Geol30_08_Lecture_13b

Geol30_08_Lecture_13b - 2/19/2008 Geology 30 History of...

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Unformatted text preview: 2/19/2008 Geology 30 History of Life -- Lecture #13 -- * Late Proterozoic life & the Cambrian Explosion p 02/19/08 *Note change in lecture title Announcements Lecture #13 title change: Snowball Earth and the Cambrian Explosion changed to Late Proterozoic life & the Cambrian Explosion Reading assignment not modified though 1 2/19/2008 Outline Snowball Earth Late Neoproterozoic life The Cambrian Explosion Isotopic (chemical) evidence for life Earliest prokaryotic body fossils Stromatolites abound (earliest ~ 3.5 Ga) Eukaryotic biomarkers Earliest eukaryotic body fossils (biological affinity?) From last time... Evidence of the origin for major algal clades Earliest animals 2 2/19/2008 Ediacaran fauna Today Snowball Earth 3 2/19/2008 Glaciation Glacier thick (many meters to km), extensive (many km2) t i ( k ), perennial ice i.e., dense ice is there through warmer months Produces distinctive features Striations linear grooves till tillites (unstratified, unsorted deposits) Glaciation normally polar (i.e. high latitudes) North Pole South Pole Occur at high latitudes (Antarctica, Greenland) and also high altitudes (Andes, Himalayas, etc.) Thus, usually limited to these cooler areas 4 2/19/2008 But, a few glaciations were really globally extensive There are times when glaciers occurred i ( near) Th ti h l i d in (or ) equatorial regions at sea-level: sea-level: ~ 710 Ma ~ 635 Ma ~ 585 Ma (less in intensity/extent) These have been called "Snowball Earth" events Snowball Earth?! 5 2/19/2008 Snowball Earth IceIce-covered ocean down to Equator ( hard snowball) (= These extreme ice age may have lasted for millions of years shutdown of photosynthesis shutdown of hydrologic cycle PostPost-Snowball Earth also climatically harsh (very warm) A "freeze-fry, double-whammy" (p Prof. Stan Awramik) "freeze- y, doubley (per ) Implication: Implication: would lead to a mass extinction Let's look at what we know...and what paleontology can tell us... N.B., N.B., In this class/lecture, we'll focus on the reconstruction. More details on HOW we know (i.e., the geological evidence) is provided in the addendum Albedo (~reflectivity) The amount of radiation absorbed depends on type of p yp material it comes into contact with Albedo = % of incoming radiation reflected back into space (~reflectivity) (~reflectivity) average albedo of Earth = 30% Ice high albedo (~90%) Ocean low albedo (~5%); light penetrates into depth Land depends on coverage albedo between ocean and ice levels 6 2/19/2008 The Freeze-Fry: The Freeze FreezeLarge land areas clustered g You'd need some at high latitudes (where glaciers begin) and some at low (to increase reflectivity) The latter would increase albedo overall energy retained by Earth's atmosphere less LongLong-term carbon cycle becomes imbalanced Atmospheric levels of CO2 levels decrease, decrease, as do global temperatures CO2 interchange between ocean (and its photosynthetic inhabitants), air, and land is disrupted The Freeze-Fry: The Freeze, cont. (1) FreezeIce began to expand and descend from the poles The Earth's albedo is further increased, increased, (more ice higher reflectivity) thus reflectivity) accelerating the cooling even more Evaporation decreased (due to cooling of oceans) Water vapor concentration decreased, decreased, weakening the greenhouse effect This accelerated the cooling even more! Runaway ice albedo effects 7 2/19/2008 The Freeze-Fry: The Freeze, cont. (2) Freeze?! Ice continued to expand until Earth's Earth s surface was frozen LongLong-term freeze: ~10 million years? BUT, continued volcanic outgassing builds up CO2 in atmosphere Due to ice cover, CO2 is not removed cover by weathering (of rocks) or by photosynthesis The Freeze-Fry: The Fry FreezeCO2 imbalance in other direction... Continued volcanism causes CO2 levels to rise Eventually, CO2 levels became very high Earth warmed up--perhaps very rapidly up--perhaps Cf. enhanced greenhouse effect Ice melted near equatorial regions Albedo decreased, accelerating the warming , g g Evaporation increased, increasing water vapor in the atmosphere 8 2/19/2008 The Freeze-Fry: The Fry, cont. FreezeGreenhouse effect became even stronger, stronger, accelerating th warming l ti the i CO2 levels now very high Atmosphere is incredibly hot (possibly ~50C = 122F) ~50 122 "Hothouse world" Various feedback systems (biogeochemical) would eventually re- t bili th E th re-stabilize the Earth system t Wouldn't all this cause a mass extinction on a grand scale? millions of years ago (Ma) 1000 900 800 700 600 Less severe? `Snowball Earth' glaciations green algae yellow-green g algae But life survived... All the documented eukaryotes that appeared before the first snowball