The Boltysh meteorite impact crater formed in the Ukrainian Shield on the margin of the Tethys Ocean a few thousand years before the Cretaceous-Paleogene boundary and was rapidly filled by a freshwater lake. Sediments filling the lake vary from early lacustrine turbidites and silts to ~300 m of fine silts, organic carbon–rich muds, oil shales, and lamenites that record early Danian terrestrial climate signals at high temporal resolution. Combined carbon isotope and palynological data show that the fine-grained organic carbon–rich lacustrine sediments preserve a uniquely complete and detailed negative carbon isotope excursion in an expanded section of several hundred meters. The position of the carbon isotope excursion in the early Danian stage of the Paleogene period, around 200 k.y. above the Cretaceous-Paleogene boundary, leads us to correlate it to the Dan-C2 carbon isotope excursion recorded in marine sediments of the same age. The more complete Boltysh carbon isotope excursion record indicates a δ 13 C shift of around -3‰, but also a more extended recovery period, strikingly similar in pattern to the highest fidelity carbon isotope excursion records available for the Toarcian and Paleocene-Eocene hyperthermal events. Changes in floral communities through the carbon isotope excursion recorded at Boltysh reflect changing biomes caused by rapidly warming climate, followed by recovery, indicating that this early Danian hyperthermal event had a similar duration to the Toarcian and Paleocene-Eocene events.

In order to better define the late Eocene clinopyroxene-bearing (cpx) spherule layer and to determine how the ejecta vary with distance from the presumed source crater (Popigai), we searched for the layer at 23 additional sites. We identified the layer at six (maybe seven) of these sites: Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Holes 592, 699A, 703A, 709C, 786A, 1090B, and probably 738B. The cpx spherule layer occurs in magnetochron 16n.1n, which indicates an age of ca. 35.4 ± 0.1 Ma for the layer. We found the highest abundance of cpx spherules and associated microtektites in Hole 709C in the northwest Indian Ocean, and we found coesite and shocked quartz in the cpx spherule layer at this site. We also found coesite in the cpx spherule layer at Site 216 in the northeast Indian Ocean. This is the first time that coesite has been found in the cpx spherule layer, and it provides additional support for the impact origin of this layer. In addition, the discovery of coesite and shocked quartz grains (with planar deformation features [PDFs]) supports the conclusion that the pancake-shaped clay spherules associated with quartz grains exhibiting PDFs are diagenetically altered cpx spherules. An Ir anomaly was found associated with the cpx spherule layer at all four of the new sites (699A, 709C, 738B, 1090B) for which we obtained Ir data. The geometric mean of the Ir fluence for the 12 sites with Ir data is 5.7 ng/cm 2 , which is ~10% of the fluence estimated for the Cretaceous-Tertiary boundary. Based on the geographic distribution of the 23 sites now known to contain the cpx spherule layer, and 12 sites where we have good chronostratigraphy but the cpx spherule layer is apparently absent, we propose that the cpx spherule strewn field may have a ray-like distribution pattern. Within one of the rays, the abundance of spherules decreases and the percent microtektites increases with distance from Popigai. Shocked quartz and coesite have been found only in this ray at the two sites that are closest to Popigai. At several sites in the Southern Ocean, an increase in δ 18 O in the bulk carbonate occurs immediately above the cpx spherule layer. This increase may indicate a drop in temperature coincident with the impact that produced the cpx spherule layer.

Impact stratigraphy is an extremely useful correlation tool that makes use of unique events in Earth's history and places them within spatial and temporal contexts. The K-T boundary is a particularly apt example to test the limits of this method to resolve ongoing controversies over the age of the Chicxulub impact and whether this impact is indeed responsible for the K-T boundary mass extinction. Two impact markers, the Ir anomaly and the Chicxulub impact spherule deposits, are ideal because of their widespread presence. Evaluation of their stratigraphic occurrences reveals the potential and the complexities inherent in using these impact signals. For example, in the most expanded sedimentary sequences: (1) The K-T Ir anomaly never contains Chicxulub impact spherules, whereas the Chicxulub impact spherule layer never contains an Ir anomaly. (2) The separation of up to 9 m between the Ir anomaly and spherule layer cannot be explained by differential settling, tsunamis, or slumps. (3) The presence of multiple spherule layers with the same glass geochemistry as melt rock in the impact breccia of the Chicxulub crater indicates erosion and redeposition of the original spherule ejecta layer. (4) The stratigraphically oldest spherule layer is in undisturbed upper Maastrichtian sediments (zone CF1) in NE Mexico and Texas. (5) From central Mexico to Guatemala, Belize, Haiti, and Cuba, a major K-T hiatus is present and spherule deposits are reworked and redeposited in early Danian (zone P1a) sediments. (6) A second Ir anomaly of cosmic origin is present in the early Danian. This shows that although impact markers represent an instant in time, they are subject to the same geological forces as any other marker horizons—erosion, reworking, and redeposition—and must be used with caution and applied on a regional scale to avoid artifacts of redeposition. For the K-T transition, impact stratigraphy unequivocally indicates that the Chicxulub impact predates the K-T boundary, that the Ir anomaly at the K-T boundary is not related to the Chicxulub impact, and that environmental upheaval continued during the early Danian with possibly another smaller impact and volcanism.