GOYENECHE AREQUIPA,
II. PLANTEAMIENTO TEÓRICO
2. MARCO CONCEPTUAL
One of the spectral biosignatures directly associated with vegetation on Earth is the “red edge” reflectance signature (see Table 1.2). This is caused by the infrared reflectance of land vegetation; hence, as vegetation coverage decreases, the strength of this signature
Microbe type Rate of gas consumption or production Source Rate from source Rate (g cell−1 yr−1)
Ammonia oxidisers Nitrosomonas species: 28.1 pmol - 0.2 fmol NH3 oxidised cell−1 d−1. Produces: NO 3×10−11 - 4×10−6 Boyd et al. (2011)
Carboxydotrophs Roseobacter species: Max. of 1.1-2.3×10−10 nmol CO oxidised cell−1 h−1
(2.7−5.6)×10−14 Tolli et al.
(2006)
Methanogens Wetland study:
273-665 µg CH4 pro-
duced per kg soil per day with 1.07-8.29×109 cell per gram soil
(2.9−9.1)×10−14 Liuet al. (2011) Sulphur oxidisers Thiooxidans species: (2.5−9.9)×10−4 µg S ox- idised cm−2 d−1 (0.9−3.6)×10−18 Smith et al. (2012) Anaerobic methanotrophs Reaction chamber: 8-33 nmol CH4 consumed
per gram sediment d−1
(4.7−19.4)×10−14 Girgius et al. (2003) Aerobic methanotrophs Landfill sites: 3-6.4 mmol CH4 con-
sumed per kg soil d−1
(1.8−23)×10−11 Kallistova et al.
(2005) Anaerobic iron
reducers
Anaerobic sediment sites: 9-130 nmol Fe reduced per g sediment h−1 Produces: CO2 (Other
iron reduction products: H2O or O2 (aquifer soils;
Sawyer et al., 1967), depending on electron donor; NB: using H2O
as an electron donor requires energy input)
(4.4−64)×10−12 Sørensen et al. (1982) Aerobic iron oxidisers Leptospirillum ferrooxi- dans: 10−5µmol Fe oxidised cell−1 d−1 No gases produced 2.1×10−7 Schrenk et al. (1997) Hydrogen oxidisers
Isolates of soil hydrogen oxidising bacteria:
0.08-0.92 µmol H2 oxi-
dised h−1 cm−3
(1.4−20)×10−12 Maimaiti et al.
(2007)
Anammox Anoxic water column site:
(1534−2228)×10−9mmol N cell−1 yr−1
(2.0−3.0)×10−11 Dalsgaard et al.
(2003)
Table 3.2: Production rates of biosignature gases from model species representing the metabolic pathways that are likely to be favourable on the far-future Earth.
Gas Present-day flux to atmosphere Source Flux g yr−1 Gas source
CH4 5.3×1013 Volcanoes Mud volcanoes Hydrocarbon seepage Burtonet al. (2013) CO2 3×1014 3×1014 Volcanoes Degassing Burtonet al. (2013) NO2 7×1012 Fixation by light- ning Tieet al. (2002)
SO2 (15−21)×1012 Volcanoes Halmer et al. (2002)
H2O 6.5×1014 Volcanoes Fischer (2008) CO CO:CO2 flux of 0.03-0.12 2−3×1014 Volcanoes Photochemical production Wardellet al. (2004) Zuo & Jones (1996)
Table 3.3: Modelled abiotic gas fluxes. Given reduced tectonic activity on the far-future Earth, volcanic gas fluxes are reduced to 20% of the present-day fluxes to account for the assumption that the only remaining volcanoes are hotspot volcanoes, which make up 20% of present-day active subaerial volcanoes. The photochemical CO flux is linked to the dissolved organic carbon content of atmospheric water droplets and so is assumed to be negligible once plant and animal life becomes extinct. Anthropogenic fluxes of gases such as CH4 and CO2 (of the order 1×1014 g yr−1 and 1×1015g yr−1, respectively) have a major impact of the present day climate. However, these effects occur over a geologically short period of time and so will not impact the Gyr-scale predictions of this model.
