peter's Flashcards
(112 cards)
1
Q
What is an ichnofacies?
A
A temporally and spatially recurrent association of trace fossils and sedimentary structures
2
Q
What are the 4 general types of ichnofacies?
A
Trypanites (rocky coast) Skolithos (sandy shore) Cruziana (Neritic zone) Zoophycos/Nereites (Bathyal and abyssal zones)
3
Q
What is the ichnofacies characterised by on-shore boring trace fossils?
A
Trypanites e.g. lithophager (bivalve)
4
Q
What trace fossils characterise the Skolithos ichnofacies?
A
Diplocraterion yoyo (U shaped burrow) Thalassinoides (2-3m vertical shafts, horizontal galleries) Ophiomorpha nodosa (“ “ + nodular secretions)
5
Q
What trace fossils characterise the Cruziana ichnofacies?
A
Rhizocorallium (similar to Diplocraterion but parallel to bedding rather than perpendicular)
6
Q
What trace fossils characterise the Zoophycos/Nereites ichnofacies?
A
Zoophycos (corkscrew structure) Palaeodictyon (series of tightly, v. geometrically defined polygonal galleries, vertical shafts off joints)
7
Q
What is Alpha diversity?
A
The richness of taxa at a single locality/community. It measures community “packing” and resource partitioning.
8
Q
What is Beta diversity?
A
The differentiation of biota between sites/communities. It measures turnover along environmental gradients and habitat specialization.
9
Q
What is Gamma diversity?
A
The differentiation between regions. It measures endemicity/provincialism.
10
Q
Define euxinic
A
Where sulphate reduction is so intense that the bi-products (H₂S) extend into the overlying waters from the anoxic sediment.
11
Q
Define hypoxic
A
Where the oxygen levels are low enough to be toxic
12
Q
Define poikilaerobic
A
Where dissolved oxygen concentrations are seasonally variable. Most low oxygen environments are probably poikilaerobic
13
Q
Define the following terms: anoxic, oxic and dysoxic
A
Anoxic: sea water of extremely low oxygen concentration (C 0.1ml/l) Oxic: of high oxygen concentration (1ml/l C) Dysoxic: of very low oxygen concentration (0.1ml/l C 1ml/l)
14
Q
Explain how sediment lithification might have impacted upon estimates of Phanerozoic biodiversity.
A
It is easier to collect identifiable fossils from non-lithified sediments than from lithified sediments because lithified sediments are much harder.
15
Q
Explain how rock outcrop area might have impacted upon estimates of Phanerozoic biodiversity.
A
If we have more marine sedimentary rock outcrop from the Cretaceous and we have higher palaeodiversity does this mean that there were more creatures alive at that time or that we simply have more rock to collect from.
16
Q
Explain how palaeolatitude might have impacted upon estimates of Phanerozoic biodiversity.
A
Diversity in the modern is greater at the equator, if we apply a uniformitarian approach we would expect the same to be true in the past. Most fossils have been collected from the USA and Europe. These continents were largely at the tropics in the Palaeozoic but in mid-temperate latitudes thereafter. Thus one might consider that the estimate of Palaeozoic diversity was inflated by the palaeolatitudinal availability of some sedimentary rock.
17
Q
How do you pyritize fossil soft parts?
A
In an OXIC/dysoxic environment, as an organism decomposes within the sediment it uses up the oxygen around it producing an anoxic micro-environment depleted in oxygen. The bacteria become sulphate reducers and sulphide is produced as the organic material decays. Dissolved Fe in the pore-water diffuses along the conc. gradient to the rotting carcass (the sink) and soft-part pyritization can occur.
18
Q
What is the proportion of species and individuals in a normal marine environment that are wholly soft bodied?
A
2/3rds They are thus unlikely to be preserved
19
Q
What is taphonomy and how do we get a taphonomic bias?
A
Taphonomy is the study of fossilization, and comes from the word “taphos” meaning burial Taphonomic bias occurs as a result of decay, transport and mineralization
20
Q
Define Lagerstatten
A
Deposits capable of being mined for fossils due to their exceptional preservation
21
Q
Give an example of a Konservat Lagerstatten
A
Burgess Shale, Canadian Rockies, British Columbia Middle Cambrian
22
Q
Give an example of Konzentrat Lagerstatten
A
Blue Lias Lyme Regis, Dorset Lower Jurassic Big ammonites in concentration due to sediment starvation
23
Q
What causes Konservat Lagerstatten to be preserved?
