Methodology:

For detailed lineage information, see the Lineage_README file.

Data collection:

Major ions (here Ca2+, Mg2+, Na+, K+, Cl-, NO3-, SO42-) were determined by ion chromatography on a Dionex ion chromatograph, DX 90, with a detection limit of 0.02 mg l-1 for the lowest, undiluted analysis. The precision errors were <5 % for the mid-range standards.

Trace metals were determined on 5 ml sample acidified with 50 ul reagent grade HNO3- (AnalaR 65% Normapur, VWR, IL, USA) , using Inductively Coupled Plasma Mass Spectrometry or ICPMS (PerkinElmer Elan DRC II, MA, USA). The precision errors for repeat analyses of mid-range standards were <5%, and the detection limits were 1.0 ug l-1. Concentrations were corrected for the blanks and normalised to the pore water content.

Water isotopes. The samples were analysed at the University of East Anglia, using a Picarro 1102i analyser, by direct injection of 2.6 microlitres of water. Samples were measured together with 2 USGS standards: USGS 64444 and USGS 67400 and UEA NTW internal laboratory standard. Using the calibration line defined by the USGS standards the true composition of the samples was calculated.

Volatile Fatty Acids. The Eppendorf tubes containing the samples were centrifuged at 13000 rpm for 5 minutes. 1 ml of the supernatant was pipetted into a GC vial. The samples, blanks and standards were acidified with 15 ul analytical grade orthophosphoric acid to ensure that the acid/salt ratio (i.e. acetic acid versus acetate ions) was weighted towards the acid. The Thermo ScientificTM TRACETM 1300 Gas Chromatograph was calibrated using dilutions of Acetic acid as well as dilutions of a Volatile Acid Mix (46975-U, Sigma) composed of Acetic acid, Propionic acid, Isobutyric acid, Butyric acid, Isovaleric acid, Valeric acid, Isocaproic acid, Caproic acid and Heptanoic acid. The R2 for all calibrations was >0.99, and the detection limits, calculated as the standard deviation of the 100 uM standard (n=5), were 15.8 uM for acetate, 30.2 uM for propionic acid, 26.8 uM for isobutyric acid, 26.7 uM for butyric acid, 26.7 uM for isovaleric acid, 31.2 uM for valeric acid, 29.0 uM for isocaproic acid, 32.7 uM for caproic acid and 36.7 uM for heptanoic acid.

Methane and carbon dioxide samples were measured on a Shimadzu GC-2014 Gas Chromatograph, using a 100 ul sample loop.

Sulphate and methane isotopes. The dry samples were weighed into tin or silver capsules for analysis of &#948;34SSO4 and &#948;18OSO4 respectively. Isotopic analysis was undertaken by Elemental Analyser (Elementar Pyrocube) linked to a continuous flow isotope ratio mass spectrometer (Isoprime 100 with dual inlet capability for injection of monitoring gases). For methane, an Agilent 7890B GC attached to an Isoprime100 Isotope Ratio Mass Spectrometer (IRMS) was employed.

Data quality:

Missing values (NAs) in this dataset spreadsheet relate to parameters that were not measured using a particular sampling strategy, e.g. the Macro Rhizons were not used for measuring dissolved gas concentrations, but the spike sampler was.