10-9 Molecular-derived fire-vegetation dynamics in CA & paleoclimate implications

Session: Geochemistry and Mineralogy (Posters)

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Abstract:

The transition from peak glacial conditions of MIS4 to the weakened glacial state of MIS3 between 60-57 kyr BP was associated with large temperature oscillations of the Dansgaard-Oeschger 18 event and widespread cooling associated with Heinrich Stadial 6.  These global climate fluctuations involved significant shifts in temperature and hydroclimate in western North America, yet vegetation dynamics and fire activity during this interval remain poorly constrained. Here, we present two records of plant- and fire-derived biomarkers from a well-dated stalagmite in McLean’s Cave (ML2), Sierra Nevada foothills, California, spanning D-O stadial and interstadial intervals 15 – 18 from 55 – 67 ka.

We analyze polycyclic aromatic hydrocarbons (PAHs; indicators of fire activity), anhydrosugars (low-temperature biomass burning), diterpenoids (conifer vegetation inputs), and pristane/phytane ratios (redox-sensitive organic matter preservation), together with published carbonate stable isotopes and speleothem reflectance, to reconstruct vegetation structure and fire regimes in response to millennial-scale climate variability. Fire and conifer biomarkers spiked during D-O interstadials 17 and 18, suggesting that millennial-scale hydroclimate variability modulated both fire activity and vegetation structure. The longer-term transition from MIS 4 to MIS 3 drove a pronounced shift towards higher conifer abundance in Central California, which intensified during Heinrich Event 6, evidenced by a sharp increase in diterpenoids. This shift to increased conifer abundance is followed by an increase in low-intensity fires, then intensified wildfire activity, evidenced by elevated PAH concentrations. High-intensity fire markers covary with stalagmite reflectance, δ¹³C_carb and NGRIP δ¹⁸O, indicating that global climate states exerted a first-order control on regional hydroclimates, vegetation, and wildfire regimes.

These results demonstrate that while short-term D-O oscillations modulated fire-vegetation dynamics, the baseline glacial–interglacial reorganization during MIS 3/4 was the dominant driver of ecosystem change. Our findings highlight the sensitivity of western North American forests to long-term hydroclimate variability and provide new insights into vegetation–fire feedbacks during glacial periods. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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