Archean Atmosphere Faint young Sun paradox presents dilemma Faint young Sun paradox presents dilemma  1) What is the source for high levels of greenhouse gases in Earth’s earliest atmosphere?  2) How were those gases removed with time?  Models indicate Sun’s strength increased slowly with time  Geologic record strongly suggests Earth maintained a moderate climate throughout Earth history (i.e., no runaway greenhouse like on Venus)

Source of Greenhouse Gases Input of CO 2 and other greenhouse gases from volcanic emissions Input of CO 2 and other greenhouse gases from volcanic emissions  Most likely cause of high levels in Archean

Is Volcanic CO 2 Earth’s Thermostat? If volcanic CO 2 emissions provided Archean greenhouse, has volcanic activity continuously slowed through geologic time? No, but… If volcanic CO 2 emissions provided Archean greenhouse, has volcanic activity continuously slowed through geologic time? No, but… Carbon input balanced by removal Carbon input balanced by removal  Near surface carbon reservoirs Stop all volcanic input of CO 2 Stop all volcanic input of CO 2  Take 270,000 years to deplete atmospheric CO 2  Surface carbon reservoirs (41,700 gt) divided by volcanic carbon input (0.15 gt y -1 )  Rate of volcanic CO 2 emissions have potential to strongly affect atmospheric CO 2 levels on billion-year timescale

Volcanic CO 2 inputs? No geologic, geophysical or geochemical evidence indicates that rates of tectonism decreased slowly through Earth history No geologic, geophysical or geochemical evidence indicates that rates of tectonism decreased slowly through Earth history  Rates of volcanic CO 2 input did not change slowly with time  Volcanic CO 2 emissions did not moderate Earth climate through geologic time  If not inputs, what about a change in removal rate of atmospheric CO 2 ?

Removal of Atmospheric CO 2 Slow chemical weathering of continental rocks balances input of CO 2 to atmosphere Slow chemical weathering of continental rocks balances input of CO 2 to atmosphere Chemical weathering reactions important Chemical weathering reactions important  Hydrolysis and Dissolution

Hydrolysis Main mechanism of chemical weathering that removes atmospheric CO 2 Main mechanism of chemical weathering that removes atmospheric CO 2 Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Summarized by the Urey reaction Summarized by the Urey reaction  CaSiO 3 + H 2 CO 3  CaCO 3 + SiO 2 + H 2 O  Atmospheric CO 2 is carbon source for carbonic acid in groundwater  Urey reaction summarizes atmospheric CO 2 removal and burial in marine sediments  Accounts for 80% of CO 2 removal

Dissolution Kinetics of dissolution reactions faster than hydrolysis Kinetics of dissolution reactions faster than hydrolysis Dissolution reaction neither efficient nor long term Dissolution reaction neither efficient nor long term Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean  CaCO 3 + H 2 CO 3  CaCO 3 + H 2 O + CO 2  Although no net removal of CO 2  Temporary removal from atmosphere

Atmospheric CO 2 Balance Slow silicate rock weathering balances long-term build-up of atmospheric CO 2 Slow silicate rock weathering balances long-term build-up of atmospheric CO 2  On the million-year time scale  Rate of chemical hydrolysis balance rate of volcanic emissions of CO 2  Neither rate was constant with time  Earth’s long term habitably requires only that the two are reasonably well balanced

What Controls Weathering Reactions? Chemical weathering influenced by Chemical weathering influenced by  Temperature  Weathering rates double with 10°C rise  Precipitation  H 2 O is required for hydrolysis Increased rainfall increases soil saturationIncreased rainfall increases soil saturation  H 2 O and CO 2 form carbonic acid  Vegetation  Respiration in soils produces CO 2 CO 2 in soils x higher than atmospheric CO 2CO 2 in soils x higher than atmospheric CO 2

Climate Controls Chemical Weathering Precipitation closely linked with temperature Precipitation closely linked with temperature  Warm air holds more water than cold air Vegetation closely linked with precipitation and temperature Vegetation closely linked with precipitation and temperature  Plants need water  Rates of photosynthesis correlated with temperature

Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that reduces the intensity of climate warming Chemical weathering can provide negative feedback that reduces the intensity of climate warming

Chemical Weathering: Earth’s Thermostat? Chemical weathering can provide negative feedback that reduces the intensity of climate cooling Chemical weathering can provide negative feedback that reduces the intensity of climate cooling

