on the geologic structure at the epicenter of the 1886 charleston south carolina earthquake the study focuses on evidence of cenozoic faulting in the epicentral area of the 1886 charleston south carolina earthquake and its connection with mesozoic structure the seismic data consist of several reflection profiles collected near summerville south carolina in the period 1975 1983 reprocessing of the data reveals an extensive early mesozoic extensional basin approximately 20 km in width between summerville and charleston the basin is delineated by the geometry of reflections that image early mesozoic volcanic and sedimentary rocks and by positive magnetic and gravity anomalies cenozoic compressional reactivation of mesozoic extensional faults is imaged in the interior of the basin the northwestern boundary of the basin is marked by a sharp gradient in the magnetic field folded cretaceous and tertiary atlantic coastal plain sediments in association with diffractions and truncated reflections from the early mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted instrumentally located earthquakes are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern basin margin modeling of magnetic and gravity data indicates that the upper crust beneath the seismically imaged structural basin is composed largely of mafic rocks to a depth of at least 4 km we propose that the charleston earthquake occurred due to compressional reactivation of a mesozoic fault in a localized zone of intense early mesozoic continental rifting

petrophysical characterization of the south georgia rift basin for supercritical co2 storage a preliminary assessment the triassic jurassic south georgia rift sgr basin buried beneath coastal plain sediments of southern south carolina southeastern georgia western florida and southern alabama consists of an assemblage of continental rift deposits popularly called red beds and mafic igneous rocks basalt flows and diabase sills the red beds are capped by basalts and or diabase sills and constitute the target for supercritical co2 storage as part of a department of energy funded project to study the feasibility for safe and permanent sequestration the purpose of this research is to evaluate subsurface suitability for underground co2 storage in terms of the local and regional distribution of porous and permeable reservoirs in addition unlike shale capped co2 reservoirs very little is known about the ability of basalts and diabase sills to act as viable seals for co2 storage new results demonstrate the presence of confined porous rocks that may be capable of storing significant quantities of supercritical co2 reservoir thicknesses as high as 420 m and an average porosity as high as 14 were obtained the sgr basin manifests distinct porosity permeability regimes that are influenced by the depositional environments these are a high porosity medium low permeability zone associated with lacustrine deposits a medium low porosity low permeability zone dominated by fluvial fine to very fine grained sandstone and an extremely low porosity and permeability zone characterized by fluvial and alluvial fan deposits analyses further show that the basalt flows and diabase sills are characterized by low porosity as well as high seismic velocities and densities that are favorable to caprock integrity

marine derived 14c calibration and activity record for the past 50 000 years updated from the cariaco basin abstract an expanded cariaco basin 14c chronology is tied to 230th dated hulu cave speleothem records in order to provide detailed marine based 14c calibration for the past 50 000 years the revised high resolution cariaco 14c calibration record agrees well with data from 230th dated fossil corals back to 33 ka with continued agreement despite increased scatter back to 50 ka suggesting that the record provides accurate calibration back to the limits of radiocarbon dating the calibration data document highly elevated 14c during the glacial period carbon cycle box model simulations show that the majority of observed 14c change can be explained by increased 14c production however from 45 to 15 ka 14c remains anomalously high indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today additional observations of the magnitude spatial extent and timing of deep ocean 14c shifts are critical for a complete understanding of observed glacial 14c variability

the δ15n of nitrate in the southern ocean consumption of nitrate in surface waters we report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern indian and pacific sectors of the southern ocean focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton the 15n of nitrate increases by 1 2 from deep water into the antarctic summertime surface layer due to kinetic isotopic fractionation during nitrate uptake estimation of the nitrate uptake isotope effect from antarctic depth profiles yields values in the range of 5 6 in east indian sector and 4 5 in the east pacific sector surface transect data from the pacific sector also yield values of 4 5 the major uncertainty in the profile based estimation of the isotope effect involves the 15n of nitrate from the temperature minimum layer below the summertime antarctic surface layer which deviates significantly from the predictions of simple models of isotope fractionation for the subantarctic surface it is possible to distinguish between nitrate supplied laterally from the surface antarctic and nitrate supplied vertically from the subantarctic thermocline because of the distinctive relationships between the 15n and concentration of nitrate in these two potential sources our subantarctic samples collected during the summer and fall indicate that nitrate is supplied to the subantarctic surface largely by northward transport of antarctic surface water isotopic data from the pacific sector of the subantarctic suggest an isotope effect of 4 5 indistinguishable from the antarctic estimates in this sector

evidence and implications of an extensive early mesozoic rift basin and basalt diabase sequence beneath the southeast coastal plain regional seismic reflection profiles across the georgia and south carolina coastal plain provide a detailed view into the internal structure and stratigraphy of the buried triassic jurassic south georgia basin reflection data indicate that the basin is a complex composite of smaller individual basins which vary drastically along strike these basins are relatively wide along north south profile maximum 100 km but appear to be more restricted along strike in cross section the basins are asymmetric being typically bounded on their south sides by an interpreted north dipping master normal falult the thickness of interpreted basin fill below the cretaceous tertiary overlap also varies drastically locally reaching 6 km analysis and correlation of the reflection profiles when integrated with drill hole and offshore reflection data suggest the presence of an areally extensive basalt flow and or diabase sill sequence of probable early middle jurassic age covering a large part of the triassic jurassic south georgia basin the sequence is traceable on seismic reflection profiles across much of the south georgia basin and may extend as far as from western georgia to offshore south carolina the implied areal extent of this sequence would be more than 100 000 km 2 ranking it among the great basalt flows and or sills of the world on seismic reflection profiles over the south georgia basin the basalt diabase sequence is also important in marking a post rift unconformity which separates tilted and apparently faulted lower mesozoic rift related strata below from a thinner more widespread and generally unfaulted sequence above this configuration is interpreted to mean that basalt extrusion and or hypabyssal intrusion occurred after the main episode of late triassic early jurassic normal faulting and associated localized subsidence syn rift phase and prior to a later jurassic episode of more widespread uniform subsidence post rift phase accompanied by relatively minor normal faulting this two stage evolutionary sequence is analogous to that of many rift basins throughout the world and suggests a hypothesis that the south georgia basin was an area of incipient crustal spreading associated with the early formation of the central atlantic ocean

glacial interglacial variability in denitrification in the world s oceans causes and consequences the late quaternary history of water column denitrifcation of the eastern pacific margins and the arabian sea is reconstructed using sedimentary 15n measurements the 15n values in six piston cores raised from these regions show remarkably similar cyclic variations being heavy 9 10 5 during the interglacials and 2 3 lighter during the glacials this implies that denitrification in these regions decreased substantially during the glacial periods the glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone since water column denitrification in these areas accounts for about half of the fixed nitrogen loss in the modern ocean the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods because nitrate is a limiting nutrient oceanic productivity and attendant changes in co2 may therefore have been modulated on glacial interglacial timescales by variations in the oceanic no3 content

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

millennial scale oscillation of denitrification intensity in the arabian sea during the late quaternary and its potential influence on atmospheric n2o and global climate the intensity of denitrification in the arabian sea during the last 65 kyr is reconstructed using high resolution 15 n records of three sediment cores in conjunction with other geochemical tracers for water column oxygenation and productivity the results reveal a close link to the greenland ice core record with low or absent water column denitrification during the last glacial maximum the stadials and at the time of the heinrich events including the younger dryas in contrast denitrification was high during the holocene and the interstadials the intensification of denitrification is related to stronger sw monsoonal upwelling which enhances organic matter flux and degradation resulting in a strengthening of the midwater oxygen deficiency such a combination of enhanced upwelling and denitrification has also been implied for the eastern tropical north pacific etnp where these events occur during the holocene and to some extent during the interstadials too today the arabian sea and the etnp together contribute substantially to the global marine water column denitrification and a significant fraction of the ocean atmosphere n 2 o flux originates from these areas changes in n 2 o emissions from these areas could thus have effected the recently described stadial interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores moreover denitrification is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory which in turn influences global primary productivity and co 2 sequestration by the biological pump short term switches between a nondenitrification mode and a denitrification mode in these marine regions therefore have an impact on global climate

