Fossil record of fire

The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago,^[1] permitting the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire.^[2] Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants.^[3]^[4] Apart from a controversial gap in the Late Devonian, charcoal is present ever since.^[4] The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire.^[5] Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago;^[6] this kindling provided tinder which allowed for the more rapid spread of fire.^[5] These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.^[5]

Fossil evidence

The fossil evidence of paleo-wildfire comes mainly from charcoal—the earliest charcoal dates to the Silurian period.^[7] Charcoal results from organic matter exposed to high temperatures, which drives off volatile elements, resulting in refractory black carbon. Charcoal differs from coal, which is a combination of fossilized plants, and inertinite or fossilized charcoal. Inertinite can be burned to yield heat but, unlike the non-charcoal components of coal, does not yield significant volatile fractions during coking. Coking is a refractory process separating coal gas from coal to make coke.

The main inertinite submacerals are fusinite, semifusinite, micrinite, macrinite, and funginite, with semifusinite being the most common. Compositional percentages of fusinite and semifusinite indicate paleo-wildfire magnitude, frequency, and type.^[8] Charred plants in fusinite and semifusinite can be preserved in detail with original cell structures preserved in three dimensions.^[8] Images can be recovered using scanning electron microscopy.^[9] Fragments can be distributed some distance. Black carbon-rich alluvial deposits in river deltas can provide a 'time-averaged' record of fire activity in the river's catchment (and up-wind) area.^[8]

The loss of volatile elements during combustion means that charred remnants are shrunk in volume. The refractory nature of the charcoal enhances preservation potential.^[8]

Evidence of lightning strikes is usually difficult to link to specific fires; occasionally, they may scorch trees, but fulgarites - fused sediments where a strike has melted together the soil - are occasionally preserved in the geological record from the Permian onwards.^[8] Fossilized trees that survived fires can also provide evidence of fire frequency - especially when scorch marking can be related to the annual growth rings of the affected tree. These are more useful for relatively recent times. There is a report of this phenomenon in pre-Tertiary strata.^{[note 1]}^[8]

Charcoal - a fire proxy

The distribution and occurrence of charcoal in sediments is widely used to understand paleo-wildfire events.^[10] Wildfires are natural events that burn wildlands - forests, grasslands, and prairie - leading to the destruction of ecosystems and can be traced through charcoal records. Charcoal is the largest product of fire by size and often deposited in layers of sedimentary rocks and lakes. Charcoal is formed from incomplete combustion of terrestrial ecosystems (vegetation) ^[11]^[12] and its evidence describes fire disturbance regimes - frequency and intensity (magnitude) - and the rate of accumulation through time.^[12]^[13] Studies on charcoal as a fire proxy are scale-dependent and are broadly grouped into macroscopic charcoal (this refers to local wildfire events) and microscopic charcoal (describes the regional scale of wildfire events). Evidence of charcoal records are important in studies of paleoecology and archived in the Global Charcoal Database, which has been used to aid the reconstruction of fire history, test hypotheses, and model fire-vegetation-climate relationship.^[14] This evidence of past fire regimes has been used to understand past vegetation changes in vegetation, evolution of plants on Earth, and other ecological processes driving the terrestrial and aquatic ecosystems. To examine the intensity and frequency of local fire regimes, charcoal is processed using microscopes.

Macroscopic charcoal processing

Macroscopic charcoal remains (>125 μm)^[15] are processed using standard protocols in the laboratory and involves three stages. Stage 1: Measurement of sediment samples and treating with 25 ml of 6% hydrogen peroxide to oxidize the organic matter, and heating for 24 hours. This process leaves the charcoal intact but breaks down any organic matter present; Stage 2: Sieving, washing, and rinsing the samples with deionized water, and drying onto a petri dish for 12 – 48 hours. Charcoal remains are sieved to sizes ranging from micro- (>125 μm but <180 μm), meso- (>180 μm but <250 μm) to macro-charcoal (>250 μm).^[11]^[12]^[15] Stage 3: Charcoal counting using the stereo microscope.^[15] When counting the charcoal, identifying them into different morphotypes is sometimes done because it helps to understand the vegetation type and plant habit (for instance woody or grassy) present at a given geologic time.^[16]^[17] The common morphotype classification of charcoal for plant record include dark, cellular, porous, fibrous, sponge, branched, lattice, branched pits etc.^[17]^[18] Processing and analyzing charcoal records are methodical approaches that stand to demonstrate charcoal occurrences and distribution, and also boost our understanding of fire-related concepts.

