stable organic carbon isotope stratigraphy across oceanic anoxic event 2 of demerara rise western tropical atlantic ocean drilling program odp leg 207 recovered expanded sections of organic carbon rich laminated shales on demerara rise western tropical atlantic high resolution organic carbon isotope and total organic carbon toc records are presented which span the cenomanian turonian boundary interval ctbi including the oceanic anoxic event oae 2 from four sites oriented along a nw striking depth transect these records represent the first high resolution carbon isotope records across oae 2 from the south american margin of the tropical atlantic due to the scarcity of age significant fossils the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the ctbi across the depth transect and to tie this to biostratigraphically well defined sections in the western interior basin pueblo usa boreal shelf seas eastbourne england and western tethys oued mellegue tunisia all four sections studied document a 6 increase of 13corg values at the base of the ctbi which is followed by an interval of elevated 13corg values and a subsequent decrease our results supply an important stratigraphic base for subsequent paleoceanographic studies on late cenomanian to early turonian sediments from demerara rise and elsewhere

age and origin of earliest adakitic like magmatism in panama implications for the tectonic evolution of the panamanian magmatic arc system abstract 40 20 ma marks a fundamental interval in the evolution of the 70 0 ma panamanian magmatic arc system during this period there is no evidence of panamanian magmatic arc activity to the east of the panama canal basin while to the west and in localized regions to the east of the panama canal basin a phase of intrusive only activity is recorded fundamentally geochemical and geochronological evidence presented herein indicate that this intrusive activity was predominantly adakitic like and becomes younger from west to east along an approximately w e striking lineament granodiorites of the petaquilla batholith western panama yield lam icp ms 206 pb 238 u zircon ages of 29 0 0 7 0 6 ma 28 5 0 7 0 5 ma 28 3 0 5 0 4 ma and 26 2 0 5 0 9 ma to the east of the panama canal basin zircons from a hypabyssal diorite of the mainly intermediate maje subvolcanic suite cedes a mean 206 pb 238 u age of 18 9 0 4 ma relative to other 70 5 ma panamanian magmatic arc lavas and intrusives maje and petaquilla intrusives yield adakitic like major and trace element abundances e g 15 wt al 2 o3 generally 3 5 wt na 2 o 400 ppm sr 120 and strongly fractionated hree patterns these 30 26 ma petaquilla and 19 ma maje suites are also compositionally similar to a subvolcanic suite of rare circa 25 ma adakitic like andesitic intrusives which occur within the panama canal basin midway between petaquilla and maje and at the same approximate latitude as petaquilla and maje collectively the geochemical and geochronological data for the adakitic like intrusives arc consistent with formation via partial melting of lowermost mafic crust above a sub horizontal slab tear that propagated from the west petaquilla to the east maje between 30 and 19 ma our new tectonic model postulates that collision between the panamanian magmatic arc system and an indentor e g a tract of thickened buoyant oceanic crust or plateau occurred at about 40 ma a time of which coincides with the initiation of left lateral offset of the early i e 70 40 ma arc system this collision resulted in the shutdown of the early arc system possible steepening of the subducting farallon slab and ultimately slab break off and the phase of mainly adakitic like intrusive activity subsequent to slab removal by 20 ma ne dipping subduction jumped to the south and initiated production of the later i e post 20 ma arc system soon thereafter

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

sedimentary organic matter preservation a test for selective degradation under oxic conditions the authors report here a test of the hypothesis that the extent of organic matter preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water sediment interface oxygen penetration depths organic element compositions and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the washington continental shelf slope and adjacent cascadia basin individual amino acid sugar and pollen distributions were analyzed for a 11 to 12 cm horizon from each core and sup 14 c based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites sediment accumulation rates decreased and dissolved o sub 2 penetration depths increased offshore along the sampling transect as a result oxygen exposure times oet increased seaward from decades mid shelf and upper slope to more than a thousand years outer cascadia basin organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location in particular organic carbon surface area ratios decreased progressively offshore and with increasing oet three independent compositional parameters demonstrated that the remnant organic matter in farther offshore sediments is more degraded both concentrationmore and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible oxic effect oet helps integrate and explain organic matter preservation in accumulating continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere less

