2,3,6-/2,3,4-Aryl Isoprenoids in Paleocene Crude Oils from Chinese Jianghan Basin: Constrained by Water Column Stratification

Abstract

2,3,4-Trimethyl aryl isoprenoid (AIP) hydrocarbons as well as their 2,3,6 homologues were found in most Paleocene crude oils from the Jianghan Basin, China. We interpret their concurrence as evidence for the presence of both purple sulfur bacteria (PSB, family of Chromatiaceae) and green sulfur bacteria (GSB, family of Chlorobiaceae) in the water body during deposition. The GSB and PSB contributions are also evidenced by the presence of the monoaromatic carotenoid derivatives chlorobactane and okenane as well as C 40 diaromatic carotenoid derivatives, including isorenieratane and renierapurpurane. The presence of related biomarkers of gammacerane, phytane and methyl trimethyltridecyl chromans (MTTCs) in the oil samples implies that these prolific photosynthetic sulfur bacteria grew in a mesosaline condition, which is consistent with their niches above and below the oxic−anoxic chemocline in a stratified water column. Thus, the general presence of 2,3,6-AIPs, chlorobactane, and isorenieratane sourced from GSB is consistent with sulfidic (euxinic) conditions in the photic zone of the water column, while their concurrence with 2,3,4-AIPs and okenane further indicates a permanent stratified water column with sulfidic (euxinic) conditions. In such a paleowater-column redox state with ideal growing conditions for the photosynthetic bacteria, PSB likely lived beneath the GSB layer and/or co-existed in the same layer with a symbiotic relationship and/or even as a "Chlorochromatium" aggregate with an endosymbiont relationship. This ecosystem model helps further explain the concurrence of similar amounts of 2,3,6-and 2,3,4-AIPs in the Jianghan Basin oils.

A Mesoproterozoic iron formation

“…The 2,3,6-TMAI biomarkers are breakdown products of the carotenoid pigments of "brown" strains of anoxygenic phototrophic green sulfur bacteria (GSBs). Their presence, therefore, is typically ascribed to the presence of an anoxygenic phototrophic population in an anoxic water-column setting (73,74).…”

Current Status and Challenges of Methyltrimethyltridecylchromans Research in Source Rocks and Crude Oils

“…MTTCs were mainly identified in the evaporite-related source rocks and crude oils (Figure a). The evaporite-related source rocks contain varying lithology types including mudstone, black shale, marl, halite deposits, etc. − The organic matter abundances of these evaporite-related source rocks vary greatly from organic-rich to organic-lean and contained type I, type II, and type III kerogens. − Based on previous studies, we found that source rocks containing MTTCs were not widely distributed globally; they were found mainly in salt-bearing sedimentary basins in China and Europe. , By comparing Figure a and Figure b, we found that evaporite-related source rocks were globally distributed, while MTTCs were not detected in most of these evaporite-related organic sediments. There are two possible reasons that may explain this situation.…”

“…23−31 Based on previous studies, we found that source rocks containing MTTCs were not widely distributed globally; they were found mainly in salt-bearing sedimentary basins in China and Europe. 32,33 By comparing Figure 2a and Figure 2b, we found that evaporite-related source rocks were globally distributed, 34 while MTTCs were not detected in most of these evaporite-related organic sediments. There are two possible reasons that may explain this situation.…”

“…(a) MTTCs were detected in the Early Cambrian-Quaternary sediments or crude oils, except for the Carboniferous, Silurian, and Ordovician sediments or crude oils. − (b) MTTCs were mainly detected in samples with a low degree of thermal maturity. − …”

Acidic molecular fossils: origin, isolation and applications

“…Figure 23 gives a general illustration of euxinic environments with the presence of sulfur bacteria, the main source for specific aromatic carotenoids (e.g., isorenieratane) in sedimentary environments and are molecular indicators of photic zone euxinia (PZE). Other important occurrences of carotenoids are in sponge species; since these organisms cannot biosynthesize such compounds, their presence is due to the symbiotic relationship (LU et al, 2015).…”