The evolution of oxygenic photosynthesis fundamentally altered the global environment, but the history of this metabolism prior to the Great Oxidation Event (GOE) at ca. 2.4 Ga remains unclear. Increasing evidence suggests that non-marine microbial mats served as localized “oxygen oases” for hundreds of millions of years before the GOE, though direct examination of redox proxies in Archean lacustrine microbial deposits remains relatively limited. We report spatially distinct patterns of positive and negative cerium (Ce) anomalies in lacustrine stromatolites from the 2.74 Ga Ventersdorp Supergroup (Hartbeesfontein Basin, South Africa), which indicate that dynamic redox conditions within ancient microbial communities were driven by oxygenic photosynthesis. Petrographic analyses and rare earth element signatures support a primary origin for Ce anomalies in stromatolite oxides. Oxides surrounding former bubbles entrained in mats (preserved as fenestrae) exhibit positive Ce anomalies, while oxides in stromatolite laminae typically contain strong negative Ce anomalies. The spatial patterns of Ce anomalies in Ventersdorp stromatolites are most parsimoniously explained by localized Ce oxidation and scavenging around oxygen bubbles produced by photosynthesis in microbial mats. Our new data from Ventersdorp stromatolites supports the presence of oxygenic photosynthesis ~300 m.y. before the GOE, and add to the growing evidence for early oxygen oases in Archean non-marine deposits.The release of free oxygen via oxygenic photosynthesis has shaped Earth's surface for billions of years, yet the onset and extent of early oxygen production remains uncertain. The youngest estimate for the origin of oxygenic photosynthesis is provided by the Great Oxidation Event (GOE) at ca. 2.4 Ga, when sedimentary and geochemical proxies indicate oxidizing surface environments (e.g., Farquhar, 2000; Holland, 2002). Two prevailing hypotheses exist regarding the relationship between early oxygen production and the GOE: (1) oxygenic photosynthesis evolved hundreds of millions of years before the GOE, but nutrient limitation, ecological competition, and redox buffering by abundant reducing compounds in Archean environments prevented oxygen accumulation in the atmosphere and oceans until ca. 2.4 Ga (Kump and Barley, 2007; Hao et al., 2020), or (2) oxygenic photosynthesis evolved at ca. 2.4 Ga, with high rates of microbial oxygen production overpowering global redox buffers to produce the GOE relatively rapidly (Fischer et al., 2016; Slotznick et al., 2022).Models of early oxygenic photosynthesis have proposed the presence of “oxygen oases”, areas where oxygen production locally overpowered redox buffers to produce transient oxidative events in the Archean rock record (Kasting, 1991; Olson et al., 2013; Riding et al., 2014). Non-marine microbial mats are frequently