Description: Cold seeps in the Aleutian deep-sea trench support prolific benthic communities and generate carbonate precipitates which are dependent on carbon dioxide delivered from anaerobic methane oxidation. This process is active in the anaerobic sediments at the sulfate reduction-methane production boundary and is probably performed by archaea working in syntrophic co-operation with sulfate-reducing bacteria. Diagnostic lipid biomarkers of archaeal origin include irregular isoprenoids such as 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI) as well as the glycerol ether lipid archaeol (2,3-di-O-phytanyl-sn-glycerol). These biomarkers are prominent lipid constituents in the anaerobic sediments as well as in the carbonate precipitates. Carbon isotopic compositions of the biomarkers are strongly depleted in 13C with values of δ13C as low as −130.3‰ PDB. The process of anaerobic methane oxidation is also reflected in the carbon isotope composition of organic matter with δ13C-values of −39.2 and −41.8‰ and of the carbonate precipitates with values of −45.4 and −48.7‰. This suggests that methane-oxidizing archaea have accumulated within the microbial community, which is active at the cold seep sites. The dominance of crocetane in sediments at one station indicates that, probably due to decreased methane venting, archaea might no longer be growing, whereas high amounts of crocetenes found at other more active stations may indicate recent fluid venting and active archaea. Comparison with other biomarker studies suggests that various archaeal assemblages might be involved in the anaerobic consumption of methane. The assemblages are apparently dependent on specific conditions found at each cold seep environment. Selective conditions probably include water depth, temperature, degree of anoxia, and supply of free methane.

Description: Saanich Inlet has been a highly productive fjord since the last glaciation. During ODP Leg 169S, nearly 70 m of Holocene sediments were recovered from Hole 1034 at the center of the inlet. The younger sediments are laminated, anaerobic, and rich in organic material (1–2.5 wt.% Corg), whereas the older sediments below 70 mbsf are non-laminated, aerobic, with glacio-marine characteristics and have a significantly lower organic matter content. This difference is also reflected in the changes of interstitial fluids, and in biomarker compositions and their carbon isotope signals. The bacterially-derived hopanoid 17α(H),21β(H)-hop-22(29)-ene (diploptene) occurs in Saanich Inlet sediments throughout the Holocene but is not present in Pleistocene glacio-marine sediments. Its concentration increases after ∼6000 years BP up to present time to about 70 μg/g Corg, whereas terrigenous biomarkers such as the n-alkane C31 are low throughout the Holocene (〈51 μg/g Corg) and even slightly decrease to 36 μg/g Corg at the most recent time. The increasing concentrations of diploptene in sediments younger than ∼6000 years BP separate a recent period of higher primary productivity, stronger anoxic bottom waters, and higher bacterial activity from an older period with lesser activity, heretofore undifferentiated. Carbon isotopic compositions of diploptene in the Holocene are between −31.5 and −39.6‰ PDB after ∼6000 years BP. These differences in the carbon isotopic record of diploptene probably reflect changes in microbial community structure of bacteria living at the oxic–anoxic interface of the overlying water column. The heavier isotope values are consistent with the activity of nitrifying bacteria and the lighter isotope values with that of aerobic methanotrophic bacteria. Therefore, intermediate δ13C values probably represent mixtures between the populations. In contrast, carbon isotopic compositions of n-C31 are roughly constant at −31.4±1.1‰ PDB throughout the Holocene, indicating a uniform input from cuticular waxes of higher plants. Prior to ∼6000 years BP, diploptene enriched in 13C of up to −26.3‰ PDB is indicative of cyanobacteria living in the photic zone and suggests a period of lower primary productivity, more oxygenated bottom waters, and hence lower bacterial activity during the earliest Holocene.

Description: Sites 1033 and 1034 of ODP Leg 169S in Saanich Inlet have an unusual diagenetic system, that has the appearance of being depth reversed, i.e. a bacterial methane accumulation zone underlain by a sulphate reduction zone. During the late Pleistocene grey, undifferentiated, glacio-marine clays were deposited with low Corg contents (〈0.4 wt.%), and interstitial fluids replete in SO4 (ca. 27 mM), devoid of CH4 and low in nutrients. This indicates oxic conditions are present, reflecting the open exchange of waters with Haro Strait during the Pleistocene before the Saanich Peninsula emerged. In the earliest Holocene (ca. 11,000 years BP) the inlet was formed, severely restricting water circulation, and leading to the presence of anoxic bottom waters. The sediments are laminated and show a dramatic rise to high Corg, Norg and Stot contents (up to 2.5, 0.4, 1.4 wt.%, respectively) over a period of ca. 1000 years. The nutrient concentrations are especially high (TA, NH4, PO4 up to 115 meq/l, 20 mM and 400 µM, respectively), SO4 is exhausted and CH4 is prolific. Stable carbon isotope ratio measurements of CH4 and co-existing CO2 indicate that methanogenesis is via carbonate reduction (delta13C-CH4 ca. -60 to - 70 per mil, delta13C-CO2 ca. +10 per mil). At the sulphate-methane interfaces, both at the near-surface and at 50 mbsf (Site 1033) and 80 mbsf (Site 1034) methane consumption by sulphate reducing bacteria is intensive.

Description: Deep subsurface sediments from the Nankai Trough, Japan Sea, ODP Leg 190, sites 1173, 1174, 1177, and near-surface sediments from Hydrate Ridge, NE-Pacific have been analysed by high performance liquid chromatography (HPLC)–electrospray ionisation (ESI)-mass spectrometry (MS). The main objective was to utilize the presence of intact phospholipids as a direct indicator of viable microorganisms. The extracts of Nankai Trough sediments were found to contain a variety of phospholipid (PL) structures, well-known to stem from microorganisms, to depths as great as 745 mbsf and in situ temperatures as high as 85 °C. In addition, high relative amounts of lysophospholipids (e.g. lysophosphatidylcholines) exceeding those of the regular phospholipids were detected. Diglyceride mass fragments of various PLs have been assigned to fatty acyl side-chains of typical chain length (C14, C16, C18, C20) and degree of unsaturation (zero, one or two double bonds). Similar results were obtained for the phospholipid distribution in extracts of organic matter-rich Hydrate Ridge sediments. To date, the enhanced occurrence of lysophospholipids cannot be explained completely but a response to increasing thermal and ecological stress seems probable.

Description: Saanich Inlet has been a highly productive fjord since the last glaciation. During ODP Leg 169S, nearly 70 m of Holocene sediments were recovered from Hole 1034 at the center of the inlet. The younger sediments are laminated, anaerobic, and rich in organic material (1-2.5 wt.% Corg), whereas the older sediments below 70 mbsf are non-laminated, aerobic, with glacio-marine characteristics and have a significantly lower organic matter content. This difference is also reflected in the changes of interstitial fluids, and in biomarker compositions and their carbon isotope signals. The bacterially-derived hopanoid 17alpha(H),21beta(H)-hop-22(29)-ene (diploptene) occurs in Saanich Inlet sediments throughout the Holocene but is not present in Pleistocene glacio-marine sediments. Its concentration increases after ~6000 years BP up to present time to about 70 µg/g Corg, whereas terrigenous biomarkers such as the n-alkane C31 are low throughout the Holocene (〈51 µg/g Corg) and even slightly decrease to 36 µg/g Corg at the most recent time. The increasing concentrations of diploptene in sediments younger than ~6000 years BP separate a recent period of higher primary productivity, stronger anoxic bottom waters, and higher bacterial activity from an older period with lesser activity, heretofore undifferentiated. Carbon isotopic compositions of diploptene in the Holocene are between ~31.5 and ~39.6 per mil PDB after ~6000 years BP. These differences in the carbon isotopic record of diploptene probably reflect changes in microbial community structure of bacteria living at the oxic-anoxic interface of the overlying water column. The heavier isotope values are consistent with the activity of nitrifying bacteria and the lighter isotope values with that of aerobic methanotrophic bacteria. Therefore, intermediate delta13C values probably represent mixtures between the populations. In contrast, carbon isotopic compositions of n-C31 are roughly constant at ~31.4 ± 1.1 per mil PDB throughout the Holocene, indicating a uniform input from cuticular waxes of higher plants. Prior to ~6000 years BP, diploptene enriched in 13C of up to -26.3 per mil PDB is indicative of cyanobacteria living in the photic zone and suggests a period of lower primary productivity, more oxygenated bottom waters, and hence lower bacterial activity during the earliest Holocene.