Greenhouse vs. Faint Young Sun Cold surface temperatures created by the faint young Sun compensated by stronger atmospheric CO 2 greenhouse effect Cold surface temperatures created by the faint young Sun compensated by stronger atmospheric CO 2 greenhouse effect

Archean Volcanism & Weathering Early Archean volcanism probably produced more atmospheric CO 2 Early Archean volcanism probably produced more atmospheric CO 2  Counteracted lower radiant energy and warmed our planet  Volcanism did not slow at same rate as Sun increase in strength Earth Earth probably still cold Earth Earth probably still cold  Weathering slow  Continents small  Continental crustal rocks silica-poor (basaltic)  Stoichiometry of Urey reaction different  Less efficient CO 2 removal from atmosphere

Greenhouse vs. Faint Young Sun When solar luminosity strengthen, chemical weathering increased and helped transfer atmospheric CO 2 into sediments When solar luminosity strengthen, chemical weathering increased and helped transfer atmospheric CO 2 into sediments

Phanerozoic Volcanism & Weathering As solar luminosity increased As solar luminosity increased  Earth warmed and became wetter  Chemical weathering increased  CO 2 levels dropped Continental crust grew during Pre- Cambrian Continental crust grew during Pre- Cambrian  Became more siliceous (granitic) Slow warming of Earth Slow warming of Earth  Caused changes in chemical weathering  Moderated Earth’s climate

Other Greenhouse Gases? Why not other greenhouse gases? Why not other greenhouse gases?  CH 4 and NH 3  Oxidize rapidly in atmosphere  Are biologically utilized  H 2 O  Detritial sediments indicate liquid water present on Earth for last 4 by  H 2 O(v) in atmosphere provides positive climate feedback

Gaia Hypothesis Biology affects geochemical processes that influence climate Biology affects geochemical processes that influence climate Gaia hypothesis Gaia hypothesis  Life has regulated Earth’s climate  All evolution occurred to keep Earth habitable (extreme interpretation)  Life affected atmospheric O 2 evolution  Plants can affect chemical weathering  Marine carbonate organisms sink for carbon  Photosynthesis and burial of organic matter can affect atmospheric CO 2

Record of life Critics of Gaia Critics of Gaia  Life evolved late in Earth history  Early life forms too primitive to affect geochemical cycles  CaCO 3 shells appeared only 0.6 bya Supporters of Gaia Supporters of Gaia  Antiquity of bacteria  Development of atmospheric O 2  Life became more complex when Earth needed it  Countered the faint young Sun

Gaia Hypothesis unproven Hypothesis unproven  Extent to which life regulated climate unknown  Life plays active roles in biogeochemical processes  Must contribute to the thermostat that regulates Earth’s climate

Plate Tectonics and Climate Position of continents, volcanic CO 2 emissions and continental elevation Position of continents, volcanic CO 2 emissions and continental elevation

Evidence for Climate Change Geologic record reveals record of long-term climate change Geologic record reveals record of long-term climate change Is the timing of “ice house” intervals on Earth related to Is the timing of “ice house” intervals on Earth related to  Continental configuration and position?  Related to a tectonic control on atmospheric CO 2 ?  Change in CO 2 supply?  Changes in weathering?

Polar Position Hypothesis Ice sheets appear on continents when they are in polar positions Ice sheets appear on continents when they are in polar positions No ice should appear on Earth if continental landmasses are equatorial No ice should appear on Earth if continental landmasses are equatorial  No world-wide change in climate only on the slow tectonic movement of continents  Testable hypothesis

Test of Polar Position Hypothesis Assembly of Gondwana carried large continental masses across the South Pole Assembly of Gondwana carried large continental masses across the South Pole  Were ice sheets present?

Polar Positions and Ice Sheets Parts of Gondwana lay over the South Pole for ~100 my Parts of Gondwana lay over the South Pole for ~100 my Evidence for glaciations exist Evidence for glaciations exist Ordovician (~430 my) glaciations lasted less than 10 my and probably less than 1 my Ordovician (~430 my) glaciations lasted less than 10 my and probably less than 1 my

Polar Position Hypothesis Presence of continents in polar positions does guarantee glaciations (question of preservation) Presence of continents in polar positions does guarantee glaciations (question of preservation) Another factor is required to regulate climate on tectonic time scales Another factor is required to regulate climate on tectonic time scales

