Archean Atmosphere n Faint young Sun paradox presents

Archean Atmosphere n Faint young Sun paradox presents dilemma u 1) What is the source for high levels of greenhouse gases in Earth’s earliest atmosphere? u 2) How were those gases removed with time? t Models indicate Sun’s strength increased slowly with time t Geologic record strongly suggests Earth maintained a moderate climate throughout Earth history (i. e. , no runaway greenhouse like on Venus)

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

Is Volcanic CO 2 Earth’s Thermostat? n n n If volcanic CO 2 emissions provided Archean greenhouse, has volcanic activity continuously slowed through geologic time? No, but… Carbon input balanced by removal u Near surface carbon reservoirs Stop all volcanic input of CO 2 u Take 270, 000 years to deplete atmospheric CO 2 t Surface carbon reservoirs (41, 700 gt) divided by volcanic carbon input (0. 15 gt y-1) u Rate of volcanic CO 2 emissions have potential to strongly affect atmospheric CO 2 levels on billion-year timescale

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

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

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

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

Atmospheric CO 2 Balance n Slow silicate rock weathering balances long -term build-up of atmospheric CO 2 u On the 1 -100 million-year time scale u Rate of chemical hydrolysis balance rate of volcanic emissions of CO 2 t Neither rate was constant with time u Earth’s long term habitably requires only that the two are reasonably well balanced

What Controls Weathering Reactions? n Chemical weathering influenced by u Temperature t Weathering rates double with 10°C rise u Precipitation t H 2 O is required for hydrolysis • Increased rainfall increases soil saturation t H 2 O and CO 2 form carbonic acid u Vegetation t Respiration in soils produces CO 2 • CO 2 in soils 100 -1000 x higher than atmospheric CO 2

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

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

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

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

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

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

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

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

Gaia Hypothesis n n Biology affects geochemical processes that influence climate Gaia hypothesis u Life has regulated Earth’s climate t All evolution occurred to keep Earth habitable (extreme interpretation) u Life affected atmospheric O 2 evolution u Plants can affect chemical weathering u Marine carbonate organisms sink for carbon u Photosynthesis and burial of organic matter can affect atmospheric CO 2

Record of life n n Critics of Gaia u Life evolved late in Earth history u Early life forms too primitive to affect geochemical cycles u Ca. CO 3 shells appeared only 0. 6 bya Supporters of Gaia u Antiquity of bacteria u Development of atmospheric O 2 u Life became more complex when Earth needed it t Countered the faint young Sun

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

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

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

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

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

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

Polar Position Hypothesis n n Presence of continents in polar positions does guarantee glaciations (question of preservation) 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 n Change in atmospheric CO 2 important u BLAG model t Driven by changes in CO 2 input that result from sea floor spreading u T. C. Chamberlain or Raymo/Ruddiman Model t Driven by changes in the rate of uplift and weathering n

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

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

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

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

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

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

Burial of Organic Matter and d 13 C n n Burial of 13 Cdepleted organic matter leaves remaining DIC enriched in 13 C Increases in d 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 n n Input to model u Record of d 13 C variations in marine carbonates u Proxy for rate of organic carbon burial Output from model u Variation in atmospheric CO 2 and O 2 u Weathering rates through time t Atmospheric CO 2 controls temperature • Precipitation and reaction rates t Atmospheric O 2 can affect weathering

Model Evaluation n Model works pretty well

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

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

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

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

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

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