Transcript Slide 1

Pollution of Lakes and Rivers
Chapter 4:
Retrieving the sedimentary archive and
establishing the geochronological clock:
collecting and dating sediment cores
Copyright © 2008 by DBS
Contents
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Collecting and dating sediment cores
Retrieving and sampling sediment profiles
Setting the time scale: geochronological methods
Correlating multiple cores from the same basin
The ‘top/bottom approach’: snapshots of environmental change
The paleolimnologist’s option of setting the most appropriate scale
Retrieving the Sedimentary Archive
Collecting and Dating Sediment Cores
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What is required?
– A sedimentary sequence that is the correct length, resolution and quality
– An accurate depth-time profile
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
• Fieldwork – what is it like?
• Requires planning
• Criteria for good recovery
– No disturbance of structure
– No change in water content or void ratio
– No change in constituent chemistry
• Dealing with variable composition
– Sites differ markedly in their morphology/climate etc.
– Composition may vary within the lake (basin to basin)
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
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Commonly used approaches:
Glew, J., Smol, J.P. and Last, W.M. (2001) Sediment core collection and extrusion. In
Last, W.M. and Smol, J.P. (eds.), Tracking Environmental Change using Lake
Sediments, Volume 1, Basin Analysis, Coring, and Chronological Techniques. Kluwer
Academic Publishers, Dordrecht, pp. 73-105.
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Long cores:
Leroy, S.A.G. and Colman, S.M. (2001) Coring and drilling equipment and
procedures for recovery of long lacustrine sediment sequences. In Last, W.M. and
Smol, J.P. (eds.), Tracking Environmental Change using Lake Sediments, Volume 1,
Basin Analysis, Coring, and Chronological Techniques. Kluwer Academic Publishers,
Dordrecht, pp. 107-135.
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Choosing the coring site
• Would like continuous, representative and undisturbed samples
• Single cores - take many weeks/months to analyze
• Not an issue since replicate cores show very good reproducibility (Charles
et al., 1991)
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Ideal: flat, central and deep basin
– Bathymetric maps
– Technology (depth soundings etc.)
Things to avoid
– Steep morphometric gradients (slumping)
– Shallow areas (prone to mixing)
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Coring platform
• Cores are taken through the water column
• From a boat, platform or ice cover
• Coring equipment
Figure 4.1. Collecting surface lake sediments
using a small diameter Glew (1991) gravity
corer from the pontoons of a helicopter in
Arctic Canada.
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Coring equipment
• Last 100 years contained in 50 cm (N. America) (cf. fast accumulation)
• More ancient histories contained in cores 2 m or longer
• Recent sediments are ‘unconsolidated’ > 90 % water
Short Cores (Surface Sediments)
• Open-barrel gravity corers
(plastic tubes)
– Close-on-contact type
(line tension)
– Messenger-operated
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Surface sediments
• Freeze-crust samplers (Renberg, 1981, Verchuren, 2000)
• Designed to preserve chemistry of the sediment-water interface
Figure 4.3. General operation of a freezecrust sampler.
Inset: Corer chamber is filled with dry ice and
a coolant, such as alcohol. The corer top is
secured.
A: Corer is lowered through the water column
B: Corer is lowered into the sediment,
sediment freezes to the corer
C: Sediment-encrusted corer brought to the
surface.
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Long Cores
• Livingstone corer
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Rod driven piston corer
Figure 4.4. Simplified diagram showing the basic principles used in piston coring.
A) To recover an undisturbed core sample, the corer is positioned at the sediment surface
B) The piston held stationary, while the core tube is pushed past it into the sediment using the coring rod.
C) The core section is recovered to the surface, with the core tube and piston locked together. The sealing of the piston in
the core tube prevents any tendency for the sample to slide out or for the core material to be deformed.
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
• Kullenberg
– Cable-operated Livingstone corer
– For deep water
Figure 4.5. General operation of a Livingstone-type piston
corer, showing the lowering (A), sampling (B), and
withdrawal (C) of a sediment sequence. The operator uses
the drive rods to push the corer into the sediment to the
desired depth. A cable, also held by the operator, keeps the
piston in place. From Glew et al. (2001); used with
permission.
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Long Cores
• Mackereth compressed air piston corer (Mackereth, 1958; 1969)
• See Glew et al., 2001
Retrieving the Sedimentary Archive
Retrieving and Sampling Sediment Profiles
Sediment Sectioning
• Photography
• Choice of division
may be based on
visible mixing
• Should be sectioned
lake-side / ASAP
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
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Radioisotopic techniques
– Measure decay of naturally occurring radioisotopes
– 210Pb (t1/2 = 22.3 yrs) most commonly used (for recent sediments ~150 yrs)
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210Pb
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Dating
Supported 210Pb (in-situ decay of 238U)
Unsupported 210Pb (from atmosphere) (= total 210Pb – Supported 210Pb)
Assumptions:
Rate of deposition of unsupported 210Pb is constant
Transfer of 210Pb from the water column to sediments
No disturbances: mixing/additions from catchment inflows
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
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210Pb
dating example
ln (unsupported 210Pb activity)
Pbx
Pb0
0
x
Depth (cm)
t = 1 ln(210Pb0 / 210Pbx)
λ
Slope; m = - λ / a
Where a = sedimentation rate
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
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Other radioisotopic indicators
– 137Cs from bomb tests and Chernobyl (also 241Am)
– Peaks in 1954, 1958, 1962 (Test ban treaty, 1986)
Appleby, 2001
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
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14C
used for longer sequences
production via cosmic rays
1 n
0
+ 147N → 146C + 11H
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Atmospheric 14C is found in 14CO2
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Incorporated into plants where it decays
– Whilst alive 14C/12C ratio is constant
– After death 14C no longer replaced from
environment
– Useful for about 7 half-lives
t1/2 = 5,730 yr
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
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Pollen chronologies
– Introduction, proliferation and demise
of plant species
– e.g. Ambrosia (ragweed) ~ European
settlement
Episodic events
– e.g. volcanic ash from known eruptions
(Tephrachronology)
– e.g. Charcoal from forest fires
Other anthropogenic time markers
– Chemicals from anthropogenic activity
Retrieving the Sedimentary Archive
Setting the Time Scale: Geochronological methods
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Annually laminated sediments (varves)
– Similar to tree rings
– Very few varve forming lakes
Figure 4.9. Annual couplets (varves) of sediment from Nicolay
Lake, Nunavut, Arctic Canada. Photograph taken by S.
Lamoureux.
Retrieving the Sedimentary Archive
The ‘Top/Bottom Approach’: Snapshots of Environmental Change
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Time consuming to analyze every layer
With multiple lakes + cores the no. of
samples increases
“before and after” type questions do not
require entire core
e.g. are lakes currently more
acidic than 1850’s?
Cumming et al., 1992
Retrieving the Sedimentary Archive
The Paleolimnologist’s Option of Setting the Most Appropriate Time Scale
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Ecosystems change and respond to stress over varying time-scales
Paleolimnology allows the scientists to set their own time-scale
Retrieving the Sedimentary Archive
Summary
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Collection and dating is a crucial first part of any study
210Pb most common dating technique for recent sediments
Top/bottom approach may be used if detailed analysis not required
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