Chicxulub: La Palinologia

Una ricerca pubblicata da PRAUSS rilancia la discussione sulla precisa datazione dell´evento di estinzione C-P e l´impatto di Chicxulub. Secondo l´analisi di un carotaggio dei dintorni di Brazos (Texas) in una fase di trasgressione marina un orizzonte al di sotto del limite del Cretaceo marca una pronunciata regressione, seguita da un forte aumento di spore nel sedimento.
Queste spore dimostrano che tra i due eventi la sedimentazione era lenta e continua. La ricerca conclude che già prima del limite di transizione si osserva pesanti cambiamenti negli ecosistemi marini, l´orizzonte di regressione potrebbe coincidere con l´impatto, é questo giocare un ruolo nell´estinzione, ma non essere la causa conclusiva. Nessuna variazione nel contenuto di spore é stata osservata invece nel strato di argilla che da KELLER é stato suggerito come rappresentare un - o l´impatto di Chicxulub.

"The quantitative marine palynology of a 25.36 m long upper Maastrichtian to lower Danian core from Brazos, Texas, is compared and integrated with planktonic foraminiferal stratigraphy and, in the upper part, with stable oxygen and carbon isotopes from benthic foraminifera. The K/Pg boundary, defined by the base of planktonic foraminiferal zone P0 and the onset of a negative 13C shift, closely corresponds to the appearance of lowermost Danian organic-walled dinocysts. Based on various palynological proxies as well as on sedimentologic features, a sequence stratigraphic subdivision of the section is proposed. Accordingly, from the base to the top, a second order relative sea-level rise is inferred, modified by several third order cycles. The most prominent change in dinocyst assemblages is related to a major transgression pulse at about 18.50 m core depth. The so-called "event deposit" (ED), a siliciclastic unit located about 97 cm below the base of the P0 foraminifera zone in the present section, possibly represents a combination of episodic relative sea-level low and lag deposit due to initial transgression. Between the base of the ED and the K/Pg boundary, the gradual increase to peak abundance of trilete spores renders a redeposition of this intervening sedimentary unit improbable and demonstrates a significant time lag between these two horizons. Throughout the section, significant fluctuations of both oxygen- and carbon isotopes are present. Suggested warm episodes correlate with increased dinocyst proportions indicative of open neritic, warm-temperate surface waters, whereas cool episodes largely correlate to an increase of the tropical-subtropical dinocyst fraction. This correlation is considered primarily a function of relative sea-level change, restricting surface water circulation largely to the subtropics during sea-level lows and cooler climates and vice versa. In addition, cool climates largely correlate to heavier 13C values and an increased ratio of peridinioid to gonyaulacoid dinocysts (p/g ratio), suggestive of significant fluctuations in marine primary productivity preceding the K/Pg boundary. By contrast, the lower Danian is characterised by the onset of both persistent warm conditions and a drawdown of marine primary productivity. These data suggest that prominent, high frequency palaeoenvironmental changes precede the K/Pg boundary, which is inconsistent with a single "catastrophic" impact as the cause for the K/Pg boundary event. However, according to the onset and distinct distribution of the peak abundance of trilete spores, the base of the ED may actually reflect the Chicxulub impact, which probably contributed significantly to K/Pg boundary crisis within the biosphere. By contrast, no significant changes within palynologic proxies are observed across the time equivalent of a yellow clay horizon recently discovered from a single outcrop at Brazos, which has been suggested as the actual Chicxulub impact fallout."

BIBLIOGRAFIA:
PRAUSS (2010): Marine palynology of upper Maastrichtian to lowermost Danian strata from the Mullinax-1 core, Brazos River, Texas, USA, - evidence for palaeoenvironmental changes. Palaeogeography, Palaeoclimatology, Palaeoecology Article in Press; doi:10.1016/j.palaeo.2010.03.035