Cyclic fluctuations of anoxia during Cretaceous time in the South Atlantic Ocean
T. Jacquin Centre des Sciences de la Terre (UA 157 CNRS), Universit6 de Bourgogne, 6 Bd. Gabriel, 21000 Dijon, France
and P. Ch. de Graciansky Ecole Nationale Sup6rieure des Mines de Paris, 60 Bd. St. Michel, 75272 Paris Cedex 06, France
Received 13April 1987; revised 26 May 1988; accepted29 May 1988
The holes of the DSDP-IPOD program in the South Atlantic Ocean document two major anoxic events during Oxfordian to middle Albian times and secondly from late Cenomanian to Santonian times. The black shales formed during these two anoxic events differ in their rhythmicity and origin•
During Lower Cretaceous time, the anoxic conditions resulted from the confined, euxinic nature of the basins. The rhythmicity of these black shales probably does not result from a global phenomenon (climatic or tectono-eustatic), but from local conditions resulting from the slender dimensions of the young ocean basin(s). The diversity and the diachroneity of the deposits from the south to the north precisely reflect the dynamics of the oceanic spreading.
During Upper Cretaceous time, the anoxic conditions fluctuated in relation to a mid-water oxygen-minimum zone. The rhythmicity of black shale deposition seems to result from a global phenomenon, because of the widespread occurrence of the event. In the South Atlantic ocean, the cyclic fluctuations of anoxia were due to cyclic variations in the depth of the mid-water oxygen-minimum zone. There is no simple process to explain such rhythmicity. It probably results from the interplay of the three main variables which characterize the oceans at the time of the Cenomanian-Turonian boundary: the increased rate of sea floor spreading, high sea-levels and low water-circulation.
The occurrence of organic-carbon-rich strata (black shales) in certain portions of Jurassic and Cretaceous sequences has been well documented from Deep Sea Drilling Project sites in the South Atlantic ocean. These DSDP sites were located in the Angola basin (sites 364 and 530), on the Walvis-Rio Grande ridge (sites 363 and 356), in the Cape basin (site 361) and on the Falkland Plateau (sites 327,330 and 511) (Figure 1). The diversity of these organic-carbon-rich sediments is well known and the problem of their origin has been under discussion for about ten years (Thiede and Van Andel, 1977; Dean et al., 1978; Gardner et al., 1978; Herbin and Deroo, 1979; Arthur and Natland, 1979; Arthur et al., 1987; Graciansky et al., 1986; Jacquin, 1987).
Our study had three objectives:
• To describe the lithology, the distribution and the organic-carbon content (% and nature) of the black shales and associated sediments; in particular to distinguish between autochthonous and redeposited sediments.
• To define the sedimentary rhythmicity and to explain the cyclic preservation of organic matter (variation of the sedimentary supply or variation of the redox conditions on the sea-floor and in the sea-column).
• To interpret the sedimentary environments in terms of the interplay between local variables and more global ones, such as major discontinuities, periods of anoxia, eustatic transgressions and oceanic spreading.
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Cretaceous anoxia fluctuations: T. dacquin and P. Ch. de Graciansky
F l u c t u a t i o n o f t h e a n o x i a d u r i n g C r e t a c e o u s t i m e delay from the south to the north, beginning as early as
Studies and analysis of South Atlantic DSDP cores allowed us to define the age of two oceanic anoxic events: OA l (Ti thonian-Neocomian) and OA 2 (Cenomanian-Turonian) .
Tithonian and Lower Cretaceous According to their location south (Cape basin and Falkland plateau) or north (Brazil and Angola basins) of the Walvis Rio Grande ridge, organic-carbon-rich sediments were deposited in different environments and at different times (Figure 2).
To the south, the anoxic deposits occurred as early as Oxfordian time, whereas to the north they only began to accumulate in Albian times on Aptian evaporites. The cessation of anoxic conditions shows a similar
Late Aptian time on the Falkland plateau, at Early Albian time in the Cape basin and only during Middle Albian time in the Angola basin. In each case studied, the anoxia decreased as the sea-floor deepened from south to north.