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

constraints on the structure and tectonic development of the early mesozoic south georgia rift southeastern united states seismic reflection data processing and interpretation seismic reflection data collected by the consortium for continental reflection profiling cocorp over the buried triassic jurassic south georgia rift reprocessed in order to improve the upper 5 s of reflection time provide a window into the regional deep structure of the rift and delineate variations in basin structure and depositional styles located at the southernmost extent of the north american east coast early mesozoic rift system the structure of the south georgia rift basin has previously been known only from scattered drilling and potential field interpretation leaving the internal and boundary as well as the deep structure unknown the reflection data reveal that the south georgia basin consists of a broad complex terrane of isolated synrift grabens with intervening structural highs in contrast with earlier concepts of a single narrow trough or an amorphous sag basin two main subbasin depocenters are observed in west and west central georgia the largest of which is more than 100 km in width and the deepest at least 7 km at its inferred base analysis of the reflection data indicates that the distinct structural and seismic stratigraphic character of the reflection packages is analogous to other better studied extensional provinces e g north american basin and range the reprocessing of the reflection data provides a view of contrasting styles of faulting and subsidence within the rift the structural style of the subbasins varies drastically along strike changing from 1 a narrow asymmetric graben bordered by a planar and more deeply penetrating normal fault to 2 a broad and roughly symmetric graben bordered by a shallower dipping possibly listric fault along which extension may have been accommodated at depth by a low angle detachment or by a more complex system of high angle faults only limited crustal extension is suggested by the narrower graben and planar fault 1 whereas much greater extension would be expected from the wider graben bordered by a listric low angle fault 2 such a dual style of basin formation is predicted by segmented plate models of variable extension along continental margins and intracratonic rifts and may imply a significant change in crustal rheology along the developing rift integration of the interpretation of the regional structure of the south georgia basin with that of the deeper crustal structure as inferred from the 16 s reflection time cocorp data indicates that the synrift basins developed mainly over the upper plate of the alleghanian late paleozoic suture between north america and west africa the major subbasin border faults dip northward in an antithetic relation to the predominantly northward vergence of the suture zone the border faults thus do not appear to have reactivated antecedant structure although the development of the south georgia rift appears to have approximately coincided with early mesozoic thinning of the crust in the southeastern united states no horizontally layered or laminated lower crust is observed as has been associated with other more fully rifted terranes e g north american basin and range continental shelf around british isles however the moho discontinuity appears to have formed as a dynamic feature superimposed on late paleozoic compressional structure probably associated with early mesozoic rifting these findings thus support the interpretation that the south georgia rift developed as a major early mesozoic depocenter and possible incipient spreading center associated with the initial formation of the central atlantic ocean

detailed sedimentary n isotope records from cariaco basin for terminations i and v local and global implications 1 for the last deglaciation and termination v the initiation of mis 11 at around 430 ka we report high resolution sedimentary nitrogen isotope 15n records from cariaco basin in the caribbean sea during both terminations the previously reported interglacial decrease in 15n clearly lags local changes such as water column anoxia as well as global increases in denitrification by several thousand years on top of the glacial interglacial change several 15n peaks were observed during the last deglaciation the deglacial signal in cariaco basin can be best explained as a combination of 1 local variations in suboxia and water column denitrification as the reason for the millennial scale peaks 2 a deglacial maximum in mean ocean nitrate 15n and 3 increasing n2 fixation in response to globally increased denitrification causing the overall deglacial 15n decrease in the holocene much of the decrease in 15n occurred between 6 and 3 ka coinciding with an expected precession modulated increase in african dust transport to the tropical north atlantic and the caribbean this begs the hypothesis that n2 fixation in this region increased in response to interglacial maxima in denitrification elsewhere but that this response strengthened with increased mid holocene iron input it remains to be seen whether the data for mis 11 support this interpretation

correlated millennial scale changes in surface hydrography and terrigenous sediment yield inferred from last glacial marine deposits off northeastern brazil abstract the stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern brazil stable oxygen isotope records of the planktonic foraminiferal speciesglobigerinoides sacculiferandglobigerinoides ruber pink reveal sub milankovitch changes in sea surface hydrography during the last 85 000 yr warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high xrf intensities of ti and fe and which suggest humid conditions in northeast brazil these tropical signals correlate with climatic oscillations recorded in greenland ice cores dansgaard oeschger cycles and in sediment cores from the north atlantic heinrich events trade winds may have caused changes in the north brazil current that altered heat and salt flux into the north atlantic thus affecting the growth and decay of the large glacial ice sheets

enhanced ventilation of the north atlantic subtropical gyre thermocline during the last glaciation the ocean s intermediate1 6 and deep7 8 circulation are both known to have differed during the last glaciation from those of today but little is known about the history of the subtropical gyres variations in gyre processes should be closely linked to variations in global climate as gyre circulation is driven by air sea interactions and reflects the climate at the ocean surface the gyres are also a significant reservoir of carbon and nutrients so that gyre processes affect the distribution of carbon and nutrients in the oceans and thus atmospheric co2 here we use measurements of 13c and 18o in foraminifera from the bahamas to produce a detailed reconstruction of nutrient and temperature profiles in the thermocline during the last glaciation the thermocline of the glacial north atlantic subtropical gyre lacked the oxygen minimum that is characteristic of the modern ocean and was depleted in nutrients indicating greater more uniform thermocline ventilation at that time thus not only intermediate waters1 5 but the entire upper water column within the glacial north atlantic was depleted of nutrients moreover glacial thermocline waters were cooler had a steeper temperature gradient and a shallower base both the ventilation and the thermal structure of the glacial thermocline are consistent with what is known of the glacial climate9 14

arctic sea ice variability on a timescale of weeks and its relation to atmospheric forcing abstract northern hemisphere sea ice concentration 500 hpa height sea level pressure and 1000 500 hpa thickness at 7 day intervals are examined for the period 1972 1989 with emphasis on the winter season the temporal variability of sea ice concentration is largest along the climatological mean ice edge where its frequency distribution is strongly bimodal with ice free and ice covered conditions being observed much more frequently than partial ice cover these results confirm impressions based on visual inspection of satellite imagery that most of the variability in these regions is associated with the advance and retreat of the ice edge relationships between large scale patterns of atmospheric variability and sea ice variability are investigated making use of singular value decomposition of the temporal covariance matrix the analysis is conducted separately for the atlantic and pacific sectors in agreement with earlier studies based upon monthly mean data on sea ice concentration the strongest

glacial interglacial modulation of the marine nitrogen cycle by high latitude o2 supply to the global thermocline 1 an analysis of sedimentary nitrogen isotope records compiled from widely distributed marine environments emphasizes the global synchrony of denitrification changes and provides evidence for a strong temporal coupling of these variations to changes in nitrogen fixation as previously inferred we explain the global coherence of these records by a simple physical control on the flux of dissolved oxygen to suboxic zones and the coupling to fixation via the supply of phosphorus to diazotrophs in suitable environments according to our hypothesis lower glacial stage sea surface temperature increased oxygen solubility while stronger winds in high latitude regions enhanced the rate of thermocline ventilation the resultant colder rapidly flushed thermocline lessened the spatial extent of denitrification and consequently n fixation during warm periods sluggish circulation of warmer less oxygen rich thermocline waters caused expansion of denitrification zones and a concomitant increase in n fixation local fluctuations in export productivity would have modulated this global signal

stratigraphic record of the early mesozoic breakup of pangea in the laurasia gondwana rift system rift basins of the central atlantic margins cam of north america and morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early mesozoic the best known is the newark basin of new jersey new york and pennsylvania where an astronomically calibrated magnetic polarity time scale is developed lacustrine cycles of milankovitch origin are commonly present in cam basins with the period changing from 10 ky paleoequator with coals to 20 ky 4 10n to perhaps 40 ky northward with evaporites cycles of100 ky 413 ky and2 my are also important four mostly unconformity bounded tectonostratigraphic sequences are present the anisian ts i is fluvial and eolian ts ii ts iv late triassic to early jurassic consist of tripartite lacustrine sequences caused by extension pulses the newark basin accumulation rate history allows comparison with quantitative rift basin models the north american plate s slow northward drift resulted in a relative shift of climate although the rapid humidification during the latest triassic and early jurassic is associated with a sea level rise the triassic jurassic mass extinction is of independent origin plausibly impact related

coupling of millennial scale changes in sea surface temperature and precipitation off northeastern brazil with high latitude climate shifts during the last glacial period 1 high resolution records of alkenone derived sea surface temperatures and elemental ti ca ratios from a sediment core retrieved off northeastern brazil 4 s reveal short term climate variability throughout the past 63 000 a large pulses of terrigenous sediment discharge caused by increased precipitation in the brazilian hinterland coincide with heinrich events and the younger dryas period terrigenous input maxima related to heinrich events h6 h2 are characterized by rapid cooling of surface water ranging between 0 5 and 2 c this signature is consistent with a climate model experiment where a reduction of the atlantic meridional overturning circulation amoc and related north atlantic cooling causes intensification of ne trade winds and a southward movement of the intertropical convergence zone resulting in enhanced precipitation off northeastern brazil during deglaciation the surface temperature evolution at the core site predominantly followed the antarctic warming trend including a cooling prior to the younger dryas period an abrupt temperature rise preceding the onset of the bolling allerod transition agrees with model experiments suggesting a southern hemisphere origin for the abrupt resumption of the amoc during deglaciation caused by southern ocean warming and associated with northward flow anomalies of the south atlantic western boundary current

