Download The Early Earth

The Early Earth
If we are looking for evidence of life beyond Earth (either
past or present), it is important to understand the
boundary conditions.
How might life originate?
What does life need to thrive/survive?
What signatures does life leave behind for us to examine?
How might such signatures change over geologic timescales?
What can we learn from Earth’s history in this context?
What is Needed for Life?
For all the diversity of life on Earth, the basic biochemistry of life isn’t really that diverse at all.
To make life possible, you need water, energy, and the right elements (CHNOPS).
3
A Simple Test
4
Can we even prove that life exists on Earth using
remote sensing techniques?
This is a fairly easy
question given the
abundance of life on
Earth, but it’s still an
important question to
ask (and answer).
Fortunately, Carl Sagan
and colleagues asked
this very question back
in 1993 using images/
data from the Galileo
spacecraft as it headed
to the Jupiter system.
A Simple Test
The Galileo data of Earth showed the clear presence of water,
oxygen, surface pigments (vegetation), and atmospheric methane.
The latter has a short lifespan and must have an active source.
5
Life: The Early Years
For today: ages >540 Ma
Rise of Oxygen
(“Great Oxygenation Event”: GOE)
Banded Iron Formation (BIF)
“Snowball” Earth
Earliest evidence for life &
preservation potential
6
The Rise of Oxygen: Mass Independent Fractionation of Sulfur
S can be oxidized to form SO2 and SO4.
UV photolysis can produce MIF in atmospheric SO2 but only if O2 levels are
low.
MIF is present in sedimentary sulfide and sulfate minerals prior to ~2.45 Ga (below).
Origin of this MIF is debated, but it may be driven more by volcanic outgassing than
microbial processes (e.g., sulfate-reducing bacteria).
7
Rise of Oxygen and Banded Iron Formation
8
Rise of oxygen, snowball Earth, banded iron formations are apparently linked.
Biological C cycle
Biological S cycle
Stromatolites
Unambiguous microfossils
Unambiguous evidence of cyanobacteria
Abundant cyanobacteria-like microfossils
Gunflint-type microfossils
Iron Formation
Surface Ocean
Oxygen Minimum Zone
Hadean
4567
No O2
Low O2
Fe (locally H2S)
Archean
4000
High O2
O2
Proterozoic
2500
Phanero
542
Electron donors
Electron acceptors
0 (Ma)
Fe3+
SO42NO3O2
H2O
Fe2+
H2S
Organics
Rise of Oxygen and Banded Iron Formation
BIF in Upper Peninsula (Michigan)
9
Rise of Oxygen and Banded Iron Formation
In Absence of Oxygen
Fe2+ is prominent in oceans (weathering of oceanic basalts, input from
hydrothermal sea vents).
Fe2+ is rather soluble in water, so it can accumulate and get concentrated if
oxygen is not present.
Bottom of oceans can be anoxic even if surface ocean is oxic.
In Presence of Oxygen
Fe2+ oxidizes to Fe3+.
Fe3+ is NOT very soluble: forms iron oxides and is removed from water
Silica is also present, so chert & clays are often found with the iron minerals
10
Rise of Oxygen and Banded Iron Formation
Why did BIF formation end??
It has been proposed that the Sudbury impact event at 1.85 Ga caused
mixing in the ocean, bringing oxygen to the deeper anoxic zones.
This could have shut down later formation of BIF.
Sudbury
impact
11
Rise of Oxygen and Banded Iron Formation
Impactor ~10-15 km in diameter.
Original crater may have been ~160 km across.
Ejecta found as far as ~800 km away.
12
Rise of Oxygen and Banded Iron Formation
Impactor ~10-15 km in diameter.
Original crater may have been ~160 across.
Ejecta found as far as ~800 km away.
Impact-generated spherules from Sudbury in
Upper Peninsula (Marquette, MI)
13
Rise of Oxygen and Banded Iron Formation
But what caused the BIFs to come back in the first
place if Earth already had more O2 by 2.5 Ga?
Sudbury
impact
14
Rise of Oxygen and Banded Iron Formation
RESEARCH LETTER
a
Iron formations
Or are the younger BIFs
associated with crustal growth
and enhanced submarine
volcanism?
