Geochemistry of post-extinction microbialites as a powerful tool to assess the oxygenation of shallow marine water in the immediate aftermath of the end-Permian mass extinction

Rapid and profound changes in earth surface environments and biota across the Permian–Triassic boundary are well known and relate to the end-Permian mass extinction event. This major crisis is demonstrated by abrupt facies change and the development of microbialite carbonates on the shallow marine shelves around Palaeo-Tethys and western Panthalassa. Microbialites have been described from a range of sites in end-Permian and basal Triassic marine sedimentary rocks, immediately following the end-Permian mass extinction. Here, we present geochemical data primarily focused on microbialites. Our geochemical analysis shows that U, V, Mo and REE (Ce anomaly) may be used as robust redox proxies so that the microbialites record the chemistry of the ancient ambient sea water. Among the three trace metals reputed to be reliable redox proxies, one (V) is correlated here with terrigenous supply, the other two elements (U and Mo) do not show any significant authigenic enrichment, thereby indicating that oxic conditions prevailed during the growth of microbialites. REE profiles show a prominent negative Ce anomaly, also showing that the shallow marine waters were oxic. Our geochemical data are consistent with the presence of some benthic organisms (ostracods, scattered microgastropods, microbrachiopods and foraminifers) in shallow marine waters that survived the mass extinction event.A. Lethiers, F. Delbès, A. Michel and B. Villemant, Q. Feng, J. Haas, K. Hips and Erdal Kosu

Impact des terrils houillers sur la qualité des eaux souterraines (bassin minier Nord-Pas-de-Calais, France) : approche géochimique et isotopique