event and still around today Thus, a lot of lineages survived red algae 9 2/19/2008 ...and in fact flourished just after 0.1 mm After the second snowball, some of the first see signs of abundant multicellular life (~600 Ma), including animals 0.5mm Animal embryos Also, microfossil assemblages before the 710 Ma-glacial Maevent are strikingly similar to an assemblage associated with and after the glaciation The same could be said for the 635 Ma-event Ma- Snowball Earth and Life: Conclusions Snowball Earth conditions did not markedly affect life: No mass extinction was exhibited in the fossil record Diversity was largely not affected Complex communities persisted `Slushball' Earth Still could be tough times for life Band of ice-free ocean in the vicinity of the equator iceNote: this is still a hotly debated issue, do not assume that my "conclusions" listed above are the last word on the subject More info in addendum and in the reading assignments on-line 10 2/19/2008 Late Neoproterozoic Life Biotic trends in the latest Neoproterozoic (~600 Ma to 542 Ma) Recall that from the origin of life to the end of the Precambrian, Precambrian life was in general microscopic was, general, Bacteria/archaea, acritarchs & early algae (possibly others?) Exception, carbonaceous compressions First possible animals ~600 Ma, Early animals (embryos, burrows) ~575 Ma, Complex soft-bodied forms are found soft(Edi Ediacaran f fauna...) fauna...) 11 2/19/2008 Ediacaran fauna Made famous from the Pound Quartzite, Ediacara Hills, South Australia Hills, Strange preservation, primarily impressions in sandstone soft bodied Controversial: What are they? Do they represent living phyla, or are they different? Plural of phylum Crown vs. stem groups An important distinction crown Stem groups can have their St h th i own distinct autapomorphies, in autapomorphies, addition to the synapomorphies synapomorphies they share with crown groups RIP stem RIP RIP Thus, they will look strange with mixtures of characters Note: early eukaryotic fossils likely represented stem group eukaryotes 12 2/19/2008 Ediacaran fauna, cont. Abundant, widespread Abundant widespread, diverse ~575 Ma through 575 to end of Proterozoic Disappear at Precambrian/Cambrian boundary Extinction? Preservational issue? What are they? Animals (crown groups) Separate `kingdom' a failed experiment in multicellularity Lichens, fungi, jellyfish, corals...? Likely a variety of things--mostly stem group animals things--mostly Ediacaran `discs' ? Stem group cnidaria (= jellyfish, corals, anemones) "Stranded jellyfish?" 13 2/19/2008 Ediacaran `frond-like animals' `frondFrond = "leaf like" http://en.wikipedia.or g/wiki/Pennatulacea A modern sea pen ("soft coral") Dickinsonia Charnia More stem group cnidaria? Charniodiscus We'll return to these groups later Ediacaran `other' Stem mollusk? Stem arthropod? Stem trilobite? Parvancorina Kimberella Spriggina 14 2/19/2008 Prelude to the Cambrian Explosion There are plenty of Precambrian p y fossils, which indicate a long history during which life evolved (score one for Darwin!) There are even Precambrian animals And shelled animals (i e hard (i.e. parts) were around before the Cambrian First Shelly Fossils Cloudina Just before the Cambrian (~544 Ma) Carbonate (CaCO3) tube ?Worm ?Cnidarian Taxonomic affinities in dispute 100 cross-section reconstruction 15 2/19/2008 Small Shelly Fossils Increasing diversity & abundance g y starting ~544 Ma mmmm-size skeletal fossils Tubes, cones, plates Many are parts of larger skeletons Mostly problematic (animals, but what kind?) Probably stem groups of modern phyla Cambrian radiation Many modern phyla of animals appeared in the Early pp y Cambrian Many included taxa that evolved skeletons Not a uniform radiation (throughout the Cambrian) Lowermost: mostly simple skeletal fossils Tommotoian fauna (small shelly fossils, but more complex) Within a few tens of millions of years, animals diversify at the phylum, class, and order level = adaptive radiation (more on this in a bit) And larger animals: some belong to phyla that have survived to the present 16 2/19/2008 Cambrian Lagersttten (Lagersttten = fossil deposits of extraordinary richness or completeness) Cambrian Lagersttten preserves soft-bodied softorganisms The Burgess Shale and Chengjiang record the Cambrian Explosion Let's look into this preservation in a little more detail... Burgessochaeta, an annelid worm from the Middle Cambrian Burgess Shale, Canada Burgess Shale (~505 Ma) SoftSoft-bodies preserved Preservation: thin carbon or clay films on bedding planes of shale Organisms tumbled off a reef escarpment, fell into deep anoxic waters below, buried 17 2/19/2008 Opabinia Fossil Reconstruction 5 eyes, vertical tail fin (probably swam), central grasping organ Wiwaxia ~2 cm Moved along seafloor; tiny scales and long spines 18 2/19/2008 Anomalocaris ? Anomalocaris ? = bodies of shrimp-like organisms? 