Identification of clathrites in the geological record
Research project funded by National Science Centre (no. 2019/35/B/ST10/01332)
Principal investigator: Maciej Bojanowski
Main collaborators: University of Torino (Italy), University of Modena and Reggio Emilia (Italy), GEOMAR (Germany)
Methane (gas) hydrates are composed of crystalline H2O that encages CH4 gas. They are found under the seafloor along continents at specific pressure and temperature conditions (Fig. 1). Changes of pressure/temperature at the seafloor, such as ocean warming or sea-level drop, may destabilize hydrates.
Fig. 1. A) Gas hydrate of type structure I. B) Stability field of methane hydrate at normal seawater salinity. Intersections of temperature profiles (stippled lines) with phase boundary (heavy line) define the area of the gas hydrate stability zone (GHSZ). C) Inferred thickness of the GHSZ in sediments at a schematic continental margin assuming a typical geothermal gradient. A-C after Bohrmann & Torres (2006)
Widespread dissociation of hydrates may lead not only to regional disasters, such as giant slope failures, but also to global warming, as CH4 is a much more effective greenhouse gas than CO2. Indeed, some of the most severe greenhouse conditions, such as the Paleocene-Eocene Thermal Maximum and mass extinctions, e.g. the end-Permian extinction, are linked to worldwide dissociation of hydrates (Fig. 2).
Fig. 2. The end-Permian mass extinction (after Brand et al., 2016). A) δ13C excursion, faunal disappearances and gas compositions. B) Sequence of events before and during the extinction. Upper panel shows extensive Siberian Trap Flood volcanism and emission of CO2 leading to climate warming, lower panel shows the subsequent release of massive amounts of CH4 hydrate and increased global warming.
Methane-derived authigenic carbonates (MDACs) precipitate due to anaerobic oxidation of methane at cold seeps. Gas hydrates commonly occur in genetic association with and directly within MDACs, which are termed “clathrites”. Since hydrates are not preserved in the rock record, ancient clathrites represent the only direct and tangible evidence of former hydrate occurrence, so they are particularly valuable research material. Yet, reports of fossil clathrites are very rare (none in the intervals representing hyperthermal events supposedly associated with hydrate breakdown) and their recognition is clearly underestimated. The main objective of this project is to diagnose the causes of this underrepresentation and provide solutions that would enhance the recognition of hydrate-associated authigenic carbonates in the geological record.
Fig. 3. Study areas and locations of the most important sections (blue stars): Hydrate Ridge on the northwestern continental margin of North America (A - modified from Bohrmann et al., 2002), Outer Carpathians (B - modified from Bojanowski, 2014), Piedmont Basin (C - modified from Dela Pierre et al., 2012) and Northern Apennines (D -modified from Conti et al., 2010).
The research will be carried out on MDACs from one modern (Hydrate Ridge) and three ancient sections (Piedmont, Apennines, Carpathians) through international cooperation with the researchers working on MDACs in these areas (Fig. 3). Petrographic (optical microscopy, CL, SEM, EMP) and bulk δ13C and δ18O analyses will chiefly result in identification of potential ancient clathrites, for which advanced geochemical investigations will subsequently be employed. Application of ion microprobe, which enables in-situ δ13C and δ18O analyses, will allow reconstruction of growth mechanism and fluid sources for successive cement generations separately, some of which may be related to hydrates. Clumped-isotope analysis will provide the paleotemperature and δ18O values of parent fluids, which will verify which cements were related to hydrates. In-situ U-Pb dating will provide a timeframe of these processes, including those related to hydrate formation or dissociation. It will also allow constraining the depth and relative timing of carbonate precipitation at different levels of paleo-seep systems examined.
Direct outcomes of the project:
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Bohrmann G., Torres M., 2006. Gas hydrates in marine sediments. In Schulz H.D., Zabel M. (eds) Marine Geochemistry, 2nd ed. Springer, Heidelberg, 481-512.
Bojanowski M.J., 2014. Authigenic dolomites in the Eocene–Oligocene organic carbon-rich shales from the Polish Outer Carpathians: evidence of past gas production and possible gas hydrate formation in the Silesian basin. Mar. Petrol. Geol. 51, 117-135.
Brand U. et al., 2016. Methane Hydrate: Killer cause of Earth's greatest mass extinction. Palaeoworld 25, 496-507.
Conti S. et al., 2010. A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy). Geo-Mar. Lett. 30, 449-460.
Dela Pierre F. et al., 2012. Messinian carbonate-rich beds of the Tertiary Piedmont Basin (NW Italy): microbially-mediated products straddling the onset of the salinity crisis. Paleogeogr., Palaeoclimatol., Palaeoecol. 344-345, 78-93.
https://drive.google.com/drive/u/0/folders/1IDSq5WmSp8r9bXlSDKzW-jeUfRi7llTX (access for collaborators)