| Aquitard |
Low permeability material that prevents or greatly retards water movement between or within an aquifer. |
| Biogeochemical facies |
A geological unit with specific biogeochemical properties such as redox chemistry or reactivity. |
| Bray-Curtis |
Multivariate statistic that uses OTU relative abundances to quantify the difference in community composition between a pair of communities. |
| Ecological selection |
Selection is the result of biotic and abiotic pressures causing variation in reproductive success across individuals and species. |
| Fermentation |
Energy yielding reaction in which organic compounds are the primary electron donor and the ultimate electron acceptor. |
| Fluvio-lacustrine |
Sediments deposited by ancient rivers or lakes. |
| Framboid |
Spherical micromorphology of pyrite (FeS2) common to some anoxic sediments. |
| Lithofacies |
A geologic unit with specific petrological characteristics such as grain size and mineralogy. |
| β-mean nearest taxon distance (βMNTD) |
Multivariate statistic that (i) uses the phylogenetic distance between each OTU in one community and its closest relative in a second community to (ii) quantify the average phylogenetic distance between two communities. |
| β-nearest taxon index (βNTI) |
Quantifies the difference between observed βMNTD and the mean of the βMNTD null distribution, in units of standard deviations. |
| Nonmetric multidimensional scaling (NMDS) |
Statistical method used to collapse high-dimensional data onto a small number of axes to facilitate interpretation and visualization. |
| Null model |
Randomization used here to generate βMNTD values expected if community assembly was not influenced by ecological selection; repeating the randomization many times provides a distribution of null βMNTD values, used in the calculation of βNTI. |
| Operational Taxonomic Unit (OTU) |
16S rRNA gene sequences similar enough to each other—based on a pre-defined threshold—to be grouped together prior to statistical analysis. |
| Redox |
Contraction of oxidation-reduction; net oxidation state of sediment based on the oxidation state of individual elements (i.e., Mn, Fe, S). |
| OTU richness |
The number of unique OTUs present in a sample or community. |
| Terminal electron acceptor (TEA) |
Oxidized form of an element used by microorganisms for the biochemical oxidation of organic carbon or reduced inorganic compounds (electron donors) to generate energy. |
| Transition zone |
Spatial domains where properties of interest (e.g., redox) change dramatically over distances much shorter than the scale of the whole system. |
