a middle miocene relative paleointensity record from the equatorial pacific abstract we present a high resolution magnetostratigraphy and relative paleointensity rpi record derived from the upper 85 m of iodp site u1336 an equatorial pacific early to middle miocene succession recovered during expedition 320 321 the magnetostratigraphy is well resolved with reversals typically located to within a few centimeters resulting in a well constrained age model the lowest normal polarity interval from 85 to 74 87 m is interpreted as the later part of chron c6n 18 614 19 599 ma thirty three other magnetozones occur from 74 87 to 0 85 m which are interpreted to represent the continuous sequence of chrons onset of chron c5er 18 748 ma to the end of chron c5an 1n 12 014 ma we identify three putative previously unrecognized subchrons within chron c5cn 1n chron 5bn 1r and c5abn sedimentation rates vary from about 7 to 15 m myr with a mean of about 10 m myr we observe rapid apparent changes in the sedimentation rate at geomagnetic reversals between 16 and 19 ma that indicate a calibration error in geomagnetic polarity timescale atnts2004 the remanence is carried mainly by non interacting particles of fine grained magnetite which have forc distributions characteristic of biogenic magnetite given the relative homogeneity of the remanence carriers throughout the 85 m thick succession and the fidelity with which the remanence is recorded we have constructed a relative paleointensity rpi record that provides new insights into middle miocene geomagnetic field behavior the rpi record indicates a gradual decline in field strength between 18 5 ma and 14 5 ma and indicates no discernible link between rpi and either chron duration or polarity state

ecological and evolutionary response of tethyan planktonic foraminifera to the middle eocene climatic optimum meco from the alano section ne italy abstract the enigmatic middle eocene climatic optimum meco is a transient 500 kyr warming event that significantly interrupted at 40 ma the long term cooling through the middle and late eocene eventually resulting in establishment of permanent antarctic ice sheet this event is still poorly known and data on the biotic response are so far scarce here we present a detailed planktonic foraminiferal analysis of the meco interval from a marginal basin of the central western tethys alano section northeastern italy the expanded and continuous alano section provides an excellent record of this event and offers an appealing opportunity to better understand the role of climate upon calcareous plankton evolution a sapropel like interval characterized by excursions in both the carbon and oxygen bulk carbonate isotope records represents the lithological expression of the post meco event in the study area and follows the 18 o negative shift interpreted as representing the meco warming high resolution quantitative analysis performed on both 38 m and 63 m fractions reveals pronounced and complex changes in planktonic foraminiferal assemblages indicating a strong environmental perturbation that parallels the variations of the stable isotope curves corresponding to the meco and post meco intervals these changes consist primarily in a marked increase in abundance of the relatively eutrophic subbotinids and of the small low oxygen tolerant streptochilus chiloguembelina and pseudohastigerina at the same time the arrival of the abundant opportunist eutrophic jenkinsina and pseudoglobigerinella bolivariana typical species of very high productivity areas also occurs the pronounced shift from oligotrophic to more eutrophic opportunist low oxygen tolerant planktonic foraminiferal assemblages suggests increased nutrient input and surface ocean productivity in response to the environmental perturbation associated with the meco particularly critical environmental conditions have been reached during the deposition of the sapropel like beds as testified by the presence of common giant and or odd morphotypes this is interpreted as evidence of transient alteration in the ocean chemistry the enhanced surface water productivity inferred by planktonic foraminiferal assemblages at the onset of the event should have resulted in heavier 13 c values the recorded lightening of the carbon stable isotope preceding the maximum warmth therefore represents a robust indication that it derives principally by a conspicuous increase of p co 2 the increased productivity of surface waters also supported by geochemical data may have acted as mechanism for p co 2 reduction and returned the climate system to the general eocene cooling trend the oxygen depleted deep waters and the organic carbon burial following the peak of the meco event represent the local response to the meco warming and suggest that high sequestration of organic matter if representing a widespread response to this event might have contributed to the decrease of p co 2 as well though the true mechanisms are still obscure several lines of evidence indicate a potential pressure on planktonic foraminiferal evolution during the meco event including permanent changes besides transient and ecologically controlled variations

reevaluation of the caco3 size index paleocarbonate ion proxy measurements of the caco3 size index broecker and clark 1999 and of whole foraminifera shell weights lohmann 1995 on glacial age sediment from a series of water depths on the ceara rise in the west equatorial atlantic clearly demonstrate that the relationship between size index and pressure normalized carbonate ion concentration established for holocene equatorial sediments does not apply to glacial sediments rather it appears that the ratio of fine to coarse caco3 raining to the seafloor was greater during glacial time than during the holocene this finding squelches our previous enthusiasm regarding the application of the size index as a paleocarbonate ion proxy

significant southern ocean warming event in the late middle eocene a prominent middle eocene warming event is identified in southern ocean deep sea cores indicating that long term cooling through the middle and late eocene was not monotonic at sites on maud rise and the kerguelen plateau a distinct negative shift in 1 8 o values 1 0 is observed ca 41 5 ma this excursion is interpreted as primarily a temperature signal with a transient warming of 4 c over 600 k y affecting both surface and middle bathyal deep waters in the indian atlantic region of the southern ocean this isotopic event is designated as the middle eocene climatic optimum and is interpreted to represent a significant climatic reversal in the midst of middle to late eocene deep sea cooling the lack of a significant negative carbon isotope excursion as observed during the paleocene eocene thermal maximum and the gradual rate of high latitude warming suggest that this event was not triggered by methane hydrate dissociation rather a transient rise in pco 2 levels is suspected possibly as a result of metamorphic decarbonation in the himalayan orogen or increased ridge arc volcanism during the late middle eocene

