Transcript 1. River Input

Chapter 3. Sources and composition
of Marine Sediments
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The sediment cycle
Sources of sediment
Sediments and sea water composition
Major sediment types
Lithogenous sediments
Biogenous sediments
Non-skeletal carbonates
Hydrogenous sediments
Sedimentation rates
3.1 The Sediment Cycle
Fig.3.1 Sources, transport,
and destination of marine
sediments
3.2 Sources of Sediment- 1. River Input
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The dissolved and particulate load of rivers
Particulate load; continental shelf & slope deposits
Dissolved load; precipitates (CaCO3 and opal,
SiO2.nH2O)
Deep sea sedimentation rate = 1~20mm/kyr
Slope sedimentation rate = up to 100mm/kyr
Taking the value of 100mm/kyr for 10% of the ocean,
and a value of 5mm/kyr for the rest; the average is near
15 mm/kyr.
1. River Input
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The ocean floor covers more than twice the area of the
land.
Hence, the continents, if they are the ultimate source of
all sediment, must wear down at a rate of near 30
mm/kyr.
Another rough calculation may be based on an estimated
sediment supply from rivers of 12 km3/yr.
If we distribute this amount on the 362 million km2 of
sea floor, we get a sedimentation rate of about 30
mm/kyr, and a corresponding erosion rate of 60 mm/kyr
for the continents.
1. River Input
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Mechanical weathering dominates in high latitudes.
Chemical weathering (leaching) is favored by high
rainfall and temperatures and dominates in tropical
areas.
We live in a highly unusual period.
High mountain ranges and the powerful abrasive action
of the continental ice masses have greatly increased
mechanical erosion for the last several million years.
3.2 Sources of Sediment – 2. Glacier Input
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Fig.3.2 Distribution of ice-rafted materials in the
North Atlantic. {resent limits of drift ice (normal
and extreme ) run from around Newfoundland
toward Greenland and Iceland. During the last ice
age, this limit went form New York straight across to
Portugal. Triangles Surface samples; dashes dredge
samples; circles core samples. [H. R. Kudrass, 1973,
Meteor Forschungserg Reihe C 13:1]
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In high latitude, the immense
masses of outwash material
are brought to the shore.
Calving glaciers can
transport both fine and very
coarse material far out to sea.
Around Antarctica this type
of transport reaches about
40°S.
In the North Atlantic the
transport boundary roughly
follows the present boundary
b/w very cold and temperate
waters.
At present about 20% of the
sea floor receives at least
some ice-transported
sediments.
3.2 Sources of Sediment – 3. Input from Wind
Fig.3.3 Abundance of haze
from dust, over the Atlantic
Ocean. Numbers are % of
observations. [ G. O. S.
Arrhenius, 1963, in M. N. Hill,
Sea 3: 695]
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Wind can only move
the fine material.
The rate of dust fall
from the air can best
be measured in
snowfields and ice
cores.
Even in the Antarctic and in Greenland, far from desert sources, the rate is
quite appreciable; 0.1~1 mm/kyr.
Exactly how much dust is falling is not known.
Some estimate suggest that much or most of the deep sea clay is derived
from wind input.
Such clay accumulates at 1 mm/kyr in the North Pacific and at 2.5 mm/kyr
in the Atlantic.
3.2 Sources of Sediment – 4. Volcanic Input
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A substantial amount of
material is delivered by
volcanoes, especially
those associated with
active oceanic margins.
Volcanoes have a short
live, and single
eruptions are flash-like
events.
Therefore, ash layers
can be used for regional
stratigraphic purposes
Tephrachronology
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백두산
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Fig.3.4 Distribution of volcanic ash produced by two large
volcanic explosions in the Aegean Sea, Presumably of
Santorini. The lower ash layer marks a prehistoric event (>
25000 yrs.). The upper layer is less than 5000 yrs old; the
volcanic explosion creating it may be the one which brought
catastrophe to the Minoan culture, sine 36000 year ago. [D.
Ninkovich, B. C. Heezen, Nature London 213: 1968. Oceans,
Prentice-Hall, New Jersey]
3.3 Sediments and Seawater
Composition
3.3.1 Acid-Base Titration
 Sea water is a solution of sodium chloride (NaCl).
 Sodium and chloride make up 86% of the ions
present by weight.
 Since the major cations form strong bases, but
bicarbonate forms a weak acid, the ocean is slightly
alkaline, with pH of near 8.
 Cl- > Na+ > SO42- > Mg2+ > Ca2+> K+ > HCO3-
3.3 Sediments and Seawater Composition
Table 3.1 Comparison between seawater and river water
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The salty ocean may be understood as the
product of emission of acid gases from
volcanoes (hydrochloric, sulfuric and carbonic
acid)
and the leaching of common silicate rocks
whose minerals have the form [MeSiaAlbOc]
where Me stands for the metals, Na, K, Mg,
and Ca, and the remainder makes insoluble
silica-aluminum oxides, clay minerals.
How stable was the composition of
seawater through geologic time?
