Review Sheet for ATMO 451A at UA
Review Sheet for ATMO 451A at UA
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Date Created: 02/06/15
insight overview The longterm carbon cycle fossil fuels and atmospheric composition Robert A Bemer D artmentabe 111111620 h m llet111112731 NewHaven Connecticut z ealw USAeem21 Z raberzbemerezygle edit The longterm carbon cycle operates over millions of years and involves the exchange of carbon between rocks and the Earth s surface There are many complex feedback pathways between carbon burial nutrient cycling atmospheric carbon dioxide and oxygen and climate New calculations of carbon uxes during the Phanerozoic eon the past 550 million years illustrate how the longterm carbon cycle has affected the burial of organic matter and fossilfuel formation as well as the evolution of atmospheric composition e carbon cycle is a variety ofprocesses that take place over timescales ranging from hours to mile lions ofyears processes occurring over shorter periods include photosynthesis respiration airrsea exchange of carbon dioxide and humus accumulation in soils However it is the longrterm carbon cycle occurring over mill39ons ofyears that is of interest when considering the or in offossil fuels The longrterm cycle shown in Fig 1 is distinguished by the exchange of carbon between rocks and the sur cial system which con sists of the ocean atmosphere biosphere and soils The longrterm carbon cycleisthe main controllerofthe concenr tration ofatmospheric carbon dioxide and alongwith the sulphur cycle atmospheric oxygen over a geological timescale It can be represented succinctly by the generalr ized reactions co2 CaSiOa lt gt Cacoa Sio2 1 CO2 H20 lt gt CHZO 02 2 These reactions summarize many intermediate steps Equation 1 going from left to right represents the uptake of atmospheric carbon dioxide during the weathering on land of calcium and magnesium silicates with the dis solvedweathering products Ca2 Mg and Hco delive ered to the oceanand precipitatedthereascalciumand magr nesium carbonates in sediments Going from right to left equation 1 represents the deepburial and thermal decomr Photosynthesis l Burial i Metamorphisrn Diagenesis Weathering Volcanisrn 7 W l Subductlon NATURE l VOL 425 l 20 NOVEMBER mos l w nature comnature silicate weathering Metamorphisrn Diagenesis position of carbonates with the liberated carbon dioxide released into the atmosphere and oceans Equation 2 is of particular interest for this review Goingfrom left to right it representsnet global photosynr thesis photosynthesis minus respiration as manifested by the burial oforganic matter CHZO in sediments The buried organic matter is eventually transformed mostly to kerogen but some becomes oil gas and coal Equation 2 going from right to left represents the oxidative weathering oforganic matter exposed by erosion on the continentsandto alesserextentthecombinationoftherr mal decomposition of organic matter to reduced gases with consequent oxidation of the gases in the atmos phere Theburning offossil fuelsbyhumanshascaused a large increase in the rate oforganic matter oxidation come pared with that of the natural weathering process This increase on the basis ofthe data we present in this review and that of the 1pcc is an acceleration by a factor of about 100Thushumanshave greatly perturbed thelongr term carbon cycle This review shows that numerical modelling based on thelongrterm carbon cycleallowsthecalculationofglobr a1 ratesoforganic matterburial over geological time and that the resulting rate distribution bears a resemblance to timesofformation ofoil and gas source rocksandcoal1n addition the complex feedbacks that govern organic burr ial are shown to be intimately tied to oceanic nutrients the carbon dioxide and oxygen content ofthe atmosphere andclimate Figure l A rnodel orthe longrtenn carbon cycle The deposition or carbonates derlvedfrom the weathering or carbonates is not shown because these processes essentlallybalarlce one another werthe long terrn as rar as carbon dioxide is concerned However carbonate deposition derlvedfrom carbonate weathering leads to additional Caco3 deposition Carbonate C m sedlmems degassing orcarbon dioxide upon deep burlal and tnerrnal decornposition oiagenesls chemical changes at low temperatures durlllg burlal The cycle Summon can be subdlvldedlntotwosubcycles lnvoivlng organic rnatterlertside