Upper Cretaceous
The differentiation into distinct basins became blurred. The maximum of the anoxia during Early and Middle Turonian time occurred synchronously both in the South Atlantic Ocean and in the North Atlantic and even on its margin (Arthur et al., 1987; Gracianky et al., 1986). This maximum is the well documented 'Oceanic Anoxic Event ' of the Cenomanian-Turonian boundary.
LITHOLOGICAL FEATURES AND
CHARACTERIZATION
OF THE
ORGANIC MATTER (after Herbin et al., 1987)
SILICICLASTIC TURBIDITES : Site 361 l xt • oo~ / d .t
/ Black shale L ~ ,~ ~ . (marine organic M)~,o,iJ
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REDEPOSITED LIMESTONES: Site 364 ~ - - ~
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I VARIATION IN PLANKTONIC PRODUCT : x l / .
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364
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S.. ! l Black shale '~i ire ~ (marine organic M ~ '~I ~ [~o
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I I I
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Figure3 Different types of Late Jurassic and Lower Cretaceous occurrences due to variations in sedimentary supply within a silled stratified basin. Rock-Evil pyrolysis data after Herbin eta/. (1987). Size of the circles is proport ional to the total organic carbon content. (HI: hydrogen index mg HC/g TOC, O1: oxy£~.n index mg CO2/g TOC)
Mar ine and Pet ro leum Geology, 1988, Vol 5, N o v e m b e r 361
Cretaceous anoxia fluctuations: 7". Jacquin and P. Ch. de Graciansky
This anoxic event was preceeded by a sedimentary gap of ocean-wide extent (event E 2 of Graciansky et al., 1982). In the South Atlantic, the hiatus is more marked in the south (Falkland plateau), where Cenomanian and Turonian sediments are absent, than in the north (Angola and Brazil basins), where only Early and Middle Cenomanian sediments are missing (Magniez-Jannin and Muller, 1987).
The end of intense anoxic conditions during Late Turonian-Lower Coniacian time also coincides with a hiatus or a very low sedimentation rate (less than 1 m/My). Anoxia ultimately disappeared from the northwestern portions of the South Atlantic during Santonian time, when further deepening of the sea floor occurred.
Rhythmic bedding and deposit ional environments during the first anoxic event (Oxfordian-Alb ian)
The South Atlantic anoxic series are never dark and organic-carbon-rich for any great thickness, but on the contrary, consist of redox cycles of interbedded more and less reduced lithologies with variable amounts of organic matter.
Three cases have been recognized. They show that the rhythmic bedding is due to variations in sedimentary supply within a silled stratified basin (Figure 3):
Intercalation of siliciclastic turbidites within a strongly anoxic environment (Site 361 example - - Cores 49 to 29 - - Cape Basin - - Early to Late p.p. Aptian time)
The redeposited sediments are turbiditic sandstones rich in terrigenous material comprising terrestrial
organic matter (OM) with preserved coal particles and large amounts of chlorite, kaolinite and illite. The hemipelagic intervals of the sequences consist of finely laminated black shales richer in marine OM (up to 18% TOC) with an HI reaching 800 mg HC/gTOC (Herbin et al., 1987) and richer in mixed-layer clays (Illite-Smectite or Chlorite-Smectite).
The lack of bioturbation and a benthic fauna, the good preservation of organic matter (large amount with high hydrogen index) and the accumulation of pyrite, other sulphide minerals and sodium minerals in black shales (Natrojarosite, Analcime, Na-Smectite) suggests a very confined azoie, depositional environment.
