A Multi-Frequency Acoustic Investigation of Seafloor Methane Seep Sites at Omakere Ridge, Hikurangi Margin, New Zealand

<p>Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the Hikurangi Margin, east of the North Island of New Zealand, and is an area of active seafloor methane seepage associated with an extensive gas hydrate province. Methane seep sites on the Hikurangi Margin are characterised by localised buildups of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor otherwise characterised by soft, muddy sediments and provide a unique window into the workings of the gas hydrate system. Seafloor methane seeps sites on Omakere Ridge have been successfully imaged using three newly-acquired acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60 Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom profiler dataset (4 kHz). Seafloor seismic amplitude and similarity maps have been derived from a preliminary shipboard post-stack migrated data cube. A pronounced acquisition artifact is manifest in the seafloor horizon slice as high- and low-amplitude stripes that alternate periodically in the crossline direction. This artifact has been removed from the seafloor horizon slice using 2D spatial frequency filtering, followed by direct sampling and stochastic removal of the very-low-frequency components in the spatial domain. The seismic amplitude map has then been transformed into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics have been created after correcting for beam pattern effects and angular variation in backscatter after taking into account the bathymetry. Several backscatter mosaics were incorporated into a stacked mosaic over the study area to attenuate random noise. The ParasoundTM sub-bottom profiler data were processed to display instantaneous amplitude and separated into 43 lines over the study area. Comparison of 3D seismic attributes, multibeam backscatter intensity and shallow subsurface reflection characteristics provides new insights into the previously unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge. Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter intensity are interpreted as carbonate formations of at least 6–7 m thickness, while areas exhibiting low seismic reflection coefficient and moderate/high sonar backscatter intensity are interpreted as areas where the carbonates are less developed. Anomalous high-amplitude subsurface reflections beneath the seeps in the ParasoundTM data are interpreted as buried carbonates and may indicate a previously unknown earlier phase of seepage at Omakere Ridge, but could also be caused by gas or gas hydrates. The extent of authigenic carbonates is directly related to the duration of seepage and thus provides a new proxy for the chronology of seepage at Omakere Ridge, which has proved consistent with an existing hypothesis based on the abundance of deceased and live chemosynthetic fauna at the seep sites.</p>

A Multi-Frequency Acoustic Investigation of Seafloor Methane Seep Sites at Omakere Ridge, Hikurangi Margin, New Zealand

<p>Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the Hikurangi Margin, east of the North Island of New Zealand, and is an area of active seafloor methane seepage associated with an extensive gas hydrate province. Methane seep sites on the Hikurangi Margin are characterised by localised buildups of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor otherwise characterised by soft, muddy sediments and provide a unique window into the workings of the gas hydrate system. Seafloor methane seeps sites on Omakere Ridge have been successfully imaged using three newly-acquired acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60 Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom profiler dataset (4 kHz). Seafloor seismic amplitude and similarity maps have been derived from a preliminary shipboard post-stack migrated data cube. A pronounced acquisition artifact is manifest in the seafloor horizon slice as high- and low-amplitude stripes that alternate periodically in the crossline direction. This artifact has been removed from the seafloor horizon slice using 2D spatial frequency filtering, followed by direct sampling and stochastic removal of the very-low-frequency components in the spatial domain. The seismic amplitude map has then been transformed into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics have been created after correcting for beam pattern effects and angular variation in backscatter after taking into account the bathymetry. Several backscatter mosaics were incorporated into a stacked mosaic over the study area to attenuate random noise. The ParasoundTM sub-bottom profiler data were processed to display instantaneous amplitude and separated into 43 lines over the study area. Comparison of 3D seismic attributes, multibeam backscatter intensity and shallow subsurface reflection characteristics provides new insights into the previously unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge. Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter intensity are interpreted as carbonate formations of at least 6–7 m thickness, while areas exhibiting low seismic reflection coefficient and moderate/high sonar backscatter intensity are interpreted as areas where the carbonates are less developed. Anomalous high-amplitude subsurface reflections beneath the seeps in the ParasoundTM data are interpreted as buried carbonates and may indicate a previously unknown earlier phase of seepage at Omakere Ridge, but could also be caused by gas or gas hydrates. The extent of authigenic carbonates is directly related to the duration of seepage and thus provides a new proxy for the chronology of seepage at Omakere Ridge, which has proved consistent with an existing hypothesis based on the abundance of deceased and live chemosynthetic fauna at the seep sites.</p>

Methane-Derived Authigenic Carbonates on the Seafloor of the Laptev Sea Shelf

Seafloor authigenic carbonate crusts are widespread in various oceanic and marine settings, excluding high-latitude basins that are corrosive to carbonate precipitation. Newly formed carbonate formations are relatively rare in modern Arctic marine sediments. Although the first-order principles of seep carbonate formation are currently quite well constrained, little is known regarding the duration or mode of carbonate formation in the Siberian Arctic shelf. Large (massive slabs or blocks) and small crusts that were micrite cemented have been recently discovered on the seafloor of the Siberian Arctic seas within the area of known seep activity in the outer Laptev Sea shelf. Cold methane seeps were detected in the area due to the presence of an acoustic anomaly in the water column (gas flares). Microbial mats, methane gas bubbles, and carbonate crusts were observed using a towed camera platform. Here, we report new geochemical and mineralogical data on authigenic shallow Siberian Arctic cold-seep carbonate crusts to elucidate its genesis. The Laptev Sea carbonate crusts mainly consist of high-Mg calcite (up to 23 mol % MgCO3). The δ13C values in carbonates range significantly (from –40.1 to –25.9‰ VPDB), while the δ18O values vary in a narrow range (+4.4 ± 0.2‰ VPDB). The δ13C values of Corg that was determined from carbonates range from –40.2 to –31.1‰ VPDB. Using the isotope data and taking into account the geological setting, we consider that not only microbial but possibly thermogenic methane participated in the authigenic carbonate precipitation. Carbonate crust formation occurred below the water/sediment interface of the shallow Siberian Arctic shelf as a result of gas hydrate dissociation during Holocene warming events. The studied carbonate crusts were exhumated after precipitation into shallow subsurface shelf sediments.

