evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

high resolution late maastrichtian early danian oceanic 87sr 86sr record implications for cretaceous tertiary boundary events a high resolution late maastrichtian early danian seawater 87 sr 86 sr reference curve is constructed from two cretaceous tertiary boundary k t boundary sections bidart france and el kef tunisia the 87 sr 86 sr curve shows maxima at 0 3 0 4 ma before the k t boundary and at the k t boundary the first maximum could mark the onset of a major outflow of the deccan traps the second maximum a rapid 0 000 06 87 sr 86 sr shift extends from 3 4 m below to 1 m above the k t boundary this profile probably results from diagenetic smoothing of an originally sharp k t boundary 87 sr 86 sr anomaly rather than from a gradual process the sharp shift could result from 1 the vaporization of the chicxulub target rocks 2 global wildfires and 3 acid rain leaching of soils and sialic surface rocks of these three possibilities only sr release by soil leaching combined with increased rainfall associated with the k t event appears to be sufficiently large to produce the observed k t 87 sr 86 sr anomaly

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

paleomagnetic evidence for dextral strike slip motion in the pyrenees during alpine convergence mauleon basin france abstract the paleomagnetic study of the aptian albian black marls from the inverted early cretaceous mauleon basin in the northern pyrenees reveals two components of magnetization that both yield negative fold tests the carriers of each component show different unblocking temperatures indicating that ferrimagnetic iron sulphides are the main carriers of the intermediate temperature component itc unblocks around 360 400 c and magnetite is the main carrier of the high temperature component htc unblocks around 460 500 c both components show a clockwise deviation in declination with respect to the reference field directions itc di and htc di both in in situ coordinates the reference direction for stable europe for the cenozoic is d i 001 50 d 5 i 3 however the reversal test for both of the magnetization components in situ is positive the structural evolution of the basin involves transtensional deformation during the main stage of basin formation and inversion during compression with migration of hydrocarbons and development of thrusts and folds with axial planar cleavage secondary magnetizations were acquired at the latest stages of the evolution of the inverted basin post folding as the result of a chemical mechanism and they were blocked during two different and likely close spaced in time polarity chrons the blocking of the two different magnetizations with different carriers implies chemical changes in the environment from anoxic sulphate reduction to oxic inferred clockwise rotation of the paleomagnetic vectors can be interpreted in terms of dextral shear across the north pyrenean zone during the tertiary probably eocene according to the plate tectonic scenario this distributed dextral shear was probably favored by discrete shear along cleavage planes as shown by geologic markers of consistent dextral strike slip motion in non competent units with strong cleavage development there is no evidence of such dextral movement in the south pyrenean zone thus indicating a strong partitioning of deformation during the pyrenean orogeny

the cretaceous tertiary boundary stratotype section at el kef tunisia how catastrophic was the mass extinction abstract the cretaceous tertiary k t boundary stratotype section at el kef tunisia represents the most complete and expanded sedimentary record across this important mass extinction horizon presently known high resolution analysis of planktic foraminifera in two outcrops el kef i stratotype and el kef ii along with comparisons between planktic and benthic foraminifera calcareous nannofossils ostracods pollen and spores and dinoflagellates indicate that major changes across the k t boundary are registered only in benthic and planktic foraminifera and calcareous nannofossils biotic changes in benthic foraminifera are unique to el kef and similarly shallow continental shelf sections and appear to be the result of a sea level regression in the latest maastrichtian followed by a sea level rise across the k t boundary that was accompanied by expansion of the local oxygen minimum zone omz biotic changes in planktic foraminifera appear partly related to these conditions also but in general reflect more global oceanographic changes for instance species extinctions are gradual and selective as observed in k t sections worldwide rather than random and abrupt although there is a 69 decline in species richness between 25 cm below and 10 cm above the k t boundary only rare species disappeared their combined relative abundance constitute less than 20 of the total population about 52 of these extinct taxa 8 of the population are large ornate morphologically complex tropical subtropical forms that lived at or below the thermocline no planktic foraminifera from this depth range survived the k t boundary event all survivor taxa were surface dwellers living within the photic zone their relative abundance 80 dominates both cretaceous and early tertiary populations these data indicate that the k t biotic record in the shallow continental shelf section at el kef was significantly influenced by local conditions which combined with the latest maastrichtian sea level regression and subsequent sea level rise resulted in shallowing of the local omz relative to the sea surface shallowing of the local omz lead to the selective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers the selective nature of species extinctions however appear to be related partly to long term global oceanographic changes which were accelerated at the k t boundary possibly by a bolide impact

cyclostratigraphy and astronomical tuning of the late maastrichtian at zumaia basque country northern spain the standard geological time scale for the cretaceous is still largely based on seafloor anomaly profiles combined with radio isotopic tie points the astronomical tuned time scale with its much higher resolution and accuracy has recently been extended to the k pg boundary and is being extended into the cretaceous to construct such a time scale for the cretaceous we selected the upper maastrichtian of the zumaia section in the basque country northern spain which contains a cyclic alternation of limestones and marls deposited in a hemipelagic setting the paleogene portion of the zumaia section has previously been studied for a joint cyclostratigraphic radioisotopic intercalibration of the age of the k pg boundary here we present a high resolution cyclostratigraphic framework for the upper maastrichtian latest cretaceous of the zumaia section in the basque country northern spain with new biostratigraphic and magnetostratigraphic data bed to bed correlation with the nearby sopelana section provides additional bio and magnetostratigraphic constraints the stacking pattern of the lithologies shows a hierarchy that reflects the combined influence of the orbital parameters of precession and eccentricity this is confirmed by time series analyses of lithological and geochemical data indicating a strong influence of eccentricity modulated precession on latest cretaceous climate the expression of the 405 kyr eccentricity cycle serves as primary signal for astronomical tuning we provide two tuning options depending on absolute k pg boundary ages of 65 56 and 65 97 ma the logged part of the section encompasses nine and a half 405 kyr cycles in total and spans 3 9 myr the acquired cyclostratigraphic framework provides ages for characteristic planktonic foraminiferal events magnetic reversals and carbon isotope excursions and resolves the late maastrichtian time scale in unprecedented detail with relative age uncertainties o 100 kyr the high resolution and large amplitude of shifts in d 13 c on the 405 kyr and 1 2 myr scales allow for global