Intercalation of organic-poor limestone within a strongly anoxic environment (Site 364 example - - Cores 46 to 26 - - Angola basin - - Early Albian time)
The redeposited sediments are white dolomitic and micritic limestones, sometimes fossiliferous, but always poor in organic matter (TOC < 0.8%). The black shales were deposited within a strongly anoxic environment (12<TOC<19% - - HI>600), but are sometimes autochthonous (finely laminated) and sometimes redeposited (sharp basinai contacts and small-scale grading lamination). Anoxia would have had to be very strong on the sea floor to preserve organic matter from oxidation during redeposition or high-velocity currents.
Cyclic variation in planktonic productivity (nannofossils, radiolarians and foraminifers) above anoxic bottom-sea-water (Site 511 example - - Falkland plateau - - Cores 62 to 59
LITHOLOGICAL FEATURES AND
CHARACTERIZATION
OF THE
ORGANIC MATTER (after Herbin et al., 1987)
o q
z F O R A M . .<
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• ~ ' . . . . ~ Burrowea grayish to ~ ' ~ " . . . . . . . . . . | w h i t i s h m a r l ( w i t h o u t O M ) ~ J :
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I !I Figure 4 Types of occurences of early Albian t ime (site 511) and middle Albian t ime (site 364) deposited during cessation of anoxia
362 Marine and Petroleum Geology, 1988, Vol 5, November
Cretaceous anoxia fluctuations: T. Jacquin and P. Ch. de Graciansky
L I T H O L O G I C A L F E A T U R E S AND
z FORAM. C H A R A C T E R I Z A T I O N ~ ~ ,~
O F THE ~ ,,~ < ~- .c
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CYCLIC F L U C T U A T I O N S IN C A R B O N A T E
D I S S O L U T I O N : Si te 364
~ V arilol|t:::: (~ ~¢~ $ t o 13.e r.,
' ~ ' - - - ~ . . t Bur rowed whi t i sh marl ~ ( ' ~ I ( w i t h o u t OM) " ~
_--.---- Green c l a y s t o n e ( a l t e r e d OM)
b u r r o w e d c o n t a c t
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, ' ' ! I , ' I ,
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Figure 5 Types of occurrences of Upper Cretaceous time due to cyclic fluctuations in carbonate dissolution within a mid-water oxygen-minimum zone. Gradational contacts between oxic layers (burrowed whitish marl) and anoxic layers (varicoloured claystone and black shale) suggest in situ deposition (without redeposition) and changes from oxic to anoxic conditions in the bottom-sea water
- - B a r r e m i a n ? to Early Aptian time; Site 364 example - - Angola basin - - Cores 41 to 39 - - Middle Albian time).
Finely laminated black sapropelic shales ( 3 < T O C < 1 6 % ) are interbedded with laminated grey marls always containing some organic matter ( T O C < 3 % ) . The contacts between the two lithologies are transitional. Benthic foraminifers and benthic metazoans are absent even in the calcareous sediments. By contrast, the gray marls contain only planktonic species (calcareous nannofossils, foraminifers or radiolarians). The rhythmic bedding is thus due to variations of high surface water productivity above anoxic bottom-water , without enhanced oxygenation of the sea floor.
This type of occurrence without discontinuity and sharp contact, is very common in the Lower Cretaceous sequences of the Atlantic ocean and has been well documented in the Late Jurassic-Lower Cretaceous series of the Falkland plateau, the Middle Albian sections of the Angola basin and also in the eastern and western basins of North Atlantic (Dean et al., 1978; Dean and Gardner , 1982; Graciansky et al., 1982; Arthur et al., 1984; Cotillon and Rio, 1984; Jacquin, 1987).
When the first anoxic event ended, the sedimentary sequences reacted to enhanced oxygenation of the sea-water in two ways (see Figure 4):
Calcareous sediment supply within an increasingly oxygenated basin (Site 511 example - - Falkland plateau - - Cores 58 to 56
- - Late Aptian - - Early Albian time)
Calcareous allochthonous sediments, which are bioturbated and rich in benthic and plankotonic foraminifers, occur more and more frequently. The organic matter of the autochthonous shales becomes altered and of terrestrial origin (low TOC content and low HI), showing the disappearance of anoxic conditions.
