Life at Middle Age - Biology & Environments on the
mid-Proterozoic Earth
Andy
Knoll (Harvard University, Cambridge, Massachusetts, USA)
63rd Annual Bownocker Lecture, Department
of Geological Sciences,
Ohio State University, Columbus, Ohio, USA
28 April 2005
Check
out Knoll (2003) - Life on a Young Planet.
Looking
at the mid-Proterozoic - Earth’s middle age.
The
initial rise in pO2 in Early Proterozoic resulted in ice ages.
Another rise in pO2 in the Late Proterozoic resulted in ice ages.
There’s
been lots of interest in the Early Proterozoic, when O2 began to
accumulate, based on chemical evidence. There’s also been lots of
interest in the Early Proterozoic ice ages. There’s also been lots of
interest in the Late Proterozoic rise in pO2 and the Late
Proterozoic multiple global ice ages.
These
are benchmarks in the history of life as well. The rise in pO2
allowed life forms to use O2.
The
stretch between the Early Proterozoic and the Late Proterozoic has received
less attention (= Earth’s middle age).
Looked
at the Roper Group (northern Australia) and the Bil’yakh Group (Siberia).
The
Bil’yakh Group is a thick carbonate succession exposed along the Catukan River
[sp.?]. It's a ~2000 m succession of platform/shelf carbonates, dated to
1.5 b.y. It includes silicified peritidal carbonates - includes
microbially laminated tidal flat sediments. Early diagenetic silica has
preserved micron-level features, including biological features - microfossils.
Ex: 10 micron diameter
Eoentophysalis balls - indistinguishable from living Entophysalis
(a cyanobacterium) in the same environments today.
So,
an introduction to mid-Proterozoic life is an introduction to cyanobacteria.
Cyanobacteria
are heroes of the environmental revolution - they invented photosynthetic
pathways & released O2. Most other photosynthetic
organisms use cyanobacteria symbiotically.
Cyanobacteria
include simple forms, but they mainly include complex colony forms & have
complex cell differentiation.
Middle
Proterozoic microfossils can be identified as cyanobacteria.
The
polysaccharide extracellular envelope of cyanobacterial cells don’t decay
easily - they are likely to fossilize. So, they are easy to
recognize and they are easy to preserve. But, we want to go on beyond
morphologies and inferred lifestyles, to broad environmental settings.
Ex: extensive Entophysalis
mats in Abu Dhabi coastal settings.
Looking
at Earth’s middle age - from 1 to 2 b.y. We’re finding the same things
elsewhere from 1 to 2 b.y. Lots of cyanobacteria in northern Siberia
rocks. Found trichomes (= filamentous cyanobacteria) - some are short -
had recently germinated from resting cells. Also get sausage-shaped cells
(60-70 microns long) in filaments consisting of smaller cells. This is
like modern nostocalean cyanobacteria. See Archaeoellipsoides.
So, we’re seeing cell differentiation at this time.
Nostocalean
cyanobacteria have 3 types of cells - including akinetes, heterocysts.
A
cyanobacterial phylogeny has shown that all cyanobacteria that have
differentiated cells are part of one clade. Traditionally, two groups of
cyanobacteria have been recognized - both with cell differentiation.
Recent phylogenies show that both groups are in one clade.
By
1500 m.y., cell differentiating cyanobacteria (= derived cyanobacteria) were present.
So, most of cyanobacterial diversity was present by 1 to 2 b.y.
~2400-2100
m.y. is the date for the appearance of this clade.
Most
of the cyanobacterial phylogenetic tree is known from mid-Proterozoic rocks.
Proterozoic
cyanobacteria were closely related to modern forms - similar physiology,
etc.
So,
a relatively modern bacterial biota was present in Earth’s middle age.
There
were also widespread stromatolites - including large reefs - built by
microbes. Microbes could build reefs comparable in size to reefs seen
today. Microbial reefs were widespread then in the photic zone. Can
see a change in stromatolite form through time. See stromatolite
similarity in rocks of the same age in different localities. But,
different-aged stromatolites show different forms. Traditionally, this
change was attributed to evolution - changes in mat-building biota, resulting
in a crude stromatolite biostratigraphy. The Russians have noticed this
for a long time. But, instead of evolution, the change in stromatolite
form corresponds with changes in seawater chemistry.
At
1-2 b.y., peritidal areas had rapid cementation. On the timescale of
bacterial cell decay, surrounding sediments had already
cemented/lithified. See casts/molds of cyanobacterial cells. There
was nearly instantaneous lithification in peritidal settings. See little fans
of carbonate cement crystals. This isn’t seen in the Late
Proterozoic. So, stromatolite form change is in concert with
environmental change.
But,
which is the dominant role? Biological evolution or physical-chemical
factors?
Looked
at northern Australia (Northern Territories), near the Gulf of
Carpentaria. There’s an extensive mid-Proterozoic succession -
well-preserved sediments. The mid-Proterozoic basins here have been well
drilled - looking at ~pristine samples of the Roper Group in drill cores.
