Complex Geological Problems: Integration of Physical, Chemical and Biological Factors

1. Complex Geological Problems: Integration of Physical, Chemical and Biological Factors

2. Continued from last class: • Sediment becomes lithified (hardened) by: • compaction • 2. cementation (sediment particles “glued ” together by minerals precipitated from sediment porewaters)

3. An Example of Using Integrated Approach to Interpreting Geological Events: Concretion-Hosted Shell Clusters in The Upper Cretaceous Bearpaw Formation of southern Alberta Concretions (hard, cemented bodies of sediment) common In many mud-dominated successions. Concretions are formed after sediment is deposited

4. First attempt at explanation: Clumped appearance of shells due to preferential preservation within concretions In other words, the original shell concentration may have comprised a continuous bed, but were not preserved outside the concretions because they were dissolved

5. Problems with dissolution idea: • Shells can be found (in lower abundances) in the • host sediment of the “event” horizons. • 2. In some cases, shell concentrations extend outside • of concretions then pinch out • 3. In some cases, shell concentrations pinch out within • concretions Verdict: Cam’s a dumbass The clustering habit of the shells is real, and was produced before the concretions formed

6. Basic Aspects of Sedimentary Environment: • Succession dominated by claystone (mud) indicating generally low energy conditions (i.e. water calm enough to permit settling of mud) • Sediment type appears to be deep subtidal, mudstones merge laterally into sandier sediments toward the west where sedimentary characteristics indicate nearshore (beach/delta) to onshore (river deposits) envrionment • 3. Mud-dominated succession generally has low diversity shelly assemblages, forms typical of normal-salinity. • 4. Thick intervals with few or no shelly faunal elements, and trace fossils indicating low oxygen • So, in general, the depositional environment was probably characterized by: • Relatively deep water • Low energy conditions with little water circulation • Dysoxic bottom waters

7. Additional Observations: Concretion-hosted shell clusters commonly occur in horizons indicating higher energy sedimentary processes Beds and lenses of silt/sand preserving: A) hummocky cross-stratification / B) rhythmite lamination / C) ripples/ D) load casts

8. Tentative (Second) Interpretation: The depositional environment was generally quiet, with low oxygen conditions on seafloor Storm waves / currents occasionally influenced the sea floor, concentrating shells and other coarser-grained sedimentary particles and washing away the mud Patchy distribution of coarser grained particles (silt, sand and shells) due to low availability of sediment In other words, coarser grained sediments (silt, sand and shells) formed isolated patches Rather than continuous beds during storms

9. Another problem: Organisms found in shell clusters often completely different from those found in mud Complete shells in the clusters dominated by those of suspension feeders Possible solution: Isolated silty/sandy bedforms (ripples) were sites of preferred colonization by suspension feeders

10. Model for second attempt at explanation Isolated patches of sediment formed during storms Extension of idea: Isolated silty/sandy bedforms (ripples) were sites of preferred colonization by suspension feeders

11. Problems with bedform idea: 1. Some of the clusters are nearly globular in shape, and obviously do not conform to the cross sectional shape of a bedform. 2. Bioturbation probably not capable of redistributing shells to form this shape because burrowing tends to disperse particles rather than concentrate them. Verdict: Cam’s a dumbass The clustering must reflect something else

12. Critical Observation: In examples where bioturbation is low (i.e. where primary sedimentary structures are not obscured by churning of burrowers) sharp contact is preserved under shell concentration Shell concentrations are therefore scour infills Note: scour in C is associated with a burrow

13. So… another idea Clustering is due to infilling of storm-produced Scours Scours preferentially sited in areas of surface Roughness on the seafloor (due to generation of turbulence) Could be negative elements (e.g. burrows) or positive elements (individual shells or clumps of shells) Note: clumping is common among suspension-feeding bivalves

14. Incorporate observations on faunal assemblages • Main Observations: • Clustered assemblages occur in distinct horizons marking storm events • Individual clusters often comprise shells of single species not generally found in muds • Shells in least disturbed clusters tend to be those derived from suspension feeders.