or flgttre and sllicate weatherlng and carbonate deposition right side or flgttre an 2003 NaturePublishing Group insight overview Organic matter burial and source rocks There are two principal methods for estimating the global rate of burial of organic matter over a geological timescale The first involves determining the mean concentration of organic carbon in sedimene tary rocks and correcting for the loss ofthe volume of rocks withtime owing to erosion metamorphism transformation by natural agencies such as heat andpressure and subduction the sideways and downwards movement of plates of the Earth s oceanic crust5 The second reported here is based onthe use ofthe carbon isotopic com position of seawater Duringphotosynthesis 12C is taken up preferene tially instead of 1 3C suchthat the organic matter derived from organ isms is about 20o lower in 13C than the carbon in atmospheric car bon dioxide or that in dissolved inorganic carbon DIC of sea water A compilation of6 3C of carbonate fossils versus time has been per formed7 where 513C0 13C ZCsmPl 3C Cmndm711000 Changes in 613C of carbonates with time are assumed to record the average 613C value for DIC in sea water and also for atmospheric car bon dioxide which is about 7o lower in 13C than DIC The rate of preferential removal from the ocean and atmosphere of isotopically light organic carbon relative to total carbon is recorded bythe value I or lt2 T l I Sourceerock areas 2 6 Global carbon 40 3 burial E 3 P 5 60 E o w z a o g 4 l 20 a x o o l v S E 3 I u l l l 10 8 D 5 O i i 2 I I I I I U 7600 7500 7400 7300 7200 7100 0 TImeMyr Figure 2 Plot of global organlc carbon burlal durlng tne Pnanerozow eon compared WItn tne tlrnes ol deposltlon of major Oll and gas source rocks Carbon burlal rate rnodltled trorn ref it sourcerrock areas trorn ref l 4 Unltslor sourcerrock area are In percentages of the total of tne slx areas Sourcerrock abundances at other tlrnes are very rnlnor Tne very nlgn carbon burlal values centred around 300 Myr ago are due predornlnantlytoterrestrlal carbon burlal and coal torrnatlonS 20 100 38 15 llTypellltotal 075 E E w 2 10 I 050 o l a D 2 5 025 0 I I I I I 7600 7500 4100 7300 7200 7100 0 TImeMyr Figure 3 Plots of organlc carbonpynte sulpnur C S burled In sedlrnents versus tlrne cornpared WItn tne traction of Oll and gas source rockstnat are of type lll Includlng coal C Svaluestrorn ref it source rock data trornret 14 324 of 613C For example an increase in 613C for seawater normally repre sents increased removal of light carbon and thus greater burial of organic matter in sediments By performing massebalance calcula tions for both total carbon and 13C for input and output ofcarbon to the oceans plus the atmosphere it is possible by using the 613C isoe topic data to calculate the global rate of burial over time of organic carbon in sediments both of marine and nonemarine originz g39 A plot of global organic burial rate during the Phanerozoic eon is shown in Fig 2 and the pattern is a crude guide to the occurrence of fossile iel source rocks as will be seenlater The burial curve is calcue lated using variable isotope fractionation during photosynthesis quot 2 new data on 613C for the late Permian period13 and data onthe rapid recycling of carbon in younger rockss l 1 The estimated error is 50 of the rate plotted The most prominent feature is abroad maximum centred around 300 Myr ago PermoeCarboniferous period This maximum is a direct consequence of the very high 613C of the ocean and atmosphere at this time7 Prominent minima occur before 470 Myr ago early Paleozoic era and around 2207180 Myr ago TriassiciEarly Jurassic period whereas secondary maxima are at 440410 Myr ago Silurian period and 907120 Myr ago Middle Cretaceous period The times of maximum modelecalculated organic carbon burial coincide with independent estimates of the times of major oil source rock deposition The relative areas of the six principal sourceerock periods expressed as a percentage ofthe total for all areas coincide in time with the maxima in the calculated carbon burial curve Fig 2 Also the minima in the carboneburial curve coincide with periods of very low occurrence ofsource rocks for oil and coalms This agree ment suggests that a major factor in oil generation is increased global deposition of organic matter Klemme and Ulmishek have divided their six sourceerock peri ods into two