Thierry Adatte

Phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2

Four sections documenting the impact of the late Cenomanian oceanic anoxic event (OAE 2) were studied in basins with different paleoenvironmental regimes. Accumulation rates of phosphorus (P) bound to iron, organic matter, and authigenic phosphate are shown to rise and arrive at a distinct maximum at the onset of OAE 2, with an associated increase in δ 13 C values. Accumulation rates of P return to preexcursion values in the interval where the δ 13 C record reaches its fi rst maximum. An offset in time between the maximum in P accumulation and peaks in organic carbon burial, hydrogen indices, and Corg/Preact molar ratios is explained by the evolution of OAE 2 in the following steps. (1) An increase in productivity increased the fl ux of organic matter and P into the sediments; the preservation of organic matter was low and its oxidation released P, which was predominantly mineralized. (2) Enhanced productivity and oxidation of organic matter created dysoxic bottom waters; the preservation potential for organic matter increased, whereas the sediment retention potential for P decreased. (3) The latter effect sustained high primary productivity, which led to an increase in the abundance of free oxygen in the ocean and atmosphere system. After the sequestration of CO 2 in the form of black shales, this oxygen helped push the ocean back into equilibrium, terminating black shale deposition and removing bioavailable P from the water column.

Multiple impacts across the Cretaceous–Tertiary boundary

Abstract The stratigraphy and age of altered impact glass (microtektites, microkrystites) ejecta layers from the Chicxulub crater are documented in Late Maastrichtian and Early Danian sediments in Mexico, Guatemala, Belize and Haiti. In northeastern Mexico, two to four ejecta layers are present in zone CF1, which spans the last 300 ky of the Maastrichtian. The oldest ejecta layer is dated at 65.27±0.03 Ma based on sediment accumulation rates and extrapolated magnetostratigraphy. All younger ejecta layers from the Maastrichtian and Early Danian Parvularugoglobigerina eugubina zone Pla(l) may represent repeated episodes of reworking of the oldest layer at times of sea level changes and tectonic activity. The K/T boundary impact event (65.0 Ma) is not well represented in this area due to widespread erosion. An Early Danian Pla(l) Ir anomaly is present in five localities (Bochil, Actela, Coxquihui, Trinitaria and Haiti) and is tentatively identified as a third impact event at about 64.9 Ma. A multiimpact scenario is most consistent with the impact ejecta evidence. The first impact is associated with major Deccan volcanism and likely contributed to the rapid global warming of 3–4 °C in intermediate waters between 65.4 and 65.2 Ma, decrease in primary productivity and onset of terminal decline in planktic foraminiferal populations. The K/T boundary impact marks a major drop in primary productivity and the extinction of all tropical and subtropical species. The Early Danian impact may have contributed to the delayed recovery in productivity and evolutionary diversity.

Yucatán subsurface stratigraphy: Implications and constraints for the Chicxulub impact

Much of the discussion about the effects of an end-of-Cretaceous impact by a large extraterrestrial body in northwestern Yucatan has been done in the context of limited and partly erroneous published data on the Mesozoic stratigraphy of that area. Reexamination of cores and geophysical logs taken in several Pemex wells has produced improved lithologic and biostratigraphic correlation of the Jurassic to Maastrichtian section across the northern Yucatan peninsula. These data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near Chicxulub. The only unusual lithologic unit is polymict breccia, which apparently was penetrated at or near the top of the Cretaceous section in all the deep wells of northern Yucatan. This breccia in Pemex wells Yucatan 1, 2, 4, 5A, and 6 is composed predominantly of detrital dolomite, limestone, and anhydrite clasts set in dolomitized carbonate mud matrix, which contains upper Maastrichtian foraminifers. These constituents, mixed with fragments of altered glass or melt rock, shocked quartz and feldspar, and basement rock, suggest an impact as the most likely origin for the breccia. The timing of brecciation is poorly constrained by biostratigraphic data. There is some evidence, however, that the breccia unit is overlain by about 18 m of uppermost Maastrichtian marls, suggesting an impact before the Cretaceous-Tertiary boundary. In addition, there may have been more than one episode of breccia deposition.

