Earth’s climate has varied substantially, with several periods in the past experiencing greenhouse climates similar to future climate projections (Scotese et al., 2021). For example, during the Cretaceous period, the Earth was in a greenhouse climatic state (Berner, 2006), characterized by atmospheric CO2 levels at 112% of modern levels (Huber et al., 2018). However, modern anthropogenic activities are causing unprecedented and accelerated climate changes (Anderegg et al., 2010), prompting significant responses from ecological systems, including more severe and frequent forest fires, as well as shifts in plant ecological communities (McCarthy, 2001; Carvalho et al., 2011; Flannigan et al., 2000). These changes have the potential to alter species compositions, nutrient cycling, and habitat structures, thereby impacting ecosystem services such as carbon sequestration, water filtration, and biodiversity (Carvalho et al., 2011; Malhi et al., 2020; Jolly et al., 2015). Furthermore, the increased frequency and severity of forest fires can lead to substantial economic losses due to property damage, firefighting costs, and healthcare expenses from degraded air quality (Lee et al., 2015). Fire regimes and plant communities of the Cretaceous can provide insights into future ecological dynamics under changing climatic conditions (Marlon, 2020; Hay, 2011). The Cretaceous is a reasonable analog to this potential climatic state because of its high atmospheric CO2 and lack of permanent polar ice caps, among other factors (Scotese et al., 2021; Berner, 2006; Huber et al., 2018). But it should be noted that the Cretaceous is not a perfect analog because of stark differences in ocean circulation mechanisms (Hay, 2008), as well, the role of anthropogenic activities being exclusive to modern times. To better understand the mechanisms driving fire in greenhouse climate states and conditions, comprehensive examination of paleoecological records from analogous periods like the Cretaceous are needed.
Paleoecological reconstructions allow us to understand how ecological communities respond to climate changes, with each proxy having benefits and biases. Paleofire records provide a useful means of assessing fire-climate relationships as well as informing predictions of how fire will respond to changing climate scenarios (Marlon, 2020; Whitlock et al., 2010; Sayedi et al., 2024; Conedera et al., 2009). Palynological records and macrofossil records have been traditionally used to assess ecological shifts and plant community dynamics (Braman and Koppelhus, 2005; Webb III et al., 1981). Sedimentary charcoal can be used as a proxy for both paleofire reconstruction and ecological analysis (Brown et al., 2023). The proxy is independent from many biases that persist in the palynological and fossil records (Adrian and Westrop, 2003), such as an overrepresentation of angiosperm pollen and an underrepresentation of fern spores (Harris, 1981; Crane et al., 1995 Braman and Koppelhus, 2005)). These biases are preservational; each proxy provides limited perspectives on their respective ecosystems (Adrian and Westrop, 2003; Ferguson, 1985).
Although much research has developed paleofire methods (Conedera et al., 2009; Remy et al., 2018), the bulk of this work has focused on the Holocene (Power et al., 2010). The climate of the Holocene was relatively cold (as compared to a greenhouse climate) in the context of geologic time (Willis and MacDonald, 2011), meaning that the research methods developed within this field are predominantly applicable to climatically cool deciduous and boreal forests (Leys et al., 2018; Rehn et al., 2021; Vachula et al., 2024); not the tropical/subtropical forests as would have dominated climatically hot periods (Levin and King, 2016). Therefore, much of the interpretation-focused, charcoal-based paleofire research is not directly applicable to deeper geologic time scales (Crawford et al., 2018). To apply paleofire methodology to new ecosystems, it is necessary to undertake experimental proxy calibration (Leys et al., 2018; Rehn et al., 2021; Blarquez et al., 2013).
Charcoal morphology and morphometry are two common paleofire methods. Both techniques focus on the classification and quantification of macroscopic (> 125 µm charcoal)(Vachula, 2019) particles isolated from sediment samples (Crawford et al., 2018), sediment cores (Feurdean et al., 2022), or experimental charcoal samples (Vachula et al., 2024). Charcoal morphology refers to the practice of classifying individual particles based on qualitative shape and structure characteristics, whereas morphometry refers to the practice of measuring physical variables of particles (e.g., L:W, rectangularity, etc.) (Vachula et al., 2021). Morphology provides inferences on fuel type, fire extent, and combustion temperature (Enache and Cumming, 2006; 2009). Morphometry can provide perspective on fuel type, burn temperature, and taxonomic classification (Feurdean, 2021; Feurdean et al., 2023).
Previous studies have used experimental charcoal production of known plants to inform the interpretation of charcoal particles found in sediments samples. These studies have focused their experimental burns on individual plant tissues (leaf, needle, etc.) (Feurdean, 2021; Vachula et al., 2024). Previous morphometric studies have been primarily focused on one measured variable, aspect ratio (L:W)(Vachula et al., 2021; 2024; Crawford and Belcher, 2014; Ogura, 2007; Pereboom et al., 2020; Umbanhowar and McGrath, 1998; Zhang and Lu, 2005; Li et al, 2019; Feurdean 2021; Feurdean et al., 2023). This variable has shown reliable accuracy in fuel type delineation, specifically in identification of graminoid fuels (Feurdean, 2021). A guiding framework was developed by Vachula et al. (2021) that values over 3.5 L:W indicate graminoid/herbaceous material while values less than 2.5 correspond to woody fuels. In addition to L:W values, other morphometrics such as rectangularity, circularity, and particle size (feret diameter) have shown promise in paleoecological research (Frank-DePue et al., 2022; Vachula et al., 2024), but have not been examined in great detail.
This paper presents a modern case study and experimental proof-of-concept for the application of sedimentary charcoal morphometry analyses to Cretaceous sediments. We conducted an extensive experimental charcoal production analysis, focusing on sub-tropical trees, ferns, and aquatic plants that have all been historically underrepresented in Holocene focused studies and are more relevant to the Cretaceous period (and other greenhouse climates) than those traditionally tested. This experiment included 23 taxa and quantified several novel morphometrics (rectangularity, circularity, and feret diameter) to explore diverse ways of differentiating charcoal particulates by source fuel. We also conducted novel dissections of plant tissue components (e.g., leaf vein, leaf petiole, etc.) to a finer scale than previous studies. And the results of burning these dissections shed light on the cause of charcoal morphological and morphometric variations. Last, we contextualize our results for fuel type delineation in subtropical ecosystems relevant to the Cretaceous period and other deep time contexts.