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Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO2 pressure
EPSL ELSEVIER
Earth and Planetary Science Letters 137 (I 996) 7 I - 82
Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO, pressure Crayton J. Yapp *, Harald Poths 2 Depurtment
of Earth
und Plunetury Sciences. University ofNew Mexico. Alhuyueryur,
NM 87131. USA
Received I9 June 1995; accepted 3 November I995
Abstract Abundance and carbon isotope data from an Fe(CO,)OH component in apparent solid solution in oolitic goethites have been used to infer ancient atmospheric CO, pressures. A test of the validity of these estimates might be comparisons of the carbon isotope compositions of Fe(CO,)OH in oolitic goethites with time-equivalent pedogenic calcites. Temporal trends of the oolitic goethite and pedogenic calcite 6 13C values are generally similar, but time-equivalent samples from each of these two groups are not common in the existing data. To facilitate discussion of the concept, comparisons were made of available goethite and calcite samples even though ages of the compared samples in each pair were not identical. In four out of the five comparisons, Fe(CO,)OH abundance and 6 13C data were combined with pedogenic calcite 6 13C data to calculate physically reasonable soil CO, concentrations for the ancient calcitic soils. This suggests that the compared oolitic goethite and pedogenic calcite systems were responding to the same global scale phenomenon (i.e., atmospheric CO,). Atmospheric P coI as determined from the goethites in these four “well-behaved” cases ranged from values indistinguishable from modem (within analytical uncertainty) to values up to approximately 16 times modem (modem atmospheric PC0 was taken to be 1O-3.5 atm). One interpretation of the fifth, “anomalous”, comparison is that atmospheric CO, levels incieased from about 3 times modem to about 18 times modem from the Triassic into the Early Jurassic. This inferred value for the PC0 of the Early Jurassic atmosphere is not uniquely constrained by the existing data and needs to be substantiated. However, eten considerably lower Early Jurassic atmospheric PC0 values of 6 to 9 times modem (i.e., l/3 to l/2 of the estimated value of 18 times modem) would still indicate significant hifferences between the global carbon cycles then and now. These results highlight the need for more research on the behavior of the atmosphere during and after the Triassic-Jurassic transition.
1. Introduction Carbon dioxide (CO,) is one of the “greenhouse” gases. It participates in the radiative energy balance
’ Present address: Department of Geological Sciences, Southern Methodist, University, Dallas, TX 75275, USA. ‘Present address: CST 7, Mail Stoo J 514. Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
of the Earth’s atmosphere and thereby contributes to the maintenance of the atmospheric temperatures characteristic of the global climate system. The effeet of higher concentrations of CO, on global climate is a question of considerable interest, because of the increase in atmospheric CO, levels since the beginning of the Industrial Revolution [I]. The comparatively brief period of time represented by this interval and the difficulty of reconstructing globally significant climatic fluctuations from the contempo-
0012-821X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00213-8
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and Plunetary Science Letters 137 (1996) 71-82
raneous but rather fragmentary instrumental temperature data complicate the task of determining direct causality between changing atmospheric chemistry and changing global climate. Adding to the complexity is the uncertain, but potentially important, role of variations in solar activity [2]. The approximately 25% increase in atmospheric CO, that has occurred over the last 170 years appears to be unprecedented in its rapidity, but not in its magnitude [3-51. Evidence from ancient surficial geological systems suggests that atmospheric CO, levels up to 16 times higher than modern existed in different epochs over the past 440 Myr [4-61 and may have influenced global climate [7,8]. This evidence is preserved in two minerals from continental weathering environments that have shown promise as CO, indicators, goethite and calcite. The common mineral goethite (a-FeOOH) contains an Fe(CO,)OH component in apparent solid solution. The amount and 6 13C value of this component appear to reflect the partial pressure and 6 j3C value, respectively, of the CO, present in the ambient environment at the time of goethite crystallization [9,10]. Goethite which formed in subaerial
weathering systems could preserve information on the partial pressure of CO, in the Earth’s ancient atmosphere [5,11]. Such goethites may be present in well-preserved ooids from ironstone deposits formed at various times in the Phanerozoic [ 121. In the current paper, we present analyses of the amount and 6 “C values of the Fe(CO,)OH component in oolitic goethites from ironstones of Ordovician, Devonian, Jurassic, and Cretaceous age. These data were interpreted in terms of ancient atmospheric CO, pressures and are compared with published carbon isotope data from pedogenic calcites. Through this comparison with oolitic goethites from wet environments, concentrations of ancient soil CO, were calculated for the relatively dry, calcite-forming soils.
2. Samples The oolitic goethites of this study are from a wide geographic range. Their locations, approximate numerical ages, and the analyzed amounts and S 13C values of the Fe(CO,)OH component in each are listed in Table 1. One non-oolitic, lateritic goethite
Table 1 Oolitic goethite data
_Fe(CO,)OH_ @C Sample
Location
Epoch
IRMWis-I
Wisconsin, USA
Late Ordovician
Ma 440
x 0.00645
Nebraska, USA
Late Devonian
360
Israel
Early Jurassic
195
Boice-
I
SIsra-1
France
IIJCS,,
Ornanic carbon
ooid
6°C (So)
s”c, (960)
P(cod*
-17.6
ooid9 --26.7
coals
155
notie
-20.0
16
0.0175
57
-21.5
-28.0
-28.0
-21.3
1
0.0097
103
-15.3
-23.5
-23.4
-16.8
18 18
LorFra- 1
Lorraine,
Early Middle Jurassic
185
0 0088
114
-15.0
-23.5
-23.3
-16.8
MSwitz-I
Switzerland
Late Middle Jurassic
165
0.0120
83
-17.2
-24.2
-23.3
-17.5
1
MSwitz-3
Switzerland
Late Middle Jurassic
165
0.0099
101
-17.3
-23.6
-23 3
-16.9
1
CHAlb-I
Alberta. Canada
Late Cretaceous
CBraz- 1
Carajas. Brazil
Late Tertiary
80 5
00103
97
-19.7
-26.7
-26.2
-20.0
4
0.0108
93
-21.7
-28.4
-27.2
NA
1
“Ma” indicates millions of years before the present and refers to the approximate ages of the stratigraphic units. Other terms in the column headings are defined in the text. IRMWis-I organic carbon was calculated from the 6°C value obtained by extrapolation of the Neda Fm array [I 11. Boice-1 organic carbon was calculated using the 250°C fraction and the 850°C fraction from MHD-1424. SIsra-1 organic carbon was assumed to be the same as LorFra-I organic carbon. LorFra-1 organic carbon was calculated from the final 230°C fraction and the 850°C fraction of MHD- 1354. MSwitz- 1 organic carbon was calculated using the weighted average Fe(CO,)OH and 850°C fraction from MHD- 1324. MSwitz-3 organic carbon was calculated using MSwitz-3 data from MHD-1450 (in the same manner as for MSwitz-1). CHAlb-1 and CBraz-1 organic carbon were measured as discussed in the text. CBraz-1 is a non-oolitic, lateritic goethite.
CJ. Yupp. H. Paths/Earth
und Planetury Science Letters 137 (1996) 71-82
of late Tertiary age was analyzed to serve as a near-modem comparison with the older ooids. The individuals who provided samples are listed in the acknowledgments. These individuals also provided time-stratigraphic information on the samples. IRMWis-1 is of Ashgillian age (Late Ordovician). Boice-1 is of Fammenian age (Late Devonian). SIsra-1 is of the Early Jurassic epoch, but we have no information on its stratigraphic age within that epoch. LorFra-I is of Aalenian age (early Middle Jurassic). MSwitz-1 and MSwitz-3 are of Callovian age (late Middle Jurassic). CHAlb-1 is Late Cretaceous, but its stratigraphic age within the Late Cretaceous is not known. The stratigraphic epoch of each oolitic ironstone is listed in Table 1. Time-stratigraphic units provide information about the relative antiquity of the samples. However, because the time-stratigraphic units of SIsra-1 and CHAlb-1 are resolved here for “epochs” rather than “ages”, their assigned numerical ages would be expected to have a larger range of possible values than would those of the other samples. This leads to larger uncertainty about the magnitude of numerical age differences between other samples and either SIsra-I or CHAlb-1. For example, although from a relative stratigraphic point of view SIsra-1 is older than LorFra- 1, it might be as little as l-2 Myr older, or as much as 20 Myr older. For consistency, SIsra-1 and CHAlb-I were assigned numerical ages which correspond approximately to the centers of their respective epochs. The approximate numerical ages of each sample are listed in Table 1 and are from the Geological Society of America’s geologic time scale.