19 2/19/2008 Anomalocaris ! Animal ~1m = 3 ft. long Anomalocaris et al. Opabinia Wiwaxia ? 20 2/19/2008 Hallucigenia 2 mm Hallucigenia, 180o 2 mm 21 2/19/2008 Chengjiang Fauna (~520 Ma) Discovered more recently Located in Yunnan China Yunnan, Preservation: thin carbon or clay films on bedding planes of shale, subsequently oxidized Many of the same organisms g as the Burgess Shale Phylum Chordata well represented Chengjiang outcrop Yunnanozoan, Yunnanozoan, an early chordate; likely ancestral to vertebrates (our lineage) Adaptive radiation Within a few tens of millions of years, animals diversify at the phylum, class, and order l h l l d d level l A large-scale adaptive radiation largeadaptive Most phyla had few only species This phenomenon is unique (but adaptive radiations are not) What about these new phyla? Kingdom Phylum Class Order Family Genus Species 22 2/19/2008 S.J. Gould on the Burgess Shale Bizarre Burgess Shale creatures = new phyla h l Phyla tend to have distinct body plans Thus, more *morphological disparity in Cambrian than any other High *morphological disparity = things look really different from each other Contingency An event that may occur; a possibility Current events are contingent (~dependent) upon past events Changes in the past have lasting repercussions in the future Mass extinctions give opportunities for new groups to become dominant But perhaps even more important... The Cambrian may have been an particularly important time for a specific body plan to survive and/or become dominant 23 2/19/2008 Addendum: Snowball Earth The evidence (Note: some slides repeated from lecture material for clarity) The Snowball Earth idea Joe Kirschvink (Cal Tech) in 1992 introduces term "Snowball Earth" Paleomagnetic analyses on glacial deposits indicated glaciers at the equator Postulated glaciers covered the entire Earth including tropics He had no good explanation for initiation and demise of Snowball Earth 24 2/19/2008 The Snowball Earth Hypothesis Paul Hoffman (Harvard) in 1998 proposed a `unified' Snowball Earth hypothesis Earth experienced vary high temperature oscillations in the Neoproterozoic, from very cold ("snowball Earth") to very hot Neoproterozoic ( snowball Earth ) Let's look at the geological evidence... Neoproterozoic glacial deposits Neoproterozoic g p glacial deposits occur on all modern continents Some ages less certain than others 25 2/19/2008 Tillites and cap carbonate Glacial deposits, tillites, are a mixture of different sediment sizes and types (unsorted, unstratified) Tillites are directly overlain by carbonate rocks - This is uncommon to have such a fast transition (Implies very cold then very hot conditions--see next slide) Cap C carbonate b t Glacial deposit (tillites) The CO2 cycle as a thermostat CO2 is more soluble in colder water CO2 is less soluble in warmer water Carbonate minerals form faster at higher ocean temperatures Thus If Earth warms up a bit, carbonate minerals form in the oceans at a more rapid rate If the Earth cools a bit, carbonate minerals form more slowly in the oceans. http://www.elmhurst.edu/~chm/ vchembook/174temppres.html 26 2/19/2008 BIFs back? In the first snowball event, banded iron formations occur (not seen since ~ 2.3 Ga!) The Freeze-Fry: The Freeze FreezeLongLong-term carbon cycle becomes imbalanced c cle Large land areas clustered at low latitudes--but latitudes-- they still spread to higher latitudes Would have increased albedo less overall energy retained Photosynthesis continues to draw down CO2 Weathering continues to draw down CO2 As atmospheric levels of CO2 levels decrease, decrease, so do global temperatures on Earth This leads to more ice inc. albedo more ice, etc. A positive, reinforcing feedback 27 2/19/2008 The Freeze-Fry: The Fry FreezeCO2 imbalance in other direction... Continued volcanism causes CO2 levels to rise Eventually, CO2 levels became very high Earth warmed up--perhaps very rapid up-- Cf. enhanced greenhouse effect Ice melted near equatorial regions Albedo decreased, accelerating the warming , g g Evaporation increased, increasing water vapor in the atmosphere The Freeze-Fry: The Fry, cont. FreezeGreenhouse effect became even stronger, stronger, accelerating the warming CO2 levels now very high Atmosphere is incredibly hot (possibly ~50C = 122F) ~50 122 "Hothouse world" 28 2/19/2008 Snowball Earth: Explaining the geology Carbonates directly over tillites, tillites, Explanation: Snowball conditions rapidly changed to hothouse conditions Return of BIFs, Explanation: Ocean was capped by ice for ~10 million years Photosynthesis effectively shut down & seawater became anoxic 29 ...
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This document was uploaded on 06/04/2008.

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