Organic Matter Content Distribution Research Articles

Spatial patterns of organic matter content in the surface soil of the salt marshes of the Venice Lagoon (Italy)

Abstract. Salt marshes are crucial eco-geomorphic features of tidal environments as they provide important ecological functions and deliver a wide range of ecosystem services. Being controlled by the interplay between hydrodynamics, geomorphology, and vegetation, the contribution of both organic matter (OM) and inorganic sediments drives salt marsh vertical accretion. This allows marshes to keep pace with relative sea level rise and likewise capture and store carbon, making them valuable allies in climate mitigation strategies. Thus, soil organic matter (SOM), i.e. the organic component of the soil, plays a key role within salt marsh environments, directly contributing to soil formation and supporting carbon storage. This study aims at inspecting spatial patterns of OM in surface salt marsh soils (top 20 cm), providing further insights into the physical and biological factors driving OM dynamics that affect salt marsh survival and carbon sink potential. Our results reveal two scales of variations in SOM content in marsh environments. At the marsh scale, OM variability is influenced by the interplay between surface elevation and changes in sediment supply linked with the distance from the marsh edge. At the system scale, OM content distribution is dominated by the gradient generated by marine and fluvial influence. The observed variations in SOM are explained by the combination of inorganic and organic input, preservation conditions, and sediment grain size. Our results highlight the importance of marshes as carbon sink environments, further emphasising that environmental conditions within a tidal system may generate strongly variable and site-specific carbon accumulation patterns, enhancing blue carbon assessment complexity.

Organic matter content and type variation in the sequence stratigraphic context of the Upper Devonian New Albany Shale, Illinois Basin

Organic matter quantity and type are important parameters in conventional source rock evaluation and unconventional shale oil/gas reservoir characterization. Understanding the stratigraphic distribution of organic matter content and type in black shale successions is critical for identifying potentially productive intervals, because organic matter can adsorb large amounts of oil and gas. Detailed examination of total organic carbon content, organic petrographic composition, and high-resolution (8 cm spacing) geochemical proxies in a sequence stratigraphic framework were conducted on an early mature (0.55% Ro) New Albany Shale core of the Illinois Basin to study the influence of relative sea-level fluctuations on paleoproductivity, clastic supply, bottom-water redox conditions, and their combined control on total organic carbon distribution pattern and organic maceral variation.Marine organic matter including amorphous organic matter and alginite (mainly derived from Tasmanites cysts) is the dominant organic matter in the New Albany Shale. Terrestrial organic matter accounts for <10% of total organic matter. Within a sequence stratigraphic context, total organic carbon content increases in transgressive systems tracts, reaches a maximum before the maximum flooding surface, and shows relatively low values in highstand systems tracts. Comparatively, low total organic carbon contents at maximum flooding surfaces reflect a combination of low sedimentation rates, elevated bottom-water oxygenation, and high biogenic silica dilution.Stratigraphically, amorphous organic matter content increases in transgressive systems tracts, reaches a maximum near the maximum flooding surface, and decreases in highstand systems tracts. Enrichment of broken Tasmanites cysts and their detrital infills are an indicator of high-energy environments and could possibly indicate lowstand systems tracts. The stratigraphic distribution of organic matter content and type may result in cyclic stratigraphic variations of hydrocarbon generation potential and oil saturation, and influence the development of secondary organic pores when the New Albany Shale is within the oil and gas windows.

Acoustical methodology for determination of gas content in aquatic sediments, with application to Lake Kinneret, Israel, as a case study

The spatial distribution of gaseous methane in the top layer of the sediment of Lake Kinneret (the Sea of Galilee), Israel is quantified using a novel acoustical methodology. Measurements were carried out with low-frequency sound pulses (200 Hz–2 kHz) for different depths of the lake along the offshore transects. The methodology is based on the correlation of gas content with sound speed and, in turn, with the reflection coefficient of sound waves from sediment. Variations in the free gas content in sediments with water depth obtained using the proposed method show a remarkable agreement with the distribution of organic matter content in and methane fluxes from sediment, both revealed by the preceding studies. This spatially variable pattern can be explained by the specific biogeochemical, hydrodynamic, and bathymetric characteristics of Lake Kinneret. The suggested method is especially suitable for characterizing gassy sediments and highlighting the location of potential methane emissions.