categories source rocks dominated by type I and tyqpe II kerogen sedimentary organic matter that is high in hydrogen and low in oxygen derived mainly from aquatic planktonic organisms and those dominated by tyqpe III kerogen lower in hydrogen higher in oxygen and derived mainly from plants with coal included with type III as a source rock mainly for gas The ratio oftype III kerogen to total kerogen for each period is plotted in Fig 3 The broad maxi mum centred about 300 Myr ago illustrates the importance of coal deposition at that time The rise of land plants between 380 and 350 Myr ago during the Devonian period created a new source of rel atively nonebio degradable organic matter lignin which when buried in sediments gave rise to increased global burial of organic carbon inthe Carboniferous and Permian 3507250 Myr ago Fig 2 Burial of lignin and other plant products gave rise to the great Permie an and Carboniferous coal basins and to alesser extent tyqpe III keroe gen for oil and gas formation By far the most abundant coal reserves are from the Permo 7Carboniferous periodl 5 even thoughthese older rocks have been subjected to erosion for longer times than younger deposits Other periods ofhigh type III kerogen formation Fig 3 are the Middle Cretaceous 907120 Myr ago and mideTertiary 30750 Myr ago These are also periods of abundant coal deposi tion15 It is notable that the time when both global organic burial and source rock deposition were at theirlowest centred around 200 Myr ago in Fig 2 is also the time whenverylittle coalwas deposited The temporal distribution of the different kerogen types corree lates somewhat with the loci of organic matter deposition over time The latter can be estimated bymeans ofisotope massebalance model ling for both carbon and sulphurB This is because of the different sulphur content which is mainly found in pyrite FeSZ of organic rich sediments deposited in different sedimentary environments The global rate of burial of sedimentary pyrite can be calculated in a manner analogous to that used for the burial of organic matter The sulphur isotopic composition of sea water as recorded in deposits of gypsum and anhydrite and traces of sulphate in carbonates genere ally re ect changes in the rate of preferential removal of 32 from sea water as pyrite Pyrite is lower in 3 8325 relative to seawater sulphate NATURE l VOL426 l 20 NOVEMBER 2003 l wwwnaturecomnature 2003 Nature Publishing Group because the reduction of sulphate to HZS from which pyrite forms involves a large isotopic fractionation Sulphur isotopic data for the ocean combined with massibalance calculations for total sulphur and 3 S enable the calculation ofrates ofpyrite burial over time Ratios of the burial rates of organic carbon and pyrite sulphur CS versus time are also shown in Fig 3 The changes with time of OS agree withwhat is known about pyrite formation in modern sedi imentsB Predominant burial in organicirich marine sediments for instance in basins with anoxic bottom water such as the Black Sea involves abundant sulphate reduction and pyrite formation leading to low CS ratios Burial in freshwater environments such as coal swamps results inveryhigh CS ratios This is because ofthe verylow sulphate content of fresh water compared with that of sea water which minimizes pyrite formationB Burial in oxygenated marine bottom waters with bioturbation results in intermediate CS values but this environment is not as favourable for oil formation Thus for oiliprone marine source rocks deposited under anoxic bottom water conditions one would expect low CS ratios and for coal deposits one would expect high CS ratios The correlation between type III kerogen relative to total kerogen and OS ratio Fig 3 agrees withthis prediction for several sourceirockperiods Fig 3 The verylowvalues ofCS forthe early Paleozoic before 370 Myr ago suggest greater deposition in unusually organicirich and pyrite rich marine rocks This is in agreement with the widespread occur rence of the graptolite black shale facies of this era which has been interpreted to have been deposited in abundant anoxic regions of the ocean The verylow CS values also re ect the relative lack ofdepoi sition in fresh water owing to the absence of large land plants at that time The argument that there is little difference in organic pres ervai tion between oxygenated and nonioxygenated bottom waters