Knoll Chapter 3

Location of rocks studied

Spitsbergen, an isolated island halfway between Norway and the North Pole.

Age of Spitsbergen rocks

600-800 million years old.

Geological Era of Spitsbergen rocks

Upper Proterozoic, well before the Cambrian Explosion.

Animal traces in Spitsbergen rocks

Completely absent; no skeletons, carcasses, tracks, or trails.

Microfossils found in Spitsbergen rocks

Cyanobacteria, algae, and protozoans.

Stromatolites

Reeflike macroscopic structures built by microbial communities.

Chemical signals in Spitsbergen rocks

Signatures imparted by microbial metabolisms (photosynthesis and sulfate reduction).

Akademikerbreen Group

A 7,000-foot-thick succession of limestones and related rocks deposited near the edge of an ancient ocean.

Total thickness of Spitsbergen sedimentary pile

More than 20,000 feet.

Ancient environment of Spitsbergen deposition

A tropical seaway.

Gunflint Formation

Nearly 2-billion-year-old rocks in western Ontario where bacterial cells were reported in 1954.

Elso Barghoorn

The father of Precambrian paleontology.

Uniformitarianism

The foundational geological principle stating "the present is the key to the past".

Limitation of Uniformitarianism

The Earth's surface state (ocean chemistry, geography, and climate) has not been invariant through time.

Dolomite

A rock-forming mineral consisting of calcium magnesium carbonate.

Limestone

Sedimentary rock composed of calcium carbonate particles cemented together.

Laminae

Millimeter-thick rock layers resembling puff pastry, formed by microbial mats trapping and binding fine sedimentary particles.

Ancient mud cracks

Networks of polygonal prism-shaped cracks that formed when wet coastal muds were exposed to the sun and dried.

Tepee structures

Wigwam-shaped ridges in rock layers that formed just above high tide when the growth of carbonate and gypsum crystals caused surface beds to buckle.

Intertidal rock characteristics

Alternating sandy/muddy layers, microbial mats, and angled laminae forming a herringbone texture due to flowing tides.

Ooids

Tiny spheres of concentrically laminated carbonate that form where coastal waves repeatedly suspend particles in warm, lime-rich waters.

Size of modern marine ooids

A maximum diameter of about one millimeter, roughly the size of sand grains.

Size of ancient Akademikerbreen ooids

The size of garden peas.

Reason for larger ancient ooids

The Spitsbergen seaway was more highly charged with calcium and carbonate ions than modern oceans, causing faster accretion.

Cause of Spitsbergen's arctic location

Plate tectonic processes (continental drift) transported the landmass poleward over millions of years.

Alfred Wegener

The German meteorologist who proposed the hypothesis of continental drift early in the twentieth century.

Benefit of Spitsbergen's arctic location

The Pleistocene Ice Age deep freeze left the rocks beautifully exposed and largely unaltered by surface weathering.

Chert (flint)

An extraordinarily hard rock made up of tiny interlocking crystals of quartz (crystalline silica).

Properties of Chert

It is hard enough to withstand tectonic deformation and impermeable enough to shield delicate contents from corroding fluids.

Formation of Chert nodules

They formed early within the sediments, before burial initiated the compaction that bent encompassing sediments around them.

Cause of black color in chert

The presence of included organic matter.

Micron

Extremely short unit of measurement equaling one-thousandth of a millimeter, or forty-millionths of an inch.

Cyanobacteria fossils in high-tide cherts

Appear as 10-micron, thick-walled tubes that represent the extracellular sheaths of filamentous cyanobacteria.

Cyanobacteria ecological role

The working-class heroes of the Precambrian Earth; main primary producers and source of oxygen.

Polybessurus bipartitus

Spitsbergen fossil consisting of 10 to 30 micron spheroidal cells atop stalks made of extracellular secretions.

Function of Polybessurus stalks

Enabled cells to maintain their position at the sediment-water interface despite an influx of lime mud.

Polybessurus bipartitus life cycle

Cells settle, secrete stalks, divide repeatedly without growth to form small cells, and then disperse to settle again.

Modern analogue for Polybessurus bipartitus

Living cyanobacteria that build small black crusts on the lonely western edge of Andros Island in the Bahama Banks.

Boring cyanobacteria

Microbes that bore into ooid grains, possessing exact fossil counterparts in silicified Proterozoic ooids.

Environmental challenges of tidal flats

Desiccating glare of the sun at low tide, osmotic trial of salty water during dry weather, and fresh water during storms.

Cyanobacterial defense mechanism

Secretion of an extracellular envelope to protect the cells inside.

Paleontological importance of cyanobacterial envelopes

Unlike the cells within, the envelope is like a microbial clamshell that strongly resists postmortem bacterial decay.

Reason for lack of heterotrophic bacteria fossils

Most tidal flat bacteria lacked preservable walls/envelopes and have simple shapes that frustrate biological interpretation.

Rare larger microfossils in Spitsbergen chert

Microfossils greater than 100 microns, shaped like miniature vases or studded with spines, washed into channels from offshore.

Black shales

Organic-rich sedimentary rocks that accumulated in quiet subtidal environments beyond the ooid shoal.

Decay inhibition in shales

The rocks are packed with clay minerals that actively inhibit organic decay.

Method for studying shale fossils

Dissolving the mineral fabric in strong acid to gently free the delicate organic remains.

Fossils found in Spitsbergen black shales

Diverse eukaryotic organisms, vase-shaped fossils, large spiny cells, and multicellular algae (small seaweeds).

Extinct Precambrian algae

Certain seaweed fossils that have no close modern counterparts and went extinct like dinosaurs.

Formation of stromatolites

Formed discontinuously but repetitively as microbial mats colonize, trap/bind fine particles, and induce carbonate precipitation via bacterial decay deeper in the mat.

Shapes of stromatolites

Can be planar, domal, conical, or cylindrical.

Biomarkers

Biological molecules of known origin (mostly lipids) that have survived a gauntlet of bacterial decay and can be extracted from rock.

Molecules unlikely to survive as biomarkers

Nitrogen- and phosphorus-rich molecules like DNA.

Grand Canyon shale biomarkers

Preserve molecular signatures of archaeans, bacteria, protozoans, and algae that left no recognizable microfossils.

Isotopes

Varieties of an atom distinguished solely by molecular weight (differing numbers of neutrons).

Carbon-12

Makes up about 99 percent of all carbon; contains six protons and six neutrons.

Carbon-13

Makes up most of the remaining 1 percent of carbon; contains an extra neutron.

Carbon-14

A radioactive isotope that decays to nitrogen on a timescale of millennia, rendering it useless for very old rocks.

Isotopic preference of photosynthesis

Organisms more readily incorporate Carbon dioxide containing the lighter Carbon-12 isotope.

Kinetic Isotope Effect (Fractionation)

The quantitative difference in the ratio of Carbon-13 to Carbon-12 between organic matter and surrounding carbonate minerals.

Magnitude of carbon fractionation

Typically about 25 to 30 parts per thousand.

Spitsbergen carbon fractionation value

Consistently differs by about 28 parts per thousand, indicating active photosynthesis.

Sulfate-reducing bacteria metabolism

They complete the marine carbon cycle by using sulfate ions to respire organic molecules, creating hydrogen sulfide.

Pyrite (Fool's Gold)

The mineral formed when biologically produced hydrogen sulfide combines with iron.

Isotopic preference of sulfate reduction

A chemical preference for the lighter Sulfur-32 over the heavier isotope Sulfur-34.

Sulfate reduction chemical signature

Results in sedimentary pyrite that is enriched in Sulfur-32 relative to gypsum formed from the same water body.