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Geochimica et Cosmochimica Acfa Vol. 56, pp. 3281-3295 Copyright 0 1992 Pcrgamon Pres Ltd. F-rimed in U.S.A.

0016-7037/92/S%oO + 03

Sedimentary carbonates through Phanerozoic time

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 carbonates 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

IN A PAPER published in 1969 ( GARRELS and MACKENZIE, 1969), Bob Garrels and Fred Mackenzie initially presented hypotheses for the meaning of the temporal variation in proportions of sedimentary rock types remaining today in the geologic column. In the book Evolution of Sedimentary Rocks ( 197 la), they further developed models of the sedimentary rock mass-age distribution and expanded on two hypotheses related to that distribution: ( 1) geochemical uniformitarianism and (2) differential cycling. Geochemical uniformitarianism implies that the total mass of sediments of all ages existing at any given time in the geologic past may have had about the same ratios of rock types that we observe today. A corollary to this hypothesis is that the fluxes of chemical con- stituents to the oceans have not varied greatly, at least during Phanerozoic time. Differential cycling implies that because of differences in relative erosional resistances or tectonic set- ting, various components of the sedimentary rock mass cycle at different rates. This factor, along with diagenesis, may lead to differences in the ratios of rock types as a function of geologic age in the sedimentary rock mass existing today.

These concepts and others involving the sedimentary rock mass-age distribution have been explored and expanded on

by various authors (e.g., GREGOR, 1970, 1985; MACKENZIE, 1975; GARRELS et al., 1976; VEIZER and JANSEN, 1979, 1985; VEIZER, 1988; GREGOR et al., 1988; WOLD and HAY, 1990). Further compilations and interpretations of the Phanerozoic sedimentary carbonate rock mass-age distribution have ap- peared in the literature in the 1980s (RONOV, 1980; HAY, 1985; WILKINSON and WALKER, 1989; WILKINSON and AL- GEO, 1989; Boss and WILKINSON, 1991). In this paper, as a tribute to and in remembrance of Bob Garrels, we explore in more detail the concepts of geochemical uniformitarianism and differential cycling relevant to interpretations of the sedimentary carbonate rock mass-age distribution through Pha- nerozoic geologic time. This study reflects one of Bob’s principal interests in the later stages of his career.

THE DATA BASE

The calculations and interpretations of this paper are based on data from a number of sources. There is still some dis- agreement in the literature concerning the best estimates of mass-age relationships within the major global carbonate reservoirs (e.g., HAY, 1985; WILKINSON and WALKER, 1989). We will clearly indicate in the following discussions the data sources and our manipulations of the data base. To provide

3281

3282 F. T. Mackenzie and J. W. Morse

Table 1. Phanerozoic carbonate 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~OIIS y-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 (WILKINSON and WALKER, 1989; WILKINSON and ALGEO, 1989), carbonate mass-age data will be given in units of 10 r3 g Ca y-l. This convention introduces an overestimation of the cation mass of about 2% for each 10% of dolomite found in a rock mass interval (WILKINSON and WALKER, 1989).

The distribution of Phanerozoic System total sedimentary masses with geologic age was obtained from the estimates of GREGOR ( 1985). The mass of carbonate rock in each system was calculated from the estimates of CO, found in Phanero- zoic carbonates as given by RONOV ( 1980) and amended by HAY ( 1985 ) . Dolomite and limestone abundances were calculated using the GIVEN and WILKINSON ( 1987) compilations ofdata on the composition (Mg/Ca or MgC03/CaC03 ratios) of Phanerozoic carbonate rock samples. Table 1 gives the resuhs of our calculations. A tentative mass-age dist~bution of sedimentary carbonates and sandstones plus shales is given in Fig. 1. The total carbonate mass makes up about 30% of Phanerozoic sediments in Fig. 1, perhaps a slightly high es-

P 180% DotwaIte

EOSO . *

[ 0.40 u c j 0.30

a I 10 0.20 Z AVHI~~ w&on& rack _______--___-_---_- -----------

I! O*‘O

Runrm PI&&ml

0 8 !5 4 3 2 1 0

Time (10'~)

100 400 1000 zaoo 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 VINOGRADOV and RONOV, 1956a.b; and CHILINGAR, 1956; from Table 1. figure modified from GARRELS and MACKENZIE, 1971a).

timate because of the use of Hay’s CO* data (WILKINSON

and WALKER, 1989).

For several decades it has been assumed that the Mg/Ca ratio of carbonate rocks increases with increasing Phanerozoic rock age. An early portrayal of this trend in carbonate rocks from the Russian Platform and North America is shown in Fig. 2. The trend represents a general, but erratic, decline in the calcium content and increase in the magn~ium content of these rocks with incising age (see VIN~CRAD~V and RONOV, 1956a,b; CHILINGAR, 1956). The magnesium content is relatively constant in these carbonates for about 100 million years, then increases gradually. The magnesium content of North American and Russian Platform continental carbonate rocks appears to increase at a geologic age that is very close to, if not the same as, the age of the beginning of the general increase in the Mg content of pelagic limestones f 100 million years before present; RENARD, 1986). The dolomite content of deepsea sediments also increases erratically with increasing age back to about 125 million years before

Geologic cycles of carbonate rocks 3283

So C Schmoker el al. (1997)

MglCa=0.14

Vinogradov and Ronov (1956b)

Mg/Ca=0.14

N=l7,353

B Sperber et al. (1994)

MglCa=0.23

D Chlllngar (1996)

MglCe=O.lS

F Given and Wilkinson (1987)

MglCa=O.23 N=607

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 content of carbonate rocks with increasing age into at least the Early Cretaceous appears to be a global phenomenon, and to a first approximation, is not lithofacies related.