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Seawater salt in the ancient salt deposits
Their composition indicates that sea salt did not vary
much over the last 600 million years.
Paleontologic evidence certainly agrees with this.
Already in the early Paleozoic there were organisms
whose closest modern relatives have rather narrow
salt tolerances; radiolarians, corals, brachiopos,
cephalopods, echinoderms.
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On comparing the average composition of river water
with that of seawater, one notes drastic difference.
River water is a solution of calcium bicarbonate and
silicic acid, with a small admixture of the familiar
salts.
The river influx is irrelevant to the make-up of sea
salt.
But steady-state condition
3.3.2 Interstitial Water and Diagenesis
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Fine-grained sediments; porosities of 70~90%, while
sands have around 50%.
Chemical reactions; diagenesis
Redox reactions
These reactions are the more intense the more organic
carbon is present.
Stripping oxygen from dissolved nitrate and from
solid iron oxides and hydroxides
Sufate reduction
Methanogenesis
There reactions are all mediated by bacteria.
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In the process, gases are produced (carbon dioxide, ammonia,
hydrogen sulfide), as well as iron sulfide (pyrite).
Methane can react with water (low temperature and high pressure)
to make clathrates.
The massive escape of gases from organic-rich muds can produce
mud volcanoes.
Fig.3.5 Mud volcano.
Side scan record from
the Black Sea bottom.
( Side scan principle
see Fig.4.15). [Courtesy
Dr. Glunow, Moscow,
UNESCO-IMSNewsletter 61, Paris]
3.3.3 Residence Time
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For steady-state conditions, output must equal
input.
The seawater has to rid itself of all new salt
coming in, in the same proportions as they are
added.
Sinks
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Calcium carbonate; calcareous skeletons built by
organisms
Silica; opaline skeletons
Metals leave the ocean as newly formed minerals
such as authigenic clay, oxides, and sulfides and as
zeolites, also by hydrothermal alteration at the ridge
crest
Sulfur; heavy metal sulfides in anaerobic sediments
Salt; pore waters in the sediments
Residence time
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The average time a seawater component remains in
the water, before going out as sediment
T = A/r
Where A is the amount present, r is the input
Salt age for the ocean, under the assumption that the
ocean started out fresh and retained all sodium since
… near 100 million years!
3.4 Major Sediment Types
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Lithogenous, Hydrogenous, Biogenous
Fig.3.6 a, b. Familiar examples of two major types of beach sand a medium sand,
lithogenous, La Jolla b coarse sand, biogenous, Hawaii. (Photos: W.H.B.)
3.5 Lithogenous Sediments
Fig.3.7 Heavy mineral provinces of the Gulf of Mexico, based on typical mineral associations. Ⅰ East Gulf; Ⅱ
Mississippi; Ⅲ Central Texas; Ⅳ Rio Grande; ⅤMexico. carbonate particles are dominant off Mexico and
Yucatan. The heavy mineral patterns contain clues about the sources and transport paths of terrigenous sediments.
[D. K. Davies, W, R. Moore, 1970, J Sediment Petrol 40: 339]
3.6 Biogenous Sediments
Fig.3.8 Recent hemipelagic sediment from Continental Rise off NW Africa (Cape Verde Rise).
SEM photos, bar 20μm. Left Fine-silt fraction (2-6μm) predominantly composed of coccoliths (c)
and of some detrital mica (m) and quartz (q); Right coarse silt fraction, mainly detrital grains (q
quartz) and foraminifera tests and fragments (f). [Photo courtesy D. Futterer]
3.7 Nonskeletal Carbonates
Fig.3.9. Calcareous oolites, produced
within tidal zone by algal activity.
Upper Air photo of oolite sand bars,
Bahama Banks. Note tidal
channels.[Photo courtesy D. L.
Eicher].
Lower SEM photos of ooids, scale bar
5μm; left slightly etched section.
Three secondarily filled borings (b)
intersect the concentric laminae of
primary oolite coating; right close-up
of oolite laminae, showing acicular
aragonite needles. [Photos courtesy D.
Futterer]
Fig.3.10 Present-day dolomite
formation. Lagoon on the
southern coast of the
Persian/Arabian Gulf (a), floor
covered with calcareous mud.
Intertidal zone (b) and algal
mats of uppermost littoral
(c) rim the lagoon.
Intermittently flooded evaporite
flats (d, sabkhas) follow
landward [After L. V, Illing et al
1965, Soc Econ Paleontol
Mineral Spec Publ 13:89,
Simplified]
Hydrogenous Sediment - Marine evaporites
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Form on evaporation of seawater
Where?
Coastal lagoon
Salt seas on the shelves
Early rift oceans in the deep sea
How much salt can be produced by evaporating a
1000m-high column of seawater?
Salt constitutes 3.5% of the weight of the column, its
density is about 2.5 times that of water.
