Palaeogeographv, Palaeoclimatology, Palaeoecology, 108 ( 1994): 165 181 165

Elsevier Science B.V., Amsterdam

Geochemical study of organic-matter rich cycles from the

Kimmeridge Clay Formation of Yorkshire (UK): productivity
versus anoxia

N i c o l a s - P i e r r e T r i b o v i l l a r d a, A l a i n D e s p r a i r i e s a , E l i s a b e t h L a l l i e r - V e r g 6 s b, P h i l i p p e B e r t r a n d b.
N i c o l e M o u r e a u a, A b d e l k a d e r R a m d a n i a a n d L a l a n i r i n a R a m a n a m p i s o a b
aUniversitO Paris Sud et URA CNRS 723, Laboratoire de G8ochimie des Roches SOdimentaires, b6tirnent 504, 91405
Orsay cedex, France
bURA CNRS 724, UniversitO d'OrlOans, Laboratoire de GOologie de la MatiOre Organique, BP 6759, 45067 Orh')ans cedex
2, France
(Received February 18, 1993; revised and accepted August 18, 1993)

ABSTRACT

Tribovillard, N.-P., Desprairies, A., Lallier-Verges, E., Bertrand, P., Moureau, N., Ramdani, A. and Ramanampisoa, L., 1994.
Geochemical study of organic-matter rich cycles from the Kimmeridge Clay Formation of Yorkshire (UK): productivity
versus anoxia. Palaeogeogr., Palaeoclimatol., Palaeoecol., 108:165 181.

In this contribution, we study two meter-scale cycles from the Kimmeridge Clay Formation (cored near Marton, Yorkshire)
which shows cyclic organic matter (OM) distribution. Our aim is to try to understand the factors responsible for OM
accumulation. The first cycle, called lower cycle, shows a total organic carbon (TOC) content fluctuating between 1 and 10%
whereas the :second, called upper cycle, shows a TOC content varying between 5 and 35%. The geochemical composition
(major elements and trace elements), the organic geochemistry (TOC, HI, palynofacies) and mineralogy of the sediments have
been determined. In both cycles, the cyclicity is expressed through variations in the nature and in the relative abundance of
the xarious types of organic-matter constituents. Furthermore, dilution effects by inorganic components of the sediment cannot
account for the TOC cyclicity. For the lower cycle, the Mo, V and U content is low and little variable as is the intensity of
the oxidation which OM suffered from. This indicates that variations in phytoplanktonic productivity may be hekl responsible
for the cyclicity in steady and mildly reducing redox conditions. In the upper cycle, cyclicity also appears to depend on
productivity but variations in the concentration of Mo, V and U and in the oxidation state of the OM suggest the environment
was temporarily more reducing. It is proposed that larger amounts of HzS were released into marine bottom waters as a result
of initial OM decomposition, forced the oxic-anoxic boundary to rise in the water column and thus favoured OM storage.
The main driving force for variations in the OM concentration was the productivity of organic-matter-walled phytoplankton.
Redox conditions of the depositional environment could have had a positive action, but only by acting as a positive feed-
back effect.

Introduction c o m m o n l y t h o u g h t t h a t the a b s e n c e or the deple-

t i o n o f d i s s o l v e d o x y g e n in b o t t o m w a t e r s foster
A n i n c r e a s i n g n u m b e r o f studies is d e v o t e d to the p r e f e r e n t i a l a c c u m u l a t i o n a n d p r e s e r v a t i o n o f
the n a t u r e o f o r g a n i c - m a t t e r d i s t r i b u t i o n in the o r g a n i c m a t t e r ( O M ) in m a r i n e s e d i m e n t s , b e c a u s e
geological record. T h e key p o i n t o f m a n y d e b a t e s it p r e v e n t s O M f r o m d e g r a d a t i o n by o x y g e n -
is to d e c i p h e r w h i c h factor, o r g a n i c - m a t t e r p r o d u c - dependant organisms. However, an alternate
tivity or p r e s e r v a t i o n / d e g r a d a t i o n , is the critical perspective has b e e n p r o p o s e d by P e d e r s e n
factor r e s p o n s i b l e for o r g a n i c - m a t t e r a c c u m u l a t i o n a n d C a l v e r t (1990) a n d C a l v e r t et al. (1992),
(Stein, 1986a, b; J a c q u i n , 1987; H u c , 1988; w h o suggested t h a t the settling flux o f o r g a n i c
P e d e r s e n a n d C a l v e r t , 1990; C a l v e r t a n d P e d e r s e n , c a r b o n , closely l i n k e d to the rate o f p r i m a r y p r o -
1992, B o r d e n a v e , 1993, a m o n g m a n y others). It is d u c t i o n , f u r n i s h e s the m a i n c o n t r o l o n lhe a c c u m u -

1)031-0182/94/$07.00 (~:) 1994 - - Elsevier Science B.V. All rights reserved.

SSDI 0031-0182(93)E0134-F
166 N. T R I B O V I L L A R D ET AL.

lation of OM irrespective of bottom water

oxygen values.
Many Mesozoic geological formations, depos-
ited in (hemi-) pelagic environments, are enriched
in organic matter. These organic-matter-rich for-
mations frequently show a cyclic organic carbon
distribution versus depth. Examples for such for-
mations include the Cretaceous Western Interior
in USA (Pratt, 1984), the Cretaceous of the
Northern Atlantic (Herbin et al., 1986), the Late
Jurassic Terres Noires Formation, the
Aptian-Albian Marnes Bleues Formation from SE
France (Tribovillard, 1988) and the Kimmeridge
Clay Formation (KCF), among many others. This
latter formation has been subjected to a number
of studies (Cox and Gallois, 1981; Myers and
Wignall, 1987; Scotchman, 1987; Oschmann, 1988;
Huc, 1988; Wignall, 1989; Miller, 1990).
For this study, we have chosen the KCF which
has a cyclic distribution of organic carbon and a
strong variation of the organic carbon content
(5-35%). We studied one of the wells drilled by
the Institut Fran~ais du P6trole and the British KimmeridgeClay ~ 7 ~ Areaofnon-
Geological Survey through the KCF in the Formationoutcrops deposition
Cleveland Basin; this well is located near Marton ~ KimmeridgeClay
(Fig. 1). The detailed work by Herbin et al. (1991) Formationsubcrops SwellAxis
on the mineralogy, sedimentology, paleontology,
geochemistry and stratigraphy of these wells,
together with preliminary studies by
J Fauks
Ramanampisoa et al. (1992) and Pradier and Fig. 1. Locationof the KimmeridgeClay Formation outcrops
Bertrand (1992), served as the background against and subcrops. The studiedboreholeis locatednear Marton.
which the present geochemical study was carried
out. mechanisms responsible for organic-matter
In the Marton well as in the other ones, total accumulation.
organic carbon (TOC) shows a cyclic distribution Rock Eval pyrolysis and palynofacies prepara-
and at least two orders of cyclicity are identified: tions were performed to characterize and quantify
megacycles grouping elementary ones. The m e a n OM. Infrared spectroscopy was carried out on
duration of elementary cycles was estimated to be kerogens to measure the degree of oxidation of
c a . 30,000 years by Herbin et al. (1991). We chose
OM. Major and trace element contents were mea-
two elementary cycles in the Eudoxus zone located sured primarily in order to determine the prevailing
at the extremities of a megacycle, called the lower redox conditions, during the deposition of the
cycle and upper cycle, respectively (Fig. 2). For two cycles.
the lower cycle, TOC content fluctuates between 1
and 10 wt%; for the upper one, TOC values fluc- Geological background
tuate between 5 and 35 wt%. The organic and
inorganic contents of both cycles were analyzed The deposition of the KCF took place during a
and compared in order to try to understand the period of maximum eustatic rise and transgression,
OR(;ANI('-MATTER RICH CYCLES FROM KIMMERIDGE CLAY OF YORKSHIRE: PRODUCTIVITY VERSUS ANOXIA 1(~7

