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by: Aurore MacGyver Sr.


Aurore MacGyver Sr.
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This 14 page Class Notes was uploaded by Aurore MacGyver Sr. on Wednesday September 9, 2015. The Class Notes belongs to OCEAN 450 at University of Washington taught by Staff in Fall. Since its upload, it has received 14 views. For similar materials see /class/192145/ocean-450-university-of-washington in Oceanography at University of Washington.




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Date Created: 09/09/15
news feature Snowball ghts Did the world freeze over some half a billion years ago Two Harvard scientists think so but convincing other climatologists is proving dif cult Naomi Lubick tracks the latest twists and turns in the snowball Earth debate aul Hoffman and Daniel Schrag have had a bus few In 1998 the two arvard University geologists rekinr dled a radical idea that on at least one occar sion between 580 million and 750 million years ago the Earth lay entirely encrusted in ice for tens of millions ofyears 39Ih39 ball rth39 h othesis seemed to sone puzzling geological data But it was a u a t u Ihebigfreez didrapidgrr of the ire raps env the entireplanetr ing more recent ice ages But Budyko39s theory 1 in39 wn example m o m me ho es it at or eventuallycaused the ice to thaw 5 In 1992 Jose h Kirschvink a geologist at the California Institute of Technology in Pasadena 0 ide lanation of how the ice could have recededz u a u in sign of lettingu Sceptics first asked how the Earth could freeze and thaw in such a short geological time Climate modellers have since quesr tioned whether icesheets could have leached the Equator nd lastyearcameana ult on Hoffmanand Schrag39s central line of geologe ical evidence The propon of snowball Earthitseensar o thedefensiveoncemore e idea of a global glaciation was firs cool slightly prompting an size of th e ga on m Budy o39s ideas explained puzzling evie denoe including signs of scouring of rocks ice tha d y iers neachedthe Equator onat leasttwo occasions between 580 rrlillio nd 750 million years ago towards the end of the Neoproterozoic period This was baffling because ice sheets Furnne 7 12 which play carbon dioxide fromthe atmosphere have shut down if 39 oc noes would then olcanic coZ may h caused angnhou effectthat freed mowball Earth from its ic ice had covered bon dioxide released by volca build up in the atmos wou se 8 age Kirschvinki s owball Earth39 reale an important role in removing 1d the ice sheets phere eventually creating enough green house warrnin to 39 Kirschvink also pointed out that a snows ball Earth could another strange geological deposit 7 ironerich rocks that forned near the end of the Neoproterozoic Iron isadded to the ocean at geothermal vents 39 the sea oor and precipitates out of sea waterwhen itcomes into contactwith oxygen But if the oceans had been capped with ice gen e es ate up Oxygen when the ice amouan of iron 0 in dissolved iron would have built levels would have increased melted causing large precipitate outandfalltothesea oor Six years later Hoffman and Schrag together with colleagues at Harvard pub lished the t thrust the hypothesis back into the limelights They had studied ratios of carbon isotopes in rocks formed when carbonrcontaining compoun e7 cipitated out of sea water Photosynthetic mari microorganisns take up carbon preferring the lighter carboanZ isotope to the heavier carbonel39S 7 so photosynthesis ca carbonelz levels in water to fall leaving lessofthat isotope to precipitate out But when Hoffman and Schrag looked at cap carbonata 7 sed39 nts that were deposited towardsthe end ofthe Neoproteror zoic glaciations 7 they found surprisingly high levels of carbonrlz In fact the ratio of carbo m which the rocks precipitated This the reasoned wasexa what would occur if ice hadcovened the ocean and starved it of light Journals39 correspondence columns were MORGUNBLADETAF D SCI IRAQ soon buzzing with debate over the paper Some critics pointed out that there was no evidence for a lowering of sea level that would be expected to accompany the freeze ing4 Hoffman countered by disputing the reasoning behind claims that sea level should fa115 The pattern was set critics have contin7 ued to assail the snowball Earth theory while Hoffman and to a lesser extent Schrag have defended the idea With some of the most serious criticisms of their idea arising in the past year the pair are as busy as ever Securing the evidence Most researchers now accept that ice reached equatorial regions 7 but only on land Evie dence for this comes from the deposits that once puzzled geologists The alignment of magnetic particles in some rocks can be used to determine the latitude at which they formed as the Earth s magnetic field 7 which pulls the particles into line as the rock forms 7 is inclined