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Geochemical tracers• Tracers – talking more about theory than application.• Geochemical tracers trace:
– Circulation – Climate – Environmental conditions (e.g. nutrient levels and productivity)
• Geochemical tracers are proxies for past environmental conditions...
• and trace changes in these conditionsꜛ
The record of climate events and ocean change recorded chemically
• Tools: the substrate– What do we measure the tracers in?
• The Tools: Tracers – Geochemical tracers…..
• the application – How we really do it– what are the issues
Trace metals Organic compoundsOxygen isotopesCarbon isotopes
Measure the microfossils…Foraminifera
Coccolithophores Coccoliths
diatoms
Sediment
… it’s not just mud
The sediment record: what’s in your mud?
Benthic and planktonic
Surface and bottom
Mud to microfossils
corals
The record of climate events and ocean change recorded chemically
• Tools: the substrate– What do we measure the tracers in?
• The Tools: Tracers – Geochemical tracers…..
• the application – How we really do it– what are the issues
The Tracer tools Geochemical tracers!
Isotopes: δ13C – circulation δ18O – temperature
Organic matter:TOC – productivityBiomarkers – 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
Trace metal inclusions in CaCO3
Trace metals that are of similar size and charge as Ca substitute into the matrix….. or as impurities
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
CaCO3 Calcium carbonate … can have substitutions
CaCO3 substitutions are controlled by:
Temperature– inclusions are more common when temperatures are higher.
Nutrients – many trace metal concentrations mimic nutrient distributions
Many environmental parameters (thermodynamics, kinetics, biology)
Organic markers or “molecular markers”
• Total organic carbon (TOC)– productivity
• Biomarkers (individual compounds) – Trace organisms– Different environments and conditions– temperature (relative productivity)
Biomarkers persist in the sediments and contain information about the presence of past organisms
mananimals
fungi flagellates
diplomonads
microsporidiaplants
ciliatesslime molds
green sulfurbacteria
grampositives
protcobacteriacyanobacteria
avobacteria
SulfolobusDesulfurococcus
Pyrodictium
ThermofilumThermoproteus
PyrobaculumPyrococcus
MethanobacteriumMethano-thermus
Methanoplanus
ArchaeoglobusHalococcus
Halobacterium
MethanosarcinaMethanospirillum
Methanopyrus
1 23 4
Methanococcus1 jannaschii
4 vanniellii
2 igneus3 thermolithotrophicus
BACTERIA
EUCARYA
ARCHAEA
Thermocrinis
Aquifex
Thermotoga
OH
OH
OH
OH
OO
OH
HO
>2.7 Ga
>2.7 Ga
HO
HO
HO
R
CH2OCHOR
CH2OR
COOH
Algal Steroids•Encode a variety of age-diagnostic signatures –
C-isotopes + steroids from algae & plants
chlorophyceans
diatoms
chrysophytes
dinoflagellates H
H
HO
HO
HHO
HHO
C29
C28
C30
C30
‘bio’ ‘geo’
Alkenones as a tracer for sea surface temperature
Alkenones record SST….
in their saturation levels
Alkenones are made only by Prymnesiophytes…
Sea Surface temperature change: New Zealand case study
Climate warming since the last glaciation
Alkenones record information about climate
The ratio of alkenones in a sample records past temperature similar to foraminifera
Southwest Pacific ~25,000 yr record
Warmer location from north proxies disagree
Cooler location from further south proxies agree better...
Last ice age
All proxies are responding to more than phenomenon you want to assess
• Many proxies are based on biological substrates with different– biochemistry– seasonality
• There are competing geochemical... – signals– controls
Geochemical tracers
Two isotopes of interest:
Oxygen isotopes:
δ18O
Carbon isotopes
δ13C
Geochemical tracers
Oxygen is:16O=99.759%17O=0.037%18O=0.204%
δ notation is simply the ratio of 16:18 in the sample relative to a standard …..
“del” notation
δA= (RA -1) x1000RSt
which is the same as (18/16O)CaCO3 sample –1) x1000(18/16O)CaCO3 std
The result is we use “per mil” ‰ as our unitsand we always speak about the data in reference to: the isotope of interest
Fractionation of oxygen isotopes
Emerson and Hedges 2008
δ18O as a tracer of the short term isotope record
Oxygen δ18O in carbonates is controlled both by:
How much δ18O is in the water – this is affected by precipitation (salinity)
and
By the temperature of the water it is precipitated in
So on short time scales:
It traces both temperature and precipitation (not ice volume)
The temperature effect on δ18O
0 ‰ ____‰
____‰
at 10°C
0 ‰ ____‰
____‰
at 20°C
0 ‰
evaporation precipitation
____‰
____‰
____‰
____‰
The precipitation effect on δ18O
Galapagos Corals
A 5 point moving average of annual δ18O for Galapagos corals
←co
ld
war
m →
δ18O as a tracer of temperature: a review on Oxygen isotopes…. δ18O
δ18O in carbonates is controlled both by:
How much δ18O is in the water –
By the temperature (and salinity) of the water it is evaporated and precipitated in.
And the extent to which that distillation has gone on.
On long time scales:
It traces both temperature and ice volume
oxygen isotopes in the climate record
Ice -35‰
snow
Emerson and Hedges 2008
The record of δ 18O in the sediment record
The distillation is controlled by temperature (salinity) and ice volume.
The record is primarily a function of temperature and ice volume Emerson and Hedges 2008
The marine isotopic record
Emerson and Hedges 2008
Is a climatic “standard”
The marine isotopic record
Emerson and Hedges 2008
Is a climatic “standard”
Col
der
War
mer
Le
ss ic
e M
ore
ice
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