The Biotic Enhancement of Weathering and the Long Term Carbon Cycle

The biotic enhancement of weathering (BEW) is defined as how much faster the silicate weathering carbon sink is under biotic conditions than under abiotic conditions at the same atmospheric pCO2 level and surface temperature. If BEW is significantly greater than 1, on an abiotic Earth the steady state would occur with higher atmospheric carbon dioxide levels and surface temperatures doing the 'work' generating an equal flux as the outgas-sing flux, than the biotic cover on land does at lower atmospheric carbon dioxide levels and temperatures by the multifold processes included in BEW.

The likely contributors to the present BEW with forest and grassland ecosystems/soils as the main sites include:

1. soil stabilization (likely the most important, and likely more than a tenfold contribution to reactive mineral surface);

2. pCO 2 elevation in soil from root/microbial respiration and decay (Lovelock and Watson's original BEW);

3. organic acids/chelators in soil, the multifold interactions in the rhizosphere including biogeophysical weathering, biogeophysical/biochemical weathering by soil fungi leading to the breakup and digestion of CaMg silicate mineral particles;

4. evapotranspiration contribution to maintaining soil water and runoff;

5. diffusion of oxygen into soil contributing to oxidation of CaMgFe silicates and production of sulfuric and nitric acids;

6. water retention by soil organics;

7. turnover of soil by ants, earthworms, mixing organic and mineral particles (e.g., Darwin's pioneering research); and

8. biotic sink effect for mineral nutrients.

And if the cumulative global BEW is on the order of 10-100, then frost wedging contributing to greater surface area of reactive minerals is a component as well, since the progressive increase of BEW over geologic time cooled the climate so much that by about 1.5 Ga ice wedging emerged as a factor (see later discussion of Earth's surface-temperature history).

There are also biotic effects that likely slow down weathering:

1. accumulation of thick, depleted soils acting as a barrier to water flow to fresh bedrock;

2. macropores in soils that allow water flow bypassing saturated pore space.

Lichen coverage of bedrock in most cases promotes chemical denudation, so it is not an example of biotic retardation of weathering (BRW).

But even with these retarding effects, BEW dominates globally. An abiotic Earth surface has no soil, little rego-lith, so as a first approximation the land surface potential reacting with water/carbon dioxide is two-dimensional. The likely cumulative BEW at present is on the order of 100, with vascular plant ecosystems adding roughly an order of magnitude on the previous BEW of microbial/ lichen/bryophyte Earth (there is a good case for lichen weathering being a BEW of at least 10 times).

Since the emergence of higher plant ecosystems in the Paleozoic, positive feedbacks may have temporarily dominated the negative even on geologic timescales on the order of a million years. For example, this may have been the case when very warm global climates occurred, such as during the Triassic-Jurassic boundary, leading to potential lethal leaf overheating (e.g., several pathways such as a-b-c-d-u-q-r-g in Figure 4). On medium time-scales (>100 to 100 000 years), both negative and positive feedbacks are also possible.

Another cycle, the carbonate (see eqn [4]), with biotic mediation involved in the precipitation of marine calcium carbonate and the weathering of limestone and marble on land, also has a role in regulating atmospheric carbon dioxide levels and surface temperature on timescales less than c. 100 000 years. Any process which increases the rate of marine calcium carbonate deposition thereby leads to an increase in atmospheric carbon dioxide (eqn [4]), but on the longer timescale the carbonate-silicate cycle dominates with its negative feedback creating a steady-state atmospheric carbon dioxide level. The carbonate cycle is likely important in the glacial/interglacial cycle (e.g., during the Pleistocene), along with changes in oceanic circulation and terrestrial organic C cycling.