A
Inhibited decay
24
Q
What causes Konzentrat Lagerstatten to be preserved?
A
Where usually hard parts accumulate in fissures, through current sorting or sediment starvation
25
Why does anaerobic decay not occur in oxic environments?
Aerobic bacteria out compete the anaerobic bacteria, because they are more efficient at decomposing aerobically than the anaerobic bacteria.
26
Is anoxia necessary to fulfil soft part preservation?
Not as far as decay goes, it isn't. During aerobic decay, One mole organic carbon requires 106 moles of oxygen; 1 g dry weight requires 671 cc of oxygen; Thus because of mass/surface area constraints all carcasses decay anaerobically anyway Anoxia is important to preservation, but not because it slows down decay in any way (only by 2-3 times)
27
Does the decay process stop when oxygen is removed?
No, a series of alternative oxidants are used by bacteria in anoxic environments during anaerobic decay, with the reactions layered according to their free energy yield as follows: Aerobic (highest yield) Manganese reduction Nitrate reduction Iron reduction Sulphate reduction Methanogenesis (lowest yield)
28
State the Preservation Model
Organic carbon is deposited under anoxic conditions. Thus there are no benthic organisms, lamination is preserved. Anoxia leads to slower decay thus carbon is able to accumulate.
29
State the Productivity Model
Organic carbon is deposited in quantity. Decay of carbon depletes the oxygen dissolved in seawater. Thus there are no benthic organisms to bioturbate and lamination is preserved.
30
Can preservation quality be used as an indicator of duration or intensity of transport?
No, just because something is preserved well doesn't imply it's in situ. Although transport does promote dis-articulation, fragmentation and abrasion, it is decay that controls degree of articulation and not duration or intensity of transport. Tumbling barrel experiments show that freshly killed worms and shrimps can remain undamaged; while the same animals after decaying for weeks with no transport were falling apart. Thus, rapid burial is important to prevent decay
31
What controls the preservation of hard parts?
Mineralization The original mineralogy controls degree of preservation in hard parts - whether something gets dissolved or not
32
What is the order of preservation potential in hard parts, from highest to lowest?
Phosphate (bone) and silica (sponges) Low Mg Calcite (brachiopods) High Mg Calcite (echinoderms) Aragonite (many bivalves)
33
Why is aerobic decay bad for hard part preservation?
CO2 formed as waste product which can react with water in the sea and the pore-waters will become more acidic, promoting the dissolution of skeletons.
34
How is the presence of calcite or siderite (iron carbonate) used as a first order approximation of differentiating between marine and fresh water paleo-environments?
During anaerobic decay (iron reduction) bicarbonate (HCO3) is produced in vast quantities as a waste product. This makes pore waters alkaline and if in a marine system will react will Ca to produce calcite, or if in a fresh water system like a lake or a delta, will react with Fe present in pore waters to produce siderite.
35
How is pyrite formed in seawater?
During anaerobic decay, reactive Fe produced from Fe reduction and hydrogen sulphide (H2S) produced from sulphate reduction react together to eventully produce pyrite
36
What is the evidence to suggest that the pyritization process happens quickly?
Periostricum on bivalves pyritized while it was still alive and bivalves only live between 1 and 10 years. So there's your evidence, brah.
37
What are metazoans?
Multi-cellular life
38
Do metazoans actually need O2?
Yes, oxygen dissolved in water is a bio-limiting element and with the possible exception to some worms all metazoans are obligate aerobic and perish without oxygen.
39
How does temperature and salinity affect waters ability to carry oxygen?
Cold, fresh water carries more oxygen than warm saline water Warm, saline water de-gases/de-volatiles
40
How is oxygen distributed in the open ocean?
Surface waters are in contact with air, are wave-agitated and typically well oxygenated. Decay of planktonic organisms removed oxygen at depth. This dysoxic zone called the Oxygen Minimum zone (OMZ) (500-1000m depth) Deeper waters are ventilated by cool water sourced from poles and so are slightly more oxygenated Note that this model does not apply for most rocks that we see, that are of epi-continental origin.