glacial to interglacial mineral magnetic and palaeoceanographic changes at chatham rise sw pacific ocean core chat 1k from 3556 m depth east of new zealand shows marked changes in magnetic susceptibility correlating very well with 18o of foraminifera over the last 150 ka susceptibility is high during interglacial stages 1 and 5 and substages 5a to 5e are well marked the susceptibility is shown to be enhanced during interglacials and is neither an artefact of glacial carbonate dilution nor due to an interglacial increase of terrigenous material mineral magnetic property measurements arm sirm s ratio also show the same well defined glacial interglacial variability interpreted as an interglacial increase of single domain magnetite grains less than 0 1 m in size examination by tem of magnetic extracts reveals abundant cubic to octahedral grains of 0 02 0 2 m size often arranged in long chains characteristic of bacterial magnetite this material is much more abundant in interglacial than glacial samples it is suggested that during interglacials with lower productivity and organic carbon input to the bed the oxic zone in the sediments which can be colonised by magnetotactic bacteria is thicker and therefore is colonised for longer by magnetite producers this yields a climate controlled stratigraphy which leads that of infaunal benthic foraminifera by no more than a few centimetres

middle eocene to late oligocene antarctic glaciation deglaciation and southern ocean productivity during the eocene oligocene transition earth cooled significantly from a greenhouse to an icehouse climate nannofossil assemblages from southern ocean sites enable evaluation of paleoceanographic changes and hence of the oceanic response to antarctic ice sheet evolution during the eocene and oligocene a combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes a cooling trend started in the middle eocene was interrupted by warming during the middle eocene climatic optimum and by short cooling episodes the cooling episode at 39 6 ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from 39 1 to 36 2 ma we suggest that oligotrophic conditions were associated with increased water mass stratification low nutrient contents and high efficiency of the oceanic biological pump that in turn promoted sequestration of carbon from surface waters which favored cooling after 36 2 ma we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance this turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient antarctic ice sheet and or to iron delivery to the sea surface eutrophic conditions were maintained throughout the oligocene which was characterized by a nannofossil assemblage shift toward cool conditions at the eocene oligocene transition finally a warm nannofossil assemblage in the late oligocene indicates a warming phase

constraints on ocean acidification associated with rapid and massive carbon injections the early paleogene record at ocean drilling program site 1215 equatorial pacific ocean massive amounts of 13c depleted carbon rapidly entered the ocean more than once during the early paleogene providing a geological framework for understanding future perturbations in carbon cycling

transient drying during the paleocene eocene thermal maximum petm analysis of paleosols in the bighorn basin wyoming abstract the paleocene eocene thermal maximum petm was a transient episode of global warming that occurred at the paleocene eocene boundary the event is marked by a sharp decline in carbon isotope values that is globally recognized from marine and continental sections the negative isotope excursion has been attributed to the release of methane hydrates from oceanic sediments although studies concur that temperatures rose during the petm changes in precipitation are not well understood alluvial paleosols in the bighorn basin that span the petm interval contain a continuous and highly resolved record of climate including information on precipitation they show a significant but transient decrease in precipitation at the onset of the petm but a gradual return to pre petm levels by the end of the interval the paleosols also show additional although less dramatic wet dry cycles within the petm interval that may correspond to precessional cycles that have been identified in the marine record of the petm this study counters interpretations of increased precipitation for wyoming at this time and shows the importance of detailed case studies of continental strata to test climatic generalizations and models that have been developed for petm precipitation patterns

rock magnetic artifacts on long term relative paleointensity variations in sediments 1 long term changes of geomagnetic field intensity including possible dependence on lengths of polarity intervals provide fundamentally important information for understanding the geodynamo a positive correlation between paleointensity and polarity interval length was previously suggested from an oligocene ca 23 34 ma relative paleointensity record at deep sea drilling program site 522 in the atlantic ocean which is the only continuous paleointensity data set published so far for this age interval we have conducted a paleomagnetic study of eocene to oligocene sediments at three sites in the eastern equatorial pacific ocean our objectives include revisiting the issue of the paleointensity polarity length correlation magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation although short wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites long term changes are inconsistent apparent positive correlation between normalized intensity and polarity length was observed but the normalized intensity has an obvious anti correlation with the ratio of anhysteretic remanent magnetization arm to isothermal remanent magnetization irm which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals furthermore the normalized intensity correlates with sedimentation rate these facts indicate a lithological contamination on the normalized intensity records the dependence on arm irm and sedimentation rate is also evident at site 522 it is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization it is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

middle eocene late oligocene climate variability calcareous nannofossil response at kerguelen plateau site 748 a major deterioration in global climate occurred through the eocene oligocene time interval characterized by long term cooling in both terrestrial and marine environments during this long term cooling trend however recent studies have documented several short lived warming and cooling phases in order to further investigate high latitude climate during these events we developed a high resolution calcareous nannofossil record from odp site 748 hole b for the interval spanning the late middle eocene to the late oligocene 42 to 26 ma the primary goals of this study were to construct a detailed biostratigraphic record and to use nannofossil assemblage variations to interpret short term changes in surface water temperature and nutrient conditions the principal nannofossil assemblage variations are identified using a temperate warm water taxa index twwt from which three warming and five cooling events are identified within the middle eocene to the earliest oligocene interval among these climatic trends the cooling event at 39 ma cooling event b is recorded here for the first time variations in fine fraction 18o values at site 748 are associated with changes in the twwt index supporting the idea that significant short term variability in surface water conditions occurred in the kerguelen plateau area during the middle and late eocene furthermore odp site 748 calcareous nannofossil paleoecology confirms the utility of these microfossils for biostratigraphic paleoclimatic and paleoceanographic reconstructions at southern ocean sites during the paleogene

