Reconstruction of Palaeoenvironmental Change during Times of Mid-Cretaceous Black Shale Deposition: Two Examples from the Vocontian Basin (SE France) (original) (raw)
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Palaeogeography Palaeoclimatology Palaeoecology, 2006
Detailed records of spore-pollen assemblages, particulate organic matter (OM), dinoflagellate cysts and calcareous nannofossils provide new insights into the palaeoclimatic and palaeoceanographic conditions during formation of Early Cretaceous black shales in the Vocontian basin (southeastern France). The early Aptian Niveau Goguel, which corresponds to the OAE1a, and the regionally distributed late Aptian Niveau Jacob have been studied with regard to changes in terrestrial vegetation patterns, terrigenous inputs and palaeofertility conditions. Palynological results from both black shale intervals exhibit a rich and stable floral pattern, dominated by various ferns, different types of cycads, bennettites as well as by several conifer families. Dinoflagellate cyst assemblages and the calcareous nannofossil-based nutrient index show no prominent changes in surface water productivity across the two studied intervals in the Vocontian basin. Significant variations are observed in terrestrial detrital input indicated by changes in absolute abundances of marine and terrestrial palynomorphs. According to our results, the laminated, OM-rich horizons of the Niveau Goguel interval reflect deposition during times of reduced siliciclastic input. Episodes of pronounced condensation were accompanied by anoxic conditions preventing degradation of the predominantly marine-derived OM. In contrast, the Niveau Jacob is characterised by a strong increase in terrestrial palynomorphs, most probably reflecting an abrupt increase in riverine runoff. The enhanced terrestrial OM input may have triggered oxygen-depletion in bottom waters, resulting in turn in increased OM preservation. Our results highlight the variety of processes that controlled the accumulation of OM in the Vocontian basin and they illustrate that enhanced surface water productivity is not an indispensable prerequisite for the formation of mid-Cretaceous black shales. D
Palaeogeography Palaeoclimatology Palaeoecology, 2003
Calcareous nannoplankton, palynomorph, benthic foraminifera, and oxygen isotope records from the supraregionally distributed Niveau Paquier (Early Albian age, Oceanic Anoxic Event 1b) and regionally distributed Niveau Kilian (Late Aptian age) black shales in the Vocontian Basin (SE France) exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid-Cretaceous low latitudes. To quantify surface water productivity and temperature changes, nutrient and temperature indices based on calcareous nannofossils were developed. The nutrient index strongly varies in the precessional band, whereas variations of the temperature index reflect eccentricity. Since polar ice caps were not present during the mid-Cretaceous, these variations probably result from feedback mechanisms within a monsoonal climate system of the mid-Cretaceous low latitudes involving warm/ humid and cool/dry cycles. A model is proposed that explains the formation of mid-Cretaceous black shales through monsoonally driven changes in temperature and evaporation/precipitation patterns. The Lower Albian Niveau Paquier, which has a supraregional distribution, formed under extremely warm and humid conditions when monsoonal intensity was strongest. Bottom water ventilation in the Vocontian Basin was diminished, probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes. Surface water productivity in the Vocontian Basin was controlled by the strength of monsoonal winds. The Upper Aptian Niveau Kilian, which has a regional distribution only, formed under a less warm and humid climate than the Niveau Paquier. Low-latitude deep water formation was reduced to a lesser extent and/or on regional scale only. The threshold for the formation of a supraregional black shale was not reached. The intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale. At least in the Vocontian Basin, the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity. ß
International Symposium, "The Origin and Evolution of Natural Diversity". 1–5 October 2007. Sapporo, Japan. Biomarker and total sulfur (TS) analyses were carried out on laminated black shales and massive marls collected from Goguel level (OAE1a) and the Kilian and the Paquier levels (OAE1b) in the Vocontian Basin in southeast France. Total sulfur content and concentrations of thiophenes are abundant in the Goguel level, whereas furans and retene, which are originated from terrestrial plants, are predominant in the Kilian and the Paquier levels. These results suggests the expansion of intense anoxic waters to deep ocean during the deposition of Goguel level rather than during the periods covered by the Kilian and the Paquier levels. In contrast, the Kilian and the Paquier sediments are characterized by a large amount of terrestrial input. Therefore, it is inferred that OAE1a was associated with supplying a large amount of sulfur to the ocean, possibly by active volcanisms, ...