MARTON 87 and especially sedimentation rate have been sup-

posed to exert a control on the organic matter and
T.0.C °°
O2 10 20 30 carbonate content of the mudstones (Myers and
i i I L I I I J i i i t l i
-~ 10.20
Wignall, 1987; Wignall, 1989).
] WHEATLETENSI$ Deposition was strongly controlled by the basin
_~ 30rO0 topography which ranged from rapidly subsiding
i
$CITULUS rift basins with anoxic conditions and high sedi-
43.10
mentation rate in the North Sea graben system to
ELEGANS
the onshore sections where anoxic conditions were
65.20 achieved in restricted areas only. Here dysoxic
conditions with moderate to low sedimentation
ITISSIODORENStS rate generally prevailed in the shelf and swell areas
(Hallam and Sellwood, 1976). Onshore basin
topography in the southern and eastern outcrops
and subcrops ranged from a shallow swell in the
South Midlands to the Dorset coast sections of
the Central Channel Basin and the fault-controlled
Cleveland Basin where the organic-matter-rich
mudstones dominate the succession (Scotchman,
1989).
169.90
Samples and methods
~ m
In the cores stored at the "lnstitut Franqais du
Depth ( m )
P6trole" (Rueil Malmaison), both cycles show a
F i g . 2. L o c a t i o n of the two cycles studied, the upper one and
monotonous lithology consisting of black mud-
the lower one, within the Marton borehole (from tlerbin
et al., 1991). stones that are finely stratified or laminated
(Pradier and Bertrand, 1992), although the
Kimmeridge Clay usually shows cycles of mud-
with relatively deep water (up to 100 m) covering stone and fissile black shales, even when observed
much of N W Europe (Gallois, 1976). Due to the on cores (Wignall, pers. comm., 1993). The lower
limited, probably silled, nature of connections to cycle is 120 cm long and was split into 90 samples.
the open ocean and high biological productivity, The upper cycle is 62 cm long and was split into
widespread development of oxygen depletion 47 samples.
occurred with the deposition of organic-matter- The CaCO3 content was calculated from the
rich to carbonate-rich mudstone cycles (Tyson, volume of (702 measured after HC1 digestion.
1987). In this context, a climate-induce stratifica- Mineralogical assemblages were determined using
tion of the water column (presence of thermocline) X-ray diffraction, on bulk rocks and on the clay
was the predominant control on the formation of fractions ( < 2 pm). Major element contents were
the different lithologies (Myers and Wignall, 1987; determined using SEM-EDS analysis upon pressed
Wignall, 1989). The vertical cyclic variation in powder (Table 1 and 3); sulfur content was also
mudstone lithology appears to be due to the measured with a LECO apparatus. Trace-element
vertical movement of the oxic-anoxic interface contents have been measured on bulk rocks
upwards through sediment column and the sedi- (Table 2 and 3) by isotope dilution, inducively
ment water interface into the bottom waters coupled plasma mass spectrometry (ICP-MS) using
(Tyson et al., 1979; Irwin, 1979; Myers and the VG Instrument Plasma Quad 2 ~ at the Service
Wignall, 1987) and its occasional storm-driven Central d'Analyses du CRNS (Vernaison, Rh6ne).
overturn (Wignall, 1989). Depositional features The precision of the method is 5 10%. Sediments
168 N. TRIBOVILLARD ET AL.

TABLE 1

Listing o f some m a j o r - e l e m e n t contents of the samples from the lower cycle (plus TOC), used in this study. Each
value is expressed in w t % .

Depth (m Si02 % AI203 % Fe % S% TOC % CaC03°A Depth{rn Si02 % i Al203 % Fe % S% TOC % iCaC03°J
128,14 4 6 , 9 5 22,18 3,22 0,56 2,85 17,58 128,69 50,22 22,60 4,14 0,72 1,74 12,90
128,15 47,27 23,01 3,16 0,51 2,85 16,24 128,71 51,65 24,73 3,10 0,54 1,85 9,g~
128,16 4 9 , 5 9 24,88 2,80 0,65 3,34 10,89 128,72 52,37 24,38 3,32 0,84 2,00 9,42
128,17 48,10 24,05 3,37 0,81 3,38 13,07 128,73 53.24 24,71 2,95 0,59 1,88 8,52
128,19 4 5 , 8 6 21,78 3,33 0,50 2,89 18,81 128,74 54r49 24r96 4r29 0~69 2,21 8,02
128,20 4 5 , 2 0 22,28 3,26 0,40 2,54 19,80 128,76 48,70 25,64 3,68 0,80 1,84 11,03
128,21 4 5 , 3 4 22.26 3,61 0,58 2,88 18,31 128,79 49,80 22,49 4,41 0,77 1,76 12,97
128,22 49,09 25,31 2,96 1,07 4,55 10,61 128,80 47,90 21,79 4,59 0,61 1,73 15,68
128,24 50,64 24,48 3,23 0,50 2,74 11,06 128,81 52,38 24,57 3,61 0,74 1,84 9,02
128,25 48,18 24,35 2,84 0,60 2,96 14,57 128,82 49,70 23,57 3,80 0,53 1,67 13,03
128,26 49,31 24,42 3,06 0,69 3,15 12,06 128,84 49,99 23,68 3,52 0,63 1~68 12,90
128.27 46,37 22.28 3.47 0,54 2,72 17,32 128,85 52,56 25,21 2,90 0,58 2,06 9,09
128,28 4 7 , 2 4 23,56 3,40 0,60 2,65 15,23 128,86 50,88 23,60 3,30 0,55 1,71 12,90
128,30 4 7 , 3 9 23,89 2,92 0,60 2,68 15,58 128,87 50,67 23,21 3,99 0,64 1,67 12,74
128,31 4 4 , 3 8 23,25 3~46 2,40 7,57 13,07 128,89 50,89 24,19 3,42 0,66 2,57 10,47
128,32 4 6 , 2 3 22,30 2,98 0,50 2,54 18,81 128,90 49,95 24,06 3,26 0,64 1,85 12,66
128,33 48,86 23,88 3,37 0,92 3,61 12,06 128,91 49,90 23,29 4,41 0,66 1,68 11,90
128,35 45,31 22,90 4,10 2,90 7,46 11,03 128,92 47,10 23,15 3,13 0,66 2,99 15,84
128,36 48,20 23,33 4,18 2,65 7,00 13,64 128,94 46,69 22,54 3,56 0,44 2,82 16,83
128,37 39,08 19,48 4110 2,75 5,76 23,06 128195 47,74 24,04 3,08 1,46 6,09 10,47
128,38 4 7 , 7 6 23,33 3,26 0,55 3,00 15,23 128,96 49,21 24,57 2,60 0,92 4,56 10,97
128,40 4 9 , 8 0 24,48 2,74 0,61 3,43 12,18 128,97 46,53 22,46 3,37 0,52 2,70 17,82
128,41 4 8 , 0 6 24,79 2,73 1,46 5,12 10,97 128,99 48,18 24,17 2,57 0,92 5,14 12,12
128,42 48,93 23.35 3,21 1,31 4,82 11,11 129,00 48.50 24,47 2.96 1,02 5.50 11.11
128,43 42,53 21,35 3.25 1,43 5,28 20,00 129,01 50104 25,23 3,09 0,94 3,55 10,05
128,45 45,12 23,36 2,83 1,33 6,44 14,57 129,02 44,29 22,53 3,77 2,69 7,70 13,13
128,46 49,65 24,43 3,19 0,72 3,48 11,06 129,04 46,71 22,65 3,56 0,48 2,91 16,83
128,47 49,06 25,21 3,06 1,31 4,86 9,55 129,05 50,58 25,75 2,79 0,73 4,25 8,50
128,48 48,08 24,45 2,82 1,19 5,34 12,00 129,06 51,00 25,92 2,73 0,73 3,91 9,14
128.50 4 0 , 9 8 21,67 3,60 2,53 7,99 17,00 129.06 44,42 21,86 4,32 2,27 7,15 13,15
128,51 4 9 , 2 8 23,17 3,51 1,25 4,91 10,78 129,09 46,41 24,00 2,67 1,04 5,64 14,03
128,52 5 0 , 8 8 25,51 2,77 0,65 3,90 9,48 129,10 47,26 23,51 3,18 0,70 3,22 15,08
128.53 50,24 24.08 2,71 0,52 2,34 12,17 129,11 45,61 22,35 3,07 1,61 6,46 14,12
128,55 50,94 23,21 3,85 0,71 1,90 12,03 129,13 51,47 24,46 3.65 0,56 2,71 9,40
128,56 40,34 19,74 4,92 4,59 9,51 15,03 129,14 51,42 25,37 2,64 0,68 4,00 9,14
128,57 50,61 22,96 3,69 0,51 1,47 12,74 129,15 49,75 24,99 2,79 0,96 4,66 10,05
128,58 48,63 22,43 2,96 0,97 4,56 13,96 129,16 40,66 20,22 2,72 1,51 8,59 20,95
128,59 50,28 23,30 3,60 0,57 1,76 12,74 129,18 47,39 22,91 3,48 0,67 2,89 15,58
128,61 5 1 , 9 8 23,80 3,59 0,55 3,38 9,80 129,19 43,95 21.83 3,15 1,21 6,71 16,56
128,62 5 1 , 4 7 24,24 3~24 0,62 I r81 11,27 129120 46,03 22,58 3,23 0161 3,18 17,76
128,63 5 1 , 6 0 24,89 3,23 0,67 2,84 9,09 129,21 40,11 20,07 4.09 3,28 6,91 19.00
128,64 51,11 23,52 3,32 0,67 2,01 11,90 129,23 42,63 21,44 4,80 4,06 8,84 11,70
128,66 49,26 23,03 3,93 0,68 1,75 13,65 129,24 51,10 23,12 2,23 0,30 2,89 14,07
128,67 51,85 23,86 3,62 0,68 2.62 10,42 129,25 49,51 24,41 3,62 0,71 3,11 11,38
128,68 49,42 23,08 4,09 0,85 1,74 12,74 129,26 50,56 25.01 2,96 1,30 5,21 8,0e

