a sulfate control on marine mid depth euxinia on the early cambrian ca 529 521 ma yangtze platform south china abstract given the high oxygen demands of complex metazoans a sound understanding of marine redox conditions is essential to reconstruct the factors that lead to the dramatic increases in ecosystem complexity and biological diversity in the early cambrian in this study we report new fe mo c s geochemical data from black shale rich strata of the early cambrian continental margin yangjiaping section in the yangtze platform south china integrated fe speciation and mo abundance data from the yangjiaping section in combination with previously reported data from shatan inner shelf songtao slope and longbizui basinal sections suggest mid depth euxinic waters coexisting with ferruginous deep waters and oxic surface waters this redox structure is similar to conditions previously suggested for the ediacaran yangtze platform and other proterozoic marine basins indicating that proterozoic layered ocean redox structure in some basins persisted into the early cambrian furthermore the fe mo data demonstrate that the mid depth euxinia was widespread during a marine transgression but shrank gradually along with widespread shelf oxygenation when the basin evolved into subsequent regression excess highly reactive iron fe hr in deep water with a hydrothermal origin may have had a control on the mid depth euxinia given geochemical and geological evidence for basinal hydrothermal activities on the other hand a sulfate limitation on bacterial sulfate reduction bsr and sulfide generation was suggested by a weak correlation r 2 0 22 between pyrite iron versus fe hr and total organic carbon toc form anoxic deep water sections and high toc averagely 6 in the deepest ferruginous water longbizui section furthermore a spatial gradient was observed in sulfur isotopic composition of pyrite averagely from 0 2 to 21 0 consistent with a lateral gradient of sulfate in concentration or and in isotope from shore to distal waters on the transgressive yangtze platform we accordingly proposed that two distinct sources riverine input and ocean reservoir provided sulfate for bsr dominantly in near shore and distal areas respectively and the euxinic retreat observed for the regression can be explained as a response to the progressive sulfate consumption of the ocean reservoir by intensive bsr and pyrite formation during preceding transgression therefore our results support a sulfate control on the spatiotemporal distribution of mid depth marine euxinia on the yangtze platform in early cambrian when complex metazoans rapidly evolved

a basin redox transect at the dawn of animal life multiple eukaryotic clades make their first appearance in the fossil record between 810 and 715 ma molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation animals require oxygen to fuel their metabolism and low oxygen levels have been hypothesized to account for the temporal lag between metazoan origins and the cambrian radiation of large ecologically diverse animals here paleoredox conditions were investigated in the fifteenmile group ogilvie mountains yukon canada which hosts an 811 ma ash horizon and spans the temporal window that captures the inferred origin and early evolution of animals iron based redox proxies redox sensitive trace elements organic carbon percentages and pyrite sulfur isotopes were analyzed in seven stratigraphic sections along two parallel basin transects these data suggest that for this basin oxygenated shelf waters overlay generally anoxic deeper waters the anoxic water column was dominantly ferruginous but brief periods of euxinia likely occurred these oscillations coincide with changes in total organic carbon suggesting euxinia was primarily driven by increased organic carbon loading overall these data are consistent with proposed quantitative constraints on proterozoic atmospheric oxygen being greater than 1 of modern levels but less than present levels comparing these oxygen levels against the likely oxygen requirements of the earliest animals both theoretical considerations and the ecology of modern oxygen deficient settings suggest that the inferred oxygen levels in the mixed layer would not have been prohibitive to the presence of sponges eumetazoans or bilaterians thus the evolution of the earliest animals was probably not limited by the low absolute oxygen levels that may have characterized neoproterozoic oceans although these inferred levels would constrain animals to very small sizes and low metabolic rates

relationships between extratropical sea level pressure variations and the central pacific and eastern pacific types of enso abstract this study examines the linkages between leading patterns of interannual sea level pressure slp variability over the extratropical pacific 20 60 n and the eastern pacific ep and central pacific cp types of el nino southern oscillation enso the first empirical orthogonal function eof mode of the extratropical slp anomalies represents variations of the aleutian low and the second eof mode represents the north pacific oscillation npo and is characterized by a meridional slp anomaly dipole with a nodal point near 50 n it is shown that a fraction of the first slp mode can be excited by both the ep and cp types of enso the slp response to the ep type is stronger and more immediate the tropical extratropical teleconnection appears to act more slowly for the cp enso during the decay phase of ep events the associated extratropical slp anomalies shift from the first slp mode to the second slp mode as the second slp mode grows subtropical sst anomalies are induced beneath via surface

global adjustment of the thermocline in response to deepwater formation the global adjustment of the thermocline in response to deepwater formation is studied in a single mode model on a beta plane the signal is carried from ocean to ocean by kelvin waves which travel equatorward along western boundaries eastward across the equator poleward at the eastern boundaries and then eastward around the southern tip of continents into the next ocean basin the interior is filled by rossby waves emanating from eastern boundaries stronger weaker deepwater formation induces an upward downward motion of the main thermocline in the world oceans the adjustment is completed on centennial time scales

