Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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EROSION AND SEDIMENT YIELD
255
material develops into a soil. Transport limited settings
typically have flat to gentle topography. If soils are deep and
permeable such that even intense rains generate little surface
runoff,
even steeper slopes may be transport limited. If erosion
is weathering limited and if the rate limiting step is chemical
weathering, then the rates and chemistry of chemical and
physical erosion products can be linked in models (Stallard
1995a,b),
Several rules of thumb can be applied to sediments produced
during this more classical erosion. Where physical erosion and
transport predominate, sediments are rich in primary minerals
with only a small contribution from weathering products. The
greater the role of chemical weathering the more depleted the
sediments are in alkalis (sodium, potassium, etc) and alkali-
earth (magnesium, calcium, etc). The most intense chemical
weathering leaves quartz, kaolinite, and aluminum and iron
sesquioxides. Chemical weathering is greatest under a warm,
wet climate and transport-limited erosion.
Episodic erosion and the role of organisms
Many aspects of the breakdown of parent material, its
processing within soil and its protection from immediate
erosion are influenced by a huge suite of organisms (Stallard,
2000),
The supply of loose material to physical transport is
often strongly mediated by biological processes. The top of the
soil profile is often reworked and churned by an ensemble of
biological processes. Included are the actions of soil fauna, the
growth and decay of roots, the toppling of trees, and in
agricultural lands, plowing. Depths of this zone range from a
few centimeters to a few meters. Moreover, animal burrowing
and the formation and decay of roots increases the perme-
ability of this zone, increasing infiltration of intense rain and
reducing surficial runoff and physical transport, Surficial
erosion processes, such as, raindrop impact, sheet wash, and
rilling detach material not anchored by vegetation (roots and
fungal mycorrhizae) and biotic crusts (lichens, mosses,
liverworts) and coverings (litter, duff), moving it downslope.
Roots and fallen trunks can dam surficial runoff promoting its
infiltration and reducing its erosiveness.
Major, often catastrophic erosional events are often
associated with the development and collapse of plant and
animal communities, often defining what is often referred to as
a "disturbance regime," The growth and failure of vegetation
as a soil anchor is related to dramatic episodic erosion when
the anchoring fails for a variety of reasons. Two types of
episodic erosion stand out: post-fire erosion (fire/flood events)
and soil avalanches (landslides). Of these, post-fire erosion
appears to have the shortest repeat times. Post-fire erosion
requires an intense forest fire that is capable of burning away
the organic duff or litter and the root network that protects the
soil. The intense heat of the fire may also drive hydrophobic
organic compounds into the soil as these materials burn. The
heat also drys the soil making it hydrophobic. Whatever the
immediate cause of this hydrophobicity, the frequent observa-
tion if that if an intense rain follows a major fire by a year or
two,
typically the soil fails to soak up the rain, which instead
flows overland producing dramatic sheet erosion, rilling, and
channel excavation (McNabb and Swanson, 1990), So much
erosion occurs that major debris flows form. Much of the
eroded material is temporarily stored in alluvial fans and
within low gradient streams (Meyer etal., 1995), Regrowing
vegetation eventually anchors material on the hillslope, but
it often takes several hundred years before enough burnable
plant material (fuels) accumulates for a fire to be sufficiently
hot to repeat the process. During the time between fires,
chemical and physical breakdown of bedrock produces the
soils and other loose material that will erode in a subsequent
fire.
Soil avalanehing is remarkably similar in its requirement
that vegetation anchor a developing soil. In contrast to fire
erosion, soil avalanehing requires a prolonged and continuous
protection of soil such that surficial erosion does not balance
the production of loose material, and the soil profile thickens.
During this time, the erosion regime appears to be transport
limited. Rooting depths are limited, and eventually roots are
predominately in less cohesive parts of the soil profile. If the
slope is sufficiently steep, and if there is a triggering event that
cause the soil to shear internally, such as a major rainfall or an
earthquake, then a soil avalanche forms, and a mass of
hillslope is hurtles toward the valley bottom, where it is in turn,
eroded by fluvial processes. Subsequently, for a number of
years,
surficial erosion removes more loose material from the
scar. The slide scar eventually revegetates and soil profiles
develop and thicken once more (Scott, 1975; Stallard, 1985),
Because a critical accumulation of soil is require, typically a
meter or more, the repeat time for landslides is much longer
than for major fires.
In setting where episodic erosion operates, it contributes
much of the sediment delivered to rivers, often well in excess of
surficial physical erosion by sheet wash and rilling. In
protected forests of eastern Puerto Rico, landslides may
contribute 80 percent to 90 percent of the sediment in rivers.
In parts of Yellowstone National Park, fire-related erosion
may contribute half of the sediment. Sediments generated by
episodic erosion are derived from the entire loose soil profile.
Typically, this material has lost a larger proportion of sodium
and calcium-bearing minerals than potassium and magnesium
bearing minerals.
Human activities
Human activities are responsible for mobilizing vast quantities
of material for physical erasion. Humans accelerate erosion by
removing plant cover and altering hillslope profiles, A feature
that distinguishes this erosion from fire-related erosion or soil
avalanehing is the human erosion is almost never in
equilibrium with the processes that generate loose material,
and the erosion proceeds by degrading soils or exposing to
transport processes vulnerable substrates that had long been
protected by vegetation. Many rural land-use practices
promote or accelerate erosional processes that already operate
on a landscape. In the case of landslides and debris flow, this
phenomena has cause great loss of life and property. In 1998
more than 10,000 people died during Hurricane Mitch and in
1999,
15,000 to 30,000 people died following rains along the
north coast of Venezuela, Many have argued that fire related
erosion has accelerated in the American West because a
century of fire suppression has allowed for sufficient buildup of
fuels that eventual fires are so intense that all biomass burns
leaving especially unprotected and vulnerable hydrophobic
soils,
Stratigraphic data from deposits in parts ofthe American
West indicate that fire-related erosion was operating at similar
rates long before European settlement. In terms of eroded
mass,
the most intense human-related erosion appears to result
from the destruction of natural ground cover on vulnerable
256
EROSION AND SEDIMENT YIELD
substrates such as loess terrains (Yellow River, central
Mississippi Valley) and deeply weathered tropical soils.
