The Merced River Chronosequence: A Natural
Experiment for Understanding Weathering and
Biogeochemical Cycling
The Merced River Chronosequence: A Natural
Experiment for Understanding Weathering and
Biogeochemical Cycling
• Geological history of the chronosequence
• Soil chemical weathering studies
• Ecosystem consequences of weathering
• Importance of region for biogeodiversity
River Terraces: “Escalators through
time”
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Location and Formation
• Sierra dominantly granitic
• East SJ Valley is dominantly
glacial outwash
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Merced River :
Alluvial fans (downslope) and inset terraces (reverse topography) upslope of
apex of deposition.
Apex
erosion
Present river profile
deposition
The Inset Fans of SJ Valley (on all major rivers) can be explained by oscillating
cycles of variations in stream capacity and sediment delivery rates.
China Hat mem
Turlock Lake
(ash)
Post Modesto
(floodplain)14C
Modesto
14C
Riverbank
North Merced
Gravels
Landform Evolution and Mima Mounds/Vernal Pools
• Presence of pools is largely related to landform age
– As landforms age, soils become more impervious to water
due to clay rich layers and Si-cemented horizons
• Arkley and Brown (1954) hypothesized a pocket gopher origin
coupled with soil development
Mima Mounds
•Actively (?)
maintained
mounds
overlying
impervious
layer
• Should
disappear in
~104 yr via
erosion
•Are gophers a
“keystone”
species for the
entire
ecosystem?
Hardpan or claypan
Mounds on China Hat member of Laguna formation (> 2 Ma)
Geochemical Changes in Soils vs. Time
Mass Balance
Model
Mass gains/losses
Brimhall et al. (1992)
C
Concentration of mobile element in soil
j ,s
C i,s
 
1
C j, p
C i,s
Concentration of immobile element in soil
Concentration of mobile element in parent material
Concentration of immobile element in parent material
Using Zr as reference:
Volumetric Changes

V
Vp
Epsilon to 100 cm

 p C i, p
 sC i,s
Merced Chronosequence
40

E psilo n to 100 cm
20
0
-20
y = 148.71 - 30.859log(x) R= 0.83407
-40
-60
1
10
100
1000
10
Age (yrs)
4
10
5
10
6
10
7
1.
Initial volumetric
expansion
2.
Long term volumetric
collapse (up to 60%)
Epsilon to 100 cm
Merced Chronosequence
40
Initial volumetric expansion due
to:
20
E psilo n to 100 cm
• organic matter accumulation
0
• bioturbation and reduction of BD
- plants
-20
- animals
y = 148.71 - 30.859log(x) R= 0.83407
-40
-60
1
10
100
1000
10
Age (yrs)
4
10
5
10
6
10
7
• gains of structural water in
hydrated minerals
Elemental Losses with Time
C
 
•Silicon losses ≈ Al losses
j ,s
C i,s
1
C j, p
C i,s
•P large losses
Tau P to 100 cm
Tau Si to 10 0 cm