Great Oxidation Event
b Volcanogenic massive
sulphide deposits
c
Crustal growth
(emplacement ages
of juvenile crust)
d Crustal growth
(Hf model ages)
8
e
Δ33S
6
4
Mass-independent
fractionation
signature in sulphur
(sedimentary sulphides)
2
0
–2
–4
1–10 f
<<10–3
Atmospheric oxygen
(percentage of present
atmospheric levels)
g Iron speciation data
Water depth
(Precambrian shales)
h
4.0
H2S
Fe2+
O2
Ocean chemistry
Fe2+
3.5
3.0
2.5
Age (Gyr)
emanation of vast volumes of ferrous iron and other reduced species
(such as H2, CH4 and Mn21) during a short-lived period of intense
submarine volcanism and hydrothermal activity. A peak in the accumulation of VMS deposits at 1.88 Gyr ago5,19 shows that large volumes
of hydrothermal iron were emitted and subsequently deposited as iron
sulphides near vents, removing sulphur compounds from sea water.
The concentration of seawater oxygen and sulphate at about 1.88 Gyr
ago was significantly lower than that of today3,4, so that the enhanced
influx of reduced species delivered by hydrothermal plumes to the
oceans may have overwhelmed both oxidants in sea water29, greatly
reducing the spatial extent of euxinic and oxygenated zones in the
oceans.
Our findings provide a simple explanation for the apparent conundrum of the sudden reappearance of iron formations after the first
major rise in atmospheric oxygen. Specifically, mantle-driven processes at about 1.88 Gyr ago not only acted as a source of dissolved
iron, but also modified ocean chemistry and redox state by releasing a
large flux of reductants that temporarily overwhelmed the supply of
oxidants to the deep ocean from shallow waters and atmosphere. This
led to the development of a largely ferruginous water column beneath
an oxygenated surface zone (Fig. 2h). Iron-oxyhydroxides accumulated on the sea floor in shallow marine, coastal environments, where
Fe21 was oxidized in clastic-starved settings, forming extensive deposits
of granular iron formation.
Although the chemistry of the deep ocean apparently returned
to Archaean-like conditions at 1.88 Gyr ago, atmospheric oxygen
apparently did not fall to pre-Great Oxidation Event levels (Fig. 2e,
f). The decoupling between the redox state of the atmosphere and
ocean may be explained by the release of reductants into the bottom
of the ocean, where they were buffered by the seawater oxidants, largely
restricting anoxia to the ocean. Our findings also provide a mechanism
for the cessation of iron formation deposition. Rather than being a
consequence of changing surface conditions, we propose that the
demise of iron formations corresponds with the termination of this
short-lived interval of global mantle-driven magmatism and crustal
growth. After 1.88 Gyr ago, the deposition of iron formations was no
longer favoured because the flux of oxidants from the atmosphere and
surface ocean significantly exceeded the long-term rate of delivery of
[Rasmussen
et parts
al., 2012]
hydrothermal Fe and reductants
to the deeper
of the oceans.
Major iron formations did not reappear until the late Neoproterozoic
H2S
Surface water
Intermediate water
Fe2+
Deep ocean
2.0
1.5
1.0
Younger occurrences coincide
with a number of factors:
increased magmatic activity,
increased input of volcanic
outgassing, increased input of
Fe2+ into ocean from
hydrothermal systems (?)
15
Rise of Oxygen and Banded Iron Formation
What about youngest BIFs?
Sudbury
impact
Snowball Earth may explain how O2 was shut down in the oceans:
no longer in contact with atmosphere!
16
Snowball Earth
Paleogeographic extent of continental ice sheets and
permanent sea ice cover over the last 800 Myr
(red lines indicate major mass extinctions)
17
Snowball Earth
18
Snowball Earth
19
Snowball Earth
20
Snowball Earth
Glacial diamictite in Death Valley (~700-800 Ma)
Kingston Peak Formation
21
Snowball Earth
glacial diamictite in Death Valley
22
Snowball Earth
Some organisms clearly persisted through these
global-scale glacial events.
23
Life As We Know It
Bacteria - One of the three domains on the phylogenetic tree of life; this domain used to be
synonymous with ‘prokaryote’ (no nucleus), but then archaea were identified as a distinct group.