Recently, the accepted truism that dolomite abundance increases relative to limestone with increasing Phanerozoic age has been challenged by GIVEN and WILKINSON ( 1987). They reevaluated all the existing data on the composition of Phanerozoic carbonates and concluded that dolomite abundances do vary significantly throughout the Phanerozoic but may not increase systematically with age. Figure 3 is a sum- mary of the global data set for Phanerozoic dolostone abundances 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 dolomite 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 with the mass-age relationships of sedimentary carbonates compiled on an epoch by epoch basis. These data were compiled by WILKINSON and WALKER ( 1989) for continental, oceanic, and global carbonate reservoirs and are tabulated in

Table 2 and shown in Fig. 5. WILKINSON and WALKER ( 1989) developed mass-age models for these various carbonate reservoir distributions similar to those used by GARRELS and MACKENZIE ( 197 la). In a later section, these mass-age relationships are discussed in detail.

DIFFERENTIAL CYCLING OF THE CARBONATE MASS

Sedimentary rocks are formed by depositional processes involving principally the agents of water and wind and are destroyed when eroded or transformed chemically into other kinds of rocks like paragneiss. The sedimentary rock mass (including volcanogenic sediments) today, as estimated from

5 4 3 lime (lo8 yP

1 0

FIG. 4. The Phanerozoic sedimentary carbonate mass distribution as a function of geologic age. Period masses of calcite and dolomite and the Period mass ratios of calcite/dolomite are also shown.


Table 2. Carbonate sediment masses for various stratigraphic intewals.

b

ContinentaI Gvbollate Total Carbonate Reconstructed rhbomte ‘Missing” Flux Deep-Sea Cahmati

St&graphic Duration Midpoint Interval Mttss (1) Remaining Mass (1) Remaining Sediment Flux Mass (1) Px?swved Jntervals (10% wy) (lolag & Y“) (1O’fg Gl 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

:olumn headings A-H are referred to in text. “Missing” flux is column (F) minus column (D).

geochemical mass balance methods, is 25-30,000 X 102’ g (cf. CARRELS and MACKENZIE, 197 la, 1972; LI, 1972; VEIZER, 1988), with 86% of the mass lying within the continental and shelf region of the globe and 14% a part of the deep ocean floor ( RONOV, 1980; cited in VEIZER, 1988).

A Global Carbonatea

0 Ocemk Crbmtn

. Conthntd Cirbomtel

% 0

,oooo,

200 400 600

Age (lO*y BP)

FIG. 5. Mass-age relationships of sedimentary carbonates. The global mass is shown as open triangles when representing the sum of pelagic and cratonic masses and as half-solid rectangles when entirely continental. Solid rectangles are cratonic masses when the global mass consists of both continental and deep oceanic carbonate. (From WILKINSON and WALKER, 1989).

This rock mass estimate includes the classic lithologies of sandstone, shale, and carbonate, as well as their metamorphic equivalents of quartzite, slate, phyllite, low-grade schist, and marble. The current mass is that preserved, not the total mass deposited throughout geologic time. Total sedimentary deposition over the last 3.5 billion of years of earth history has been at least 130,000 X 10 2o g.

Like human populations, sedimentary rock masses can be assigned birth rates and death rates, and they can be subdi- vided into age groups (CARRELS et al., 1976). GREGOR

( 1985) demonstrated that the sedimentary mass-age distribution for Carboniferous and younger sediments has a log 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 Period, and t is the median age of the mass (in units of IO* years; see also CARRELS and MACKENZIE, 1971b, for discussion of survival rate).

We have calculated the survival rates of the carbonate and dolomite masses for different Phanerozoic systems (Table 1); these are plotted in Fig. 6, together with the GRECOR ( 1985) plot for the total sediment mass. The difference between the survival rate of the total carbonate mass and that


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 sedimentary mass. It implies a 130 million year half-life for the post-Devonian mass, and for a constant sediment mass with a constant probability of destruction, a mean sedimentation rate since post-Devonian time of about 100 X 10 I4 g y-l. The modem global erosional flux is about 200 X 10 I4 g y-r ( GARRELS and MACKENZIE, 197 la; MARTIN and MEY- BECK, 1979)) of which about 15% is particulate and dissolved carbonate. Although the data are less reliable for the survival rate of Phanerozoic carbonate sediments than for the total sedimentary mass, a best log linear fit to the Post-Permian preserved mass of carbonate rocks is

log S,,,, = 9.55 - 0.36t (r = 0.96, cr = 0.32) (2)

with Sin units of lo9 metric tons and t in units of lo8 years. This corresponds to a half-life for the post-Permian carbonate mass of 86 million years, and a mean sedimentation rate of these sediments of about 35 X 10 I4 g carbonate per year; the present-day carbonate flux is 30 X 10 I4 g y-’ (MORSE and MACKENZIE, 1990). The difference in half-lives between the total sedimentary mass, which is principally sandstone and shale, and the carbonate mass probably is a consequence of the more rapid recycling (throughout this paper the term recycling refers to the half-life of the reservoir in one form

This is not an unlikely situation. With the advent of abundant carbonate-secreting, planktonic organisms in the Juras- sic, the site of carbonate deposition shifted significantly from shallow-water areas to the deep sea. A graduate shift in carbonate deposition from shallow-water environments to the deep sea would increase still further the rate of destruction (by eventual subduction) of the global carbonate mass relative to the total sedimentary mass from Jurassic time on. Table 3 shows the removal rate of pelagic carbonate from the oceanic realm by subduction and by transfer to accretionary wedges, as obtained from different models. This removal rate represents potential loss of carbonate from the sedimentary lithosphere. Furthermore, the recycling rate of oceanic crust (the b” values of VElzER and JANSEN, 1985; VEIZER, 1988) exceeds that of the continental basement by a factor of 17. Also, SOUTHAM and HAY ( 198 1 ), using a half-life of 100 million years for pelagic sediment, estimated that as much as 50% of all sedimentary rock formed by weathering of ig- neous rock may have been lost by subduction during the past 4.5 billion years.