proposed as oxygen oases to reconcile Neoarchean evidence for oxidative continental weathering under a reducing atmosphere (Stüeken et al., 2012; Lalonde and Konhauser, 2015). Modern analogues for lacustrine oxygen oases occur in benthic microbial communities from Antarctic lakes, where cyanobacteria increase oxygen concentrations in mats without oxidizing the overlying water column (Sumner et al., 2015). Microbial mats preserved as stromatolites are more likely to record primary signatures for oxygenic photosynthesis than non-biogenic deposits, including oxygen bubbles in cyanobacterial cones preserved as rounded fenestrae at the tips of conical stromatolites (Bosak et al., 2009, 2010). Archean lacustrine stromatolites therefore provide clear targets to test for oxygen oases (Buick, 1992; Wilmeth et al., 2019).The ca. 2.74 Ga Hartbeesfontein Basin (Rietgat Formation, Ventersdorp Supergroup) in South Africa (Fig. 1) is an isolated half-graben of the Ventersdorp continental rift; its lacustrine setting is further supported by frequent, meter-scale facies shifts and intercalation with subaerial volcanic deposits (Karpeta, 1989; Wilmeth et al., 2019). The basin contains extensive stromatolitic chert beds with exquisite microbial textures including abundant rounded fenestrae that are interpreted as former bubbles formed by microbial gas production (Figs. 1 and 2) (Wilmeth et al., 2019). Fenestrae occur throughout the stromatolites and are not localized at cone apices (Wilmeth et al., 2019); similar mat textures can form through various metabolisms producing oxygen, methane, or other gases (Hoehler et al., 2001; Mata et al., 2012). Therefore, while Hartbeesfontein fenestrae indicate microbial gas production, additional evidence is required to determine the specific gases. We present new major and rare earth element (REE) data from Hartbeesfontein fenestral stromatolites to investigate the presence or absence of oxygenic photosynthesis in Archean lacustrine mats. We show distinct patterns of cerium (Ce) anomalies in fenestral and laminated stromatolite textures indicating localized oxygen production in microbial communities. Positive Ce anomalies around fenestrae are interpreted as oxidative Ce scavenging onto oxides around former bubbles, and negative anomalies in laminae indicate oxide precipitation where Ce has already been removed from oxidizing solutions.Hartbeesfontein stromatolites contain three distinct oxide assemblages: in bubble fenestrae, stromatolite lamination, and weathering surfaces. Fenestral oxides appear orange and white in reflected light and reside at the contacts between fenestral walls and megaquartz, pore-filling cements, indicating early emplacement before cementation (Fig. 2; Fig. S3 in the Supplemental Material1; Wilmeth et al., 2019). Elemental composition of fenestral oxides measured by electron