The earliest evidence of wildfire is documented from two localities: the early mid-Silurian Pen-y-lan Mudstone, Rumney, Wales (UK), and the late Silurian Winnica Formation, Winnica, Poland. Nematophytes dominate both charcoal assemblages. Reflectance data indicate low-temperature fires with localized intense conditions. Fire temperatures are greater in the older and less evolved assemblage. These charcoal assemblages and others, new and previously documented, from the Silurian and earliest Devonian are compared to box models of atmospheric oxygen concentration (pO2). Based on modern charring experiments, these data indicate pO2 is divergent from the broad trends predicted by the COPSE-revisited and GEOCARBSULFOR models. Sustained burns require a minimum pO2 threshold of 16%, or ~0.75 present atmospheric level. This threshold was first met and, our charcoal data indicate, was exceeded in the mid-Silurian and then, later in the Silurian, attained again repeatedly.Atmospheric oxygenation has played a fundamental role in Earth systems since the Great Oxygenation Event (ca. 2.5–2.3 Ga; Bekker et al., 2004). Geochemical proxies indicate a soil-forming microbial cover, established by ca. 850 Ma (Kump, 2014), contributed to increasing atmospheric oxygen concentration (pO2). Atmospheric oxygen levels remained low until the mid-Ordovician advent of embryophytes, as evidenced by phytodebris (Wellman and Strother, 2015). These spore producers coexisted in a predominantly microbial biome (Wellman and Strother, 2015), where fungi acted as both decomposers and mycorrhizal symbionts, potentially accelerating the photosynthetic potential of embryophytes and green algae (Berbee et al., 2020). Nonvascular and vascular land plants evolved gradually in this landscape. The earliest vascular plant, Cooksonia, is preserved in the mid-Silurian (Wenlock, 433.4–427.4 Ma) rocks of Ireland (Edwards and Feehan, 1980) and the Czech Republic (Libertín et al., 2018). These diminutive (<10 cm high), leafless, dichotomously branched plants terminated in sporangia and grew alongside an enigmatic, globally distributed group, nematophytes.Nematophytes vary from sub-millimetric to multi-meter-long structures (e.g., Prototaxites). All are united anatomically by aggregations of tubes and cuticles in a bound plant body. Their function and ecological role are unclear. Some are considered ascomycetes (fungi), and others are considered lichenized fungi (Wellman and Ball, 2021). If fungal, then nematophyte chitinous cell-wall chemistry would have differed from the cellulose, hemicellulose, and lignin of embryophytes. Most charring experiments focus on embryophytes (e.g., Belcher et al., 2010), though data exist from bracket fungi (Scott and Glasspool, 2005), with which Prototaxites has been compared (Boyce et al., 2007). Fungal cells must be charred at higher temperatures to produce the same reflectance values found in embryophytes (see the Supplemental Material1).The geobiosphere was altered fundamentally through embryophyte diversification and feed-backs on Earth systems (Kenrick et al., 2012). Increased