Me rced Ch em ical Gr ap h Dat a
10
y = 108.37 - 25.34 5log(x)
Me rced Ch em ical Gr ap h Dat a
20
R= 0.75 549
y = 75.013 - 17.144 log(x)
R= 0.74 618
0
10
-10
T au P t o 1 00 c m
T au Si t o 10 0 c m
0
-20
-30
-40
-10
-20
-50
-30
-60
-70
-40
0
5 10
5
1 10
6
1.5 10
6
2 10
6
Age (yrs )
2.5 10
6
3 10
6
3.5 10
6
4 10
6
0
5 10
5
1 10
6
1.5 10
6
2 10
6
Age (yrs )
2.5 10
6
3 10
6
3.5 10
6
4 10
6
Summary of geochemical data
• Enormous losses of Si, Al, P, and major cations etc. with time
(following initial period of rapid gains of C, N)
• Short term volumetric expansion followed by long term
volumetric collapse
Consequences of Weathering to Ecosystem
Processes
• Plant chemistry is: C,H,O,N,S, P,…
• Atmospherically derived elements
–
–
–
–
H, O (water, …)
C (CO2)
N (atm deposition (NO3, NH4, org N)
S (SO4)
• Soil/rock derived elements
– P (apatite)
– metals
• P is a key control on long term ecosystem productivity….
Model for N and P vs time: Walker and Syers (1976)
N limited
P limited
Merced Soil N vs. P
• Total
Soil N (and C) decline with soil
age
• Total soil P is reduced by ~ 60% in
old soils
Harden, 1986
• Loss of apatite
1.2
Apatite (%)
1
A patite (% )
0.8
0.6
0.4
0.2
0
-0.2
0.1
•
1
10
Age (Ka)
100
1000
Merced N
• Nitrate increases in soil water with age
• Increasing fraction of nitrate in total N loss
drives soil 15N to higher values.
• N becomes an “excess” element from
biological perspective
WHITE AND
BRANTLEY(1995)
Summary
• Merced chronosequence is becoming one of most studied on
Earth (after Hawaii)
• Weathering ultimately affect biota
– Reduction in P
– Reduced NPP
– Clay and silica cements induce mima mounds
• Merced chronosequence is the complex geological foundation
for unique vernal pool ecosystem…..and in a vastly reduced
state….
Merced River Vernal Pools and Soil Preservation
• Need old soils for Mima mounds/vernal pools
• Old soils are rare --- and becoming rarer
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Mima Mounds used to cover > 500,000 acres in Great Valley
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Agricultural
Encroachment and
Loss of Mima
Mounds
• leveling and
dynamite
• modern farm
implements
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Now a rare and highly fragmented ecosystem
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Newest threat:
urban expansion
• “Farming on the
Edge” hotspots of
prime farmland loss
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San Joaquin soil is made “state soil” in 1997
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Soil Diversity in the United States: soil series = biological species
Rare soils < 50,000 ha,
Endangered soils = rare soils w/ > 50% use,
Extinct soils = 100% use
Soil Diversity and Conservation Planning
•
•
•
•
•
Not all soils are equal
Rare soils appear to harbor rare plants
The Great Valley is an endangered soil hotspot
Role of pocket gophers as “keystone” species??
Merced River chronosequence is a scientific, and a
biogeodiversity, treasure
Atacama: hyperarid
Atacama: hyperarid
Atacama: arid
Merced
Hawaii: arid
Mendocino
Hawaii: humid
Hawaii: hyperhumid
600
y = 25.033 + 25.033log(x) R= 1
y = 20.852 + 20.852log(x) R= 1
y = -0.22413 - 0.22413log(x) R= 1
y = 32.307 - 22.339log(x) R= 0.8516
y = 243.41 - 96.72log(x) R= 0.85315
y = 56.548 - 33.381log(x) R= 0.94594
y = 99.317 - 55.793log(x) R= 0.9794
y = 663.93 - 217.33log(x) R= 0.98633
50
y = 58.202 - 51.785log(x) R= 0.80085
Atacama: north
500
S lop e of C o llap se (lo g m od el)
0
400
E psilo n to 100 cm (% )
Atacama: central
300
200
100
Atamcama: south
Merced
-50
Hawaii:humid
Hawaii:arid
-100
Y = M0 + M1*x + ... M8*x
-150
-200
0
8
+ M9*x
9
M0
-6.6079
M1
M2
-0.014048
-8.7495e-06
R
0.87487
Hawaii:Hyperhumid
-100
-250
0.1
1
10
100
Soil Age (Kyr)
1000
10
4
1
10
100
Annual Rain (mm)
1000
10
4
Geochronology of Terraces
• Weathering Rate = total weathering/time
• Generally poorly known
– Post Modesto, Modesto constrained by 14C (few dates
though)
– Turlock Lake dated by one ash
– U-trend ages, though used, are now not considered credible
by U-series community
•
10Be
was used in 1980’s
Geochronology of Terraces
• Weathering Rate = total weathering/time
• Ages poorly known
– Post Modesto, Modesto constrained by 14C (few dates
though)
– Turlock Lake dated by one ash
– Correlations based on fossils, etc
•
10Be
was used in 1980’s
10Be
•
•
•
•
(“garden variety”): Pavich et al. (1986) GCA.
Produced via cosmic rays in atmosphere
Half-life = 1.5 x 106 years
Delivery rate to soils ~ .5 to 1.5 x 106 atoms cm-2 yr-1
Considered highly immobile in soils (attaches to negatively
charged clays)
• Has z/r (ionic potential) similar to Al (~ 6), indicating it should
form sparingly soluble oxides in aqueous solutions
• Can be used for various dating or transport studies
Interpretation of Merced Terrace 10Be
- assumed assigned age are correct
- determined if measured 10Be = amount that should be there
- authors needed erosion to reconcile data
• Constant input, no physical loss
dN
 q  N
dt
N  q(1  e
 t
)/
• Constant input, erosional loss
dN

 q   N  Em
dt
N  (q  Em )(1  e
 t
)/
• Constant input, chemical loss

dN
dt
 q  (  kw )N
N  q(1  e
 ( k   )t
) /(   k w )
Trends in 10Be:
• total amount increases with
age
• depth trends and amount
correlate with clay content
• modern river alluvium
~100x106 at/gm
Analysis of Data Assuming Chemical Loss
Rationale for Chemical Loss:
• Nearly 60% of Al is lost from upper 1m of soils
• Significant losses of Ti
• Assume 10Be loss is proportional to amount present (first order
reaction)
Interpretations of Chemical Loss Model
1.
Young terraces (where little erosion or chemical loss might occur),
should give reasonably accurate ages
a. Modesto fm yields older ages than U trend
2.
Assuming chemical losses, China Hat fm. yields ages within reasonable
ranges
a. Turlock Lake, with high physical erosion history, can not be
reconciled.
terrace
M12
WITH LOSS
W/O LOSS
corrected Be
accepted age
calculated age
calculated age
atom cm-2
(yrs)
(yrs)
(yrs)
40000
61021.36782
60451.74998
R9
31000000000
1.31E+11
250000
279935.4107
267830.6257
R10
1.09E+11
250000
228522.0757
220472.0979
T6
1.48E+11
600000
321100.7089
305148.9964
T11
1.35E+11
600000
289505.0835
276553.7104
CH1
5.39E+11
3000000
2066319.763
1410771.458
CH2
4.78E+11
3000000
1591850.624
1196616.653
Topography of Fans/Terraces and Erosion
China Hat fm
• level w/
enormous Mima
Mounds
Turlock Lake fm
• highly
undulating
Riverbank fm
Modesto fm
level
•Levelundulating
•Mima mounds
• lots of gravel