3. Experimental Prior to analysis, ooids were physically separated from matrix material. The analyses were then performed according to published procedures [ 10,l 1,131. The amount of Fe(CO,)OH is reported as a mole fraction ( X) of the total FeOOH + Fe(CO,)OH system in each sample. 6 13C is the conventional representation of the per mil relative difference beween the sample ‘“C/ 12C ratio and that of the intemational PDB standard [ 141. The analytical uncertainty in the reported value of X is about f0.0002, while for 6 13C it is about k 0.2 per mil. The ooids from
73
the Late Devonian (Boice-1) ironstone contain a relatively small amount of goethite (about 6 wt%). Because of the small sample size, the uncertainty in the X and 613C values of the Fe(CO,)OH component in this sample may be larger. 3.1. Soil CO, mixing model Steady-state solutions to the one-dimensional Fickian diffusion equation provide good representations of depth-dependent CO, concentrations in modem soils [ 151. In these models, the Earth’s atmospheric CO, is the upper boundary condition, and there is a depth-dependent CO, production term which describes the oxidation of organic carbon in the soil [15]. Thus, the CO, in soil gas can generally be regarded as a mixture of two endmembers (atmospheric CO, and CO, derived from the oxidation of biological material) [4,11,16]. For the Fe(CO,)OH component in pedogenic goethite as a proxy for soil CO,, the two-endmember mixing equation can be expressed as follows [5,11]: 6’3C,
= (s’?,
- 6’3C,)X,(
l/X,)
+ 6’3Co (1)
where S 13Cm is the measured S 13C value of the Fe(CO,)OH component in goethite; 6 13CA is the 6 13C value the Fe(CO,)OH component would have if atmospheric CO, were the only CO, in the soil: 613Co is the 613C value the Fe(CO,)OH component would have if CO, from oxidation of biological material were the only CO, in the soil; X, is the measured value of X for the Fe(CO,)OH component in goethite; and X, is the value of X for the Fe(CO,)OH component if atmospheric CO, were the only CO, in the soil. Goethite samples from the shallower portions of any particular soil would be expected to yield a linear data array in a plot of 6 13C,,, vs. l/X,, if Eq. (1) is valid as a descriptor of ancient weathering systems [51. Such an apparently linear array was measured for the Late Ordovician Neda Formation oolitic ironstone by Yapp and Poths [5,11]. According to Eq. (11, the slope of such an array will be equal to (6’3C, - S’3Co)X,. If (S13C, - S13Co) can be independently determined, the remaining unknown (X,) can be calculated from the slope of the
74
CJ. Yupp, H. Poths/Eurth
and Planetary Science Letters 137 (1996) 71-82
line. The Henry’s Law expression of Yapp and Poths [5] might then be used to calculate an apparent P(C0,) for the Earth’s ancient atmosphere. This approach was used by Yapp and Poths to estimate that the P(C0,) of the Earth’s atmosphere in the Late Ordovician was about 16-17 times higher than the P(C0, ) in the Earth’s modem atmosphere [5,1 I]. This value is in approximate agreement with a value recently calculated by Bemer for the Late Ordovician using a model of the global carbon flux [3]. The shallower portions of ancient weathering profiles are not commonly preserved in oolitic goethite systems. Therefore, a single-profile data array of S 13Cm vs. l/X, can not usually be measured in these deposits. However, if the value of S 13Co were known for a particular deposit (i.e., the intercept on a S 13C vs. l/X plot), it could be combined with measured S”C, and l/X, values from a single, ,well-preserved sample in that deposit to produce a relevant two-point straight line. Prior to the incremental vacuum dehydration of goethite to extract CO, from the Fe(CO,)OH component at 200-3OO”C, samples were treated with concentrated hydrogen peroxide (30%) at room temperature to remove accessible, admixed organic carbon [lo]. For the late Tertiary sample (CBraz- l), the S 13C value of the accessible organic carbon was determined from material balance by measuring the abundance and S 13C values of total carbon before and after H,O, treatment. One aliquot of the Late Cretaceous sample (CHAlb-1) was not treated with H,O,. The S 13C value of the organic carbon in this aliquot was measured after removal of goethite by dissolution in concentrated hydrochloric acid (- 15 N) at room temperature. The organic carbon S 13C values thus measured for these two samples are listed in Table 1. With increasing age of the deposits, however, the possibility increases that the accessible organic carbon is comprised, at least in part, of younger, adventitious organic matter whose S 13C values are different from those of the original soil biological carbon. For some s,amples, there was isotopic evidence for a small amount of organic matter remaining in the H,O,-treated goethite. This comparatively inaccessible, residual organic matter was not measurably re.moved during low-temperature (< 300°C) incremental vacuum dehydration, but appears to be oxidized
to CO, in the terminal 850°C step [ 101.The “refractory” organic carbon manifests itself through increased amounts (normalized to hydrogen) and decreased S 13C values of the CO, recovered in the 850°C step [IO]. The inaccessibility of this refractory organic carbon suggests that it could be a remnant of the biological carbon present at the time of goethite formation. Its S 13Cvalue was calculated by material balance. The calculation assumed that the measured molar CO,/H,O ratio (F) in the 850°C step increased because of the addition of CO, originating from the oxidation of the refractory organic carbon. It was also assumed that a small amount of residual goethite was the only source of hydrogen in the 850°C increment. An example of results from an incremental dehydration is shown in Fig. 1 for the oolitic goethite MSwitz-1. The relative increase in F and decrease in S 13C of the 850°C increment is readily apparent in this sample and is attributed to the refractory organic matter. Three oolitic goethite samples were not suitable for determination of organic carbon S 13C values in the 850°C increment. Because of the presence of significant kaolinite in the SIsra-1 sample, it was not possible to use the measured CO,/H,O ratio of the 850°C dehydration step to calculate a credible S13C value for the organic carbon. The S 13C value determined for the refractory organic matter in LorFra-1 was assumed to represent organic carbon S 13C values in SIsra-1, because LorFra-1 is the closest in age to Slsra-1. In addition, because of excessive error introduced by extremely small amounts of organic carbon in the 850°C increments, no organic carbon S13C values were determined for the Late Ordovician (IRMWis-1) and Late Cretaceous (CHAlb-1) ooids by this mass balance method. However, the aforementioned linear array permitted extrapolation to an endmember S 13Co value for IRMWis-1 from which the organic carbon S 13Cvalue was calculated [ 111.The organic carbon in CHAlb-1 was determined as discussed previously. These various S ‘3C values for the organic carbon in oolitic goethite are listed in Table 1. With present analytical methods, S 13C values of refractory organic carbon, calculated from material balance, have an uncertainty of about f0.5 per mil. The precision is about +0.2 per mil for the S 13C values measured for the “accessible” organic carbon in the CHAlb-1 and CBraz-1 samples.
CJ. Yupp, H. Poths/Eurth
and Planetary Science Letters 137 (1996) 71-82
MSwitz-1 (MHD-1324)
O.l? WC
-Jo.10
-
0.03
F - 0.06
-18
- 0.04
- 0.02
-24' 0
1 02
0.4
0.6
0.8
lO.cQ I
UH3
Fig. 1. Plots of S13C vs. X,(H,) and F vs. X,(H,) are shown for the incremental dehydration of oolitic goethite MSwitz- I. The 613C value is that of the CO, recovered during an increment of the dehydration. F is the molar ratio of CO, to H,O recovered in a particular increment. X,(H,) is a progress variable which indicates the degree of conversion of goethite to hematite as measured by the loss of hydrogen (reported as hydrogen removed as a mole fraction of the total initial hydrogen, see [ 1 I]). X,(H,) = 0 when no goethite has been converted, and X,(H,)= 1 for complete conversion of goethite to hematite. The “plateau” values of 613C and F are presumed to represent the CO, recovered from the Fe(CO,)OH component in goethite during the vacuum dehydration steps at temperatures < 300°C. The range of X,(H,) encompassed by the four increments considered to constitute these plateaus is indicated by the double arrow. The terminal 850°C step is presumed to have a more negative 6 13C value and a higher value of F, because of a contribution of CO, from the oxidation of “refractory” organic carbon.
An attempt was also made to estimate ancient continental organic carbon S 13C values from an independent source. Because of the large quantities of organic carbon present in coal deposits, their S 13C values may be less susceptible to modification by addition of younger contaminants. Therefore, S 13C data from ancient coal deposits might provide some general insight into the isotopic composition of biological carbon from relatively wet continental environments at different times in the Phanerozoic. C.W. Holmes of the US Geological Survey in Denver, Colorado, provided access to his 6 13C data on coal
7s
deposits from the Devonian to the Tertiary. A sixthorder polynomial was fit by least squares to those coal data. That curve was used to estimate S 13C values of coal for times corresponding to the ages of the oolitic goethites. The uncertainty of these 613C values as estimates of the average ancient organic matter on the continents is probably no better than f0.5 per mil. The nominal S 13C values of the relevant coals are listed in Table 1 and plotted against corresponding 6 13C values of organic carbon from oolitic goethite in Fig. 2. Although there is scatter in the data, the 613C values of the coal and the S 13C values of the organic carbon in goethite generally covary. Unless it is accidental, this approximate covariance suggests that the 6 13C values of organic carbon from the oolitic goethite may be representative of the biological carbon present in the ancient weathering profiles. These organic carbon 613C values from the oolitic goethite were used to determine the 6 13Co values which are required by Eq. (1). The 613Co values were calculated using the expression of Yapp and Poths [l 11, which incorporates the isotopic fractionation associated with diffusive loss of biologically-derived CO, from soils to the atmosphere [17] and the isotopic fractionation between the Fe(CO,)OH component and soil CO,. The Late Ordovician S 13Co value was taken directly
-22
Fig. 2. A plot of the estimated 6 13C values of coal from different times in the Phanerozoic against the corresponding S13C values of organic carbon associated with the goethites of this study (see also Table 1). The dashed reference line is y = x. Estimated uncertainties in the 6 13C values are also depicted.
76
CJ. Yupp, H. Poths/Eurth
. Ancient
Oolitic
urul Planetury Science Letters 137 (1996) 71-82
.._.----
Gxtbites
-15 __--a
-%&a
Fig. 3. A plot of 6 13C vs. l/X for oolitic goethites of different ages and locales. The analytical uncertainties for S 13C and 1/X lie within the dimensions of the filled squares representing each data point. The straight lines are drawn between the measured values (H) and the presumed value of 6 13Co (at 1/X = 0) at the times of formation of the oolitic goethites. These lines represent two-endmember CO, mixing in ancient soils between biologically-derived CO, and CO, from the Earth’s atmosphere. A steeper slope should correspond to higher partial pressures of atmospheric CO, in the past. The numbers in parentheses are the approximate numerical ages of the ironstone deposits represented by the data points of each line (see Table 1).
from Yapp and Poths [I 11. There are a number of uncertainties associated with the use of the S 13Co values of Table 1, not the least of which is the assumption that these values reflect the original or-
ganic carbon in the ancient soils. Nevertheless, in the absence of better information, these assumptions will be adopted as working hypotheses. 6 13C,,, is plotted against l/X, in Fig. 3 (data from Table 1). The lines in Fig. 3 are the straight lines drawn between the measured goethite data points and the corresponding S 13Co values from Table 1. The single line drawn through the data points for the samples from Israel and France is shown with an age of 190 Ma, which is the mean of the approximate ages of these two deposits (Table 1). The S 13C,,, and nominal 6 13Co values of Boice-1 and MSwitz-3 (Table 1) suggest that lines with slightly negative slopes should be drawn for these samples. No such lines were drawn, since that would imply negative values for the partial pressure of CO, in the Earth’s atmosphere. Moreover, with the uncertainty in 613Co values, the apparent atmospheric P(C0,) values for Boice-1 and MSwitz-3 cannot be distinguished from a value equivalent to the modem CO, pressure. Preliminary temperatures of formation have been determined for the samples from Wisconsin, Israel and France from oxygen isotope data obtained in this laboratory. These calculated temperatures are all within three degrees of 25°C (i.e., within analytical error, they are the same). A temperature of 25°C will be adopted as the temperature of formation of all the deposits of Table 1. Such a temperature is consistent
1
Atmqhenc CO, (fmn colitic gcethibs)
16 x &
12
8
MO
450
403
350
303
250
Ma (B.P.)
