oceanic anoxic cycles orbital prelude to the bonarelli level oae 2 abstract the prelude to the second global oceanic anoxic event of the cretaceous oae 2 as recorded in the classic sections of the scaglia bianca formation in the umbria marches region of italy is characterized by an oscillation between black to gray organic rich cherts and white pelagic limestones spectral analysis of these lithologic alternations reveals a robust set of cycles whose periods are remarkably similar to the orbital cycles calculated by laskar et al laskar j robutel p joutel f gastineau m correia a c m levrard b 2004 a long term numerical solution for the insolation quantities of the earth astronomy and astrophysics 428 261 285 bandpass filtering shows that the 140 80 kyr signal approximates the envelope of the 26 16 kyr signal just as expected if these components represent responses to orbital eccentricity and precession matching the presumed eccentricity signal to the theoretical eccentricity of laskar et al yields an age for the base of the bonarelli level onset of oae 2 in italy of 94 21 ma 93 72 ma provides an alternative consistent with radiometric constraints from a correlative section in the western united states given the strong orbital control on the cyclic deposition of the organic rich precursors of oae 2 within the scaglia bianca formation we hypothesize that the timing of oae 2 itself is similarly controlled by astronomical variations indeed the occurrence of oae 2 coincides with a protracted period of unusually low insolation variation owing to simultaneous nodes in all orbital parameters we speculate that other oaes similarly coincided with insolation nodes and dampened seasonality from this perspective some but perhaps not all of the oaes may be periodic events reflecting cycles of oceanic anoxia that predictably punctuated a 50 myr period of cretaceous climate

geochemical evidence for enhanced productivity during s1 sapropel deposition in the eastern mediterranean the geochemistry of the youngest mediterranean sapropel layer suggests changes in productivity and water column oxygen conditions during sapropel deposition the ba enriched interval is broader than the organic carbon rich interval of this sapropel we suggest that the ba enriched horizon records the original thickness of the sapropel prior to subsequent partial oxidation the main carrier of ba is barite as microcrystals 0 5 5 m having a morphology characteristic of marine barite particularly abundant beneath high productivity regions ba concentrations do not change at the sapropel layer oxidation front and diagenetic barite crystals are absent thus the ba enriched layer reflects original oceanic conditions of increased biological productivity during sapropel deposition and not diagenetic ba remobilization paleoredox indicators point to restricted oxygenated bottom water but not to fully anoxic conditions detrital elements within this layer indicate a lower eolian terrigenous input enhanced humidity and increased precipitation runoff thus likely higher nutrient supply supporting table is available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered by mail from agu 2000 florida ave n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

late pleistocene dark laminated mud layers from the korea plateau western east sea japan sea and their paleoceanographic implications abstract a variety of approaches were applied to a 11 1 m long sediment core ebp 4 from the korea plateau one of the bathymetric highs in the western part of the east sea japan sea to understand late pleistocene paleoenvironmental change the chronostratigraphy of the core was established using a radiocarbon age dark laminated mud dlm layers with known ages the oxygen isotope 18 o curve of planktonic foraminifera biogenic opal and total organic carbon toc contents downcore changes in the carbon nitrogen c n ratio as well as their isotopic ratio 13 c and 15 n in organic matter although interrupted by several pumice lapilli layers the core bottom represents marine isotope stage mis 8 and spans more than 250 kyr of paleoceanographic history of the western east sea japan sea paleoenvironmental conditions during mis 2 and mis 6 are clearly distinguishable represented by the negative shift of 18 o values of planktonic foraminifera resulting from surface water freshening due to basin isolation caused by eustatic sea level lowering the corresponding dark laminated mud dlm i ii a ii b and iii layers formed during glacial maxima are characterized by strong mo and total sulfur ts enrichment in anoxic conditions which indicates a stagnant and strongly stratified water column structure biogenic opal contents can be a useful indicator to confirm the chronostratigraphic assessment and can also identify periods of high surface water productivity in the east sea japan sea particularly during interglacial times in contrast glacial paleoproductivity might be low compared to the interglacial because of insufficient nutrient availability resulting from limited vertical ventilation the 13 c and 15 n values of sedimentary organic matter also emphasize increased terrestrial input during the glacial maxima exhibiting highly depleted 13 c and 15 n values in all dlm layers sea level induced paleoenvironmental changes associated with glacial interglacial variations are identified from the korea plateau confirming these events occurred throughout the whole east sea japan sea

