Laurence Froget

Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event

Abstract Magnetic spinels produced by oxidation of extraterrestrial objects in the atmosphere have a composition distinct from terrestrial spinels. They are characterized by a high iron oxidation state, arising from crystallization under high oxygen fugacities, and a high nickel concentration due to the relatively high abundance of this element in extraterrestrial material. The iron oxidation state increases from micrometeorites, to meteoroid ablation material and to impact-generated products. This reflects a progressive increase of the oxygen fugacity, corresponding to decreasing altitudes of crystallization. Spinels found at the Cretaceous-Tertiary boundary are similar to those that crystallized from meteoroid ablation material and impact-generated products, supporting the view that a collisional event did occur at the end of the Cretaceous.

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

Abstract Ni-rich spinels have been found throughhout the world at the Cretaceous-Tertiary boundary. These minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris, suggesting that the Cretaceous-Tertiary boundary spinels result from an extraterrestrial event. Their stratigraphic distribution, which has been observed at the Cretaceous-Tertiary boundary sites of El Kef (Tunisia), Caravaca (Spain) and Hole 761C (ODP Leg 122, Indian Ocean), are markedly different from site to site and are all much less extended than the Ir anomalies. Regional differences in Ni-rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions. The wider stratigraphic dispersion of Ir is attributed to postdepositional chemical processes. At El Kef, the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate, the 1–2 mm thick Ni-rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years. This upper limit is consistent only with the collision of an asteroid or a comet at the close of the Cretaceous.

The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area

Abstract The K-T boundary at Beloc in Haiti is characterized by a bed containing more or less spheroidal globules composed of either smectite or calcite. A small fraction of these objects still contains well-preserved glass particles which have been interpreted as the remains of tektites resulting from a bolide impact in the Caribbean area. However, Beloc glasses are markedly different from tektites and microtektites: their structure and composition suggest that they are not impact-generated products, but are likely of volcanic origin. On the other hand, in sections which have probably preserved their original stratigraphy, the globule bed, showing well-developed grading, appears first in the sedimentary column followed by the ubiquitous Cretaceous-Tertiary boundary (KTB) clay layer containing Ir- and Ni-rich spinels: granulometric considerations indicate that the globule bed and the Ir- and spinel-rich clay layer correspond to two distinct events. We conclude that the globules at the KTB depositional sequence of Beloc represent a local volcanic event slightly preceding the great KTB cosmic catastrophe. Our observations do not provide any unequivocal evidence that a cratering event occurred nearby.

The K/T boundary at Beloc (Haiti): Compared stratigraphic distributions of the boundary markers

At Beloc, Haiti the Cretaceous-Tertiary boundary (KTB) is characterized by a spherule bed containing glass particles. These particles are considered by some authors as remains of tektites resulting from a nearby impact. However, because of the stratigraphic complexity of the Beloc sections the genetic link between the KTB cosmic event and the spherule bed is not obvious. In this paper, we report new data on shocked quartz and Ni-rich spinels at Haitian KTB sites. The detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence. The first and largest shocked quartz is found in the upper part of the spherule layer. They are abundant and size graded over the 25–30 cm of carbonate-rich sediments overlying this layer. The first Ni-rich spinels, which are also rich in Cr, appear in the carbonate sediments. The size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event. Although different from typical impact glasses (tektites), Beloc glass particles must be considered as impact-derived products. The enormous fluence of shocked quartz (≈104 grains/cm2) is consistent with a proximal event. The upper part of the sequence is more complex. A second distribution of shocked quartz associated with Ni-rich spinels of different compositions appears in the layer containing the maximum Ir concentration. We propose that these features, which are not easily explained by a sedimentary artifact, result from a second collisional event.

Datation par thermoluminescence de la base du gisement paléolithique de Tabun (mont Carmel, Israël)

Abstract In this paper, we present new thermoluminescence dates for burnt flints from the lowest archaeological deposits of Tabun Cave (Mt. Carmel, Israel), which is often used as reference for all of Levant [3] , [11] . We discuss the impact of these dates on the chronostratigraphy and show that the uplift of Mt. Carmel (estimated at 10 cm·ka–1) might be responsible for some of the sedimentary collapses.

Luminescence dates for the palaeolithic site of Piekary IIa (Poland) comparison between TL of burnt flints and OSL of a loess-like deposit

We report the thermoluminescence age-estimates for burnt flints excavated at the palaeolithic site of Piekary IIa and discuss attempts to date quartz grains extracted from the embedding deposit using the OSL technique with the SAR protocol. The results indicate that this loess-like deposit contains a mixture of grains, well bleached at different periods in the past, and that the study of small aliquots seems to be the only way to discriminate between the different fractions.

