Fossil Ecosystems of North America: The Gunflint Chert

In this Archean Lagerstätten we are seeing some of the very first, single-celled life forms on Earth.

Following, you will find a quick summary and some sample questions you should be able to answer after studying this book chapter. These questions should help you and should guide you through the reading, and are also supposed to provide you with an indication of what kind of information you need to retrieve.

These questions are just examples, and are by no way a complete indication on what could appear in your quiz.

part 1 - BACKGROUND: THE ORIGIN OF LIFE AND THE FIRST 3 BILLION YEARS

This section introduces you to the rest of the chapter by reviewing the early history of life on Earth. It also explains why we would not concentrate on the most ancient fossil record we have found, but rather move on to the 2 billion-year-old (2 by) Gunflint Chert of Northwest Ontario, in Canada (not far from the Minnesota border).

After Earth's surface turned solid (around 4 by ago) evidence shows that life arose around 3.5 billion years ago. That early life consisted only of bacteria belonging to two different kingdoms, the Eubacteria (also known simply as Bacteria) and the Archaebacteria (also known as Archaea).

Bacteria also include photosynthesizing organisms, which arrived on the scene after Archaea. Archea could (and can today) survive in extreme conditions, including absence of sunlight, very hogh or very low temperatures, very hogh concentration of salt, under extremely high pressure, under very high levels of nuclear radiation, and also immersed in what would be (to us) a "cocktail of toxic chemicals".

These last conditions are still existing today in what is a very unfamiliar place for us: around the black smokers forming around the hydrothermal vents that are present in the rift valley of mid-ocean ridges. Archea exist today at these locations, and it has been suggested that life has originated in such a setting. This would make Archaea our oldest ancestors. While both Bacteria and Archaea are Prokaryotes (very simple cells), more advanced Eukaryotes developed around 1.2 by ago (that is, a good 2.3 billion years after the first fossils, and 3.4 billion years after Earth's origin).

This chapter deals with prokaryotic life and its record. This record is scant and sparse, and relies mostly on the evidence provided by two kinds of deposits: stromatolites and cherts. The record of stromatolites and fossiliferous cherts only becomes totally agreed upon for the 2 by old Gunflint Chert.

What is the difference between Prokaryotes and Eukaryotes?
What is the difference between Bacteria and Archaea?
When did life start on Earth?
Where do you find chemosynthetic organisms today?
When did eukaryotic life start?
What are stromatolites?
What are cherts?
The Gunflint Chert is not the oldest prokaryotic fossil. Why then do not we prefer to use an older record for our studies?

part 2 - HISTORY OF DISCOVERY OF THE GUNFLINT CHERT

A Formation is a body of rock that is distinct from surrounding bodies of rocks. We will see more in the future about this concept, but for now the name of a Formation (such as Gunflint) is defined based on the locality where it is first described. The Gunflint Formation comprises alternations of ironstones (a generic name for a sedimentary rock rich in iron, such as hematite; we have seen hematite when we discussed chemical weathering in class) and cherts, chemical sedimentary rocks made of silica (SiO₂). Cherts include layers of stromatolites, and can be red, yellow or black. Red indicates further oxidation of iron, present in minor amounts within the chert, but black indicates the presence of organic matter. These rocks showed evidence of fossils: thousands of minute spheres and stars, with branched and unbranched filaments, all of them less than 10 microns in size (a micron is a 1/1000 of a millimeter, so 10 microns are equivalent to 0.01 millimeters; it takes 25.4 millimeters to make an inch, so these guys are about 1/2500 of an inch in size).

Where is the area where the Gunflint Chert has been found?
What rocks are found in the Gunflint Formation?
What does it mean if your cherts are black instead of red?
Would you call these macrofossils or microfossils? Explain

part 3 - STRATIGRAPHIC SETTING AND TAPHONOMY OF THE GUNFLINT CHERT

From this paragraph you need to get the following information: the age (numerical and relative) of the Formation, where it is, what is made of. Also important is the idea that a Formation can be split in smaller (or shorter in time) members. You should also be able to answer the following question:

How are the Gunflint stromatolites and cherts different from those normally encountered in the Precambrian?

Under usual conditions, chert settles to the bottom of the ocean together with carbonates. While carbonates mostly remain preserved while they settle, cherts are often dissolved in the ocean waters. The silica that makes chert can become re-concentrated within limestones after deposition, forming differentshapes such as lenses or beds. Stromatolites are also usually found in carbonates and not in chert. At Gunflint though, and this is a major difference, we have layers of chert with no limestones, and even stromatolites are found just in chert. There are currently four different lines of thought, the first one being the most credited:

Stromatolites formed directly in chert; chert were so abundant that formed a straight deposit
Stromatolites have been misinterpreted and are actually inorganic (that is, not formed by organisms) deposits of silica (called sinter) like the ones found around the geysers of Yellowstone National Park
There are fossil bacteria on layers of chert, but they did not "make" the rock (like in stromatolites), they just "fell" on the rock
These are real stromatolites but they were originally made of carbonate (limestone), like today's stromatolites, and that old carbonate has been replaced by silica (chert)

part 4 - DESCRIPTION OF THE GUNFLINT CHERT BIOTA

Read this section to familiarize yourself with the kind of organisms that make this early Lagerstätten, but I will not ask you any question out of this section.

part 5 - PALEOECOLOGY OF THE GUNFLINT CHERT

This is a very important paragraph because it outlines the ancient geography of the area where these rocks were deposited and the rocks characteristics.

What do you need to know?

The deposition of the Gunflint Formation occurred on an open shelf (we discussed shelves in class, and I am including a picture below for you to review; the picture also shows where different sediments form).
These rocks make up for "Banded Iron Formations", or BIFs. These are alternating layers of reddish hematite (Fe₂O₃) and dark gray chert (SiO₂) that were only abundant in a very narrow time interval of the Precambrian (see picture below). They begin to become abundant in the rock record around 2.8 billion years ago, peak by 2.5 billion years ago, and finally virtually disappear by 2 billion years ago.

Banded Iron Formations (BIFs) from the Soudan Underground Mine State Park, Tower, Minnesota
What is the possible origin of BIFs? (discussed in class, and explained in detail on pages 21 and top page 22)
Based on what you read, is there any place today where these conditions still exist?
Review the concept of chemosynthesis based on what you read in the left column of page 22
Read the right column on page 22 for completeness of information, but I will not ask any question out of this section.
Be able to explain why these Gunflint-type bacteria almost became extinct at the end of this period.

part 6 - COMPARISON OF THE GUNFLINT CHERT WITH OTHER PRECAMBRIAN MICROBIAL BIOTAS

Read this section to familiarize yourself with other older and younger Lagerstätten from around the world (North Pole, Australia, being the most celebrated), but I will not ask any question out of this section.

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