photosynthesis impacted the oxygen cycle, raising pO2 to near present atmospheric level (PAL) (Kump, 2014; Lenton et al., 2016). Agreement exists that terrestrialization impacted pO2, but no consensus exists as to the timing of its rise to near PAL. Predictions vary from the Neoproterozoic (>550 Ma; Och and Shields-Zhou, 2012) to the Phanerozoic (300 Ma; Krause et al., 2018). The Phanerozoic transition is marked by a switch from widespread marine anoxia to oxygenation (435–392 Ma; Dahl et al., 2010). However, for elevated pO2 to have persisted, a basic and continued change must have occurred to either increase the generation of free oxygen or decrease its sink (see Kump, 2014; Lenton et al., 2016). Evidence of fire (charcoal) is used as a proxy for pO2 (Glasspool et al., 2015) and provides an absolute minimumburn threshold (Belcher and McElwain, 2008). Hence, fire is important for interpreting changes in atmospheric oxygenation.Charcoal constrains pO2 in the rang ~0.7–1.4 PAL (Belcher et al., 2010). Its earliest occurrence is synchronous with a Silurian–Devonian shift of the molybdenum-isotope record (Dahl et al., 2010). Intervals, such as the Devonian “charcoal gap”, exist where charcoal data are scarce (Fig. 1; Scott and Glasspool, 2006; Lu et al., 2021). Another such interval is the Silurian. Three widely used pO2 box models (GEOCARBSULF: Berner, 2009; COPSE-revisited: Lenton et al., 2018; GEOCARBSULFOR: Krause et al., 2018) support a steep pO2 rise to a peak initiated in the mid-Ordovician. A steep decline is predicted beginning in the Lochkovian (Krause et al., 2018, their figure 3). The rise is linked to the spread of cryptogamic cover (Lenton et al., 2018) and is consistent with reports of fire through this interval (Glasspool et al., 2015).We report charred phytoclast (Gastaldo, 1994) remains from the Homerian and Ludlow (mid-to late Silurian) extending the burn record back 10 m.y. We used data from the Pen-y-lan Mudstone at Rumney, Wales (UK), and the Winnica Formation at Winnica, Poland (Fig. 1; see the Supplemental Material), to examine pO2 box models. We demonstrate that pO2 reached sufficient levels to sustain wildfire in the mid-to late Silurian and that it implies elevated concentrations.Nematophytes and Pachytheca (Fig. 2B) dominate the organic residues of the Rumney specimen morphotypes (Table S5 in the Supplemental Material). Vitrinite from eight samples of the 314.71–316.3 m borehole interval (Rumney Borehole, drilled to 317.39 m by the British Geological Survey in 1978; 51°30'23.9″N, 3°08'18.6″W) indicate a mean random reflectance of Ro = 1.08% (medium to high volatile bituminous A), and that from 15 clasts of the 316.3–317.39 m interval indicate Ro = 1.10%; the mean of both data sets is Ro = 1.10%.Fire-impacted reflectances range from near those of vitrinite to as high as Ro = 5.61%; 20 samples have Ro ≥ 2%. Charred fragments indicate fire temperatures (T) ranging from w410–w730 °C or g440–g940 °C (calibrated against wood [prefix w, Equation S1 in the Supplemental Material] and the fungus Ganoderma [prefix g, Equation S2]; Table