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

3.3. Reworked spherules in event deposit

We demonstrated that (1) spherules in the event deposits are not the original ejecta fallout – but reworked from an older layer as evident by the lithified clasts{1} with well-preserved spherules, and (2) the original Chicxulub ejecta layer is present in the altered impact glass spherule layer (cheto smectite) 45-60 cm below the event deposit [1]. Schulte et al. misrepresent our data and interpretation with a mock argument asserting that we proposed the clasts were eroded from the cheto smectite clay. Clearly, the clasts originated from a lithified unit of the original spherule layer. This unit was probably deposited in very shallow waters{2}, lithified and subsequently eroded, as suggested by the cracks infilled with spherules that suggest subaerial erosion. This is also indicated by the highly negative (-7 to -9‰) δ13C values of the clasts that suggest secondary calcite precipitated from isotopically light meteoric water. It is only in the lithified clasts that spherule preservation is good,{3} which in now way contradicts the clay altered (cheto smectite) original impact spherule layer{4} as Schulte et al. argue (see below). Contrary to their argument, the three upward fining spherule-rich units of the event deposit in Mull-1 show characteristic re-sedimentation and dilution by glauconite, phosphate grains, broken shells and detritus{5}.

1) Of course the spherules are reworked (they occur in a sandstone) but reworked from the same layer, not from some yellow layer below. Such reworked 'lithified clasts' occur in Brazos, as well as many sections in eastern Mexico, and are rip-up clasts from upper Maastrichtian Corsicana marls and limey layers. These clasts are poorly lithified when redeposited, so they pick up spherules when rolling over the seafloor in and on the spherule layer and become armoured mudballs. Moreover, some display cracks that are filled with squeezed in spherules. Lithified clasts exclusively consisting of spherules do not occur at Brazos.

2) A funny argument. In the Keller 2007 paper (which we criticized in Schulte 2008), the Keller et al refer to Culver2003 as "independent" proof that at Brazos river the Maastrichtian shallows towards the KT boundary in support of their sealevel lowering scenario. Culvers (2003) paper is a review of benthics, and cites for Brazos river exclusively the Keller 1991 paper where the KT boundary is drawn too high. Talking about circular reasoning. Final wrap-up: No shallowing occurs below the event bed.

3) ?? Nonsense. In many places (Darting Minnows Creek) at Brazos the loose individual spherules are (in shape) very well preserved (although altered from glass), as shown in Keller (2007) own figure 9 (!). In the yellow layer no clearly visible spherules occur (yes, circular root traces) but not solid evidence.

4) The claim of "original spherule layer" should be based on more than a vague picture. The claymineral arguments are pointless, because these to smectite altered spherules and/layers don't prove or disprove anything.

5) This dilution (at Darting Minnows creek about 50%) is easily explained when freshly fallen spherule ejecta are reworked through tsunami surges together with local seafloor detritus (yes, glauconite, phosphate, bioclasts). However, our criticism was about the feasibility how to do the same after 200.000 years! As clast, may be, but we doubt the presence of those. But to reach reworked abundances of spherules over 200.000 years requires more than a miracle, especially when this is required to explain the presence of >90% spherule abundances in several locations in Mexico (Penon, Loma Cerca)