Biodetrital carbonates on the Adriatic continental shelf imprinted by oxidation of seeping hydrocarbons

Author(s)

Marco Taviani, Fulvio Franchi, Lorenzo Angeletti, Annamaria Correggiari, Matthias Lopèz-Correa, Vittorio Maselli, Claudio Mazzoli, Jörn Ludwig Peckmann

Abstract

The gas-productive Bonaccia area located at ca. 80-90 m below sea level offshore Ancona (Central Adriatic Sea) is site of hydrocarbon-derived carbonate production. The carbonates include large dm-sized slabs (bryozoan limestone), smaller concretionary aggregates, mudstones and pipes. The mudstones and botryoidal aragonite cements within limestones show δ

¹³C values as low as -47.8‰ VPDB, consistent with seepage of isotopically light hydrocarbons (e.g. methane). These hydrocarbon-derived carbonates commonly incorporate abundant shell remains, deriving from the prevalently coarse bioclastic-rich muddy deposits from post-glacial transgressive units. It is, therefore, hypothesized that hydrocarbon-rich fluids permeated the post-glacial sediments, resulting in seafloor seeps that were inhabited by chemosymbiotic lucinid bivalves and burrowing callianassid shrimps; fossils and traces of which have been found in the Bonaccia carbonates. Microbial oxidation of the reduced compounds contained in the seep fluids led to a locally patchy carbonate cementation of the sediments at the seep sites. The pipes are interpreted as decapod burrows that subsequently served as conduits for hydrocarbon leakage. Seepage is probably still active at present as testified by gas production at the study site. Interestingly, seep carbonates exhumed by erosion served as hard substrate for fouling benthos (i.e., bryozoans, oysters and red algae) in the Holocene. The main products of the processes at the Bonaccia study site are composite bryozoan-dominated limestones, whose multi-step and complex history was unfold thank to radiocarbon dating of key components and precise stratigraphic control. The Bonaccia case-study can serve as a model for the interpretation of ancient analogs, such as bryozoan dominated limestones and mudstones of Paleozoic to Mesozoic age, which are not uncommon in the geological record. It further calls for caution in assuming that the presence of dominant macrobenthic fossil in old hydrocarbon-derived limestones implies its ecological connection to active seepage.

Organisation(s)

Department of Geology

External organisation(s)

Institute of Genetics and Biophysics "Adriano Buzzati-Traverso", CNR, Woods Hole Oceanographic Institution, German University of Technology in Oman, University of Padova

Journal

Marine and Petroleum Geology

Volume

66

Pages

511-531

No. of pages

21

ISSN

0264-8172

DOI

https://doi.org/10.1016/j.marpetgeo.2015.03.015

Publication date

09-2015

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry, 105121 Sedimentology

Keywords

ASJC Scopus subject areas

Economic Geology, Geology, Geophysics, Oceanography, Stratigraphy

Portal url

https://ucrisportal.univie.ac.at/en/publications/biodetrital-carbonates-on-the-adriatic-continental-shelf-imprinted-by-oxidation-of-seeping-hydrocarbons(5e229fc7-fce5-496a-9711-2df32aa103ff).html