Formation of seep carbonates along the Makran convergent margin, northern Arabian Sea and a molecular and isotopic approach to constrain the carbon isotopic composition of parent methane.

Author(s)

Tobias Himmler, Daniel Birgel, Germain Bayon, Thomas Pape, Lu Ge, Gerhard Bohrmann, Jörn Ludwig Peckmann

Abstract

Authigenic carbonate deposits have been sampled with the remotely operated vehicle 'MARUM-QUEST 4000m' from five methane seeps between 731 and 1823m water depth along the convergent Makran continental margin, offshore Pakistan (northern Arabian Sea). Two seeps on the upper slope are located within the oxygen minimum zone (OMZ; ca. 100 to 1100m water depth), the other sites are situated in oxygenated water below the OMZ (below 1100m water depth). The carbonate deposits vary with regard to their spatial extent, sedimentary fabrics, and associated seep fauna: Within the OMZ, carbonates are spatially restricted and associated with microbial mats, whereas in the oxygenated zone below the OMZ extensive carbonate crusts are exposed on the seafloor with abundant metazoans (bathymodiolin mussels, tube worms, galatheid crabs). Aragonite and Mg-calcite are the dominant carbonate minerals, forming common early diagenetic microcrystalline cement and clotted to radial-fibrous cement. The δ

¹⁸O

_carbonate values range from 1.3 to 4.2‰ V-PDB, indicating carbonate precipitation at ambient bottom-water temperature in shallow sediment depth. Extremely low δ

¹³C

_carbonate values (as low -54.6‰ V-PDB) point to anaerobic oxidation of methane (AOM) as trigger for carbonate precipitation, with biogenic methane as dominant carbon source. Prevalence of biogenic methane in the seepage gas is corroborated by δ

¹³C

_methane values ranging from -70.3 to -66.7‰ V-PDB, and also by back-calculations considering δ

¹³C

_methane values of carbonate and incorporated lipid biomarkers. These calculations (δδ

¹³C

_{methane-carbonate}, δδ

¹³C

_ANME-methane, δδ

¹³C

_MOX-methane) prove to be useful to assess the carbon stable isotope composition of seeping methane if this has not been determined in the first place; such an approach represents a useful tool to reconstruct fluid composition of ancient seeps. AOM is also revealed by lipid biomarkers of anaerobic methane oxidizing archaea such as crocetane, pentamethylicosane (PMI), and sn2-hydroxyarchaeol strongly depleted in

¹³C (δ

¹³C values as low as -127‰ V-PDB). Biomarkers of sulphate-reducing bacteria are also abundant, showing slightly less negative δ

¹³C values, but still significantly

¹³C-depleted (average values as low as -101‰). Other bacterial biomarkers, such as bacteriohopanepolyols (BHPs), hopanols, and hopanoic acids are detected in most carbonates, but are particularly common in seep carbonates from the non-OMZ sites. The BHP patterns of these carbonates and their low δ

¹³C values resemble patterns of aerobic methanotrophic bacteria. In the shallower OMZ sites, BHPs revealed much lower contents and varying compositions, most likely reflecting other sources than aerobic methanotrophic bacteria.

²³⁰Th/U carbonate ages indicate that AOM-induced carbonate precipitation at the deeper non-OMZ seeps occurred mainly during the late Pleistocene-Holocene transition, i.e. between 19 and 15ka before present, when the global sea level was lower than today.

Organisation(s)

Department of Geology

External organisation(s)

Institut français de recherche pour l'exploitation de la mer (IFREMER), Nanjing University, Universität Bremen, Universität Hamburg

Journal

Chemical Geology

Volume

415

Pages

102-117

No. of pages

16

ISSN

0009-2541

DOI

https://doi.org/10.1016/j.chemgeo.2015.09.016

Publication date

11-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology, Geology

Portal url

https://ucrisportal.univie.ac.at/en/publications/formation-of-seep-carbonates-along-the-makran-convergent-margin-northern-arabian-sea-and-a-molecular-and-isotopic-approach-to-constrain-the-carbon-isotopic-composition-of-parent-methane(5e451232-a0bd-4d47-a8c4-45e387422b17).html