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App 3: Dating River Incision (Burial dating method (Examine cave structure…
App 3: Dating River Incision
Problematic
Lotsa recycling fluvial material
A)
Dating incision via terraces
Problem:
Lotsa recycling fluvial material
Solutions
Sample lotsa different clasts
Depth profiles
Isochron burial dating
B)
Dating bedrock river incision
Example: Taroka Gorge, Taiwan
Marble bedrock ∴ sampled lotas Cl-36
Vertical profile canyon walls
Taroka Gorge assumptions
No gorge wall retreat after exposure (incision)
∴ Maximum:
estimate for erosion rates
exposure age
Since if background erosion occurred
would take longer to get that Cl conc
∴ age would be
younger
Burial Dating
Samples have exposure history prior to burial
Once buried, sample = shielded from cosmic rays
∴ Radioactive decay of cosmogenic nuclides = useful
C)
Incision from burial dating
(Orange 2001)
Dated sediments in Mammoth Caves, Kentucky
Samples sand & gravel from depo & erosional areas
Measured Al-26 & Be-10
Orange & Muzikar (2001)
Fluvial sed washed into Mammoth Cave system
When bury something
Start to see radioactive decay signal
∴ Want to date burial event
Burial History Unknowns
How long exposed before in fluvial system
How long buried for in cave
Burial dating method
Examine cave structure
areas deposition vs incision vs roof collapse
Measure Al-26 + Be-10 concs
2 nuclides with
known production ratio
sufficiently different half-life
Model pre-burial nuclide conc
If previously exposed sample is shielded
preferential loss shorter lived isotope occurs (Al-26)
Granger Findings
Pre-burial erosion rates ~ 1.5 - 1.7 mm/yr
Mammoth cave
Burial ages ~ 0 - 3.3 Ma
Green River History
Inferred from geomorphology & incision rates/burial ages
7 events alternating btwn
river aggradation & cave sedimentation
incision
App 5: Detecting landscape transience
Using C-14 & Be-10
Some event - causes rapid ↑ erosion rates
C-14 has shorter half-life ∴ responds faster to erosion perturbation
Look @ C-14 : Be-10 to detect transience
Problems detecting landscape transience
Suited to point msmts
C-14 = sensitive to burial
∴ spatial averaging method = less suitable
∴ Limit - up to 10 ka after burial
can detect small change
using C-14:B-10 ratios
(if much larger change, poss recorded for longer)
Source - sink problem
cannot show all sed gets to sink
Meteoric Be-10
Forms in atmosphere
Sticks to soil grains
Can use to estimate modern erosion rates