First Microfossils Flashcards
(24 cards)
4 obstacles with looking for early life
1- Archean rock rare
2- old rocks often poorly preserved and can be highly metamorphosed
3- sedimentary rock best for preserving life, but Archean rocks found today are mostly igneous
4 - early life may not be recognizable
Challenges with finding a microfossil (which would be the remains of early life)
1 - smaller than the eye can see
2 - only 30 well accepted known instances of Archean microfossils (3000 in Proterozoic)
How microfossil is formed
rapid (induced) mineralization
usually only the cell sheath or cell wall (outside of cell)
benefit of rapid (induced) mineralization
no time for post mortem degradation
when does rapid (induced) mineralization occur
silica super-saturated waters
biologically induced mineralization
minerals forming as a byproduct of metabolic activity or passive processes
biologically controlled mineralization
organism specifically precipitates minerals to serve a physiological purpose (ex: shell)
how does metabolism impact (induced) mineralization
alters aqueous chemistry/saturation states (whole system) or immediately outside of the cell (microenvironments) through
-excretion of metabolites
-depletion of nutrients
passive processes that impact (induced) mineralization
surface reactions: cell walls have functional groups that can interact with dissolved ions allowing for ion concentration and nucleation/precipitation
which type of processes is more common with induced biomineralization involving silica precipitation
passive processes
three types of passive processes involving silica precipitation
hydrogen bonding
cation bridging
direct electrostatic interactions
hydrogen bonding in biomineralization of silica
hydroxyl (OH) groups interacting with silica oligomers concentrates silica around a cell sheath
cation bridging with silica
outside of a cell wall (no sheath) with negatively charged functional groups will interact with positively charged cations with hydroxyl groups - which then use hydrogen bonding to precipitate silica
direct electrostatic interactions with silica
passive process outside a positively charged cell wall that interact directly with the negatively charged silica oligomers to allow for silica precipitation
carbonate induced biomineralization in cyanobacteria - passive processes
negative charged ligands on cell wall will concentrate CA2+ cations on the surface, promoting nucleation of gypsum (from sulfate SO4^2-) or calcite (from carbonate CO3^2-)
carbonate induced biomineralization in cyanobacteria - metabolic processes
organic carbon fixation produces OH-, which is pumped out of system, increasing pH of surroundings, creating more carbonate, which leads to more calcite to form on the surface (see passive processes)
Low temp pyrite formation
organic carbon reduces Fe III to Fe II, SRBs have reduced sulfate (SO4^2-) to sulfide (S2-) and the sulfide combines with Fe II to form pyrite
stromatolite
mounds of layered accretionary material in shallow waters formed by trapping and binding sedimentary grains in biofilms of microorganisms (lithification via induced biomineralization)
oldest possible microfossils
Apex Chert, Warrawoona, Australia
filamentous structures that look like cells in a sheath
3.45 Ga
debunked - could be formed from abiotic mechanisms
Possible oldest microfossils - debated
Sulphur Springs, Australia
3.2 Ga
filaments in hydrothermal subsurface metal deposits,
could be response to stimulus in reducing, anaerobic environment that contained sulfur - not confirmed evidence, but possible SRBs?
oldest putative microfossils
Transvaal SuperGroup, Southern Africa
2.6 Ga
dividing cells, complexity, familiar microbial shapes similar to modern microbes
oldest stromatolite - disputed
3.7 Ga - Isua Greenland
heavily contested, has other possible explainations, if it is - it is highly altered
oldest stromatolite, well accepted
3.4 Ga, Warrawoona Group, Australia
many layers with karogen (lithified organic carbon), flexture, likely biogenic
age of multiple uncontroversial evidence of biogenic stromatolites
3.0 Ga
2.7 Ga in Tumbiana formation, Australia
