Transcript Slide 1
Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole
Catchment Soil Respiration. Jeremy B. Jones, Jr. and Patrick J. Mulholland. Ecosystems (1998) 1:
183–196
There are 7 common ions in freshwater:
Cations: Ca+2, Mg+2, Na+1, K+1; Anions: Cl-1, SO4-2, HCO3-1
Sources of ions in freshwater
From the atmosphere.
The principal atmospheric source of ions is from sea spray. Accordingly, the ratios of the
common ions in rain and snow reflect the ratios in seawater: Na and Cl are often the most
abundant ions in rain. (The concentrations of ions in rain are of course much lower than the
concentrations in seawater.)
From rock weathering.
Watersheds contain soils and rock which consist of a wide variety of different minerals.
Weathering of the soil and rock minerals produces runoff containing the common ions (and
many other ions in lesser concentration). Some sample reactions which illustrate the general
nature of weathering:
Na-feldspar weathering
2 NaAlSi3O8 + 2 CO2 + 3 H2O <----> 4 SiO2 + Al2Si2O5(OH)4 + 2 Na+1 + 2 HCO3-1
Calcite weathering
H2CO3 + CaCO3 <----> Ca+2 + 2 HCO3-1
There are of course a host of other soil and rock minerals subject to similar weathering
reactions. In general, weathering reactions can be characterized as a weak acid (carbonic
acid) slowly dissolving basic minerals. Weathering reactions result in runoff containing
dissolved ions, and commonly, undissolved particles of less soluble minerals such as clays.
Concentration patterns in surface water (the Gibbs 1970 "boomerang" plot).
Atmospheric dominance
Lakes which receive rainwater and snowmelt (without much influence by rock weathering) contain water
with low concentrations of ions. Because of the source of atmospheric ions, from sea spray, chloride and
sodium are relatively more abundant than the other ions. (The relative abundance of salts is not precisely
the same as sea water because there is some fractionation in evaporation and transport of sea salts.)
In addition, rainwater is in equilibrium with atmospheric CO2. In effect, rainwater is (approximately) a dilute
solution of carbonic acid with an admixture of a small amount of sea salt.
Rock Dominance
Water more or less in equilibrium with the materials in the drainage basin is characterized by higher
concentrations of ions and an increased significance of Ca, Mg, and bicarbonate ions. Many of the worlds
great rivers discharge water which can be characterized as rock-dominated.
Evaporation-Precipitation
Evaporation and fractional precipitation represent the third mechanism identified by Gibbs. The world
ocean is the end of this process. Some rivers and lakes have a similar chemistry if they occur in regions
which are arid. The overall concentration of ions increases with evaporation until selected minerals begin
to precipitate because their solubility product is exceeded. Because CaCO3 is commonly the first mineral
to precipitate, the relative concentration of these ions begins to decline with evaporation. With further evaporation, other minerals, such as gypsum (CaSO4) will also precipitate. As a consequence, the relative concentrations of Na+, K+, and Cl- increase.
Comment: Recent studies have produced data sets which are not consistent with Gibbs boomerang. For example, Kilham (1990, L&O, vol 35:80-83)
presented data for African waters. For his data set, the "rainwater" arm of Gibbs boomerang is missing, possibly because of very rapid rock weathering
or because of terrigenous dust contributing to precipitation chemistry. Most of the African samples are above 100 ppm total dissolved salts, and Ca and
Cl are low. In contrast, Eilers and Brakke (1992, L&O, vol 37:1335-1337) report data for Oregon-Washington and other locations which are generally of
very low overall ionic concentration, but with a range of Na/Ca ratios which span most of Gibbs plot. Eilers and Brakke argue that the Gibbs model is
misleading. Gibbs reply (1992, L&O, vol 37:1338-1339) discusses Eilers comments and defends the original model as still the most suitable for
"major" water bodies.
The Carbonate System
The carbonate system is important for a number of reasons. Natural waters are buffered with respect to pH
mostly because of the content of inorganic carbon species. In turn, pH is an important controlling variable for
many important geochemical reactions (e.g. solubility of carbonates). Many important biochemical reactions,
such as photosynthesis and respiration, interact with the pH and the carbonate system.
1. The chemical species of interest.
The carbonate species: CO2, H2CO3, HCO3-, CO3-2
Water and its ionization products: H2O, H+, OH-
The other common ions: Na, K, Ca, Mg, Cl, SO4
2. Some definitions.
Carbonate alkalinity is defined as: A = [HCO3-] + 2[CO3-2] + [OH-] - [H+]
(where [species] implies the concentration, in moles/liter, of the species in question.)
