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The Merced River Chronosequence: A Natural Experiment for Understanding Weathering and Biogeochemical Cycling
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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
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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.
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Apex
Present river profile
deposition
erosion
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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.
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Post Modesto
(floodplain)14C
Modesto
14C
Riverbank
Turlock Lake
(ash) North Merced Gravels
China Hat mem.
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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
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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
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Mounds on China Hat member of Laguna formation (> 2 Ma)
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Geochemical Changes in Soils vs. Time
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Mass Balance Model
Mass gains/losses
Brimhall et al. (1992)
€
τ =
C j,s
Ci,s
C j,p
Ci,s
−1
Concentration of mobile element in soil
Concentration of immobile element in soil
Concentration of mobile element in parent material
Concentration of immobile element in parent material
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-60
-40
-20
0
20
40
1 10 100 1000 104 105 106 107
Merced Chronosequence
Epsilon to 100 cm
y = 148.71 - 30.859log(x) R= 0.83407
Epsilon to 100 cm
Age (yrs)
Using Zr as reference: Volumetric Changes
1. Initial volumetric expansion
2. Long term volumetric collapse (up to 60%)
€
ε =ΔV
Vp
=ρ pCi,p
ρ sCi,s
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-60
-40
-20
0
20
40
1 10 100 1000 104 105 106 107
Merced Chronosequence
Epsilon to 100 cm
y = 148.71 - 30.859log(x) R= 0.83407
Epsilon to 100 cm
Age (yrs)
Initial volumetric expansion due to:
• organic matter accumulation
• bioturbation and reduction of BD
- plants
- animals
• gains of structural water in hydrated minerals
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-70-60-50-40-30-20-10010
05 1051 1061.5 1062 1062.5 1063 1063.5 1064 106
Merced Chemical Graph DataTau Si to 100 cmy = 108.37 - 25.345log(x) R= 0.75549
Tau Si to 100 cmAge (yrs)-40-30-20-1001020
05 1051 1061.5 1062 1062.5 1063 1063.5 1064 106
Merced Chemical Graph DataTau P to 100 cmy = 75.013 - 17.144log(x) R= 0.74618
Tau P to 100 cmAge (yrs)
•Silicon losses ≈ Al losses
•P large losses
€
τ =
C j,s
Ci,s
C j,p
Ci,s
−1
Elemental Losses with Time
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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
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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….
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Model for N and P vs time: Walker and Syers (1976)
N limitedP limited
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Merced Soil N vs. P
• Total Soil N (and C) decline with soil age
• Total soil P is reduced by ~ 60% in old soils
• Loss of apatite
•
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0.1 1 10 100 1000
Apatite (%)
Apatite (%)
Age (Ka)
Harden, 1986
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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)
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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….
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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
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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
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-100
0
100
200
300
400
500
600
0.1 1 10 100 1000 104
Merced
Hawaii: humidHawaii: hyperhumid
Hawaii: aridMendocino
Atacama: hyperaridAtacama: hyperaridAtacama: arid
y = 32.307 - 22.339log(x) R= 0.8516
y = 99.317 - 55.793log(x) R= 0.9794 y = 663.93 - 217.33log(x) R= 0.98633
y = 243.41 - 96.72log(x) R= 0.85315 y = 56.548 - 33.381log(x) R= 0.94594
y = 25.033 + 25.033log(x) R= 1 y = 20.852 + 20.852log(x) R= 1 y = -0.22413 - 0.22413log(x) R= 1
Epsilon to 100 cm (%)
Soil Age (Kyr)
-250
-200
-150
-100
-50
0
50
1 10 100 1000 104
Atacama: north Atacama: central
Atamcama: south
Hawaii:arid
Merced
Hawaii:humid
Hawaii:Hyperhumid
y = 58.202 - 51.785log(x) R= 0.80085
Slope of Collapse (log model)
Annual Rain (mm)
Y = M0 + M1*x + ... M8*x8 + M9*x9
-6.6079M0-0.014048M1
-8.7495e-06M20.87487R
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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
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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
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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
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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
• Constant input, erosional loss
• Constant input, chemical loss
€
dN
dt= q − λN
N = q(1− e−λ t ) /λ
€
dN
dt= q − λN − Em
N = (q − Em)(1− e−λt ) /λ
€
dN
dt= q − (λ + kw )N
N = q(1− e−(k +λ )t ) /(λ + kw )
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Trends in 10Be:
• total amount increases with age
• depth trends and amount correlate with clay content
• modern river alluvium ~100x106 at/gm
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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)
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WITH LOSS W/O LOSSterrace corrected Be accepted agecalculated age calculated age
atom cm-2 (yrs) (yrs) (yrs)
M12 31000000000 40000 61021.36782 60451.74998R9 1.31E+11 250000 279935.4107 267830.6257R10 1.09E+11 250000 228522.0757 220472.0979T6 1.48E+11 600000 321100.7089 305148.9964T11 1.35E+11 600000 289505.0835 276553.7104CH1 5.39E+11 3000000 2066319.763 1410771.458CH2 4.78E+11 3000000 1591850.624 1196616.653
Interpretations of Chemical Loss Model1. 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.
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Topography of Fans/Terraces and Erosion
Modesto fm
level
Riverbank fm
•Level-undulating
•Mima mounds
Turlock Lake fm
• highly undulating
China Hat fm
• level w/ enormous Mima Mounds
• lots of gravel