Journal of the Geological Society, 2005
The Niveau Breistroffer black shale succession in the Vocontian Basin (SE France) is the regional equivalent of the widely distributed Late Albian Oceanic Anoxic Event 1d. The studied black shale-rich interval at the Col de Palluel section is 6.28 m thick and comprises four black shale units with up to 2.5 wt% total organic carbon (TOC) intercalated with marlstones. Calcareous nannofossil, palynomorph, planktic Foraminifera and stable isotopic data from the Niveau Breistroffer succession suggest that short-term climate changes influenced its deposition, with relatively warm and humid climate during black shale formation in comparison with relatively cool and dry climatic conditions during marlstone deposition. An increase in the terrigenous/marine ratio of palynomorphs indicates enhanced humidity and higher runoff during black shale formation. A nutrient index based on calcareous nannofossils and the abundance pattern of small (63-125 ìm) hedbergellid Foraminifera show short-term changes in the productivity of the surface water. Surfacewater productivity was reduced during black shale formation and increased during marlstone deposition. A calcareous nannofossil temperature index and bulk-rock oxygen isotope data indicate relative temperature changes, with warmer surface waters for black shale samples. At these times, warm-humid climate and reduced surface-water productivity were accompanied by greater abundances of 'subsurface'-dwelling calcareous nannofossils (nannoconids) and planktic Foraminifera (rotaliporids). These taxa presumably indicate more stratified surface-water conditions. We suggest that the formation of the Niveau Breistroffer black shales occurred during orbitally induced increase in monsoonal activity that led to increasing humidity during periods of black shale formation. This, in turn, caused a decrease in low-latitude deep-water formation and probably an increase in surface-water stratification. The combination of these two mechanisms caused depleted O 2 concentrations in the bottom water that increased the preservation potential of organic matter.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2004
Geochemical and petrographic analysis of an interval of black shales between the Huron Shale Member of the New Albany Shale (Upper Devonian, Famennian) and the Sunbury Shale (Lower Mississippian, Tournaisian) suggests that multiple controls influenced the accumulation of organic matter. Stratigraphic trends were determined following detailed analysis of a core from the central Appalachian Basin (D6, Montgomery County, KY). Specifically, C-S-Fe relationships, Mo, V/(V+Ni), Ni/Co, and V/Cr were used to assess paleo-redox conditions during sediment accumulation; the role of clastic input was evaluated using Ti/ Al, K/Al, and Si/Al; and C/P was used as a proxy for paleoproductivity. The terrestrial-to-marine maceral ratio (T/M)-(vitrinite plus inertinite)/(alginite plus bituminite)-was used to evaluate changes in OM source. C-S-Fe relationships and trace-element ratios suggest anoxic conditions prevailed during deposition of much of the upper part of this interval. However, bottom-water conditions were intermittently dysoxic during accumulation of the Huron Shale Member, and possibly dysoxic to marginally oxic during accumulation of the lowermost Huron Shale Member. Productivity appears to have been an important factor in the accumulation of C org , and high C/P ratios are consistent with a productivity-anoxia feedback mechanism. In addition, nutrients from increased terrestrial weathering may have contributed to enhanced productivity. Variations in sediment influx may also have influenced C org concentrations through sediment starvation. One of the more significant observations in this work is an increase in terrestrial OM (inertinite and vitrinite) up-section, which may reflect vegetative changes occurring on land during this time interval. Most of this increase is due to inertinite, much of which appears to be fusinitic in origin and hence may be fossil charcoal. This latter observation has implications for the expansion of land plants into drier niches and for Late Devonian atmospheric oxygen levels. Introduction of more resistant terrestrial OM to anoxic basins may have been an important factor in the preservation of high-carbon (N10%)
Marine and Petroleum Geology, 2009
