Geochimica et Cosmochimica Acfa Vol. 56, pp. 3281-3295 0016-7037/92/S%oO + 03 Copyright 0 1992 Pcrgamon Pres Ltd. F-rimed in U.S.A.
FRED T. MACKENZIE’and JOHN W. MORSE’ ‘Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA ‘Department of Oceanography, Texas A&M University, College Station, TX 77843, USA
(Received March 19, 199 1; accepted in revisedfirm January 15, 1992)
Abstract-Plate tectonic processes play a critical role in the origin and distribution of sedimentary car- bonates through Phanerozoic time. The Phanerozoic age distribution of sedimentary properties like calcite/ dolomite ratio, inferred o&d and cement mineralogy, and survival rate of continental carbonates is cyclic. The cycles appear to be coupled to plate tectonic processes that give rise to global sea level change and changes in the properties of the ocean-atmosphere system. First-order changes in sea level are driven by the accretion of mid-ocean ridges: high accretion rate, high sea level; low accretion rate, low sea level. Although correlations between sea level and sedimentary carbonate properties are not strong, high sea level over an extended period of time appears to be correlated with low calcite/dolomite ratios, lack of inferred aragonite oiiids and cements, and maxima in the survival rate of continental carbonates. The opposite is true for extended periods of low mid-ocean ridge accretion rates and global sea levels. The lack of strong correlations may reflect an insufficient data base and the possibility of lags between sea level change and change in carbonate properties. Furthermore, the survival rate of continental carbonates appears to be affected by differential cycling and, therefore, may not be directly related to accumulation rate. It appears that the environmental conditions for early dolomitization and calcite oSid and cement formation are best met during extended times of high sea level when atmospheric CO2 levels are high and the saturation state of seawater with respect to carbonate minerals relatively low. During low sea levels, early dolomitization is less favored, and aragonite precipitates are more abundant because of low atmospheric CO2 levels and enhanced seawater carbonate saturation states. Differential cycling has modified the Phanerozoic sedimentary carbonate mass-age distribution. Because of erosion of younger units within continental carbonate cycles, it may be difficult to derive an unequivocal record of the partitioning of carbonate between the deep-sea and shallow-water realms of deposition during the Phanerozoic. This difficulty must be considered in further quantification of geochemical models describing the geologic history of atmospheric CO* and climate change.
INTRODUCTION by various authors (e.g., GREGOR, 1970, 1985; MACKENZIE, 1975; GARRELSet al., 1976; VEIZERand JANSEN,1979, 1985; IN A PAPERpublished in 1969 ( GARRELS and MACKENZIE, VEIZER, 1988; GREGOR et al., 1988; WOLD and HAY, 1990). 1969), Bob Garrels and Fred Mackenzie initially presented Further compilations and interpretations of the Phanerozoic hypotheses for the meaning of the temporal variation in pro- sedimentary carbonate rock mass-age distribution have ap- portions of sedimentary rock types remaining today in the peared in the literature in the 1980s (RONOV, 1980; HAY, geologic column. In the book Evolution of Sedimentary Rocks 1985; WILKINSONand WALKER, 1989; WILKINSONand AL- ( 197 la), they further developed models of the sedimentary GEO, 1989; Boss and WILKINSON, 1991). In this paper, as a rock mass-age distribution and expanded on two hypotheses tribute to and in remembrance of Bob Garrels, we explore related to that distribution: ( 1) geochemical uniformitari- in more detail the concepts of geochemical uniformitarianism anism and (2) differential cycling. Geochemical uniformi- and differential cycling relevant to interpretations of the sed- tarianism implies that the total mass of sediments of all ages imentary carbonate rock mass-age distribution through Pha- existing at any given time in the geologic past may have had nerozoic geologic time. This study reflects one of Bob’s prin- about the same ratios of rock types that we observe today. A cipal interests in the later stages of his career. corollary to this hypothesis is that the fluxes of chemical con- stituents to the oceans have not varied greatly, at least during THE DATA BASE Phanerozoic time. Differential cycling implies that because of differences in relative erosional resistances or tectonic set- The calculations and interpretations of this paper are based ting, various components of the sedimentary rock mass cycle on data from a number of sources. There is still some dis- at different rates. This factor, along with diagenesis, may lead agreement in the literature concerning the best estimates of to differences in the ratios of rock types as a function of mass-age relationships within the major global carbonate res- geologic age in the sedimentary rock mass existing today. ervoirs (e.g., HAY, 1985; WILKINSONand WALKER, 1989). These concepts and others involving the sedimentary rock We will clearly indicate in the following discussions the data mass-age distribution have been explored and expanded on sources and our manipulations of the data base. To provide
3281 3282 F. T. Mackenzie and J. W. Morse
Table 1. Phanerozoiccarbonate mass distribution.
Muss Survival Rate
Period Duration TOM TOM Total Calcite/ Total I-%S Total Logs TOtZ4 Logs (lO~-s) carbonate Dolomite Cslcite Dolomite Dolomite Dolomite C&i& Calcite CarboMtc CariYonatc (1~) (lO?oM) (lO%ms) Ratio flti y-l) (tons y’) (W~OIISy-l) (tons yz) (Wtons y-I) (tons y’) re&Iy 65 19.42 2.92 16.50 5.65 4.42 a.65 25.0 9.40 29.42 9.47 cretaceous 66 lo.48 3.88 6.60 1.70 5.88 8.71 10.0 9.00 15.88 9.20
consistency with the works of Wilkinson and colleagues timate because of the use of Hay’s CO* data (WILKINSON (WILKINSON and WALKER, 1989; WILKINSONand ALGEO, and WALKER, 1989). 1989), carbonate mass-age data will be given in units of 10 r3 For several decades it has been assumed that the Mg/Ca g Ca y-l. This convention introduces an overestimation of ratio of carbonate rocks increases with increasing Phanerozoic the cation mass of about 2% for each 10% of dolomite found rock age. An early portrayal of this trend in carbonate rocks in a rock mass interval (WILKINSONand WALKER, 1989). from the Russian Platform and North America is shown in The distribution of Phanerozoic System total sedimentary Fig. 2. The trend represents a general, but erratic, decline in masses with geologic age was obtained from the estimates of the calcium content and increase in the magn~ium content GREGOR ( 1985). The mass of carbonate rock in each system of these rocks with incising age (see VIN~CRAD~V and was calculated from the estimates of CO, found in Phanero- RONOV, 1956a,b; CHILINGAR, 1956). The magnesium con- zoic carbonates as given by RONOV ( 1980) and amended by tent is relatively constant in these carbonates for about 100 HAY ( 1985 ) . Dolomite and limestone abundances were cal- million years, then increases gradually. The magnesium con- culated using the GIVEN and WILKINSON ( 1987) compilations tent of North American and Russian Platform continental ofdata on the composition (Mg/Ca or MgC03/CaC03 ratios) carbonate rocks appears to increase at a geologic age that is of Phanerozoic carbonate rock samples. Table 1 gives the very close to, if not the same as, the age of the beginning of resuhs of our calculations. A tentative mass-age dist~bution the general increase in the Mg content of pelagic limestones of sedimentary carbonates and sandstones plus shales is given f 100 million years before present; RENARD, 1986). The do- in Fig. 1. The total carbonate mass makes up about 30% of lomite content of deepsea sediments also increases erratically Phanerozoic sediments in Fig. 1, perhaps a slightly high es- with increasing age back to about 125 million years before
P 180% DotwaIte
[ 0.40 u c j 0.30 a I 100.20 Z ______--___-_---_-AVHI~~w&on& rack ------RunrmPI&&ml I! O*‘O 0 8 !5 4 3 2 1 0 100 400 1000 zaoo Time (10'~) Age (104~BP)
FIG. 1. Mass-age distribution of carbonate rocks and other sedi- FIG. 2. Magnesium to calcium weight ratios in Russian Platform mentary rocks plotted as survival rate (S) versus age. Total rock mass and North American carbonate rocks as a function of age. (Data data from GREGOR ( 1985) and estimates of carbonate rock mass from VINOGRADOVand RONOV, 1956a.b; and CHILINGAR,1956; from Table 1. figure modified from GARRELSand MACKENZIE,1971a). Geologic cycles of carbonate rocks 3283
B Sperber et al. (1994) MglCa=0.23
So C Schmoker el al. (1997) D Chlllngar (1996) MglCa=0.14 MglCe=O.lS
Vinogradov and Ronov (1956b) F Given and Wilkinson(1987) Mg/Ca=0.14 MglCa=O.23 N=607 N=l7,353
Age (106 y BP) FIG. 3. Estimates of percent dolomite in Phanerozoic cratonic carbonate rocks as a function of age. (After WILKINK~N and ALGEO,1989).
