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

Abstract

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.

• author: Cronholm, Anders
• supervisor: Schmitz Birger
• opponent: Professor Alan, Hildebrand, University of Calgary, Canada
• organization: Lithosphere and Biosphere Science
• publishing date: 2009
• type: Thesis
• publication status: published
• subject: Geology
• keywords: L-chondite, sediment-disperesed, chromite, break-up event, Arenig-Llanvirn
• in: Litholund theses
• issue: 17
• pages: 104 pages
• defense location: Lithosphere and Biosphere Science, Geocentrum II, Pangea (room 229)
• defense date: 2009-06-03 13:15:00
• ISSN: 1651-6648
• ISSN: 1651-6648
• language: English
• LU publication?: yes
• id: 1b8099da-1fd0-4cc0-a005-bb69d6b7e808 (old id 1393615)
• date added to LUP: 2016-04-04 09:03:42
• date last changed: 2019-05-21 15:02:37