at different angles at dif7 ferent latitudes Some Neoproterozoic rocks with equatorial magnetic signatures appear to show signs of scouring from moving ice But Michael Arthur a geochem ist at Penn7 sylvania State University in University Park who is sympathetic to the snowball Earth the ory says that there still is not enough evidence to demonstrate that the tropical oceans froze over The nature of the glaciation is not well known he says ls it a big ice sheet Or are they just mountain glaciers coming down Others share his doubts In January this year geologist Daniel Condon and colleagues at the University of St Andrews UK pub7 lished an analysis of rocks formed on the sea oor during the period when Hoffman and Schrag claim the oceans were iced over These rocks contained layers of stony debris of the kind carried by glacierse If the debris reached the sea oor then the ice must have melted And the layers are so close together claims Condon that the oceans must have been Paul Hoffman left and Daniel Schrag claim that snowball Earth left its hallmark on rocks unfrozen for at least some time each year Climate modellers are also reluctant to embrace snowball Earth It s very very diffi7 cult to simulate says Chris Poulsen a mode eller at the University of Southern California in Los Angeles Last year he published a sim7 ulation showing that the ice sheets would have stopped at northern Europe during the late Neoproterozoic7 One problem say the modellers is that oceans contain too much heat for them to freeze over completely Even the evidence from cap carbonates which is considered by Hoffman and Schrag to be the smoking gun for snowball Earth has recently come under attack Last May Martin Kennedy a palaeoclimatolo gist at the University of California Riverside proposed that the unusual carbon isotope ratios in the cap carbonates were caused by a sudden release of methane gas8 Methane matter Kennedy together with Nicholas Christiee Blick and Linda Sohl of the LamontrDoherty Earth Observatory in Palisades New York noted that large amounts of organic matter would have been trapped under ice sheets on land during the Neoproterozoic Methane released when this matter decomr posed would have remained trapped in this ice When the ice started to melt ice sheets close to the oceans would have been ooded by rising sea levels and the methane slowly released But this methane like the organic matter that produced it would have con tained high levels of carbonelZ and could have caused the skewed isotope ratios seen in the cap carbonates Last December Kennedy and Christier Blick teamed up with Anthony Prave of the University of St Andrews to publish a paper that cast doubt on another piece of evidence central to the snowball Earth hypothesis In an analysis of samples of carbonate rocks from Namibia that formed before the melt7 but Martin Kennedy argues that the sediments carbon signature was left by a methane release NATURElVOL 417 l 2 MAY 2002 lwwwnaturecom 2002 Macmillan Magazines Ltd news feature Chris Poulsen s simulation of temperatures on Neoproterozoic Earth suggest that polar ice sheets white lines did not reach low latitudes ing of the land ice9 they found little evidence of the odd isotope ratios that Hoffman and Schrag found in their cap carbonates The results indicate a very normal ocean says Kennedy There was clearly no ice sheet Hoffman and Schrag have not taken such criticisms lying down and their rebuttals are scattered throughout the geological literature of the past few years In March this year for example Hoffman took on what he described as Kennedy s creative methane hypothesis Kennedy had pointed to several unusual features of cap carbonates such as tuber shaped cavities as evidence that methane gas was released into the rocks as they were formed But Hoffmann counters that these features are absent from other cap carbonates in the same area and also disputes whether there would have been sufficient organic mat ter to generate the methane To the modellers he points out that the oceans have reached freezing point during the coldest periods of the past 10000 years Hoffman sometimes with Schrag has managed to publish a reply to almost every article on snowball Earth Although the debate remains polarized most geologists and climate researchers believe that elements of many of the compete ing ideas are likely to be correct But few palaeoclimatologists back the hard version of snowball Earth with its iceecovered oceans A slushball Earth with oceanic ice sheets extending to middle latitudes seems to be gaining broader support But Hoffman remains unperturbed arguing that radical ideas always face a bare rage of scepticism Both plate tectonics and evolution by natural selection faced similar battles for acceptance for example I like to say I m Kirschvink s bulldog he laughs I aomi lubick is a ireelanoe writer in