The accelerated erosion of uplands by human activities has
not translated to a major export of sediment to the ocean
(Milliman and Mead, 1983), Instead much of the sediment
appears to be trapped as colluvium, alluvium, and lacustrine
deposits. The construction of dams worldwide, about 70,000 in
the United States alone has promoted lacustrine deposition.
The storage of carbo in these deposits may be a signiticant
component in today's carbon cycles (Stallard, 1998),
Role of storage and remobilization
The highest rates of physical erosion typically are measured on
hillslopes in steep upland catchments. Frequently, because
flows are less energetic on less steep slopes near valley bottoms,
loose material is deposited as colluvium. Then, as streams
leave uplands and gradients become less steep and flows less
Hashy, sediment is deposited in channel systems as alluvium,
Colluvial, alluvial, and lacustrine sedimentation can intercept
and store sediment coming from upland regions over long,
even geologic, timeseales. For example, vast quantities of
sediment is stored in alluvial fans and foreland basins, A large
fraction of the sediment generated by human-induced physical
erosion is stored as colluvium, alluvium, and lacustrine
sediment in lakes and reservoirs.
Chemical weathering in alluvium can completely change the
composition of sediment migrating downriver. For example,
sands crossing the Llanos alluvial plane in Venezuela change
from lithic arenites to quartz arenites (Stallard, etal., 1991),
Glacial erosion
During the past approximately two million years, the Earth
has experienced episodic glaciations with repeat times of a bit
more than 100,000 years. Associated with these ice ages has
been the development of large continental glaciers, the largest
being the Laurentide Ice Sheet, along with global cooling and
drying. The flow of glacial ice is often associated with
profound physical erosion, particularly where glaciers have
warm bases, where liquid water exists at the glacial bed. While
exact mechanisms are uncertain, freezing and thawing near
the glacial bed and perhaps more narrow shear zones appear
to enhance the breakdown of bedrock. Cold-based glaciers,
where the base is frozen, can be remarkably nonerosive,
sometimes preserving pre-glacial soils. Marine cores indicate
that the continental ice sheets were quite erosive, lowing the
Canadian Shield, for example, more than would chemical and
physical weathering operating over the same period of time.
The chemical weathering associated with glaciation is quite
distinctive, reflecting a high ratio of fresh rock to water. Of
note is the large contribution from easily weathered minerals
that are disseminated through most siliceous rocks. These
include carbonates (dissolved calcium), sulfides (dissolved
sulfate), and micas (dissolved potassium). The exposure of
large amounts of biotite to water and oxidative breakdown
produces and releases large amounts of vermiculites to soils
and sediments (Stallard, 2000),
Ice ages are times of exceptional wind erosion and
deposition. The wind-borne dust forms great deposits of loess
around the peripheries of the glacierized landscapes. Ice Age
samples from deep ice cores from Greenland show far more
dust than was deposited after the Ice Ages ended.
The study of erosion and sediment production is one of the
most interdisciplinary fields of investigation in the Earth
sciences. As such it still presents many barely-explored
frontiers for researchers. Of particular note is; (1) the role of
organisms, especially plants, in the erosion process; (2) the
chemical and physical changes of sediments deposited on land;
and (3) the details of erosion under continental ice sheets. An
understanding of these issues is critical for the reconstructing
sedimentary record of the history of the Earth,
Robert F, Stallard
Bibliography
Carson, M,A,, and Kirkby, M,J,, 1972, Hillslope. Eorm and Process.
Cambridge University t^ress,
Garrels, R,M,, and Mackenzie, F,T,, 1971, Evolution of Sedimeniary
Rocks. New York: W,W, Norton,
McNabb, D,H,, and Swanson, F,J,, 1990, Effects of fire on soil
erosion, tn Walstad, J,D,, Radosevich, S,L,, and Sandberg, D,V,
(eds,).
Natural and
Prescribed Eire in
the
Pacific
Northwest. Corvallis,
Oregon: Oregon State University Press, pp, 159-176,
Meyer, G,A,, Wells, S,G,, and Jull, A,J,T,, 1995, Fire and alluvial
chronology in Yellowstone National Park: climatic and intrinsic
controls on Holocene geomorphic processes.
Geological
Society of
America Bulletin, 107: 1211-1230,
Milliman, J,D,, and Meade, R,H,, 1983, World-wide delivery of river
sediment to the oceans, Journalof Geology, 91: 1-21,
Moody, J,A,, and Martin, D,A,, 2001, Initial hydrologic geomorphic
response following a wildfire in the Colorado front range. Earth
Surface
Processes
and Landforms, 26: 1049-1070,
Scott, G,A,J,, 1975, Relationships between vegetation cover and soil
avalanehing in Hawaii, Proceedings ofthe Association of American
Geographers, 7: 208-212,
Stallard, R,F,, 1985, River chemistry, geology, geomorphology, and
soils in the Amazon and Orinoco basins. In Drever, J,I, (ed,).
The Chemistry of Weathering. Dordrecht, Holland: D, Reidel,
pp,
293-316,
Stallard, R,F,, 1988, Weathering and erosion in the humid tropics. In
Lerman, A,, and Meybeck, M,, (ed,). Physical and Chemical
Weath-
ering
in
Geoehemical
Cycles.