Common fire concepts and their interpretations

Fire triangle - wildfire require the combination of three components namely: oxygen, ignition, and fuel.^[19] Oxygen is the most abundant element on planet Earth and a major contributing factor to fire events and life on Earth. Throughout geologic time, oxygen levels have changed significantly particularly before the Cretaceous period but have not changed much since the Cretaceous.^[20] Ignition is the combustion of fuel materials in the presence of oxygen. Sources of ignition include heat, lightning, people (humans), volcanoes, wildlife, and spontaneous combustion. Fuel is any organic or inorganic material that reacts to heat (energy), and can be influenced by the quality and quantity of material, moisture content, decomposition rate, climate, biomass, and connectivity of materials (see ref.^[19] for details). These three sources of fire often leave behind charcoal remains which are evidence of fire history, disturbances ^[12] and are indicators of past ecological and biological processes.^[21]
Fire regimes - this refers to the spatial and temporal patterns and characteristics of fire activity. Wildfire regimes are typically driven by the vegetation type (for instance grasses) and weather (climatic patterns). Wildfire regimes are defined by the frequency, intensity, and size of fire activities.^[13]^[22] Other factors that affect fire regimes include previous fire records, succession stage after previous disturbances (for example fire activity), moisture content, pest management and so on.^[23]^[24]
Fire frequency - this refers to the number of times fire occurs in a given area under a defined geologic time. The concept of fire frequency is often applied to local fire events.^[25]
Fire intensity - also known as fire severity or magnitude is the degree of fire or the magnitude of fire event. Fire intensity is categorized into low fire intensity and high fire intensity. Low fire intensity typically destroys understory forest vegetation and affects dispersal events while high fire intensity destroys forest sub-canopy and canopy ^[23] and can cause ghost forests.
Fire interval - this refers to the time period between one fire event and another fire event within a given area. The time period for such wildfire event can be driven by vegetation type and temperature.^[26] Fire interval can also be known as fire rotation or fire cycle.

Geochemical evidence

The amount of oxygen in the atmosphere is the major control on the abundance of fire; this can be approximated by a number of proxies.^[27]

Development through time

Fires among the low, scrubby, wetland plants of the Silurian can only have been limited in scope. Not until the forests of the Middle Devonian could large-scale wildfires really gain a foothold.^[8] Fires really took off in the high-oxygen, high-biomass period of the Carboniferous, where the coal-forming forests frequently burned; the coal that is the fossilised remains of those trees may contain as much as 10-20% charcoal by volume. These represent fires which may have had approximately a 100-year repeat cycle.^[8]

At the end of the Permian, oxygen levels plummeted, and fires became less common.^[8] In the early Triassic, after the largest naturally-caused extinction event in Earth's history at the end of the Permian, there is an enigmatic coal gap, suggesting a very low biomass;^[28] this is accompanied by a paucity of charcoal throughout the entire Triassic period.^[8]

Fires again become significant in the late Jurassic through the Cretaceous. They are especially useful as charcoalified flowers provide a key piece of evidence for tracking the origin of the angiosperm lineage.^[8] Contrary to popular perception, there is no evidence of a global inferno at the end of the Cretaceous, when many lineages were driven to extinction, most notably all non-avian dinosaurs; the record of fire after this point is somewhat sparse until the advent of human intervention around half a million years ago, although this may be biased by a lack of investigations from this period.^[8]

Ecological effects of wildfire

The effects of wildfire on Earth are broad and can be traced back to the Silurian period.^[29] Some common effects of wildfires are summarized as follows:

Disturbance - wildfire is a naturally occurring agent of disturbance that affects the natural environment and leads to the destruction of terrestrial ecosystems. Fire systems have contributed significantly in shaping the vegetation structure on Earth.^[13]^[30] For instance, the first record of fire disturbance on ecosystem functions dates back to 10 million years ago during which increased fire activity destroyed woodland vegetation and created favorable space for the spread of C4 plants. This activity destroys the vegetation structure (species composition) and consequently decreases ecosystem functions and services.^[21]^[31]

Ghost forests - high fire events in forest vegetation can lead to ghost forests. Ghost forests are former forest regions that are composed of dead and dried trees with declining ecosystem functions and services.

Increased erosion - fire activities cause destruction of vegetation cover - grassland - leading to the exposure of the topsoil to agents of erosion such as water and wind. This process (erosion) wash away the topsoil, nutrients, minerals and other biological organisms. Thereby leaving the soil poor in nutrients and not suitable for agricultural and domestic activities.

Heterogeneous landscape - increased fire activity can cause the destruction and alteration of homogeneous landscapes, leading to a heterogeneous landscape. For example, the destruction of 20% of a forest area by fire promotes the spread of grasses and change in the forest structure and species composition. Similarly, the destruction of a grassland by fire activity facilitates the washing of the topsoil by erosion and a loss of plant community and ecosystem services.

Loss of moisture - the frequency and intensity of fire activity can cause a loss in plant moisture content and decrease moisture availability for plant uptake due to vegetation and habitat destruction.

Destruction of forest canopy and understory vegetation - fire activities of low intensity (also known as surface fire) can cause the destruction of forest understory vegetation particularly grasses and herbaceous plants. The effects of such low fire intensity is more on the forest understory vegetation but less on forest trees (only affects tree barks). On the other hand, wildfire activities can cause the destruction of forest canopies which opens a window for understory vegetation to flourish given the amount of direct sunlight received.

Notes

^ From a Triassic tree in Antarctica

^{[clarification needed]}

References

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