were analyzed after dissolution of 200 mg sample whilst Mn is usually depleted. These elements
with 2 ml H F plus 2 ml HCIO4 plus 5 ml HNO3. however may have different behaviours. Ni, Co,
The entire sample was then dried, taken up in 5 ml Cu and Zn are fixed on organic-matter species
HNO3 and diluted into 50 ml. (e.g. Calvert et al., 1985; Pruysers et al., 1991).
For some samples the trace-metal content was The more abundant the reactive OM, the more
measured (Table 2 and 3). The elements which transition metals (present in sea water) will
have been studied are those being frequently form complexes with organic products and the
reported as linked to black shales, i.e. Ni, Co, Cu, more they will be trapped into the sediment.
Zn, U, V, Mo, Cr which are usually enriched Differing from the latter trace-elements, V, U, Mo
O R G A N I ( ' - M A V I E R RICH ( YCLES F R O M K I M M E R I I ) G F ('LAY OF Y O R K S H I R E : P R O D I ! ( ' I ' I \ ' I I S ' \ ' E R S U S ANOXIA 1(~9

TABLE 2

Trace-element contents (expressed in ppm) of the samples from the lower cycle (plus CaCO3 SiO2. AI~O 3. Fe. S and T()C in wt"'~,
and Ill in mg hydrocarbon per g TOC).

o.°olc.co31s,o.l .o31F, i sl.ocl.,looPi

(meters) (%} (%) (%) (%) (%) (%)
v
I ppm)
c,l.° I .,icolcol
(ppm) (ppm) (ppm) (ppm) (ppm) (ppm)
l.lo.
(ppm) (ppm)
lc0
(ppm)
.o I z. l u Th
(ppm) (ppm) (ppm) (ppm)
128,18 17,32 46,37 22,28 3,47 0,54 2,72 443 0,16 161 135 208 73 17 47 58 688 266 1 128 8 17
128,40 15,23 47,76 23,33 3,26 0,55 3,00 443 139 132 62 43 73 300 7 6 16
126,50 9,14 51,42 25.37 2,64 0,68 4,00 425 0,63 184 130 110 86 19 67 61 599 280 0,6 134
120,52 10.05 50,24 25,23 3,09 0,94 3,55 632 113 131 58 52 58 280 0,6 6 7 16
126,56 8,08 50,56 25,01 2,96 1,30 5,21 575 263 123 99 109 0,3 17
128,61 10,47 47,74 24,04 3,08 1,46 6,09 578 135 129 80 66 48 290 0,5 13
128,62 10,61 49,09 25,31 2,96 1,07 4,55 696 196 86 75 63 0,2 12
128,70 14.57 45,12 23,36 2,83 1,33 6,44 586 148 129 61 68 63 276 0,5 14 7 19
128,71 13,07 44,38 23,25 3,46 2,40 7,57 609 0,74 187 128 111 95 19 80 69 928 257 0,8 129
126,72 13,13 44,29 22,53 3,73 2,69 7,70 552 139 120 97 09 114 270 1,5 38 8 18
12833 11,03 45,31 22,90 4,10 2,90 7,46 545 0,79 169 115 120 109 22 80 108 1168 261 2,1 126
128,75 11,70 42,63 21,44 4,80 4,06 8,84 550 0,76 149 113 132 153 24 87 171 1022 223 3,5 109
126,76 15,03 40,34 19,74 4,92 4,59 9,51 582 0,82 144 105 146 171 25 84 431 1060 212 3,3 103
126,77 17.00 40,98 21,67 3,60 2.53 7.99 563 0.68 162 115 144 134 23 84 114 1196 248 1,9 109
128,79 20,00 42,53 21,35 3,25 1,43 5,28 705 0,60 171 127 133 99 19 69 61 1150 272 0,7 115 8 17
126,80 20,95 40,66 20.22 2,72 1,51 6,59 676 0,53 177 124 147 112 20 71 79 1234 269 1,3 113 8 16
126,82 2 3 , 0 6 39,06 19,48 4,10 2,75 5,76 565 0,58 166 111 166 143 24 58 233 1499 247 1,3 108
128,83 14,12 45,61 22,35 3,07 1,61 6,46 594 0,50 188 133 144 102 19 74 161 1111 271 1,5 146 7 17
128,86 19,00 40,11 20,07 4,09 3,28 6,91 665 0,63 170 114 154 175 26 104 243 1326 257 3,6 110
128,88 1 3 , 6 4 40,20 23,33 4,18 2,65 7,00 623 0,55 185 123 144 158 22 89 196 1996 271 7,3 117
129,03 11,11 40,93 23,35 3,21 1,31 4,82 539 143 133 75 54 66 280 1 15 8 20
129,06 12,74 50,61 22,96 3,69 0,51 1,47 355 0,24 186 143 195 71 17 42 52 550 289 0,6 141 6 19
129,23 9,02 52,38 24,57 3,61 0,74 1,84 316 132 145 54 36 84 300 0,6 4 7 20

and Cr are fixed after reduction due to oxydo- acids mainly (Pilipchnuk and Volkov, 1974;
reduction reactions at the expense of OM (Disnar, Calvert and Morris, 1977; our data from ongoing
1980; Disnar and Trichet, 1983). Vanadium study). Manganese is an element sensitive to redox
behaves in the same way as U and Mo. Reduction, conditions. When the host-sediment is submitted
adsorption and complexation of dissolved vana- to reducing conditions, Mn may be solubilized and
date favour addition of vanadium to organic- then may migrate upward, back to the water
matter-rich sediments (Brumsack and Gieskes, column. So, black shales usually arc depleted in
1983; Brumsack, 1986: Breit and Wanty, 1991): Mn relative to normal marine shales.
dissolved vanadate-V(V)-species are dominant in We also calculated the degree of pyritisation
oxic seawater and are reduced to vanadyl ions by (DOP), based on a sequential leach procedure
organic-matter compounds (or HzS in euxinic envi- (Berner, 1970; Raiswell et al., 1988; Huerta-Dias
ronments). Vanadyl ions, V(IV) are adsorbed to and Morse, 1990, 1992). The DOP is a means for
particle surface and are consequently incorporated estimating the extent to which the original poten-
into the sediment as particles settle. Thus, the more tially reactive iron has been transt2~rmed to pyrite.
reducing the marine environmental conditions, the A full description of the sequential extraction
larger vanadium accumulation into the sediment. procedure wilt be given elsewhere (ongoing study
During early diagenesis, V is later incorporated, on trace-metal incorporation into the different
V(IlI), into silicates (Wanty and Goldhaber, 1992). phases of the sediment in various Mesozoic black
V(III) forms by reaction of V(IV) with H2S or, shales). Briefly, it is designed to obtain the
possibly, through reaction with residual OM at following three operationally-detined fractions:
high temperature (Breit and Wanty, 1991). The 1. Reactive iron: obtained after digestion of
early diagenesis of molybdenium is like that of V sediment samples with HCI (12N), according to
but it is latter incorporated into sulfides and humic Berner (1970) or Raiswell ct al. (1988). This step
170 N. T R I B O V I L L A R D ET AL.

TABLE 3

Major and trace-element contents of all the samples from the upper cycle, expressed respectively in wt%
and ppm. HI are expressed in mg hydrocarbon per g TOC.