S6). The mean temperature (mT) of 48 phytoclasts is w490 °C (g540 °C). Recorded temperatures are greater than those at Winnica. The temperature range has a long, sparsely populated positive skew. Thirteen (13) specimens record T > w500 °C (g540 °C), five record T > w600 °C (g750 °C), and three record T > w700 °C (g900 °C) (Table S5). The T > w/g500 °C specimens are Pachytheca, nematophytes, or prototaxodioids (where specimens cannot be differentiated between Prototaxites and Nematasketum, we term them “prototaxodioids”, e.g., Fig. 2A). The T > g700 °C materials are prototaxodioids or detritus of probable prototaxodioids.The majority of phytoclasts from Winnica (specimens collected in ca. 2013 from small outcrops on either side of the Słupianka stream near Winnica, in the Kielce region of Poland; 50°52'24.5″N, 21°6'16.5″E) are nematophytic (79%); none are demonstrably tracheophytic (Table S5). Taxa include undifferentiated prototaxodioids (Nematasketum or Prototaxites), nematothalloids (including Nematothallus or Tristratothallus; Fig. 2C), and aggregates of different-sized tubes (Fig. 2D; Table S5). In incident light, fossils differ in color (brown to matte-black gradient) and luster, and exhibit characteristics of charring (see the Supplemental Material). Scanning electron microscopy and reflected light microscopy confirm these observations (Table S5). Vitrinite in strew mounts indicate Ro = 0.93% (high volatile bituminous A; Teichmüller, 1987). Of the phytoclasts with anatomy, 32 had Ro ≥ 2% (maximum 4.78%) which, if attributable to rank maturation, approaches that of meta-anthracite (Teichmüller, 1987). However, this charcoal burned at predominantly low temperatures; 82 specimens calibrated against both wood and Ganoderma indicate a mT of w480 °C (g520 °C) (see Tables S1–S6 and Figs. S2–S5B). The greatest T recorded in a specimen is w640 °C (g820 °C), while 30% of the samples record T ≥ w500 °C (g540 °C) (Table S5).Fire plays a multifaceted role in Earth history. Increasing Paleozoic pO2 was integral in metazoan evolution and radiation (Lenton et al., 2014). Pinpointing the timing of thresholds for sustaining life on land is critical to understanding the biosphere. Elevated pO2 allows aerobes to prosper through the enzymatic combustion of organics, though free radicals are deleterious. Cyanobacterial nitrogenase is particularly sensitive to oxygen and may have driven the early symbiotic evolution of cyanobacteria (e.g., Nematothallus, a lichenized fungus; Wellman and Ball, 2021), notably if pO2 was elevated (Rikkinen, 2017). Fire data (Fig. 3) are indicative of pO2 elevated to or above PAL at points from the Wenlock to earliest Devonian. Elevated pO2 may justify the proliferation of lichenized fungi documented from the latest Silurian.Fire had a strong effect on pO2 through actions on weathering, runoff, erosion, and organic carbon burial, affecting phosphorous input to oceans (Kump, 2014). Several coarseresolution pO2 box models explore when such thresholds might have been attained (e.g., Brand et al., 2021). The charcoal record contributes to