Terrestrial Biogeochemistry
The study of the biological, geological and chemical factors that
control the distribution and abundance of elements on land
Introduction - Chapter 1
Studying the earth system is complicated
--large spatial and temporal scales
--feedback effects
--no true replication
…but we do the best we can
--plot-scale experimental work
--regional scale studies
--modeling to extrapolate in space and time
--remote sensing of major earth processes
Introduction - Chapter 1
Fluctuations are the KEY to most element cycles
Finzi Teaches First Biogeochemistry Course
Finzi Graduates High School
Finzi Born
Source: Luthi et al. (2008) Nature 453:379-382
Source: Pearson and Palmer (2000). Nature 406:695-699
QUIZ
1.
What links obesity in America to annual fish kills in the Gulf of Mexico?
2.
What do the deserts of Asia and the wet tropical forests of Hawaii share
in common?
3.
If you wanted to reduce the concentration CO2 in the Earth’s
atmosphere, what the best long-term storage reservoir?
Origins - Chapter 2
Six elements constitute 95% of mass of biosphere (i.e.
living organisms)
--C, H, N, O, P and S
20 Other elements are critical to life
All elements have mass < iodine (atomic mass 53)
--Life is driven by “light” elements
Origins - Chapter 2
Origins of the Elements
What is the distribution of elements in our solar system?
(1) Except for Li, Be, and B, light elements (atomic number <30) are
more abundant than heavy elements;
(2) Elements with even AN are more abundant than odd AM
(Figure 2.1)
How did these elements form?
“Big Bang” ~13.7 Billion Years Ago (BYA)
Fusion of “quarks” into protons (1H) and neutrons
And…fusion of protons and neutrons to form simple atoms: 2H, 4He,
Origins - Chapter 2
Origins of the Elements
What is the distribution of elements in our solar system?
(1) Except for Li, Be, and B, light elements (atomic number <30) are
more abundant than heavy elements;
(2) Elements with even AN are more abundant than odd AM
(Figure 2.1)
How did these elements form?
“Big Bang” ~13.7 Billion Years Ago (BYA)
Fusion of “quarks” into protons (1H) and neutrons
And…fusion of protons and neutrons to form simple atoms: 2H, 4He,
Origins - Chapter 2
But…temperature and pressure ↓ declined rapidly
…formation of heavier elements could not occur
……until the formation of stars (>1 BY)
Stars: whirling clouds of gas and dust
2H + 2H → 4He
Core: Hydrogen “burning”
As star ages, H is consumed, star collapses inward under own
gravity…
Origins - Chapter 2
Collapse ↑ core temperature and pressure
Resulting in He burning…which generates carbon!
+ 4He ↔ 8Be (unstable, rapid decay)
8Be + 4He → 12C
4He
But also…
+ 12C → 16O OXYGEN!
&
12C + 12 C → 24Mg MAGNESIUM!
4He
Which can decay to…
24Mg → 20Ne + 4He (alpha particle)
Origins - Chapter 2
Planetary “formation” model explains elements up to AM
of Fe (= 55.9)
… but a star core dominated by Fe won’t burn, causing a
supernova (catastrophic collapse and explosion)
Elements w/AM > Fe formed by capture of successive
neutrons by Fe which requires LOTS of energy
National Radio Astronomy Observatory produced
this series of images showing SN1993J as it expands
to a diameter of 1/10th of a light year in 18 months
Origins - Chapter 2
Collectively this model explains:
1.
Logarithmic ↓ in abundance of elements after H and He (original building
blocks)
--3 Phases
Big Bang (H, He)
Core Burning (C-Fe)
Supernova – Fusion (AM>Fe=56)
2.
Even AM elements are more abundant than odd AM elements
-- Formation of all elements beyond Li is based on fusion of nuclei with even
number of atomic mass
--Odd AM elements formed by fission of heavier elements and they are less
stable
16O
+ 16O → 32S → 31P + 1H
Origins - Chapter 2
Why are Li, Be and B in such low cosmic abundance?
Initial fusion reactions pass over nuclei of AM 5 and skip to elements with
even AM>8;
Li, B, Be are formed by spallation—fission of heavier elements that are hit
by cosmic rays in interstellar space!
Origins - Chapter 2
Origin of the Solar System and the Earth
Our galaxy is ~12.5 billion years old
Our solar system is ~4.6 billion years
-- the remnants of a supernova;
-- all material is derived from “planetesimals” formed by
the coalescence of dust and small bodies;
-- each planet is unique because it is derived from different
portions of the solar nebula.
Inner Planets
Sun
Mercury
Venus
Earth
Mars
Mercury
Outer Planets
Jupiter
Saturn
Uranus
Neptune
Pluto
‘cool’ portions of solar nebula
‘hot’ portions of solar nebula
Origins - Chapter 2
“Inner” planets formed in “hot” areas of solar nebula;
“Outer” plants formed in “cool” areas of the solar
nebula.
