Transcript Paleooceanography and Sea-level Changes

Paleooceanography and Sealevel Changes
Chapter 6
• Information about past history of the ocean
basins comes from
– Magnetic anomalies and related data
– The seafloor sediments
• These retain a record of the events in the overlying
waters
• Also can tell us about sea level change
• The thickness of the sediments increase
with distance from the spreading ridge
– Why?
– Near the ridge they are 1-2 m thick
– On the abyssal plain they are 1 km or more
– On the continental slope-rise 10 km or more
• Sediments that settle from suspension in
the open ocean are called pelagic
sediments
• Main types
– Biogenic calcareous sediments
– Biogenic siliceous sediments
– Inorganic red clay
– Inorganic ice-rafted sediments
• Red clays are the smallest particles
• Red is from iron oxide
• Also contains volcanic ash and meteoritic
fragments
– The latter accumulate at 0.1-1.0 mm / 106 yr
• Biogenic sediments just means biological
materials accumulate at higher rates than
other materials
– Used to be called oozes
– May contain 50% or more clay
• Distribution is controlled by
– Climate and current patterns
– Nutrient (upwelling) induced biological
production
– Relative solubilities of calcium and silica
• Calcium dissolves more readily under high
pressure
• Calcium dissolves more readily in cold water
• The lower parts of sediment layers may
have been affected by warm water
solutions
– Although this ceases after the crust is about
70 Ma old.
• In early ocean basins evaporites may form
– These are later buried by sediments
– The salts are less dense and plastic
• Can be forced upward
• Evaporites form salt pillars or domes
– These are good traps for hydrocarbons
– Oil and gas are result of anaerobic
decomposition of plankton
• One of the most spectacular examples of
anticlinal fold structures lie on the north
shore of the Strait of Homuz in the Persian
Gulf.
– Located near the important city of Bandar
Abbas, these folds form the foothills of the
Zagros Mountains, which run northnorthwesterly through Iran.
– The folds were formed when the Arabian
shield collided with the western Asian
continental mass about 4 to 10 million years
ago.
• The other features that are prominent in this
photograph are the dark circular patches.
• These represent the surface expression of salt
domes that have risen diapirically from the
Cambrian Hormuz salt horizon through the
younger sediments to reach the surface.
– Only in a hot arid environment such as that of the Gulf
can the soluble salt escape rapid erosion.
American Scientist, Sept.-Oct. 1991, p.426
• In the 1980s it was discovered that
communities similar to those at
hydrothermal vents were found at the
continental margin at 1000 m deep.
– Seeps and vents of cold water along with H2S
and CH4
– Chemosynthetic sulfide-oxidizing bacteria
– The seeps are caused by dewatering of
sediments due to compaction
Sediments and Paleoceanography
• The Antarctic Circumpolar Current driven
by west winds
– Continuous around Antarctica
– Reaches depths of 3-4,000 m
• Did not always exist
– Not before all the southern continents
separated from Antarctica
Orsi A. H., T.
Whitworth and W.
D. Nowlin. 1995.
On the meridional
extent and fronts
of the Antarctic
Circumpolar
Current. DeepSea Research
42(5): 641673.
• The southern continents started moving
away as early as 170 Ma ago
– Oceanic crust did not form between South
America and the Antarctic prior to 20 Ma
• Jenkins (1978) argues about 28 Ma
– There are indications that there was a
connection prior to this based on the spread
of marine fauna
• Foram Guembelitria
• This connection is postulated to have been
due to rifting
– So true seafloor spreading had not been
initiated yet
Changes in Sea-Level
• Ocean bathymetry changes significantly only on
time scales of 106-107 years
– The geoid is the hypothetical surface of the Earth that
coincides everywhere with mean sea level and is
perpendicular, at every point, to the direction of
gravity.