nitrogen isotope dynamics of the cariaco basin venezuela 1 we report 15n 14n measurements of water column nitrate and ammonium sinking particles and sediments from the cariaco basin an anoxic marine basin off the coast of venezuela water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic anoxic interface 275 m being free of a large denitrification signal the 15n of shallow thermocline nitrate is 3 5 significantly lower than the mean deep ocean nitrate 15n of 5 this may be due to the nitrification of newly fixed n whether it occurs within the basin or in open atlantic waters that flow into the cariaco over the sill the 15n 14n of the sinking flux in the deepest trap 1250 m is similar to that of thermocline nitrate as expected given the complete consumption of nitrate in the surface layer moreover the 15n 14n of the seafloor sediment is similar to that of the sinking flux as is common in environments of high export production low o2 and good organic matter preservation thus the modern cariaco basin records the 15n 14n of the thermocline nitrate which in turn may record the input of newly fixed n to the upper ocean be it local or more regional in origin

late quaternary changes of western equatorial atlantic surface circulation and amazon lowland climate recorded in ceara rise deep sea sediments today the western tropical atlantic is the most important passage for cross equatorial transfer of heat in the form of warm surface water flowing from the south into the north atlantic circulation changes north of south america may thus have influenced the global thermohaline circulation system and high northern latitude climate here we reconstruct late quaternary variations of western equatorial atlantic surface circulation and amazon lowland climate obtained from a multiproxy sediment record from ceara rise variations in the illite smectite ratio suggest drier climatic conditions in the amazon basin during glacials relative to interglacials the 230thex normalized fluxes and the 13c 12c record of organic carbon indicate that sea level fluctuations shelf topography and changes of the surface circulation pattern controlled variations and amplitude of terrigenous sediment supply to the ceara rise we attribute variations in thermocline depth reconstructed from vertical planktic foraminiferal oxygen isotope gradients and abundances of the phytoplankton species florisphaera profunda to changes in southeast trade wind intensity strong trade winds during ice volume maxima are associated with a deep western tropical atlantic thermocline strengthening of the north brazil current retroflection and more vigorous eastward flow of surface waters

the role of north brazil current transport in the paleoclimate of the brazilian nordeste margin and paleoceanography of the western tropical atlantic during the late quaternary abstract reconstructions of surface paleoceanographic conditions of the western equatorial atlantic and past climates of the adjacent northeast brazilian the nordeste continental margin were undertaken by analyzing sediments from a piston core and associated gravity and box cores recovered from 3107 meter water depth at 0 20 n on the equatorial brazilian continental slope the record is dated by radiocarbon analysis and oxygen isotopic stratigraphy of planktonic foraminifers and spans from near modern to approximately 110 ka high resolution xrf analysis provides insight into the paleoclimate history of the nordeste during the last glacial interval several large amplitude and abrupt peaks are observed in the time series of ti ca and are usually accompanied by peaks of fe k together these record periods of increased precipitation and intense weathering on the adjacent continent and increased terrestrial sediment discharge from nordeste rivers into the atlantic within the limits of dating accuracy most ti ca peaks correlate with heinrich events in the north atlantic this record thus corroborates and extends back in time the previous record of arz et al 1998 determined on sediment cores from farther southeast along the nordeste margin stable oxygen isotopic analysis and mg ca paleothermometry on the near surface dwelling planktonic foraminiferal species globierinoides ruber find that mean sea surface temperature sst during glacial time 20 55 ka n 97 was 23 89 0 79 c and the mean sst during the late holocene 0 5 ka n 14 was 26 89 0 33 c ssts were 0 5 2 c higher and inferred sea surface salinities were lower during most of the periods of elevated ti ca thus as observed in previous studies the western equatorial atlantic was warm at least locally and the adjacent southern tropical continent was wet at the same time that the high latitude north atlantic was cold using the syntrace ccsm3 fully coupled climate model with transient forcing for the period 22 ka to present we find that decreased transport of the north brazil current co occurs with reduced atlantic meridional overturning circulation and colder than normal ssts in the north atlantic region these simulated conditions are invariably associated with significantly increased precipitation in the nordeste region

diachronous rifting drifting and inversion on the passive margin of central eastern north america an analog for other passive margins integration of new data with existing information indicates that the tectonic development of the passive margin of eastern north america between the carolina trough and scotian basin was considerably more complex than the classic two stage rift drift model first the transition from rifting to drifting was diachronous in the southeastern united states the rift drift transition occurred after the late triassic synrift deposition and before eastern north america magmatism in the earliest jurassic 200 ma in maritime canada the rift drift transition occurred after eastern north america magmatic activity and synrift deposition in the early jurassic and before postrift deposition in the early middle jurassic 185 ma second the deformational regime changed substantially after rifting on both the southern and northern segments of the margin generally northwest southeast postrift shortening replaced northwest southeast synrift extension northeast striking reverse faults formed and many of the rift basin boundary faults had reverse displacements in the southeastern united states the change in the deformational regime occurred in the late triassic early jurassic during the rift drift transition simultaneously diabase sills and dikes many striking nearly perpendicular to the trend of the rift basins intruded the continental crust and a massive wedge of volcanic or volcaniclastic rocks developed near the continent ocean boundary in maritime canada the change in the deformational regime occurred during or after the early jurassic and before or during the early cretaceous that is during the rift drift transition or early stages of sea floor spreading

on the relationship between atmospheric circulation and the fluctuations in the sea ice extents of the bering and okhotsk seas monthly and 3 day averaged nimbus 5 electrically scanning microwave radiometer sea ice data have been analyzed to document the fluctuations of the bering sea and sea of okhotsk ice covers for the four winters 1972 1973 through 1975 1976 on the short term seasonal and interannual time scales and to define those periods for which there is an out of phase relationship between the fluctuations of the two sea ice covers the sea ice fluctuations are shown to be related to changes in the position and intensity of quasi stationary high and low pressure systems associated with large scale standing wave patterns in the atmosphere through a comparison of the areal changes in ice cover with changes in the amplitude and phase of zonal harmonics 1 2 and 3 of the sea level pressure field on the seasonal time scale the sea level pressure for december the month of most rapid seasonal sea ice growth for both the bering and okhotsk seas is dominated by harmonic 2 which explains almost 60 of the total sea level pressure variance on average over the four winters the phase of harmonic 2 which is associated with the position of the aleutian low shifts from 156 e in 1972 1973 to 190 e in 1975 1976 thus providing a connection between the increasingly heavy bering sea ice cover over these four winters and the general circulation of the atmosphere on the short term episodes of midwinter bering sea ice retreat are generally preceded by or coincident with the breakdown of harmonic 1 and the intensification of harmonics 2 and 3 these episodes are also related to the coincidence of the phases of harmonics 2 and 3 with a mean position of 165 e this result is in excellent agreement with a study of the north pacific oscillation showing that the two modes of the oscillation are associated with the longitudinal position of the aleutian low it is suggested here that these two modes result from changes in the amplitudes and phases of atmospheric long waves 1 2 and 3 evidence is also presented indicating that the interaction of standing and transient components of wave 1 may play an important role in these fluctuations finally these results establish an observational connection relating climate dynamics to changes in ice covers in seasonal sea ice zones

glacial interglacial differences in circulation and carbon cycling within the upper western north atlantic we investigated glacial interglacial changes in the circulation and carbon cycling in the western north atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of bahamian benthic foraminifera potential temperature salinity depth relations show that modern thermocline 200 1000 m and deep 1000 2000 m waters in the providence channels bahamas originate in the sargasso sea and are typical of the subtropical gyre gradients in the stable isotopic compositions of late holocene planulina and cibicidoides species from the bank margins 400 to 1500 m depth reflect temperature nutrient and isotopic gradients of modern subtropical gyre waters the difference between the 18o of glacial maximum and late holocene foraminifera is 2 1 for the upper 900 m of the water column and 1 6 for deeper waters indicating that these waters were 4 c and 2 c cooler respectively during glacial time the glacial temperature gradient dt dz was similar to today while the base of the thermocline was 100 m shallower these results differ significantly from our earlier results from multiple shell 18o analyses which implied upper thermocline waters were only 1 c cooler and dt dz was greater during the glacial maximum the difference occurs because bioturbation adversely affects multiple shell analyses of glacial aged samples from shallow water depths at all depths above 1500 m foraminiferal 13c are greater during the glacial maximum than the late holocene by at least 0 1 to 0 2 as much as 0 6 in the lower thermocline indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum this suggests greater more uniform ventilation of the thermocline results of single and multiple shell 13c analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial interglacial change of 13c was small cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal surfaces outcrop including large areas of the subtropical ocean a shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water production enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of ekman downwelling

