The Merced River Chronosequence: A Natural Experiment as long as Underst in addition to ing Weatheri

The Merced River Chronosequence: A Natural Experiment as long as Underst in addition to ing Weatheri www.phwiki.com

The Merced River Chronosequence: A Natural Experiment as long as Underst in addition to ing Weatheri

Shumway, Sharon, Owner has reference to this Academic Journal, PHwiki organized this Journal The Merced River Chronosequence: A Natural Experiment as long as Underst in addition to ing Weathering in addition to Biogeochemical Cycling The Merced River Chronosequence: A Natural Experiment as long as Underst in addition to ing Weathering in addition to Biogeochemical Cycling Geological history of the chronosequence Soil chemical weathering studies Ecosystem consequences of weathering Importance of region as long as biogeodiversity River Terraces: “Escalators through time”

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Location in addition to Formation Sierra dominantly granitic East SJ Valley is dominantly glacial outwash Merced River : Alluvial fans (downslope) in addition to 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 in addition to sediment delivery rates. Post Modesto (floodplain)14C Modesto 14C Riverbank Turlock Lake (ash) North Merced Gravels China Hat mem. L in addition to as long as m Evolution in addition to Mima Mounds/Vernal Pools Presence of pools is largely related to l in addition to as long as m age As l in addition to as long as ms age, soils become more impervious to water due to clay rich layers in addition to Si-cemented horizons Arkley in addition to 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 as long as the entire ecosystem Hardpan or claypan Mounds on China Hat member of Laguna as long as mation (> 2 Ma)

Geochemical Changes in Soils vs. Time Mass Balance Model Mass gains/losses Brimhall et al. (1992) 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 Using Zr as reference: Volumetric Changes Initial volumetric expansion Long term volumetric collapse (up to 60%)

Initial volumetric expansion due to: organic matter accumulation bioturbation in addition to reduction of BD – plants – animals gains of structural water in hydrated minerals Silicon losses Al losses P large losses Elemental Losses with Time Summary of geochemical data Enormous losses of Si, Al, P, in addition to 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 as long as N in addition to P vs time: Walker in addition to Syers (1976) N limited P limited Merced Soil N vs. P Total Soil N ( in addition to C) decline with soil age Total soil P is reduced by ~ 60% in old soils Loss of apatite 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 in addition to silica cements induce mima mounds Merced chronosequence is the complex geological foundation as long as unique vernal pool ecosystem in addition to in a vastly reduced state . Merced River Vernal Pools in addition to Soil Preservation Need old soils as long as Mima mounds/vernal pools Old soils are rare — in addition to becoming rarer

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Mima Mounds used to cover > 500,000 acres in Great Valley Agricultural Encroachment in addition to Loss of Mima Mounds leveling in addition to dynamite modern farm implements Now a rare in addition to highly fragmented ecosystem

Newest threat: urban expansion “Farming on the Edge” hotspots of prime farml in addition to 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

Analysis of Data Assuming Chemical Loss Rationale as long as 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 Young terraces (where little erosion or chemical loss might occur), should give reasonably accurate ages a. Modesto fm yields older ages than U trend 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 in addition to 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

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