adjustment of the ocean under buoyancy forces part i the role of kelvin waves abstract the early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model the initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the north atlantic the main adjustment in the time examined here the first few months is by coastal kelvin waves the first baroclinic mode dominates so the behavior proves to be very similar to that in a shallow water model also the dynamics is locally close to that for an f plane but account must be taken of the way f varies with latitude the essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin especially at the level where the first mode has the maximum effect this study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations namely in a model with too coarse a horizontal resolution and an artif

oxygenation of ediacaran ocean recorded by iron isotopes abstract the increase in atmospheric oxygen during the late neoproterozoic era ca 800 542 ma may have stimulated the oxygenation of the deep oceans and the evolution of macroscopic multicellular organisms however the mechanism and magnitude of neoproterozoic oxygenation remain uncertain we present fe isotopes fe species and other geochemical data for two sections of the doushantuo formation ca 635 551 ma deposited after the nantuo glacial episode in the yangtze gorge area south china it is highlighted that highly positive 56 fe values reflect a lower oxidation rate of fe ii aq under ferruginous conditions and in turn near zero 56 fe values indicate oxidizing conditions our study suggests that during the deposition of the bottom of member ii of the doushantuo formation the shallow seawater was oxic but the deep water was characterized by ferruginous conditions which is consistent with a redox chemical stratification model subsequent anoxic conditions under shallow seawater represented by positive 56 fe and negative 13 c carb excursions should be ascribed to the upwelling of fe ii aq and dissolved organic carbon doc rich anoxic deep seawater the oxidation of fe ii aq and doc rich anoxic deep seawater upon mixing with oxic shallow water provides an innovative explanation for the well known negative 13 c carb excursions enc2 and positive 56 fe excursions in the middle of doushantuo formation meanwhile the upwelling fe ii aq and doc rich anoxic deep seawater could increase photosynthetic activity the balance between oxygen consumption and production was most important criteria for the oxygenation of early ediacaran ocean and diversity of eukaryotic organisms

the buoyancy and wind driven ventilated thermocline abstract an analytical study of the combined wind driven and buoyancy driven thermocline problem is presented the analysis is an extension of the ventilated thermocline model of luyten et al an exact solution for the ventilated region of the subtropical gyre is found under the condition that the ratio of the buoyancy flux to ekman pumping is a function of latitude alone it demonstrates the general westward shift of streamlines when the subtropical gyre is heated and the enlargement of the eastern shadow zone the flow in the shadow zone is described in most instances for the case of relatively small buoyancy flux it is shown that the shadow zone now in motion due to buoyancy forcing splits into two zones there is a broad eastward zone in which the deep flow is northeastward and a narrow zone between this branch and the ventilated fluid in which the shadow zone flow is returned in a relatively swift current nearly preserving potential vorticity for buoyancy ekman flux ratios which are nearly inde

upb and smn edvidence for eburnian and pan african high grade metamorphism in cratonic rocks of southern cameroon abstract u pb analyses of zircons and sm nd mineral and whole rock analyses of samples from southern cameroon document three distinct rock forming or high grade metamorphic events 2900 ma 2050 ma and 600 ma zircons from the ebolowa charnockite in southern cameroon yield an age of 2896 7 ma consistent with published rb sr total rock data for the region sm nd crustal formation ages for these rocks are 3000 ma indicating that the granitoids and ntem series gneisses in the north end of the congo craton formed 2900 ma ago and do not contain major contributions from older archean crust a second metamorphic event of eburnian age is identified from re equilibration of sm and nd in gneisses and metadolerites of the ntem complex at 2050 ma zircons from the younger nyong series gneisses are a complex mixture of cores and overgrowths the overgrowths in most cases formed during a high grade metamorphic event between 2100 and 2000 ma and clearly postdate the ebolowa charnockite the ages of the cores are not precisely known but we believe most represent detrital zircons from 2900 ma protoliths sm nd crustal formation ages for samples of the nyong series gneisses are generally 2500 to 2900 ma indicating that their precursors were apparently derived mainly from the archean craton with variable amounts of younger component sm nd garnet total rock isochrons for metamorphosed mafic rocks in the nyong series yield ages of 2050 ma and zircons from one of them yield an upper intercept age of 2037 10 ma further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 ma we interpret the nyong series metasedimentary gneisses as post 2900 ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 ma eburnian orogeny we cannot define precise limits on the ages of sedimentation preliminary data on metamorphosed intrusive rocks associated with the yaounde series yield u pb concordant ages around 620 ma consistent with sm nd garnet whole rock ages from the same series this dates the pan african tectono metamorphic activity in the southern end mobile belt the pan african lower intercepts of zircons and sm nd garnet whole rock ages from thnyong series are the consequences of this event which is further represented by a 590 ma zircon age from a late syenitic pluton results from cameroon are consistent with recent results from the western part of the congo craton farther south and from the northeastern part of the sao francisco craton in brazil one interpretation of these results is that the congo and sao francisco cratons were joined during the ebumian trans amazonian orogeny 2100 ma and are bordered to the north by a late mesoproterozoic rift basin that was closed during the pan african brasiliano orogeny 600 ma

