GEOL 102 Historical Geology

Spring Semester 2017
The Archean Eon

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.

(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.)

During Archean:

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:

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:

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:

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:

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:

As fuel supply decreased, various new forms appeared from among the proto-organisms:

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

To Lecture Notes.

Last modified: 18 January 2017