Lecture 6: The Faint Young Sun Problem, Part 1-- Solar evolution/CO 2 feedbacks/Geochemical constraints Abiol 574

Solar luminosity versus time See The Earth System, ed. 2, Fig The fundamental problem of long-term climate evolution: the Sun was ~30% dimmer when it formed and has brightened more or less linearly since that time

Stellar nucleosynthesis Most of the Sun’s energy is produced by the proton-proton chain (at right) Overall reaction is 4 1 H + 2 e --> 4 He + 2 neutrinos + 6 photons Wikipedia: wiki/Image:FusionintheSun.png

Why the Sun gets brighter with time H fuses to form He in the core Core becomes denser Core contracts and heats up Fusion reactions proceed faster More energy is produced  more energy needs to be emitted

Now, consider the implications for Earth’s climate This problem was first pointed out in 1972 by Carl Sagan and George Mullen

The faint young Sun problem Kasting et al., Scientific American (1988) T e = effective radiating temperature = [S(1-A)/4  ] 1/4 T S = average surface temperature

Question: How did Earth remain habitable during its early history when the Sun was less bright? Was it Gaia, or was it something else?

First presented in the 1970s by James Lovelock The Gaia hypothesis

According to this hypothesis, organisms (plants and algae pulled CO 2 out of the atmosphere at precisely the right rate to offset increasing solar luminosity CO 2 + H 2 O  CH 2 O + O 2 Is this idea teleological?

Before trying to solve the faint young Sun problem, let’s look briefly at Earth’s long- term climate history 

Phanerozoic Time First shelly fossils Age of fishes First vascular plants on land Ice age First dinosaurs Dinosaurs go extinct Ice age (Pleistocene)

Geologic time Warm (?) Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Ice age

Now, let’s return to the FYS problem…

Possible solutions to the FYS problem Think back to the planetary energy balance equation  T e 4 = S (1 – A) 4 Albedo changes? –A possible feedback involving biogenic sulfur gases and cloud condensation nuclei has been suggested (Rosing et al., Nature, 2010), but it is probably not strong enough to solve the problem Geothermal heat? –Too small (0.09 W/m 2 vs. ~240 W/m 2 from absorbed sunlight

Possible solutions to the FYS problem Increasing the greenhouse effect also works T s = T e +  T g Possible greenhouse gases –NH 3 : Doesn’t work very well (photolyzes rapidly) –CO 2 : Works! (supplied by volcanoes) –CH 4 : Also works (probably requires life) – H 2 : This is a new (and surprising!) idea Sagan and Mullen (1972) liked methane (CH 4 ) and ammonia (NH 3 ) because they were aware that O 2 levels were low on the early Earth. We’ll return to that thought later.

To get a handle on climate evolution, it’s important to understand climate feedbacks For this, it is useful to define some systems notation…

Systems Notation = system component = positive coupling = negative coupling

Positive Feedback Loops (Destabilizing) Surface temperature Atmospheric H 2 O Greenhouse effect Water vapor feedback (+)

The faint young Sun problem Kasting et al., Scientific American (1988) Less H 2 O More H 2 O

Positive feedback loops (destabilizing) Surface temperature Snow and ice cover Planetary albedo Snow/ice albedo feedback (+) If this feedback were included in models of early climate, the FYS problem would be even worse

So, we need a negative feedback loop to stabilize Earth’s long-term climate What could this be?

The carbonate-silicate cycle (metamorphism) Silicate weathering slows down as the Earth cools  atmospheric CO 2 should build up

Negative feedback loops (stabilizing) The carbonate-silicate cycle feedback (−)(−) Surface temperature Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect

CO 2 vs. time if no other greenhouse gases (besides H 2 O) J. F. Kasting, Science (1993) Snowball Earth events

Is CO 2 the solution to the FYS problem? Thus, high CO 2 levels could, in principle, have solved the FYS problem Unfortunately, geochemists have made this problem more difficult by attempting to measure paleo-CO 2 concentrations J. F. Kasting, Science (1993)

pCO 2 from paleosols (2.8 Ga) Rye et al., Nature (1995 ) According to Rye et al., the absence of siderite (FeCO 3 ) places an upper bound on pCO 2 That limit depends on surface temperature, because siderite stability is temperature- dependent Today’s CO 2 level (3  atm) CO 2 upper limit

pCO 2 from paleosols (2.8 Ga) Rye et al., Nature (1995 ) Again, according to these authors, a reasonable upper bound on pCO 2, given the uncertainties, is 0.03 bar, or about 100 PAL Today’s CO 2 level (3  atm) CO 2 upper limit

(2006 ) Rye et al. Rye et al. recalculated Berthierine However, the thermodynamic data have changed over the past 15 years -- Free energy of formation of siderite is lower than previously thought ( P. Benezeth et al., Chem. Geol. (2009) According to Sheldon (2006), recalculation of the Rye et al. constraint using modern thermodynamic values yields pCO 2 well below today’s value (pCO 2  3  atm)

