Carbon Cycle 2023 PDF

Buffers and Bedrocks Organic vs Inorganic Carbon Organic Carbon • reduced carbon • stores energy • carbon atom is combined with • other carbon, hydrogen, or nitrogen atoms • biomass, fossil fuel, your kitchen compost, you and me Inorganic carbon • oxidized carbon • carbon is combined with oxyg...

Buffers and Bedrocks Organic vs Inorganic Carbon Organic Carbon • reduced carbon • stores energy • carbon atom is combined with • other carbon, hydrogen, or nitrogen atoms • biomass, fossil fuel, your kitchen compost, you and me Inorganic carbon • oxidized carbon • carbon is combined with oxygen e.g., CO2 gas, CaCO3 minerals • its production releases energy Carbon Reservoirs near the Earth Surface ~1995 inorganic carbon CO32CaCO3 HCO3CaCO3 textbook Figure 8-3 Inorganic Carbon Cycle CO2 combines with water to from carbonic acid: H 2O + CO2 « H 2CO3 This process occurs in rainwater and seawater. Whether a part of the surface ocean is a source or sink of CO2 for the atmosphere • depends on the circulation and productivity regime • CO2 diffuses along a concentration gradient from higher to lower concentrations • somewhat modulated by the winds Annual mean sea-to-air CO2 flux (mol m-2 yr-1) ingassing Gruber et al. 2023, Nature Reviews outgassing mean for the period 1990 to 2020 Geology 101 Carbonates: CaCO3, limestone • simple composition • chiefly contain C and O main minerals are calcite and dolomite • form at Earth’s surface Silicates: CaSiO3 • complex compositions • chiefly contain Si and O • many different minerals • abundant in igneous and metamorphic rocks • we use wollastonite (CaSiO3 to represent them all Chemical Weathering CaCO3+ H 2CO3 ↔ Ca + 2HCO 2+ 3 CaSiO3 + 2H 2CO3 ↔ Ca + 2HCO + SiO 2 + H 2O 2+ 3 Carbonic acid eats away at carbonate and silicate rocks to produce bicarbonate ions, dissolved silica, and calcium ions. Carbonate Mineral Formation 2HCO + Ca 3 2+ ↔ CaCO3 + H 2CO3 Dissolved bicarbonate becomes solid thanks to organisms that build solid CaCO3 shells in the ocean (e.g., corals, shellfish, foraminifera, coccolithophorids..) Carbonate Mineral Formation and Ocean Acidity Carbonic acid dissociates and produces a bicarbonate and a hydrogen ion. H2CO3 <-> HCO3- + H+ The bicarbonate ion further dissociates and produces another hydrogen ion and a carbonate ion. HCO3- <-> H+ + CO32- The carbonic acid produced during carbonate mineral formation lowers ocean pH Carbonate Mineral Formation and atmospheric CO2 The presence of carbonic acid also enhances dissolved CO2 concentrations. H 2O + CO2 « H 2CO3 This promotes diffusion of CO2 back to the atmosphere. Confused? Carbonate mineral formation promotes diffusion of CO2 back to the atmosphere but... • many carbonate mineral producers are also phytoplankton • They photosynthesize organic matter in the upper ocean, which removes dissolved CO2 from the surface water The majority of phytoplankton are NOT carbonate mineral producers • • so the overall effect of primary production is to reduce CO2 concentrations in the upper ocean Carbonate Mineral Accumulation Carbonate minerals accumulate on bathymetric highs (MOR) 2 O i S , m o t Dia 3 O C a C , r e f i n i m a r Fo Co cc ol ith op ho re ,C aC O3 Ra dio lar ian , SiO 2 Air-sea gas exchange is rapid and the atmospheric CO2 reservoir is close to equilibrium with the surface ocean CO2 reservoir. Chem. weathering converts atmospheric CO2 into bicarbonate ions. Marine organisms precipitate carbonate mineral shells by combining bicarbonate and calcium ions. 40% of this material becomes buried as carbonate sedimentary rock (= limestone). textbook Fig. 8-16 ¡ The net effect of limestone weathering on land and carbonate mineral formation & burial in the ocean is zero (…when the system is at state state). ¡ However, silicate rock weathering removes CO2 from the atmosphere. ¡ Two molecules of CO2 (in the from of carbonic acid) are needed to weather silicate rocks. CaSiO3 + 2H 2CO3 ↔ Ca + 2HCO + SiO 2 + H 2O 2+ 3 both bicarbonate ions will eventually be used to from carbonate shells • Net removal of atmospheric CO2 occurs by the combined processes of silicate weathering on land and carbonate mineral deposition in the ocean. • The rate is small: ~ 0.03 Gt (C)/yr • On long time-scales, the CO2 removal must be balanced by CO2 input. Plate tectonic processes provide the return flow CaCO3 + SiO2 -> CaSiO3 + CO2 textbook Fig. 8-17 On long time-scales: • CO2 removal from silicate weathering is roughly balanced by CO2 release from volcanoes associated with MORs and subduction zones. Earth has supported life for 3.5 billion years Earth Mars Venus Surface: 288 K Surface: 730 K Surface: 218 K very thick & dense: <- Atmosphere -> thin & low density Runaway Greenhouse 96% CO2 textbook Fig. 8-18 Negative feed back loops diminish the effect of disturbances textbook Fig. 8-18