magnetotactic bacteria and their magnetofossils in sediments abstract living magnetotactic bacteria from freshwater environments show under natural and laboratory conditions a great variety of morphological appearances their magnetosomes vary in number shape and size one species of bacteria yields up to 1000 magnetosomes per cell individual particles reach a size of up to 200 nm as a rule they form elongated chains in bacteria which are held under laboratory conditions loop shaped arrangements and nearly unordered clusters are also found during the laboratory experiments different species of bacteria periodically dominated freshwater samples always exhibited a great variety of different magnetosomes on the other hand samples from marine environment clearly showed the preponderance of one type of particle these observations may reflect specific properties of the physico chemical milieu in which the bacteria grew at the end of our experiments north and south seeking bacteria co existed in roughly equal quantities quaternary unconsolidated sediments from the ammersee bavaria and the antarctic quaternary to tertiary deep sea sediments from the atlantic and pacific and jurassic limestones from the sonnwendgebirge tyrol austria were analyzed for fossil magnetosomes we found that fossil magnetosomes were morphologically similar to those from living bacteria but seemed to be corroded in some cases the observed differences in quantity size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites but are not determined by geographical latitude

integrated magnetobiostratigraphy of the middle eocene lower oligocene interval from the monte cagnero section central italy the monte cagnero sedimentary section which crops out in the northeastern apennines near urbania in the umbria marche basin italy contains well exposed strata spanning the middle eocene to lower oligocene interval we use an integrated magnetobiostratigraphic approach to generate a high resolution age model for the monte cagnero section with the goal of obtaining a reliable chronostratigraphic framework for studying eocene oligocene palaeoceanographic changes during the switch from greenhouse to icehouse conditions the studied sediments consist of alternating reddish and greenish limestones and marlstones a new integrated age model for the section is based on high resolution palaeomagnetic analyses combined with detailed planktonic foraminiferal and calcareous nannofossil biostratigraphic results rock magnetic measurements show that the magnetic mineralogy is dominated by a mixture of highand low coercivity minerals probably representing a combination of hematite and magnetite a robust magnetostratigraphic signal together with the identification of key planktonic foraminiferal and nannofossil biostratigraphic events allows construction of a detailed age model for the section based on these results we infer that the section spans a continuous interval within magnetochron resolution from the middle eocene to lower oligocene c 41 27 ma chrons c18r c12r the monte cagnero section therefore represents a sequence that is suitable for studying the impact of the neo tethyan gateway closure on subtropical eocene circulation and determining the nature and timing of palaeoceanographic changes in the tethys through the late middle eocene to early oligocene interval the cenozoic era which represents the last 65 million years myr from the end of the cretaceous to the present is a transformational time in the evolution of earth s palaeoclimate benthic foraminiferal oxygen isotope do records from all major ocean basins indicate a cooling trend over the past 50 myr since the early eocene climatic optimum c 53 51 ma e g zachos et al 2001 2008 the middle eocene to early oligocene was a key interval for earth s transition from warmer greenhouse to cooler icehouse conditions this interval is characterized by the expansion of antarctic large ice sheets to 50 the size of the present day volume e g zachos et al 1996 lear et al 2000 coxall et al 2005 katz et al 2008 coxall wilson 2011 culminating in the eocene oligocene from jovane l herrero bervera e hinnov l a housen b a eds 2013 magnetic methods and the timing of geological processes geological society london special publications 373 79 95 first published online march 25 2013 http dx doi org 10 1144 sp373 13 the geological society of london 2013 publishing disclaimer www geolsoc org uk pub ethics 2013 at universita degli studi di urbino carlo bo on july 29 http sp lyellcollection org downloaded from

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

seismicity gravity anomalies and lithospheric structure of the andaman arc ne indian ocean abstract the andaman arc in the northeastern indian ocean defines nearly 1100 km long active plate margin between the india and burma plates where an oblique benioff zone develops down to 200 km depth several east trending seismologic sections taken across the andaman benioff zone abz are presented here to detail the subduction zone geometry in a 3 d perspective the slab gravity anomaly computed from the 3 d abz configuration is a smooth long wavelength and symmetric gravity high of 85 mgal amplitude centering to the immediate east of the nicobar island where a prominent gravity high follows the nicobar deep the slab residual gravity anomaly srga and mantle bouguer anomaly mba maps prepared for the andaman plate margin bring out a double peaked srga low in the range of 150 to 240 mgal and a wider cum larger mba low having the amplitude of 280 to 315 mgal demarcating the andaman arc trench system the gravity models provide evidences for structural control in propagating the rupture within the lithosphere the plate margin configuration below the andaman arc is sliced by the west andaman fault waf as well as by a set of sympathetic faults of various proportions often cutting across the fore arc sediment package some of these fore arc thrust faults clearly give rise to considerably high post seismic activity but the seismic incidence along the waf further east is comparatively much less particularly in the north although the lack of depth resolution for many of the events prohibits tracing the downward continuity of these faults tectonic correlation of the gravity derived models presented here tends to favour the presence of oceanic crust below the andaman nicobar outer arc ridge

biochemical vs detrital mechanism of remanence acquisition in marine carbonates a lesson from the k t boundary interval abstract an apparently complete carbonate rich cretaceous tertiary boundary interval in odp section 119 738c 20r 5 from the southern kerguelen plateau provides a unique insight into processes of magnetization acquisition in marine carbonates the boundary interval is characterized by a 1 m thick clay rich zone distinct depositional lamina are preserved within the basal 15 cm of this zone the upper part is bioturbated previous studies have demonstrated that the bulk of the detrital fraction in the laminated and bioturbated carbonates has the same local source and hence the two intervals likely had similar initial detrital assemblages magnetic properties of these rocks however differ significantly the laminated sediments have a higher content of non silicate bound iron yet approximately an order of magnitude lower intensity of the natural remanent magnetization compared to the bioturbated rocks our detailed rock magnetic study indicates that in the bioturbated interval the dominant iron bearing phase is single domain magnetite likely of biogenic origin in the laminated interval apart from a small ferromagnetic fraction with multi domain like behavior non silicate bound iron is mainly sequestered in paramagnetic phases likely poorly crystalline oxyhydroxides it appears that a shut down of biological productivity after the k t event allowed preservation of the initial detrital early authigenic iron phases that are dominated by reactive iron oxyhydroxides with recovery of the normal biological activity as evidenced by resumption of bioturbation the oxyhydroxides had been replaced with biogenic magnetite thus produced biochemical magnetization led to a several fold increase in the remanence our results suggest that in areas where bioavailable iron constitutes a significant part of the detrital input such as in pelagic marine environments distant from clastic sources the biochemical remanent magnetization may be the dominant process of magnetization acquisition