enhanced productivity led to increased organic carbon burial in the euxinic north atlantic basin during the late cenomanian oceanic anoxic event 1 three cenomanian turonian c t 93 5 ma black shale sections along a northeast southwest transect in the southern part of the proto north atlantic ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in 13c values of both bulk organic matter om and molecular fossils of algal chlorophyll and steroids all three sites show an increase in marine organic carbon oc accumulation rates during the c t oceanic anoxic event oae the occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria lack of bioturbation and high abundance of redox sensitive trace metals indicate sulfidic conditions periodically reaching up into the photic zone before as well as during the c t oae during the c t oae there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and mo al ratios the presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the c t oae however bottom water conditions did not dramatically change as indicated by more or less constant v al and zn al ratios at site 367 this suggests that the increase in oc burial rates resulted from enhanced primary productivity rather than increased anoxia which is supported by stable carbon isotopic evidence and a large increase in ba al ratios during the c t oae the occurrence of the productivity event during a period of globally enhanced organic carbon burial rates i e the c t oae points to a common cause possibly related to the formation of a deep water connection between north and south atlantic basins

modelling oceanic carbon and phosphorus fluxes implications for the cause of the late cenomanian oceanic anoxic event oae2 a two box ocean model with a coupled carbon and phosphorus cycle is used to test which processes can realistically generate a rapid positive 2 13c excursion in the global carbon reservoir internal oceanic processes enhanced preservation of organic matter under anoxic conditions enhanced upwelling cannot produce a substantial 13c excursion under the assumption that burial rates of c and p are coupled loss of nutrients to the sediment acts as a negative feedback mechanism that prevents a substantial perturbation of the carbon cycle a large and renewable source of nutrients is needed either from increased continental weathering or from enhanced regeneration of sedimentary organic phosphorus under anoxic conditions measurements of total phosphorus and organic carbon in sediments from the tarfaya basin morocco suggest that a large decrease in p burial occurred relative to organic carbon at the base of the cenomanian turonian oceanic anoxic event oae2 with the right magnitude to quantitatively explain a 2 shift in 13c it is concluded that the main underlying mechanism that allowed and sustained enhanced carbon burial during the mid cretaceous was a perturbation of the oceanic phosphorus cycle

water column anoxia enhanced productivity and concomitant changes in δ13c and δ34s across the frasnian famennian boundary kowala holy cross mountains poland the investigation of the trace element and organic geochemistry of the frasnian famennian boundary section at kowala holy cross mountains poland shows that the lower water column was oxygen deficient during late frasnian and early famennian times the abundance and carbon isotopic composition of diaryl isoprenoids biomarkers indicative for green sulfur bacteria prove that euxinic waters reached into the photic zone at least episodically total organic carbon toc contents show two maxima that are time equivalent to the kellwasser horizons deposited in shallower water settings enhanced toc concentrations are explained by a higher primary productivity presumably as a consequence of an enhanced nutrient supply from the continent the increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to toc increased at the frasnian famennian transition the sulfur isotopic composition of pyritic and organically bound sulfur shows a 27 excursion across the boundary the observation that the 34s values of organic bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source likely early diagenetic sulfide a change in the 13c of total dissolved inorganic carbon as a consequence of an enhanced burial of 12c enriched organic carbon is indicated by a 3 excursion measured for toc as well as for individual n alkanes and isoprenoids the burial of large amounts of organic carbon is expected to result in a decrease in pco2 and should affect the photosynthetic carbon isotope fractionation ep the fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values as a consequence of high atmospheric and oceanic dissolved co2 concentrations during the devonian

sediment geochemical records of productivity and oxygen depletion along the margin of western north america during the past 60 000 years teleconnections with greenland ice and the cariaco basin abstract many sediment records from the margins of the californias alta and baja collected in water depths between 60 and 1200 m contain anoxic intervals laminated sediments that can be correlated with interstadial intervals as defined by the oxygen isotope composition of greenland ice dansgaard oeschger d o cycles these intervals include all or parts of oxygen isotope stage 3 ois3 60 24 cal ka the bolling allerod warm interval b a 15 13 cal ka and the holocene this study uses organic carbon corg and trace element proxies for anoxia and productivity namely elevated concentrations and accumulation rates of molybdenum and cadmium in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom water oxygen bwo conditions on open highly productive continental margins the main conclusion from these proxies is that during the last glacial interval lgi 24 15 cal ka and the younger dryas cold interval yd 13 11 6 cal ka productivity was lower and bwo levels were higher than during ois3 the b a and the holocene on all margins of the californias the corg and trace element profiles in the lgi b a holocene transition in the cariaco basin on the margin of northern venezuela are remarkably similar to those in the transition on the northern california margin correlation between d o cycles in greenland ice with gray scale measurements in varved sediments in the cariaco basin also is well established synchronous climate driven changes as recorded in the sediments on the margins of the californias sediments from the cariaco basin and in the gisp 2 greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka millennial scale cycles in productivity and oxygen depletion on the margins of the californias demonstrate that the california current system was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern north pacific ocean it is likely that the pacific and atlantic oceans were linked through the atmosphere warmer air temperatures during interstadials would have strengthened hadley and walker circulations which in turn would have strengthened the subtropical high pressure systems in both the north pacific and the north atlantic producing increased rainfall over the cariaco basin and increased upwelling along the margins of the californias

