Algeo et al. Changes in ocean denitrification during Late Carboniferous glacial-interglacial cycles NGS

Transcription

1 Algeo et al. Changes in ocean denitrification during Late Carboniferous glacial-interglacial cycles NGS Supplementary Methods Crossplots of total organic carbon (TOC) versus total nitrogen (TN) can be useful in identifying sources of nitrogen in sediments (Nijenhuis and De Lange, 2; Calvert, 24). TOC and TN covary with a correlation coefficient (r) of >+.99 (p( ) <.1) in all three study units (Figure 1). Regression-line slopes (m) of.41 to.51 indicate the presence of C and N in the organic fraction in an average molar ratio between 23:1 (Muncie Creek) and 28:1 (Hushpuckney). Regression-line Y-intercepts (b; TOC = %) are between.6 and.8% TN, representing the average amount of N present in non-organic (mineral) phases in each study unit. For samples from the black shale facies, in which TOC is >2.5% and TN values average ~.8-1.%, organic N comprises the bulk of total nitrogen and mineral N represents only a minor component. For samples from the gray shale facies, the amount of mineral N may exceed the amount of organic N. Similar relationships have been observed in modern marine sediments (Nijenhuis and De Lange, 2; Calvert, 24), and on this basis bulk-sample δ 15 N values are frequently reported as δ 15 N org ; analyses in this study are given as δ 15 N tot but are equivalent to δ 15 N org values in some published studies. Organic matter provenance is commonly evaluated using Rock-Eval pyrolysis, as developed by Espitalié et al. (1977, 1985). The hydrogen index (HI) represents the ratio of hydrocarbons generated (S2 peak) to total organic carbon (TOC); it correlates with the elemental H/C ratio of kerogen in the sample. The oxygen index (OI) represents the ratio of CO 2 generated to TOC; it correlates with the elemental O/C ratio of kerogen in the sample. High HI-low OI values are interpreted to reflect algal sources of organic matter, whereas low HI-high OI values are associated with organic matter derived from higher land plants and/or extensive postdepositional alteration (oxidation) of algal remains. In this study, pyrolysis was performed using a ROCK EVAL II-PLUS analyzer (Vinci Technologies) at the University of Köln, Germany. Samples from the black shale facies exhibited limited variation in HI and OI, ca mg H g -1 TOC and 5-2 mg O g -1 TOC, respectively (Figure 2a). In contrast, samples from the gray shale facies yielded lower and more variable HI and OI values, probably as a consequence of strong oxidation of a mixture of terrigenous and marine OM. TOC versus S2 crossplots are also used to evaluate OM provenance (Figure 2b). Both crossplots suggest that OM in the study units is of mixed provenance, with roughly subequal (but somewhat variable) proportions derived from algal (marine) and higher land-plant (terrigenous) sources. Although HI-OI and S2-TOC crossplots are useful for general characterization of OM provenance, they are not generally used to quantify the proportions of the marine and terrigenous fractions in a sample. Petrographic point counts of organic maceral (fragment) types are a means of generating quantitative data regarding OM provenance (Hutton, 1987). OM of marine origin can be broadly classified as alginite (well-preserved algal OM) or bituminite (highly bacterially degraded OM of amorphous character). OM of higher land-plant origin can be broadly classified as vitrinite or inertinite, which differ in their degree of oxidation (higher for inertinite). Because of the refractory character of terrigenous OM (i.e., its resistance to bacterial degradation), vitrinite and inertinite are generally well-preserved and easy to recognize in thin section, and their point-count frequencies are likely to reflect their actual abundance in each sample. In the study units, the

