CHAPTER 6: GEOCHEMICAL CYCLES Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017 THE EARTH: ASSEMBLAGE OF ATOMS OF THE 92 NATURAL ELEMENTS Most abundant elements: oxygen (in solid earth!), iron (core), silicon (mantle), hydrogen (oceans), nitrogen, carbon, sulfur The elemental composition of the Earth has remained essentially unchanged over its 4.5 Gyr history Extraterrestrial inputs (e.g., from meteorites, cometary material) have been relatively unimportant Escape to space has been restricted by gravity Biogeochemical cycling of these elements between the different reservoirs of the Earth system determines the composition of the Earth s atmosphere and oceans, and the evolution of life
HISTORY OF EARTH S ATMOSPHERE N 2 CO 2 H 2 O oceans form CO 2 dissolves O 2 O 2 reaches current levels; life invades continents Outgassing Life forms in oceans Onset of photosynthesis 4.5 Gy B.P 4 Gy B.P. 3.5 Gy B.P. 0.4 Gy B.P. present
Comparing the atmospheres of Earth and Venus Venus Earth Radius (km) 6100 6400 Surface pressure (atm) 91 1 CO 2 (mol/mol) 0.96 3x10-4 N 2 (mol/mol) 3.4x10-2 0.78 O 2 (mol/mol) 6.9x10-5 0.21 H 2 O (mol/mol) 3x10-3 1x10-2
BIOGEOCHEMICAL CYCLING OF ELEMENTS: examples of major processes Physical exchange, redox chemistry, biochemistry are involved Surface reservoirs
OXIDATION STATES OF NITROGEN N has 5 electrons in valence shell 9 oxidation states from 3 to +5 Increasing oxidation number (oxidation reactions) -3 0 +1 +2 +3 +4 +5 NH 3 Ammonia NH 4 + Ammonium N 2 Dinitrogen N 2 O Nitrous oxide NO Nitric oxide HONO Nitrous acid NO 2 - Nitrite NO 2 Nitrogen dioxide HNO 3 Nitric acid NO 3 - Nitrate R 1 N(R 2 )R 3 Organic N Decreasing oxidation number (reduction reactions)
THE NITROGEN CYCLE: MAJOR PROCESSES ATMOSPHERE N 2 combustion lightning NO oxidation HNO 3 BIOSPHERE orgn assimilation biofixation decay NH 3 /NH 4 + denitrification nitrification deposition NO 3 - burial weathering LITHOSPHERE
Questions 1. Denitrification seems at first glance to be a terrible waste for the biosphere, jettisoning precious fixed nitrogen back to the atmospheric N 2 reservoir. In fact, denitrification is essential for maintaining life in the interior of continents. Can you see why? 2. Although volcanoes don't emit O 2 they do emit a lot of oxygen (as H 2 O and CO 2 ). Both H 2 O and CO 2 photolyze in the upper atmosphere. Photolysis of H 2 O results in production of atmospheric O 2 but photolysis of CO 2 does not. Why this difference?
BOX MODEL OF THE NITROGEN CYCLE Inventories in Tg N Flows in Tg N yr -1
Population, billions Global human perturbation of nitrogen cycle Global anthropogenic N fixation now exceeds natural: 8 160 J.N. Galloway, UVa 6 4 2 Natural 0 0 1850 1900 1950 2000 2050 120 80 40 Nr Creation, Tg N/yr Population Haber Bosch C-BNF Fossil Fuel agricultural biofixation Resulting N deposition (NH 4+, NO 3- ) modifies ecosystem function, C storage Annual N deposition Atmosphere NH 3 NO 40 30 Tg N a -1 fossil fuel critical load Zhang et al. [2012]
N 2 O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION Important as source of NO x radicals in stratosphere greenhouse gas Main anthropogenic source: agriculture IPCC [2014]
FAST OXYGEN CYCLE: ATMOSPHERE-BIOSPHERE Source of O 2 : photosynthesis nco 2 + nh 2 O (CH 2 O) n + no 2 Sink: respiration/decay (CH 2 O) n + no 2 nco 2 + nh 2 O CO 2 Net photosynthesis by green plants: 200 Pg O/yr orgc 1.2 10 6 Pg O O 2 O 2 lifetime: 6000 years litter orgc decay
but abundance of organic carbon in biosphere/soil/ocean reservoirs is too small to control atmospheric O 2 levels
SLOW OXYGEN CYCLE: ATMOSPHERE-LITHOSPHERE O 2 : 1.2x10 6 Pg O O 2 lifetime: 3 million years Photosynthesis decay O 2 CO 2 Fe 2 O runoff 3 H 2 SO 4 O 2 CO 2 weathering FeS 2 orgc OCEAN orgc CONTINENT SEDIMENTS burial orgc CO 2 microbes FeS 2 Compression subduction Uplift orgc: 1x10 7 Pg C FeS 2 : 5x10 6 Pg S
http://www.esrl.noaa.gov/gmd/ccgg
ATMOSPHERIC CO 2 INCREASE OVER PAST 1000 YEARS Intergovernmental Panel on Climate Change (IPCC), 2007
ANTARCTIC ICE CORE RECORD OF TEMPERATURE AND CO 2 CO 2 and temperature are strongly correlated through glacial-interglacial cycles
IPCC [2014] CO 2 over the last 60 million years Eocene
INTERANNUAL TREND IN CO 2 INCREASE Pg C yr -1 On average, only 60% of emitted CO 2 remains in the atmosphere but there is large interannual variability in this fraction
ATMOSPHERE CO 2 (g) UPTAKE OF CO 2 BY THE OCEANS OCEAN K H = 3x10-2 M atm -1 CO 2. H 2 O K 1 = 9x10-7 M CO 2. H 2 O HCO 3- + H + K 2 = 7x10-10 M HCO 3 - CO 3 2- + H + CO 2. H 2 O HCO 3 - CO 3 2-
EQUILIBRIUM PARTITIONING OF CO 2 BETWEEN ATMOSPHERE AND GLOBAL OCEAN Equilibrium for present-day ocean: F N ( g) 1 NCO2( g) + NCO2( aq) VocPKH K1 KK 1 2 1+ 1 2 N + + + + a [H ] [H ] CO2 = = = 0.03 only 3% of total inorganic carbon is currently in the atmosphere But CO 2 (g) [H + ] F positive feedback to increasing CO 2 Pose problem differently: how does a CO 2 addition dn partition between the atmosphere and ocean at equilibrium (whole ocean)? f dn ( g) 1 CO2 = = = dnco2( g) + dnco2( aq) VocPKHK1K2 1+ + 2 Naβ H 28% of added CO 2 remains in atmosphere! 0.28
ADDITIONAL LIMITATION OF CO 2 UPTAKE: SLOW OCEAN TURNOVER (~ 200 years) Inventories in 10 15 m 3 water Flows in 10 15 m 3 yr -1 Uptake by oceanic mixed layer only (V OC = 3.6x10 16 m 3 ) would give f = 0.94 (94% of added CO 2 remains in atmosphere)
MEAN COMPOSITION OF SEAWATER
2.1 2.0 1.9 1.8 1.6 1.4 4 3 2 8.6 8.4 Equilibrium calculation for [Alk] = 2.3x10-3 M [CO 2. H 2 O]+[HCO 3- ] +[CO 3 2- ], 10-3 M [HCO 3- ], 10-3 M [CO 3 2- ], 10-4 M Ocean ph 8.2 100 200 300 400 500 pco 2, ppm LIMIT ON OCEAN UPTAKE OF CO 2 : CONSERVATION OF ALKALINITY Charge balance in the ocean: [HCO 3- ] + 2[CO 3 2- ] = [Na + ] + [K + ] + 2[Mg 2+ ] + 2[Ca 2+ ] - [Cl - ] 2[SO 4 2- ] [Br - ] The alkalinity [Alk] [HCO 3- ] + 2[CO 3 2- ] = 2.3x10-3 M is the excess base relative to the CO 2 -H 2 O system It is conserved upon addition of CO 2 uptake of CO 2 is limited by the existing supply of CO 3 2- : CO 2 (g) + CO 32 + H 2 O 2HCO 3 - Increasing Alk requires dissolution of sediments: CaCO 3 Ca 2+ + CO 3 2- which takes place over a time scale of thousands of years
LAND-ATMOSPHERE CARBON CYCLING: MAJOR PROCESSES
LAND-ATMOSPHERE CARBON CYCLING: BOX MODEL Inventories in PgC Flows in PgC a -1
Reforestation in action: Harvard Forest in Petersham, central Mass. then and now
Decrease in O 2 as constraint on land uptake of CO 2 IPCC [2014]
EVIDENCE FOR LAND UPTAKE OF CO 2 FROM TRENDS IN O 2, 1990-2000
Current net uptake of CO 2 by biosphere (1.4 Pg C yr -1 ) is small residual of large atmosphere-biosphere exchange
NET PRIMARY PRODUCTIVITY (NPP): The tropics dominate
Observed latitudinal gradient of atmospheric CO 2 shows that the missing sink is at northern mid-latitudes but we still don t know what continent
Carbon budget, 1750 present IPCC, 2014
Projected future trends in CO 2 emissions
PROJECTED FUTURE TRENDS IN CO 2 UPTAKE BY OCEANS AND TERRESTRIAL BIOSPHERE