Meteorite impacts as responsible for lunar retreat deceleration in the Cambrian boundary. Proterozoic clues and the role of Puncoviscana rhythmites data, northwestern Argentina

VANINA LOPEZ DE AZAREVICH; MIGUEL AZAREVICH

Puerto Madryn

Congreso; XIV Reunión Argentina de Sedimetología; 2014

Asociación Argentina de Sedimentología

Statistical analyses with Fast Fourier Transforms (FFT) carried out on tidal rhythmites from ElatinaFormation-Australia (620 Ma, Sonett et al. 1996, and literature therein) allowed calculation of an Earth-Moondistance of 3.709 e08 meters. Results obtained on Puncoviscana Formation?s rhythmite sequences in NWArgentina (523 Ma, López de Azarevich, 2010; López de Azarevich et al., 2010) determined a separation of3.7608 e08 m between the Earth and the Moon with less than 5% error.The calculated lunar retreat is 6.48 cm/yrfor Ediacaran-Early Cambrian range. Results highlight a slightly deceleration of the lunar retreat from thecalculated value of 7.01 cm/yr from rhythmite data of the 900 Ma Big Cottonwood sequences together withthose from Elatina Formation during the current research. Deceleration was in order of 0.5 cm/year previous toPuncoviscana sedimentation, evidenced by an inflection point in the lunar retreat graphic (Fig. 1).This astronomic configuration of the Earth-Moon system was consequence of great magnitude episodeoccurred within the celestial alignment in the 100 Ma time span among deposition of Elatina (Ediacara,Australia) and Puncoviscana (Neoproterozoic-Early Cambrian, Argentina) sequences. A major astronomicepisode is registered on Earth by that time due to collision of the Beaverhead (600 Ma), Acraman (590 Ma) andSpieder (~570 Ma) meteorites (Compston et al. 1987; Grieve 1991). The impact craters are allocated in USA andSouth Australia respectively. Acraman generated a 160 km-diameter crater, Spider and Beaverhead were muchsmaller with 13 and 15 km-diameter (Grieve, 1991).The impact energy seems to have positioned the Earth closer to the Moon, stabilizing the balance amongthe planet and its satellite, and diminishing the lunar retreat. This means an inverse situation that happened at900 Ma when the Moon was pushed back (López de Azarevich 2010; López de Azarevich et al., 2010). The totalretreat is conformed then by the sum of the lunar retreat in one sense and the Earth advance in the oppositesense, to the resultant 6.48 cm/year.The Acraman Crater ranks among the largest 4% of known terrestrial impact structures (Earth ImpactDatabase 2009), and the impact would have severely perturbed the Ediacara environment causing a globalcatastrophe (Toon et al., 1997; Williams and Wallace, 2003; Williams and Gostin, 2010). Storms evidences fromprimary sedimentary structures and corresponding facies associations would affect platforms and coastal settingson a drastic sense by 590 Ma, with perturbation of the atmospheric conditions caused by dust ejected to the sky.Declining of surficial temperatures also contributed to installation of the Gaskiers Glaciation (~582 Ma),generating extinction of Early Ediacara Donshuato Fauna which had not evolved towards the Cambrianboundary (Xiao and Laflamme, 2009). Some authors hold that Ediacarans have been largely exterminated by anutrient crisis (Kotlin Crisis, Brasier 1992 among others) immediately prior to the Ediacara-Cambrian boundary.Now the fact of the Acraman impact (sum to other bolides) allows interpretation of a new and relevant factorinvolved in the extinction of most of the early Ediacara fauna.The post-glacial Ediacara was warm to hot and relatively arid at low and middle latitudes, confirming thatthe Neoproterozoic cycle with meteorite catastrophes and extreme climates was finished and the secondoxygenation event installed (NOE, Och and Shields-Zhou, 201 2). Then life colonized Earth platforms and latelyland.Meteorite impacts on Earth surface related to changes in lunar recession detected for the latest Ediacara(Acraman, Beaverhead, Spieder) can be read from raw data processed with FFT methodology of a tidalrhythmite series. The extinction event in the Ediacara-Cambrian boundary was caused by a sum of factors inwhich meteorite impacts have not to be discarded. Stabilization of the recession velocity of the Earth?s satellitefrom the Cambrian boundary results in the generation of longer astronomic cycles, less amplitude tides and theconsequent extension of more benign environments along platforms. This last gave the proper conditions fordevelopment, proliferation and specialization of species in the Cambrian Explosion.On the other hand, wave?s activity associated to cap carbonates development after Neoproterozoicglaciations helped mixing the hydrosphere-atmosphere interphase introducing oxygen to seas along extendedplatforms. This process contributes to life rice during the generation of the NOE. New impact craters are to be found on Earth surface that would bring more data to comprehension of impact event?s role in the planetarygeodynamics.