Geological history of the chronosequence Soil chemical weathering studies

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

2. 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

3. River Terraces: “Escalators through time”

4. Location and Formation • Sierra dominantly granitic • East SJ Valley is dominantly glacial outwash

5. Merced River : Alluvial fans (downslope) and inset terraces (reverse topography) upslope of apex of deposition.

6. Apex erosion Present river profile deposition

7. 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.

8. China Hat mem. Turlock Lake (ash) North Merced Gravels Post Modesto (floodplain)14C Riverbank Modesto 14C

9. 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

10. 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

12. Mounds on China Hat member of Laguna formation (> 2 Ma)

13. Geochemical Changes in Soils vs. Time

14. 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

15. Using Zr as reference: Volumetric Changes Initial volumetric expansion Long term volumetric collapse (up to 60%)

16. Initial volumetric expansion due to: • organic matter accumulation • bioturbation and reduction of BD - plants - animals • gains of structural water in hydrated minerals

17. Elemental Losses with Time • Silicon losses ≈ Al losses • P large losses

18. 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

19. 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….

20. Model for N and P vs time: Walker and Syers (1976) N limited P limited

21. 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 • Harden, 1986

22. WHITE AND BRANTLEY(1995) 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

23. 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….

24. Merced River Vernal Pools and Soil Preservation• Need old soils for Mima mounds/vernal pools• Old soils are rare --- and becoming rarer

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

26. Agricultural Encroachment and Loss of Mima Mounds • leveling and dynamite • modern farm implements

27. Now a rare and highly fragmented ecosystem

28. Newest threat: urban expansion • “Farming on the Edge” hotspots of prime farmland loss

29. San Joaquin soil is made “state soil” in 1997

30. 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

32. 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

35. 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

36. 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

37. 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

38. 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

39. Trends in 10Be: • total amount increases with age • depth trends and amount correlate with clay content • modern river alluvium ~100x106 at/gm

40. 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)

41. 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.

42. Topography of Fans/Terraces and Erosion China Hat fm • level w/ enormous Mima Mounds • lots of gravel Turlock Lake fm • highly undulating Riverbank fm •Level-undulating •Mima mounds Modesto fm level