Cosmogenic nuclide concentrations in Sinclair, H.D.1 .../media/shared/documents... · PowerPoint...

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)