is controversial and does not agree with other studies of modern sedii ments or with geological observations 0 The presence of only a moderately high proportion of tyqpe Ill kerogen in PermoACarboniferous source rocks compared with the veryhigh concurrent values of OS maybe because the compilation of Klemme and Ulmishek was oriented towards oil and gas formation not coal abundance Also the poor correlation between the ratio of tyqpe III to total kerogen and OS for the midiTertiary 40720 Myr ago is explained by the deposition of much terrestrially derived type 111 organic matter in marine estuarine and deltaic environments where abundant pyrite formation is supported by sulphateirich sea water Organic burial nutrients and atmospheric composition The burial of organic matter as it affects sourceirock formation is intimatelytied to the chemical composition ofthe oceans and atmos 7 phere For the atmosphere as shown by equation 2 the burial of organic matter involves the removal of carbon dioxide and the addi7 tion of oxygen lnthis way organicburial on a geologicaltimescale is a major controller of atmospheric composition Atmospheric com position in turn exerts a major control on organic matter burial and acts in a feedback capacity A major factor linking organic burial in the marine environment and atmospheric composition is the level of essential nutrients in sea water principally phosphorus and nitrogen This is because biological production is normallylimited bynutrients Changes in atmospheric carbon dioxide and oxygen can affect the availability of nutrients which in turn affect organic pro duction and burial This leads to a variety of biogeochemical feed backs that control both atmospheric composition and organic bur ial Because ofthis complexweb of feedbacks a simple explanation of the various changes over time in rates of organic burial and source rock formation Figs 2 and 3 is difficult with the exception of the effect of the rise oflarge land plants on terrestrial carbon burial The interaction of nutrients and atmospheric carbon dioxide and oxygen with organic burial can be represented by systemsianalysis diagrams An example is shown in Fig 4 A complete coverage of all suggested linkages is beyond the scope of this review 7 for more NATURE lVOL 426 l 20 NOVEMBER 2003 l wwwnaturecomnature Climate Tand pptn eathering total P Ocean 39 circulation Nutrient aqueous Organic C sediment burial Marine nitrogen xation burial Figure 4 Systemsranalysis diagram showng some of the feedback relationships between marine organic carbon burial nutrients climate atmospheric carbon diogtltide and oxygen and ocean circulation The upward dashed arrow at h represents disagreement concerning the effect of oxygen on organic burial See text for detailed explanation Solid arrows represent positive responses and arrows With bullseyes represent negative responses A loop With an even number of arrows With bullseyes or With no bullseyes represents a positive feedback loop A loop With an odd number of bullseyes represents negative feedback FeR phosphorus associated With ferric ogtltide details consult refs 23725 Plain arrows between entities represent positive responses This means for example if organic burial increases oxygen production and therefore oxygen level increases Arrows with attached bullseyes refer to negative responses For example if organic burial increases carbon dioxide the ultimate source of the carbon decreases By following the diagram one can deduce whether various loops lead to positive or negative feedback A loop with an even number of arrows with bullseyes or no bullseyes repre7 sents positive feedback A loop with an odd number ofbullseyes rep resents negative feedback Because atmospheric oxygen and carbon dioxide have not varied considerably over geological timescales interest has focused on negative feedback pathways which provide stabilization 40 02 30 R002 N O 0 CE 20 N 0 10 0 i i i i i 7600 7500 7400 300 7200 7100 0 Time Myr Figure 5 Plots of RC0Z the ratio of the mass of carbon diogtltide in the atmosphere in the past to that for the prerlndustrial present and 02 during the PhanerOZOic eon Values of RCOzfrom the GEOCARB lll modelg g values of Ozfrom ref it using the 3C data of refs 1 2 and l 3 Estimated errors are 150for RC0Z and 17 for O2 325 2003 Nature Publishing Group
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