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Indian Ocean sea surface temperatures impact precipitation across the basin through coupled ocean‐atmosphere responses to changes in climate. To understand the hydroclimate response over the western Indian Ocean and equatorial east Africa to different forcing mechanisms, we present four new proxy reconstructions from core VM19‐193 (2.98°N, 51.47°E) that span the last 250 ky. Sub‐surface water temperatures (Sub‐T; TEX86) show strong precessional (23 ky) variability that is primarily influenced by maximum incoming solar radiation (insolation) during the Northern Hemisphere spring season, likely indicating that local insolation dominates the upper water column at this tropical location over time. Leaf waxes, on the other hand, reflect two different precipitation signals: δ13Cwax (in phase with boreal fall insolation) is likely reflecting vegetation changes in response to local rainfall over east Africa, whereas δDprecip (primarily driven by boreal summer insolation) represents changes in regional circulation associated with the summer monsoon. Glacial‐interglacial changes in ocean temperatures support glacial shelf exposure over the Maritime Continent in the eastern Indian Ocean and the subsequent weakening of the Indian Walker Circulation as a mechanism driving 100 ky climate variability across the tropical Indo‐Pacific. Additionally, the 100 ky spectral power in δDprecip supports a basin‐wide weakening of summer monsoon circulation in response to glacial climates. Overall, the proxy records from VM19‐193 indicate that both precession and glacial‐interglacial cycles exert control over hydroclimate at this tropical location. Key Points: New multi‐proxy reconstructions from the tropical western Indian Ocean show both precession and glacial‐interglacial variabilityA new precipitation isotope record likely reflects large‐scale Indian summer monsoon circulation changes over timeGlacial‐scale changes in upper water column structure across the Indian Ocean support shelf exposure as a primary hydroclimate influence [ABSTRACT FROM AUTHOR]

We present downcore records of redox-sensitive authigenic uranium (U) and manganese (Mn) concentrations based on five marine sediment cores spanning a meridional transect encompassing the Subantarctic and Antarctic zones in the southwestern Indian Ocean covering the last glacial cycle. These records signal lower bottom water oxygenation during glacial climate intervals and generally higher oxygenation during warm periods, consistent with climate-related changes in deep-ocean remineralized carbon storage. Regional changes in the export of siliceous phytoplankton to the deep sea may have entailed a secondary influence on oxygen levels at the water–sediment interface, especially in the Subantarctic Zone. The rapid reoxygenation during the deglaciation is in line with increased ventilation and enhanced upwelling after the Last Glacial Maximum (LGM), which in combination conspired to transfer previously sequestered remineralized carbon to the surface ocean and the atmosphere, contributing to propel the Earth's climate out of the last ice age. These records highlight the still insufficiently documented role that the Southern Indian Ocean played in the air–sea partitioning of CO2 on glacial–interglacial timescales. [ABSTRACT FROM AUTHOR]

Although the global environmental impact of Laurentide Ice-Sheet destabilizations on glacial climate during Heinrich Events is well-documented, the mechanism driving these ice-sheet instabilities remains elusive. Here we report foraminifera-based subsurface (~150 m water depth) ocean temperature and salinity reconstructions from a sediment core collected in the western subpolar North Atlantic, showing a consistent pattern of rapid subsurface ocean warming preceding the transition into each Heinrich Event identified in the same core of the last 27,000 years. These results provide the first solid evidence for the massive accumulation of ocean heat near the critical depth to trigger melting of marine-terminating portions of the Laurentide Ice Sheet around Labrador Sea followed by Heinrich Events. The repeated build-up of a subsurface heat reservoir in the subpolar Atlantic closely corresponds to times of weakened Atlantic Meridional Overturning Circulation, indicating a precursor role of ocean circulation changes for initiating abrupt ice-sheet instabilities during Heinrich Events. We infer that a weaker ocean circulation in future may result in accelerated interior-ocean warming of the subpolar Atlantic, which could be critical for the stability of modern, marine-terminating Arctic glaciers and the freshwater budget of the North Atlantic. The mechanism driving past Laurentide Ice-Sheet instabilities remains elusive Here, the authors present a sediment record from the subpolar western North Atlantic and show that massive warming of the upper interior ocean was the likely trigger for repeated collapses of the Laurentide Ice-Sheet and iceberg discharge into the North Atlantic, known as Heinrich Events. [ABSTRACT FROM AUTHOR]

