Cosmogenic 10Be depth-profile chronology of Late Pleistocene Alluvial Fan Deposits, Baja California, Mexico
JOSE LUIS ANTINAO (1), ERIC MCDONALD (1), JOHN C. GOSSE (2) AND SUSAN ZIMMERMANN (3)
(1) Division of Earth and Ecosystem Sciences, Desert Research Institute, Reno, NV 89512 (jantinao@dri.edu)
(2) Dalhousie University, Halifax, NS B3H 4J1, Canada; (3) Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore CA 94550.
Geomorphology, sedimentology, soils data
10°N
Monsoon region
Baja
California
140°W
120°W
24° N
1 km
San Jose
del Cabo
10°N
Tropical Cyclones
160°W
azaro
Fig. 2. Landsat images showing the effects of Hurricane Juliette (2001)
in San José del Cabo, southernmost peninsula (Fig.1). Note aggradation
in the alluvial channel marked by bright sediment reflection, the breaching
of the beach ridge and progradation of the coast.
5 4
10 km
100°W
SL-III
EAO-2
1 SJ pits
2
10
Fig. 7. Sampling pits, indicating location of Be sediment
samples. Compare soil development with Figure 5.
soil/chronology pits
Our main objective is to gain knowledge about the
alluvial fan chronology in the area most affected by
tropical cyclones during present-day conditions
(Baja California; Figs. 1-3).
5 km
1
Basement
planned sedimentology section
Qlit
Jurassic gneiss
Qt3
Qm
Jurassic gneiss/granodiorite
Qt5
AMS 14C sample
OSL sediment sample
EAO-3
Cretaceous granite
Miocene volcanics
Qt6
The high energy depositional environment of
these alluvial sediments is evidenced by
sedimentological features like horizontal
upper-flow regime bedding, antidunes and
transverse gravel bars (Fig. 6). These features
preclude developing an absolute chronology by
14
means of C dating of organic remains.
We used 10Be depth-profile cosmogenic nuclide
chronology (e.g. Hidy et al., 2010) to assess
abandonment of alluvial fan surfaces near La Paz,
at Ejido Alvaron Obregon (EAO; Fig. 4). In this work,
we report preliminary ages for the two older units
(Q2 and Q3 in Fig. 4).
0
EAO-3
cumulative
-150
-250
D
6C
5
Erosion rate vs. χ2
−1
min χ2
0
−0.5
0
0.5
Age distribution
-100
χ2 value
200
50
60
-150
1.5
2
2.5
70
Age (ka)
80
90
0
60
0.5
1
2
300
200
100
0
min χ2
0
60
40
0.5
1
−1
0
1
2
300
200
100
0.5 1
1.5 2
2.5
5
concentration (10 atoms / g)
0.7
0.75
0.8
Inheritance (10
5
2
15u
1
15-20u
0.5
20-30
10u
5m
NE
boulders
LEGEND
coarse layers, sandy gravel ~7-15 mm
20u
imbrication, dip, u: upstream, otherwise downstream
sandy gravel horizons 2-5 mm
10
bed apparent dip, u: upstream, otherwise downstream
fine horizons, 0.25-2 mm sand
SIMPLIFIED
UNITS
5m
SW
-0.8
−0.5
0
1
2
0.85
2
A
Fig. 6. Sedimentological section SL-III (Fig. 4) highlighting upper-flow regime features. This section is part of San José Basin, Unit Qt4. Sandy gravel is the major
component unles noted otherwise. Note the low angle bedding dipping both upstream and downstream of the lower portion of the section (A in the inset), and the
long (~15 m) wavelength structure in both the lower and upper portions of the section (A and B). The upper portion (B) displays also a transverse gravel bar on top.
Although depositional breaks can be inferred from sedimentological differences between units A, B and C, the lack of soil structure developed on the breaks
suggests these do not reporesent long periods. The observed structures are interpreted as antidunes. The wavelength of these antidune trains corresponds
to the wavelength of the standing waves with which they were associated in the flow. It is estimated that for the San Lazaro bedforms, a discharge of up to
~13,000 m3/s was reached, considering a channel width similar to the present-day. These discharges are associated only with major tropical cyclones.
100
120
140
160
4
5
3
4
5
0.9
−1
atoms g )
0.95
1
4
5
2
Inheritance vs. χ
1
min χ2
0.9
0.8
0.7
0
1
2
2
3
χ value
Conclusions and Future Work
Our results indicate that in this region seasonal high intensity
precipitation coupled to rapid weathering of bedrock is generating
cyclic alluvial fan aggradation in response to millennial-scale shifts
in tropical climate parameters.