present ( LUMSDEN,1985 ) . Thus, the increase in magnesium Table 2 and shown in Fig. 5. WILKINSON and WALKER( 1989) content of carbonate rocks with increasing age into at least developed mass-age models for these various carbonate res- the Early Cretaceous appears to be a global phenomenon, ervoir distributions similar to those used by GARRELS and and to a first approximation, is not lithofacies related. MACKENZIE( 197 la). In a later section, these mass-age re- Recently, the accepted truism that dolomite abundance lationships are discussed in detail. increases relative to limestone with increasing Phanerozoic age has been challenged by GIVEN and WILKINSON( 1987). DIFFERENTIAL CYCLING OF THE CARBONATE MASS They reevaluated all the existing data on the composition of Sedimentary rocks are formed by depositional processes Phanerozoic carbonates and concluded that dolomite abun- involving principally the agents of water and wind and are dances do vary significantly throughout the Phanerozoic but destroyed when eroded or transformed chemically into other may not increase systematically with age. Figure 3 is a sum- kinds of rocks like paragneiss. The sedimentary rock mass mary of the global data set for Phanerozoic dolostone abun- (including volcanogenic sediments) today, as estimated from dances from WILKINSON and ALGEO ( 1989). The percent dolomite in the Phanerozoic carbonate mass as obtained by other authors is shown for comparison, The meaning of these abundance curves, and indeed their actual validity, is still controversial ( ZENGER, 1989). However, as mentioned pre- viously, we used the GIVEN and WILKINSON( 1987) dolomite abundances to obtain estimates of carbonate rock masses for the Phanerozoic and for their relative calcite and dolomite contents (Table 1). Figure 4 illustrates the distribution of Phanerozoic carbonate rock masses and their calcite and do- lomite contents on a Period-averaged basis. It can be seen that, as with the total sedimentary mass (GARRELS and MACKENZIE, 197 la,b), the mass of carbonate rock preserved is pushed toward the front of geologic time within this trend. The Tertiary, Carboniferous, and Cambrian periods are times of significant carbonate preservation, whereas the preservation of Silurian and Triassic carbonates is minimal. The final set of data used in this paper is that concerned 5 4 3 1 0 with the mass-age relationships of sedimentary carbonates lime (lo8 yP compiled on an epoch by epoch basis. These data were com- FIG. 4. The Phanerozoic sedimentary carbonate mass distribution piled by WILKINSON and WALKER ( 1989) for continental, as a function of geologic age. Period masses of calcite and dolomite oceanic, and global carbonate reservoirs and are tabulated in and the Period mass ratios of calcite/dolomite are also shown. 3284 F. T. Mackenzie and J. W. Morse
Table2. Carbonatesediment masses for variousstratigraphic intewals. b
ContinentaI Gvbollate Total Carbonate Reconstructed rhbomte ‘Missing” Flux Deep-Sea Cahmati St&graphic DurationMidpoint Interval Mttss(1) Remaining Mass (1) Remaining SedimentFlux Mass (1) Px?swved Jntervals (10% wy) (lolag & Y“) (1O’fgGl y.‘) (lolag cp y’) (lo”g cp y’) (lO’$ c!a y-1) Modem 24 12 48 Pleistocene u (1) Pliocene 3.1 3.5 33 (38) 73 (73) (76) (38) 32 (40) Miocene 18.4 14.5 43 89 56 Oligocene 12.9 30.2 32 51 20 I(2) Eocene 21.2 41.2 55 (52) 92 (83) (93) (41) 38 (33) P&X‘Ztle 8.6 62.1 45 60 21 (3) L. cret&x0us 28.6 77.0 67 (67) 95 (95) (111) (44) 30 (30) (4) E. Cretaceous 40.0 115.0 46 (46) 48 (48) (60) (14) 4 (4)
(5) L. Jurassic 17.0 143.5 52 (48) 53 (49) (68) (20) 4 (4) M. Jurassic 28 166.0 50 52 4 E. Jurassic 2s 192.5 42 44 4
:olumnheadings A-H are referredto in text. “Missing”flux is column(F) minuscolumn (D).