2 abundance of vitrinite plus inertinite commonly exceeded 2% of sample volume. In contrast, alginite and, especially, bituminite are difficult to recognize in thin section owing to their small size and diffuse distribution through the rock matrix, and, as a consequence, these maceral types are frequently underestimated by point-counting techniques. In the study units, the abundance of alginite plus bituminite rarely exceeded 2% of sample volume. The apparent dominance of terrigenous OM reflected in maceral point counts is inconsistent with Rock-Eval data indicating subequal proportions of marine and terrigenous OM in many samples (Figure 2). We have developed a new procedure for quantifying the proportions of OM of marine and terrigenous provenance in organic-rich sediments. The method makes use of a combination of TOC concentrations and point-count data for terrigenous macerals (which can be reliably determined, as discussed above). The procedure is based on the observation that, in a crossplot of TOC versus the sum of vitrinite plus inertinite, almost all samples are distributed within a triangular field the hypotenuse of which is defined by the equation V+I = 2.47TOC (Figure 3). We interpret this pattern to indicate that (1) for samples plotting along the hypotenuse, ~1% of OM is of terrigenous origin, and (2) for samples plotting below the hypotenuse, the fraction of OM of terrigenous origin (F terr ) is equal to (V+I) samp / 2.47TOC samp. The coefficient in this relationship (2.47) is equal to the volume-to-weight ratio of OM in a shale consisting of OM and clay minerals with densities of 1. and 2.5 g cm -3, respectively, and, hence, has physical significance. Figure 3 shows that some samples yielding high TOC values contain little or no vitrinite and inertinite; logically, these samples must contain large amounts of (most petrographically unrecognizable) marine OM. The method developed here allows the amount of OM of marine origin to be estimated as (1-F terr ) x TOC. Using this technique, average F terr values for the study units are.5-.6, indicating subequal proportions of terrigenous and marine OM, which is consistent with inferences based on Rock-Eval data (Figure 2). Crossplots of F terr versus C org :N and 15 N can provide information about the elemental and isotopic compositions of the marine and terrigenous OM fractions of the study units. F terr and C org :N exhibit statistically significant positive covariation (r = +.26; p( )<.1; n = 13), from which the C org :N ratios of the pure marine and terrigenous organic endmembers can be estimated at and 21-25, respectively (Figure 4a). 15 N shows no relationship to F terr for samples of the LBS and UBS facies, but statistically significant negative covariation for samples of the LBS(N) facies (r = -.42; p( )~.5; n = 2; Figure 4b; n.b., LBS(N) designates the portion of the lower black shale facies of each study unit characterized by a positive N-isotopic excursion). This pattern implies that the N-isotopic composition of the terrigenous organic endmember is about +6 to +8 in all facies, but that the N-isotopic composition of the marine organic endmember shifts from about +5 to +7 in the LBS and UBS facies to +1 to +12 in the LBS(N) facies. These observations are strong evidence that the N-isotopic excursions in the lower black shale facies of the study units are associated with the marine organic fraction and, hence, may record isotopic variation in the seawater reservoir of bioavailable nitrogen. Degree-of-pyritization (DOP) is a widely employed paleoredox proxy based on the ratio of sulfide-hosted Fe to total reactive Fe in a sample (Raiswell et al., 1988). It is a labor-intensive procedure that requires wet-chemical extraction and measurement of both sulfide S (to calculate sulfide Fe assuming a 2:1 S:Fe stoichiometry) and reactive Fe. DOP T, which is the ratio of pyrite Fe (calculated from total S) to total Fe, can be used in place of true DOP, if pyrite S comprises the bulk of total S and reactive Fe comprises the bulk of total Fe (conditions that are true of many organic-rich sediments including the study units). The advantage of DOP T over true DOP is that

3 values can be generated rapidly for large numbers of samples using XRF and C-S elemental analyses. DOP T values can be calibrated to a true DOP scale if a subset of samples is analyzed for the latter parameter. In the present study, DOP T was calibrated to true DOP using the Cruse and Lyons (24) dataset for the Hushpuckney Shale (and its lateral equivalent, the Tacket Shale), from which both DOP T and true DOP values could be determined. These data yielded a 2nd-order polynomial regression (y =.65x x +.16) with r 2 =.89 and a mean deviation from the regression line ( (DOP samp -DOP regr )) of.3 (Figure 5). Based on this relationship, DOP T values for samples of the present study were converted to an estimate of true DOP (DOP est ) with an uncertainty of ±.3. Fe T /Al is also useful as a paleoredox proxy (Lyons and Severmann, 26), and, because total Fe appears in the numerator rather than in the denominator of the equation (as for DOP T ), it is an independent estimator of paleoredox conditions. Fe T /Al values higher than that of the detrital fraction (typically ~.4) imply elevated Fe concentrations owing to the presence of syngenetic pyrite (i.e., pyrite formed within the water column) in the sample. Supplementary References Calvert, S. E. Beware intercepts: interpreting compositional ratios in multi-component sediments and sedimentary rocks. Org. Geochem. 35, (24). Cruse, A. M. & Lyons, T. W. Trace metal records of regional paleoenvironmental variability in Pennsylvanian (Upper Carboniferous) black shales. Chem. Geol. 26, (24). Espitalié, J., La Porte, J. L., Madec, M., Marquis, F., Le Plat, P., Paulet, J. & Boutefeu, A. Méthode rapide de caractérisation des roches mères de leur potentiel pétrolier et de leur degré d`évolution. Rev. Instit. Franç. Pétrole 32, (1977). Espitalié, J., Deroo, G. & Marquis, F. Rock Eval pyrolysis and its applications. Rev. Instit. Franç. Pétrole, Part I: 32, ; Part II: 4, ; Part III: 41, (1985). Hutton, A. C. Petrographic classification of oil shales. Internat. J. Coal Geol. 8, (1987). Lyons, T. W. & Severmann, S. A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins. Geochim. Cosmochim. Acta 7, (26). Nijenhuis, I. A. & De Lange, G. J. Geochemical constraints on Pliocene sapropel formation in the eastern Mediterranean. Palaeogeogr. Palaeoclimatol. Palaeoecol. 163, (2). Raiswell, R., Buckley, F., Berner, R. A. & Anderson, T. F. Degree of pyritization of iron as a paleoenvironmental indicator of bottom-water oxygenation. J. Sed. Pet. 58, (1988).