changes in the hydrological cycle in tropical east africa during the paleocene eocene thermal maximum the paleocene eocene thermal maximum petm at ca 55 8 ma is one of the most studied instances of past greenhouse gas induced global warming as such it provides a rich opportunity to examine the impact of such global change on local climates the effects of increased continental and sea surface temperatures on local precipitation and humidity during the petm remain poorly constrained and studies reveal complex regional differences whilst some localities appear to experience a net increase in humidity others exhibit the opposite crucially there are few records of hydrological change from tropical regions recent onshore drilling expeditions in tanzania have yielded expanded sedimentary sections deposited in a marine environment that span much of the late cretaceous and paleogene and show exceptionally good preservation of both calcareous microfossils and organic matter the petm interval has previously been constrained by both biostratigraphy and carbon isotopic records and spans ca 7 m of section lipid distributions including various terrestrial marine and bacterial biomarkers and their hydrogen isotopic compositions as well as mineralogy were used to examine east african vegetation and hydrological responses to the global change occurring at the petm although total organic carbon contents decrease the concentrations of both higher plant n alkanes n alkanoic acids and soil bacterial glycerol dialkyl glycerol tetraethers biomarkers increase dramatically at the onset of the petm negative carbon isotope excursion cie suggesting an increased discharge of fluvial sedimentary organic matter similarly mineralogical indicators of terrestrial input including ti al and si al ratios quartz contents and notably the proportion of kaolinite also increase at the onset of the cie however higher plant leaf wax n alkanes c27 c29 and c31 become more deuterium enriched throughout the same interval suggesting a more arid and or hotter rather than a more humid environment this evidence collectively suggests an east african early petm climate characterised by overall hot and arid conditions punctuated by intense perhaps seasonal precipitation events these data match observations from other locations at mid latitudes suggesting that the humid climate often suggested for the petm was not globally widespread

non steady state magnetic mineral reduction chemical lock in and delayed remanence acquisition in pelagic sediments abstract magnetic hysteresis data collected from pelagic sediment sites in the western equatorial pacific ocean define a progressive deepening of modern redox boundaries with decreased organic carbon input paleomagnetic data from the site with the highest organic carbon shows marked changes in the natural remanent magnetization at the modern fe redox boundary suggesting a type of chemical lock in process in which some magnetic grains form in situ above the boundary and freely orient in the field the depth of chemical lock in suggests paleomagnetic data will be smoothed on time scales ranging from less than 40 kyr to more than 425 kyr the higher values are compatible with models calling for a delayed remanence acquisition to account for the saw tooth pattern observed in some sedimentary paleointensity records

environmental magnetism of pleistocene sediments in the north pacific and ontong java plateau temporal variations of detrital and biogenic components 1 an environmental magnetic study was conducted to investigate temporal variations of the sources of magnetic minerals in the pacific ocean using sediment cores of pleistocene age the proportion of interacting i to noninteracting n i single domain components was estimated from first order reversal curve diagrams and the proportion of middle m to low l coercivity components was determined from decomposition of isothermal remanent magnetization irm acquisition curves the i and m components are interpreted to be carried by terrigenous maghemite while the n i and l components represent biogenic magnetite an inverse correlation between the ratio of anhysteretic remanent magnetization susceptibility arm to saturation irm sirm and the i n i ratio was confirmed for sediments across wide expanses in the pacific ocean this implies that arm sirm can be used to estimate the relative abundances of biogenic and terrigenous components in sediments in the north pacific the relative abundance of the i and m components increases in glacial periods variations of arm sirm and s ratio s 0 1t resemble each other and decrease in the same time intervals these variations reflect increases of terrigenous input in glacial periods as eolian dust on the ontong java plateau in the western equatorial pacific on the other hand minima in arm sirm and s 0 1t occur at glacial to interglacial transitions and the relative abundance of the i and m components increases in these periods terrigenous material in this region is considered to be transported mainly from the new guinea and solomon islands by the equatorial undercurrent the higher proportion of the terrigenous component at glacial to interglacial transitions is coeval with carbonate preservation maxima reported from the ontong java plateau suggesting a linkage between the two

new biostratigraphic magnetostratigraphic and isotopic insights into the middle eocene climatic optimum in low latitudes the middle eocene climatic optimum meco was a warming event that interrupted the long term eocene cooling trend while this event is well documented at high southern and mid latitudes it is poorly known from low latitudes and its timing and duration are not well constrained because of problems of hiati microfossil preservation and weak magnetic polarity in key sedimentary sections here we report the results of a study designed to improve the bio magneto and chemostratigraphy of the meco interval using high resolution records from two low latitude sections in the atlantic ocean ocean drilling program odp sites 1051 and 1260 we present the first detailed benthic foraminiferal stable isotope records of the meco from the low latitudes as well as the biostratigraphic counts of orbulinoides beckmanni and new magnetostratigraphic results our data demonstrate a 750 kyr long duration for the meco characterized by increasing 13c and decreasing 18o with minimum 18o values lasting 40 kyr at 40 1 ma coincident with a short lived negative 13c excursion thereafter 18o and 13c values recover rapidly the shift to minimum 18o values at 40 1 ma is coincident with a marked increase in the abundance of the planktonic foraminifera o beckmanni consistent with its inferred warm water preference o beckmanni is an important eocene biostratigraphic marker defining planktonic foraminiferal zone e12 with its lowest and highest occurrences lo and hos our new records reveal that the lo of o beckmanni is distinctly diachronous appearing 500 kyr earlier in the equatorial atlantic than in the subtropics 40 5 versus 41 0 ma we also show that at both sites the ho of o beckmanni at 39 5 ma is younger than the published calibrations increasing the duration of zone e12 by at least 400 kyr in accordance with the tropical origins of o beckmanni this range expansion to higher latitudes may have occurred in response to sea surface warming during the meco and subsequently disappeared with cooling of surface waters