understanding when pO2 reached this threshold.Latest Silurian fire was documented first from Ludford Lane, Welsh Borders (Glasspool et al., 2004; Fig. 3). However, material shown by Niklas and Smocovitis (1983, their figures 20 and 22–25) from the Massanutten Sandstone, Passage Creek, Virginia (USA), may be charred (this and other localities are discussed in the Supplemental Material). If confirmed, this record would extend fire activity to the early Llandovery (ca. 441 Ma; Tomescu et al., 2009). Other Silurian and earliest Devonian (e.g., North Brown Clee Hill; Glasspool et al., 2006) charcoal exists in the Welsh Borders, and these and more regional data are herein confirmed (Fig. 3; see the Supplemental Material discussion). Our Rumney and Winnica data add to this record, pushing the fire threshold back to at least the early Wenlock (ca. 430 Ma).High-amplitude carbon isotopic excursions, including the Wenlock to Přídolí, indicate the global carbon cycle was more frequently and severely perturbed during the Silurian than during any other Phanerozoic period (Frýda et al., 2021). Given the carbon and pO2 feedbacks (Lenton et al., 2018), pO2 oscillations could be expected due to the scale of carbon perturbations. The general trends of pO2 models (Fig. 3; GEOCARBSULF: Berner, 2009; COPSE-reloaded: Lenton et al., 2018; GEOCARBSUL-FOR: Krause et al., 2018) are roughly comparable. These models have coarse resolution, and predictions of pO2 are typically binned at 10 m.y. intervals. Hence, such binning cannot resolve high-frequency events such as the global carbon cycle perturbations predicted by Frýda et al. (2021). Conversely, single-point data, such as individual fire events, do not disprove model predictions at their broad 10 m.y. scale, though they do not preclude high-frequency fluctuations. However, fire regime data across longer intervals can cast doubt on model predictions. The data herein begin to establish this regime.Box models show discrepancies in predictions of Late Ordovician to Early Devonian pO2 amplitude. During this interval, the COPSE-reloaded model's best estimate predicts pO2 < 15%, a level discordant with the record of Silurian and earliest Devonian fire at this time (Fig. 3). For a dry natural fuel, a 16% pO2 threshold is required for ignition and self-sustaining combustion (Belcher et al., 2010). The record of fire is also discrepant with the GEOCARBSULFOR model's best estimate (Krause et al., 2019), which models pO2 at 420 Ma <18%, a predicted threshold for the ignition and support of small fires but only where rainfall is “very low” and/or the fuel “sea-sonally very dry” (Belcher et al., 2010). Fire is not precluded at pO2 <18% but, given the diminutive and hydrophilic nature of the Silurian fuel, it is highly unlikely. Early Homerian charcoal from Rumney and from the Gorstian of Nant Cwm-Ddu, Wales (under preliminary investigation; see the Supplemental Material), representing a time when Baragwanathia was an embryophytic “giant” (Gensel et al., 2020), is more incongruent with pO2 predictions for this earlier interval, which fall at or below 