Accumulation and transformation of organic substances in marine sediments 3

Accumulation of organic matter in bottom sediments

Unicellular eukaryotes known as benthic foraminifera have sophisticated survival mechanisms and ecological environmental indicators. Ten surface samples from the Yellow River Delta's coastal region were taken for this study to analyze the distribution of foraminifera in relation to environmental factors and to examine their microbial diversity. We looked at the physicochemical aspects of the environment at each sampling site (such as pH, TN, TOC, EC, δ¹³C, δ¹⁵N, PAHs, etc.), and we employed 16S rRNA high-throughput sequencing analysis to look into the interactions between microbial communities and foraminiferal species. The findings demonstrated that the Yellow River Delta's advantage benthic foraminiferal species were Quinqueloculina complanata, Ammonia beccarii, and Ammonia aomoriensis, and that TOC and TN were significant determinants of the distribution of benthic foraminiferal communities. PAH enrichment in the coastal zone effects on microbial communities and benthic foraminifera are not yet readily apparent. Sediment organic carbon and nitrogen isotope analysis revealed that marine plankton may be the dominant source of organic matter in the coastal zone sediments in the research area, which was made up of both land-based and marine organic matter. It implies that foraminifera have some environmental indicators, especially when combined with the distributional traits of the benthic foraminiferal assemblages and their considerable association with environmental parameters. The findings revealed that Q. complanata was found in the Yellow River water-affected estuarine coastal zone, A. aomoriensis was significantly influenced by sediment organic matter content and reflected the estuarine and nearshore environments and the combination of Ammonia beccarii-Ammonia aomoriensis-Elphidium excavatum assemblage indicated a semi-open intertidal shallow marine environment. Additionally, benthic foraminifera showed a responsive association with microorganisms, indicating that microbial diversity may be one of the driving forces behind benthic foraminifera's ability to adjust to environmental changes. The findings of this study will open up new avenues for research into the environmental importance of coastal zone ecosystems and for understanding how benthic foraminiferal communities survive.

Glacier-fed lakes are characterized by cold temperatures, high altitudes, and nutrient-poor conditions. Despite these challenging conditions, near-surface sediments of glacier-fed lakes harbor rich microbial communities that are critical for ecosystem functioning and serve as a bridge between aquatic ecology and the deep subsurface biosphere. However, there is limited knowledge regarding the microbial communities and their assembly processes in these sediments, which are highly vulnerable to climate change. To fill this knowledge gap, this study systematically analyzed environmental variables, microbial communities, diversity, co-occurrence relationships, and community assembly processes in the near-surface sediments of a glacier-fed lake in the Tibetan Plateau. The results revealed distinct vertical gradients in microbial diversity and subcommunities, highlighting the significant influence of selection processes and adaptive abilities on microbial communities. Specifically, specialists played a crucial role within the overall microbial communities. Microbial assembly was primarily driven by homogeneous selection, but its influence declined with increasing depth. In contrast, homogenizing dispersal showed an opposite pattern, and the bottom layer exhibited heterogeneous selection and undominated processes. These patterns of microbial assembly were primarily driven by environmental gradients, with significant contributions from processes associated to ammonium and organic matter deposition, as well as chemical precipitation in response to a warming climate. This study enhances our understanding of the microbial communities and assembly processes in the near-surface sediments of glacier-fed lakes and sheds light on geo-microbiological processes in climate-sensitive lacustrine sediments.

Radiocarbon ages from contemporaneous samples of varying types can show large age differences (Nakamura et al., 2016; Raymond and Bauer, 2001), and changes in offsets between these ages (δage) within a core have been scarcely examined. Changes in δage have previously suggested disturbances or shifts to the depositional setting (Nakamura et al., 2016; Ishiwa et al., 2021; Yokoyama et al., 2019). Radiocarbon dating of ocean sediment cores from near Ube City, Yamaguchi prefecture, Japan revealed periods of slower and rapid sedimentation which aligned with, respectively, post-Holocene High Stand (HHS, ∼7–4 cal ka BP (Yokoyama, 2021)) conditions and sea level fall, and pre-HHS conditions with rising sea levels.

δage patterns implied that the reliability of sample types as depositional age indicators may depend on the depositional environment. Radiocarbon ages and relative sea level (RSL) curves suggested that local inundation may have occurred up to ∼ 200 years earlier than previously reported. In addition to constraining the relative timing of sea transgression and regression during the HHS, this study highlights the need to examine the depositional setting to accurately represent depositional ages.

