Nitrate reduction, sulfate reduction, and sedimentary iron isotope evolution during the Cenomanian‐Turonian oceanic anoxic event

Abstract

[1] Organic carbon-rich shales from localities in England, Italy, and Morocco, which formed during the Cenomanian-Turonian oceanic anoxic event (OAE), have been examined for their total organic carbon (TOC) values together with their carbon, nitrogen, and iron isotope ratios. Carbon isotope stratigraphy (d 13 C org and d 13 C carb ) allows accurate recognition of the strata that record the oceanic anoxic event, in some cases allowing characterization of isotopic species before, during, and after the OAE. Within the black shales formed during the OAE, relatively heavy nitrogen isotope ratios, which correlate positively with TOC, suggest nitrate reduction (leading ultimately to denitrification and/or anaerobic ammonium oxidation). Black shales deposited before the onset of the OAE in Italy have unusually low bulk d57 Fe values, unlike those found in the black shale (Livello Bonarelli) deposited during the oceanic anoxic event itself: These latter conform to the Phanerozoic norm for organic-rich sediments. Pyrite formation in the pre-OAE black shales has apparently taken place via dissimilatory iron reduction (DIR), within the sediment, a suboxic process that causes an approximately À2% fractionation between a lithogenic Fe(III)oxide source and Fe(II) aq . In contrast, bacterial sulfate reduction (BSR), at least partly in the water column, characterized the OAE itself and was accompanied by only minor iron isotope fractionation. This change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from $10 to $7 mm. The gradual, albeit irregular increase in Fe isotope values during the OAE, as recorded in the Italian section, is taken to demonstrate limited isotopic evolution of the dissolved iron pool, consequent upon ongoing water column precipitation of pyrite under euxinic conditions. Given that evidence exists for both nitrate and sulfate reduction during the OAE, it is evident that redox conditions in the water column were highly variable, in both time and space.

Dominant eukaryotic export production during ocean anoxic events reflects the importance of recycled NH 4 +

“…Indeed, enhancement of cyanobacterial production during many episodes of ocean anoxia has been proposed based on increased burial of 2-methylhopanoids (11)(12)(13), as these compounds are thought to be markers for cyanobacteria (14). Because such biomarker indices are only qualitative indicators of change and cannot provide quantitative estimates of export flux, complementary data generally include isotope ratios of total sedimentary nitrogen (δ 15 N TN ) (11,(15)(16)(17), as diazotrophy also affects the nitrogen isotopic budget of the ocean (18)(19)(20).…”

“…In contrast, sediments from OAE 2 record striking nitrogen isotopic depletion. They are characterized by values of δ 15 N TN consistently <−1‰, and often <−3‰ (11,16,17,23). Expression of these negative values of δ 15 N TN varies consistently by depositional location, with the average value of δ 15 N TN for OAE 2 horizons of the Bonarelli section (Gubbio and Furlo, Italy) being −3.3‰ and the South Ferriby formation (England) being −2.8‰ (16); whereas the average value for the South Atlantic is −1.9‰ (23), for the proto-North Atlantic is −1.8‰ (11,17, this work), and for the Tarfaya Basin, Morocco is −1.7‰ (between 45-60 m in section) (16).…”

“…They are characterized by values of δ 15 N TN consistently <−1‰, and often <−3‰ (11,16,17,23). Expression of these negative values of δ 15 N TN varies consistently by depositional location, with the average value of δ 15 N TN for OAE 2 horizons of the Bonarelli section (Gubbio and Furlo, Italy) being −3.3‰ and the South Ferriby formation (England) being −2.8‰ (16); whereas the average value for the South Atlantic is −1.9‰ (23), for the proto-North Atlantic is −1.8‰ (11,17, this work), and for the Tarfaya Basin, Morocco is −1.7‰ (between 45-60 m in section) (16). Such differences thus reflect regional heterogeneity of water masses, phototroph ecology, and/or nutrient biogeochemistry; and it has been suggested that patterns of intrabasinal upwelling intensity and nutrient concentrations correspond directly to regional patterns of sedimentation (8,23,24).…”

“…16). This is different from a modern-ocean scenario, in which denitrification associated with the spreading of anoxic zones leads to progressively higher (positive) values of δ 15 N NO 3 − that are then propagated to δ 15 N TN (21,22).…”

Geomicrobial functional groups: A window on the interaction between life and environments

“…These GFGs convert most of the biologically available nitrogen (NH 3 , NH 4 + , NO 3  , NO 2  ) to inert nitrogen gas in euxinic conditions. This means that euxinic oceans would be deficient in bioavailable nitrogen and would promote microbial nitrogen fixation dominantly driven by cyanobacteria [178,179]. Some archaea with unknown functions also could play important roles in development of euxinic oceans [180,181].…”