How Do The Differences In Clay Mineral Assemblages And Geochemical Signatures Between The Paleogene And Neogene Formations In The North American Cordillera Reflect Changes In Orogeny-driven Weathering Regimes And Paleo-atmospheric CO2 Levels, And What Implications Do These Findings Have For Refining Existing Paleoclimate Reconstructions Based On The Proxy Records From The Western Interior Seaway?

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The differences in clay mineral assemblages and geochemical signatures between Paleogene and Neogene formations in the North American Cordillera provide valuable insights into changes in weathering regimes and paleo-atmospheric CO2 levels, which can refine paleoclimate reconstructions from the Western Interior Seaway.

  1. Clay Mineral Assemblages and Weathering Regimes:

    • Paleogene: Higher concentrations of kaolinite suggest a warm, humid climate with intense chemical weathering. This is supported by higher Chemical Index of Alteration (CIA) values, indicating more alteration under elevated CO2 levels, which promote warmer and more humid conditions.
    • Neogene: Shift to minerals like illite and chlorite points to drier, cooler conditions, possibly due to lower CO2 levels and reduced weathering intensity. This could also reflect changes in orogeny, with lower mountain relief affecting rainfall patterns.

  2. Implications for Paleo-atmospheric CO2 Levels:

    • The transition from high to lower CIA values and clay mineral changes suggest a decline in CO2 levels from Paleogene to Neogene. This aligns with global cooling trends and can help quantify CO2 effects on climate.

  3. Relevance to Western Interior Seaway:

    • Sediments in the Seaway reflect weathering conditions in the surrounding regions, including the Cordillera. Paleogene sediments with more kaolinite and higher CIA indicate a humid, warm climate, while Neogene sediments suggest drier, cooler conditions.
    • These findings enhance paleoclimate reconstructions by providing detailed weathering and CO2 data, improving the accuracy of climate models and our understanding of past climates.

  4. Integration with Other Proxies:

    • Combining clay mineral data with other proxies (e.g., fossil leaves, oxygen isotopes) offers a more comprehensive view of climate evolution, despite potential complicating factors like sea level changes or drainage patterns.

In conclusion, the analysis of clay minerals and geochemistry in the Cordillera, alongside considerations of CO2 levels and orogeny, provides critical data to refine paleoclimate models, offering a nuanced understanding of climate change dynamics.