41
How are CS ratios used as a geochemical metric to infer palaeo-oxygenation?
In a normal marine system we expect that as the organic C content increases, so does the sulphur content. The more organic C, the more sulphate reduction. However, this doesn't apply to fresh water systems because there's no sulphate and doesn't apply to euxinic systems because hydrogen sulphide is present right throught the water column. This use of this proxy is also dependent on the values of % organic C because there are overlapping fields at low values of organic C. Samples cannot be weathered or metamorphosed.
42
How is DOP used as a geochemical metric to infer palaeo-oxygenation?
Degree of pyritization - the degree to which Fe minerals pyritized: Low DOP values indicate fresh water environments. DOP values of between 0.5 and 0.75 form under low oxygen conditions. DOP values greater than 0.75 are indicators of euxinic systems.
43
Define uniformitarianism
The present is the key to the past
44
What are the 3 main limitations of uniformitarianism through time?
Carbonate productivity and reef distribution through time Calcite and aragonite seas Distribution of epi-continental seas through time
45
How does carbonate productivity and reef distribution through time affect application of uniformitarianism?
Different organisms have been reef builders at different points in Earth History. Thus, the topographic structure of the sea-floor has changed through time. Corals were big reef producers in latter part of the tertiary, the Jurassic and parts of the Ordovician. This is important because the environmental tolerances of the organisms would have been different e.g. corals more tightly environmentally controlled than Rudist bivalves.
46
How does the limitation of proxy data affect how we use carbonate productivity and reef distribution through time as an application of uniformitarianism?
Data suggests that carbonate accumulation was much slower in the past - 1-2 orders on mag less than today, of similar orders to the accumulation made by cold water carbonates today. Limited by time slices of thousands of years that field geologists measure, against direct measurements of today which have to be multiplied to have a comparison. BIAS interp. If carb accumulation was actually slower then this will strongly affect the geometries of platforms/cont margins
47
Explain how calcite-aragonite seas have changed through time.
Continental spreading rates affect the Mg concentration of sea water. When the Mg conc was high aragonite would precipitate as it was the more stable phase, and when the Mg conc was lower calcite would get precipitated. Aragonite seas correlate with icehouse times, and calcite seas with greenhouse times. This means that the preservation potential of an aragonite shell today will be different to that in a calcite sea during the past.
48
What is the biggest and most pervasive limit to uniformitarianism and how we interpret ancient environments?
Stratification, productivity and anoxia in epi-continental seas
49
How is the distribution of epi-continental seas through time such a pervasive limit to uniformitarianism?
Almost all sedimentary rocks that we see would have been deposited in an epicontinental sea. But there was more epicontinental sea during some points of Earth History than others e.g. during the Cretaceous there were vast, 1000'skm2 epicontinental seas. However, these seas are shallow (10-100m deep) and are not affected by ocean circulation and so had the tendency to become anoxic stratified.
50
How do we get stratification in epicontinental seas?
Density differences in the water column with lower more dense layer overlain by a less dense layer. Density differences in the marine realm are due to variations in T and salinity. Mixing in the upper layers of the water column prevents stratification and requires water-body movement. In shallow coastal waters mixing is due to fluid flow (tides, waves and flow-topography interaction)
51
How to you get anoxia associated with stratification?
Overlain layer of warm, circulated water is constantly resupplied with oxygen. At ~80-100m depth we get thermal stratification and shearing of the water as we enter a colder, denser bottom layer isolated from solar heating and has its own circulation cell. This bottom layer has a fixed amount of O2 and so if organics, and ONLY if organics are present in this bottom layer will we get anoxia
52
Why do we care about stratification?
1) Stratification can impact upon bottom water T and carbonate grain types (cool/warm water carbs) 2) If there is a lot of organics (plankton) in the system decaying aerobically this removes a lot of O2 3) It impacts upon fossil preservation 4) Elevated source rocks (hydrocarbons) 5) Anoxia associated with extinction events 6) Helps us understand atmospheric gas (pCO2) through time - impacts upon storage of and capture of natural carbon
53
What does much of the geological literature reference as causes for stratification?
Solar forcing (heating the water), salinity variations (fresh water flows on top of salt water) and silled basins (epicontinental seas are not uniformly flat, and silled basins impact upon local circulation)
54
Why is the Black Sea an exception when applying a uniformitarian view to stratification?