200
lx!
loo
M
_ 1 0
Fig. 4. P(CO,)* is plotted against the age of the goethite deposit from which the P(CO,)* value was determined. P(C0,) * is the ratio of the partial pressure of CO, in the Earth’s ancient atmosphere to the partial pressure of CO, in the modem atmosphere. Pco, for the modem atmosphere was taken to be 10-a 5 amt. Vertical lines associated with the data points represent the esimated uncertainties in the P(C0,) * values arising from the uncertainties in the respective values of S 13Co.
CJ. Yapp. H. Poths/Eurth
and Plunetary
with the conditions of formation of modem laterites summarized by Tardy et al. [18]. The value of ( S13C, - 6 13Co) in Eq. (1) is assumed to be the same at all times in the Phanerozoic for C, photosynthesis. A value of + 16 per mil was adopted, because it represents the approximate difference between the estimated Si3C values of pre-industrial atmospheric CO, (- 6.5) and recent C, continental biota (- 271, after the latter value is adjusted for the diffusive 4.4 per mil increase in 6 13C in the biologically-derived CO, of the soil [4,17]. Although Popp et al. [19] have suggested that the 6 13C values of terrestrial, vascular C, plants will be relatively insensitive to the partial pressure of atmospheric CO,, the 6 13C values of this ancient organic matter might have varied in concert with 6 13C values of atmospheric CO, [20]. This provides some support for the assumption that (6 13CA 6 ‘3Co) was approximately constant throughout the time of interest. Furthermore, evidence from fossils indicates that C, photosynthesis did not become important (and then principally in warm, dry climates) until the Miocene epoch [4]. Therefore, the ancient environments represented by the goethite systems in this study were probably dominated by C, photosynthesis (as found in coal-forming plants). The different slopes of the lines drawn in Fig. 3 imply that atmospheric PC0 values were different at different times in the Phankrozoic. As indicated by Eq. (11, higher slopes suggest higher values of X, and, therefore, higher values of atmospheric PC0 . Using a temperature of 25°C and the Henry’s Law equation of Yapp and Poths [5], numerical values of ancient atmospheric PC0 were calculated from the slopes of the lines in Fig! 3. These calculated values were normalized to the modem atmospheric PC0 and designated as P(CO,)* (modem Pco, wai taken to be 10-3.5 atm). P(CO,)* values and estimated uncertainties are plotted against age in Fig. 4. The single P(C0,) * value shown for 165 Ma in Fig. 4 represents the single line in Fig. 3 drawn through the data points for MSwitz-1 and MSwitz-3, whose ages and S13C values are analytically indistinguishable. For reference, an additional data point is shown in Fig. 4 representing modem atmospheric = 1). The nominal ancient CO, (i.e., P(C0,)” P(CO,)* values in Fig. 4 range from about 1 to 18. Atmospheric CO, seems to have decreased from 16
Science Letters 137 (1996)
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77
times modem at 440 Ma to about modem values at approximately 360 Ma. The nominal P(C0,) * value of 18 in the Early Jurassic is the highest value for Phanerozoic atmospheric CO, reported thus far from studies of geochemical indicators of P(C0,). It is also much higher than the value calculated by Bemer [3] for the Early Jurassic. The fact that oolitic goethites from two widely separated locales (France and Israel) seem to indicate the same apparently high value for atmospheric CO, about 190 Myr ago lends credence to the result. Yet, because of uncertainty about the “true” value for 6 13Co, the interpretation of these Jurassic oolitic goethite data in terms of atmospheric CO, is not uniquely constrained. 3.2. Isotopic comparison of oolitic goethite and pedogenic calcite 613C values of pedogenic calcite have been used by several investigators to estimate atmospheric CO, abundances from different times in the Phanerozoic [4,6,21-241. No measurement equivalent to goethite’s X, value can be made for calcite, because of the fixed activity of the carbonate molecule in calcite. However, a comparison of 6 13C values for timeequivalent pedogenic calcite and oolitic goethite samples could be especially interesting because of the contrasting environments of formation. Warm, very wet environments produce authigenic goethite [18], while pedogenic calcite forms in dry climates [25]. The relatively small number of published 6 13C data from both ancient oolitic goethites and pedogenie calcites precludes the comparison of precisely time-equivalent samples. However, general comparisons can be made for the data sets plotted in Fig. 5. The 6 13C data for the Fe(CO,)OH component in oolitic goethite are from Table 1. Pedogenic calcite data in Fig. 5 are from the literature [4,6,21-241. The range of calcite 6 13C values for different samples of the same age could be a consequence of different temperatures of formation, different 6°C values of organic matter, and/or different concentrations of ancient soil CO, that was derived from oxidation of organic matter [4]. Lower calcite 613C values might imply higher concentrations of soil CO, from organic matter and possibly soil environments with relatively more moisture [4]. To simplify the comparison of pedogenic calcite and oolitic goethite of
CJ.
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0
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can be formulated for goethite and calcite-bearing soils, respectively:
I
-5
S’3Crn(G0’= ( ~‘3C*(G0) - ~‘3Co(o,,)(C*/Cs),, + 6 ‘3CO(G0)
-10
S13CMcc) =
g " % -15
6’3C
+s13c
n.
-20 ocelute
-25 SW
(
400
300
200
Ma(B.P.) Fig. 5. Measured 6 “C values of the Fe(CO,)OH component in the oolitic goethites of this work (m ) plotted against approximate age of formation. A = published 613C values of pedogenic calcites [4,6,21-241. There appears to be an overall correspondence of the temporal 6 13C variations of oolitic goethites and pedogenic calcites. The goetbite data point at ca. 5 Ma is from Brazil and was provided by M. Bird. It was included as an example of a non-oolitic goethite from a lateritic weathering envi“young”, ronment. It is not compared with “young” pedogenic calcites, because of the common influence of C, plants in post-Miocene calcitic soils [4,22].
(2)
A(cc)- S'3c0(cc))(CA/CS)cc O(CC)
(3)
The 613C values subscripted with “m”, “A”, and “0” are defined in the manner discussed for Eq. (1). For Eq. (2). the subscript “(Go)” refers to the Fe(CO,)OH component in goethite. The subscript “(cc)” in Eq. (3) indicates calcite. C, refers to the concentration of CO, gas in the soil (goethitic or calcitic as indicated) if the only contribution to soil CO, were from the Earth’s ancient atmosphere. Cs was the actual concentration of ancient soil CO, gas from both sources (atmosphere and organic matter). If goethitic and calcitic soils formed at the same time in two different locales, they would have the same C, value for a globally well-mixed atmosphere (i.e., CA(oo’= C+’ ). For such time-equivalent samples, Eqs. (2) and (3) could be combined to yield the following expression:
6 13cm(cc) = ! S’3Cm(oo) - 6 ‘3CO(Go))(CS(Co)/CS(cc)) similar ages, the most negative published calcite S 13C value will be considered in each case. This could give more credibility to the assumption that the organic matter in the compared goethitic (wet environments) and calcitic soils had the same S 13C values. A dashed line connects these calcite data in Fig. 5. Such a line also connects the goethite data of Fig. 5. No line was drawn across the gap in goethite data between 190 and 360 Ma. There is an overall similarity in the temporal changes of the calcite and goethite data as depicted by the dashed lines in Fig. 5. This may indicate that these contrasting environments were responding to the same global-scale phenomena through much of the Phanerozoic. The possibility can be explored further with the aid of the two-endmember soil CO, mixing model described previously. The following two-component mixing equations
+ s13c O(G0)+ A’3Co(cc- Go)
(4)
where A13COccc_Goj is 8’3Co(ccj - 6’3Co(oo’. If the 6 13C values of soil organic matter are the same for time-equivalent samples, and if both soil systems are dominated by steady-state diffusive transport of CO, out of the soil, the value of A’3Co(,,_Goj is approximately the difference (in per mil) between the calcite-CO, and Fe(CO,)OH-CO, fractionation factors. At equilibrium and 25°C calcite is enriched in 13C by about 10 per mil relative to gaseous CO, 11261,while Fe(CO,)OH is enriched in 13C by about 2.5 per mil [ 111.Therefore, the value of A13C,,, _ Goj is expected to be about +7.5 at 25°C. This value will be used in Eq. (4). For the data in Fig. 5, there are only two instances in which approximate timeequivalence of oolitic goethite and pedogenic calcite samples is assumed (Late Devonian and Late Creta-
CJ. Yapp, H. Paths/Earth
19
and Planetary Science Letters 137 (1996) 71-82
soil CO, that originated from oxidation of ancient biological carbon. C, for the calcitic soils is plotted in Fig. 6, as is C, for the goethitic systems. The latter quantity was calculated by subtracting atmospheric CO, concentrations from CS(ooj values which were determined from X,. The C, values for the calcitic soils are plotted at the same ages as the oolitic goethites with which they were compared. These are the hypothetical C, values that calcitic soils (with these 6 13C values) would have possessed, if they were truly time-equivalent with the oolitic goethites and had organic matter with the same isotopic composition. The high values of C, for the ancient oolitic goethites in Fig. 6 are also found in modem, tropical, lateritic soils [27]. The commonly observed C a range for modern calcitic soils [4] is enclosed by the horizontal dashed lines in Fig. 6. Of the five ancient calcitic C B values, four are associated with this modem calcitic soil range. Such consistency could indicate that the assumptions used to determine calcitic C B values were approximately correct. The fifth calcitic C, value at 190 Ma is about 40,000 ppmV
ceous). However, to further explore the implications of Eq. (4) more pairs are needed. Therefore, the following comparisons were made with the assumption that the partial pressures of atmospheric CO, and S 13C values of soil organic carbon were identical for the two members of a particular pair of compared deposits: (1) Late Ordovician goethite with Late Silurian calcite; (2) Early Jurassic goethite with Late Triassic calcite; and (3) late Middle Jurassic goethite with Early Cretaceous calcite. These inexact temporal comparisons are not satisfying and might not be valid, but are necessitated by the limited amount of isotopic data. 8’3Cm(ccj and S’3C,,o,, in Eq. (4) are the measured quantities in Fig. 5. 6 13c oGoj values to be used here are those in Table 1. As stated, A’3CO(cc_Gojis taken to be + 7.5 per mil. CS(Go) values can be calculated from measured goethite X, values using the Henry’s Law equation of Yapp and Poths [5]. With all of these values Eq. (4) can be solved for Cscccj. The apparent atmospheric CO, contribution (Fig. 4) to the total soil CO, was subtracted from each calculated Cscccj value. This subtraction yielded the concentration CC,> of
Biologically-derived
Soil CO,
0
x)0
4.50
400
350
3ca
2.50
200
150
100
M
0
Ma (B.P.) Fig. 6. Concentration (C,) in ppmV of ancient soil CO, that was derived from oxidation of biological carbon. C, values in the ancient oolitic goethite systems (0) were determined from the measured X values (see Table 1). The C, value of the young, non-oolitic Brazilian goethite of Fig. 5 is also shown (0). The C, values of the calcitic soils (A) were determined by a comparison of the measured oolitic goethite 8 “C and X data with the published pedogenic calcite S13C data of “similar” ages. The horizontal dashed lines define the commonly observed range of C, values in modem calcitic soils [4]. Lower values of Ca in modem calcitic soils could also be expected. Four of the five calcitic C, values are associated with the horizontal dashed lines. The fifth calculated calcitic C, value is anomalously high.