Mercury (“inner”) dominated by Fe and other elements formed at
high temp; high bulk density 5.4g cm-3
VS.
Jupiter (“outer”) dominated H and He (BD = 1.25 g cm-3) w/overall
composition same as cosmic abundance of elements
Earth (“inner”) dominated by silicate materials due to intermediate
temperatures (BD = 5.5 g cm-3); low abundance of light elements
relative to solar abundance.
Other <1%
Aluminum 1.1%
Calcium 1.1%
Sulfur 1.9%
Nickel 2.4%
100
90
Magnesium 13%
80
Silicon 15%
70
Y Data
Percent
60
Oxygen 30%
50
40
30
Iron 35%
20
10
0
Relative abundance of
elements in whole earth
What is the elemental composition of the earth?
--Analysis of total vs. crustal composition
suggests differentiation
Other <1%
Aluminum 1.1%
Calcium 1.1%
Sulfur 1.9%
Nickel 2.4%
100
Other <1%
Sodium 2.1%
Potassium 2.3%
Calcium 2.4%
Magnesium 4%
90
Magnesium 13%
Iron 6%
80
Aluminum 8%
Silicon 15%
70
Silicon 28%
Y Data
Percent
60
Oxygen 30%
50
40
30
Oxygen 46%
Iron 35%
20
10
0
Relative abundance of
elements in whole earth
Relative abundance of
elements in earth's crust
Origins - Chapter 2
What is the distribution of elements on the earth?
--Analysis of crustal composition vs. total elemental composition
suggests differentiation (Figure 2.3)
Theories:
Homogenous Accretion:
1. All elements arrived early in formation (~100MY);
2. NRG—collision of planetesimals and radioactive decay—melts
Fe, Ni etc…and form magma;
3. Density separation of elements
- Heavy core, light crust;
4. As earth cools lighter elements solidify on surface (Figure 2.4)
Origins - Chapter 2
Heterogeneous Accretion:
1. Planetesimals and other materials not consistent through earths
formation.
--Core constituents arrived earlier than mantle
2. Late arrival of light elements in carbonaceous chondrites
Which theory is correct? Well, not mutually exclusive… but late veneer of light
elements seems probable
20Ne
can provide some clues…
-Noble gas, no reaction w/crust
-Too heavy to leave atmosphere
-Not product of radioactive decay
(…no transformation)
(…no losses)
(…no/low rate of new input)
…abundance of 20Ne in atm. today ≈ abundance in solar cloud
Origins - Chapter 2
Assume other elements were delivered simultaneously, then
Mass of Element Z =
on Earth
Qty of Z in Solar Cloud
x
Qty of 20Ne in Solar Cloud
Qty 20Ne on Earth
Consider the 14N:
Ratio of 14N/20Ne in Solar Cloud = 0.91
Mass of 20Ne on Earth = 6.5 x 1016g
Predict:
5.9 x 1016g N
Observe:
39 x 1020g N
…suggesting a heterogeneous accumulation of N on Earth!
A tentative chronology of the Earth’s accretion.
F Albarède Nature 461, 1227-1233 (2009) doi:10.1038/nature08477
Chronometers shown in brown. Accretion of planetary material was interrupted by energetic electromagnetic radiation (T
Tauri phase) sweeping across the disk within a few Myr of the isolation of the solar nebula. Runaway growth of
planetesimals produces Mars-sized planetary embryos, which, collision after collision, form the planets with their modern
masses. The last of these 'giant' collisions left material orbiting the Earth that later reassembled to form the Moon. The
182Hf–182W chronometer dates metal–silicate separation. The identical abundance of radiogenic 182W between the Earth
and the Moon indicates that either the Moon formed after all the short-lived 182Hf had disappeared (>60 Myr) or, rather, the
Moon-forming impact and terrestrial core segregation took place simultaneously 30 Myr after isolation of the solar nebula.
Addition of a late veneer of chondritic material coming from beyond 2.5 au provides a strong explanation for the modern
abundances of siderophile and volatile elements in the terrestrial mantle. This material also contained water and other
volatile elements, which account for the origin of the terrestrial ocean. Such a model indicates that most of the terrestrial Pb
and Xe was delivered by the asteroids that constituted the late veneer, and therefore that the young Pb–Pb and I–Xe ages
of the Earth date, not the Earth, but events that affected the asteroids. It is suggested here that these events are those of
the accretion to the Earth of the late veneer.
Origins - Chapter 2
Origins of Atmosphere and Oceans
Atmosphere:
Carbonaceous chondrites major sources of C and N
0.5 – 3.6 % Carbon
0.01 – 0.28% Nitrogen
Comets likely sources of C,N, H, O and other volatiles
Meteors inputs in 1st billion years accounts for earths mass
Early Atmospheric Composition
--Most volatiles degassed from rocks delivered to mantle
--Volcanoes emit light elements  degassing ongoing
--H2O, CH4, SO2, HCl, CO2, N2
Origins - Chapter 2
Oceans:
When Earth hot (>100 oC) volatiles in atmosphere.