the little ice age and 20th century deep pacific cooling proxy records show that before the onset of modern anthropogenic warming globally coherent cooling occurred from the medieval warm period to the little ice age the long memory of the ocean suggests that these historical surface anomalies are associated with ongoing deep ocean temperature adjustments combining an ocean model with modern and paleoceanographic data leads to a prediction that the deep pacific is still adjusting to the cooling going into the little ice age whereas temperature trends in the surface ocean and deep atlantic reflect modern warming this prediction is corroborated by temperature changes identified between the hms challenger expedition of the 1870s and modern hydrography the implied heat loss in the deep ocean since 1750 ce offsets one fourth of the global heat gain in the upper ocean

oceanic teleconnections remote response to decadal wind forcing abstract the transhemispheric and interbasin response to time dependent wind forcing localized in the northern hemisphere of a single basin is examined using the reduced gravity shallow water equations in domains of simple geometry on decadal timescales the pressure on the eastern boundary fluctuates synchronously in both hemispheres and thus communicates a signal to latitudes distant from the forcing the signal then penetrates into the interior through westward radiation of rossby waves associated with the eastern boundary pressure fluctuation is a time dependent mass flux across the equator that in a single basin is balanced by a storage of mass in the unforced hemisphere two oceanic basins connected by a reentrant channel at the high latitude edge of the southern hemisphere are then considered again the forcing is confined to the northern hemisphere of one basin only in this geometry the time dependent mass flux across the equator of the forced basin is not entirely balanced within the same ba

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

contrasting eastern pacific and central pacific types of enso abstract surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of el nino southern oscillation enso in the tropical pacific an eastern pacific ep type and a central pacific cp type an analysis method combining empirical orthogonal function eof analysis and linear regression is used to separate these two types correlation and composite analyses based on the principal components of the eof were performed to examine the structure evolution and teleconnection of these two enso types the ep type of enso is found to have its sst anomaly center located in the eastern equatorial pacific attached to the coast of south america this type of enso is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical indian ocean in contrast the cp type of enso has most of its surface wind sst and subsurface anomalies confined in the central pacific and tends to onset develop and decay in situ

iron isotope composition of some archean and proterozoic iron formations abstract fe isotopes can provide new insight into redox dependent biogeochemical processes precambrian iron formations if are deserving targets for fe isotope studies because they are composed predominantly of authigenic fe phases and record a period of unprecedented iron deposition in earth s history we present fe isotope data for bulk samples from 24 archean and proterozoic if and eight phanerozoic fe oxide rich deposits these data reveal that many archean and early paleoproterozoic iron formations were a sink for isotopically heavy fe in contrast to later proterozoic and phanerozoic fe oxide rich rocks the positive 56 fe values in if are best explained by delivery of particulate ferric oxides formed in the water column to the sediment water interface because if are a net sink for isotopically heavy fe there must be a corresponding pool of isotopically light fe in the sedimentary record earlier work suggested that archean pyritic black shales were an important part of this light sink before 2 35 billion years ago ga it is therefore likely that the persistently and anomalously low 56 fe values in shales are linked with the deposition of isotopically heavy fe in if in the deeper parts of basins if deposition produced a residual isotopically light dissolved fe pool that was captured by pyritic fe in shales local dissimilatory fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced fe isotope heterogeneity in marine sediments and an iron shuttle may have transported isotopically light fe from shelf sediments to the basin nevertheless water column processing of hydrothermally delivered fe likely had the strongest influence on the bulk iron isotope composition of archean and paleoproterozoic iron formations and other marine sediments

the transient response of the southern ocean pycnocline to changing atmospheric winds 1 the vertical density structure of the southern ocean is dynamically linked to wind stress at the surface but the nature of this coupling is not fully understood observations from the last several decades show a significant increase in the strength of westerly winds over the southern ocean but an appreciable change in the tilt of constant density surfaces isopycnals has not yet been detected there using a combination of theory and idealized numerical simulations we show that the response of the density structure occurs on centennial timescales making it difficult to detect significant changes with a few decades of hydrographic observations dynamic coupling between the circumpolar current and northern basins regulates the slow adjustment of the density structure our results provide a new interpretation for recent observations and highlight the importance of the interaction between regional southern ocean dynamics and global ocean circulation citation jones d c t ito andn s lovenduski 2011 thetransientresponseofthe southern ocean pycnocline to changing atmospheric winds geophys res lett 38 l15604 doi 10 1029 2011gl048145

opposing decadal changes for the north atlantic meridional overturning circulation hydrographic properties of the north atlantic ocean have changed significantly over the past decades a combination of changes in seawater density calculated from observed properties of sea water and a numerical ocean model reveals that the strength of the meridional overturning circulation has changed in different directions in the subpolar and subtropical basins respectively