15% (Lenton et al., 2018; Krause et al., 2019; cf. Belcher et al., 2010, their figures 2 and 4). At Rumney, the proportional volume of charcoal recovered from a marine setting indicates the extensive propagation of fire. This is concordant with pO2 levels that approached PAL earlier than the latest box models predict (Lenton et al., 2018; Krause et al., 2019) and are in line with GEOCARBSULF (Berner, 2009).Most Rumney data indicate low-temperature fire (x̄ [mean]: Ro 2.31% = w490 °C; Table S6), but 30% of the inertinite is calculated to have formed at > w500 °C, and 6% at > w700 °C. Calibration of fire temperature using data from Hudspith et al. (2014; Tables S3–S4) increases this to 79% > w500 °C, 11% > w800 °C, and 4% > w900 °C. Experimental charring of the chitinous bracket fungus Ganoderma (Scott and Glasspool, 2007; Table S2), compared with composite lignin-rich data from Scott and Glasspool (2005; Table S1), indicates that this fungus requires higher burn temperatures (40–85 °C greater) to generate comparable reflectance values (Table S5). At Rumney, the highest reflectances are recorded in fragmented nematophytes. Early Homerian fire temperatures > g700 °C may seem improbable due to the impoverished potential embryophyte fuel load. However, very high reflectances for nematophytes, a group with demonstrable fungal and lichenized-fungal affinities (Wellman and Ball, 2021), lend support that these temperatures were attained.Trends for pO2 based on Silurian halites suggest atmospheric levels rose then fell between 442 and 422 Ma (Brand et al., 2021; Fig. 3). Back-calculated measurements for pO2 at 442 Ma are 14.3% ± 1.7%; at 425 Ma, 20.4% ± 6.6% and 24.7% ± 3.7%; and at 422 Ma, 19.8% ± 2.1%. The charring events at Winnica (Ludlow, ca. 424 Ma; mT = w480 °C; Table S6) and Ludford Lane (Přídolí, ca. 423 Ma; Ro 1.03%–2.74%; x̄ = 1.67%; mT = w450 °C; Table S6) align with these data. The occurrence of fire in the Gorstian at Nant Cwm-Ddu also aligns with these data. The locally intense early Silurian burns are discordant with the coarse amplitudes of GEOCARBSULFOR (Krause et al., 2018) and COPSE-reloaded (Lenton et al., 2018) through this time interval.At present, in most tropical settings, phosphorus is the major limiting nutrient, while this role falls to nitrogen at higher latitudes (Du et al., 2020). The predominant drivers of these phenomena are mean annual temperature and precipitation, seasonality of temperature and precipitation (climatically driven), and soil-clay fraction (Du et al., 2020). The same criteria would have impacted our low-latitude Silurian sites (Fig. 1), where phosphorus could be predicted to have been the limiting factor on primary productivity. Fires would have mobilized phosphorus from the terrestrial to marine realms, promoting algal photosynthesis and increased levels of pO2 (Kump, 2014). However, intense or frequent fires impact biocrusts in modern cold desert ecosystems, resulting in complete loss of the bacterial community and recovery times of decades for the pre-fire flora (Aanderud et al., 2019). In the Silurian, these