Uniformitarianism suggests that with the exception of the Black Sea (deep silled basin) it is very difficult to maintain year-round permanent stratification in any modern coastal water. Stratification is usually seasonal. But ancient epicontinental seas have no modern suitable sized counterpart.
55
Black shales and source rocks are impacted upon by what factors?
Productivity, preservation and sediment supply/dilution
56
Productivity requires nutrient replenishment. Explain how phosphorous is replenished in the oceans.
Phosphorous is a limiting element - it is essential for producing biomass (lipids and ATP). Productivity is high in shallow water, and here organisms die and fall to the sea floor and decompose which enriches the lower layers of water in phosphorous. This water is then brought back to the surface in certain "hot spots" with elevated productivity by oceanic upwelling. You also get hot spots for productivity off-shore from major rivers which deliver weathered nutrients to the water.
57
Why is the Black Sea an exception when applying a uniformitarian view to stratification?
Uniformitarianism suggests that with the exception of the Black Sea (deep silled basin) it is very difficult to maintain year-round permanent stratification in any modern coastal water. Stratification is usually seasonal. But ancient epicontinental seas have no modern suitable sized counterpart = NON-UNIFORMITARIAN.
58
What % of global marine productivity is restricted to shallow shelf waters?
30% of global marine productivity is restricted to shallow shelf waters which represents 8% of modern marine area
59
Can the Mississippi-Atchafalaya River be used as a model for anoxia in the geologic past?
No. The Mississippi drains the equivalent of 5% of the Earth's land surface and is loaded with anthropogenic nutrient runoff. And although the annual "Dead Zone" associated with the Mississippi is around 7000km2, the condensed Kimmeridge Clay basin is in excess of 500,000km2. The scales are non-comparable.
60
Rocks deposited in epicontinental seas include:
the vast majority of the archive of marine biodiversity, record of climate change and much of the world's hydrocarbons. but they are non-uniformitarian systems because they are so different from the way that modern systems behave
61
What makes water bodies more prone to stratification?
1) Heat - when it's warmer, it expands floats and allows stratification process to begin. 2) Wind (or lack of) - winds determine depth of stratification. Turbulent water kills stratification due to mixing
62
How are the seas around the UK prone to seasonal stratification?
In places like the N sea, during summer when water is warm we get stratification away from the coast (waves interact with sea bed producing turbulence/plumes - get mixing, kills stratification). The water gets cold again in winter and stratification ceases. Baltic Sea i protected from the tides and so it is warm and gets stratified.
63
Why were epicontinental seas of the past (Lower Jurassic of Europe and Cretaceous) so prone to stratification?
Waves get damped by frictional drag and since these epicontinental seas were so big this caused a lot of damping down of the waves, and so they were largely devoid of mixing. Tidal flow may have been elevated between islands generating some local mixing. Mixing occurs in modern epicontinental seas because they are much smaller = less friction
64
Why is there no record of what's going on in the marine realm prior to the Jurassic?
All oceanic sediments older than the Jurassic have been reworked/metamorphosed
65
In big Black Shales, what is the source of productivity?
Source of productivity probably not due to some source of nutrients coming into the system from land but down to a particular mixing regime in the sea. It is easy to make sea-ways seasonally stratified. Could be for a few months every few years. This is an intermittently stratified and intermittently mixed system.
66
Only under what condition can a water body become permanently stratified?
Possible only if hyper-saline at bottom of water column. This allows for evaporation and the production of salt deposits. The salt then works it's way down into the water column. Increasing the water depth also makes water bodies more prone to stratification.
67
To what extent are the multi-disciplinary approaches used to determine environmental parameters such as oxygenation in ancient systems limited to particular time scales? E.g. which approach might be best for identifying a brief oxygenation event in an otherwise anoxic basin?
Geochemical analyses assume that the oxygenation is evenly distributed through time. A hand specimen for example could represent 5-6kyrs and will be analysed for DOP, CS ratios and the answer will be the average value over that time period (i.e. it measures the temporal precision) which could be that that system was anoxic or dysoxic or euxinic. But then upon further examination of the rock we may see thin shell plasters of bivalves that are 5-6mm across which tell of an oxic system. So these two data sets superficially contradict but this isn't the case. What's being said here is that over the 5-6kyrs represented by the rock, overall, on average it was an (e.g.) anoxic system but with brief periods of oxygenation.