a three layer model for wind driven circulation in a subtropical subpolar basin part i model formulation and the subcritical state abstract a three layer model is formulated and run numerically in order to understand the highly nonlinear structure of the wind driven circulation in a basin including both a subtropical and a subpolar gyre the third layer is assumed to be infinitely deep and motionless we emphasize the strong nonlinearity associated with large layer thickness change a broad range of parameters has been tested to explore possible flow patterns for cases of weak wind forcing and large amounts of water in the upper layer the upper layer covers the whole basin the second layer is stagnant except near the western boundary for stronger forcing closed potential vorticity isolines appear within the second layer and there is a strong nonlinear coupling between the first and second layers the numerical results show some flow patterns highly asymmetric with respect to the zero wind curl line the potential vorticity distribution in the second layer shows a strong competition between potential vorticity advection interfa

localization of abrupt change in the north atlantic thermohaline circulation 1 recent climate model experiments as well as paleoclimate records suggest that the meridional overturning circulation or thermohaline circulation in the atlantic ocean could change abruptly as a result of global warming and that this could have a significant impact on european climate we use a reduced gravity model to investigate the response of the atlantic overturning circulation to changes in forcing we find that variability at decadal and higher frequencies is confined to a single hemisphere this implies that a overturning variability resulting from high frequency changes in buoyancy forcing in the labrador and greenland seas will be limited to the north atlantic and b any observed decadal and higher frequency fluctuations in north atlantic overturning can only result from changes in the surface fluxes within the north atlantic basin itself these results suggest that southern ocean wind forcing is not important for north atlantic overturning on decadal and shorter timescales

reconstruction of early cambrian ocean chemistry from mo isotopes the neoproterozoic cambrian transition was a key time interval in the history of the earth especially for variations in oceanic and atmospheric chemical composition however two conflicting views exist concerning the nature of ocean chemistry across the precambrian cambrian boundary abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late ediacaran while other studies provide seemingly conflicting evidence for anoxic deep waters with ferruginous conditions fe ii enriched persisting into the cambrian here two early cambrian sedimentary platform and shelf slope sections in south china were investigated to trace early cambrian ocean chemistry from mo isotopes the results reveal that early cambrian sediments deposited under oxic to anoxic euxinic conditions have 98 95mo values ranging from 0 28 to 2 29 which suggests that early cambrian seawater may have had 98 95mo values of at least 2 29 similar to modern oceans the heaviest and relatively homogeneous 98 95mo values were recorded in siltstone samples formed under completely oxic conditions which is considered that mn oxide free shuttling was responsible for such heavy 98 95mo value further combined with fe species data and the accumulation extent of mo and u the variation of 98 95mo values in the two studied sections demonstrate a redox stratified ocean with completely oxic shallow water and predominantly anoxic even euxinic deeper water having developed early on which eventually became completely oxygenated this suggests that oceanic circulation at the time became reorganized and such changes in oceanic chemistry may have been responsible for triggering the cambrian explosion of biological diversity

reconstructing marine redox conditions for the transition between cambrian series 2 and cambrian series 3 kaili area yangtze platform evidence from biogenic sulfur and degree of pyritization the transition between series 2 and series 3 of the cambrian period was a time of environmental perturbations and biological innovations a generally positive sulfur isotopic composition of sedimentary pyrite with delta s 34 pyrite values between 1 7 and 373 is consistent with a biological origin via reduction of strongly s 34 enriched seawater sulfate no substantial changes in the redox conditions of the water column are discernible from the dop record and the sulfide sulfur isotopic composition is independent of dop in particular an excursion towards less is enriched values immediately preceding the proposed level of the cambrian series 2 to cambrian series 3 transition this level is also characterized by a negative carbonate carbon isotope excursion believed to reflect the transgressive flooding of the shelf with basinal anoxic waters low dop values would suggest that these might have been ferruginous rather than sulfidic c 2013 elsevier b v all rights reserved

iron speciation and isotope fractionation during silicate weathering and soil formation in an alpine glacier forefield chronosequence abstract the chemical weathering of primary fe bearing minerals such as biotite and chlorite is a key step of soil formation and an important nutrient source for the establishment of plant and microbial life the understanding of the relevant processes and the associated fe isotope fractionation is therefore of major importance for the further development of stable fe isotopes as a tracer of the biogeochemical fe cycle in terrestrial environments we investigated the fe mineral transformations and associated fe isotope fractionation in a soil chronosequence of the swiss alps covering 150 years of soil formation on granite for this purpose we combined for the first time stable fe isotope analyses with synchrotron based fe exafs spectroscopy which allowed us to interpret changes in fe isotopic composition of bulk soils size fractions and chemically separated fe pools over time in terms of weathering processes bulk soils and rocks exhibited constant isotopic compositions along the chronosequence whereas soil fe pools in grain size fractions spanned a range of 0 4 in 56 fe the clay fractions 56 fe for all soils the lightest isotopic composition was observed in a 1 m nh 2 oh hcl 25 acetic acid extract targeting poorly crystalline fe iii hydr oxides compared with easily leachable fe in primary phyllosilicates 0 5 m hcl extract and fe in residual silicates the combination of the fe isotope measurements with the speciation data obtained by fe exafs spectroscopy permitted to quantitatively relate the different isotope pools forming in the soils to the mineral weathering reactions which have taken place at the field site a kinetic isotope effect during the fe detachment from the phyllosilicates was identified as the dominant fractionation mechanism in young weathering environments controlling not only the light isotope signature of secondary fe iii hydr oxides but also significantly contributing to the isotope signature of plants the present study further revealed that this kinetic fractionation effect can persist over considerable reaction advance during chemical weathering in field systems and is not only an initial transient phenomenon