The isotopic composition of Si in biogenic silica (BSi), such as opal buried in the oceans' sediments, has changed over time. Paleorecords suggest that the isotopic composition, described in terms of δ30Si, was generally much lower during glacial times than today. There is consensus that this variability is attributable to differing environmental conditions at the respective time of BSi production and sedimentation. The detailed links between environmental conditions and the isotopic composition of BSi in the sediments remain, however, poorly constrained. In this study, we explore the effects of a suite of offset boundary conditions during the Last Glacial Maximum (LGM) on the isotopic composition of BSi archived in sediments in an Earth System Model of intermediate complexity (EMIC). Our model results suggest that a change in the isotopic composition of Si supply to the glacial ocean is sufficient to explain the observed overall low(er) glacial δ30Si in BSi. All other processes explored trigger model responses of either wrong sign or magnitude or are inconsistent with a recent estimate of bottom water oxygenation in the Atlantic Sector of the Southern Ocean. Caveats, mainly associated with generic uncertainties in today's pelagic biogeochemical modules, remain. Key Points: We simulate the isotopic composition of biogenic silica archived in the ocean's sedimentOur simulations with an Earth System Model compare modern climate to glacial conditionsOut of several hypotheses tested, altering the isotopic composition of runoff during the LGM is most consistent with proxies [ABSTRACT FROM AUTHOR]

The article informs on the emergence of an equatorial mode of climate variability in the Indian Ocean (IO). It mentions that simulations indicate that under greenhouse warming and Last Glacial Maximum conditions, the IO can exhibit increased sea surface temperature variability in the eastern equatorial IO.

The present-day ocean ventilation in the Arctic Mediterranean (Nordic Seas and Arctic Ocean), via transformation of northward inflowing warm Atlantic surface water into cold deep water, affects regional climate, atmospheric circulation and carbon storage in the deep ocean. Here we study the glacial evolution of the Arctic Mediterranean circulation and its influence on glacial climate using radiocarbon reservoir-age reconstructions on deep-sea cores from the Fram Strait that cover the late glacial period (33,000–20,000 yr ago; 33–20 ka). Our results show high Benthic-Planktic 14C age differences of ∼1500 14C years 33–26.5 ka suggesting significant water column stratification between ∼100–2600 m water depth, and reduction and/or shoaling of deep-water formation. This phase was followed by break-up of the stratification during the Last Glacial Maximum (LGM; 26–20 ka), with Benthic-Planktic 14C age differences of ∼250 14C years, likely due to enhanced upwelling. These ocean circulation changes potentially contributed to the final intensification phase of glaciation via positive cryosphere-atmosphere-ocean circulation-carbon cycle feedbacks. Our data also do not support 'extreme aging' of >6000 14C years in the deep Arctic Mediterranean, and appear to rule out the proposed outflow of very old Arctic Ocean water to the Nordic Seas during the LGM and to the subpolar North Atlantic Ocean during the deglacial period. • Strong ocean stratification in the Fram Strait during the late glacial (33–26 ka). • Breakup of ocean stratification during the LGM (26–20 ka) due to enhanced upwelling. • No extreme aging of >6000 yr in the Arctic Mediterranean as previously suggested. [ABSTRACT FROM AUTHOR]

At least two global Neoproterozoic (Cryogenian) glaciation events ("Snowball Earth") occurred 710 Ma (Sturtian) and 630 Ma (Marinoan) ago, during which the oceans were either completely frozen or at most a few small open water oases remained. Neither the cause of the onset of the glaciations, their extent and duration, nor the cause for the deglaciations, are currently fully understood. In this study we try to follow up earlier studies by our group that indicated the presence of an extraterrestrial signature in rock units that mark the transition between the glacial diamictites and the overlying cap carbonates. This study deals with the suggestion that a large-scale impact event might have triggered the rapid deglaciation. Here we discuss the search for impact signatures using mineralogical and geochemical methods in samples from the Chuos/Rasthof (Sturtian) and Ghaub/Maieberg (Marinoan) postglacial transition at the Neoproterozoic Otavi Group (Namibia). However, no geochemical and mineralogical signatures that would be characteristic of an impact, such as an enrichment in siderophile element abundances or the presence of Cr-spinels or shocked minerals, were detected in our samples. Thus, there is no indication yet that an impact event was involved in the formation of the studied transition layers. [ABSTRACT FROM AUTHOR]

