Causes for negative carbon isotope anomalies in Mesozoic marine sediments : Constraints from modern and ancient anoxic settings

Abstract

Oceanic Anoxic Events (OAEs) were short periods in Earth history (˜0.5-1 Ma) characterized by atypically high burial rates of organic carbon in marine sediments worldwide. OAEs reflect increased marine primary production and/or enhanced organic matter preservation under anoxic water column conditions, and caused the global atmospheric and marine carbon reservoirs to become enriched in 13C. Isotopic enrichments are typically recorded in carbonate and organic-rich sedimentary successions as positive δ13C excursions. The Toarcian and Early Aptian OAEs also exhibit relatively rapid negative shifts in bulk carbon isotope sediment records of both marine and terrigenous origin, suggesting a shift in the 13C-content of atmospheric CO2 by substantial input of 13C-depleted carbon. Elucidating such anomalies can therefore shed light on the environmental consequences of episodic perturbations in the global carbon cycle. The "Küspert model" for the Toarcian OAE postulates processes of recycling of organic-derived CO2 in stratified marine settings, and is supported by 13C-depleted dissolved inorganic carbon (DIC) in extant analogues (e.g., stratified lakes, fjords). Studies of a Norwegian fjord through the annual cycle revealed low δ13C values (-19%) and high concentrations of DIC below the chemocline. Seasonal availability of respired DIC to algae was indicated by a decrease of δ13CDIC values in the oxygenated surface water in autumn and spring, via storm mixing. However, δ13C values of particulate organic carbon and taxon-specific algal compounds do not decrease concurrently with δ13C DIC values. This can be explained by the observed seasonal fluctuations in the biological carbon isotope fractionation (εp), possibly caused by variations in the nutrient supply. The εp of isorenieratene, a carotenoid specific to photosynthetic green sulphur bacteria, does not display significant seasonal change. Therefore, δ13C records of isorenieratane, bulk organic and inorganic carbon can be collectively used to estimate respired CO2 contributions at the chemocline. Such estimated contributions for the Kyllaren fjord are about 40%, whilst for the Toarcian OAE of the Paris Basin do not exceed 10%. It is concluded that the "Küspert" model alone is unlikely to account for the δ13C shifts recorded during the Toarcian OAE. Molecular studies of negative δ13C excursions in Aptian OAE 1a sections of Italy and the Mid-Pacific, suggest a concomitant change in organic carbon sources from allochthonous to autochthonous. However, 4% isotopic shifts in continuous algal and terrigenous (n-alkanes) records, require a perturbation of the global carbon cycle. The terrigenous record in particular, makes a strong case for a source that injects 13C-depleted carbon into the atmosphere (e.g., combustion of terrestrial organic carbon reservoirs, destabilization of gas hydrates, etc). Although the exact source could not be further constrained, it is likely that a mechanism other than the "Küspert" mechanism caused the Aptian negative δ13C excursions. Finally, biomarker evidence in conjunction with 15N-depletions in organic matter, point towards the widespread presence of nitrogen-fixing cyanobacteria during both OAE-1a and OAE2 (Cenomanian/Turonian), implying that N2-fixation supplied an important part of nutrient N for primary production during both OAEs. Therefore, long-term stratification must have characterized early Aptian depositional environments, despite little molecular evidence for photic-zone anoxia.