Fire Distinguishers: Molecular and isotopic tools for identifying grassland burning in deep time
Abstract
Fire is crucial to maintaining modern subtropical grasslands, yet our understanding of the origins and evolution of this association on geologic timescales is limited, due in part to few coeval records of both grass and fire proxies in the geologic record. The ambiguity of past ecosystem-fire dynamics impedes our ability to predict how these critical ecosystems may respond to the current changing climate. Polycyclic aromatic hydrocarbons (PAHs) are a complex suite of molecules that can derive both from the burning of terrestrial vegetation, as well as thermal maturation of organic matter. Distinguishing burn signatures with these compounds will make it possible to link paleo-fire and vegetation change in the past. Here, we evaluate fire signatures in PAHs during a time of significant changes in terrestrial biome structure, the expansion of C4 grassland ecosystems in the Mio-Pliocene, and propose multi-molecule proxies for fire and vegetation using samples from Bengal fan sediments (ODP Leg 116).
A major challenge to quantifying paleo-fire inputs in sediments is that PAHs can be sourced from weathered fossil carbon (i.e., a petrogenic source) as well as from aerosols and particles derived from burned terrestrial biomass (i.e., a pyrogenic source). We overcome this obstacle through principle components analysis (PCA) of alkylated phenanthrene homologs to distinguish between predominately pyrogenic and petrogenic derived samples. Then, we use nonmetric multidimensional scaling (NMDS) to identify the non-alkylated structures of pyrene, fluoranthene, ideno[1,2,3-cd]pyrene and fluorene as PAHs associated with pyrogenic inputs. We focus on these compounds' abundance patterns alongside records of δ13C of leaf waxes, and make preliminary novel measurements of δ13C values of these deep time pyrogenic PAHs to test the association of burned biomass with the rise of C4 vegetation in the Mio-Pliocene. Fire-derived PAH relative abundances jump in association with a rise in grass inputs through time. Our results link fire to the opening of grassy landscapes, and support disturbance as a critical mechanism of terrestrial biome transitions that can and should be quantified in deep time. These new tools can be used to examine paleo-fire and vegetation interactions for other geographic regions and climactic transitions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMGC43D..05K
- Keywords:
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- 0315 Biosphere/atmosphere interactions;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCESDE: 0468 Natural hazards;
- BIOGEOSCIENCESDE: 1631 Land/atmosphere interactions;
- GLOBAL CHANGE