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