NOAA/WDS Paleoclimatology - Polar Ice Core 42-77ka Excess Methane Data

Ice core records of atmospheric methane (CH4) and its isotopic composition provide important information about biogeochemical cycles in the past. Interpreting these data requires that they faithfully record the composition of the atmosphere. In this study, we describe anomalies of up to 30-40 ppb CH4 that are only observed in dust-rich (>~60 ng Ca/g ice), glacial-period ice measured with standard melt-refreeze methods. The stable isotopic composition of CH4 is also significantly affected. Results from the GISP2 and NEEM ice cores from Greenland show that excess CH4 is either released or produced in the presence of liquid water in amounts which are highly correlated with the abundance of Ca2+ and mineral dust in the sample. Additional experiments show that excess CH4 is unaffected by the addition of HgCl2 (a microbial inhibitor) and is not related to ice core storage time. Dust concentrations in Antarctic ice cores are an order of magnitude lower than in Greenlandic ice cores and no excess CH4 was observed in samples from the Antarctic WAIS Divide (WD) and South Pole (SPICE) ice cores. While the overall structure of the ice core atmospheric methane history is minimally impacted by excess CH4, the impacts on the isotopic record and on inverse models used to reconstruct CH4 sources are greater. We propose three potential mechanisms to explain the presence of excess CH4: (1) that CH4 is adsorbed on dust particles prior to deposition on the ice sheet and is slowly desorbed during the melt-extraction step of sample analysis; (2) that dust acts as a micro-environment within the ice sheet for methanogenic extremophiles; or (3) that excess CH4 is a product of abiotic degradation of organic compounds during the melt-extraction step of sample analysis.

大气甲烷（CH₄）及其同位素组成的冰芯（ice core）记录，可为过去的生物地球化学循环研究提供关键信息。对这些数据进行解译，需以其能够真实记录大气组成为前提。本研究中，我们报道了最高可达30～40 ppb的甲烷异常信号：此类异常仅在采用标准熔冻法测定的、富尘（钙含量>~60 ng钙/克冰）的冰期冰芯样品中被观测到。甲烷的稳定同位素组成亦会受到显著影响。来自格陵兰岛GISP2与NEEM冰芯的分析结果显示，液态水存在条件下会产生或释放过量甲烷，其生成/释放量与样品中钙离子（Ca²+）及矿物粉尘的丰度呈高度相关。额外对照实验表明，添加氯化汞（HgCl₂，微生物抑制剂）对过量甲烷的产生无影响，且该现象与冰芯的储存时长无关。南极冰芯中的粉尘浓度比格陵兰冰芯低一个数量级，且南极威兰斯冰架分冰芯（WAIS Divide，WD）与南极点冰芯（SPICE）的样品中未观测到过量甲烷信号。尽管过量甲烷对冰芯记录的大气甲烷历史整体框架影响极小，但其对同位素记录以及用于反演甲烷源重建的反演模型的影响则更为显著。我们提出三种潜在机制以解释过量甲烷的产生：（1）甲烷在沉积至冰盖前便吸附于粉尘颗粒之上，并在样品分析的熔样提取步骤中缓慢解吸；（2）粉尘可为冰盖内部的产甲烷极端微生物提供微生境；（3）过量甲烷是样品分析熔样提取步骤中，有机化合物发生非生物降解的产物。