microprobe analyses (EMPA) revealed goethite, titanite, and a Mn-rich phase (Table S1). In contrast, oxides in stromatolite laminae are composed of isolated hematite crystals with associated goethite, as identified by Raman spectroscopy, EMPA, and reflected light microscopy (Fig. 3; Fig. S3; Table S1). Laminar oxides (10−30 µm in diameter) are visually distinct from fenestral oxides and exhibit metallic luster and variable black, red, and yellow coloration (Fig. 3). EMPA backscatter imaging revealed hexagonal and rhombohedral hematite crystal habits with partial dissolution features frequently containing goethite, supported by Raman spectra and light microscopy (Fig. 3; Fig. S3; Table S1). Recent surficial weathering surfaces also contain abundant red-orange iron oxides that cross-cut through primary fabrics (Fig. 3F).Laser ablation–high-resolution–inductively coupled plasma–mass spectrometry (LA-HR-ICP-MS) of Hartbeesfontein stromatolites revealed distinct trace element signatures that differentiate fenestral, laminar, and surficial oxides (Fig. 4; Fig. S1). Recent weathering surfaces display flat REE signatures with mild negative Ce anomalies when normalized to post-Archean Australian shales (PAAS) and are an order of magnitude higher in total REE than crustal abundances (Fig. 4A). By contrast, laminar oxides deeper in stromatolite samples are REE-poor (~0.01–0.5 × crustal values) and show strong heavy REE (HREE)/light REE (LREE) enrichments (Fig. 4A). Fenestral oxides show intermediate total REE concentrations (~0.5–5 × crustal values) and lesser degrees of HREE enrichment.Remarkably, laminar oxides frequently exhibit strong negative Ce anomalies, as low as 0.2 Ce/Ce*, with corresponding Pr/Pr* as high as 1.74 (Fig. 4B; calculations as per Bau and Dulski, 1996). Ce/Ce*, calculated on a log-linear scale according to Lawrence and Kamber (2006), reaches 0.21. In contrast, fenestral oxides contain significant positive Ce anomalies that reach 1.56 Ce/Ce* (Pr/Pr* as low as 0.70), corresponding to 2.02 Ce/Ce* on the same log-linear scale. One ICP-MS laser shot yielded an incomplete REE data set from which Ce anomalies could not be evaluated, and three anomalous laser shots contained significantly elevated LREE and Pb concentrations, which are not included in the figures but are noted in Table S2 and discussed in the Supplemental Material.Previous petrographic analyses of Hartbeesfontein stromatolites (Wilmeth et al., 2019), such as paragenetic sequences of fenestral cements, indicate that fenestral and laminar oxides are in situ, derived from precursor minerals precipitated in Archean microbial mats. However, subsequent greenschist-grade metamorphism (Crow and Condie, 1988) and surficial weathering must be addressed before considering the environmental interpretations of REE signatures in oxide minerals. For example, hematite in