denitrification under glacial and interglacial conditions a physical approach 1 recent analysis of sedimentary 15n records over the last 200 000 years has shown an expansion of water column denitrification zones during warmer periods and a shrinkage of these oxygen poor regions during glacial periods two different mechanisms could be responsible for driving the changes in the denitrification records variations in local productivity leading to a change in remineralization rates and or changes in the ocean circulation and ocean temperature resulting in variations of the physical supply of dissolved oxygen here we focus on the supply mechanism by using a fully coupled atmosphere ocean sea ice land surface scheme model the university of victoria earth system climate model to examine oxygen supply under varying physical conditions while maintaining an invariant biological oxygen utilization scheme we show that circulation changes can be the cause for the observed changes in dissolved oxygen in the glacial ocean lower temperatures and enhanced formation of antarctic intermediate water and north pacific intermediate water during glacial periods increase the physical supply of oxygen and therefore decrease water column denitrification in our last glacial maximum simulations the change in water characteristics in the eastern tropical pacific is important enough to reduce denitrification by 46 to 65 compared to present day conditions depending on the wind fields used as boundary conditions the consequences of our findings could be substantial for the near future with a warming climate denitrification zones could expand leading to changes in the biological pump and the flux of n2o into the atmosphere

local and regional geochemical signatures of surface sediments from the cariaco basin and orinoco delta venezuela we have analyzed the chemical compositions of 87 samples from the cariaco basin and 20 samples from the orinoco delta venezuela in order to better characterize the fluvial sources of material to the cariaco basin we observe distinctive regional patterns in composition shelf sediments found near the tuy river manzanares river the araya margarita region and in the northern portion of the unare platform record reproducible compositional differences however linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin average trap values are best approximated by mixing upper crust 30 50 with local river sources 50 70 of either the tuy unare or neveri rivers and eolian dust

tectonic significance of porosity and permeability regimes in the red beds formations of the south georgia rift basin abstract a simple new porosity permeability depth profile was developed from available laboratory measurements on triassic sedimentary red beds sandstone from parts of the south georgia rift sgr basin in order to investigate the feasibility for long term co2 storage the study locations were sumter berkeley dunbarton clubhouse crossroad 3 cc 3 and norris lightsey wells as expected both porosity and permeability show changes with depth at the regional scale that was much greater than at local scale the significant changes in porosity and permeability with depth suggest a highly compacted deformed basin and potentially a history of uplift and erosion the permeability is generally low both at shallow less than 1826 ft 556 56 m and deeper depths greater than 1826 ft 556 56 m both porosity and permeability follow the normal trend decreasing linearly with depth for most parts of the study locations with the exception of the norris lightsey well a petrophysical study on a suite of well logs penetrating the norris lightsey red beds at depths sampled by the core derived laboratory measurements shows an abnormal shift by 50 in the acoustic travel time and or in the sonic derived p wave velocity that indicates possible faulting or fracturing at depth the departure of the norris lightsey s porosities and permeabilities from the normal compaction trend may be a consequence of the existence of a fault fracture controlled abnormal pressure condition at depth the linear and non linear behaviors of the porosity permeability distribution throughout the basin imply the composition of the sgr red beds and by extension analog similar triassic jurassic formations within the eastern north american margin have been altered by compaction uplift erosion and possible faulting that have shaped the evolution of these triassic formations following the major phase of rifting

seaward dipping reflectors offshore the southeastern united states seismic evidence for extensive volcanism accompanying sequential formation of the carolina trough and blake plateau basin deep penetration multichannel seismic reflection profiles off the southeastern united states reveal widespread seaward dipping reflectors sdrs similar features have been imaged and sampled on other north atlantic rifted margins where voluminous volcanism has accompanied continental breakup beneath the carolina trough are two sets of sdrs one along a basement hinge zone and another seaward of the east coast magnetic anomaly axis the hinge sdrs lie beneath and apparently developed prior to a flood basalt that erupted at 184 3 ma and is marked by a prominent reflector j beneath the northern blake plateau basin only the hinge sdrs are observed but they developed after j we suggest that the inferred north to south age difference of sdr emplacement implies a heretofore unrecognized time transgressive breakup of northwest africa and north america during the early middle jurassic

intermittent existence of a southern californian upwelling cell during submillennial climate change of the last 60 kyr 1 application of a high resolution multiproxy approach to a sedimentary section drilled at ocean drilling program site 1017 located under a highly active upwelling cell off point conception california provides clear evidence for surface ocean productivity shifts on submillennial timescales during the last 60 kyr the proxies include bulk sediment major and minor elements organic carbon and carbonate concentrations 15n and planktonic foraminiferal species assemblage and carbon isotope determinations the collective results demonstrate that marine productivity in this area was not simply linearly related to cold and warm cycles except during the millennial scale climate oscillations of marine isotope stage mis 3 during that interval the upwelling cell and resulting high productivity were active during warm interstadial events and were largely inactive during cool stadial events however the last glacial maximum was also relatively productive productivity increased dramatically during the bolling warm interval while the allerod and younger dryas were much less productive high coccolithophorid abundance commenced during the earliest holocene after 10 ka the complexity of the productivity response was probably related to interplay between local winds as well as california undercurrent strength

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

coccolithophores as palaeoecological indicators for shifts of the itcz in the cariaco basin during the late quaternary coccoliths were studied from the odp hole 1002c and core pl07 39pc in the cariaco basin increases in emiliania huxleyi are synchronous with decreases of gephyrocapsa oceanica and vice versa a new index gex based on the relative abundances of these two taxa is proposed and correlates with various other proxies it is shown that gex can serve as upwelling proxy this confirms that the intertropical convergence zone shifted north during the bolling allerod south during the younger dryas and back north during the preboreal the upwelling proxy shows few discrepancies with the terrigenous record copyright 2008 john wiley sons ltd

nitrogen fixation in the south atlantic gyre and the benguela upwelling system 1 dinitrogen n2 fixation is recognized as an important input of new nitrogen n to the open ocean gyres contributing to the export of organic matter from surface waters however very little n2 fixation research has focused on the south atlantic gyre where dust deposition of iron fe an important micronutrient for diazotrophs is seasonally low recent modeling efforts suggest that n2 fixation may in fact be closely coupled to and greatest in areas of denitrification as opposed to the oceanic gyres one of these areas the benguela upwelling system lies to the east of the south atlantic gyre in this study we show that n2 fixation in surface waters across the south atlantic gyre was low overall 1 5 nmol n l 1 d 1 with highest rates seen in or near the benguela upwelling system up to 8 nmol n l 1 d 1 surface water dissolved fe dfe concentrations were very low in the gyre 0 3 nm or lower while soluble reactive phosphorus srp concentrations were relatively high 0 15 m n2 fixation rates across the entire sampling area were significantly positively correlated to dfe but also to srp and no3 thus high no3 concentrations did not exclude n2 fixation in the upwelling region which provides evidence that n2 fixation may be occurring in previously unrecognized waters specifically near denitrification zones however the gene encoding for a nitrogenase component nifh was not detected from known diazotrophs at some stations in or near the upwelling where n2 fixation was greatest suggesting the presence of unknown diazotrophs in these waters

modern climate forcing of terrigenous deposition in the tropics cariaco basin venezuela one approach to deciphering tropical quaternary paleoclimate records has been to study the composition of terrigenous material in order to infer past changes in hydrologic and atmospheric conditions here we present the inorganic geochemistry major and trace elements of modern sediment traps and shelf sediments in the tropical cariaco basin venezuela in order to characterize seasonal variation in deposition of the terrigenous component and to document linkages within the ocean atmosphere climate system we show that variation in the chemistry of terrigenous input to the basin is a sensitive monitor of the annual meridional migration of the intertropical convergence zone itcz additionally seasonal terrigenous relative abundances are decoupled from terrigenous absolute fluxes showing that abundances are not directly related to terrigenous delivery changes in mixing between local fluvial sediments fluvial and or eolian mafic material and saharan dust explains the observed seasonality the modern seasonal variations are significantly smaller than those observed in the glacial interglacial paleorecord indicating that climate sensitivity at the two time scales is very different a striking contrast in the behavior of ti al between the longer glacial interglacial time scale and the modern seasonality also suggests that long term terrigenous deposition is not linked simply to 100kyr shifting of the itcz the modern data can be reconciled with the glacial interglacial record by invoking generally drier conditions during glacial periods over tropical latitudes combined with intermittent wet conditions caused by precessional forcing 2007 elsevier b v all rights reserved