Search for impact remains at the Frasnian–Famennian boundary in the stratotype area, southern France

In order to detect whether the end-Frasnian worldwide biotic crisis is related to an extraterrestrial impact, the global stratotype section of the Frasnian-Famennian boundary and auxiliary sections within the stratotype area have been examined for impact indicators: iridium. Ni-rich spinel bearing spherules and glassy microtektites. This area is particularly well suited to the search for discrete events because it exhibits biostratigraphically continuous sections of sedimentologically homogenous off-shore deposits. Different environmental settings on oxygenated deep-water seamounts, such as the stratotype section at Coumiac, and in oxygen-depleted depressions (La Serre section) are available. The latter is investigated in more detail because it is the least condensed across the boundary, which is determined by the first occurrence of the typical morphotype of Palmatolepis triangularis, the indicator of the first Famennian conodont biozone. Samples from the biostratigraphically defined boundary and adjacent levels failed to provide significantly high Ir values and no Ni-rich spinel or microtektite has been recovered. This is in contradiction with the results of earlier investigations carried out by H. Geldsetzer on the same section. In contrast, the values of Ir concentrations that we measured are always very low or not detectable. The small overabundances observed in some samples, which are about two orders of magnitude lower than what is currently observed at the Cretaceous-Tertiary boundary, are probably due to the accumulation of the normal flux of cosmic dust during periods of relatively low depositional rates or to a terrestrial origin. At present, we have no evidence that an extraterrestrial impact occurred at the F-F transition.

The K/T stratotype section of El Kef (Tunisia): Events and biotic turnovers

Abstract At El Kef (NW Tunisia), the absence of bioturbation and the high sedimentation rate preserve a high resolution record of the Cretaceous-Tertiary transition. In the upper part of the Maastrichtian (Mayaroensis zone) planktonic foraminifer assemblages show a clear marine regression while the variations of spore-pollen contents show a trend related to a climatic cooling. In addition to these gradual changes, one can see a sharp discontinuity at the top of the Mayaorensis zone. This discontinuity is marked by a thin millimeter-scale brownish goethite layer containing Ir and Ni-rich spinels, the remains of the K/T cosmic catastrophe. This event occurred in coincidence with a drastic reduction (90–95%) of calcareous microfossils (coccoliths, foraminifers, ostracods,…). In contrast, the noncalcareous algae (dinoflagellates) seem to have been insensitive to the K/T event. Moreover, spores and pollen grains from the nearby lands do not indicate such a change in the continental vegetation. The selectivity of the extinctions can be explained neither by the absorption of solar radiations by stratospheric dusts produced by the K/T impact nor by an heat flash burning everything on the Earth. All species requiring solar light for their development should have been affected. The severity of the crisis for calcareous marine species seems due to the, difficulty of microorganisms to fix carbonate. It is well known that trace elements play a fundamental role in the development of micro-organisms, particularly in the construction of their carbonate shells and tests. A significant infall of meteoritic material in the oceans could have severely disturbed the normal processes. The disappearance of Coccoliths should have produced disastrous consequences in surface oceanic warm waters, where they are the main providers of oxygen. Judging from El Kef data, the crisis was fatal to all highly specialized taxa; the others, more adaptable, could find refuge in less disturbed places and recovered when normal conditions prevailed again. In the continental vegetation, it seems that the angiosperms, newly arrived in the vegetal realm and consequently having greater adaptative ability, cross the K/T boundary without very catastrophic extinctions.

Characteristics and origin of the glass spherules from the Paleocene flood basalt province of western Greenland

Abstract Glass spherules have been found in several Paleocene sandstone beds in central Nuussuaq, western Greenland. These beds are a mixture of glass, smectite, and calcite with rare occurrence of graphite and metal. The glass varies from a basaltic to a basic-andesitic composition and displays, on a micrometer scale, Mg, Al, Ca, Fe, Ni, and Cu enrichments relative to the bulk composition. These enrichments are associated with resorbed silicate crystals (plagioclase, pyroxene, and olivine) and partly oxidized metal-sulphide inclusions. The spherules have high Co, Ni, Cu, and Ir concentrations positively correlated with Fe. The high Cu content of the glass and the nonmeteoritic Ir/Cu and Ir/Ni (or Ir/Co) ratios of the spherules exclude a meteoritic origin for these elements. These compositional characteristics indicate a purely terrestrial origin, and likely result from the contamination of a basaltic-andesitic melt by sulphide liquid formed through reduction of a magma by C-rich sediments. This mechanism has already been proposed to account for the occurrence of Fe-bearing basalts and andesites in the Nuussuaq and Disko area. This origin is further supported by the high Cr content of the glasses, consistent with a highly magnesian tholeiitic picrite basalt for the parent magma, and the presence of metal, sulphide, and graphite in the spherule layers. The occurrence in the glass of rounded magnetite resulting from the oxidation of sulphide, and of dendritic spinel crystals characteristic of a rapid growth from a high temperature melt, show that the spherules were quenched in an oxidizing environment. We conclude that Nuussuaq glass spherules result from rapid and incomplete oxidation of liquid droplets generated by one or several episods of lava fountains.