sulfur isotopes track the global extent and dynamics of euxinia during cretaceous oceanic anoxic event 2 the mesozoic era is characterized by numerous oceanic anoxic events oaes that are diagnostically expressed by widespread marine organic carbon burial and coeval carbon isotope excursions here we present coupled high resolution carbon and sulfur isotope data from four european oae 2 sections spanning the cenomanian turonian boundary that show roughly parallel positive excursions significantly however the interval of peak magnitude for carbon isotopes precedes that of sulfur isotopes with an estimated offset of a few hundred thousand years based on geochemical box modeling of organic carbon and pyrite burial the sulfur isotope excursion can be generated by transiently increasing the marine burial rate of pyrite precipitated under euxinic i e anoxic and sulfidic water column conditions to replicate the observed isotopic offset the model requires that enhanced levels of organic carbon and pyrite burial continued a few hundred thousand years after peak organic carbon burial but that their isotope records responded differently due to dramatically different residence times for dissolved inorganic carbon and sulfate in seawater the significant inference is that euxinia persisted post oae but with its global extent dwindling over this time period the model further suggests that only 5 of the global seafloor area was overlain by euxinic bottom waters during oae 2 although this figure is 30 greater than the small euxinic fraction present today 0 15 the result challenges previous suggestions that one of the best documented oaes was defined by globally pervasive euxinic deep waters our results place important controls instead on local conditions and point to the difficulty in sustaining whole ocean euxinia

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

iron isotope and trace metal records of iron cycling in the proto north atlantic during the cenomanian turonian oceanic anoxic event oae 2 1 the global carbon cycle during the mid cretaceous 125 88 million years ago ma experienced numerous major perturbations linked to increased organic carbon burial under widespread possibly basin scale oxygen deficiency and episodes of euxinia anoxic and h2s containing the largest of these episodes the cenomanian turonian boundary event ca 93 5 ma or oceanic anoxic event oae 2 was marked by pervasive deposition of organic rich laminated black shales in deep waters and in some cases across continental shelves this deposition is recorded in a pronounced positive carbon isotope excursion seen ubiquitously in carbonates and organic matter enrichments of redox sensitive often bioessential trace metals including fe and mo indicate major shifts in their biogeochemical cycles under reducing conditions that may be linked to changes in primary production iron enrichments and bulk fe isotope compositions track the sources and sinks of fe in the proto north atlantic at seven localities marked by diverse depositional conditions included are an ancestral mid ocean ridge and euxinic intermittently euxinic and oxic settings across varying paleodepths throughout the basin these data yield evidence for a reactive fe shuttle that likely delivered fe from the shallow shelf to the deep ocean basin as well as 1 hydrothermal sources enhanced by accelerated seafloor spreading or emplacement of large igneous province s and 2 local scale fe remobilization within the sediment column this study the first to explore fe cycling and enrichment patterns on an ocean scale using iron isotope data demonstrates the complex processes operating on this scale that can mask simple source sink relationships the data imply that the proto north atlantic received elevated fe inputs from several sources e g hydrothermal shuttle and detrital inputs and that the redox state of the basin was not exclusively euxinic suggesting previously unknown heterogeneity in depositional conditions and biogeochemical cycling within those settings during oae 2

