Analyses of shocked quartz from the K-P boundary

The precise cause and timing of the Cretaceous-Paleocene (K-P) mass extinction 65 Ma ago remains a matter of debate. Many advocate that the extinction was caused by a meteorite impact at Chicxulub, Mexico, and a number of potential kill-mechanisms have been proposed for this. Although we now have good constraints on the size of this impact and chemistry of the target rocks, estimates of its environmental consequences are hindered by a lack of knowledge about the obliquity of this impact. An oblique impact is likely to have been far more catastrophic than a sub-vertical one, because greater volumes of volatiles would have been released into the atmosphere. The principal purpose of this study was to characterize shocked quartz within distal K-P ejecta, to investigate whether the quartz distribution carried a signature of the direction and angle of impact. Our analyses show that the total number, maximum and average size of shocked quartz grains all decrease gradually with paleodistance from Chicxulub. We do not find particularly high abundances in Pacific sites relative to Atlantic and European sites, as has been previously reported, and the size-distribution around Chicxulub is relatively symmetric. Ejecta samples at any one site display features that are indicative of a wide range of shock pressures, but the mean degree of shock increases with paleodistance. These shock- and size-distributions are both consistent with the K-P layer having been formed by a single impact at Chicxulub. One site in the South Atlantic contains quartz indicating an anomalously high average shock degree, that may be indicative of an oblique impact with an uprange direction to the southeast +/- 45°. The apparent continuous coverage of proximal ejecta in this quadrant of the crater, however, suggests a relatively high impact angle of >45°. We conclude that some of the more extreme predictions of the environmental consequences of a low-angle impact at Chicxulub are probably not applicable.

(Table 1) Stable oxygen isotope ratios of Globigerinoides sacculifer of ODP Hole 130-806B

A detailed Pliocene oxygen isotope record from the Ontong Java Plateau, based on measurements of the surface-dwelling planktonic foraminifer Globigerinoides sacculifer, was produced for the period from 5 to 2 Ma. The record documents major long- and short-term climate changes. The results show periods of enhanced ice volume at 4.6 to 4.3 Ma and after 2.85 Ma, a long-term warming trend from 4.1 to 3.7 Ma, and a distinct cooling trend that was initiated at 3.5 Ma and progressed through the initiation of large-scale Northern Hemisphere glaciation after 2.85 Ma (according to the time scale of Shackleton and others proposed in 1990). Periods of high average ice volumes also show the highest d18O amplitudes. The pattern of climate cyclicity changed markedly at about 2.85 Ma. Earlier times were marked by high-frequency variability at the precessional frequencies or even higher frequencies, pointing to low-latitude processes as a main controlling factor driving planktonic d18O variability in this period. The high-frequency variability is not coherent with insolation and points to strong nonlinearity in the way the climate system responded to orbital forcing before the onset of large scale Northern Hemisphere glaciation. After 3 Ma, stronger 41-k.y. cyclicity appears in the record. The shift in pattern is clearest around 2.85 Ma (according to the time scale proposed by Shackleton and others in 1990), 100-200 k.y. before the most dramatic spread of Northern Hemisphere ice sheets. This indicates that high-latitude processes from this point on began to take over and influence most strongly the d18O record, which now reflects ice-volume fluctuations related to the climatic effects of obliquity forcing on the seasonality of high-latitude areas, most probably in the Northern Hemisphere. The general Pliocene trend is that high-latitude climate sensitivity and instability was increasing, and the causal factors producing the intensified glacial cyclicity during the Pliocene must be factors that enhance cooling and climate sensitivity in the subarctic areas.

10Be and barium concentrations of sediment core GeoB1008-3

Particle reactive elements are scavenged to a higher degree at ocean margins than in the open ocean due to higher fluxes of biogenic and terrigenous particles. In order to determine the influence of these processes on the depositional fluxes of 10Be and barium we have performed high-resolution measurements on sediment core GeoB1008-3 from the Congo Fan. Because the core is dominated by terrigenous matter supplied by the Congo River, it has a high average mass accumulation rate of 6.5 cm/kyr. Biogenic 10Be and Ba concentrations were calculated from total concentrations by subtracting the terrigenous components of10Be and Ba, which are assumed to be proportional to the flux of Al2O3. The mean Ba/Al weight ratio of the terrigenous component was determined to be 0.0045. The unusualy high terrigenous 10Be concentrations of 9.1 * 10**9 atoms/g Al2O3 are either due to input of particles with high10Be content by the Congo River or due to scavenging of oceanic 10Be by riverine particles. The maxima of biogenic 10Be and Ba concentrations coincide with maxima of the paleoproductivity rates. Time series analysis of the 10Be and of Ba concentration profiles reveals a strong dominance of the precessional period of 24 kyr, which also controls the rates of paleoproductivity in this core. During the maxima of productivity the flux of biogenic Ba is enhanced to a larger extent than that of biogenic 10Be. Applying a model for coastal scavenging, we ascribe the observed higher sensitivity of Ba to biogenic particle fluxes to the fact that the ocean residence time of Ba is approximately 10 times longer than that of 10Be.