68
What is a trace fossil and what is palaeoethology?
Fossilized behaviour The study of ancient behaviour
69
What is the bioturbation index and what does it tell you about depositional environment?
A bioturbation index reflects the extent to which primary sedimentary fabric has been destroyed by biological activity and exists on a scale of no bioturbation to complete cross cutting of the rock face by trace fossils. A high bioturbation index can suggest a low sediment input into the depositional environment as this gives organisms more time to delaminate the sediment, and vice-versa.
70
Onshore-offshore trends of Zoophycos are a critic to the ichnofacies concept how?
Zoophycos in Paleozoic present in all environments. Zoophycos in Cenzoic and latter part of Tertiary restricted to just deeper water environments. Therefore, the Zoophycos ichnofacies only deep water indicator in Cenozoic and latter part of Tertiary.
71
What is autochthoneity?
If something is autochthonous it is in place. 99.9% of trace fossils are autochthonous because they are essentially part of the sediment and were created at the same time that the sediment was deposited. Sometimes trace fossils can be transported when mineral ppt occurs on the inside of a burrow for eg
72
Equalble climates were typical of greenhouse worlds. What is an equable climate?
A climate where there is a reduced T gradient between the equator and the poles
73
How can cycles of change be used to predict what rocks we see in the field?
Milankovitch driven variations allow us to predict for example, in the Carboniferous, we know it was an Icehouse world so we expect to see rapid, high frequency 100m sea level variations of Black Shales through to coals over a period of 50kyrs. We can apply the same knowledge of Greenhouse worlds to say the Cretaceous, where there would have been eustacy driven variations of 10m over 50kyrs. So the figure has predictive value.
74
What kind of effect do orbital variations have on climate through time?
Smaller scale climate variations are presented by the ~20ky cycle of Precession. This allows for light to be concentrated on different parts of the Earth for different durations of time giving variations of seasonality. But over say a Myr period these variations are negligible.
75
What kind of effect do orbital variations have on the sedimentary record?
Due to the short term climate perturbations of between 40-100kyr, these cyclic variations in climate impact on the way that sedimentary rock gets deposited in that it: (i) produces variations of organic productivity which in turn causes variations in oxygenation; (ii) variations in planktonic productivity e.g. shelly plankton; and (iii) subtle variations in run-off on 100kyr scale
76
What controls the global distribution of terrestrial bioomes?
Largely controlled by latitude and altitude. Latitudinal biomes are constrained by T and rainfall, and then altitude modifies/moderates the latitudinal pattern. E.g. at equator at low altitude have a warm climate and tropical rainforests but at higher altitude, you move through temperate rainforests into alpine pastures.
77
What's the distinction between weather and climate?
One storm is an indicator of weather. A tendency for storminess is an indicator of climate.
78
Why are the biggest storms documented in the sedimentological record associated with strongly stratified water bodies?
Upper layers of the water column would be hotter which in turn accelerates the storm. Associated with greenhouse times when equatorial latitudinal zone would be expanded. Tropical storms require a sea-surface T of 27 degrees C and currently occur within 15degrees of the equator.
79
What's the potential of a rock for capturing a storm?
Great. Even in temperate latitudes where mid-latitude storms occur (between 40-50degN and S of equator e.g. London 51deg N) infrequently, over a 40kyr time-scale represented by a rock 1000's of stroms may be recorded. Thus one environment could be a storm documented env on geologic time-scale but not in real-time. Bigger storms are better preserved in the rock record.
80
How does climate impact upon the formation of terrestrial sedimentary rocks?
This is not all about T and rainfall: the relationship between evaporation and ppt is a very important factor. E.g. when you find evaporite deposits, doesn't mean that it was an arid env, just that EVAP\>\>PPT
81
What is evidence of a fossil fire in the rock record?
Charcoal
82
The Old and New Red S.stns of the UK are examples of what climate?
EVAP\>\>PPT Continental scale sand seas (sandy deserts e.g. Sahara) where evap is far in excess of ppt Chemical weathering is minimal, Millet sand grains tremendously well rounded and well sorted by wind.
83
Under what climates do we form evaporites?