Dordrecht, Holland: Kluwer, pp, 225-
246,
Stallard, R,F,, 1995a, Relating chemical and physical erosion. In
White, A,F,, and Brantley, S,L, (eds,). Chemical
Weathering
Rates
of Silicate Minerals. Washington, DC: Mineralogical Society of
America, pp, 543-564,
Stallard, R,F,, 1995b, Tectonic, environmental, and human aspects of
weathering and erosion: a global review using a steady-
state perspective. Annual Review of Earth and Planetary Sciences,
12:
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Stallard, R,F,, 1998, Terrestrial sedimentation and the carbon cycle:
coupling weathering and erosion to carbon burial. Global Biogeo-
ehemical Cycles, 12: 231-252,
Stallard, R,F,, 2000, Tectonic processes and erosion. In Jacobson,
M,C, etal. (eds,). Earth System Seienee:
Erom
Biogeoehemieal Cycles
to Global Change. San Diego: Academic Press, pp, 195-229,
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processes and the composition of inorganic material transported
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Geoderma, 51: 133-165,
Cross-references
Climatic Control of Sedimentation
Cyclic Sedimentation
Debris Flow
Earth Flows
Mass Movement
Sands, Gravels and their Lithified Equivalents
Tectonic Controls of Sedimentation
Weathering, Soils, and Paleosols
EVAPOKITES
257
EVAPORITES
Introduction
Evaporites
are
sedimentary deposits chemically precipitated
from slanding bodies
of
seawater (maritie evaporites)
or
lake-
waters (nonmaritie evaporites)
as a
result
of
evaporative
concentration.
The
major minerals
of
evaporites
are
salts
of
sodium, potassititn, calcitim. magnesium, chloride, sulfate,
bicarbonate
and
carbonate ions (Table
El).
Evaporite deposits
are found
in
stratigraphic units
of all
ages back
to the
Late
Precambrian, Indirect evidence
of
evaporite deposition
is
found
in the
ibtTii
of
trtolds
and
casts
of
saline minerals such
as hahte
and
gypsum
in
rocks
as old as
Archean.
The
scientific
importance
of
evaporites lies mainly
in the
information they
carry iibout:
(1) the
chernistry
of
ancient seawaters
and
lakcwaters;
(2) the
distribution
in
space
and
titne
of
hydro-
logically-restricted seditnetitary basins with arid climates;
and
(3)
the
early stages
of
continental breakup
and the
fortnation
of "proto-oceans"' when thiek
and
extensive saline deposits
accumulated (e.g,.
the
huge thieknesses
of
Jurassic
and
Cretaceous salt deposits preserved
in
initial rift valleys beneath
the continental margins
of
both
the
North
and
South Atlantic
Oceans).
Evaporite deposits
are of
major economic importance,
mined
on all
continents
as
basic materials
for the
manufacture
of inorganic chetnicals used extensively
in the
chemical,
agricultural
and
construction industries. Principal mining
targets
are
halite (NaCI, common table salt), gypsum
(CaSO4 2H2O, used
to
make •'drywall"). potash salts {such
as sylvite.
KCI.
known
as
"muriate
of
potash"
in
industry),
sodium carbonate salts (such
as
trona. NaHCOyNa^CO.-*
-2H2O from which "soda
ash"'
used
in the
glass, paper
and
pulp industries
is
made),
and
sodium sulfale salts (such
as
mirabilite. Na^SOa-lOHiO from which "salt cake"
or
"Glauber's salt"*
is
made). Modern surface
and
subsurface
saline brines
(the
latter
are
known
as
basinai brines
or
oilfield
brines), both primary
and
secondary
(the
latter formed
by
dissolution
of
buried evaporites).
are
major sources
of
itnportant minor chemical elements such
as
bromine, iodine,
lithium,
etc.
Evaporite deposits also play
a
significant role
as
impermeable seals
of
petroleum reservoirs. Despite
the
fact
!hat evaporites constitute less than
5
percent ofthe volume
of
seditnentary rocks
in the
geologic record, they
cap
about half
the world's known
oil and t'as
resources.
Common table salt
has
been
a
critical trading commodity
for tnillenia. Roman soldiers were given money
to buy
salt,
a
payment that became known
as a
salariutn. from which
the
English word "salary""
was
derived.
As
early
as 500 B,C. the
Roman government took over control
of
salt works near
the
River Tiber, Three hundred years later
the
Roman governtnent
imposed ta,xes
on
salt
to
raise money
to
finance
the
Second
Punic
War.
Medie\al Erance copied this
old
Rotnan practice
of salt tnonopolies
and
taxes.
The
leaders
of the
French
Revolution tnade
the
repea!
of the
salt
tax a
major objective,
which they achieved
in 1790
(Multhauf,
R.P..
1978).
Environmental conditions required
for the
formation
of
evaporite deposits
First
and
foremost, evaporites require
for
their formation:
(1)
an
aridelinutte, that
is,
annual rate
of
evaporation >annual
rate
of
inflow
of
water, where inllow
=
surface sources
(seawater and/or river water
+
rainfall
+
surface spring
in-
flow) + subsurface input
of
groundwater:
and (2) a
hydrotogi-
cally closed or re.striclecJ ha.sin (outllow<inflow). These
two
basic
conditions will ensure that
any
standing water
on the
tloor
of
the deposiiionai basin will uitdergo
the
evaporative concentra-
tion necessary
to
induce evaporile minerals
to
precipitate.
However,
for
accumulation
of any
measurable thieknesses
of
evaporites there
are two
additional requiretnents;
(3) the
closed
arid basin must
be
either
a
deep receptacle before evaporite
accumulation
or
must actively subside during evaporite
accumulation:
and (4) the
inflow
of
solutes into
the
basin
tnust
be
substantial over
a
long period
of
titne.
If seawater
is the
pritnary inflow water then
the
evaporating
basin tnust
be a
coastal depression (shallow
or
deep) with
a
constricted, very shallow inlet channel
or a
"leaky"" sill
of
some
kind. Such
a
hydrologic system must
be
fundamentally
dependent
on the
position
of sea
level.
The
absence
of any
major tnarine evaporite depocenters today
is
testimony
to
this
particular constraint.