500 year average NINO3
Qt4 (La Paz)
Qt3 (EAO-2)
The expression of tropical variability is probably in the form of more
recurrent approach and landfall of tropical cyclones, which perform
most of the sediment transport.
Ongoing OSL (e.g., Rhodes et al., 2012) and cosmogenic dating of
the two younger (Q4, Q5) alluvial units in the area should provide
additional constraints on the observed relation to tropical variability.
120
100
80
60
40
20
0
Time [ka]
transverse gravel bar (transverse ribs)
B
80
0
-0.6
10
60
min χ2
-0.4
20-30u
40
Erosion rate vs. χ2
0
20u
3
χ value
400
0.65
20
Age [ka]
-0.2
5-10u
20
0
0.2
Figure 6C
30
χ value
Inheritance distribution
Qt2 (EAO-3)
40
Age vs. χ2
Erosion rate (cm ka )
0.4
transverse gravel bar (transverse ribs)
10
50
χ value
80
100
Erosion rate distribution
0
-3
Figure 6B
2m
Fig. 11. Cosmogenic depth-profile model ages versus model
inheritance for Providence and Muggins Mts. (closed circles,
unpublished data), compared to this study (open circles).
min χ2
1
100
400
0
40
-300
density (g cm )
Figure 6A
Qt3
1
Inheritance vs.χ2
0.8
1.1
χ2 value
Discussion
flow from clast orientation: N50, N80, N10
(i.e., from right to left)
Fig. 5. The alluvial fan soil chronosequence for La Paz. Older units
have well developed Bt and Btk horizons, with carbonate morphology
at different stages. The younger units have thinner soil horizons, no
carbonates in the profiles, and some of them are subject to episodic
flooding during tropical storm approach (Figs. 2, 3).
1
10
Cosmogenic Be inheritance modeled for both
depth-profiles (Figs. 9-10) are lower by at least
a factor of 2 compared to values of Mojave and
northern Sonoran deserts (Fig. 11).
1
10
-250
1
SAN LAZARO III SECTION
29.5 m, strike: N30
Qt5 / Qt6
0.9
Inheritance (10 atoms g )
0.6
Qt4
0
0.8
0.5
Fig. 10. Model Distributions for Age, Landform Erosion, Inheritance (EAO-3)
-200
-200
0.8
Qt2
Age (ka)
Frequenc y
2.5
0
−0.5
Mio-Pliocene sediments
6B
6A
0.5
Erosion rate (cm ka−1)
-50
-300
Fig. 3. Oblique aerial photograph taken shortly after hurricane Liza
(1976, Sept. 30) struck La Paz (Fig. 4). The flow destroyed an earth
dam (arrow) aiming to protect the urban area, which has been built
mostly on top of units Q5/Q6 (Fig. 4) Credits: INEGI, Mexico.
2
5
measured
-100
Qt2
TCN depth profile/stream sediment sample
1.5
concentration (10 atoms / g)
C
-50
sedimentology section
Ql
1
Fig. 8. The
concentration distribution can be modeled (Hidy et al., 2010)
solving simultaneously for age, surface erosion and inheritance (Figs. 9, 10).
Best-fit model curves (B, D) are shown (model results for 10,000 simulations).
0
Qt1
Qt4
0
500
10Be
San José
planned soil/chronology pits
Geochronology
200
0
−0.5
0
0.5
Inheritance distribution
0.5
2.5
50
min χ2
Concentration vs. depth
Setting
Late Pleistocene to Holocene alluvial fans in
the southern tip of the Baja California peninsula
(Fig. 3, 4) are characterized by thick sedimentary
sequences that display a well-developed soils
chronosequence (Fig. 5) that allow mapping
discrete sedimentary units.
2
Density vs. depth
Alluvial fan units and other
Quaternary sediments
Qt0
Soil pits and sections
1.5
-3
3
2
-150
-250
1
30
40
Age (ka)
Erosion rate distribution
-200
density (g cm )
LEGEND
If tropical variability is expressed in periods of
enhanced transport of tropical moisture towards
the mid-latitudes, we should be able to observe
geomorphic (alluvial) activity during these periods.
One of several mechanisms for transport of
moisture is landfall of tropical cyclones (Fig. 1).
