evolution of the chemistry of our earth’s oceans “weathering”portion based on lecture by david...
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EVOLUTION OF THE CHEMISTRY OF OUR EARTH’S OCEANS
“Weathering”portion based on lecture by David Montgomery
EVOLUTION OF THE CHEMISTRY OF OUR EARTH’S OCEANS
HAVE THE OCEANS ALWAYS BEEN THE SAME?
WHAT MIGHT CAUSE DIFFERENCESIN OCEAN COMPOSITION
THROUGH TIME ?
“IN THE BEGINNING” OCEAN WATER PROBABLY
WAS DIFFERENT FROM PRESENT
BUT IT IS DIFFICULT TO SAY HOW DIFFERENT….
OUR RECORD REALLY IS ONLY VALID FROM THE NEOPROTEROZOIC
(IN RELATIVLY UNALTERED MATERIALS)
Conservative vs. Nonconservative Elements
ConservativeNon-reactiveLong residence time
Major ions (conservative)Na+, K+, Mg2+, Sr2+, Cl-, Br-
Chemical composition of modern seawaterMajor ions in seawater of salinity 35
Symbol Name % total (wt) mmol/kg g/kg seawater seawater
Cl- Chloride 55.29% 545.87 19.353
Na+ Sodium 30.81% 469.07 10.784
SO42- Sulfate 7.75% 28.24 2.712
Mg2+ Magnesium 3.67 52.82 1.284
Ca2+ Calcium 1.18% 10.28 0.412
K+ Potassium 1.14% 10.21 0.399
Total 99.84%
A GOOD RECORD COMES FROM PRIMARY FLUID INCLUSIONS
IN UNDEFORMED HALITE
BUT
THERE IS NO UNDEFORMED HALITEOLDER THAN THE NEOPROTEROZOIC
Secular variation in marine aragonite ooids, early marine aragonite cements, and MgSO4- bearing potash evaporites. Histogram is based on data by Sandberg (1983, 1985a, 1985b) with additional Vendian ooid data from Singh (1987). Evaporite data are based on 62 potash deposits (Zharkov, 1984; Hardie, 1990)
FIRST POSSIBLE CONTROLSARE BASED ON THE IONS COMING
FROM THE EVOLUTION OF OCEANIC AND CONTINENTAL
(granitic) IGNEOUS ROCKS
(Hardie, 1996)
Spencer and Hardie (1990) showed thatthe composition of modern seawater can beaccounted for by steady-state mixing of thetwo major contributors to ocean chemistry,river water (RW) and hydrothermal brinesfrom Mid-Ocean-Ridges (MOR)
Regional map of the Juan de Fuca Plate showing the location of the“Three Bares” and ODP sites 1026 and 1027. Dashed lines representthe basement topographic high of 2 ridges.
spring
From Wheat and Mottl,, 2000
Based on results of Alt,
Paragonite = yellow/green mineral of the mica group. Usually formed by hydrothermalalteration. The rock, here termed paragonite, is similar to talc. (NaAl 2(AlSi 10)(OH)2)
Central Atlantic, mid-ocean ridgeTeagle et al., 1998, Chem. Geol. V. 149
.
Northwest-southeast cross-section of a MOR mound (TAG). The stratigraphy of TAG-5 (borehole) has been projected onto the mound cross-section
Humphris et al., 1995
Alt et al., 1986, J.G.R. v. 91
ADDITIONALLY
We must consider surface and atmospheric processes
HOW DID WE BEGIN TO TEASE APART ALL THE SEGMENTS
OF MARINE EARTH HISTORY?
FIRST: GEOLOGISTS NOTICED THAT FOSSIL AND OTHER CARBONATE GROUPINGS CHANGED THROUGH TIME
example. Reefs were made up of bryozoan, then corals, then rudistids. Then back again to corals (different families however). Why?
WAS THE OCEAN WATER ALWAYS THE SAME?
HOW DID WE BEGIN TO TEASE APART ALL THE SEGMENTS
OF MARINE EARTH HISTORY?
FIRST: GEOLOGISTS NOTICED THAT FOSSIL AND OTHER CARBONATE GROUPINGS CHANGED THROUGH TIME
example. Reefs were made up of bryozoan, then corals, then rudistids. Then back again to corals (different families however). Why?
SECOND: WE NOTED THAT OOLITES SOMETIMES WERE RADIAL AND AT OTHER TIMES TANGENTIAL. Why?
WAS THE OCEAN WATER ALWAYS THE SAME?
HOW DID WE BEGIN TO TEASE APART ALL THE SEGMENTS
OF MARINE EARTH HISTORY?
FIRST: GEOLOGISTS NOTICED THAT FOSSIL AND OTHER CARBONATE GROUPINGS CHANGED THROUGH TIME
example. Reefs were made up of bryozoan, then corals, then rudistids. Then back again to corals (different families however). Why?
SECOND: WE NOTED THAT OOLITES SOMETIMES WERE RADIAL AND AT OTHER TIMES TANGENTIAL. Why?
THIRD: WE HAD ENOUGH SALT SAMPLES FROM DIFFERENT AGES TO EXAMINE SALT TYPES, THEIR PRIMARY FLUID AND SOLID INCLUSIONS, AND EVEN SOME ISOTOPIC COMPOSITIONS
WAS THE OCEAN WATER ALWAYS THE SAME?
High Mg/Ca ratio
produces an ARAGONITE
SEA
High Mg/Ca ratio produces an ARAGONITE SEA
Lowenstein et al., 2003
CAN WE SET UP A DEFINITIVE PROOF IN AMODERN CONTROLLED SETTING?