will decrease. The strength of this signature can be linked to the leaf area index (LAI) - a remote sensing term used as an estimate of vegetation coverage, defined as green leaf area/surface area. It has been shown by Filella & Pen˜nuelas (1994) that the area of the red edge peak in spectra changed with LAI, because LAI determines the ratio of near-IR and red reflectance. On Earth, sub-tropical dry zones tend to be sparsely vegetated and therefore would exhibit a much reduced red-edge signature. Warming temperatures would lead to the poleward expansion of Hadley cells - atmospheric circulation cells between 0◦ and±30◦latitude caused by warming air rising from equatorial regions and being deflected polewards by the tropopause temperature inversion. Most of the moisture carried by this warm air is lost as condensation during the upward phase of the circulation, which leads to the descending cooler air being dry (Trenberth & Stepaniak, 2004). This air descends at about±30◦ latitude, which is why much of the land area around these latitudes is arid or semi-arid (Johanson & Fu, 2009). The expansion of the extent in latitude of equatorial
Hadley cells,φH, with temperature can be approximated using φH ≈ 5gHt∆h 3Ω2R2 pθ0 12 , (3.5)
wheregis the acceleration due to gravity, Ht is the height of the tropical tropopause, ∆h is the equator-to-pole difference in radiative-equilibrium (i.e. under a constant radiation flux) potential temperature1, Ω is the angular rate of rotation of the Earth, Rp is the radius of the Earth andθ0 is the global mean temperature (Held & Hou, 1980; Showman
et al., 2010).
For animal biosignatures, predictions are made in Chapter 5, via inferences about the rates of extinctions. These can be predicted from factors like the upper temperature tolerances of different species, their tolerances to lower atmospheric oxygen concentrations, their abilities to move to more habitable environments when conditions in their present habitat become more hostile and their tolerances to reduced food supplies due to factors such as changes in vegetation cover (predicted from Hadley cell expansion). The by- products from animal decomposition will also be investigated.
For biosignatures associated with a declining microbial biosphere, the atmospheric gas reservoir from the microbial extinction model can be sampled at any time point. Initially set to the expected levels of biosignature gases after the extinction of all plants and animals, these gases evolve as described in Section 3.1.2. Knowing the mass of a gas in the atmosphere, alongside the general composition of the atmosphere, allows its detectability to be assessed.
3.3
Summary
By combining predictions of regional temperature evolution with methods for predicting the associated atmospheric gas compositions and the types of life able to exist in a given environment, the long-term future of a global biosphere can be outlined. By modelling how life interacts with an atmosphere, the biosignatures a global biosphere would produce and how these change over time can also be predicted, providing a tool-kit for modelling the biosignature changes with time of an Earth-like biosphere from conditions on the 1
For a parcel of air at pressure, the potential temperature is the temperature that parcel would acquire if adiabatically brought to a standard reference pressure.
4
Refuges for life near the end of the Earth’s
habitable lifetime
B
Y using the temperature model described in Chapter 2, the mean temperature evolution of the Earth at local scales, from the present to 3 Gyr into the future, was simulated. This enabled the type, abundance and extent of future habitats to be evaluated and also allowed a timespan for the remainder of Earth’s habitable lifetime to be set.4.1
Far-future temperature evolution
Figure 4.1 shows the general mean surface temperature trends predicted by the model for the next 2.5 Gyr. The steep increase in temperature at around 1 Gyr from the present represents the onset of rapid ocean evaporation. Assuming an upper temperature bound for life of 420 K (allowing some increase over the currently known upper temperature tolerance of thermophiles, cf. Chapter 3) and assuming no changes to obliquity or eccen-
250 300 350 400 450 500 550 600 650 0 0.5 1 1.5 2 2.5 3 Temperatur e (k)
Time from present (Gyr)
Poles Equator
Figure 4.1: Change in global mean temperatures over time with increasing solar luminosity. The dashed line represents equatorial temperature and the solid line represents polar temperature. After about 1 Gyr a moist greenhouse begins when temperatures reach 330 K allowing the water vapour content of the stratosphere to increase rapidly. When temperatures reach approximately 420 K life would likely no longer be able to survive. A runaway greenhouse regime begins after approximately 2.8 Gyr. Initially, the poles warm noticeably less rapidly than the equator; however, as the planet heats up, the equator-to-pole temperature gradient decreases due to an increased latitudinal heat diffusion coefficient, caused by the increase in atmospheric pressure. Note that the initial polar and equatorial temperatures are the same. This is a consequence of the initial temperature of the model being set to the present global mean temperature at all latitudes. The system then evolves away from this initial state.
tricity cycles, life could persist 0.7 Gyr longer at surface levels at polar latitudes than at the equator.
Rapid ocean loss as a result of a moist greenhouse effect would likely represent the end- point of an Earth-like planet’s habitable lifetime. Assuming ocean loss was not uniform across the globe due to regional temperature variations, there could potentially be pockets of liquid water that remain for a brief time before total loss of liquid surface water occurs. A source of liquid water is a prerequisite for life as we know it; hence, these last pools of water would represent the final habitable regions on a dying planet. In this section, potential locations for these last habitable regions are discussed. Precession effects (e.g. changes to longitude of perihelion) can be ignored in this case, as this simply changes the hemisphere that receives the most insolation, which will not influence the conclusions made over Gyr time-spans.