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

rapid change in strontium isotopic composition of sea water before the cretaceous tertiary boundary a critical aspect of the debate over the origin of the chemical biological and climatological perturbations that characterize the cretaceous tertiary boundary k t boundary is the timescale and detailed pattern of the extinctions and geochemical changes whether the development of the anomalies was catastrophic over tens to hundreds of years or more gradual 103 to 106 years whether the anomalies occur exactly at the boundary or precede it and whether the chemical and biological signatures represent one event or can be resolved into several discrete events are issues fundamental to identifying the cause of the marked changes to determine the timing and rate of change of strontium isotope evolution of sea water during the five million years preceding the k t boundary we analysed foraminifera from an exceptionally thick palaeontologically well characterized marine k t section exposed at bidart southwest france we found a rapid increase in 87sr 86sr of ocean water 1 5 to 2 3 myr before the boundary the increase may be explained by a 10 increase in continental strontium flux to the oceans over a period of about 1 myr

an iridium rich layer at the cretaceous tertiary boundary in the bidart section southern france we have measured an anomalous iridium concentration in the clays of the cretaceous tertiary boundary in the bidart section thus confirming the planetary character of this anomaly the iridium anomaly is synchronous with a very sharp minimum of the 13c 12c ratio and there is no evidence of any other iridium enrichment in marly concentrations away from the boundary the integrated iridium content is of the same order of magnitude as that calculated from deep sea cores in the pacific ocean in spite of a careful search we have found no evidence of any exotic material

the relation of a coastal environment to early diagenetic clinoptilolite zeolite formation new data from the late cretaceous european basin abstract clinoptilolite an authigenic mineral of the zeolite group is commonly considered as a product of rhyolitic volcanic glass transformation during diagenesis of carbonate pelagic hemipelagic sediments of cretaceous paleogene age another subordinate way of clinoptilolite formation that has been suggested in the literature is its precipitation during diagenetic transformation of carbonate sediments enriched with biogenic silica the observations concerning transformation of the latter into zeolite are scarce and mostly based on cretaceous pelagic hemipelagic and shallow marine deposits and post cretaceous sediments in which there was no volcanic material the mechanism of diagenetic clinoptilolite formation in volcanic free sediments as well as the role of the depositional setting in its precipitation is not well understood in carbonates of the nw part of the european basin west side of the ysogory dobrogea archipelago studied here the clinoptilolite is associated with opal ct from the dissolution of sponge spicules and a detrital clays are documented for the first time in a cretaceous coastal environment the carbonates were deposited in a coastal environment which was under the influence of weak currents delivering sponge spicules biogenic opal from the offshore zone and the terrestrial input was transported by rivers from a neighboring island the lush plant vegetation with numerous pteridophytes and conifers within a warm sub tropical or tropical and humid climate enhanced the terrestrial weathering of clays and feldspars this unique combination of elevated silica concentrations from sponge spicules dissolution and aluminum availability from terrestrial weathering of feldspars in a subtropical climate during the early diagenesis combined with alkalinity triggered by organic matter decomposition enhanced the clinoptilolite formation a few centimeters below the seabed the investigation demonstrates a novel route by which pore fluids enriched with silica and aluminum initiate clinoptilolite formation during the very early stage of diagenesis a few centimeters below the seafloor the clinoptilolite distribution in late cretaceous silica rich deposits with no volcanic input of the european basin is always associated with opal ct from sponge spicules and elevated terrestrial input clays taking into account the distinctive campanian maastrichtian sedimentation in the epicontinental european basin composed almost exclusively of pure pelagic carbonates the depositional settings in which the silica and aluminum were available in pore waters was restricted to the part of the basin influenced by terrestrial input this leads to the conclusion that clinoptilolite formation in volcanic free sediment is an indicator of not only enhanced sponge silica availability but also the close proximity of the source area

middle eocene to early oligocene paleoceanography of the antarctic ocean maud rise odp leg 113 site 689 change from a low to a high productivity ocean the sediment record from the middle eocene about 45 ma to the early oligocene about 33 ma on maud rise site 689 weddell sea paleodepth 1400 1650 m has been studied by means of a coarse fraction analysis in order to reconstruct variations in surface water productivity and bottom water masses productivity has been estimated using carbonate dissolution and accumulation rates of radiolaria diatoms benthonic and planktonic foraminifers sponge debris ostracods and echinoids accumulation rates of bolboforma spp vary parallel to these proxies productivity increased with time three episodes were found in which productivity increased sharply and productivity varied cyclically with a period of 430 ky the middle eocene ca 45 44 37 26 ma was a period of generally low productivity no opal was deposited and carbonate preservation was excellent during periods of a slight increase in productivity accumulation rates of benthonic and planktonic foraminifers ostracods and echinoids increased in spite of an increase in carbonate dissolution in the period ca 41 62 43 07 ma productivity rose in three distinct pulses clinoptilolite formed from biogenous opal is a common mineral in these sediments these three productivity maxima coincide with a mica rich unit mica probably originated from erosion on antarctica and was transported by an intermediate water depth current in the late eocene ca 36 9 34 95 ma productivity increased considerably and opal skeletons were deposited carbonate dissolution increased but planktonic foraminifer accumulation rates were not markedly decreased in periods of higher productivity because productivity increase exceeded or equalled rises in dissolution rates increasing opal accumulation rates coincided with increases in the rates for benthonic foraminifers sponge debris ostracods and echinoids in the late eocene early oligocene period 34 95 33 05 ma a further increase in productivity led to a strong dissolution of planktonic foraminifers in high productivity periods the productivity increase is probably due to the increasing pole equator temperature gradient which led to stronger atmospheric and oceanic turnover and mixing in the oceans in the uppermost late eocene two unusual layers suggest changes in bottom water masses affecting the seafloor at site 689 a sediment interval with especially well preserved carbonate microfossils underlies an interval that has poor carbonate preservation despite evidence of low productivity these intervals suggest a change in bottom water conditions from warm saline deep water wsdw which favors carbonate preservation to antarctic bottom water aabw which promotes carbonate dissolution cooling of surface waters over a period of 300 ky led to a thickening of the deep aabw layer and an upward shift of the overlying wsdw until the aabw finally reached the top of the maud rise in the area of site 689 this period of aabw influence on top of the maud rise lasted 200 ky

climatic and sea level control of jurassic pliensbachian clay mineral sedimentation in the cardigan bay basin llanbedr mochras farm borehole wales

authigenic zeolites in late pleistocene sediments of the south atlantic angola basin two types of authigenic zeolites from late pleistocene hemipelagic sediments of the south atlantic were examined by xrd and sem analysis the xrd characteristics and the thermal behaviour of the zeolites suggest that type 1 is composed of clinoptilolite type 2 was identified as mordenite based on morphological features and qualitative elemental composition their authigenic nature is suggested from the shape of the crystals and their growth on and within foraminifera tests volcanic ash material acting as precursor minerals to zeolite formation could not be found nor were the zeolites directly related to siliceous remains an authigenesis from pore waters must be assumed mineral stabilities for amorphous sio2 and zeolites were calculated with the computer program solmineq88 interstitial waters proved to be oversaturated throughout the core in respect to all zeolites considered biogenic opal represents the limiting solid phase for si concentrations in pore waters therefore the dissolution of opal is regarded as the main source of si for interstitial waters below 9 m core depth where opal is only present in traces detrital feldspars and the zeolites themselves are affected by dissolution this may indicate the metastable nature of the zeolites also corroborated by the presence of two almost isochemical members of the same zeolite group

ichnological analysis of the bidart and sopelana cretaceous paleogene k pg boundary sections basque basin w pyrenees refining eco sedimentary environment abstract ichnological analysis of two cretaceous paleogene boundary sections at bidart se france and sopelana n spain has been conducted in order to refining eco sedimentary environment and to a make comparison with previous interpretations based on microfossils in both sections trace fossil assemblage is low diverse consisting of chondrites planolites thalassinoides trichichnus zoophycos and phycosiphon ascribed to the zoophycos ichnofacies however with distinct differences in the bidart section early danian dark filled trace fossil assemblage is more abundant in large thalassinoides zoophycos and larger chondrites and less abundant in trichichnus and small chondrites in comparison to the sopelana section sopelana is thus interpreted as a more offshore deeper section than bidart although both were located in the upper bathyal zone of the basin oxygenation below the sediment water interface was enough to favour a generalized total bioturbation of sediments the presence of chemichnia and trace fossils which tracemakers sequestered sediments from the sea floor in deep burrows zoophycos suggests at least periodical food deficiency the distribution and content of trace fossils across the k pg boundary do not change significantly revealing minor incidence of the k pg boundary event on the macrobenthic environment the k pg boundary transition including the boundary layer is nearly totally bioturbated hence a redistribution of microbiotic and abiotic components by tracemakers is possible

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

multiproxy approach for determination of nature and origin of carbonate micro particles so called micarb in pelagic sediments abstract carbonate fractions of pelagic and hemipelagic sediments may contain significant proportions of microcrystalline 6 m calcitic particles with no specific structure so called micarb depending on the authors this term is related to different descriptions and yet there is no clear consensus on their origin although micarb can dominate the sediment compositions in the pelagic realm their isolation from the other carbonate particles is not easy due to their small size the use of a granulometric separation method of the main carbonate particles allows the characterization and quantification of the compositions of the carbonate phases of the sediments and gives access to the geochemical signatures oxygen and carbon stable isotopes and mineralogical characteristics of each type of carbonate particles in the sediments this approach was applied to three sections the mid pliocene punta piccola section south sicily the upper campanian bidart section sw france and the aptian sediments from la marcouline quarry se france the quantifications showed quite important proportions up to 70 of the carbonated phases of micarb in the sections studied excepted in the mid pliocene punta piccola samples however depending on the sedimentary context it appears that those fine calcitic particles may result from fine biogenic fragmentation aptian la marcouline quarry authigenic precipitation in the water column mid pliocene punta piccola section or they may have an allochthonous origin upper campanian bidart section based on these interpretations it is then possible to estimate precisely the composition of the carbonate fractions in each studied section this work shows that except for the bidart sediments which are dominated by fine exported carbonate mud the other sections mainly consist in biogenic carbonates

clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower miocene carbonate sediments of the bahamas platform isotopic and thermodynamic evidence abstract massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor mbsf in lower miocene well compacted carbonate periplatform sediments from the great bahama bank ocean drilling program odp leg 166 site 1007 the diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix as well as mg smectites in carbonate rich deposits the formation of the zeolite requires a supply of silica thus the objective of the study is to determine the origin of the silica supply its diagenetic evolution and consequently the related implications on interpretation of the sedimentary record in terms of local or global paleoceanographic change for lack of evidence for any volcaniclastic input or traces of si enriched deep fluids circulation an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow water carbonate sequences geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite as well as its contemporaneous formation with the carbonate deposition sr model ages of 19 6 23 2 ma the test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage seawater rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica which reacts with clay minerals the amount of silica opal a involved in the reaction has to be significant enough at least 10 wt to account for the observed content of clinoptilolite occurring at the most zeolite rich level modeling also shows that the observed amount of clinoptilolite 19 reflects an in situ and short term reaction due to the high reactivity of primary biogenic silica opal a until its complete depletion the episodic occurrence of these well lithified zeolite rich levels is consistent with the occurrence of seismic reflectors particularly the p2 seismic sequence boundary located at 1115 mbsf depth and dated as 23 2 ma the age range of most zeolitic sedimentary levels biostratigraphic ages of 21 5 22 ma correlates well with that of the early miocene glaciation mi 1 and mi 1a global events thus the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply or in other sensitive marine areas is potentially a significant new proxy for paleoproductivity and oceanic global events such as the miocene events which are usually recognized in deep sea pelagic sediments and high latitude deposits

new chronostratigraphy of the cretaceous paleogene boundary interval at bidart france abstract the cretaceous paleogene boundary section at bidart french basque coast was prevented for a long time to be considered as a reference section for the k t boundary due to a suspected hiatus comprising most of chron c29r in our new magneto and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small but consistent intensity excursion during laboratory demagnetization trends magnetite and maghemite are the main carriers of remanence with contribution of hematite to the magnetic mineralogy of the red brown lowermost danian and maastrichtian marly limestones magnetite is the main magnetic mineral in the white to pink danian limestones the nrm of marls and marly limestones is characterized by 1 a present day field overprint 2 a medium temperature 300 400 c reversed component interpreted as a relic of the characteristic remanent magnetization chrm carried by magnetite grains which are not completely oxidized into maghemite and 3 a high temperature up to 590 c normal component due to late diagenesis hematite growth the nrm of danian limestones is characterized by 1 a present day field overprint and 2 a chrm component of normal or reversed polarity the chrm directions allow a new correlation of the magnetic polarity sequence with the geomagnetic polarity time scale gpts chron c29r corresponds to the maastrichtian unit and extends up to 3 5 m above the k t boundary and chron c29n occurs at the top of the section there is no evidence for a hiatus at or below the k t boundary and the section does not extend up to chron c28r as previously suggested these new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper our study demonstrates that the section of bidart can be considered as a key section to study the k t boundary event s

new and revisited paleomagnetic data from permian triassic red beds two kinematic domains in the west central pyrenees abstract new paleomagnetic results of permian triassic red beds from bielsa aure and somport anayet sectors in the pyrenean axial zone are presented and combined with revisited and reviewed paleomagnetic data from the west central pyrenees the paleomagnetic data from the red beds vary between different sectors that share similar structural position i e whilst all sectors share the existence of a characteristic prefolding component a cenozoic postfolding component is found only in one area but not in other areas with similar structural position and kinematics previous paleomagnetic data to the west of the studied zone reveal a pre turonian remagnetization component but this secondary component is not found in an area that shares a similar structural position aure this variability suggests that the paleomagnetic behavior is the result of a series of factors related to the particular tectonic history of the region sedimentation burial and deformation during basin inversion and therefore paleomagnetic data demands careful assessment in order to unravel the kinematics of areas with equivalent structural positions in orogens however important kinematic implications that hold in the pyrenees can be inferred from the restoration of the permian triassic characteristic magnetizations to positions previous to the paleomagnetic rotations recorded by cretaceous or cenozoic paleomagnetic data the restoration reveals on one hand the lack of large rotations in the south pyrenenan zone except in the nogueras area according to bates 1989 and on the other hand the contrasting rotations recorded to the west of the studied zone in the paleozoic basque massifs pbm and the south pyrenean zone indicating the existence of two different tectonic domains in the west central sector of the pyrenees the strong clockwise rotations postdating the early cretaceous remagnetization that are recorded in the pbm and in the north pyreneean zone suggest a late dextral shear deformation affecting areas near the north pyrenean fault zone

catastrophic mass extinction and assemblage evolution in planktic foraminifera across the cretaceous paleogene k pg boundary at bidart sw france abstract a high resolution biostratigraphic analysis of planktic foraminifers confirms that the bidart section at the eastern margin of the atlantic ocean exhibits a continuous and complete cretaceous paleogene k pg transition interval the biozones and subzones recorded in this section are less expanded than their equivalent in tunisian sections el kef global stratotype section and point gssp for the k pg boundary and elles auxiliary section but they are sufficiently thick to allow a detailed analysis of the evolution of the planktic foraminiferal assemblages across the k pg transition throughout the uppermost 4 m maastrichtian the planktic foraminiferal assemblages are highly diversified containing up to 72 species these maastrichtian assemblages are rich in cosmopolitan taxa 70 dominated by small biserial morphotypes which belong mainly to the genus heterohelix which coexist with less abundant but highly diverse tropical and subtropical species the extinction pattern at the bidart section suggests a sudden catastrophic mass extinction at the k pg boundary which affected at least 53 out of 72 species the species becoming extinct include globotruncanids e g contusotruncana spp globotruncana spp globotruncanita spp and complex heterohelicids e g racemiguembelina spp pseudotextularia spp gublerina spp at the bidart section only archaeoglobigerina cretacea disappears 2 m below the k pg boundary event specimens of 18 small and even tiny maastrichtian species are found at the lowermost danian only a few of these species belonging to the genera of guembelitria hedbergella and heterohelix are considered to be real cretaceous survivor species whereas the specimens belonging to the rest are most probably reworked because they differ in their preservation throughout lowermost danian the planktic foraminiferal assemblages are dominated by opportunistic species of the genus guembelitria these opportunists are associated to small and poorly diversified pioneer globigerinids palaeoglobigerina spp and parvularugoglobigerina spp these assemblages became progressively more diversified across the early danian containing species with cancellate walls eoglobigerina spp parasubbotina spp subbotina triloculinoides and praemurica spp and new taxa of biserial heterohelicids woodringina spp and chiloguembelina spp suggesting a paleoenvironmental recovery

maastrichtian climate productivity and faunal turnovers in planktic foraminifera in south atlantic dsdp sites 525a and 21 abstract stratigraphic faunal and isotopic analyses of the maastrichtian at dsdp sites 525a and 21 in the south atlantic reveal a planktic foraminiferal fauna characterized by two major events an early late maastrichtian diversification and end maastrichtian mass extinction both events are accompanied by major changes in climate and productivity the diversification event which occurred in two steps between 70 5 and 69 1 ma increased species richness by a total of 43 and coincided with the onset of major cooling in surface and bottom waters and increased surface productivity the onset of the terminal decline in maastrichtian species richness began at 67 5 ma and the first significant decline in surface productivity occurred at 66 2 ma coincident maximum cooling to 13 c in surface waters and the reduction of the surface to deep temperature gradient to less than 5 c major climatic and moderate productivity changes mark the mass extinction and the last 500 kyr of the maastrichtian between 200 and 400 kyr before the k t boundary surface and deep waters warmed rapidly by 3 4 c and cooled again during the last 100 kyr of the maastrichtian surface productivity decreased only moderately across the k t boundary species richness began to decline during the late maastrichtian cooling and by k t boundary time the mass extinction had claimed 66 of the species viewed within the context of maastrichtian climate and productivity changes the k t mass extinction could have resulted from extreme environmental stress even without the addition of an extraterrestrial impact

does large igneous province volcanism always perturb the mercury cycle comparing the records of oceanic anoxic event 2 and the end cretaceous to other mesozoic events mercury hg is increasingly being used as a sedimentary tracer of large igneous province lip volcanism and supports hypotheses of a coincidence between the formation of several lips and episodes of mass extinction and major environmental perturbation however numerous important questions remain to be answered before hg can be claimed as an unequivocal fingerprint of lip volcanism as well as an understanding of why some sedimentary records document clear hg enrichment signals whilst others do not of particular importance is evaluating the impact of different volcanic styles on the global mercury cycle as well as the role played by depositional processes in recording global hg cycle perturbations here new mercury records of cretaceous oceanic anoxic event 2 oae 2 94 ma and the latest cretaceous 67 66 0 ma are presented oae 2 is associated with the emplacement of multiple predominantly submarine lips the latest cretaceous with subaerial volcanism of the deccan traps both of these connections are strongly supported by previously published trends towards unradiogenic osmium os isotope values in globally distributed sedimentary records hg data from both events show considerable variation between different locations attributed to the effectiveness of different sediment types in registering the hg signal with lithologically homogeneous records documenting more clear hg enrichments than sections with major changes in lithology such as limestones to claystones or organic rich shales crucially there is no geographically consistent signal of sedimentary hg enrichment in stratigraphic records of either oae 2 or the latest cretaceous that matches os isotope evidence for lip emplacement indicating that volcanism did not cause a global hg perturbation throughout the entire eruptive history of the lips formed at those times it is suggested that the discrepancy between os isotope and hg trends in records of oae 2 is caused by the limited dispersal range of hg emitted from submarine volcanoes compared to the global scale distribution of os a similar lack of correlation between these two proxies in uppermost cretaceous strata indicates that although subaerial volcanism can perturb the global hg cycle not all subaerial eruptions will do so these results highlight the variable impact of different volcanogenic processes on the efficiency of hg dispersal across the globe factors that could influence the impact of lip eruptions on the global mercury cycle include submarine versus subaerial volcanism volcanic intensity or explosivity and the potential contribution of thermogenic mercury from reactions between ascending magma and surrounding organic rich sediments

a multi proxy approach to decode the end cretaceous mass extinction mass extinctions generally involve a complex array of interrelated causes and are best evaluated by a multi proxy approach as applied here for the end cretaceous mass extinction this study documents and compares the planktic foraminiferal records carbonate dissolution effects stable isotopes and magnetic susceptibility in france bidart austria gamsbach and tunisia elles in order to explore the environmental conditions during the uppermost maastrichtian plummerita hantkeninoides zone cf1 leading up to the mass extinction planktic foraminiferal assemblages at bidart and gamsbach appear to be more diverse than those at elles with unusually high abundance 20 30 and diversity 15 species of globotruncanids in the two deep water sections but lower abundance 10 and diversity 10 species at the middle shelf elles section oxygen isotopes in zone cf1 of elles record rapid climate warming followed by cooling and a possible return to rapid warming prior to the mass extinction the onset of high stress conditions for planktic foraminifera is observed 50 60 cm below the ktb at bidart and gamsbach and 4 5 m below the ktb at elles due to much higher sediment accumulation rates these intervals at bidart and gamsbach record low magnetic susceptibility and high planktic foraminiferal fragmentation index fi at elles bidart and gamsbach an increased abundance of species with dissolution resistant morphologies is also observed at gamsbach the correlative interval in india records significantly stronger carbonate dissolution effects in intertrappean sediments between the longest lava flows ending with the mass extinction based on current evidence this widespread dissolution event stratigraphically coincides with the climate cooling that follows the late maatrichtian global warming and may be linked to ocean acidification due to deccan volcanism the estimated 12 000 28 000 gigatons gt of co2 and 5200 13 600 gt of so2 introduced into the atmosphere likely triggered the carbonate crisis in the oceans resulting in severe stress for marine calcifiers leading to mass extinction

diagenesis of upper jurassic concretions from the antarctic peninsula abstract early diagenetic calcite concretions are widespread in mudstone tuff sequences deposited in late jurassic oxygen deficient basins of the antarctic peninsula although obscured or destroyed in host rocks original components and sedimentary structures are well preserved in concretions in spite of significant mineralogical and geochemical change during growth early diagenesis led to a rapid and almost total variation in the mineralogy within the concretions independently of the original host lithologies siliceous particles were replaced by chlorite and zeolites in suboxic alkaline and moderately reducing conditions on the sea floor and in the first centimeters of burial these conditions also led to localized early formation of phosphate concretions pyrite developed subsequently because of high sulfide abundance in the sulfate reduction zone most calcite precipitation took place in the lower part of the sulfate reduction zone in a strongly alkaline environment where sulfide content had been lowered by pyrite precipitation in this stage the remaining siliceous elements were dissolved or replaced by calcite later diagenetic changes include enlargement of zeolites minor changes in clay minerals and the formation of calcite veins the wide range of 13c values in cement and vein calcite 1 47o oo and 20 23o oo pdb is explained by mixed carbon sources from organic matter oxidation and dissolution of calcareous shells the low 18o of calcite 2 34o oo to 19 72o oo pdb is the result of recrystallization in contact with high temperature diagenetic or hydrothermal fluids flushed through micro fractures in the concretions which also produced extensive vein formation

degree of supersaturation and precipitation of organogenic dolomite this paper integrates the degree of carbonate saturation into a general model for the origin of dolomite in organic rich marine sediments degradation of organic matter by sulfate reducing bacteria promotes early dolomite precipitation by simultaneously increasing the carbonate alkalinity and lowering the sulfate ion concentration to near zero the interstitial water in organic rich sediments of several deep sea drilling project sites where dolomite is believed to be precipitating is up to 10 sup 3 times supersaturated with respect to near ideal dolomite and up to 10 times supersaturated with respect to calcite the highly supersaturated interstitial waters appear to overcome low temperature kinetic barriers to precipitate a nonideal fine grained calcian dolomite the degree of supersaturation with respect to dolomite decreases and the interstitial water approaches carbonate equilibrium with increasing burial depth at several of these sites removal of sulfate ion is important because sulfate ion may inhibit dolomite precipitation a calcareous sediment is not required but the presence of calcite or aragonite appears to greatly enhance dolomitization by providing an additional source of ca and carbonate ions

the dan c2 hyperthermal event at gubbio italy global implications environmental effects and cause s abstract the dan c2 event is an early danian transient 100 kyr hyperthermal episode centered at 65 2 ma which records shifts in carbon reservoirs and ocean warming in the northwestern and southeastern sectors of the atlantic ocean here we present and discuss high resolution biochronostratigraphic and magnetic susceptibility data and geochemical records from the western tethyan contessa highway section gubbio italy which provide the first direct evidence of the dan c2 event beyond the atlantic ocean and point to the supra regional possibly global significance thereof at contessa highway the dan c2 event exhibits stressed ecological responses among calcareous nannoplankton and foraminifera which highlight marked environmental perturbation affecting the geobiosphere and resulting in enhanced eutrophication of the sea surface waters and carbonate dissolution as well as lowered oxygen content along the water column and at the sea bottom as for other early paleogene hyperthermal events the cause of the dan c2 event might likely to be found in changes potentially astronomically paced in the distribution of carbon within surface biosphere reservoirs however the role played by the concurrent third and last phase of deccan volcanism with its huge release of greenhouse gases into the atmosphere would be also taken into account in addition here we provide first evidence of a further short lived 38 kyr hyperthermal event not known up to now and that we term the lower c29n event

dolomite mountains and the origin of the dolomite rock of which they mainly consist historical developments and new perspectives beginning in the late 18th century the dolomite mountains in northern italy have been the location for major sedimentological developments from the discovery of the mineral dolomite to the formulation of the coral reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region further the dolomite mountains have inspired voluminous research into the origin of dolomite questioning whether dolomite is a primary precipitate or a secondary replacement product recently with the recognition that microbes can mediate dolomite precipitation a new geomicrobiological approach combining the study of modern natural environments with bacterial culture experiments is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record this three pronged methodology applied to the study of dolomite formation holds great promise for future research into the dolomite problem and provides a new impetus to revisit the dolomite mountains in the 21st century

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

paleoceanographic and paleoclimatic evolution in the weddell sea antarctica during the middle eocene late oligocene from a coarse sediment fraction and clay mineral data odp site 689 abstract paleoceanographic and paleoclimatic variations from the middle eocene to the late oligocene in the antarctic area atlantic sector have been deduced from a coarse fraction and clay mineral analysis of odp site 689 leg 113 sediments in the middle eocene smectite rich sediments were a consequence of warm climate with alternating wet and dry seasons on the antarctic margins productivity in the maud rise area of the southern ocean was low no opal was deposited preservation of carbonate is very good carbonate preservation shows cyclic changes clinoptilolite is a common mineral at the middle late eocene boundary 40 8 ma productivity suddenly increased enough to allow preservation of opal skeletons clinoptilolite disappeared where opal appeared illite appearance points to cooling of antarctic climates the presence of detrital kaolinite indicates a change in oceanic circulation in the late eocene cyclic changes in productivity cycle length 0 42 ma were associated with clay mineral compositional variations sediments from colder highly productive periods generally contain more illite and less smectite than warmer ones the early oligocene was a period of very high productivity highest opal accumulation rates correspond to strongest carbonate dissolution cyclic changes in productivity with a mean cycle duration of 0 4 0 5 myr were associated with cyclic changes in clay mineral composition highest productivity intervals are generally those with highest smectite content because of strong erosion of sediments of the antarctic margins sediments from less productive warmer periods most of the time contain more illite in the late oligocene the intensity of physical weathering on antarctica was stronger as documented by increasing fluxes of illite muscovite amphiboles and talc ice expanded on the east antarctic continent lower productivity than in the early oligocene is indicated by lower accumulation rates of opal and weaker carbonate dissolution cyclic variations in productivity correlate to cyclic variations in illite and smectite contents as in the early oligocene

lower maastrichtian cyclostratigraphy of the bidart section basque country sw france a remarkable record of precessional forcing abstract cyclostratigraphic analysis of the maastrichtian limestone marl alternations of bidart sw france allows the hypothesis of orbital control on lithological cycles to be evaluated magnetic susceptibility ms oxygen and carbon isotope measurements sampled at a high resolution are analyzed using various cyclostratigraphic tools a statistically significant orbital signal is detected with a remarkable record of the precession corresponding to the limestone marl couplets this well expressed orbital forcing allows the building of a relative cyclostratigraphic time scale for the ms and 13 c records based on the 100 kyr eccentricity cycle the total duration of the section is estimated at 1 44 0 22 myr correlation based on calcareous nannofossil biostratigraphy and comparison of the scaled bidart 13 c record to the astronomically calibrated 13 c signal of odp hole 762c shows that the studied section extends from 71 5 to 70 ma covering the upper part of chron c32n 1n and 2 3 of chron c31r oxygen isotope data suggest a 2 c cooling of sea surface temperatures during the studied interval when placed on the long term 18 o trend of the bidart section this interval is here recognized as the onset of the early maastrichtian cooling event with its excellent record of the precessional forcing the bidart section along with other sections of the basque country is a useful tool for the refinement of the maastrichtian timescale

asteroid impact vs deccan eruptions the origin of low magnetic susceptibility beds below the cretaceous paleogene boundary revisited abstract the respective roles of an asteroid impact and deccan traps eruptions in biotic changes at the cretaceous paleogene k pg boundary are still debated in many shallow marine sediments from around the world the k pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter thick low susceptibility zone a previous study of the gubbio section italy lowrie et al 1990 attributed low magnetization intensity in this interval to post depositional dissolution of ferrimagnetic minerals dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the k pg event we compare the magnetic properties of sediments from the gubbio section with those of the bidart section in southern france the two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone when compared to background cretaceous sediments the low susceptibility zone in both sections is marked by an absence of biogenic magnetite a decrease in total ferrimagnetic mineral content and a preferential loss of magnetite with respect to hematite features that are consistent with reductive dissolution however unlike the gubbio section where the low susceptibility zone starts immediately below the boundary clay the low susceptibility zone and the clay layer at bidart are separated by a 4 cm carbonate interval that contains abundant biogenic magnetite such separation casts doubt on a causal link between the impact and sediment bleaching more likely the low susceptibility layer marks a different environmental event that preceded the impact an episode of increased atmospheric and oceanic acidity associated with deccan traps volcanism that occurred well before the k pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals

extinction survivorship and evolution of planktic foraminifera across the cretaceous tertiary boundary at el kef tunisia an expanded sediment record at el kef shows that the k t boundary extinctions of planktic foraminifera extend over an interval from 25 cm below the geochemical boundary ir anomaly to 7 cm above species extinctions appear sequential with complex large ornate forms disappearing first and smaller less ornate forms surviving longer the 14 species extinctions below the boundary appear unrelated to an impact event cretaceous species survivorship is greater than previously assumed about 10 species survive 22 into subzone p1a globigerina eugubina all cretaceous survivors are small primitive forms which are generally smaller than their ancestors in cretaceous sediments species evolution after the k t event occurs in two pulses the first new paleocene species evolve in the basal black clay zone po immediately after the major cretaceous extinctions evolving species are small and primitive similar to cretaceous survivors the second pulse in species evolution occurs in the lower part of subzone p1b with the appearance of larger more diverse species the first major increase in carbonate sedimentation and productivity occurs at this time and signals the recoveyr of the ecosystem nearly 300 000 years after the k t event the species extinctions prior to the generally assumed impact event implied by the ir anomaly and the long recovery period of the ecosystem thereafter cannot be explained by a single impact but suggest that multiple causes may be responsible such as climatic changes volcanism a sea level drop production of warm saline bottom water and the chemical consequences associated with increased salinity

authigenic zeolites and their relation to silica diagenesis in odp site 661 sediments leg 108 eastern equatorial atlantic at odp site 661 from the continental margin of northwest africa authigenic clinoptilolites were investigated in detail based on chemical data crystal size and shape and their intergrowth with associated mineral phases two types of clinoptilolite type a and b occur representing different diagenetic conditions under which the formation of zeolites took place clinoptilolite type a was found in a sediment section comprises numerous hiatuses and stratigraphically condensed sediments and could have been formed during early diagenesis in contrast clinoptilolite type b was precipitated in campanian sediments during later diagenesis after the transformation from opal a to opal ct had taken place at site 661 the silica source for the authigenesis of both clinoptilolite types is biogenic opal volcanic ash material occurs but in low amounts and thus is not a prerequisite for the formation of clinoptilolite at site 661

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

an expanded cretaceous tertiary transition in a pelagic setting of the southern alps central western tethys abstract an integrated micropalaeontological planktonic foraminifera and calcareous nannofossils mineralogical and stable isotope investigation was carried out across the cretaceous tertiary k t boundary in a previously undescribed hemipelagic section forada section cropping out in the piave river valley southern alps belluno province ne italy our results suggest that an apparently complete k t transition with an expanded basal danian is preserved in the forada section the k t boundary is marked by a ca 1 5 cm thick clay interval intercalated in the pelagic carbonates of the scaglia rossa formation the boundary clay characterised by an iridium anomaly is constituted by a basal 1 2 mm thick green layer and an overlying red layer below the boundary clay the calcareous nannofossil micula murus zone and the planktonic foraminiferal plummerita hankteninoides zone that characterise the terminal maastrichtian in complete sections have been identified virtually all the calcareous plankton biohorizons normally utilised in the early danian have been recognised interestingly the ranking of the various biohorizons is the same observed elsewhere if considering the single plankton groups separately while several inconsistencies emerge if considering the relative succession of the two calcareous plankton groups simultaneously we caution to infer hiatuses on a section based only on the spacing of the biohorizons of a single group because of the risk of diachroneities in the stressed early danian environment by comparing the distance of the key biohorizons i e the first occurrence fo of cruciplacolithus primus and the last occurrence lo of parvularugoglobigerina eugubina from the boundary clay at forada and in other complete classical sections we have inferred that the early danian is expanded in the forada section the section seems ideally suited for studying the recovery pattern of the pelagic ecosystem and calcareous plankton communities in the aftermath of the k t catastrophe in a pelagic setting of the western central tethys the adaptive radiation of planktonic foraminifera was very rapid in contrast with the delayed diversification of coccolithophores the latter show a ca 21 cm thick dead zone that is followed by a long lasting survival interval ca 180 cm thick that might represent some 200 kyr dominated by persistent cretaceous coccolithophores and calcareous dinoflagellate thoracosphaera spp the temporal pattern in recovery of the coccolithophore and planktonic foraminiferal communities is decoupled as well pointing to their varying sensitivities to the k t perturbation it appears that the ecospace occupied by coccolithophores in the terminal maastrichtian was occupied in the initial danian by the calcareous dinoflagellates thoracosphaera spp that remained a significant component of the assemblage up to the base of the prinsius dimorphosus zone of romein romein a j t 1979 lineages in early paleogene calcareous nannoplankton utrecht micropaleontological bulletins 22 231 pp probably in the early chron c29n on the contrary planktonic foraminifera seem to reach stable conditions earlier in the late cruciplacolithus primus zone the early danian interval considered is also characterised by low sr ca ratio of the biogenic carbonates that would suggest dramatically low coccolithophore productivity conditions in agreement with previous flux studies we speculate that the altered ecological structure of phytoplankton community and the low productivity of coccolithophores severely affected the efficiency of the transfer of organic carbon from the photic zone to the deep ocean thus supporting the early danian living ocean model of d hondt et al d hondt s donaghay p zachos j c luttenberg d lindinger m 1998 organic carbon fluxes and ecological recovery from the cretaceous tertiary mass extinction science 282 276 279

a new sedimentary benchmark for the deccan traps volcanism 1 the origin of the cretaceous paleogene boundary kpb mass extinction is still the center of acrimonious debates by opposing partisans of the bolide impact theory to those who favored a terrestrial origin linked to the deccan traps volcanism here we apply an original and high resolution environmental magnetic study of the reference bidart section france our results show that the kpb is identified by an abrupt positive shift of the magnetic susceptibility ms also observed by others at the kpb elsewhere in addition an anomalous interval of very low ms carried by an unknown cl bearing iron oxide similar to specular hematite is depicted just below the kpb grain size and morphology of the cl iron oxide are typically in the range of hematitic dust currently transported by winds from sahara to europe this discovery is confirmed in the referenced gubbio section italy suggesting a global scale phenomenon as a conjecture we suggest an origin by heterogeneous reaction between hcl rich volcanic gas and liquid solid aerosols within buoyant atmospheric plumes formed above the newly emitted deccan flood basalts based on this hypothesis our discovery provides a new benchmark for the deccan volcanism and witnesses the nature and importance of the related atmospheric change

constraining magnesium cycling in marine sediments using magnesium isotopes abstract magnesium concentrations in deep sea sediment pore fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust to better characterize and differentiate these processes we have measured magnesium isotopes in pore fluids and sediment samples from ocean drilling program sites 1082 1086 1012 984 1219 and 925 that span a range of oceanographic settings at all sites magnesium concentrations decrease with depth at sites where diagenetic reactions are dominated by the respiration of organic carbon pore fluid 26mg values increase with depth by as much as 2 because carbonates preferentially incorporate 24mg low 26mg the increase in pore fluid 26mg values at these sites is consistent with the removal of magnesium in mg carbonate dolomite in contrast at sites where the respiration of organic carbon is not important and or weatherable minerals are abundant pore fluid 26mg values decrease with depth by up to 2 the decline in pore fluid 26mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore fluid the identity of this enriched magnesium sink is likely clay minerals using a simple 1d diffusion advection reaction model of pore fluid magnesium we estimate rates of net magnesium uptake removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites independent estimates of magnesium isotope fractionation during dolomite precipitation from measured 26mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites suggesting that local exchange of magnesium between sediment and pore fluid at these sites can be neglected our results indicate that the magnesium incorporated in dolomite is 2 0 2 7 depleted in 26mg relative to the precipitating fluid assuming local exchange of magnesium is minor at the rest of the studied sites our results suggest that magnesium incorporated into clay minerals is enriched in 26mg by 0 to 1 25 relative to the precipitating fluid this work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources sinks in low temperature aqueous systems

changes in the pelagic fine fraction carbonate sedimentation during the cretaceous paleocene transition contribution of the separation technique to the study of bidart section abstract the bidart section southwestern france displays an almost complete late maastrichtian early danian transition as the cretaceous paleocene k p crisis mainly affects carbonate producers the aim of this study is to precisely evaluate the evolution of pelagic sediments composition across the boundary a new protocol for separation of fine carbonates into discrete size fractions was used 1 to precisely evaluate in weight of carbonate and not only number of particles per gram of rock the contribution of each carbonate component type foraminifera calcareous nannofossils and nonbiogenic carbonates calcite microparticles and calcareous macrocrystals and 2 to measure their isotopic signature and thus constrain the origin of these carbonate particles the maastrichtian pelagic carbonates are mostly biogenic 70 of the bulk carbonate but also contain nonbiogenic carbonates calcite microparticles 4 m due to their isotopic signature calcite microparticles cannot result from fragmentation and partial dissolution of biogenic remains and the most likely origin appears to be early diagenetic processes and or export from shallow water platforms the calcareous macrocrystals derive from an early diagenetic process the lowermost danian sediments are dominated by nonbiogenic carbonates between 50 and 80 of the bulk carbonate nevertheless two kinds of biogenic components are present thoracosphaera operculata and maastrichtian like calcareous nannofossils thoracosphaera operculata is an opportunistic calcareous dinoflagellate which frequently occurs at high abundances in basal danian sediments and is certainly in situ this is confirmed by it showing a danian isotopic signature the maastrichtian like calcareous nannofossils are nannofossils of taxa that indisputably occur in the maastrichtian but which do not occur consistently in later danian strata they show a maastrichtian type isotopic signature which strongly supports the inference that they are mainly reworked from the maastrichtian this study shows that nonbiogenic carbonates are important components of the pelagic carbonate even during periods of normal sedimentation in the crisis interval the biogenic carbonate production drops sharply while calcite microparticles and carbonate macrocrystals remain almost unaffected and represent the major part of the carbonate in the earliest danian sediments finally the sediments return to a composition similar to that of the maastrichtian in m pseudobulloides foraminifera zone

holocene coastal carbonates and evaporites of the southern arabian gulf and their ancient analogues abstract the holocene sediments of the coast of the united arab emirates in the southeastern arabian gulf are frequently cited in the literature as type examples for analogous assemblages of carbonates evaporites and siliciclastics throughout the geologic record this paper is intended as a convenient single source for the description of sediments of this region providing information on how to reach the classic localities and some of the analogs the holocene sediments of the region accumulate over an area that is 500 km long and up to 60 km wide the sediments collecting offshore are predominantly pelecypod sands mixed with lime and argillaceous mud with these latter fine sediments increasing as the water deepens the pelecypod rich sediments also collect east of abu dhabi island both in the deeper tidal channels between the barrier island lagoons and in deeper portions of the protected lagoons west of abu dhabi island the shallow water margin is the site of coral reefs and coralgal sands whereas to the east oolites accumulate on the tidal deltas of channels located between barrier islands grapestones accumulate to the lee of the reefs and the oolite shoals where cementation becomes more common they are particularly common on the less protected shallow water margins of the lagoons west of abu dhabi island pelleted lime muds accumulate in the lagoons in the lee of the barrier islands of the eastern abu dhabi lining the inner shores of the protected lagoons of abu dhabi and on other islands to the west are cyano bacterial mats and mangrove swamps landward of these a prograding north facing shoreline is formed by supratidal salt flats sabkhas in which evaporite minerals are accumulating this paper describes the localities associated with 1 the mangrove swamps of the west side of the al dhabaiya peninsula 2 the indurated cemented carbonate crusts cyanobacterial flats and sabkha evaporites on the shore of the khor al bazam south of qanatir island 3 the reef and oolitic sand flats on the coast just east of jebel dhana and 4 the marine travertine and aragonite coats associated with the beach sediments in a small bay south of jebel dhana and 5 the sabkha mutti between jebel barakah and al sila similar sedimentological associations of carbonate and evaporites to those of the holocene of the united arab emirates are to be found in the tertiary and mesozoic sedimentary rocks of the immediate subsurface in the arabian gulf other analogs to this setting include the paleozoic carbonates of the western usa europe and asia mesozoic carbonates of the gulf of mexico europe and middle east and tertiary sedimentary rocks in the middle east

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

diagenetic clinoptilolite and opal ct from the middle eocene wittering formation isle of wight u k abstract thinly interbedded estuarine sandstones and mudstones in the middle eocene wittering formation bracklesham group isle of wight contain authigenic clinoptilolite opal ct and possibly authigenic smectite over a thickness of 8 5 m the clinoptilolite and opal ct cements have resulted in these beds being the only ones tightly cemented in the wittering formation the rest being soft sands and muds the unit contains abundant siliceous sponge spicules many of which are preserved as voids and were the likely source of the opal ct the presence of an organic rich rooted paleosol histosol immediately overlying the unit is believed to have contributed significantly to the early diagenetic history of the estuarine sediments in particular the formation of zeolite although the ph and dissolved silica of unmodified seawater are too low for zeolite formation high concentrations of dissolved silica are believed to have resulted from dissolution of siliceous sponge spicules opal a and formation of silica organic acid complexes organic acid dissolution of feldspars absent from the zeolite bearing interval is the most probable source of alumina and cations for zeolite formation ree data show that volcanic ash was never present and cannot therefore have been the source of the zeolite this investigation therefore demonstrates a novel route by which the pore fluid chemistry required for zeolite formation can be achieved

integrated paleocene calcareous plankton magnetobiochronology and stable isotope stratigraphy dsdp site 384 nw atlantic ocean abstract at deep sea drilling site 384 j anomaly ridge grand banks continental rise nw atlantic ocean paleocene nannofossil chalks and oozes 70 m thick are unconformably disconformably underlain 168 m upper maastrichtian and overlain 98 7 m upper lower eocene by sediments of comparable lithologies the chalks are more indurated in stratigraphically higher levels of the paleocene reflecting increasing amounts of biosiliceous radiolarians and diatoms components this site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy we report the results of a magnetostratigraphic study which when incorporated with published magnetostratigraphic results reveals an essentially complete magnetostratigraphic record spanning the interval from magnetochron c31n late maastrichtian to c25n partim late paleocene thanetian integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of and constrain the estimated duration of a short hiatus 0 9 my within the younger part of chron c29r including the k p boundary and an 6 my hiatus separating upper paleocene magnetozone c25n and upper lower eocene magnetozone c22r sediments some 30 planktonic foraminiferal datum levels including the criteria used to denote the paleocene planktonic foraminiferal sub tropical zonal scheme of berggren and miller micropaleontology 34 4 1988 362 380 and berggren et al sepm spec publ 54 1995 129 212 geol soc am bull 107 11 1995 1272 1287 and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology planktonic foraminiferal subzones p4a and p4b of upper paleocene zone p4 are emended redefined based on the discovery of a longer stratigraphic extension of acarinina subsphaerica into at last magnetozone c25n stable isotope stratigraphies from benthic foraminifera and fine fraction

mercury anomaly deccan volcanism and the end cretaceous mass extinction reply the contribution of the deccan traps west central india volcanism in the cretaceous paleogene kpg crisis is still a matter of debate recent u pb dating of zircons interbedded within the deccan lava flows indicate that the main eruptive phase 1 1 106 km3 of basalts initiated 250 k y before and ended 500 k y after the kpg boundary however the global geochemical effects of deccan volcanism in the marine sedimentary record are still poorly resolved here we investigate the mercury hg content of the bidart france section where an interval of low magnetic susceptibility ms located just below the kpg boundary was hypothesized to result from paleoenvironmental perturbations linked to the paroxysmal deccan phase 2 results show hg concentrations 2 orders of magnitude higher from 80 cm below to 50 cm above the kpg boundary maximum 46 6 ppb and coincident with the low ms interval increase in hg contents shows no correlation with clay or total organic carbon contents suggesting that the hg anomalies resulted from higher input of atmospheric hg species into the marine realm rather than organic matter scavenging and or increased runoff the hg anomalies correlate with high shell fragmentation and dissolution effects in planktic foraminifera suggesting correlative changes in marine biodiversity this discovery represents an unprecedented piece of evidence of the nature and importance of the deccan related environmental changes at the onset of the kpg mass extinction

magnetostratigraphy and rock magnetism of the ilerdian stratotype at tremp spain abstract the first magnetic polarity stratigraphy of the ilerdian stratotype locality the tremp section spain has been established in total 315 samples from 66 levels have been demagnetized both thermally and by alternating field various types of rock magnetic behaviors and low and high temperature magnetization components have been distinguished and have been correlated with the three sampled rock types clays marls and fine grained sandstones and coarse grained sandstones the paleomagnetic directions have been classified according to their reliability in representing a primary magnetization the magnetostratigraphy of the tremp section is characterized by a long reversed polarity zone interrupted by a normal zone we conclude that the normal polarity interval found in the ilerdian stratotype may represent chron 24 2 the lower and upper reversed intervals of the section are then respectively chrons 24 1r and 24r