Abstract: Dinoflagellate cyst (dinocyst), coccolith and planktic foraminiferal assemblages from a core in the western Iceland Sea were used to reconstruct and compare the surface ocean developments of the Holocene and the Last Interglacial (Marine Isotopic Stage or MIS 5e). While increasing subpolar planktic foraminifers from ~10ka indicate subsurface warming peaking around 7.5ka, the dinocyst data suggest that the uppermost ocean remained dominated by cold waters until ~6.5ka. A reduced advection of cold polar waters through the East Greenland/East Icelandic Current thereafter entailed warmest and most saline Holocene conditions between 6.5 and 5ka, in turn followed by a general cooling trend. By contrast, both planktic foraminifers and dinocysts show an increased presence of Atlantic (-type) waters from ~122ka onward resulting in a MIS 5e thermal optimum around 120.5ka. Nonetheless, occasional freshwater input from melting drift ice created stratified but also seasonally variable conditions during this first half of MIS 5e. This stratification signature disappeared at ~120ka when a marked repositioning of the oceanographic fronts occurred. Slightly colder conditions are indicated by both phyto- and zooplankton from there on until the end of MIS 5e around ~117ka. A late MIS 5e cooling at the Iceland Plateau is opposite to a late MIS 5e optimum observed in the eastern Nordic seas. This regional difference is likely explained by various feedback mechanisms following the major reorganisation of the oceanic fronts at ~120ka. The Holocene and MIS 5e interglacial variability is not reflected in the (quasi-monospecific) coccolith assemblages and illustrates the low sensitivity of living coccolithophore communities to subtle temperature changes in the low-temperature regions of the Nordic seas. Overall, quite different surface water properties appear to have characterised both interglacial intervals, with a higher share of warm, Atlantic elements in the planktic communities during MIS 5e. This suggests a higher contribution of Atlantic waters in the southwestern Nordic seas probably due to a more northward expansion of the Irminger Current under weakened polar East Greenland/East Icelandic currents. Such a reduced influence of polar waters in the southwestern Nordic seas may thus explain other evidence for relatively warm conditions in MIS 5e all around southern Greenland. [Copyright &y& Elsevier]

The article discusses a study on the metabolic model of glacial-interglacial energy supply to the upper ocean. It mentions the application of a two-state two-box ocean model for simulating carbon dioxide signal during the glacial-interglacial changes in the earth system. It also notes that the capability of the model to reproduce long-term oscillations and identify the planet's metabolic rate with net autotrophic primary production in the ocean.

The last glacial period was punctuated by rapid climate shifts, known as Dansgaard–Oeschger events, with strong imprint in the North Atlantic sector suggesting that they were linked with the Atlantic meridional overturning circulation. Here an idealized single-hemisphere three-dimensional ocean–atmosphere–sea ice coupled model is used to explore the possible origin of the instability driving these abrupt events and to provide a plausible explanation for the relative stability of the Holocene. Focusing on the physics of noise-free millennial oscillations under steady external (solar) forcing, it was shown that cold climates become unstable, that is, exhibit abrupt millennial-scale transitions, for significantly lower freshwater fluxes than warm climates, in agreement with previous studies making use of zonally averaged coupled models. This fundamental difference is a direct consequence of the weaker stratification of the glacial ocean, mainly caused by upper-ocean cooling. Using a two-hemisphere configuration of a coupled climate model of intermediate complexity, it is shown that this result is robust to the added presence of a bottom water mass of southern origin. The analysis reveals that under particular conditions, a pronounced interdecadal variability develops during warm interstadials. While the nature of the instability driving the millennial oscillations is identical to that found in ocean models under mixed boundary conditions, these interstadial–interdecadal oscillations share the same characteristics as those previously found in ocean models forced by fixed surface fluxes. The wind stress forcing is shown to profoundly affect both the properties and bifurcation structure of thermohaline millennial oscillations across a wide range of variation of freshwater forcing. In particular, it is shown that the wind stress forcing favors the maintenance of thermally direct meridional overturning circulations during the cold stadial phases of Dansgaard–Oeschger cycles. [ABSTRACT FROM AUTHOR]

