Precambrian carbonates lacrustine carbonates submarine springs and seeps (29 30 31)

The carbonate factory in the rpecambrian consisted of what?

• No skeletal or calcareous algae

• only organisms that could matt

• This means that any stromatolites that develop are unaltered and intact since nothing could modify them

What was the dominant reef builder in the precambrian and the domuinant grains

• The stromatolite and thrombolite

• Ooids, peloids (from bacteria) and intraclasts

What are the two main theories for why the stromatolites look the way that they do?

• Either microbially detrmined

• or environment determined (SL speed of waves etc)

What is consistent between modern reefs and precambrian reefs?

• The make up and model of the reefs are very consistent

• Same envirnments then and now, just difefrent organisms that fill those environments

What is the difference between stromatolites and thrombolites

• Stromatolites are laminated and thrombolites are not laminated

What is the importance of seeps in the reef building environment?

• There is large overlap between reef building environments ans seeps

• where there are seeps there will be life

• seeps provide warm water, nutrients

• in exinctions when majority of life gets wiped out, seeps will eb the first place where life comes back to

What is the chemical environment of the ocean after a mass extinction?

• Low O2 levels, high Co2, this leads to ocean acidification

• the CCD rises as less calcite can be in solid state in the ocean because the pH is too low

How does CO2 impact ocean pH?

Co2 goes into ocean, reacts with carbonate creates carbonic acid which lowers pH

released H+ ions which further lower pH

What was the lead up to the P-T extinction

• Anoxia of the oceans with high CO levels (can see a redoc boudnary in the svedrup basin)

What caused the P-T extinction?

• The explosion of the Siberian traps

• Dissolevd carbonates from increasing ocean acidification

• Spiculite rocks went crazy

• This change also rapidly raised ocean temeprature, futher decreasing the viability of carbonates

What was observed int he svedrup basin during the p-T extinction and pangea formation?

• Major upwelling of ocean which brough to the surface organic rich water which carbonates also dont like

• spiculite facies began to grow in abundance again

• from the early permian to late permian - the ccd depth kept rising until it was basically any CAco3 had to be dissolved due to unfavourable conditions

What happens to carbonates post extinction?

• return to microbial carbonates near seeps

• domal stromatolites

• microbial boundstones

• temporary until more complex reef builders evolve again

What is a lake?

• A temporary stroage for water on continents and with no pernamnent outflow

• Can be open ( through flowing lake) where water can outflow sometimes lake level is stable

• can be closed ( where has an inflow but no water leaves ) Lake level is unstable

What impacts lake formation?

• CLimate

• Geology

• Time

• Hydrology

• Biology

What is needed to preserve lakes/ carbonates

• Subsidence, tectonic and basin stability

• some lake occur from tectonically active events such as faulting

What is a thermocline

The seasonal layering of water related to temperature that prevents layers of water from mixing due to density differences

• Summer stratiphication (prevents oxygenation of bottom sediments)

• autmmun mixing

• winter homogenous

How does a pernament therocline form

• Chemoclines where water is always a different denisty, creating a very anoxic lake floor

Where is phosphate levels the lowest in a lake?

• At the surface as there is the most organism useage

• same with water for the most part (lowest in the middle of the lake)

Where do lakes get their carbonates?

• Surficial deposists from mountains etc provide carbonate ions for lakes

• many sources, just in smaller quantities than in ocean settings

What are some typical features in lake environements

• Grainstones

• Laminates

• Turbidites

• Oncoids

• stromatolites

• algal mats

• beach rocks

• thrombolites

What are beach rocks?

• Early lithified carbonate rocks

What are charophytes

• Plants that life in fresh water that have their stems calficied

• their spores are calcified as well

When do dessicationc racks occur in lacrustine environments?

• When evaporation is greater than precipitation

• mud cracks

What are the evaporation intense minerals?

• Trona and Nahcolite

• Sodium rich carbonate minerals

Fort vermillion case study

• AN example of a brackish carbonate environment

What is a spring carbonate and where do they from?

• Form from geothermal water rich in Caco3

• The degass at the surface (co2) which causes Caco3 to precipitate when spring waters cool at the surface

What are the common features and deposits of spring carbonates

• Dendritic deposits (Caco3 crystals formed around the hot water outflows of the springs)

• Aragnoite cyrtsal fans: rapid precipitation creates aragonite crystals

• Mini dams and pools: continual precipitation of Caco3 creates small pools and carbonate structures

What is a submarine seep and spring

• This a place where fluids rich in nutrients and minerals seep from the subsurface and into the ocean

• They create specialized ecosystems that rely on chemosynthesis and not on light

• come from hot springs and black smokers

• precipiatte H2S

Why are chemosynthetic ecosystems life osases?

• High density low diversity fauna

• Feed on microbial mats or have chemosynthetic relationships with microbes that provide them with energy

Biogeochemical cycling at methane seeps

• aeorobic and anaerobic with advective flow

• creates dolomite and aragonite

• H2S CH4 bubbles

• emthane oxidizing bacteria and sulphate reducing bacteria dominate

• dolomite precipitation is often associated with ths

• pyrite forms in deeper layers

What is the anaerobic oxidation of methane

• Sulphate methane transiiton zone of the shallow subsurface

• Authigenic carbonate and pyrite

• Creates HS- and Water

What is the difefrence between methane seeps andvents

• seeps are cold slow of methane

• vents (blakc smokers) are hot rapid mineral rich fluid discharge

What are thw carbon isotope signatures associated with the seeps

• Have very negative carbon signature

• this distiguishes from marine carbonates as the signature is much less negative

Seep systems through time

• Miocene in new zealand Low fluc and high flux

• Cretaceous svedrup basin

• Carboniferous methane near salt domes in the svedrup basin

How do you recognize seeps in the rock record

• unique fossils like the tube worm

• special fibourous aragonite and high macgnesium calcite

• depletion of 13C

• Plumbing system that allows transport of fluid

How do you know when methane has influence

• Mainly the very negative 13C

• Methane is very enriched in 12C which is the lighter isotope

• THe organims prefer 12C as it is the lighter isotope and easier to break bonds with

• 13C means very enriched in 12C and all that 12C is being used

How do you tell the difefrence between a seep and reef

• different food sources (methane and chemosynthesis vs photosynthesis)

  • swan hill sis a reef with seep like features

    This is due to faulting and dolomitization