geochemical mass balance methods, is 25-30,000 X 102’ g This rock mass estimate includes the classic lithologies of (cf. CARRELS and MACKENZIE, 197 la, 1972; LI, 1972; sandstone, shale, and carbonate, as well as their metamorphic VEIZER, 1988), with 86% of the mass lying within the con- equivalents of quartzite, slate, phyllite, low-grade schist, and tinental and shelf region of the globe and 14% a part of the marble. The current mass is that preserved, not the total mass deep ocean floor ( RONOV, 1980; cited in VEIZER, 1988). deposited throughout geologic time. Total sedimentary de- position over the last 3.5 billion of years of earth history has been at least 130,000 X 10 2og. Like human populations, sedimentary rock masses can be A Global Carbonatea assigned birth rates and death rates, and they can be subdi- 0 Ocemk Crbmtn . Conthntd Cirbomtel vided into age groups (CARRELS et al., 1976). GREGOR ( 1985) demonstrated that the sedimentary mass-age distri- bution for Carboniferous and younger sediments has a log % 0 linear relationship such that
log&s = 10.01 - 0.24t (1)
where S is survival rate, defined as mass in metric tons of a System divided by duration (in years) of the corresponding ,oooo, Period, and t is the median age of the mass (in units of IO* 200 400 600 years; see also CARRELS and MACKENZIE, 1971b, for dis- Age (lO*y BP) cussion of survival rate). FIG. 5. Mass-age relationships of sedimentary carbonates. The We have calculated the survival rates of the carbonate and global mass is shown as open triangles when representing the sum dolomite masses for different Phanerozoic systems (Table of pelagic and cratonic masses and as half-solid rectangles when en- 1); these are plotted in Fig. 6, together with the GRECOR tirely continental. Solid rectangles are cratonic masses when the global mass consists of both continental and deep oceanic carbonate. (From ( 1985) plot for the total sediment mass. The difference be- WILKINSON and WALKER, 1989). tween the survival rate of the total carbonate mass and that Geologic cycles of carbonate rocks 3285 of dolomite is the mass of limestone surviving per interval or another) of the carbonate mass at a rate about 1.5 times of time. the total mass. Equation ( 1) is the log linear relationship for the total This is not an unlikely situation. With the advent of abun- sedimentary mass. It implies a 130 million year half-life for dant carbonate-secreting, planktonic organisms in the Juras- the post-Devonian mass, and for a constant sediment mass sic, the site of carbonate deposition shifted significantly from with a constant probability of destruction, a mean sedimen- shallow-water areas to the deep sea. A graduate shift in car- tation rate since post-Devonian time of about 100 X 10 I4 g bonate deposition from shallow-water environments to the y-l. The modem global erosional flux is about 200 X 10 I4 deep sea would increase still further the rate of destruction g y-r ( GARRELS and MACKENZIE, 197 la; MARTIN and MEY- (by eventual subduction) of the global carbonate mass relative BECK, 1979)) of which about 15% is particulate and dissolved to the total sedimentary mass from Jurassic time on. Table carbonate. Although the data are less reliable for the survival 3 shows the removal rate of pelagic carbonate from the rate of Phanerozoic carbonate sediments than for the total oceanic realm by subduction and by transfer to accretionary sedimentary mass, a best log linear fit to the Post-Permian wedges, as obtained from different models. This removal rate preserved mass of carbonate rocks is represents potential loss of carbonate from the sedimentary lithosphere. Furthermore, the recycling rate of oceanic crust log S,,,, = 9.55 - 0.36t (r = 0.96, cr = 0.32) (2) (the b” values of VElzER and JANSEN, 1985; VEIZER, 1988) exceeds that of the continental basement by a factor of 17. with Sin units of lo9 metric tons and t in units of lo8 years. Also, SOUTHAM and HAY ( 198 1 ), using a half-life of 100 This corresponds to a half-life for the post-Permian carbonate million years for pelagic sediment, estimated that as much mass of 86 million years, and a mean sedimentation rate of as 50% of all sedimentary rock formed by weathering of ig- these sediments of about 35 X 10 I4g carbonate per year; the neous rock may have been lost by subduction during the past present-day carbonate flux is 30 X 10 I4 g y-’ (MORSE and 4.5 billion years. MACKENZIE, 1990). The difference in half-lives between the Thus, it appears, as originally suggested by GARRELS and total sedimentary mass, which is principally sandstone and MACKENZIE ( 1969, 197 la, 1972), that the carbonate com- shale, and the carbonate mass probably is a consequence of ponent of the sedimentary rock mass may have a cycling rate the more rapid recycling (throughout this paper the term different than that of the total sedimentary mass. These au- recycling refers to the half-life of the reservoir in one form thors argued that the differential recycling rates of the different components of the sedimentary lithosphere were related to their resistance to chemical weathering and transport. Evap- orites are the most easily soluble; limestones are next, followed by dolostones, and shales and sandstones are the most inert. Although resistance to weathering may play some role in the selective destruction of sedimentary rocks, it is likely that differences in the recycling rates of different tectonic regimes in which sediments are deposited are more important. The carbonate mass distribution and calcite/dolomite ratios will be discussed further in the following section.
PHANEROZOIC DOLOMITE /CALCITE RATIOS
Voluminous research on the “dolomite problem” (see, e.g., HARDIE, 1987, for discussion) has shown that the reasons for the high magnesium content of carbonates are diverse 8.0 and complex. Some dolomitic rocks are primary precipitates; others were deposited as CaC03 and then converted entirely or partially to dolomite before deposition of a succeeding 0 TotalSedimentary Mass A CarbonateWss layer; still others were dolomitized by migrating underground W DolomiteMass waters tens or hundreds of millions of years after deposition. It is exceedingly important to know the distribution of the 7(3. ._ I % rO,S,D,CrP,R,J,K,T< 6 5 4 3 2 10 calcite/dolomite ratios of carbonate rocks through geologic Time (1OSY) time. This information has a bearing on the origin of dolo- mite, as well as on changes in atmosphere-hydrosphere en- FIG. 6. Phanerozoic sedimentary rock mass-age relationships ex- pressed as the logarithm of the survival rate in tons y-’ versus time. vironmental properties through geologic time (GIVEN and The straight lines are best fits to the total mass data (solid line; see WILKINSON, 1987; WILKINSONand ALGEO, 1989; BERNER, GREGOR, 1985) and to the carbonate mass data (dashdot line) for 1990; MORSEand MACKENZIE,1990). For example, it could particular intervals of Phanerozoic time. The difference between the be argued that if the dolomite/calcite ratio progressively in- logarithm of S for the carbonate mass and that of the dolomite mass is the survival rate of the calcite mass. The black star is the value of creases with increasing age of the rock units through geologic the present day total riverine flux to the ocean, whereas the open star time (Fig. 2), this trend principally reflects increased suscep- is the value of today’s chemical and detrital inorganic carbonate flux. tibility of older rock units to processes of dolomitization. The 3286 F. T. Mackenzie and J. W. Morse
Table 3. Removal rate of pelagiccarbonate from the oceanicrealm by subductionand by transfer to accretionary ridges (units of 10smetric tons per year).
