Benthic foraminifera from the deep-water Niger delta (gulf of Guinea): Assessing present-day and past activity of hydrate pockmarks
|Author(s)||Fontanier Christophe1, 2, Koho K. A.3, 4, Goni-Urriza M. S.5, Deflandre Bruno6, Galaup S.7, Ivanovsky A.6, Gayet Nicolas8, Dennielou Bernard1, Gremare Antoine6, Bichon S.6, Gassie C.5, Anschutz P.6, Duran R.5, Reichart G. J.4|
|Affiliation(s)||1 : Ctr Brest, IFREMER, Lab Environm Sedimentaires, F-29280 Plouzane, France.
2 : Univ Nantes Angers Le Mans, CNRS, LPG UMR 6112, F-44322 Nantes 3, France.
3 : Univ Utrecht, Fac Geosci, Dept Earth Sci Geochem, NL-3508 TA Utrecht, Netherlands.
4 : Royal Netherlands Inst Sea Res, NL-1797 SZ T Horntje, Netherlands.
5 : Univ Pau & Pays Adour, CNRS, IPREM Equipe Environm & Microbiol 5254, F-64013 Pau, France.
6 : Univ Bordeaux, CNRS, UMR 5805, F-33400 Talence, France.
7 : IPB ENSEGID, Ecole Ingenieurs Environm Georessources & Ingn De, F-33607 Pessac, France.
8 : Ctr Brest, IFREMER, Lab Environm Profonds, F-29280 Plouzane, France.
|Source||Deep-sea Research Part I-oceanographic Research Papers (0967-0637) (Pergamon-elsevier Science Ltd), 2014-12 , Vol. 94 , P. 87-106|
|WOS© Times Cited||19|
|Keyword(s)||Niger delta, Hydrate pockmark, Foraminifera, Stable isotopes, Bacteria|
|Abstract||We present ecological and isotopic (δ18O and δ13C) data on benthic foraminifera sampled from 4 deep-sea stations in a pockmark field from the deep-water Niger delta (Gulf of Guinea, Equatorial Atlantic Ocean). In addition, a series of sedimentological and (bio)geochemical data are shown to back up foraminiferal observations. All stations are located within 1.2 km of each other, so prevailing oceanographic conditions can be assumed to be similar at each site. Two of the sites (GMMC-01 and GMMC-02) are located in a pockmark (named “pockmark A”) where current methane seepages were recorded by ROV observations. A third station (GMMC-03) is located in the topographic depression interpreted as a collapsed pockmark (named “pockmark B”). The fourth site (GMMC-04) is a reference station, without evidence of past or present seepages. Our observations show that degraded organic matter with low bio-availability is present at all stations with a preferential burial of organic compounds in topographic depressions (GMMC-03 station). Authigenic aragonite is abundant in surface sediments at stations GMMC-01 and -02. Its precipitation is likely related to high rates of methane oxidation during past seep events in episodically active pockmark A. In contrast, the absence of anaerobic methanotrophic Archaea (ANME) during the sampling period (November 2011) suggests that only moderate sulphide and methane oxidation take place close to the sediment-water interface. Compared to the reference site GMMC-04, living foraminifera at the collapsed and episodically active pockmarks show minor changes in terms of diversity, standing stocks and faunal composition. However, the δ13C signal of living and dead (but well-preserved) foraminiferal species (Ceratobulimina contraria, Melonis barleeanus, Uvigerina peregrina) is depleted in the episodically active pockmark A compared to the other stations. Overgrowth of authigenic carbonate on altered foraminifera generates an important shift to lower δ13C values. Dead faunas carry a complex time-averaged message, integrating taphonomic gains and losses related to the temporal variability of gas emission. They reveal major faunal differences that may be useful to detect gas hydrate seepages in different pockmark stages.|