Geoscience Reference
In-Depth Information
Figure 6.1. the letters and digits here were written with the graphite in metamorphosed
sedimentary rocks from Isua, greenland. the text is a nod to the typical isotopic composi-
tion of organic carbon in sediments. Photo kindly provided by minik rosing.
have. On close inspection the oldest rocks, and we don't have many of
these, are also in pretty bad shape. By this I mean that many of them
have been heated to high temperatures, in some cases more than once.
Th is heating changes the original minerals in the rock to other forms
through metamorphism. In a rather telling example, the original or-
ganic matter in some of our oldest examples, the ∼ 3.8 billion year old
rocks from Isua, Greenland, has been completely converted to graphite
through heating. You can actually write with these rocks (fi g. 6.1). In
the rocks from Greenland, and in many of the others, fl uids have also
fl owed through the rocks when they were deeply buried in Earth's crust,
in some cases redistributing the chemical constituents.
6
My point here
is not to be discouraging but rather to be realistic about the limitations
of the geologic record, particularly when we are looking way back in
time. Nevertheless, despite the diffi culties, there are some things we can
say about both the chemistry and biology of earliest Earth. We will begin
by exploring early Earth biology. In this chapter, we will be particularly
interested in looking for any signs of cyanobacteria. Remember, they
were the fi rst oxygen producers on Earth and were the stars of
chapter 4.
Without cyanobacteria, oxygen could not have accumulated into the
atmosphere. We will focus in the next chapter on chemical evidence for
early Earth oxygen.
We begin our quest with the ∼ 3.8- billion- year- old graphite- rich rocks
from Isua, Greenland. My friend and colleague Minik Rosing from the
Geological Museum in Denmark (we briefl y met Minik in
chapter 2
)
has spent much of his career exploring the rocks of Greenland. In fact,
Greenland holds a special place for Minik as he was born there, and his
father, Jens Rosing, was a well-known Greenlandic painter, illustrator,
jewelry designer, and author. After many years of fi eldwork, Minik dis-
covered these graphite-rich sedimentary rocks and immediately recog-
nized them as good candidates for revealing the nature of early life on
Earth. Th e sediments themselves, exposed on an outcrop not much big-