Source Model Linear (as for ocean floor, Exponential Sclater et al., 1981) Table 2, cot. H 5.8 8.3 Wilkinsonand Walker (1989) 11.5 (mass fit) 15.5 (flux fit) Gregor(1985). assuming 70% of all 5.4 8.1 pelagic sediment is carbonate .I___-n 1. - -J 0
trend is a secondary feature of the sedimentary carbonate was high or declining from its Ordovician maximum, the rock mass due to progressive diagenesis (GARRELS and calcite/dolomite ratio remains about I- 1.5, increasing MACKENZIE, 197 la; MACKENZIE, 1975). However, if the sharply in the Permian to about 13. It then decreases into trend in the calcite/dolomite ratio is cyclic in nature, this the Triassic, which has a ratio of 0.5. As global sea level rises cyclicity could be interpreted as representing environmental toward the maximum Cretaceous sea level transgression, the change in the ocean-atmosphere system (WILKINSON and calcite/dolomite ratio remains low, but tends toward the ALGEO, 1989). For our discussion, we have accepted the data higher ratios of the Tertiary and Quaternary as sea level falls, of GIVEN and WILKINSON (1987) on the Ca/Mg ratio of Pha- and dolomite becomes less and less abundant in the se.di- nerozoic sedimentary carbonates (Fig. 3) to calculate the mass mentary record. MACKENZIE and AGEGIAN ( 1986, 1989) and ratio of these carbonate components as a function of Pha- GIVEN and WILKINSON( 1987) were the first to suggest this nerozoic age (Table 1; Figs. 4, 7). It should be emphasized possible cyclicity in the calcite/dolomite ratio during the once more, however, that the data on the calcite/dolomite Phanerozoic, and LUMSDEN( 1985) observed a secular de- ratio of carbonate rocks through geologic time are still a mat- crease in dolomite abundance in deep marine sediments from ter of dispute ( ZENGER, 1989). the Cretaceous to Recent, corresponding to the general fall It can be seen in Fig. 7 that the Period-averaged mass ratio of sea level during this time interval. These cycles in calcite/ of calcite to dolomite is relatively high for Cambrian, Permian, dolomite ratios correspond crudely to the FISCHER ( 1984) and Tertiary System rocks, whereas this ratio is low for Or- two Phanerozoic super cycles and the MACKENZIEand PI- dovician through Carboniferous age sediments and rises in GOTT ( 198 1) oscillatory and submergent modes. value from the Triassic through the Recent. The generalized The cyclic pattern found in the calcite/dolomite ratio of sea level curve of VAIL et al. ( 1977), and also that of HALLAM Phanerozoic carbonates crudely correlates with the distri- ( 1984), appears to correlate crudely with the calcite/dolomite bution of inferred carbonate oiiid and cement mineralogy ratio through Phanerozoic time. The Phanerozoic starts out through Phanerozoic time ( SANDBERG, 1975, 1985; MACK- with sea level rising, and Cambrian carbonate strata are en- ENZIE and PIGOTT, 1981; WILKINSON et al., 1985). The riched in calcite. For much of the Paleozoic when sea level Permian, Tertiary and younger, and Cambrian high calcite/ dolomite ratios correspond to times of abundance of inferred aragonite oiiids and cements relative to calcite phases. The low calcite/dolomite ratios of much of the Paleozoic and those of most of the Mesozoic appear to be intervals domi- DOLOMITE nated by calcite oaids and cements. Thus, both the cyclic pattern in calcite/dolomite ratios and the inferred mineralogy of carbonate oiiids and cements roughly track the global sea level curve. The reasons for these relationships are not totally clear. A number of investigators (e.g., MACKENZIEand PIGOTT, 198 1; SANDBERG,1985; WILKINSONet al., 1985; WILKINSONand GIVEN, 1986; WILKINSON and ALGEO, 1989; MACKENZIE and AGEGIAN, 1989) concluded that these observations are the result of changing atmosphere-hydrosphere environmen- tal conditions through the Phanerozoic. However, it should be pointed out that some investigators (e.g., BATES and 6 5 :irn, 3 2 1 0 (lo*y) BRAND, 1992) argue that the observations are not statistically significant or the trends are not proven. Although we might FIG. 7. The Phanerozoic distribution of the mass ratio of calcite/ be building a “house of cards,” it appears that the observations dolomite (black dots) in sedimentary carbonates as a function of can be tied to a number of environmental conditions that age. The relative sea level curve is that of VAILet al. ( 1977). In general, times of low sea level seem to be times of high caIcite/do- changed during the Phanerozoic. These conditions are inti- lomite ratios in sedimentary carbonates. mately linked to plate tectonics. Geologic cycles of carbonate rocks 3287
The first-order changes in sea level are driven by the ac- Figure 8 shows WILKINSONand WALKER’S( 1989) model cretion of ridges: high accretion rate, high sea level; low ac- fits to global (a), continental (b), and oceanic (c, pelagic cretion rate, low sea level. Extended times of global high sea and slope-rise) sedimentary carbonate masses. Recently, level appear to be times of high atmospheric COz levels, high WILKINSONand ALGEO ( 1989) have modified slightly these temperatures, probably lower Mg/Ca ratios and saturation model fits and expressed the preserved sedimentary mass in states of seawater, and, consequently, relatively shallow car- terms of “extant” carbonate flux. They also attempted to bonate compensation depths (CCD; cf. VAN ANDEL, 1975; separate slope-rise and pelagic mass-age data and model them MACKENZIEand PIGOTT, 1981; BERNERet al., 1983; SAND- separately. Of importance here is the conclusion that conti- BERG, 1985; WILKINSONand ALGEO, 1989; BERNER, 1990; nental and oceanic reservoirs of sedimentary carbonates have Boss and WILKINSON, 199 1). The converse is true for first- different patterns of exponential decay of mass with increasing order global sea level low stands. It appears that the conditions rock age. They cycle at different rates. for early dolomitization and calcite ooid and cement for- WILKINSONand WALKER ( 1989) concluded that the total mation are best met during extended times of global high sea sedimentary carbonate mass comprises about 3500 X 10” level when atmospheric CO* levels are high and the saturation state of seawater with respect to carbonate minerals relatively low. This state would favor precipitation of less soluble car- bonate minerals, low magnesian calcite oiiids, and cements, rather than high magnesian calcite and aragonite phases, in regions of carbonate deposition. Dolomitization of precursor calcite and aragonite phases either in marine waters or in mixed continental-marine waters would be enhanced under these conditions. This conclusion is the same as that reached by WILKINSONand ALGEO ( 1989 ) . Furthermore, the poten- tial lowered pH of marine waters during times of high at- mospheric Pco, (see BLAG model; BERNER et al., 1983; LASAGA et al., 1985) would favor syndepositional or later dolomitization in mixed marine-meteoric waters, because the range of seawater-meteoric compositional mixtures over which calcite could be dissolved and dolomite precipitated is expanded (PLUMMER, 1975). Thus, it appears that the apparent trends in Phanerozoic carbonate mineralogy are related to changes in atmosphere- P hydrosphere conditions that are driven by plate tectonic E mechanisms. However, further substantiation of this conclu- i! sion requires collection of more data on the detailed chemistry t and mineralogy of carbonate sequences worldwide.