0..1. c.c03 ,,0, .,,03 " 1 ' v I c, ...,ico z.ls.i u ic, .o

(meters) %) (%) %) (%) (%) (%) (ppm) (ppm)(ppm (ppm) ppm) ppm (pprn)(ppm ppm ppm
121,60 1 0 , 2 6 47,01 20,66 2,26 0,67 4,73 443
121,59 9,74 4 3 , 9 1 19,08 2,37 0,72 4,43 470 0,82 437 107 98 86 63 63 367 6 2 8
121,58 1 0 , 2 6 53,45 23,56 2,71 0,82 4.99 458
121,57 1 0 , 7 7 42,49 19,29 2,69 0,94 5,53 458
121,56 1 2 , 8 2 37.94 17,20 3,40 2,09 9,65 555 0,82
121,54 1 3 , 8 5 36,81 16,51 3,41 2,11 9,19 535
121,53 1 4 , 8 7 35,13 1636 4,14 2,45 9,40 549
121,52 14,87 33,66 15,75 2,88 1,44 8,13 536 0,86 420 126 101 121 99 86 300 8 4 40
121,51 13,33 34,21 16,62 3,91 2,34 8.87 694
121.50 17,43 33,74 16,20 3,70 2,29 10,15 493
121,49 19,23 33,02 16,45 3,62 2.20 9,46 479
121.48 20,51 32,16 15,43 3,68 2.36 12.30 507 0,92 402 103 115 213 93 628 257 14 14 160
121,47 15,38 32,14 15,29 4,62 3,32 14,79 665
121,46 9,33 32,31 15,91 5,20 4,09 14,05 694
121.44 9,33 24.56 11,83 5,34 3,59 20,04 674 0,96 328 85 131 230 90 408 199 15 10 220
121,43 8.29 17,26 7,88 6,13 3,83 28,47 628 0,41
121,42 8,29 15,60 7,12 6,07 3,64 28.94 642 0,97 242 60 118 195 86 327 167 18 8 200
121,40 10,36 17,44 7,91 4.99 3.23 27,87 669
121.38 10,88 15,69 7,35 6,83 3,74 31,37 650 0,97 223 58 112 169 77 204 169 12 8 200
121.37 12,95 29,94 14,65 3,96 3,12 17.50 722 0,94 418 106 133 183 83 263 207 12 6 130
121,35 18,65 26.50 12,68 4,39 2.91 13,66 730 0,92 376 98 138 193 88 288 210 12 7 140
121,30 11,85 29,82 14,04 4,00 2,78 16.86 626
121,29 11,85 27.21 13,29 3,96 2,58 13,81 647 0,90 410 116 119 160 99 219 235 7 6 120
121,28 9,28 22,49 10,77 5,39 2,82 9,18 636 0,67 0,69 396 120 111 169 83 247 251 8 6 160
121,27 17,01 29,03 14,30 4,21 2,63 13,08 622
121,26 14,43 27,68 13.38 3,79 2,49 15,62 616 0,92 362 111 114 141 77 200 267 8 5 110
121,24 12,37 25,69 12.54 4,46 2,36 15,06 808 0,87
121,23 12,89 31,94 15.83 3,63 2,34 12,76 591
121,22 12,89 30,84 1S,32 3,43 2,05 11,51 587 0,91 389 129 103 115 92 128 284 8 7 60
121.21 10,31 32,18 15,51 3,54 1.95 9,49 557
121,19 6,70 34,56 16,63 3,62 1,76 8,09 539 0,45
121,18 7,22 34.69 17,56 3,14 1,38 7,39 525
121,17 6,07 33,95 16,47 3,46 1,40 7,39 503
121,15 8,25 37,35 18,21 3,69 1,88 7,67 512 0,90 409 146 97 104 83 121 269 6 3 30
121,14 8,76 36.32 17,90 3,59 1,74 7,76 535
121,12 10.82 36,54 18,11 3,26 1,68 6,72 632
121,11 9,79 34,35 16,96 3,11 1,49 7,68 530
121,10 7,73 34.31 16,58 3,32 1,76 8,14 565 0,64
121,09 7,22 30,38 16,42 3,24 1,20 6,37 524
121,08 7,22 39,40 19,40 3,38 2,17 9,07 620 0,93 394 135 98 125 78 105 263 6 2 60
121,07 8,25 32,66 15,32 4,18 3.26 17,02 634
121,05 8,76 30,03 14,84 4,16 2,49 12,21 575 0,71
121,04 10,82 33.49 16,08 3,98 2,47 8,58 809
121,02 12.89 29,56 14,69 4,23 3,10 14,19 689 0,96 359 109 119 230 96 410 276 12 10 200
121,01 9.28 31,97 15,70 4,05 2,13 9.28 594
121,00 7,18 34,47 17,69 8,39 6,67 5.39 722 0,97 327 109 77 107 62 189 148 6 8 70
120.98 8,76 27,64 13.96 4.69 2,59 t2,11 641 0,67

allows the extraction of iron from oxyhydroxides 2. Silicate: extracted after leaching with HF for
and minor removal of silicate-bound iron and of 16 hours (Huerta-Diaz and Morse, 1992); this
monosulfide-bound iron if still present (Lyons and fraction comprises clays.
Berner, 1992). For some samples, this digestion 3. Pyrite: obtained after digestion of the silicate-
was made using 1N HCI, according to the methods fraction residu with concentrated nitric acid for 2
of Huerta-Diaz and Morse (1990), the same results hours (Huerta-Diaz and Morse, 1992). This frac-
were obtained. tion comprises pyrite.
O R G A N I ( -MATTER RICH CYCLES F R O M K I M M E R I D G E CLAY OF Y O R K S H I R E : P R O I ) U C T I V I T Y VERSUS ANOXIA l "7[

In each step, iron was measured using an ICP- The analytical protocol is fully described in
OES spectrometry (Varian, model Liberty 200). Benalioulhaj and Trichet (1990). The samples were
DOP of iron is defined as D O P = analysed in the form of pellets with 1.5 mg of
( P y r i t e - F e ) / ( P y r i t e - Fe + H C 1 - soluble Fe) , kerogen dispersed in 1 g KBr. The spectra are
using steps 1 and 3. represented in absorbance as a function of the
Rock Eval pyrolysis have been performed wavenumber (cm L) in the 4000-600 cm 1 range.
upon bulk rocks; Rock Eval parameters such The assignment of the main infi'ared bands
as total organic carbon (TOC), Hydrogen Index (Table 4) was made using the works ot' Robin et al.
(HI) or Tmax are defined in Espitali~ et al. (1977), Benalioulhaj and Trichet (1990) and
(1985, 1986). Rochdi et al. (1991). The differences between the
Infrared Fourier Transform spectroscopy (using relative intensities of infrared absorption bands
a Perkin Elmer model 1600 FTIR) was performed can be interpreted as the mark of more or less
upon kerogens, extracted according to standard intense oxido-reduction processes having affected
procedures (Combaz, 1980) but without oxidative the organic matter during its sedimentation and
acid attack (Gorin and Feist-Burkhardt, 1990). diagenesis. This may be done provided all the
samples studied contained OM of the same origin
and had endured the same burial effects, which is
TABLE 4
the case here (see below). Oxidation of kerogens
Assignment of absorption bands in IR spectra from kerogens is marked by a decrease in aromatic and aliphatic
(from Tribovillard et al., 1992). CH band intensity and by an increase in OH and
C = C + C = O band intensity (Rochi et al., 1991).
Function Peak Integration
(cm-1) range In order to compare the oxidation state of kero-
gens, the ratio:
OH 3500-3000 3700-3100 Index of oxidation= l.ox.= (180(1 1520 cm
1630 bands)/(3000 2800 cm 1 bands)
was chosen (Tribovillard et al., 1992). This index
Aromatic CH 3050-3000 3100-3000 is derived from the one of Benalioulhaj and Trichet
(1990). I.ox. represents the ratio between the rela-
875
tive importance of, from one hand, C = O
825 920-720
(1710cm 1), C = C , C = O , COO (1610cm J)
755
bounds and aliphatic ones (C-H bounds,
2900cm l) from the other hand. The values of
O-I 3 2960 3000-2800
this index are reported in Tables 2 and 3.
2870 2880-2820
1475 1475-1390
1380 1390-1300 Results

CH2 2920 3000-2800 Mineralogy

2850
1475 1475-1390 The bulk sediment consists of calcite, clay miner-
als (kaolinite, illite, ordered and randomly inter-
C = O esters 1745 stratified illite-smectite mixed layers, plus very little
C = O carbonyls 1710 1800-1520 chlorite), quartz, pyrite (traces of feldspar) and
C = O carboxyls 1710 organic matter. Traces of gypsum are visible in
C = O con jug. 1650 the upper cycle; gypsum is thought to derive from
ketones pyrite oxidation. In the lower cycle, calcite content
is 8-25%, whereas in the upper cycle, this content
C=C 1610 & 1500 1520-1475
is 6 21%. No qualitative variation was detected
in the clay assemblage. Minor quantitative varia-
172 N. T R I B O V I L L A R D ET AL.

tions exist within each cycle and between the two TOC (%) CaCO3 (%)
cycles, as shown by the K/A1 and illite to kaolinite Depth (m)
ratios (Fig. 3). For the upper cycle, SiO2 and 0 10 20 30 40 0 10 20 30
-120,9. I , I , I , I , I i . , . i . I

A120 3 correlate very well ( r = 0.96, n = 47). The

correlation is poorer but still good for the lower
cycle (r = 0.63, n = 90). This indicates a homogen- _e -121,1
eous terrigenous detrital supply, as is also sup-
ported by the X-ray diffraction results (Figs. 3 to

and 5). Q. -121,3

In the two cycles, the main source of carbonates O,.

is coccoliths as was already reported by Herbin

et al., 1991). We have also observed rare foramini- -121,5
fers and diatoms as well as scarce shell fragments
but no calcispheres. Thus, the CaCO3 content
represents almost entirely the productivity of car- -121,7.
bonate-walled plankton. In both cycles, TOC and
0 10 20 30 40 e 10 20 30
CaCO3 contents do not have exactly the same -128,0. i . , . J , I i I , . i . i . I

distribution with depth (Fig. 4). Coccoliths have

-128,2.
been observed with SEM from samples throughout e
both cycles. Variation has neither been observed -128,4.
in the taxonomic composition nor in the the preser- t_

vation state of the coccoliths. Only -128,6.