The
rising Holoeene
sea
level
has
overrun
all the
preexisting restricted marine basins
in
arid
settings, such
as the Red Sea and the
Mediterranean
Sea,
During
the
Miocene these
two
basins were essentially isolated
from direct surface inflow
of
seawater
as sea
level dropped
to
or below
the
shallow sills
at
their "tnouths"'. During this
period
of
basin restriction, very thick (km-scale) deposits
of
tnarine evaporites accumulated
on the
floors
of the Red and
Mediterranean Seas,
as has
been revealed
by
drilling.
Table
El
Chemiial composition
of
some
of the
common saline
ot'
evaporites
Anhydrite
Antarctieite
Aragonite
Bassanite
Bischolile
Biocdite
Calcite
Carnallite
Dolomite
Epsomite
Glaiiberite
Gypsum
Halite
ILvKahvdrite
CaS04
CaCl.
CaCO,
CaSO4
MgCl:
Na-iSO
CaCOi
MgCN
MgCO
MgSO,^
CaSO4
CaSO4
NaCI
MgSO,.
6H:O
•
I/2H2O
6H:d
4-
MgSO4f)
KCI
6H.0
, CaCO,
r7H.O
•
Na.SO4
-2H:O
,-6H-O
kainite
kieserite
laiigheinite
mirabilile
naheolite
natron
polyhalite
rinneite
short ite
sylvite
tachyhydrite
thenardite
thertnonatrite
trona
MgSO4-KCl-ll/4H:O
Na-,SO4 IDH.O
NaHCO.
Na.COvlOH.O
)4 2H2O
FeCl. NaCI 3KCI
2CaC0,
•
Na.CO,
KCI
CaCl. 2MgC!: I2H:O
Na^CO,
H^O
NaHCOi Na.CO,
2H.O
258
RVAPORITI-S
For notitnaritie basitis. the iiillow waters may be meteoric
waters (dilute river waters or groundwaters). upwelling basinai
brines, upwelling hydrothenrtal brines, upwelling aeid volca-
nogenic waters, or any combination of these source waters. Of
particular significance is that many modern nonmarine
evaporite deposits are accumulating in active continental rift
and strike-slip basins. The floors of these tectonic basins arc
fed by groundwaters convected upward along fault and
fracture zones by thertnaliy-induced density instabilities (e,g,,
generated by magmatic heat sources beneath the rifts) or
forced to the surfaee by large topographic gradients resulting
from the high relief between the basin floor and the
surrounding uplifted fault-block tnountains.
Types of evaporite deposits
Marine evaporites (formed by evaporation of seawater).
There arc two tnajor groups, relaled lo their tectonic setting:
(1) Shaltow.stielf marine evaporiies deposited along the coasts
of eontinental shelves (e,g,, modern Lake MacLeod, Western
Australia) or foreland basin ratnps (e.g., modern Persian Gulf)
in arid settings. Stratigraphically. these evaporites are typically
made up of stacks of thin (tiieter seale). sheet-like bodies
deposited in baek-barrier tidal flats and shallow lagoons.
The tidal flat (or sabkha, an Arabic term for salt flat) tnarine
evaporites are supratidal deposits that cap subtidal-iiuertidal
marine detrital deposits (silicielastic or carbonate composi-
tion),
producing a classic "shallowing-upward cycle". Each
cycle is formed by seaward progradation of the shoreline, with
the vertical thickness dictated by the tidal range and the depth
ofthe subtidal lagoon (typically a few tneters).
The lagoonal evaporites are subtidal deposits thai consist
principally of layers of subaqueous gypsum and halite crystal
accumulations of: (1) "pelagic" crystals whieh nucleated at the
brine-air interfaee then settled gravitationaliy to the lagoon
floor: and (2) arrays of vertically-oriented crystals grown on
the lagoon floor. These gypsum-halite units may be punctuated
by thin interbeds of marine detrital sediment that record
"freshening" events when new seawater entered the lagoon.
(2) Rift ha.sin marine eyapvriles deposited in active teetonic
pull-apart basins Ibrmed during the early stages of continental
breakup and development of oeean basins by seafloor spread-
ing (e.g.. Cretaceous of Brazil and Gabon, proto-Atlantic
Ocean: Miocene beneath the Red Sea). These evaporites are
eotntnonly very thick (10"m to lO'm) deposits, fringed by
alluvial fan deposits. The type of evaporite depositional
environment ean vary from tidal flat to lagoonal to deep
perennial basin. This latter setting is eharaeterized by the
deposition of "pelagic"" gypsum and halite precipitated from
the evaporating surface brines and by incursions of gypsiferous
turbidity flows derived from the "gypsum factories'" on the
shallow fringing shelves.
Nonmarine evaporites (formed by evaporation of
continental waters)
Ancient nonmarine evaporite deposits arc uncommon. The
Eocene Green River Fotmation of Wyoming, a major souree
of sodium carbonate and oil shale, is one ofthe best known of
the nonmarine type of evaporite deposit, renowned for its huge
inventory of unusual minerals and spectacular speeitnens of
fossil plants, flsh. insects, birds, turtles, crocodilians, lizards,
snakes, bats, and a host of invertebrates.
Mtieh of what we know about deposition of nonmarine
evaporites eomes from the study of active modern systems.
The vast majority of such depositional settings are hydro-
logieally closed basins fonned by continental rifting (e.g.. Lake
Magadi. East African rift) or strike-slip faulting (e,g.. Dead
Sea),
Importantly, the high relief of the upfaulted mountains
enelosing these two types of eontinental teetonie basins can act
as rainfall snowfall traps and barriers, making the basin floors
local rain shadow deserts independent of latitude ("oro-
graphie deserts"". e,g,. the modern Basque Lakes evaporites
are located in western Canada at latitude 50^'N). For those
tectonic basins outside of the dry "horse-latitudes" the
atmospheric precipitation trapped by the enclosing high
mountains provides a signilicant source of inflow waters
(e,g,. modern Death Valley. California: modern Great .Salt
Lake. L'tah), Sueh basins are capable of aeeutnulating very
thick nonmarine evaporite deposits.
