Recent wildfire activity in semi-arid regions like Western North America surpasses historical records, necessitating a deeper understanding of the interplay between climate, vegetation, and fire regimes beyond the limited scope of tree-ring data. High-resolution paleoclimate archives, such as stalagmites, offer a valuable tool for reconstructing past environmental conditions. This study focuses on the California Coast Range, a region experiencing a shift towards shorter, drier rainy seasons and increased wildfire risk. Climate models predict further intensification of this trend with anthropogenic warming, leading to increased year-to-year precipitation volatility, or 'climate whiplash,' characterized by oscillations between extreme wetness and aridity. This whiplash effect can exacerbate fire recurrence by increasing autumn dryness and promoting the growth of flammable vegetation. Speleothems, such as stalagmites, provide multiple geochemical proxies that record precipitation changes, offering insights into past hydroclimatic variability. Previous studies using stalagmites from White Moon Cave (WMC) in the California Coast Range documented enhanced precipitation volatility during the 8.2 kyr event, a prominent Holocene cold period. However, the understanding of how wildfires are recorded in stalagmites is still developing. This study aims to investigate this gap by analyzing levoglucosan (a marker of biomass burning) and LOPs (indicators of vegetation composition) in a WMC stalagmite to understand the relationship between climate whiplash and wildfire activity during the 8.2 kyr event. The research question is how are climate whiplash and fire activity coupled in California during the early Holocene. The study's importance lies in its potential to improve the understanding of past fire regimes and project future wildfire risk under climate change. The purpose is to use a novel multi-proxy approach to reconstruct fire activity and vegetation during the early Holocene in California and to link these changes to hydroclimate variability.

Existing literature highlights the increasing risk of wildfires in California due to a shorter, drier rainy season and increased precipitation volatility. Studies using tree-ring data have shown the sensitivity of fire activity to summer temperatures and hydrological drought. Other research using lake sediments and pollen records have established links between increased fire activity and summer drought in the Pacific Northwest during the Holocene. However, these studies often have limitations in temporal resolution or geographical coverage. Prior work on stalagmites from WMC has demonstrated the potential of these archives to reconstruct past hydroclimatic variability, particularly during the 8.2 kyr event. While some studies have examined the response of cave systems to wildfires, the use of levoglucosan and LOPs in stalagmites to reconstruct fire history and vegetation change remains relatively unexplored.

The study analyzed levoglucosan and lignin oxidation products (LOPs) in a stalagmite from White Moon Cave (WMC) in the California Coast Range. The stalagmite segment analyzed encompassed the 8.2 kyr event (8600–6900 years BP). Levoglucosan, an anhydrosugar formed during cellulose combustion, serves as a direct indicator of fire activity. LOPs, derived from the oxidative digestion of lignin, provide insights into vegetation composition (woody vs. non-woody, angiosperm vs. gymnosperm). The authors used advanced analytical techniques, including high-resolution mass spectrometry, to quantify levoglucosan and LOPs. To interpret the paleorecords, the authors compared their results with previously published stable carbon and calcium isotope data from the same stalagmite, which reflect changes in precipitation amount, source, and seasonality. Furthermore, they analyzed levoglucosan and LOPs in modern dripwater and calcite samples from the cave to understand how surface signals are transported and incorporated into the stalagmite. Statistical analyses, including changepoint analysis of the δ¹⁸O record, helped identify changes in the amplitude and frequency of hydroclimate variability. The methodology involved meticulous sample preparation, including cleaning, pulverizing, and extracting levoglucosan and LOPs using specific solvents and techniques (e.g., solid-phase extraction, high-performance liquid chromatography coupled with high-resolution mass spectrometry). The method involved several steps including: 1. Speleothem Sample Massing Preparation: Stalagmite samples were cleaned, dried, pulverized, and stored. 2. Levoglucosan Extraction: Samples were spiked with levoglucosan standard, extracted with methanol, filtered, and evaporated. 3. LOP Analysis: Dried samples were digested with HCl, and LOPs were extracted using solid-phase extraction and analyzed via HPLC-HRMS. 4. Cave Dripwater Analysis: Water samples were collected and analyzed for levoglucosan. 5. Statistical Analyses: Changepoint analysis was used to identify changes in δ¹⁸O records. An age model was constructed for levoglucosan and LOP samples using U-Th dates and stalagmite aging function in R.

The stalagmite record revealed elevated levoglucosan concentrations between 8217 ± 23 and 7847 ± 20 years BP, indicating increased fire activity during the 8.2 kyr event. The highest levoglucosan concentrations were found between 8167 ± 20 and 8019 ± 15 years BP. LOP analysis showed a shift from non-woody to woody vegetation between 8300 and 8200 years BP, followed by a return to more non-woody vegetation between 8000 and 7000 years BP. The period of increased woody vegetation coincided with the elevated levoglucosan concentrations. Analysis of modern calcite samples showed higher levoglucosan concentrations than Holocene samples, potentially indicating increased fire activity in the mid-to-late Holocene. The C/V and S/V ratios from modern calcite were consistent with the current mixed evergreen forest above the cave, similar to speleothem values from 7800 to 2200 years BP when woody vegetation was more abundant. The study's findings suggest a clear link between increased hydroclimate volatility (as shown by δ¹⁸O and δ⁴⁴Ca), a shift toward more woody vegetation (shown by LOP ratios), and elevated fire activity (shown by levoglucosan concentrations) during the 8.2 kyr event and its precursor event around 8300 years BP. The changepoint analysis revealed shifts in mean and variance of the δ¹⁸O record coinciding with the onset and end of the precursor event and the 8.2 kyr event. These findings highlight the strong coupling between hydroclimate volatility and fire activity in California during the early Holocene.

The findings demonstrate a strong link between hydroclimate volatility, vegetation change, and fire activity in California during the early Holocene, consistent with patterns observed in modern tree-ring records and Holocene lake sediment records. The increased hydroclimate variability during the 8.2 kyr event and its precursor, likely driven by changes in atmospheric circulation patterns, promoted shifts in vegetation composition and increased fire frequency. The shift towards more woody vegetation possibly created conditions favoring more extensive and intense fires. This highlights the importance of considering both precipitation variability and temperature changes when assessing wildfire risk, particularly in the context of anthropogenic climate change. The results are consistent with other studies showing similar links between fire, fuel, and climate in western North America across different timescales. The results emphasize the importance of considering the interplay between climate variability and vegetation dynamics in predicting future wildfire behavior. The study’s results suggest a significant influence of climate-driven changes in vegetation and fire regimes that goes beyond the effects of land-use changes, such as fire suppression.

This study provides novel insights into the interplay between climate, vegetation, and fire activity in California during the early Holocene using a multi-proxy approach involving levoglucosan and LOPs in a stalagmite. The findings highlight the strong coupling between climate whiplash and fire activity, a relationship projected to intensify under future climate change. The study demonstrates the potential of using speleothems to reconstruct past fire regimes and improve predictions of future wildfire risk. Future research should focus on expanding the spatial and temporal coverage of such studies, integrating other fire and vegetation proxies, and refining the understanding of the transport and preservation of levoglucosan in cave systems.

The study is limited to a single stalagmite from one location in the California Coast Range, limiting the generalizability of the findings to the broader region. The resolution of the stalagmite record may not capture interannual variability in fire activity, only broader trends over several decades. The interpretation of LOP ratios relies on existing vegetation-LOP relationships, which may need further refinement. The study does not directly address the role of human activities (if any) in shaping fire activity during the study period.