No abstract

No abstract

Microbial communities and their associated metabolic activities, methanogenesis and anaerobic oxidation of methane (AOM), are the key components of carbon biocycles in continental margin sediments. The composition and diversity of microbial communities in a methanic environment have been widely investigated, but identifying direct correlations between microbial communities and their activities remains a challenge. Here, we investigated shifting microbial communities that performed methanogenesis and AOM in long-term incubations (the longest is up to 199 days). AOM, methanogenesis, sulfate reduction and iron reduction occurred during the incubation, and 16S rRNA gene sequencing showed that some bacteria were maintained or even enriched during the incubation compared to the environmental samples. In contrast, archaeal diversity was reduced, and only some uncultured archaea belonging to the phylum Bathyarchaeota were enriched after treatment with a high sulfate concentration (29.38 mM), suggesting that sulfate might promote their enrichment. Well-known anaerobic methanotrophic archaea (ANME) were not detected, and SEEP-SRB1, which is in syntrophy with ANME, decreased to approximately zero after the incubation. The abundance of known methanogens, such as genera Methanococcoides and Methanosarcina, increased slightly in some incubations but was still present at a low relative abundance (<0.15%). Incubation with a lower sulfate concentration (4 mM) and higher iron content resulted in greater δ13CO2 accumulation, indicating that iron may be the additional electron acceptor for AOM. Based on these results, other unknown or unconventional phylotypes or pathways of methanogenesis and AOM may occur during the incubation. Thus, the diversity of methanogens and anaerobic methanotrophs warrants further investigation.

We integrated multiple geochemical analysis of a 13.75 m-long core 973-4 recovered from the northeastern South China Sea (SCS) to detect the response of deep-sea sediment archives to sea-level change spanning the last 42 kyr. The age-depth model based on AMS 14C dating, together with the sediment grain size, shows an occurrence of turbidity current at around 14 kyr, which was associated with submarine landslides caused by gas hydrate dissociation. A dominantly terrigenous sediment input was supplied from southwestern Taiwan rivers. By synthesizing environment-sensitive indexes, four distinct stages of paleoenvironmental evolutions were recognized throughout the studied interval. Well-oxygenated condition occurred during the stage I (42.4-31.8 kyr) with low sea-level stand below -80 m, accompanied by flat terrigenous input. The largest amounts of terrigenous sediment input occurred during the late phase of stage II (31.8-20.4 kyr) with the lowest sea-level stand below -120 m because of a short distance from paleo-Taiwan river estuaries to the core location. An occurrence of Ca-enriched turbidity current disturbed the original sediments during the stage III (20.4-13.9 kyr). The stepwise elevated sea-level stand resulted in an enclosed (semi-enclosed) system and contributed to a relatively low-oxygen environment in deep ocean during the stage IV (13.9 kyr—present). Temporal variations of TOC and CaCO3 display contrary pattern synchronously, indicating a decoupled relationship between organic carbon burial and carbonate productivity. Our results highlight that these sedimentary records as reflected in the paleoenvironmental changes in the northeastern SCS were mainly driven by sea-level fluctuations and later, since the mid-Holocene, the strengthening East Asian summer monsoon (EASM) overwhelmed the stable sea level in dominating the environmental changes.

Authigenic carbonate in seep environments, as a direct byproduct of sulfate-driven anaerobic oxidation of methane (SD-AOM), is usually absent within the sediment column because of the requirement of a strict formation condition. In this case, the lack of a reliable carbon signal may impede the identification of SD-AOM and methane leakage. Here, carbon and oxygen isotopes, elemental compositions, AMS 14C dates in sediments, and porewater geochemistry were investigated from two sites (A27 and SH1) of the Shenhu area, northern South China Sea (SCS), to discuss how SD-AOM affects the carbon isotope in methane-affected marine sediments. Porewater results at both sites indicate the occurrence of methane diffusion from the sulfate-methane transition zone (SMTZ) below. The carbon isotopes of bulk-sediment carbonate and foraminifera show no distinctly negative excursion, reflecting that these signals are invalid in response to SD-AOM in the investigated sites. Then, a mass balance model is adopted to evaluate the δ13C value of authigenic carbonate (δ13CAC). Consequently, three intervals (A2 and A3 from site A27 and S2 from site SH1) are identified, featuring negative δ13CAC values, high TS/TOC ratios, and enhanced contents of authigenic carbonate, which are most likely influenced by SD-AOM. Considering the current SMTZ located at deeper layers, intervals A2 and S2 represent the locations of paleo-SMTZ, while interval A3 is thought to be influenced by the current methane diffusion. Interestingly, the δ13C values of total organic matter (δ13CTOC) show positive excursions within the paleo-SMTZs, which can be explained herein by the diagenetic modification. In the course of SD-AOM at the SMTZ, high rate of (methylotrophic) methanogenesis preferentially consumes lighter carbon atoms in organic matter, with the remainder being gradually more positive. Our results indicate that the exploration of a reliable methane-carbon response in systems lacking seep carbonates plays an important role in constraining SD-AOM and methane release.