CYCLING OF PHANEROZOIC SEDIMENTARY CARBONATES
WILKINSONand WALKER ( 1989) approximated the mass- age relations for the Phanerozoic sedimentary carbonate mass (Fig. 5 ) by use of exponential decay functions for which two major assumptions were employed. The first was that the rate of destruction of carbonate mass per unit time by erosion or metamorphism is proportional to the mass of rock present. Preferential preservation was not considered important over 200 400 6 the time periods involved. The second assumption was that Age (lOsy BP) each mass-age unit cycles at the same rate; that is, a single decay constant describes the rate of recycling of the entire FIG. 8. Mass-age relationships for global (a), cratonic (b), and carbonate mass. Models may be constructed in which the pelagic (c) sedimentary carbonates (Symbols as in Fig. 5; solid ex- mass of total carbonate rock remains constant or grows lin- ponential, “mass fit”; dashed exponential, “flux-fit”). The exponential fit in (b) is that of WILKINSON and WALKER ( 1989), and is based early (constant mass and linear accumulation models of on the best statistical approximation of total mass (“mass fit”). The GARRELS and MACKENZIE, 1971a), or for a continuum of straight lines represent linear, least-squares fits of Epoch-interval data growth possibilities (power law models of VEIZER and JAN- from WILKINSON and WALKER ( 1989) for the last 100 million years SEN, 1979). The models may be constrained to yield the best ( 1), Carboniferous through Early Triassic (2), and Cambrian to Late estimate of reservoir mass or the best estimate of the present Silurian (3). The equations of the lines are ( 1) y = 3.105 + 0.0378~; r = 0.80, c = 1.34, (2) y = -4.250 + 0.02871; r = 0.69, Q = 1.79; rate of cycling (mass-fit and flux-fit approximations of (3) y = -2.857 + 0.0117t; r = 0.88, 0 = 0.67; where y = 10” g Ca WILKINSONand WALKER, 1989). y-’ and t is median age of epoch in IO6 years. 3288 F. T. Mackenzie and J. W. Morse g, expressed as calcium, with a range of estimates of 3 170- Mass Remaining (10” g Ca y-l) 3850 X 10” g. We estimate for the Phanerozoic (Table 1) a carbonate mass of about 2660 X lo*’ g, expressed as cal- cium. The global carbonate mass cycles at a rate of 8.6 X 102’ g Ca per million years and has a decay constant of 0.0025 ma-‘. The present global sedimentary flux of carbonate, ac- cording to WILKINSONand WALKER ( 1989), is between 7.7 and 9.5 X 10” g Ca ma-‘. MORSE and MACKENZIE( 1990) estimate the dissolved calcium and magnesium flux as car- bonate to the seafloor today corresponds to accumulation rates over a million year time span of 6.4 X 1020 g Ca and 0.07 X 10” g Mg. If we add today’s particulate carbonate fluxof0.7 X 10Zogma-‘, expressed as Ca, the resultant total carbonate flux is close to the lower estimate of WILKINSON and WALKER (1989). In contrast to the total sedimentary carbonate mass, oceanic carbonate oozes have a decay constant of 0.02 ma-‘, about ten times that of the global carbonate mass. According to WILKINSONand WALKER ( 1989), the pelagic oozes com- pose about 7% of the global sedimentary carbonate mass but presently account for about 60% of carbonate deposition. The exponential model fit for continental carbonates, as shown in Figure 8b, is not good. These rocks dominate the sedimentary carbonate mass older than 100 million years. One can fit the pre-Cretaceous continental carbonate data with a negative exponential function having a decay constant of0.0025 ma-‘, as done by WILKINSONand WALKER( 1989; see our Fig. 8b). However, from analysis of the mass- 570 0 10 20 30 40 50 60 70 age data of Fig. 5, we interpret the continental carbonate Percent Flooded Central N. American Craton mass-age data differently. There appear to be within the Phanerozoic three major FIG. 9. Phanerozoic continental freeboard curve of the North and distinct continental carbonate mass-age cyclic trends, as American craton (WISE,1974) compared with Epoch-interval survival rate of cratonic carbonates calculated as mass of carbonate rock in shown in Fig. 9, representing Cambrian through early De- a Series divided by duration of Epoch (data from WILKINSONand vonian, Devonian through early Triassic, and Triassic to WALKER, 1989). Dark triangle is present-day accumulation rate of present time. The oldest cycle is incomplete but probably shallow-water carbonates (see Fig. I I ) . extends back into the low sea level interval of Early Cambrian- Late Precambrian. These cycles approximately coincide in time with the Caledonian, Hercynian, and Alpine tectonic ganisms and increasing continental freeboard (relative ele- cycles. vation of continents with respect to sea level) since the late The later portions of the three cycles shown in Fig. 9 exhibit Mesozoic. Only a finite reservoir of carbonate components a nearly linear, but slightly erratic, decrease in mass per Epoch is available for deposition; if the area of shoal-water accu- with decreasing age (Fig. 8b, least-squares fits). The Cenozoic mulation is limited and a new sink in marine calcareous trend, which extends back to about Late Cretaceous time, plankton provided, the preserved mass per unit time of con- has been interpreted to represent a decline in cratonic car- tinental sedimentary carbonate will decrease. BOSS and bonate deposition and transposition of that deposition to the WILKINSON( I99 1) also concluded that both biological and deep sea. For the Cenozoic portion of the trend, there are eustatic processes are important in determining the locus of two possible reasons for this change in locus of carbonate carbonate accumulation. deposition (see also, e.g., WILKINSONand WALKER, 1989). The half-life of 86 million years for post-Permian sedi- First, planktonic, shelled marine protists in the Jurassic may mentary carbonates gives a value of the rate constant for have replaced shallow-water calcareous organisms as the cycling of 0.008 ma-‘, which is about 1.5 times that for the principal sink of carbonate. Second, the general decline of post-Devonian total sedimentary mass of 0.0054 ma-’ sea level since the late Mesozoic would have reduced the ( GREGOR, 1985). This difference is due to the fact that the geographic extent of shoal-water carbonate deposition and site of carbonate deposition since the Jurassic has moved decreased areas of warm, carbonate-saturated seas. This progressively toward the deep sea. WILKINSONand WALKER change would lead to transfer of carbonate from shallow to ( 1989) estimate that shallow-water limestone deposition has deep environments. It is likely that the Cenozoic increase in been decreasing at a rate of 0.04 X 102’ g Ca ma-’ for the carbonate accumulation in the deep sea and decrease in rates past 100 million years. Because it is very likely that the cation of accumulation in cratonic and other shoal-water settings fluxes of Ca and Mg have not varied greatly during the Pha- are a result of both evolution of planktonic calcareous or- nerozoic (CARRELSand MACKENZIE,197 la, 1972), and that Geologic cycles of carbonate rocks 3289 the oceans have been saturated with respect to calcite for all bonate deposition increased on slopes and in the deep sea, Phanerozoic time, this rate reflects, to a first approximation, carbonate oijlite and ironstone deposition on shelves and the increased rate of accumulation of deep-sea carbonate banks nearly ceased. Therefore, it appears that global eustasy ooze. plays a strong role in controlling the distribution of sedi- The other trends of decreasing epoch mass with decreasing mentary components between deepsea and shallow-water age of continental sedimentary carbonates (Figs. 8, 9) for realms of deposition. This conclusion for sedimentary car- Cambrian to early Devonian and Carboniferous to early bonates and the influence of differential cycling on carbonate Triassic strata correlate very well with the continental free- partitioning between the shallow and deep ocean are discussed board curve (Fig. 9) for the cratonic interior of the United in more detail in the following section. States and southern Canada. The preserved mass distribution of continental sedimentary carbonates expressed as Epoch CARBONATE PARTITIONING AND ‘I-HE mass preserved per year (survival rate) tracks reasonably well CO&LIMATE CONNECTION the freeboard curve. The terminations of the major cycles Modern Carbonate Cycle are very close in time to the end of the Tippecanoe and Ab- saroka cratonic sequences as defined by SLOSS ( 1963). These In the previous section, we demonstrated that the parti- sequence boundaries appear to be marked by regional, and tioning of carbonate burial between shoal-water and deep probably global, emergences and unconformities. SLOSS sea realms has probably varied in a cyclic pattern through ( 1976) has also demonstrated that the sedimentary mass per Phanerozoic time. The variation in the magnitudes of the unit time of a cratonic sequence decreases with decreasing fluxes of ( Ca,Mg)C03 to the two environments through time sequence age as the end of the sequence is approached. is difficult to assess; even today’s fluxes are probably not It is tempting to argue that there are three phases of de- known within a factor of 2. In Fig. 11 a tentative model of creasing Epoch continental carbonate sedimentary mass with the carbonate carbon cycle in the world’s oceans is shown. decreasing rock age in the Phanerozoic. The origin of the About 18 X 10 I2 moles of Ca2+ and Mg2+ (equivalent to youngest was discussed above. The origin of the two older 2 16 X lo6 metric tons of C) accumulate yearly as carbonate phases may be related to one of the assumptions underlying minerals (MORSE and MACKENZIE, 1990), mainly as bio- the construction of mass-age models; that is, the assumption logical precipitates. Of this flux about 6 X 10” moles are that there is a single decay constant for the continental car- deposited as calcium and magnesium carbonates in shoal- bonate mass that applies equally well to any unit mass of this water areas (cf. MILLIMAN, 1974; SMITH, 1978), and the carbonate reservoir. It seems likely that this assumption of remainder accumulates as calcareous oozes in the pelagic equal jeopardy of the continental carbonate rock reservoir realm. The 12 X 10 I2 moles of carbonate accumulated an- to destruction may be incorrect. Within each cycle, the younger units of the cycle may be more susceptible to de- struction by erosion and thus cycle at a faster rate than the I I I I 1 older units. It is also possible that with decreasing freeboard as shown for the Caledonian and Hercynian tectonic cycles, there was increased transport of shoal-water carbonate derived from platform margins to the deeper ocean, where its susceptibility to destruction by subduction would be enhanced. Also, it is likely that, in order to maintain a steady state ocean with respect to Ca2+ and dissolved inorganic carbon, times of low sea level within these earlier cycles may have coincided with a deepened CCD and, perhaps, enhanced inorganic precip nation of carbonate in the deep sea. These inorganic lime- stones may be represented in the stratigraphic record by some of the occurrences of dark limestone and rhythmically layered marble associated with Paleozoic ophiolites (cited by BOSS and WILKINSON, 199 1). The cycling rate of sediments in rise and deepocean tectonic regimes is greater than that in cra- tonic settings. This mechanism is not unlike what happened during the last 100 million years of the Alpine cycle; but in that case, carbonate transfer was significantly influenced by biological evolution.