Organic shale supply within a yet oxygenated basin (Site 364 example - - Angola basin - - Cores 39 to 37 - - Middle Albian time)
The organic shales are laminated with small-scale grading. They show sharp basinal contacts with other lithologies and an organic matter content enriched in terrigenous material. All this suggests evidence of turbiditic processes or high velocity currents during deposition, which were unconnected with the deterioration of anoxia.
In these two cases, the establishment of oxic conditions occurred with a full renewal of benthic microfauna and also corresponded to a rapid deepening of the sea floor (Magniez-Jannin and Jacquin, 1986).
Rhythmic bedding and depositional environments during the second anoxic event (Late Cenomanian-Santonian)
Homogenous and widespread character of black shales o c c u r r e n c e s
The Cenomanian-Turonian black shales are well
Marine and Petroleum Geology, 1988, Vol 5, November 363
Cretaceous anoxia fluctuations: T. Jacquin and P. Ch. de Graciansky known for their wide distribution, their low thickness and high OM content in comparison to the Barremian-Albian anoxic series (the latter represent less than 30% of core sections against sometimes 90% in the former case).
The type of sequence is always the same at each site, suggesting a homogenous depositonal environment (Figure 5). Black shales (marine organic matter and the highest TOC values measured in South Atlantic, up to 29% with HI reaching 700 mg/gTOC, Herbin et al., 1987) are interbedded with greenish claystones (low TOC content, altered and terrestrial OM) and white bioturbated limestone (sites 356 and 364) or brick-red claystones (sites 361 and 530) according to the depth of deposition.
Rhythmic bedding as a result of redox cycles
The proposed model attempts to explain 1) carbonate-clay cycles by periodic carbonate dissolution of sediments within a mid-water oxygen-minimum zone; 2) cyclic preservation of organic matter by an upward fluctuation of the oxygen-minimum layer.
1. Carbonate-clay cycles: an effect of carbonate dissolution within the mid-water oxygen-minimum zone
We illustrate this using the Late Cenomanian to Santonian redox cycles from site 364 (Angola basin). Similar redox cycles also occur in many Early to Late Cretaceous series recovered by DSDP in the eastern part of North Atlantic, off the African continental margin (sites 367-368; Dean et al., 1978; Dean and Gardner, 1982) and in the western part of the Bermuda rise (sites 386-387: Graciansky et al., 1982).
The carbonate-clay cycles have been attributed mainly to (a) periodic dilution by terrigenous material, (b) variation in productivity of calcareous plankton, (c) cyclic fluctuation in carbonate dissolution within the water column, on the sea-floor or during diagenesis (Dean et al., 1978; Einsele, 1982; Wetzel, 1982; Arthur et al. , 1984).
la. Periodic dilution by terrigenous material If this occurred, the dilution by terrigenous material
would have to have resulted from an enhanced influx of terrigenous minerals during the deposition of the clay-rich intervals. There are two reasons why this cannot have caused the cyclicity in carbonate/clay content. Firstly there is no relationship between the clay-mineral assemblages and the thickness of carbonate/clay rich intervals. Secondly the terrigenous clay supply occurs only during Coniacian-Santonian time, when the claystone beds are thinnest.
lb. Variation in productivity of calcareous planktonic species
This mechanism must also be dismissed. If it had occurred, the sedimentary cycles would show a positive relationship between carbonate content and the richness of calcareous species.
