The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.
The Paleocene to early Eocene is punctuated by several transient, ∼20-200 ky lasting hyperthermal events of which the Paleocene-Eocene Thermal Maximum (PETM) was the most prominent one. Abrupt shallowing of the lysocline/CCD, negative carbon isotope excursions, and benthic faunal turnover all imply a major perturbation of the ocean system during these events. Our recent research at the Southern Tethyan shelf suggests the presence of an additional hyperthermal event associated with sea-level fluctuations, the Latest Danian Event (LDE; Speijer, 2003; Bornemann et al., 2009). At Zumaia, Northern Spain, a negative ∼0.5 per mil carbon isotope excursion is present in the uppermost Danian that may correlate to the LDE (Arenillas et al. 2008). Moreover, cyclostratigraphic studies have shown that several deep-sea sites are characterized by a prominent peak in both Fe and MS data at cycle Pc100-38 in the uppermost Danian: this applies to all Walvis Ridge (Atlantic) and Shatsky Rise (Pacific) sites as well as Site 1001 in the Caribbean Sea (Top Chron C27n Event; Westerhold et al., 2007). These results suggest that the LDE in the Tethys and the Top Chron C27n Event in the Atlantic may be correlative. We have conducted mineralogical, geochemical, and micropaleontological investigations to characterize this event in the Western Atlantic. Our first results from ODP Leg 165 Site 1001 show that the Top Chron 27n Event shown by Westerhold el al. (2008) corresponds to a ∼12 cm thick clay layer. Mineralogical analyses reveal a sharp ∼50% drop of the carbonate content in the clay layer and a disproportionally high increase of the phyllosilicate content in the insoluble residue compared to the quartz and illite content. Bulk rock isotope analyses show an abrupt negative ∼0.6 per mil carbon isotope excursion at the onset of the clay layer, followed by a 1-m thick interval where carbon isotopes shows a tailing back to pre-event values. The magnitude and pattern of the carbon isotope excursion is very similar to the results for the LDE in the Tethys and at Zumaia. In conclusion, our results demonstrate a supra-regional transient perturbation of the carbon cycle during the LDE in the Tethyan realm, the Atlantic, and possibly the Pacific Ocean.
Understanding the depositional sequence and composition of impact ejecta is critical for the interpretation of timing and effects of the Chicxulub impact regarding the mass extinction at the Cretaceous-Paleogene (K-Pg) boundary. Preliminary investigations have shown that the shallow La Popa and Parras foreland basins in northeastern Mexico both feature outstanding and continuous 3D exposures of the Chicxulub ejecta-rich, K-Pg boundary event deposit (Lawton et al., 2005). The m-thick sand-siltstone interval directly underlying the ejecta-rich mass flows shows evidence of slumping and liquefaction, locally leading to complete disorganization and disruption of the pre-impact late Cretaceous sedimentary sequence. The subsequent ejecta-rich sequence consists of an up to one m-thick basal carbonate-rich bed that discontinuously fills a valley-like topography. Besides abundant silicic and carbonate ejecta spherules (up to 50%) that are excellently preserved, this bed includes abundant mollusks and gastropod shells, as well as vertebrate bones and teeth. The conglomeratic bed is overlain by a series of alternating fineto medium grained calcareous sandstones with shell debris and ejecta that were deposited by repeated currents / mass flow events incorporating varying source areas. Hummocky-cross-stratified strata that mark the return to a normal out-shelf depositional regime conformably overly these sandstones. We interpret this sequence as evidence for presumably seismic-induced sediment liquefaction followed by a series of impact-related tsunami deposits. The specific depositional sequence and Fe-Mg-rich ejecta composition as well as the petrography of the sandstones all closely link the K-Pg boundary sequence in the La Popa and Parras basin to the well-known deep-water K-Pg sites in the Gulf of Mexico (e.g. El Mimbral; Smit et al., 1996; Schulte and Kontny, 2005).
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