YES! AN ARAGONITIC CODIACIAN ALGA, GROWN IN LOWERED Mg
WATER, WILL CAUSE SLOWER GROWTH AND WEAKENED ALGAE (prone to disease etc) AND THIS COMMON ARAGONITE-
PRODUCING FORM WOULD DIE OUT IF THE Mg/Ca RATIO WOULD SHIFT PERMANENTLY
J. B. Ries, 2006J.S.R. v.76, 515-523
If there are so many variations in seawater with time, what actually makes it happen?
1. Changes in climate especially due to thegeographic redistribution of continents
2. Removal of components due to sedimentation
3. Result of increased/decreased rates of seafloor spreading with an influx/decrease of new components
4. Topographic changes leading to more or less land surface available for weathering.
Aqueous trace-metal concentrationprofiles for the modernopen ocean
OMZ=OxygenMinimumZone
Long term climate change and ocean chemistry. On million year time scales, climate is driven by theInput of CO2 to the atmosphere by plate tectonics. The atmospheric CO2 reservoir is small & wouldraise global temperatures unchecked without a chemical weathering carbon feedback. This globalMg cycle shares some similarities with the carbon cycle. Elderfield, 2011: Coggon et al. 2010
WEATHERING PROCESSES&
THE ORIGIN OF SEDIMENTSAND
THE CHANGES IN SEAWATEROVER EARTH’S HISTORY
Weathering
Weathering: the disintegration, or breakdown of rock material
Rates of weatheringRates of weathering• Climate
– Temperature and moisture characteristics
– Mechanical weathering
• Enhanced where there are frequent freeze-thaw cycles
– Chemical weathering
• Most effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weathering
• Least effective in polar regions (water is locked up as ice) and arid regions (little water)
Weathering
Rates of weatheringRates of weathering• Climate
– Temperature and moisture characteristics
– Mechanical weathering
• Enhanced where there are frequent freeze-thaw cycles
– Chemical weathering
• Most effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weathering
• Least effective in polar regions (water is locked up as ice) and arid regions (little water)
Weathering
CLIMATE/TEMPERATURE CONTROL
(Jurassic)
WEATHERING THROUGH TIME
DAVID MONTGOMERY
AlaskaAlaska
Seattle
Altiplano Amazon
DAVID MONTGOMERY
Role of Physical Weathering
1) Reduces rock material to smaller fragments that are easier to transport
2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering
Mechanical Weathering
Physical breakup• pressure release• water: freeze - thaw cycles• crystallization of salt in cracks• thermal expansion and contraction
All this increases the total surface area exposed to weathering processes.
Surface Area and Weathering
Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints
Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces
Chemical Weathering
Definition: transformation/decomposition of one mineral into another
Mineral breakdown• carbonate dissolves• primary minerals --> secondary minerals (mostlyclays)
Net loss of elements retained in the soil (leached).
• Water is the main operator:– Dissolution
• Many ionic and organic compounds dissolve in water
– Silica, K, Na, Mg, Ca, Cl, CO3, SO4
– Acid Reactions• Water + carbon dioxide <---> carbonic acid• Water + sulfur <---> sulphuric acid• H+ effective at breaking down minerals
Chemical Weathering
Chemical Weathering: breakdown as a result of chemical reactions
One example:
CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-
Bowen’sReaction
Series
GoldrichStabilitySeries
First toCrystallize
Last toCrystallize
SlowWeathering
FastWeathering
Resistance to Weathering
Solution: process by which rock is dissolved in water
• Is strongly influenced by pH and temperature• When water becomes saturated, chemicals may precipitate out
forming carbonate and evaporite deposits.• Calcium carbonate (calcite, limestone), sodium chloride (salt),
and calcium sulfate (gypsum) are particularly vulnerable to solution weathering.
Chemical Weathering
• Oxidation– Oxygen dissolved in water promotes
oxidation of sulfides, ferrous oxides, native metals POST ARCHEAN
• Organic Activity– Plant material makes H+ ions available
Chemical Weathering
• Hydration: attachment of water molecules to crystalline structure of a rock, causing expansion and weakness
• Hydrolysis: combination of hydrogen and oxygen in water with rock to form new substances
Chemical Weathering
Olivine/pyroxene to clay
+ H2CO3 (acid)
Feldspars to clay
+ H2CO3 (acid)
Quartz to quartz (!)
+ anything
Calcite to …….
nothing
+ anything
Weathering controlled by both mechanical and chemical processes.
Spheroidal weathering in the presence of water with extensive freeze and thaw. Marli Miller, Oregon
A basaltic segment of a lava layer (modern), weathered in ahumid climate. Much of the iron and magnesium in theoriginal rock has been leached out and the residualsilicate converted to clays. Late Archean weathering, witha higher CO2 content than todays atmosphere (~100X+) also may produce associated siderite (FeCO3) in the residue (brown color), as well as the iron oxides we associate with iron. (See Rye et al., 1995, Nature 378)
Biological Weathering
Can be both chemical and mechanical in nature.
• roots split rocks apart
• roots produce acids that dissolve rocks.
• burrowing animals
Combined effects of weathering
– Fracturing, disintegration caused by mechanical weathering exposes more surface area.
– Greater surface area, means more places for chemical action to occur.
– Special effects- solubility, pH of waters etc.
Flux of oxygen and acidic fluids (represented by H2SO4)in soil profiles
Chigira + Oyama, 1999
EFFECTS OF WEATHERING IN TEMPERATE, HUMIDCLIMATE ON A GRANODIORITE
Adapted from Wahlstrom, 19488
WHAT HAPPENS CHEMICALLY TO THE GRANODIORITE WITH WEATHERING