Constraining the sources of archaeal tetraether lipids in multiple cold seep provinces of the Cascadia Margin

Archaeal isoprenoid glycerol dialkyl glycerol tetraether lipid (iGDGT) abundance profiles and carbon isotopic compositions reflect the relative distributions of archaeal sources, including planktonic, benthic, and methane-cycling contributions. Here, we analyze the carbon isotope ratios of iGDGTs purified from sediments of three different cold seep sites in Cascadia Margin, off the coast of Washington, USA. Together with relative abundance and glycerol configurations, we use the carbon isotope ratios to estimate the contributions of multiple archaeal sources to the sedimentary iGDGT assemblages and their impact on values of the TEX86 and methane indices. Using a Bayesian mixing model, we robustly characterize three potential endmembers by determining their characteristic lipid distributions, inferred contributions to the total sediment inventory, and carbon isotopic signatures. Despite the geographic proximity of the sample locations, we find site-specific heterogeneity in relative iGDGT abundances and δ13C values. Planktonic and benthic methane-cycling sources predominate in all cases (contributing > 98% of iGDGTs), while benthic non-methane cycling archaea contribute minimally to the sedimentary lipid pool. Environments with higher methane influence show an increased presence of anti-parallel iGDGTs, indicating that methane-cycling archaea may dominantly or exclusively synthesize iGDGTs in this configuration. Our results quantify the relationship between the methane index (MI) and methane impact in systems dominated by planktonic and benthic methane-cycling archaea. Within the framework of the TEX86 temperature proxy, this permits a quantitative demonstration that it is overly simplistic to apply a MI cutoff threshold as a binary indicator to determine methane influence, and caution is needed when taking this approach in paleoclimate reconstructions.

古菌类异戊二烯甘油二烷基甘油四醚脂质（iGDGT）的丰度分布和碳同位素组成反映了古菌来源的相对分布，包括浮游生物、底栖生物以及参与甲烷循环的贡献。在此，我们分析了从美国华盛顿海岸外卡斯卡迪亚大陆边缘三个不同冷泉地点的沉积物中提纯的iGDGT的碳同位素比值。结合相对丰度和甘油构型，我们利用碳同位素比值来估算多种古菌来源对沉积iGDGT组合的贡献以及它们对TEX86和甲烷指数数值的影响。利用贝叶斯混合模型，我们通过确定其特征脂质分布、对总沉积物存量的推断贡献以及碳同位素特征，有力地描述了三个潜在的端元。尽管样本位置在地理上相近，但我们发现iGDGT相对丰度和δ13C值存在特定地点的异质性。在所有情况下，浮游生物和底栖生物中参与甲烷循环的来源占主导地位（对iGDGT的贡献>98%），而底栖非甲烷循环古菌对沉积脂质库的贡献极小。受甲烷影响较大的环境中，反平行iGDGT的含量增加，这表明参与甲烷循环的古菌可能主要或完全以这种构型合成iGDGT。我们的研究结果量化了在以浮游生物和底栖生物中参与甲烷循环的古菌为主的系统中甲烷指数（MI）和甲烷影响之间的关系。在TEX86温度指标的框架内，这可以定量地证明将MI临界阈值作为一个二元指标来确定甲烷影响是过于简单化的，在古气候重建中采用这种方法时需要谨慎。