In fact, the richness of calcareous species in the claystone beds of the sedimentary cycles depends on the organic matter content. In addition, microfauna and calcareous nannoplankton from clay-rich beds manifestly are dissolved when the TOC is high. The black shale beds with their large amount of well-preserved OM are azoic, the associated greenish
and reddish claystones with low TOC values contain some calcareous and agglutinated benthic foraminifers; in addition the varicoloured marlstones contain some planktonic foraminifers.
lc. Cyclic fluctuation in carbonate dissolution From Late Cenomanian to Middle Turonian times,
the increased frequency of occurrence of argillaceous rocks and preservation of organic matter suggests the two phenomena have a common cause. Either the cyclic preservation of organic matter was induced by terrigenous supply produced by high river discharge, as suggested by RossignoI-Strick et al. (1982), or else the cyclic fluctuation in carbonate dissolution was induced by CO2 production within the anoxic waters during periods of enhanced biogenic productivity.
To prove that different argillaceous rocks result from the calcium carbonate dissolution of a single starting sediment, they must have the same mineralogical and the same geochemical characteristics. This is shown to be the case because:
• There are no mineralogical and geochemical differences, except the organic matter content, between the black shales and the reddish or greenish claystones (Figure 6).
• Sedimentary contacts between black shales and greenish claystones on the one hand, and between greenish and reddish claystones on the other, are always transitional. Some reddish beds arc due apparently to late oxidation of greenish levels in contact with white oxic limestones.
• Geochemical characteristics of the claystones (especially their low Mn and high Fe contents) suggest they were deposited under anoxic conditions (they are therefore laminated).
• A final point concerns the preservation state of different sedimentary particles in the reddish and greenish claystones. Calcium carbonate dissolution, manganese fleetingness, iron accumulation, poor preservation of organic matter are higher in the reddish claystones than in the greenish ones. All this may result from a lower sedimentation rate during reddish claystone deposition, which would have increased the residence time of sedimentary particles in the zone of oxic consumption and bacterial sulphate reduction (Curtis, 1988). It would have been different for the black shales with their higher OM contents because there is evidence of strong redox conditions which influenced the carbonate dissolution and also the accumulation of some trace elements like zinc, copper, chromium, nickel (Brumsack, 1984; Jacquin, 1987).
In summary, the alternations of claystones and marlstones, which were deposited from Late Cenomanian to Santonian time, off the African continental margin (sites 364 and perhaps 367 and 368}, result from the cyclic fluctuations in the calcium carbonate dissolution.
It is clear this dissolution cannot be attributed to an upward excursion of the CCD. The cyclic aspect of the process, in addition to the low average thickness of each redox cycle (less than 30 cm), is inconsistent with CCD fluctuations.
Further, the calcium carbonate dissolution cannot be attributed to an early diagenetic process. This would have produced a large volume change that is
364 Marine and Petroleum Geology, 1988, Vol 5, November
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BLACK SHALE
- No bioturbation
-No benthic foraminifers
- Fe*>l .6 Mn*<0
~, [ l ~ O - 0 - No bioturbation
~ ~ [ ; 2 ~ -Only some arenaceous benthic oz,, ~ 2 0 .. ~ i ~ ~ - foraminifers _ . _ _ _ [ % = 0 - F e * > l , 6 M n * < 0
: ! ' ,' i 02= 0 - Some arenaceous and calcareous 02~b, ~ ~ ; ~' benthic foraminifers
. . . . . . . . . . - -=r 02= 0 - F e * > l , 6 Mn*<0 RED CLAY
~ ~ I -Strongly burrowed -Benthic and planktonic
0 2 l ~ j foraminifers rich
~ _ a . _ * * __L_.'~ ~ - F e * < l . 6 Mn*>0
WHITE MARL
Figure 7 Models illustrating the variations of the depth of the mid-water oxygen-minimum zone above sea-floor (02 = 0 isopleth). The redox state in the bottom-sea water had to change episodically to explain the alteration of oxic and anoxic l ithologies without sharp contacts. The high content of aquatic OM indicate anoxic conditions on the sea-floor, whereas the presence of benthic foraminifers, burrowing, positive Mn* value and low Fe* value show more oxidizing conditions
inconsistent with the preservation of delicate structures and perfect forms of burrows at the claystone-marlstone interface. In addition, during early diagenetic dissolution, strontium frequently accumulates with the insoluble residue (Maillot, 1983), but here we did not find any preferential increase of Sr in the claystone beds in comparison to the calcareous beds.
