Greenhouse worlds & Oceanic anoxic events Flashcards
What rock type can be used to show anoxic events in the past?
Black shales
What are the seasonal cycles like with photosynthesis at present day?
High near equator where strong upwelling
High in NH summer
Where are large algae populations likely to be found in the marine environment?
Continental margins due to run off
What is the main control on phytoplankton productivity?
Nutrient levels
What are the major nutrients needed by phytoplankton?
N
P
Si (diatoms with silica shell)
What minor nutrients are needed by phytoplankton?
Fe
Zn
Cu
How are phytoplankton supplied their nutrients?
Deep ocean via upwelling
River runoff along coasts
Aeolian dust deposition
Recycling in the upper ocean/mixed
Where are algal blooms concentrated depthwise?
In the photic zone (top 100m)
What happens to nutrients used by phytoplankton?
Absorbed then when organism dies either eaten or sink to sea floor
What is biopump?
Nutrients absorbed by phytoplankton then poo or die and then upwelling brings it back to surface
What happens to oxygen levels when phytoplankton release energy form their food?
Oxygen used up and co2 increased (oxygen depletion) creates oxygen depletion zones
What is an example of an unnatural anoxic event?
Deepwater horizon 2010
What was the cause of the deepwater horizon?
Failure to control oil pressure so well blew out of sea floor
How did deepwater horizon cause anoxia?
plume spread out to the west Microbes were eating it and using oxygen this created very low oxygen zones which was worst for larger organisms which need high oxygen level
What levels of oxygen is needed for water to be “saturated”?
7-8 mgl
Below what oxygen level is water hypoxic?
2 mgl
What is hypoxia and anoxia?
Hypoxia- low oxygen
Anoxia - no oxygen
What is an example of an area which is always hypoxic?
Black sea
What is an example of an area with seasonal hypoxia?
Baltic sea
What is an example of an area with temporary hypoxia?
Areas affected by el nino i.e. east south American coast
What are the causes of hypoxia in modern oceans?
Limited connections to the open ocean
High nutrient loading from land
Highly stratified
How can stratification affect oxygen levels?
If highly stratified by temp (warm on top cold below) there will be very little mixing
How many “dead zones” are there and what area di they occupy?
405
250,000 sq km
What is the future threat of warming temp to hypoxia?
Increased air temp will increase surface water temp intensifying stratification
How can carbon be used to analyse past conditions?
lighter c12 will be taken up more readily
More C12 = negative c13 and vice versa
How does ocean anoxia affect carbon levels?
C12 in organisms sinks and locked in sediment as black shale resulting in + C13 value
What is the Selli event?
Oceanic anoxic event 1a
Early Aptian = ~120 ma
What were the outcomes from the Selli event?
Widespread black shale deposition across the Tethys
Large positive δ13 C excursion;
Long duration ~1 Ma
Increase in high-nutrient phytoplankton groups
What is the Bonarelli event?
Oceanic anoxic event 2
Age: Cenomanian/Turonian
What is special with mesozoic oceans?
Early opening of Atlantic
Restricted Tethys seaway -> limited ocean circulation & ventilation
What is special about the mesozoic climate?
Period of global warmth (~ice free poles)
Warm polar and high latitude regions
Oceans that are warm, sluggish circulation and low in oxygen
What is the temp gradient between the poles and tropics like in the mesozoic?
Very weak gradient so little circulation
Why is there increased burial of organic carbon in oceanic anoxic events?
Reduced oxygen content in deep ocean during OAEs ->
Less organic matter break down in deep ocean sediments by bacteria and animals that need oxygen ->
More organic carbon being buried rather than “eaten”
What was the source of CO2 for the Selli event?
Ontong Java Plateau volcanism
What are oceanic anoxia events often linked to?
Large igneous provinces
How can anoxia affect evolution?
Belemnites were originally deep water but OAE forced them to live in surface waters where they evolved to live there as Oxygen levels returned their habitat radiated form surface to deep water