dilution of the northern north atlantic ocean in recent decades declining salinities signify that large amounts of fresh water have been added to the northern north atlantic ocean since the mid 1960s we estimate that the nordic seas and subpolar basins were diluted by an extra 19 000 5000 cubic kilometers of freshwater input between 1965 and 1995 fully half of that additional fresh water about 10 000 cubic kilometers infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year patterns of freshwater accumulation observed in the nordic seas suggest a century time scale to reach freshening thresholds critical to that portion of the atlantic meridional overturning circulation

a critical look at iron paleoredox proxies new insights from modern euxinic marine basins enrichments in reactive iron occur under euxinic marine conditions that is where dissolved sulfide is present in the water column these enrichments result primarily from the export of remobilized iron from the oxic shelf which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments strongly elevated ratios of highly reactive iron to total iron fehr fet and total iron to aluminum fet al and high degrees of pyritization dop are each products of this enrichment process these paleoredox proxies are among the most faithful recorders of ancient euxinia contrary to previous arguments iron enrichment is decoupled from biogenic sediment inputs but it does appear to be a uniquely euxinic phenomenon in other words we can rule out a major contribution from preferential physical transport of fehr rich detrital sediment to the deep basin which could also operate under oxic conditions furthermore enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins elevated fet al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net fehr addition these conclusions are supported by observations in the modern black sea orca basin and effingham inlet fet al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration however this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism resulting in only intermediate dop values for euxinic sediments and fet al ratios that mimic the oxic shelf such dilution effects are well expressed in black sea basinal turbidites and rapidly accumulating muds on euxinic basin margins under conditions of persistent euxinia varying extents of fehr enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation the magnitude of enrichment is a function of the source shelf to sink ocean basin areal ratio suggesting that iron proxies can also record ocean scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia for the first time manganese data are interpreted in light of the redox shuttle model as for the iron data the black sea orca basin and effingham inlet show enrichments in total manganese in the deep euxinic basin suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin the black sea data reveal iron and manganese enrichment across the broad deep euxinic basin suggesting efficient lateral transport and deep water mixing tied to the physical properties of the water column

time dependent adjustment in a simple model of the mid depth meridional overturning cell abstracta recently proposed reduced gravity model of the warm water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence the model describes the balance between gain of warm water through northward ekman advection across the circumpolar current loss of warm water through eddy fluxes southward across the current net gain or loss of warm water through diabatic processes north of the current and changes in the thickness of the warm water layer the steady solutions are the same as those found previously when the previous parameterization of diabatic fluxes is used time dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed an initial adjustment occurs through a combination of boundary and equatorial adjustment followed by planetary wave propagation the longer term adjustment to equilibrium consists p

adjustment of the ocean under buoyancy forces part ii the role of planetary waves abstract a numerical ocean general circulation model is used to investigate the early stages in the adjustment to equilibrium of an ocean initially at rest with imposed uniform meridional potential temperature gradients which yield density gradients representative of those observed in the north atlantic the main feature of the adjustment during the early stages the first year is the formation and decay of a subtropical warm core and a subpolar cold core density gyre the gyres are formed by the irreversible winding up of the initially zonal isotherms by first baroclinic mode coastally trapped dissipative kelvin waves this phase and the north south asymmetry arising from the variation in viscous diffusive kelvin wave properties with latitude were discussed in part i of this series within a month effects become significant especially in the evolution of the southern gyre which develops a distinct east west asymmetry through western intensification and long planetary wave propagation of b

a simple model of the response of the atlantic to the north atlantic oscillation the response of an idealized atlantic ocean to wind and thermohaline forcing associated with the north atlantic oscillation nao is investigated both analytically and numerically in the framework of a reducedgravity model the nao related wind forcing is found to drive a time dependent leaky gyre circulation that integrates basinwide stochastic wind ekman pumping and initiates low frequency variability along the western boundary thisissubsequentlycommunicated togetherwith the stochasticvariability induced bythermohaline forcing at high latitudes to the remainder of the atlantic via boundary and rossby waves at low frequencies the basinwide ocean heat content changes owing to nao wind forcing and thermohaline forcing are found to oppose each other the model further suggests that the recently reported opposing changes of the meridional overturning circulation in the atlantic subtropical and subpolar gyres between 1950 70 and 1980 2000 may be a generic feature caused by interplay between the nao wind and thermohaline forcing