Thus, it appears, as originally suggested by GARRELS and MACKENZIE ( 1969, 197 la, 1972), that the carbonate com- ponent of the sedimentary rock mass may have a cycling rate different than that of the total sedimentary mass. These authors 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.

8.0

7(3

0 Total Sedimentary Mass A Carbonate Wss W Dolomite Mass

I % rO,S,D,CrP,R,J,K,T< . ._ 6 5 4 3 2 10

Time (1OSY)

FIG. 6. Phanerozoic sedimentary rock mass-age relationships expressed as the logarithm of the survival rate in tons y-’ versus time. The straight lines are best fits to the total mass data (solid line; see GREGOR, 1985) and to the carbonate mass data (dashdot line) for particular intervals of Phanerozoic time. The difference between the 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 the present day total riverine flux to the ocean, whereas the open star is the value of today’s chemical and detrital inorganic carbonate flux.

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 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 layer; still others were dolomitized by migrating underground waters tens or hundreds of millions of years after deposition. It is exceedingly important to know the distribution of the calcite/dolomite ratios of carbonate rocks through geologic time. This information has a bearing on the origin of dolomite, as well as on changes in atmosphere-hydrosphere environmental properties through geologic time (GIVEN and WILKINSON, 1987; WILKINSON and ALGEO, 1989; BERNER, 1990; MORSE and MACKENZIE, 1990). For example, it could be argued that if the dolomite/calcite ratio progressively increases with increasing age of the rock units through geologic time (Fig. 2), this trend principally reflects increased susceptibility of older rock units to processes of dolomitization. The


Table 3. Removal rate of pelagic carbonate from the oceanic realm by subduction and by transfer to accretionary ridges (units of 10s metric tons per year).

Source Model

Linear (as for ocean floor, Exponential Sclater et al., 1981)

Table 2, cot. H 5.8 8.3 Wilkinson and 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 rock mass due to progressive diagenesis (GARRELS and MACKENZIE, 197 la; MACKENZIE, 1975). However, if the trend in the calcite/dolomite ratio is cyclic in nature, this cyclicity could be interpreted as representing environmental change in the ocean-atmosphere system (WILKINSON and ALGEO, 1989). For our discussion, we have accepted the data of GIVEN and WILKINSON (1987) on the Ca/Mg ratio of Pha- nerozoic sedimentary carbonates (Fig. 3) to calculate the mass ratio of these carbonate components as a function of Pha- nerozoic age (Table 1; Figs. 4, 7). It should be emphasized once more, however, that the data on the calcite/dolomite ratio of carbonate rocks through geologic time are still a mat- ter of dispute ( ZENGER, 1989).

It can be seen in Fig. 7 that the Period-averaged mass ratio of calcite to dolomite is relatively high for Cambrian, Permian, and Tertiary System rocks, whereas this ratio is low for Or- dovician through Carboniferous age sediments and rises in value from the Triassic through the Recent. The generalized sea level curve of VAIL et al. ( 1977), and also that of HALLAM

( 1984), appears to correlate crudely with the calcite/dolomite ratio through Phanerozoic time. The Phanerozoic starts out with sea level rising, and Cambrian carbonate strata are en- riched in calcite. For much of the Paleozoic when sea level

DOLOMITE

6 5 :irn, 3 2 1 0 (lo*y)

FIG. 7. The Phanerozoic distribution of the mass ratio of calcite/ dolomite (black dots) in sedimentary carbonates as a function of age. The relative sea level curve is that of VAIL et al. ( 1977). In general, times of low sea level seem to be times of high caIcite/dolomite ratios in sedimentary carbonates.

was high or declining from its Ordovician maximum, the calcite/dolomite ratio remains about I- 1.5, increasing sharply in the Permian to about 13. It then decreases into the Triassic, which has a ratio of 0.5. As global sea level rises toward the maximum Cretaceous sea level transgression, the calcite/dolomite ratio remains low, but tends toward the higher ratios of the Tertiary and Quaternary as sea level falls, and dolomite becomes less and less abundant in the se.di- mentary record. MACKENZIE and AGEGIAN ( 1986, 1989) and GIVEN and WILKINSON ( 1987) were the first to suggest this possible cyclicity in the calcite/dolomite ratio during the Phanerozoic, and LUMSDEN ( 1985) observed a secular decrease in dolomite abundance in deep marine sediments from the Cretaceous to Recent, corresponding to the general fall of sea level during this time interval. These cycles in calcite/ dolomite ratios correspond crudely to the FISCHER ( 1984)

two Phanerozoic super cycles and the MACKENZIE and PI- GOTT ( 198 1) oscillatory and submergent modes.

The cyclic pattern found in the calcite/dolomite ratio of Phanerozoic carbonates crudely correlates with the distribution of inferred carbonate oiiid and cement mineralogy through Phanerozoic time ( SANDBERG, 1975, 1985; MACK-

ENZIE and PIGOTT, 1981; WILKINSON et al., 1985). The 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- 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., MACKENZIE and PIGOTT, 198 1; SANDBERG, 1985; WILKINSON et al., 1985; WILKINSON and GIVEN, 1986; WILKINSON and ALGEO, 1989; MACKENZIE and AGEGIAN, 1989) concluded that these observations are the result of changing atmosphere-hydrosphere environmental conditions through the Phanerozoic. However, it should be pointed out that some investigators (e.g., BATES and BRAND, 1992) argue that the observations are not statistically significant or the trends are not proven. Although we might be building a “house of cards,” it appears that the observations can be tied to a number of environmental conditions that changed during the Phanerozoic. These conditions are inti- mately linked to plate tectonics.