Throughout Earth's history, the oceans have played a dominant role in the climate system through the storage and transport of heat and the exchange of water and climate-relevant gases with the atmosphere. The ocean's heat capacity is ≈1,000 times larger than that of the atmosphere, its content of reactive carbon more than 60 times larger. Through a variety of physical, chemical, and biological processes, the ocean acts as a driver of climate variability on time scales ranging from seasonal to interannual to decadal to glacial-interglacial. The same processes will also be involved in future responses of the ocean to global change. Here we assess the responses of the seawater carbonate system and of the ocean's physical and biological carbon pumps to (i) ocean warming and the associated changes in vertical mixing and overturning circulation, and (ii) ocean acidification and carbonation. Our analysis underscores that many of these responses have the potential for significant feedback to the climate system. Because several of the underlying processes are interlinked and nonlinear, the sign and magnitude of the ocean's carbon cycle feedback to climate change is yet unknown. Understanding these processes and their sensitivities to global change will be crucial to our ability to project future climate change. [ABSTRACT FROM AUTHOR]

The factors driving glacial changes in ocean overturning circulation are not well understood. On the basis of a comparison of 20 climate variables over the past four glacial cycles, the SPECMAP project proposed that summer insolation at high northern latitudes (that is, Milankovitch forcing) drives the same sequence of ocean circulation and other climate responses over 100-kyr eccentricity cycles, 41-kyr obliquity cycles and 23-kyr precession cycles. SPECMAP analysed the circulation response at only a few sites in the Atlantic Ocean, however, and the phase of circulation response has been shown to vary by site and orbital band. Here we test the SPECMAP hypothesis by measuring the phase of orbital responses in benthic δ13C (a proxy indicator of ocean nutrient content) at 24 sites throughout the Atlantic over the past 425 kyr. On the basis of δ13C responses at 3,000–4,010 m water depth, we find that maxima in Milankovitch forcing are associated with greater mid-depth overturning in the obliquity band but less overturning in the precession band. This suggests that Atlantic overturning is strongly sensitive to factors beyond ice volume and summer insolation at high northern latitudes. A better understanding of these processes could lead to improvements in model estimates of overturning rates, which range from a 40 per cent increase to a 40 per cent decrease at the Last Glacial Maximum and a 10–50 per cent decrease over the next 140 yr in response to projected increases in atmospheric CO2 (ref. 4). [ABSTRACT FROM AUTHOR]

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods, but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal. Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that—together with previously published results—show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000–15,000 years ago), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition ∼14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration. We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic, which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir, but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions. [ABSTRACT FROM AUTHOR]

Climate change has reduced the extent and thickness of sea ice in the Arctic, making international shipping in the Northwest Passage a virtual certainty in the foreseeable future. Such future shipping raises the question of whether the Passage is or might become an international strait, with the consequent right of transit passage. This article examines the two possible legal bases for Canada's claim that the waters of the Canadian Arctic Archipelago are internal waters: a historic title and straight baselines. It also addresses the issue of the possible internationalization of the Passage, if Canada does not take preventive measures. Some such measures are recommended in the last part of the article. [ABSTRACT FROM AUTHOR]

A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced "heartbeat" in the global carbon cycle and periodicity of glaciations. This heartbeat consists of 405,000-, 127,000-, and 96,000-year eccentricity cycles and 1.2 million-year obliquity cycles in periodically recurring glacial and carbon cycle events. That climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle, solar forcing, and glacial events. Box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass. The pronounced 405,000-year eccentricity cycle is amplified by the long residence time of carbon in the oceans. [ABSTRACT FROM AUTHOR]

Abstract: A method for simulating vessel movement in an oceanic setting is described and used to provide temporal and spatial constraints to potential pre-historic transoceanic crossings into the Americas. The technique is based on current and wind output from numerical models and on leeway drift estimates from the United States Coast Guard. Simulations were carried out for present day (PD) and last glacial maximum (LGM) wind and current conditions. The fastest crossing time for the PD no-paddling experiments occurs between Australia and New Zealand in 23 days. The fastest transoceanic crossings occur between Central Europe and Iceland/Greenland in 72 days, Japan and North America in 83 days and northern Africa and South America in 91 days. The crossing with the highest probabilities of occurrence under 180 days occurs between southern Africa and South America (13%). This increases to 18% when paddling is allowed. Crossings from northern Africa to South America, from Australia to New Zealand and from New Zealand to South America have probabilities of occurrence . Differences between LGM and PD crossing times and rates of occurrence range between 25% and 30%, indicating that the PD results serve as a rough estimate of LGM crossings. Mid-latitude crossings are of shorter duration and more probable during the LGM. [Copyright &y& Elsevier]