wind stress curl and enso discharge recharge in the equatorial pacific discharge and recharge of the warm water volume wwv above the 20 c isotherm in an equatorial pacific ocean box extending across the pacific from 156 e to the eastern ocean boundary between latitudes 5 s and 5 n are key variables in enso dynamics a formula linking wwv anomalies zonally integrated wind stress curl anomalies along the northern and southern edges of the box and flow into the western end of the box is derived and tested using monthly data since 1993 consistent with previous work a wwv balance can only be achieved if the 20 c isotherm surface is not a material surface that is warm water can pass through it for example during el nino part of the wwv anomaly entering the box is cooled so that it is less than 20 c and therefore passes out of the bottom of the box the 20 c isotherm surface the observations suggest that the anomalous volume passing through the 20 c isotherm is approximately the same as t the anomalous wwv entering the western end of the box therefore the observed wwv anomaly can be regarded as being driven by the anomalous wind stress curl along the northern and southern edges of the box the curl anomaly changes the wwv both by divergent meridional flow at the edges of the box and vortex stretching that is the sverdrup balance does not hold in the upper ocean a typical amplitude for the rate of change of wwv for the 5 s 5 n box is 9 6 sv sv 10 6 m 3 s 1 the wind stress curl anomaly and the transport anomaly into the western end of the box are highly correlated with the el nino index nino 3 4 the average sea surface temperature anomaly ssta over the region 5 s 5 n 170 120 w and nino 3 4 leads minus the wwv anomaly by one quarter of a cycle based on the preceding results a simple discharge recharge coupled enso model is derived only water warmer than about 27 5 28 c can give rise to deep atmospheric convection so unlike past discharge recharge oscillator models the west central rather than eastern equatorial sstas are emphasized the model consists of two variables t the ssta averaged over the region of strong enso air sea interaction in the west central pacific equatorial strip 5 s 5 n 156 e 140 w and d the 20 c isotherm depth anomaly averaged over the

iron isotopes in the seawater of the equatorial pacific ocean new constraints for the oceanic iron cycle abstract this study presents the isotopic compositions and concentrations of dissolved and particulate iron from two seawater profiles of the western and central equatorial pacific ocean sampled during the eucfe cruise most of the 56 fe values are positive relative to irmm 14 from 0 01 to 0 58 in the dissolved fraction dfe and from 0 02 to 0 46 in the particulate fraction pfe the mean measurement uncertainty of 0 08 2sd allows the observation of significant variations most of the isotope variations occur in the vertical and not in the horizontal direction implying that each isotope signature is preserved over long distances within a water mass the thermocline waters of the papua new guinea png area mostly influenced by sedimentary inputs display a mean 56 dfe value of 0 37 0 15 2sd this isotopic signature suggests that the process releasing dissolved iron into the seawater in this area is a non reductive dissolution of sediments discharged by local rivers and likely re suspended by strong boundary currents rather than dissimilatory iron reduction dir within the sediment characterized by negative 56 dfe these positive 56 dfe values seem to be the result of a mean isotopic fractionation of 56 fe dfe pfe 0 20 0 11 2sd produced by the non reductive dissolution at 0 n 180 e the fe isotope signature of the equatorial undercurrent euc waters is identical to that of the png station within the range of the uncertainty this suggests that the dissolved iron feeding the euc and ultimately the eastern pacific high nutrient low chlorophyll area is of png origin likely released by a non reductive dissolution of terrigenous sediments significant fe removals are observed within the thermocline and the intermediate waters between the png and the open ocean stations the corresponding isotopic fractionations appear to be small with 56 fe removed sw fe values of 0 30 0 31 to 0 18 0 12 2sd for dfe removal and of 0 10 0 04 to 0 05 0 31 2sd for pfe removal in the chlorophyll maximum of the open ocean station the isotopic fractionation associated with biological uptake is estimated at 56 fe phyto dfe 0 25 0 10 to 0 13 0 11 2sd although these fractionations are based on a limited dataset and need to be further constrained they appear to be small and to limit the transformations of the iron source signatures within the ocean

determination of the fe ii aq magnetite equilibrium iron isotope fractionation factor using the three isotope method and a multi direction approach to equilibrium abstract magnetite is ubiquitous in the earth s crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical fe cycling magnetite is also the most abundant fe oxide in banded iron formations bifs that have not been subjected to ore forming alteration magnetite is therefore an important target of stable fe isotope studies and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable fe isotope fractionation factors for magnetite relative to fluids and other minerals in this study we utilized the three isotope method 57fe 56fe 54fe to explore isotopic exchange via an enriched 57fe tracer and natural mass dependent fractionation using 56fe 54fe variations during reaction of aqueous fe ii fe ii aq with magnetite importantly we employed a multi direction approach to equilibrium by reacting four 57fe enriched fe ii solutions that had distinct 56fe 54fe ratios which identifies changes in the instantaneous fe isotope fractionation factor and hence identifies kinetic isotope effects we find that isotopic exchange can be described by two 56fe 54fe fractionations where an initial rapid exchange 66 isotopic mixing within 1 day involved a relatively small fe ii aq magnetite 56fe 54fe fractionation followed by slower exchange 25 isotopic mixing over 50 days that was associated with a larger fe ii aq magnetite 56fe 54fe fractionation this later fractionation is interpreted to approach isotopic equilibrium between fe ii aq and the total magnetite all four fe ii solutions extrapolate to the same final equilibrium 56fe 54fe fractionation for fe ii aq magnetite of 1 56 0 20 2 at 22 c additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous fe ii yield final 56fe 54fe fractionations that are identical to those of the exchange experiments our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for fe h 2 o 6 2 from rustad et al 2010 and stoichiometric magnetite from mineev et al 2007 and these relations may be combined with the experimental constraints to determine the temperature dependence of the fe ii aq magnetite fractionation factor 10 3 ln f e i i aq magnetite 0 145 0 002 10 6 t 2 0 10 0 02 where t is in k part of the reason for large discrepancies in calculated fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies given the significant effect of octahedral versus tetrahedral fe isotope fractionation that has been calculated when our results are applied to bif genesis our experimentally determined fe ii aq magnetite fractionation factor indicates that magnetite siderite mineral pairs in 2 5 ga bifs did not form in fe isotope equilibrium with each other or with ancient seawater iron oxides in such bifs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans indicating that such bif minerals may not be suitable proxies for ancient paleoenvironments

