Paleooceanography and Sea-level 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 north-northwesterly 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. Deep-Sea 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)

• 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 value means that the sample contains more of the heavy isotope than the standard;

• A negative 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