Specific microbial biomarker signatures from Congo fan sediments of ODP site 175-1075, Congo soils, wetland and estuary sediments@en

Methane (CH4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of -30 per mil to -40 per mil for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.甲烷（CH₄）是一种强效温室气体，既往研究表明其可扰动全球气候，当在短时间内从陆源或海源大量释放时，这种影响尤为显著。因此，理解并尽可能量化这些多变的甲烷源对自然气候变率的单独与联合响应，至关重要。 然而，过往温室气体储库的稳定性变化仍不明确，且地质证据对其约束能力有限。 本研究报道了来自刚果扇的一则高特异性生物标志物记录，该标志物针对好氧甲烷氧化（aerobic methane oxidation, AMO）的细菌藿烷多醇（bacteriohopanepolyol, BHP）——35-氨基藿烷-30,31,32,33,34-五醇（aminopentol, 氨基五醇），可将增强的好氧甲烷氧化活动的离散时段追溯至1.2 Ma。 氨基五醇及其他35-氨基BHP的浓度波动呈现与晚第四纪冰期-间冰期气候旋回相关的模式，在暖期浓度达到峰值。 我们结合刚果河流域背景，探讨了氨基五醇的潜在来源以及作为其前体的甲烷氧化菌所代谢消耗的甲烷，其中涵盖了来自陆地流域及/或海源（天然气水合物及/或深部地下气藏）的甲烷氧化标志物输入。 大洋钻探计划（Ocean Drilling Program, ODP）1075站位的BHP化合物特异性碳同位素值介于-30‰至-40‰之间，且该岩芯的BHP特征与刚果河口以及刚果盆地内陆泛滥平原湿地的表层沉积物特征高度相似，这支持了扇沉积物中检出的35-氨基BHP具有甲烷氧化菌起源且大概率为陆源的结论。 这一新证据证实了热带深海扇的海洋沉积物BHP记录与补给河流集水区的湿地环境之间存在因果关联，进而与热带大陆水文循环紧密相关。 仍需开展进一步研究以更好地约束甲烷排放的不同来源与途径。不过本研究证实了氨基BHP（尤其是氨基五醇）在示踪过往甲烷源及其通量方面的巨大潜力，一旦完成更精准的校准与机理阐释，便可实现对陆源乃至潜在海源甲烷排放的定量分析。