Cyclogelosphaera margereli and Watznaueria sp. O
-I -128,8.
have been identified (Fig. 5).
-129,0.

-129,2.
Illite/kaolinite K/AI
Depth (m) -129,4.
o 2 3 0,10 0,15 0,20
-121,0 I ,
,q
, I . I I i l i I Fig. 4. Carbonate and TOC-content distribution with depth for
the two cycles.

-121,1
Organic matter
-121,2

-121,3 1 Lower cycle

Figure 6 illustrates the distribution of TOC and
HI values with depth (see also Tables 1 and 2).
-121,4 According to the classification of Tissot and Welte
(1984), based on Rock Eval parameters, all the
-121,5 samples observed here contain type-II OM. This
type generally is of marine origin (to some extent,
type-II OM can also be derived from leaves and
-121,6 spores, i.e. terrestrial origin). Polished rock section
analysis showed in the most organic-matter-rich
-121,7 part of the lower cycle the unique occurrence of
Fig. 3. Distribution of the values of the lllite/kaolinite and brown-coloured algal bodies which cannot be seen
K/A1 ratios with depth for the upper cycle. elsewhere in this cycle (Pradier and Bertrand,
ORGANI('-MAIIER RICH CYCLES FROM KIMMERIDGECLAY OF YORKSHIRE:PRODUCTIVITYVERSUSANOXIA 173

I Terrestrial influx ] I Redox conditions I l Organic matterl

Si/AI K/AI V (ppm) Mo (ppm) Mn (ppm) Lox. TOC (wt%)
Depth (m)
3 4 5 01 02 031t00200 300400500 0 100 200 300 16~ . 1100
. . 140
. . 180
. . .220
. . 0,4
. . 0,6
. . 0,8 1,0 O 10 20 30 40
-120,9- ~ = ' ' . . . . . . . . . . . . . . . . . . . .

-121,1 "

-121,3"

°
-121,5"

-121,7"
-128,0 . .3 . . 4 5 0;1 , 0;2 , Or3 1 .,~o .,~o ~,, 19 ,2oo~4. .°IB 1,o 30
1
= -129,2t ~

O~ -128,8 •

-129,2 ] •

-129,4. J
Fig. 5. Distribution profiles of geochemical parameters representative of terrestrial influx (Si/A1, K/AI), oxygenation conditions (V,
Mo, Mn contents and l.ox. values) and organic matter (TOC).

1992). Palynofacies assemblage is dominated by all the kerogen in the cycle. I.ox. values are almost
an overwhelming, amorphous OM. The most org- constant (Fig. 5), suggesting that the conditions
anic-matter-rich part of the lower cycle contains for OM preservation were constant during the
also a particular type of amorphous, orange- deposition of this cycle.
coloured, OM. This orange, amorphous OM has
not been seen in other parts of the cycle, which Upper cycle
have a palynofacies dominated by a brown amor- Rock Eval parameters indicate that OM is again
phous OM (Ramanampisoa et al., 1992). When of type II (Fig. 6, Table 3). This second cycle, with
observed with transmission electronic microscope high TOC and HI values, is dominated by the
or TEM (Boussafir, pers. comm.), the orange brown-coloured algal bodies except in the least
amorphous OM of the palynofacies appears as a organic-matter-rich parts, where Tasmanacea and
structureless gel whereas the brown amorphous yellow algal bodies dominate (polished rock sec-
OM shows a so-called "ultralaminar" structure tion observation). The palynofacies assemblage is
(Largeau et al., 1990). the same as in the lower cycle, but more samples
The different types of spectra obtained from show the abundant presence of orange, amor-
infrared spectroscopy are shown in Fig. 7. Very phous, OM. It is again gel-like when observed
little variation exists between the various spectra under the TEM.
of the lower cycle: the nature and the state of The different types of spectra obtained for the
preservation of the OM seem to be the same for upper cycle from infrared spectroscopy are shown
174 N. TRIBOVILLARDET AL.

TOC (%) HI (mg HC/g TOC)

Depth (m)
0 10 20 30 40 100 300 500 700 900
-1209, . . .. . . . . . .= , , , q , i
, ~ UPPERCYCLE

-121,1 l

-121,3]
==.
::} _g

n~

0 10 20 30 40 100 300 500 700 900

-128,0 . . . . . . . . ~ . . . . . . ~,
4000 3000 2000 1000
-128,2. Wavenumbe(cm
r -1)

-128,4.
_¢
O
~>~ -128,6.
O
I,,,, LOWER CYCLE
-128,8.
3
O
.-I -129,0.
28-I
-129,2.

-129,4

Fig. 6. Hydrogen index (HI) and TOC-content distribution .g

with depth for the two cycles. HI are expressed in milligrams
of hydrocarbon per gram of total carbon organic, TOC values
are in weight percents.

in Fig. 7. Some difference can be seen between the

intensities of the absorption bands. This indicates
that not all the kerogens suffered~ from the same
oxidation. This is also supported-by Lox. values 4000 3000 2000 1000
which are more varied than in the lower cycle, but Wave number (cm -1)
show here no clear trend (Fig. 5). However, the
Fig. 7. Infrared spectra of kerogens extracted from samples
most organic-matter-rich sample shows the lowest located along the lower cycle and the upper one.
Lox. value, thus suggesting that the maximum of
OM accumulation occurred in the least oxidizing facies preparations, or with brown algal body
conditions. abundance, seen on polished rock sections. Thus,
For both cycles, TOC values correlate well with the TOC acts as an indication of the productivity
orange amorphous OM abundance, seen in palyno- of the organic-matter-walled plankton.
ORGANI('-MATTER RICH C Y ( ' L E S F R O M K I M M E R I D G E CLAY OF: Y O R K S H I R E : P R O D L ! ( ' I IVI'I'Y V E R S U S &.NOXIA 175

SM/i*r and sulfides be recognized: for population I, the sulfur content

is nearly invariable whatever the iron content: for
For both cycles, organic-matter sulfur has been population II, a correlation exists between
measured for on selected samples representing the S-content and Fe-content. Both these populations
range of values of TOC (Bertrand and Lallier- are also indicated in Fig. 9. For all the samples of
Verges, 1993). It turns out that, in the two cases, the lower cycle for which sequential leach pro-
organic-matter sulfur represents less than 2% of cedure was performed, iron content, measured
the bulk sulfur. So, in what is following, bulk from the silicate fraction (step 2) is almost constant
sulfur may be regarded as representative of reduced (between 1.20 and 1.88%, i.e. below 2%). This
sulfur (sulfide-bound). means that bulk Fe variations are not affected by
the composition of clay minerals.
Iron-sulfur relationship - - P o p u l a t i o n I. For those samples, the quantity
of sulfur is constant ( < 1 wt%). HI values vary
Lower (3'cle Figure 8 illustrates the relations between 240 and 550 (Fig. 9). There is no positive
between the contents in iron and in sulfur, relation between S and Fe (expressed in wt%). The
expressed in weight percent. Two populations can quantity of sulfur (pyrite) is constant whatever the
iron content. There is no relation between sulfur
content and TOC values or HI values either .
S(bUlk,wt%) [Lower cycle I - - P o p u l a t i o n II. For those samples, the sulfur-
8 content shows some correlation with the iron-
content (Fig. 8). This second population contains
the samples belonging to the most organic-matter-
[populationII] rich part of the TOC curve (Fig. 9), i.e. these
o
o samples show high TOC and HI values.
o
O~°o Upper cycle The samples from this cycle show a
good correlation between S-content and Fe-
2. ~ Oe~V-'-'-'-'-[populationI]
0
2
•
4 6 8 10
Lower cycle
TOC (wt%)
Fe (bulk, wt %) HI (mg HC/gTOC)
800