Hartbeesfontein stromatolites is interpreted as the metamorphic product of metastable iron oxide precursors such as ferrihydrite, and goethite in laminar oxides is interpreted as secondary oxy-hydroxide precipitation in partially dissolved hematite crystals during surficial weathering.REE patterns provide a powerful metric for distinguishing chemical signatures inherited during primary iron mineral precipitation (as indicated by petrography) versus secondary alteration (as indicated by mineralogy), as REEs are relatively immobile under most post-depositional conditions. Cerium anomalies are particularly attractive as a redox proxy, as oxidation from Ce(III) to Ce(IV) renders Ce more immobile relative to its neighbors, leading to its enhanced removal onto reactive particles such as Fe- and Mn-hydroxides (Byrne and Sholkovitz, 1996). The removal of Ce in oxidizing solutions produces negative Ce anomalies in associated chemical sediments, while Ce-scavenging particles themselves will contain positive Ce anomalies (Sholkovitz et al., 1994).Alteration can produce secondary Ce anomalies after deposition (Hayashi et al., 2004; Bonnand et al., 2020; Planavsky et al., 2020). For example, exposed rock is more likely to contain Ce anomalies than less-weathered drill core (Planavsky et al., 2020), and Ce anomalies generated by Cenozoic alteration have been demonstrated in Paleoarchean rocks (Hayashi et al., 2004; Bonnand et al., 2020). Post-depositional negative Ce anomalies are best explained by secondary LREE enrichment from Ce-depleted aqueous solutions (Hayashi et al., 2004; Bonnand et al., 2020). Laminar and fenestral oxides have low LREE/HREE ratios (~0.1–0.01), which argues against significant post-depositional REE mobility. In contrast, secondary weathering surfaces on Hartbeesfontein stromatolites have negative Ce anomalies but are clearly differentiated from laminar and fenestral oxides by elevated REE concentrations (~10× crustal values) and high LREE/HREE ratios (Fig. 4A; Table S2). Therefore, while the current mineralogy of iron oxides in Hartbeesfontein stromatolites indicates a certain degree of secondary alteration, REEs in specific stromatolite textures (laminae and fenestrae) appear to maintain primary signatures of Archean lake chemistry.The REE signatures of fenestral and laminar oxides in Hartbeesfontein stromatolites support previous interpretations of a lacustrine depositional environment (Karpeta, 1989). Marine Archean deposits typically contain yttrium/holmium ratios of >45 g/g (Kamber et al., 2004; Bolhar and van Kranendonk, 2007), and positive Eu anomalies indicate elevated hydrothermal input (Derry and Jacobsen, 1990). Both features are absent in Hartbeesfontein REE data (Fig. 4A) and other non-marine Archean deposits such as the Fortescue Group in Australia (Bolhar and van Kranendonk, 2007). Unlike Fortescue