51. INORGANIC AND ISOTOPIC GEOCHEMISTRY OF SEDIMENTS FROM SITES 549 TO 551, NORTHEASTERN NORTH ATLANTIC 1 Robert Cunningham and Peter M. Kroopnick, Exxon Production Research Company 2 ABSTRACT Thirty eight samples from DSDP Sites 549 to 551 were analyzed for major and minor components and trace element abundances. Multivariate statistical analysis of geochemical data groups the samples into two major classes: an organic carbon rich group (> 1% TOC) containing high levels of marine organic matter and certain trace elements (Cu, Zn, V, Ni, Co, Ba, andCr) and an organic carbon lean group depleted in these components. The greatest organic and trace metal enrichments occur in the uppermost Albian to Turanian sections of Sites 549 to 551. Carbon isotopic values of bulk carbonate for the middle Cenomanian section of Site 550 (2.35 to 2.70‰) and the upper Cenomanian Turonian sections of Sites 549 (3.35 to 4.47‰) and 551 (3.13 to 3.72‰) are similar to coeval values reported elsewhere in the re gion. The relatively heavy δ 13 C values from Sites 549 and 551 indicate that this interval was deposited during the global "oceanic anoxic event" that occurred at the Cenomanian/Turonian boundary. Variation in the δ 18 θ of bulk carbonate for Section 55OB 18 1 of middle Cenomanian age suggests that paleosalinity and/or paleotemperature variations may have occurred concurrently with periodic anoxia at this site. Climatically controlled increases in surface water runoff may have caused surface waters to periodically freshen, resulting in stable salinity stratification. INTRODUCTION Thirty eight samples from DSDP Sites 549 to 551 were analyzed for major and minor components and trace el ement abundances. About half of the samples were col lected from black, laminated, organic carbon rich mud stone and marl intervals occurring in the mid Cretaceous lithologic units at each site. The other samples came from light colored, bioturbated, organic carbon lean intervals from Tertiary to Lower Cretaceous units at the sites. The primary objective of this study was to determine the effect of redox conditions on the geochemical variability (both inorganic and organic) of the sediments. In addi tion to elemental analysis, δ 13 C and δ 18 θ measurements on bulk carbonate were obtained for cyclic interbeds of laminated, organic carbon rich and bioturbated, organ ic carbon lean chalky marls from Unit 5 at Site 550 and an organic carbon rich mudstone in Unit 5 at Sites 549 and 551. These data were used to assess the origin of the organic enrichment at each of the sites. METHODS The sediment samples were analyzed for 19 elements using DC arc plasma optical emission spectroscopy (DCP OES). Details of the ana lytical methods are described elsewhere (Bernas, 1968; Foster, 1971). In addition total organic carbon (TOC) and inorganic carbon analyses were conducted on the samples using LECO combustion. Inorganic car bon was assumed to be in the form of CaCO 3 . This was verified by the high correlation of Ca to inorganic carbon (r 2 = 0.99). Stable isotope analyses of the carbonate fraction of some of the samples were also obtained. CO 2 gas was prepared by reaction of carbonate with phos phoric acid at 50° C (Killingley and Berger, 1979), and analyses for carbon and oxygen stable isotopic compositions were carried out on a V.G. 602 mass spectrometer. The data are reported in delta (δ) nota tion relative to the PDB standard. RESULTS AND DISCUSSION Inorganic Chemistry Site 549 Graciansky, P. C. de, Poag, C. W., et al., Init. Repts. DSDP, 80: Washington (U.S. Govt. Printing Office). 2 Address: Exxon Production Research Company, P.O. Box 2189, Houston, TX 77001. Two lithologic units at Site 549, Units 5 and 10, were sampled for geochemical analyses (Fig. 1). The results are given in Table 1. Unit 5 consists of gray and greenish gray nannofossil chalks, with a black carbonaceous mudstone interval occurring in Core 27. Two samples taken from this inter val were found to be enriched in organic matter (average 5.7% TOC) of predominantly marine origin (Cunning ham and Gilbert, this volume). Elements enriched in these sediments include Cu, Zn, Fe, V, Ni, Cr, and most notably Ba (concentrations up to 3,500 ppm). The con centration of these trace elements in organic carbon rich sediments and rocks is well established (see Arrhenius, 1952; Wedepohl, 1964; Calvert and Price, 1970; Vine and Tourtelot, 1970; Chester et al., 1978). The presence of biogenic