age and geochemistry of host rocks of the cobre panama porphyry cu au deposit central panama implications for the paleogene evolution of the panamanian magmatic arc abstract the cobre panama porphyry cu au deposit located in the petaquilla district of central panama is hosted by a sequence of medium to high k calc alkaline volcanic and sub volcanic rocks new crystallisation ages obtained from a granodiorite petaquilla batholith and associated mineralised diorite to granodiorite porphyry stocks and dikes at cobre panama indicate that the batholith was emplaced as a multi phase intrusion over a period of 4 million years from 32 20 0 76 ma to 28 26 0 61 ma while the porphyritic rocks were emplaced over a 2 million year period from 28 96 0 62 ma to 27 48 0 68 ma both the volcanic to sub volcanic host rocks and intrusive rocks of the cobre panama deposit evolved via fractional crystallisation processes as demonstrated by the major elements e g al2o3 fe2o3 tio2 and mgo displaying negative trends with increasing sio2 the petaquilla intrusive rocks including the diorite granodiorite porphyries and granodiorite batholith are geochemically evolved and appear to have formed from more hydrous magmas than the preceding host volcanic rocks as evidenced by the presence of hornblende phenocrysts higher degrees of large ion lithophile element lile and light rare earth element lree enrichment and heavy rare earth element hree depletion and higher sr y and la yb values however the degree of lree enrichment hree depletion and la yb values are insufficient for the intrusive rocks to be considered as adakites collectively the volcanic and intrusive rocks have lile ree and mobile trace element concentrations similar to enriched miocene age cordilleran arc magmatism found throughout central and western panama both the petaquilla and cordilleran arc magmatic suites are geochemically more evolved than the late cretaceous to eocene chagres bayano arc magmas from northeastern panama as they display higher degrees of lile and lree enrichment the geochemical similarities between the petaquilla and cordilleran arc magmas suggest that evolved calc alkaline arc magmatism may extend to the late eocene at least 10 million years earlier than previously estimated the crystallisation ages for intrusive rocks associated with mineralisation at cobre panama imply that the deposit formed in the early oligocene between a period of late cretaceous to eocene magmatism ca 66 42 ma chagres bayano arc and cordilleran arc magmatism 22 7 ma similarities in the timing of intrusive suite emplacement and the fingerprinting of magmatic fractionation processes between the cobre panama porphyry deposit and the cerro colorado porphyry deposit in western panama ca 5 3 ma suggest that these features provide favourable geodynamic and geochemical prerequisites for the formation of porphyry deposits along the panamanian magmatic arc during the cenozoic

transition from the farallon plate subduction to the collision between south and central america geological evolution of the panama isthmus this paper presents new geological constraints on the collision of southern central america with south america and the resulting deformational episodes that have affected the panama isthmus since the late cretaceous the panama isthmus is located in southwestern central america and it represents the zone of contact between the two land masses central america and south america this collision event is still active today it has resulted in regional uplift since the late miocene pliocene and is responsible for the great american biotic interchange between south and north america depending on the methods of investigation used and due to the lack of data available the time when this collision began is still widely debated and poorly constrained to better constrain this age we have studied the rock formations and the tectonic deformations in central and eastern panama that have occurred since the late cretaceous this study presents new rock ages field work documentation and analyses and seismic line interpretations and it is complemented by spatial images for the eastern panama area during the middle eocene a number of changes suddenly appeared in the geological records that were synchronous with the break up of southern central america into two smaller blocks chorotega and choco our main results identify the prevalence of an extensional tectonic regime from the middle eocene to the middle miocene that caused the formation of horst and graben structures with thick sedimentary basin fills and a synchronous clockwise block rotation here we propose that these geologic events are associated with the initiation of the oblique collision of southern central america with south america the first contact of the southeastern extremity of central america occurred around 40 ma to 38 ma and then propagated northwestwards we describe here this long term collision episode in relation to the history of the panama isthmus

terrestrial or waterlain glacial diamictites three case studies from the late precambrian and late ordovician glacial drifts in west africa abstract three examples of extensive diamictites within glacial sequences well exposed in the sedimentary cover of the west african cratonic platform are described they belong to the late precambrian and upper ordovician glacial periods and the paleogeography is summarized the late precambrian diamictites of the jbeliat group in adrar of mauritania displays characteristics of tillite deposited as lodgement or melt out till detail studies on structure clast shape nature and fabric texture in thin section clay mineralogy and carbonate content are given and discussed in order to delimitate useful criteria for tillite identification compared to this first example the two other diamictites from the late precambrian bakoye group in western mali and from the upper ordovician tichit group in southeastern mauritania have obviously been deposited in an aquatic environment waterlain glacial diamictites by the action of ice rafting bottom current and flow processes these examples show that for such ancient deposits sedimentological interpretations are partly solved by broad scale stratigraphical relationships however detailed sedimentological analyses remain essential for precise facies identification especially structural and petrographic studies must be encouraged in the lights of modern glaciodynamic processes