80
CJ. Yupp, H. Paths/Earth
and Planetary Science Letters 137 (1996) 71-82
and is anomalously high. In fact, such high C, values in modern soils would generally be associated with conditions which were too wet to preserve pedogenic calcite [4]. This anomalous value suggests that one (or more> of the assumptions used to calculate C, at 190 Ma was incorrect. One possibility is that the S 13C values of the organic matter were different in the calcitic and goethitic soils. Another possibility is that atmospheric PCO, was not the same in the Late Triassic as in the Early Jurassic. If so, this would invalidate Eq. (4) in a comparison of the pedogenic calcite and oolitic goethite, respectively, from these times. Using the most negative 613C value for Late Triassic calcite of -7.6 [4]; a S 13C value for soil organic carbon (from the coal data) of -23.9 (which implies a S13C,,,, value of about -9.7); and a soil CO, concentration of 7500 ppmV [4]; an approximate Late Triassic P(CO,)* value of 3 was calculated with Eq. (3). An increase in ancient atmospheric CO, levels from about 3 to about 18 times modem from the Triassic into the Jurassic would have required a net increase in the global CO, input to the Earth’s atmosphere relative to the global output. The Early Jurassic was a time of active flood basalt volcanism on the continents [28], and subaerial volcanism seems to discharge most of its CO, to the atmosphere [29]. At the same time, continental relief was low and the weathering of Ca,Mg silicates that serve as a sink for atmospheric CO, was at an ebb 131. Furthermore, global sea level was near an apparent Phanerozoic minimum in the Early Jurassic [30]. This suggests lower rates of seafloor spreading [3], less production of oceanic basalts, etc., at midocean ridges, and perhaps a reduction in the effectiveness of the lowtemperature submarine basaltic weathering recently proposed as an important sink for atmospheric CO, [31]. Whether or not these, or other unknown geologic phenomena could have acted at that time to produce such high levels of atmospheric CO, remains to be determined. Additional information is required including: (1) reliable 6 13Cdata from continental organic carbon throughout the Triassic and Jurassic periods, (2) S i3C and X data from goethitic weathering systems, (3) 6 13C data from pedogenic calcites, and (4) perhaps other as yet unrecognized sources information. Knowledge of the 6 13C values of organic matter on the continents is of particular
importance. For example, if 6 13Co for the Early Jurassic were only 0.7 per mil more positive than the value in Table 1, the partial pressure of atmospheric CO, for that epoch (as calculated from the oolitic goethite) would be 9 times higher than modem, rather than 18 times. However, it is noteworthy that even the relatively “conservative” estimate of 9 times modem is still a high level of atmospheric co*. A recent estimate of Late Cretaceous (Maastrichtian) atmospheric P,, was made by Andrews et al. [24] using the 6 13Cvllues of pedogenic calcite from an ancient soil in India. Their results suggest that the Late Cretaceous atmospheric Pco, was no greater than about 4 times modem and, with associated uncertainties in model parameters, could have been similar to modem. Cerling [pers. commun., 199.51 obtained comparable results on a different calcitic soil of the same geologic age. Our results from the Late Cretaceous oolitic goethite (CHAlb-1) yielded a nominal atmospheric PC0 which was 4 times modem (Table I and Fig. 4). however, as shown in Fig. 4, the uncertainties associated with this particular estimate encompass atmospheric PC0 values indistinguishable from modem. Because the relative age of the Late Cretaceous oolitic goethite is resolved for stratigraphic “epoch” rather than “age”, it is not known, at present, if the estimates for the atmospheric Pco, from the two types of indicators provide temporally overlapping information. Several assumptions were associated with this effort to determine ancient atmospheric P(CO,)* values from oolitic goethite data. It is notable that in the application of these model assumptions to comparisons of S 13C data from oolitic goethite and pedogenic calcite, four out of the five pairs of compared samples yielded physically reasonable calculated concentrations for CO, in calcitic soils. This suggests that the ancient atmospheric P(CO,)* values determined from the oolitic goethites for the four “well-behaved” intervals might be taken seriously within the analytical uncertainties. The interpretation of the “anomalous” interval in the Late Triassic and Early Jurassic is tentative, but, if it should prove to be correct, could have interesting implications for such things as ancient climate and evolutionary biology [32]. Apart from the need for more information on the intervals discussed here, data need to be
CJ. Yupp, H. Paths/Earth
urul Plmetury
obtained from the large temporal “gaps” for which paired oolitic goethite and pedogenic calcite results do not presently exist (Fig. 5).
Acknowledgements
We thank the following individuals for providing samples: M. Bird (CBraz-I), W. Carlson (Boice-I), A. Gehring (MSwitz-1, MSwitz-3), F. Van Houten (SIsra-I, LorFra-1, CHAlb-I), and B. Witzke (IRMWis-I). We appreciate comments provided by Thure Cerling, Miriam Kastner and an anonymous reviewer. This research was supported by NSF grants EAR-92043 13 and EAR-9596206. [ MK ]
References [ll C.D. Keeling, R.B. Bacastow,
A.F. Carter, SC. Piper, T.P. Whorf, M. Heimann, W.G. Mook and H. Roeloffzen, A three dimensional model of atmospheric CO, transport based on observed winds: 1. analysis of observational data, in: Aspects of Climate Variability in the Pacific and Western Americas, D.H. Peterson, ed., Am. Geophys. Union Geophys. Monogr. 55, 165, 1989. 121E. Friis-Christensen and K. Lassen, Length of the solar cycle: an indicator of solar activity closely associated with climate, Science 254, 698, 1991.
[31 R.A. Bemer, Geocarb II: a revised model of atmospheric CO, over the Phanerozoic, Am. J. Sci. 294, 56, 1994. evidence [41 T.E. Cerling, Carbon dioxide in the atmosphere: from Cenozoic and Mesozoic paleosols, Am. J. Sci. 291, 377, 1991. 151 C.J. Yapp and H. Poths. Ancient atmospheric CO, pressures inferred from natural goethites, Nature 355, 342, 1992. 161 C.I. Mora, A.G. Driese and P.G. Seager, Carbon dioxide in the Paleozoic atmosphere: evidence from carbon-isotope compositions of pedogenic carbonate, Geology 19, 1017, 1991. [7] E.J. Barron and W.M. Washington, Warm Cretaceous climates: high atmospheric CO, as a plausible mechanism, in: The carbon cycle and Atmospheric CO,: Natural Variations Archean to Present, E.T. Snndquist and W.S. Broecker, eds., Am. Geophys. Union Geophys. Monogr. 32, 546, 1985. [8] T.J. Crowley and S.K. Baum, Reconciling Late Ordovician (440 Ma) glaciation with very high CO, levels, J. Geophys. Res. 100, Dl 1093, 1994. [9] C.J. Yapp, Oxygen and hydrogen isotope variations among goethites ( rY-FeOOH) and the determination of paleotemperatures, Geochim. Cosmochim. Acta 5 I, 335, 1987. [IO] C.J. Yapp and H. Poths, “C/“C ratios of the Fe(I11) carbonate component in natural goethites, in: Stable Isotope
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Geochemistry: A Tribute to Samuel Epstein, H.P. Taylor, Jr., J.R. O’Neil and I.R. Kaplan, eds., Geochem. Sot. Spec. Publ. 3, 257, 1991. [I 1I C.J. Yapp and H. Poths, The carbon isotope geochemistry of goethite (a-FeOOH) in ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA: implications for early Paleozoic continental environments, Geochim. Cosmochim. Acta 57, 2599, 1993. [I21 A. Siehl and J. Thein, Minette-type ironstones, in: Phanerozoic Ironstones, T.P. Young and W.E.G. Taylor, eds., Geol. Sot. Spcc. Publ. 46, 176, 1989. [ 131 C.J. Yapp, Stable hydrogen isotopes in iron oxides-isotope effects associated with the dehydration of a natural goethite. Geochim. Cosmochim. Acta 47. 1277, 1983. [14] H. Craig, Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analysis, Geochim. Cosmochim. Acta 12, 133. 1957. [I51 D.K. Soloman and T.E. Cerling, The annual carbon dioxide cycle in a montane soil: observations, modeling and implications for weathering, Water Resour. Res. 23. 2257, 1987. [161 C.B. Parada, A. Long and S.N. Davis, Stable-isotopic composition of soil carbon dioxide in the Tucson basin,Arizona, USA, lsot. Geosci. 1, 219, 1983. 171 T.E. Cerling, D.K. Soloman, J. Quade and J.R. Bowman, On the isotopic composition of carbon in soil carbon dioxide, Geochim. Cosmochim. Acta 55, 3403, 199 I 181 Y. Tardy. B. Kobilsek, C. Roquin and H. Paguet, Influence of Periatlantic climates and paleoclimates on the distribution and mineralogical composition of bauxites and ferricretes, Chem. Geol. 84, 179, 1990. 191 B.N. Popp, R. Takigih, J.M. Hayes, J.W. Louda and E.W. Baker, The post-Paleozoic chronology and mechanism of ‘“C depletion in primary marine organic matter, Am. J. Sci. 289, 436, 1989. [201 P.L. Koch, J.C. Zachos and P.D. Gingerich, Correlation between isotope records in marine and continental carbon reservoirs near the Paleocene/Eocene boundary, Nature 358, 319, 1992. [211 C.1. Mora, D.E. Fastovsky and S.G. Driese, Geochemistry and Stable Isotopes of Paleosols, 66 pp., Geol. Sot. Am. Short Course, 1993. I221 T.E. Cerling, Use of carbon isotopes in paleosols as an indicator of the P(CO,) of the paleoatmosphere, Global Biogeochem. Cycles 6, 307, 1992. [231 K. Ferguson, T.H. Lehman and R.T. Gregory, C-and O-isotopes of pedogenic soil nodules from two sections spanning the K-T transition in west Texas, Geol. Sot. Am. Abstr. Prog. 23, A302, 1991. [24] J.E. Andrews, S.K. Tandon and P.F. Dennis, Concentration of carbon dioxide in the Late Cretaceous atmosphere, J. Geol. Sot. London 152, I, (1995). 1251 T.E. Cerling. The stable isotopic composition of modem soil carbonate and its relationship to climate, Earth Planet. Sci. Lett. 71, 229, 1984. [261 I. Friedman and J.R. O’Neil, Data of geochemistry, sixth edition, Compilation of stable isotope fractionation factors of geochemical interest, Geol. Surv. Prof. Pap. 44@KK, 1977.