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

modern iron isotope perspective on the benthic iron shuttle and the redox evolution of ancient oceans the increase in atmospheric oxygen ca 2 4 ga had a significant impact on the geochemical cycling of fe the history of environmental oxygenation may be recorded in the fe isotope composition of archean and proterozoic sediments but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations here we present fe isotope data and iron aluminum ratios from the black sea oxic shelf and euxinic basin the isotope data demonstrate that shelf fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin we propose that there is net transport of isotopically light fe from sediments of the shelf to those of the distal anoxic basin consistent with enrichments in reactive fe seen in the deep basin the low 56 fe benthic fe flux is generated during the coupling of microbial fe iii reduction or sulfidization with fe 2 aq oxidation low 56 fe values reported previously from late archean sedimentary pyrites may be an isotopic fingerprint of analogous fe redox cycling in the late archean oceans this interpretation implies shallow water fe redox recycling in the late archean we predict that the light isotopic compositions of the late archean will prove to be distinct from those of the early archean before fe redox cycling became an important process and we infer that this difference may be related to the presence of oxygen in the surface ocean

global teleconnections of meridional overturning circulation anomalies there is a wide range of evidence from both models and palaeoclimatic data that indicates the possibility of abrupt changes in the oceanic meridional overturning circulation moc however much of our dynamical understanding of the moc comes from steady state models that rely upon the assumption of thermodynamic equilibrium and are therefore only valid on millennial time scales here a dynamical model for the global teleconnections of moc anomalies on annual to multidecadal time scales is developed it is based on a linear theory for the propagation of zonally integrated meridional transport anomalies in a reduced gravity ocean and allows for multiple ocean basins connected by a circumpolar channel to the south the theory demonstrates that the equator acts as a low pass filter to moc anomalies as a consequence moc anomalies on decadal and shorter time scales are confined to the hemispheric basin in which they are generated and have little impact on the remainder of the global ocean the linear theory is compared with the results of a global nonlinear numerical integration which it reproduces to a good approximation

multidecadal sea level and gyre circulation variability in the northwestern tropical pacific ocean abstractsea level rise with the trend 10 mm yr 1 has been observed in the tropical western pacific ocean over the 1993 2009 period this rate is 3 times faster than the global mean value of the sea level rise analyses of the satellite altimeter data and repeat hydrographic data along 137 e reveal that this regionally enhanced sea level rise is thermosteric in nature and vertically confined to a patch in the upper ocean above the 12 c isotherm dynamically this regional sea level trend is accompanied by southward migration and strengthening of the north equatorial current nec and north equatorial countercurrent necc using a 1 layer reduced gravity model forced by the ecmwf reanalysis wind stress data the authors find that both the observed sea level rise and the nec necc s southward migrating and strengthening trends are largely attributable to the upper ocean water mass redistribution caused by the surface wind stresses of the recently strengthened atmospheric walker circulation based on the lo

the thermohaline modes of the global ocean abstractthe first 2000 m of the global thermohaline structure of the ocean are statistically decomposed into vertical thermohaline modes using a multivariate functional principal component analysi

a two level model for the wind and buoyancy forced circulation abstract the classic theory of deep circulation by stommel and arons is extended to a two level model instead of specifying the interfacial upwelling a priori it is calculated as part of the solution the model shows strong upwelling along the southern and eastern boundaries while the upwelling in the interior ocean is much weaker with a medium rate of ekman pumping the basin is separated into western and eastern regions by critical characteristics it is shown that across the boundary between the different regions temperature and velocity are continuous but vorticity or some higher order quantity is discontinuous

circumpolar response of southern ocean eddy activity to a change in the southern annular mode 1 analysis of satellite altimeter data reveals anomalously high eddy kinetic energy eke in the antarctic circumpolar current acc during the period 2000 2002 around 2 3 years earlier 1998 the circumpolar eastward wind stress as quantified by the southern annular mode sam showed a significant positive peak and we have shown previously that the acc peaked around 1998 in response an eddy resolving ocean model is used to investigate the delay between wind forcing and the eddy response and demonstrates that the lag is due to the time taken to influence the deep circulation of the acc winds over the southern ocean have shown a strong climatic increase over the past few decades if this increase in winds is also reflected as an increase in eddy activity as our analysis suggests it might then the increased poleward heat flux may have played a significant role in the observed warming of the southern ocean citation meredith m p and a m hogg 2006 circumpolar response of southern ocean eddy activity to a change in the southern annular mode geophys res lett 33 l16608 doi 10 1029 2006gl026499