Chemically and mineralogieally. non-marine evaporites are
far more varied than marine evaporites beeause of the very
wide range of ehemica! compositions displayed by eontinental
waters (Table E2), Nonetheless, non-marine evaporites can be
separated into two major chetnical groups, an alkaline group
and a neutral group.
The alkaline group of cotnposition Na-K-CO^-SOj-CI are
characterized by sodiutn carbonate minerals such as trona.
Table E2 Representative (hemicjl compositions of inflciw waters to modern evnpnrite basins (concentrations in
p.irts
per million)
SiOi
Ca
Mg
Na
K
HCO,
SO4
a
TDS
pH
1
15,0
13,0
4,?
8,4
3,5
72,0
6.9
3,K
148,0
7.0
2
49,0
13,0
9,0
6,6
2,8
88,0
4,9
6.9
IS9,0
7,7
3
1.1
43,0
1.2
1,5
0.7
133.0
3.2
2.2
199.0
7.3
4
10,4
13,4
3.4
5,2
1,3
52,0
8.3
5.8
99,6
—
5
369.0
6.5
0.0
243.0
61,0
0.0
454.0
408.0
1.570,0
2,5
6
<50
32.800
6,440
51,500
2,460
0
140
160,000
256.000
7
400
28.000
54
50.400
17.500
500
5
155,000
258,000
5,2
8
805
0
11.725
1.009
0
0
20,530
34,069
413
1.296
10.800
407
137
2.717
19.010
35.000
8,1
I, GrouiK|y.ater Igranue. White elal..
THWC I.#7):
2, GriHin(.tw;ucr (basalt. White
i-lal..
Table 2. #8):
1<.
Grounilwuler (limestone. White flai.. Table ft, #2);
4,
"Average" modern river wiuer iSpenuer iintt Hurdie. tWO. Table I. #9): 5, Acid voluinic hot spring (White elal.. Table 1'). #l|; 6, Basinai brine (While eial..
Table I.', #7): 7. Coniinental hydrothcrmal brine (Salton Sea,
Calif,.
Hardie. 1*591, Table 6|; 8. Mid-ocean ridge hydrolhcrniiil brine (Spcneer and Hardie. IWCl,
Table I, #4); 4 ^ Modern >eawater (Spencer and Hardie. l')90, Titble I. #1),
EVAPORITfS 259
+1 -
0 -
Wi
o
polyhalite consumed
(solution no loDger
GYPSUM
I ANHYDRITE
HALITE
GLAUBERITE
POLYHALITE
HP-HX-KS
CARNALLITE
BiSCHOFITE
cr
Figure E2 Evaporative concentration of modern seawater at 25'C and t atm. pressure simul<ited using the computer program of Harvie vt dl.
(19801,
The curves show the progressive changes in major ion concentrations in the seawater brine as evaporation and mineral precipitation
and resorplion proceed (initial mass of seawdter contained 1 kg of HjO) (from Hardie, 1990).
naheolite and natron (Table Fl) precipitated fVom high pH
alkaline brines. The parent waters of this alkaline group are
river waters and shallow continental groutidwaters, which are
classic bicarbonate-rich meteoric waters formed by chenitcal
weathering of coniinental bedrocks by CO^-charged rainwater.
Evaporative concentration of these bicarbonate-rich meteoric
waters produces alkaline brines rich in sodium and carbonate
ions and poor in calcium and magnesium ions {see next section
on brine evolution).
The neutral group of cotnposition Na-K-Ca-Mg-S04-CI are
characterized by gypsum, halite, and a variety of sodium and
magnesium suifate minerals, among others (Table El),
precipitated from near-neittral pH sulfate-chloride brines.
The chemical evolution of these bicarbonate/carbonate-poor
brines during evaporative concentration pivots around the
early precipitation of gypsum, which detertnines whether the
brines will becotne sulfate-poor or calcium-poor (the so-called
gypsum "chemical divide", see next section on brine evolu-
tion).
This leads to two main chemical subgroups, an Na-K-
Mg-Ca-Ct subgroup ("CaCh evaporites", characterized by
potash tninerals such as sylvite and rare CaCI^ minerals such
as lachyhydritc. Table El) and an Na-K-Mg-S04-Cl subgroup
(••MgSO4-NaSO4 evaporites". characterized by minerals such
as epsomite, thenardite and glauberite. Table El). These
subgroups owe their chemical signatures to the influenee of
unusual bicarbonate-poor inflow waters, such as S04-rich acid
260
FVAPORITES
volcanogenic waters, SO4-rich meteoric waters derived by
weathering of sulfide ore-bearing bedrocks, or CaCU-rich
basinai brines and hydrothermal brines (Table E2),
Brine evolution and evaporite mineral sequences:
geoehemical principles
The reluctance of sparsely soluble minerals to preeipitate from
aqueous solutions at Earth surface tetnperatures is well-
known. Marine carbonates, so common in the sedimentary
record, are classic examples of non-equilibrium precipitation
of such sparsely soluble minerals. Modern marine cements are
typically aragonite and high-inagnesian calcite (HMC). miner-
als that are theoretically metastable at Earth surface tempera-
tures and pressures with respect to calcitc and mixtures of
dolomite + calcite respectively. Experimental work has shown
that the Mg/Ca ratio, the tetnperature and the salinity ofthe
parent waler determines whether calcitc. HMC + aragonite. or
aragoniie will precipitate, in agreement with the observation
that metastabie aragonite + HMC are the phases that prefer-
entially nucleate from modern seawater with its high Mg/Ca
mole ratio of 5.1. In contrast to all this, thermodynamics
predicts that the only stable carbonate phase in equilibrium
with modern seawater should be ordered stoichiometric
dolomite. However, experimental work has failed to nucleate
ordered stoichiometric dolotnite in typical surface waters such
as modern seawater at Earth surfaee temperatures, supporting
the long recognized absence of ordered dolomite as the
primary sedimentary precipitate in bolh modern and ancient
carbonate deposits (the "dolomite problem"). Clearly, kinetics
rather than equilibrium thertnodynamies controls the impor-
tant alkaline earth carbonate system in tnodern and ancient
sedimentary systetns. including evaporites.
