XI Life and the planet.

Transcription

1 XI Life and the planet

2 11.0 Introduction Life interacts with the planet, and changes the properties of a planet and its atmosphere. The most obvious example is the oxygenation of the Earth s atmosphere. The Earth can be thought of as comprising two inter-linked systems: living (biotic, or the biosphere), and non-living (abiotic). The abiotic system is further composed of the hydrosphere (liquid water), lithosphere (crust), atmosphere, and cryosphere (ice, frozen zones). The presence of a biosphere can (significantly) alter the properties of abiotic systems.

3 11.1 Biogeochemical cycles A biogeochemical cycle is a route through which a chemical (element or molecule) passes through the different systems of a planet. Life alters the properties of a planet it provides new routes for cycles, produces chemicals that would otherwise not be present, and... Life is out of equilibrium with its surroundings therefore a planet with life is not in equilibrium (at least the parts where there is significant life in our case the surface layers). That life changes the global properties of a planet is the key to allowing life to be detected by astronomers.

4 11.2 The carbon cycle One of the most interesting and important biogeochemical cycles is the carbon cycle. The carbon cycle controls the passage of carbon between its reservoirs: In the biosphere carbon is mainly stored as a component of living things, In the hydrosphere carbon is dissolved in the oceans, In the lithosphere carbon is mainly stored in sedimentary rocks, In the atmosphere carbon is in the form of CO 2.

5 11.2 The carbon cycle The carbon cycle is NOT about the fixing of atmospheric carbon by plants. If I see this in an essay you will be heavily penalised. The carbon cycle is the cycle in which carbon dioxide is released by volcanoes is then locked into carbonate rocks, subducted, and rereleased.

6 11.2 Carbon reservoirs Oceanic carbon ~3900 Gt, stored mostly as bicarbonate: CO 2 + H 2 O -> H 2 CO3 -> HCO H + in intermediate and deep ocean storage. Biospheric carbon ~2100 Gt, stored in biomass (600 Gt in vegetation, 1500 Gt in soils and dead organic matter). Lithospheric carbon ~90 million Gt (+ 150 Gt in surface sediments and Gt in recoverable fossil fuels). Atmospheric carbon ~750 Gt almost all in CO 2 (some CH 4 )

7 11.2 Carbon reservoirs

8 11.2 Primary sources and sinks of carbon On geological timescales the CO 2 content of the atmosphere is controlled by the balance between weathering, subduction and volcanic output. The primary source of CO 2 into the atmosphere is volcanic outgassing. Most of this outgassing is of carbon stored in the lithosphere when the Earth was formed. However, much (most) of the atmosphere's initial CO 2 content is now stored in sedimentary rocks (esp. coal and black shale in the form of undecayed biomass).

9 11.2 Weathering Some CO 2 is removed from the atmosphere by dissolving it in water CO 2 + H 2 O -> H 2 CO 3 (Carbonic acid) however, the atmosphere is in approximate equilibrium with the dissolved CO 2 and this does not remove CO 2. But, carbonic acid weathers silicate (igneous) rocks: H 2 CO 3 + H 2 O + silicates -> HCO + + cations(ca ++, Fe ++, Ne +, etc.) + clays how rapidly this occurs depends on the temperature. Calcium carbonate is then precipitated from the bicarbonate in seawater marine organisms can speed this up, but are not crucial Ca HCO 3 - -> CaCO 3 (limestone) + CO 2 + H 2 O (Note 2 CO 2 initially release 1 CO 2 ).

10 11.2 Weathering The carbon cycle acts to moderate the Earth's temperature: For example, if volcanic outgassing increases (e.g. the Deccan Traps), CO 2 levels rise and the global temperature increases. More CO 2 means more carbonic acid (as they are in equilibrium), and more heat means more weathering. More weathering means more CO 2 removed from the atmosphere into carbonates, and a lowering of the greenhouse effect. If global temperatures fall, less weathering will occur (as weathering is temperature dependent) and so global CO 2 levels will rise as volcanoes continue to outgas. Temperatures are regulated on geological timescales.

11 11.3 Pre-requisites To have a carbon cycle regulating the surface temperature a planet needs: 1. Plate tectonics to drive volcanism and also subduction and reprocessing (needs oceans?). 2. An ocean in which to precipitate the carbonates. 3. Weatherable silicate rocks above the surface of the sea this means continent building associated with plate tectonics.

12 11.3 Plate tectonics Revision from PHY106: The Earth s crust is made of 7 major and several minor plates : solid blocks of crust (mostly igneous rock) that can move over the athenosphere (the outer mantle which is very fluid due to mixing with water). New crust is formed where plates separate (e.g. mid-atlantic Ridge), and crust is destroyed in subduction zones (e.g. along the Pacific coast of South America).

