subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

error analysis of paleosalinity calculations a detailed error analysis is performed for the calculation of open ocean salinity changes from carbonate oxygen 18 proxy data measurement errors are generally negligible however errors due to the goodness of fit of various regressions used in the calculation are significant depending on the method used and on the proxies available error bars 1 for the salinity range from 0 6 to 1 8 in particular expected errors for the midlatitude and tropical salinity differences at the last glacial maximum are around 1 1 to 2 5 the largest errors occur because of the differences between temporal and spatial seawater oxygen 18 salinity relationships and through the use of nonconcurrent temperature measurements methods of improving paleosalinity estimates should focus on eliminating these large sources of uncertainty

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

salinity bias on the foraminifera mg ca thermometry correction procedure and implications for past ocean hydrographic reconstructions 1 mg ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures here we show however that the mg ca temperature relationship of the planktonic species globigerinoides ruber is significantly affected by seawater salinity with a 1 psu change in salinity resulting in a 1 6 c bias in mg ca temperature calculations if not accounted for such a bias could lead for instance to systematic overestimations of mg ca temperatures during glacial periods when global ocean salinity had significantly increased compared to today we present here a correction procedure to derive unbiased sea surface temperatures sst and 18osw from g ruber tmg ca and 18of measurements this correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the last glacial maximum lgm in the western pacific warm pool while uncorrected tmg ca data indicate a 3 c warming of the western pacific warm pool since the lgm the salinity corrected sst result in a stronger warming of 4 c

systematic change of foraminiferal mg ca ratios across a strong salinity gradient abstract the mg ca ratio of foraminiferal calcite is an important proxy for estimating past ocean temperatures used in conjunction with 18 o of foraminiferal calcite it allows deconvolution of temperature and ice volume signals to infer past ocean temperatures and salinities assuming the relationship between seawater 18 o and salinity is known such work assumes that temperature is the only or at least the dominant environmental controller of foraminiferal mg ca the semi enclosed mediterranean sea where salinity varies from 36 to 40 psu over a seasonal temperature range of between only 5 c to 8 c provides a relevant setting to test this assumption outside the laboratory in this study planktonic foraminifera o universa g siphonifera g bulloides and g ruber white and pink were picked from 11 box core tops spanning the mediterranean salinity gradient and analysed for their trace element concentrations mg ca ratios are higher for the associated calcification temperatures than in other regions where calibrations have been conducted and correlate poorly with calcification temperature mg ca ratios are particularly high for samples from the eastern mediterranean where salinity is unusually high correlations of mg ca with the calcification salinity are statistically significant with mg ca changing by 15 59 per psu suggesting that salinity may act as a control on mg ca ratios in addition to the dominant temperature control we show that contamination by non carbonate material and diagenetic high mg carbonate overgrowths cannot account for the observed trend of increasing mg ca with salinity a relationship between mg ca and salinity is also suggested by re analysis of calibrations from open ocean settings these new mediterranean results are from a region with unusually high salinity but suggest that the effects of salinity on the mg ca palaeothermometer should be considered even in open ocean settings particularly where large salinity changes occurred in the past

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

persistent positive north atlantic oscillation mode dominated the medieval climate anomaly the medieval climate anomaly mca was the most recent pre industrial era warm interval of european climate yet its driving mechanisms remain uncertain we present here a 947 year long multidecadal north atlantic oscillation nao reconstruction and find a persistent positive nao during the mca supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker nao conditions into the little ice age lia globally distributed proxy data suggest that this nao shift is one aspect of a global mca lia climate transition that probably was coupled to prevailing la nina like conditions amplified by an intensified atlantic meridional overturning circulation during the mca

determination of radiogenic isotopes rbsr smnd and pbpb in fluid inclusion waters an example from the bluebell pb zn deposit british columbia canada abstract rb sr pb nd and sm isotopes were determined in fluid inclusion waters extracted from quartz samples of the bluebell pb zn deposit british columbia the results demonstrate that radiogenic isotopes in fluid inclusion waters can constitute useful geochronological and genetic tools when applied in conjunction with geological geochemical and other geochronological studies rb sr isotopes of fluid inclusion waters and of mineralized marbles hosting the deposit indicate a tertiary mineralization age of 19 2 5 9 ma and an initial 87 sr 86 sr ratio of 0 7143 this rb sr date is in general agreement with a cenozoic age suggested by geologic field relations described by irvine 1957 and fyles 1967 pb isotope data suggest two isotopically distinct fluids one less radiogenic and similar to values measured on galenas and the other more radiogenic this is in agreement with ohmoto and rye s 1970 hypothesis of two meteoric hydrothermal fluids and lecouteur s 1973 convection cell model nd and sm isotope also suggest mixing of fluids both pb pb and nd sm data suggest very old lower and upper crustal materials as possible source of the metals