15. Another approach: How do assemblages of clusters differ from those of the more “typical” muddy sediments ? Classify into “background” and “event” assemblages

16. Background community: dominated by Deposit feeding clams, predators/scavengers, And specialized suspension feeders adapted to Living on/in soupy mud

17. Event community – dominated by suspension feeders (in this case, bivalves), generally adapted to more stable substrata

18. Eureka !: To establish event community, must have: 1. Relatively stable substrate (“primed” by storms) 2. Elevation in oxygen 3. High larval production to permit establishment of high-density communities • …and clustering is due to interaction of • waves/currents during storms with presence of • living things on seafloor due to turbulence • generated around areas of surface roughness • on sea floor • e.g. burrows, shells, naturally patchy clumps of • organisms

19. General Model for Shell Clusters Storm brings in sand/silt New substrate during high Larval productivity allows Establishment of suspension-feeding community During next storm, shells locally generate turbulence to produce Scour (shells themselves act as tools in the scouring process) Sand/rains out as storm abates Primary sed. Structures generally erased due to bioturbation after storm

20. How are multiple events preserved in the Geological record ? Consider a slot machine

21. Elaboration of Scour Idea

22. So where do concretions fit in ? These concretions are cemented mostly by calcite (calcium carbonate) In anoxic part of the sediment column, a few cm below sediment-water interface, sulphate reducing and methane oxidizing bacteria eat up organic matter in sediment, releasing bicarbonate and sulphide. If porewaters become saturated with bicarbonate, the abundant calcium ions combine with bicarbonate and calcite is precipitated. Verdict: Cam’s still a dumbass, but managed to figured this one out.

23. The more technical version: Sulphate Reduction 2CH2O + SO42- 2HCO3- + H2S Anaerobic Methane Oxidation CH4 + SO42- HCO3- + HS- + H2O Calcite Precipitation Ca 2+ + 2HCO3- CaCO3 + H20 + CO2

24. Concretion Seeding • A basic principle: in a supersaturated solution, minerals • can precipitate more readily if crystals of that mineral • are present in the environment (.e.g the effect of dropping • a grain of salt in a glass of salt water) • Clusters of shells made of calcium carbonate acted as • seeds for the precipitation of calcium carbonate cements • from sediment porewaters. • Thus the association of shell clusters and calcite-cemented • concretions makes complete sense. • So…concretion-hosted shell clusters formed due • a coincidence of factors: • Establishment of suspension-feeding community due to • Priming of sediment by storm • Event of high larval production • 2. Formation of scours during later storm due to surface • roughness provided by organisms as depressions or • areas of positive relief and infill of scours by skeletal remains • 3. Seeding of concretions by virtue of calcium carbonate • Composition of skeletal remains in clusters

25. Single or Multiple Causes ? Any other applications of this approach ? Plenty ! Example: Mass Extinctions

26. K-T boundary: General Trends See detail (next slide)

27. Detail at K-T Boundary (detail of previous slide)

28. Mass Extinctions: Summary of “The Big Five” 5. Cretaceous-Tertiary extinction (65 Ma) -bolide impact -flood volcanism (Deccan Traps in India) -cooling, rapid sea level fall ? victims: 47 % marine genera 4. Triassic-Jurassic extinction (199 - 214 Ma) - flood basalt volcanism (central Atlantic) victims: 52 % of marine genera. 1. Permian-Triassic extinction (251 Ma) -bolide impact ? -flood basalt volcanism in Siberia ? -assembly of Pangaea (continents interconnected) -global cooling, major sea level fall victims: 84 % marine genera; 95 % all species on Earth ! 3. Late Devonian extinction (364 Ma) - global cooling (note: coincident with expansion of land plants) ? victims: 57 % of marine genera. 2. Ordovician-Silurian extinction (439 Ma) - global cooling, then rapid warming - rapid sea level fall followed by rapid sea level rise victims: 60 % percent of marine genera. Different mass extinctions, different causes ?

29. K-T boundary debate: oh, the fights The Fights Luis Alvarez (to Dewey McLean – opponent of Impact Hypothesis) “If the president of the college had asked me what I thought about Dewey McLean, I’d say he’s a weak sister. I thought he’d been knocked out of the ball game and had just disappeared, because nobody invites him to conferences anymore.” Walter Alvarez (to Dewey McLean) “Dewey, count them, 24 are with us. You are all alone. If you continue to oppose us, you will wind up being the most isolated scientist on this planet.” Dewey McLean (to David Raup, proponent of Impact Hypothesis) “Now, for your “Just So” comments in a Mosaic article (1981, v. 12) that you “know nothing about.” The one that got asteroid opponents lumped into an insulting blue–outlined box titled “Just So,” with the note that “some of the proponents abide in lofty isolation.” David, I believe you introduced the “Just So” epithet into the K–T.” David Raup to Cam Tsujita (2000) “In his recent paper, Tsujita makes parallels between a multicausal model of shell beds and mass extinction. Nice touch. Tsujita does, however, remain an outspoken opponent to the consensus that asteroid impact is the one and only possible cause of the end-Cretaceous mass extinction. His attempts to construct a “just so” story are poignant.”