Effects of the Discovery
In the years following, the work of Hildebrand and his colleagues would be tested to further identify the importance of Chicxulub as the site of a K/T boundary impact. The following aspects became of particular interest:

Confirming the Time of Impact
An impact winter results from sudden drop in temperature as impact debris in the atmosphere blocks the sun's radiation from reaching the Earth's surface. The freezing of the reproductive stages of aquatic fossil plants at specific points in their reproductive cycle has constrained the timing of the impact to approximately early June (5). Despite the precision of this estimate it is difficult to constrain the time of impact to within a 500,000 year period. Estimates of impact time show slight variance from study to study, but those of Virgil Sharpton and colleagues (6) are complementary with other studies. They obtain an age of 65.2+/-0.1 Myr for the formation of the crater's melt rocks from 40Ar/39Ar dating. The melt rocks are also constrained by chron 29R (the time bracket when the Earth's magnetic field was the same as that preserved in the melt crystals when they formed) (6). This lasted from 64.68 to 65.37 million years ago. These ages are synonymous with the age of tektite glasses (impact ejecta) from Haiti (7, 8, 9), and Mimbral, northeast Mexico (10).

Whilst these dates show close correlation to the 65 Myr old Cretaceous/Tertiary boundary layer, W. C. Ward and colleagues (11) find it difficult to declare the Chicxulub crater as the site of the K/T boundary impact crater using present biostratigraphic data; dating of the fossils found within drill hole cores taken across the Yucatan peninsula show approximately 18m of late Cretaceous (Maastrichtian) marl overlying the impact breccia. The exact relation of the Chicxulub crater to K/T boundary sediments remains unresolved.

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Rejecting Manson crater, Iowa, USA, as the K/T impact site
Three differences have been investigated between the Manson and Chicxulub crater; age, size, and melt rock composition relative to K/T impact glass chemistry.

According to Joel Blum and colleagues (12), within analytical uncertainties, both craters have been dated with ages of 65Myr (40Ar-39Ar dating). In 1994, however, the Manson impact structure would be dated at 74 Myr (13), distinguishing it from the 65Myr K/T boundary layer.

The diameter of the Chicxulub crater is at least 170km (see below), compared to only 35km for the Manson crater. It is difficult to explain the size of the extinctions, and the volume and distribution of ejecta at the boundary, from this lesser impact.

Whilst both craters were formed in a target rock of similar composition (sandstones and carbonates), the compositions of their melt rocks are very different (Fig. 3). The Chicxulub melt rocks are isotopically indistinguishable from K/T impact glasses, whilst the composition of the Manson melt rocks are significantly different (12).

Fig. 3 Plot showing the similarity in chemical composition between the Chicxulub melt rocks and K/T impact glasses, compared to the Manson crater melt rocks (12). The composition of the mantle derived mantle rocks is shows that the sampled rocks have not come from the mantle. ie. they may have an extraterrestrial origin.

All of this evidence rejects Manson crater as the site of the K/T impact.

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Multiple Impacts?
Evidence for more than one meteorite impact around 65Myr ago is limited. However, whilst the Chicxulub crater explains most, but not all, K/T boundary phenomena, some unexplained physical characteristics are better understood by multiple impacts at the time. The few inferences for multiple impacts have been used to explain the following:

Despite some support for this topic, Sharpton and colleagues document how the Chicxulub impact crater can account for all the globally distributed ejecta deposited at the K/T boundary, without the need for multiple impacts (6).

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Size and Shape of Chicxulub Crater
The Chixculub site is a multi-ringed impact crater with estimated diameters ranging from 170 to 300km. Hildebrand and colleagues (15) opt for a 170km diameter, as suggested by gravity profiles, and the location of rings of cenotes (mainly water-filled holes found in limestone areas), thought to have formed along fault lines where the steep edges of the crater have slumped (Fig. 4). In contrast, Sharpton and colleagues (16) identified two more-distant rings in their gravity profiles, and they interpret a 300km crater. The depth of the crater is more difficult to determine, and although it is estimated at 15-25km (17) for a vertical impact, a lower impact angle would form a shallower crater - see 'Modelling Impacts' below. The size of the crater remains unresolved.

Fig. 4 Gravity map over the southwestern part of the Chicxulub crater (15). The star represents the centre of the crater. The number lines (1-6) are gravity peaks that show the multi-ringed structure of the crater. The dots represent cenotes. These are particularly evident between 4 and 5, where they are thought to result from slumping at the crater rim. Note the linear features in some of the outer rings to the southwest.

Understanding the internal structure of the crater is also problematic. The northern half is not well defined by gravity data, and in the southwest, the crater's rings show some linearity (Fig. 4). Deep-seated rocks beneath the crater may cause these effects (15).

The crater also shows possible asymmetry (4,16). A central elongate topographic high may be present extending NNW (Fig. 6 below). The diagram represents variations in rock density, however, and inferring crater asymmetry assumes direct correlation to topography, which may be unlikely. See 'Modelling Impacts' below.

