Download calcium - Mark W. Williams, Ph.D

Climate change and calcium
• Calcium from geochemical weathering
causes climate to become cooler
• Calcium from dust does not have a climate
change effect
Carbon Reservoirs
• Reservoirs are:
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Atmosphere
Terrestrial biosphere
Oceans
Fossil fuels
• Reservoirs or components of reservoirs that
release carbon to other reservoirs are called
sources
• Those that absorb carbon are called sinks
• This overall network of storage or absorption and
transfer is known as the carbon cycle
Silicate Weathering
• Precipitation from the atmosphere falls to Earth as
bicarbonate acid rain, with a natural acidity
• CO2 (gas) + H2O (liquid) --> H2CO3 (carbonic acid)
• Carbonic acid weathers the rocks on the Earth's surface,
releasing ions of calcium (Ca++) and bicarbonate (HCO-3)
into the oceans
• An example reaction is shown on the next slide
Urey Reaction
• Named after Harold Urey, Nobel-prize
winning geochemist
• This reaction is an example of the
conversion of silicates to carbonate
• CaSiO3 + CO2 = SiO2 + CaCO3
• Wollasonite + carbon dioxide =
quartz + limestone
Harold C. Urey,
1893-1981
Carbonate-Silicate Cycle Diagram
Precipitation of Limestone
• Soluble carbonates may be converted to solid form by
inorganic chemical reactions, or by organisms which
extract lime and use it to build shells
• Limy muds or biochemical shells sink to the ocean floor,
where the lime may be converted to limestone
• This stores large amounts of carbon on the ocean floor
Carbon Sequesterization
• Limestone added to the ocean flow
sequesters carbon
• The oceans become sinks for carbon
• The atmosphere, which was the source of
the carbon, is a source
• Carbon is depleted in the atmosphere
Carbonate – Silicate Cycle
• Cycle combines the Carbon Cycle with
Silicate Weathering
• A steady state is reached, where the fluxes
into and out of each reservoir are constant
• Changes to the system can upset the balance
• There have been claims that this cycle
provides a self-regulating mechanism for
CO2 in the atmosphere
Example: Plate Collisions
• When India hit Asia, the Himalayan
mountains were pushed up
• Mountain ranges increase precipitation
• The extra rain weathered the newly exposed
silicate minerals, and started the process of
carbon sequesterization in the ocean
• This removed carbon dioxide from the
atmosphere
Himalayas caused global cooling
Ice Age Trigger
• Eldridge Moores, 1996 president
of the Geological Society of
America, has suggested that the
uplift of the Himalayas may have
triggered the Pleistocene glacial
advances
Eldridge Moores
DUST: MORE AROUND
THAN YOU THINK
DUST STORM IN FEBRUARY
Determination of Sources of Ca
to Surface Waters in the Rocky
Mountains using mineralogic
and Sr-isotopic data
David Clow1, Thomas D. Bullen2,
Mark W. Williams3, John A.
Fitzpatrick2, and Julie K. Sueker4
PROBLEM STATEMENT:
Surface water in granitoid terrain
in the Rocky Mountains often has
higher Ca/Na ratios than can be
explained by weathering of major
primary minerals in local
bedrock.
APPROACH
• Chemical analysis of calcium cannot
discriminate sources of calcium
– There is not a unique signature of calcium from
chemical analysis
• Strontium substitutes for calcium in
chemical reactions
• Different sources of strontium have
different isotopic ratios
HYPOTHESES
• (1) nonstoichiometric weathering of
plagioclase
• (2) weathering of eolian carbonates
• (3) weathering of trace calcite in granitoid
bedrock.
METHODS
• Rock and stream water samples were
collected from sites in the Wind River
Range of Wyoming, and the Park and Front
Ranges of Colorado. Soil samples were
collected from a subset of sites in the Front
Range.
nonstoichiometric weathering of
plagioclase
• evaluated by analyzing the zonation characteristics
and chemical composition of plagioclase crystals
in 40 rock samples collected from 12 recently
glaciated basins underlain by Proterozoic granitoid
bedrock in the Wind River Range of Wyoming, the
Front Range of Colorado, and the Park Range of
Colorado.
• Thin sections were made from the core of each
rock sample, and 10 plagioclase crystals in each
thin section were examined using a petrographic
microscope.
Plagioclase zonation
Plagioclase zonation
• No zonation of plagioclase found
• Reject hypothesis one
• Reject Williams et al. 1993
weathering of eolian carbonates
• evaluating patterns in 87Sr/86Sr of
exchangeable cations in soils collected from
three different landscapes: forest, alpine,
and near glaciers. These landscapes
correspond to progressively younger and
fresher soils.
weathering of trace calcite in
bedrock
• tested by measuring the carbonate content
of the rock samples used in the thin-section
analysis. Cores cut from the interior of the
rock samples were crushed into powder, and
the carbonate content of the powder was
measured by coulometric titration
87Sr/86Sr
of exchangeable cations
in soil
inorganic carbon concentrations
(as CaCO3) in rock samples
Scatterplot of Ca/Na in stream
water against % CaCO3 in rocks
Scatterplot of Ca/Na in stream
water against fraction of basin
area with slope ≥30°
87Sr/86Sr
Scatterplot of
in stream
water against Ca/Na in stream
water
Modeling strontium content
• Multivariate stepwise linear regression was
performed
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basin slope
basin size
amount of Pleistocene glacial till
granite/gneiss ratio
• explanatory variables were normalized to 1
Scatterplot of predicted 87Sr/86Sr
against observed 87Sr/86Sr in stream
water
Ca behavior may be summarized
as follows: weathering of trace
calcite occurs primarily in areas
with steep slopes (≥ 30°) because
that is where physical weathering
is most active.
Eolian deposition of carbonates is
important in cirques on the lee (east) side
of the continental divide. Small glaciers
and perennial snowfields occur in those
locations because prevailing westerly
winds deposit snow that is carried from
the west side of the divide there; eolian
deposition probably follows a similar
pattern.