Geoscientist Online

C. Chicxulub - Yaxcopoil-1

1. Foraminifera are not dolomite crystals

Smit 'strongly contests' that the images shown by Keller from the micritic limestones from the Yax-1 core (Fig. 9) are planktic foraminifera. He states that after 30 years of looking at hundreds of thin sections (he must have started as an undergraduate) he would rather abstain from identifying species in thin sections. He also cites Jose Antonio Arz as a foraminiferal specialist who could not identify any specimens in these dolomitized micritic limestones. What he fails to mention is that Arz is a young PhD and to my knowledge Smit has not published any papers on foraminiferal studies since l982.

But overall, his protestations also sound very familiar - he couldn't find the foraminifera, therefore they don't exist. He has used the same argument to contest every piece of empirical evidence that doesn't fit his impact-tsunami hypothesis, even going as far as claiming that a limestone consists of welded marl clasts, or injected toothpaste-like, that multiple spherule layers are fluidized injections, that stratified Mendez marls are really fluidized slumps, that glauconite is really altered impact glass, and micritic and dolomitic limestones are really sandstones.

This time Smit raises a red herring by claiming that the dolomite crystal of his Figure n resembles a foraminiferal test.

Figure 21. Late Maastrichtian globotruncanids (probably G. insignis) from Yax-1 sample 19, 10cm above suevite breccia, and Smit's figure n, which he says resembles a foraminifer.

Some of the late Maastrichtian planktic foraminifera shown in Fig. 9 of the previous round, and an earlier set shown at the April EUG-AUG meeting in Nice, including the ones reproduced in Fig. 21 have been identified as unequivocally Maastrichtian foraminifera various foraminiferal experts, including I. Premoli Silva, V. Luciani, S. Abramovich and H. Oberhansli. Even non-experts will see that the specimens in Figure 21 in no way resemble Smit's corroded dolomite crystal and no self-respecting foraminifer expert would call a dolomite rhomb a foraminferal test. The crystal clearly looks like a corroded crystal, nothing more nor less. A scale would help to show that this crystal is much smaller than any globotruncanid species. Moreover, there are no foraminiferal shells preserved in dolomites.

Smit states that Keller's Late Maastrichtian planktic foraminifera identified from the micritic limestone above the suevite breccia are 'just fortuitous combinations of smaller and larger dolomite rhombs'. Would he also argue that normal foraminiferal tests are fortuitous combinations of calcite crystals? Smit backs this assertion up with Figure o where he compares the micritic limestone specimens from Yax-1 with well-preserved foraminifera from Lajilla and Gubbio.

This is like comparing a rotten apple with a fresh one and saying they are not the same because they don't taste the same. They are still both apples in shape and form - merely the preservation has changed. It is the same with the foraminifera from the Yaxl-1 core as compared with Lahilla and Gubbio. The preservation at Lajilla and Gubbio is good, because these are marls and marly limestones. Smit for some reason calls these 'true micrites'.

Figure 22. Late Maastrichtian planktic foraminifera compared with micritic limestones and dolomite rhombs all from the 50cm interval between the suevite breccia and the K/T boundary.

Dolomite rhombs and micrites from the 50cm interval between the impact breccia and the K/T boundary are shown in Figure 22 along with the planktic foraminifera from the same interval at similar magnification. This illustrates that there is no similarity between these as anyone can see. The micritic crystals are tiny and even the largest dolomite crystals are much smaller than any foraminifera. If it were just a fortuitous aggregation of crystals, why are they fortuitously looking like planktic foraminiferal species? Why are no dolomite rhombs present in any of these foraminiferal shapes? It is because these are foraminifera with the original test calcite replaced by micrite.

The true reason behind denying the existence of the obvious comes down once again to science being driven by the impact-hypothesis. The presence of late Maastrichtian planktic foraminifera is incompatible with a K/T age for the Chicxulub impact.

2. There must be sand in sandstone

Smit argues that the micritic limestone between the breccia and K/T boundary is really sandstone. He further argues that any coarse grains, like dolomite rhombs, qualify as sandstone. This is news to us - and many others I'm sure. We still teach our students that sandstone contains sand, a limestone is carbonate, and a dolomite is a dolomite, not sandstone.

Sediment particles are characterised by grain sizes, and particles from 2mm to 0.0625mm are considered sand, lithified as sandstones / arenites. Usually particles are of clastic origin, and most are quartz. That's why we are talking of a siliciclastic deposit. Obviously, there are sand-sized bioclasts grains of reworked limestone, or dolomite, clacarenites, doloarenites, but these are rare. A micritic limestone cannot be a sandstone, because micrite particles consist of lime mud, and are just a few microns in diameter.

The argument here concerns the 50cm-thick laminated micritic limestone interval between the impact breccia and the K/T boundary. Thin section analysis clearly shows these laminations to consist of fine micrite, alternating with larger dolomite rhombs as a diagenetic feature. There is no quartz present. Four thin layers of glauconite are present. Smit views these micrite/dolomite alternations as coarse and fine-grained laminated sandstones consisting entirely of dolomite and limestone grains. But the coarser grained layers of dolomite are a diagenetic feature, which he has recognized himself. This makes them neither sandstone nor a high-energy coarse-grained deposit.

3. Glauconite from impact glass?!

From a mineralogical perspective Smit seems to me to lack some fundamental expertise in clay minerals. Glass alteration will produce mainly Cheto smectite as observed in Belize and southern Mexico (Debrabant et al. 1999; Keller et al., 2003b). Illite or chlorite are not likely glass-alteration products, as claimed by Smit, nor glauconite, especially not the type which impregnates the K/T layer at Yaxcopoil-1.

Smit claims that the green mineral from the K/T boundary that we show in Fig. 11 of the last round of debate is typical for glauconite or chlorite. Evidently our point didn't get across and so we will elaborate on glass alteration products and glauconite formation.

By definition, glauconite is a poorly crystallized illite enriched in K and Fe (Odin, 1988) with a 001 peak located close to 10Å (8.9°). XRD analyses of the K/T interval at Yax-1 shows a typical glauconite and NOT a chlorite, which is characterized by different 00l peaks (compare Figures 23 and 24), and it is NOT an illite, which is characterized by a well developed 002 peak at 5Å (Fig.3) (contrary to glauconite).

Figure 23. Typical XRD profile of the glauconite layer that marks the K/T boundary transition at Yaxcopoil-1 core.

Figure 24. Typical XRD profile of a Cheto smectite from altered glass in the suevite breccia of the Yaxcopoil-1 core.

The composition of the Yax-1 glauconite is also completely different from that observed in NE Mexico spherules layers (Fig.25). For example, the clay minerals of the spherule layers are very different from those observed in southern Mexico, Haiti, Belize and Guatemala. In NE Mexico the spherule layers contain more irregular mixed layers of illite smectite (IS;10-15%) and chlorite smectite (CS;5%), than smectite (<7%). Mica and chlorite are also abundant with 30 to 50% and 40-50% respectively. All these clay minerals are already present in the Mendez marls, which do not significantly differ from the spherule layers.

Figure 25. Typical XRD profile of the spherule layers in NE Mexico. Note how different these are from the cheto smectite or glauconite of the Yax-1 core.

Smectite is poorly crystallized in NE Mexico sections and never constitutes a single and very well crystallized phase, as observed at Yax-1, Bochil, Armenia and Albion Island quarry and to a lesser extent at Haiti (Keller et al., 2003b). Nevertheless, each SRD layer is characterized by a small but significant increase in smectite (from 0-2% to 5-15%), derived probably from glass alteration. But in this case, the glass alteration product appears to have been combined and/or diluted by other clay minerals, such as illite and chlorite, which are common byproducts of weathering reactions with low hydrolysis typical of warm and/or dry climates. The low smectite contents derived from glass alteration in these spherule layers appears strongly diluted by the more dominant illite, mica, CS and IS (enriched in Al,Mg, Fe). These minerals are probably derived from increased erosion linked to the coeval Sierra Madre Oriental uplift. Hence, the composition of the NE Mexico spherules is in no way related to the glauconite observed in the Yax-1 core.

Smit likens the glauconitic spherules found at or near the K/T boundary in the Apennines by Montanari to those of the Yax-1 core. But these spherules are not the result of glass alteration. They formed by winnowing of glauconitic hardgrounds, which are common in the Apennines before, at and after the K/T boundary. Such glauconitic spherules commonly form during periods of very condensed sedimentation, especially at times of sea level rises. Contrary to Smit's interpretation, glauconite formation does not depend on tectonic stress - glauconite forms on the sea floor.

Most glauconite deposits correlate with hiatuses or very slow sedimentation and generally consist of four successive stages that follow one another at the sediment-water interface, provided suitable conditions persist for at least 105years (Chamley, 1989).

The significance of having glauconite in the critical 50cm interval between the impact breccia and the K/T boundary in the Yax-1 core is the fact that it represents deposition over a very long time period. We identified four thin glauconite layers within this interval. Each layer represents a prolonged period of non-deposition and erosion. This means that this interval could not have been deposited as a result of backwash, reworking, or any other high energy or short-term deposition related to an impact event. In other words, there was a prolonged time interval of normal low energy deposition and glauconite formation between the time of the Chicxulub impact and the K/T boundary event, which means that the Chicxulub impact must predate the K/T boundary.

Conclusions

Science progresses from hypotheses to empirical records to testing hypotheses. As more evidence is discovered, hypotheses need to be revised or discarded. Even the most popular hypothesis is unlikely to withstand the test of time and accumulating evidence. Popular hypotheses, such as the K/T Chicxulub impact-tsunami mass extinction, tend to get a cult following and hence a life of their own that resists change even in the face of overwhelming evidence. While some parts of the impact hypothesis are still valid, as for example that a major impact occurred at the K/T boundary, others are clearly not and need to be jettisoned. The parts of the theory in question include:

(a) that the Chicxulub impact is of K/T age,

(b) that the Chicxulub impact caused the K/T mass extinction,

(c) that the Chicxulub impact generated a major impact-tsunami

(d) that the Ir anomaly at the K/T boundary is due to the Chicxulub impact.

The empirical evidence that Chicxulub predates the K/T boundary and did not cause a mass extinction is multifaceted and very strong.

1. The age of the oldest Chicxulub impact ejecta spherule layer in NE Mexico predates the K/T boundary by 300,000 years.

• Evidence includes multiple spherule layers with the oldest one near the base of zone CF1, which spans the last 300 kyr of the Maastrichtian.
• The spherule layers are interbedded in undisturbed bedded marls of the Mendez Formation, which reveals the age of deposition.
• Spherule layers are correlatable over great distances.
• The stratigraphically lowest and oldest spherule layer consists of almost pure spherules and only rare clasts, which indicates rapid deposition after the impact and no significant bottom currents.
• Subsequent spherule layers contain variable amounts of reworked clasts indicating erosion and re-deposition.
• The absence of major tectonic disturbance, including major slumps, faults or fluidized sediments.
• The stratigraphically highest and youngest spherule layer occurs just below the siliciclastic deposit and is known as spherule unit 1. It contains the most abundant reworked shallow water debris and mud clasts, indicating transport from shallow shelf areas.
• A sandy limestone layer within spherule unit 1 indicates that deposition occurred in two phases separated by hemipelagic deposition.
• The K/T boundary, Ir anomaly and mass extinction occurs above the siliciclastic deposit and represents the true K/T impact event.

2. The Chicxulub impact-tsunami hypothesis is invalid

This hypothesis was designed to explain the presence siliciclastic deposit between the K/T boundary above it and the Chicxulub spherule ejecta below. This hypothesis is invalid for many reasons, but the major ones include:

• There are various horizons of bioturbation within units 1, 2 and 3 of the siliciclastic deposit, which indicate repeated colonization of the ocean floor during sedimentation and hence rules out a tsunami deposition event.
• There are various fine-grained layers, often bioturbated within unit 3 that indicate normal hemipelagic sedimentation alternating with rapid deposition.
• Two bentonite layers in unit 3 are correlatable across NE Mexico and indicate periods of volcanic influx and normal deposition.
• A sandy limestone layer within spherule unit 1 is burrowed and also indicates a period of hemipelagic deposition.

3. The age of the Chicxulub impact breccia at Yax-1 predates the K/T boundary.

Critical evidence is in the 50cm interval between the impact breccia and the K/T boundary and includes the presence of:

• Low energy laminated micrites and dolomitic limestones between the impact breccia and the K/T boundary.
• Four thin layers of glauconite formation within this interval that indicates very low sedimentation over a very long time (l05 yrs).
• Bioturbation within these sediments that indicates an active bottom dwelling fauna during deposition.
• Late Maastrichtian planktic foraminiferal assemblages of zone CF1, indicative of deposition during the last 300Ka, similar to NE Mexico.
• Palaeomagnetic chron 29r that marks the last 500Ka of the Maastrichtian.
• Carbon isotope values characteristic of late Maastrichtian sediments and without evidence of erratic changes that would indicate reworking.
• Absence of impact breccia clasts, or reworked clasts from lithologies below the impact breccia.
• Absence of reworked fossils from older sediments.
• Absence of high-energy deposition, backwash, slumps, crater infill.
• Absence of Cheto smectite that would indicate presence of altered impact glass.

The evidence presented here indicates that the Chicxulub impact predates the K/T boundary by about 300Ka. No mass extinction is associated with this time interval. This should not be surprising since the estimate of the Chicxulub crater size has been steadily shrinking to where it is now only about 120km to at most 150km in diameter. The 100km Popigai crater caused no mass extinctions, and not even any significant species extinctions. However, a major global warming occurred between 200-400Ka before the K/T boundary, which is generally attributed to major Deccan volcanism, though the Chicxulub impact may also have contributed.

The K/T boundary impact, known from the global Ir distribution, still remains to be found. Considering the global distribution of Ir and the mass extinction, we would expect this impact crater to be much larger than Chicxulub. The K/T boundary mass extinction needs to be re-evaluated with respect to the multiple impacts combined with volcanic effects on biota.

There is evidence of a third post-K/T impact event in the early Danian zone Pla (middle of P. eugubina zone), which is recognized by an Ir anomaly in sections from Haiti, Guatemala, Mexico and the Gulf of Mexico. This impact event has yet to be fully investigated. This early Danian impact may have been responsible for the delayed recovery after the impact event.

References

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