Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

Abstract

Understanding of the processes of petroleum source rock (SR) accumulation in lacustrine rift basins and the behavior of lake systems as long-term carbon sinks is fragmentary. Investigation of an 800 m thick (500 m core and ~ 300 m outcrop), deep-lacustrine, Oligocene section in Vietnam, provides a rare insight into the controls and deposition of organic carbon (OC) and SR formation in continental rift basins. A multidisciplinary dataset, combining elemental data, inorganic and organic geochemistry with sedimentology, shows that the richest alginite-prone, sapropelic SR developed during periods of relative tectonic quiescence characterized by moderate primary productivity in a mainly dysoxic lacustrine basin. Increased rift activity and further development of graben morphology intensified water column stratification and anoxia, which hindered nutrient recycling. Sapropelic organic matter (OM) continued to accumulate, but with increasing amorphous OM content and decreasing total OC values. Periods of increased seasonality were characterized by thermocline weakening, enhanced mixing of water columns, increased primary productivity and diatom blooming. The results suggest that a change from dysoxia towards anoxia or extreme primary productivity does not necessarily enhance OC burial and SR quality. External nutrient input from a phosphate-rich hinterland is sufficient for sapropel formation, whereas the main limiting factor is methanogenesis.

Redox conditions and ecological resilience during Oceanic Anoxic Event 2 in the Western Interior Seaway

“…12-13) when compared to biotic crises documented from more open ocean settings, such as the end-Triassic mass extinction (~0-0.03: Kasprak et al, 2015), and Toarcian OAE (0.01-0.04, French et al, 2014). Values are more in line with those found during the CTB at other WIS sites, such as west-central Alberta (0.07-0.21 : Furmann et al, 2015) and central Texas (up to 0.58 : French et al, 2019), alongside those values for lacustrine deposits (0.04-0.42 (Chen and Summons, 2001;Huang et al, 2020;Rizzi et al, 2020;Yin et al, 2020). This suggests that the WIS experienced particularly stratified conditions during the CTB, although local variation exists (e.g., values <0.03 in Utah; Boudinot et al, 2020).…”

Thermal Maturity of the Grajcarek Unit (Pieniny Klippen Belt): Insights for the Burial History of a Major Tectonic Boundary of the Western Carpathians

“…The latter was dominated by hemipelagic sedimentation of black shales (e.g., [21]). The active rifting phase typically causes profound changes in factors controlling organic matter (OM) production and preservation (e.g., [105]).…”