By convention, [H2CO3*] = [H2CO3] + [CO2]
Total inorganic carbon, CT = [H2CO3*] + [HCO3-] + [CO3-2]
The carbonate system may be completely described by 6
equations:
(1) [H+][HCO3-]/[H2CO3*] = K1 = 10-6.3
(2) [H+][CO3-2]/[HCO3-] = K2 = 10-10.25
(1st dissociation of carbonic acid)
(2nd dissociation of carbonic acid)
(3) [H2CO3*] = pCO2 x kCO2
(Henry’s law for CO2)
(pCO2=10 -3.5 atm, KH = 10-1.5)
(4) [H+][OH-] = Kw = 10-14
(Dissociation of water)
(5) [Ca+2][CO3-2] = KSo = 10-8.35
(Solubility of calcite)
(6) [H+]+[Na+]+[K+]+2[Ca+2]+2[Mg+2] = [Cl-]+2[SO4-2]+[HCO3-]+2[CO 3-2]+[OH-]
(Requirement for electroneutrality)
The ionization fractions (top panel of figure) are defined algebraically as:
[H2CO3*] = CT x a 0; [HCO3-] = CT x a 1; [CO3-2] = CT x a 2
The sum of the a ’s: a 0 + a 1 + a 2 = 1
The numerical value of the a ’s are a function of pH alone. They are defined as:
a0 = {1 + K1/[H+] + K1K2/[H+]2}-1
a1 = { [H+]/K1+ 1 + K2/[H+]}-1
a2 = {[H+]2/K 1K2 + [H+]/K2 + 1}-1
Definitions of equivalence points
a. Endpoint for an alkalinity titration: point x This endpoint is defined by the proton condition:
[H+] = [HCO3 -] + 2 [CO3-2] + [OH-] (Or, a pure solution of CO2)
b. Endpoint for "phenolphthalein" alkalinity titration: point y:
[H2CO3*] + [H+] = [CO3-2] + [OH-] (Or, a pure solution of NaCO3)
c. Point z:
2[H2CO3*] + [HCO3-] + [H+] = [OH-] (Or, a pure solution of Na2CO3)
Notice that for all three of these points (x, y, and z), the pH at which the endpoint occurs is a
function of CT. For example, the pH for x decreases as CT increases.
Alkalinity
Alkalinity is a measure of the ability of a water system to resist changes in pH** when acid is added to water.
A stream that has a high alkalinity is well buffered so that large inputs of acid (from acid rain for instance) can
be made with little affect on the stream pH. A stream that has a low alkalinity is poorly buffered and may
undergo large, sudden drops in pH in response to acid inputs.
•
Alkalinity is roughly equivalent to the imbalance of cations and anions:
Alkalinity = 2[Ca2+] + 2[Mg2+] + [Na+] + [K+] + [NH4+] - 2[SO42-] – [NO3-] – [Cl-]
•
The charge imbalance is “corrected” by changes in equilibrium in the DIC system.
•
Thus: Alkalinity = [HCO3-] + 2[CO32-] + [OH-] - [H+]
•
Alkalinity increases by processes that consume SO42-, NO3- or other anions, or that release DIC.
•
Alkalinity decreases by processes that consume cations or DIC.
•
Acid rain decreases alkalinity due to additions of H+ and SO42-.
An alternative definition of alkalinity
The current tendency is to use "acid neutralizing capacity", or ANC, instead of "alkalinity" to describe the
acid buffering capacity of a natural water. This usage is justified because of the method used to measure
ANC: an acid titration to a particular pH endpoint (but note the use of the Gran plot)
An alternate definition of alkalinity (or ANC) can be developed from the requirement for electroneutrality.
Beginning with the electroneutrality equation the terms may be rearranged to give:
[HCO3-]+2[CO3-2]+ [OH-]-[H+ ]=[Na+]+[K+]+2[Ca+2]+2[Mg+2]-[Cl-]-2[SO4 -2]
Thus, Alkalinity (or ANC) can be defined as:
Alk = [Na+] + [K+] + 2 [Ca+2] + 2[Mg+2] -[Cl-] -2[SO 4-2]
This version of alkalinity explains why it typically behaves as a "conservative" quantity. Samples may be
collected, stored, and analyzed as much as 6 months later. The conceptual definition based on carbon
species would seem to imply that alkalinity should be rather volatile. The alternative definition based on
common ions also draws attention to conditions under which alkalinity would not be conservative. Some
examples:
Redox processes which involve important buffering species will alter alkalinity. Examples are: oxidation
of ammonia to nitrate (nitrification), oxidation of sulfide or reduction of sulfate, oxidation or reduction of iron.
Precipitation or dissolution of minerals involving participating species will alter alkalinity. For example, the
precipitation or dissolution of calcium carbonate can alter alkalinity.
Other participating species, such as organic acids, may influence ANC, and may not behave conservatively.
Note also that CT and pH do not behave conservatively. They must be measured as quickly as possible to
obtain an accurate estimate of the makeup of a water sample at the time of collection.
