What is time?
The geological timescale covers more than 4,000 million years (Fig. 1). This timescale is far beyond a human lifespan and therefore challenging to comprehend.
In order to scale the duration of geological processes, geologists make use of various reoccurring patterns in Earth history, including overarching mechanisms like for example plate tectonics (Wilson cycle), the Earth’s orbital movements (Milanković cycles), and the occurrence and disappearance of fossils (biostratigraphy).
In addition, radiometric dating of minerals reveals the timing of diverse regional and local processes, for instance the formation of igneous rock bodies, volcanic eruptions, sediment deposition, weathering, and authigenic mineral precipitation.
NORCRUST uses radiometric dating of carbonate minerals, specifically aragonite: One objective of the NORCRUST project is to decipher the chronology of seabed methane venting along the Norwegian continental margin.
A consequence of seabed methane venting is the formation of methane-derived authigenic carbonate crusts (MDAC) which can be dated. Where methane rich fluids ascend toward the seafloor, methane-oxidizing archaea and sulphate-reducing bacteria mediate the anaerobic oxidation of methane (AOM) in shallow sediments. The production of bicarbonate increases the carbonate alkalinity in the pore fluid, resulting in the precipitation of authigenic carbonate:
AOM: CH4 + SO42− → HCO3− + HS− + H2O
Carbonate precipitation: 2HCO3−+ Ca2+→ CaCO3 + CO2 + H2O
With the help of remotely operated vehicles (ROV), NORCRUST samples MDAC from the seafloor (Fig. 2). In addition, NORCRUST studies MDAC samples from drill cores down to ~24 meters below the seafloor.
In collaboration with the Geochronology and tracers facility of the British Geological Survey in Keyworth (U.K.), NORCRUST applies uranium–thorium (U/Th) dating of aragonite in order to reveal the timing of MDAC formation, hence past seabed methane venting.
In brief, this technique uses the radioactive decay and ingrowth relationships among 234U, and 230Th in aragonite crystals that grow from seawater. In oxygenated seawater, 234U is readily dissolved and gets incorporated into the aragonite crystal lattice, whereas 230Th is insoluble and thus is devoid in aragonite. Consequently the measured 230Th within the aragonite crystals derived from the radioactive decay of its mother nuclide 234U.
In particular, MDAC samples with relatively large, up to 2 cm thick, aragonite cement crusts were sampled using the micromill at BGS. By using the micromill equipped with a drill bit of 0.2 mm diameter a high-precision and high-resolution dataset is obtained (Fig. 3). After measuring the concentrations of 234U, 232Th, and 230Th the age of the MDAC sample can be calculated using the respective decay constants.
Knowing the U–Th ages of the MDAC samples allows for reconstructing the chronology of seabed methane venting. This chronology permits the assessment of the spatial and temporal evolution of seabed methane vents and indicates potential environmental drivers of methane release at the Norwegian seabed.