The influence of life on marine calcium carbonate precipitation also plays a role in the long-term C cycle by its influence on the site of this precipitation, the continental shelves, or deep ocean floor. Calcium carbonate deposited in the deep ocean is more likely to be subducted as the oceanic crust with its sediment cover plunges down the oceanic trenches. There, decarbonation occurs, thus releasing carbon dioxide to the atmosphere and increasing the source flux in the carbonate-silicate cycle. With the spread of the deep-water protists, cocco-lithophores, and foraminifera in the Mesozoic, this deep-sea deposition increased, thereby setting the stage for an increased source flux of carbon dioxide to the atmosphere, raising the steady-state atmospheric level of this greenhouse gas.

Carbonate Silicate Cycle

Figure 4 Geophysiological feedbacks between plants and carbon dioxide. It applies only when times of potential overheating of leaves due to high CO2 levels may have occurred. Arrows originate at causes and end at effects. Arrows with bull's-eyes represent inverse responses. Arrows without bull's-eyes represent direct responses. Letters adjacent to arrows designate paths followed by feedback loops. The timescales over which the paths operate are as follows: a, b, i, k, p, q, s, and u, 1-10 years; cand d, 10—103 years; f-h, j, m, n, r, and t, 103-106 years; e, >106 years. From Beerling DJ and Berner RA (2005) Feedbacks and coevolution of plants and atmospheric CO2. Proceedings of the National Academy of Sciences of the United States of America 102: 1302-1305.

Figure 4 Geophysiological feedbacks between plants and carbon dioxide. It applies only when times of potential overheating of leaves due to high CO2 levels may have occurred. Arrows originate at causes and end at effects. Arrows with bull's-eyes represent inverse responses. Arrows without bull's-eyes represent direct responses. Letters adjacent to arrows designate paths followed by feedback loops. The timescales over which the paths operate are as follows: a, b, i, k, p, q, s, and u, 1-10 years; cand d, 10—103 years; f-h, j, m, n, r, and t, 103-106 years; e, >106 years. From Beerling DJ and Berner RA (2005) Feedbacks and coevolution of plants and atmospheric CO2. Proceedings of the National Academy of Sciences of the United States of America 102: 1302-1305.

Biogeochemical Cycles of Oxygen, Phosphorus, and Sulfur and Their Influences on the Coevolution of Life and Climate

The biogeochemical carbon cycle is linked to the oxygen cycle and of course to the cycles of all the necessary nutrient elements for life. The rise of atmospheric oxygen by 1.9 Ga to approximately 20% present atmospheric level (PAL) predated the emergence of Metazoa, which may have required less than ^2% PAL (e.g., mud-dwelling nematodes). This history is consistent with a temperature constraint on the timing of the emergence of Metazoa. On the other hand, the atmospheric oxygen level plausibly constrained the emergence of megascopic eukaryotes, particularly Metazoa, as originally argued by the paleo-biologist Preston Cloud 30 years ago, with the explanation being the diffusion barrier of larger organisms. The rise of atmospheric oxygen in the Carboniferous, driven by a burst of organic carbon burial (hence organic carbon not oxidized) leading to the great coal deposits of this age, was by no coincidence the epoch of giant insects. Likewise, the apparent doubling of atmospheric oxygen levels in the last 200 million years has been linked to the evolution and increase in size of placental mammals.

Changes in the level of atmospheric carbon dioxide may also be linked to events in the history of the Earth's biota. An atmospheric pCO2 constraint on evolution has been suggested for the emergence of leaves (megaphylls) in Devonian triggered by sharp decline in atmospheric pCO 2 as well as for macroevolutionary events in marine fauna in the Phanerozoic and emergence of terrestrial C4 photosynthesis in the Paleogene. The rise of a methane-dominated greenhouse and concomitant drop in atmospheric carbon dioxide level at about 2.8 Ga has been proposed as the trigger for the emergence of cyanobacteria.