Finally because of the foregoing remarks, the relationship between the organic matter preservation and the intensity of planktonic calcium carbonate dissolution, we propose that the dissolution must have occurred in the water column, within a mid-water oxygen-minimum layer.
2. Redox/carbonate cycles: a result of upward fluctuations in the mid-water oxygen-minimum layer depth (Figure 7). • When the base of the oxygen-minimum zone
reached down to the sea-floor, undersaturation with respect to calcium carbonate occurred in the oxygen-depleted water column, at the sediment-water interface and within the sediment. Marine organic matter was well preserved (high TOC and high HI), benthic metazoan activity ceased and no benthic formaminifers were present (black shales example).
• When the mid-water oxygen-minimum zone did not impinge on the bottom, but anoxic conditions existed below the sediment-water interface, the calcium carbonate dissolution could also occur in the
Graciansky water column. Only terrigenous organic matter was preserved because of the slightly oxic conditions at sea-floor. Benthic metazon activitv was always scarce, but some agglutinated benthic foraminifers were present (greenish claystones example). When oxic conditions prevailed on the sea-floor and in the surficial sediments, with a mid-water oxygen-minimum layer at mid-depths, calcium carbonate dissolution occurred as described above, but any organic matter was preserved. Burrowing and benthic foraminifers were scarce (reddish claystones example).
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02 02
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Figure 8 Reconstitution of depositional environments related to a mid-water oxygen-minimum layer during the Second Anoxic Event• A) The oxygen minimum water zone is at middle depth; 13) the base of the anoxic water layer is sinking; C) maximum extension of the anoxic conditions; D) disappearance of the anoxic conditions at every depths of the basin.
366 Marine and Petroleum Geology, 1988, Vol 5, November
Cretaceous anoxia fluctuations: T. Jacquin and P. Ch, de Graciansky
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Marine and Petroleum Geology, 1988, Vol 5, November 367
Cretaceous anoxia fluctuations: T. Jacquin and P. Ch. de Graciansky
• When oxic conditions prevailed in the water column, on the sea-floor and within the sediments, white bioturbated marlstones or reddish claystones were laid down according to the depositional depth (above or below the CCD).
Rhythmic bedding in terms of space and time
A comparison of DSDP data from sites 364 and 530, located in the Angola basin at different paleodepths (upper and lower bathyal zone), shows that the mid-water oxygen-minimum zone oscillated four times (Figure 8):
1) Late Cenomanian: the mid-water oxygen-minimum zone is at middle depth, the organic rich beds of site 530 (lower bathyal zone) were redeposited from the flanks of the basin where autochthonous anoxic conditions occurred;
2) End o f Late Cenomanian: plunging of the base of the anoxic water layer;
3) Early and Middle Turonian: maximum extension of the anoxic conditions because the anoxic beds with high TOC contents and marine OM have been recorded both at shallow depth and in the deeper parts of the basin;
3) Late Turonian to Santonian time: disappearance of anoxia at every depth in the basin.
The Late Cenomanian-Santonian series can be broken up into three hierarchical cycles (Figure 9): first order cycles consist of alternation of argillaceous and calcareous beds; second-order cycles group together first-order cycles according to the frequency and the thickness of anoxic layers; the third-order cycle consists of the whole anoxic event of the Cenomanian- Turonian boundary (and after until the Santonian).
• It is no longer necessary to prove the widespread extent of the third-order cycle, although its duration is not exactly known because of sedimentary gaps or submarine erosion which often cut into Cenomanian and Late Turonian sediments.