siliceous debris and fine laminations in con junction with enrichment in the above components sug gests that these sediments were deposited under oxygen deficient conditions possibly associated with high sur face water productivity (Waples and Cunningham, this volume). Unit 10 consists of interbedded calcareous and non calcareous mudstones to sandstones. Elemental analyses were conducted on three samples enriched in organic mat ter (average 3.4% TOC) of terrestrial origin. Burrow mottling and the absence of marine organic matter sug gest that these sediments were deposited under well oxy genated conditions (Waples and Cunningham, this vol ume). Compared to the overlying organic carbon rich sediments in Unit 5, these mudstones contain relatively low levels of Cu, V, Ni, Cr, and Ba. Concentrations of the other elements are similar to those in Unit 5. 1073
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51. INORGANIC AND ISOTOPIC GEOCHEMISTRY OF SEDIMENTS FROM SITES 549 TO 551,NORTHEASTERN NORTH ATLANTIC1
Robert Cunningham and Peter M. Kroopnick, Exxon Production Research Company2
ABSTRACT
Thirty-eight samples from DSDP Sites 549 to 551 were analyzed for major and minor components and trace elementabundances. Multivariate statistical analysis of geochemical data groups the samples into two major classes: an organic-carbon-rich group (> 1% TOC) containing high levels of marine organic matter and certain trace elements (Cu, Zn, V,Ni, Co, Ba, and Cr) and an organic-carbon-lean group depleted in these components. The greatest organic and tracemetal enrichments occur in the uppermost Albian to Turanian sections of Sites 549 to 551. Carbon-isotopic values ofbulk carbonate for the middle Cenomanian section of Site 550 (2.35 to 2.70‰) and the upper Cenomanian-Turoniansections of Sites 549 (3.35 to 4.47‰) and 551 (3.13 to 3.72‰) are similar to coeval values reported elsewhere in the re-gion. The relatively heavy δ13C values from Sites 549 and 551 indicate that this interval was deposited during the global"oceanic anoxic event" that occurred at the Cenomanian/Turonian boundary. Variation in the δ 1 8θ of bulk carbonatefor Section 55OB-18-1 of middle Cenomanian age suggests that paleosalinity and/or paleotemperature variations mayhave occurred concurrently with periodic anoxia at this site. Climatically controlled increases in surface-water runoffmay have caused surface waters to periodically freshen, resulting in stable salinity stratification.
INTRODUCTION
Thirty-eight samples from DSDP Sites 549 to 551 wereanalyzed for major and minor components and trace el-ement abundances. About half of the samples were col-lected from black, laminated, organic-carbon-rich mud-stone and marl intervals occurring in the mid-Cretaceouslithologic units at each site. The other samples came fromlight-colored, bioturbated, organic-carbon-lean intervalsfrom Tertiary to Lower Cretaceous units at the sites.The primary objective of this study was to determine theeffect of redox conditions on the geochemical variability(both inorganic and organic) of the sediments. In addi-tion to elemental analysis, δ13C and δ 1 8 θ measurementson bulk carbonate were obtained for cyclic interbeds oflaminated, organic-carbon-rich and bioturbated, organ-ic-carbon-lean chalky marls from Unit 5 at Site 550 andan organic-carbon-rich mudstone in Unit 5 at Sites 549and 551. These data were used to assess the origin of theorganic enrichment at each of the sites.
METHODS
The sediment samples were analyzed for 19 elements using DC-arcplasma optical emission spectroscopy (DCP-OES). Details of the ana-lytical methods are described elsewhere (Bernas, 1968; Foster, 1971).In addition total organic carbon (TOC) and inorganic carbon analyseswere conducted on the samples using LECO combustion. Inorganic car-bon was assumed to be in the form of CaCO3. This was verified by thehigh correlation of Ca to inorganic carbon (r2 = 0.99). Stable-isotopeanalyses of the carbonate fraction of some of the samples were alsoobtained. CO2 gas was prepared by reaction of carbonate with phos-phoric acid at 50° C (Killingley and Berger, 1979), and analyses forcarbon and oxygen stable-isotopic compositions were carried out on aV.G. 602 mass spectrometer. The data are reported in delta (δ) nota-tion relative to the PDB standard.
RESULTS AND DISCUSSION
Inorganic Chemistry
Site 549
Graciansky, P. C. de, Poag, C. W., et al., Init. Repts. DSDP, 80: Washington (U.S.Govt. Printing Office).