13 11.3 Plate tectonics The Earth has two types of crust: oceanic and continental. Continental crust is less dense and so can sit on old oceanic crust and be above the oceans. Initially the Earth will have had only oceanic crust and so little (or no) land above the ocean. It is unclear how quickly continents were built.

14 11.4 Paleoclimate The Sun has got 20-30% more luminous over the past 4.5 Gyr, but the Earth s temperature has remained roughly stable at an average of about 20C (note we are currently in an ice age, so global temperatures are a bit lower than normal). To counter the increased energy input from the Sun the Earth must have changed its properties, in particular the greenhouse effect must be less carbon dioxide removed from the atmosphere by the carbon cycle (plus changes in other greenhouse albedo and maybe albedo).

15 11.4 Paleoclimate For the Earth to have remained habitable, the carbon cycle seems critical: this means volcanism, plate tectonics and liquid water. The runaway greenhouse on Venus might have been due to a lack of a carbon cycle, or maybe it was not efficient enough and was stopped by the evaporation of the oceans? The runaway icehouse on Mars was caused by the cooling of the planet stopping volcanism, so not introducing any new atmosphere resulting in a constantly decreasing greenhouse effect. How delicate is the carbon cycle? What is needed to start it? How difficult is it to keep active? Without a carbon cycle is there any way to avoid a runaway icehouse or greenhouse?

16 11.5 The oxygen cycle The oxygen cycle controls the movement of oxygen between the lithosphere (~99.5% of O), atmosphere (~0.49%) and biosphere (~0.01%). Lithospheric oxygen is very tightly bound into oxides (ferric and silicate mainly). Its main source for biological activity is from volcanic CO 2 with is converted by photosynthesis (with a small amount created by photolysis). Of the ~10 18 kg of O in the atmosphere, every year photosynthesis adds ~3x10 15 kg, and biological activity (mainly respiration) removes ~95% of this (the rest is removed by the combustion of fossil fuels, weathering, and several other sinks).

17 11.5 The nitrogen cycle Most of the Earth's nitrogen is stored in the atmosphere. Nitrogen needs to be converted from the relatively inert atmospheric N 2 into forms usable by life (called nitrogen fixation). Biological nitrogen fixation occurs in specialist bacteria using specialised nitrogenase enzymes which act as a catalyst in the reaction N 2 + 6H + energy(atp) -> 2NH 3 the details are quite nasty. The important point is that a biological process is required to create biologically useful nitrogen, without nitrogenase there would be much less life on Earth as there would not be a significant source of ammonia.

18 11.5 The phosphorus cycle As well as the CHON elements, phosphorus is a vital element for life. P is a vital nutrient (used in nucleotides in DNA and for energy storage in ATP for example). The P-cycle unusually does not involve the atmosphere, but moves between the lithosphere, hydrosphere and biosphere.

19 11.6 Limiting nutrients If a particular chemical is in short supply it can cause huge problems, limiting growth and expansion. One theory about the Cambrian Explosion was that scarce nutrients (in particular P) rapidly became available at the break-up of a supercontinent. The elemental composition of plants is: C (45%), O (45%), H (6%), N (1.5%), K (1%), Ca (0.5%), P (0.2%), Mg (0.2%), S (0.1%) Important trace elements in order (> 0.001%): Cl, Fe, Mn, Zn, B. A shortage of any of these elements will limit the size of the biosphere.

20 11.7 The effect of life on the planet The presence of life on Earth significantly changes a number of the Earth's global planetary characteristics, in particular: The atmospheric composition is largely determined by biological activity. The presumably largely CO 2 atmosphere of the young planet has been converted by the carbon cycle and biological activity into an oxygen-nitrogen atmosphere (without life renewing it the oxygen would disappear in ~4500 yrs). Life modifies the lithosphere. Some rocks are entirely the result of biological activity (e.g. Limestone, fossil fuel deposits).

21 Summary Terrestrial life modifies the Earth, life is a vital component in a number of cycles, hence biogeochemical. The carbon cycle seems crucial (but is not bio, although life can assist). It depends on tectonic activity to recycle lithospheric carbon, without tectonic activity could life exist on Earth? Would all usable sources of carbon become exhausted leaving a nitrogen atmosphere? Note that plate tectonics requires large quantities of water to 'lubricate' the plates. This regulation has been taken by some to suggest that the entire planet acts as a single organism (the Gaia hypothesis).