Laterally extensive black shales were deposited on the São Francisco craton in southeastern Brazil during low-latitude Neoproterozoic glaciation ∼740 to 700 million years ago. These rocks contain up to 3.0 weight % organic carbon, which we interpret as representing the preserved record of abundant marine primary productivity from glacial times. Extractable biomarkers reflect a complex and productive microbial ecosystem, including both phototrophic bacteria and eukaryotes, living in a stratified ocean with thin or absent sea ice, oxic surface waters, and euxinic conditions within the photic zone. Such an environment provides important constraints for parts of the "Snowball Earth" hypothesis. [ABSTRACT FROM AUTHOR]

Abstract: We report C-isotopic data from the cap carbonate overlying the terminal Proterozoic Nantuo diamictites in south China. The C-isotopic measurements reveal a large C-isotopic gradient of ∼3‰ along paleoenvironmental transect from shelf to deep basinal sedimentary facies. The C-isotopic stratification may attribute to the effect of biological pumping in a postglacial ocean. Our results confirm that the sea-level rise was rapid and that atmospheric CO2 was significantly high immediately following deglaciation. [Copyright &y& Elsevier]

Abstract: We address the problem of reconstructing a global field from proxy data with sparse spatial sampling such as the MARGO (multi-proxy approach for the reconstruction of the glacial ocean surface) SST (sea-surface temperature) and O (oxygen-18/oxygen-16 isotope ratio preserved in fossil carbonate shells of planktic foraminifera) data. To this end, we propose to `assimilate'' these data into coupled climate models by adjusting some of their parameters and optimizing the fit. In particular, we suggest to combine a forward model and an objective function that quantifies the misfit to the data. Because of their computational efficiency, earth system models of intermediate complexity are particularly well-suited for this purpose. We used one such model (the University of Victoria Earth System Climate Model) and carried out a series of sensitivity experiments by varying a single model parameter through changing the atmospheric concentration. The unanalyzed World Ocean Atlas SST and the observed sea-ice concentration served as present-day targets. The sparse data coverage as implied by the locations of 756 ocean sediment cores from the MARGO SST database was indeed sufficient to determine the best fit. As anticipated, it turned out to be the 365ppm experiment. We also found that the 200ppm experiment came surprisingly close to what is commonly expected for the Last Glacial Maximum ocean circulation. Our strategy has a number of advantages over more traditional mapping methods, e.g., there is no need to force the results of different proxies into a single map, because they can be compared to the model output one at a time, properly taking into account the different seasons of plankton growth or varying depth habitats. It can be extended to more model parameters and even be automated. [Copyright &y& Elsevier]

Secular δ13Ccarb and Ceanom profiles in the post-glacial Neoproterozoic and Early Cambrian interval are reported from Guizhou on the southeasten border of the Yangtze platform, South China. Overall, the δ13Ccarb profile drifts to negative values in the post- glacial Nantuo and lower to middle Doushantuo Formations, and then to positive values in the upper Doushantuo and Dengying Formations of Neoproterozoic, followed by negative values in the Lower Cambrian Gezhongwu and Niutitang Formations. A detailed investigation of the relationship between the δ13Ccarb, and Ceanom profiles reveals that the main anoxic and oxic episodes are coupled with negative and positive δ13Ccarb values, respectively. This may suggest a control of alternation between ocean stratification and mixing on variations in 13C abundance in the ancient ocean of the investigated areas. [ABSTRACT FROM AUTHOR]

The spatial distribution of warm surface water in the Atlantic Ocean reflects the state of the thermohaline circulation. The Azores Current/Front, which is a recirculation of the Gulf Stream, marks the northeastern boundary of the North Atlantic subtropical gyre. Its position is therefore diagnostic of the width of the Atlantic warm water sphere. Here we report high resolution stable isotope and faunal abundance records of planktonic foraminifera in a sediment core from the Gulf of Cadiz (southwest Spain) which reflects shifting of the Azores Front since the Last Glacial Maximum (LGM). Today, the Azores Front does not penetrate into the Gulf of Cadiz, even though the front resides at the same latitude as the Gulf of Cadiz in the Atlantic. Our results indicate that the Azores Front is a robust feature of the Atlantic surface circulation, and that is present both in interglacial times and during the LGM at roughly the same latitude. However, during the LGM prior to 16 ka BP and during the Younger Dryas, the Azores Front did penetrate eastward into the Gulf of Cadiz. [Copyright &y& Elsevier]