Archaea - Prokaryotes that are distinct from bacteria. Originally thought to only occur in extreme
environments, we now know that they are widespread in many habitats.
Eukaryote - An organism with a complex cell structure and a membrane-bound cell nucleus that
contains genetic material; plants, animals, fungi, etc.; most (but not all) of the members utilize oxygen.
24
The Explosion of Life
25
Preservation Through Geologic Time
Preservation is key, not everything will remain
in the geologic record.
26
Preservation Through Geologic Time
What factors will favor preservation?
What types of environments might
we associate with these conditions?
27
Searching for Life on Early Earth
Search for compositional indicators of past or present life
Chemical
Isotopes,
Redox gradients
δ13C
δ33,34S
Molecular
Morphological
28
What Gets Preserved?
29
Microfossils of bacteria have been a focus of assessing ancient life on Earth.
Burial is important!!
Need to limit post-depositional interaction with O2.
Easier to accomplish before the advent of bioturbating organisms.
Only ‘tough’ parts get preserved (bacterial walls, envelopes).
[Knoll, 2015]
What Gets Preserved?
30
Mineralization is even better....can preserve 3-D form of bacteria.
Pyrite
Calcium
Carbonate
Opal / Chert
Chert (SiO2) is excellent at
preserving materials, and it was
important in the oceans on early
Earth prior to the advent of
silica-secreting organisms.
What Gets Preserved?
Cyanobacteria are often what is found in ancient rock record.
Abundant and diverse through much of Earth’s history.
Relatively large cells (easy to see w/ optical microscope).
Utilize photosynthesis.
Responsible (or at least linked) to the rise of oxygen.
Common in environments that favor preservation (e.g., peritidal environs.)
31
Stromatolites: Evidence for Life on Early Earth
Layered, accretionary structures
found in the most ancient
sedimentary rocks (commonly in
carbonate or chert).
The stromatolite type
"Conophyton" (below) is hailed as a
biogenic stromatolite, formed by
cyanobacteria ~3.4 billion years ago
....but are these
morphologies definitive of
biological activity?
Or could they instead be
formed by an abiotic
process??
32
Stromatolites: Evidence for Life on Early Earth
Stromatolites can span a
huge range in size (see
left, notebook for scale).
They are believed to be
formed by the trapping and
binding of sediment by
biofilms composed of
microorganisms (e.g., algae).
Their shapes are quite diverse
and they were apparently
much more common on the
ancient Earth.
Extremely rare on Earth today,
yet algae/bacteria are
common...
33
Searching for Life on Early Earth
Search for compositional indicators of past or present life
Chemical
Isotopes,
Redox gradients
δ13C
δ33,34S
Molecular
Morphological
34
Molecular Signs of Life
Proteins and nucleic acids are not easily preserved in Precambrian rocks (>541 Ma).
By contrast, lipids can be preserved over billions of years.
Lipids: Chemical compound that is insoluble in water but soluble in organic
solvents. These compounds make up cellular membranes.
Example: Cholesterol
35
Molecular Signs of Life
Hopanoids: geologically stable lipid formed by many, many bacteria
(start out as more complex molecules, but they are broken down to simpler
forms over time; hopane forms the core of hopanoids)
Hopane
(5 rings)
Similar to cholesterol, hopanoids
can be inserted into baceterial
membranes to provide rigidity
and stability.
Hopanoid
(chains, etc.
added)
36
Molecular Signs of Life
37
Hopanoids were detected in ~2.7 Ga rocks from Australia, and the specific
compounds were suggested to be linked to cyanobacteria.
Major implications! This would suggest photosynthetic bacteria were already
around prior to the rise of oxygen.
However.....later work has shown that there may not be a direct link between these
specific hopanoids and cyanobacteria; they may be produced under anoxic conditions.
Current goal is to try and understand if/how certain molecules (“biomarkers”) can
be linked to specific types of bacteria and other organisms.
Bottom Line
Understanding the origin of life on Earth is very complex, but it is
ultimately an intimate story of biology and geology.
As our ability to understand these processes on Earth improves, and thus
our understanding of life’s origin and evolution, so do our prospects for
searching for life beyond Earth.