Diazotrophy Drives Primary Production in the Organic-Rich Shales Deposited Under a Stratified Environment During the Messinian Salinity Crisis (Vena del Gesso, Italy)

Abstract

Density stratification between freshwater and brine is periodically formed during massive evaporation events, which often associates deposition of organic-rich sediments. Here, we investigated phototrophic communities and nitrogen cycle during the deposition of two organic-rich shale beds of gypsum-shale alternation, representing the initial stage of the Messinian salinity crisis (Vena del Gesso, Northern Apennines, Italy). The structural distributions and the carbon and nitrogen isotopic compositions of geoporphyrins show a common pattern in the two shales, indicating the predominance of a particular phototrophic community under freshwater-brine stratified conditions. The ∼6 difference in δ 13 C of total organic carbon between PLG 4 and 5 shales was associated with similar shift in δ 13 C of the porphyrins derived from chlorophyll c, suggesting that the eukaryotic algae producing chlorophyll c were the major constituent of the phototrophic community. Importantly, these porphyrins show δ 15 N values (−7.6-−4.7 ) indicative of N 2 -fixation. We suggest that nitrate-depletion in the photic zone induced the predominance of diazotrophic cyanobacteria, which supplied new nitrogen for the chlorophyll c-producing eukaryotic algae. The large difference in the δ 13 C values of porphyrins and total organic carbon between PLG 4 and 5 shales are interpreted to reflect the depth of the chemocline, which fluctuates in response to changes in the regional evaporation-precipitation balance. Such variation in the chemocline depth may have dynamically changed the mode of the nitrogen cycle (i.e., nitrificationdenitrification-N 2 -fixation coupling vs. phototrophic assimilation of ammonium) in the density-stratified marginal basins during the Messinian salinity crisis.

The response of water column and sedimentary environments to the advent of the Messinian salinity crisis: insights from an onshore deep-water section (Govone, NW Italy)

“…These results agree with recent hydrological models (de Lange & Krijgsman, 2010;Simon & Meijer, 2017;García-Veigas et al 2018). In contrast, only a few geochemical data are available for the sediments deposited during the early stages of the MSC (Kenig et al 1995;Sinninghe Damsté et al 1995b;Isaji et al 2019a). These studies all describe organic-rich shales of the PLG unit, depicting a stratified basin that received freshwater from rivers (Natalicchio et al 2017(Natalicchio et al , 2019Sabino et al 2020) and/or low-salinity water from the Paratethys (Grothe et al 2020).…”

Evaluation of δ13C and δ15N Uncertainties Associated with the Compound-Specific Isotope Analysis of Geoporphyrins

“…Our method has the capacity to determine δ 13 C and δ 15 N values of both major and minor geoporphyrin types found in ancient sediments, especially for nitrogen, which only requires 0.15 μgN to achieve the analytical precision described earlier. The isotopic compositions of minor but more source-specific geoporphyrins, such as that derived exclusively from chlorophyll c or those derived from bacteriochlorophyll d or e homologues, , provide a much more detailed snapshot of the primary producers and the carbon and nitrogen cycles of past environments. , Furthermore, recent studies have incorporated ecosystem models to quantitively interpret the isotopic data . The δ 13 C and δ 15 N uncertainties comprehensively evaluated for our analytical method will thus be the basis for future integration of compound-specific isotope analysis and model-based studies that would enable quantitative discussion on past biogeochemical cycles.…”

Integrated isotopic and organic geochemical constraints on the depositional controls and source rock quality of the Neogene Kalamaki sedimentary successions (Zakynthos Island, Ionian Sea)

“…The following parameters were determined: free and residual pyrolizable hydrocarbons (S 1 and S 2 , mg HC/g rock), oxygenated organic compounds content (S 3 , mg HC/g rock), hydrogen index (HI, 100*S 2 /TOC mg HC/g C org ), oxygen index (OI, 100*S 3 /TOC mg CO 2 /g C org ), T max (ºC), total organic carbon (TOC) content (wt%), hydrocarbon potential (SP, S 1 + S 2 , mg HC/g rock), and production index (PI, S 1 / S 1 + S 2 ) ( Table 1). The minimum TOC value for a sample to be characterized at least as an immature source rock is 0.5 wt% (Hunt 1979;Tissot and Welte 1984). Bitumen extraction was achieved using a Soxhlet apparatus, with a chloroform methanol mixture (90:10 v/v).…”

“…Comparing the C 25 with the neighboring n-C 20 alkane it may be observed that the upper Kalamaki succession (evaporitic and Lago Mare) contains much more abundant middle-chain n-alkanes than the lower pre-evaporitic part of the section. This observation could indicate that the organic matter originates from grasses growing in marine and brackish coastal waters (Hunt 1996). This would imply that the upper Kalamaki section had an additional contribution of eukaryotes (mainly zooplankton, algae) and/or prokaryotes (bacteria) beyond the higher plants as organic matter input.…”