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Paleolightning refers to the remnants of ancient lightning activity studied in fields such as historical geology, geoarchaeology, and fulminology. Paleolightning provides tangible evidence for the study of lightning activity in Earth's past and the roles lightning may have played in Earth's history. Some studies have speculated that lightning activity played a crucial role in the development of not only Earth's early atmosphere but also early life. Lightning, a non-biological process, has been found to produce biologically useful material through the Redox of inorganic matter. Research on the impact of lightning on Earth's atmosphere continues today, especially with regard to feedback mechanisms of lightning-produced nitrate compounds on atmospheric composition and global average temperatures.
Detecting lightning activity in the geologic record can be difficult, given the instantaneous nature of lightning strikes in general. However, fulgurite, a glassy tube-like, crust-like, or irregular mineraloid that forms when lightning fuses soil, , clay, rock, biomass, or caliche is prevalent in electrically active regions around the globe and provides evidence of not only past lightning activity, but also patterns of convection. Since lightning channels carry an electric current to the ground, lightning can produce magnetic fields as well. While lightning-magnetic anomalies can provide evidence of lightning activity in a region, these anomalies are often problematic for those examining the magnetic record of rock types because they disguise the natural magnetic fields present.
Lightning strikes are short-lived, high-intensity electrical discharges that can reach temperatures five times hotter than the surface of the Sun. As a result, as a lightning channel travels through the air, ionization occurs, forming nitrogen oxide (NOx) compounds within the lightning channel. Global production as a result of lightning is around 1–20 Tg N yr−1. Some studies have implied that lightning activity may be the "greatest contributor to the global nitrogen budget", even larger than the burning of . With anywhere between 1500 and 2000 thunderstorms and millions of lightning strikes occurring daily around the Earth, it is understandable that lightning activity plays a vital role in nitrogen fixation. While nitrogen oxide compounds are produced as a lightning channel travels toward the ground, some of those compounds are transferred to the geosphere via wet or dry deposition. Variations of nitrogen in terrestrial and oceanic environments impact primary production and other biological processes. Changes in primary production can impact not only the carbon cycle, but also the climate system.
Fulgurites are indicative of ; the distribution of fulgurites can hint at patterns of lightning strikes. Sponholz et al. (1993) studied fulgurite distributions along a north–south cross section in the south central Saharan Desert (Niger). The study found that newer fulgurite concentrations increased from north to south, which indicated not only a paleo-monsoon pattern, but also the demarcation for thunderstorms as they progressed from a northern line to a southern location over time. By examining the outcrops in which the fulgurite samples were found, Sponholz et al. (1993) could provide a relative date for the minerals. The fulgurite samples dated back approximately 15,000 years to the mid to upper Holocene. This finding was in agreement with the of the region, as this period of the Holocene was particularly wet. A wetter climate would suggest that the propensity for thunderstorms was probably elevated, which would result in larger concentrations of fulgurite. These results pointed to the fact that the climate with which the fulgurite was formed was significantly different from the present climate because the current climate of the Saharan Desert is arid. The approximate age of the fulgurite was determined using thermoluminescence (TL). Quartz sands can be used to measure the amount of radiation exposure, so if the temperature at which the fulgurite was formed is known, one could determine the relative age of the mineral by examining the doses of radiation involved in the process.
Fulgurites also contain air bubbles. Given that the formation of fulgurite generally takes only about one second, and the process involved in the creation of fulgurite involves several chemical reactions, it is relatively easy to trap gases, such as , within the vesicles. These gases can be trapped for millions of years. Studies have shown that the gases within these bubbles can indicate the soil characteristics during the formation of the fulgurite material, which hint at the paleoclimatology. Since fulgurite is almost entirely composed of silicon dioxide with trace amounts of calcium and magnesium, an approximation of the total amount of organic carbon associated with that lightning strike can be made to calculate a carbon-to-nitrogen ratio to determine the paleoenvironment.
Evidence of lightning activity can often be found in the paleomagnetism record. Lightning strikes are the result of tremendous charge buildup in clouds. This excess charge is transferred to the ground via lightning channels, which carry a strong electric current. Because of the intensity of this electric current, when lightning hits the ground, it can produce a strong, albeit brief, magnetic field. Thus, as the electric current travels through soils, rocks, plant roots, etc., it locks a unique magnetic signature within these materials through a process known as lightning-induced remanent magnetization (LIRM). Evidence of LIRM is manifested in concentric magnetic field lines surrounding the location of the lightning strike point. LIRM anomalies normally occur close to the location of the lightning strike, usually encapsulated within several meters of the point of contact. The anomalies are generally linear or radial, which, just like actual lightning channels, branch out from a central point. It is possible to determine the intensity of the electric current from a lightning strike by examining the LIRM signatures. Since rocks and soils already have some preexisting magnetic field, the intensity of the electric current can be determined by examining the change between the "natural" magnetic field and the magnetic field induced by the lightning current, which generally acts parallel to the direction of the lightning channel. Another characteristic feature of an LIRM anomaly compared to other magnetic anomalies is that the electric current intensity is generally stronger. However, some have suggested that the anomalies, like other characteristics in the geologic record, might fade over time as the magnetic field redistributes.
LIRM anomalies can often be problematic when examining the magnetic characteristics of rock types. LIRM anomalies can disguise the natural remanent magnetization (NRM) of the rocks in question because the subsequent magnetization caused by the lightning strike reconfigures the magnetic record. While investigating the soil attributes at the 30-30 Winchester archeological site in northeastern Wyoming to discern the daily activities of prehistoric people that had once occupied that region, David Maki noticed peculiar anomalies in the magnetic record that did not match the circular magnetic remnant features of the ovens used by these prehistoric groups for cooking and pottery. The LIRM anomaly was significantly bigger than the other magnetic anomalies and formed a dendritic structure. To test the validity of the assertion that the magnetic anomaly was indeed the result of lightning and not another process, Maki (2005) tested the soil samples against known standards indicative of LIRM anomalies developed by Dunlop et al. (1984), Wasilewski and Kletetschka (1999), and Verrier and Rochette (2002). These standards include, but are not limited to: 1) Average REM (ratio between natural remanent magnetization to a laboratory standard value) greater than 0.2, and 2) Average Koenigsberger ratio (ratio between natural remanent magnetization and the natural field created by Earth's magnetic field). The findings indicated the evidence of LIRM at the archaeological site. LIRM anomalies also complicated the determination of the relative location of the poles during the late Cretaceous from the magnetic field record of basaltic lava flows in Mongolia. The presence of LIRM-affected rocks was determined when calculated Koenigsberger ratios were drastically higher than other magnetic signatures in the region.
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