Wildfire thermochronology Peter Reiners, UofA Geosciences, in collaboration with Stuart Thomson (Yale) and others Thermochronology is typically used to infer timing and rates of cooling due to erosion or tectonic exhumation. In this project we focus instead on resetting of the (U-Th)/He and fission-track thermochronometric systems by wildfire. Shallow soils and exposed bedrock experience thermal histories involving temperatures soil: 82% of grains of c. 300-600 oC for minutes or tens of minutes in typical wildfires. soil: 92% of grains fully reset
T [oC] 650
500
400
300
200
1 myr
(F Ts ys te m )
g( FT sy ste m tin et
1 kyr
ien tr es et tin g
ll r es
m
1 yr
(H es ys te m )
in
cip
es ys te (H g tin et
8 typical wildfire thermal histories
creek sand: 29% of grains fully reset
100 hr
in cip ie nt re se tti ng
13
AFT fully reset, (U-Th)/He partially reset
ll r es
18
)
fu
23
fu
ln(t) [ln(s)]
28
duration
For wildfire events, a kinetic inversion occurs, in which fission-track ages reset faster than (U-Th)/He ages. This leads to a diagnostic signature of shortduration, high-T events in detrital and exposed bedrock samples that can be used to trace wildfire through the landscape.
colluvial hollow: 70% of grains fully reset
100
)
32
2.0
1.5 AHe, a=100 μm AHe, a=50 μm
3.0
2.5
103/T [K-1]
f = 0.99 (”full”) or 0.01 (”incipient”)
colluvial hollow: 46% of grains fully reset
2001 fire limit
Antelope Lake area, northern Sierra Nevada
Ketcham et al. (1999) curvilinear r = 0.93 (”full”) or 0.55 (”incipient”)
90
Sierra Nevada (excluding Fort Sage)
70 soil
% fully reset grains
80
60
col luv i low al hol low -or hig der ch h-o a rde nnel r ch ann el
100
The proportion of wildfirereset grains decreases systematically with basin area. This reflects the fact that most apatite in river sand comes from landslides in a few highslope areas. Fluvial apatite is therefore highly spatially fractionated with respect to its distribution in the landscape.
Antelope Lake, Sierra
50
Adams Peak, Sierra
40
Cas c
30
ade
Grizzly Dome, Sierra
s
Fort Sage, Sierra
20
Kings River, Sierra
10
San Joaquin River, Sierra
0 0.8
Icicle Creek, Cascades
10-3
10-2
10-1
100
101
basin area [km2]
102
interior bedrock: 0% of grains fully reset
Most apatite grains in soils are completely reset for the fission-track system and partially reset for the He system. Samples in colluvial hollows and creeks have abundant but fewer reset grains. Rivers have very few.
1 hr
1 min
3 1.0
fully reset
103
104