Group carbonates, Hartbeesfontein oxides are relatively depleted in LREE (~0.01–0.1× crustal values; Fig. 4A). While LREE depletion is more frequently observed in marine than non-marine deposits, lakes can exhibit a wide variety of LREE/HREE ratios, such as LREE-depleted waters with negative Ce anomalies in Lake Tanganyika, Africa (Barrat et al., 2000).Positive Ce anomalies in Hartbeesfontein fenestral oxides indicate Ce sorption onto mineral surfaces under oxidizing conditions (Byrne and Sholkovitz, 1996). Oxidative Ce scavenging is especially prevalent on Mn- and Fe-Mn oxide surfaces (Byrne and Sholkovitz, 1996; De Carlo et al., 1997), and positive Ce anomalies in fenestral oxides are positively correlated with Mn concentration in Hartbeesfontein stromatolites (Fig. 4C). Conversely, negative Ce anomalies in laminar oxides (Fig. 4) indicate mineral precipitation in lake waters where Ce and Mn had previously been scavenged. The presence of positive and negative Ce anomalies in different stromatolite fabrics has two potential explanations, both of which require the presence of oxygenic photosynthesis in Archean lakes.The presence of positive and negative Ce anomalies in Hartbeesfontein stromatolites could represent Ce shuttling across a redoxcline in the surrounding water column. In modern environments, Fe-Mn oxides remove Ce from oxygen-rich surficial waters and sink to lower depths (German et al., 1991; Byrne and Sholkovitz, 1996). Oxide dissolution in reducing deeper zones releases Ce back into surrounding waters, and chemical precipitates in such Ce-enriched areas will exhibit positive Ce anomalies (Glasby et al., 1987). In such a scenario, Hartbeesfontein stromatolites record a dynamic lacustrine chemocline that shifted above and below benthic microbial mats over time, where Ce-depleted laminar oxides precipitated in oxidizing lake waters, while Ce-enriched fenestral oxides formed in reducing zones as chemoclines shifted above mats.Alternatively, the specific distribution of positive Ce anomalies in Hartbeesfontein fenestrae is more parsimoniously explained by localized Mn oxidation around oxygen bubbles entrained in microbial mats (Figs. 2E and 3E). Mn and Ce sequestration in fenestral oxides would correspondingly deplete concentrations in the surrounding layers of stromatolites, producing negative Ce anomalies in laminar oxides farther away from fenestrae. In modern mats, oxygen bubbles form similar loci for oxide precipitation in cyanobacterial communities (Raudsepp, 2012; Wilmeth et al., 2019). In anaerobic experiments simulating Archean environments, Raudsepp (2012) noted a wide variability in Mn/Fe ratios from oxides that precipitated in the same microbial mat (between <1:1 to >5:1). Therefore, Mn and Ce concentrations in stromatolites can vary due to redox gradients in microbial mats and do not necessarily require