Transcript Weathering and Elemental Cycling

RIVERS: Major Components
1) water
2) suspended inorganic matter - major elements are Al,
Fe, Si, Ca, K, Mg, Na and P
3) dissolved major spp. - HCO3-, Ca2+, SO42-, H4SiO4,
Cl-, Na+, Mg2+,K+
a) no gaseous phase - Ca2+, Cl-, H4SiO4, Na+,
Mg2+ ,K+
b) with gaseous phase - HCO3-, SO42-
4) dissolved nutrient elements - N, P - Si
5) suspended and dissolved organic matter
6) trace metals
• Runoff
ratio - avg. river runoff / avg. rainfall
• World average - 0.46
• Thus, 50% of rainwater returned to atmosphere
by evaporation
Major dissolved components:
• CHLORIDE in rocks very soluble - not reactive with
other ions - good tracer of water mass
1) main source to river - sea salt - rain - dry fallout
2) dissolved during weathering of halite (NaCl)
evaporites
3) thermal and mineral springs in volcanic areas
4) saline crusts in desert basins (not primary)
5) pollution - oil well brines, road salt, sewage
• SODIUM
- seawater input to atmosphere; Na in
halites - sedimentary rock - brine, road salts, etc.
• POTASSIUM
1) 90% weathering of silicate minerals - feldspar,
orthoclase, mica (biotite)
2) 3/4 sedimentary rocks
3) 1/4 igneous and metamorphic (also fertilizer)
• CALCIUM
& MAGNESIUM - rock
weathering
1) Ca - carbonate rocks: calcite(CaCO3),
dolomite(CaMg(CO3)2)
2) 65% of Ca2+ in river water -- dolomite is
main source of Mg
•HCO3- - rock weathering
1) soils: CO2 + H2O + CaCO3 ---> Ca2+ + 2HCO3-
2) soils: 2CO2 + 11H2O + 2NaAlSi3O8 ---> 2Na+ +
2HCO3- + Al2Si2O5(OH)4 + 4H4SiO4
3) some weathering by sulfuric acid formed by
oxidation of pyrite: H2SO4 + 2CaCO3 ---> 2Ca2+ +
2HCO3- + SO42H2SO4 + 9H2O + NaAlSi3O8---> 2Na+ + SO42- +
Al2Si2O5(OH)4 + 4H4SiO4
• SILICA -
silicate weathering
1) 20% Si relative to HCO3- (carbonate
weathering dominant)
2) 8% chert in carbonate rocks
3) more dissolved silica in tropics -4) Si minerals weather to kaolinite - 1.5 times
dissolved Si versus smectite in temperate
systems; gibbsite even more in tropics
5) biogenic source not important as in oceans
• SULFATE
-
1) 2% from salt, 33% weathering, 54%
pollution, 8% volcanic, 3% biological
2) pyrite FeS2, gypsum CuSO4· H2O,
anhydrite CuSO4
3) FeS2 weathers to H2SO4 which then reacts
with Si and Ca minerals
4) pyrite - derived SO42- in river water
Organic Carbon:
• Average river dissolved organic carbon (DOC)
- 5.3 mg/l; global is 200Tg DOC/yr (avg. global
DOC/total dissolved substances (TDS) - 1:19)
• POC - 172 Tg/yr; on average 1% of TSS is
CARBON
• TOC = POC + DOC
•Black Water River - pH is 4.3 due to dissociation of
humic carboxyl groups (R-COOH)--->(R-COO)- + H+
• DOC 24 mg/l -- DOC (humics and fulvics) / TDS
-- 1:1
• HCO3- is low
• HCO3- + H+ ---> H2O + CO2
• (R-COOH) + HCO3- ---> (R-COO)- + H2O + CO2
• High DOC rivers also have high Fe and Al
• Using total concentration of dissolved ions in river
water one can calculate the chemical denudation rate
of a drainage basin, continent - even the whole world.
Nutrients
Atmospheric Nutrients:
• C, N, P, S, S, K, Mg, Na, Ca, Fe, Mn, Zn, Cu, Mo,
Co, B
• macronutrients, micronutrients
• NITROGEN - oxid-states - NO3-, (+5 state) (of N) to
NH4+, (-3 state) (of N)
• organic N highly reduced; urea, amino acids
• PHOSPHORUS - PO43- (+5 state) (of P)
• 3 structural configurations - ortho, para, meta.
However, can occur in (+4 state) = PO42-
• SILICON
• detrital quartz - crystalline silica,
aluminosilicate clays, dissolved silicon
• oceans > opal - biogenic silica - amorphous
silica polymer
• silicic acid - H4SiO4
• SiO42- (+4 state)
• There
is significant competition for nutrients
between bacteria and algae
• Uptake kinetics - organisms with a halfsaturation coefficient (ks) for a given nutrient
will have greater affinity for that nutrient
• Organisms with large ks can take greater
advantage of large pools
• Plant
Redfield Ratio (1934) - C:N:P - 106:16:1
• Redfield 1963 - uptake of N, P - 16:1 ratio
• Phytoplankton organic matter
• 106 CO2 + 122 H2O + 16 HNO3 + H3PO4 --->
C(H2O)106 + (NH3)16 + H3PO4 + 138 O2
•Nutrient regeneration via decomposition will
return
• N2 fixation - 78% of atmospheric N2
• bacteria and cyanobacteria “fixing” N2 into the
inorganic salt ammonium
• N2 + 3H2 ---> 2NH3
• Nitrogenase inhibited by oxygen
• Heterocysts - maintain anoxic conditions
• Clostridium,
Azobacter, Pseudomonas
• Heterocystic cyanobacteria - Trichodesmium,
Oscillatoria, Calothrix
• P enhances N2 fixation, inhibited by NH4+
• can use acetylene reduction
• 15N - labelled N2
• Nitrogen fixation - high in lakes, not in estuaries
1) NH4+ inhibited, or:
2) SO4 may inhibit uptake of molybdate - needed to
make nitrogenase
• Important in marshes, ~5% of NH4+ needed for
Spartina growth
Nitrification and Denitrification:
• Nitrification - oxidation of NH4+ to NO3under aerobic conditions
• Nitrifying bacteria use NH4+ as energy source
to fix CO2 into organic matter
Nitrification - 2 steps:
• Nitrosomonas, Nitrocystis
• NH4 + 3/2 O2 ---> HNO2 + H2O
• Nitrobacter - HNO2 + 1/2 O2 ---> HNO3
• can measure with 15N - labeled NH4+ substrate
• sewage outfalls - high nitrification rates -- due
to NH4+ concentrations
Denitrification - bacteria use NO3- as e- acceptor to
oxidize organic matter anaerobically, releasing N2
gas
5C6H12O6 + 24HNO3 ---> 30CO2 + 42H2O + 12N2
• Pseudomonas - also, N O produced in reaction
• C6H12O6 + 6HNO3 ---> 6CO2 + 9H2O + 3N2O
• denitrification - limited by NO3 -