Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: <i>p</i>CO<sub>2</sub> and paleo productivity

Abstract

Abstract. A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ13Cphytane), a biomarker for photosynthetic algae. The δ13Cphytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7‰) compared to existing C/T proto-North Atlantic δ13Cphytane records (3–6‰). Overall, the amplitude of the excursion of δ13Cphytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ13Cphytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ13Cphytane-records from Site 1260, we subsequently reconstructed pCO2 levels across the OAE-2 interval. Accordingly, pCO2 decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO2. Whereas the onset of OAE-2 coincided with increased pCO2, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO2 related to perturbations of the global carbon cycle.

Imbalanced nutrients as triggers for black shale formation in a shallow shelf setting during the OAE 2 (Wunstorf, Germany)

“…Stable carbon isotope ratios of chlorophyllderived phytane support that pCO 2 was high during OAE 2 (Freeman and Hayes, 1992;van Bentum et al, 2012) with a pCO 2 almost 5 times higher than modern values (maximizing at 1750 ppm). As a key aspect explaining the enhanced pCO 2 , massive CO 2 emissions due to the eruption of the Caribbean Large Igneous Province (CLIP) were suggested (Sinton and Duncan, 1997;Turgeon and Creaser, 2008).…”

“…10). In these samples, the aromatic parts of the kerogen may be interpreted in two ways or a mixture of both origins: mainly as an input of allochthonous degraded terrestrial organic matter, which is commonly highly aromatic due to the input from lignin (e.g., van de Meent et al, 1980). An allochthonous source may indeed explain parts of the aromatics, particularly because maturities calculated from MPI-1 are too high to fully reflect immature autochthonous biomacromolecules from microalgae (Table 1) and because of differences between biomarker distributions in kerogen and extracts (e.g., lack of dinosterane and 2-methyl hopanes in HyPy products).…”

“…Thus, it is simple to derive the OM, e.g., from the Bohemian Massive. There, expanded estuary and fluvial valleys drowned during the Upper Cenomanian transgression (Ulicný et al, 1997), and the transgressive surge might have reworked considerable amount of organic matter.…”

Cyanobacteria Proliferation in the Cenomanian‐Turonian Boundary Interval of the Apennine Carbonate Platform: Immediate Response to the Environmental Perturbations Associated With OAE‐2?

“…However, considering temporal equivalency, the second dark laminite interval at MC and RdP clearly lags the timing of the PCE, which can be located chemostratigraphically using a prominent decrease in δ 13 C carb (Figure 2). In association with this hypothesis, the sea surface temperature cooling during the PCE (interval B of the C-isotope curve; Figures 2 and 8) could have introduced thermal instability in the water masses, thus stimulating enhanced mixing and recycling of nutrients to the surface fueling productivity (Flögel et al, 2011;Forster et al, 2007;Jarvis et al, 2011;Jenkyns et al, 2017;Sweere et al, 2018;van Bentum et al, 2012). Most likely the renewed increase of pCO 2 and temperature after the PCE reflected continued volcanogenic degassing from the Caribbean LIP (Jarvis et al, 2011, and references therein;Du Vivier et al, 2014) which has been calculated to last for~200-250 kyr after the onset of OAE-2 (Du Vivier et al, 2014Vivier et al, , 2015.…”

“…Most likely the renewed increase of pCO 2 and temperature after the PCE reflected continued volcanogenic degassing from the Caribbean LIP (Jarvis et al, 2011, and references therein;Du Vivier et al, 2014) which has been calculated to last for~200-250 kyr after the onset of OAE-2 (Du Vivier et al, 2014. In association with this hypothesis, the sea surface temperature cooling during the PCE (interval B of the C-isotope curve; Figures 2 and 8) could have introduced thermal instability in the water masses, thus stimulating enhanced mixing and recycling of nutrients to the surface fueling productivity (Flögel et al, 2011;Forster et al, 2007;Jarvis et al, 2011;Jenkyns et al, 2017;Sweere et al, 2018;van Bentum et al, 2012). The latter was probably sustained by continued remobilization of P from sediments, favored by anoxic deep waters (Kraal et al, 2010;Monteiro et al, 2012;Tsandev & Slomp, 2009;van Cappellen & Ingall, 1994) which would have reduced the N:P ratio in surface waters giving a further advantage to N-fixing cyanobacteria (Monteiro et al, 2012).…”