Palo Alto Caliiomia BudykoMl Te11u5216117619 1969 Kirschvink Lin The Proterozoic Biosphere eds Schopf W 8 Klein C 51752 Cambridge Univ Press NewYork 1992 HoffmanP F Kaufman A Halverson GP 8 Schrag D P ScienceZSl 134271346 1998 Williams GE Nature398 5557556 1999 HoffmanP F Nature 400 708 1999 Condon D Prave A R 8 Benn D l Geology30 35738 2002 Poulsen C Pierrehumbert R T ampacob R L Geophys Res Lett 28 157571578 2001 Kennedy M ChristieeBlick N 8 SohlL E Geology 4437446 2001 9 Kennedy M ChristieeBlick N 8 Prave A R Geology 29 113571138 2001 10HoffmanP F Halverson GP 8 CrotzingerP Geology30 2867287 2002 NH Neme 9 oo 13 M J KENNEDY Early Earth History r carbon Solar system began about 46 Gy ago Started with several supernova explosions in the local neighborhood Sun formation from accretionary disk Roughly 500 planetoids about size of moon in region of inner planets Collisions ofthese planetoids produced Venus Earth and Mars all with inventories of water vapor and Key Reference Nisbet and Sleep The habitat and nature of early life Nature 409 10831091 2001 There may have been early oceans on all three ofthese planets 1 w years ago In discussing geological time 1 Gyr is 109 years 1 Myr is 105 years the ago39 is implicit and often omitted such that G and Myr refer to both 39 e before present and duration There are four aeons The Hadean is taken here as the time from the formation of the Solar d a ly accr n ofthe G to the of life probably sometime around 40201 Gyr The Archaean or time of the beginnlng of life is from about 4 25 Gyr the Proterozoic from 25 Gyr to about 056 Gyr and the Phanerozoic since then Earth in the Hadean A 9 e I For 500 to 800 My Earth was bombarded by large meteorites adding to earth39s mass also adding heat Hot spinning preearlh mass melted caused differentiation of materials according to densit Distinct earth layers begin to form Dense 39 n and nickel migrate to center core silicate material moves out to man e Spreading Plume Rising Plume cms s mhusphmic quotunite Old Plume Hot Spot amennspmm ve u uday No terrestrial geology uquot quotmy record ofthis data taken Bumhlrdmant nu Rate rd from dating of lunar impacts Many impacts had suf cient energy to boiloff 03039 53quotquot me oceans wtm Oceans TmetGa Fr Eada JL HuWLWe Began Eanh andPtanelawSmenceLettestZE znmwews Prlblotlc ProRNA RNA nNAmmn Epoch World World World Fri 1 m tmplrl hi nry at my Elly my and m Vanws agzx nwnhcd m m mum undcvahtion mm mm m 212 Waler evapamed m 3 2 Tuna Gyv Figure 2 The legal bond impact on me Eank am the Mann 0pm hm H mm mm bonsai115ml Guy lme inferred Emh impxmswnz Dmd Mn is dplh at man Vapmzni by mm m refers m m memsmmm mpl ndTy refers m be hum cnxarTychamm Slug quot2131939 HADEAN EON odest ode5t ARCHEAN EON Earth mineral continental accretion grain rocks Assembly of 2500 y Columbia Assembly of Rodinia PHANEROZOIC Orogenies associated AlpineHimalayan 0 with the assembly orogeny of Pangaea Figure 1019 Understanding Earth Fifth Edition a 2007 w HFreeman and Company A palaeotemperature curve for the Precambrian based on silicon isotopes in cherts nae oceans Fiancuis Ruben K Mai Chaussidun New 2 I Precl l bmn Pmmmic Armaezn 39i MW a mm o a o o o 0 on sadoms o azmaledlu 9i Cretaceous hot house 100 My ago Temperature of Planet Earth 91mm morsncw1x39wu m WWM W mhm39 1 A may r I m q 0521 W 25a 0 5 1 a 15 Thousand Years Sabre Present 0 11 The Faint Young Sun Paradox The Sun s interiorthrough out the history of its existence 455 Byr has been the site of ongoing nuclear reaction H to He fusion This nuclear reaction process has caused our Sun to expand and gradually become 39 hese models indicate that the earliest Sun shone 25 to 30 more faintly than today This is a problem for climate scientists A decrease ofjust few percentage in our Sun s present strength would caus all the water on Earth to 39eeze despite the warming effect of our present day greenhouse gases A positive feedback would be caused by their high albedo and it would never get warm Climate models suggest that an early Earth with so weak Sun and present level greenhouse gases would have remained 39ozen forthe rst 3Byr of its existence J V 07 08 09 1 50 25 0 25 Solar luminosity Temperature C relative to present value Evidence of running water in sedimentary rocks formed during the early Earth s history zircons means it was not frozen solid First evidence of icedeposited sediments occurs in rocks dated to about 23 Byr ago probably due to glaciations localized in polar regions as on Earth today and are not an indication of completely frozen planet This conclusion also supported by the continued presence of life 1 on Earth Primitive life forms date back at least 35 Byr ago The Problem Vl th so weak a Sun why wasn t Earth frozen for the rst twothirds of its history This is known as The faint young Sun paradox


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