Finally, this tripartite cyclicity is also seen in the frequency flme(106y) of occurrence of Phanerozoic ironstones and oijlites (Fig. 10). As sea level withdrew from the continents and conti- FIG. 10. Number of occurrences of Phanerozoic ironstones (upper nental freeboard increased, shallow-water areas with the req- diagram; data from VAN HOUTON and BHATTACHARYYA, 1982) and of iiolitic limestones (lower diagram; data from WILKINSON et uisite environmental conditions necessary to form oiilite and al., 1985) as a function of geologic age. The relative sea level curve ironstone deposits decreased in extent. Thus, as calcium car- is that of HALLAM ( 1984). 3290 F. T. Mackenzie and J. W. Morse
where S is the mass remaining of the original sediment flux Ocean at time t that would be observed today after t years of cycling Production Production Rsef-Bsnk-Shsll at a constant destructional rate (erosion + metamorphism) of k and a constant depositional rate of A. A plot of log S (g Ca y-‘) against age (m.y.) gives log S = 14.87 - 0.00086t; Riven , r = 0.68 and g = 0.20. A is 74.5 X lOI g Ca y-’ and k 15” (rounded) is 0.0009. This corresponds to a half-life of the Phanerozoic carbonate mass of 350 million years. The re- constructed carbonate accumulation flux is obtained from Accumulation Ackhtion the relation: 1 reconstructed flux = observed mass remaining X e”‘. These fluxes are given in Table 2 (column F) and plotted in Fig. 12. The similarities in the mass-age curves for the survival rate of continental carbonates and the reconstructed total car- FIG. 11.Tentative model of global carbonate cycle. Fluxes are in bonate fluxes for various stratigraphic intervals are obvious units of lOI moles C y-’ as (Ca,Mg)C03. aA~~~~~~ et al. (1988); in Fig. 12 and expected. The continental carbonate mass bM~~~~~~ (1979); ‘SMITH (1978) and MILLIMAN (1974); dominates the total surviving carbonate mass from Cambrian dCfB~~~~~~~and PENG (1982); 8B~~~~~~~ and PENG (1982), through Early Cretaceous time and is a substantial part of MILLIMAN( 1974), and HAY and SOUTHAM( 1977); hW~~~~~~ and MACKENZIE,(1983). the total carbonate mass of younger rocks. Thus, the calcu- lation of total carbonate flux is strongly influenced by the survival rates of the continental carbonates. nually in the deep sea are only about 17% of the annual The reconstructed total carbonate fluxes have varied by carbonate production rate of 72 X 10 I2 moles of the open no more than a factor of 70% around their mean of 75 X 10 ” ocean photic zone. g Ca y-’ during Phanerozoic time. This variation, while sig- This efficient recycling of carbonate carbon in the open nificant, is not great, and argues for the hypothesis of geo- ocean water column and at the sediment-water interface is a chemical uniformitarianism; that is, fluxes of carbonate con- well-known feature of the marine carbon cycle (e.g., stituents to the ocean may not have varied greatly during the BROECKER and PENG, 1982). It is important to note that Phanerozoic. This variation could even be smaller, if the re- much shoal-water carbonate production ends up in sediments constructed fluxes do not represent accumulation rates. This of reefs, banks, etc., so that, in contrast to the pelagic realm, proviso is discussed more fully in a later section. production rate more closely approximates sedimentation In general, there is a good correlation between the first- rate. There is, however, escape of carbonate sediment from order changes in the sea level curves of both VAIL et al., 1977 shoal-water areas to the deep sea, where it is deposited or (Fig. 7) and HALLAM 1984 (Fig. 10) and the survival rates dissolved (e.g., LAND, 1979). The magnitude of this flux is of continental carbonates and total carbonate fluxes through poorly known but may affect the chemistry of open-ocean the Phanerozoic from the Neogene to the Devonian. Periods regions owing to dissolution of the carbonate debris ( DROX- of global low sea level, like those of today and the Permo- LER et al., 1988; AGEGIAN et al., 1988). Furthermore, its Triassic, appear to correlate with minima in the extant mass accumulation on the slopes of banks may act as a record of of continental carbonates and the reconstructed total car- paleoenvironmental change ( DROXLER et al., 1983). bonate fluxes, whereas the Cretaceous sea level transgression correlates with a maximum in these variables. The correlation Phanerozoic Carbonate Fluxes roughly extends back into the Devonian, beyond which it appears to break down. The early Paleozoic sea level high is It is difficult to obtain the carbonate flux and its partitioning represented by minimum values in the survival rate of con- between deep-sea and shallow-water areas of deposition tinental carbonates and calculated total carbonate flux. throughout Phanerozoic time. One approach is to use the Other relationships appear to exist between the mass-age methodology of TARDY et al. ( 1989) and WOLD and HAY curves of Fig. 12 and sedimentary attributes. WOLD and HAY ( 1990) to reconstruct sediment fluxes and to apply it to the ( 1990), from their analysis of the mass-age curve for the reconstruction of carbonate accumulation rates. To do so, total Phanerozoic sedimentary mass distribution, argued for the Epoch-interval data of WILKINSON and WALKER ( 1989) a 150 million year cycle in the mass distribution. They sug- were grouped into thirteen time intervals (Table 2). This gested that mass maxima at 540,390,240,90, and 6.6 million grouping increases the time constant, improving the resolu- years before present (BP) reflected real variations in the global tion of the time series and leading to an improvement in the rates of erosion and sedimentation. The mass maxima were mass-age correlation. The total carbonate masses remaining thought to represent periods of high sedimentation (see ar- expressed in units of 10I3 g Ca y-’ (survival rate) for the rows, Fig. 12). Furthermore, TARDY et al. (1989), in their thirteen stratigraphic intervals given in Table 2 (column E) analysis of the global water cycle and continental erosion were fit by a simple exponential decay curve having the form through Phanerozoic time, concluded that global runoff was S = Ae-k’, (3) high in the Cretaceous ( 100 m.y. BP), the Siluro-Devonian Geologic cycles of carbonate rocks 3291
1 1 I 4 I I LSL H6L L&L I\ HSL LSL 120 LRO 1 ’ \ I ! \ I I t /HSR HRO LRO
-t t t t t o~ no2+uKJ,LKI CR ,111 PI,PIMI ~Isl o,I c 1~1 0 200 400 , IO
Age WY BP) FIG. 12. Survival rate of continental carbonates and reconstructed global, total sedimentation flux of carbonates through Phanerozoic time. Arrows show times of high, total (chemical plus detrital ) , global sedimentation rates (after WOLD and HAY, 1990). HSL is high sea level, whereas LSL is low sea level (cf. VAIL et al., 1977; HALLAM, 1984). HRO and LRO are, respectively, times of high and low runoff, and HSR and LSR are, respectively, times of high and low total sedimentation rate (after TARDYet al., 1989). Open and closed triangles are, respectively, present-day total carbonate accumulation rate and shallow-water carbonate accumulation rate in the oceans.