• Second-order cycles can appear as a succession of elementary redox cycles of constant wavelength, which oscillate on both sides of a threshold separating anoxic sediments from others (Figure 9) (House, 1985). Their wavelength seems to decrease progressively from Late Cenomanian to Santonian time. During Coniacian-Santonian time, it occurred with a 11)0-200 kyr period (with an average sedimentation rate of 20 m/103 year). Unfortunately, because of the bad recovery of the cores, it is not possible to assign precise thicknesses and, in particular, precise durations to the wavelength of Cenomanian-Turonian cycles.
• First-order cycles cross thresholds for red, greenish or black shales, according to the interbedded lithology. This may correspond to the depth fluctuations of the mid-water oxygen-minimum zone above sea floor. Their wavelength is more or less constant: 10-20 kyr with the same sedimentation rate noted above.
Conclusions
The first anoxic event The diversity of black shales occurrences from Oxfordian time in the South to Middle Albian time in the North, is consistent with the diverse range of basinal settings developed during the terminal phases of rifting and early spreading, ie, from restricted basins to a spreading ocean, from a few hundred metres to a few kilometres water-depth and from siliciclastic to carbonate sedimentation as oceanic spreading progressed from South to North.
The rhythm of turbidites and other redeposited sediments within the confined environments, including sandstone-turbidites, mud-turbidites and redeposition of shallow-water carbonate material, has to be uncertain since there is no evidence of pronounced cyclicity, even after analysis.
Productivity cycles may be the result of periodic variations in global parameters such as climate (Arthur etal., 1984; Cotillon and Rio, 1984), but they are scarce because of the overprinting effects of redcposition. In addition, their diachronieity across the Atlantic Ocean is incompatible with global factors but confirms the pronounced influence of local ones.
The second anoxic event In contrast to the first anoxic event, a large body of evidence proves the global character of the Late Cretaceous anoxic event, eg, (1) the type of sequence is always the same at every site, suggesting a more homogenous depositional environment; (2) the synchronous occurrence of anoxia at the Cenomanian-Turonian boundary over a wide range of environments, around the Atlantic and Tethyan oceans; (3) the rhythm of anoxic layers is not random, but shows a cyclic aspect which may correspond to depth fluctuations of the oxygen-minimum zone above the sea-floor. Because of the various uncertainties in establishing the frequency of observed redox cycles, assumptions made in comparing the observed rhythmicity to orbital forcing are unfounded.
The anoxic conditions causing the two events do not have the same origin. During Early Cretaceous time, the euxinic conditions were faw)ured because of the geometry of the basins and the transgressive nature of the oceans which increased organic productivity and evaporitic deposition by providing better organic and salt supply. During Upper Cretaceous time, the origins are global and independent of the local depositional environment. The Cenomanian-Turonian black-shale horizon may simply be related to the high stand sea-level of eustatic origin (Haq et al., 1987). However, another global mechanism can be considered to explain the widespread and synchronous distribution of the organic-rich sediments (Herbin et al., 1987). Additional effects of tectono-magnetic processes cannot be dismissed (Arthur, Dean and Schlanger, 1985); because throughout the South Atlantic Ocean the end of anoxia begins with an abrupt deepening of the sea-floor. Another effect of the intense mid-ocean ridge activity may be the large amounts of silica, manganese and magnesium found in the late Albian series, just prior to the anoxic event. It is plausible that the increased mineral supply due to high hydrothermal activity also caused high primary productivity.
368 Marine and Petroleum Geology, 1988, Vol 5, November
Cretaceous anoxia fluctuations: T. Jacquin and P. Ch. de Graciansky Acknowledgements Redescription of cores and appropriate sampling have been accomplished at the Lamont Doherty Geological Observatory: the authors thank the National Science Foundation and the curatorial staff of the East Coast Repository for their approval and their help. Laboratory studies were performed at the Institut Franqais du P6trole (organic geochemistry) under a contract with the Comit6 d'Etudes P6troli~res Marines (CEPM), at the Ecole Nationale Sup6rieure des Mines de Paris (mineralogy) and at the Centre des Sciences de la Terre of Dijon University (sedimentology ~nd mineral geochemistry).
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