Background in Biogeochemistry

1. Background in Biogeochemistry Definition of Ecological Biogeochemistry--study of the biological and chemical transformations and fluxes of elements and compounds in populations, communities, ecosystems of the Earth.

2. Background in Biogeochemistry Elements can be separated into reservoirs or pools and transfers among these pools are measured by fluxes. The content of the pool is controlled by fluxes in and out of the reservoirs and transformations within the reservoir. The actual delineation of reservoirs and fluxes depends on scale of measurement both with respect to time and pace. Some definitions: Reservoir (box, compartment)--An amount of matter or energy defined by certain physical, chemical ,or biological characteristics that, under the particular consideration, can be considered as reasonably homogenous. Flux--The amount of matter or energy transferred from one reservoir to another per unit time.

3. Background in Biogeochemistry Source--The flux of matter or energy into a reservoir. Sink--The flux of matter or energy out of a reservoir. Budget--A balance of all sources and sinks of a reservoir.

4. Background in Biogeochemistry A mean residence time “” can be defined such that: = amount in reservoir/sum of all fluxes (pool/mass input or mass output). If the reservoir amount is constant, it is in steady state. The assumption of steady state is often very important and useful especially for very large reservoirs. It is very difficult to detect divergence from steady state of large reservoirs over a short period. The analysis or depiction of the interconnections reservoirs and fluxes of a biogeochemical component is termed a cycle.

5. Background in Biogeochemistry

6. Background in Biogeochemistry Some aspects of element composition and behavior are illustrated in Table 1. The major elements include Si, C, Al and Ca. T = pool/mass input or mass output

8. Background in Biogeochemistry (cont.) Most of the major elements are largely found in the lithosphere (solid part of the Earth including the crust and outer mantle ~ 100 km) and exhibit long residence times in this pool. For the most part, the atmosphere (the mass of air surrounding the Earth) and biosphere (regions of the surface and atmosphere of the Earth where living organisms exist) are minor element pools. Only for N is the atmosphere a significant pool. For some elements, a significant fraction of the mass is found in the oceans (Na, Cl, S). Some elements exhibit more than one oxidation state and therefore can participate in redox (oxidation-reduction) reactions (e.g. C, Fe, S, N, O).

9. Acid-Base Chemistry Acid—a substance that increases [H+] in water. Base—a substance that increases [OH-] in water. pH = -log10 [H+] in moles L-1 For pure water at 24C: [H+] [OH-]= [10-7][10-7] = 10-14 pH = 7

10. pH Scale

11. Acid-Base Chemistry (cont.) Major ionic solutes Cations (positively charged ions) Basic - Ca2+, Mg2+, K+, Na+ largely derived from cation supply in the lithosphere Acidic - H+, Al3+, Fe3+ occurs under acidic conditions Reduced - Fe2+, Mn2+ occurs under reducing conditions (sediments, wetlands)

12. Acid-Base Chemistry (cont.) Anions (negatively charged ions) Strong acid anions - SO42-, NO3-, Cl- (strongly dissociated with H+) Weak acid anions - HCO3-, CO32-, An- (organic anions) Basic - OH- CB - sum of basic cations (often expressed in eq L-1 or molc L-1) = 2[Ca2+] + 2[Mg2+] + [Na+] + [K+] CA - sum of strong acid anions = 2[SO42-] + [NO3-] + [Cl-]

13. Acid-Base Chemistry (cont.) Dissolved inorganic carbon (DIC) = CT [H2CO3*] + [HCO3-] + [CO32-] The distribution of inorganic carbon species is a function of pH (see figure). H2CO3* = H+ + HCO3- ; pKa1 = 6.3 HCO3- = H+ + CO32- ; pKa2 = 10.3

15. Acid-Base Chemistry (cont.) An important measurement of the acid-base status of waters is acid neutralizing capacity or alkalinity. ANC = [HCO3-] + 2[CO32-] + n[An-] + [OH-] - [H+] = the ability of a system to neutralize inputs of strong acid. HCO3- + H+ = H2CO3* CO32- + 2H+ = H2CO3 OH- + H+ = H2O ANC = CB - CA

16. Acid-Base Chemistry (cont.) An important concept is electroneutrality. All solutions (and systems) must be electrically neutral. ci - concentration of ionic solute zi - charge 2[Ca2+] + 2[Mg2+] + [Na+] + [K+] + [H+] = 2[SO42-] + [NO3-] + [Cl-] + [HCO3-] + 2[CO32-] + n[An-] + [OH-] Rearranging ANC= [HCO3-] + 2[CO32-] + n[An-] + [OH-] - [H+] = 2[Ca2+] + 2[Mg2+] + [Na+] + [K+] - 2[SO42-] - [NO3-] - [Cl-] = CB - CA

17. Acid-Base Chemistry (cont.) Increases in Ca2+ by weathering of minerals increases ANC. CaCO3 + H+ = Ca2+ + HCO3- Inputs of H2SO4 from acid rain decreases ANC. H2SO4 = 2H+ + SO42- Any process which affects the concentration of ionic solutes changes ANC. There is a non-linear relationship between ANC and pH (see figure). e.g., ANC production is an important indicator of the abiotic fixation or removal of CO2.

19. Acid-Base Chemistry (cont.) Weathering is an important process by which CO2 is fixed from the atmosphere. NaAlSi3O8(s) + H2O + CO2 = Na+ + HCO3- + Al(OH)3(s) + 3H4SiO4 (albite) This HCO3- is transported by rivers to the oceans. A budget for bicarbonate of the ocean is shown in Table 2 (from Berner and Berner).

20. Table 2. The Oceanic Bicarbonate Budget (rates in Tg HCO3- yr-1) Note: Tg = 1012g. Replacement time for HCO3 (river input only) is 83,000 years.

21. Acid-Base Chemistry (cont.) There are two budgets presented. The first is the current budget. The second is the budget over the past 25 MY. Note that the major inputs of HCO3- are riverine inputs and biogenic pyrite formation. About 7% of the total HCO3- inputs is associated with SO42- reduction. 2CH2O + SO42- = H2S + 2HCO3- Also remember: Ca2+ + HCO3-  CaCO3 + H+

22. Organic S Compounds Organic Matter Bacteria H2S Dissolved SO42- Bacteria Iron Minerals Pyrite FeS2

23. Acid-Base Chemistry (cont.) The sink of HCO3- inputs to the oceans is precipitation of CaCO3. Ocean water is not oversaturated with respect to the solubility of CaCO3 over the entire depth. The upper waters are oversaturated while the lower waters are undersaturated. The reason for this pattern is that the solubility of CaCO3 increases with increasing depth due to increases in pressure (see figure). Also, solubility increases with temperature. At the average ocean depth, the pressure is 400 atm. Under these conditions, CaCO3 is about twice as soluble as at the surface.

25. Acid-Base Chemistry (cont.) Also, the production of CO2 from respiration of organic matter facilitates the dissolution of CaCO3. What happens with increasing [CO2]? About 83% of the precipitated CaCO3 redissolves at depth. CO2 + H2O + CaCO3 = Ca2+ + HCO3-

26. Acid-Base Chemistry (cont.) Note that the current ocean is not at steady-state with respect to inputs of HCO3-. Over the short-term, HCO3- is being depleted, sediment deposition exceeds inputs. At the current rate of deposition, all of the HCO3- in the ocean would be removed in 200,000 yr. This condition would never exist, as the ocean would eventually become undersaturated with respect to the solubility of CaCO3 and precipitation would stop. The condition of elevated CaCO3 deposition to sediments has only been occurring during the past 11,000 yr. This condition is due to the rapid post-glacial rise of sea level over the continent of shelves. Role of the biota: coral reefs, forminifera, etc.

28. Redox Chemistry Redox reactions involve the transfer of electrons. All redox reactions must be coupled and require an electron donor and electron acceptor. e.g. CH2O + H2O = CO2 + 4e- + 4H+ O2 + 4e- + 4H+ = 2H2O CH2O + O2 = CO2 + H2O Redox reactions are often characterized in stoichiometric half reactions. e.g. Fe3+ + e- = Fe2+

29. Redox Chemistry (cont.) This reaction can be written as a mass law. where K is a thermodynamic equilibrium constant. If we take the logarithm of this expression. or pe = - log[e-] and is the indicator of the redox status of the system. oxidizing conditions - high or positive pe values (high K) reducing conditions - low or negative pe values (low K)

30. Redox Chemistry (cont.) Only a few elements dominate redox reactions in natural waters. Major redox elements - C, N, O, S, Fe, Mn The most important electron donor in natural waters is organic matter. The electron acceptor that is coupled with the electron donor (organic matter) is variable. It depends on the quantity and energetics of the electron acceptor. Redox reactions in the natural environment can be thought of as an electron titration. The source of electrons is the electron donor (organic matter). These electrons are released to electron acceptors in order of their electron affinity or energetics.

31. peo = standard electron activity Peow = standard electron activity in water at pH =7.0 for 25C

32. Redox Chemistry (cont.) The energy yield of these electron acceptor reactions decreases as a function of energetic yield (electron activity). So, O2 is the preferred electron acceptor. When O2 is consumed, electrons are transferred to NO3- and so on. Note that when the electron acceptor O2 is in excess, conditions are aerobic (i.e. Earth's surface). When the quantity of electron donor (organic matter) exceeds the quantity of electron acceptor (O2), then anaerobic conditions result. These conditions occur in wetlands or in lake sediments.

34. Redox Chemistry (cont.) If photosynthetic products were oxidized completely by respiration, the atmosphere would be devoid of O2. Photosynthesis is a critical process in generating O2. Light energy is converted to chemical energy. The electron cycle is responsible for the partitioning of an oxidizing atmosphere and reducing lithosphere.

36. Methods Water Column Monitoring – 1981, 1989, 1990, 1991, 2000 O2 NO3- NH4+ SO42- (H2S)T Fe2+ CH4 Alkalinity pH DIC – calculated from alkalinity, pH, temperature Sediment Traps – 10m – 1989, 1990, 1991 POC

39. Rates of Solute Accumulation (+) or Loss (-) in the Hypolimnion of Onondaga Lake * Includes ebullitive loss, assumed to be 33% of total; soluble component in parentheses.

40. Effective Equilibrium Constants of Aquatic Redox Couples

41. Oxygen Reduction DIC Equivalents per Mole O2: -1 (CH2O)106(NH3)16H3PO4 + 106 O2 106 CO2 + 16 NH3 + H3PO4 + 106 H2O Denitrification DIC Equivalents per Mole NO3-: -1.25 (CH2O)106(NH3)16H3PO4 + 84.9 HNO3 106 CO2 + 16 NH3 + H3PO4 + 42.4 N2 + 148.4 H2O Manganese Reduction DIC Equivalents per Mole Mn: 0.5 (CH2O)106(NH3)16H3PO4 + 212 MnO2 + 424 H+ 106 CO2 + 16 NH3 + H3PO4 + 212 Mn2+ + 318 H2O Iron Reduction DIC Equivalents per Mole Fe2+: 0.25 (CH2O)106(NH3)16H3PO4 + 424 FeOOH + 848 H+ 106 CO2 + 16 NH3 + H3PO4 + 424 Fe2+ + 742 H2O Sulfate Reduction DIC Equivalents per Mole H2S: -2 (CH2O)106(NH3)16H3PO4 + 53 SO42- 106 CO2 + 16 NH3 + H3PO4 + 53 S- + 106 H2O Iron & Sulfate Reduction DIC Equivalents per Mole SO42-: -2.25 (CH2O)106(NH3)16H3PO4 + 47.1 FeOOH + 47.1 SO42- + 94.2 H+ 106 CO2 + 16 NH3 + H3PO4 + 47.1 FeS + 176.6 H2O Methanogenesis DIC Equivalents per Mole CH4: 1 (CH2O)106(NH3)16H3PO4 53 CO2 + 16 NH3 + H3PO4 + 53 CH4 Fermentation DIC Equivalents per Mole C: 0.5 (CH2O)106(NH3)16H3PO4 35.3 CO2 + 16 NH3 + H3PO4 + 35.3 C2H5OH Humification DIC Equivalents per Mole C: 0.5 (CH2O)106(NH3)16H3PO4 35.3 CO2 + ( C2H5OH)35.3 (NH3)16 H3PO4

44. Organic Carbon Budget 1987-92

45. Hypolimnion Electron Budget 1987-92

46. Comparison of Hypolimnetic Electron Budgets During Summer Stratification