wobbly ocean conveyor circulation during the holocene abstract compared to the last glacial period the climate of the holocene has been quite stable however the record does suggest that at high latitudes in the north temperatures have cooled at least since the middle holocene and that this cooling has been modulated by millennial duration fluctuations that culminated in the medieval warm little ice age oscillation in this paper we explore the possibility that these variations were associated with concurrent changes in the strength of the atlantic s conveyor circulation three lines of evidence are considered one involves records that relate directly to ocean operation a second involves the tie between sea ice extent in the northern atlantic and the extent of mountain glaciers in the european alps and in the cordillera of western north america and a third involves evidence for a bipolarity in climate changes in the northern and southern polar regions in addition we look into clues provided by the atlantic multidecadal oscillation amo although at this point no firm conclusion can be drawn the case for ocean involvement is sufficiently strong to merit further attention

an ocean model s response to north atlantic oscillation like wind forcing the response of the atlantic ocean to north atlantic oscillation nao like wind forcing was investigated using an ocean only general circulation model coupled to an atmospheric boundary layer model a series of idealized experiments was performed to investigate the interannual to multi decadal frequency response of the ocean to a winter wind anomaly pattern overall the strength of the sst response increased slightly with longer forcing periods in the subpolar gyre however the model showed a broad response maximum in the decadal band 12 16 years

progress in paleosalinity overview and presentation of a new approach this paper concerns the use of stable oxygen isotope ratios 18o and stable hydrogen isotope ratios 2h or d to derive paleosalinity first i evaluate the potential and limitations of existing methods based on 18o and d separately next i propose a new theoretical framework for the combined use of 18o and d to constrain the impact of the hydrological cycle on the surface waters and to thus characterize changes in surface water salinity this new method is accompanied by an error propagation exercise to demonstrate its limitations and areas of potential improvement a feasible strategy is outlined for achieving reconstructions of paleosalinity change with an uncertainty of 1 practical salinity unit or less with the newly proposed method especially in regions with high deuterium excess values

gulf stream density structure and transport during the past millennium the gulf stream transports approximately 31 sv 1 sv 10 6 m 3 s 1 of water and 1 3 10 15 w of heat into the north atlantic ocean the possibility of abrupt changes in gulf stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries given the limited length of the instrumental record our knowledge of gulf stream behaviour on long timescales must rely heavily on information from geologic archives here we use foraminifera from a suite of high resolution sediment cores in the florida straits to show that the cross current density gradient and vertical current shear of the gulf stream were systematically lower during the little ice age ad 1200 to 1850 we also estimate that little ice age volume transport was ten per cent weaker than today s the timing of reduced flow is consistent with temperature minima in several palaeoclimate records implying that diminished oceanic heat transport may have contributed to little ice age cooling in the north atlantic the interval of low flow also coincides with anomalously high gulf stream surface salinity suggesting a tight linkage between the atlantic ocean circulation and hydrologic cycle during the past millennium

1400 yr multiproxy record of climate variability from the northern gulf of mexico a continuous decadal scale resolution record of climate variability over the past 1400 yr in the northern gulf of mexico was constructed from a box core recovered in the pigmy basin northern gulf of mexico proxies include paired analyses of mg ca and 18 o in the white variety of the planktic foraminifer globigerinoides ruber and relative abundance variations of g sacculifer in the foraminifer assemblages two multi decadal intervals of sustained high mg ca indicate that gulf of mexico sea surface temperatures ssts were as warm or warmer than near modern conditions between 1000 and 1400 yr b p foraminiferal mg ca during the coolest interval of the little ice age ca 250 yr b p indicate that sst was 2 2 5 c below modern sst four minima in the mg ca record between 900 and 250 yr b p correspond with the maunder sporer wolf and oort sunspot minima suggesting a link between changes in solar insolation and sst variability in the gulf of mexico an abrupt shift recorded in both 18 o calcite and relative abundance of g sacculifer occurred ca 600 yr b p the shift in the pigmy basin record corresponds with a shift in the sea salt sodium ssna record from the greenland ice sheet project 2 ice core linking changes in high latitude atmospheric circulation with the subtropical atlantic ocean