Mixed carbonate-siliciclastic record on the North African margin (Malta)—coupling of weathering processes and mid Miocene climate

We investigated rock outcrops spanning the middle Miocene, global climate-cooling step on the Maltese Islands in order to reconstruct continental weathering rates and terrigenous fluxes, as well as to explore the coupling between these later, regional climate and carbonate accumulations. Sedimentation at this location was dominated during the Oligocene and early Miocene by a transitional platform to slope carbonates but progressively switched to a clay-rich carbonate slope system in the middle Miocene. Around 13 Ma, an abrupt change toward clay-dominated marls occurred, and marl deposition persisted until the Tortonian (ca. 12 Ma), when a shallow-water carbonate ramp was reestablished. Clay mineralogy and bulk-rock oxygen isotope analyses suggest that the deposition of the Blue Clay formation was mainly caused by global climate change and related change in the rate of continental weathering. A significant negative correlation (R 2 = 0.65) exists between the carbonate content and the δ 1 8 O record. This, combined with the variation of mass accumulation rate of terrigenous material, suggests that shorter-term periods of globally cooler climate (Mi events) were associated with higher rates of accumulation in continental-derived material. Since during the Miocene Malta was attached to the North African Margin, we propose that the observed trends were due to a regional increase in rainfall during cooler periods, which consequently increased continental weathering and runoff. We further suggest that this pattern was linked to the perturbation of atmospheric fronts due to an increased thermal gradient during the Miocene. Thus, regional increase in rainfall might have been linked to the northward migration of the Intertropical Convergence Zone (ITCZ).

Deposition of channel deposits near the Cretaceous-Tertiary boundary in northeastern Mexico: Catastrophic or normal sedimentary deposits?

Investigation of Cretaceous-Tertiary (K-T) boundary channel deposits in three sections in northeastern Mexico reveal three lithologically and mineralogically distinct units separated by three or more disconformities. The units include a spherule-rich layer (unit 1), laminated sandstone (unit 2), and rippled sand and shale layers (unit 3). On the basis of paleontological, mineralogical, and geochemical evidence, the K-T boundary is just above the top of unit 3. Although the deposits have previously been interpreted as a tsunami deposit related to a presumed K-T impact at Chicxulub on Yucatan, we have found no evidence of a nearby impact. We suggest that deposition occurred in an incised valley setting and over an extended period of time.

Spherule deposits in Cretaceous-Tertiary boundary sediments in Belize and Guatemala

Large spheroid deposits at Albion Island and Armenia in northern and central Belize and the spherule deposits of southern Belize and eastern Guatemala have the same glass origin based on the presence of almost pure Cheto smectite derived from alteration of impact glass from the Chicxulub impact on Yucatan, Mexico. The same origin has also been determined for altered glass spherules in Mexico, Haiti and the Caribbean. However, the spherule layers have variable ages as a result of erosion and redeposition, with an early Danian (Parvularugoglobigerina eugubina) zone Pla(1) age in southern Belize, Guatemala, Haiti, southern Mexico and the Caribbean, and a pre-K–T (Plummerita hantkeninoides) zone CF1 age of 65.27 ± 0.03 Ma in NE Mexico. A pre-K–T age for the Chicxulub impact has now also been determined from the new Yaxcopoil 1 core drilled in the impact crater. These data show that Chicxulub was not the K–T impact that caused the end-Cretaceous mass extinction, but an earlier impact event. A multiple impact hypothesis, volcanism and climate change appears the likely scenario for the end-Cretaceous mass extinction.