We have conducted elemental, isotopic, and Rock-Eval analyses of Cenomanian–Santonian sediment samples from ODP Site 1138 in the southern Indian Ocean to assess the origin and thermal maturity of organic matter in mid-Cretaceous black shales found at this high-latitude location. Total organic carbon (TOC) concentrations range between 1 and 20 wt% in black to medium-gray sediments deposited around the Cenomanian–Turonian boundary. Results of Rock-Eval pyrolysis indicate that the organic matter is algal Type II material that has experienced modest alteration. Important contributions of nitrogen-fixing bacteria to the amplified production of organic matter implied by the high TOC concentrations is recorded in δ15N values between −5 and 1‰, and the existence of a near-surface intensified oxygen minimum zone that favored organic carbon preservation is implied by TOC/TN ratios between 20 and 40. In contrast to the marine nature of the organic matter in the Cenomanian–Turonian boundary section, deeper sediments at Site 1138 contain evidence of contributions land-derived organic matter that implies the former presence of forests on the Kerguelen Plateau until the earliest Cenomanian.
1] Profiles of Mo/total organic carbon (TOC) through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, United Kingdom, and the Posidonia shale of Germany and Switzerland reveal water mass restriction during the interval from late tenuicostatum Zone times to early bifrons Zone times, times which include that of the putative Early Toarcian oceanic anoxic event. The degree of restriction is revealed by crossplots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm/%) of 0.5 and 17. The slope of 0.5 applies to sediment from the upper semicelatum and exaratum Subzones. This value, which is one tenth of that for modern sediments from the Black Sea (Mo/TOC regression slope 4.5), reveals that water mass restriction during this interval was around 10 times more severe than in the modern Black Sea; the renewal frequency of the water mass was between 4 and 40 ka. The Mo/TOC regression slope of 17 applies to the overlying falciferum and commune subzones: the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years. The more restricted of the two intervals has been termed the Early Toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to NW Europe. Crossplots of Re, Os, and Mo against TOC show similar trends of increasing element concentration with increase in TOC but with differing slopes. Together with modeling of 187 Os/ 188 Os and d 98 Mo, the element/TOC trends show that drawdown of Re, Os, and Mo was essentially complete during upper semicelatum and exaratum Subzone times (Mo/TOC regression slope of 0.5). Drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3% in d 98 Mo and up to 0.6% for 187 Os/ 188 Os. Restriction thereby compromises attempts to date Toarcian black shales, and possibly all black shales, using Re-Os chronology and introduces a confounding influence in the attempts to use d 98 Mo and initial 187 Os/ 188 Os for palaeo-oceanographic interpretation.
Paleoceanography, 2008
1] Profiles of Mo/total organic carbon (TOC) through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, United Kingdom, and the Posidonia shale of Germany and Switzerland reveal water mass restriction during the interval from late tenuicostatum Zone times to early bifrons Zone times, times which include that of the putative Early Toarcian oceanic anoxic event. The degree of restriction is revealed by crossplots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm/%) of 0.5 and 17. The slope of 0.5 applies to sediment from the upper semicelatum and exaratum Subzones. This value, which is one tenth of that for modern sediments from the Black Sea (Mo/TOC regression slope 4.5), reveals that water mass restriction during this interval was around 10 times more severe than in the modern Black Sea; the renewal frequency of the water mass was between 4 and 40 ka. The Mo/TOC regression slope of 17 applies to the overlying falciferum and commune subzones: the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years. The more restricted of the two intervals has been termed the Early Toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to NW Europe. Crossplots of Re, Os, and Mo against TOC show similar trends of increasing element concentration with increase in TOC but with differing slopes. Together with modeling of 187 Os/ 188 Os and d 98 Mo, the element/TOC trends show that drawdown of Re, Os, and Mo was essentially complete during upper semicelatum and exaratum Subzone times (Mo/TOC regression slope of 0.5). Drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3% in d 98 Mo and up to 0.6% for 187 Os/ 188 Os. Restriction thereby compromises attempts to date Toarcian black shales, and possibly all black shales, using Re-Os chronology and introduces a confounding influence in the attempts to use d 98 Mo and initial 187 Os/ 188 Os for palaeo-oceanographic interpretation.
Palaeogeography Palaeoclimatology Palaeoecology, 2010
A high-resolution study focussing on the distribution of calcareous nannofossils and carbon isotopes was carried out to improve the understanding of mid-Cretaceous black shale formation. The studied interval of the early Late Aptian is characterized by two major black shale couplets, the Niveau Noire 4 (NN4) and Niveau Noire Calcaire 2 (NNC2), of the Serre Chaitieu section in the Vocontian Basin . This interval occurs within a long-term negative carbon isotope excursion of N 1.5‰ following the Early Aptian Oceanic Anoxic Event 1a (OAE1a). In contrast to the local NN4 black shales, the black shales of NNC2 are of regional significance and occur at the end of the major negative carbon isotope excursion of the early Late Aptian. Time equivalent black shales are suggested to be coeval with black shales found in the Western Tethys and Atlantic Ocean . Calcareous nannofossil analyses and carbon isotopes indicate higher surface water productivity (mesotrophic), warmer surface water, and higher sea-level during the formation of the NN4 black shales. In contrast, the formation of the NNC2 black shales took place during a cooler phase, lower surface water productivity, and lower sea-level. A sea-level fall may cause a restriction of water mass exchange between the open-marine Western Tethys and the Vocontian Basin. This resulted in a longer residence time of the bottom water, decreased ventilation and less mixing of surface waters and thus enhanced preservation of organic matter at the sea floor. Our results indicate that the black shale formation of NN4 and NNC2 was caused by different processes such as increased surface water productivity and enhanced preservation of organic matter at the sea floor. Thus, we emphasize the role of different forcing factors which control the formation of local and regional black shales. The most important factors are sealevel fluctuations, increasing productivity, and changes in precipitation and evaporation rates.
Basinal Restriction, Black Shales, and the Early Toarcian (Jurassic) Oceanic Anoxic Event
Agu Fall Meeting Abstracts, 2008
Profiles of Mo/TOC through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, UK, and the Posidonia Shale of Germany and Switzerland, reveal water-mass restriction during the interval from late Tenuicostatum Zone times to early Bifrons Zone times. In this interval, faunal diversity was reduced because of the long period of restriction. The degree of restriction is revealed best by cross-plots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm per %) of 0.5 and 17. The slope of 0.5 is one-tenth that for modern sediments from the Black Sea (Mo/TOC 4.5) and reveals severe water-mass restriction during upper-semicelatum and exaratum Sz. times, a period often known as the Early Toarcian oceanic anoxic event. The frequency of water-mass renewal was between 4 and 40 kyrs, a frequency ten time lower than in the current Black Sea. The Mo/TOC regression slope of 17 for the overlying falciferum and commune Szs. shows lesser restriction and renewal times of 10 - 130 years i.e. substantially more frequent than before. The putative Early Toarcian oceanic anoxic event (OAE) is thus shown not to be global but to reflect an event local to NW Europe caused by basin restriction. Cross-plots of Re/TOC, Os/TOC, and Mo/TOC are similar and, together with modelling of 187Os:188Os and del98Mo, show that drawdown of Re, Os, and Mo, was essentially complete during the more restricted interval (Mo/TOC slope 0.5). Drawdown sensitized the restricted water-mass to isotopic change so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3 in del98Mo and up to 0.6 for 187Os:188Os. Restriction thereby compromises attempts to date Toarcian black shales, and possibly all black shales, using Re-Os chronology, and introduces a confounding influence in the attempts to use del98Mo and initial-187Os/188Os for palaeo-oceanographic interpretation of organic- rich strata.