2 Address: Exxon Production Research Company, P.O. Box 2189, Houston, TX 77001.
Two lithologic units at Site 549, Units 5 and 10, weresampled for geochemical analyses (Fig. 1). The results aregiven in Table 1.
Unit 5 consists of gray and greenish gray nannofossilchalks, with a black carbonaceous mudstone intervaloccurring in Core 27. Two samples taken from this inter-val were found to be enriched in organic matter (average5.7% TOC) of predominantly marine origin (Cunning-ham and Gilbert, this volume). Elements enriched inthese sediments include Cu, Zn, Fe, V, Ni, Cr, and mostnotably Ba (concentrations up to 3,500 ppm). The con-centration of these trace elements in organic-carbon-richsediments and rocks is well established (see Arrhenius,1952; Wedepohl, 1964; Calvert and Price, 1970; Vine andTourtelot, 1970; Chester et al., 1978). The presence ofbiogenic siliceous debris and fine laminations in con-junction with enrichment in the above components sug-gests that these sediments were deposited under oxygen-deficient conditions possibly associated with high sur-face-water productivity (Waples and Cunningham, thisvolume).
Unit 10 consists of interbedded calcareous and non-calcareous mudstones to sandstones. Elemental analyseswere conducted on three samples enriched in organic mat-ter (average 3.4% TOC) of terrestrial origin. Burrowmottling and the absence of marine organic matter sug-gest that these sediments were deposited under well-oxy-genated conditions (Waples and Cunningham, this vol-ume). Compared to the overlying organic-carbon-richsediments in Unit 5, these mudstones contain relativelylow levels of Cu, V, Ni, Cr, and Ba. Concentrations ofthe other elements are similar to those in Unit 5.
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R. CUNNINGHAM, P. M. KROOPNICK
Site 550
Lithologic Units 1 to 5 at Site 550 were sampled forgeochemical analyses (Fig. 1). The results are given inTable 1.
Units 1 to 3 are represented mainly by marly nanno-fossil chalks and mudstones. The minor amount of or-ganic matter in these sediments (average <0.1% TOC)is composed exclusively of terrestrial components (Cun-ningham and Gilbert, this volume). No exceptional ele-mental enrichment is found in these units except for therelatively high concentrations of Sr (average ~ 1,900 ppm)in the nannofossil chalks of Unit la. These values aresimilar to those reported for coccolith tests that haveundergone relatively little diagenesis (Turekian, 1964).The concentrations of the other elements are fairly uni-form in the samples from Units 1 to 3. However, Ti, Fe,Al, V, and Ba are enriched three to five times in the car-bonate-poor samples (those with less than about 50%CaCO3).
Unit 4 is represented by pale green nannofossil chalksand siliceous mudstones. A sample from a faintly lami-nated interval, rich in biogenic siliceous fragments (main-ly radiolaria) and zeolites, was geochemically analyzed. Itwas shown to be enriched in CaCO3 (69%), lean in or-ganic matter, and generally low in trace elements exceptfor Ba, which is exceptionally high at 5,296 ppm. Pre-sumably this interval was deposited in conjunction withrelatively high productivity in the surface waters but with-out the bottom-water oxygen depletion necessary to pre-serve the organic fraction.
Unit 5 occurs as a black, organic-carbon-rich mud-stone (up to 10.94% TOC), with marine-derived compo-nents comprising about 30% of the organic matter (Cun-ningham and Gilbert, this volume). The two samplesfrom this unit are highly enriched in Cu, Zn, Fe, V, Ni,and Ba in conjunction with their high TOC contents.Laminated fabrics, abundant biogenic siliceous materi-al, and organic trace element enrichment indicate thatthe sediments were deposited in oxygen-depleted watersprobably associated with high productivity.
Unit 6 is composed of white to pale yellow-orangechalk. Geochemical results on a sample from this unit in-dicate low organic content (0.04% TOC), high CaCO3
content (78%), and relatively low levels of trace ele-ments.
Grouping of Geochemical Components
Geochemical data recorded for the 38 samples wereanalyzed using the multivariate techniques of principalcomponents analysis (factor) and cluster analysis. Fac-tor analysis groups the variables (organic and inorganicgeochemical data) into a smaller number of factors inwhich specific variables are highly correlated. Cluster anal-ysis groups samples that have similar distributions ofvariables.