the sequence of events surrounding termination ii and their implications for the cause of glacial interglacial co2 changes events surrounding termination ii as preserved in the vostok ice core provide a number of clues about the mechanisms controlling glacial to interglacial climate change antarctic temperature and the atmosphere s co2 content increased together over a period of 8000 years this increase is bounded by a drop in dust flux at its onset and by a drop in the 18o of trapped air at its finish a similar lag between dust flux and foraminiferal 18o is seen in a southern ocean marine record suggesting that the 18o in air trapped in vostok ice is a valid proxy for ice volume the synchronous change of atmospheric co2 and southern hemisphere temperature thus preceded the melting of the northern hemisphere ice sheets this observation coupled with the fact that nutrient reorganization in the north atlantic occurs with or after the sea level rise eliminates many scenarios proposed to explain the co2 rise including those which rely on sea level change conveyor related nutrient redistribution or north atlantic cooling southern ocean scenarios become the front runners but the most popular mechanism iron fertilization has two problems in explaining the co2 rise before termination ii first much of the dust demise occurs prior to the change in co2 so if iron is the villain a threshold value of its supply must be called upon above which productivity does not continue to increase second the co2 rise continues for some 4 5 kyr after the dust flux has fallen to close to zero these problems may be solved if the increased iron supply in dust caused higher rates of nitrogen fixation during the glacial periods in this case the residence time of oceanic nitrate of a few thousand years would enable decreasing productivity to be a global rather than a local phenomenon and would explain the slow rampup of atmospheric co2

bromine counts from xrf scanning as an estimate of the marine organic carbon content of sediment cores 1 xrf sediment core scanning technology is increasingly used to quantify sediment composition the overall good correlation between biophilic halogen bromine br and sedimentary total organic carbon toc potentially allows the fast estimation of down core toc profiles by xrf scanning in order to test this approach we present data from the arabian sea and a mediterranean brine basin comparing xrf core scanning br data with discrete sample toc analyses overall br counts and toc show a clear correlation except when stable carbon isotope and c n data indicate intervals characterized by enhanced input of terrestrial organic matter hence solid phase br is exclusively associated with marine organic matter moc and can be used as a direct estimate of this parameter after a calibration is established high pore water br in the brine core steepens the br toc correlation but after salt correction shows a nearly identical gradient to that of the arabian sea core

the impact of the siberian high and aleutian low on the sea ice cover of the sea of okhotsk comparison of monthly averaged sea ice distributions in the sea of okhotsk with atmospheric pressure data during the four winters having passive microwave sea ice coverage from the nimbus 5 satellite 1973 76 revealed a strong apparent relationship between the extent of the sea ice cover and the influence of the siberian high atmospheric pressure system examination of data for the years 1978 86 having passive microwave coverage from the nimbus 7 satellite reveals that the strong correspondence found for 1973 76 between okhotsk sea ice extents and the siberian high was not maintained in the 1978 86 period a weaker correspondence continued however between the sea ice and the combined siberian high aleutian low system a siberian high aleutian low index was created and the correlation coefficient between that index and sea ice extents in the midwinter month of february is 0 97 for the 1973 76 period and 0 52 for the 1978 86 period primary reasons for the lack of a consistently strong monthly averaged ice atmosphere correspondence are the various oceanographic influences on the sea ice cover the failure of monthly averages to reflect fully the important shorter term interactions between the ice and the atmosphere and the fact that ice conditions in one month are influenced by ice conditions in previous months

reconstructing marine productivity of the cariaco basin during marine isotope stages 3 and 4 using organic walled dinoflagellate cysts 1 an organic walled dinoflagellate cyst analysis was carried out on sediment core md03 2622 retrieved from the cariaco basin the core comprises the 73 30 ka interval down core changes in cyst abundance accumulation rate and composition of cyst assemblages were used to identify climatic and oceanographic changes at orbital and millennial time scales in this near equatorial seasonal upwelling area throughout the sequence dinoflagellate cyst assemblages were dominated by heterotrophic dinocysts mainly brigantedinium spp with the exception of four autotrophic dominated mainly spiniferites ramosus intervals around 58 53 46 and 37 ka at orbital time scales changes in the dinoflagellate productivity seem to follow low latitude insolation with the highest productivities coinciding with maximum february insolation 47 38 ka at millennial scales cyst accumulation rates appeared to coincide with dansgaard oeschger d o variability with significant increments occurring during warm interstadials the opposite was true during stadials short periods of high nutrient availability and stratified conditions followed heinrich events h4 h5 h5a and h6 and concurred with enhanced river runoff spectral analyses confirm the existence of these and other higher frequency periodicities and support the idea of a tightly coupled terrestrial marine and tropical high latitude climate system during the last glacial period

crustal structure of the southeast georgia embayment carolina trough preliminary results of a composite seismic image of a continental suture and a volcanic passive margin new deep penetration multichannel seismic reflection data combined with refraction results and magnetics modeling support a hypothesis that the carolina trough is a mesozoic volcanic passive margin exhibiting a seaward dipping wedge and associated underplating the structure of carolina platform continental crust is consistent with the late paleozoic continental collision that produced the appalachians but imbrication has had no obvious effect on shallower structures produced by mesozoic extension and volcanism the origin of prominent magnetic anomalies crossing the southeast georgia embayment can be explained by processes attending mesozoic separation of africa and north america and is not related to a paleozoic continental suture as previously postulated

a 0 6 million year record of millennial scale climate variability in the tropics a 600 kyr long scanning x ray fluorescence record of redox variability from the cariaco basin venezuela provides insight into rapid climate change in the tropics over the past five glacial interglacial cycles variations in the sediment accumulation of the redox sensitive element molybdenum mo can be linked to changes in intertropical convergence zone migration and reveal that millennial scale variability is a persistent feature of tropical climate over the past 600 kyr including during periods of interglacial warmth this new record supports the idea that high frequency tropical climate variability is not controlled solely by ice volume changes with implications for the role of high latitude forcing of intertropical convergence zone position and tropical hydrology on millennial timescales

variability in the mean latitude of the atlantic intertropical convergence zone as recorded by riverine input of sediments to the cariaco basin venezuela abstract changes in the strength and position of the intertropical convergence zone itcz are an important component of climate variability in the tropical atlantic the cariaco basin located on the northern margin of venezuela is sensitive to tropical atlantic climate change and its sediments provide a record of past itcz behavior today the cariaco basin experiences two distinct seasons that reflect the annual migration of the atlantic itcz between january and march when the itcz lies south of the equator northeasterly trade winds sit directly over cariaco basin and strong coastal upwelling and dry conditions dominate beginning in june july as the itcz moves north local rainfall reaches a maximum and the upwelling diminishes or disappears here we summarize new and previously published data on the river derived terrigenous fraction of cariaco basin sediments as well as comparisons to other paleoclimate records which together suggest a coherent climatologic response in the tropical atlantic triggered by a pattern of itcz migration that mimics the seasonal cycle during periods of cooler north atlantic ssts on time scales ranging from the little ice age to the younger dryas to the cold stadials of the last glacial decreased detrital delivery to cariaco basin from local rivers suggests a southward shift in the mean latitudinal position of the itcz during warm interstadials and periods of holocene and deglacial warmth northward shifts in itcz position and its belt of convective rainfall are inferred from increased detrital delivery to the basin whether the rapid shifts in itcz position and precipitation recorded by cariaco basin sediments and other regional records reflect a response to forcing originating in the high latitude atlantic or to forcing potentially sourced in the tropics is a key question yet to be fully answered

influence of okhotsk sea ice extent on atmospheric circulation an atmospheric general circulation model was used to investigate the influence of okhotsk sea ice on large scale atmospheric circulation significant differences in the model responses between heavy and light ice cover are evident not only around the sea of okhotsk but also downstream towards north america in the form of a stationary wavetrain in the troposphere because the wave activity flux associated with the wavetrain emanates downstream from the sea of okhotsk this remote response is regarded as a stationary rossby wave excited thermally through an anomalous surface heat flux over the sea of okhotsk the wavetrain is thermally reinforced downstream by the anomalous heat flux associated with wind anomalies over the ocean surface which are induced by the wavetrain itself