• The geoid is used as a reference surface for astronomical
measurements and for the accurate measurement of
elevations on the Earth's surface
• Sea level fluctuations can occur on scales of
decades to centuries
• Sea-level is the level to which the ocean
basins between the continents are filled at
any particular time
– Since all the basins are interconnected, filling
one would also fill the others
– This is known as eustatic sea-level change
• The equilibrium level is determined by
– Volume of water in the oceans
• Inputs – precipitation, rivers and groundwater,
melting of ice
– Shape of the container
• Global thickness and area of continental crust
• Relative thermal states of ocean and continental
crusts, volume of large igneous provinces
• Mass of water and sediment load on the oceanic
crust
• Water expansion is 2.1 x 10-4ºC-1
• How change in sea level would a 1ºC
increase cause? Avg depth = 3.7 km
• 3.7 km x 1000m/km x 1ºC x 2.1 x 10-4ºC-1
= 0.777 m
Time Scales
• Sea-level is subject to numerous local and
short term changes
– Some of great magnitude (10s of meters)
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Tidal fluctuations
Wind waves
Barometrically induced surges
Tsunamis
Freshwater floods
Ship wakes
• Other factors affecting local sea level
– wind and ocean currents that can "pile up" the
ocean water locally, temperature anomalies
like El Niño, local gravity wells of ice sheets
and land masses, and regional salinity levels
that alter the water's density.
– Measurement of these levels is further
complicated by changes in land height as the
Earth's crust moves up or down from tectonic
motion and rebounds after long and recently
ended glaciation, although these
complications are avoided by using satellite
measurements.
• Mean sea level is defined from long-term
averages
• Changes of 1 mm yr-1 can be detected
• Most analysis of recent past use tide
gauge records
– Need to account for seasonal, tidal and
episodic events
• Atmospheric pressure changes can affect
sea level
– 1 mbar of pressure change affects sea level
by about 1 cm
• Tide gauge data is now supplemented by
deep sea pressure gauges
• All earthbound gauges suffer from
possible crustal movement
– Cause local rise or fall
– These are called isostatic sea level
changes
Satellite Sea-Level
• Satellite altimeters have been used to
measure the marine geoid
• TOPEX/Poseidon measured sea surface
from 1992
– Cover globe from 66ºS to 66ºN every 10 days
– TOPEX visualization
• Most changes however are short term
– Like the El Nino on the next slide
• A global increase in sea level was
detected in the TOPEX record
• Can Thermal expansion explain the
increase?
• Have to consider the mixed layer
– Reasonable to assume 100m
• Assume it warmed 0.2 ºC in 1993-95
– 100 m x (2.1 x 10-4ºC-1) x 0.2 ºC = 4.2 mm
– = 2.1 mm per year
• Short term thermal expansion plays an
obvious role
• The sea level rise has been going on for
the past 20,000 years
– Why is that?
• The rate has increased due to human
activities in the last century
Post-Glacial Rise in SL
• Several times in Earth history that there
have been major ice sheets at high
latitudes
– Most recent was in the Quaternary (1.6 Ma
ago) and it may not be over yet
– Within this period there have been several
glaciations
• The most recent from 120,000 to 20,000 years ago
• So there may be another one on the way
• The initial rise in sea level was rapid from
18,000 years ago to about 6,000 years
ago
• Things are complicated due to isostatic
factors
• Additional loading on the oceanic crust
can depress it
• There is no easy way to separate local
isostatic adjustments from eustatic
change
Measuring Quaternary Changes
• We don’t have sufficient evidence to
evaluate isostatic adjustments to sea level
change in the or Quaternary.
• Only evidence of the static changes
• The technique used to past marine
temperatures is also value to studying sea
level fluctuations
• It relies on differential incorporation of the
18O and 16O isotopes into calcium
carbonate
– 99.763% of natural oxygen is 16O and 18O
– The ratio can be measured with a mass
spectrometer
• Marine organisms incorporate oxygen
isotopes into their skeletal parts in different
proportions depending on temperature
– To lower the temperature at the higher the
18O:16O ratio
– The ratio depends somewhat on species
• Forams are ideal because they are
abundant widespread and have hard parts
of calcium carbonate
• A complication arose when benthic forams
showed as great a variation in oxygen
isotope ratios as surface forams
– This seemed improbable because bottom
waters should be more consistent in
temperature
• How can this be explained then?
• It has to do with evaporation and
precipitation
• Water vapor tends to be enriched in
molecules containing the lighter isotopes,
relative to the liquid from which it
evaporated
• With water vapor condenses there is
fractionation in the other direction
– Condensed water is enriched in the heavier
isotope
• Because of the different temperatures at
which evaporation and condensation
occurred. The isotopic fractionation is
greater during evaporation.