Different reconstructions of glacial sea-surface temperatures (SST) are used to force a hybrid coupled atmosphere–ocean model. The resulting glacial states differ in global salinity and temperature distributions, and consequently in the strength of the thermohaline circulation. Stability analysis of the Atlantic Ocean circulation, by means of freshwater-flux hysteresis maps, reveals mono-stability for each glacial background state, which appears to be a robust feature of the glacial ocean. We show that this behaviour is directly linked to the hydrological cycle. A monotonic relation between the freshwater input necessary for reaching the off-mode and the hydrological budget in the Atlantic catchment area, accounts for the sensitivity of the ocean’s circulation. The most sensitive part of the hydrological balance appears to be in the tropical and subtropical regions suggesting that the ‘Achilles heel’ of the global conveyor belt circulation is not restricted to the northern North Atlantic where convection occurs. [ABSTRACT FROM AUTHOR]

Analyzing simulations of glacial ocean circulation and radiocarbon distribution I show that increased sea ice cover over the Southern Ocean reduces ventilation and radiocarbon content of the deep ocean. Two simulations, one present-day and one glacial, tuned to have similar rates of North Atlantic Deep Water formation are used. Insulation from air–sea gas exchange due to more extended sea ice cover increases Southern Ocean radiocarbon ages by more than 100 yr. Higher rates of sea ice formation and export from high southern latitudes lead to a salinification of Antarctic Bottom Water (AABW), making it the most saline deep water mass of the glacial world oceans. This increases the density of AABW and hence its rate of formation. Mass and radiocarbon fluxes to the deep ocean are thus decoupled. Both older Southern Ocean waters and a stronger flux of AABW increase radiocarbon ages of glacial bottom waters by up to 300 yr. Available reconstructions of glacial bottom water properties are broadly consistent with the simulation. These results question previous inferences from radiocarbon distributions on glacial deep water formation rates. [Copyright &y& Elsevier]

Two hypotheses have been put forward to explain the large and abrupt climate changes that punctuated glacial time. One attributes such changes to reorganizations of the ocean's thermohaline circulation and the other to changes in tropical atmosphere-ocean dynamics. In an attempt to distinguish between these hypotheses, two lines of evidence are examined. The first involves the timing of the freshwater injections to the northern Atlantic that have been suggested as triggers for the global impacts associated with the Younger Dryas and Heinrich events. The second has to do with evidence for precursory events associated with the Heinrich ice-rafted debris layers in the northern Atlantic and with the abrupt Dansgaard-Oeschger warmings recorded in the Santa Barbara Basin. [ABSTRACT FROM AUTHOR]

We use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from Ocean Drilling Program cores to reconstruct salinity and temperature of the deep ocean during the Last Glacial Maximum (LGM). Our data show that the temperatures of the deep Pacific, Southern, and Atlantic oceans during the LGM were relatively homogeneous and within error of the freezing point of seawater at the ocean's surface. Our chloride data show that the glacial stratification was dominated by salinity variations, in contrast with the modern ocean, for which temperature plays a primary role. During the LGM the Southern Ocean contained the saltiest water in the deep ocean. This reversal of the modern salinity contrast between the North and South Atlantic implies that the freshwater budget at the poles must have been quite different. A strict conversion of mean salinity at the LGM to equivalent sea-level change yields a value in excess of 140 meters. However, the storage of fresh water in ice shelves and/or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level. [ABSTRACT FROM AUTHOR]