Sheldon’s pCO 2 estimate from paleosols Sheldon (2006) criticized the Rye et al. paper –Can’t use thermodynamic arguments when the entire suite of minerals is not present He presented an alternative analysis of paleosols based on mass balance arguments (efficiency of weathering) Sheldon’s calculated pCO 2 at 2.2 Ga is also about bar, or 28 PAL, with a quoted uncertainty of a factor of 3 New estimate for pCO 2 at 2.7 Ga (Driese et al., 2011): PAL N. Sheldon, Precambrian Res. (2006) Driese et al., 2011

New paleosol paper (under review) Kanzaki and Murakami (GCA, submitted) have done a lengthy reanalysis of all the old paleosol data. They find much higher pCO 2 levels than did Sheldon’s group. These data are from relatively recent paleosols. Sheldon/Driese

New paleosol paper (under review) Kanzaki & Murakami, GCA (submitted) If the new paleosol analysis is correct, then CO 2 could have been high enough to solve the faint young Sun problem by itself

(2010) Another attempt to constrain Archean CO 2 concentrations has been made using banded iron-formations, or BIFS We’ll talk more about BIFs later in the semester, because they tell us things about O 2

Banded iron- formation or BIF (>1.8 b.y ago) Fe +2 is soluble, while Fe +3 is not BIFs require long- range transport of iron  The deep ocean was anoxic when BIFs formed

Rosing et al. place constraints on pCO 2 based on the mineralogy of banded iron-formations, or BIFs Siderite (FeCO 3 ) and magnetite (Fe 3 O 4 ) are found within the same units  pCO 2 should lie near the phase boundary The implied atmospheric CO 2 concentrations are really low, about bar, or 3 PAL (2010)

The methanogen constraint on pH 2 comes from the metabolic reaction CO H 2  CH H 2 O According to the Nernst equation  G R =  G 0 + RT ln Q Where Q = pCH 4 / (pCO 2  pH 2 4 ) Methanogens and other anaerobes are able to pull down H 2 until  G R  9-15 kJ/mol (2010)

Rosing et al.: CO 2 from BIFs J. F. Kasting Nature (2010) Rye et al. (old) Ohmoto Sheldon von Paris et al. If the new CO 2 constraints are correct, then other warming mechanisms are clearly needed (but the BIF constraints are almost certainly wrong!) Rosing et al. suggest a reduced cloud albedo caused by the absence of biogenic sulfur gases, but this doesn’t work if the climate was as warm or warmer than today (Goldblatt and Zahnle, Climate of the Past, 2010)

There are reasons to question the Rosing et al. analysis, however Conversion of magnetite into siderite requires a reductant, for example, hydrogen: Fe 3 O CO 2 + H 2  3 FeCO 3 + H 2 O But, if you go through the math on this, there was not enough H 2 available to make this work Nature, 2011 Dauphas and Kasting, Nature (2011)

Conversion of magnetite into siderite requires a reductant Fe 3 O CH 2 O + 2 H 2 O  3 FeCO H 2 Light carbon isotopes in the siderite indicate that the reductant was organic carbon –If you run out of organic carbon, then magnetite would remain stable even under high pCO 2 –Jim Walker foresaw this in a 1987 Nature paper entitled “Was the Archean biosphere upsidedown?” Nature, 2011

The upsidedown Archean biosphere Today The biosphere is based on oxygenic photosynthesis CO 2 + H 2 O  CH 2 O + O 2 Organic carbon is less mobile than O 2, so sediments are reduced and the atmosphere is oxidized

The upsidedown Archean biosphere In the Archean, however, the situation was reversed The Archean biosphere was based on various forms of anoxygenic photosynthesis, e.g. 4 Fe ++ + CO H 2 O  4 Fe(OH) 3 + CH 2 O + 8 H + Organic carbon can then decay into methane via fermentation and methanogenesis, which sum to 2 CH 2 O  CO 2 + CH 4 Iron oxides are less mobile than CH 4 ; hence, during the Archean, sediments were oxidized and the atmosphere was reduced

BIF depositional environment (Early Proterozoic, Kuruman Iron Formation, South Africa) Klein & Beukes, Econ. Geol. (1989) Siderite forms in the near-shore environment Hematite forms off-shore Magnetite forms somewhere in-between

Possible Archean BIF depositional environment 4 Fe ++ + CO H 2 O  4 Fe(OH) 3 + CH 2 O + 8 H + pH 2  atm Fe 2 O 3 Fe 3 O 4 FeCO 3 Iron-oxidizing bacteria (photosynthetic) Iron-reducing bacteria 3 Fe(OH) H 2  Fe 3 O H 2 O pH 2  3  atm pO 2  0 Hydrogen minimum zone