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Soil Conditions and Plant Growth, 9th ed., Wiley, New York, 1961. [28] F.J. Fitch and J.A. Miller, Dating Karoo igneous rocks by the conventional K-Ar and 40Ar/39Ar age spectrum methods, Geol. Sot. S. Afr. Spec. Pub]. 13, 247, 1984. 1291 S. Amorsson and S.R. Gislason, CO, from magmatic sources in Iceland, Miner. Mag. 58A, 27, 1994. [30] W.T. Holser, Gradual and abrupt shifts in ocean chemistry
during Phanerozoic time, in: Patterns of Change in Earth Evolution, H.D. Holland and A.F. Trendall, eds., pp. 123, Springer, Berlin, 1984. 1311 P.V. Brady, Looking behind the wizard’s curtain: mineral surface controls on long term climate, Geol. Sot. Am. Abstr. Prog. 26, A-287, 1994. [32] C.T.S. Little and M.J. Benton, Early Jurassic mass extinction: A global long-term event, Geology 23, 495, 1995.
Earth and Planetary Science Letters 137 (I 996) 7 I - 82
Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO, pressure Crayton J. Yapp *, Harald Poths 2 Depurtment
of Earth
und Plunetury Sciences. University ofNew Mexico. Alhuyueryur,
NM 87131. USA
Received I9 June 1995; accepted 3 November I995
Abstract Abundance and carbon isotope data from an Fe(CO,)OH component in apparent solid solution in oolitic goethites have been used to infer ancient atmospheric CO, pressures. A test of the validity of these estimates might be comparisons of the carbon isotope compositions of Fe(CO,)OH in oolitic goethites with time-equivalent pedogenic calcites. Temporal trends of the oolitic goethite and pedogenic calcite 6 13C values are generally similar, but time-equivalent samples from each of these two groups are not common in the existing data. To facilitate discussion of the concept, comparisons were made of available goethite and calcite samples even though ages of the compared samples in each pair were not identical. In four out of the five comparisons, Fe(CO,)OH abundance and 6 13C data were combined with pedogenic calcite 6 13C data to calculate physically reasonable soil CO, concentrations for the ancient calcitic soils. This suggests that the compared oolitic goethite and pedogenic calcite systems were responding to the same global scale phenomenon (i.e., atmospheric CO,). Atmospheric P coI as determined from the goethites in these four “well-behaved” cases ranged from values indistinguishable from modem (within analytical uncertainty) to values up to approximately 16 times modem (modem atmospheric PC0 was taken to be 1O-3.5 atm). One interpretation of the fifth, “anomalous”, comparison is that atmospheric CO, levels incieased from about 3 times modem to about 18 times modem from the Triassic into the Early Jurassic. This inferred value for the PC0 of the Early Jurassic atmosphere is not uniquely constrained by the existing data and needs to be substantiated. However, eten considerably lower Early Jurassic atmospheric PC0 values of 6 to 9 times modem (i.e., l/3 to l/2 of the estimated value of 18 times modem) would still indicate significant hifferences between the global carbon cycles then and now. These results highlight the need for more research on the behavior of the atmosphere during and after the Triassic-Jurassic transition.
1. Introduction Carbon dioxide (CO,) is one of the “greenhouse” gases. It participates in the radiative energy balance
’ Present address: Department of Geological Sciences, Southern Methodist, University, Dallas, TX 75275, USA. ‘Present address: CST 7, Mail Stoo J 514. Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
of the Earth’s atmosphere and thereby contributes to the maintenance of the atmospheric temperatures characteristic of the global climate system. The effeet of higher concentrations of CO, on global climate is a question of considerable interest, because of the increase in atmospheric CO, levels since the beginning of the Industrial Revolution [I]. The comparatively brief period of time represented by this interval and the difficulty of reconstructing globally significant climatic fluctuations from the contempo-
0012-821X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00213-8
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and Plunetary Science Letters 137 (1996) 71-82
raneous but rather fragmentary instrumental temperature data complicate the task of determining direct causality between changing atmospheric chemistry and changing global climate. Adding to the complexity is the uncertain, but potentially important, role of variations in solar activity [2]. The approximately 25% increase in atmospheric CO, that has occurred over the last 170 years appears to be unprecedented in its rapidity, but not in its magnitude [3-51. Evidence from ancient surficial geological systems suggests that atmospheric CO, levels up to 16 times higher than modern existed in different epochs over the past 440 Myr [4-61 and may have influenced global climate [7,8]. This evidence is preserved in two minerals from continental weathering environments that have shown promise as CO, indicators, goethite and calcite. The common mineral goethite (a-FeOOH) contains an Fe(CO,)OH component in apparent solid solution. The amount and 6 13C value of this component appear to reflect the partial pressure and 6 j3C value, respectively, of the CO, present in the ambient environment at the time of goethite crystallization [9,10]. Goethite which formed in subaerial
weathering systems could preserve information on the partial pressure of CO, in the Earth’s ancient atmosphere [5,11]. Such goethites may be present in well-preserved ooids from ironstone deposits formed at various times in the Phanerozoic [ 121. In the current paper, we present analyses of the amount and 6 “C values of the Fe(CO,)OH component in oolitic goethites from ironstones of Ordovician, Devonian, Jurassic, and Cretaceous age. These data were interpreted in terms of ancient atmospheric CO, pressures and are compared with published carbon isotope data from pedogenic calcites. Through this comparison with oolitic goethites from wet environments, concentrations of ancient soil CO, were calculated for the relatively dry, calcite-forming soils.
2. Samples The oolitic goethites of this study are from a wide geographic range. Their locations, approximate numerical ages, and the analyzed amounts and S 13C values of the Fe(CO,)OH component in each are listed in Table 1. One non-oolitic, lateritic goethite
Table 1 Oolitic goethite data
_Fe(CO,)OH_ @C Sample
Location
Epoch
IRMWis-I
Wisconsin, USA
Late Ordovician
Ma 440
x 0.00645
Nebraska, USA
Late Devonian
360
Israel
Early Jurassic
195
Boice-
I
SIsra-1
France
IIJCS,,
Ornanic carbon
ooid
6°C (So)
s”c, (960)
P(cod*
-17.6
ooid9 --26.7
coals
155
notie
-20.0
16
0.0175
57
-21.5
-28.0
-28.0
-21.3
1
0.0097
103
-15.3
-23.5
-23.4
-16.8
18 18
LorFra- 1
Lorraine,
Early Middle Jurassic
185
0 0088
114
-15.0
-23.5
-23.3
-16.8
MSwitz-I
Switzerland
Late Middle Jurassic
165
0.0120
83
-17.2
-24.2
-23.3
-17.5
1
MSwitz-3
Switzerland
Late Middle Jurassic
165
0.0099
101
-17.3
-23.6
-23 3
-16.9
1
CHAlb-I
Alberta. Canada
Late Cretaceous
CBraz- 1
Carajas. Brazil
Late Tertiary
80 5
00103
97
-19.7
-26.7
-26.2
-20.0
4
0.0108
93
-21.7
-28.4
-27.2
NA
1
“Ma” indicates millions of years before the present and refers to the approximate ages of the stratigraphic units. Other terms in the column headings are defined in the text. IRMWis-I organic carbon was calculated from the 6°C value obtained by extrapolation of the Neda Fm array [I 11. Boice-1 organic carbon was calculated using the 250°C fraction and the 850°C fraction from MHD-1424. SIsra-1 organic carbon was assumed to be the same as LorFra-I organic carbon. LorFra-1 organic carbon was calculated from the final 230°C fraction and the 850°C fraction of MHD- 1354. MSwitz- 1 organic carbon was calculated using the weighted average Fe(CO,)OH and 850°C fraction from MHD- 1324. MSwitz-3 organic carbon was calculated using MSwitz-3 data from MHD-1450 (in the same manner as for MSwitz-1). CHAlb-1 and CBraz-1 organic carbon were measured as discussed in the text. CBraz-1 is a non-oolitic, lateritic goethite.
CJ. Yupp. H. Paths/Earth
und Planetury Science Letters 137 (1996) 71-82
of late Tertiary age was analyzed to serve as a near-modem comparison with the older ooids. The individuals who provided samples are listed in the acknowledgments. These individuals also provided time-stratigraphic information on the samples. IRMWis-1 is of Ashgillian age (Late Ordovician). Boice-1 is of Fammenian age (Late Devonian). SIsra-1 is of the Early Jurassic epoch, but we have no information on its stratigraphic age within that epoch. LorFra-I is of Aalenian age (early Middle Jurassic). MSwitz-1 and MSwitz-3 are of Callovian age (late Middle Jurassic). CHAlb-1 is Late Cretaceous, but its stratigraphic age within the Late Cretaceous is not known. The stratigraphic epoch of each oolitic ironstone is listed in Table 1. Time-stratigraphic units provide information about the relative antiquity of the samples. However, because the time-stratigraphic units of SIsra-1 and CHAlb-1 are resolved here for “epochs” rather than “ages”, their assigned numerical ages would be expected to have a larger range of possible values than would those of the other samples. This leads to larger uncertainty about the magnitude of numerical age differences between other samples and either SIsra-I or CHAlb-1. For example, although from a relative stratigraphic point of view SIsra-1 is older than LorFra- 1, it might be as little as l-2 Myr older, or as much as 20 Myr older. For consistency, SIsra-1 and CHAlb-I were assigned numerical ages which correspond approximately to the centers of their respective epochs. The approximate numerical ages of each sample are listed in Table 1 and are from the Geological Society of America’s geologic time scale.
