Brazos river comment and reply

Comments (here below) to the Reply by Gerta Keller, Thierry Adatte, Gerald Baum, Zsolt Berner, a reply to Comment by Schulte et al., Earth and Planetary Science Letters (2008) in press, doi: 10.1016/j.epsl.2007.12.025

2.1. Mass extinction and First Danian Species (quoted text from Keller et al)

On p. 352 [1] we cautioned that "the mass extinction of all tropical and subtropical planktic foraminifera is diminished in the Brazos region because this species group is extremely rare or absent in the very shallow, low oxygen depositional environment", which leads to the absence of the sudden mass extinction. Schulte et al. took the phrase "absence of the sudden mass extinction" out of context to argue that if the mass extinction is not as sudden as in open marine tropics, then there is no mass extinction. This simplistic and erroneous view is best dispelled with the new data shown in Figure 1. Most species survived up to and just across the K-T boundary, similar to the tropics. Their presence in zone P0 may be due to reworking or survivorship. The extinction pattern is gradual rather than sudden and the mass extinction diminished as a result of the lower species diversity in shallow environments. Similar patterns have been documented for other Brazos sections, as well as shallow environments in Denmark, southern Tunisia and Egypt [19-21].

Schulte et al. seem fixated on the abrupt mass extinction pattern of condensed deep-sea sections without realizing that this pattern is often due to incomplete records, and does not apply to shallow water environments such as Brazos. Thus, they argue that our biozonation is inappropriate because "Keller et al. do not record the simultaneous extinction of Cretaceous taxa, (therefore) the base of biozone P0 cannot be established based on these data." At Brazos, as well as El Kef and Elles in Tunisia and complete sections worldwide there is an overlap of Cretaceous species with the evolution of Danian species in zone P0 and even into P1a [18-19, 22-23] and this overlap is independently confirmed by the d13C shift (Fig. 1). Inexplicably Schulte et al. wrongly assert that no evolutionary first appearances of species of any biotic group coincide with the base of the Danian in expanded continuous sequences. Since only planktic foraminifera evolved immediately in the aftermath of the mass extinction, other biotic groups (e.g., dinocysts and nannofossils) are immaterial to this argument. At the stratotype and co-stratotype sections of El Kef and Elles in Tunisia, the boundary clay (zone P0) is 50 cm thick and the first appearances of Woodringina hornerstownensis, Parvularugoglobigerina extensa and Globoconusa daubjergensis occur in the basal 1-10 cm [17-18, 22]. Only the larger morphotypes of G. daubjergensis first appear in zone P1a [23]. Therefore, it is ironic that Schulte et al. argue that Keller et al. [22] placed the first appearance of G. daubjergensis in zone P1a and use this argument to support their placement of the K-T boundary at the base of the event deposit at Brazos- 1 [2, Fig. 1].

We are more than happy to share this fixation with the vast majority of geologists. Please tell all the little children who believe in dinosaurs that the mass-extinction did not happen at the KT boundary.(top)

Talking about out of context! Keller et al fail to observe the sarcasm in our wording where we repeat the sentence by Keller et al. Of course we did not argue that the mass-extinction at Brazos is as sudden as in the tropics. (top)

Keller et al consistently confuse a local disappearance with a mass-extinction. The absence of many species because the area is too shallow, does not mean that the mass-extinction is diminished, but only that the local expression is subdued. (top)

This is simply not true. Keller et al must have used samples in the lowest 1-20 cm of the Danian of el Kef that are contaminated with foraminifers from higher levels. Contamination is easy if one does not clean the outside of those samples, or the cracks in the sample, because above 30 cm the Danian marls are extremely foram-rich with small specimens, and those are easily washed downslope. Another source of contamination could be from burrows that penetrate from above. However, those can be minimized if the samples are thouroughly inspected, which is apparently not the case here. (top)

G. daubjergensis first appears higher in the Danian, appreciably above the fad of G. eugubina . Keller et al probably confused examples of Guembelitria cretacea with the spiny and more evolved forms of G. daubjergensis