on the north atlantic ocean heat content change between 1955 70 and 1980 95 theupper oceanheatcontentofthenorthatlantichasundergonesignificantchangesoverthelast50years but the underlying physical mechanisms are not yet well understood in the present study the authors examine the north atlantic ocean heat content change in the upper 700 m between the 1955 70 and 1980 95 periods consistent with previous studies the large scale pattern consists of warming of the tropics and subtropics and cooling of the subpolar ocean however this study finds that the most significant heat content change in the north atlanticduring these two time periods is the warmingofthe gulfstream region numerical experiments strongly suggest that this warming in the gulfstream region is largely driven by changes of the large scale wind forcing furthermore the increased ocean heat content inthe gulfstream regionappears tofeedbackon tothe atmosphere resulting in warmer surface air temperature and enhanced precipitation there

deep water dissolved iron cycling and reservoir size across the ediacaran cambrian transition abstract the majority of the deep ocean was likely under ferruginous conditions during the first four billion years of earth s history as the atmosphere was gradually oxygenated the sources sinks redox cycling and reservoir size of dissolved iron in the deep ocean are likely to have changed dramatically whether deep water was thoroughly oxygenated by the time of the ediacaran cambrian transition and the relationship of this oxygenation to the cambrian explosion remains debated to explore the degree of oceanic oxygenation and its effect on cambrian explosion we measured the iron isotopic composition 56fe of bulk rock i e cherts and mudstones shales through the piyuancun and hetang formations using samples collected from the chunye 1 core on the lower yangtze block in western zhejiang the limited variation in 56fe values 0 55 during the end ediacaran and cambrian stages 1 3 to

long rossby wave basin crossing time and the resonance of low frequency basin modes the ability of long wave low frequency basin modes to be resonantly excited depends on the efficiency with which energy fluxed onto the western boundary can be transmitted back to the eastern boundary this efficiency is greatly reduced for basins in which the long rossby wave basin crossing time is latitude dependent in the singular case where the basin crossing time is independent of latitude the amplitude of resonantly excited long wave basin modes grows without bound except for the effects of friction the speed of long rossby waves is independent of latitude for quasigeostrophic dynamics and the rectangular basin geometry often used for theoretical studies of the wind driven ocean circulation is such a singular case for quasigeostrophic dynamics for more realistic basin geometries where only a fraction of the energy incident on the western boundary can be transmitted back to the eastern boundary the modes have a finite decay rate that in the limit of weak friction is independent of the choice of frictional parameters explicit eigenmode computations for a basin geometry similar to the north pacific but closed along the equator yield basin modes sufficiently weakly damped that they could be resonantly excited

gyres driven by combined wind and buoyancy flux abstract the combined effects of buoyancy forcing and wind in a gyre scale steady ocean circulation are modeled using discrete layers with an interfacial flux not necessarily vertical the equations for vorticity conservation of the geostrophic flow in this system are fully nonlinear involving a jacobian for the layer thicknesses these equations are written in a form which can be solved by the method of characteristics the form of these equations invites the interpretation that the geostrophic baroclinic flow is driven by buoyancy and steered by the wind two examples are solved and discussed a subtropical gyre with heating and a subpolar gyre with cooling in each case there are distinct regimes of flow depending upon whether the characteristics originate at the eastern or western boundaries of the model a simple geometrical argument illustrates that the difference between these two regimes the direct and indirect cells depends upon the sign of the true vertical velocity relative to the interf

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

heaving modes in the world oceans part of climate changes on decadal time scales can be interpreted as the result of adiabatic motions associated with the adjustment of wind driven circulation i e the heaving of the isopycnal surfaces heat content changes in the ocean including hiatus of global surface temperature and other phenomena can be interpreted in terms of heaving associated with adjustment of wind driven circulation induced by decadal variability of wind a simple reduced gravity model is used to examine the consequence of adiabatic adjustment of the wind driven circulation decadal changes in wind stress forcing can induce three dimensional redistribution of warm water in the upper ocean in particular wind stress change can generate baroclinic modes of heat content anomaly in the vertical direction in fact changes in stratification observed in the ocean may be induced by wind stress change at local or in the remote parts of the world oceans intensification of the equatorial easterly can induce cooling in the upper layer and warming in the subsurface layer the combination of this kind of heat content anomaly with the general trend of warming of the whole water column under the increasing greenhouse effect may offer an explanation for the hiatus of global surface temperature and the accelerating subsurface warming over the past 10 15 years furthermore the meridional transport of warm water in the upper ocean can lead to sizeable transient meridional overturning circulation poleward heat flux and vertical heat flux thus heaving plays a key role in the oceanic circulation and climate