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The flux of extraterrestrial matter to Earth as recorded in Paleogene and Middle Ordovician marine sediments

by Cronholm, Anders, PhD

Abstract (Summary)
This thesis aims at reconstructing events in the solar system, mainly collisional events in the asteroid belt, by searches for extraterrestrial minerals in Paleogene and Middle Ordovician marine sediments on Earth. Recent empirical evidence show that Earth has experienced a few brief periods during the Phanerozoic when the flux of extraterrestrial matter significantly increased. The most prominent of these occurred at approximately 470 Ma, as a consequence of the massive break-up of the L-chondrite parent body in the asteroid belt. The finds of more than 87 fossil L chondritic meteorites (Ø = 1-21 cm) in mid-Ordovician strata at Thorsberg, Kinnekulle, give testimony to the spectacular flux of meteorites that followed the break-up event. The fossil meteorites are almost completely pseudomorphed, with the exception of chromite, an exceptionally resistant accessory mineral (~0.25 wt%) in ordinary chondrites. Extraterrestrial chromite (EC) is distributed in the immediate surrounding limestones beds of the fossil L chondrites, indicating that most meteorites that reached the sea floor were dissolved, dispersing the EC grains in the contiguous sediments. The distribution of EC has previously been studied at mid- Ordovician sections in Sweden. The goals of this thesis are threefold: (1) establish the normal background distribution of EC to corroborate the extraordinary circumstances recorded during the mid-Ordovician; (2) investigate the global pattern of the EC distribution during the mid-Ordovician, by studying a remote site; (3) study variations in the marine osmium isotope (187Os/188Os) record across the EC-rich interval at Hällekis, Kinnekulle. The Paleogene marine sediments at Gubbio and Massignano, Italy, were analysed for EC content, yielding 7 EC in a total of 377 kg whole-rock (0.019 EC kg-1). This result is very similar to previously studied mid-Ordovician strata, forming prior to the L-chondritic breaking event, in Sweden and China (0.009-0.013 EC kg-1). In addition, the low EC content at Massignano contradicts a proposed ordinary (L) chondritic meteorite shower in the late Eocene. The general trend in the distribution of sediment-dispersed EC in Swedish strata during the mid-Ordovician has been reproduced in the coeval stratigraphic interval at Puxi River, central China. At this time, the Chinese section was positioned at mid-latitudes on the southern hemisphere, a few 1000 km east of the Swedish sites. The EC-rich interval at Puxi typically has 1-4 EC grains per kg rock, equivalent to previous results for coeval Swedish limestone. Consequently, a global correlation has been established for the EC distribution across the Arenig-Llanvirn transition. A close temporal correlation has also been suggested for the main phase of the Great Ordovician Biodiversification Event and the disruption of the L-chondrite parent body at ~470 Ma, based on bed-by-bed records of EC, 187Os/188Os and invertebrate fossils in Middle Ordovician sediments in Baltoscandia and China. The intense species radiation and level of change in biodiversity of this event changed the biological composition of the Earth’s oceans forever. The causes of the event remain elusive, although influences of extraterrestrial origin cannot be excluded, and further studies are warranted. At Hällekis, the first appearance of common EC grains is marked by a negative 187Os/188Os excursion in the strata, verifying an increased influence of unradiogenic osmium. This source is most likely extraterrestrial in origin, as corroborated by stable strontium isotope ratios from late Arenig to early Llanvirn. In all, 665 kg of Paleogene and Middle Ordovician sediments from Italy and China has been searched for EC grains in this thesis work. The composite background material from the Italian and Chinese sections represents 487 kg of rock, and yielded only 8 EC altogether. The EC-rich Ordovician interval, representing the sequential L. variabilis, Y. crassus and M. hagetiana conodont zones, yielded a total of 290 EC grains in 178 kg of limestone, signifying an average 1.63 EC per kg rock. This clearly shows a two orders-of-magnitude increase in the flux of L-chondritic matter during the mid-Ordovician. In conclusion, the largest documented break-up event in the asteroid belt has left a prominent signature in the coeval sediments on Earth, and this thesis corroborates the significance and global consequences of this event.
Bibliographical Information:

Advisor:

School:Lunds universitet

School Location:Sweden

Source Type:Doctoral Dissertation

Keywords:NATURAL SCIENCES; Earth sciences; sediment-disperesed; L-chondite; chromite; break-up event; Arenig-Llanvirn

ISBN:

Date of Publication:01/01/2009

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