Long-term decline in carbon dioxide supersaturation in rivers across
the contiguous United States
Jeremy B. Jones Jr., Emily H. Stanley, Patrick J. Mulholland
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 10 2003
1. Introduction
The concentration of CO2 is an important property of
aquatic ecosystems, reflecting both internal carbon dynamics
and external biogeochemical processes in the terrestrial
ecosystem [Cole et al., 1994; Jones and Mulholland, 1998a;
Richey et al., 2002]. CO2 and dissolved inorganic carbon
(DIC) concentrations in rivers and streams result from an
interplay between inorganic carbon fixation via aquatic
primary production, organic matter decomposition, import
via ground waters, and exchange with the atmosphere
[Hope et al., 2001; Palmer et al., 2001]….. (nice discussion)
2. Methods
2.1. Dataset
2.2. Chemical Analyses
Table 1. Long-term Trends in the Partial Pressure of CO2 (pCO2),
Dissolved Inorganic Carbon (DIC), and Associated Chemical and
Physical Parameters Across the Contiguous United States
Variable
Mean
pCO2 (ppmv)
Alkalinity
(mEq l1)
Trend
(y1)
2109
2622
Trend
standard
error
-78.4
0.9
4. Conclusions
[16] The decline in pCO2, given the lack of change in alkalinity, eqDIC, or eqH+, points to a reduction
or alteration in the quantity and/or quality of carbon import from terrestrial ecosystems. These
changes could be caused by a myriad of factors but we suggest that either a decline in soil
respiration or alteration in the nature of the hydrologic connections between terrestrial and aquatic
ecosystems are the likely explanations. Baseflows (i.e., groundwater fluxes to streams) have declined
in the southeastern U.S. over the past 30 years [Lins and Slack, 1999], which could reduce
the import of terrestrial carbon to streams [Jones and Mulholland, 1998b]. However, baseflow
increased in other parts of the country where we also observed CO2 declines. Terrestrial production
appears to have increased given trends in North America of increased plant growth [Myneni
et al., 1997], afforestation [Delcourt and Harris, 1980; Dixon et al., 1994], and nitrogen deposition
[Townsend et al., 1996], which may lead to increased soil respiration and CO2. More recent evidence,
however, suggests that increased nitrogen deposition may lead to reduced soil
CO2; results from experimental additions of nitrogen to ….
…forest soils have demonstrated a reduction in the rate of organic matter decomposition
[Berg and Matzner, 1997; Berg and Meentemeyer, 2002]. The reduction in riverine pCO2 may
have also been caused by a decline in riparian and wetland habitat. Wetlands and nearstream environments can affect in-river metabolism and pCO2 by releasing organic matter
and contributing water supersaturated in CO2 from soil respiration. Indeed, respiration rates
in riparian soils can exceed those in adjacent forest and cropland ecosystems [Tufekcioglu et
al., 2001]. High rates of primary production and soil respiration of wetlands and riparian
zones, coupled with dramatic losses of these habitats over the past century [Mitsch and
Gosselink, 1993] should have a strong and direct effect on stream pCO2. Regardless of the
cause, the trend in pCO2 indicates that gaseous carbon losses from terrestrial ecosystems
via aquatic pathways have likely declined across much of the contiguous U.S. and that
ecosystem functioning significantly shifted during the latter part of the 20th century.
Increase in the Export of Alkalinity from North America’s Largest River
Peter A. Raymond and Jonathan J. Cole
4 JULY 2003 VOL 301 SCIENCE
In terrestrial systems, there are two major
processes that sequester atmospheric CO2:
Organic carbon produced during photosynthesis
can be stored on land or exported in
rivers, or terrestrial alkalinity (carbonate and
bicarbonate ions) produced during chemical
weathering in soils can be fluvially exported.
The fluvial export of terrestrial alkalinity is
also the major source of oceanic alkalinity
and is a key regulator of the CaCO3 saturation
state of the oceans (1). During chemical
weathering, the atmosphere provides the reservoir
of CO2 either directly, or indirectly
through the respiration of plant-derived organic
matter. The lithosphere converts some
of this CO2 to dissolved bicarbonate or carbonate
through the weathering of the parent
rock material, and the hydrologic cycle transports
this dissolved inorganic carbon, as alkalinity,
to rivers and ultimately to the ocean.
Regional chemical weathering rates are controlled…..,
………Finally, these data demonstrate an increase in the export of alkalinity from the Mississippi
watershed over the past five decades, which demands an increase in the supply of protons to mineral
surfaces or an increase in the rate of chemical weathering due to warming. The acidity of rain has not
increased in recent decades (38), and therefore the increase in proton delivery does not appear to be
linked to acid rain. Potential mechanisms for an increase in proton delivery include an increase in
atmospheric CO2, an increase rainwater throughput, or an increase in plant and microbial production
of CO2 and organic acids in soils due to biological responses to increased rainfall and temperature. Plant
productivity responses to increased temperature and rainfall are already documented for the United
States (39, 40). Determining the relative contribution of these mechanisms, which will vary in importance
regionally, is a critical component of future research. In particular, because nitrogen loading has
also increased in the Mississippi watershed over the study period and is linked to agricultural
practices (41, 42), studies must also determine the relationship between nitrogen loading and alkalinity
export. Nitrogen loading could stimulate both soil respiration (43) and nitrification (44), the latter
of which represents a source of acid to weather minerals that does not consume atmospheric CO2 and
was found to be responsible for 6% of the bicarbonate generation in a carbonate-rich, extensively
farmed watershed (45).