The first-order changes in sea level are driven by the accretion of ridges: high accretion rate, high sea level; low accretion rate, low sea level. Extended times of global high sea level appear to be times of high atmospheric COz levels, high temperatures, probably lower Mg/Ca ratios and saturation states of seawater, and, consequently, relatively shallow carbonate compensation depths (CCD; cf. VAN ANDEL, 1975; MACKENZIE and PIGOTT, 1981; BERNER et al., 1983; SAND-

BERG, 1985; WILKINSON and ALGEO, 1989; BERNER, 1990; Boss and WILKINSON, 199 1). The converse is true for first- order global sea level low stands. It appears that the conditions for early dolomitization and calcite ooid and cement formation are best met during extended times of global high sea 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 carbonate 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 WILKINSON and ALGEO ( 1989 ) . Furthermore, the potential lowered pH of marine waters during times of high atmospheric 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- hydrosphere conditions that are driven by plate tectonic mechanisms. However, further substantiation of this conclusion requires collection of more data on the detailed chemistry and mineralogy of carbonate sequences worldwide.

CYCLING OF PHANEROZOIC SEDIMENTARY CARBONATES

WILKINSON and 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 the time periods involved. The second assumption was that each mass-age unit cycles at the same rate; that is, a single decay constant describes the rate of recycling of the entire carbonate mass. Models may be constructed in which the mass of total carbonate rock remains constant or grows lin- early (constant mass and linear accumulation models of GARRELS and MACKENZIE, 1971a), or for a continuum of growth possibilities (power law models of VEIZER and JAN-

SEN, 1979). The models may be constrained to yield the best estimate of reservoir mass or the best estimate of the present rate of cycling (mass-fit and flux-fit approximations of WILKINSON and WALKER, 1989).

Figure 8 shows WILKINSON and WALKER’S ( 1989) model fits to global (a), continental (b), and oceanic (c, pelagic and slope-rise) sedimentary carbonate masses. Recently, WILKINSON and ALGEO ( 1989) have modified slightly these model fits and expressed the preserved sedimentary mass in terms of “extant” carbonate flux. They also attempted to separate slope-rise and pelagic mass-age data and model them separately. Of importance here is the conclusion that continental and oceanic reservoirs of sedimentary carbonates have different patterns of exponential decay of mass with increasing rock age. They cycle at different rates.

WILKINSON and WALKER ( 1989) concluded that the total sedimentary carbonate mass comprises about 3500 X 10”

P E i! t

200 400 6 Age (lOsy BP)

FIG. 8. Mass-age relationships for global (a), cratonic (b), and pelagic (c) sedimentary carbonates (Symbols as in Fig. 5; solid exponential, “mass fit”; dashed exponential, “flux-fit”). The exponential fit in (b) is that of WILKINSON and WALKER ( 1989), and is based on the best statistical approximation of total mass (“mass fit”). The straight lines represent linear, least-squares fits of Epoch-interval data from WILKINSON and WALKER ( 1989) for the last 100 million years ( 1 ), Carboniferous through Early Triassic (2), and Cambrian to Late 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; (3) y = -2.857 + 0.0117t; r = 0.88, 0 = 0.67; where y = 10” g Ca y-’ and t is median age of epoch in IO6 years.


g, expressed as calcium, with a range of estimates of 3 170- 3850 X 10” g. We estimate for the Phanerozoic (Table 1) a carbonate mass of about 2660 X lo*’ g, expressed as calcium. 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, according to WILKINSON and 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 carbonate 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 WILKINSON and 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 WILKINSON and WALKER ( 1989; see our Fig. 8b). However, from analysis of the mass- age data of Fig. 5, we interpret the continental carbonate mass-age data differently.

There appear to be within the Phanerozoic three major and distinct continental carbonate mass-age cyclic trends, as shown in Fig. 9, representing Cambrian through early De- vonian, Devonian through early Triassic, and Triassic to present time. The oldest cycle is incomplete but probably 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 cycles.

The later portions of the three cycles shown in Fig. 9 exhibit a nearly linear, but slightly erratic, decrease in mass per Epoch with decreasing age (Fig. 8b, least-squares fits). The Cenozoic trend, which extends back to about Late Cretaceous time, has been interpreted to represent a decline in cratonic carbonate deposition and transposition of that deposition to the deep sea. For the Cenozoic portion of the trend, there are two possible reasons for this change in locus of carbonate deposition (see also, e.g., WILKINSON and WALKER, 1989). First, planktonic, shelled marine protists in the Jurassic may have replaced shallow-water calcareous organisms as the principal sink of carbonate. Second, the general decline of sea level since the late Mesozoic would have reduced the geographic extent of shoal-water carbonate deposition and decreased areas of warm, carbonate-saturated seas. This change would lead to transfer of carbonate from shallow to deep environments. It is likely that the Cenozoic increase in carbonate accumulation in the deep sea and decrease in rates of accumulation in cratonic and other shoal-water settings are a result of both evolution of planktonic calcareous or-

Mass Remaining (10” g Ca y-l)

570 0 10 20 30 40 50 60 70

Percent Flooded Central N. American Craton

FIG. 9. Phanerozoic continental freeboard curve of the North American craton (WISE, 1974) compared with Epoch-interval survival rate of cratonic carbonates calculated as mass of carbonate rock in a Series divided by duration of Epoch (data from WILKINSON and WALKER, 1989). Dark triangle is present-day accumulation rate of shallow-water carbonates (see Fig. I I ) .

ganisms and increasing continental freeboard (relative ele- vation of continents with respect to sea level) since the late Mesozoic. Only a finite reservoir of carbonate components is available for deposition; if the area of shoal-water accumulation is limited and a new sink in marine calcareous plankton provided, the preserved mass per unit time of continental sedimentary carbonate will decrease. BOSS and WILKINSON ( I99 1) also concluded that both biological and eustatic processes are important in determining the locus of carbonate accumulation.