4 Supplementary Figures Figure 1. TOC versus TN for the Hushpuckney, Stark, and Muncie Creek shales. Regression line parameters shown in inset box. Figure 2. Rock Eval data for the Hushpuckney, Stark, and Muncie Creek shales. (a) HI vs. OI. The shaded field represents the HI-OI compositional range of OM in the black shale facies, and the dashed arrow represents the evolutionary trajectory of OM in the gray shale facies. (b) TOC vs. S2. Regression lines are shown for samples of the LBS and UBS facies. Facies: GS = gray shale, LBS = lower black shale, LBS(N) = lower black shale (high 15 N values), UBS = upper black shale; symbols apply to both panels. Note that LBS(N) designates the portion of the lower black shale facies of each study unit characterized by a positive N-isotopic excursion. OM types I, II, and III from Espitalié et al. (1977, 1985). Figure 3. TOC vs. vitrinite+intertinite (i.e., total terrigenous macerals) for the Hushpuckney, Stark, and Muncie Creek shales. F terr values (dashed lines) are calculated as (V+I) samp / 2.47TOC samp. Facies abbreviations as in Figure 2; regression lines (solid) are shown for samples of the LBS, LBS(N), and UBS facies, and correlation coefficients (r) are given in the upper left (all are significant at p( )<.1). Note that samples from the LBS(N) facies generally contain smaller proportions of terrigenous OM and, hence, larger proportions of marine OM than samples from the LBS and UBS facies. This observation is consistent with the inference that the N-isotopic excursions documented in this study record a marine signal. For all three facies, the fraction of terrigenous OM declines as TOC values decrease. Figure 4. F terr versus (a) C org :N and (b) 15 N for the Hushpuckney, Stark, and Muncie Creek shales. Regression lines (solid) are shown for samples of the LBS, LBS(N), and UBS facies; facies abbreviations as in Figure 2, and F terr values calculated as in Figure 3. Figure 5. DOP T versus true DOP for the Hushpuckney and Upper Tacket shales (data from Cruse and Lyons, 24). Although DOP T values are systematically lower than true DOP values (dashed line = 1:1 ratio), there is a strong relationship between the two parameters. A secondorder polynomial regression (solid line) yields y =.65x x +.16 with r 2 =.89 and a mean deviation ( (DOP samp -DOP regr ) of.3. Thus, the discriminatory power of DOP T as an estimator of true DOP is good, especially at mid to high DOP values. Redox fields are from Raiswell et al. (1988).

5 sample pos. C org N tot C org :N 13 C 15 N DOP est Fe Al Fe T /Al no. (cm) % % mol % % MUNCIE CREEK SHALE MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X MC-X EDM EDM EDM EDM EDM EDM EDM EDM EDM MC-X MC-X DMB MC-X DMB DMB MC-X DMB DMB MC-X DMB MC-X MC-X DMB MC-X DMB DMB DMB

6 DMB DMB DMB DMB DM MC-X MC-X MC-X DM MC-X MC-X DM DM DM DM DM DM DM DM DM DM DM DM DM DM DM STARK SHALE S-ED-X S-ED-X S-ED-X EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X

7 S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X S-ED-X HUSHPUCKNEY SHALE EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM EDM

8 EDM EDM EDM EDM EDM EDM DHB DHB DHB EDM DHB DHB EDM DHB DHB DH-1 (BS) DH DH DH DH DH DH DH DH DH

9 2. core shale m b r 1.6 Muncie Creek Stark Hushpuckney Muncie Creek 1.2 Stark.8 TN (%).4.4 Hushpuckney TOC (%) Supplementary figure 1

10 5 type I type II a facies HI (mg HC / g TOC) GS UBS LBS LBS(N) type III OI (mg CO 2 / g TOC) b 15 type I (marine) type II (mixed) LBS S2 (mg HC / g) 1 5 UBS type III (terrigenous) TOC (%) Supplementary figure 2

11 8 facies GS UBS r +.59 V+I = 2.47 TOC 1..8 LBS Fterr vitrinite + inertinite (vol %) LBS LBS(N) UBS LBS(N) TOC (wt %) Supplementary figure 3

12 Corg:N LBS +LBS(N) UBS GS a facies UBS LBS LBS(N) F terr 14 b N tot LBS+UBS LBS(N) F terr Supplementary figure 4

13 1..8 Hushpuckney Shale Upper Tacket Shale.6 DOPT.4.2 "aerobic" "restricted" "inhospitable" DOP Supplementary figure 5