The Biogeochemistry of Soils: Soils from Stars

•Composition of soils on earth is arguably unexpected

•Soils, and Earth, not reflective of chemistry of Universe

•Soils reflect chemical fractionation processes since beginning of universe:

–Big Bang–Subsequent star formation/collapse–Chemical differentiation during formation of solar system–Chemical differentiation during formation of Earth–Late cometary additions to Earth

Chemistry of Solar System

•Exponential decline in abundance w/ atomic number (number of protons)

•Sawtooth pattern

•Elements from Fe have passed through stars

•Solar system is dominantly H and He -6

-4

-2

0

2

4

6

0 20 40 60 80 100

H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

Si

Cl

Ar

K

Ca

Sc

Ti

V

CrMn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

Rb

Sr

Y

Zr

Nb

MoRu

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

In

Xe

Cs

Ba

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

Ta

W

Re

OsIrPt

QuHgTl

Pb

Bi

Th

U

log (solar system mass fraction)

atomic number

Crust vs. Solar System

•Depleted in volatiles (as other inner planets)•Noble gases (group VIIIA)•H, C, N

•Core formation depleted crust in siderophile elements (group VIIIB..)

•Crust also reflects late stage cometary additions of light elements, etc. including water

-8

-6

-4

-2

0

2

4

6

0 20 40 60 80 100

H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

AlSi

P

S

Cl

Ar

K

CaScTi

V

Cr

MnFe

Co

Ni

CuZn

Ga

Ge

As

Se

Br

Kr

RbSrY

ZrNb

Mo

RuRhPd

AgCdIn

SnSb

Te

I

Xe

Cs

BaLaCePrNdSmEuGdTbDyHoErTmYbLu

HfTa

W

Re

Os

Ir

Pt

Au

Hg

TlPb

Bi

ThU

log (crust/solar system)

atomic number

period 2 period 3 period 4 period 5 period 6

lanthanides actinides

crust depleted

crust enriched

Soil vs. Crust

•Soil enriched in biochemically impt elements (C, N, S, Se)

•Soil depleted in alkali and alkine earths, Si, ….

•Date normalized to a relatively immobile element (Zr)

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 20 40 60 80 100

Li

Be

B

C

N

O

FNa

Mg

AlSiP

S

Cl

KCaSc

TiVCrMnFe

Co

Ni

CuZnGa

Ge

As

Se

Br

Rb

Sr

YZr

Nb

MoAg

Cd

In

Sn

Sb

I

CsBa

La

CePrNd

SmEuGdTbDy

HoEr

TmYbLu

HfTaW

Au

Hg

Tl

PbBi

ThU

log(soil

Zr

/crust

Zr)

atomic number

soil enriched

soil depleted

Methods of (reasons for) Normalization to Index Element

100 kg @ t=0 80 kg @ t=X

1) Mineralogical composition of parent material and soil

Parent material Soil49 kg quartz (SiO) 49 kg quartz50 kg Ca silicate (CaSiO) 30 kg Ca silicate1 kg zircon (ZrSiO) 1 kg zircon

2) Molecular weights of elements and minerals

Molecular Wts (g/mole) Formula WtsSi=28.1 SiO=44.1O=16.0 CaSiO=84.2Ca=40.1 ZrSiO=135.3Zr=91.2

3) Sum of elemental mass in parent material and soils

Σ Σiparent material = (31.2)quartz+(16.7)Ca silicate+(0.2)zircon= 48.1Σ Σisoil = (31.2)quartz+(10.0)Ca silicate+(0.2)zircon= 41.4

Σ Caparent material = (23.8)Ca silicateΣ Casoill = (14.3)Ca silicate

Σ Zrparent material/soil = (.7)zirl

Original land surface

20% weatheringloss

Parentmaterial

Soil

4) Concentrations in parent material and soil

Parent material Soil Enrich/Deplete

Si = .481 .518 enrichCa = .238 .179 depleteZr=.007 .009 enrich

5) Elemental ratios w/ and w/o normalization

No normalization Ratio Zr Normalization RatioCasoil/Caparent material .75 (Ca/Zr)soil/(Ca/Zr)pm .60Sisoil/Siparent material 1.08 (Si/Zr)soil/(Si/Zr)pm .86

Weathering Losses of Elements from Soils

•As might be expected, water enriched relative to crust via chemical reactions

•Relative concentration related to chemical nature of elements and their reactivity in water and type of bonds they form in crust

-1

0

1

2

3

4

5

6

0 20 40 60 80 100

LiBe

B

C

N

FNaMg

Al

SiP

S

Cl

Ar

K

Ca

ScTi

V

Cr

MnFeCoNi

CuZn

Ga

As

Se

Br

Rb

Sr

ZrNb

Mo

Ag

Cd

Sn

Sb

I

Cs

Ba

LaCe

SmEu

Tb Yb

Lu

HfTa

W

AuHg

PbBi

Ra

Th

Ulog(water

Zr

/crust

Zr)

atomic number

Plant Composition and Soil Chemistry

Plants reflect water chemistry (with some selectivity) and photosynthesis/N fixation

