Macro-charcoal accumulation in floodplain wetlands: Problems and prospects for reconstruction of fire regimes and environmental conditions

Abstract

Floodplain wetland ecosystems respond dynamically to flooding, fire and geomorphological processes. We employed a combined geomorphological and environmental proxy approach to assess allochthonous and autochthonous macro-charcoal accumulation in the Macquarie Marshes, Australia, with implications for the reconstruction of fire regimes and environmental conditions in large, open-system wetlands. After accounting for fluvial macro-charcoal flux (1.05 ± 0.32 no. cm-2 a-1), autochthonous macro-charcoal in ~1 m deep sediment profiles spanning ~1.7 ka were highly variable and inconsistent between cores and wetlands (concentrations from 0 to 438 no. cm-3, mean accumulation rates from 0 to 3.86 no. cm-2 a-1). A positive correlation existed between the number of recent fires, satellite-observed ignition points, and macro-charcoal concentrations at the surface of the wetlands. Sedimentology, geochemistry, and carbon stable isotopes (δ13C range -15 to -25 ‰) were similar in all cores from both wetlands and varied little with depth. Application of macro-charcoal and other environmental proxy techniques is inherently difficult in large, dynamic wetland systems due to variations in charcoal sources, sediment and charcoal deposition rates, and taphonomic processes. Major problems facing fire history reconstruction using macro-charcoal records in these wetlands include: (1) spatial and temporal variations in fire activity and ash and charcoal products within the wetlands, (2) variations in allochthonous inputs of charcoal from upstream sources, (3) tendency for geomorphic dynamism to affect flow dispersal and sediment and charcoal accumulation, and (4) propensity for post-depositional modification and/or destruction of macro-charcoal by flooding and taphonomic processes. Recognition of complex fire-climate-hydrology-vegetation interactions is essential. High-resolution, multifaceted approaches with reliable geochronologies are required to assess spatial and temporal patterns of fire and to reconstruct in order to interpret wetland fire regimes.

Hydrogeomorphic Response of Charcoals During River Transits and Its Impact on the Carbon Cycle

“…Moreover, the absence of a relationship between the sediment grain size and charcoal storage in bedload suggests that the charcoal migration or accumulation occurs as an individual entity and is not co‐genetically linked with the process of the storage of pyrogenic biomolecules (Evan et al., 1990; Oen et al., 2006). The finer substrate plays a crucial role in carbon storage in the transient floodplain areas with limited variation in disturbance and modification through post‐depositional processes (Brown & Pasternack, 2004; Gao et al., 2017; Graves et al., 2019; Whitlock & Larsen, 2002). Such observation suggests that the higher finer sediment accumulation in the lowland floodplain can aid in the pre‐aging of charcoal and allows temporary storage of PyC across intermediate reservoirs.…”

“…Natural transportation processes would impart a wide range of imposed energy as well as duration on charcoal particles during active hydrological transport across different geomorphic regimes of riverine transects (Crawford & Belcher, 2014; Graves et al., 2019). The stream power, discharge, slope, sediment load, resultant channel sinuosity and braid‐channel ratio have an integrated influence on the development of distinct geomorphic regimes and associated physical processes in fluvial environments (Bawa et al., 2014; Bizzi & Lerner, 2015; Sinha et al., 2005).…”

“…In the lowland regions (alluvial plains), the hydraulic action of sediments on charcoal particles is expected to have a limited impact during the transient transport. Active sedimentation, development of stable landforms and varying hydrodynamic conditions could influence the transport and deposition of sediments, including charcoal particles, across the lowland riverine deposit (Clark & Royall, 1996; Graves et al., 2019). Such distinctly varying regional geomorphic conditions (sediment load, slope, discharge) might induce differences in spatial distribution and morphology in charcoal particles.…”

Rivers up in smoke: impacts of Australia’s 2019–2020 megafires on riparian systems

“…However, to what extent riparian systems were burned and with what regime is unknown (Enright and Thomas 2008). Nevertheless, it is unlikely that ubiquitous, widespread and severe burning like 2019-2020 has occurred before (Black and Mooney 2006;Mooney et al 2011;Graves et al 2019).…”

Thinking outside the Continent and outside the Box: Cross-Continental Comparative Studies Can Enrich Studies of Pre-Columbian Raised-Field Agriculture

“…For example, a study of present-day African RF by our group showed that, within fewer than 40 years, bioturbation almost completely removes the initial stratigraphic evidence of how fields are constructed and maintained with the periodic addition of organic matter (Rodrigues et al 2020). In addition, archaeobotanical proxies can often under-represent certain crops (e.g., manioc produces few diagnostic phytoliths [Chandler-Ezell et al 2006]), may be affected by post-depositional modifications and taphonomic processes (e.g., macro-charcoal in wetlands [Graves et al 2019]), or are not even well preserved in tropical soils, as occurs with starch and pollen grains (Pearsall 2015).…”