a shifting redoxcline in lake waters.In Hartbeesfontein stromatolites, distributions of positive Ce anomalies around bubble fenestrae, surrounded by mat textures containing negative Ce anomalies, support the interpretation of fenestrae as loci of intense oxygenic photosynthesis in lacustrine microbial mats. While oxygen produced by the subaqueous stromatolites in our study is unlikely to have affected the oxidation of nearby terrestrial surfaces (e.g., Sumner et al., 2015), evidence for oxygenic photosynthesis in Archean lakes supports the presence of microbial mats in terrestrial environments contributing to early signals of oxidative weathering (Stüeken et al., 2012; Lalonde and Konhauser, 2015).Geochemical, petrographic, and sedimentary data from 2.7 Ga Hartbeesfontein stromatolites support the evolution of oxygenic photosynthesis at least 300 m.y. before the GOE, which substantiates previous evidence from non-biogenic sedimentary deposits and molecular clocks. Localized oxygenic photosynthesis in lacustrine stromatolites also strengthens hypotheses of non-marine benthic mats as oxygen oases before the GOE, as predicted by geochemical models, observations of modern microbial mats, and phylogenetic analyses of cyanobacteria. The presence of sub-meter–scale oxygen oases at ca. 2.7 Ga helps to reconcile previous Neoarchean evidence for localized, oxidative continental weathering under a reducing atmosphere.This work was supported by a U.S. National Science Foundation Graduate Research Fellowship, the Lewis and Clark Fund for Exploration and Field Research, and a student research grant from the Geological Society of America to D.T. Wilmeth; the European Union's Horizon 2020 Research and Innovation Program (716515) to S.L. Lalonde, a NASA Astrobiology Rock Powered Life Grant and Yellowstone Center for Resources for Research Permit (5664) to J.R. Spear, and a NASA Exobiology grant (80NSSC19K0479) to F.A. Corsetti. We thank the University of Johannesburg (South Africa) and Alyssa Bell for fieldwork support in summer 2014, Sami Nabhan for Raman assistance and discussion of iron diagenesis, and the engineers of the Pôle Spectrométrie Océan (Plouzané, France; Bleuenn Guéguen and Marie Laure Rouget) and Microsonde Ouest (University of Brest, France; Jessica Laglande) for their analytical support.

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新太古代湖相叠层石中产生底栖氧气的证据

含氧光合作用的演变从根本上改变了全球环境，但这种新陈代谢的历史发生在大约 20 年的大氧化事件 (GOE) 之前。2.4 Ga 仍不清楚。越来越多的证据表明，非海洋微生物垫在 GOE 之前的数亿年里一直作为局部“氧气绿洲”，尽管对太古代湖相微生物沉积物中氧化还原代理的直接检查仍然相对有限。我们报告了来自 2.74 Ga Ventersdorp 超群（南非哈特比斯方丹盆地）的湖相叠层石中铈 (Ce) 正负异常的空间不同模式，这表明古代微生物群落内的动态氧化还原条件是由含氧光合作用驱动的。岩石学分析和稀土元素特征支持叠层石氧化物中 Ce 异常的主要来源。垫层中夹带的前气泡周围的氧化物（保留为窗孔）表现出正的 Ce 异常，而叠层石层中的氧化物通常包含强烈的负 Ce 异常。Ventersdorp 叠层石中 Ce 异常的空间模式最简洁地解释为局部 Ce 氧化和清除微生物垫中光合作用产生的氧气泡周围。我们来自 Ventersdorp 叠层石的新数据支持在 GOE 之前约 300 米的氧光合作用的存在，并为太古宙非海洋沉积物中的早期氧绿洲增加了越来越多的证据。