• Therefore rainwater and snow are richer in
16O than the sea water from which the
water vapor came
• The result is that when snow and ice
accumulates to form glaciers and ice caps,
the ice will be relatively depleted in 18O
– A low 18O:16O ratio
• The oceans on the other hand, will
become relatively enriched and 18O
– A high 18O:16O ratio
• As icecaps grow, the proportion of 16O
removed from seawater increases
• The oxygen isotope ratios of forams is
therefore a reflection of the volume of
water locked up in the icecaps and
glaciers, not just a direct temperature
effect.
• There is also temperature effect, though,
because the lower the global surface
temperature, the more ice we would
expect in the icecaps
• Lastly there is also the biological effect
• Different species have different isotope
ratios
– But for any given species the isotope ratio is
always greater in cold water than in warm
water
• Isotopic ratios are calibrated against
standards
– Standard Mean Ocean Water (SMOW)
– Pee Dee Belemnite (PDB)
• Replaced in 1995
– Vienna Standard Mean Ocean Water
(VSMOW)
• These represent average isotopic
composition of current normal seawater
• Isotopic ratios are conventionally recorded as
delta (δ18O) values expressed as ppt (‰),
commonly called “per mil”.
–
– where "R" is the ratio of the heavy to light isotope in
the sample or standard
• A positive d value means that the sample
contains more of the heavy isotope than
the standard;
• A negative d value means that the sample
contains less of the heavy isotope than the
standard.
• To lower the temperature evaporation, the
greater the enrichment of 16O in water
vapor
• In the tropics δ18O values close to zero
• Polar snow and ice are very negative
– δ18O = -30 Greenland
– δ18O = -50 South Pole
• Comparison of oxygen isotope ratios of
forams at the peak of the last glaciation
with composition of modern forams allows
a direct relationship to be established
between isotopic composition and sea
level
– The difference in δ18O of 0.1 per mil is found
to be equivalent to a 10 m change in sea level
• The Quaternary sea level fluctuations of
100 m and more were the result of about
50 x 106 km³ of water being alternately
withdrawn from and returned to the
oceans
• The remains about 30 x 106 km³ of ice in
the polar icecaps
– Total melting would lead to a further increase
of 60 m in sea level
Growth of a ice sheet
• The evidence for the rate of growth of the
Antarctic ice sheet comes mainly from the
nature of the sediments on the seafloor
– Supported by oxygen isotope analysis of
forams
• Seafloor sediments
– Exotic rock fragments ice rafted from Antarctic
– Poor sorting
– Quartz grains typical of glaciation
• Presence of ice transported debris of late
Eocene (40 Ma ago) indicate the region
may have been partially glaciation at the
time
– Oxygen isotope ratios, however show no
large accumulation of ice
• Definite glacial characteristics seen in the
late Oligocene (25 Ma ago)
• Since then glacial sediments have become
more widespread
– Reaching maximum extent in the Quaternary
• The Antarctic are chic began to rapidly
developed in mid Miocene
• This could’ve been linked to the final
separation of South America from
Antarctica
– Initiation of the Antarctic circumpolar current
– Thermal isolation of the Antarctic continent
from warmer waters to the north
• This time, glacial conditions became more
widespread in the northern hemisphere in
mountainous areas
– The great continental ice sheets did not
appear until more recently, about 3 million
years ago
• The cooling trend was not uniform, and
there were great climatic fluctuations
between cold warm periods
• About 5 million years ago these
fluctuations were associated with a
remarkable consequence of sea level
change
Salinity crisis in the Mediterranean
• Early in the Miocene (20 Ma ago) the
Arabian plate and Eurasian plate collided
blocking the link between the
Mediterranean and the Indian Ocean to
the East
• The Mediterranean became almost totally
landlocked
– Only a shallow connection to the Atlantic
• The connection to the Atlantic had a
tendency to close this Africa moved
northward relative to Europe
• Loss of calm activity led to drier climatic
conditions throughout the Mediterranean
– Evidenced by a thick evaporites in the Red
Sea of Miocene age, over a period of about
700,000 yrs.
