The Merced River Chronosequence: A Natural Experiment for Understanding Weathering and...

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

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River Terraces: “Escalators through time”

Location and Formation

• Sierra dominantly granitic

• East SJ Valley is dominantly glacial outwash



Merced River :

Alluvial fans (downslope) and inset terraces (reverse topography) upslope of apex of deposition.

Apex

Present river profile

deposition

erosion

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.

Post Modesto

(floodplain)14C

Modesto

14C

Riverbank

Turlock Lake

(ash) North Merced Gravels

China Hat mem.

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)

€

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

-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

-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

-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

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 limitedP 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

• 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

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





Mima Mounds used to cover > 500,000 acres in Great Valley



Agricultural Encroachment and Loss of Mima Mounds

• leveling and dynamite

• modern farm implements



Now a rare and highly fragmented ecosystem



Newest threat: urban expansion

• “Farming on the Edge” hotspots of prime farmland loss



San Joaquin soil is made “state soil” in 1997

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

-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

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

• 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 )

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)

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.

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

The Merced River Chronosequence: A Natural Experiment for Understanding Weathering and...

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