EVAP\>\>PPT Potassium and sodium salts such as halite and sylvite only occur naturally in large scale where net evaporation rates are very high Modern e.g. Great Salt Lake of Utah
84
Under what conditions does gypsum ppt?
EVAP\>PPT Where evaporation rates are high but in semi-arid env. Can form a clean gypsum sheet at sed-air interface. Can ppt at paleo-water-table interface - the ppt crystals push sediment apart, until eventually is only apparent as thin wisps. This is 'chicken-wire' fabric.
85
How big to dessication cracks need to be to be an indicator of climate?
2-5m across, cracks 0.5-1m deep and 10cm wide. Not like small ones you get in the garden. Indicative of arid-semi-arid env, where PPT\>\>EVAP, PPT\>EVAP
86
How and when can calcium carbonate ppt in soils?
EVAP\>PPT. Ppt is brought about by changing the volume of CO2 dissolved in the pore-water. Loss of CO2 causes carbonate to be ppt from pore-water. This can result from changes in T and as a result of photosynthesis in plants. Vertisols (lineations) may form seasonally with repeated wetting/drying, as CaCO3 nodules expand horizontally when wet (not to oppose gravity) and fracture and fold into pseudo-anticlines upon drying.
87
What are tap roots and what are they indicative of in the sediment record?
Calcified tap roots that go vertically down to the paleo-water table are long roots of a tree indicative of evolving in semi-arid environments. e.g. Sherwood Sandstone, Jurassic
88
How do karsts form?
When you warm a wet soil, chemical reactions between the water and organic material in the soil occur rapidly producing acids which seep into the underlying limestone, running through joints and fracture surfaces. These surfaces are then characteristically open up by weathering.
89
How can karsts be of great geologic importance?
In some instances they can be indicative of sub-aerial expose and thus are able to be locally correlated. If a limestone is formed, and then a regression occurs and the limestone is karsted and then a transgression results in another limestone being deposited on top, this produces a characteristic Klint (block) and Greic (crack/fracture infilled with overlying limestone) structure. This also gives way-up.
90
Are reefs an indicator of climate?
NO. Reefs can form in temperate climates as well as tropical ones because they are not always composed of corals. Corals ARE however and indicator of climate.
91
Define 'biodiversity'
The variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems, and the ecological complexes of which they are a part; this includes diversity within species, between species and of ecosystems.
92
How do we measure modern diversity?
Usually based on estimates of numbers of higher taxa
93
Explain how some people argue that volume of sedimentary rock has impacted upon estimates of Phanerozoic biodiversity
Superficially, there is an elevated volume in Devonian rocks compared to Carboniferous and Silurian rocks which correlates with Devonian being the seemingly most diverse period in the Paleozoic, but is this peak in species richness due to there actually being more creatures alive at that time or is it just that there is a greater volume of rock to collect from. Some might argue that rock outcrop/map area is more important because no matter how great the volume is, if you can't access it to get the fossils you can't make an estimate.
94
Explain how Palaeontologist interest units might impact upon estimates of Phanerozoic biodiversity.
There is a peak in the number of Palaeontologists working on Devonian rocks which correlates with a peak in biodiversity during the Devonian. Is this peak in biodiversity because there was actually more creatures alive at that point in time or is it because more fossils have been collected from rocks of that age relative to other age rocks producing a bias dataset.
95
Explain how a collection bias might impact upon estimates of Phanerozoic biodiversity.
There are areas of the Earth today that haven't been adequately collected from e.g. parts of the world like in Africa where access is difficult perhaps due to transport, political issues or the climate is difficult. Whereas the UK has been extensively collected from. North America is also beautifully exposed but not mapped in as much detail as the UK. Therefore we just don't know what fossils are preserved in the less excavated areas of the Earth, thus not capturing the full diversity signal of fossils in rocks.
96
Explain how sea-level change through time might have impacted upon estimates of Phanerozoic biodiversity.
From The Mesozoic to the Cenozoic represents an interval in geologic time where more flooded continent (epicontinental sea) existed, in comparison to other times. Most of the rock we see in the field, particularly in the USA and Europe, is of epicontinental origin and so this is where we get most of our fossils. Because more sedimentary rock is deposited when sea-level is high, thus we have more (e.g.) Cretaceous rock to collect from. Then if we have a higher palaeodiversity is this because there was actually more creatures alive or because we have more rock to collect from.