Factor analysis shows that a five-factor model explainsmost (81 %) of the variance in the data. Much of the vari-ability (61.1%), however, is controlled by just two fac-tors. Cross-plotting the factor scores for the variables inFactors 1 and 2 causes geochemically significant group-
ings of the data to emerge (Fig. 2). Factor 1 shows a strongpositive association of noncalcareous, terrigenous, clas-tic and biogenic siliceous components that is opposite toa strong negative association of biogenic, calcareouscomponents rich in Ca, CaCO3, Mg, and Sr. Factor 2shows a moderately strong, positive association of or-ganic matter, marine-derived kerogen, and associatedinorganic elements (Cu, Zn, P, V, Ni, Co, Y, and Ba)that is opposite to a moderately strong, negative associ-
Bluish white to light greenishgray nannofossil chalk.
Brown, yellow, and gray marlynannofossil and nannofossilchalk.
Light-colored nannofossil chalk
Greenish gray nannofossil chalk.
2 Gray calcareous siltstoπe.
Figure 1. Lithologic and stratigraphic summary of Sites 549 to 551.
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INORGANIC AND ISOTOPIC GEOCHEMISTRY
Site 549 continued Site 550
Lithologic description
Gray calcareous siltstone
Red sandy dolosparite
Reddish, yellowish, and graycalcareous and sandy calcareoumudstones.
Hard, gray grainstones.
Interbedded calcareous and noncalcareous sandy mudstonesand mudstones.
Light olive brown foliatedmicaceous sandstone.
Site 551
Lithologic description
Pale brown to white foraminifenannofossil ooze and calcareou:
Yellow to light brown calcaiooze and calcareous mudsto
Light gray calcareous oozechalk with white mottling.
White to pale green nannofossilchalk and siliceous mudstone.Black shale rich in organic mattei.^White andj>ale yeNow chalks^-.
Brownish, reddish and grayaltered pillow basalts with pinkand white calcareous sedimentinfillings.
3b
Lithology
I I
Lithologic descriptii
Light bluish and greenish graynannofossil oozes and chalks.
Yellow-brown, green and olivenannofossil chalk, marly nanno-fossil chalk and mudstone; minoisiliceous biogenic component.
-Win
Brownish and grayish marlynannofossil chalk.
Brownish, gray and olive siliceousnannofossil chalk and mudstone.
White and pink nannofossilchalks, grayish green and palebrown marly nannofossil chalkswith interbedded, often graded,green, pink and white calcareou!turbidites.
[Homogeneous, carbonate free, Jdark, massive mudstones with ^no bioturbation.Interbedded dark mudstone, cal-careous mudstones and chalks.
Interbedded light (gray, reddishand brownish) bioturbated cal-careous mudstone and finelylaminated, dark gray to black,calcareous mudstone.
Dark green to black wih red,brown, and pink thin, indurated,calcareous sediment interbeds.
Figure 1. (Continued).
1075
oas
Table 1. Major and minor components (in wt.%) of samples from Sites 549 to 551.
* MAR represents the percentage of total organic carbon (TOC) that is of marine origin based on optical characterization of kerogen. It is calculated as % amorphous +b Analyses provided by P. C. de Graciansky for 549-27-1, 50-51 cm and 551-5-2, 68-70 cm.
algal round bodies.
INORGANIC AND ISOTOPIC GEOCHEMISTRY
GeochemistrySites 549-551
0.5 -
0 -
-0.5 -
FeCr
A l ' ' ^Ti /•
W
,'Zn"foc'Ni NN
/ Cu JP 0f Ba ^-MA*
"~~ Co—"
-0.5Principal component 2
0.5
Figure 2. Cross-plot of factor scores for variables in principal compo-nents 1 arid 2.
ation of terrigenous clastic components rich in Ti, Al,Mg, K, and Na. Fe and Cr plot midway between the ter-rigenous clastic and organic matter groupings, appar-ently indicating equal affinity for both of these sedi-mentary components.
The major groups of variables defined by factor anal-ysis can be further differentiated by cluster analysis. A
six-cluster model provided the most geologically mean-ingful clusters of samples (Table 2). Clusters 1, 2, 3, and5 are composed of trace metal-enriched samples gener-ally containing high levels of organic matter (mean TOC1.7-9.3%). The organic-carbon-rich, black mudstonesand marls from Unit 5 at Sites 549 to 551 are grouped inClusters 1 to 3. Differences in the concentrations of traceelements (especially Ba) control the nature of these group-ings. Cluster 5 represents samples from Unit 5 at Site 550.These black marly chalks have lower mean TOC andtrace metal concentrations than Clusters 1 to 3.