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

on the interannual variability of surface salinity in the atlantic the mechanisms controlling the interannual variability of sea surface salinity sss in the atlantic are investigated using a simulation with the echam4 opa8 coupled model and for comparison the ncep reanalysis and an observed sss climatology anomalous ekman advection is found to be as important as the freshwater flux in generating sss anomalies in contrast to sea surface temperature sst anomalies which are primarily caused by surface heat flux fluctuations since the surface heat flux feedback does not damp the sss anomalies but generally damps existing sst anomalies sss anomalies have a larger characteristic time scale as a result they are more influenced by the mean currents and the geostrophic variability which dominate the sss changes at low frequency over much of the basin the link between sss anomalies and the dominant patterns of atmospheric variability in the north atlantic sector is also discussed it is shown that the north atlantic oscillation generates sss anomalies much more by ekman advection than by freshwater exchanges at least in the coupled model there is little one to one correspondence between the main atmospheric and sss anomaly patterns unlike what is found for sst anomalies

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

calibration of mg ca thermometry in planktonic foraminifera from a sediment trap time series 1 paired mg ca and 18o measurements on planktonic foraminiferal species g ruber white g ruber pink g sacculifer g conglobatus g aequilateralis o universa n dutertrei p obliquiloculata g inflata g truncatulinoides g hirsuta and g crassaformis from a 6 year sediment trap time series in the sargasso sea were used to define the sensitivity of foraminiferal mg ca to calcification temperature habitat depths and calcification temperatures were estimated from comparison of 18o of foraminifera with equilibrium calcite based on historical temperature and salinity data when considered together mg ca mmol mol of all species except two show a significant r 0 93 relationship with temperature t c of the form mg ca 0 38 0 02 exp 0 090 0 003 t equivalent to a 9 0 0 3 change in mg ca for a 1 c change in temperature small differences exist in calibrations between species and between different size fractions of the same species o universa and g aequilateralis have higher mg ca than other species and in general data can be best described with the same temperature sensitivity for all species and pre exponential constants in the sequence o universa g aequilateralis g bulloides g ruber g sacculifer other species this approach gives an accuracy of 1 2 c in the estimation of calcification temperature the 9 sensitivity to temperature is similar to published studies from culture and core top calibrations but differences exist from some literature values of pre exponential constants different cleaning methodologies and artefacts of core top dissolution are probably implicated and perhaps environmental factors yet understood planktonic foraminiferal mg ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures using g ruber pink g ruber white g sacculifer n dutertrei p obliquiloculata etc the existence of a single mg thermometry equation is valuable for extinct species although use of species specific equations will where statistically significant provide more accurate evaluation of mg ca paleotemperature

temperature δ18o relationships of planktonic foraminifera collected from surface waters abstract most of the isotopic paleotemperature equations used for paleoceanographic reconstructions have been derived from culture experiments or inorganic precipitates of calcium carbonate to test these equations in the modern ocean we measured the oxygen isotope composition of planktonic foraminifera globigerinoides ruber globigerinoides sacculifer globigerina bulloides and neogloboquadrina pachyderma collected from atlantic and southern ocean surface waters and added published plankton tow data from the pacific indian and arctic oceans the resulting species specific regression equations of the temperature 18o relationships for g ruber g sacculifer and g bulloides are statistically indistinguishable the equations derived for g sacculifer and g bulloides agree with relationships obtained from laboratory experiments in which these species were cultured at ph values close to modern surface waters the equation derived from n pachyderma has a significantly lower slope and offset than the other three species but produces a regression equation that is nearly identical to the one for the epifaunal benthic foraminifer cibicides sp our work on plankton tow and pumped samples indicates that culture derived equations appear to be more appropriate for predicting the absolute 18o of the species examined compared to equations derived from inorganic precipitates however over the oceanic temperature range the slopes of the equations we derive for living species agree with the slopes obtained from inorganic precipitates

florida current surface temperature and salinity variability during the last millennium surface water d 18 ow near dry tortugas increased 0 4 during the course of the little ice age lia 1200 1850 a d equivalent to a salinity increase of 0 8 1 5 on the great bahama bank where surface waters are influenced by the north atlantic subtropical gyre d 18 ow increased by 0 3 during the last 200 years although a portion 0 1 of this shift may be an artifact of anthropogenically driven changes in surface water sco2 the remaining d 18 ow signal implies a 0 4 1 increase in salinity after 200 years b p the simplest explanation of the d 18 ow data is southward migration of the atlantic hadley circulation during the lia scaling of the d 18 ow records to salinity using the modern low latitude d 18 ow s slope produces an unrealistic reversal in the salinity gradient between the two sites only if d 18 ow is scaled to salinity using a high latitude d 18 ow s slope can the records be reconciled variable atmospheric 14 c paralleled dry tortugas d 18 ow suggesting that solar irradiance paced centennial scale migration of the inter tropical convergence zone and changes in florida current salinity during the last millennium citation lund d c and w curry 2006 florida current surface temperature and salinity variability during the last millennium paleoceanography 21 pa2009 doi 10 1029 2005pa001218