late cretaceous to early quaternary organic sedimentation in the eastern equatorial atlantic abstract reconstructing the long term evolution of organic sedimentation in the eastern equatorial atlantic odp leg 159 provides information about the history of the climate ocean system sediment accumulation and deposition of hydrocarbon prone rocks the recovery of a continuous 1200 m long sequence at odp site 959 covering sediments from albian to the present day about 120 ma makes this position a key location to study these aspects in a tropical oceanic setting new high resolution carbon and pyrolysis records identify three main periods of enhanced organic carbon accumulation in the eastern tropical atlantic i e the late cretaceous the eocene oligocene and the pliocene pleistocene formation of upper cretaceous black shales off west africa was closely related to the tectono sedimentary evolution of the semi isolated deep ivorian basin north of the cote d ivoire ghana transform margin their deposition was confined to certain intervals of the last two cretaceous anoxic events the early turonian oae2 and the coniacian santonian oae3 organic geochemical characteristics of laminated coniacian santonian shales reveal peak organic carbon concentrations of up to 17 and kerogen type i ii organic matter which qualify them as excellent hydrocarbon source rocks similar to those reported from other marginal and deep sea basins a middle to late eocene high productivity period occurred off equatorial west africa porcellanites deposited during that interval show enhanced total organic carbon toc accumulation and a good hydrocarbon potential associated with oil prone kerogen deposition of these toc rich beds was likely related to a reversal in the deep water circulation in the adjacent sierra leone basin accordingly outflow of old deep waters of southern ocean origin from the sierra leone basin into the northern gulf of guinea favored upwelling of nutrient enriched waters and simultaneously enhanced the preservation potential of sedimentary organic matter along the west african continental margin a pronounced cyclicity in the carbon record of oligocene lower miocene diatomite chalk interbeds indicates orbital forcing of paleoceanographic conditions in the eastern equatorial atlantic since the oligocene miocene transition a similar control may date back to the early oligocene but has to be confirmed by further studies latest miocene early pliocene organic carbon deposition was closely linked to the evolution of the african trade winds continental upwelling in the eastern equatorial atlantic ocean chemistry and eustatic sea level fluctuations reduction in carbonate carbon preservation associated with enhanced carbon dissolution is recorded in the uppermost miocene 5 82 5 2 ma section and suggests that the latest miocene carbon record of site 959 documents the influence of corrosive deep waters which formed in response to the messinian salinity crisis furthermore sea level related displacement of higher productive areas towards the west african shelf edge is indicated at 5 65 5 6 5 55 5 2 4 8 ma in view of humid conditions in tropical africa and a strong west african monsoonal system around the miocene pliocene transition the onset of pronounced toc cycles at about 5 6 ma marks the first establishment of upwelling cycles in the northern gulf of guinea an amplification in organic carbon deposition at 3 3 ma and 2 45 ma links organic sedimentation in the tropical eastern equatorial atlantic to the main steps of northern hemisphere glaciation and testifies to the late pliocene transition from humid to arid conditions in central and western african climate aridification of central africa around 2 8 ma is not clearly recorded at site 959 however decreased and highly fluctuating carbonate carbon concentrations are observed from 2 85 ma on that may relate to enhanced terrigenous eolian dilution from africa

the role of a longshore pressure gradient in pacific northwest coastal dynamics abstract in this paper we demonstrate the importance of the seasonal barotropic longshore pressure gradient force to pacific northwest coastal dynamics values of the seasonal longshore pressure gradient corrected for gauge height relative to a level surface hickey and pola 1982 and for year to year variations enfield and alien 1980 were included in the two dimensional time dependent baroclinic finite difference model of hamilton and rattray 1978 as an external force observed wind stress stratification and bottom topography were included in the model and comparisons were made with current meter data in each of the three seasonal situations observed in the northwest pressure gradient force southward opposing local wind stress winter pressure gradient force northward opposing local wind stress summer and pressure gradient force and local wind stress both southward spring three important features of the seasonal circulation are shown to depend on the existence of the pressure gradient