While the kinetically-controlled alkaline earth carbonates
are the typical minerals thai precipitate during the early stages
of evaporative concentration of natural waters, evaporite
deposits are characterized by the more soluble salts such as
gypsum and halite that for the most part behave in accordance
with thermodynatnic predictions. Using a combination of
equilibriutn thermodynamics and experimentally detennined
solubilities of evaporite minerals, it is possible to predict
quantitatively: (1) the chemical evolution of waters undergoing
evaporative concentration at a given tetnperature and I attn.
pressure; and (2) the sequence of minerals (and their masses)
precipitated during this evolution (Harvie and Weare. 1980;
Harvie et at., 1980). Figure E2 illustrates this approach as
applied to evaporative concentration of tnodern seawater (see
Table E2 for composition of modern seawater). Figure E2
clearly highlights the critical steps in the chemical evolution of
modern seawater as it undergoes progressive evaporative
concentration. These critical steps occur at those points where
new minerals Iirst precipitate. The progressive crystallization
ofa new mineral causes drastic changes in the concentrations
in the brine of those ionic species making up that mineral.
Halite provides a simple but powerful example (Figure E2). As
the evaporating brine iirst reaches saturation with halite, the
equilibriutn between halite and brine Is defined by the chemical
reaction:
-NaCI,
lEc],
hcicnt) ofthe components are related as follows:
J —
JNUCI/'IN^,''^ti'
(tq^ -)
where K;i is the equilibrium constant for the T and P of
interest. Since halite is a pure crystalline phase the activity of
the component NaCI in halite is unity by definition. Thus, as
long as equilibrium is maintained between halite and the brine
during evaporative concentration, the product ofthe activities
of Na and Cl ions in the brine is fixed even though these ions
are being extracted from the brine to produce halite crystals. It
follows, then, that during halite precipitation, as Na and Cl
ions are removed in equat motar proportions while the ion
activity product (aM^i^aci) remains constant, the concentrations
of Na and Cl ions must ehatige antipattietieatty. Typically, as
evaporative concentration proceeds the ion with the lower
concentration at the initial point of mineral precipitation will
progessively decrease in concentration while that of the other
ion will progessively increase (but at a much reduced rate).
This type of critical step in brine evolution driven by
evaporative concentration (see Hardie and Eugster, 1970) bas
been dubbed a "ctiemicat divide" by Drever (1982. pp. 201-
209).
This behavior is clearly illustrated in Figure E2 for both
halite and gypsum. Experimental work and observations of
natural saline lakes and comtnercial salt pans have verified this
predicted behavior.
The significance of the principle of "chemical divides" is
that it explains the origin of the following three major
compositional types of brines (Hardie and Eugster, 1970):
(1) Na-K-Ca-Mg-CI brines ("calcium chloride brines", e.g.,,
hydrothermal brines, basina! brines)
(2) Na-K-Mg-SO4-Cl brines ("magnesium sulfate brines",
e.g,. modern seawater bitterns)
(3) Na-K-C0i-S04-CI brines ("alkaline brines", e.g,. soda
lake brines).
As illustrated in Figure E3. the initial precipitation of
CaCOi (as either calcite or aragonite) during the very early
stages of evaporative concentration of either seawater or
lakewaters determines whether the resulting brine will become
Evaporative Concentration
\ TM
Precipitation caC03
01
CaC03 I Divide
HCO3-
MgSO4 Brine
Na-K-Mg
Ci-SO4
CaCl2 Brine
Na-K-Mg-Ca
Ci
At equilibrium between halite and the brine, the activities
("thermodynamic concentrations" = molality x activity
coef-
Figure E3 "Chemical divides" and the chemical evolution of the three
major types of brines during evdpurative concentration of surface
waters (after Hardie and Eugster, 1970).
FVAPORITFS
261
an "alkaline brine" (rich in HCO^-^COi ions, poor in Ca
ions) or a "neutral brine" (poor in HCO^ + COi ions, rich in
Ca ions). The outcome is determined by the relative
concentrations of Ca and HCOi ions at the initial point of
precipitation of CaCOv if Ca>HCOi then the resulting brine
will be a "neutral brine", if HC^O3>Ca then the brine will be an
"alkaline brine" (Step I in Figure E3), Further evaporation of
the alkaline brine will lead to remo\al of essentially all the Ca
ions while the CO? ion concentration and the pH of the brine
both increase, ultimately producing an Na-K-COrSO4-Cl type
brine. For those waters initially richer in Ca than HCO3 ions
(Table E2), evaporation beyond the "CaCOi divide" leads to a
second evolutionary step, the "gypsum divide" (Step 2 in
Figure F.3). The outcome at this "gypsum divide" is
determined by the relative concentrations of Ca and SO4 ions:
if Ca^'SO4 then the resulting brine will be a "CaCN brine"
(Na-K-Ca-Mg-Cl type), if SO4 > Ca then the brine will be an
"MgS04 brine" (Na-K-Mg-SOj-Cl type) (Figure F.'^).
The exact pathways followed on evaporative concentration
of a given water belonging to any one of the three tnain types
depends mainly on the particular ion ratios at each chemical
divide but other factors that can alter the chemical evolution of
ihc evaporating brine may come inlo play. In this latter tegard.
two of the more important laciors are: (1) nucleation of
metastable instead of stable minerals, e.g.. precipitation of
magnesian calcitc instead of calcite will inliuence the ultimate
Mg/Ca ratio ofthe brine: (2) equilibrium brine evolution may
depend on certain early formed minerals lhat hack-reaei with
the chetTiically evolving brine, i.e.. the reaction
2CaSO4
predicted for modern seawater. In this exatnpte. if the early
fortned gypsutn becotnes separated frorn the mother-liquor by
settle-out or by brine migration then both the chemical
evolution of the brine and the tnineral suecession will be
signtftcantly different from those tnvolvtng back-reaction
(Hardie, 1984, figure E4),
Evaporites and the chemistry of
phanerozoic seawater
Ancient "potash evaporites", rich in economically important
potassium minerals, have for more than a century been
assumed to be of tnarine origin, fonned by evaporation of
seawater with the chemistry of tnodern ocean water. Indeed.
