That fire facilitated the late Miocene C4grassland expansion is widely suspected but poorly documented. Fire potentially tied global climate to this profound biosphere transition by serving as a regional-to-local driver of vegetation change. In modern environments, seasonal extremes in moisture amplify the occurrence of fire, disturbing forest ecosystems to create niche space for flammable grasses, which in turn provide fuel for frequent fires. On the Indian subcontinent, C4expansion was accompanied by increased seasonal extremes in rainfall (evidenced by δ18Ocarbonate), which set the stage for fuel accumulation and fire-linked clearance during wet-to-dry seasonal transitions. Here, we test the role of fire directly by examining the abundance and distribution patterns of fire-derived polycyclic aromatic hydrocarbons (PAHs) and terrestrial vegetation signatures in
Fire dynamics potentially account for the asynchronous timing of the expansion of C4grasslands throughout the Mio‐Pliocene world. Yet how fire, climate, and ecosystems interacted in different settings remain poorly constrained because it is difficult to quantify fires and fuel source over these timescales. Here, we apply molecular proxies for fire occurrence alongside records of vegetation change and paleohydrology in Bengal Fan sediments (ODP Leg 116) to examine fire feedbacks on the south Asian continent. We employ abundances of polycyclic aromatic hydrocarbons (PAHs) to reconstruct fire occurrence and δ13C measurements of pyrogenic PAHs to constrain fuel source and grassland burning. This combination allowed us to test whether: (1) a fire‐seasonality forcing facilitated the expansion of grassland ecosystems and (2) a fire‐C4grass burning feedback maintained these systems. PAHs can be sourced from weathered fossil carbon (i.e., a petrogenic source) and from burned terrestrial biomass (i.e., a pyrogenic source). Alkylated and non‐alkylated structure abundance data distinguished pyrogenic from petrogenic sourced samples. A sharp increase in pyrogenic PAHs along with increases in δ2H and δ13C values of plant waxes at 7.4 Ma indicates increased fire coincided with the onset of C4expansion and hydrologic change in South Asia. The correlated13C enrichment in PAHs,13C enrichment in plant waxes, and increased abundances of PAHs suggest burning of C4grasslands likely maintained open ecosystems. Our results link fire to the initial opening of grassland ecosystems on a subcontinental‐scale and support disturbance as a critical mechanism of terrestrial biome transition.
more » « less- NSF-PAR ID:
- 10449750
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 36
- Issue:
- 4
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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n -alkane carbon isotopes from paleosol samples of the Siwalik Group (Pakistan). Two million years before the C4grassland transition, fire-derived PAH concentrations increased as conifer vegetation declined, as indicated by a decrease in retene. This early increase in molecular fire signatures suggests a transition to more fire-prone vegetation such as a C3grassland and/or dry deciduous woodland. Between 8.0 and 6.0 million years ago, fire, precipitation seasonality, and C4-grass dominance increased simultaneously (within resolution) as marked by sharp increases in fire-derived PAHs, δ18Ocarbonate, and13C enrichment inn -alkanes diagnostic of C4grasses. The strong association of evidence for fire occurrence, vegetation change, and landscape opening indicates that a dynamic fire–grassland feedback system was both a necessary precondition and a driver for grassland ecology during the first emergence of C4grasslands. -
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n ‐alkanes andn ‐alkanoic acids, and quantification of lignin phenols, charcoal, and pollen. We analyze the hydrogen isotopic composition (δD) of plant wax to reconstruct precipitation δD. We use the Branched and Isoprenoid Tetraether (BIT) index to diagnose shifts between terrestrial versus marine lipid inputs between turbidite and hemipelagic sediments. We reconstruct ocean temperatures using the TEX86index only where marine lipids dominate. We find evidence for the late Miocene grassland expansion in both facies, confirming this was a regional ecosystem transformation. Turbidites contain dominantly terrestrial matter from the Indus catchment (D‐depleted plant wax), delivered via fluvial transport as shown by the presence of lignin. In contrast, hemipelagic sediments lack lignin and bear D‐enriched plant wax consistent with wind‐blown inputs from the Indian peninsula; these show a 7.4–7.2 Ma expansion of C4grasslands on the Indian subcontinent. Within each facies, we find no clear change in δD values across the late Miocene C4expansion, implying consistent distillation of rainfall by monsoon dynamics. Yet, a cooling in the Arabian Sea is coincident with the C4expansion. -
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Abstract C4grasslands proliferated later in Australia than they did on other continents (∼3.5 Ma vs. 10–5 Ma). It remains unclear whether this delay reflects differences in climate conditions or ecological feedbacks, such as fire, that promote C4ecosystems. Here, we evaluated these factors using terrestrial biomarkers from marine sediments off western Australia. Fire‐derived polycyclic aromatic hydrocarbons (PAH) indicate fire ecology did not substantially change during or following C4expansion. The presence of fire‐adapted C3woody vegetation likely diminished the role of fire and delayed C4expansion until it was prompted by climate drying between 3.5 and 3.0 Ma. At the same time, mass accumulation rates of weathered PAHs increased 100‐fold, which indicates a significant loss of soil carbon accompanied this ecosystem shift. The tight couplings between hydroclimate and carbon storage altered boundary conditions for Australian ecosystems, and similar abrupt behavior may shape environmental responses to climate change.