simulations of ecosystem response during the sapropel s1 deposition event abstract a one dimensional ecosystem numerical model is used to simulate the ecosystem changes that could have occurred in the open ocean areas of the eastern mediterranean sea during the climatic optimum interval 9500 6000 b p mercone et al mercone d thomson j croudace i w siani g paterne m troelstra s 2000 duration of s1 the most recent sapropel in the eastern mediterranean sea as indicated by accelerator mass spectrometry radiocarbon and geochemical evidence paleoceanography 15 336 347 in this period the s1 sapropel was deposited s1 is the most recent sapropel in the succession of organic carbon rich layers intercalated in normal neogene sedimentary sequences different theories have been invoked in order to explain the deposition of this peculiar layer our simulations seem to indicate that the modified thermohaline circulation supplying oxygen only in the first 500 m of the water column is responsible for the sapropel deposition when higher productivity is allowed in the euphotic zone the model shows the importance in this process of bacteria that consume oxygen by decomposing the particulate organic matter pom produced in the upper water column the sinking velocity of pom partially regulates the timescale of the occurrence of anoxia at the bottom and in the whole water column allowing the relatively rapid onset of sapropel deposition

insolation control on the late cretaceous hydrological cycle and tropical african climate global climate modelling linked to marine climate records data from late cretaceous paleoclimate simulations and their linkage to a geological model derived from long term high resolution proxy data indicate and support strong relationships between african climate and tropical atlantic sedimentation here we present results from an interdisciplinary study by varying only one parameter in the set of orbital boundary conditions of the numeric model we focus on the climatic impact of precessional forcing on the global and regional climate system as a result new insights to the internal dynamics of climate the different compartments and fluxes of the hydrological cycle and finally the sedimentary response within the oceanic realm during greenhouse conditions have been approached the climate models suggest that insolation changes at 25 55 s are the trigger for cyclic variations of the tropical hydrological cycle of northern africa between the various models a maximum difference in insolation at the top of the atmosphere of 14 w m2 is needed to produce the documented changes in the hydrological cycle first of all the simulations do not suggest any substantial latitudinal movement of the itcz over the course of one precessional cycle the models rather indicate cross latitudinal variation of pressure systems and variation in the magnitude and direction of surface winds linking tropical africa to the mid southern latitudes this cross latitudinal atmospheric teleconnection denotes a reduced role of the tropics as driver of cretaceous climate system therefore the linkage of proxy based geological and numeric models rather supports the idea that tropical atlantic black shale formation in the late cretaceous was ultimately triggered by climate change in mid southern latitudes with precipitation and river discharge being the transport mechanisms as a hypothesis that will be tested in the near future we speculate that the mid latitudes represent the ultimate region of climate signal formation during times of extreme global warmth

phanerozoic organic carbon rich marine sediments overview and future research challenges one of the major obsessions of many early workers to the mid 1900s was the application of uniformitarian principles to deposi tional models for black shales arthur and sageman 1994 the purpose of this overview of organic carbon oc rich marinesediments is to provide a brief but current summary of the historicaldevelopments principle concepts and remaining challenges in inte grated sapropel and black shale research as such it provides asubstantive introduction to the special issue of palaeogeography palaeoclimatology palaeoecology on organic carbon rich sedimentsthrough the phanerozoic processes progress and perspectives giventhis focused scope the overview does not aim to be comprehensive orcomplete but to provide a solid setting for the fourteen individualresearch papers that constitute this special issue and two previousspecial issues meyers and negri 2003 negri et al 2006 thatcoverresearch aspects complementary to this one like the individualcontributions this introduction and overview is organized intocenozoic mesozoic and palaeozoic units the cenozoic and palaeozoicunitsaredeliberatelylargerthanthemesozoicunit acknowledgingthatmuch ground on mesozoic black shale is already covered in the twoprevious special issues because the late cenozoic sapropels of themediterraneanbasinhavebeenextensivelystudied understandinghowthese near modern analogs of ancient black shales were depositedprovidesagoodfoundationforunderstandinghowtheoldersequencesmayhaveevolved incontrast becausefarlessisgenerallyknownabouttheblackshalesofthepalaeozoicthancomparableoc richsequencesofeither the cenozoic or the mesozoic a more comprehensive summaryand comparison of the more ancient sequences is particularly appro priate to the theme of this special issue 1 introductionsediments rich in organic carbon are restricted to small areas alongcontinental margins in the modern ocean and have rarely accumulatedduringthecenozoic yetorganic rich sediments werewidely depositedduring multiple intervals of mesozoic palaeozoic time and even earlierduringtheproterozoic archean globalmarinedepositsdocumentthatepisodes of accumulation of oc rich sediments occurred in differentregions and at different times these episodes were linked to climaticandpalaeoceanographicperturbationsthat resulted inmassive uctua tions in hydrologic and nutrient cycles and in oceanchemistryand thatrecurred throughout geologic time however the paucity of survivingpalaeozoic and earlier black shale sections makes it dif cult toimpossible to recognize the internal structure of global events that arecommon inyounger oc rich sedimentary sequences asunderlinedbelow themarinesectionsinmanylocalitiesconsistof intercalated carbonates and shales that may grade into purecarbonate or black shale sedimentation often re ecting shallowing

origin sequence stratigraphy and depositional environment of an upper ordovician hirnantian deglacial black shale jordan the upper ordovician succession of jordan was located 60 s less than 100 km from the hirnantian ice sheet margin new graptolite dates indicate glaciation ended in jordan in the late hirnantian persculptus biozone the succession records two glacial advances within the ammar formation and the subsequent deglaciations organic rich black shales batra formation form part of the final deglacial transgressive succession that in filled an existing low stand glacial continental shelf topography the base of the black shale is coincident with the maximum flooding surface during transgression interfluves and sub basin margins were breached and black shale deposition expanded rapidly across the region the top of the black shales coincides with peak highstand the expanding puddle model sensu wignall for black shale deposition adapted for the peri glacial setting provides the best explanation for this sequence of events we propose a hypothesis in which anoxic conditions were initiated beneath the halocline in a salinity stratified water column a fresher surface layer resulted from ice meltwater generated during early deglaciation during the initial stages of marine incursion nutrients in the monimolimnion were isolated from the euphotic zone by the halocline increasing total organic carbon toc and 13corg up section indicates the organic carbon content of the shales was controlled mainly by increasing bioproductivity in the mixolimnion the strakhov model mixolimnion nutrient levels were sustained by a continual and increasing supply of meltwater derived nutrients modulated by obliquity changes in high latitude insolation anoxia was sustained over tens to hundreds of thousands of years the formation of black shales on the north gondwana shelf was little different to those observed in modern black shale environments suggesting that it was the nature of the ordovician seas that pre disposed them to anoxia

sapropel burn down and ichnological response to late quaternary sapropel formation in two 400 ky records from the eastern mediterranean sea two approximately 400 ky long sedimentary records from the eastern mediterranean sea were used to study the factors determining why some sapropels are more or less completely erased while others are preserved x ray radiographs bulk density organic carbon content fe al and mn al ratios were used to reconstruct initial thickness and burn down ratios of sapropels s1 to s11 the results show that at the low sedimentation rate open ocean settings studied initial carbon content of the sapropel as well as post sapropel sedimentation rate and carbon flux have had only limited influence on the preservation of sapropels in contrast the regional position relative to the deep water source controlling bottom water current intensity and bottom water oxygenation played a major role in the preservation of the sapropels furthermore orbitally controlled variations in the intensity of subsequent summer insolation maxima also seem to play an important role in the preservation potential of sapropel layers subsequent strong insolation maxima resulting in thick sapropels tend to act as a lid preventing further burn down of the older sapropels below the long and well dated records also offer an opportunity to study the response of the benthic fauna to the drastic environmental changes related to the formation of sapropels the sediment in the two cores is generally characterized by mottled burrows with only scarce occurrences of distinct trace fossils such as scolicia thalassinoides chondrites and trichichnus the meager ichnofauna is attributed to the well oxygenated and warm bottom waters in an oligotrophic environment typical for non sapropel times the sparse ichnofauna and the lack of tiered ichnocoenosis render existing models of bioturbational response to changes in bottom water oxygenation unsuitable for this kind of environments however observations on individual traces support earlier studies that suggest that the trace fossils trichichnus and chondrites are the result of chemosymbiotic activities that benefit from reducing conditions in the pore water and that the producers of scolicia have a preference for well oxygenated conditions and sandy substrates c 2006 elsevier b v all rights reserved

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

a dynamic explanation for the origin of the western mediterranean organic rich layers the eastern mediterranean sapropels are among the most intensively investigated phenomena in the paleoceanographic record but relatively little has been written regarding the origin of the equivalent of the sapropels in the western mediterranean the organic rich layers orls orls are recognized as sediment layers containing enhanced total organic carbon that extend throughout the deep basins of the western mediterranean and are associated with enhanced total barium concentration and a reduced diversity dysoxic but not anoxic benthic foraminiferal assemblage consequently it has been suggested that orls represent periods of enhanced productivity coupled with reduced deep ventilation presumably related to increased continental runoff in close analogy to the sapropels we demonstrate that despite their superficial similarity the timing of the deposition of the most recent orl in the alboran sea is different than that of the approximately coincident sapropel indicating that there are important differences between their modes of formation we go on to demonstrate through physical arguments that a likely explanation for the origin of the alboran orls lies in the response of the western mediterranean basin to a strong reduction in surface water density and a shoaling of the interface between intermediate and deep water during the deglacial period furthermore we provide evidence that deep convection had already slowed by the time of heinrich event 1 and explore this event as a potential agent for preconditioning deep convection collapse important differences between heinrich like and deglacial like influences are highlighted giving new insights into the response of the western mediterranean system to external forcing

euxinia and primary production in late cretaceous eastern equatorial atlantic surface waters fostered orbitally driven formation of marine black shales 1 oceanic anoxic events oae in the cretaceous represent major perturbations in the global climate and ocean system characterized by widespread deposition of organic carbon in the ocean the causes and effects of these events are poorly constrained mainly because of the lack of high resolution marine records here we report the distribution of molecular markers from a millennial scale record of coniacian santonian black shale oae 3 from ocean drilling program site 959 in the deep ivorian basin in the eastern equatorial atlantic highly branched isoprenoids and alkenone derived organic compounds indicate that diatoms and calcareous nanoplankton were important primary producers changes in redox sensitive trace metal accumulation and biomarkers of green sulfur bacteria provide evidence for extreme variations in redox conditions with intervals of lower photic zone euxinia pze accordingly oxygen in the coniacian santonian tropical atlantic was absent as during the cenomanian turonian boundary oae 2 but over more restricted geographic area and more limited time intervals we hypothesize that pze was a common phenomenon typical in large areas of tropical continental margins these conditions fostered sequestration of atmospheric co2 and thus helped cause the positive excursion in 13c of carbonate documented in higher latitude marine records

tropical warming and intermittent cooling during the cenomanian turonian oceanic anoxic event 2 sea surface temperature records from the equatorial atlantic oceanic anoxic event 2 oae 2 occurring during the cenomanian turonian c t transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 myr however the impact of this major perturbation on and interaction with global climate remains unclear here we report new high resolution records of sea surface temperature sst based on tex86 and 18o of excellently preserved planktic foraminifera and stable organic carbon isotopes across the c t transition from black shales located offshore suriname french guiana demerara rise ocean drilling program leg 207 site 1260 and offshore senegal cape verde basin deep sea drilling project leg 41 site 367 at site 1260 where both sst proxy records can be determined a good match between conservative sst estimates from tex86 and 18o is observed we find that late cenomanian ssts in the equatorial atlantic ocean 33 c were substantially warmer than today 27 29 c and that the onset of oae 2 coincided with a rapid shift to an even warmer 35 36 c regime within the early stages of the oae a marked 4 c cooling to temperatures lower than pre oae conditions is observed however well before the termination of oae 2 the warm regime was reestablished and persisted into the turonian our findings corroborate the view that the c t transition represents the onset of the interval of peak cretaceous warmth more importantly they are consistent with the hypotheses that mid cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide co2 and that major oaes were capable of triggering global cooling through the negative feedback effect of organic carbon burial led co2 sequestration evidently however the factors that gave rise to the observed shift to a warmer climate regime at the onset of oae 2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 myr

organic carbon accumulation in bannock basin evaluating the role of productivity in the formation of eastern mediterranean sapropels abstract three sediment cores from bannock basin a hypersaline anoxic basin in the eastern mediterranean have been studied to evaluate the importance of productivity on organic carbon accumulation and sapropel formation in this region organic carbon accumulation rates were determined for two cores from within the anoxic hypersaline zone and for one core from the oxygenated portion of this basin organic carbon accumulation rates are higher by an order of magnitude in the surficial sediments of the anoxic zone due to better preservation an empirically derived preservation factor which allows calculation of paleoproductivity during the deposition of the most recent sapropel si indicates that productivity increased by an order of magnitude during the formation of si we conclude that increased productivity was coupled with a density stratification and a circulation reversal in the eastern mediterranean which together led to sapropel formation

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

redox sensitivity of p cycling during marine black shale formation dynamics of sulfidic and anoxic non sulfidic bottom waters abstract a high resolution geochemical record of a 120 cm black shale interval deposited during the coniacian santonian oceanic anoxic event 3 odp leg 207 site 1261 demerara rise has been constructed to provide detailed insight into rapid changes in deep ocean and sediment paleo redox conditions high contents of organic matter sulfur and redox sensitive trace metals cd mo v zn as well as continuous lamination point to deposition under consistently oxygen free and largely sulfidic bottom water conditions however rapid and cyclic changes in deep ocean redox are documented by short term 15 20 ka intervals with decreased total organic carbon toc s and redox sensitive trace metal contents and in particular pronounced phosphorus peaks up to 2 5 wt p associated with elevated fe oxide contents sequential iron and phosphate extractions confirm that p is dominantly bound to iron oxides and incorporated into authigenic apatite preservation of this fe p coupling in an otherwise sulfidic depositional environment as indicated by fe speciation and high amounts of sulfurized organic matter may be unexpected and provides evidence for temporarily non sulfidic bottom waters however there is no evidence for deposition under oxic conditions instead sulfidic conditions were punctuated by periods of anoxic non sulfidic bottom waters during these periods phosphate was effectively scavenged during precipitation of iron oxyhydr oxides in the upper water column and was subsequently deposited and largely preserved at the sea floor after 15 25 ka sulfidic bottom water conditions were re established leading to the initial precipitation of cds zns and pyrite subsequently increasing concentrations of h2s in the water column led to extensive formation of sulfurized organic matter which effectively scavenged particle reactive mo complexes thiomolybdates at site 1261 sulfidic bottom waters lasted for 90 100 ka followed by another period of anoxic non sulfidic conditions lasting for 15 20 ka the observed cyclicity at the lower end of the redox scale may have been triggered by repeated incursions of more oxygenated surface to mid waters from the south atlantic resulting in a lowering of the oxic anoxic chemocline in the water column alternatively sea water sulfate might have been stripped by long lasting high rates of sulfate reduction removing the ultimate source for hs production

paleoclimatic and paleoceanographic conditions leading to development of sapropel layer s1 in the aegean sea abstract sapropel s1 occurs as 25 35 cm thick black weakly laminated muds in aegean sea cores s1 was deposited between 9600 and 6400 yr b p during a period of isotopically depleted and relatively cool surface waters micro faunal and floral data indicate a major reduction in surface waters salinity during the deposition of s1 and oxygen isotopic data show a northerly fresh water source relatively light 13 c org and high pollen spore concentrations in s1 suggest increased influx of terrestrial organic carbon probably supplied by major rivers draining into the northern aegean sea benthic foraminifera indicate high nutrient low oxygen bottom waters for s1 and together with silt sized hematite and manganese coatings suggest that during the deposition of s1 surface sediments were oxic visual and xrd evidence of pyrite in s1 together with enrichments in s cu zn as ni cr and fe suggest that subsurface conditions were sufficiently reducing for so 4 2 reduction to occur probably by diffusion from surface oxic into subsurface anoxic sediments palynomorphs in s1 show large increases in terrestrial pollen and spores with the floral assemblage indicating significant influx from northern european rivers and minor african components associated with increased summer monsoonal rain abundance of dinoflagellates and amorphogen suggests some increase in primary productivity in response to increased influx of humic compounds however there is no evidence of upwelling the clay fraction in s1 shows notable decreases in smectite and kaolinite and reciprocal increases in illite and chlorite the combined data suggested that the evolution of s1 in the aegean sea largely resulted from stagnation of the surface waters during the final disintegration of the continental ice sheets rather than an increase in primary productivity and higher preservation of organic carbon on the sea floor

cretaceous black shales as active bioreactors a biogeochemical model for the deep biosphere encountered during odp leg 207 demerara rise abstract a transport reaction model was designed to identify the combination and importance of biogeochemical processes operating in four sites drilled during odp leg 207 demerara rise equatorial atlantic almost 100 ma after their deposition deeply buried cretaceous black shales still act as active bioreactors in great sediment depths and control the biogeochemical reaction network of the whole sediment column according to a model calibrated at the four drill sites through inverse modeling techniques methanogenesis could be identified as a key process that dominates not only organic matter degradation but also sulfate availability through the anaerobic oxidation of methane above the black shales a complete depletion of sulfate within the black shale sequences promotes the remobilization of biogenic barium that reprecipitates as authigenic barite at the top of the sulfate depletion zone temporal dynamics of degradation processes caused continuous shifts of the barite precipitation zone during burial thus inhibiting the formation of an authigenic barite front or causing the dissolution of earlier formed fronts major deviations of pore water sulfate profiles from a linear gradient coincide with depths of decelerated or accelerated transport caused by local porosity minima or maxima model determined reaction rates are by far lower than those found in shallower sediments due to the low metabolic activities that are characteristic for the deep biosphere but even after almost 100 ma changing organic matter quality still influences the degradation within the black shale sequences as it is indicated by model results

deposition of organic matter in the norwegian greenland sea during the past 2 7 million years abstract variations in the amount and composition of sedimentary organic matter in glacial and interglacial deposits of the last 2 7 myr correlate with late cenozoic climatic and oceanographic changes in the norwegian greenland sea these variations are predominantly caused by the changing supply of terrestrial and reworked organic matter the highest amounts of terrestrial organic particles macerals and of reworked coal clasts in glacial and early deglacial diamictons are closely related to glacial erosion of mesozoic strata that crop out along the scandinavian shelf the first occurrence of coal clasts at 2 53 myr demonstrates an initial advance of continental ice margins to these source areas the establishment of anoxic conditions at the sea floor during diamicton deposition probably reflects an increased vertical flux of labile organic matter due to lithogenic adsorbtion followed by an almost complete mineralization at the water sediment interface the content of marine organic matter was continuously low over the past 2 7 myr except for past interglacial highstands i e isotopic event 5 5 1 suggesting persistent diagenetic degradation of the labile organic fraction the marine organic matter exclusively consists of residual dinoflagellate cysts and fragments of them

the mystery of the sapropels sapropels are layers of sediment on the sea floor that are rich in organic carbon the way in which they were formed has been a matter of intense debate with researchers usually invoking one of two alternative explanations work in the mediterranean now allows those explanations to be reconciled

simulation of multifrequency borehole resistivity measurements through metal casing using a goal oriented hp finite element method the authors simulate multifrequency through casing resistivity tool measurements operating at different frequencies in a borehole environment for the assessment of rock formation properties rock formations are assumed to exhibit axial symmetry around the axis of a vertical borehole the simulations are performed with a goal oriented hp adaptive finite element method that delivers exponential convergence rates in terms of the quantity of interest for example the second vertical difference of the electric potential against the cpu time numerical results confirm the efficiency and accuracy of the method allowing for high accuracy and reliable simulations of borehole logging measurements in the presence of highly conductive steel casing the study of different tool configurations shows the advantages of using calibrated instruments with toroid antennas located on the borehole wall the agreement between the numerical and analytical results when the latter is available is quantified errors on the simulations are consistently below 0 1

review and new aspects concerning the formation of eastern mediterranean sapropels abstract in this paper it is proposed that the formation of eastern mediterranean sapropels occurred in an anti estuarine type of circulation which was to some degree weakened relative to the present in response to reduction of the eastern mediterranean excess of evaporation over freshwater input this reduction of excess evaporation would have been imposed by intensifications of 1 the indian ocean summer sw monsoon influencing the eastern mediterranean via increased nile discharge and 2 the system of mediterranean depressions an element of the westerly atlantic system causing increased precipitation and decreased evaporation both the indian ocean summer monsoon and the westerly atlantic system would be intensified in response to the occurrence of distinct minima in the cycle of precession it is demonstrated that reduced excess evaporation whether or not coinciding with global phases of deglaciation would lead to reduction of surface water salinities in the eastern mediterranean causing isolation of previously formed high salinity cooler deep water thus mixing was severely reduced possibly even restricted to eddy diffusion with only occasional convective events that because of the existing density gradient hardly ever would reach the deepest parts of the basin the consequently diminished oxygen advection down from a few hundred meters in the water column favoured preservation of the sinking organic matter this would however suffice only to enable the formation of sapropels with low organic carbon contents the high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production the described scenario accounts for previously reported 1 increases of organic carbon content with increasing depth of deposition within individual sapropels and 2 asymmetrical sequences of sapropel deposition characterized by gradual build up and rather abrupt ending it is furthermore in agreement with 3 isotopic and faunal reconstructions of the history of exchange transports through both the strait of sicily and the strait of gibraltar and 4 faunal and floral reflections of the presence or absence of a distinct deep chlorophyll maximum with its associated increases in export production indicating the presence or absence of a shallow pycnocline within the euphotic layer moreover the described scenario 5 is in no way conflicting with the reports of sapropels in the western mediterranean and 6 seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation amidst anoxic benthic desert levels a situation that has been observed in a few quaternary and pliocene sapropels

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day