Changing Atmospheric CO 2 Polar position alone does not explain climate variations over last 500 my Polar position alone does not explain climate variations over last 500 my Change in atmospheric CO 2 important Change in atmospheric CO 2 important  BLAG model  Driven by changes in CO 2 input that result from sea floor spreading  T. C. Chamberlain or Raymo/Ruddiman Model  Driven by changes in the rate of uplift and weathering

BLAG The rate of global average seafloor spreading The rate of global average seafloor spreading  Controls delivery of CO 2 to atmosphere  Direct injection from rock reservoir  Changes in atmospheric CO 2 control climate

Carbon Cycle Model Seafloor spreading the driver of change Seafloor spreading the driver of change  Model relies on feedback through chemical weathering  Transport of carbon to oceans  Burial of carbon in sediments  Return of carbon from mantle through volcanism

Carbon Cycling Carbon cycles continuously between rock reservoir and atmosphere Carbon cycles continuously between rock reservoir and atmosphere  CO 2 removed from atmosphere by chemical weathering, deposited in ocean sediments, subducted and returned by volcanism

Organic Carbon Burial Affect CO 2 If the rate of organic carbon burial increases, less organic matter available for decomposition and less carbon returned to the atmosphere as CO 2 If the rate of organic carbon burial increases, less organic matter available for decomposition and less carbon returned to the atmosphere as CO 2  Atmospheric CO 2 reservoir shrinks

Organic Carbon Burial Affect O 2 If the rate of organic carbon burial increases, less organic matter available for decomposition and less oxygen is used during decomposition If the rate of organic carbon burial increases, less organic matter available for decomposition and less oxygen is used during decomposition  Atmospheric O 2 reservoir grows

Why carbon Isotopes? Carbon isotopes tell us when carbon cycle not in balance Carbon isotopes tell us when carbon cycle not in balance

Burial of Organic Matter and  13 C Burial of 13 C- depleted organic matter leaves remaining DIC enriched in 13 C Burial of 13 C- depleted organic matter leaves remaining DIC enriched in 13 C Increases in  13 C of marine carbonates indicates an increase in the rate of burial of organic matter in ocean or on land Increases in  13 C of marine carbonates indicates an increase in the rate of burial of organic matter in ocean or on land

BLAG Input and Output Input to model Input to model  Record of  13 C variations in marine carbonates  Proxy for rate of organic carbon burial Output from model Output from model  Variation in atmospheric CO 2 and O 2  Weathering rates through time  Atmospheric CO 2 controls temperature Precipitation and reaction ratesPrecipitation and reaction rates  Atmospheric O 2 can affect weathering

Model Evaluation Model works pretty well Model works pretty well

Competing Hypothesis? Uplift Weathering Hypothesis Uplift Weathering Hypothesis  Chemical weathering is the active driver of climate change  Rate of supply of CO 2 constant, rate of removal changes  Global mean rate of chemical weathering depends on availability of fresh rock and mineral surfaces  Rate of tectonic uplift controls/enhances exposure of fresh rock surfaces

Tectonic Uplift and Weathering Uplift causes several tectonic and climatic effects that affects weathering by fragmenting fresh rock Uplift causes several tectonic and climatic effects that affects weathering by fragmenting fresh rock

Testing the Hypothesis Times of continental collision coincide with times of glaciations Times of continental collision coincide with times of glaciations  Uplift weathering hypothesis consistent with geologic record

What is the Difference? Key factors controlling weathering differ Key factors controlling weathering differ  BLAG – chemical weathering is a negative feedback  Moderates climate change driven by volcanic CO 2 inputs  Uplift weathering – chemical weathering is the driver of climate change  Physical fragmentation and exposure of fresh material during uplift Removes atmospheric CO 2Removes atmospheric CO 2

Weathering in Amazon Basin Chemical weathering is more intense in the Andes Mountains Chemical weathering is more intense in the Andes Mountains  80% of the ions that reach the Atlantic Ocean from eastern Andes  20% from the Amazon basin lowlands  Lowlands intensely weathered quickly

Academic Arguments? Processes of uplift and exposure are linked to volcanic CO 2 emissions Processes of uplift and exposure are linked to volcanic CO 2 emissions  Plate tectonics  Both processes are important factors affecting global geochemical cycles  One or the other may be more important at any given time  Explain better geologic observations  Neither explanation fully incorporates biological influences  Life plays active roles in biogeochemical processes