Why have plants been implicated in the Ordovician and Devonian glaciations, but not the Proterozoic glaciations?

Question

A.  Proterozoic plants used anoxygenic photosynthesis.
B.  Proterozoic plants had simple root structures that did not increase rock weathering.
C.  Land plants had not yet evolved during the Proterozoic.
D.  The non-vascular plants present during the Proterozoic did not contribute to global nutrient cycles.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
 
Gather Content
· What do you already know about glaciation?
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  Land plants had not yet evolved during the Proterozoic.

Clarify Question
· What is the key concept addressed by the question?
        o This question addresses glaciation.
· What type of thinking is required?
        o This question is asking you to take what you already know and apply it to this unfamiliar situation.
 
Gather Content
· What do you already know about glaciation?
        o There is growing evidence that plants contributed to two glaciations.
        o The initial colonization of land by plants was followed by gradual cooling and abrupt glaciation 488 to 444 mya in the Ordovician period. The early land plants lacked roots, but studies using extant relatives show they released organic acids that can increase rock weathering.
        o The most convincing proposal is that phosphorous, an essential nutrient for plant growth, was released from the rocks through weathering and entered the ocean where it supported extensive algal growth. The rapid growth of the photosynthetic algae led to increased uptake of CO2 for photosyn-thesis, decreasing atmospheric CO2 levels and triggering glaciation.
        o A second glaciation was concurrent with the diversification of vascular plants 400 to 360 mya during the Devonian period. Vascular plants’ extensive root systems increased weathering of rocks, which decreased atmospheric CO2 levels.
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o The Proterozoic Eon lasted from 2500 to about 500 mya.
        o Multicellular plants originated shortly before the Cambrian period (~500 mya), and did not colonize land until the Ordovician period (~450 mya).
        o Therefore, before the Ordovician there could not have been increased rock weathering due to plants, leading to glaciation.
        o In short, the Proterozoic predates the evolution of land plants.
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o The question required you to take what you already know and apply it to this unfamiliar situation.
        o Did you recognize that the Proterozoic is the older time period that predates the Cambrian?
        o Did you recognize that the role of plants in glaciation involves rock weathering?