An examination of the late Miocene C4 grass expansion through sedimentary charcoal morphometrics and morphotypes

Abstract

The C4 Fire Hypothesis posits that an increase in fire was the catalyst for the late Miocene C4 grass expansion by clearing forest canopies to create and maintain open landscapes preferred by C4 grasses, creating a positive feedback loop. However, to date, few analyses have directly resolved whether the fire feedback is supported, though several studies have identified support for components of the Fire Hypothesis. We reconstructed the fuel type of the late Miocene fire regime by analyzing the morphological and morphometric characteristics of charcoal found in Ocean Drilling Program Leg 184 Site 1146 (South China Sea; l19°27.40´N, 116°16.37´E) legacy cores and compared it to previously published paleoclimate data to critically examine the Fire Hypothesis and understand the extent of the role of fire in the C4 grass expansion. The sedimentary charcoal interpretations reflect fire activity primarily from the Pearl River Basin and Southeast China. From 8 to 5 Ma, the fuel type per morphology type shifted from woody to grassy fuel, the opposite of what would be expected in the Fire Hypothesis. Charcoal accumulation rates show an increase of fire around the time of the expansion (7.3–6.9 Ma). Influence diagnostics, leave-one-out refitting, jackknife and bootstrap resampling, and robust regression all show that the 7.3–6.9 Ma increase in CHAR is stable and not driven by individual data points. However, fuel-indicative characteristics of the charcoal (morphology and L:W ratio) show no discernable transition in fuel type during the period of increased fire activity, suggesting neither C4 nor C3 vegetation were preferentially burnt at this time. The observed increase in fire activity aligns with regional climatic changes including a decrease in monsoon intensity and increase aridification starting around 7 Ma. Our findings partially support the Fire Hypothesis in that we see evidence for an increase in fire activity, but not preferential burning recorded in the South China Sea as initially argued.