3. Experimental Prior to analysis, ooids were physically separated from matrix material. The analyses were then performed according to published procedures [ 10,l 1,131. The amount of Fe(CO,)OH is reported as a mole fraction ( X) of the total FeOOH + Fe(CO,)OH system in each sample. 6 13C is the conventional representation of the per mil relative difference beween the sample ‘“C/ 12C ratio and that of the intemational PDB standard [ 141. The analytical uncertainty in the reported value of X is about f0.0002, while for 6 13C it is about k 0.2 per mil. The ooids from
73
the Late Devonian (Boice-1) ironstone contain a relatively small amount of goethite (about 6 wt%). Because of the small sample size, the uncertainty in the X and 613C values of the Fe(CO,)OH component in this sample may be larger. 3.1. Soil CO, mixing model Steady-state solutions to the one-dimensional Fickian diffusion equation provide good representations of depth-dependent CO, concentrations in modem soils [ 151. In these models, the Earth’s atmospheric CO, is the upper boundary condition, and there is a depth-dependent CO, production term which describes the oxidation of organic carbon in the soil [15]. Thus, the CO, in soil gas can generally be regarded as a mixture of two endmembers (atmospheric CO, and CO, derived from the oxidation of biological material) [4,11,16]. For the Fe(CO,)OH component in pedogenic goethite as a proxy for soil CO,, the two-endmember mixing equation can be expressed as follows [5,11]: 6’3C,
= (s’?,
- 6’3C,)X,(
l/X,)
+ 6’3Co (1)
where S 13Cm is the measured S 13C value of the Fe(CO,)OH component in goethite; 6 13CA is the 6 13C value the Fe(CO,)OH component would have if atmospheric CO, were the only CO, in the soil: 613Co is the 613C value the Fe(CO,)OH component would have if CO, from oxidation of biological material were the only CO, in the soil; X, is the measured value of X for the Fe(CO,)OH component in goethite; and X, is the value of X for the Fe(CO,)OH component if atmospheric CO, were the only CO, in the soil. Goethite samples from the shallower portions of any particular soil would be expected to yield a linear data array in a plot of 6 13C,,, vs. l/X,, if Eq. (1) is valid as a descriptor of ancient weathering systems [51. Such an apparently linear array was measured for the Late Ordovician Neda Formation oolitic ironstone by Yapp and Poths [5,11]. According to Eq. (11, the slope of such an array will be equal to (6’3C, - S’3Co)X,. If (S13C, - S13Co) can be independently determined, the remaining unknown (X,) can be calculated from the slope of the
74
CJ. Yupp, H. Poths/Eurth
and Planetary Science Letters 137 (1996) 71-82
line. The Henry’s Law expression of Yapp and Poths [5] might then be used to calculate an apparent P(C0,) for the Earth’s ancient atmosphere. This approach was used by Yapp and Poths to estimate that the P(C0,) of the Earth’s atmosphere in the Late Ordovician was about 16-17 times higher than the P(C0, ) in the Earth’s modem atmosphere [5,1 I]. This value is in approximate agreement with a value recently calculated by Bemer for the Late Ordovician using a model of the global carbon flux [3]. The shallower portions of ancient weathering profiles are not commonly preserved in oolitic goethite systems. Therefore, a single-profile data array of S 13Cm vs. l/X, can not usually be measured in these deposits. However, if the value of S 13Co were known for a particular deposit (i.e., the intercept on a S 13C vs. l/X plot), it could be combined with measured S”C, and l/X, values from a single, ,well-preserved sample in that deposit to produce a relevant two-point straight line. Prior to the incremental vacuum dehydration of goethite to extract CO, from the Fe(CO,)OH component at 200-3OO”C, samples were treated with concentrated hydrogen peroxide (30%) at room temperature to remove accessible, admixed organic carbon [lo]. For the late Tertiary sample (CBraz- l), the S 13C value of the accessible organic carbon was determined from material balance by measuring the abundance and S 13C values of total carbon before and after H,O, treatment. One aliquot of the Late Cretaceous sample (CHAlb-1) was not treated with H,O,. The S 13C value of the organic carbon in this aliquot was measured after removal of goethite by dissolution in concentrated hydrochloric acid (- 15 N) at room temperature. The organic carbon S 13C values thus measured for these two samples are listed in Table 1. With increasing age of the deposits, however, the possibility increases that the accessible organic carbon is comprised, at least in part, of younger, adventitious organic matter whose S 13C values are different from those of the original soil biological carbon. For some s,amples, there was isotopic evidence for a small amount of organic matter remaining in the H,O,-treated goethite. This comparatively inaccessible, residual organic matter was not measurably re.moved during low-temperature (< 300°C) incremental vacuum dehydration, but appears to be oxidized
to CO, in the terminal 850°C step [ 101.The “refractory” organic carbon manifests itself through increased amounts (normalized to hydrogen) and decreased S 13C values of the CO, recovered in the 850°C step [IO]. The inaccessibility of this refractory organic carbon suggests that it could be a remnant of the biological carbon present at the time of goethite formation. Its S 13Cvalue was calculated by material balance. The calculation assumed that the measured molar CO,/H,O ratio (F) in the 850°C step increased because of the addition of CO, originating from the oxidation of the refractory organic carbon. It was also assumed that a small amount of residual goethite was the only source of hydrogen in the 850°C increment. An example of results from an incremental dehydration is shown in Fig. 1 for the oolitic goethite MSwitz-1. The relative increase in F and decrease in S 13C of the 850°C increment is readily apparent in this sample and is attributed to the refractory organic matter. Three oolitic goethite samples were not suitable for determination of organic carbon S 13C values in the 850°C increment. Because of the presence of significant kaolinite in the SIsra-1 sample, it was not possible to use the measured CO,/H,O ratio of the 850°C dehydration step to calculate a credible S13C value for the organic carbon. The S 13C value determined for the refractory organic matter in LorFra-1 was assumed to represent organic carbon S 13C values in SIsra-1, because LorFra-1 is the closest in age to Slsra-1. In addition, because of excessive error introduced by extremely small amounts of organic carbon in the 850°C increments, no organic carbon S13C values were determined for the Late Ordovician (IRMWis-1) and Late Cretaceous (CHAlb-1) ooids by this mass balance method. However, the aforementioned linear array permitted extrapolation to an endmember S 13Co value for IRMWis-1 from which the organic carbon S 13Cvalue was calculated [ 111.The organic carbon in CHAlb-1 was determined as discussed previously. These various S ‘3C values for the organic carbon in oolitic goethite are listed in Table 1. With present analytical methods, S 13C values of refractory organic carbon, calculated from material balance, have an uncertainty of about f0.5 per mil. The precision is about +0.2 per mil for the S 13C values measured for the “accessible” organic carbon in the CHAlb-1 and CBraz-1 samples.
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and Planetary Science Letters 137 (1996) 71-82
MSwitz-1 (MHD-1324)
O.l? WC
-Jo.10
-
0.03
F - 0.06
-18
- 0.04
- 0.02
-24' 0
1 02
0.4
0.6
0.8
lO.cQ I
UH3
Fig. 1. Plots of S13C vs. X,(H,) and F vs. X,(H,) are shown for the incremental dehydration of oolitic goethite MSwitz- I. The 613C value is that of the CO, recovered during an increment of the dehydration. F is the molar ratio of CO, to H,O recovered in a particular increment. X,(H,) is a progress variable which indicates the degree of conversion of goethite to hematite as measured by the loss of hydrogen (reported as hydrogen removed as a mole fraction of the total initial hydrogen, see [ 1 I]). X,(H,) = 0 when no goethite has been converted, and X,(H,)= 1 for complete conversion of goethite to hematite. The “plateau” values of 613C and F are presumed to represent the CO, recovered from the Fe(CO,)OH component in goethite during the vacuum dehydration steps at temperatures < 300°C. The range of X,(H,) encompassed by the four increments considered to constitute these plateaus is indicated by the double arrow. The terminal 850°C step is presumed to have a more negative 6 13C value and a higher value of F, because of a contribution of CO, from the oxidation of “refractory” organic carbon.
An attempt was also made to estimate ancient continental organic carbon S 13C values from an independent source. Because of the large quantities of organic carbon present in coal deposits, their S 13C values may be less susceptible to modification by addition of younger contaminants. Therefore, S 13C data from ancient coal deposits might provide some general insight into the isotopic composition of biological carbon from relatively wet continental environments at different times in the Phanerozoic. C.W. Holmes of the US Geological Survey in Denver, Colorado, provided access to his 6 13C data on coal
7s
deposits from the Devonian to the Tertiary. A sixthorder polynomial was fit by least squares to those coal data. That curve was used to estimate S 13C values of coal for times corresponding to the ages of the oolitic goethites. The uncertainty of these 613C values as estimates of the average ancient organic matter on the continents is probably no better than f0.5 per mil. The nominal S 13C values of the relevant coals are listed in Table 1 and plotted against corresponding 6 13C values of organic carbon from oolitic goethite in Fig. 2. Although there is scatter in the data, the 613C values of the coal and the S 13C values of the organic carbon in goethite generally covary. Unless it is accidental, this approximate covariance suggests that the 6 13C values of organic carbon from the oolitic goethite may be representative of the biological carbon present in the ancient weathering profiles. These organic carbon 613C values from the oolitic goethite were used to determine the 6 13Co values which are required by Eq. (1). The 613Co values were calculated using the expression of Yapp and Poths [l 11, which incorporates the isotopic fractionation associated with diffusive loss of biologically-derived CO, from soils to the atmosphere [17] and the isotopic fractionation between the Fe(CO,)OH component and soil CO,. The Late Ordovician S 13Co value was taken directly
-22
Fig. 2. A plot of the estimated 6 13C values of coal from different times in the Phanerozoic against the corresponding S13C values of organic carbon associated with the goethites of this study (see also Table 1). The dashed reference line is y = x. Estimated uncertainties in the 6 13C values are also depicted.
