cosmogenic nuclide concentrations in sinclair, h.d.1 .../media/shared/documents... · powerpoint...
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Cosmogenic nuclide concentrations in Neogene rivers of the Great Plains
reveal the evolution of fluvial storage and recycling
Sinclair, H.D.1 McCann, L.1 , Stuart, F.2,
& Zui Tao1
1 – School of GeoSciences, University of
Edinburgh, UK 2 – Scottish Universities Environmental Research
Centre (SUERC), East Kilbride, UK
Surface residence Time (yrs)
Surface residence Time (yrs)
Cosmogenic nuclides generated by interaction of cosmic rays with the nucleus of the atom
http://www.ams.ethz.ch/research/ams/insitu/source.png?hires
Widespread application in geomorphology and Quaternary geology
Surface residence Time (yrs)
Surface residence Time (yrs)
21Ne concentration
21Ne concentration
Modern and Quaternary processes Radiogenic nuclides mainly 10Be and 26Al
Stratigraphic record throughout Earth history
Stable nuclides such as 21Ne
Neogene of Great Plains - a thin (~100m) succession of fluvial sediments known as the Ogallala Group sourced
from the Rockies. Modern rivers incised into Plains.
Duller et al., 2012. Geology.
Modern North Platte
Non-cosmogenic 21Ne can be identified using a three isotope plot of 20Ne, 21Ne and 22Ne.
Modern and ancient river sediment
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1 2 3 4 5 6 7 8 9 10
21
Ne
(at
/g)
Miocene and Pliocene river sediment
Mean sediment concentration – 1.5E+08
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1 2 3 4
21
Ne
(at
/g)
Shielded quartzite
granite gneiss
Mean granite concentration - 8.3E+07 Mean quartzite concentration - 8.0E+06
‘Shielded’ bedrock samples of source area eg. roadcuts
0.095
0.1
0.105
0.11
0.115
0.12
0.125
0.13
0.00 0.01 0.01 0.02 0.02
22
Ne
/20
Ne
21Ne/20Ne
0.095
0.1
0.105
0.11
0.115
0.12
0.125
0.13
0.00 0.01 0.01 0.02 0.02
22
Ne
/20
Ne
21Ne/20Ne
0.095
0.1
0.105
0.11
0.115
0.12
0.125
0.13
0.00 0.01 0.01 0.02 0.02
22
Ne
/20
Ne
21Ne/20Ne
Miocene
Pliocene
Modern
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1 2 3 4 5 6 7 8 9 10
Exce
ss 2
1N
e (
at/g
)
Mean = 4.3E+07 σ = 1.5E+07
1.00E+06
1.00E+07
1.00E+08
1 2 3 4 5 6 7 8 9 10
Exce
ss 2
1N
e (
at/g
)
Mean = 2.6E+07 σ =4.6E+07
1.00E+06
1.00E+07
1.00E+08
1 2 3 4 5 6 7 8 9 10 11 12 13 14
exc
ess
21
Ne
(at
/g)
Mean 1.8E+07 σ =1.7E+07
Upper Miocene
Pliocene
Modern
Extra 21Ne above source rock values
Mean 950 Kyr Range 240-7100 Kyr
Mean 580 Kyr Range 0-1520 Kyr
Mean 400 Kyr Range 242-1380 Kyr
Surface residence times
Conclusions
• Stable cosmogenic nuclides such as 21Ne can be used to derive palaeo-surface residence times of sediment throughout the stratigraphic record
• In the Great Plains of Nebraska sampled pebbles indicate a reduction in surface residence times from Miocene to present
• Pebbles resided near the surface for up to 7 Myr in Miocene times
http://rmgsc.cr.usgs.gov/uas/products.shtml
Condon, 2005
Miocene drainage
Condon, 2005
Pliocene drainage
Results
0.095
0.1
0.105
0.11
0.115
0.12
0.00 0.01 0.01 0.02 0.02
22
Ne
/20
Ne
21Ne/20Ne
Modern
0.095
0.1
0.105
0.11
0.115
0.12
0.125
0.00 0.01 0.01 0.02 0.02 0.03 0.03
22
Ne
/20
Ne
21Ne/20Ne
Pliocene
0.095
0.1
0.105
0.11
0.115
0.12
0.125
0.00 0.01 0.01 0.02
22
Ne
/20
Ne
21Ne/20Ne
Miocene
0.095
0.1
0.105
0.11
0.115
0.12
0.00 0.00 0.00 0.01 0.01 0.01 0.01
22
Ne
/20
Ne
21Ne/20Ne
Bedrock
Evidence of shortening rates Terrace data
0.03 0.02 0.02 0.03
0.02
0.03
0.02
0.09
0.07
0.07
0.08
0.07
New data
Munack et al., 2014
Erosion rates (mm/yr) derived from detrital 10Be in quartz
0.04
0.02
0.02
Dortsch et al., 2011
0.006, 0.005, 0.007
0.013, 0.009
Summit erosion rate (Reynhout, 2011)