Abstract

Abstract. Azooxanthellate cold-water corals (CWCs) have a global distribution and have commonly been found in areas of active fluid seepage. The relationship between the CWCs and these fluids, however, is not well understood. This study aims to unravel the relationship between CWC development and hydrocarbon-rich seepage in Pompeia Province (Gulf of Cádiz, Atlantic Ocean). This region is comprised of mud volcanoes (MVs), coral ridges and fields of coral mounds, which are all affected by the tectonically driven seepage of hydrocarbon-rich fluids. These types of seepage, for example, focused, scattered, diffused or eruptive, is tightly controlled by a complex system of faults and diapirs. Early diagenetic carbonates from the currently active Al Gacel MV exhibit δ13C signatures down to −28.77 ‰ Vienna Pee Dee Belemnite (VPDB), which indicate biologically derived methane as the main carbon source. The same samples contain 13C-depleted lipid biomarkers diagnostic for archaea such as crocetane (δ13C down to −101.2 ‰ VPDB) and pentamethylicosane (PMI) (δ13C down to −102.9 ‰ VPDB), which is evidence of microbially mediated anaerobic oxidation of methane (AOM). This is further supported by next generation DNA sequencing data, demonstrating the presence of AOM-related microorganisms (ANMEs, archaea, sulfate-reducing bacteria) in the carbonate. Embedded corals in some of the carbonates and CWC fragments exhibit less negative δ13C values (−8.08 ‰ to −1.39 ‰ VPDB), pointing against the use of methane as the carbon source. Likewise, the absence of DNA from methane- and sulfide-oxidizing microbes in sampled coral does not support the idea of these organisms having a chemosynthetic lifestyle. In light of these findings, it appears that the CWCs benefit rather indirectly from hydrocarbon-rich seepage by using methane-derived authigenic carbonates as a substratum for colonization. At the same time, chemosynthetic organisms at active sites prevent coral dissolution and necrosis by feeding on the seeping fluids (i.e., methane, sulfate, hydrogen sulfide), allowing cold-water corals to colonize carbonates currently affected by hydrocarbon-rich seepage.

Highlights

  • Cold-water corals (CWCs) are a widespread nonphylogenetic group of cnidarians that include hard skeleton scleractinian corals, soft-tissue octocorals, gold corals, black corals and hydrocorals (Roberts et al, 2006, 2009; Cordes et al, 2016)

  • This study aims to unravel the relationship between cold-water corals (CWCs) development and hydrocarbon-rich seepage in Pompeia Province (Gulf of Cádiz, Atlantic Ocean)

  • This study aims to elucidate the linkage between the present-day formation of methane-derived authigenic carbonates (MDACs) and CWCs development by testing whether CWCs are non-chemosynthetic fauna or, harbor chemosynthetic symbionts, which allow them to consume some of the reduced compounds in sites of active emission of under seafloor fluids

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Summary

Introduction

Cold-water corals (CWCs) are a widespread nonphylogenetic group of cnidarians that include hard skeleton scleractinian corals, soft-tissue octocorals, gold corals, black corals and hydrocorals (Roberts et al, 2006, 2009; Cordes et al, 2016). Large vertical mounds and elongated ridges formed by episodic growth of scleractinian corals (mainly Lophelia pertusa) are, for instance, widely distributed along the continental margins of the Atlantic Ocean (Roberts et al, 2009). These systems are of great ecological value since they offer sites for resting, breeding and feeding for various invertebrates and fishes (Cordes et al, 2016, and references therein)

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