76
CJ. Yupp, H. Poths/Eurth
. Ancient
Oolitic
urul Planetury Science Letters 137 (1996) 71-82
.._.----
Gxtbites
-15 __--a
-%&a
Fig. 3. A plot of 6 13C vs. l/X for oolitic goethites of different ages and locales. The analytical uncertainties for S 13C and 1/X lie within the dimensions of the filled squares representing each data point. The straight lines are drawn between the measured values (H) and the presumed value of 6 13Co (at 1/X = 0) at the times of formation of the oolitic goethites. These lines represent two-endmember CO, mixing in ancient soils between biologically-derived CO, and CO, from the Earth’s atmosphere. A steeper slope should correspond to higher partial pressures of atmospheric CO, in the past. The numbers in parentheses are the approximate numerical ages of the ironstone deposits represented by the data points of each line (see Table 1).
from Yapp and Poths [I 11. There are a number of uncertainties associated with the use of the S 13Co values of Table 1, not the least of which is the assumption that these values reflect the original or-
ganic carbon in the ancient soils. Nevertheless, in the absence of better information, these assumptions will be adopted as working hypotheses. 6 13C,,, is plotted against l/X, in Fig. 3 (data from Table 1). The lines in Fig. 3 are the straight lines drawn between the measured goethite data points and the corresponding S 13Co values from Table 1. The single line drawn through the data points for the samples from Israel and France is shown with an age of 190 Ma, which is the mean of the approximate ages of these two deposits (Table 1). The S 13C,,, and nominal 6 13Co values of Boice-1 and MSwitz-3 (Table 1) suggest that lines with slightly negative slopes should be drawn for these samples. No such lines were drawn, since that would imply negative values for the partial pressure of CO, in the Earth’s atmosphere. Moreover, with the uncertainty in 613Co values, the apparent atmospheric P(C0,) values for Boice-1 and MSwitz-3 cannot be distinguished from a value equivalent to the modem CO, pressure. Preliminary temperatures of formation have been determined for the samples from Wisconsin, Israel and France from oxygen isotope data obtained in this laboratory. These calculated temperatures are all within three degrees of 25°C (i.e., within analytical error, they are the same). A temperature of 25°C will be adopted as the temperature of formation of all the deposits of Table 1. Such a temperature is consistent
1
Atmqhenc CO, (fmn colitic gcethibs)
16 x &
12
8
MO
450
403
350
303
250
Ma (B.P.)
200
lx!
loo
M
_ 1 0
Fig. 4. P(CO,)* is plotted against the age of the goethite deposit from which the P(CO,)* value was determined. P(C0,) * is the ratio of the partial pressure of CO, in the Earth’s ancient atmosphere to the partial pressure of CO, in the modem atmosphere. Pco, for the modem atmosphere was taken to be 10-a 5 amt. Vertical lines associated with the data points represent the esimated uncertainties in the P(C0,) * values arising from the uncertainties in the respective values of S 13Co.
CJ. Yapp. H. Poths/Eurth
and Plunetary
with the conditions of formation of modem laterites summarized by Tardy et al. [18]. The value of ( S13C, - 6 13Co) in Eq. (1) is assumed to be the same at all times in the Phanerozoic for C, photosynthesis. A value of + 16 per mil was adopted, because it represents the approximate difference between the estimated Si3C values of pre-industrial atmospheric CO, (- 6.5) and recent C, continental biota (- 271, after the latter value is adjusted for the diffusive 4.4 per mil increase in 6 13C in the biologically-derived CO, of the soil [4,17]. Although Popp et al. [19] have suggested that the 6 13C values of terrestrial, vascular C, plants will be relatively insensitive to the partial pressure of atmospheric CO,, the 6 13C values of this ancient organic matter might have varied in concert with 6 13C values of atmospheric CO, [20]. This provides some support for the assumption that (6 13CA 6 ‘3Co) was approximately constant throughout the time of interest. Furthermore, evidence from fossils indicates that C, photosynthesis did not become important (and then principally in warm, dry climates) until the Miocene epoch [4]. Therefore, the ancient environments represented by the goethite systems in this study were probably dominated by C, photosynthesis (as found in coal-forming plants). The different slopes of the lines drawn in Fig. 3 imply that atmospheric PC0 values were different at different times in the Phankrozoic. As indicated by Eq. (11, higher slopes suggest higher values of X, and, therefore, higher values of atmospheric PC0 . Using a temperature of 25°C and the Henry’s Law equation of Yapp and Poths [5], numerical values of ancient atmospheric PC0 were calculated from the slopes of the lines in Fig! 3. These calculated values were normalized to the modem atmospheric PC0 and designated as P(CO,)* (modem Pco, wai taken to be 10-3.5 atm). P(CO,)* values and estimated uncertainties are plotted against age in Fig. 4. The single P(C0,) * value shown for 165 Ma in Fig. 4 represents the single line in Fig. 3 drawn through the data points for MSwitz-1 and MSwitz-3, whose ages and S13C values are analytically indistinguishable. For reference, an additional data point is shown in Fig. 4 representing modem atmospheric = 1). The nominal ancient CO, (i.e., P(C0,)” P(CO,)* values in Fig. 4 range from about 1 to 18. Atmospheric CO, seems to have decreased from 16
Science Letters 137 (1996)
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77
times modem at 440 Ma to about modem values at approximately 360 Ma. The nominal P(C0,) * value of 18 in the Early Jurassic is the highest value for Phanerozoic atmospheric CO, reported thus far from studies of geochemical indicators of P(C0,). It is also much higher than the value calculated by Bemer [3] for the Early Jurassic. The fact that oolitic goethites from two widely separated locales (France and Israel) seem to indicate the same apparently high value for atmospheric CO, about 190 Myr ago lends credence to the result. Yet, because of uncertainty about the “true” value for 6 13Co, the interpretation of these Jurassic oolitic goethite data in terms of atmospheric CO, is not uniquely constrained. 3.2. Isotopic comparison of oolitic goethite and pedogenic calcite 613C values of pedogenic calcite have been used by several investigators to estimate atmospheric CO, abundances from different times in the Phanerozoic [4,6,21-241. No measurement equivalent to goethite’s X, value can be made for calcite, because of the fixed activity of the carbonate molecule in calcite. However, a comparison of 6 13C values for timeequivalent pedogenic calcite and oolitic goethite samples could be especially interesting because of the contrasting environments of formation. Warm, very wet environments produce authigenic goethite [18], while pedogenic calcite forms in dry climates [25]. The relatively small number of published 6 13C data from both ancient oolitic goethites and pedogenie calcites precludes the comparison of precisely time-equivalent samples. However, general comparisons can be made for the data sets plotted in Fig. 5. The 6 13C data for the Fe(CO,)OH component in oolitic goethite are from Table 1. Pedogenic calcite data in Fig. 5 are from the literature [4,6,21-241. The range of calcite 6 13C values for different samples of the same age could be a consequence of different temperatures of formation, different 6°C values of organic matter, and/or different concentrations of ancient soil CO, that was derived from oxidation of organic matter [4]. Lower calcite 613C values might imply higher concentrations of soil CO, from organic matter and possibly soil environments with relatively more moisture [4]. To simplify the comparison of pedogenic calcite and oolitic goethite of
CJ.
78
Yapp, H. Paths/
Earth and Planetary
0
Science Letters 137 (1996)
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can be formulated for goethite and calcite-bearing soils, respectively:
I
-5
S’3Crn(G0’= ( ~‘3C*(G0) - ~‘3Co(o,,)(C*/Cs),, + 6 ‘3CO(G0)
-10
S13CMcc) =
g " % -15
6’3C
+s13c
n.
-20 ocelute
-25 SW
(
400
300
200
Ma(B.P.) Fig. 5. Measured 6 “C values of the Fe(CO,)OH component in the oolitic goethites of this work (m ) plotted against approximate age of formation. A = published 613C values of pedogenic calcites [4,6,21-241. There appears to be an overall correspondence of the temporal 6 13C variations of oolitic goethites and pedogenic calcites. The goetbite data point at ca. 5 Ma is from Brazil and was provided by M. Bird. It was included as an example of a non-oolitic goethite from a lateritic weathering envi“young”, ronment. It is not compared with “young” pedogenic calcites, because of the common influence of C, plants in post-Miocene calcitic soils [4,22].
