Objectives: Review: Remember Earth Systems and cycles? This lecture will be about interactions between the physical Earth and biosphere and how those interactions have changed over time. We'll focus on five major events:
The Origin of life:

Environmental conditions at time of life origins.

Photosynthesis: Organisms change Earth chemistry

For a while, organisms got away with eating the organic materials that were floating around in the ocean. As these started to get scarce, one group, the cyanobacteria, came up with a new method of capturing energy from the environment - Photosynthesis,

6 CO2 + 6 H2O + energy (sunlight)---> C6H12O6+ 6 O2

Note that oxygen is a product.

We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for the rock record


Banded Iron formations (BIFs) :

Late Archean - Early Proterozoic (~3.0 - 1.8 g.a.) Cherts with alternation of gray and rust red bands of hematite (Fe2O3).

The effect: Life remodeled its environment to its own benefit.

Stromatolites (The Age of Slime): Beginning about 3.0 g.a., we begin to see fossil stromatolites - laminated bacterial mats.


Multicellularity:

Until the Late Proterozoic, life had chugged along just fine as simple prokaryotic (bacterium-like) single cells. At this point, that changed:

Hard parts: Life makes rocks

Hard parts: At the very end of the Proterozoic, we see the first hard animal parts, the "small shelly faunas" of Siberia." The shells were small and enigmatic, but for the first time, living things were contributing directly to the formation of rocks. There are two major hypotheses for why this happened-

Cambrian explosion:

During the Early Cambrian, the diversity of readily fossilizable animals with large hard parts explodes. By the end of the Cambrian, almost all of the familiar major modern animal phyla with hard parts are represented and predators like Anomalocaris were most definitely present. At this point, living things start contributing large volumes of calcite, opal, silicate, and phosphatic material to the formation of biogenic sedimentary rocks.

Result: A totally new depositional environment - the carbonate shelf.

Reef builders: Some organisms modify their environment even more radically, forming reefs: fixed upright calcareous structures that trap other sediments and create a complex environment for other organisms. Throughout the Phanerozoic Eon, reef builders have been at work, although the exact critter acting as the dominant reef former changed over time.

The effect: Life remodeled its environment to its own benefit.

Land Plants

One final issue. During the Proterozoic and early Paleozoic, the physical topography of the Earth seems to have been different from what we know today in two related ways:

Arguably, these were effects of the total absence of any kind of vegetation on land to bind sediment to the ground and limit erosion. The appearance of land plants, therefore, had a profound effect on rates of erosion, transport, and sedimentation.



Again and as always, life altered its environment to its own advantage. (Of course, if it didn't, we wouldn't be having this conversation.)


Key concepts and vocabulary: