Chapter 13: The Global Sulfur Cycle

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Chapter 13 The Global Sulfur Cycle
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The Global Sulfur Cycle

• Sulfur is found in valence states ranging from 6
  (SO42-)
• to -2 (sulfides)
• Original S pool was in igneous rocks, largely as
  pyrite (FeS2)
• Weathering of crust exposes pyrite to O2, causing
  it to oxidize to SO42- (and H)
• SO42- then leaches to oceans

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The Global Sulfur Cycle

• Sulfur is found in valence states ranging from 6
  (SO42-)
• to -2 (sulfides)
• Original S pool was in igneous rocks, largely as
  pyrite (FeS2)
• Weathering of crust exposes pyrite to O2, causing
  it to oxidize to SO42- (and H)
• SO42- then leaches to oceans

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The Global Sulfur Cycle

• Sulfur in plants is found in proteins and as
  SO42- (the latter not being noted in the book)
• Microbial transformations between the valence
  states drive the S cycle
• Under anaerobic conditions, (SO42- is reduced to
  sulfide forms (H2S, FeS2, others)
• Anaerobic conditions can also lead to S-based
  photosynthesis

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The Global Sulfur Cycle

• No sulfur gas is a long-lived, major constituent
  of the atmosphere
• Thus, global S cycles must explain the fate of S
  inputs and outputs from the atmosphere
• Fig 13.1

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The Sulfur Cycle and Acid Rain

• 1960 Eriksson and Junge found that
  SO42-deposition in Sweden was much greater than
  could be accounted for from SO42- / Cl- ratios in
  seawater
• Must be other sources
• Dust gypsum (CaSO4) (8 x 1012 g S yr-1)
• Volcanic eruptions (5 x 1012 g S yr-1)
• Fossil fuel combustion SO2.
• The largest source (90 x 1012 g S yr-1)
• But going down - 40 reduction in the US, more
  in Europe

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The Sulfur Cycle and Acid Rain

• Fossil fuel combustion
• SO2 1/2O2 --gt SO3
• SO3 H2O --gt H2SO4 --gt H SO42-

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The Global Sulfur Cycle

• Transport in rivers
• 28 to 50 of 130 x 1012 g S yr-1 thought to be
  due to human activities (acid rain, mining, etc.)
• Some due to rainfall inputs, some due to natural
  pyrite oxidation

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Sulfur Cycle in the Oceans

• Seasalt cycles back and forth at a high rate (144
  x 1012 g
• yr-1)
• Precip and dryfall 180 x 1012 g yr-1
• Biogenic gases (e.g., DMS) net export of 16 x
  1012 g yr-1
• Fig 9.22

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Sulfur Cycle in the Oceans

• Sinks/outputs
• Biogenic gases 16 x 1012 g yr-1
• Seasalt transport to land 4 x 1012 g yr-1
• Hydrothermal vents 96 x 1012 g yr-1
• Pyrite formation 39 x 1012 g yr-1
• Pore-water burial 3 x 1012 g yr-1
• Total 168 x 1012 g yr-1

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Sulfur Cycle in the Oceans

• Inputs
• Rivers 130 x 1012 g yr-1
• Transport from land 20 x 1012 g yr-1
• Precip dryfall 180 x 1012 g yr-1
• Total 320 x 1012 g yr-1
• Net accumulation in oceans 152 x 1012 g yr-1
• Hard to document because oceans contain 13 x 1020
  g yr-1

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The Global Sulfur Cycle Temporal Perspective

• In primordial earth, volcanic activity causes
  much outgassing of S
• As oceans condensed, it dissolved and converted
  to SO42-
• Sulfides probably precipitated

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The Global Sulfur Cycle Temporal Perspective

• Isotope ratios help unravel what happened over
  time
• When ?34S hit near 30 o/oo, pyrite deposition
  must have greatly exceeded sulfide mineral
  oxidation on land
• Later, during the Carboniferous and Permian, ?34S
  dropped to near 10 o/oo, indicating less
  importance of SO42-, sulfate reduction, and
  pyrite deposition were less important as primary
  production occurred in freshwater swamps
• (Figures 13.2 and 13.3)

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The Sulfur Cycle and Acid Rain

• The current oxidation of S from fossil fuels
  probably represents an unprecedented change in
  global S cycling

• Fossil fuel combustion
• SO2 1/2O2 --gt SO3
• SO3 H2O --gt H2SO4 --gt H SO42-
• Normal rain pH is 5.6 (in equilibrium with CO2)
• CO2 H2O --gt H2CO3 lt--gt H HCO3-
• Acid rain pH can be 4.5 or less

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Acid Rain - now called Acidic Deposition

• Agricultural activities (fertilization, manure)
  releases large amounts of NH3 in soils or on
  foliage
• NH3 H2O ? NH4 OH- (initially basifying)
• Later acidifying, as discussed above
• The pH of rainfall in high ammonium areas (e.g.,
  Holland) can be neutral but high levels of
  ammonium can cause extreme acidification
• Rainfall pH is NOT a good indicator of acidifying
  potential of atmospheric deposition!!!!

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Acid Rain - now called Acidic Deposition

• Effects on Soils and Forests
• Swedish report to the UN in 1971 predicted soil
  Ca loss and forest decline based on correlation
  between exchangeable Ca and volume growth (next
  slide)
• However, Ca rarely limits forest growth
• Sulfur rarely limits forest growth
• Nitrogen usually limits
• Acidification effect may (or may not) reduce
  growth.

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Acidic Deposition

• How serious is the problem is acid rain in
  forests?
• Total devegetation and major soil acidification
  near major point-sources of s SO2, such as
  unregulated copper smelting plants at Copper
  Hills, TN, Sudbury, ONT
• Direct effects of SO2 and sulfuric acid on plants
  not distinguishable from acidification or soil or
  trace metal deposition in such cases

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Acidic Deposition

• How serious is the problem is acid rain in
  forests?
• Could the impacts of high concentration problems
  be extrapolated to chronic, low-concentration
  situations?
• No. There are effects of high concentration that
  will not be manifest at slow inputs at low
  concentration.
• Forest growth in Europe was increasing all the
  time there were cries of forest decline!!!
• Bottom line There was no regional forest decline
  in Europe, therefore acid rain did not harm
  forest growth!

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Acidic Deposition

• How serious is the problem is acid rain in
  surface waters?
• Early predictions (e.g., all Adirondack lakes
  dead by 1990) not supported
• Some acidification in areas with poor buffering
  (Norway, NE US and Canada)
• Reduced S emissions have NOT led to surface water
  pH increases, but only SO42- increases
• The lack of recovery in pH is blamed on long-term
  soil acidification - this is possible only if
  soil acidification exactly kept pace with
  reductions in mobile acid anions

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Acidic Deposition

• Conclusions on Acid Rain
• Forest Effects not shown, despite fervent beliefs
  by many during the 1980s
• During the, 1980s when scientists, politicians,
  and the public were very concerned about acid
  rain and forest decline, the overall growth of
  the forests of Europe was increasing at
  unprecedented rates.

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Acidic Deposition

• Conclusions on Acid Rain
• How did our perceptions of forest decline deviate
  so strongly from reality?
• In part, normal dynamics in forests were suddenly
  thought to be very abnormal
• Thin canopies are common on suppressed trees, and
  Norway spruce trees apparently experience crown
  thinning as a result of drought or other factors,
  and then recover

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Acidic Deposition

• Conclusions on Acid Rain
• The quick belief in forest decline was a complex
  social phenomenon, and may be explained in part
  by a few key points (see Innes, 1993)
• Unhealthy trees exist, and it is often difficult
  to assign a cause
• Greenhouse studies can demonstrate nutritional
  problems that resemble field symptoms
• Assessments of forest canopy conditions are
  difficult to perform and interpret (most trees
  in a forest look poorer than the dominant trees,
  for example)
• .

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Acidic Deposition

• Conclusions on Acid Rain
• Perhaps a major contributor to the mismatch
  between perception and reality was largely
  sociological.
• The risk of a Type II error (failing to believe
  real impacts of pollution) were frightening
• Some scientists predicted massive growth
  reductions
• and even deforestation
• Many people were very upset by such prospects.

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Carbonyl Sulfide (COS)

• Potential for effects on global warming
• Main source of sulfate aerosols in the
  stratosphere
• The most abundant S gas in the atmosphere
• Concentrations are increasing (sources greater
  than sinks Vg Table 13.3)
• Major anthropogenic contribution (?)
• Fossil fuels 0.02 x 1012 g yr-1
• Biomass burning 0.07 x 1012 g yr-1
• Oxidation of CS2 0.18 x 1012 g yr-1
• Total antropogenic 0.27 x 1012 g yr-1 , or 40
• Out of 0.68 x 1012 g yr-1 total