history of our planet week 4.2

evolution of plants and carbon silicate cycle interactions

Could life have played a role?

Microfossils

Molecular clocks

Carbon isotope signature of photosynthetic microbial communities

a new genetic code

Lots of phosphorous: involved in the base pairs and in metabolic processes

Fertilisers rich in P as it’s a limiting nutrient – without it cannot run metabolisms, undergo cellular reproduction

Important in plants and all life

Leaky bucket metaphor: to increase plant productivity make sure there’s sufficient P – add it into something, productivity and activity usually goes up

Biological amplification of weathering: -why

Source of nutrients, especially P

Biological amplification of weathering: how

Organic acids, carbonic acid, chelating agents, soil stabilisation, enhanced hydrological cycle, rock splitting

Biological amplification of weathering: global consequences

Release of P, increased organic carbon burial, source of O2

Release of Ca and Mg from silicates sink of CO2 with formation of marine carbonates, planetary cooling

Byproduct of plant wanting phosphorous and releasing calcium and magnesium

hypothesis for global cooling

Nuclear winter didn’t last very long – injected lots of ash and sulphur but only approx. 10 yrs – but this was long enough to kill large animals

This theory:

Colonisation of and by plants increases silicate weathering and export of calcium magnesium to the ocean and therefore carbonate formation and burial which decreases carbon dioxide concentrations in the atmosphere which reduces global temps.

Right now that might be self limiting as if uts gets really cold then its going to be too cold for photosynthesis. The positive feedback loop results in snowball earth. The ice sheets snap close – lots of photosynthesis stops. Theres no bio weathering etc as its all ice. Eventually co2 builds up and up until its so warm that snowball earth begins to melt

evolution of land plants

Organisms that needed to live in water – undergone series of evolutionary adaptations – can move further and further away from water, get bigger and higher, undertake more primary productivity

 

The phylogenetic ‘tree’ of plants:

First land plants were bryophytes

Liverworts

Hornworts

Mosses

Tied to moist habitats to complete their life cycle

Earliest land plants (ordrovician):

cryptospores ~475 Ma

First fragments of plants ~460 Ma

Bryophytes

trilete spores ~445 Ma

Found in groups of 4 (tetrads)

Trilete (‘Y’) marks indicate dessication-resistant spores

These are the first type of seeds of a plant that doesn’t need water in which to reproduce

Vascular plants (mid silurian):

Fossils from 425 Ma

Maintain internal water pressure (homiohydric)

No leaves or roots

Tube like structures

Could live on surface of rocks and regulate internal water pressure

Leaves and roots (late Silurian)

Fossils from 420 Ma

Baragwanathia related to modern clubmosses

Small leaves (microphylls) and proper roots

Come areas such up nutrients

Other areas become more specialised for photosynthesis

first trees (mid Devonian):

Wattieza ~385 Ma

Stumps have been known since 1870 from Gilboa, new York

Recently reunited with the crown of the tree

Up to 8m high

For spore dispersal

No proper leaves

Rignin – hard material in trees and plants – to build hard, stiff structure – go higher – beet other plants for sunlight etc.

 

The first forests (late Devonian):

Archaeopteris ~375 ma

Up to 10m high with 1.5 m trunk diameter

Widespread forests

New source of organic carbon for burial (lignins)

Must also have increased weathering

Large forests important for hydrological cycle – form rich soil deposits – hold water and co2 – amplify weathering

First forests would’ve had local climatic impacts

Rain forests today generate their own water

Consequences of extinctions:

Origination rates increase after extinctions as empty ecological niches are filled

After the end of the cretaceous extinction mammals diversified, rapidly increasingly in average size filling many of the niches vacated by dinosaurs