Changes
The Ice Ages
 Swiss-American geologist Louis Agassiz (1807-1873) recognized that
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Lecture 6
glaciers larger than the present ones carved many features of Switzerland.
Similar features recognized in N. America
 But what caused the glaciations? What was the chronology of events?
Milankovitch proposed that variations in the following orbital parameters
caused the Ice Ages:
Axial Obliquity (Tilt)
 Tilt of the Earth’s rotational axis relative to the orbital plane varies from about
21 1/2˚ to about 24 1/2˚ on a time scale of 40,000 years.
Orbital Eccentricity
 Earth’s orbit varies from more circular to less circular on a time scale of
about 100,000 years.
Precession
 Precession refers to the changing orientation of the rotational axis and its
relation to orbital ellipse.
 Time-scale: 23,000 years.
Testing the Theory
Harold Urey proposed that this theory could be tested by measuring oxygen
isotope ratios in calcareous oozes
What are isotopes?
 Atoms of the same element that different numbers of neutrons in the nucleus
and therefore different masses. The mass difference leads to very slight
difference in chemical behavior
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 H2 O evaporates from the ocean more readily than H2 O; The extent of this
effect increases with decreasing temperature in a predictable way.
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 When foraminifera produces their shells of calcite (CaCO3),
O is used in
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preference to O; The extent of this effect depends on temperature – it is
greater at lower temperature.
 In the 1970’s and 1980’s oceanographers John Hayes and John Imbrie
conclusively showed a correlation between O isotope ratios and
Milankovitch forcing.
Need for a feedback mechanism and amplification
 The Milankovitch insolation changes are too small to produce the climate
change observed.
Possible Feedbacks
 Albedo
 CO2
 Changes in ocean circulation
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CO2 and the Greenhouse Effect
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Although a trace gas, CO2 strongly affects transmission of radiation through
the atmosphere.
In ice cores from both Antarctica and Greenland, there is a strong
correlation between CO2 and temperature: CO2 was lower
during glacial periods. But why?
The most likely theory involves wind, dust, nutrients, plankton, and the
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Lecture 6
biological pump.
Ice Ages and Atlantic Circulation
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The formation of North Atlantic Deep Water (NADW) and the Gulf Stream
play an important role in warming the North Atlantic (particularly Europe).
Without them, Europe would have a substantially cooler climate.
During glacial periods cooler temperatures in the North Atlantic may have
reduced the Gulf Stream flow, and shut down associated NADW
production, cutting off the flow of thermal energy to the North Atlantic.
Some Conclusions about Climate
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Earth’s climate has varied dramatically in the past.
These variations are recorded by various proxies in marine sediments
This variation occurs on all time scales
From a few years (El Niño)
To 100 million years
Some of this variation is due to external forcing (e.g., Milankovitch cycles)
Some is due to other factors
On all these scales, the oceans play an important role in climate change.
Some Study Questions
Explain why the principal of superposition should hold – in other
words, why should sediments get older downward?
Give an example of one kind of shell that makes up a siliceous
ooze? What kind of organism produces this shell?
Where are siliceous oozes typically found?
Why are manganese nodules only found in areas of very low
sedimentation rate?
What are the Milankovitch parameters
Explain how does variations in the Earth’s Axial Obliquity affect
climate?
Does global average insolation change as a result of Milankovitch
variations?
Explain how 18O and 16O atoms differ
Explain how the ratio of 18O to 16O in seawater changes when
glaciers build up on continents.
Define the term “albedo”. Explain how albedo operates as a
climate feedback mechanism.
What is the evidence that atmospheric CO2 concentrations and
temperature correlate over the last 150,000 years?
Explain how changes in ocean circulation, particularly in the North
Atlantic might result in drastic changes in climate.
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