User:Tmahseredjian/sandbox

 Gunflint Chert 

What is the gunflint microbiota? & History of discovery


 * First microbiota ever discovered (mid 1950s)


 * Some of the oldest microfossils


 * At the time of discovery:
 * Oldest life on Earth
 * First appearance of photosynthesis?
 * Indicator of oxygen in the environment


 * First described by Cloud, Barghoorn and Tyler in 1965 (both in Science)
 * Cloud focused on the paleoecological significance of the microfauna, taking a big picture perspective
 * Barghoorn and Tyler focused on characterizing the microorganisms that comprise the microfauna, looking at the individual players

Gunflint Iron Formation: Animikie Group


 * First described on the shores of Lake Superior


 * Extends from Thunder Bay, ON to N. Minnesota

Stratigraphic setting:


 * Four major members:
 * Lower Cherty
 * Lower Slaty
 * Upper Cherty
 * Upper Slaty


 * Overlying black slate layer
 * Underlying andesitic layer with pillow lavas

Composition


 * Banded Iron Formation!


 * Composed predominantly of dense chert layers, and some carbonate (ankerite) layers


 * Chert can be subdivided into black (organics, pyrite) red (hematite), and green (siderite)


 * Microfossils: stromatolites, algal filaments, sporelike spheroids, and ooids with organic matter

Age Constraints


 * Mid-late Paleoproterozoic (approximately 1.878 Ga ± 1.3 Ma)


 * Determined using Rb/Sr, K/Ar dating techniques


 * Big deal considering the oldest life known at the time was the Ediacaran fauna

Modern methods that have been used to study Gunflint microfauna


 * Samples collected from Schreiber Beach from Thunder Bay in Ontario, Canada


 * Samples thin sectioned into over 800 slides


 * Acid maceration (HF) utilized to dissolve chert and leave behind organics


 * Lots and lots of light microscopy


 * Age determined via Rb/Sr, K/Ar dating technique

Preservation Modes:


 * Association with organic residues and films
 * A film of light-to-dark brown material outlines organic structures, acts as stain
 * Preserves filaments, sporelike bodies, carbonate rhombs within chert
 * Fine-grain pyritization
 * Association of fine-grained pyrite with organic matter is the most common type of preservation in algal cherts - stippling pattern
 * Coarse-grain pyritization
 * Pyrite occurs in chert as individual crystals (fine-grain: micrometer scale) and coarse aggregates (coarse-grain: millimeter scale)
 * Carbonate association
 * Filaments, sporelike bodies, and other organic structures can be preserved by carbonate filaments (<1μm in diameter) imbedded in a chert matrix
 * Carbonate can form a continuous body outlining filamentous remains
 * Carbonate can occur as a series of lenses along the axis of a filament
 * Often seen as carbonate trails tailing pyrite crystals
 * Due to the thickness of filaments and the fact that they are encased in silica, it is difficult to determine the type of carbonate
 * Hematite preservation
 * Less common, filamentous fossils can be preserved by hematite filaments (<1μm in diameter)
 * Usually found at black algal chert / red algal jasper interfaces
 * Often outlined by carbonaceous films and pyrite grains
 * This type of preservation thought to indicate hematite replacement of original organic matter

Key Micro Players / New Discoveries


 * Most abundant organisms in Gunflint are filaments ranging from 0.5 to 6.0μm in diameter and up to several hundred microns in length
 * Overall, there were 3 new genera and four new species of filamentous forms discovered
 * Filaments
 * Filaments represent a mixed population of blue-green algal species, and/or remnants of iron bacteria
 * On they outcrop scale, they form stromatolitic domes
 * New genus Gunflintia: septate filaments of various sizes
 * Subdivided by filament diameter and regularity of cell size
 * New taxon Animikiea septata: finely septate filaments
 * New taxon Entosphaeroides amplus: non-septate filaments which may or may not contain internal spores
 * New taxon Archaeorestis schreiberensis: branched, nonseptate, coarse or rope-like filaments
 * Nod to Schreiber Beach locality where Gunflint has been well characterized
 * Spheroids
 * Spheroidal sporelike bodies are found irregularly distributed through the chert matrix ranging from 1 to 16μm in diameter
 * Shape ranges from ellipsoidal to spherical
 * Wall thickness is highly variable between spheroids
 * Membrane ranges from diaphanous to coarsely reticulate
 * Spheroids are thought to be several things:
 * Unicellular cyanobacteria (initially described in the 1960s as blue-green algae)
 * Endogenously produced endospores of blue-green algae
 * Free-swimming dinoflagellates
 * Fungus spores
 * New genus Huroniospora: spherical to ellipsoid sporelike bodies with various wall textures
 * Subdivided based on wall-sculpturing pattern
 * Nod to Lake Huron Gunflint localities
 * New taxon Eosphaera tyleri: sphere within a sphere sporelike body that may or may not exhibit a connecting tubercle
 * Number of tubercles ranges from 1 to 15
 * Nod to Stanley Tyler's role in characterizing Gunflint microfauna
 * New genus Eoastrion: spherical to ellipsoid sporelike bodies with various wall textures
 * Subdivided based on the form of the radiating filament

Paleoenvironmental Implications


 * Gunflint microbiota represented the first autotrophic mode of existence
 * Chemosynthesis v. photosynthesis?


 * Microbiota could inhabit a wide range of temperature and pH conditions comparable to present-day averages (non-glacial setting)
 * Salinity not as well constrained


 * Microbiota presumably inhabited shallow water conditions with active water movement within the photic zone (probably marine)
 * At the time of Gunflint’s discovery, the state of the Precambrian atmosphere wasn’t well characterized
 * Oxygen levels not well understood
 * Subsequent effect of oxygen levels on the radiation of life
 * Evolution of photosynthesis
 * Relationship between different redox conditions
 * Gunflint geochemistry provides evidence for oxidizing and reducing conditions
 * Widespread oxidized iron formation were interpreted as evidence of a highly oxidative atmosphere
 * A reducing atmosphere was required to transport large quantities of iron seen in the Gunflint formation in a soluble ferrous state
 * A few hypotheses arose regarding the state of oxygen:
 * Gunflint represents an overarching oxygenation of the atmosphere, with some local reducing environments
 * Pulses of oxygen: oxygenation of the environment ceased between 1.7 and 1.2 Ga, then later resumed

Modern and Historical Significance


 * While not the earliest evidence for life on Earth, Gunflint was the oldest known microbiota when first described


 * Contains fossil evidence of some of the earliest potentially photosynthetic life on Earth


 * Spurred scientists to contemplate ancient atmospheres and to consider the role of oxygen, different redox conditions


 * Older fossils have since been discovered, but Gunflint opened the floodgates to seeking older and older microscopic life on Earth
 * Since the publication of the original Cloud and Tyler and Barghoorn papers:
 * Microbial phylogeny and ecosystems are better characterized
 * Rise of oxygen is better understood
 * Hydrothermal sources have been described
 * Unique ocean redox conditions are better understood (and still complicated)

To cite:


 * Cloud 1965
 * Tyler and Barghoorn 1965
 * Awramik and Barghoorn 1977
 * Goodwin 1956
 * Shapiro and Konhauser, 2015
 * Planavsky et al., 2009