通过氧光合作用释放的游离氧已经塑造了地球表面数十亿多年来，然而，早期氧气产生的开始和程度仍然不确定。对含氧光合作用起源的最年轻估计是由 ca. 的大氧化事件 (GOE) 提供的。2.4 Ga，当沉积物和地球化学指标表明地表氧化环境时（例如，Farquhar，2000；Holland，2002）。关于早期氧气产生与 GOE 之间的关系存在两个普遍的假设：（1）氧气光合作用在 GOE 之前数亿年进化，但太古宙环境中丰富的还原性化合物的营养限制、生态竞争和氧化还原缓冲阻止了氧气的积累在大气和海洋中，直到 ca。2.4 Ga (Kump and Barley, 2007; Hao et al., 2020)，或 (2) 含氧光合作用在 ca 进化。2.4 镓，微生物的高产氧率压倒了全球氧化还原缓冲液，从而相对快速地产生了 GOE（Fischer 等人，2016 年；Slotznick 等人，2022 年）。早期含氧光合作用模型提出了“氧气绿洲”的存在，其中氧气产生局部超过了氧化还原缓冲液，从而在太古宙岩石记录中产生了短暂的氧化事件（Kasting，1991；Olson 等人，2013；Riding 等人，2014）。非海洋微生物垫经常被提议作为氧气绿洲，以调和新太古代在还原气氛下大陆氧化风化的证据（Stüeken 等，2012；Lalonde 和 Konhauser，2015）。湖泊氧绿洲的现代类似物存在于南极湖泊的底栖微生物群落中，其中蓝藻增加垫子中的氧气浓度而不氧化上覆的水柱（Sumner 等人，2015 年）。作为叠层石保存的微生物垫比非生物沉积物更有可能记录含氧光合作用的主要特征，包括在锥形叠层石尖端保存为圆形窗孔的蓝藻锥中的氧气泡（Bosak 等，2009, 2010）。因此，太古宙湖相叠层石为测试氧绿洲提供了明确的目标（别克，1992 年；Wilmeth 等人，2019 年）。南非2.74 Ga Hartbeesfontein盆地（Rietgat组，Ventersdorp Supergroup）（图1）是Ventersdorp大陆裂谷的一个孤立的半地堑；频繁的米级相移和与陆上火山沉积物 (Karpeta, 1989; Wilmeth 等人，2019）。该盆地包含广泛的叠层石燧石床，具有精致的微生物结构，包括丰富的圆形窗孔，这些窗孔被解释为由微生物产气形成的前气泡（图 1 和 2）（Wilmeth 等人，2019 年）。窗孔出现在整个叠层石中，并不局限于锥顶（Wilmeth 等人，2019 年）；类似的垫子纹理可以通过产生氧气、甲烷或其他气体的各种新陈代谢形成（Hoehler 等人，2001；Mata 等人，2012）。因此，虽然 Hartbeesfontein 窗孔表明微生物气体的产生，但需要额外的证据来确定特定的气体。我们提供了来自 Hartbeesfontein 窗状叠层石的新的主要和稀土元素 (REE) 数据，以研究太古代湖相垫中是否存在含氧光合作用。我们在窗孔和叠层叠层石纹理中显示出明显的铈 (Ce) 异常模式，表明微生物群落中局部产氧。窗孔周围的正 Ce 异常被解释为氧化 Ce 清除到前气泡周围的氧化物上，薄片中的负异常表明 Ce 已经从氧化溶液中去除的氧化物沉淀。Hartbeesfontein 叠层石包含三种不同的氧化物组合：在气泡窗中，叠层石层压，和风化表面。窗孔氧化物在反射光下呈橙色和白色，存在于窗孔壁和巨型石英、孔隙填充胶结物之间的接触处，表明胶结前早期就位（图 2；补充材料 1 中的图 S3；Wilmeth 等人，2019） . 通过电子探针分析 (EMPA) 测量的窗孔氧化物的元素组成揭示了针铁矿、钛铁矿和富锰相（表 S1）。相比之下，叠层石层中的氧化物由孤立的赤铁矿晶体和相关的针铁矿组成，如拉曼光谱、EMPA 和反射光显微镜所确定的（图 3；图 S3；表 S1）。层状氧化物（直径 10-30 µm）在视觉上与窗状氧化物不同，具有金属光泽和可变的黑色、红色和黄色颜色（图 3）。EMPA 背散射成像揭示了六方和菱面体赤铁矿晶体习性，具有部分溶解特征，通常含有针铁矿，得到拉曼光谱和光学显微镜的支持（图 3；图 S3；表 S1）。最近的地表风化表面还含有丰富的橙红色氧化铁，它们穿过原始织物（图 3F）。 Hartbeesfontein 叠层石的激光烧蚀-高分辨率-电感耦合等离子体质谱法 (LA-HR-ICP-MS)揭示了区分窗状、层状和表面氧化物的不同微量元素特征（图 4；图 S1）。当归一化为太古代后澳大利亚页岩（PAAS）时，最近的风化表面显示出平坦的 REE 特征，具有轻微的负 Ce 异常，并且总 REE 比地壳丰度高一个数量级（图 4A）。相比之下，叠层石样品中较深的层状氧化物缺乏稀土元素（~0.01-0.5×地壳值），并显示出强烈的重稀土（HREE）/轻稀土（LREE）富集（图4A）。窗孔氧化物显示中间的总 REE 浓度（~0. 5-5 × 地壳值）和较小程度的 HREE 富集。值得注意的是，层状氧化物经常表现出强烈的负 Ce 异常，低至 0.2 Ce/Ce*，相应的 Pr/Pr* 高达 1.74（图 4B；计算根据 Bau 和 Dulski，1996）。根据 Lawrence 和 Kamber (2006) 在对数线性标度上计算的 Ce/Ce* 达到 0.21。相比之下，窗孔氧化物包含显着的正 Ce 异常，达到 1.56 Ce/Ce*（Pr/Pr* 低至 0.70），对应于相同对数线性标度上的 2.02 Ce/Ce*。一次 ICP-MS 激光照射产生了一个不完整的 REE 数据集，无法从中评估 Ce 异常，三个异常激光照射包含显着升高的 LREE 和 Pb 浓度，这些未包含在图中，但在表 S2 中进行了说明，并在补充材料。先前对 Hartbeesfontein 叠层石的岩相学分析（Wilmeth 等人，2019 年），例如窗孔胶结物的共生序列，表明窗孔和层状氧化物是原位的，源自太古宙微生物垫中沉淀的前体矿物。然而，在考虑对氧化物矿物中 REE 特征的环境解释之前，必须解决随后的绿片岩级变质作用（Crow 和 Condie，1988 年）和地表风化。例如，Hartbeesfontein 叠层石中的赤铁矿被解释为亚稳态氧化铁前体（如水铁矿）的变质产物，层状氧化物中的针铁矿被解释为表面风化过程中部分溶解的赤铁矿晶体中的二次氢氧化物沉淀。REE 模式提供了一个强大的度量标准，用于区分在初级铁矿物沉淀（如岩相学所示）与二次改变（如矿物学所示）期间遗传的化学特征，因为 REE 在大多数沉积后条件下相对固定。铈异常作为氧化还原代理特别有吸引力，因为从 Ce(III) 到 Ce(IV) 的氧化使 Ce 相对于其相邻元素更加固定，导致其在反应性颗粒（如 Fe-和 Mn-氢氧化物）上的去除增强（Byrne 和肖尔科维茨，1996）。