97
What is bio-molecular innovation and explain how it is suspected to have changed taphonomic bias through time.
The evolution of the materials from which organisms are constructed: Some organic molecules and skeletons are more preservable than others and this has changed with time. The appearance of specific bio-molecules such as lignin, cutan and sporopollenin has potentially imparted decay resistance to plants, increasing their preservation potential over time. Lignin is considered to be particularly decay-resistant in oxygen deficient regimes.
98
Explain how secular trends/variations in ocean chemistry and skeletal mineralogy has the potential to impart a temporally and variable taphonomic over-print on the fossil record.
Ocean chemistry has changed through time and this has influenced the relative preservation of calcite and aragonite. The Ca/Mg ratio of seawater influenced the mineralogy of marine carbonates during the Phanerozoic causing oscillations between “calcite and aragonite seas” which impacted upon calcareous skeletal mineralogies and affected cementation rates over time, strongly affecting the preservation of primary reef structures.
99
Explain how biological evolution is suspected to have changed taphonomic bias through time.
the evolution and diversification of burrowing organisms would disturb and potentially degrade carcasses that were buried. This bias can be expected to have increased as the depth of burrowing/bioturbation (paleontological grave disturbance) has increased with time, from low levels in the Cambrian to where burrows reached modern depths of one meter or more at the end of the Paleaozioc. The exceptional preservation of soft tissues in Cambrian Lagertatte such as the Burgess Shale exist because of the low depth and extent of bioturbation, leading to greater chance of preservation. The extent of bioturbation (which includes scavenging) increased significantly post-Cambrian. Equally as biodiversity has increased, organisms have evolved whose ecology promotes the direct destruction of biogenic remains (e.g. insects, fungi and microbes that destroy plant material in the terrestrial realm; diverse borers that degrade shelly remains in aquatic habitats, reducing their preservation potential and subsequently impacting upon hard-part preservation through time). The impact of predator-prey escalation (e.g. durophagous marine vertebrates) could potentially lead to bias against fossils preservation but equally it may have led to the evolution of defence mechanisms that include stronger, more robust shells that were more likely to be preserved
100
Explain how temporal trends in conserving environments is suspected to have changed taphonomic bias through time.
Fossil Lagerstätten occur in preservational windows that are unevenly distributed in time and space and clearly reflect temporal trends in fossilization and impact upon estimates of global diversity through time. There are also times in Earth history where we get more lagerstatten than others. E.g. conserving environments such as lacustrine (lake) deposits of the Cenozoic and Burgess Shale type lagertatte are restricted to certain time-zones through time and preserve more of one type of fossil than another. The occurrence of phosphate and silica replacement as well as konservat-Lagertatten is time restricted
101
What is taphonomy and how does it impact upon fossil preservation?
Taphonomy is the study of the fossilization process. • Taphonomic processes exert a profound and widespread bias to the fossil record, and there are few, if any fossils that are preserved bias-free. The most profound example of preservational bias is the rarity of fossilized soft parts and soft bodied organisms. In “normal” marine near-shore communities such organisms can account for about two thrids of individuals and yet they are rarely preserved.
102
Give an example which underscores the pervasive nature of taphonomic bias.
•Soft tissues are preserved in exceptional circumstances, but it would be fallacious to assume that the preservation of soft tissues implied a minimal taphonomic bias e.g the Iron-Age peat bogs of Europe preserve human carcasses that include exquisite preservation of soft tissue, enhanced by the action of organic acids in the peat which also promoted mineral dissolution to the extent that some carcasses are devoid of bone •This preservation of soft parts in preference to bone in this instance underscores the pervasive nature of taphonomic bias.
103
What are the factors that impact upon the decay of organic soft-tissues in sediments?
Oxidant supply Temperature Nature of the Carbon molecule Depositional setting Rate of burial
104
How does the oxidant supply impact upon the decay of organic soft-tissues in sediments?