crusts included cyanobacteria, algae, lichenized fungi, and early, diminutive embryophytes (Wellman and Ball, 2021). We propose that the locally intense burn temperatures, documented at Rumney in the early Homerian and to a lesser extent at Winnica in the Ludlow, demand levels of pO2 equivalent to, or possibly above, PAL. At such levels, fires must have been a significant global phenomenon, having a strong negative effect on the photosynthetic biocrustose flora. It seems probable that the relationship between pO2 and other Earth system processes continued throughout the Silurian until the development of a more embryophytically dominant flora.Charcoal from early Homerian strata at Rumney, Wales, extends the earliest record of fire on Earth back a further 10 m.y. to ca. 430 Ma. The frequency of charcoal data from Silurian sequences indicates that fires were not rare but an established part of the terrestrial biome from at least the Wenlock onward. Compiling the reflectance data of our study with that of Ludford Lane (Welsh Borders; Glasspool et al., 2004) indicates the most intense Silurian fires occurred in the least-evolved ecosystems. The Devonian charcoal gap conforms to box-model forecasts of pO2 decline to <16% prior to the Late Devonian rise of higher vascular plants. This coincidence may imply the feed-backs moderating pO2 within the fire window differed in the Silurian. Our data further indicate that geochemical models for the Silurian still require refinement before they accurately reflect the amplitude of pO2 at that time.We thank the following colleagues for project assistance: W. Kozłowski, A. Bodzioch, A. Gorski, S. Schultka, D. Edwards, C. Eble, L. Cherns, L. Axe, P. Hayes, S. Renshaw, A. Scott, and K. Robak. The research was supported by U.S. National Science Foundation grant EAR-1828359. We gratefully acknowledge the help of Lee Kump and two anonymous reviewers whose comments greatly improved this manuscript.

中文翻译：

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志留纪野火代理和大气氧气

最早的野火证据记录在两个地方：早期的中期志留纪 Pen-y-lan Mudstone，Rumney，威尔士（英国）和晚期志留纪 Winnica 组，波兰温尼卡。线虫在两种木炭组合中占主导地位。反射率数据表明存在局部强烈条件的低温火灾。在较老且进化较少的组合中，火灾温度更高。这些来自志留纪和最早泥盆纪的木炭组合和其他新的和以前记录的组合与大气氧浓度 (pO2) 的箱模型进行了比较。基于现代炭化实验，这些数据表明 pO2 与 COPSE 重新审视和 GEOCARBSULFOR 模型预测的广泛趋势不同。持续烧伤需要 16% 的最低 pO2 阈值，或约 0.75 的当前大气水平。首次达到此阈值，并且，我们的木炭数据表明，在中志留世超过了，然后，在志留世后期，再次重复达到。自大氧合事件（约 2.5-2.3 Ga；Bekker 等人）以来，大气氧合在地球系统中发挥了重要作用., 2004)。地球化学代理表明形成土壤的微生物覆盖物，由 ca 建立。850 Ma (Kump, 2014)，有助于增加大气中的氧气浓度 (pO2)。直到中奥陶世胚胎植物出现之前，大气中的氧气水平一直很低，植物碎片就是证明（Wellman 和 Strother，2015 年）。这些孢子生产者共存于一个以微生物为主的生物群落中（Wellman 和 Strother，2015 年），其中真菌既是分解者又是菌根共生体，有可能加速胚芽植物和绿藻的光合作用潜力（Berbee 等人，2020 年）。非维管和维管陆生植物在这片土地上逐渐进化。最早的维管植物 Cooksonia 保存在爱尔兰（Edwards 和 Feehan，1980 年）和捷克共和国（Libertín 等人，2018 年）的中志留纪（Wenlock，433.4-427.4 Ma）岩石中。这些小型（<10 厘米高）、无叶、分枝的植物终止于孢子囊，并与一个神秘的、全球分布的线虫类群一起生长。线虫类的结构从亚毫米到数米长不等（例如，原紫杉类）。所有这些都通过结合植物体中的管和角质层的聚集在解剖学上结合在一起。它们的功能和生态作用尚不清楚。有些被认为是子囊菌（真菌），有些被认为是地衣真菌（Wellman and Ball，2021）。如果是真菌，那么线虫几丁质的细胞壁化学就会不同于胚芽植物的纤维素、半纤维素和木质素。大多数炭化实验都集中在胚胎植物上（例如，Belcher 等人，2010），尽管存在来自支架真菌的数据（Scott 和 Glasspool，2005），与 Prototaxites 进行了比较（Boyce 等人，2007）。