Life and the Planet – Part 2

23 05 2011

Lynn Margulis, who as Lovelock said earlier “put the flesh and bones” on Gaia, spoke on ‘Evolutionary novelty in the Proterozoic eon: Symbiogenesis in Gaia’. She described a sequence of evolutionary events involving eubacteria and thermobacteria coming together to form the first eukaryotes. This occurred not through random mutations but through symbiosis occurring over evolutionary time scales, or symbiogenesis. While Lynn is often credited with the theory of symbiogenesis she emphatically states that others preceded her in this idea, particular a Russian scientist, Boris Mikhaylovich Kozo-Polyansky, who in 1924 published a book “Symbiogenesis: A New Principle in Evolution”. Still, Lynn undoubtedly put the “flesh and bones” on the theory of Symbiogenesis as well.

Nicholas Butterfield spoke on ‘Multicellularity in deep time’ where he described the early fossil record of various multicellular life forms. He pointed out that by ~1 Ga ago there is evidence for clonal colonies of cyanobacteria, coenobial and filamentous green algae, and branched multicellular filaments of red algae. There is even a 850 Ma old fungus-like fossil with complex multicellular vesicles/hyphae. He states, however, that at this time there is “not a whisper of land plant fossils”. Doubting that this is a preservation issue, he left open the question of plant and animal life on land in the Proterozoic.

Speaking on ‘Neoproterozoic glaciation: Microbes at work in terrestrial oases’ Ian Fairchild acknowledged that even under the most extreme conditions of Snowball Earth life must have persisted and even flourished in places. He described stratigraphic sequences from northern Svalbard which bear units of sandstone, rhythmites, and carbonates which appear to owe their origin, in part, to microbial mats of cyanobacteria. He concludes that “extremophile” life flourished at this time and provided a geochemical record of the Cryogenian (Snowball Earth) period. Unfortunately, he offered no ideas on possible biological feedbacks on the climate.

One of the biological feedbacks that must be considered in any discussion of continental glaciation is the moisture transport problem in initiating the buildup of an ice cap in the dry interior of a continent. The presence of extensive sponge-like microbial mats (proto-peatlands if you will, or “slimeball earth”) would readily solve the transport problem, as well as provide sources of moisture and condensation nuclei for cloud formation. The organic acids and sulfur gases emitted by these mats would increase the acidity of rainfall and groundwater, thus enhance weathering of nutrients, especially iron that would flow to the ocean and stimulate phytoplankton production. In my view, peat bogs are a somewhat more complex modern-day analogs of these early ecosystems. My own findings indicate that there are a number of significant feedbacks involving peatland dynamics and the most recent (Pleistocene) glaciations.

Graham Shields-Zhou covered ‘The Neoproterozoic Oxygenation Event’ (not to be confused with the earlier Great Oxidation Event), focusing on fossil and geochemical evidence linking this event to the rise of animals. He surmised from carbon isotopes that the oxygenation was accompanied by are large burial of carbon (mmmm, proto-peatlands?). He also pointed out that there were significant excursions in the carbon isotopes, indicating a very dynamic carbon cycle that quickly changed modes (mmmm, like in a living system?). Still, no mention of biological feedbacks.

Andy Ridgwell’s talk ‘Evolution and revolution in marine (carbonate) carbon cycling’ described the results of a mathematical modeling study that shows how carbonate uptake via biomineralization by planktic calcifiers enhanced the stability of the climate system. The feedback mechanism acts to “buffer ocean carbonate-ion concentration” via “saturation-dependent preservation of carbonate in sea-floor sediments”. He found bioturbation to have a significant buffering effect, according to the model.

Dr. Ridgwell may well have made an important contribution to the science of Gaia in this work. I think this mechanism deserves some deeper investigation using empirical studies. I am always suspicious of modeling studies. Mathematical models are the constructs of humans, not nature. Thus, the models have knobs (parameters) that can be adjusted to provide whatever image we wish to see.