700 .t
4 9
S (bulk, wt%) I Upper cycle t
600 1
8
500 -~ '7
1
6. 400 /

1 =
5
3001
4. o

o oo °e 2001 "3

2- °~ °° 100t
04 , , 1

~o°~; °- 128,1 1 2 8 , 3 128,5 128,7 128,9 129,1 129,3

i i
2 4 6 8 10 HI Depth (m)
TOC
Fe (bulk, wt%)
Fig. 9. TOC and HI values vs. depth crossplot, showing the
Fig. 8. Bulk Fe-bulk S crossplots for the two cycles. Lower two populations from the lower cycle, identified thanks to the
cycle: two populations may be identified (see also Fig. 9). Fe vs. S crossplot (Fig. 8).
 176 N. T R I B O V I L L A R D ET AL.

content (Fig. 8). They also have high TOC and HI Cu (ppm)
values (Table 3). 120
--Degree of pyritization. For the lower cycle,
DOP values fluctuate between 0.16 and 0.82 100, 0
whereas, for the upper cycle, they are very strong,
o•
close to 1, whatever the TOC content (Fig. 10).
This suggests that, in the latter case, nearly all the 80
00_ 0 •
reactive iron has been transformed to pyrite, being
eventually a limiting factor for pyrite formation. 60
0o
Trace-metal contents 0
40 0 o o Cu lower cycle ]
0
• Cu upper cycle
Lower cycle
These samples show an enrichment in transition 20
metals, such as Ni, Co, Cu and Zn, which is more 0 10 20 30 40
or less proportional to the increase of the TOC TOC (wt%)
values (Fig. 11). Contents in V, Mo or U are (very) NI (ppm)
low compared to average concentrations for black 300.
shales reported in the literature (e.g. Brumsack,
1986; Klinkhammer and Palmer, 1991; Hatch and
Leventhal, 1992). Furthermore the trace-metal
200
concentrations show little variability, in the lower
cycle (Fig. 5).
o°t ..
Upper cycle I O0
The samples of this cycle are, by far, richer in 9' ©
organic-matter carbon. Nevertheless, they do not o Ni lower cycle I
• Ni upper cycle
show any enrichment in transition metals propor-
i i J
tional to TOC values (e.g. Cu or Ni), suggesting a
0 10 20 30 40
TOC (wt%)

DOP Fig. 11. Diagrams of copper content and nickel content plotted
against T O C for the two cycles studied.
1,0
• •• QO¢O • • • •
threshold effect (Fig. 11). Conversely, these
0,8 • % •O
samples are enriched in V and Mo, reaching usual
O contents of black shales (Mo-enrichment is propor-
0,6 tional to TOC content). Uranium does the same
0
0 to a lesser extent. On the other hand, sediments
0,4 from the upper cycle are a little depleted in Mn
relative to those from the lower cycle (Fig. 5).
o ] • DOP upper cycle I Cadmium is also enriched in the upper cycle
0,2
o o DOP lower cycle relative to the lower one.

0,0 i ! i
Barium
0 10 20 30 40
Barium is considered to be a marker of biogenic
TOC (wt%)
sediments or productivity (Papavassiliou and
Fig. 10. D O P - T O C crossplot for the two cycles studied. Cosgrove, 1982; Hite-Prat, 1985; Schmitz, 1987;
()I~.(}ANIC-MATI[!R RICH (ZYCLES F R O M K I M M E R I D G E ('LAY O F Y O R K S H I R E : P R O I ) [ C T I V I I Y V E R S U S A N O X I A 177

Arthur and Dean, 1991). Here, Ba correlates nei- Nevertheless, environmental conditions, being
ther with TOC nor CaCO 3 but with AI20 3. This dysoxic and not anoxic, may not have been reduc-
suggests that Ba was supplied to the basin together ing enough to allow the reduction and the adsorp-
with terrigenous inputs (feldspar or clays: Ba may' tion, and thus active accumulation, of abundant
replace K in illite). V, Mo, Cr or U.
In the upper cycle, the peak of the TOC also
Discussion corresponds to the abundant presence of the same
type of algal OM, as identified in the lower cycle.
1. The very small variation of the elemental The content of Mo, V and U is more variable as
ratios such as Si/AI, K/A1 and illite/kaolinite (Figs. well as the values of I.ox. This suggests that the
3 and 5) suggests that the terrestrial detrital influx redox conditions of the depositional environment
was very constant in composition. However, with were probably less steady than during the depos-
the present data, we cannot exclude that some ition of the lower cycle. Furthermore, the samples
variations in the accumulation rate of the terrige- from the upper cycle are relatively enriched in V,
nous components occurred within the cycles or Mo and U, and depleted in Mn, compared to the
from one cycle to the other. lower cycle. This indicates that the sediment of the
2. The productivity of the carbonate-walled upper cycle have encountered reducing conditions
plankton (represented by the CaCO3 content) and more drastic than experienced by the sediments of
of the organic-matter-walled plankton (represented the lower cycle. An alternative explanation would
by the TOC values) do not fluctuate in phase nor be that the sedimentation rate had been higher
show an inverse relationship. This means that the during the deposition of the lower cycle than
cyclic TOC distribution cannot be explained by during the deposition of the upper one, which
dilution effects caused by the carbonate pro- could have influenced trace-metal accumulation.
duction. It also implies that both these productivit- The V/Ni ratio may be used as an indicator of
ies did not react to the same factors or stimuli of the relative importance of active accumulation and
the milieu. passive accumulation of trace metals. Passive accu-
3. The lower cycle shows important differences mulation occurs for metals being trapped with OM
in the nature and relative abundance of the org- (complexes with Ni, Co, Cu, or Zn). Active accu-
anic-matter components. The peak of the TOC mulation occurs for metals whose fixation depends
content coincides with the abundant appearance on reducing conditions. So, V/Ni gives relative
of a specific," type of algae. The low content of indications on oxygenation conditions. In both
trace metals such as Mo, V, U and their regular cases, the V/Ni ratio values are correlated nega-
distribution (Fig. 5) indicate that environmental tively with TOC (Fig. 12). This means that the
conditions were only slightly reducing and must increase in the OM content is followed by the Ni-
have been steady during the deposition of the concentration but not by the V-concentration. It
sediments of the lower cycle. Thus, the variations might be deduced that the trace-metal enrichment
observed in the composition and distribution of was primarily determined by the OM accumulation
the organic matter are probably due to variations (by complexes formed with Ni, Cu, Zn, Co) and,
in productivity and not to preservation conditions secondly, by redox-conditions (favouring V, Mo
in the depositional environment. This is also con- and U precipitation),
firmed by the consistency of the degree of oxidation DOP values fl'om the upper cycle are higher
of kerogens (I. ox.). For the samples of the lower than those from the lower cycle ll::ig. 10). This
cycle, a good correlation exists between TOC indicates that during deposition of the upper cycle,
values and Ni-, Ca-, Co-content. It is suggested sulfate-reduction reactions were more etticient,
that within the water column and at the wat- producing much H2S and transforming most of
er sediment interface, the presence of large quanti- the reactive iron into sulfides. In other words,
ties of algal OM favoured the complexation and conditions were more anoxic and favorable for
thus, the passive accumulation of these metals. sulfate reduction. For the upper cycle, DOP values
178 N. T R I B O V I L L A R D ET AL.

V/N i if little or no H2S had been produced by their

6" decomposition. A possible explanation is that the
ultralaminar structure of this OM made it resistant
O V/Ni lower cycle to the attacks of sulfate-reducing bacteria (see also
• V/Ni(lupper cycle
Derenne et al., 1991 and Largeau et al., 1990 for
data on bioresistant ultralaminar amorphous OM).
4¸ The samples from the upper cycle show an
abundant, orange amorphous OM (gel-like ultra-
3 structure). The same relation is found between the
0 S- and Fe-content as for population II of the
lower cycle.
2' %¢
o~ Conclusions

