What is time?

Publisert: 23. mai 2018 | Sist endret: 7. juni 2019

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The geological timescale covers more than 4,000 million years.

We are establishing a chronology of methane-release from the Norwegian seabed.

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.

Figure 1: The geological time is well organized in eons, eras, periods, and epochs by the keepers of time - The International Commission on Stratigraphy. (From http://www.stratigraphy.org/)

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.

Figure 2: Sampling of a methane-derived authigenic carbonate crust from the seabed in 1200 m water depth with the ROV manipulator arm.

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.

Using the micromill at BGS for high-precision sampling of methane-derived authigenic carbonates. Please note the drill bit used is 0.2 mm in diameter.

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.