temporal trends of magnetic dissolution in the pelagic realm gauging paleoproductivity abstract magnetic hysteresis data from the western equatorial pacific suggest that the preservation of magnetic remanence in pelagic carbonates is controlled by paleoproductivity hysteresis excursions 100 k y intervals are linked here to greater magnetic dissolution associated with increased glacial organic carbon supply changes in the redox boundary coupled to glacial terminations result in anomalous magnetic features which are preserved in the sedimentary record these high frequency variations are superimposed on a longer term signal of magnetic dissolution which may reflect decreased productivity since the miocene

shelf and open ocean calcareous phytoplankton assemblages across the paleocene eocene thermal maximum implications for global productivity gradients abrupt global warming and profound perturbation of the carbon cycle during the paleocene eocene thermal maximum petm ca 55 ma have been linked to a massive release of carbon into the ocean atmosphere system increased phytoplankton productivity has been invoked to cause subsequent co2 drawdown cooling and environmental recovery however interpretations of geochemical and biotic data differ on when and where this increased productivity occurred here we present high resolution nannofossil assemblage data from a shelf section the u s geological survey usgs drill hole at wilson lake new jersey and an open ocean location ocean drilling program odp site 1209 paleoequatorial pacific these data combined with published biotic records indicate a transient steepening of shelf offshelf trophic gradients across the petm onset and peak with a decrease in open ocean productivity coeval with increased nutrient availability in shelf areas productivity levels recovered in the open ocean during the later stages of the event which coupled with intensified continental weathering rates may have played an important role in carbon sequestration and co2 drawdown

nannofossil assemblage fluctuations during the paleocene eocene thermal maximum at sites 213 indian ocean and 401 north atlantic ocean palaeoceanographic implications the paleocene eocene thermal maximum petm was an abrupt global warming event at f55 ma this event had profound effects on the biosphere quantitative investigations of calcareous nannofossil assemblages were performed across the petm at deep sea drilling project site 213 in the tropical indian ocean and site 401 in the bay of biscay north atlantic ocean at a temperate latitude calcareous nannofossil assemblages from both sites show major changes before during and after the petm fluctuations at site 213 are affected by selective dissolution of the least robust taxa by contrast assemblages in the lower part of the petm at site 401 show less diagenetic alteration and display high abundances of genera such as discoaster fasciculithus and ericsonia these taxa reflect relatively warm and probably oligotrophic surface waters abundances of the genus chiasmolithus an indicator of relatively cool and eutrophic waters decreased dramatically the upper portion of the petm is characterized by an increase in the abundance of toweius spp and zygrhablithus bijugatus interpreted as a return to cooler and more eutrophic conditions the elimination of fasciculithus was probably due to competition with the holococcolith species z bijugatus which occupied the same ecological niche the occurrences of long armed and asymmetrical discoaster species such as discoaster araneus discoaster anartios and discoaster okadai and rhomboaster species such as rhomboaster calcitrapa and rhomboaster cuspis are characteristic of the petm interval d 2004 elsevier b v all rights reserved

enhanced primary productivity and magnetotactic bacterial production in response to middle eocene warming in the neo tethys ocean abstract earth s climate experienced a warming event known as the middle eocene climatic optimum meco at 40 ma which was an abrupt reversal of a long term eocene cooling trend this event is characterized in the deep southern atlantic pacific and indian oceans by a distinct negative 18 o excursion over 500 kyr we report results of high resolution paleontological geochemical and rock magnetic investigations of the neo tethyan monte cagnero mca section northeastern apennines italy which can be correlated on the basis of magneto and biostratigraphic results to the meco event recorded in deep sea sections in the mca section an interval with a relative increase in eutrophic nannofossil taxa and decreased abundances of oligotrophic taxa spans the culmination of the meco warming and its aftermath and coincides with a positive carbon isotope excursion and a peak in magnetite and hematite goethite concentration the magnetite peak reflects the appearance of putative magnetofossils while the hematite goethite apex is attributed to an enhanced detrital mineral contribution likely as aeolian dust transported from the continent adjacent to the neo tethys ocean during a drier more seasonal climate during the peak meco warming based on our new geochemical paleontological and magnetic records the meco warming peak and its immediate aftermath are interpreted as a period of high primary productivity sea surface iron fertilization is inferred to have stimulated high phytoplankton productivity increasing organic carbon export to the seafloor and promoting enhanced biomineralization of magnetotactic bacteria which are preserved as putative magnetofossils during the warmest periods of the meco event in the mca section together with previous studies our work reinforces the connection between hyperthermal climatic events and the occurrence or increased abundance of putative magnetofossils in the sedimentary record

composite analysis of large scale environments conducive to western pacific polar subtropical jet superposition abstractalthough considerable research attention has been devoted to examination of the northern hemisphere polar and subtropical jet streams relatively little has been directed toward understanding the circumstances that conspire to produce the relatively rare vertical superposition of these usually separate features this study investigates the structure and evolution of large scale environments associated with jet superposition events in the northwest pacific an objective identification scheme using ncep ncar reanalysis 1 data is employed to identify all jet superpositions in the western pacific 30 40 n 135 175 e for boreal winters djf between 1979 80 and 2009 10 the analysis reveals that environments conducive to western pacific jet superposition share several large scale features usually associated with east asian winter monsoon eawm northerly cold surges including the presence of an enhanced hadley cell like circulation within the jet entrance region it is further demonstrated that s

magnetostatic interactions in deep sea sediments inferred from first order reversal curve diagrams implications for relative paleointensity normalization 1 understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization arm acquisition and hence the ratio of arm susceptibility arm to saturation isothermal remanent magnetization sirm a magnetic grain size proxy widely used for environmental applications can be controlled by magnetostatic interactions relative paleointensity estimations may also be influenced by magnetostatic interactions because arm is often used as a normalizer to correct for the difference in magnetizability of sediments in this study characterization of magnetic grains in deep sea sediments from the north pacific ocean was conducted using first order reversal curve forc diagrams and irm acquisition curves the forc diagrams indicate that the magnetic grains consist mainly of a noninteracting single domain sd component and an interacting sd component and additionally of a multidomain md component the relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields hu and through a peak in the coercivity hc three components were fitted assuming a gaussian distribution of interaction fields hu the irm acquisition curves can be described by two dominant components assuming a log gaussian distribution the mean coercivities of the two components are 40 and 100 mt respectively it is estimated that the former component is carried by biogenic magnetite and roughly corresponds to the noninteracting sd component derived from the forc diagrams and that the latter component is carried by eolian maghemite corresponding to the interacting sd component the arm sirm ratio decreases with increasing concentration of the interacting sd component this implies that the arm sirm ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size the effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood but if it is similarly sensitive to the sirm normalization by arm in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature

transient dwarfism of soil fauna during the paleocene eocene thermal maximum soil organisms as recorded by trace fossils in paleosols of the willwood formation wyoming show significant body size reductions and increased abundances during the paleocene eocene thermal maximum petm paleobotanical paleopedologic and oxygen isotope studies indicate high temperatures during the petm and sharp declines in precipitation compared with late paleocene estimates insect and oligochaete burrows increase in abundance during the petm suggesting longer periods of soil development and improved drainage conditions crayfish burrows and molluscan body fossils abundant below and above the petm interval are significantly less abundant during the petm likely because of drier floodplain conditions and lower water tables burrow diameters of the most abundant ichnofossils are 30 46 smaller within the petm interval as burrow size is a proxy for body size significant reductions in burrow diameter suggest that their tracemakers were smaller bodied smaller body sizes may have resulted from higher subsurface temperatures lower soil moisture conditions or nutritionally deficient vegetation in the high co2 atmosphere inferred for the petm smaller soil fauna co occur with dwarf mammal taxa during the petm thus a common forcing mechanism may have selected for small size in both above and below ground terrestrial communities we predict that soil fauna have already shown reductions in size over the last 150 years of increased atmospheric co2 and surface temperatures or that they will exhibit this pattern over the next century we retrodict also that soil fauna across the permian triassic and triassic jurassic boundary events show significant size decreases because of similar forcing mechanisms driven by rapid global warming

morphometry of the eocene nannofossil reticulofenestra umbilicus lineage and its biochronological consequences abstract morphometruc measurements of eocene and early oligocene reticulofenestrids from dsdp site 523 in the south atlantic show that a 90 size increased led to the appearance of reticulofenestra umbilicus this increase occurred between about 45 2 ma and 42 9 ma the attempts made to recognize r umbilicus during this period of change as a distinct subpopulation among the reticulofenestrid assemblages did not yield any tenable conclusions however using a lower size limit of 14 m for recognition of the r umbilicus morphovariant resulted in an easily identified first appearance event which occurred during chron 19r at an age of about 44 4 44 5 ma from a morphometric point of view the two morphovariants r hillae and r umbilicus could not be distinguished as two distinct subpopulations the oligocene assemblages show markedly smaller 10 central opening placolith length ratios as compared to the eocene forms but for several reasons it seems difficult to use this decrease biostratigraphically

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

the response of calcareous nannofossil assemblages to the paleocene eocene thermal maximum at the walvis ridge in the south atlantic compositions and abundances of calcareous nannofossil taxa have been determined in a ca 170 kyrs long time interval across the paleocene eocene boundary at 1 cm to 10 cm resolution from two odp sites 1262 1263 drilled along the flank of the walvis ridge in the south atlantic the results are compared to published data from odp site 690 in the weddell sea the assemblages underwent rapid evolution over a 74 kyrs period indicating stressed unstable and or extreme photic zone environments during the petm hyperthermal this rapid evolution which created 5 distinct stratigraphic horizons is consistent with the restricted brief occurrences of malformed and or weakly calcified morphotypes the production of these aberrant morphotypes is possibly caused by major global scale changes in carbon cycling in the ocean atmosphere system affecting also photic zone environments no marked paleoecologically induced changes are observed in abundances of the genera discoaster fasciculithus and sphenolithus at the walvis ridge sites surprisingly there is no significant correlation in abundance between these three genera presumed to have had a similar paleoecological preference for warm and oligotrophic conditions

oligocene miocene magnetic stratigraphy carried by biogenic magnetite at sites u1334 and u1335 equatorial pacific ocean 1 sediments from the equatorial pacific ocean at the integrated ocean drilling program sites u1334 and u1335 record reliable magnetic polarity stratigraphies back to 26 5 ma late oligocene at sedimentation rates usually in the 5 20 m myr range putative polarity subchrons that do not appear in current polarity timescales occur within chrons c5acr c5adn and c5bn 1r at site u1335 and within chrons c6aar 2r c6br c7ar and c8n 1n at site u1334 subchron c5dr 1n 17 5 ma is recorded at both sites supporting its apparent recording in the south atlantic ocean and has an estimated duration of 40 kyr the oligocene miocene calcareous oozes have magnetizations carried by submicron magnetite as indicated by thermal demagnetization of magnetic remanences the anhysteretic remanence to susceptibility ratio and magnetic hysteresis parameters transmission electron microscopy of magnetic separates indicates the presence of low titanium iron oxide magnetite grains with size 50 100 nm and shape similar to modern and fossil bacterial magnetite supporting other evidence that biogenic submicron magnetite is the principal remanence carrier in these sediments in the equatorial pacific ocean low organic carbon burial arrests microbial pore water sulfate reduction thereby aiding preservation of bacterial magnetite

three million years of monsoon variability over the northern sahara we present a 3 million year record of aeolian dust supply into the eastern mediterranean sea based on hematite contents derived from magnetic properties of sediments from ocean drilling program site 967 our record has an average temporal resolution of 400 years geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the sahara we deduce that the aeolian hematite in eastern mediterranean sediments derives from the eastern algerian libyan and western egyptian lowlands located north of the central saharan watershed 21 n in corroboration of earlier work we relate dust flux minima to penetration of the african summer monsoon front to the north of the central saharan watershed this would have enhanced soil humidity and vegetation cover in the source regions in agreement with results from green sahara climate models our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years as would be expected this orbital precession scale mechanism is modulated on both short 100 kyr and long 400 kyr eccentricity time scales we also observe a strong expression of the 41 kyr obliquity cycle which we discuss in terms of high and low latitude mechanisms that involve southern hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics respectively we also observe a marked increase in sub milankovitch variability around the mid pleistocene transition 0 95 ma which suggests a link between millennial scale climate variability including monsoon dynamics and the size of northern hemisphere ice sheets

origin of magnetic mineral concentration variation in the southern ocean 1 in the southern ocean magnetic mineral concentration increases in glacial periods the variation pattern closely resembles eolian dust flux records from antarctic ice cores but the cause of the linkage remains unclear as the dust flux is too small for the source of terrigenous materials in the southern ocean we have conducted an environmental magnetic study of late pleistocene sediments from the south indian ocean to investigate the origin of the magnetic concentration changes biogenic magnetites can be detected using the characteristics of almost no magnetostatic interactions and narrow coercivity distribution reflecting occurrence of single domain magnetites in a chain we interpret that a non interacting component on first order reversal curve diagrams and low coercivity components with small dispersion from isothermal remanent magnetization irm component analyses represent biogenic magnetites and that the interacting and middle coercivity components represent terrigenous maghemites the ratio of anhysteretic remanent magnetization susceptibility to saturation irm reflects relative abundance of the biogenic and terrigenous components it was revealed that biogenic magnetites are a dominant constituent of the magnetic minerals in glacials the abundance of both biogenic and terrigenous components increased with increased proportions of the latter increased ocean productivity in glacials is suggested from increased proportions of biogenic magnetites with elongated morphologies indicative of less oxic conditions and increased sedimentation rates these observations suggest that the increased magnetic concentration in glacials in the southern ocean may be explained by iron fertilization the production of biogenic magnetites was enhanced associated with increased ocean productivity which was fueled by increased eolian dust flux

detection of live magnetotactic bacteria in south atlantic deep sea sediments abstract since they were first descrbed in 1975 1 magnetotactic bacteria have been observed in a wide variety of environments including soils 2 rivers freshwater lakes and coastal marine sediments 3 4 and in one case at a water depth of about 600 m in a hemipelagic sediment 5 here we report the first systematic findings of magnetotactic bacteria in pelagic and hemipelagic sediments of the eastern south atlantic different morphologies were identified cocci spirilla vibrionic and rod shaped forms at water depths to about 3000 m on the african continental margin between the equator and 30 s and on the walvis ridge in a pelagic environment about 1400 km off the coast no actively swimming magnetotactic bacteria could be detected in deeper waters magnetotactic bacteria were observed only in the upper 10 cm of the sediments with the highest concentrations always found in a well defined narrow subsurface layer the depth of this layer is dependent on the input and turnover rates of organic matter in the sediment measurements of oxygen and nitrate concentrations 6 7 provide an assignment of depth distributions of magnetotactic bacteria to the stratification of terminal electron acceptors in the sediment the data show clearly that the majority of marine magnetotactic bacteria live in the anaerobic zone in contrast to former assumptions that they may inhabit microaerobic layers 8 11

eocene oligocene calcareous nannofossils from maud rise and kerguelen plateau antarctica paleoecological and paleoceanographic implications abstract the evolution of the southern ocean climate during the late eocene late oligocene interval is examined through high resolution quantitative calcareous nannofossil analyses on samples from the southern ocean sections on maud rise and kerguelen plateau we determined the abundance patterns of the counted species to clarify the biostratigraphy which we correlated with high resolution magnetostratigraphy roberts a p bicknell s j byatt j bohaty s m florindo f harwood d m 2003a magnetostratigraphic calibration of southern ocean diatom datums from the eocene oligocene of kerguelen plateau ocean drilling program sites 744 and 748 in florindo f cooper a k o brien p a eds antarctic cenozoic palaeoenvironments geologic record and models palaeogeogr palaeoclimatol palaeoecol 198 145 168 florindo f roberts a p in press eocene oligocene magnetobiochronology of odp sites 689 and 690 maud rise weddell sea antarctica geol soc am bull and used this data to interpret paleoceanographic changes through the late eocene to late oligocene percentage plots of the individual species compared with r mode principal component and cluster analysis results allowed us to divide the assemblages into three groups temperate water taxa cool water taxa and no temperature affinity taxa we attempt correlations between these paleoecological groups and the major sea surface temperature sst variations with tectonic and paleoceanographic changes in the southern ocean during the late eocene the nannofossil assemblage data reveal that there were several minor sst decreases coolings from 36 to 34 ma before the eocene oligocene e o boundary a sharp cooling event dated at 33 54 ma earliest oligocene occurred about 160 kyr after the e o boundary which is dated at 33 7 ma relatively stable cool conditions are interpreted to persist until the latest oligocene when an increase in abundance of temperate water taxa which corresponds to an antithetical decrease in abundance of cool water indicators is recorded on the basis of our dating the opening of the drake passage allowing shallow water circulation began by 33 54 ma at the latest while the establishment of deep water connections through the tasmanian gateway occurred at 33 ma as suggested by exon et al proc odp init rep 189 2001 1

correction to evidence of surface water oligotrophy during the paleocene eocene thermal maximum nannofossil assemblage data from ocean drilling program site 690 maud rise weddell sea by timothy j bralower