Late Maastrichtian and K/T paleoenvironment of the eastern Tethys (Israel): mineralogy, trace and platinum group elements, biostratigraphy and faunal turnovers

The late Maastrichtian to early Danian at Mishor Rotem, Israel, was examined based on geochemistry, bulk rock and clay mineralogies, biostratigraphy and lithology. This section contains four red clay layers of suspect impact or volcanic origin interbedded in chalk and marly chalks. PGE anomalies indicate that only the K/T boundary red layer has an Ir dominated PGE anomaly indicative of an impact source. The late Maastrichtian red clays have Pd dominated PGE anomalies which coincide with increased trace elements of terrigenous and volcanogenic origins. Deccan or Syrian-Turkey arc volcanism is the likely source of volcanism in these clay layers. Glauconite, goethite and translucent amber spherules are present in the clay layers, but the Si-rich spherules reported by Rosenfeld et al. [l989] could not be confirmed. The absence of Cheto smectite indicates that no altered impact glass has been present. The red layers represent condensed sedimentation on topographic highs during sea level highstands. In the Negev area, during the late Maastrichtian, the climate ranged from seasonally wet to more arid conditions during zones CF3 and CF2, with more humid wet conditions in the latest Maastrichtian zone CF1 and in the early Danian, probably linked to greenhouse conditions. Planktic foraminifera experienced relatively high stress conditions during this time as indicated by the low species richness and low abundance of globotruncanids. Times of intensified stress are indicated by the disaster opportunist Guembelitria blooms, which can be correlated to central Egypt and also to Indian Ocean localities associated with mantle plume volcanism. Marine plankton thus support the mineralogical and geochemical observations of volcanic influx and reveal the detrimental biotic effects of intense volcanism.

Application of organic matter and clay mineral studies to the tectonic history of the Abruzzo-Molise-Sannio area, Central Appenines, Italy

Abstract The Abruzzo-Molise-Sannio region is part of the Central Apennines foreland fold and thrust belt, a structure that formed during the Neogene as a result of the continental collision between the European and Adria plates. Certain areas of this crust experienced anomalously high temperatures due to deep burial, while others did not. This burial has been investigated using optical indicators of organic matter (OM) maturity and clay mineralogy. The maximum depths of burial of Tertiary surface sequences have been established based on the vitrinite reflectance (Ro) and thermal alteration index (TAI) of OM dispersed in synorogenic sediments and from clay mineralogy of pre-thrusting clayey deposits. The synorogenic deposits show low levels of organic maturity having Ro values less than 0.6% indicating early mature to mid-mature hydrocarbon generation. The pre-thrusting deposits are shales characterised by a very high percentage of clay minerals with lesser amounts of quartz, K-feldspar, plagioclase and calcite. The dominant minerals are highly smectitic interstratified illite/smectite (RIS). Both Ro and I/S ratios were used to calibrate burial and thermal evolution of this chain sector by numerical modelling. The results indicate that the structural units which outcrop in the study area (particularly the Molise Unit) were never overthrust by significant thicknesses of rocks during mountain building, at least before Pliocene times; the hypothesis that more internal units overthrust the presently exposed units also seems to be unlikely.

Recent changes in sedimentary organic matter from Lake Neuchâtel (Switzerland) as traced by Rock-Eval pyrolysis

Recent changes in the composition of sedimentary organic material (OM) from a well-oxygenated lake (Lake Neuchâtel, Switzerland) were investigated using the new generation Rock-Eval pyrolysis (Re6). Although Re6 offers a range of new analytical parameters, the classical hydrogen and oxygen indices (HI and OI) prove to be most useful for the kind of material analysed. For a rapid screening of lake sediment cores we propose an additional parameter, POI=S3/(S 1+S2a), which reflects most of the observed changes in the nature of the OM, but can be calculated from the pyrolysis step alone.

Chicxulub impact: The origin of reservoir and seal facies in the southeastern Mexico oil fields: Comment and Reply

[Grajales-Nishimura et al. (2000)][1] suggested that “offshore oil-producing breccias and seals from oil fields in the Campeche marine plateform are of K-T boundary age and that their mode of formation is probably related to the K-T impact event at Chicxulub.” The basis for this interpretation