-4

-2

0

2

4

6

0 20 40 60 80 100

H

Li

Be

B

CN

FNa

Mg

Al

Si

P

SCl

KCa

Sc

TiV

CrMn

Fe

Co

Ni

CuZn

Ga

AsSe

Br

Rb

Sr

YZr

MoAgCd

Sn

Sb

I

Cs

Ba

La

Ce

Sm

Eu

TbDy

YbLu

W

Au

Hg

TlPbBi

Th

U

log(plant

Zr

/crust

Zr)

atomic number

Soil Biogeochemistry Highlights

•Biological group•Alkali/alkaline earths•Halogens•Rare earths•Ti group•Si, Al, Fe, P

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 20 40 60 80 100

Li

Na

K

RbCs

Be

MgCa

SrBa

F

Cl

BrI

CePrNd

SmEuGdTbDg

HoEm

TmYbLu

TiZr

Hf

C

N

S

SiPAl

FeThUlog (soil

Zr

/crust

Zr)

atomic number

Soil Mineralogy: Primary Minerals

•Minerals are associations of elements

•Mineralogical composition a function of elemental behavior and abundances

–O 474,000 mg/kg–Si 277,000–Al 82,000–Fe 41,000–Ca 41,000–Na 23,000–Mg 23,000–K 21,000

•Relative abundance and behavior leads to reality that soils are dominated by aluminosilicates (O,Si, Al).

Structure of Silicates

•Silica tetrahedron–Net charge–Role of Al

•Covalent bonds (Si-O, Al-O) vs. ionic bonds (cations-O)–Bond type based on electronegativity differences and tendency to attract electrons

•Big differences lead to ionic bonds•Similar electronegativities lead to covalent bonds

•Linage of tetrahedra dictate classes of silicates and their chemical behavior

– Nesosilicates–Inosilicates–Phyllosilicates–Tectosilicates

Electronegativities of the Elements

•Electonegativities dictated by position on table: elements with outer shells almost filled highly electonegative, those just starting new shell not. •Si-O form mainly covalent bond

The Silica Tetrahedron

•1 Si, 4 O = -4 net charge

•Tetrahedra can be linked by sharing O, thereby reducing net negative charge.

•Class of silicate is determined by number of shared O, and need for cations to neutralize net negative charge

Nesosilicates: Singe Tetrahedra Linked with Cations

Foresterite

•Single tetra linked with Mg+2

•Other minerals in group have all Fe+2

•Highly susceptable to chemical weathering via ejection of cations by acid (H+)•Products then form secondary silicates and oxides

Inosilicates: Chains

Diopside:•Single chains

Tremolite:•Double chains

Phyllosilicates: SheetsMuscovite•‘dioctahedral w/ Al+3

Phlogopite•‘trioctahedral’ w/ Mg+2

•K+ strongly adsorbed in cavities

Tectosilicates: Framework

Anorthite (Ca)•50% Al for Si substition

Albite (Na)•25% Al substition

Quartz•No substition/O charge

Primary Silicate Summary

SilicateClassifiication

TetrahedronArrangement

Examples Chemical Formula ofSpeicific Minerals

(+)chargeper 100Oxygen

MeltingTemperature(C)

Nesosilicates independenttetrahedra

olivine series (foresterite)Mg2SiO4 100 1890 (1)

(fayalite)Fe2SiO4 100 1205 (1)Inosilicates single chains pyroxene

group(augite)Ca(Mg,Fe,Al)(Al,Si)2O6

66 (2) ~1200 (1)

double chains amphibolegroup

(hornblende)NaCa2(Mg,Fe,Al)5(Si,Al)8

O22(OH)2

55(1)

Phyllosilicates sheets mica group (biotite)(Mg,Fe)3(AlSi3O10)(OH)2

80 ~1100 (1)

(muscovite)KAl2(AlSi3O10)(OH)2

80 ~980(3)

Tectosilicates framework plagioclasegroup

(anorthite) CaAl2Si2O8 100 1550 (1)

(albite) NaAlSi3O8 50 1100 (1)feldspar group (orthoclase) KalSi3O8 50 1150 (1)silica group (quartz) SiO2 0 867 (1)

(1) Data from W.A. Deer, R.A. Howie, and J. Zussman, An Introductionto the Rock Forming Minerals,Longman Group, Ltd., London (1966)(2) Value for endmember with no Al substitution for Si. Value will decrease in proportion to added Al.(3) From ranges reported in: D.S. Fanning, V.A. Keramidas, and M.A. El-Desoky, Micas. Chap. 12 in:J.B. Dixon and S.B. Weed (eds), Minerals in Soil Environments, 2nd Ed., Soil Science Society of America,Madison, WI (1989).

Mineralogical Composition of Igneous Rocks

Stability of Primary Minerals in Soils

•Increasing Si/O ratio increases stability–More covalent bonds–Fewer ionic bonds–Less susceptable to acids

•Decreasing Si/Al ratio reduces stability–Al creates charge imbalance and need for cations

•Presence of Fe+2 reduces stability–Fe+2 oxidizes to +3

–Size and charge altered and Fe is expelled