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Subsurface Stratigraphy
Although analysis of the well logs was documented by Hildebrand and colleagues (4) in their original paper, the stratigraphy was not analysed in detail, and in 1995, Ward and colleagues (11) decided to re-examine them (Fig. 5). They improved correlation of the stratigraphy and biotic zones across the Yucatan platform, and constrained the main disturbance of strata to within a 100km radius of the crater centre. The whole stratigraphy is a mixture of limestones and marls, dolomite, shales, sandstones and evaporite deposits.

Fig. 5 Stratigraphy shown through a section of the crater. The left diagram is based on data from Ramos and Hildebrand and colleagues (1994). The right diagram has been modified in the light of re-examination of the subsurface stratigraphy (11). The main difference is the presence of the breccia across the whole area. Blue=limestone and marl, Yellow=mainly dolomite (a magnesium containing carbonate rock), Green=Evaporite beds with some thiner dolomite units.

In addition to the melt rocks found within the crater, an overlying, poorly sorted breccia, found right across the Yucatan Peninsula, was of particular interest. It contains shocked quartz and feldspar grains (4, 6, 18), anomalously high iridium levels in isolated rock fragments (6), fragments of melt rock and basement rock (19), and glasses similar to those found in the K/T boundary layer.

Their improved stratigraphy further supported the impact theory, and provided constraints for the time of impact.

Further drilling has also been undertaken to retrieve more core samples, and further investigate the crater stratigraphy (20).

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Modelling Impacts
Modelling is used to help understand the mechanism of impact and the distribution of ejected material. Investigation has shown a high impact angle to be unlikely due to the lack of crystalline basement material in the ejecta; an excavation depth of between 15 and 25km, easily within the depth to the basement rock, has been estimated for a vertical impact (17). Evidence for a shallow impact angle from the southeast is given by thick down-range ejecta blankets to the northwest, and widening of the crater rim transverse to the impact trajectory (although thick ejecta blankets to the northwest can also be explained by ejecta angle - see below) (17). Experimental modelling suggests an impact angle for the Chicxulub crater of between 20 and 30 degrees (17).

Gravity measurements also support the possibility of a shallow impact. The digitised gravity data, represented as relief in Fig. 6, show an asymmetric pattern, with an elongate central gravity high trending northwest from the centre of the crater (4,16). The gravity data are effectively density contrasts between surrounding rocks, and this gravity high suggests either the presence of a dense, melt-filled central depression, or an elongate central uplift (17). Studies of known asymmetric craters show elongate central topographic highs to be a common characteristic of oblique impact craters (17).

Fig. 6 Gravity data has been mapped across the Chicxulub crater. The data has been given relief corresponding to gravity field intensity. A central gravity high extends from the centre of the crater to the northwest. This shape suggests either a dense, melt-filled central depression or an elongate central uplift, the latter being characteristic of oblique impact craters (17). Plot courtesy of Buck Sharpton.

Considering ejection angles and velocities of material expelled from the crater, modelling of ejecta patterns has also been undertaken. A three-phase model of ejecta distribution has been proposed (Fig. 7) (21):

Fig. 7 Proposed three-phase model of ejecta distribution following impact (21).

Using this model, Walter Alvarez and colleagues (21) were able to solve three problems concerning the distribution of shocked quartz grains:

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Accounting for Mass Extinction
Whilst many theories relating global extinctions to meteorite impacts had been proposed prior to the discovery of the Chicxulub crater, ideas were now able to incorporate physical constraints controlled by its formation. Considering their belief in an oblique impact from the southeast, Peter Shultz and colleagues (17) demonstrated that the resulting concentration of ejecta deposits in the northern hemisphere, and particularly the northwest interior of North America, led to greater extinction (and subsequent recovery) in these areas (Table 1).

Region
Number of
Extinction
taxa that
(%)
disappeared at
K-T boundary
North America (various)
47 of 136
35
North Africa (Tunisia)
11 of 67
16
Antarctica (Seymour Island)
2 of 53
4

Table 1 Extinction estimates from studies of K-T bounary fossils.
(After Shultz & D'Hondt (17). The North American records are from
Terrestrial sediments, whilst the others are from marine sediments.

Shultz and colleagues also suggested that an oblique impact would exaggerate global extinction by enhancing the release of SO2, CO and CO2 from the shallow anhydrite and carbonate beds in the target rock. Guangqing Chen and colleagues (22), considering Chicxulub target geology, have estimated the implications of these emissions. A significant global temperature decline is envisaged due to sulphuric acid (H2SO4) vapour, formed from the SO2 and SO3 release, blocking the sun's rays from reaching the Earth. It has been estimated that the cold period would have lasted no more than tens of years, however, as the sulphur oxides would be rained out of the atmosphere over this time. The abundance of CO2 in the atmosphere would also cause global warming, reducing the cooling effects still further, and showing that volatilisation of sulphates was not a major extinction mechanism at the K/T boundary. In contrast, Kevin Pope and colleagues (23) calculate an 8-13yr cooling period, induced by sulphuric acid (H2SO4) production, to be more severe, far exceeding the temperature increases caused by CO2 production (global warming). They conclude that the impact winter may have been a major cause of the K/T extinctions.

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