– Virtually complete drying up of the
Mediterranean
• It’s hard to picture how this could happen,
especially since it is now more than 3000
m deep in places with an average depth of
a 1500 m
• Let’s look at how reasonable, this is
• The surface area of the Mediterranean is about
2.5 x 106 km2
• Average depth is 1.5 kilometers
• Therefore the volume is 3.75 x 106 km3
• Evaporation is considerably larger than
precipitation in the region
– Present-day evaporation is 4.7 x 103 km3/yr
– Present-day precipitation is 1.2 x 103 km3/yr
• The E – P = 3.5 x 103 km3/yr
• Other inputs from rivers and the Black Sea
amount to 0.25 x 103 km3/yr
• The net loss is 3.25 x 103 km3/yr
• How long will it take to dry up?
• T = 3.75 x 106 km3 / 3.25 x 103 km3/yr
• T = 1,153.8 yr
• Evidence that it did dry comes from
– buried River gorges a 1000 m or so below the
valleys of the Nile and Rhone
– The death right deposits over 1000 m thick
• Salt domes are evident in many places
• The thickness of the salt poses somewhat
of a problem
– The volume of salt derived from the volume of
the Mediterranean Sea, would not produce a
1 km thick layer of salt
• Salt = 35 g/l x 1012 l/km3 x 3.75 x 106 km3
• Salt = 130 x 1018 g or 1.3 x 1017 kg
– ρ = 2 x 103 kg/m-3
• Vsalt = 1.3 x 1017 kg / 2 x 103 kg/m-3
• Vsalt = 6.5 x 1013 m3 or 6.5 x 104 km3
• The surface area is 2.5 x 106 km2
– But assume that 20% is littoral (shallow)
• How thick a layer would our salt form?
• 6.5 x 104 km3 / 2 x 106 km2 = 0.0325 km
• = 32.5 m
• How many Mediterraneans would it take to
make 1000 m thick layer?
• 1000 m / 32.5 m = 30.8
• This implies that there must have been
influxes of Atlantic
• Since the Messinian “salinity crisis” lasted
over 700,000 years the Atlantic connection
could easily have been broken and
reconnected several times (at 1000 yrs to
dry up)
– The average rate of supply of Atlantic waters
still have to be less than the evaporation rate
throughout the deposition of the evaporites
• The sea water supply began to exceed the
amount of evaporation about 4.8 Ma ago
– This restored normal marine conditions and
deposition reverted to muds and deep water
carbonate sentiments
• The Atlantic connection became deep
enough for Coldwater to enter the
Mediterranean throughout most of the
Pliocene.
– About one million years ago to Gibraltar sill
was uplifted, and the deep supply ceased
• Oxygen isotope can be used to relate
changes in the volume of glaciers and
polar icecaps, to changing sea levels that
we responsible for the Messinian salinity
crisis
– Two distinct periods of evaporation
• 5.5 Ma ago, the Mediterranean became
isolated through combined tectonic uplift
and global fall in sea level
– Successive inundations and desiccations due
to fluctuations in sea levels because of
changes in ice volume
• These are seen in oxygen isotope measurements
Migration of climatic belts
• How come the Mediterranean stayed
warm while icecaps were growing?
• This again can be deduced from psychotic
organisms
• It turns out that climate zones became
compressed especially at mid latitudes
Present
18,000 yrs ago
Effective plate tectonic processes
on sea level
• Two major processes that may be
important in raising sea level
– The rate of production of new oceanic crust
due to heat content can cause sea level rise
• Probably happened in the upper Cretaceous about
90 Ma ago when sea level was some 300-400 m
higher than today
– During periods of Conrail break up the overall
elevation of confidence is reduced due to
local crustal thinning and increased erosion
• Lowering of sea level occurs when
continents collide
– Large amounts of sediments become piled up
next to the Continental margin and
Continental blocks to come second, and
isostatically elevated
Major transgressions of
regressions
• Those that occurred before the Quaternary
cannot be documented in the same detail
as those that occurred during it
– Sedimentary record is not complete enough
• Five ice ages in the past 900 Ma
• There have also been longer-term marine
transgressions and regressions lasting
millions of years
– These had nothing to do with Ice ages
– e.g. late Cambrian, Cretaceous
• The Cretaceous transgression coincides
with the breakup of Pangea
– Expect a larger proportion of the ocean basins
to be occupied by young, hot ridges
– The eruption of lavas of the Ontong-Java
Plateau occurred then
• High sea level of the Ordovician did not
coincide with the breakup of a
supercontinent
– May be related to an increase in spreading
rates or formation of large igneous provinces
which have disappeared