J.H, van't
Hoff,
the first Noble Prize winner for Chemistry
spent the later years of his career (1896 1908) experimentally
investigating the phase relations among the saline minerals in
the system Na-K-Ca-Mg'SO4-Cl-H2O at temperatures trom (J
to liO'C at atmospheric pressure. His objective was to better
understand the origin ofthe economically important Stassfurt
potash evaporites of Germany. His extensive series of studies
(carried out with the aid of 33 collaborators) constitutes the
first systematic application of chemical experiments to the
solution of a geological problem, and gave birth to modern
experimental geochemistry (Eugster, 1971).
Unfortunately, van"t Hoff s pioneering experimental efforts
were unable to resolve the fundamental dilemma presented by
the stratigraphic record of evaporite deposits, namely, that the
majority of ancient potash evaporites thought to be of marine
SO^
•
K2SO4
•
2H2
550 500 450 400 350 300 250 200 t50 100 50
Time (Ma)
Predicted
MgSO4
KCI
•] KCI ll-lMgso*
Observed
(see Table
b)
KCI KCI
"8SO4
Figure E4 Plot of MOR/RW t'lux ratio versus geologic time over the
past ,550m.y, using the new cakuldlion molhod of Hardie
(2000},
The
horizont,il line .il MOK/RW = 1,45 is the chemical divide that
^epar,ites |).ik'osfawnters from which either KCI or Mg,SO4 evaporites
will tnrm on eva[)orative concentration. Comparison ofthe predicted
and observed potash evaporite types is shown in the bars below ihe
graph.
origin are of the "KCl-typc" (formed from CaCN brines of
the system Na-K-Ca-Mg-Cl, see above) rather than the
"MgS04-type" (formed from brines of the system Na-K-Mg-
SO4-CI, lo which modern seawaler belongs) (Hardie, 1990.
Tables I and 2), If the major ion chemistry of seawaler has
remained close to its present composition for the last
1-2 billion years, as has been assumed by almost all 20th
Century geologists and paleontologists as well as aqueous
geochemists (e.g.. Rubcy. 1951: Mackenzie and Garrels. 1966:
Sillen. 1967: Holland, 1972), then how are we to explain Ihe
preponderance of KCI over MgSO4 marine potash evaporites
in the Phanerozoic record? A variety of explanations have
been proposed, mainly involving penecontemporaneous or
later burial diagenesis of primary MgSO4 potash evaporites,
contempotaneous alteration of seawater entering a marginal
marine basin by dolomitization of early formed CaCOi
deposits, by the activity of sulfate-reducing bacteria, or by
mixing of seawater with Ca-HCOrrich ri\cr waters, A radical
solutioti to this dilctnma has been suggested by Spencer and
Hardie (1990), They proposed that the chemistry of global
seauater over much of geologic time has been controlled
principally by simple mixing of the two principal sources of
major ions to the ocean, namely, river water (RW) and mid-
ocean ridge (MOR) hydrothermal brine. They presented
quantitative predictions of secular variations in the major
ion chemistry of seawaler as a function of hypothetical secular
variations in the ratio of the input lluxes of MOR brine and
river water. Their calculations indicated that relatively stnall
changes in the MOR/RW flux ratio would result in seawater
chemistry switching from the Na-K-Mg-S04-Cl type (the
modern seawater type, from which MgS04 potash evaporites
would precipitate) to the Na-K-Ca-Mg-Cl type (IVom which
KCI potash evaporites would precipitate), Hardie (1996, 2()0()).
262 EVA PO RITES
ARAGONITE +
HIGH-Mg CALCITE
550 500 450 400 350 300 250 200 150 100 50 0
Time (Ma)
Predicted
Aragonite H ^
AregonHe
Calcite
Calcite
Stta
Observed (Sandberg.
1983.
1985)
Calcite Sea ^Aragonite Sea :<i| f Calcite Sea
^^ Ar«gonlt»
Aragonlte
Figure E5 Plot of predictttl oscilkilions in Mg/Ca mole ratio in seawater over the past 550 m,y, using the new tiikuiation method ot Hardie
(2000), The horizontal hne at Mg/Ca = 2,00 is the divide that separates waters from which either calcite or aragonite i +high-Mgl are predicted to
precipitate (Stanley and Hardie, 1998, 1999). Comparison of the predicted and observed mineralogies of nonskeletal limestones is shown in the
bars below Ihe graph.
using the SpHJticer-Hardie seawater approach and the assump-
tion lhat secular variations in the MOR brine flux were
propoi'lional to secular variations in ocean floor production,
showed that the temporal reeords of oscillations during the
Phanerozoic in KCI versus MgS04 marine potash t-vaporites,
and in "aragonite seas" versus "calcite seas'" delertnined by
Sandberg
(1983,
1985) are in very good agreement with those
predicted by this simple '"mixing model"' (see Figures E4
and E5. which are based on the improved mixing model of
Hardie. 2000). Furthermore, the predictions are in full accord
with the observed synehroneity of KCI potash evaporites with
calcite seas and MgS04 potash evaporites with afagonite seas.
Chemical analyzes of fluid inclusions in ancient halite crystals
from evaporite deposits ranging in age from Late Precambrian
to Late Mioeene earried out by a number of workers using
several different teehniques are also in very good agreement
with the predicted oscillations in global seawater chemistry
based on the MOR brine-river water mixing model (e.g,.
