Contents and stable carbon isotope compositions of biomarkers indicative for AOM-communities dominated by different ANME groups in two carbonate samples from Hydrate Hole pockmark

Different types of seep carbonates were recovered from the 'Kouilou pockmarks' on the Congo deep-sea fan in approximately 3100 m water depth. The carbonate aggregates are represented by pyritiferous nodules, crusts and slabs, tubes, and filled molds. The latter are interpreted to represent casts of former burrows of bivalves and holothurians. The nodules consisting of high-Mg-calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs. Nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic ANME-1 group, whereas aragonitic crusts resulted from the activity of archaea of the ANME-2 cluster. Evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates, showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules. Formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts, probably reflecting higher porosities of the host sediment during carbonate formation. This finding is supported by lower d18O values of crusts, agreeing with precipitation from pore waters similar in composition to seawater. The aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate-rich pore waters similar to seawater. Moreover, the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that ANME-1 archaea tend to occur deeper in the sediment than members of the ANME-2 group. Methane represents the predominant carbon source of all carbonates (d13C values as low as -58.9 per mil V-PDB) and the encrusted archaeal biomarkers (d13C values as low as -140 per mil V-PDB). Oxygen isotope values of some nodular carbonates, ranging from + 3.9 to + 5.1per mil V-PDB, are too high for precipitation in equilibrium with seawater, probably reflecting the destabilization of gas hydrates, which are particularly abundant at the Kouilou pockmarks.

Pore water (SO4, CH4, alkalinity, HS-, Ca, Sr, Mg, Ba) and solid phase (Ca, Sr, Mg, Ba) data measured in pockmark sediments of the Northern Congo Fan

This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1-4), 93-109, doi:10.1016/j.margeo.2007.03.007]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600 yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.

High-resolution pore-water and solid-phase analyses of sediment cores GeoB6520-3 (Hydrate Hole) and GeoB6521-2 (Worm Hole) of the northern Congo Fan

The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates "from above". As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.

Studies of the microbial diversity on analyzed samples from from Peru Margin, Equatorial Pacific, Hydrate Ridge, and Juan de Fuca Ridge

Marine sediments harbor an enormous quantity of microorganisms, including a multitude of novel species. The habitable zone of the marine sediment column begins at the sediment-water interface and probably extends to depths of several thousands of meters. Studies of the microbial diversity in this ecosystem have mostly relied on molecular biological techniques. We used a complementary method - analysis of intact polar membrane lipids - to characterize the in-situ microbial community in sediments covering a wide range of environmental conditions from Peru Margin, Equatorial Pacific, Hydrate Ridge, and Juan de Fuca Ridge. Bacterial and eukaryotic phospholipids were only detected in surface sediments from the Peru Margin. In contrast, deeply buried sediments, independent of their geographic location, were dominated by archaeal diether and tetraether lipids with various polar head groups and core lipids. We compared ring distributions of archaeal tetraether lipids derived from polar glycosidic precursors with those that are present as core lipids. The distributions of these related compound pools were distinct, suggestive of different archaeal sources, i.e., the polar compounds derive from sedimentary communities and the core lipids are fossil remnants from planktonic communities with possible admixtures of decayed sedimentary archaea. This in-situ production of distinct archaeal lipid populations potentially affects applications of the TEX86 paleotemperature proxy as demonstrated by offsets in reconstructed temperatures between both pools. We evaluated how varying cell and lipid stabilities will influence the sedimentary pool by using a box-model. The results are consistent with (i) a requirement of continuous inputs of freshly synthesized lipids in subsurface sediments for explaining the observed distribution of intact polar lipids, and (ii) decreasing lipid inputs with increasing burial depth.