on the three dimensional propagation of stationary waves abstract a locally applicable nonzonally averaged conservation relation is derived for quasi geostrophic stationary waves on a zonal flow a generalization of the eliassen palm relation the flux which appears in this relation constitutes it is argued a useful diagnostic of the three dimensional propagation of stationary wave activity this is illustrated by application to a simple theoretical model of a forced rossby wave train and to a northern hemisphere winter climatology results of the latter procedure suggest that the major forcing of the stationary wave field derives from the orographic effects of the tibetan plateau and from nonorographic effects diabatic heating and or interaction with transient eddies in the western north atlantic and north pacific oceans and siberia no evidence is found in the data for wave trains of tropical origin forcing by the orographic effects of the rocky mountains seems to be of secondary importance

millennial scale sea surface temperature variability in the western tropical north atlantic from planktonic foraminiferal census counts planktonic foraminiferal census counts are used to construct high resolution sea surface temperature sst and subsurface thermocline temperature records at a core site in the tobago basin lesser antilles the record is used to document climatic variability at this tropical site in comparison to middle and high latitude sites and to test current concepts of cross equatorial heat transports as a major player in interhemispheric climate variability temperatures are estimated using transfer function and modern analog techniques glacial maximum cooling of 2 5 3 c is indicated maximum cooling by 4 c is inferred for isotope stage 3 the sst record displays millennial scale variability with temperature jumps of up to 3 c and closely tracks the structure of ice core dansgaard oeschger cycles sst variations in part of the record run opposite to the sst evolution at high northern latitude sites pointing to thermohaline circulation and marine heat transport as an important factor driving sst in the tropical and high latitude atlantic both on orbital and suborbital timescales

seismic velocity structure of the crust and shallow mantle of the central and eastern united states by seismic surface wave imaging seismic surface waves from the transportable array of earthscope s usarray are used to estimate phase velocity structure of 18 to 125 s rayleigh waves then inverted to obtain three dimensional crust and upper mantle structure of the central and eastern united states ceus down to 200 km the obtained lithosphere structure confirms previously imaged ceus features e g the low seismic velocity signature of the cambrian reelfoot rift and the very low velocity at 150 km depth below an eocene volcanic center in northwestern virginia new features include high velocity mantle stretching from the archean superior craton well into the proterozoic terranes and deep low velocity zones in central texas associated with the late cretaceous travis and uvalde volcanic fields and beneath the south georgia rift which contains jurassic basalts hot spot tracks may be associated with several imaged low velocity zones particularly those close to the former rifted laurentia margin

modeling the water masses of the atlantic ocean at the last glacial maximum 1 we produced gridded monthly sea surface boundary conditions for the atlantic ocean at the last glacial maximum lgm based on the sea surface temperature reconstruction of the glamap project we used an ocean general circulation model ogcm subject to these sea surface boundary conditions and a corresponding wind stress field from an atmospheric general circulation model to study the differences in the distribution of the main water masses between the lgm and the present our global ogcm is characterized by high vertical resolution low vertical diffusion and isopycnal mixing and hence allows for a realistic representation of the hydrology and circulation of the modern atlantic ocean according to a series of lgm experiments with an increasing sea surface salinity anomaly in the weddell sea the ventilated thermocline was colder than today by 2 3 c in the north atlantic ocean and in the experiment with the largest anomaly 1 0 beyond the global anomaly by 4 5 c in the south atlantic ocean its depth was reduced by 50 m on average most notably in the tropics in the north atlantic ocean the outcrop locations of the thermocline isopycnal surfaces migrated southward by 5 10 and the ventilation increased in the south atlantic ocean the mixed layer and thermocline water masses were dominated by cold water originating from drake passage and the import of warm water from the indian ocean was reduced to about 4 sv or 40 of its modern value antarctic intermediate water was colder by 3 4 c and could be traced as far as 10 n the meridional overturning rates of north atlantic deep water nadw and antarctic bottom water aabw in the atlantic ocean were similar to those of the present day experiment 9 10 sv and 4 sv respectively however nadw cooled by 2 5 c and aabw by 1 c aabw was near the freezing point of seawater at the surface and the saltiest water mass in the atlantic ocean even saltier than nadw we show that the differences between the lgm and the present day experiments can be traced back to the changes in the subpolar and interhemispheric sea surface density gradients

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

climate induced variations in productivity and planktonic ecosystem structure from the younger dryas to holocene in the cariaco basin venezuela a high resolution molecular organic geochemical study of sediments in the anoxic cariaco basin indicates significant changes in primary productivity and planktonic community structure associated with the transition from the younger dryas to the holocene variations in climate conditions over the past 12 14c kyr have induced large scale changes in upwelling intensity which directly affected levels of primary productivity as reflected in accumulation rates of bulk productivity proxies concentrations and accumulation rates of sterol and alkenone biomarkers have been used to identify how productivity changes affected the structure of the planktonic ecosystem a shift in the dominant primary producer from diatoms younger dryas to coccolithophores holocene is identified if productivity and ecosystem variations like those identified in the tropical upwelling zone of the cariaco basin region occur throughout the tropical oceans they have the potential to affect global climate through perturbations in the biogeochemical cycle of carbon

the acceleration of oceanic denitrification during deglacial warming the marine nitrogen cycle was altered during the transition from glacial to interglacial conditions an analysis of 15n records throughout the world s oceans suggests that rates of denitrification in the water column accelerated during the last deglaciation

glacial interglacial variations in production and nitrogen fixation in the cariaco basin during the last 580 kyr the effect of sea level change on nutrient supply to the anoxic cariaco demonstrates the fundamental importance of nitrogen n2 fixation and phosphate to oceanic production as n2 fixation produces biomass of low 15n and has been reported to be an important component of the nitrogen cycle in the modern cariaco basin we propose that it contributes to the light interglacial 15n 2 3 values observed in the ocean drilling program odp site 1002 sediment record during the glacials the sediments are bioturbated oxic conditions with low total organic carbon toc contents and sedimentary 15n values of 5 suggesting that nitrogen n2 fixation contributed little to the n nutrition of cariaco surface waters the most plausible explanation for the inferred glacial interglacial changes in n2 fixation in the cariaco is that they have occurred in response to variations in the n p ratio of the nutrient supply driven by changes in denitrification

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

large fluctuations of dissolved oxygen in the indian and pacific oceans during dansgaard oeschger oscillations caused by variations of north atlantic deep water subduction a s and s h were supported by the paleoclimate program of the national science foundation as part of the paleovar project atm 0602395

deglacial nitrogen isotope changes in the gulf of mexico evidence from bulk sedimentary and foraminifera bound nitrogen in orca basin sediments 1 constraining variations in marine n2 fixation over glacial interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate yet paleo records from n2 fixation regions are sparse here we present new nitrogen isotope 15n records of bulk sediment and foraminifera test bound fb nitrogen extending back to the last ice age from the oligotrophic gulf of mexico gom previous studies indicate a substantial terrestrial input during the last ice age and early deglacial for which we attempt to correct the bulk sediment 15n using its observed relationship with the c n ratio both corrected bulk and fb 15n reveal a substantial glacial to holocene decrease of 15n toward holocene values of around 2 5 similar to observations from the caribbean this 15n change is most likely due to a glacial to holocene increase in regional n2 fixation a deglacial peak in the fb 15n of thermocline dwelling foraminifera orbulina universa probably reflects a whole ocean increase in the 15n of nitrate during deglaciation the 15n of the surface dwelling foraminifera globigerinoides ruber and the corrected bulk 15n show little sign of this deglacial peak both decreasing from last glacial values much earlier than does the 15n of o universa this may indicate that g ruber and bulk n reflect the euphotic zone signal of an early local increase in n2 fixation our results add to the evidence that during the last ice age the larger iron input from dust did not lead to enhanced n2 fixation in this region rather the glacial to holocene decrease in 15n is best explained by a response of n2 fixation within the atlantic to the deglacial increase in global ocean denitrification

the deglacial history of the western tropical atlantic as inferred from high resolution stable isotope records off northeastern brazil abstract the western south atlantic boundary currents represent a sensitive system within the global thermohaline circulation thc we investigated the impact of deglacial thc changes on the western tropical atlantic studied in six high resolution sediment cores from the upper continental slope of brazil the stratigraphy of the cores is mainly based on 14c ams dating of monospecific foraminiferal samples changes in the upper layer tropical ocean during the deglaciation are inferred from stable oxygen isotope measurements on planktic and benthic foraminifera variations in the 18o residuals are assumed to be mainly temperature related during the oldest and younger dryas cooling periods two major deglacial thc disturbances are reported from north atlantic sediment cores concomitant to the repeated thc slowdown we observe an upper layer warming in the tropical ocean a reduced northward heat export from the tropical areas during these periods weak north brazil current is additionally reflected by low meridional gradients in the stable oxygen records this generally agrees with results from coupled ocean atmosphere models