(400 m.y. BP), and the Cambrian (500 m.y. BP), whereas High runoff and sedimentation rates mark the Cretaceous the present-day and the Permo-Triassic (200 m.y. BP) were and Devonian maxima in survival rate, whereas low runoff thought to be dry periods. Sedimentation rates were shown and sedimentation rates are coincident with the low survival to correlate moderately with global runoff, with the Cambrian, rate of the Permo-Triassic. Thus, the continental carbonate Devonian, Cretaceous, and Tertiary periods having the largest mass survival rate appears to bear some relationship to the sedimentation rates, and the Carboniferous and Permo- sea level curve, and runoff and sedimentation fluxes from Triassic exhibiting the lowest rates (see Fig. 12). The abnor- Neogene to Devonian time. Exceptions to this statement are mally high erosion rates of the Neogene and the present-day the low runoff and survival rates, but the high sedimentation probably reflect increased tectonic activity and, during more rate of the Tertiary, and the relatively high sedimentation recent times, may be a consequence of deforestation, agri- rate of the late Paleozoic-early Mesozoic ( WOLD and HAY, cultural practices, and other activities of humankind. Gla- 1990)) a time of low sea level. Furthermore, for the Neogene ciation has also played a role. TARDY et al. ( 1989) and WOLD to Devonian interval, the calculated total carbonate flux curve and HAY ( 1990)) in their analyses of the sedimentary mass- tracks the survival rate curve. These relationships suggest, to age distribution, assume that the present-day mass-age dis- a first approximation, that the survival rate curve of conti- tribution of extant Phanerozoic sedimentary rock can be fitted nental carbonates reflects changes in the total sedimentation by a single exponential decay curve such as that of Eqn. (3). flux of shallow-water carbonates through time and that these This implies a single rate constant for decay of the sedimen- changes are related to global eustasy (the “calcite push” model tary mass and a steady-state constancy of this mass throughout of WILKINSON and WALKER, 1989). High sea levels give rise the Phanerozoic. This is the approach used by us to construct to high total fluxes of shallow-water carbonates, whereas low the total carbonate mass fluxes (reconstructed fluxes) shown sea levels result in transposition of carbonate deposition to in Fig. 12. deeper depths in the ocean and reduced total shallow-water It is apparent from the above discussion that the continental carbonate fluxes. carbonate mass survival curve tracks the first-order changes It is usually thought that the times of first-order changes in sea level from the Neogene to Devonian. During this in- in the Phanerozoic sea level curve reflect changes in sea-floor terval, minima in the survival rate curve correlate with periods spreading rates, which lead to changes in mid-ocean ridge of low sea level and maxima with periods of high sea level. system volumes and subsequent sea level rise or fall. Thus, 3292 F. T. Mackenzie and J. W. Morse plate tectonic mechanisms control sea level on the time scale approximation of the total flux, this difference represents of the first-order changes in the Phanerozoic sea level curve. continental carbonate destroyed by erosion or carbonate that Also, times of rapid seafloor spreading, increased ridge volume accumulated in the deep sea. and ocean water displacement, and flooding of continents The progressive decrease of the “missing” flux through a appear to be times of high atmospheric CO2 levels and con- long period of high sea level suggests that this trend may sequently high temperature ( BERNER et al., 1983; LASAGA represent post-depositional destruction by erosion of the et al., 1985; BERNER, 1990), as well as shallow CCD depths. continental carbonate mass. The same relationships are ob- It might be anticipated that because of high tectonic activity served during sea level withdrawal from the continents during and high temperatures, these periods were times of high total the latter part of the Hercynian tectonic cycle. It appears that sedimentation rate and high fluxes of carbonate components in both cycles, the younger continental carbonates were pref- to the ocean. Calcite and dolomite weathering rates are erentially eroded relative to older carbonates, which were strongly influenced by temperature: the higher the temper- covered by overlying sediments and perhaps protected in the ature, the more rapid the weathering rate (HARMON et al., deeper regions of tectonic basins. Notice that for the Late 1975; BERNERet al., 1983). Total runoff from continents, a Cretaceous and Tertiary part of the Alpine tectonic cycle the function of both continental position and climatic factors, calculated “missing” flux is close to the deep-sea extant car- fluctuated considerably in the Phanerozoic from 0.35 X 10’” bonate mass (Table 2, column H). This similarity suggests g Y-’ in the Triassic to 0.65 X lo*’ g y-’ in the Devonian that for this period of time, the “missing flux” is the carbonate (TARDY et al., 1989). During the Cretaceous and Devonian deposited in the deep sea. In other words, erosion of post- high sea level stands, sufficient continental area was located Cretaceous continental carbonates has not been great. Thus, at humid climatic latitudes to lead to greater runoff, whereas it appears that the overall trend of decreasing survival rate during Triassic time a greater portion of continental area was of continental carbonates for the latter part of the Alpine located in semi-arid and arid climatic regions. tectonic cycle is due to sea level fall and transposition of It appears that the higher carbonate weathering rates ac- carbonate accumulation to the deep sea. However, for the companying the higher atmospheric CO2 levels, and hence latter portion of the Hercynian cycle, and particularly for the temperature of the Cretaceous and Devonian coupled with latter portion of the Caledonian cycle, survival rate and hence higher runoff rates during these periods, led to enhanced fluxes calculated total carbonate flux (reconstructed flux) appear of carbonate components to the ocean and increased car- to be strongly influenced by differential cycling by erosion of bonate depositional rates. For much of the Permo-Triassic, the younger carbonate strata in the cycles. If this assertion is when sea level and atmospheric CO2 were low and interior true, it may be difficult to determine accurately the accu- deserts widespread, global runoff, riverine carbonate flux, and mulation rates of shallow-water and deep-sea carbonates hence carbonate sedimentation rate were subdued. Again in through much of Phanerozoic time. the Tertiary, comparable conditions prevailed (see TARDY et al., 1989; WOLD and HAY, 1990). Survival Rate and Atmospheric COZ We are now in a position to ask the question: Why does the relationship between sea level and continental carbonate Both VOLK (1989) and BERNER (1990) have discussed survival rate appear not to hold for pre-Devonian rocks? The the sensitivity of atmospheric CO*, and hence climate, to the survival rate is at a minimum near the Siluro-Devonian partitioning of carbonate burial between the shallow- and boundary and rises gradually toward the early Cambrian, as deep-ocean realms. In both models it was shown that changes does the reconstructed global carbonate flux curve (Fig. 12 ). in the CO2 degassing rate, because of the Urey reaction However, Paleozoic