Wetland soils are among the largest carbon reservoirs in terrestrial ecosystems and are vulnerable to climate change. Here we elucidate the developmental history of a wetland soil located on the Konsen Plateau in Hokkaido, Japan, during the Holocene. Through radiocarbon dating of sediment samples, we revealed that continuous sedimentation persisted over the past 4000 years. This onset of peat sedimentation was simultaneous with those reported in other wetlands in eastern Hokkaido. We hypothesized that cooling associated with the end of climatic optimal may have contributed to peat preservation. Elemental analyses also confirmed the formation of a stable wetland condition during the late Holocene. The C/N ratios were approximately 15–25 throughout the core, indicating that the organic materials in the sediments were steadily derived from terrestrial plants. The C/S ratios of the core varied between 43 and 97, suggesting that the sediments are free from marine influences. Our results provided continuous and stable record of wetland sedimentary environment in eastern Hokkaido over the past 4000 years.

Re-analysis of the lithological, paleontological, and geochemical data of three lagoonal sediment cores reveal the tsunami overwash history from 8000 to 6500 cal BP along the northern Suruga Bay coast, facing the eastern Nankai Trough. The coring sites are located in a transcect 0.8, 1.5, and 1.7 km inland, respectively, from the coastline at the time the overwash events occurred. The washover sand beds are distinguished from the lagoonal muds by the presence of quartz and mica, which are abundant on the outer (marine) side of the lagoon, as well as marine shell fossils. The increased C/S ratio often associated with sand beds, reflects input of open seawater to the reductive lagoon, and supports this interpretation. A maximum of eight tsunami deposits were identified between ∼7800 and 6500 cal BP, many of which can be correlated between the three cores. The magnitude of tsunami overwash implied by the thickness and grain size of tsunami deposits, and increase of C/S ratio following their deposition is relatively large at ∼7500-7200 cal BP during the post-glacial sea level rise and decreases with time during the subsequent high sea level stand. This occurrence mode (timing and scale of formation) of tsunami washover is explained by temporal changes in coastal geomorphology, rather than differences in the magnitude of individual tsunamis. Around 7500-7200 cal BP, sea level rise equilibrated with or exceeded the growth of the bay mouth barrier, and tsunamis could easily cross the small barrier and enter the lagoon, leaving behind thick and coarse-grained deposits. As the barrier becomes larger during the subsequent sea-level highstand, only the highest-amplitude waves exceeded the height of the barrier. By ∼6500 years ago, the barrier was high enough to prevent tsunamis from entering the lagoon. Similarly, the occurrence mode of tsunami deposits reported along the coast of Suruga Bay and Enshu-nada over the past 6000 years can be explained in relation to the growth of coastal barriers.

Eight sediment multi-cores samples collected from the Bay of Bengal and the Andaman Sea were examined to evaluate organic matter (OM) sources to the marine setting for a large sediment and carbon sink. Total organic carbon (TOC) for sediments from the northern Bay of Bengal (NBB), eastern Bay of Bengal (EBB) and the northern Andaman Sea (NAS) range from 0.34 to 0.79 wt%, 0.84 to 1.06 wt%, and 0.20 to 1.18 wt%, respectively, with higher concentrations in fine silts than in the sandy fractions suggesting hydrodynamic sorting and surface area control TOC. The TOC to total nitrogen (TN) ratio (C/N) (NBB: 5.31–8.52; EBB: 5.51–6.97; NAS: 6.44–11.92) indicates OM is a mixture of dominant contributions from marine plankton and lesser contributions from land plants. The organic carbon (δ¹³C_org) isotopic values display a similar range of values in each of the regions of the NBB (−12.0 to −19.9‰), EBB (−10.1 to −20.2‰) and NAS (−8.3 to −20.3‰). δ¹³C_org values may derive from marine phytoplankton, terrestrial C₄ plants and/or aquatic macrophytes. The total nitrogen (δ¹⁵N) isotopic compositions (NBB: 3.7–4.1; EBB: 4.1–5.9; NAS: 4.2–6.5‰) diagnose input from planktonic sources or aquatic macrophytes. Hydrocarbons from the sediments have a bimodal distribution with high relative abundance of mid-chain n-alkanes (C₂₃ and C₂₅ peaks) and long-chain n-alkanes (C₂₉ and C₃₁). Average chain length (ACL_25-35), carbon preference index (CPI_25-35), and grassland (P_grass) ratios suggest contributions from terrestrial vegetation including grasses. High TOC to total sulphur (TS) ratios (C/S) observed suggest well-oxygenated interval associated with vertical mixing of seawater about 1000 years ago. The average sedimentation rates of Bay of Bengal and Andaman Sea are about 0.4 mm yr⁻¹ and 0.07 mm yr⁻¹, respectively. The organic carbon accumulation rate of the Bay of Bengal is found to be several times higher than that of Andaman Sea. Rapid deposition of both marine and terrestrial OM in the Bay of Bengal could be a favourable sedimentary system for methane fermentation to form methane hydrate in this regime.