annual variability of the subtropical recirculations in the north atlantic and north pacific a topex poseidon study abstract using sea surface height data collected by geosat and topex poseidon the seasonal annual gyre circulations in the regions of the gulf stream and the kuroshio extension were studied the seasonal gyre circulation is roughly confined to the regions where the annual mean subtropical recirculations exist associated with this seasonal gyre circulation the surface transports of the kuroshio and the gulf stream are found to be maximum in the late fall and minimum in the late spring using historical data the authors demonstrated that these seasonal gyre circulations are mostly confined to the mixed layer a simple diagnostic calculation of the buoyancy balance associated with the seasonal gyre circulations shows that they are driven primarily by local buoyancy flux heating and cooling while contribution from advection by large scale ocean circulation is negligible even though the seasonal gyre circulation is primarily driven by local buoyancy forcing it is in the opposite sense to that origin

cooling of northwest atlantic slope waters during the holocene 2 greenland ice cores depict a holocene climate that was warm and stable compared to the preceding 100 kyr yet most elements of the climate system were undergoing large secular changes during the last 11 kyr summer insolation in the northern hemisphere declined to last glacial maximum lgm levels atmospheric co2 increased 20 ppmv and the carbonate ion concentration of the deep sea decreased 5 10 mmol kg broecker et al 1999 signaling a large shift in the global carbon cycle the oxygen isotope composition of atmospheric o2 an indicator of global productivity and hydrology increased 0 5 retracing roughly 50 of the non ice volume deglacial decrease bender et al 1999 the monsoons of asia and

regional climate variability in the western subtropical north atlantic during the past two millennia 1 western subtropical north atlantic oceanic and atmospheric circulations connect tropical and subpolar climates variations in these circulations can generate regional climate anomalies that are not reflected in northern hemisphere averages assessing the significance of anthropogenic climate change at regional scales requires proxy records that allow recent trends to be interpreted in the context of long term regional variability we present reconstructions of gulf stream sea surface temperature sst and hydrographic variability during the past two millennia based on the magnesium calcium ratio and oxygen isotopic composition of planktic foraminifera preserved in two western subtropical north atlantic sediment cores reconstructed sst suggests low frequency variability of 1 c during an interval that includes the medieval climate anomaly mca and the little ice age lia a warm interval near 1250 a d is distinct from regional and hemispheric temperature possibly reflecting regional variations in ocean atmosphere heat flux associated with changes in atmospheric circulation e g the north atlantic oscillation or the atlantic meridional overturning circulation seawater d 18 o which is marked by a fresher mca and a more saline lia covaries with meridional migrations of the atlantic intertropical convergence zone the northward advection of tropical salinity anomalies by mean surface currents provides a plausible mechanism linking carolina slope and tropical atlantic hydrology

the medieval climate anomaly and little ice age in chesapeake bay and the north atlantic ocean abstract a new 2400 year paleoclimate reconstruction from chesapeake bay cb eastern us was compared to other paleoclimate records in the north atlantic region to evaluate climate variability during the medieval climate anomaly mca and little ice age lia using mg ca ratios from ostracodes and oxygen isotopes from benthic foraminifera as proxies for temperature and precipitation driven estuarine hydrography results show that warmest temperatures in cb reached 16 17 c between 600 and 950 ce common era centuries before the classic european medieval warm period 950 1100 ce and peak warming in the nordic seas 1000 1400 ce a series of centennial warm cool cycles began about 1000 ce with temperature minima of 8 to 9 c about 1150 1350 and 1650 1800 ce and intervening warm periods 14 15 c centered at 1200 1400 1500 and 1600 ce precipitation variability in the eastern us included multiple dry intervals from 600 to 1200 ce which contrasts with wet medieval conditions in the caribbean the eastern us experienced a wet lia between 1650 and 1800 ce when the caribbean was relatively dry comparison of the cb record with other records shows that the mca and lia were characterized by regionally asynchronous warming and complex spatial patterns of precipitation possibly related to ocean atmosphere processes