Carbon and the Early Anthropocene Flashcards
First Atmosphere
- All carbon originally brought to the planet by meteorite accretion, but was in the geology
- All carbon from meteorites
- Therefore, all carbon on Earth is finite
- Changes in carbon isotopes over time are good indication of internal change as amount of carbon not changing
- Composition - Probably H2, He
- These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth’s history because Earth’s gravity is not strong enough to hold lighter gases
2nd Atmosphere
• Produced by volcanic outgassing
• Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2) and NH3 (ammonia) and CH4 (methane)
• No free O2 at this time (not found in volcanic gases)
• Ocean Formation - As the Earth cooled, H2O produced by outgassing could exist as liquid in the Early Archaean allowing oceans to form
o Know due to presence of pillow basalts
3rd Atmosphere
- Cyanobacteria absorbed CO2 and produced oxygen
- Initially consumed immediately by oxidising rocks and atmospheric methane (producing CO2)
- Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in the atmosphere
- During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10 %
- Present levels of O2 were probably not achieved until ~400 Ma
- Stromatolites present
Other planets atmosphere (infrared spectra):
- Ozone (O2) absent from other planets
- O3 on spectra shows possibility for extra-terrestrial life
- If present, then something must be producing it
- Carbon added to atmosphere constantly by volcanoes…the Earth’s atmosphere would resemble Venus’s if not for carbon SINKS
Carbon isotopes revision
- Organic carbon is enriched in 12C relative to 13C – it is isotopically “light” and d13C values are highly negative: generally, -25‰ PDB
- Inorganic calcite shells secreted by organisms are in equilibrium with sea water, d13C ~0, plus or minus a few points ‰.
- Changing the global volume of organic carbon within rock reservoirs affects the d13C of sea water, and hence the d13C of shells of forams etc. This is what produces the global d13C “wiggle curve” – see next slide.
- d13C = 0 is just the value assigned to the sample used as the global standard (the Pee Dee Belemnite); it has no unique significance
Variation in d13C in marine sediments implicates carbon cycle
- Bulk marine carbonate d13C >0, means more carbon stored in rocks or plants than there was at the time of the standard (when by definition d13C = 0)
- d13C <0, means less carbon stored in rocks or plants than there was at the time of the standard (when by definition d13C = 0)
- In the Late Cenozoic, since ~18 Ma, d13C has declined
- C12 being sequestered on land means more C13 in shells and vice versa
Organic carbon cycle
• Most carbon in plants or plankton is quickly reduced in the ocean-atmosphere system by oxidation
• Only a minute fraction of the organic carbon formed by photosynthesis ends up in rocks
• Organic carbon is returned from rocks by:
o Oxidation of organic rich rocks
o Thermal cracking of organic molecules with depth and the consequent release of CO2
• Because 20% of Earth’s carbon is included in this sub-cycle it has over geological timescales the potential to have large amplitude effects on the global carbon balance
• Organic carbon cycle (under certain conditions) has potential to react more rapidly than carbonate subcycle
Carbon Sink #1
• Silicate weathering • CaSiO3 + 2CO2 + H2O à Ca+2 + 2HCO3 + SiO2 • Dissolved ions transported to the ocean • Precipitation of calcite and silica • Ca+2 + 2HCO3 CaCO3 + CO2 + H2O • SiO2 SiO2 (opal) • Net reaction: CaSiO3 + CO2 à CaCO3 + SiO2
Carbon removed from atmosphere and buried in geology
Carbon and climate Globally
- ~80% of all CaCO3 precipitated in open ocean (~ 1Gt) dissolves in water column
- ~ 1Gt accumulates as CaCO3 sediment
- 1 Gigatonne = 1015 grams = 1 Petagram
- This CO2 must be replaced annually if climate system is to remain in stasis
Carbon Sink #2: Organic carbon burial
Early Carboniferous (340 mya)
• Abundance of shallow seas
• Coal swamps
• Ice sheet
• Mud rocks deposited under anoxic conditions preserve organic carbon
o (Source rocks for oil and gas deposits; coal measures)
Carbon Sink #3: Forests
- Trees only act as sink while they are growing
- Fully sized, old trees are no longer growing and therefore not sequestering CO2
- Once tree dies microbial breakdown returns C to atmosphere
- C is only ‘permanently’ sequestered if tree C buried and stored in the geology
- Vegetation does accelerate silicate weathering
LGM vs Holocene: An enigma
- LGM drier and vast areas covered by ice
- Land contained 300-700 Pg less C during LGM!!
- If land has less C, and the atmosphere has less C, where is all the C?
Possibilities:
• More CO2 degassing from warmer ocean
• More iron fertilisation of ocean due to more dust influx to oceans
• Increased ventilation rates of deep ocean
- Gross terrestrial primary production during the Last Glacial Maximum was about 40 Pg C per year, half that of the pre-industrial Holocene
- Despite the low levels of photosynthesis, estimate that the late glacial terrestrial biosphere contained only 330 Pg less carbon
- Large, inert reservoir of carbon somewhere on land
Evolution of the global carbonate cycle - ‘Strangelove Oceans’-
- Sharp negative d13C anomaly interpreted as mass extinction event
- Oceans devoid of life
- Part of Snowball Earth?
- ‘Type’ example is in Neoproterozoic
- Plants metabolise 12C easier than 13C – so plant material has lower d13C
- Biology therefore is a major reservoir of 12C
- Mass extinction releases a pulse of 12C
‘Strangelove’ ocean
- Geochemistry-ruled CaCO3 cycling, biogenic precipitation of CaCO3 is essentially absent.
- Characterized by high-supersaturation and generally inorganic (at most partly bacterially mediated) formation of carbonates.
- Attempt to explain cap carbonates predating Snowball Earth? Term used at other times, such as post K-T impact
Neritan ocean
- Cambrian biomineralization permitted considerable biologically controlled carbonate precipitation in shallow-water (neritic) environments.
- Dominant mode of Ca2+ and CO32− removal from seawater is biogenic, neritic carbonate deposition.
- Neritan ocean saturation state is highly susceptible to changes shallow-water calcifier populations.
