Geochemical tracers

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Geochemical tracers

• Tracers talking more about theory than
  application.
• Geochemical tracers trace
• Circulation
• Climate
• Environmental conditions
• Geochemical tracers are proxies for past
  environmental conditions (and trace changes)
• Signals in sediments or other substrates
• Isotopes (composition of the sediments)
• Trace metals (in fossils)
• Organic compounds

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The record of climate events and ocean change
recorded chemically

• The Tools Tracers
• Geochemical tracers..
• the application
• How we really do it
• what are the issues

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How do we trace the movement of components in the
ocean?

• Nutrient controls on productivity.
• they are trace-able
• With major components like carbon, how can we
  track the cycling of that?
• How can we trace changes in important properties
  in the past?

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The biological pump.
How do we know it changed in the past? How do we
trace carbon movement through the system?
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The tools
Geochemical tracers! Isotopes ?13C
circulation ?18O temperature Organic
matter TOC productivity Biomarkers
temperature (relative productivity) Anthropogenic
compounds - pollution tracers Trace metals-
Trace metal ratios in calcite (CaCO3) Cd/Ca
(phosphate), Ba/Ca (nutrients), Mg/Ca and
Sr/Ca temperature
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Trace metal inclusions in CaCO3
Trace metals that are of similar size and charge
as Ca substitute into the matrix..
interpretation is based on the simple principle
that if there is more in the water there is more
in the calcite Empirically, we have found their
chemistry is controlled by Cd/Ca (phosphate),
Ba/Ca (productivity) Mg/Ca and Sr/Ca
temperature
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CaCO3 Calcium carbonate can have impurities
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CaCO3 impurities are controlled by
Many environmental parameters (thermodynamics,
kinetics, biology)
Temperature inclusions are more common when
temperatures are higher. Nutrients many trace
metal concentrations mimic nutrient distributions

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Organic markers or molecular markers

• Total organic carbon (TOC)
• productivity
• Biomarkers (individual compounds)
• Trace organisms
• Different environments and conditions
• temperature (relative productivity)

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Biomarkers persist in the sediments and contain
information about the presence of past organisms
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Algal Steroids

• Encode a variety of age-diagnostic signatures
• C-isotopes steroids from algae plants
• chlorophyceans
• diatoms
• chrysophytes
• dinoflagellates

geo
bio
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Alkenones as a tracer for sea surface temperature
Alkenones are made only by Prymnesiophytes
Alkenones record SST. in their saturation levels
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Alkenones record information about climate
The ratio of alkenones in a sample records past
temperature similar to foraminifera
Southwest Pacific 30,000 yr record
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Geochemical tracers
Two isotopes of concern Oxygen isotopes
?18O Carbon isotopes ?13C
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Geochemical tracers
Oxygen is 16O99.759 17O0.037 18O0.204
? notation is simply the ratio of 1618 in the
sample relative to a standard ..
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del notation
?A (RA -1) x1000 RSt which is the
same as (18/16O)CaCO3 sample 1) x1000
(18/16O)CaCO3 std The
result is we use per mil as our units and we
always speak about the data in reference
to the isotope of interest
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Fractionation of oxygen isotopes
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oxygen isotopes in the climate record
snow
Ice -35
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The record of ? 18O in the sediment record
The distillation is controlled by temperature
salinity and ice volume. The record is
primarily a a function of temperature and ice
volume
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The marine isotopic record
Is a climatic standard
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Geochemical tracer carbon isotopes
Carbon isotopes 12C98.89 13C1.11 14Csmal
l and variable
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Fractionation of carbon is primarily
biologically mediated
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Fractionation of carbon is primarily
biologically mediated
Plants discriminate against 13C during
photosynthesis. The the ultimate ?13C amount
varies with plant type and substrate.
Libes chapt. 29
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Fractionation of carbon is primarily
biologically mediated
Plants discriminate against 13C during
photosynthesis. The the ultimate ?13C amount
varies with plant type and substrate.
Libes chapt. 29
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The biological pump moves organic carbon
The biological pump moves organic matter with
its depleted isotopic signal from the surface
ocean to the deep water masses..
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The vertical movement of organic carbon can be
seen in the isotopes of Carbonate
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The conveyor belt circulation
And thermohaline circulation moves CO2 through
the system
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Controls on the fractionation of 13C
Water downwells DIC enriched Low nutrients High
O2
13C
13C
13C
12C
13C
13C
12C
12C
12C
13C
13C
DIC (in Water) more depleted with increasing age
13C
12C
12C
12C
12C
12C
Porewaters very sensitive to remineralization can
be very depleted in 13C
12C
12C
12C
12C
12C
12C
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The vertical profile of ?13C reflects the global
thermohaline circulation
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How do we use these tracers?
Evidence for the effects of Short term
perturbations (that we can verify other ways)
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Dissolved inorganic carbon change

• DIC change in the ocean is due to increasing CO2
  in the atmosphere
• How can you trace increasing CO2 input when the
  ocean is full of ?CO2 ?

Find another complimentary tracer
CFC cloro-flouro-carbons
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Anthropogenic gases
Tracers with a known history..
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Atlantic Ocean DIC CFC as a tracer of ocean
ventilation
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Pacific Ocean DIC
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Anthropogenic CO2
This is calculated via a proxy. (13C or CFCs)
using 13C we can also trace inputs directly in
the DIC..
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?13C can trace Fossil fuel input

• Fossil fuels are organic molecules
• They are derived from compounds that were
  originally produced by photosynthetic organisms
• Therefore they have a depleted 13C signal (of
  about -20 to -30 )

We can use this to measure the change in ? CO2
directly. And on long time scales.
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?13C traces the source of the DIC in the surface
ocean
Southern Ocean