Chemoherms on Hydrate Ridge — Unique microbially-mediated carbonate build-ups growing into the water column

Abstract

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly ¹³C-depleted carbon isotope values of the samples (as low as − 48.1 ‰ PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The δ¹⁸O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 ‰ PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 ‰ PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

Introduction

Authigenic carbonates have been recognized as ubiquitous precipitates at active methane-seeps in many active and passive margin settings such as the Cascadia (Kulm et al., 1986), Eastern Aleutian (Suess et al., 1998), Peruvian (Thornburg and Suess, 1990) and Costa Rica margins (Bohrmann et al., 2002, Han et al., 2004), the Gulf of Mexico (Aharon et al., 1992), the Mediterranean Ridge (Aloisi et al., 2000), the Black Sea (Thiel et al., 2001), the Florida Escarpment (Paull et al., 1992) and the Blake Outer Ridge (Pierre et al., 2000, Naehr et al., 2000). Most occurrences of authigenic seep carbonates are of limited extent and are found in the immediate vicinity of methane-seeps.

Only a few seep deposits form large mound- or pinnacle-shaped structures. At the Hydrate Ridge they reach up to 90 m above the seafloor. Such seep carbonates were first described as ‘bioherm’ structures (e.g., Sample and Reid, 1998, Carson and Westbrook, 1995). Their high carbonate content and the incorporation of shell debris suggested growth by organisms, not unlike coral reefs. It was not recognized until later that the formation of these carbonates is driven by chemosynthetic organisms that live on reduced substances like methane and sulfide. The term ‘chemoherm’ was first introduced by Aharon (1994) to distinguish these seep carbonates from bioherms or lithoherms. Aharon (1994) defined a chemoherm as a build-up of abiotic carbonates and calcareous skeletal debris of chemosynthetic fauna with an anomalously negative carbon isotope composition. Recent chemoherms have been described from the Gulf of Mexico (e.g., Roberts and Aharon, 1994, Aharon et al., 1997), the Cascadia accretionary margin (e.g., Kulm et al., 1986, Bohrmann et al., 1998) and the Yaquina Basin (Hübscher and Kukowski, 2003).

Most seep carbonates that have been described in the literature have a subseafloor or near-surface origin and were later unroofed by sediment winnowing and erosion (Roberts and Aharon, 1994). The precipitation of these carbonates is triggered by a consortium of microorganisms that oxidize methane via sulfate reduction (AOM; Boetius et al., 2000, Suess, 2002) increasing the alkalinity. Special cases are the methane-derived carbonate chimneys from the Black Sea. Peckmann et al. (2001) concluded that they are mainly formed above the seafloor in anoxic bottom waters, whereas other flat carbonate crusts in the Black Sea in contact with oxic bottom waters have precipitated only within the anoxic near-surface sediment. These observations illustrate the general importance of AOM in the formation of seep carbonates and serves to clearly define the criteria of true chemoherm structures as growing upward from the seafloor into the free water column.

Here we present new data from two recently discovered chemoherms from the Cascadia convergent margin (Pinnacle and South East-Knoll Chemoherms; Fig. 1). Based on petrographic and geochemical results and seafloor observations of the surface of the chemoherms, a conceptual model is presented that proposes an above-the-seafloor origin of the carbonate build-ups. This new model reconciles the apparently contradictory situation of carbonate formation by anoxic oxidation of methane in an oxic water environment.