The overall marine organic matter enrichment (up to70% of the TOC) and finely laminated structure of thesamples in Clusters 1 to 3 and 5 suggests that they weredeposited under anoxic conditions. The trace elementsmay have been concentrated by plankton and transport-ed to the sediments as metal-organic carbon complexes(Martin and Knauer, 1973). Preservation of marine-de-rived organic matter in these sediments could then havecontributed to their trace element enrichment. Recently,however, Holland (1979) has suggested that certain traceelements may precipitate directly from anoxic and re-ducing waters. Probably a combination of both factorsacting simultaneously is a better explanation for traceelement enrichment in these samples.
A wide variety of lithofacies from sandstones to mud-stones and calcareous oozes characterizes the sedimentsfrom Sites 549 to 551 grouped in Clusters 4 and 6. Themajor factor they all share is their lack of organic mat-ter (mean TOC 0.1-1.0%) and trace elements as com-
Table 2A. Arithmetic mean concentration of major and minor components in the statistical clusters.
pared to Clusters 1 to 3 and 5. The prevalence of biotur-bation and pervasive terrestrial organic facies character-izing these sediments suggests that they were depositedunder oxygenated bottom waters (Cunningham and Gil-bert, this volume). Therefore, trace element enrichmentby direct precipitation or burial of marine organic mat-ter probably could not have occurred.
Isotopic Analyses
Cyclically Deposited Sediments inUnit 5, Site 550
Interbeds of laminated, organic-carbon-rich and bio-turbed, organic-carbon-lean marly chalk of latest Albi-an (Vraconian) to Cenomanian age in Unit 5 at Site 550reflect rapidly varying environmental conditions. It hasbeen suggested that bottom waters periodically becamereducing at 10,000- to 100,000-yr. intervals as a result ofclimatically induced variations in productivity and cir-culation as this unit was deposited (Cunningham and Gil-bert, this volume). Indeed, organically associated tracemetals Zn, V, Ni, Co, Ba, and Cr are enriched two tofour times in the laminated (anoxic) versus bioturbated(oxic) intervals (Table 1).
In an effort to better assess the origin of cyclicity inUnit 5, stable carbon- and oxygen-isotope analyses wereconducted on a series of samples through three completeredox cycles in Section 55OB-18-1. The carbon-isotopicvalues remain fairly constant throughout the light/darkcycles at about 2.5‰, whereas the oxygen-isotopic sig-nal shows a significant shift (up to l‰) toward lightervalues in the organic-carbon-rich intervals (Fig. 3).
Scholle and Arthur (1976, 1980) and Weissert et al.(1979) argue that positive δ13C shifts in the stratigraphic
column, either locally or world-wide, may occur as aresult of paleoceanographic events that cause the burialof marine organic matter. Surface waters become depletedin 12C as marine organic matter (enriched relative to sur-face waters in 12C) is removed. Conversely, the tests ofplanktonic calcareous organisms that are in isotopic equi-librium with surface waters become enriched in 13C, thuscausing a positive δ13C shift in the isotopic record. Thelack of δ13C variability in Section 550B-18-1 suggeststhat isotopic fractionation did not occur in the surfacewaters, probably because variations in productivity and/or circulation were not great enough to change the bulkcomposition of the CO2 reservoir.
Oxygen-isotopic ratios show a definite shift from back-ground values of about -3.4‰ to maximum values ofabout — 4‰ in the organic-carbon-rich intervals. Varia-tion of oxygen-isotopic ratios may occur because of se-lective dissolution during burial diagenesis of the car-bonates (Scholle, 1977a, b) or as a result of paleotem-perature and paleosalinity variations in surface waters(Savin, 1977). If the fractionation resulting from dia-genesis was minimal, the δ 1 8 θ shifts in sediments in Sec-tion 55O-B-18-1 indicate that warmer surface-water tem-peratures or decreases in salinity may have occurred duringdeposition of the organic-carbon-rich intervals. Climat-ically controlled increases in runoff, have been suggest-ed as the cause of periodic stable salinity stratificationand bottom-water anoxia in the northeastern North At-lantic during the mid-Cretaceous (Arthur, 1979). Else-where in this volume (see Cunningham and Gilbert) it isshown that productivity may have increased concurrent-ly with the periods of bottom-water anoxia at this site.Nutrient loading of surface waters may have occurred inassociation with increased runoff, causing the periodic
1 0 -
150-1550B-18-1 25 50 75 0 1 2 3
CaCO3(%) TOC (%)
2.3 2.5
δ 1 3 c (%)2.7 -4 -3
δ 1 8 θ (%)
Figure 3. Variations in CaCO3, TOC, and carbon and oxygen isotopes for Section 550B-18-1. Beds alter-nate between finely laminated, organic-carbon-rich and bioturbated, organic-carbon-lean marls.