Phosphorus is an example of a minor nutrient element essential for several biomolecules, in particular, nucleic acids and ATP/ADP. Phosphorus plays an interesting role in the long-term C cycle (see Figure 5) because of its essentiality and its relatively insoluble state in the continental crust in phosphate minerals. Both negative and positive feedback loops are potentially important, such as A-H-Q-C (negative) and D-F-M (positive). The weathering flux of phosphorus is very small compared to the recycled flux in the biotic loop both on the land and in the sea.

The sulfur biogeochemical cycle is very complex, entailing several possible oxidation states (e.g., hydrogen sulfide, sulfates), hence linked to the oxygen cycle. In the ocean, dimethyl sulfide (DMS) produced by algae oxidizes in the

Concentric Weathering

Figure 5 Cause-effect feedback diagram for the long-term carbon cycle. Arrows originate at causes and end at effects. The arrows do not simply represent fluxes from one reservoir to another. Arrows with small concentric circles represent inverse responses; for example, as Ca-Mg silicate weathering increases, CO2 decreases. Arrows without concentric circles represent direct responses; for example, as organic C burial goes up, O2 goes up. Letters adjacent to arrows designate paths followed by feedback loops. T is temperature; pptn is precipitation. Tectonic processes include volcanic, metamorphic, diagenetic degassing; continental relief and position. Dashed lines between climate and tectonics or ocean circulation refer to complex combinations of physical processes. Nutrient aqueous P is phosphorus dissolved in natural waters that is available for uptake via photosynthesis, both continental and marine; FeP represents phosphate adsorbed on hydrous ferric oxides. Organic C and P burial includes that on the continents and in marine sediments. For diagrammatic clarity, arrows from organic carbon burial to organic weathering or degassing (i.e., recycling of carbon) are not shown. There is no arrow going directly from O2 to the weathering of organic carbon because of evidence that changes in atmospheric O2 probably do not affect organic carbon weathering rate. From Berner RA (1999) A new look at the long-term carbon cycle. GSA Today 9: 1-6.

Figure 5 Cause-effect feedback diagram for the long-term carbon cycle. Arrows originate at causes and end at effects. The arrows do not simply represent fluxes from one reservoir to another. Arrows with small concentric circles represent inverse responses; for example, as Ca-Mg silicate weathering increases, CO2 decreases. Arrows without concentric circles represent direct responses; for example, as organic C burial goes up, O2 goes up. Letters adjacent to arrows designate paths followed by feedback loops. T is temperature; pptn is precipitation. Tectonic processes include volcanic, metamorphic, diagenetic degassing; continental relief and position. Dashed lines between climate and tectonics or ocean circulation refer to complex combinations of physical processes. Nutrient aqueous P is phosphorus dissolved in natural waters that is available for uptake via photosynthesis, both continental and marine; FeP represents phosphate adsorbed on hydrous ferric oxides. Organic C and P burial includes that on the continents and in marine sediments. For diagrammatic clarity, arrows from organic carbon burial to organic weathering or degassing (i.e., recycling of carbon) are not shown. There is no arrow going directly from O2 to the weathering of organic carbon because of evidence that changes in atmospheric O2 probably do not affect organic carbon weathering rate. From Berner RA (1999) A new look at the long-term carbon cycle. GSA Today 9: 1-6.

atmosphere to form sulfate aerosols, which are cloud condensation nuclei (CCNs) over the ocean, raising cloud albedo and leading to surface cooling. The possibility of a negative feedback loop was originally proposed, where DMS production varies directly with temperature. However, the overall global feedback may be positive except during the coldest glacial episodes. The key feedback is the apparent positive link between cold water and marine productivity and DMS production. Apparently only eukaryotic algae produce DMS. Thus, in their absence, during most of the Archean, without comparable alternative sources of CCNs, the Earth's albedo might have been significantly lower, approaching a cloud-free value of around 0.1. This effect alone would have resulted in higher surface temperatures. However, there were other possible contributors to cloud albedo, such as intermittent organic haze produced by the reaction of methane and carbon dioxide.

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