EOSC 326 Module C.docx

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Earth and Ocean Sciences
EOSC 326
Michael Wheeler

Lesson 10: Fossil Preservation and Modes of Life 12/8/2012 9:40:00 AM Learning Objectives:  Describe why organisms have different potential for preservation  Compare and Contrast body and tract fossils  Compare and contrast the different methods of fossil preservation  Define and identify an external mold, a cast and an internal mold  List and define the different modes of life that have been exploited by marine organisms over time Fossil Types.  2 main types of fossils: o trace fossils  impression made on substrate which record past activities of organisms  trails, tracks, burrows, excrement o body fossils  preserve a record of actual organism or some part of it  hard parts of organism are preserved  bones, shell ,teeth  more rarely, soft parts are preserved  skin, muscles, tendons, and organs Preservation Methods 2 methods of preservation:  direct preservation: material is preserved without any chances, aside from removal of less stable organic matter (eg. Soft tissues) o common in corals and sponges  indirect preservation: original organic material is partially to fully changed into new material o occurs via carbonization, petrifaction, dissolution and replacement o carbonization: fossils preserved as thin films of carbon  material in question is buried  chemical reaction that water transforms the organic material of plant/animal into thin film of carbon  nitrogen, hydrogen and oxygen are driven off as gases, leaving a carbon outline of original organism  common in fish, insects and plants o petrifaction: new material completely fills up available pore spaces and fossil becomes a solid rock which is still partially composed of original materials  material is buried  groundwater percolates through pore spaces present in biological tissue  solution that is supersaturated in either calcium carbonate or silica precipitates minerals in the pore spaces  common in bones, some groups of plants and shells o Dissolution and Replacement  Material is buried  Groundwater seeps through sediments and original material dissolves leaving a void, which preserves the shape of the organism  Void is filled with sediment or another mineral which precipitates in the void as calcite  Common in shells and trilobites  2 different situations can occur during initial burial process:  situation 1impression of organism on the surrounding rock will be preserved  external cast, replica of original shell present  cast  when shell buried, inside of shell originally filled with soft tissue, does not get filled with sediment and remains empty  when shell dissolves, entire central cavity is empty  this space is infilled with either additional sediment or mineral precipitate  situation 2 external mold and cast of shell preserved but internal cavity of organism gets filled with sediment upon initial burial. Impression of inside of shell also preserved  internal mold  when shell buried, inside of shell originally filled with soft tissue does get filled with sediment  when original shell dissolves, only the space that was actually occupied by shell is empty and filled with either additional sediment or mineral precipitate Marine Lifestyles 2 different marine lifestyles:  pelagic: organisms that live up water column, these organisms with this lifestyle have maximum dispersal o 2 different methods of functioning in water column:  nektonic groups: swim freely and not dependent on currents  groups range widely from microscopic to many metres long  planktonic groups: move through water column by simply floating or drifting  tend to be microscopic  benthic: organism live either on or within substrate, these organisms have limited dispersal o epifaunal: live on substrate  cementing lifestyle: organisms are often sessile (do not move freely), tend to have radial symmetry or be asymmetric and often cement to their neighbors or live in holes/cavities  vagrant lifestyle: organisms move around substrate, usually bilaterally symmetric, tend to have a head o infaunal: live within substrate  burrowing lifestyle: burrow down into soft substrates like sand/mud  boring lifestyle: bore down into hard substrate such as rock, boring can cause significant damage to hard substrates, sometimes known as “bioeroders” Lesson 11: Bioherms (Reefs and Mounds) 12/8/2012 9:40:00 AM Learning Objectives:  Compare and contrast a bioherm and a mound  List and explain the controls of bioherm growth and development  List and explain the different stages of bioherm succession  Compare and contrast the 4 zones found in a reef during the diversification phase  List and define 3 major roles played by organisms that inhabit modern and fossil bioherms  Explain why biodiversity in bioherms is so high Bioherms (Reefs) and Mounds  Bioherms: rock-like structures produced by the cementing together of organisms with secreted skeletons o Almost entirely the direct or indirect production of a community of sessile benthic organisms that have grown in one place for an extended period of time  Presence of these organisms has led to higher rate of carbonate production that present in surrounding sediments  Mounds: steep cone-like piles or flat lenses of relatively small size o Typically form in quiet water environments o Principally made up of mud and lack macroscopic skeletal structure o Can also include microscopic organisms or delicate branching or encrusting organisms Reef Growth Window  restricted to reasonably clear, shallow and tropical waters  can withstand erosion by wave action and tend to be built parallel to shore but perpendicular to waves  majority of reefs limited to photic zone  grow best between temps of 25-29C o although will grow in temps ranging from 18-36C  salinity of water important for optimal reef growth  25-35 ppt o although will grow in water with salinity ranging from 22- 40ppt Walker-Alberstadt Model of Reef Succession  As reef grows  one community of reef-building organisms is replaced by another (this process known as Walker-Alberstadt Model of Reef Succession)  4 stages: stabilization, colonization, diversification and domination stages o Stabilization:  Begins with accumulation of skeletal debris from echinoderms or algae which move around on substrate and gradually accumulate into piles over time  Living organisms such as algae, plants and sometimes echinoderms accumulate around piles and stabilize the substrate by sending down roots or holdfasts  Low diversity o Colonization: in coming of reef-building organisms  Cementers cement down on the mounds and stabilize the substrate creating patch reefs  Massive and branching growth forms are common and act as framework organisms  stick up into water column and slow down passing material  this process slowly build-up the crest  these groups provide niches for other attached and cementing organisms  zone quite thin compared with size of entire reef and often monospecific or of low diversity o Diversification: reef reaches atmosphere/water interface  Lateral diversity occurs because conditions differ depending on where you are in reef  Some areas high turbulence from waves and wind  Other areas sheltered from wind/waves and are much quieter  Some areas substrate is firm and others substrate loose  due to environmental differences, organisms go where they are best adapted and lateral zonation develops in communities that inhabit the reef  leads to diverse communities that are highly variable in growth habit, producing an increase in niche space for other organisms that grow in crevices/cavities  leads to increase in debris-producing organisms o Domination:  Very low diversity  Reef dominated by only few different species  Encrusting typical growth in reefs in this stage  Change from diversification stage to domination is often sudden Reef structure during diversification phase  Lateral zonation  4 distinct but connected zones o Moving across reef in perpendicular direction away from shore Lagoonback reefreef crestfore reef  Lagoon: o Conditions calm and quiet zone blocked from wave action by main reef structure o Substrate contains a high percentage of organically produced material  due to boring organisms and wave action which break off pieces of skeletons of animals in other reef zones which are wasted back into lagoon  These small loose grains make-up substrate shift easily o due to this movement not many organisms colonize in this zone  biota is limited to some pelagic scavengers and some infaunal bivalves and worms o lagoon only present if sea level rising, if sea level falling or stable  will be filled with sediment over time  Back Reef o Quite shallow and high light intensity and high temperatures o Portions of reef exposed at low tide o Inhibit growth of corals o During heavy storms, parts of reef crest thrown into back reef creates mobile pieces over time organisms colonize these chunks, cementing them together and loose sediment forms channels between them o Growth pattern of organisms radial, no forces present to make them linear o Animals  stubby, branching or massive forms that extend above substrate  Allows them to withstand quiet muddy times and stormy agitation o Common organisms  sponges, mollusks, and crustaceans, burrowers  Reef Crest o Rigid lattice built-up to near water/atmosphere interface o Exposed at low tide o Greatest tidal action very-high energy environment due to constant wave/wind action o Cementing important in this region due to high physical erosion by waves o Calcareous algae and encrusting corals  dominate this area  Fore Reef o Slopes downward at great angle, extending from low tide mark into deep water o Very-high energy due to wave action o 2 solutions for organisms: cement down close to substrate or have arms that protrude into prevailing currentreduces chances of resistance and breakage o rich in cracks and creviceshighest number of different niches of all reef zones  highest diversity of brachiopods and bivalves o lower part much quieter and host large number of platy corals, some sponges o species diversity decreases rapidly at deeper depths Roles in Reef Communites framework, cementing and bioeroding roles  Framework: o Animals provide rigid framework or skeleton for reef that allows reef to stand above substrate o Organisms relatively large with solid structure  Cementing: o Organism fill i
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