There is an association of anoxic sediments and high organic concentration in muds with beautifully preserved fossils and so the assumption developed that anaerobic decay is slower than aerobic decay and this is why soft parts were being preserved. But anoxia doesn’t really slow decay processes down. There are very few direct rate measurements and the available data suggests that aerobic decay may only be 2-3 times more rapid than anaerobic decay – this is not enough to make any significant difference to soft-part preservation. So what does anoxia do? If the anoxic-oxic interface is above the sediment-water interface then there is no scavenging or bioturbation to destroy soft tissues, disarticulate skeletons and damage fossils. There is evidence to suggest that in some modern oxygen deficient marine environments every sediment particle present will be ingested and excreted by a worm up to 30 times before being buried beyond the bioturbation depth. This is very thorough bioturbation, just like we see in The Chalk of the Cretaceous where there are no visible primary sedimentary structures like ripples
105
How does temperature impact upon the decay of organic soft-tissues in sediments?
Microbially mediated decay is a chemical reaction and so it is impacted by temperature. A 10°C rise in temperature can roughly double the rate of decay. But there are a number of internal controls on the effect that temperature has upon the decay rate: for example, doubling the rate of decay would double the demand of the bacteria for nutrients and it would also double the rate at which their toxic, metabolic bi-products would need to be removed by diffusion or water movement. If these demands are not met then the increase in decay rate can be capped
106
How does the nature of the carbon molecule impact upon the decay of organic soft-tissues in sediments?
It is obvious that some molecules will decay faster than others. Volatile (reactive) molecules decay rapidly while refractory (non-reactive) molecules will decay more slowly. Complex polymers will act more refractory while less complex polymers will act more volatile. This also means that in an anoxic environment, where there is less bio-diversity of bacteria, very complex molecules (e.g. lignin in wood) can only be partly decomposed. Each bacterium will have its own specific enzyme that decomposes a specific part of the carcass, for example, and with less diversity of bacteria available to decompose, less of the carcass will be decomposed.
107
How does the depositional setting impact upon the decay of organic soft-tissues in sediments?
Depositional setting can effect decay in that some settings are toxic. For example, in peat bogs, tannic and fulvic acids damage the cell walls of bacteria and thus inhibit their activity. There have been many examples of human skin being preserved in peat bogs while the bones have been dissolved away by the acids (which refers back to the nature of the carbon molecule affecting preservation, except in this example it is the presumably most preservable part that has been destroyed and the typically least preservable part being fossilized. I.e. it is not always the most preservable tissues that get preserved, and the least preservable tissues that get removed). Amber in another example which has beautifully preserved the outer form of organisms but leaving a void on the inside with very few traces of muscle tissue sometimes visible.
108
How might rapid burial impact upon the decay of organic soft-tissues in sediments?
Rapid burial will essentially prevent decomposition. Decay usually takes days or weeks (a geological instant) so burial must be extremely quick to prevent it – perhaps in a mass flow.
109
How might an ichnofacies be used to determine paleo-depth?
An ichnofacies is a temporally and spatially recurrent association of trace fossils and sedimentary structures. Trace fossils vary with environment and so as environments change so do the trace fossils present within them. Therefore, a change in ichnofacies indicates a change in palaeo-environment
110
What is the first critic one might apply to using the ichnofacies concept to determine palaeo-depth?
the ichnofacies concept is based upon an idealized onshore-offshore transition and assumes that depth controls the distribution of organisms. Depth does not control the distribution of organisms, however the distrobution does depend upon and responds to a number of factors such as oxygenation, temperature, organic input and depth to storm wave base and lower limit of the photic zone. These factors are very loosely controlled by depth and there are no absolute, fixed parameters for each factor within a shallow or deep marine environment. For example, there can be strong variations in the input of organic material into a shallow marine environment according to whether it was offshore from a desert or a delta-type environment. I.e. there is a passive relationship between depth and the distribution of organisms; depths itself essentially only increases pressure which has very little control on the distribution of organisms
111
What is the second critic one might apply to using the ichnofacies concept to determine palaeo-depth?
Organisms evolve through time, forever adapting to new environments. For example, a trace fossil that was present in a shallow marine environment in the Palaeozoic could be present in a deep marine environment today, or vice-versa. Therefore, it is flawed due to its reliance on an uniformitarian concept.
112
Taphonomic bias is thought to have varied with time. This is suspected due what reasons?
Bio-molecular innovation Secular trends/variations in ocean chemistry and skeletal Biological evolution Temporal trends in conserving environments