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INORGANIC AND ISOTOPIC GEOCHEMISTRY
increases in fertility. Variations in productivity, however,were not great enough to cause carbon-isotopic fractiona-tion in the surface waters.
Cenomanian-Turonian Black Mudstones,Sites 549 and 551
Carbon- and oxygen-isotopic values are significantlymore positive in the late Cenomanian-Turonian blackmudstones in Unit 5 at Sites 549 and 551 than in themiddle Cenomanian marly chalks in Unit 5 at Site 550.The maximum δ13C values, in Unit 5 at Sites 549 and551, are +4.47 and +3.72‰, respectively, whereas thatin Section 550B-18-1 is +2.7‰ (Table 3). Maximum δ 1 8 θvalues range from -0.76 and -2.22‰ in Unit 5 at Site549 and 551, respectively, to -2.68‰ in Unit 5 at Site550 (Table 3). Carbon-isotopic values for the Cenoma-nian-Türonian black mudstones collected on Leg 80 fallinto the range noted for this horizon elsewhere in theworld (Scholle and Arthur, 1980), indicating that theyare coeval with the worldwide "oceanic anoxic event"proposed by Schlanger and Jenkyns (1976) and Arthurand Schlanger (1979).
CONCLUSIONS1. Multivariate statistical analysis of geochemical data
causes samples to be grouped into two major classes: anorganic-carbon-rich group (> 1 °/o TOC) containing highlevels of marine organic matter and certain trace elements(Cu, Zn, V, Ni, Co, Ba and Cr) and an organic-carbon-lean group depleted in these components.
2. Enrichment of the above trace elements in the or-ganic-carbon-rich samples may occur because of preser-vation of marine organic matter enriched in these ele-ments or by direct precipitation from anoxic and reduc-ing bottom waters.
3. The samples lacking in organic matter show en-richments in Ti, Si, and Al, if they contain a large frac-tion of siliciclastic material, or Ca and Sr, if they are main-ly carbonates.
4. Variation in the δ 1 8 θ of bulk carbonate for Sec-tion 550B-18-1 suggests that paleosalinity and/or pa-leotemperature variations may have occurred concur-rently with periodic anoxia at Site 550. Climatically con-trolled increases in surface-water runoff may havecaused surface waters to periodically freshen, resultingin stable salinity stratification.
Table 3. Comparison of carbon- and oxygen-isotopic values for Ceno-manian to Turonian sediments from Sites 549 to 551.
Sample(interval in cm)
549-27-1, 40-43
549-27-1, 43-46
55OB-18-1, 125-126551-5-2, 67-70
551-5-2, 103-112
Age
late Cenomanian-Tlironian
late Cenomanian-Türonian
mid-Cenomanianlate Cenomanian-
Tiironianlate Cenomanian-
Türonian
TOC(%)
9.31
2.09
0.206.24
6.66
CaCO3
(%)
25.00
1.50
43.001.33
2.33
δ 1 3 C
(‰)
+ 4.47
+ 3.35
+ 2.70a
+ 3.72
+ 3.13
δ 1 8 θ
<‰)
-1.72
-0.76
- 2 . 6 8 a
-2.22
-2.53
a Maximum value from Section 55OB-18-1.
5. Carbon-isotopic values for the black mudstones inUnit 5 at Sites 549 and 551 indicate that they were de-posited during the global "oceanic anoxic event" at theCenomanian/Turonian boundary defined and discussedby Schlanger and Jenkyns (1976) and amplified by Ar-thur and Schlanger (1979).
ACKNOWLEDGMENTS
We thank Exxon Production Research Company for the permis-sion to publish this work. A. E. Bence and P. E. Drez provided helpfulcomments on the ideas presented here. Elemental analyses were con-ducted by D. Wray of EPRCo., and δ13C and δ 1 8 θ measurements weremade by Amy Levantor and Rob Dunbar of Rice University.
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Date of Initial Receipt: May 17, 1983Date of Acceptance: July 18, 1983