The above observations allow to formulate the

0 i i i hypothesis that a regular variation in the produc-
0 10 20 30 40 tivity of organic-matter-walled plankton is the
TOC (wt%)
main driving force responsible for the cyclic TOC.
Fig. 12. V/Ni ratio values TOC crossplot for the two cycles When productivity was strong but not exception-
studied. ally high (lower cycle, TOC < 10%), the deposi-
tional environment was only slightly reducing.
nearly reach 1, even when TOC values varie widely. When productivity was extremely high (upper
This suggests that iron was a limiting factor for cycle, TOC exceeding 30%), the presence of abun-
pyrite formation. The consequence is that smaller dant, reactive OM favoured the production of very
amounts of sulfur were trapped in the sediment as large quantities of HzS through bacterial decompo-
pyrite, whereas larger amounts of H2S were sition. In that case, the availability of reactive iron
returned to the marine environment. The released was the limiting factor for iron-sulfide formation.
HzS then probably poisoned the environment at The H2S, not fixed by reaction with iron, returned
the sea floor. to the marine water, and caused the redox-cline to
4. In the lower cycle, two types of amorphous rise in the water column. This implies that beside
OM are distinguished. The first type, the orange phytoplanktonic productivity as the driving force
amorphous OM, has a gel-like ultrastructure and for OM accumulation, also variations of environ-
shows high HI values. This amorphous OM is the mental conditions influenced OM storage in turn,
dominant (if not exclusive) organic-matter product acting as a positive feedback mechanism.
of samples forming population II. Their decompo- The scheme proposed here is not intended as a
sition by sulfate-reducing bacteria produced HzS substitute for previous, convincing, models, rather
which formed sulfides with reactive iron, as sug- it aims to complement them. In a marine environ-
gested by the good correlation between S and Fe ment prone to water stratification and where varia-
contents of population II. tions in the accumulation rate may be evoked, this
The second type of OM is found in the palynofa- scheme tries to show how TOC cyclicity may be
cies of samples from population I, which are explained (at least parly) by variations in organic-
dominated by a brown amorphous OM with ultra- matter-walled phytoplankton productivity.
laminar structures. The HI values of these samples Related phenomena induced by these variations
are lower than those o f the samples from popula- can in turn affect environmental conditions. Most
tion II, although they remain relatively high authors invoke water stratification as a characteris-
(Fig. 9). For population I, there is no relation tic parameter of the depositional conditions of the
between S-content and the abundance of OM, as KCF. Here it is suggested that the water stratifica-
()RGANIt'-MATTER RICH ('YCLES FROM KIMMERIDGE (;LAY OF YORKSHIRE: PRODLCTIVIqY VERSUS ANOXIA 179

tion c o u l d find its o r i g i n in the a m o u n t o f H 2 S carbonaceous rocks: a review of geochemical controls during
deposition and diagenesis. Chem. Geol., 91:83 97.
released d u r i n g O M b a c t e r i a l d e c o m p o s i t i o n .
Brumsack, H.J., 1986. The inorganic geochemistry of
F o l l o w i n g the q u a n t i t y o f o r g a n i c - m a t t e r p r o d u c t s Cretaceous black shales (DSDP leg 41) in comparison to
a c c u m u l a t i n g a n d b e i n g d e g r a d e d by sulfate- modern upwelling sediments from the Gulf of California. In:
r e d u c i n g b a c t e r i a ( w h i c h is g o v e r n e d by p h y t o - C.P. Summerhayes and N.J. Shackleton IEditors), North
p l a n k t o n i c p r o d u c t i v i t y in s u r f a c e water), t h e Atlantic Palaeoceanography. Geol. Soc. Spec. PUN., 21:
447 462.
amount of released H2S would have fluctuated, Brumsack, H.J, and Gieskes, J.M., 1983. Interstitial water
c a u s i n g the rises a n d falls in the r e d o x cline in the trace-element chemistry of laminated sediments of the Gulf
water column. of California (Mexico). Mar. Chem,, 1 4 : 8 9 106.
The detailed study of organic matter and of Calvert, S.E. and Morris, A.J., 1977. Geochemical study of
organic-rich sediments from the Namibian shelf. II. Metal-
g e o c h e m i c a l p a r a m e t e r s such as V, M o , M n , U ,
organic association. In: M. Angel (Editor), A Voyage of
t o g e t h e r w i t h the o x i d a t i o n d e g r e e o f O M calcu-- Discovery. Pergamon, London, pp. 580 667.
lated f r o m i n f r a r e d s p e c t r o s c o p y , a l l o w s us to Calvert, S.E., Muhkerjee, S. and Morris, R.J., 1985. Trace-
decipher subtle variations of oxygenation condi- metals in fulvic and humic acids from modern organic-rich
sediments. Oceanol. Acta, 8:167 173.
tions o f d e p o s i t i o n a l m i l i e u s a n d h e l p s to distin-
Calvert, S.E., Bustin, R.M. and Pedersem T.F., 1992. Lack ot"
guish b e t w e e n the critical p a r a m e t e r s d e t e r m i n i n g evidence for enhanced preservation of sedimentary organic
OM storage. matter in the oxygen mininmm of the Gulf of California.
Geology, 20:757 760.
Acknowledgements Calvert, S.E. and Pedersen, T.E., 1992. Organic carbon accumu-
lation and preservation in marinc sediments: how important
is anoxia ? In: J.K. Whelan and J.W. Fartington (Editors),
We t h a n k I F P , T o t a l a n d S N E A ( P ) for t h e i r Organic Matter. Production, Accumulation and Preservation
p a r t n e r s h i p . W e also t h a n k J.-P. H e r b i n a n d P. in Recent and Ancient Sediments, Columbia Univ. Press,
T r e m b l a y f o r t h e i r help. T h i s w o r k b e n e f i t t e d f r o m New York, pp. 231 262.
Combaz, A., 1980. Les k~rogbnes vus au microscope. In: B.
the s t i m u l a t i n g a n d c o n s t r u c t i v e c o m m e n t s o f P.B.
Durand (Editor), Kerogen. Technip, Paris, pp. 55 -Ill.
W i g n a l l , F.S.P. Van B u c h e m a n d R . A . B e r n e r , to Cox. B.M. and Gallois, R.W., 1981. The stratigraphy of the
w h o we are g r a t e f u l . Kimmeridge Clay of the Dorset type area and its correlation
T h i s is a c o n t r i b u t i o n o f the " G r o u p e m e n t de with some other Kimmcridgian sequences. Rep. Inst. Geol.
R e c h e r c h e d u C e n t r e N a t i o n a l de la R e c h e r c h e Sci.. 80/4.
Dercnne, S., Largeau, C., CasadevM1, E., l-~crkaloff. C. and
S c i e n t i f i q u e G D R 942 C N R S : R e l a t i o n s et proces-
Rousseau, B., 199l. Chemical evidence of kerogcn formation
sus organo-min&aux en environnements in source rocks and oil shales via selective preservation of
s ( d i m e n taire ~"'. thin resistant outer walls of microalgae: origin of ultralami-
nae. Geochim. Cosmochim. Acta, 55:1(141 1050.
Disnar, J.R., 1980. Etude experimentale c~e la fixation de
References melaux par un materiel s6dimentaire actuel d'origine algaire,
I1. Fixation in vitro de U022 ~, Cu ~', Ni 2 ~, Zn 2', Pb -'~,
Arthur, M.A. :and Dean, W.E., 1991. A holistic geochemical Co 2~ ainsi que de VOf, MoO42-. Geochim. Cosmochim.
approach to cyclomania: example from Cretaceous pelagic Acta, 45:363 379.
limestone sequences. In: G, Einsele, W. Ricken and A. Disnar, J.-R. and Trichet, J., 1983. Simulation expdrimentalc
Seilacher (Editors), Cycles and Events in Stratigraphy. de la diagen~Ssede complexes organometalliques. Implications
Springer, Berlin, pp. 126 166. metallogdniques. C.R. Acad. Sci. Paris, (ll) 296:631 634,
Benalioulhaj, S. and Trichet, J., 1990. Comparative study by Espitalid, J., Deroo, G. and Marquis, F., 1985. La pyrolyse
infrared spectroscopy of the organic matter of phosphate- Rock Eval et ses applications. Part A. Rex,. Inst. Fr. P6t.,
rich (Oulad Abdoum basin) and black shale (Timahdit basin) 40(5): 563 579.
series. Org. Geochem., 16:649 660. Espitali& J., Deroo, G. and Marquis. F., 1986. La pyrolysc
Berner, R.A., 1970. Sedimentary pyrite formation. Am. J. Sci., Rock Eval et ses applications. Par! B. Rev. Inst. Fr. P~t.,
268:1 23. 40(6): 755 784.
Bertrand. P. and Lallier-Verg6s, E., 1993. Past sedimentary Gallois, R.W., 1976. Coccolith blooms in the Kimmeridge (-'lay
organic mailer accumulation and degradation controlled by and origin of the North Sea oil. Nature, 259: 473-475.
productivity. Nature, 364:786 788. Gorin, G.E. and Feist-Burkhardt, S,, 1990. Organic facies of
Bordenave, M.L.. 1993. Applied Petroleum Geochemistry. early to middle Jurassic sediments in the Jura Montains.
Technip, Paris, 524 pp. Switzerland, Rev. Paleobol. Palynol., 65:349 355.
Breit, G.N. and Wanty, R.B, 1991. Vanadium accumulation in Hallam. A. and Sellwood, B.W,, 1976. Middle Mesozoic sedi-
180 N. TRIBOVILLARDET AL.