L'exploitation du charbon dans le Nord-Pas-de-Calais (France) a engendré le dépôt d'importantes quantités de résidus miniers sous forme de terrils, essentiellement constitués de schistes houillers. Le lessivage de ces stériles par les eaux météoriques et l'oxydation des sulfures de fer contenus sont susceptibles de contribuer à l'enrichissement en sulfates et métaux associés de l'aquifère de la craie, principale ressource en eau de la région. Des analyses chimiques et isotopiques (S & C) ont été effectuées sur des prélèvements d'eau en amont et en aval hydraulique des sites d'étude ainsi que sur la fraction minérale des terrils. Afin de déterminer les modalités d'infiltration des eaux météoriques au sein des terrils, des prospections radio-magnétotelluriques, couplées à des mesures de perméabilité et de granularité ont été effectuées. Les analyses ont permis de mettre en évidence que le lessivage des terrils entraîne un flux d'ions sulfate et de carbone vers l'aquifère de la craie. L'approche géophysique a permis de mettre en évidence un phénomène d'infiltration des eaux météoriques. Elle a montré en outre l'existence de barrières de perméabilité en profondeur. Il apparaît donc que les eaux météoriques, lorsqu'elles s'infiltrent dans le terril, ne peuvent pénétrer à plus de quelques mètres de profondeur. L'existence de zones imperméables, en limitant l'infiltration des eaux en profondeur, limite également la quantité de sulfure potentiellement oxydable et donc la quantité de sulfates entraînée vers l'aquifère de la craie.In the Nord-Pas-de-Calais region (France), coal mining activity has induced a build-up of many mine tips. The tip materials are dominantly composed of siltstones, locally rich in iron sulfide. Weathering of pyrite might be expected to release sulfate ions and associated metals within the run-off waters down to the underlying aquifer, which is composed of a thick Cretaceous chalk formation. The objectives of this study were twofold: (1) to determine the possible role of the mine tips in the sulfate mineralization of the chalk aquifer and; (2) to assess the amount of waste material that can be leached and may supply sulfate ions to the water table.Two sites were selected for this study. Site 1 rests directly on the Senonian-Turonian chalk, whereas site 2 lies on sandy-clayey Cenozoic formations overlying the chalk formations (Figs. 1-2). Water samples were collected within the chalk aquifer (Fig. 3), which represents a free water table except for where the almost impermeable Cenozoic formations confine this table (site 2). Rock samples were collected at the surface and at a depth of <12 m at both sites. Various analyses were performed on these samples including mineralogical analyses carried out on both the bulk fraction and the clayey fraction, as well as elementary analyses of total carbon, total sulfur and CaCO3 contents. Elemental analyses were carried out by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES: major and minor elements) or Inductively Coupled Plasma-Mass Spectrometry (ICP-MS: trace elements). Chemical and isotopic (C, S) analyses were performed on water sampled from upstream and downstream of the mine tips. In situ measurements were also carried out during sampling. Finally, to assess the degree of rainwater seeping through the coal mine tips, two radio-magnetotelluric surveys were carried out in September and November, 1999 following rainy periods. Permeability measurements and grain-size analyses of subsurface samples were also performed at site 2.The carbon and sulfur contents showed superficial leaching on the mine tips (Fig. 4). The use of sulfur isotopes as tracers of the sulfate origin allowed identification of two sources for the two sites: a "mine tip" source with a slightly negative d34 S (-2.8‰ to -3.9‰), which corresponds to the oxidation of sulfides contained by the Carboniferous shales, and another source (d34 S=-20‰) corresponding to the gypsum of the Cenozoic formations, which was only present at site 2 (Figs. 5 & 6).This study outlined different behavior for the tips of the two sites. At site 1, where there is a free water-table zone, the mine tip leachates carry sulfate ions directly to the water table, whereas in the case of a confined aquifer zone such as the one present at site 2, a proportion of the sulfate was reduced once exported to the water table (the redox potential showed negative values; Table 1). This suggestion of bacterially-mediated reduction is supported by the d34 S of the sulfate content in the water table. The bacterial activity was fueled by the organic carbon release that accompanies the sulfur leaching on the mine tips. This carbon contribution was confirmed by the 14C activity that characterized the chalk aquifer waters at the upstream region of the mine tip and noticeably decreased downstream. The decrease is a result of the supply of "dead carbon" from the mine tips (Fig. 7).The oxidation of pyrite also results in H+ production. However, the pH decrease observed downstream from the sites was very slight. Waters derived from leaching of the mine tip seeped through the buffered environment of the chalk aquifer. The distribution of metal content showed no surface to depth gradient for samples taken from both sites. The only evidence of "neutral acid mining drainage" (NAMD) was the sulfate amounts exported, and the increase in Mg, Ca, HCO3- and Sr contents observed downstream from the sites (Table 2a-b).The decrease in the apparent resistivity of radio-magnetotelluric profiles demonstrated that rain waters could deeply penetrate in some parts of the tip at site 2 (Fig. 9). Considering the constant nature of the waste material (grain size and porosity), a decrease in resistivity accompanied by an increase in conductivity between the two surveys indicated water seepage. Permeability measurements showed the occurrence of deep permeability barriers (Fig. 10), limiting not only rainwater seepage, but also the amount of mobilizable sulfide and consequently the amount of sulfates exported to the chalk aquifer. Grain size is not the only reason for the permeable or impermeable nature of waste material - the grain ordering and the compaction of levels at depth also have a role

The Cretaceous-Palaeogene (K/P) boundary in the Aïn Settara section (Kalaat Senan, Central Tunisia): lithological, micropalaeontological and geochemical evidence