Roper Group here is 1429-1492 m.y. - a series of shallowing-upward sequences -
mostly siliciclastics: very organic-rich shales, some
siltstones/sandstones. Probable cyanobacteria have been freed up from
these shales. But also, eucaryotic cells have been retrieved - almost
1500 m.y. old. These eucaryotic fossil cells are an order of magnitude
larger than the cyanobacterial cells we’ve looked at. The eucaryote cells
have processes - variable in number, asymmetrically placed, sometimes
branching. So, these are vegetatively active cells - cytoskeleton present
- nucleated organisms. Some minor diversity in eucaryotic cells is seen
in rocks of this age. Larger cells here have surface ornament not
known in any bacterial cells. So, clearly eucaryotes. Under the
SEM, some of these eucaryotic fossils have walls composed of tessellated blocks
(hexagons making a closed pattern - not like dinoflagellate
paratabulation). So, these have cell wall ultrastructure. FTIR
(Fourier Transform Infrared spectroscopy) can give a chemical spectrum of a
single cell. Can get chemical constituents of walls of individual cells.
None can be confidently placed in modern eucaryote groups.
Note
the presence of 750 m.y. testate amoebae in the Grand Canyon, >750 m.y.
green algae from Spitsbergen, 1200 m.y. red algae, 1100 m.y. lipids from
alveolates, >1000 m.y. vaucheriacean algae (golden green algae).
So,
seeing divergence of major branches of the eucaryotic tree.
Not
until the end of the Precambrian do we see diversification (increase in
diversity) in eucaryotes.
So,
fairly early origins & limited diversity until the end of the Proterozoic.
Then, the Phanerozoic is a eucaryote world.
Soon
et al. (2004) - Molecular Biology & Evolution 21: 809 - a molecular
clock study, which ~matches the known fossil record.
Now,
looking at redox conditions in Proterozoic oceans.
Before
2.4 b.y., there’s no evidence of appreciable O2 in the surface
oceanic waters or the atmosphere. From 2.4 to 2.3 b.y., get a transition
to a world with some surface oceanic and atmospheric O2.
Mid-Proterozoic
oceans were distinct from today.
Looked
at 1.73 b.y. black shales (Wollogorang Fm.) and 1.64 b.y. black shales (Reward
Fm.) - at maximum flooding surfaces.
Looked
at degree of pyritization and reactive iron content (FeHR/FeT)
(portion of highly-reactive iron in sample compared with total iron
content). Found mid-Proterozoic oceans were like euxinic Black Sea
sediments.
Roper
Group facies - basinal sediments plot in euxinic field & inner/distal shelf
sediments plot in oxic field.
Conclusion:
low sulfate, redoxcline at moderate depth in mid-Proterozoic oceans - ~10% of
sulfate concentration of today’s oceans.
Looking
at low sulfate, limited O2 in surface ocean waters. Below
that, anoxic & sulfidic waters.
Anbar
et al. (2004) - looking at Mo isotopic trends in mid-Proterozoic interval.
Fe/Mn
nodules/crusts form in oxidizing conditions - are a sink for Mo-98.
Modern seawater has a positive d98Mo/95Mo‰. Mid-Proterozoic oceans had
a d98Mo/95Mo‰
value closer to a basalt/granite MoS2 value - consistent with a
euxinic ocean then - a not well ventilated ocean then. This is an
independent source of info., with the same conclusion.
Anbar
& Knoll (2002) - a speculative paper.
Archean
ocean - not oxygenated; high
Fe
1250-1850
m.y. old ocean - sulfidic
water below surface water having some O2; Mo scarce
Phanerozoic
ocean - well oxidized; Fe low
Mo
is the most abundant trace element in oceans today.
In
mid-Proterozoic oceans, fixed nitrogen not common. The ability of biotas
to fix nitrogen may have been limited by availability of Mo/Fe.
So,
expect procaryotes to be the dominant primary producers in
mid-Proterozoic oceans. Expect algae to be most common along the
coast, where nitrate & Mo are likely to occur in greatest abundance.
These
expectations are met by the mid-Proterozoic record, so far.
Mid-Proterozoic
biomarkers recovered:
-
2a-methylopanes and b-carotanes and g-carotanes
- from cyanobacteria.
-
3b-methylopane - from methane eating proteobacteria.
-
very low steranes - from algae
-
chlorobactane and isorenieratane - from green sulfur photobacteria (anaerobic
bacteria)
-
okenane - from purple sulfur photobacteria (anaerobic bacteria)
So,
mid-Proterozoic oceans had O2 in a surface layer and H2S
below that (sulfidic).
We’ve
been learning much about the mid-Proterozoic - cyanobacteria were
abundant, diverse, widespread, modern in diversity & biological
function.
Modern
cyanobacterial environmental distributions can be used to predict
mid-Proterozoic cyanobacterial fossil distributions.
Eucaryote
body/cell plans were starting to emerge, but they were not widespread or
diverse at this time.
Large
diversification in algae/animals only occurred in the Phanerozoic.
“The
present is the key to the past” - a statement about processes, not the state
of things. The microbial world today has roots in a world very different
from today. Keep this in mind when looking at microbes in today’s
communities.
Before
2.5 b.y., very little O2.
From
2.5 b.y. to ~600 m.y. - some O2.
Since
the beginning of the Phanerozoic - lots of O2.