氧化溶液中 Ce 的去除会在相关的化学沉积物中产生 Ce 负异常，而 Ce 清除颗粒本身将包含 Ce 正异常（Sholkovitz 等，1994）。沉积后会产生次生 Ce 异常（Hayashi 等， 2004；Bonnand 等人，2020 年；Planavsky 等人，2020 年）。例如，与风化程度较低的钻芯相比，裸露的岩石更可能含有 Ce 异常（Planavsky 等，2020），并且已在古太古代岩石中证实了新生代蚀变产生的 Ce 异常（Hayashi 等，2004；Bonnand 等等人，2020）。沉积后的负 Ce 异常最好用 Ce 耗尽的水溶液中的二次 LREE 富集来解释（Hayashi 等人，2004；Bonnand 等人，2020）。层状和窗状氧化物具有低 LREE/HREE 比率（~0.1-0.01），这与显着的沉积后 REE 迁移率相矛盾。相比之下，Hartbeesfontein 叠层石上的次生风化表面具有负的 Ce 异常，但通过升高的 REE 浓度（~10× 地壳值）和高 LREE/HREE 比率（图 4A；表 S2）与层状和窗状氧化物明显区分开来。因此，虽然目前 Hartbeesfontein 叠层石中铁氧化物的矿物学表明存在一定程度的二次蚀变，但特定叠层石结构（薄片和窗孔）中的 REE 似乎保持了太古代湖化学的主要特征。 Hartbeesfontein 中窗和层状氧化物的 REE 特征叠层石支持先前对湖相沉积环境的解释（Karpeta，1989）。海洋太古代沉积物通常含有 >45 g/g 的钇/钬比率（Kamber 等，2004；Bolhar 和 van Kranendonk，2007），和正的 Eu 异常表明热液输入升高（Derry 和 Jacobsen，1990）。Hartbeesfontein REE 数据（图 4A）和其他非海洋太古宙矿床（如澳大利亚的 Fortescue 组）中不存在这两个特征（Bolhar 和 van Kranendonk，2007 年）。与 Fortescue Group 碳酸盐不同，Hartbeesfontein 氧化物在 LREE 中相对贫化（~0.01-0.1×地壳值；图 4A）。虽然在海洋中比非海洋沉积物中更频繁地观察到 LREE 耗竭，但湖泊可以表现出各种 LREE/HREE 比率，例如非洲坦噶尼喀湖的 LREE 耗竭水域与 Ce 负异常 (Barrat et al., 2000) . Hartbeesfontein 窗孔氧化物中的 Ce 阳性异常表明 Ce 在氧化条件下吸附在矿物表面上（Byrne 和 Sholkovitz，1996 年）。氧化 Ce 清除在 Mn-和 Fe-Mn 氧化物表面上尤为普遍（Byrne 和 Sholkovitz，1996；De Carlo 等，1997），并且窗状氧化物中的正 Ce 异常与 Hartbeesfontein 叠层石中的 Mn 浓度正相关（图 1）。 4C)。相反，层状氧化物中的负 Ce 异常（图 4）表明之前已清除 Ce 和 Mn 的湖水中有矿物沉淀。在不同的叠层石结构中存在正负 Ce 异常有两种可能的解释，这两种解释都需要太古代湖泊中存在氧光合作用。Hartbeesfontein 叠层石中存在正负 Ce 异常可能代表 Ce 在太古宙中穿梭穿过氧化还原层。周围的水柱。在现代环境中，Fe-Mn 氧化物从富氧地表水中去除 Ce 并沉入较低深度（German 等，1991；Byrne 和 Sholkovitz，1996）。还原较深区域的氧化物溶解将 Ce 释放回周围水域，在这种 Ce 富集区域中的化学沉淀物将表现出 Ce 正异常（Glasby 等人，1987 年）。在这种情况下，Hartbeesfontein 叠层石记录了一个动态的湖泊化学跃层，随着时间的推移在底栖微生物垫的上方和下方移动，其中缺铈的层状氧化物沉淀在氧化的湖水中，而富含铈的窗状氧化物在还原区形成，因为化学跃层在垫子上方移动或者，Hartbeesfontein 窗孔中 Ce 正异常的具体分布更简洁地解释为微生物垫中夹带的氧气泡周围的局部 Mn 氧化（图 1-2）。2E 和 3E)。窗孔氧化物中 Mn 和 Ce 的螯合会相应地消耗叠层石周围层中的浓度，在远离窗孔的层状氧化物中产生负 Ce 异常。在现代垫子中，氧气泡在蓝藻群落中形成类似的氧化物沉淀位点（Raudsepp，2012；Wilmeth 等人，2019）。在模拟太古宙环境的厌氧实验中，Raudsepp (2012) 注意到在同一微生物垫中沉淀的氧化物的 Mn/Fe 比率存在很大差异（在 <1:1 到 >5:1 之间）。因此，叠层石中的 Mn 和 Ce 浓度可能会因微生物垫中的氧化还原梯度而变化，并且不一定需要在湖水中发生变化的氧化还原层。在 Hartbeesfontein 叠层石中，气泡窗周围的正 Ce 异常分布，被包含负 Ce 异常的垫子纹理包围，支持将窗孔解释为湖泊微生物垫中强烈的含氧光合作用的位点。虽然我们研究中水下叠层石产生的氧气不太可能影响附近陆地表面的氧化（例如，Sumner 等，2015），但太古宙湖泊中含氧光合作用的证据支持陆地环境中微生物垫的存在有助于氧化风化的早期信号（Stüeken 等人，2012；Lalonde 和 Konhauser，2015）。来自 2.7 Ga Hartbeesfontein 叠层石的地球化学、岩石学和沉积数据支持在 GOE 之前至少 300 米的含氧光合作用演化，这证实了先前的证据来自非生物沉积沉积物和分子钟。正如地球化学模型、现代微生物垫的观察和蓝藻的系统发育分析所预测的那样，湖相叠层石中的局部含氧光合作用也加强了非海洋底栖垫在 GOE 之前作为氧绿洲的假设。大约存在亚米级氧气绿洲。2.7 Ga 有助于调和以前的新太古代证据，即还原气氛下的局部氧化大陆风化。这项工作得到了美国国家科学基金会研究生研究奖学金、刘易斯和克拉克探索和实地研究基金以及来自美国地质学会致 DT Wilmeth；欧盟地平线 2020 研究和创新计划 (716515) 到 SL Lalonde，美国宇航局天体生物学岩石动力生命补助金和黄石资源中心研究许可证 (5664) 授予 JR 斯皮尔，美国宇航局外生物学补助金 (80NSSC19K0479) 授予 FA Corsetti。我们感谢约翰内斯堡大学（南非）和 Alyssa Bell 在 2014 年夏季的实地工作支持，Sami Nabhan 对拉曼的帮助和铁成岩作用的讨论，以及 Pôle Spectrométrie Océan 的工程师（法国普劳扎内；Bleuenn Guéguen 和 Marie Laure Rouget ) 和 Microsonde Ouest（法国布雷斯特大学；Jessica Laglande）的分析支持。