Coal balls are calcium carbonate accumulations that permineralized peat in paleotropical Permo-Carboniferous (∼320–250 Ma) mires. The formation of coal balls has been debated for over a century yet a widely applicable model is lacking. Two observations have been particularly challenging to explain: 1) the narrow temporal occurrence of coal balls and 2) their typical elemental (high Mg) and isotopic (low δ18O) composition that paradoxically indicate marine and freshwater origins, respectively. We evaluate a new model in which coal balls formed as CO2 escaped from the peat by diffusion through unusual air-filled spaces in the rootlets of lycopsid trees; critically, these trees were very common in paleotropical mires, and their evolutionary range matches the temporal range of coal balls. The episodic delivery of seawater and marine carbonate sediment to coastal mires is the first step in our model, although other pathways for the delivery of divalent cations are permissible. Subsequent dilution by freshwater and dissolution of these carbonates at the elevated CO2 of the mire subsurface is followed by the transport of CO2 through the rootlet airspaces and into the overlying water and atmosphere, which drives carbonate mineral precipitation in the sediment. We show that dilution by freshwater, because it minimally affects Mg/Ca ratios, can result in relatively low pore water δ18O values while allowing high-Mg calcite formation. This model thus explains the restriction of coal balls to the Permo-Carboniferous and resolves the discrepancy between elemental and isotopic compositions of coal ball carbonate minerals. Furthermore, we show with a 3D reactive transport model that CO2 could escape rapidly enough through the rootlets to fill 25 percent of the peat pore spaces with calcite before substantial burial (top decimeter of peat), explaining the exceptional preservation of coal swamp flora. Therefore, we suggest that coal balls are pedogenic in origin and that the disappearance of these pedogenically permineralized Histosols represents the first documented decrease in soil diversity on a vegetated planet. The rock record may thus provide important context for the modern loss and degradation of soils.

中文翻译：

---

CO2通过树状石蒜的根部脱气形成煤球

煤球是碳酸钙堆积物，使古热带二叠纪-石炭纪（~320-250 Ma）泥炭中的泥炭矿化。煤球的形成已经争论了一个多世纪，但缺乏一个广泛适用的模型。两个观察结果特别难以解释：1）煤球的时间范围很窄；2）它们典型的元素（高 Mg）和同位素（低 δ18O）组成，矛盾地分别表明海洋和淡水起源。我们评估了一个新模型，在该模型中，当二氧化碳通过石蜡树根部中不寻常的充满空气的空间扩散时，煤球从泥炭中逸出；至关重要的是，这些树木在古热带泥沼中非常普遍，它们的进化范围与煤球的时间范围相匹配。海水和海洋碳酸盐沉积物向沿海沼泽的间歇性输送是我们模型的第一步，尽管其他输送二价阳离子的途径也是允许的。随后被淡水稀释，这些碳酸盐在沼泽地下升高的 CO2 中溶解，随后 CO2 通过根部空间传输到上覆的水和大气中，从而推动沉积物中的碳酸盐矿物沉淀。我们表明淡水稀释，因为它对 Mg/Ca 比的影响最小，可以导致相对较低的孔隙水 δ18O 值，同时允许形成高镁方解石。因此，该模型解释了煤球对二叠纪-石炭系的限制，并解决了煤球碳酸盐矿物的元素和同位素组成之间的差异。此外，我们使用 3D 反应迁移模型显示，CO2 可以通过小根迅速逸出，在大量掩埋（泥炭的顶部分米）之前用方解石填充 25% 的泥炭孔隙空间，解释了煤沼泽植物群的特殊保存。因此，我们认为煤球起源于土壤，这些土壤透矿化组织溶胶的消失代表了植被星球上土壤多样性的首次减少。因此，岩石记录可以为现代土壤的流失和退化提供重要的背景。我们认为煤球起源于土壤，这些土壤透矿化组织溶胶的消失代表了植被星球上土壤多样性的首次减少。因此，岩石记录可以为现代土壤的流失和退化提供重要的背景。我们认为煤球起源于土壤，这些土壤透矿化组织溶胶的消失代表了植被星球上土壤多样性的首次减少。因此，岩石记录可以为现代土壤的流失和退化提供重要的背景。