GEOL 102 Historical Geology
Spring Semester 2017
The Archean Eon
Record of the oldest rocks is very poor: because of great age, is often buried and/or has been recycled (metamorphosed, melted, etc.) over the last few billion years.
- 4.03 (or 4.40) - 2.5 Ga
- By definition, the oldest rocks known
- Recently subdivided into the Eoarchean (> 3.60 Ga), Paleoarchean (3.60 - 3.20 Ga), Mesoarchean (3.20 - 2.80 Ga), and Neoarchean (2.80 - 2.50 Ga). These boundaries are arbitrarily defined, and do not correspond with particular correlative phenomena (although the upper boundary of the Neoarchean is close to the origin of modern plate tectonics).
(Note: Archean (-EAN) refers to a part of Earth history; archaean (-AEAN) refers to a type of prokaryotic organism, and is thus a branch of the Tree of Life.)
- Atmosphere extremely different: little free oxygen
- Impacts common early on, drop off by around 3.8 Ga
- No large continental masses known (however, newly dated very old zircons indicate continental-type crust present by at least 4.40 Ga!)
- Vastly more geothermal energy: radioactive materials had not decayed as much
Drip tectonics: prior to rise of proper plate tectonics, it seems that subduction zones didn't exist as such. Instead, the higher amount of radioactivity within the core and the lack of dissolved water in the mantle instead favored downward-plunging "drips" of lithospheric material sinking into the mantle. The downward-warping produces basins on the surface, which fill in with marine sediments.
Find unusual lithologies and suites of rocks in Archean:
- Komatiite: Ultramafic volcanics, very common in Archean, very rare afterwards. Require temperatures of greater than 1600 ∫C (modern lavas max out at 1350 ∫C). Hint at the extreme activity of Archean mantle.
- Banded Iron Formations (BIFs): Interbedded layers of chert and iron-rich minerals (iron sulfides, iron carbonates, etc.). Cannot form under modern levels of oxygen: indicate low levels in Precambrian. First appear 3.8 Ga; much more common in
Proterozoic. Rare after 1.9 Ga, last appear c. 720 Ma. Major iron ore.
- Granitoid-Greenstone Complex:
- THE typical Archean lithological suite.
- Pods of metamorphosed granitic rock (now gneisses) separated by greenstone belts: bands of sequences of weakly metamorphosed komatiites -> basalts -> felsic volcanics -> marine sediments (turbidites, cherts, banded iron formations, etc.).
- Oldest 4.03 Ga, disappear around 2.5 Ga.
- Cratonic Complexes: "Modern" style shallow marine and terrestrial siliciclastic deposition.
- First appear c. 3.1 Ga; common 2.5 Ga.
- Indicate presence of sizable land surfaces produced by suturing together of island arcs
- Some evidence (common age, shared position in younger land masses) that cratons in South Africa, Madagascar, Sri Lanka, India, western Australia, and parts of East Antarctica that all date from about 3.1 Ga formed the first continent: Ur
- Additionally, some evidence that various Archean provinces had actually united to form a decent-sized supercontinent Kenorland (or Superia; comprised of Ur, parts of what would become Baltica (northwestern Europe) and parts of what would become Laurentia (North America plus Greenland) by 2.7 Ga
- During Archean and Paleoproterozoic, contain detrital uranite and
pyrite (unstable under higher levels of oxygen).
Note that there does seem to be a form of mineral evolution: not minerals descending from earlier minerals, but rather large scale changes in the chemical conditions of the Earth (and before that, the proto-Earth) that allow for different suites of minerals to be formed at different periods.
Some famous localities:
- Acasta Gneiss:
- Oldest known rocks in the world (4.03 Ga)
- Granitoid-greenstone complex, NW Territories, Canada.
- Isua Formation: In Greenland, extremely ancient rocks containing oldest evidence of life.
- Pongola Supergroup:
- Oldest well-known cratonic complex.
- Sits conformably on top of greenstone belt, changes to shallow marine and non-marine sediments. (Non-marine include common gold deposits).
- Southern Africa.
- Jack Hills
- Metasediments containing oldest known terrestrial minerals (zircons dated at 4.40 Ga) indicating incredibly ancient continental rock
- Western Australia
Where did granitic material come from, if mid-ocean ridges only produce mafic materials? A "hot topic" in historical igneous studies, but seems to be related to partial melting and recycling of materials near ancient subduction zones: as material is subducted and
remelted in the presence of water, produces lots of felsic materials. These bubble up as a sort of "scum". Comparable granitic material is present in modern Iceland.