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

fluvial discharge and sea level changes controlling black shale deposition during the paleocene eocene thermal maximum in the dababiya quarry section egypt abstract the formation of black shales along the tethyan margins at the paleocene eocene thermal maximum petm 55 ma may have been an important feedback mechanism to reduce the warming by excess carbon burial however the detailed evolution of sediment redox conditions during this event is still poorly constrained we address this issue by a high resolution mineralogical and geochemical investigation of the outer neritic dababiya quarry petm section in central egypt which serves as the global boundary stratotype section and point for the base of the eocene there the base of the petm beds corresponds to the onset of sediment lamination indicative of oxygen deficiency at the seafloor the absence of calcium carbonate and a major increase in phyllosilicate abundance and change in detritus sensitive trace elements is indicative of severe carbonate dissolution in addition to enhanced fluvial input erosion of coastal low lands and deposition during low or slightly rising sea level subsequently fully anoxic conditions were established for a short period during the peak phase of the petm as recorded by the strong relative enrichment of redox sensitive trace elements and organic carbon compared to background sediments the decoupling of the fe and mo contents from the detrital fraction may even indicate that euxinic conditions existed with free hydrogen sulfide in the water column concurrent to the onset of anoxia high carbonate and quartz contents as well as high elemental ratios of si al suggest a major drop of siliciclastic sediment supply sediment starvation most likely caused by a rapid sea level rise the final recovery phase of the petm is associated with a progressive restoration of the pre petm siliciclastic sedimentation temporary seasonal anoxic to dysoxic conditions and high phosphorus enrichment followed by carbonate dominated sedimentation and the return to well oxygenated conditions we conclude that after a period of strong fluvial discharge several parameters inherent to the petm e g warming water column stratification and enhanced fluvial discharge may have contributed to the development of anoxia and increased the burial efficiency of organic carbon by reducing water column oxygenation the temporal coincidence of sediment starvation and onset of black shale deposition during the petm suggests that a rapid sea level rise was an additional important triggering factor therefore our results support not only current views that tethyan continental shelf areas may have acted as large carbon sinks during the petm but also indicate that transgressions may have provided an important feedback mechanism for organic carbon drawdown during hyperthermal events

coupled greenhouse warming and deep sea acidification in the middle eocene 1 the middle eocene climatic optimum meco is an enigmatic warming event that represents an abrupt reversal in long term cooling through the eocene in order to further assess the timing and nature of this event we have assembled stable isotope and calcium carbonate concentration records from multiple deep sea drilling project and ocean drilling program sites for the time interval between 43 and 38 ma revised stratigraphy at several sites and compilation of 18o records place peak warming during the meco event at 40 0 ma chron c18n 2n the identification of the 18o excursion at sites in different geographic regions indicates that the climatic effects of this event were globally extensive the total duration of the meco event is estimated at 500 ka with peak warming lasting 100 ka assuming minimal glaciation in the late middle eocene 4 6 c total warming of both surface and deep waters is estimated during the meco at the study sites the interval of peak warming at 40 0 ma also coincided with a worldwide decline in carbonate accumulation at sites below 3000 m depth reflecting a temporary shoaling of the calcite compensation depth the synchroneity of deep water acidification and globally extensive warming makes a persuasive argument that the meco event was linked to a transient increase in atmospheric pco2 the results of this study confirm previous reports of significant climatic instability during the middle eocene furthermore the direct link between warming and changes in the carbonate chemistry of the deep ocean provides strong evidence that changes in greenhouse gas concentrations exerted a primary control on short term climate variability during this critical period of eocene climate evolution

evidence for bacterial palaeoecological origin of mineral magnetic cycles in oxic and sub oxic tasman sea sediments abstract sediments from the tasman basin in the southwest pacific ocean show a cyclic pattern of variation in mineral magnetic properties including susceptibility and remanence over the brunhes chron the magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by tem investigation of the magnetic fraction the intervals of low bacterial magnetite concentration occurring in glacial periods are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage there is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped either as the result of variation in bottom water oxygen concentration or sedimentation rate the variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration the bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick

caco3 size distribution a paleocarbonate ion proxy lysocline reconstructions play an important role in scenarios purporting to explain the lowered atmospheric co2 content of glacial time these reconstructions are based on indicators such as the caco3 content the percent of coarse fraction the ratio of fragments to whole foraminifera shells the ratio of solution susceptible to solution resistant species and the ratio of coarse to fine caco3 all assume that changes with time in the composition of the input material do not bias the result however as the composition of the input material does depend on climate none of these indicators provides an absolute measure of the extent of dissolution in this paper we evaluate the reliability of the ratio of 63 m caco3 to total caco3 as a dissolution indicator we present here results that suggest that in today s tropics this ratio appears to be determined solely by co3 ion concentration and water depth i e the saturation state of bottom waters this finding offers the possibility that the size fraction index can be used to reconstruct co3 ion concentrations for the late quaternary ocean to an accuracy of 5 mol kg 1

what do the hirm and s ratio really measure in environmental magnetism the hard isothermal remanent magnetization hirm and the s ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations respectively of antiferromagnetic minerals hematite and goethite in mineral mixtures we demonstrate that synthetic al substituted hematite and goethite exhibit a wide range of coercivities which significantly influences the hirm and s ratio these parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite goethite to circumvent this problem we propose a new parameter the l ratio which is the ratio of two remanences after alternating field af demagnetization of an irm imparted in a 1 t field with a peak af of 100 mt and 300 mt irmaf 300mt irmaf 100mt these parameters are easily measured using modern vibrating sample or alternating gradient magnetometers changes in hirm only reflect changes in the absolute concentration of hematite and or goethite if the l ratio is relatively constant conversely l ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite goethite corresponding hirm and s ratio variations should be interpreted with caution in such cases the l ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols for example the hirm is equivalent to irmaf 300mt likewise 0 5 sirm irm 100mt where irm 100mt represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back field of 100 mt is equivalent to irmaf 100mt the hirm 0 5 sirm irm 100mt ratio is therefore a suitable substitute for the l ratio when measurements are made with a long core magnetometer the newly proposed l ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism the hirm and s ratio