the influence of rapid millennial scale climate change on nitrogen isotope dynamics of the cariaco basin during marine isotope stage 3 understanding changes to the marine nitrogen cycle on millennial and shorter time scales can help determine the influence of rapid climate change on the fixed n pool and its sources and sinks rapid changes in denitrification have been observed in the eastern tropical north pacific etnp and arabian sea however millennial scale 15n records in regions influenced by n2 fixation are sparse we present a sedimentary 15n record from the cariaco basin during marine isotope stage mis 3 35 55 ka the 15n record displays a pattern of millennial scale variability that tracks the greenland ice core dansgaard oeschger cycles with higher values observed during interstadial periods lower values during stadial periods and abrupt transitions in between conditions during interstadials are similar to those at present in the cariaco basin with the sedimentary 15n signal reflecting a combination of local processes and an imported regional signal if interpreted to reflect regional processes the interstadial 15n values average 5 1 support the argument that n2 fixation did not increase in the tropical north atlantic during the last glacial the lower 15n values during stadials when lower sea level resulted in increased physical isolation of the basin can be explained primarily by local processes in spite of the importance of local processes striking similarity is observed between the cariaco record and millennial scale 15n records from the etnp and arabian sea the apparent synchronicity of changes observed in all three regions suggests an atmospheric teleconnection between the three sites and high latitude climate forcing during mis 3

evidence for a prolonged retroflection of the north brazil current during glacial stages the importance of the role played by the tropics in driving and propagating climate change between hemispheres has long been the focus of attention in a bid to evaluate ocean atmosphere interactions on glacial interglacial timescales the amazon fan and ceara rise in the western equatorial atlantic ocean lie directly in the flowpath of the north brazil current nbc which as a conduit for the cross equatorial transport of heat and salinity is a key component for the heat budget of the north atlantic mg ca palaeothermometry and stable oxygen isotope analysis of planktonic foraminifera sampled from 15 sites across the amazon fan and ceara rise reveal variations in oceanic surface currents and of the dispersal of freshwater from the river amazon over five timeslices modern early holocene younger dryas late glacial and marine isotope stage 3 sea surface temperature reconstructions reveal progressive climatic amelioration over the last similar to 30 ka indicating a temperature increase of similar to 3 2 1 1 degrees c since the late glacial in conjunction with this warming values of delta delta o 18 a proxy for water column stratification indicates increased vertical mixing of the glacial ocean the spatial distribution of values of delta 18 w the isotopic composition of ambient seawater is used to infer surface current variations and demonstrates an oceanward shift in the river outflow plume in cold climates representing a prolongation or possibly permanent continuation in the duration of the seasonal retroflection of the nbc causing the curtailment of cross equatorial transport a prolongation of this retroflection could have resulted from a mean southward migration of the southern boundary of the itcz c 2011 elsevier b v all rights reserved

nitrogen isotope and productivity variations along the northeast pacific margin over the last 120 kyr surface and subsurface paleoceanography 1 glacial interglacial changes in sedimentary 15n over the last 120 kyr display a remarkably similar pattern in timing and amplitude in core records extending from the denitrification zone in the eastern tropical north pacific etnp where subsurface denitrification is active to the oregon margin where no denitrification occurs today low 15n values 4 6 generally characterize glacial stages 2 and 4 and higher 15n values 7 10 are representative of the holocene millennial scale periods within stage 3 and stage 5 the inferred synchroneity of 15n variations along the entire margin implies that the nitrate isotopic signal produced in the oxygen poor subsurface waters in the etnp is rapidly advected northward and recorded at sites far beyond the boundaries of the modern denitrification zone similar to 15n primary production indicators percent corg ba al and percent opal show glacial interglacial as well as millennial scale variations along the ne pacific margin with higher primary production during warm periods however the relative phasing between 15n and paleoproduction tracers within individual records changes latitudinally whereas 15n and primary production vary approximately synchronously in the midlatitudes production lags 15n in the etnp by several kiloyears this lag calls for a new understanding of the processes driving denitrification in the etnp we suggest that oxygen input by the equatorial undercurrent as well as local organic matter flux controls denitrification rates in the etnp moreover the differences in relative timing point to a time transgressive development of upwelling favorable winds along the ne pacific margin after the last glaciation with those in the north developing several kiloyears earlier

the dynamics of the marine nitrogen cycle across the last deglaciation 1 we use a geochemical box model to investigate the changes in marine n fixation and denitrification required to match the observed sedimentary 15n changes between 30 kyr b p and the late holocene this is achieved by optimizing a set of seven parameters that describe the strengths of three ocean internal n feedbacks and the response of the oceanic n cycle to four external forcings scenarios that best match the 15n constraints indicate a strong transient decrease in n fixation in the early deglacial in response to the decrease in iron input by dust around 15 kyr b p n fixation rebounds primarily in response to an abrupt increase in water column denitrification caused by an expansion of anoxia benthic denitrification is not well constrained by our model but tends to increase in sync with water column denitrification as a result of the transient imbalance between n fixation and denitrification we infer a glacial to interglacial decrease in the marine n inventory of between 15 and 50 the model diagnoses this reduction in order to simultaneously fit the data from all ocean basins requiring it to reduce the degree by which water column denitrification in the oxygen minimum zones is influencing the 15n of nitrate of the whole ocean dilution effect our optimal solution suggests a glacial n cycle that operated at nearly the same rates as that in pre industrial times but sensitivity cases with substantially lower rates fit the data only marginally worse an important caveat of our study is the assumption of an unchanging ocean circulation an initial sensitivity experiment shows that this affects primarily the magnitude of the change in the n inventory while the diagnosed deglacial dynamics with global marine n fixation taking a dip before the onset of denitrification remains a robust result

pollen and spores date origin of rift basins from texas to nova scotia as early late triassic palynological studies of the nonmarine newark supergroup of eastern north america and of rift basins in the northern gulf of mexico facilitate correlation with well dated marine sections of europe new information emphasizes the chronological link between the newark basins and a gulf of mexico basin and their common history in the rifting of north america from pangea shales from the subsurface south georgia basin are shown to be of late karnian age early late triassic the known time of earliest sedimentation in the culpeper basin is extended from norian late late triassic to mid karnian and the date of earliest sedimentation in the richmond and deep river basins is moved to at least earliest karnian perhaps ladinian the subsurface eagle mills formation in texas and arkansas has been dated palynologically as mid to late karnian the oldest parts of the newark supergroup and the eagle mills formation mostly began deposition in precursor rift basins that formed in ladinian to early karnian time in the southern newark basins sedimentation apparently ceased in late karnian but continued in the northern basins well into the jurassic until genesis of the atlantic ended basin sedimentation

surface and deep ocean variability in the northern sargasso sea during marine isotope stage 3 new geochemical proxy data from bermuda rise piston cores reveal ocean and climate conditions in the northern sargasso sea during marine isotope stage 3 using 18o on the planktonic foraminifer globigerinoides ruber we can correlate explicitly with every stadial interstadial change in greenland ice between 32 and 58 ka these data suggest repetitive changes of 4 c in the annual average sea surface temperature sst with maximum cooling comparable to or greater than sst during glacial maximum conditions the extent of sst depression is about the same for typical stadial events and for heinrich events 4 and 5 which we have identified on the bermuda rise by traces of ice rafting benthic foraminiferal 13c decreases during every stadial event consistent with reduced production of the deepest component of north atlantic deep water and shoaling of its interface with antarctic bottom water this interpretation is supported by benthic cd ca data from the climate cycle associated with interstadial 8

nitrate isotope anomalies reflect n2 fixation in the azores front region subtropical ne atlantic 1 the subtropical northeast atlantic has previously been identified as a marine environment with an apparent imbalance between low nitrate supply to the surface and concurrent high export production to better constrain the sources and fluxes of mixed layer nitrate and to assess the potential role of n2 fixation in providing new nitrogen n we investigated the depth distribution of nitrate 15n and 18o at six stations across the azores front in the ne atlantic in addition we measured the 15n of dissolved organic n don in surface waters and of sinking particulate n collected in sediment traps at 2000 m depth between 2003 and 2005 at station kiel276 the nitrate isotope profiles at the majority of the hydrographic stations displayed a decrease in the 15n from depth toward low nitrate surface waters concomitant with an increase in 18o given that nitrate uptake by phytoplankton leads to a proportional increase in nitrate 15n and 18o the observed surface water nitrate isotope anomalies 15 18 up to 6 indicate that nitrate assimilation is not the sole process controlling the isotopic composition of nitrate in the photic zone and implicate a significant addition of newly fixed n that is remineralized in surface and subsurface waters both the concentration of don and its 15n in surface water were spatially invariant showing mean values of 4 7 0 5 mol l 1 and 2 6 0 4 n 35 respectively supporting the conjecture of a mostly recalcitrant don pool the weighted biannual mean 15n of sinking particulate n 1 8 0 8 n 33 was low with respect to thermocline nitrate the anomalous dual nitrate isotope signatures together with the low 15n of export production and elevated nitrate to phosphate ratios in surface and subsurface waters strongly suggest that n2 fixation represents a substantive source of n in this part of the subtropical northeast atlantic simple isotope mass balance suggests that locally n2 fixation supplies between 56 and 259 mmol n m 2 a 1 for phytoplankton growth in the photic zone accounting for up to 40 of the estimated export production