真菌细胞必须在更高的温度下烧焦才能产生与胚芽植物相同的反射率值（参见补充材料 1）。通过胚芽植物的多样化和地球系统的反馈，地球生物圈发生了根本性的变化（Kenrick 等人，2012 年）。光合作用的增加影响了氧气循环，将 pO2 提高到接近目前的大气水平 (PAL) (Kump, 2014; Lenton et al., 2016)。一致认为，陆地化会影响 pO2，但对于其上升到接近 PAL 的时间，尚无共识。从新元古代（> 550 Ma；Och 和 Shields-Zhou，2012）到显生宙（300 Ma；Krause 等，2018），预测各不相同。显生宙过渡的标志是从广泛的海洋缺氧转变为氧合（435-392 Ma；Dahl 等，2010）。然而，要使 pO2 持续升高，必须发生基本且持续的变化，以增加游离氧的产生或减少其汇（见 Kump，2014；Lenton 等人，2016）。火灾证据（木炭）被用作 pO2 的代表（Glasspool 等人，2015 年），并提供了绝对最低燃烧阈值（Belcher 和 McElwain，2008 年）。因此，火对于解释大气氧合的变化很重要。木炭将 pO2 限制在 ~0.7-1.4 PAL 范围内（Belcher 等人，2010 年）。它最早的出现与钼同位素记录的志留纪-泥盆纪移动同步（Dahl 等，2010）。在木炭数据稀缺的地方存在泥盆纪“木炭间隙”等间隔（图 1；Scott 和 Glasspool，2006；Lu 等人，2021）。另一个这样的间隔是志留纪。三种广泛使用的 pO2 盒模型（GEOCARBSULF：Berner，2009 年；COPSE-revisited：Lenton 等人，2018 年；GEOCARBSULFOR：Krause 等人，2018 年）支持在奥陶纪中期开始的 pO2 急剧上升至峰值。预计从 Lochkovian 开始会急剧下降（Krause 等人，2018 年，他们的图 3）。上升与隐花覆盖的蔓延有关（Lenton 等人，2018 年），并且与该区间发生火灾的报道一致（Glasspool 等人，2015 年）。我们报告了烧焦的植物碎屑（Gastaldo，1994) 来自 Homerian 和 Ludlow（志留纪中晚期）的遗骸将燃烧记录延长到 10 米 我们使用了来自威尔士 Rumney（英国）的 Pen-y-lan Mudstone 和波兰 Winnica 的 Winnica 组的数据（图 1；见补充材料），检查 pO2 盒模型。我们证明 pO2 在志留纪中晚期达到了足以维持野火的水平，这意味着浓度升高。线虫和厚壁菌（图 2B）在 Rumney 标本形态类型的有机残留物中占主导地位（补充材料中的表 S5）。来自 314.71-316.3 m 钻孔间隔（Rumney 钻孔，1978 年英国地质调查局钻至 317.39 m；51°30'23.9"N，3°08'18.6"W）的八个样品的镜质体表明平均随机反射率为Ro = 1.08%（中高挥发性沥青 A），316.3-317.39 m 层段的 15 个碎屑表明 Ro = 1.10%；两个数据集的平均值为 Ro = 1.10%。火灾影响的反射率范围从接近镜质体的反射率到高达 Ro = 5.61%；20 个样品的 Ro ≥ 2%。烧焦的碎片表明火灾温度 (T) 范围为 w410–w730 °C 或 g440–g940 °C（针对木材 [前缀 w，补充材料中的方程式 S1] 和灵芝真菌 [前缀 g，方程式 S2] 进行校准；表 S6 ）。48 个植物碎屑的平均温度 (mT) 为 w490 °C (g540 °C)。记录的温度高于温尼卡的温度。温度范围有一个长的、人口稀少的正偏斜。十三 (13) 个样本记录 T > w500 °C (g540 °C)，五个记录 T > w600 °C (g750 °C)，三个记录 T > w700 °C (g900 °C)（表 S5）。T > w/g500 °C 标本为厚壁菌，线虫，或原紫杉类（标本不能区分原紫杉类和线虫类，我们称它们为“原紫杉类”，例如，图 2A）。T > g700 °C 材料是 prototaxodioids 或可能的 prototaxodioids 的碎屑。来自 Winnica 的大多数植物碎屑（约 2013 年从波兰凯尔采地区 Winnica 附近 Słupianka 溪流两侧的小露头收集的标本；50° 52'24.5”N, 21°6'16.5”E) 是线虫（79%）；没有一个是明显的气管植物（表S5）。分类群包括未分化的 prototaxodioids（Nematasketum 或 Prototaxites）、线虫类（包括 Nematothallus 或 Tristratothallus；图 2C）和不同大小的管聚集体（图 2D；表 S5）。在入射光下，化石的颜色（棕色到哑光黑色渐变）和光泽不同，并表现出炭化特征（参见补充材料）。扫描电子显微镜和反射光显微镜证实了这些观察结果（表 S5）。散布支架中的镜质体表明 Ro = 0.93%（高挥发性沥青 A；Teichmüller，1987）。在具有解剖结构的植物碎屑中，32 个的 Ro ≥ 2%（最大 4.78%），如果归因于等级成熟，则接近于间无烟煤（Teichmüller，1987）。然而，这种木炭主要在低温下燃烧。针对木材和灵芝校准的 82 个样本表明 mT 为 w480 °C (g520 °C)（参见表 S1-S6 和图 S2-S5B）。样本中记录的最大 T 为 w640 °C (g820 °C)，而 30% 的样本记录 T ≥ w500 °C (g540 °C)（表 S5）。火在地球历史中扮演着多方面的角色。增加古生代 pO2 在后生动物进化和辐射中是不可或缺的（Lenton 等人，2014 年）。确定维持陆地生命的阈值时间对于了解生物圈至关重要。升高的 pO2 允许需氧菌通过有机物的酶促燃烧繁衍，尽管自由基是有害的。蓝藻固氮酶对氧气特别敏感，并且可能推动了蓝藻的早期共生进化（例如，线虫，一种地衣真菌；Wellman 和 Ball，2021 年），尤其是在 pO2 升高的情况下（Rikkinen，2017 年）。火灾数据（图 3）表明从 Wenlock 到最早的泥盆纪期间 pO2 升高到或高于 PAL。升高的 pO2 可能证明最新志留纪记录的地衣真菌的增殖是合理的。火通过风化、径流、侵蚀和有机碳埋藏，影响磷输入海洋（Kump，2014 年）。几个粗分辨率 pO2 盒模型探索了何时可能达到了这些阈值（例如，Brand 等人，2021）。木炭记录有助于理解 pO2 何时达到此阈值。最近的志留纪火灾首先记录在威尔士边境的 Ludford Lane（Glasspool 等人，2004 年；图 3）。然而，Niklas 和 Smocovitis（1983 年，他们的图 20 和 22-25）显示的来自弗吉尼亚州 Passage Creek 的 Massanutten 砂岩的材料可能会被烧焦（补充材料中讨论了这个和其他地方）。如果得到证实，这一记录将把火灾活动扩大到早期的兰多弗里（约 441 Ma；Tomescu 等人，2009 年）。其他志留纪和最早的泥盆纪（例如，North Brown Clee Hill；Glasspool 等人，2006）木炭存在于威尔士边境，这些和更多的区域数据在此得到证实（图 3；见补充材料讨论）。