mentation in relation to tectonics in the British area. J. Geol., Oschmann, W., 1988. Kimmeridge Clay sedimentation--a new
84: 301-321. cyclic model. Palaeogeogr., Palaeoclimatol., Palaeoecol., 65:
Hatch, J.R. and Leventhal, J.S., 1992. Relationship between 217-251.
inferred redox potential of the depositional environment and Papavassiliou, C.T. and Cosgrove, M.E., 1982. The geochemis-
geochemistry of the upper Pennsylvanian (Missourian) Stark try of DSDP sediments from site 223 (Indian Ocean). Chem.
Shale Member of the Dennis Limestone (Wabaunsee County, Geol., 37: 299-315.
Kansas, USA). Chem. Geol., 99: 65-82. Pedersen, T.F. and Calvert, S.E., 1990. Anoxia vs. productivity:
Herbin, J.P., Mfiller, C., Geyssan, J.R., Melieres, F., Penn, what controls the formation of organic-carbon rich sediments
I.E. and the Yorkim group (IFP), 1991. H6t6rog~n6it6 quanti- and sedimentary rocks ? Am. Assoc. Pet. Geol., 74 (4):
tative et qualitative de la mati+re organique dans les argiles 454-466.
du Val de Pickering (Yorkshire, U.K.): cadre s6dimentolog- Pilipchuk, M.F. and Volkov, I.I., 1974. Behavior of molybde-
ique and stratigraphique. Rev. Inst. Fr. P&r., 46 (6): 675-712. nium in processes of sediment formation and diagenesis in
Herbin, J.-P., Masure, E. and Roucach6, J., 1986. Cretaceous Black Sea. In: E.T. Degens and D.A. Ross (Editors), The
formations from the lower continental rise off Cape Hatteras: Black Sea--Geology, Chemistry and Biology. AAPG Mere.,
organic geochemistry, dinoflagellate cysts and the 20: 542-553.
Cenomanian-Turonian boundary event at sites 603 (leg 93) Pradier, B. and Bertrand, P., 1992. Etude fi haute r6solution
and 105 (leg 11). In: J.E. Van Hinte, S.W. Wise Jr. et al., d'un cycle du carbone organique des argiles du Kimmeridgicn
Init. Rep. DSDP, 93: 1139-1162. du Yorkshire (G.B.): relations entre composition p6tro-
Hite-Prat, S., 1985. Les "Marnes Bleues" ~i nodules barytiques graphique du contenu organique observ~ in situ, teneur en
du Gargasien et de l'Albien de la fosse vocontienne (SE de carnobe organique et qualit6 p6trolig~ne. C.R. Acad. Sci.
la France): etude gbochimique et min6ralogique. Heritage, Paris, 315 (2): 187 192.
diagen6se and hydrothermalisme. Thesis. Univ. Lyon I, Pratt, L., 1984. Influences of paleoenvironmental factors on
1644, 143 pp. preservation of organic matter in the middle Cretaceous
Huc, A.Y., 1988. Sedimentology of organic matter. In: F.H. Green Horn Formation, Pueblo, Colorado. AAPG Bull.,
Frimmel and R.F. Christman (Editor), Humic Substances 68(9): 1146-1159.
and Their Role in the Environment. Wiley, New York, Pruysers, P.A., De Lange, G.J. and Middelburg, J.J., 1991.
pp. 215-243, Geochemistry of eastern Mediterranean sediments: primary
Huerta-Diaz, M.A. and Morse, J.W., 1990. A quantitative sediments composition and diagenetic alteration. Mar. Geol.,
method for determination of trace metal concentrations in 100: 137-154.
sedimentary pyrite. Mar. Chem., 29:119-144. Raiswell, R., Buckley, F., Berner, R.A. and Anderson, T.F.,
Huerta-Diaz, M.A. and Morse, J.W., 1992. Pyritisation of trace 1988. Degree of pyritisation as a paleoenvironmental indica-
metals in anoxic marine sediments. Geochim. Cosmochim. tor of bottom-water oxigenation. J. Sediment. Petrol., 58:
Acta, 56: 2681-2702. 812-819.
Irwin, H., 1979. On an environmental model for the type Ramanampisoa, L., Bertrand, P., Disnar, J.R., Lallier-Verges,
Kimmeridge Clay--A discussion. Nature, 279: 819. E., Pradier, B. and Tribovillard, N.-P, 1992. Etude ft. haute
Jacquin, T., 1987. Analogies et differences entre les black shales r6solution d'un cycle du carbone organique des argiles du
du Cr6tac~ inf~rieur et sup~rieur de l'Atlantique Sud. Bull. Kimmeridgien du Yorkshire (G.B.): r6sultats prSliminaires
Soc. G6ol. Fr., 8, 3: 705-714. de g6ochimie et de p6trographie organique. C.R. Acad. Sci.
Klinkhammer, G.P. and Palmer, M.R., 1991. Uranium in the Paris, 314 (2): 1493-1498.
oceans: where it goes and why. Geochim. Cosmochim. Acta, Robin, P.L., Rouxhet, P.G. and Durand, B., 1977.
55: 1799-1806. Caract6risation des k~rog6nes et de leur ~volution par
Largeau, C., Derenne, S., Casadevall, E., Berkaloff, C., spectroscopie infrarouge. In: M. Bjoroy (Editor), Advance
Corolleur, M., Lugardon, Raynaud, J.F. and Connan, J., in Organic Geochemistry 1975. ENADIMSA, Madrid,
1990. Occurrence and origin of "ultralaminar" structures in pp. 693 716.
"amorphous" kerogens of various source rocks and oil Rochdi, A., Landais, P. and Burneau, A., 1991. Analysis of
shales. Advance in Organic Geochemistry 89. Org. coal by transmission FTIR microspectroscopy: methodologi-
Geochem., 16: 889-895. cal aspect. Bull. Soc. G+ol. Fr., 162: 155-162.
Lyons, T.W. and Berner, R.A., 1992. Carbon-sulfur-iron Schmitz, B., 1987. Baryum equatorial high productivity and
systematics of the uppermost deep-water sediment of the the northward wandering of the Indian continent.
Black Sea. Chem. Geol., 99: 1-27. Paleoceanography, 2: 67-78.
Miller, R.G., 1990. A paleoceanographic approach to the Scotchman, I.C., 1989. Diagenesis of the Kimmeridge Clay,
Kimmeridge Clay Formation. In: A.Y. Huc (Editor), onshore U.K.J. Geol. Soc., 146: 285-303.
Deposition of Organic Facies. AAPG Stud. Geol., 30:13 26. Stein, R., 1986a. Organic carbon and sedimentation r a t e - -
Myers, K.J. and Wignall, P.B., 1987. Understanding Jurassic further evidence for anoxic deep water conditions in the
organic-rich mudrocks. New concepts using gamma-ray spec- Cenomanian-Turonian Atlantic Ocean. Mar. Geol., 72:
trometry and palaeoecology: examples from the Kimmeridge 199-209.
Clay of Dorset and the Jet Rock of Yorkshire. In: J.K. Stein, R., 1986b. Surface-water pateoproductivity as inferred
Leggett and G.G. Zuffa (Editors), Marine Clastic from sediments deposited in oxic and anoxic deep water
Sedimentology. Graham and Trotman, London, environments of the Mesozoic Atlantic Ocean. Mitt. Geol.
pp. 172-189. Palaeontol. Inst. Univ. Hamburg., 60:55 70.
O R G A N I ( -MA FI'ER RICH CYCLES F R O M K I M M E R I D G E CLAY OF Y O R K S H I R E : P R O D U C T I V I T Y VERSUS ANOXIA [E[

Tissot, B. and Welte, W.D., 1984. Petroleum Formation and of marine petroleum source rocks. In: ]. Brooks and
Occurrences. Springer, Berlin, 482 pp. A.J. Fleet (Editor), Marine Petroleum Source Rock. Geol.
Tribovillard, N.-P., 1988. Macanismes de la sadimenlation Soc. Spec. Publ., 26:47 67.
marneuse en milieu palagique semi-euxinique. Exemples dans Tyson, R.V., Wilson, R.C.L. and Downie, C., 1979. A stratified
lc Mesozo'/que du Sud-Est de la France et de l'Atlantique. water column environment model for the Klmmeridge Clay.
Doc. Lab. Gaol. Lyon Univ., 109, 119 pp. Nature, 277: 377-380.
Tribovillard, N.-P,, Gorin, G., Belin, S., Hopfgartner, G. and Wanty, R.B. and Goldhaber, M.B., 1992. Thermodynamics
Pichon, R., 1992. Organic-rich biolaminated facies from a and kinetics of reactions involving vanadium in natural
Kimmeridgian lagooual environment in French Southern systems: accumulation of vanadium in sedimentary rock.
Jura Mountains A way of estimating accumulation rate Geochim. Cosmochirn. Acta. 56: 1471-1483.
varialions. Palaeogeogr.. Palaeoclimatol., Palaeoecol., 99: Wignall, P.B., 1989. Sedimentary dynamics of the Kimmeridge
163 177. Clay: tempests and earthquakes. Jour. Gec,l. Soc. London.
Tyson, R.V., 1987. The genesis and palynofacies characteristics 146:273 284.