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

sea level changes forced ice breakouts in the last glacial cycle new results from coral terraces abstract sea level cycles recorded by coral terraces at huon peninsula hp in papua new guinea match rapid climate changes that occurred between 30 000 and 65 000 years ago seen in greenland ice cores and marine sediments each cycle of 6000 7000 years ended with a sea level rise of 10 15 m lasting 1000 2000 years following a longer period of falling sea level precise dating shows that each rise corresponds to a heinrich episode of ice rafted detritus in north atlantic signalling massive ice outbreak from north america sea level may be the trigger that forced near synchronous breakouts of ice from north america and eastern greenland the hp sea level changes also correspond to similar cycles in benthic oxygen isotopes reported from the north atlantic which are too large to be explained entirely in terms of ice volumes jointly the sea level and isotope records suggest that the north atlantic deep ocean cooled as sea level fell and warmed as sea level rose in each 6000 year cycle

links between tropical rainfall and north atlantic climate during the last glacial period the last glacial period was marked by dramatic climate fluctuations sediment records from the cariaco basin and the arabian sea suggest that cooling in the north atlantic region was tightly coupled with a southward displacement of the intertropical convergence zone and a weakening of the indian summer monsoon

preserved extent of jurassic flood basalt in the south georgia rift a new interpretation of the j horizon at the end of the triassic 200 m y ago the central atlantic magmatic province camp one of the largest igneous provinces in the world was emplaced within a very short period of time the flows sills and dikes that mark the event are predominantly preserved in triassic rift basins along the atlantic margins conventional wisdom implies that the areally largest of the camp flows is preserved in the south georgia rift a triassic rift basin buried beneath the atlantic coastal plain the extent of this flow has been mapped on the basis of a prominent seismic reflection referred to as the j horizon this seismic horizon has been used as a time marker for estimating the end of rifting in the southern united states and the beginning of seafloor spreading reanalysis of existing well and seismic data however shows that the extent of the flood basalt is limited to a few areas and that the j horizon coincides with the base of the coastal plain this reopens the question of how the camp relates to the rift drift transition of eastern north america

millennial scale variability during the last glacial the ice core record the oxygen isotope records from greenland ice cores show a very strong reproducible pattern of alternation between warm greenland interstadials gi and cold greenland stadials gs at millennial scale during the last glacial period here we summarise what is known about this variability from ice core records the typical cycle has a sawtooth pattern with a very rapid warming event occurring in a few decades a slow cooling trend and then a final fast cooling 25 such events have been numbered in the last glacial the recent gicc05 age scale provides the best available age scale that can be directly applied to this stratigraphy and we summarise the timing of the warming events and the length and strength of each event the greenland stratigraphy can be transferred to other records if we make assumptions about the contemporaneous nature of rapid events in different archives other parameters such as the snow accumulation rate and the concentration of terrestrial dust and sea salt recorded in the greenland cores also show a strong contrasting pattern between gi and gs methane concentrations are generally high during gi and lower during gs with the increase from gs to gi occurring within a century antarctic ice cores show a different pattern each gi has an antarctic counterpart but antarctica appears to warm while greenland is in a gs and cool during gi these changes are consistent with a mechanism involving ocean heat transport but the rapid nature of warmings poses a challenge for modellers while the rapid methane changes pose questions about the pattern of land biosphere emissions during the glacial that are also relevant for understanding glacial interglacial methane variability

changes in atmospheric circulation and ocean ice cover over the north atlantic during the last 41 000 years high resolution continuous multivariate chemical records from a central greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41 000 years these chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to greenland defined here as the polar circulation index pci and in the extent of ocean ice cover massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in pci during stadials without discharge events ocean ice cover appears to reach some common maximum level the massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

dominant northern hemisphere climate control over millennial scale glacial sea level variability abstract based on a radiocarbon and paleomagnetically dated sediment record from the northern red sea and the exceptional sensitivity of the regional changes in the oxygen isotope composition of sea water to the sea level dependent water exchange with the indian ocean we provide a new global sea level reconstruction spanning the last glacial period the sea level record has been extracted from the temperature corrected benthic stable oxygen isotopes using coral based sea level data as constraints for the sea level oxygen isotope relationship although the general features of this millennial scale sea level records have strong similarities to the rather symmetric and gradual southern hemisphere climate patterns we observe in constrast to previous findings pronounced sea level rises of up to 25 m to generally correspond with northern hemisphere warmings as recorded in greenland ice core interstadial intervals whereas sea level lowstands mostly occur during cold phases corroborated by climber 2 model results the close connection of millennial scale sea level changes to northern hemisphere temperature variations indicates a primary climatic control on the mass balance of the major northern hemisphere ice sheets and does not require a considerable antarctic contribution

controls on sedimentary nitrogen isotopes along the chile margin chilean margin sedimentary n isotope records have been the focus of paleoceanographic studies examining the extent of water column denitrification in the eastern south pacific in the past here we use 15n 14n of nitrate and surface sedimentary n along the chilean coast to investigate the relative contributions of water column denitrification and surface nitrate assimilation by phytoplankton to the sedimentary n isotope record off northern and central chile subsurface enrichment of 15no3 is associated with the lowest oxygen concentrations and maximum nitrate deficits the product of water column denitrification locally at 21 s and through the transport of denitrified waters to the south while elevated the 15n of pycnocline nitrate shows no distinct trend with distance from the omz and is nearly homogenous within the shallow subsurface layer presumably due to lateral circulation along the margin moreover an isotopically depleted and relatively uniform layer exists within the shallow subsurface possibly as a result of the remineralization of newly fixed nitrogen that may work to further homogenize the 15n of the upwelling nitrate whereas the high 15n of sedimentary n in the region is clearly a product of denitrification and its isotopic imprint on nitrate along the margin the northward increase in sedimentary 15n from higher southern latitudes also reflects the degree of surface layer nitrate consumption by phytoplankton the northward increase in sedimentary 15n corresponds to a regional decrease in the surface nitrate concentration and isotopic fractionation during nitrate assimilation is apparent in shallow nitrate 15n a comparison of the 15n in shallow subsurface nitrate and sedimentary n suggests that north of 30 s nearly complete nitrate consumption causes the 15n of sediments to converge on that of the nitrate supply such that denitrification should be the dominant signal in downcore 15n records from these latitudes moreover the lateral homogeneity of the denitrification signal in the subsurface within this region suggests that such records will provide robust reconstructions of denitrification intensity

a high resolution late quaternary upwelling record from the anoxic cariaco basin venezuela results are presented of a high resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the cariaco basin in the southern caribbean sea the cariaco basin is a small anoxic marine basin on the northern continental margin of venezuela in an area today characterized by both seasonal trade wind induced upwelling and pronounced dry and wet seasons our data indicate that large changes in the intensity of upwelling and hence trade wind strength occurred in this region during the last glacial interglacial transition and throughout the holocene during the last glacial lowstand of sea level the cariaco basin was effectively isolated from the open caribbean along its northern margin by the then largely emergent tortuga bank oxic conditions existed in the deep cariaco basin at this time and surface productivity was low about 12 600 years ago the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the laurentide ice sheet into the gulf of mexico strong upwelling between 12 600 and about 10 000 years ago may be related to intensified trade winds resulting in part from cooler sea surface temperatures in the caribbean and gulf of mexico after about 10 000 years ago upwelling intensity was reduced though highly variable a preliminary frequency domain analysis of the holocene portion of the cariaco basin time series suggests that solar forcing may explain a significant component of the century scale variability observed in the record of upwelling and trade wind strength