Clay minerals modulate early carbonate diagenesis

Early diagenetic precipitation of authigenic carbonate has been a globally significant carbon sink throughout Earth history. In particular, SO4 2– and Fe3+ reduction and CH4 production create conditions in pore fluids that promote carbonate mineral precipitation; however, these conditions may be modified by the presence of acid-base buffers such as clay minerals. We integrated the acid-base properties of clay minerals into a biogeochemical model that predicts the evolution of pore-water pH and carbonate mineral saturation during O2, Fe3+, and SO42– reduction and CH4 production. Key model inputs were obtained using two natural clay mineral–rich sediments from the Integrated Ocean Drilling Program as well as from literature. We found that clay minerals can enhance carbonate mineral saturation during O2 and SO42– reduction and moderate saturation during Fe3+ reduction and CH4 production if the pore-fluid pH and clay mineral pKa values are within ~2 log units of one another. We therefore suggest that clay minerals could significantly modify the environmental conditions and settings in which early diagenetic carbonate precipitation occurs. In Phanerozoic marine sediments—where O2 and SO42– have been the main oxidants of marine sedimentary organic carbon—clay minerals have likely inhibited carbonate dissolution and promoted precipitation of authigenic carbonate.

Novel U and Th isotopic tracers for characterization of karstic freshwater and recent tufa from the Krka River (Croatia)

&lt;p&gt;The Krka River in Croatia is a specific groundwater-fed karstic river, characterized by complex hydrology and seasonally variable diffuse subsurface recharge. It represents a unique model system, where tufa is precipitating in a turbulent stream at morphologic discontinuities and in lentic environments. Tufa is especially attracting attention as a potential environmental archive that can provide insight into water-rock interactions, hydraulic connections, recharge, and terrestrial CO&lt;sub&gt;2&lt;/sub&gt; cycling in terms of storage, evasion, and transfer to the ocean. In a dynamic karst river system with alternating lentic and turbulent lotic sections, the carbonate precipitation rarely occurs in isotopic equilibrium for either C or O isotopes. Therefore, the use of traditional isotopes in river water (d&lt;sup&gt;18&lt;/sup&gt;O, d&lt;sup&gt;2&lt;/sup&gt;H, d&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;DIC&lt;/sub&gt;), tufa, surrounding bedrock, soil (d&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;CaCO3&lt;/sub&gt;, d&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;CaCO3&lt;/sub&gt;, d&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;POC&lt;/sub&gt;) and geochemical parameters (Ca, Mg, Na, K, HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) in river/carbonate system in combination with uranium (U) and thorium (Th) isotopic composition could increase the understanding of this complex karst hydrodynamic system and help with the identification and quantification of authigenic carbonate precipitated in the river.&lt;/p&gt;&lt;p&gt;River water samples, tufa, and surrounding bedrock and soil samples were collected at 11 locations, which were selected based on the spatial distribution of bedrock types and occurrence of tufa. Measurements of U and Th isotope ratios were carried out with multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) and for assessing U and Th concentrations, triple Quadrupole ICP-MS was used.&lt;/p&gt;&lt;p&gt;The study brought a new perspective to already known data on this highly sensitive karst eco-system. U concentration and the activity ratios of &lt;sup&gt;234&lt;/sup&gt;U/&lt;sup&gt;238&lt;/sup&gt;U in the river show a decrease with the distance from the spring. U isotopic differences reflect the changing bedrock lithology and the mixing of waters from different sources. Therefore, U values show promise as a tracer for studying changes in host rock composition and hydraulic connections in the karst aquifer.&lt;/p&gt;&lt;p&gt;Tufa samples of the studied system demonstrate a much higher activity ratio of &lt;sup&gt;234&lt;/sup&gt;U/&lt;sup&gt;238&lt;/sup&gt;U compared to the bedrock and soil. The &lt;sup&gt;234&lt;/sup&gt;U/&lt;sup&gt;238&lt;/sup&gt;U ratio of carbonate in tufa is almost identical to that of the dissolved U in the river water, indicating that a majority of U present in tufa samples is co-precipitated with the carbonate from the river water. This assumption was confirmed with a much lower &lt;sup&gt;234&lt;/sup&gt;U/&lt;sup&gt;238&lt;/sup&gt;U ratio of the non-carbonate fraction of tufa, which is comparable to that of the soil and bedrock, and the d&lt;sup&gt;18&lt;/sup&gt;O and d&lt;sup&gt;13&lt;/sup&gt;C values of carbonate in tufa, which confirmed its authigenic origin.&amp;#160;&lt;/p&gt;&lt;p&gt;The Th and U concentrations and their isotope ratios in carbonate materials from our study were shown to be reliable indicators of the storage of CO&lt;sub&gt;2&lt;/sub&gt; as authigenic carbonate in tufa. Moreover, they were also useful for the determination of tufa with U bond to detrital material and consequently relevant for both the construction of the CO&lt;sub&gt;2 &lt;/sub&gt;mass balance in a karst aquifer, as well as for dating.&lt;/p&gt;