The half-life of 86 million years for post-Permian sedimentary carbonates gives a value of the rate constant for cycling of 0.008 ma-‘, which is about 1.5 times that for the post-Devonian total sedimentary mass of 0.0054 ma-’ ( GREGOR, 1985). This difference is due to the fact that the site of carbonate deposition since the Jurassic has moved progressively toward the deep sea. WILKINSON and WALKER ( 1989) estimate that shallow-water limestone deposition has been decreasing at a rate of 0.04 X 102’ g Ca ma-’ for the past 100 million years. Because it is very likely that the cation fluxes of Ca and Mg have not varied greatly during the Pha- nerozoic (CARRELS and MACKENZIE, 197 la, 1972), and that


the oceans have been saturated with respect to calcite for all Phanerozoic time, this rate reflects, to a first approximation, the increased rate of accumulation of deep-sea carbonate ooze.

The other trends of decreasing epoch mass with decreasing age of continental sedimentary carbonates (Figs. 8, 9) for Cambrian to early Devonian and Carboniferous to early Triassic strata correlate very well with the continental freeboard curve (Fig. 9) for the cratonic interior of the United States and southern Canada. The preserved mass distribution of continental sedimentary carbonates expressed as Epoch mass preserved per year (survival rate) tracks reasonably well the freeboard curve. The terminations of the major cycles are very close in time to the end of the Tippecanoe and Ab- saroka cratonic sequences as defined by SLOSS ( 1963). These sequence boundaries appear to be marked by regional, and probably global, emergences and unconformities. SLOSS ( 1976) has also demonstrated that the sedimentary mass per unit time of a cratonic sequence decreases with decreasing sequence age as the end of the sequence is approached.

It is tempting to argue that there are three phases of decreasing Epoch continental carbonate sedimentary mass with decreasing rock age in the Phanerozoic. The origin of the youngest was discussed above. The origin of the two older phases may be related to one of the assumptions underlying the construction of mass-age models; that is, the assumption that there is a single decay constant for the continental carbonate mass that applies equally well to any unit mass of this carbonate reservoir. It seems likely that this assumption of equal jeopardy of the continental carbonate rock reservoir to destruction may be incorrect. Within each cycle, the younger units of the cycle may be more susceptible to destruction by erosion and thus cycle at a faster rate than the 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 limestones 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 cratonic 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 of occurrence of Phanerozoic ironstones and oijlites (Fig. 10). As sea level withdrew from the continents and continental freeboard increased, shallow-water areas with the req- uisite environmental conditions necessary to form oiilite and ironstone deposits decreased in extent. Thus, as calcium car-

bonate deposition increased on slopes and in the deep sea, carbonate oijlite and ironstone deposition on shelves and banks nearly ceased. Therefore, it appears that global eustasy plays a strong role in controlling the distribution of sedimentary components between deepsea and shallow-water realms of deposition. This conclusion for sedimentary carbonates and the influence of differential cycling on carbonate partitioning between the shallow and deep ocean are discussed in more detail in the following section.

CARBONATE PARTITIONING AND ‘I-HE CO&LIMATE CONNECTION

Modern Carbonate Cycle

In the previous section, we demonstrated that the partitioning of carbonate burial between shoal-water and deep sea realms has probably varied in a cyclic pattern through Phanerozoic time. The variation in the magnitudes of the fluxes of ( Ca,Mg)C03 to the two environments through time is difficult to assess; even today’s fluxes are probably not known within a factor of 2. In Fig. 11 a tentative model of the carbonate carbon cycle in the world’s oceans is shown.

About 18 X 10 I2 moles of Ca2+ and Mg2+ (equivalent to 2 16 X lo6 metric tons of C) accumulate yearly as carbonate minerals (MORSE and MACKENZIE, 1990), mainly as biological precipitates. Of this flux about 6 X 10” moles are deposited as calcium and magnesium carbonates in shoal- water areas (cf. MILLIMAN, 1974; SMITH, 1978), and the remainder accumulates as calcareous oozes in the pelagic realm. The 12 X 10 I2 moles of carbonate accumulated an-

I I I I 1

flme(106y)

FIG. 10. Number of occurrences of Phanerozoic ironstones (upper diagram; data from VAN HOUTON and BHATTACHARYYA, 1982) and of iiolitic limestones (lower diagram; data from WILKINSON et al., 1985) as a function of geologic age. The relative sea level curve is that of HALLAM ( 1984).


Riven , 15”

Accumulation 1

Production Rsef-Bsnk-Shsll

Ocean

Production

Ackhtion

FIG. 11. Tentative model of global carbonate cycle. Fluxes are in units of lOI moles C y-’ as (Ca,Mg)C03. aA~~~~~~ et al. (1988); bM~~~~~~ (1979); ‘SMITH (1978) and MILLIMAN (1974); dCfB~~~~~~~ and PENG (1982); 8B~~~~~~~ and PENG (1982), MILLIMAN ( 1974), and HAY and SOUTHAM ( 1977); hW~~~~~~ and MACKENZIE, (1983).

nually in the deep sea are only about 17% of the annual carbonate production rate of 72 X 10 I2 moles of the open ocean photic zone.

This efficient recycling of carbonate carbon in the open ocean water column and at the sediment-water interface is a well-known feature of the marine carbon cycle (e.g., BROECKER and PENG, 1982). It is important to note that much shoal-water carbonate production ends up in sediments of reefs, banks, etc., so that, in contrast to the pelagic realm, production rate more closely approximates sedimentation rate. There is, however, escape of carbonate sediment from shoal-water areas to the deep sea, where it is deposited or dissolved (e.g., LAND, 1979). The magnitude of this flux is poorly known but may affect the chemistry of open-ocean regions owing to dissolution of the carbonate debris ( DROX-

LER et al., 1988; AGEGIAN et al., 1988). Furthermore, its accumulation on the slopes of banks may act as a record of paleoenvironmental change ( DROXLER et al., 1983).

Phanerozoic Carbonate Fluxes

It is difficult to obtain the carbonate flux and its partitioning between deep-sea and shallow-water areas of deposition throughout Phanerozoic time. One approach is to use the methodology of TARDY et al. ( 1989) and WOLD and HAY

( 1990) to reconstruct sediment fluxes and to apply it to the reconstruction of carbonate accumulation rates. To do so, the Epoch-interval data of WILKINSON and WALKER ( 1989)

were grouped into thirteen time intervals (Table 2). This grouping increases the time constant, improving the resolu- tion of the time series and leading to an improvement in the mass-age correlation. The total carbonate masses remaining expressed in units of 10 I3 g Ca y-’ (survival rate) for the thirteen stratigraphic intervals given in Table 2 (column E) were fit by a simple exponential decay curve having the form

S = Ae-k’, (3)

where S is the mass remaining of the original sediment flux at time t that would be observed today after t years of cycling 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; r = 0.68 and g = 0.20. A is 74.5 X lOI g Ca y-’ and k

(rounded) is 0.0009. This corresponds to a half-life of the Phanerozoic carbonate mass of 350 million years. The reconstructed carbonate accumulation flux is obtained from the relation:

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 carbonate fluxes for various stratigraphic intervals are obvious in Fig. 12 and expected. The continental carbonate mass dominates the total surviving carbonate mass from Cambrian through Early Cretaceous time and is a substantial part of 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.

The reconstructed total carbonate fluxes have varied by no more than a factor of 70% around their mean of 75 X 10 ” g Ca y-’ during Phanerozoic time. This variation, while significant, is not great, and argues for the hypothesis of geochemical uniformitarianism; that is, fluxes of carbonate con- stituents to the ocean may not have varied greatly during the Phanerozoic. This variation could even be smaller, if the reconstructed fluxes do not represent accumulation rates. This proviso is discussed more fully in a later section.

In general, there is a good correlation between the first- order changes in the sea level curves of both VAIL et al., 1977

(Fig. 7) and HALLAM 1984 (Fig. 10) and the survival rates of continental carbonates and total carbonate fluxes through the Phanerozoic from the Neogene to the Devonian. Periods of global low sea level, like those of today and the Permo- Triassic, appear to correlate with minima in the extant mass of continental carbonates and the reconstructed total carbonate fluxes, whereas the Cretaceous sea level transgression correlates with a maximum in these variables. The correlation roughly extends back into the Devonian, beyond which it appears to break down. The early Paleozoic sea level high is represented by minimum values in the survival rate of continental carbonates and calculated total carbonate flux.

Other relationships appear to exist between the mass-age curves of Fig. 12 and sedimentary attributes. WOLD and HAY ( 1990), from their analysis of the mass-age curve for the total Phanerozoic sedimentary mass distribution, argued for a 150 million year cycle in the mass distribution. They suggested that mass maxima at 540,390,240,90, and 6.6 million years before present (BP) reflected real variations in the global rates of erosion and sedimentation. The mass maxima were thought to represent periods of high sedimentation (see arrows, Fig. 12). Furthermore, TARDY et al. (1989), in their analysis of the global water cycle and continental erosion through Phanerozoic time, concluded that global runoff was high in the Cretaceous ( 100 m.y. BP), the Siluro-Devonian

Geologic cycles of carbonate rocks

t

LSL H6L LRO 1

L&L 120

HSR HRO I

/ LRO

1 1 I 4 I\

I I

’ \ HSL LSL

! \ I I

-t t t t t

o~ no2 +uKJ,LKI CR ,111 PI,PIMI ~Isl o,I c 1~1 0 200 400 ,

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 TARDY et 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 the present-day and the Permo-Triassic (200 m.y. BP) were thought to be dry periods. Sedimentation rates were shown to correlate moderately with global runoff, with the Cambrian, Devonian, Cretaceous, and Tertiary periods having the largest sedimentation rates, and the Carboniferous and Permo- Triassic exhibiting the lowest rates (see Fig. 12). The abnor- mally high erosion rates of the Neogene and the present-day probably reflect increased tectonic activity and, during more recent times, may be a consequence of deforestation, agri- cultural practices, and other activities of humankind. Gla- ciation has also played a role. TARDY et al. ( 1989) and WOLD and HAY ( 1990)) in their analyses of the sedimentary mass- age distribution, assume that the present-day mass-age distribution of extant Phanerozoic sedimentary rock can be fitted by a single exponential decay curve such as that of Eqn. (3). This implies a single rate constant for decay of the sedimentary mass and a steady-state constancy of this mass throughout the Phanerozoic. This is the approach used by us to construct the total carbonate mass fluxes (reconstructed fluxes) shown in Fig. 12.

It is apparent from the above discussion that the continental carbonate mass survival curve tracks the first-order changes in sea level from the Neogene to Devonian. During this interval, minima in the survival rate curve correlate with periods of low sea level and maxima with periods of high sea level.

High runoff and sedimentation rates mark the Cretaceous and Devonian maxima in survival rate, whereas low runoff and sedimentation rates are coincident with the low survival rate of the Permo-Triassic. Thus, the continental carbonate mass survival rate appears to bear some relationship to the sea level curve, and runoff and sedimentation fluxes from Neogene to Devonian time. Exceptions to this statement are the low runoff and survival rates, but the high sedimentation rate of the Tertiary, and the relatively high sedimentation rate of the late Paleozoic-early Mesozoic ( WOLD and HAY, 1990)) a time of low sea level. Furthermore, for the Neogene to Devonian interval, the calculated total carbonate flux curve tracks the survival rate curve. These relationships suggest, to a first approximation, that the survival rate curve of continental carbonates reflects changes in the total sedimentation flux of shallow-water carbonates through time and that these changes are related to global eustasy (the “calcite push” model of WILKINSON and WALKER, 1989). High sea levels give rise to high total fluxes of shallow-water carbonates, whereas low sea levels result in transposition of carbonate deposition to deeper depths in the ocean and reduced total shallow-water carbonate fluxes.

It is usually thought that the times of first-order changes in the Phanerozoic sea level curve reflect changes in sea-floor spreading rates, which lead to changes in mid-ocean ridge system volumes and subsequent sea level rise or fall. Thus,

3291

IO


plate tectonic mechanisms control sea level on the time scale of the first-order changes in the Phanerozoic sea level curve. Also, times of rapid seafloor spreading, increased ridge volume and ocean water displacement, and flooding of continents appear to be times of high atmospheric CO2 levels and consequently high temperature ( BERNER et al., 1983; LASAGA et al., 1985; BERNER, 1990), as well as shallow CCD depths. It might be anticipated that because of high tectonic activity and high temperatures, these periods were times of high total sedimentation rate and high fluxes of carbonate components to the ocean. Calcite and dolomite weathering rates are strongly influenced by temperature: the higher the temperature, the more rapid the weathering rate (HARMON et al., 1975; BERNER et al., 1983). Total runoff from continents, a function of both continental position and climatic factors, fluctuated considerably in the Phanerozoic from 0.35 X 10’”

g Y-’ in the Triassic to 0.65 X lo*’ g y-’ in the Devonian (TARDY et al., 1989). During the Cretaceous and Devonian high sea level stands, sufficient continental area was located at humid climatic latitudes to lead to greater runoff, whereas during Triassic time a greater portion of continental area was located in semi-arid and arid climatic regions.

It appears that the higher carbonate weathering rates ac- companying the higher atmospheric CO2 levels, and hence temperature of the Cretaceous and Devonian coupled with higher runoff rates during these periods, led to enhanced fluxes of carbonate components to the ocean and increased carbonate depositional rates. For much of the Permo-Triassic, when sea level and atmospheric CO2 were low and interior deserts widespread, global runoff, riverine carbonate flux, and hence carbonate sedimentation rate were subdued. Again in the Tertiary, comparable conditions prevailed (see TARDY et al., 1989; WOLD and HAY, 1990).

We are now in a position to ask the question: Why does the relationship between sea level and continental carbonate survival rate appear not to hold for pre-Devonian rocks? The survival rate is at a minimum near the Siluro-Devonian boundary and rises gradually toward the early Cambrian, as does the reconstructed global carbonate flux curve (Fig. 12 ). However, Paleozoic sea level reached a high sometime in late Cambrian-early Ordovician time (cf. sea level curves Of VAIL et al., 1977, and HALLAM, 1984; Figs. 7, 10). Therefore, one would expect the survival rate of continental carbonates to be high around 400-500 million years BPif survival rate were truly representative of depositional rate. The Hallam sea level curve may provide a clue to this enigma. In the Hallam curve, there is a sharp withdrawal of sea level from the continent near the Siluro-Devonian boundary at the close of the Cale- donian tectonic cycle (Fig. 10). This feature of the sea level curve, which is also seen in the details of the VAIL et al. ( 1977) curve, is coincident with one of the minima in the continental carbonate survival rate curve (Fig. 12). Thus, the downward trend of the survival rate curve from Cambrian to Silurian may reflect in part sea level fall at the close of the Caledonian tectonic cycle. Furthermore, the difference between the reconstructed total carbonate flux and the survival rate of continental carbonates decreases from Cambrian to Silurian (the “missing flux” of Table 2, column G). If the reconstructed total carbonate sediment flux is a reasonable

approximation of the total flux, this difference represents continental carbonate destroyed by erosion or carbonate that accumulated in the deep sea.

The progressive decrease of the “missing” flux through a long period of high sea level suggests that this trend may represent post-depositional destruction by erosion of the continental carbonate mass. The same relationships are observed during sea level withdrawal from the continents during the latter part of the Hercynian tectonic cycle. It appears that in both cycles, the younger continental carbonates were pref- erentially eroded relative to older carbonates, which were covered by overlying sediments and perhaps protected in the deeper regions of tectonic basins. Notice that for the Late Cretaceous and Tertiary part of the Alpine tectonic cycle the calculated “missing” flux is close to the deep-sea extant carbonate mass (Table 2, column H). This similarity suggests that for this period of time, the “missing flux” is the carbonate deposited in the deep sea. In other words, erosion of post- Cretaceous continental carbonates has not been great. Thus, it appears that the overall trend of decreasing survival rate of continental carbonates for the latter part of the Alpine tectonic cycle is due to sea level fall and transposition of carbonate accumulation to the deep sea. However, for the latter portion of the Hercynian cycle, and particularly for the latter portion of the Caledonian cycle, survival rate and hence calculated total carbonate flux (reconstructed flux) appear to be strongly influenced by differential cycling by erosion of the younger carbonate strata in the cycles. If this assertion is true, it may be difficult to determine accurately the accumulation rates of shallow-water and deep-sea carbonates through much of Phanerozoic time.

Survival Rate and Atmospheric COZ

Both VOLK ( 1989) and BERNER (1990) have discussed the sensitivity of atmospheric CO*, and hence climate, to the partitioning of carbonate burial between the shallow- and deep-ocean realms. In both models it was shown that changes in the CO2 degassing rate, because of the Urey reaction (UREY, 1956),

CaCO, + Si02 = Casio3 f CO2, (4)

are significantly affected on a multi-million year time scale by shifts in the site of carbonate deposition from platforms to the deep sea and vice versa. VOLK ( 1989) further argued that if the open ocean were not inhabited by pelagic fora- minifera and coccolithophores, the CO2 levels of today would be lower and the climate colder. Carbonates deposited in the deep sea are a warming factor on climate change over a multi- million time scale. This is because of their subsequent subduction and decarbonation, leading to increased rates of CO2 degassing to earth’s surface and a subsequent warming owing to changes in the earth’s radiation balance (“greenhouse effect”). Both VOLK ( 1989) and BERNER ( 1990) call on the plate tectonic process of subduction to transport deep-sea carbonates to depths where pressure and temperature are high enough to convert carbonate to silicate by the UREY ( 1956) reaction. However, VOLK, ( 1989) (see also MACKENZIE and PIGOTT, I98 1) also argued that metamorphic decarbonation


is a degassing source of CO2 from sedimentary piles of continental carbonates.

If the metamorphism of carbonates on subducting slabs were the principal source of CO* outgassing at plate boundaries ( MACKENZIE and PIGOTT, 198 1; VOLK, 1989; BERNER, 1990)) and if the deep-sea burial of carbonates were a warming factor ( VOLK, 1989) during the Cenozoic, then the cyclic pattern of Phanerozoic extant, shoal-water carbonate mass shown in Figs. 9 and 12 might be linked to CO* degassing rates and hence climate change. This statement implies that the preserved mass per unit time of shoal-water carbonates can be interpreted strictly as a primary feature of the sedimentary rock column; that is, the mass per unit time preserved is the mass deposited per unit time. This is surely not the case, but it can be taken as an extreme position for the sake of argument. Then, if total carbonate accumulation were constant through time, the transposition of carbonate from shoal-water areas to the deep sea during the past 100 million years occurred at a rate of 0.04 X 10” g Ca my-‘, the Mis- sissippian to Triassic rate was 0.03 X 10 I3 g Ca my-‘, and that of the Cambrian to Silurian was about 0.0 1 X 10 *’ g Ca my-’ . The present total shoal-water carbonate accumulation rate expressed as g Ca my-’ obtained by extrapolation of the nearly linear portion of the latest cycle in cratonic accumulation is equivalent to 30 X 10 I3 g y -’ ; the tentative cycle of carbonate carbon shown in Fig. 11 gives a value of 24 X 10 I3

g Ca y-’ (marked with black triangles in Figs. 9 and 12). These estimates are independent, reasonably close, and provide some feeling for confidence in the estimated values.

It is interesting to note that the Cambrian-to-Silurian and Mississippian-to-Triassic carbonate transpositions precede and overlap maxima in the BERNER ( 1990) calculated atmospheric CO2 trend for the Phanerozoic. This would be expected, because there is a varying delay between carbonate deposition on the seafloor and subduction to depths necessary for conversion of CaCO, to a calcium-bearing silicate and release of C02. This delay can be on the order of tens of milliOnSOf years( VOLK, 1989).

Because pre-Jurassic deep-sea carbonates are virtually ab- sent from the geologic record and because the mass-age distribution curve of Phanerozoic continental carbonates not only represents changes in carbonate accumulation fluxes but also differential cycling, it will be difficult to obtain quantitative estimates of the partitioning of carbonate between deep-sea and shallow-water areas of deposition through geologic time. Geochemical models (VOLK, 1989; BERNER, 1990) that attempt to describe atmospheric CO2 variations over geologic time may be constrained by this limitation, because the partitioning of carbonate between the deep and shallow ocean in these models will require parameterization and assumptions.

CONCLUSIONS

It appears that plate tectonic processes play an important role in the origin and distribution of sedimentary carbonates through Phanerozoic time. Since the late Mesozoic, the site of carbonate deposition has progressively, but erratically, moved toward the deep sea. This has resulted in differential

cycling of carbonates, because of subduction of oceanic carbonates, at a rate about two times that of the total sedimentary mass. Within the total Phanerozoic carbonate mass, the age distribution of sedimentary properties like calcite/dolomite ratio, inferred oiiid mineralogy, and survival rate of continental carbonates is cyclic. The cycles appear to be coupled to plate tectonic processes that result in sea level change and changes in the properties of the ocean-atmosphere system. However, the mass-age distribution of continental carbonates does not appear to be simply a primary feature of the sedimentary lithosphere but is governed in part by increased probability of erosion of the younger strata within the three main tectonic cycles (Caledonian, Hercynian, and Alpine). This interpretation implies that simple exponential decay models with a single recycling constant cannot be used with- out reservation to describe the global carbonate mass-age distribution. It is possible that this statement is also true for the total sedimentary rock mass-age distribution, as suggested earlier by GARRELS and MACKENZIE ( 197 lb). If true for the carbonate record, then interpretation of this record in terms of partitioning of carbonate between deep-sea and shallow- water realms during the Phanerozoic is difficult. Further quantification of geochemical models describing the geologic history of atmospheric CO2 and climate change necessitates consideration of the difficulties inherent in obtaining carbonate depositional fluxes and their partitioning through Phanerozoic time.

Acknowledgments-One of Bob Garrels’s favorite mementos was a 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 in the effort of Sisyphus, much as he saw progress in scholarship and life in general. Many of us push the stone of scholarship ahead a little at a time, building on the efforts of those who have gone before and our colleagues; Bob forged ahead. We are grateful to him for having 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 especially thank Bryan Gregor for his very thoughtful review of an earlier version of this manuscript, in particular for his suggested re- visions of Table 1 and for providing Table 3.

This research was supported in part by NSF grant EAR-8816350. The manuscript was completed while FTM held a Visiting Scientist Fellowship, Capital Region of Bruxelles, Belgium. School of Ocean and Earth Science Contribution No. 2902, University of Hawaii.

Editorial handling: H. C. Helgeson

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