我们的 Rumney 和 Winnica 数据增加了这一记录，将火灾阈值至少推回到了温洛克早期（约 430 Ma）。高振幅碳同位素偏移，包括温洛克到 Přídolí，表明全球碳循环更加频繁，并且志留纪比任何其他显生宙都受到严重扰动（Frýda et al., 2021）。鉴于碳和 pO2 反馈（Lenton 等人，2018 年），由于碳扰动的规模，可以预期 pO2 振荡。pO2 模型的总体趋势（图 3；GEOCARBSULF：Berner，2009 年；COPSE-reloaded：Lenton 等人，2018 年；GEOCARBSUL-FOR：Krause 等人，2018 年）大致相当。这些模型具有粗略的分辨率，并且 pO2 的预测通常以 10 我的间隔进行分类。因此，这种分箱无法解决高频事件，例如 Frýda 等人预测的全球碳循环扰动。（2021 年）。相反，单点数据（例如单个火灾事件）并不能在其广泛的 10 米范围内反驳模型预测，尽管它们不排除高频波动。然而，跨越较长时间间隔的火情数据可能会对模型预测产生怀疑。这里的数据开始建立这种制度。箱模型显示了晚奥陶世到早泥盆世 pO2 振幅的预测存在差异。在此期间，COPSE 重新加载模型的最佳估计预测 pO2 < 15%，这一水平与当时志留纪和最早的泥盆纪火灾的记录不一致（图 3）。对于干燥的天然燃料，点火和自持燃烧需要 16% 的 pO2 阈值（Belcher 等人，2010 年）。火灾记录也与 GEOCARBSULFOR 模型的最佳估计不一致（Krause 等人，2019 年），该模型在 420 Ma <18% 时模拟 pO2，这是点燃和支持小火的预测阈值，但仅在降雨量“非常低”和/或燃料“季节性非常干燥”（Belcher 等人，2010 年）。pO2 <18% 时不排除着火，但鉴于志留纪燃料的小型和亲水性，这种可能性极小。来自 Rumney 和威尔士 Nant Cwm-Ddu 的 Gorstian 的早期荷马木炭（正在初步调查；见补充材料），代表 Baragwanathia 是胚胎植物“巨人”的时期（Gensel 等人，2020），更加不一致对于这个较早的时间间隔的 pO2 预测，低于 15%（Lenton 等人，2018 年；Krause 等人，2019 年；参见 Belcher 等人，2010 年，他们的图 2 和图 4）。在 Rumney，从海洋环境中回收的成比例的木炭量表明火灾的广泛传播。这与比最新盒模型预测的更早接近 PAL 的 pO2 水平一致（Lenton 等人，2018 年；Krause 等人，2019 年），并且与 GEOCARBSULF（伯纳，2009 年）一致。大多数 Rumney 数据表明低温火灾（x̄ [平均值]：Ro 2.31% = w490 °C；表 S6），但计算出 30% 的惰性体在 > w500 °C 时形成，6% 在 > w700 °C 时形成。使用 Hudspith 等人的数据校准火灾温度。（2014 年；表 S3-S4）将其增加到 79% > w500 °C、11% > w800 °C 和 4% > w900 °C。几丁质支架真菌灵芝的实验炭化（Scott 和 Glasspool，2007；表 S2）与来自 Scott 和 Glasspool（2005；表 S1）的复合富含木质素的数据相比，表明这种真菌需要更高的燃烧温度（40-85° C 更大）以产生可比较的反射率值（表 S5）。在 Rumney，最高的反射率记录在破碎的线虫中。由于潜在的胚芽燃料负荷不足，荷马早期的火灾温度 > g700 °C 似乎不太可能。然而，线虫的反射率非常高，这是一个具有明显真菌和地衣真菌亲和力的群体（Wellman 和 Ball，2021 年），支持达到了这些温度。基于志留纪岩盐的 pO2 趋势表明大气水平上升然后下降在 442 和422 Ma（Brand 等人，2021；图 3）。442 Ma 时 pO2 的反算测量值为 14.3% ± 1.7%；在 425 Ma，20.4% ± 6.6% 和 24.7% ± 3.7%；在 422 Ma 时，19.8% ± 2.1%。Winnica (Ludlow, ca. 424 Ma; mT = w480 °C; Table S6) 和 Ludford Lane (Přídolí, ca. 423 Ma; Ro 1.03%–2.74%; x̄ = 1.67%; mT = w450 °C) 的炭化事件; 表 S6) 与这些数据一致。Nant Cwm-Ddu 的 Gorstian 发生火灾也与这些数据一致。局部强烈的早期志留纪燃烧与 GEOCARBSULFOR (Krause et al., 2018) 和 COPSE-reloaded (Lenton et al., 2018) 的粗振幅不一致。目前，在大多数热带环境中，磷是主要限制养分，而这一作用在高纬度地区下降到氮（Du et al., 2020）。这些现象的主要驱动因素是年平均气温和降水量，温度和降水的季节性（由气候驱动）以及土壤粘土比例（Du 等人，2020 年）。同样的标准也会影响我们的低纬度志留纪遗址（图 1），据预测，磷是初级生产力的限制因素。火灾会将磷从陆地转移到海洋，促进藻类光合作用并增加 pO2 水平（Kump，2014 年）。然而，强烈或频繁的火灾会影响现代寒冷沙漠生态系统中的生物结皮，导致细菌群落完全丧失，火灾前植物群的恢复时间长达数十年（Aanderud 等人，2019 年）。在志留纪，这些结壳包括蓝细菌、藻类、地衣真菌和早期的小型胚胎植物（Wellman 和 Ball，2021 年）。我们建议局部强烈的燃烧温度，在荷马早期的 Rumney 和在较小程度上在 Ludlow 的 Winnica 的记录中，pO2 的需求水平等于或可能高于 PAL。在这样的水平上，火灾一定是一个重要的全球现象，对光合生物甲壳糖菌群具有强烈的负面影响。pO2 与其他地球系统过程之间的关系似乎很可能在整个志留纪持续存在，直到发展出更具胚胎植物优势的植物群。来自威尔士 Rumney 早期荷马地层的木炭将地球上最早的火灾记录延长了 10 米到约 430 毫安。来自志留纪序列的木炭数据的频率表明，火灾并不罕见，而是至少从文洛克开始的陆地生物群落的一个既定部分。将我们研究的反射率数据与 Ludford Lane 的反射率数据（威尔士边界；Glasspool 等人，2004 年）相结合，表明最强烈的志留纪火灾发生在发展最少的生态系统中。泥盆纪木炭间隙符合箱模型预测，即在泥盆纪晚期更高维管植物上升之前 pO2 下降到 <16%。这种巧合可能意味着在火窗内调节 pO2 的反馈在志留纪有所不同。我们的数据进一步表明，志留纪的地球化学模型在准确反映当时 pO2 的幅度之前仍需要改进。感谢以下同事的项目协助：W. Kozłowski、A. Bodzioch、A. Gorski、S. Schultka， D. Edwards、C. Eble、L. Cherns、L. Axe、P. Hayes、S. Renshaw、A. Scott 和 K. Robak。该研究得到了美国的支持 美国国家科学基金会授予 EAR-1828359。我们非常感谢 Lee Kump 和两位匿名审稿人的帮助，他们的评论极大地改进了这份手稿。