A coupled atmosphere–ocean–sea ice–land surface climate system model developed at the Canadian Centre for Climate Modelling and Analysis (CCCma) is used to investigate the response to glacial ice-sheet topography and decreased CO2 representative of the last glacial maximum (LGM, roughly 21,000 years before present). Imposing these glacial boundary conditions leads to a strong and rapid climate adjustment, first in the atmosphere, and then in the ocean. We describe the model and boundary conditions and analyze the initial transient response. In a subsequent paper we analyze the near-equilibrium response. During the transient phase, surface air temperature rapidly decreases with time. The rapid cooling over the Laurentide and Fennoscandian ice sheets introduces a strong inter-hemispheric asymmetry. Sea surface temperature also decreases and after 80 years of integration, ocean surface temperature has decreased by 3 °C and global surface air temperature by 5 °C. The expected overall cooling is contrasted by marked regions of unexpected localized ocean surface warming and the retreat of sea ice in the northern North Atlantic and in the high latitude Southern Ocean. This strong transient behavior, associated with more vigorous oceanic convection at these high latitudes, represents the localization of the initial response in areas remote from the direct forcing change. The strength of the North Atlantic (Southern Ocean) overturning stream function, associated with the formation of North Atlantic Deep Water (Antarctic Bottom Water), increases from 12 to 36 Sv (2 to 26 Sv). There is a marked increase in the Antarctic Circumpolar Current transport as well (from 80 to 140 Sv), even though the wind stress decreases over the Southern Ocean, presumably as a consequence of the enhanced bottom pressure torque due to an increase in the density and the formation of Antarctic Bottom Water. Both precipitation and evaporation decrease on average with time and the change in the hydrological cycle modifies sea surface salinity (SSS). SSS increases in the Arctic Ocean due to a decrease in river runoff while it decreases in the North Atlantic due to an increase in river runoff and an increase in precipitation minus evaporation. The initial (and near equilibrium) climate response to LGM forcing in the fully coupled atmosphere–ocean climate model is quite different from that in the associated atmosphere-mixed layer ocean model due to very strong oceanic feedbacks. This is especially evident where oceanic convection is active. The anomalously warm surface temperature, especially in the Southern Ocean, and the marked increase in the overturning circulation are transient features of the response to imposed LGM boundary conditions which moderate or reverse as the system approaches equilibrium (as described in the companion paper). Other differences with the mixed layer simulation include a considerably colder tropical Pacific and the development of a La Niña-like response both of which are absent in the mixed layer simulations. [ABSTRACT FROM AUTHOR]

Investigates the glacial-interglacial stability of ocean hydrogen-ion concentration (pH). Inference of the stability from foraminifer dissolution rates; Role of ocean in the global carbon cycle; Absence of changes in the whole ocean pH.

ABSTRACT A simple ocean-atmosphere model suitable for long-term paleoclimate studies is presented. It consists of a three-basin zonally averaged ocean model coupled to an energy moisture-balance atmospheric model and a thermodynamic sea-ice model. The model is used to simulate the "present-day" climate conditions and the conditions resembling those of the last glacial maximum (LGM). In the first case, the model has an ocean thermohaline (THC) circulation in which the overturning in the North Atlantic is slightly more than 15 Sv. Two methods are used to spin up the model to the LGM climate. First, a cool state is obtained by running the coupled model with a reduced model greenhouse effect and increased albedo, but without making any explicit changes to the hydrological cycle. In the second approach, the model is directly set up for LGM conditions, in a manner similar to the method used to simulate the present-day climate. In both cases the computed LGM sea surface temperatures are reasonable, but only in the second case is a realistic ocean THC obtained. The LGM THC is steady and no self-sustained millennial-scale oscillations are present for conventional values of the diffusivity coefficients. The absence of these oscillations in the model and their presence in other simple climate models is discussed, and it is suggested how a combination of paleoceanographic data and modeling... [ABSTRACT FROM AUTHOR]

The climate at the Last Interglacial Maximum (125 000 years before present) is investigated with the atmosphere-ocean general circulation model ECHAM-1/LSG and with the climate system model of intermediate complexity CLIMBER-2. Comparison of the results of the two models reveals broad agreement in most large-scale features, but also some discrepancies. The fast turnaround time of CLIMBER-2 permits one to perform a number of sensitivity experiments to (1) investigate the possible reasons for these differences, in particular the impact of different freshwater fluxes to the ocean, (2) analyze the sensitivity of the results to changes in the definition of the modern reference run concerning CO2 levels (preindustrial versus “present”), and (3) estimate the role of vegetation in the changed climate. Interactive vegetation turns out to be capable of modifying the initial climate signals significantly, leading especially to warmer winters in large parts of the Northern Hemisphere, as indicated by various paleodata. Differences due to changes in the atmospheric CO2 content and due to interactive vegetation are shown to be at least of the same order of magnitude as differences between the two completely different models, demonstrating the importance of careful experimental design. [ABSTRACT FROM AUTHOR]

Looks at the climatic implications of the revised sea surface temperature field for the Last Glacial Maximum using an atmospheric general circulation model. Observations of modified seasonal air temperatures by cooler equatorial oceans; Suggestion that study results may help to resolve disagreements between oceanic and continental proxy climate data; Suggestion of a wind-related mechanism for enhancing the export of water vapour from the Atlantic to the Indo-Pacific oceans.