Abiogenesis: origin of life:
- Based on our inferences of modern life forms and of the environments that existed in the Archean, the transition from non-living matter
into living things occurred sometime late in the Hadean or early in the Archean
- Laboratory experiments show that most of the building blocks of life will form out of inorganic chemicals in a variety of situations:
near clays; near deep-sea vents; inside the spaces of rocks near deep-sea vents; even near ice
- Was likely a step-wise transformation: Origin of simple building blocks into Origin of biological monomers (simple organic materials like amino acids) into
Origin of biological polymers (more complex molecules formed of chains or other arrangements of monomers strung together) into Competition
among varied biological polymers for utilization of space, materials, and energy into Winners of this competition (the ones that more
effectively take up space, materials, and resource) produce more duplicates
- From this point on, we might consider these things "alive". They are certainly operating under the Darwinian rules of evolution:
duplication (with occasional errors) and competition
- Over time, the particular combination of features that characterize life on Earth (e.g., lipid bilayer membranes, RNA ribozymes,
proteins) develop & DNA replaces RNA as the store of genetic information
- Oldest (controversial) chemical traces of life at 3.8 Ga (Isua Fm., Greenland)
- Oldest (controversial) body fossils of prokaryote-grade organisms at 3.5 Ga
- More convincing body fossils of prokaryote-grade organisms (including contraversial cyanobacteria: blue-green algae) by 3.4 Ga, so
had definitely been around by 3.4 Ga or earlier (in western Australia & South Africa)
- Stromatolites at 3.3-3.5 Ga (Warrawoona Group, Australia)
Archean life would have very short food chains, but contained a great diversity of chemical pathways. Life would have been limited to the
water. Most of the shallow seafloors, shores, etc. would be covered in algal/bacterial slime. As photosynthesizers spread, the oceans and
atmosphere began to fill with oxygen.
Earliest life was probably exceedingly simple: far simpler than any modern form.
Would have been:
- Anaerobic: evolved outside the presence of free oxygen (free oxygen would
probably poison it!)
- Heterotrophic (a consumer, absorbing organic molecules from water) and/or Lithotrophic (both autotrophs and consumers,
using nonorganic material [aka rocks] for energy and biological synthesis)
- Prokaryotic: no nucleus nor other complex organelles
Once thought to have appeared in "quite little pond" ("primordial soup").
More likely: formed near the (then more active) oceanic vents along mid-ocean ridges:
- Abundant energy supply and mineral supply
- Far more common in Archean than now (due to higher geothermal activity)
- Ocean water would protect early life from UV rays
- Even today many exceedingly simple prokaryotes live near vents, black smokers, etc.
As fuel supply decreased, various new forms appeared from among the proto-organisms:
- Photosynthesizers first to use sunlight directly.
- 1rst order consumers: absorb other cells.
With photosynthesizers come free oxygen: begin to change global atmosphere. Has most
direct effect as the Proterozoic begins.
LUCA: the "Last Universal Common Ancestor" (i.e., the ancestor of Bacteria, Archaea, and Eukaryota) would not have been the
first living thing. Instead, there would have been a history and diversity of living forms that included the ancestors of LUCA as well as
many diverse side branches that have no living survivor. Based on genetics divergence data and biomarker traces, LUCA would have
to have been present by around 3.8-3.5 Ga (and thus around in the Paleoarchean, if not earlier).
NOTE: for many decades the earliest photosynthesizers were thought to be cyanobacteria
(aka "blue-green algae"). However, the fossils of supposed Archean cyanobacteria do not show definite cyanobacterial traits, but could
instead be any form of prokaryote. Biomarkers (chemical traces) do not show any clear presence of cyanobacteria until
the Proterozoic. Additionally, cyanobacteria are aerobic, and thus unlikely to have done well in the anaerobic conditions of
the Archean. Instead, the photosynthesizers of the Archean were anaerobes, possibly including the purple sulfur bacteria and
green sulfur bacteria (which do not release oxygen as a waste product) and the green bacteria and heliobacteria (which do).
Late Archean Events
- Stromatolites become more common, indicating diversity of photosynthesizers increasing
- Oxygen production increases
- Oldest glaciers (tillites about 2.9 Ga in southern Africa)
- Presence of steranes as biomarkers by 2.7 Ga. Steranes are only produced by eukaryotes today (but could be from archaeans on the line leading to eukaryotes)
- Regional shift towards modern tectonics
To Lecture Notes.
Last modified: 18 January 2017