We link present-day geomorphic effects of tropical
cyclones with the sedimentology and chronology of
fan units in order to compare with records of past
climatic variability in the Tropical Pacific.
-300
B11-SL1 B11-SL4
TCN
stream
sediment
sample
EAO pits
-200
-250
ST pits 2 1
San L
measured
cumulative
3
La Paz
30°N
Pacific
High
23°15’ N
-150
-100
0
20
Age vs. χ
Inheritance [104 atoms / g]
Cajoncito
Fig. 1. Storm paths affecting southwestern North
America, including Baja California Peninsula. Note
proximity of peninsula to region where tropical
cyclones originate.
winter
extratropical
cyclone
tracks
2
La Palma
North
America
30°N
ño
50
40
30
20
Age (ka)
10/26
-100
Cadua
500
Erosion rate (cm k a−1)
CAD
pits
-50
Depth (cm)
05/19
1
2
Age distribution
Inheritance (105 atoms g−1)
50°N
EAO-2
Frequenc y
80°W
REC-1 pit
B
Frequenc y
100°W
A
-50
depth (cm)
120°W
0
Depth (cm)
Aleutian
Low
140°W
B
depth (cm)
50°N
160°W
El Coyote
0
Fig. 9. Model Distributions for Age, Landform Erosion, Inheritance (EAO-2)
Frequency
A
What is the effect that climate variability in the tropics has on geomorphic systems of arid regions? We
have recently proposed that the Tropical Pacific has a relevant effect on landscape evolution of SW North
America over millennial timescales (Antinao and McDonald, submitted). This region, for example, receives
anually at least one landfalling tropical storm (Fig. 1), most of the times with significant effects on ecology,
geomorphology and hydrology. It has been shown that these storms are by far the largest agent in alluvial
activity of this region (Figs. 2, 3; Antinao and Farfan, 2013; Antinao and McDonald, 2009, 2011).
Concentration vs. depth
Density vs. depth
109°45’ W
Frequency
110°15’ W
Erosion
rate (cm ka−1)
−1
Inheritance (105 atoms g )
Fig. 4. Geological-geomorphological map of La Paz and San Jose basins.
Documenting geomorphic effects of tropical variability in arid regions
180°
Results
Frequency
Introduction
Fig. 12. The 500-year average NINO3 index (degrees Celsius) from Zebiak-Cane model (Clement et al.,1999) compared
to the distribution of ages for units Qt2, Qt3 (this study) and OSL data from Qt4 at La Paz (Busch et al., 2011; Maloney, 2009).
Compare with global ice volume curve (blue).
Preliminary assessment of the 10Be depth-profiles (Fig. 8) yield ages of 67+27-19 ka (2-sigma) and 36±6 ka (1-sigma) that are
out of phase with global glacial variability, but in phase with tropical Pacific climate variability, and particularly with modeled
positive anomalies of the NINO3 index (Clement et al., 1999) (Fig. 12). Our results replicate modern observations of the relation
between high intensity and long duration precipitation from tropical sources and ENSO variability.
References
Antinao, J.L., Farfán, L. M., 2013. Atmósfera (in press).
Antinao, J.L., McDonald, E. 2009. AGU - Fall Meeting. Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract T31A-1780.
Antinao, J.L., McDonald, E., 2011. XVIII INQUA Congress, Bern, 2011.
Antinao, J.L., McDonald. E., submitted, Quaternary Science Reviews .
Busch, M.M., Arrowsmith, J.R., Umhoefer, P.J., Coyan, J.A., Maloney, S.A., Martínez Gutiérrez, G., 2011. Lithosphere 3, 110-127.
Clement, A.C., Seager, R., Cane, M.A., 1999. Paleoceanography, 14(4):441-456.
Maloney, S. J., 2009. Unpublished M.S. thesis, Northern Arizona University, Flagstaff, Arizona.196 pp.
Rhodes, E., Brown, N.D., Antinao, J.L., Huenupi, E.C., Baker, S.E., McDonald, E., 2012. AGU - Fall Meeting, Abstract GC33D-1052.
Acknowledgements
National Science Foundation Grant NSF-EAR123481 and US Army Research
Office Contract No. DAAD19-03-1-0159 have supported this research, DRI provided
postdoctoral support funds to Antinao. Logistical support by CICESE (La Paz, BCS)
was crucial to this research. Thanks to L. Farfan, S. Meyer, R. Ortega (CICESE),
B. Finkel, T. Brown (LLNL), G. Yang (Dalhousie), E. Huenupi (DRI-DEES).