The Cretaceous-Palaeogene (K/P) boundary, until recently known as the "Cretaceous-Tertiary" or K/T boundary, is well exposed at Aïn Settara in the Kalaat Senan area (Central Tunisia), 50 km south of the El Kef section. Micropalaeontological and geochemical studies led to the identification of six main features tentatively named "events", which characterise the K/P boundary interval, and of which at least two (B and C) have global significance. The lowermost event A located at about 14 cm below the base of the Dark Boundary Clay is marked by a sudden increase in tiny bioturbations, by small nodules and a few macrofossils, a 50% drop in calcareous nannofossil abundance and an increase in Scytinascias (organic linings of foraminifera). It is thought to witness a slowdown in sedimentation. Event B is characterised by a burrowed surface, separating the ca 60-cm thick Dark Boundary Clay from the underlying Aïn Settara marls. It indicates an episode of nondeposition, just before a major change in lithology from marls to clays, corresponding to a major flooding. No substantial palaeontological changes have been recorded in relation to this event. Event C is characterised by maximum concentrations of Ir and Ni-rich spinels, which have been observed in platy nodules, similar to the level at El Kef (K/P boundary sensu ODIN, 1992). It coincides with a major extinction in planktonic foraminiferal species (71%) and a 60% drop in nannofossil abundance. The change in lithology (occurrence of small ripples and channel-like structures) recorded at event D, a few cm up-section, might be related to a locally recorded storm activity. Events E and F, which are situated higher up in the Dark Boundary Clay, are mainly determined by palaeontological changes (palynomorphs and nannofossils), probably resulting from small sea-level variations. The coincidence of the cosmic markers with the major biotic changes at event C pleads for the asteroid impact hypothesis. Their disjunction from the base of the Dark Boundary Clay shows that the change of lithology usually used to determine the K-P boundary is distinct from the major extinction (in the planktonic realm), classically referred to this boundary and linked to the presence of cosmic markers. These results argue the need for the revaluation of the K-P boundary GSSP at El Kef. It is suggested to redefine the K-P boundary at the level of coincidence of the major biotic changes and the cosmic markers

Bottom-Water Conditions in a Marine Basin after the Cretaceous–Paleogene Impact Event: Timing the Recovery of Oxygen Levels and Productivity

An ultra-high-resolution analysis of major and trace element contents from the Cretaceous–Paleogene boundary interval in the Caravaca section, southeast Spain, reveals a quick recovery of depositional conditions after the impact event. Enrichment/depletion profiles of redox sensitive elements indicate significant geochemical anomalies just within the boundary ejecta layer, supporting an instantaneous recovery –some 102 years– of pre-impact conditions in terms of oxygenation. Geochemical redox proxies point to oxygen levels comparable to those at the end of the Cretaceous shortly after impact, which is further evidenced by the contemporary macrobenthic colonization of opportunistic tracemakers. Recovery of the oxygen conditions was therefore several orders shorter than traditional proposals (104–105 years), suggesting a probable rapid recovery of deep-sea ecosystems at bottom and in intermediate waters.This research was supported by Projects CGL2009-07603, CGL2008-03007, CGL2012-33281 and CGL2012-32659 (Secretaría de Estado de I+D+I, Spain), Projects RNM-3715 and RNM 05212, and Research Groups RNM-178 and 0179 (Junta de Andalucía)

Marine oxygen production and open water supported an active nitrogen cycle during the Marinoan Snowball Earth

The Neoproterozoic Earth was punctuated by two low-latitude Snowball Earth glaciations. Models permit oceans with either total ice cover or substantial areas of open water. Total ice cover would make an anoxic ocean likely, and would be a formidable barrier to biologic survival. However, there are no direct data constraining either the redox state of the ocean or marine biological productivity during the glacials. Here we present iron-speciation, redox-sensitive trace element, and nitrogen isotope data from a Neoproterozoic (Marinoan) glacial episode. Iron-speciation indicates deeper waters were anoxic and Fe-rich, while trace element concentrations indicate surface waters were in contact with an oxygenated atmosphere. Furthermore, synglacial sedimentary nitrogen is isotopically heavier than the modern atmosphere, requiring a biologic cycle with nitrogen fixation, nitrification and denitrification. Our results indicate significant regions of open marine water and active biologic productivity throughout one of the harshest glaciations in Earth history

Sédimentologie de la matière organique en milieu marin : le point de départ de la formation des roches mères d'hydrocarbures

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