Abstract The Toarcian Oceanic Anoxic Event (T-OAE) is marked by major palaeoenvironmental and palaeoceanographical changes on a global scale, associated with a severe disturbance of the global carbon cycle and organic-rich facies deposition. Here, a multiproxy approach (petrographic and geochemical techniques) was applied to the study of the organic content of the T-OAE of the Paris Basin, whose phytoplanktonic origin has been previously inferred by its geochemical signature. The top of the tenuicostatum Zone is characterized by palynomorphs and marine phytoplankton-derived amorphous organic matter (AOM), representing a proximal marine environment with emplacement of euxinic conditions at the top (total organic carbon/sulfur content and increase in AOM). At the base of the serpentinum Zone the proliferation of bacterial biomass begins, with phytoplankton playing a secondary role. This indicates the development of stagnant and restrictive conditions in a proximal environment, with water column stratification (neohop-13(18)-ene). The majority of the serpentinum Zone is dominated by bacterial biomass, suggesting a marine environment with bottom-water stagnation, possibly related to basin palaeogeomorphology and circulation patterns, with episodic euxinia. This therefore suggests that the T-OAE organic fraction is dominated by bacterial biomass, not phytoplankton, showing the importance of an integrated approach to the determination of the organic facies.

Abstract Foraminiferal palaeoecological analyses were carried out on 124 upper Maastrichtian chalk samples from the M-10X and E-5X wells of the Danish Central Graben. The two wells demonstrate similar trends, with some notable differences. Both are strongly dominated by planktic foraminifers, of which the small, biserial Heterohelix globulosa is by far the most common species. Based on variations within five significant, benthic foraminiferal morphogroups and the plankton/benthos ratio, eight specific foraminiferal intervals have been described. The faunal and palaeoenvironmental changes observed during the late Maastrichtian period were, in most places and especially in the lower part, not very distinct, and it is believed that the palaeoenvironment during the majority of the interval was a mostly stable, deep outer-shelf environment characterized mainly by pelagic sedimentation under temperate, suboxic conditions. More unstable conditions characterized the latest Maastrichtian. The analyses show that the sediments in the M-10X well were deposited in a generally deeper palaeoenvironment than those from E-5X. The influx of common Pseudotextularia elegans (three acmes), together with scattered specimens of the typical Tethyan species Abathomphalus mayaroensis and Pseudoguembelina hariaensis (in E-5X only), indicate that relatively warm conditions prevailed, at least periodically, during the latest part of the late Maastrichtian in both areas.

This is a commentary on the preceding chapter by Ohmoto et al., in which it is suggested that oxygen concentrations have been high throughout Earth history. This is a contentious suggestion at odds with the prevailing view in the field, which contends that atmospheric oxygen concentrations rose from trace levels to a few percent of modern-day levels around 2.5 b.y. ago. This comment notes that many of the data sets cited by Ohmoto et al. as evidence for a relatively oxidized environment come from deep-ocean settings. This presents a possibility to reconcile some of these data and suggestions with the overwhelming evidence for an atmosphere free of oxygen at that time. Specifically, it is possible that deep-ocean waters were relatively oxidized with respect to certain redox pairs. These deep-ocean waters would have been more oxidized than surface waters, thus representing an “upside-down biosphere,” as originally proposed 25 years ago by Jim Walker.