As Earth’s surface cooled (<100 oC)….water started to condense and form the
OCEANS
…one
hell
of a rainstorm…
Liquid water ever since 3.8 BYA.
Atmospheric gases enter into primitive ocean (Henry’s Law):
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3HCl + H2O ↔ H3O+ + ClSO2 + H2O ↔ H2SO3
Origins - Chapter 2
Gas solubility  early on atm. dominated by N2
CO2
HCl
SO2
1.4 g/L
700 g/L
94.1 g/L
VS.
N2
Early composition of ocean is difficult to know…
--substantial qty. of Cl- (like today)
--dissolution of CO2 & HCl produce acids and “weather” crust
releasing cations (Na+, Mg2+, Ca2+)
--oceans accumulate cations until formation of precipitates
CaCO3 dominant marine sediment for BYs
…suggesting that Earth’s early oceans similar to that today.
0.018 g/L
Origins - Chapter 2
Origin of Life
1.
Reduced atm  abiotic synthesis of OC;
2.
Carbonaceous chondrites contain simple
OC and AA;
3.
Clay minerals—surface charge and
repeating structure—“string” together
OC……RNA……proteins…abiotically;
4.
Polarity of organics forms coacervates in
H20…simple membranes;
5.
In lab, organic molecules self
replicate…although replicating,
metabolizing, membrane bound structures
not yet produced ……but given billions of
years…
6.
Science 9 January 2009, Vol. 323. no.
5911, pp. 198 – 199, DOI:
10.1126/science.323.5911.198
Apparatus used in the original
Miller (1957) experiment
Origins - Chapter 2
Origin of Metabolic Pathways
3.8 BY old rocks contain fossil bacteria…oldest known life…evolved in the sea
Early heterotrophs…(today’s methanogens?)
CH3COOH →
2CO2 + CH4
(Acetate)
(Methane)
N is a key component of biochemistry yet little availability N (e.g. NO3-) in
seawater
Origin of N2 fixation 2.2-3.5 BYA
N2 + 8H+ + 8e- + 16ATP → 2NH3 + H2 + 16ADP + 16Pi
-- N≡N requires 226kcal/mol energy to break triple bond
-- low solubility of N2 in H2O
Today N fixation coupled to photosynthesis  Cynaobacteria (marine)
Symbiosis w/plants (terrestrial)
Origins - Chapter 2
...early heterotrophy inefficient given abiotic source of organic cmpds
Strong selective pressure for autotrophy…
1st Photosynthesis S-based (lower NRG of reaction, but limited S in oceans)
sunlight
CO2 + 2H2S → CH20 + 2S + H20
…probably soon thereafter, O2-based PS
sunlight
CO2 + H2O → CH20 + 02 + H20
Origins - Chapter 2
Isotopic evidence for Ps (both S based / O2 evolving)
- 13CO2 less reactive than 12CO2
- physical < biological discrimination, both present
é 13 C /12 Csample -13 C /12 Cstan dard ù
d13CO2 = ê
ú x1000
13
12
C / Cstan dard
ë
û
Standard for C = “Pee Dee” Belemnite, South Carolina
- Carbonates less depleted in 13C, 0 – 2 ‰
- Carbohydrates greatly depleted in 13C, ~20 ‰
(Overhead 2.6)
Origins - Chapter 2
Evidence for O2 releasing Ps only
1. Banded Iron Formations
-Fe2+ in sea  Fe2O3 & S2-…
- no free O2 in the atmosphere
3.5 – 2.0 BYA
2. Red Beds (transient atm. O2)
2.0 - Present
-FeS2 on land oxidized
-bands of Fe2O3 alternates w/other continental elements
3. Accumulation in atm
- Ps > O2 consumption
(Figure 2.7)
A banded iron formation from the 3.15 BY old Moories Group, Barberton
Greenstone Belt, South Africa. Red layers represent the times when oxygen
was available, gray layers were formed in anoxic circumstances.
Origins - Chapter 2
Free O2 in atmosphere fundamental change to earth
I.
Oxidizing agent in atmosphere (e.g. O3 in stratosphere and UV)
II.
Rapid evolution



III.
Eukaryotes diverge from prokaryotes ~2BYA (end of B.I.F.)
Respiration in mitochondria efficient metabolism
Ps now in specialized chloroplasts…more efficient
Evolution of new biochemical pathways

“Chemoautotrophy” (protons coupled to CO2 reduction)
Nitrification
2NH4+ + 3O2 → 2NO2- + 2H20 + 4H+
2NO2- + O2 → 2NO3-
Origins - Chapter 2
…evolution of aerobic N transformations allow evolution of anaerobic N
transformations…
Denitrification: (anaerobic…microsites)
5CH2O + 4H+ + 4NO3- → 2N2 + 5CO2 + 7H2O