tectonic and magmatic responses to the subduction of high bathymetric relief subduction of high bathymetric relief such as aseismic ridges and magmatic plateaus is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone for example the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism in addition the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum pacific subduction systems we demonstrate that flattening of the subduction dip angle is only significant in the eastern pacific where the average slab dip angle is relatively shallow in the western pacific in contrast the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction subduction of high bathymetric relief in the circum pacific is commonly associated with reduced arc volcanism and in many cases the area of ridge subduction coincides with a volcanic gap in the overriding plate ridge subduction is associated with pronounced changes in the style of deformation involving uplift reactivation of basement thrusts development of orogen perpendicular tear faults and block rotations leading to oroclinal bending the discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes and could help constraining the geodynamics of ancient subduction systems

orbitally induced limestone marlstone rhythms in the albian cenomanian cismon section venetian region northern italy sedimentology calcareous and siliceous plankton distribution elemental and isotope geochemistry abstract a multidisciplinary study of the upper albian cenomanian portion of the cismon section venetian region northern italy was undertaken in order to characterize the cyclic alternations of carbonate rich and carbonate poor layers and to investigate their possible origin and cyclic patterns limestone semicouplets are characterized by abundant radiolarians and micarbs micron sized calcitic fragments common planktonic foraminifera strong bioturbation good oxygenation as expressed by the mn and v v ni parameters high si al ratio low k al in the absence of pyrite and organic matter the marlstone semicouplets are on the contrary frequently laminated rich in pyrite and organic matter with common fish remains in the absence of radiolarians the mn and v v ni parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions which allowed the preservation of organic matter the low si al ratio and high k al ratio the latter related to abundant feldspars indicate that marlstones received a substantial contribution from a terrigenous source the abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters the enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating during marlstone deposition surface waters were less fertile than in the limestone semicouplets but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils whereas the bottom waters were oxygen depleted to accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18o depleted values in the meantime the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate the cyclic organization of limestone marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles the alternating conditions are 1 efficient mixing highly fertile surface and near surface waters and a well oxygenated seafloor to 2 enhanced water runoff or decreased evaporation or both and oxygen depleted bottom waters we suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital climatic cycles in tune with the milankovitch cycles within this scenario the latest cenomanian bonarelli level is an exceptional event marked by a large positive 13c spike it is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near surface waters the enhanced production of organic matter was in excess for the oxygen available at the bottom floor thus allowing the organic matter to accumulate in the sediments

u pb la mc icp ms zircon geochronology and geochemistry from a postcollisional biotite granite of the baja guajira basin colombia implications for late cretaceous and neogene caribbean south american tectonics abstract u pb la mc icp ms zircon ages and whole rock geochemistry from a biotite granitoid recovered at 2 1 km depth from the rancheria 2 well in the baja guajira basin southern caribbean region reveal the existence of a ca 70 ma silicic plutonic event this granitoid intrudes ca 76 80 ma intraoceanic arc fragments and continental margin sediments that experienced continental subduction and collision since ca 76 ma the major compositional shift recorded by this granitoid including light rare earth element lree enrichment documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty decl

black shale deposition atmospheric co2 drawdown and cooling during the cenomanian turonian oceanic anoxic event 1 oceanic anoxic event 2 oae2 spanning the cenomanian turonian boundary ctb represents one of the largest perturbations in the global carbon cycle in the last 100 myr the 13ccarb 13corg and 18o chemostratigraphy of a black shale bearing ctb succession in the vocontian basin of france is described and correlated at high resolution to the european ctb reference section at eastbourne england and to successions in germany the equatorial and midlatitude proto north atlantic and the u s western interior seaway wis 13c offset between 13ccarb and 13corg is shown to be a good pco2 proxy that is consistent with pco2 records obtained using biomarker 13c data from atlantic black shales and leaf stomata data from wis sections boreal chalk 18o records show sea surface temperature sst changes that closely follow the 13c pco2 proxy and confirm tex86 results from deep ocean sites rising pco2 and sst during the late cenomanian is attributed to volcanic degassing pco2 and sst maxima occurred at the onset of black shale deposition followed by falling pco2 and cooling due to carbon sequestration by marine organic productivity and preservation and increased silicate weathering a marked pco2 minimum 25 fall occurred with a sst minimum plenus cold event showing 4 c of cooling in 40 kyr renewed increases in pco2 sst and 13c during latest cenomanian black shale deposition suggest that a continuing volcanogenic co2 flux overrode further drawdown effects maximum pco2 and sst followed the end of oae2 associated with a falling nutrient supply during the early turonian eustatic highstand

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

paired organic matter and pyrite δ34s records reveal mechanisms of carbon sulfur and iron cycle disruption during ocean anoxic event 2 abstract the sulfur s isotope composition of pyrite in the sedimentary record has played an important part in our understanding of the evolution of biogeochemical cycles throughout earth history however the kinetics of pyritization are complex and depend strongly on the reactivity and mineralogy of available iron as a second major sink for sulfide in anoxic sediments organic matter om provides essential context for reconstructing the distribution and isotopic composition of environmental sulfide to first order roughly parallel pyrite and om 34 s profiles reflect changes in sulfide while independent patterns require alternative explanations including changes in iron availability or om characteristics we apply this framework to ocean anoxic event 2 oae 2 94 mya a period of enhanced burial of reduced c and s in om and pyrite that has been associated with an expansion of reducing marine conditions we present paired s isotope records for pyrite and om along with profiles of om s c ratio and s redox speciation from four well characterized lithologic sections with a range of depositional environments pont d issole cismon tarfaya basin and demerara rise to reconstruct both local redox structure and global mechanisms impacting the c s and fe cycles around oae 2 om sulfurization appears to be a major control on om preservation at all four sites similar to modern anoxic environments there is a positive correlation between om s c ratios and toc concentrations for sites with more reducing conditions implying a link between om sulfurization and burial at consistently anoxic sites like tarfaya basin and demerara rise strongly sulfurized om with a consistent s redox speciation and s isotope composition most likely formed rapidly in sinking particles before during and after oae 2 particle hosted om sulfurization may therefore have been a central mechanism facilitating the massive burial of om in anoxic environments during this and other periods of enhanced global carbon burial at the same time a nearly 25 negative shift in the 34 s values of pyrite but not om occurs at multiple globally distributed sites prior to the onset of oae 2 indicating slower pyritization reactions that likely reflect changes in iron delivery due to expanding regional or global anoxia the combination of pyrite and organic s isotopes thus provides novel constraints on the interwoven cycles of carbon iron and sulfur across a major carbon cycle perturbation

black shale deposition on the northwest african shelf during the cenomanian turonian oceanic anoxic event climate coupling and global organic carbon burial high resolution geochemical records from a depth transect through the cenomanian turonian c t tarfaya basin northwest african shelf reveal high amplitude fluctuations in accumulation rates of organic carbon oc redox sensitive and sulphide forming trace metals and biomarkers indicative of photic zone euxinia these fluctuations are in general coeval and thus imply a strong relationship of oc burial and water column redox conditions the pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher frequency forcing determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event oae2 and its duration we propose that eccentricity is the main forcing factor at tarfaya and controlled fluctuations in wind driven upwelling of nutrient rich oxygen depleted intermediate waters from the adjacent atlantic and the periodic development of photic zone and bottom water euxinia on the mid cretaceous northwest african shelf accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six fold increase in oc burial from similar to2 g m 2 yr prior to the oae2 to similar to12 g m 2 yr during the oae2 photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing mass balance calculations imply that similar to2 of the overall global excess oc burial associated with the oae2 was deposited in the tarfaya basin an area that represented only similar to0 05 of the total global c t ocean floor in fact the lateral extent of similar black shales along the african continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the oae2

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

the paleogene arcs of the northern andes of colombia and panama insights on plate kinematic implications from new and existing geochemical geochronological and isotopic data abstract the northern andes of colombia comprise two non cogenetic paleogene arcs formed in contrasting geodynamic settings including continental and oceanic domains new whole rock geochemistry and isotopic constraints together with a review of 332 geochemical 76 isotopic and 204 geochronological data from paleocene to eocene volcanic and plutonic rocks exposed in central and western colombia and panama are used to evaluate cause effect relations between regional plate kinematics and the spatio temporal distribution of the circum caribbean magmatic arcs short lived 60 45 ma arc like magmatism in the central cordillera of colombia was emplaced in a thickened continental crust due to the oblique subduction of the caribbean oceanic plate underneath south america as suggested by the high sr y ratios conversely the panama arc and its poorly explored extension in the northwestern segment of the western cordillera of colombia document a major phase of tholeiitic to calc alkaline arc magmatism between 71 ma and 34 ma mostly derived from a hydrated mantle wedge and emplaced in an oceanic plateau crust this arc record a major phase of magmatic activity between 40 ma and 50 ma that can be associated to changes in the convergence direction and subduction rates of the farallon plate farther to the south in the pacific region of colombia eocene arc related rocks from the timbiqui complex show a geochemical signature that suggests a magmatic origin from melting of both a subduction modified mantle and lower tectonically thickened crust a possible interpretation for the pacific and western colombia panama arc systems and its analogous in ecuador recall an aleutian type convergent margin in which the continental arc laterally switches to a purely oceanic system along the trailing edge of the caribbean plate as consequence of eastward subduction of the farallon plate

a study of sulphur enrichment in the humic fraction of marine sediments during early diagenesis abstract the evolution of the sulphur content of humic acids extracted from a near shore sediment core was investigated special attention was taken to avoid s contamination of the humic materials during extraction the s c ratios increased continuously with depth to values which strongly suggest s addition to the humic matrix during early diagenesis by reactions between organic matter and h2s or its oxidation products their light isotopic composition supports this view however subsequent isotopic exchange has obscured the mechanism initially involved since a large fraction of the enrichment occurred above the sulphidic zone redox boundaries such as the interface of anoxic microniches within more oxidized zones or the sulphidic suboxic boundary of the sediment column must have been important sites for s addition bioturbation by increasing the contact between sedimentary zones of a different redox regime will likely enhance such processes

rates of reaction between silicate iron and dissolved sulfide in peru margin sediments porewaters in site 680 peru margin sediments contain dissolved sulfide over a depth of approximately 70 m which at a sedimentation rate of 0 005 cm yr 1 gives a sediment exposure time to dissolved sulfide of about 1 4 106 yr reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly reactive silicate iron fraction defined as the difference between iron extracted by dithionite fed at room temperature and that extracted by boiling concentrated hcl feh normalised to the total iron content fet straight line plots are obtained for 1n feh fed fet against time of burial from which a first order rate constant of 0 29 10 6 yr 1 equivalent to a half life of 2 4 106 yr can be derived for the sulfidation of this silicate iron comparable half lives are also found for the same poorly reactive iron fraction in the nearby site 681 and 684 sediments this silicate fe fraction comprises 0 8 1 0 fe only 30 60 of which reacts even with 1 5 3 million years exposure to dissolved sulfide diagenetic models based on porewater concentrations of sulfate and sulfide and solid phase iron contents at site 680 are consistent in indicating that this poorly reactive iron fraction is only sulfidized on a million year time scale silicate iron not extracted by hcl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments