could early cenozoic thermohaline circulation have warmed the poles the paleocene and eocene epochs provide examples of oceanographic conditions when the earth had a stable climate much warmer than that of the modern world lower cenozoic sediments thus provide a way to investigate how oceanographic processes might respond to extreme warming because high latitude sea surface temperatures were significantly warmer during the early cenozoic while tropical sea surface temperatures were similar to or somewhat less than modern there may have been a much higher poleward oceanic heat transport at that time shackleton and boersma 1981 barron 1987 sloan et al 1995 here i develop a simple ocean thermohaline model to demonstrate that poleward oceanic heat transport could strongly warm the polar regions only if the strength of vertical mixing vertical eddy diffusion in the oceans is sensitive to changes in oceanic density structure if this is true the early cenozoic oceans because they had significantly less density stratification could have had a thermohaline circulation as much as 3 times as strong as that of the modern ocean despite reduced temperature gradients the model also predicts that an optimum bottom water temperature exists for maximum poleward heat transport continued warming of the earth does not continue to increase oceanic heat transport to the poles the hypothesized sensitivity of oceanic vertical mixing to the density gradient should provide a positive feedback to both the transition to ice ages and to a future transition out of them a test of the sensitivity of vertical eddy diffusion to density gradients can be studied through map and depth transects in the horizontal direction high vertical diffusion would be marked by relatively strong bottom water temperature gradients away from bottom water source regions in the vertical direction high vertical diffusion should be indicated by very small temperature gradients

abyssal circulation and benthic foraminiferal changes near the paleocene eocene boundary a major change in cenozoic deep sea benthic foraminifera occurred in the atlantic indian and pacific oceans near the paleocene eocene boundary benthic foraminiferal abundance changes began at about 61 5 ma at pacific deep sea drilling project dsdp site 577 a major extinction event followed at 58 57 ma between zones p6a and p6b and a series of first appearances continued until circa 55 5 ma zone p6c these faunal changes occurred during a 6 c warming of pacific bottom water and may indicate that the primary cause was changing temperature other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions comparison of benthic and planktonic 13c records requires no change in the ratio of oceanic phosphorous to carbon during the late paleocene to early eocene which weakens the case for but does not disprove a change in surface ocean productivity at this time interbasinal comparisons of benthic foraminiferal 13c records document that water with high 13c values filled the cape basin during the late paleocene and possibly the early eocene circa 61 57 ma but apparently did not extend into the western basins of the atlantic this pattern suggests a supply of antarctic source water for the cape basin and possible tectonic isolation of the western atlantic basins during at least part of the late paleocene carbon isotope comparisons show that bottom water supply to the cape basin was reduced in the early eocene eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the paleocene these late paleocene climatic changes bottom water warming and decreased wind intensity correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism which may have contributed to climatic warming through increased supply of co2

paleogene deepwater mass composition of the tropical pacific and implications for thermohaline circulation in a greenhouse world 1 the pacific ocean likely exerted the greatest marine influence on climate during the greenhouse interval of the paleogene the first step toward assessing the role of pacific deepwater circulation in global heat transport is a comprehensive reconstruction of deepwater mass composition and circulation patterns we present new deepwater mass nd isotope data from odp leg 199 sites 1215 1217 1219 and 1221 to augment the existing coverage provided by ferromanganese crust and fossil fish debris data the new data corroborate the notion that the pacific ocean operated in a bimodal pattern with deepwater sources in both the southern ocean and north pacific for much of the early to middle paleogene in addition the data indicate that there was little to no influence of atlantic waters on the deepwater composition of the tropical pacific the available deepwater nd isotope data provide refined constraints on the geographic and depth range of influence of both a southern ocean and north pacific deepwater mass from 65 to 45 ma the deep tropical pacific was the mixing location of deep waters sourced from the southern ocean and the north pacific the southern ocean deepwater mass extended from 1800 m to abyssal depths until 53 6 ma it circulated northward in the western pacific at least as far as the paleolocation of fe mn crust cld01 5 n paleolatitude and in the eastern portion of the pacific as far as site 1215 10 n paleolatitude a north pacific deepwater mass extended from at least 2400 m to 2900 m water depth from 65 to 54 ma south to shatsky rise paleolatitude 10 15 n and possibly as far as site 1215 700 km to the south of shatsky rise sites 1209 and 1211

silica burp in the eocene ocean the eocene was a time of greatly increased silica accumulation in the ocean and the peak was in the early middle eocene at about 50 ma the responsible geohistorical configuration included the following elements extensive volcanism about 4 m y earlier as part of the chron 24 plate reorganization early eocene warming with deep weathering to high latitudes and accumulation of the released silica in a sluggish ocean and sharp cooling in the earliest middle eocene stimulating oceanic upwelling and biosilicification it is possible on the evidence of carbon and oxygen isotopic patterns that the trigger for the exhalation of silica was a reverse greenhouse effect

neodymium isotopic reconstruction of late paleocene early eocene thermohaline circulation abstract high resolution fish tooth nd isotopic records for eight deep sea drilling project and ocean drilling program sites were used to reconstruct the nature of late paleocene early eocene deep water circulation the goal of this reconstruction was to test the hypothesis that a change in thermohaline circulation patterns caused the abrupt 4 5 c warming of deep and bottom waters at the paleocene eocene boundary the paleocene eocene thermal maximum petm event the combined set of records indicates a deep water mass common to the north and south atlantic southern and indian oceans characterized by mean nd values of 8 7 and different water masses found in the central pacific ocean nd 4 3 and caribbean sea nd 1 2 the geographic pattern of nd isotopic values before and during the petm suggests a southern ocean deep water formation site for deep and bottom waters in the atlantic and indian ocean basins the nd data do not contain evidence for a change in the composition of deep waters prior to the onset of the petm this finding is consistent with the pattern of warming established by recently published stable isotope records suggesting that deep and bottom water warming during the petm was gradual and the consequence of surface water warming in regions of downwelling

paleogene geochronology an integrated approach geochronology is the conceptual division of continuous time as measured geochronometry by the progression in an ordinal series of events this is best achieved by an approach which integrates four independent data sets magnetostratigraphy seafloor spreading magnetic lineation patterns biostratigraphy and isotopic ages this integrated approach results in an ordinal framework which can measure time with greater resolution though perhaps less accuracy than a radiometric approach alone a comparative analysis of two recently proposed paleogene geochronologic time scales is presented

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

centurial millenial ice rafted debris pulses from ablating marine ice sheets we use an ice sheet model to show that i mar gins of marine ice sheets can be expected to be frozen to the bed except where ice streams discharge ii 20 50km retreats induced by ablation rates of 2 m yr provide suffi cient debris flux through the grounding line to produce large sedimentation events such ablation would reduce ice shelf extent markedly permitting debris to reach the calving front and be transported by icebergs leading to ice rafted debris ird events ice shelf break up takes around a century start of ird pulse while the creation of warm based con ditions end of ird pulse due to upwards motion of warm ice takes a few more centuries such ird pulses are un likely to explain heinrich events which are associated with relatively cold periods within glaciations surges are not necessary conditions for the production of large ird events the model requires a band of erosion of sediment along a broad zone of the continental shelf our proposed mech anism can produce large ird events using the hein rich events as an example of a large ird event each of

seismic and geologic evidence for early paleogene deepwater circulation in the western north atlantic previous studies of paleogeography global climate and foraminiferal stable isotopes have suggested that paleogene deepwater circulation was significantly different from that of today extracting details of deepwater history from the western north atlantic has been hampered by oligocene erosion and the lack of ideally located drill holes we report the discovery of a seismic disconformity on the sw bermuda rise that we have tied to a late paleocene erosional event in deep sea drilling project dsdp boreholes from this region distribution of this new seismic marker reflector ab indicates that it was formed by cyclonically circulating bottom water entering the western north atlantic from the south we attribute this vigorous deep circulation to a southern ocean source analogous to the effects of modern antarctic bottom water this interpretation is supported by isotopic comparisons that show the paleocene southern ocean was enriched in 13c relative to the pacific we distinguish reflector ab from the overlying reflector au on the basis of superposition and ages obtained at dsdp sites and piston cores using one of the cores we have estimated that the youngest age for reflector au is earliest oligocene 36 6 34 0 ma while its oldest age has been previously established as late eocene laminated sediments beneath reflector ab are enriched in organic carbon indicating that the early paleocene deep western north atlantic was occasionally anoxic these low oxygen levels may have been the result of sluggish deep circulation and or low latitude production of warm saline deep water wsdw sediments immediately above reflector ab show that deposition resumed on the western bermuda rise site 387 by the latest paleocene to earliest eocene biochron np10 57 2 0 6 ma while deposition resumed on the central bermuda rise site 386 during the late paleocene erosion nondeposition continued across much of the bermuda rise widespread resumption of pelagic deposition in the early eocene is consistent with 13c evidence for reduced southern ocean influence at this time however lower eocene sediments record well oxygenated conditions and 13c and 18o comparisons indicate that there were two early eocene deepwater sources the southern ocean and wsdw pak and miller this issue

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

paleoceanographic conditions on the sao paulo ridge sw atlantic ocean for the past 30 million years inferred from os and pb isotopes of a hydrogenous ferromanganese crust abstract hydrogenous ferromanganese fe mn crusts can provide records of long term environmental changes during the cenozoic to understand the paleoceanographic conditions in the southwestern atlantic ocean we investigated depth profiles of major and trace element concentrations as well as os and pb isotopic compositions in a fe mn crust collected from the southern flank of the sao paulo ridge in the southwestern atlantic major and trace element data plotted on ternary mn fe 10 ni co cu and rare earth element plus yttrium rey discrimination diagrams indicate that the analyzed sample is a typical hydrogenous fe mn crust the obtained 187 os 188 os data were matched to the cenozoic seawater os isotope evolution curve reconstructed from pelagic sediments the result suggests that the fe mn crust has accreted over 30 myr with growth rates of 0 5 3 mm myr although the sample likely grew in two directions during the early stage of its growth we found no evidence of growth hiatus in the sample which may contrast with the growth histories of many pacific fe mn crusts hence the conditions favorable for the accretion of hydrogenous fe mn crusts were likely to have developed on the sao paulo ridge over the past 30 myr the pb isotopic compositions show very limited ranges e g 206 pb 204 pb 18 80 18 85 and are similar to those of pre anthropogenic seawater in the southern ocean as the sao paulo ridge is located near the vema channel which is presently a major path of antarctic bottom water we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge and played a vital role in the fe mn crust formation since 30 ma

abyssal current character determined from sediment bedforms on the nova scotian continental rise abstract bottom photographs across the nova scotian continental rise show effects of an intense bottom current at 4800 5000 m that decreases in strength upslope to tranquil near bottom conditions at 3200 4000 m apparently weak currents are present above 3200 m long term geologically significant flow parallels the bathymetric contours toward the southwest but significant short term variability in direction is indicated by orientations of small scale bedforms the western boundary undercurrent documented elsewhere along the continental margin at depths less than 4000 m has relatively little effect in modifying the sea floor on the nova scotian rise but the deep currents 4000 m have a significant effect repeated photography of small seafloor areas suggests that many bedforms especially those of smaller scale are constructed and destroyed by strong variably directed current events benthic storms on time scales of 3 6 months or less thus a significant part of the geologic response of the seabed to currents may be event controlled on time scales not very different from those in coastal environments

heat flow observations on the bermuda rise and thermal models of midplate swells the bermuda rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western north atlantic the magnitudes of the depth and geoid anomalies associated with the bermuda rise are similar to the anomalies associated with other swells surrounding recent volcanic islands e g hawaii suggesting that despite the lack of recent volcanism on bermuda the rise has a similar origin to other midplate swells results are reported from 171 new heat flow measurements at seven carefully selected sites on the bermuda rise and the surrounding seafloor off the bermuda rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict with the measured heat flow apparently reaching a uniform value of about 50 mw m 2 on 120 m y old crust on the bermuda rise the heat flow is significantly higher 57 4 2 6 mw m 2 than off the swell 49 5 1 7 mw m 2 the magnitude of the anomalous heat flux 8 10 mw m 2 is comparable to that previously found along the older portion of the hawaiian swell near midway the existence of higher heat flow on both the hawaiian swell and bermuda rise indicates that these features fundamentally have a thermal origin the differences in the shape uplift and subsidence histories of the hawaiian swell and bermuda rise can be quantitatively explained by the different absolute velocities of the pacific and north american plates moving across a distributed heat source in the underlying mantle two dimensional numerical convection models indicate that the observed depth geoid and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

paleocene to eocene benthic foraminiferal isotopes and assemblages implications for deepwater circulation early paleogene warm climates may have been linked to different modes and sources of deepwater formation warm polar temperatures of the paleocene and eocene may have resulted from either increased atmospheric trace gases or increased heat transport through deep and intermediate waters the possibility of increasing ocean heat transport through the production of warm saline deep waters wsdw in the tethyan region has generated considerable interest in addition general circulation model results indicate that deepwater source regions may be highly sensitive to changing basin configurations to decipher deepwater changes we examined detailed benthic foraminiferal faunal and isotopic records of the late paleocene through the early eocene 60 to 50 ma from two critical regions the north atlantic bay of biscay site 401 and the pacific shatsky rise site 577 these records are compared with published data from the southern ocean maud rise site 690 islas orcadas rise site 702 during the late paleocene similar benthic foraminiferal 18o values were recorded at all four sites this indicates uniform deepwater temperatures consistent with a single source of deep water the highest 13c values were recorded in the southern ocean and were 0 5 more positive than those of the pacific we infer that the southern ocean was proximal to a source of nutrient depleted deep water during the late paleocene upper paleocene reflector ab was cut on the western bermuda rise by cyclonically circulating bottom water also suggesting a vigorous source of bottom water in the southern ocean a dramatic negative excursion in both carbon and oxygen isotopes occurred in the latest paleocene in the southern ocean this is a short term 100 kyr globally synchronous event which also is apparent in both the atlantic and pacific records as a carbon isotopic excursion of approximately 1 faunal analyses from the north atlantic and pacific sites indicate that the largest benthic foraminiferal faunal turnover of the cenozoic was synchronous with the isotopic excursion lending support to the hypothesis that the extinctions were caused by a change in deepwater circulation we speculate that the southern ocean deepwater source was reduced or eliminated at the time of the excursion during the early eocene southern ocean 13c values remained enriched relative to the north atlantic and pacific however the southern ocean was also enriched in 18o relative to these basins we interpret that these patterns indicate that although the southern ocean was proximal to a source of cool nutrient depleted water the intermediate to upper deep water sites of the north atlantic and pacific were ventilated by a different source that probably originated in low latitudes i e wsdw

cenozoic north atlantic deep circulation history recorded in contourite drifts offshore newfoundland canada abstract in the north atlantic ocean contour following deep currents have created regional erosional unconformities and deposited contourite drifts that exceed 2 km in thickness and extend for 100 s of km the stratigraphic records in the drifts have been used to reconstruct variations in north atlantic deep water circulation throughout the cenozoic however uncertainties remain about certain aspects of the timing intensity depth distribution and regional impact of these currents here we use an integrated dataset of seismic reflection profiles and iodp core data lithology biostratigraphy and magnetostratigraphy to document sedimentation history and the development of current effects in the cretaceous to present sedimentary record on the j anomaly ridge and southeast newfoundland ridge offshore newfoundland canada the newfoundland ridges are in a key location lying between well studied areas in the northern and western north atlantic and under the path of both the modern deep western boundary current and the gulf stream late cretaceous through early eocene sedimentation on the ridges was dominated by biogenic pelagic sedimentation but at 47 ma near the early middle eocene boundary well developed contourite drifts began to accrete in paleo water depths of 4000 4500 m accompanied by an order of magnitude increase in terrigenous sediment mass accumulation rates from this time forward drift deposition interrupted by brief episodes of erosion continued unabated this timing for the onset of persistent deep currents is coincident with reorganization of atlantic circulation inferred from a change from biosiliceous to non biosiliceous sedimentation in the western north atlantic horizon a c and with the current eroded intra eocene unconformity ieu in the northern north atlantic a change in sedimentation style occurred within the middle eocene to upper oligocene drift sequence and it likely was related to a shift to deeper more intense currents that eroded the widespread horizon a u along the margin of eastern north america about early oligocene time beginning in the late oligocene 25 ma a thick drift exhibiting seismically laminated mudwaves was deposited in a distinct belt at 3500 4500 m paleodepth on the southeast newfoundland ridge this development correlates with widespread late oligocene through miocene pliocene drift accumulation throughout the north atlantic the most recent phase of drift deposition since late pliocene time 3 ma occurred after a shift to the modern circulation system of deeper swifter currents and it includes mixed pelagic hemipelagic sediments and ice rafted debris that reflect glacial interglacial influences on sedimentation

evolution of atlantic thermohaline circulation early oligocene onset of deep water production in the north atlantic the flow of deep water masses is a key component of heat transport in the modern climate system yet the role of deep ocean heat transport during periods of extreme warmth is poorly understood the present mode of meridional overturning circulation is characterized by deep water formation in both the north atlantic and the southern ocean however a different mode of meridional overturning circulation operated during the extreme greenhouse warmth of the early cenozoic during which time the southern ocean was the dominant region of deep water formation the combination of general global cooling and tectonic evolution of the atlantic basins over the past 55 m y ultimately led to the development of a mode of overturning circulation characterized by both southern ocean and north atlantic deep water sources the change in deep water circulation mode may in turn have affected global climate however unraveling the causes and consequences of this transition requires a better understanding of the timing of the transition new nd isotope data from the southeastern atlantic ocean indicate that the initial transition to a bipolar mode of deep water circulation occurred in the early oligocene ca 33 ma the likely cause of significant deep water production in the north atlantic was tectonic deepening of the sill separating the greenland norwegian sea from the north atlantic

abyssal current influence on the southwest bermuda rise and surrounding region abstract echograms 3 5 khz and bottom photographs reveal that the northward flowing antarctic bottom water aabw has strongly influenced the modern depositional regime on the southwest bermuda rise the spatial distribution of echo character types the orientation and nature of current controlled structures and limited current meter data show that aabw flows with varying intensities along three primary pathways around and over the southwest bermuda rise the main core of aabw flows clockwise around the eastern and western flanks of the southern bermuda rise roughly parallel to the 5400 m isobath this current bifurcates at 28 30 n 69 w where a portion flows northeast over the southwest bermuda rise and the remainder continues north along the physiographic boundary between the southwest bermuda rise and the hatteras abyssal plain secondary ribbons of aabw branch off the main core of aabw during its southerly journey along the southeastern bermuda rise and flow west through fracture zones finally a diffuse northward flowing aabw sweeps the entire southwest bermuda rise a progression of current controlled bedforms occurs beneath the main path of the aabw reflecting the spatially varying current velocities and sediment supply the main core of aabw flows west through the narrow vema gap creating erosional furrows along the border between the southwest bermuda rise and the vema gap current velocities greater than 20 cm s 1 are inferred from the bedforms in this region farther north along the southwestern edge of the bermuda rise sediment waves become more prevalent this transition from erosional to more depositional bedforms results from diminished current velocities 5 15 cm s 1 and increased sediment supply although some of these bedforms on the southwest bermuda rise appear to be relict their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs

rough smooth basement boundary in the western north atlantic basin evidence for a seafloor spreading origin a distinctive change in the character of acoustic basement has been mapped within the mesozoic keathley magnetic anomaly sequence in the western north atlantic basin this boundary known as the rough smooth basement boundary separates relatively smooth acoustic basement on the older western side of the boundary from much rougher basement to the east south of bermuda the boundary is well defined and nearly isochronous corresponding to anomaly m 12 or m 13 128 m y b p north of bermuda the boundary is diachronous and more transitional with the change in basement relief occurring in a zone between anomaly m 21 and m 16 age crust 132 to 142 m y b p both the well defined and transitional changes in basement relief found in the western basin are also identifiable in seismic reflection records from the eastern canary basin and an analysis of magnetic anomaly patterns in the central north atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge these results strongly suggest a primary seafloor spreading origin for the rough smooth basement boundary with the greater basement relief associated with slower spreading rates changing directions of relative plate motion and ridge jumps of isolated accretionary segments beginning in late jurassic early cretaceous time

visions of ice sheets in a greenhouse world abstract backstripped eustatic estimates from new jersey and the russian platform show large 25 m and rapid 18 o record highlights the enigma this was the warmest interval of the past 200 my yet it was bracketed by two inferred eustatic falls of 25 m that were associated with two large 0 75 deep sea 18 o increases 92 93 ma mid turonian and 96 ma mid cenomanian we reconcile records of warm high latitudes with glacioeustasy by proposing that late cretaceous early eocene ice sheets generally reached maximum volumes of 8 12 10 6 km 3 20 30 m glacioeustatic equivalent but did not reach the antarctic coast hence coastal antarctica hence deep water remained relatively warm even though there were significant changes in sea level as the result of glaciation unlike the oligocene and younger icehouse world these ice sheets only existed during short intervals of peak milankovitch insolation leaving antarctica ice free during much of the greenhouse late cretaceous to middle eocene these results highlight the need to re evaluate the paradigm that polar ice sheets did not exist during times of warm high latitude climates

ocean nutrient distribution and oxygenation limits on the formation of warm saline bottom water over the past 91 m y it has been proposed that deep water formation in the oceans would be quite different during geologic intervals with reduced equator to pole temperature gradients salinity rather than temperature differences might drive the deep ocean circulation saline water would tend to form at subtropical latitudes where evaporation exceeds precipitation we point out a likely consequence of warm saline bottom water formation on ocean chemistry the tendency to drive the ocean toward anoxia this effect is not due to the previously noted lower oxygen solubility with increasing ocean temperature but rather to the increased efficiency with which plankton will extract nutrients from convecting waters at low latitudes the present ocean nutrient content is sufficient to induce deep water anoxia if circulation were to change yet anoxia is rare in the paleoceanographic record a simple ocean chemical model makes explicit the tradeoffs between mean ocean nutrient content and circulation parameters that will satisfy the geologic observations of an oxygenated ocean since the mid cretaceous barring decreases of ocean phosphate on the order of 30 50 deep water formation at high latitudes was a major source of ocean ventilation in the warmer past

geophysical evidence for widespread cenozoic bottom current activity from the continental margin of nova scotia canada abstract two dimensional and three dimensional multichannel seismic reflection data are utilized to examine previously unrecognized contourite depositional systems along the continental margin of nova scotia canada prior to this study these features were thought to be of limited extent in the study area the new data show that contourite drifts are widespread with greatest drift development during the late miocene to pliocene giant sediment waves form stacked aggrading sequences of bedforms and their development is linked to pre existing seafloor morphology small sediment drifts developed locally throughout the late paleogene and neogene either southwest and down current of seafloor obstacles or form channel fills major erosional pulses form regional seismic markers first along the continental rise in the early oligocene then along the continental slope during the late miocene and pliocene three dimensional seismic data show that erosion surfaces preserve along slope amplitude anomalies two dimensional low relief sediment waves and barchanoid bedforms the geophysical evidence implies that a northeast to southwest along slope bottom current influenced cenozoic depositional patterns throughout the study area

early eocene palaeoceanography and palaeoclimatology of the eastern north atlantic stable isotope results for dsdp hole 550

ocean and climate response to north atlantic seaway changes at the onset of long term eocene cooling abstract around the early middle eocene boundary the first occurrence of contourite drift sediments and widespread deep ocean erosion indicate changes in the north atlantic ocean circulation interestingly these changes coincide with the first steps of cenozoic cooling from the paleogene greenhouse climate towards the modern icehouse the cause for this ocean circulation reorganization is poorly understood since modern water mass tracers may have worked fundamentally different in the past and the paleoceanographic proxy record is limited in both time and space as a result it is challenging to reliably reconstruct the climatic and tectonic boundary conditions e g atmospheric greenhouse gas concentration and the depth and geometry of developing and closing passages between ocean basins in this study we attempt to identify thresholds in tectonic gateway passages and atmospheric co2 concentration using the fully coupled earth system model cosmos indeed the simulation of earth s past climates can unravel the physical processes driving deep water formation in a greenhouse world specifically we use cosmos to evaluate the impact of changes in the north atlantic gateways at the early middle eocene boundary on the north atlantic deep western boundary currents under low obliquity configuration we find that northern component waters start to form when the greenland scotland ridge reaches a threshold depth of deeper than 200 m while the arctic ocean is still shut off from the north atlantic in this scenario the relatively deep greenland scotland ridge allows for sufficient inflow of warm salty atlantic surface waters into the nordic seas to initiate convection during winter cooling opening the seaway towards the arctic leads to a cessation of northern component water formation as it allows for inflow of brackish surface waters into the northern nordic seas hindering northern component water formation

deep water temperature carbonate ion and ice volume changes across the eocene oligocene climate transition 1 paired benthic foraminiferal stable isotope and mg ca data are used to estimate bottom water temperature bwt and ice volume changes associated with the eocene oligocene transition eot the largest global climate event of the past 50 myr we utilized odp sites 1090 and 1265 in the south atlantic to assess seawater 18o w antarctic ice volume and sea level changes across the eot 33 8 33 54 ma we also use benthic 13c data to reconstruct the sources of the deep water masses in this region during the eot our data together with previously published records indicate that a pulse of northern component water influenced the south atlantic immediately prior to and following the eot benthic 18o records show a 0 5 increase at 33 8 ma eot 1 that represents a 2 c cooling and a small 10 m eustatic fall that is followed by a 1 0 increase associated with oi 1 the expected cooling of deep waters at oi 1 33 54 ma is not apparent in our mg ca records we suggest the cooling is masked by coeval changes in the carbonate saturation state co32 which affect the mg ca data to account for this the bwt ice volume and w estimates are corrected for a change in the co32 of deep waters on the basis of recently published work corrected bwt at sites 1090 and 1265 show a 1 5 c cooling coincident with oi 1 and an average w increase of 0 75 the increase in ice volume during oi 1 resulted in a 70 m drop in global sea level and the development of an antarctic ice sheet that was near modern size or slightly larger

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

the onset of modern like atlantic meridional overturning circulation at the eocene oligocene transition evidence causes and possible implications for global cooling a compilation of benthic 18o from the whole atlantic and the southern ocean atlantic sector shows two major jumps in the interbasinal gradient of 18o 18o during the eocene and the oligocene one at 40 ma and the second concomitant with the isotopic event of the eocene oligocene transition eot 33 7 ma ago from previously published circulation models and proxies we show that the first 18o jump reflects the thermal isolation of antarctica associated with the proto antarctic circumpolar current acc the second marks the onset of interhemispheric northern sourced circulation cell similar to the modern atlantic meridional overturning circulation amoc the onset of amoc like circulation slightly preceded 100 300 ky the eot as we show by the high resolution profiles of 18o and 13c previously published from dsdp odp sites in the southern ocean and south atlantic these events coincide with the onset of anti estuarine circulation between the nordic seas and the north atlantic which started around the eot and may be connected to the deepening of the greenland scotland ridge we suggest that while the shallow proto acc supplied the energy for deep ocean convection in the southern hemisphere the onset of the interhemispheric northern circulation cell was due to the significant eot intensification of deepwater formation in the north atlantic driven by the nordic anti estuarine circulation this onset of the interhemispheric northern sourced circulation cell could have prompted the eot global cooling

bipolar atlantic deepwater circulation in the middle late eocene effects of southern ocean gateway openings we present evidence for antarctic circumpolar current acc like effects on atlantic deepwater circulation beginning in the late middle eocene modern ocean circulation is characterized by a thermal differentiation between southern ocean and north atlantic deepwater formation regions in order to better constrain the timing and nature of the initial thermal differentiation between northern component water ncw and southern component water scw we analyze benthic foraminiferal stable isotope 18obf and 13cbf records from ocean drilling program site 1053 upper deep water western north atlantic our data compared with published records and interpreted in the context of ocean circulation models indicate that progressive opening of southern ocean gateways and initiation of a circum antarctic current caused a transition to a modern like deep ocean circulation characterized by thermal differentiation between scw and ncw beginning 38 5 ma in the initial stages of drake passage opening in addition the relatively low 18obf values recorded at site 1053 show that the cooling trend of the middle late eocene was not global because it was not recorded in the north atlantic the timing of thermal differentiation shows that ncw contributed to ocean circulation by the late middle eocene 1 4 myr earlier than previously thought we propose that early ncw originated in the labrador sea based on tectonic reconstructions and changes in foraminiferal assemblages in this basin finally we link further development of meridional isotopic gradients in the atlantic and pacific in the late eocene with the tasman gateway deepening 34 ma and the consequent development of a circumpolar proto acc

hydrocarbon derived ferromanganese nodules in carbonate mud mounds from the gulf of cadiz mud breccia sediments and clasts as nucleation sites abstract more than 500 fe mn nodules were sampled during the anastasya 01 cruise tasyo project along the continental margin of the gulf of cadiz eastern central atlantic at the confluence of the mediterranean sea with the atlantic ocean where extensive nodule fields were discovered based on wide previous studies that included swath bathymetry multi channel and very high resolution seismic reflection gravimetry magnetism and underwater photography surveys nodules were collected at water depths ranging from 850 to 1000 m associated with hydrocarbon derived ankerite and dolomite chimneys and crusts thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties morphology colour surface texture sphericity weight and size mineralogy xrd optical and electronic microscopy and geochemistry xrf aas icp ms epma gc ms s c and o isotopes nodules show a wide range of sizes densities weights and morphologies nodules are formed of multiple millimetre thick layers of fe and mn oxyhydroxides surrounding the nucleus composed of early middle miocene plastic marl the textures developed by the fe and mn oxyhydroxides layers are massive laminated detrital and mottled to dendritic goethite lepidocrocite mn oxides 7 a manganates and 10 a manganates quartz and phyllosilicates are the main components accessory minerals are calcite dolomite siderite rhodochrosite kutnohorite pyrite chalcopyrite potassium feldspar zircon rutile ilmenite and chlorite fe mn carbonates from the siderite rhodochrosite continuous series are forming part of the marl nuclei framboidal filamentous and globular textures are observed in fe mn oxides and pyrite suggesting biogenic origin nodules show a high mean abundance of fe 38 6 moderate mn 6 0 and low contents of trace metals and rees compared to the average content of deep seabed polymetallic nodules from other oceanic areas the mn fe ratio ranges from 0 07 to 0 25 the studied nodules hold in their oxide layers hydrocarbons n alkanes derived from marine bacterial activity aromatic hydrocarbons in the form of phenanthrene are also present which is characteristic of mature petroleum the structure mineralogy and chemical composition in the studied nodules are similar to those of diagenetic hydrogenetic continental margin nodules rather than deep sea nodules we suggest that the formation of this type of nodule occurred through a combined diagenetic hydrogenetic growth process involving fluid venting from deep seated hydrocarbon reservoirs bio mineralization processes and bottom current erosion

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

a silicon depleted north atlantic since the palaeogene evidence from sponge and radiolarian silicon isotopes despite being one of earth s major geochemical cycles the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica dsi concentration through geologic time remain poorly constrained silicon isotope ratios expressed as 30si in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling in particular the 30si of siliceous sponge spicules has been demonstrated to be a useful proxy for past dsi concentrations we analysed 30si in radiolarian tests and sponge spicules from the blake nose palaeoceanographic transect odp leg 171b spanning the palaeocene eocene ca 60 30 ma our 30si results range from 0 32 to 1 67 and 0 48 to 0 63 for the radiolarian and sponge records respectively using an established relationship between ambient dissolved si dsi concentrations and the magnitude of silicon isotope fractionation in siliceous sponges we demonstrate that the western north atlantic was dsi deplete during the palaeocene eocene throughout the water column a conclusion that is robust to a range of assumptions and uncertainties these data can constitute constraints on reconstructions of past ocean circulation previous work has suggested ocean dsi concentrations were higher than modern ocean concentrations prior to the cenozoic and has posited a drawdown during the early palaeogene due to the evolutionary expansion of diatoms our results challenge such an interpretation we suggest here that if such a global decrease in oceanic dsi concentrations occurred it must predate 60 ma

neogene evolution of atlantic thermohaline circulation perspective from walvis ridge southeastern atlantic ocean 1 we present new nd isotope records from walvis ridge ocean drilling program odp sites 1262 1264 southeastern atlantic spanning the past 24 ma to investigate the neogene evolution of atlantic thermohaline circulation the new data indicate that deepwater nd t values from odp site 1262 decrease from 11 0 at 10 6 ma to 12 5 by 7 3 ma this decrease parallels the nd isotope trends contained in fe mn crust records from the northwestern atlantic however the shift at odp site 1262 4755 m water depth occurred up to 6 ma earlier than the corresponding changes in crust records from the atlantic and earlier than any atlantic site shallower than 2700 m paleowater depth recent interpretations of the rapid decrease in fe mn crust nd nd t ssvalues invoke changes in weathering inputs to the labrador sea region rather than a fundamental change in deepwater convection in the labrador sea however the new evidence for significant depth stratification of the nd isotope signal in the southeastern atlantic between 10 6 and 7 3 ma suggests that the onset of deepwater convection in the labrador sea may have played a role in the deepwater decrease in nd isotopic composition climatic conditions during the middle to late miocene likely favored an increase in the importance of glacially induced mechanical weathering while at the same time promoting deep convection in the labrador sea