(2)
A(cc)- S'3c0(cc))(CA/CS)cc O(CC)
(3)
The 613C values subscripted with “m”, “A”, and “0” are defined in the manner discussed for Eq. (1). For Eq. (2). the subscript “(Go)” refers to the Fe(CO,)OH component in goethite. The subscript “(cc)” in Eq. (3) indicates calcite. C, refers to the concentration of CO, gas in the soil (goethitic or calcitic as indicated) if the only contribution to soil CO, were from the Earth’s ancient atmosphere. Cs was the actual concentration of ancient soil CO, gas from both sources (atmosphere and organic matter). If goethitic and calcitic soils formed at the same time in two different locales, they would have the same C, value for a globally well-mixed atmosphere (i.e., CA(oo’= C+’ ). For such time-equivalent samples, Eqs. (2) and (3) could be combined to yield the following expression:
6 13cm(cc) = ! S’3Cm(oo) - 6 ‘3CO(Go))(CS(Co)/CS(cc)) similar ages, the most negative published calcite S 13C value will be considered in each case. This could give more credibility to the assumption that the organic matter in the compared goethitic (wet environments) and calcitic soils had the same S 13C values. A dashed line connects these calcite data in Fig. 5. Such a line also connects the goethite data of Fig. 5. No line was drawn across the gap in goethite data between 190 and 360 Ma. There is an overall similarity in the temporal changes of the calcite and goethite data as depicted by the dashed lines in Fig. 5. This may indicate that these contrasting environments were responding to the same global-scale phenomena through much of the Phanerozoic. The possibility can be explored further with the aid of the two-endmember soil CO, mixing model described previously. The following two-component mixing equations
+ s13c O(G0)+ A’3Co(cc- Go)
(4)
where A13COccc_Goj is 8’3Co(ccj - 6’3Co(oo’. If the 6 13C values of soil organic matter are the same for time-equivalent samples, and if both soil systems are dominated by steady-state diffusive transport of CO, out of the soil, the value of A’3Co(,,_Goj is approximately the difference (in per mil) between the calcite-CO, and Fe(CO,)OH-CO, fractionation factors. At equilibrium and 25°C calcite is enriched in 13C by about 10 per mil relative to gaseous CO, 11261,while Fe(CO,)OH is enriched in 13C by about 2.5 per mil [ 111.Therefore, the value of A13C,,, _ Goj is expected to be about +7.5 at 25°C. This value will be used in Eq. (4). For the data in Fig. 5, there are only two instances in which approximate timeequivalence of oolitic goethite and pedogenic calcite samples is assumed (Late Devonian and Late Creta-
CJ. Yapp, H. Paths/Earth
19
and Planetary Science Letters 137 (1996) 71-82
soil CO, that originated from oxidation of ancient biological carbon. C, for the calcitic soils is plotted in Fig. 6, as is C, for the goethitic systems. The latter quantity was calculated by subtracting atmospheric CO, concentrations from CS(ooj values which were determined from X,. The C, values for the calcitic soils are plotted at the same ages as the oolitic goethites with which they were compared. These are the hypothetical C, values that calcitic soils (with these 6 13C values) would have possessed, if they were truly time-equivalent with the oolitic goethites and had organic matter with the same isotopic composition. The high values of C, for the ancient oolitic goethites in Fig. 6 are also found in modem, tropical, lateritic soils [27]. The commonly observed C a range for modern calcitic soils [4] is enclosed by the horizontal dashed lines in Fig. 6. Of the five ancient calcitic C B values, four are associated with this modem calcitic soil range. Such consistency could indicate that the assumptions used to determine calcitic C B values were approximately correct. The fifth calcitic C, value at 190 Ma is about 40,000 ppmV
ceous). However, to further explore the implications of Eq. (4) more pairs are needed. Therefore, the following comparisons were made with the assumption that the partial pressures of atmospheric CO, and S 13C values of soil organic carbon were identical for the two members of a particular pair of compared deposits: (1) Late Ordovician goethite with Late Silurian calcite; (2) Early Jurassic goethite with Late Triassic calcite; and (3) late Middle Jurassic goethite with Early Cretaceous calcite. These inexact temporal comparisons are not satisfying and might not be valid, but are necessitated by the limited amount of isotopic data. 8’3Cm(ccj and S’3C,,o,, in Eq. (4) are the measured quantities in Fig. 5. 6 13c oGoj values to be used here are those in Table 1. As stated, A’3CO(cc_Gojis taken to be + 7.5 per mil. CS(Go) values can be calculated from measured goethite X, values using the Henry’s Law equation of Yapp and Poths [5]. With all of these values Eq. (4) can be solved for Cscccj. The apparent atmospheric CO, contribution (Fig. 4) to the total soil CO, was subtracted from each calculated Cscccj value. This subtraction yielded the concentration CC,> of
Biologically-derived
Soil CO,
0
x)0
4.50
400
350
3ca
2.50
200
150
100
M
0
Ma (B.P.) Fig. 6. Concentration (C,) in ppmV of ancient soil CO, that was derived from oxidation of biological carbon. C, values in the ancient oolitic goethite systems (0) were determined from the measured X values (see Table 1). The C, value of the young, non-oolitic Brazilian goethite of Fig. 5 is also shown (0). The C, values of the calcitic soils (A) were determined by a comparison of the measured oolitic goethite 8 “C and X data with the published pedogenic calcite S13C data of “similar” ages. The horizontal dashed lines define the commonly observed range of C, values in modem calcitic soils [4]. Lower values of Ca in modem calcitic soils could also be expected. Four of the five calcitic C, values are associated with the horizontal dashed lines. The fifth calculated calcitic C, value is anomalously high.
80
CJ. Yupp, H. Paths/Earth
and Planetary Science Letters 137 (1996) 71-82
and is anomalously high. In fact, such high C, values in modern soils would generally be associated with conditions which were too wet to preserve pedogenic calcite [4]. This anomalous value suggests that one (or more> of the assumptions used to calculate C, at 190 Ma was incorrect. One possibility is that the S 13C values of the organic matter were different in the calcitic and goethitic soils. Another possibility is that atmospheric PCO, was not the same in the Late Triassic as in the Early Jurassic. If so, this would invalidate Eq. (4) in a comparison of the pedogenic calcite and oolitic goethite, respectively, from these times. Using the most negative 613C value for Late Triassic calcite of -7.6 [4]; a S 13C value for soil organic carbon (from the coal data) of -23.9 (which implies a S13C,,,, value of about -9.7); and a soil CO, concentration of 7500 ppmV [4]; an approximate Late Triassic P(CO,)* value of 3 was calculated with Eq. (3). An increase in ancient atmospheric CO, levels from about 3 to about 18 times modem from the Triassic into the Jurassic would have required a net increase in the global CO, input to the Earth’s atmosphere relative to the global output. The Early Jurassic was a time of active flood basalt volcanism on the continents [28], and subaerial volcanism seems to discharge most of its CO, to the atmosphere [29]. At the same time, continental relief was low and the weathering of Ca,Mg silicates that serve as a sink for atmospheric CO, was at an ebb 131. Furthermore, global sea level was near an apparent Phanerozoic minimum in the Early Jurassic [30]. This suggests lower rates of seafloor spreading [3], less production of oceanic basalts, etc., at midocean ridges, and perhaps a reduction in the effectiveness of the lowtemperature submarine basaltic weathering recently proposed as an important sink for atmospheric CO, [31]. Whether or not these, or other unknown geologic phenomena could have acted at that time to produce such high levels of atmospheric CO, remains to be determined. Additional information is required including: (1) reliable 6 13Cdata from continental organic carbon throughout the Triassic and Jurassic periods, (2) S i3C and X data from goethitic weathering systems, (3) 6 13C data from pedogenic calcites, and (4) perhaps other as yet unrecognized sources information. Knowledge of the 6 13C values of organic matter on the continents is of particular
importance. For example, if 6 13Co for the Early Jurassic were only 0.7 per mil more positive than the value in Table 1, the partial pressure of atmospheric CO, for that epoch (as calculated from the oolitic goethite) would be 9 times higher than modem, rather than 18 times. However, it is noteworthy that even the relatively “conservative” estimate of 9 times modem is still a high level of atmospheric co*. A recent estimate of Late Cretaceous (Maastrichtian) atmospheric P,, was made by Andrews et al. [24] using the 6 13Cvllues of pedogenic calcite from an ancient soil in India. Their results suggest that the Late Cretaceous atmospheric Pco, was no greater than about 4 times modem and, with associated uncertainties in model parameters, could have been similar to modem. Cerling [pers. commun., 199.51 obtained comparable results on a different calcitic soil of the same geologic age. Our results from the Late Cretaceous oolitic goethite (CHAlb-1) yielded a nominal atmospheric PC0 which was 4 times modem (Table I and Fig. 4). however, as shown in Fig. 4, the uncertainties associated with this particular estimate encompass atmospheric PC0 values indistinguishable from modem. Because the relative age of the Late Cretaceous oolitic goethite is resolved for stratigraphic “epoch” rather than “age”, it is not known, at present, if the estimates for the atmospheric Pco, from the two types of indicators provide temporally overlapping information. Several assumptions were associated with this effort to determine ancient atmospheric P(CO,)* values from oolitic goethite data. It is notable that in the application of these model assumptions to comparisons of S 13C data from oolitic goethite and pedogenic calcite, four out of the five pairs of compared samples yielded physically reasonable calculated concentrations for CO, in calcitic soils. This suggests that the ancient atmospheric P(CO,)* values determined from the oolitic goethites for the four “well-behaved” intervals might be taken seriously within the analytical uncertainties. The interpretation of the “anomalous” interval in the Late Triassic and Early Jurassic is tentative, but, if it should prove to be correct, could have interesting implications for such things as ancient climate and evolutionary biology [32]. Apart from the need for more information on the intervals discussed here, data need to be
CJ. Yupp, H. Paths/Earth
urul Plmetury
obtained from the large temporal “gaps” for which paired oolitic goethite and pedogenic calcite results do not presently exist (Fig. 5).
Acknowledgements
We thank the following individuals for providing samples: M. Bird (CBraz-I), W. Carlson (Boice-I), A. Gehring (MSwitz-1, MSwitz-3), F. Van Houten (SIsra-I, LorFra-1, CHAlb-I), and B. Witzke (IRMWis-I). We appreciate comments provided by Thure Cerling, Miriam Kastner and an anonymous reviewer. This research was supported by NSF grants EAR-92043 13 and EAR-9596206. [ MK ]
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[31 R.A. Bemer, Geocarb II: a revised model of atmospheric CO, over the Phanerozoic, Am. J. Sci. 294, 56, 1994. evidence [41 T.E. Cerling, Carbon dioxide in the atmosphere: from Cenozoic and Mesozoic paleosols, Am. J. Sci. 291, 377, 1991. 151 C.J. Yapp and H. Poths. Ancient atmospheric CO, pressures inferred from natural goethites, Nature 355, 342, 1992. 161 C.I. Mora, A.G. Driese and P.G. Seager, Carbon dioxide in the Paleozoic atmosphere: evidence from carbon-isotope compositions of pedogenic carbonate, Geology 19, 1017, 1991. [7] E.J. Barron and W.M. Washington, Warm Cretaceous climates: high atmospheric CO, as a plausible mechanism, in: The carbon cycle and Atmospheric CO,: Natural Variations Archean to Present, E.T. Snndquist and W.S. Broecker, eds., Am. Geophys. Union Geophys. Monogr. 32, 546, 1985. [8] T.J. Crowley and S.K. Baum, Reconciling Late Ordovician (440 Ma) glaciation with very high CO, levels, J. Geophys. Res. 100, Dl 1093, 1994. [9] C.J. Yapp, Oxygen and hydrogen isotope variations among goethites ( rY-FeOOH) and the determination of paleotemperatures, Geochim. Cosmochim. Acta 5 I, 335, 1987. [IO] C.J. Yapp and H. Poths, “C/“C ratios of the Fe(I11) carbonate component in natural goethites, in: Stable Isotope
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