Lowenstein el at.. 2001), Finally. Stanley atid'hlardie (1998,
1999) have drawn attention to the possible consequences of
oscillations in the Mg/Ca ratio and the Ca concentrations of
Phanero7oic seawaters on the carbonate mineralogies of
invertebrate skeletons. For exampie. the very low Mg/Ca ratio
and very high Ca concentration predicted for Cretaceous
seawater (compared to modern seawater) provide a plausible
explanation for the spectacular production of coeeolith chalk
during this period. The progessive increase in Mg/Ca ratio and
decrease in Ca concentration predicted for the period
Paleocene to Quaternary can explain the observed progressive
loss of eoccolith mass and the flnal demise of the Discoaster
coeeolithoporids during this period (Stanley and Hardie. 1998.
1999).
The combined weight of all these results provides strong
support for the hypothesis that seawater chemistry has
oscillated significantly during the Phanerozoic Eon as a result
of oscillations in the MOR/RW Hux ratio driven mainly by
secular variations in rates of sealloor spreading.
Lawrence A. Hardie and Tim K. Lowenstein
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Lakes: Ctieniistrv. Geoto^v. Ptiysies. New York: Springer-
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Hardie. L,A,. 1984, Evaporiies: marine or nonmarine? Ameriean
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Hardie. L,A,, 2000, A new method of predicting paleoseawater
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°C,
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Sonnenfeld, P,, 1984, Brines and
Evaporites.
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Warren, J,, 1999, Evaporites: Their Evolution and Eeonomies. Blackwell
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Cross-references
Anhydrite and Gypsum
Seawater: Temporal Changes to the Major Solutes
EXTRATERRESTRIAL MATERIAL IN SEDIMENTS
Ultimately, the entire Earth is composed of "extraterrestrial"
material, which accreted (mostly in the form of kilometer-sized
and larger bodies) about 4,56 billion years ago to form our
planet. But even today, the Earth continues to accrete large
amounts of extraterrestrial matter, about 40,000 tons per year
of mostly fine dust (within the mass range
10~'
*
kg to 10"'' kg),
which is derived from asteroids, comets, and even from the
Moon, Mars (as rare meteorites), and the interstellar medium,
although proportions vary widely. This extraterrestrial matter
settles through the atmosphere and accumulates, much diluted,
in terrestrial and marine sediments, comprising one class of
extraterrestrial material in sediments. On occasion, larger
bodies (cm- to m-size) fall as meteorites, which mostly weather
away in geologically short time periods, although a few fossil
meteorites have been found in Ordovician rocks (Peucker-
Ehrenbrink and Schmitz, 2001), Even less frequent, but still
often in geological terms, asteroid- and comet-nucleus-sized
bodje? impact the Earth with cosmic velocity, leading to the
forrnation of an impact crater, and, depending on the size of
the impaetor, a regional to global distribution of impact ejecta,
being the other class of extraterrestrial material in sediments.
Extraterrestrial dust is mainly derived from the collision of
asteroids in the asteroid belt and the subsequent migration of
the dust due to the Poynting-Robertson effect toward the
Earth and Sun, The other main source is the disintegration of
cornets ("dusty snowballs"), which release dust into the inner
solar system when they get close to the sun and the ice, which
makes up the bulk of their mass, evaporates. The fiux
from both sources varies considerably over time (Peucker-
Ehrenbrink and Schmitz, 2001), An excellent tracer for
extraterrestrial material in marine sediments is the presence
of 'He, which is carried to the seafioor in the finest fraction of
interplanetary dust and is retained in some sediments for up to
several hundred million years (Farley, 2001), Such isotopic
studies of oceanic sediments provided evidence for a shower of
long-period comets at around 35 Ma,
Ejecta from large impact events on Earth comprise a
mixture of uniquely shocked and melted terrestrial material
and remnants of the extraterrestrial impaetor (Montanari and
Koeberl, 2000; Koeberl, 2001), The discovery of shock-
metamorphosed mineral grains (e,g,, quartz, feldspar) in
sediments of Cretaceous-Tertiary (K-T) boundary age
(65 Ma) from locations around the world (Bohor etal., 1987)
provided confirming evidence for the hypothesis that a large
impact event marked the end of the Cretaceous and was
responsible for the mass extinction at that time (Koeberl and
MacLeod, 2002), Impact craters with diameters of about
>80km may be associated with globally distributed impact
ejecta; the Chiexulub impact strueture, which is buried beneath
Tertiary sediments in Yucatan, Mexico, and which is the
source of the K-T boundary ejecta, has a diameter of almost
200
km,
264 EXTRArERRESTRIAL MATERIAL
IN
SEDIMENTS
Geoehemical methods can be employed to detect the
chemical signature ofthe remnant ofthe impaetor in sediments
(Koeberl, 1998), This is mainly done by searching for
anomalously high concentrations of siderophile elements,
especially the platinum-group elements (PGEs—e,g,, Ir, Os),
because these elements typically show low concentrations in
crustal rocks and sediments, and much higher abundances (by
several orders of magnitude) in most meteorites. Most often
contents of the element Ir are reported, because it is the easiest
of all PGEs to measure, Crustal Ir contents are about
0,02 ppb, whereas impact ejecta commonly have 0,2 ppb to
10 ppb. Also, the isotopic compositions of the elements Os
(Koeberl, 1998) and Cr (Shukolyukov et al., 2000) show
significant differences between terrestrial crustal rocks and
meteorites, providing very sensitive and selective tracers for
meteoritie matter. Impact ejecta are found from the beginning
of the geological history of the Earth as recorded in
sedimentary rocks, although the evidence for traces of the
so-called late heavy bombardment at 3,85 Ga is still contra-
dictory (Ryder et al., 2000), Rather unusual impact ejecta
are found in 3,4 Ga rocks from Barberton, South Africa
(Shukolyukov et al., 2000), Geoehemical evidence for an
extraterrestrial component was found in 2Ga spherule layers
from the Transvaal Supergroup, South Africa (Simonson etal.,
2000), which are similar to approximately coeval spherule
layers in Australia,
Christian Koeberl
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Cross-reference
Sediments Produced
by
Impact