About 14m of salt
Marine Evaporites
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The least soluble salts precipitates first: calcium
carbonate (aragonite) and calcium sulphate
(gypsum)
To precipitate halite, the brine needs to be
concentrated about 10 fold
Many evaporites only contain carbonate and
gypsum (or anhydrite), others have thick
deposits of halite or of the valuable (and very
soluble) potassium salts.
3.8 Hydrogenous Sediments
Fig.3.11 Possible models for marine evaporite formation. a Serial fractionation in very shallow and extended
basins. Saturation of different salts is reached in a series ocean to land. Terrigenous particles may be supplied
from land. Recent example: Adshi-darja Lagoon attached to the Caspian Sea by Kara Bogaz Inlet (chemical
conditions there are not fully comparable with open sea). b Serial fractionation and differential preservation
in deeper basins divided by sills. Saturation of differents salt is reached in a series shallow to deep water.
Derail Only gypsum is precipitated near the sill. Halite saturation is not reached, because brine sinks down to
the basin escaping further evaporation. Sill depths can be considerably reduced by carbonate and /or gypsum
precipitation. No Recent example known. [ G. Richter-Bernburg, 1955, Dtsch Geol Ges 105 [4]: 59]
Differential preservation and serial fractionation are the
key processes in controlling the chemistry of salt
deposits.
Fig. 3.12 Model of
concentric serial
fractionation in the
uppermost Miocene
underneath the Western
Mediterranean. G Gibraltar
Region, where the
connection with the Atlantic
was closed for half a million
years; M Mallorca; C
Corsica; S Sardinia [K. J.
Hsu et al., 1973 in Ryan, W.
B. F. et al. eds. Initial Repts.
DSDP 13, 695, Washington
D. C]
Phosphorites
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Phosphorites deserve special attention for the
reason of this tie-in to ocean fertility, but also
because of their economic value
Marine phosphorites vary in composition
A general formula is Ca10(PO4,CO3)6F2-3, with
increases in carbonates going parallel to increase
in fluoride
Phosphorites
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Modern phosphorites typically occur in areas of high
productivity.
Occurs as nodules or crusts up to head size and as
irregularly shaped cakes
In the geologic record, they occur as replacement of
previously deposited carbonates, or as mineralization
of pre-existing organic matter
The common depth of deposition is on the shelf and
upper slope.
Phosphorites
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The association of geologically young phosphorites deposits with
present-day regions of upwelling suggests that the source of the
phosphorous is organic matter.
Apparently the algae growing in these regions, in the surface
waters, extract the phosphorous from the water, and crustacesn and
fish concentrate it further in their bodies and excrement.
During decomposition of organic debris, much phosphate is
released to the interstitial water (and also to seawater)
Thus, interstitial waters may become saturated with the phophate
mineral apatite.
Precipitation of apatite replacement of pre-existing carbonate
mineral, and impregnation of sediment can then proceed.
Iron Compounds
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Abundant in both oceanic margin sediments and in the
deep ea since iron is one of the most abundant elements
on Earth
On the slopes, the high supply of organic matter
commonly leads to oxygen deficiency, and to sulfate
reduction by bacteria, in the uppermost sediment layers.
This process results in H2S formation, and in the
precipitation of iron sulfide (pyrite).
In the deep sea, oxygen is generally plentiful, and
essentially all iron occurs in its oxidized form, as ironoxide/hydroxide (goethite)
Iron Compounds
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An iron-mineral which has been much studies is glauconite
It is greenish silicate common in shallow marine areas.
Chemically it is a poorly crystallized mica, rich in potassium
(7-8%) and in iron (20-25%)
Apparently the association with decaying organic matter
(fecal pellets, interior of shells) is a necessary condition of
growth: part of the iron in glauconite is reduced iron.
A high concentration in interstitial waters (at conditions
intermediate between reduction of iron oxide and
precipitation of sulfide) appears to be favorable for glauconite
formation, as is the presence of the right kind of clay to
convert into the glauconite mica.
3.9 Sedimentation Rates
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High rates at the edges of the
continents, especially in
estuaries and marginal basins
with river influx
The lower rates in abyssal
regions
C. slopes = 40-200mm/kyr
Deep sea = 1-20 mm/kyr
Coral reef = 10m/kyr
Fig.3.13 Rates of vertical crustal motion, of
denudation, and of sedimentation rates. Scale
in mm/1000 years. also called "Bubnoffs"
(B), of m/million years. Postglacial sea level
rise for comparison: 100 m in 5000 yrs. Note
interaction with recently deglaciated land
(arrow left) and with coral growth rates
(arrows right).
Fig.3.14 Annual layers (varves) in the sediment of a bay in the Adriatic Sea (Mljet Island). The photo shows light and dark laminae.
One light-dark pair corresponds to one year. The lower boundary of light layers are generally sharp; they are due to precipitation of
carbonate by phytoplanktom, which bloom in early summer (record to the right) as temperature rises. In fall and winter, rains bring
terrigenous matter which- together with organic detritus- provides for dark colors. the interpretation of the varves is a complicated
matter: recent studies use statistical procedures to reconstruct climatic conditions in detail.(photo E. S.)