At the start of the second day of the meeting Liam Dolan, speaking on ‘The first land plants and their effect on the planet’, asked the question of whether the first land plants, the non-vascular mosses and liverworts, could have enhanced silicate weathering rates of rocks, thus lowering CO2. He and coauthor Tim Lenton proposed that the rhizoids (root-like structures) of bryophytes (mosses and liverworts) may play a significant role in weathering through the secretion of various organic acids (e.g. malic acid, citric acid, glyceric acid, succonic acid) into the substrate. They devised an experiment to characterize the weathering processes of mosses, growing moss on andesite rock with water and comparing that to a control container with only andesite and water. After 90 days the moss container showed a very large enhancement of magnesium and calcium and also a significant enhancement of potassium, aluminum and iron compared to the control. Applying these results to a model (COPSE) they concluded that moss-enhanced weathering rates could account for the drawdown of CO2 and cooling of the planet during the Ordovician (circa 470 Ma ago).

Of course, if there were mosses in the Ordovocian then there likely were peat mosses and peatlands as well. What about the carbon storage of these early peatlands playing a role in the drawdown of CO2? Also, the weathering would not only be limited to the rhizoid/substrate interface. Peatlands produce high levels of gaseous sulfur compounds that oxidize in the atmosphere to form sulfuric acid, which will dissolve rock hundreds of miles away. Peatlands also produce dissolved organic matter rich in sulfur and metals that flows into the streams and groundwater feeding a vast array of microbial communities, including the deep earth microbes associated with cavern formation. And then there are the stimulating effects of iron fertilization in oceans from enhanced iron chelation in the waters draining peatlands, and the associated increases in planktonic sulfur emissions leading to cloud condensation nuclei and sulfuric acid in the atmosphere, some of which falls back onto the land. None of this is included in the models of biotic enhancement of weathering. I doubt any of this could be modeled, but that is beside the point.

I found the next several talks to be unrelated to Gaia or biospheric feedbacks and I will refrain from reporting or commenting.

Tim Lenton spoke on ‘The Neoproterozoic revolution in oxygenation, biochemistry and biological complexity’ and gave an excellent overview of our understanding of the terrestrial biota of the early earth. He reiterated the importance of biotic amplification of weathering, especially the weathering of phosphorus, which is limiting to ocean productivity. He made a case for extreme glaciations, brought about by the enhanced weathering and lower CO2, as favoring the evolution of altruism.

I felt Tim’s altruism argument was a good one, and one I hadn’t considered before. However, I don’t know why there is such a fixation on linking every glaciation to greenhouse gas concentration. There are many other factors that come into play. During the question period afterwards Bill Chaloner pointed out that tectonics could also have played an important role, but that there was no mention of tectonic changes in the talk.  I pointed out that while the biotic weathering of the early lands plants was being considered in the model, the ecology also needed to be considered, especially how the sponge-like ecosystems could have altered the cloud and rainfall patterns.

Robert Foley presented on ‘Human evolution, environment and climate’, and his first point was that ancient people had little effect on the earth, though no evidence was offered. That allowed him to then pose the question, “Did climate change cause human evolution?” He pointed out that the evolution of terrestriality (habitat shifting from forest canopy to ground dwellers) coincided with a cooling trend during the Miocene and Pliocene that “caused” savanna environments. The cooling he attributed to earth orbital variations and, thus, quipped “human evolution is caused by changes in the earth’s orbit”. Foley suggests that eventually humans did have an effect on the environment, with evidence of fire-induced environmental change at about 50 Ka ago. The main environmental impact of humans came with the development of agriculture at 12 Ka ago.

I question several of the assertions in this talk, especially the idea that a cooling climate increases the extent of savanna in tropical latitudes. The evidence I have seen is that the opposite is true. Cooling climates cause an altitudinal lowering of the climate zones, with forest replacing savanna, and tropical peatlands replacing forest. I suspect that the paleo records are misinterpreting the evidence of low forest cover, classifying it as savanna when it might well be peatland. Both are low in tree cover.




One response

30 05 2011

Lee, I just wanted to say thanks for putting all this stuff up on the website – quite a lot of work I can imagine, not only writing it and checking all the facts, but also writing all your notes during the conference itself with the high degree of concentration required at all times.

Your comments about the GHG-fixation also well taken. There is a strong tendency to ascribe all environmental changes to GHG-induced global warming when we humans are intervening at so many levels, altering the Earth’s surface with farming, deforestation, (over-)grazing, urbanization, mining, river-damming, irrigation, flood control, drainage, and so on.

In many cases these other changes have a much greater and more immediate effect on the environment than GHGs. All are important.

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