sea level reached a high sometime in late (UREY, 1956), Cambrian-early Ordovician time (cf. sea level curves Of VAIL CaCO, + Si02 = Casio3 f CO2, (4) et al., 1977, and HALLAM, 1984; Figs. 7, 10). Therefore, one would expect the survival rate of continental carbonates to are significantly affected on a multi-million year time scale be high around 400-500 million years BPif survival rate were by shifts in the site of carbonate deposition from platforms truly representative of depositional rate. The Hallam sea level to the deep sea and vice versa. VOLK ( 1989) further argued curve may provide a clue to this enigma. In the Hallam curve, that if the open ocean were not inhabited by pelagic fora- there is a sharp withdrawal of sea level from the continent minifera and coccolithophores, the CO2 levels of today would near the Siluro-Devonian boundary at the close of the Cale- be lower and the climate colder. Carbonates deposited in the donian tectonic cycle (Fig. 10). This feature of the sea level deep sea are a warming factor on climate change over a multi- curve, which is also seen in the details of the VAIL et al. million time scale. This is because of their subsequent sub- ( 1977) curve, is coincident with one of the minima in the duction and decarbonation, leading to increased rates of CO2 continental carbonate survival rate curve (Fig. 12). Thus, degassing to earth’s surface and a subsequent warming owing the downward trend of the survival rate curve from Cambrian to changes in the earth’s radiation balance (“greenhouse ef- to Silurian may reflect in part sea level fall at the close of the fect”). Both VOLK (1989) and BERNER ( 1990) call on the Caledonian tectonic cycle. Furthermore, the difference be- plate tectonic process of subduction to transport deep-sea tween the reconstructed total carbonate flux and the survival carbonates to depths where pressure and temperature are high rate of continental carbonates decreases from Cambrian to enough to convert carbonate to silicate by the UREY ( 1956) Silurian (the “missing flux” of Table 2, column G). If the reaction. However, VOLK, ( 1989) (see also MACKENZIEand reconstructed total carbonate sediment flux is a reasonable PIGOTT, I98 1) also argued that metamorphic decarbonation Geologic cycles of carbonate rocks 3293 is a degassing source of CO2 from sedimentary piles of con- cycling of carbonates, because of subduction of oceanic car- tinental carbonates. bonates, at a rate about two times that of the total sedimentary If the metamorphism of carbonates on subducting slabs mass. Within the total Phanerozoic carbonate mass, the age were the principal source of CO* outgassing at plate bound- distribution of sedimentary properties like calcite/dolomite aries ( MACKENZIEand PIGOTT, 198 1; VOLK, 1989; BERNER, ratio, inferred oiiid mineralogy, and survival rate of conti- 1990)) and if the deep-sea burial of carbonates were a warm- nental carbonates is cyclic. The cycles appear to be coupled ing factor ( VOLK, 1989) during the Cenozoic, then the cyclic to plate tectonic processes that result in sea level change and pattern of Phanerozoic extant, shoal-water carbonate mass changes in the properties of the ocean-atmosphere system. shown in Figs. 9 and 12 might be linked to CO* degassing However, the mass-age distribution of continental carbonates rates and hence climate change. This statement implies that does not appear to be simply a primary feature of the sedi- the preserved mass per unit time of shoal-water carbonates mentary lithosphere but is governed in part by increased can be interpreted strictly as a primary feature of the sedi- probability of erosion of the younger strata within the three mentary rock column; that is, the mass per unit time pre- main tectonic cycles (Caledonian, Hercynian, and Alpine). served is the mass deposited per unit time. This is surely not This interpretation implies that simple exponential decay the case, but it can be taken as an extreme position for the models with a single recycling constant cannot be used with- sake of argument. Then, if total carbonate accumulation were out reservation to describe the global carbonate mass-age dis- constant through time, the transposition of carbonate from tribution. It is possible that this statement is also true for the shoal-water areas to the deep sea during the past 100 million total sedimentary rock mass-age distribution, as suggested years occurred at a rate of 0.04 X 10” g Ca my-‘, the Mis- earlier by GARRELSand MACKENZIE( 197 lb). If true for the sissippian to Triassic rate was 0.03 X 10 I3 g Ca my-‘, and carbonate record, then interpretation of this record in terms that of the Cambrian to Silurian was about 0.0 1 X 10 *’ g Ca of partitioning of carbonate between deep-sea and shallow- my-’ . The present total shoal-water carbonate accumulation water realms during the Phanerozoic is difficult. Further rate expressed as g Ca my-’ obtained by extrapolation of the quantification of geochemical models describing the geologic nearly linear portion of the latest cycle in cratonic accumu- history of atmospheric CO2 and climate change necessitates lation is equivalent to 30 X 10 I3 g y -’ ; the tentative cycle of consideration of the difficulties inherent in obtaining car- carbonate carbon shown in Fig. 11 gives a value of 24 X 10 I3 bonate depositional fluxes and their partitioning through g Ca y-’ (marked with black triangles in Figs. 9 and 12). Phanerozoic time. These estimates are independent, reasonably close, and pro- vide some feeling for confidence in the estimated values. Acknowledgments-One of Bob Garrels’s favorite mementos was a It is interesting to note that the Cambrian-to-Silurian and figure of Sisyphus rolling a heavy stone up a hill, only to have it roll back down again. However, I (FTM) always felt that Bob saw progress Mississippian-to-Triassic carbonate transpositions precede in the effort of Sisyphus, much as he saw progress in scholarship and and overlap maxima in the BERNER ( 1990) calculated at- life in general. Many of us push the stone of scholarship ahead a little mospheric CO2 trend for the Phanerozoic. This would be at a time, building on the efforts of those who have gone before and expected, because there is a varying delay between carbonate our colleagues; Bob forged ahead. We are grateful to him for having deposition on the seafloor and subduction to depths necessary set the tone and foundation for this paper, and to our many colleagues whose works we have used as a foundation for our thoughts. We for conversion of CaCO, to a calcium-bearing silicate and especially thank Bryan Gregor for his very thoughtful review of an release of C02. This delay can be on the order of tens of earlier version of this manuscript, in particular for his suggested re- milliOnSOf years( VOLK, 1989). visions of Table 1 and for providing Table 3. Because pre-Jurassic deep-sea carbonates are virtually ab- This research was supported in part by NSF grant EAR-8816350. The manuscript was completed while FTM held a Visiting Scientist sent from the geologic record and because the mass-age dis- Fellowship, Capital Region of Bruxelles, Belgium. School of Ocean tribution curve of Phanerozoic continental carbonates not and Earth Science Contribution No. 2902, University of Hawaii. only represents changes in carbonate accumulation fluxes but also differential cycling, it will be difficult to obtain quan- Editorial handling: H. C. 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