Jean-Louis Tison

High-resolution record of Northern Hemisphere climate extending into the last interglacial period

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

One-to-one coupling of glacial climate variability in Greenland and Antarctica.

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth’s climate dynamics. For the last glacial period, ice core studies have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard–Oeschger events in Greenland through the Atlantic meridional overturning circulation. It has been unclear, however, whether the shorter Dansgaard–Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland, the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard–Oeschger events by the bipolar seesaw6. The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

Segregation of solutes and gases in experimental freezing of dilute solutions: implications for natural glacial systems

Low ionic strength waters containing significant calcium and bicarbonate are common in nature, but little literature exists on their behaviour during freezing. Modelling indicates that freezing-induced concentration of solutes (in a closed-system) would lead to progressive increase in calcite saturation index, despite rising partial pressure of CO2 (PCO2), but the consequences of CaCO3 precipitation for the distribution of matter between solid, liquid, and gas phases required experimental investigation. We studied the effects of variations in the rate of advance of an ice-water interface and in the initial degree of saturation for calcite on the behaviour of the system. Downward growth of ice in a 24-cm diameter cylindrical vessel was achieved at a constant linear rate of 3 or 8 mm/h by the progressive cooling of an overlying alcohol reservoir, and the expansion of volume accommodated by regular water sampling through side ports, together with a small expansion chamber. Initial air-saturated solutions (initial PCO2 in the range 10−3 to 10−3.2) were prepared to reflect a range from strongly undersaturated to supersaturated for calcite. Comparative blank experiments were run using deionized water. Ice growth led to enrichment in solutes at the ice-water interface and the creation of a diffusive boundary layer, calculated to be 0.6 mm thick, truncated below by convecting fluid. The first-formed ice (stage 1), was relatively solute-rich because of initial rapid ice nucleation. Where solutions were not strongly supersaturated for calcite this was followed by formation of a solute-poor (stage 2) ice. Ice-interface water segregation coefficients of stage 2 ice were calculated to be 0.0004–0.003 for various solute ions. The relative magnitude of segregation coefficients (Mg2+ > Ca2+ > Sr2+) is attributed to interstitial incorporation (coupled with HCO3−) in the ice lattice, and controlled by ion size. Air bubbles nucleated once nitrogen supersaturation had reached values of 2–2.5 in the boundary layer and were incorporated into ice. These gas inclusions had dissolved air compositions modified by the differential diffusion of O2, N2, and CO2 out of the boundary layer, an O2/N2 ratio of 0.4 being characteristic. Freezing of solutions strongly supersaturated for calcite led to formation of impure (stage 3) ice in which ions are incorporated in similar proportions to those of the parent aqueous solution. Stage 3 ice contains both solid CaCO3 and aqueous (solute-rich) inclusions, associated with an irregular ice-water interface. Gas inclusions were invariably rich in CO2, up to 63% by volume, yet represented only a small proportion of the CO2 generated as a by-product of CaCO3 precipitation. These data allow a better understanding of the expected chemical characteristics of ice that has formed from freezing of bulk water, including river icings, basal ice of glaciers, and local refrozen layers in firn and glacier ice. Generation of CO2-rich gas bubbles by re-freezing is a powerful mechanism for modification of CO2 compositions of bulk gaseous inclusions in ice.

Modeling brine and nutrient dynamics in Antarctic sea ice: The case of dissolved silica

Sea ice ecosystems are characterized by microalgae living in brine inclusions. The growth rate of ice algae depends on light and nutrient supply. Here, the interactions between nutrients and brine dynamics under the influence of algae are investigated using a one-dimensional model. The model includes snow and ice thermodynamics with brine physics and an idealized sea ice biological component, characterized by one nutrient, namely, dissolved silica (DSi). In the model, DSi follows brine motion and is consumed by ice algae. Depending on physical ice characteristics, the brine flow is either advective, diffusive, or turbulent. The vertical profiles of ice salinity and DSi concentration are solutions of advection-diffusion equations. The model is configured to simulate the typical thermodynamic regimes of first-year Antarctic pack ice. The simulated vertical profiles of salinity and DSi qualitatively reproduce observations. Analysis of results highlights the role of convection in the lowermost 5–10 cm of ice. Convection mixes saline, nutrient-poor brine with comparatively fresh, nutrient-rich seawater. This implies a rejection of salt to the ocean and a flux of DSi to the ice. In the presence of growing algae, the simulated ocean-to-ice DSi flux increases by 0–115% compared to an abiotic situation. In turn, primary production and brine convection act in synergy to form a nutrient pump. The other important processes are the flooding of the surface by seawater and the percolation of meltwater. The former refills nutrients near the ice surface in spring. The latter, if present, tends to expell nutrients from the ice in summer

Natural iron fertilization of the Atlantic sector of the Southern Ocean by continental shelf sources of the Antarctic Peninsula

In large parts of the Southern Ocean, primary production is limited due to shortage of iron (Fe). We measured vertical Fe profiles in the western Weddell Sea, Weddell-Scotia Confluence, and Antarctic Circumpolar Current (ACC), showing that Fe is derived from benthic Fe diffusion and sediment resuspension in areas characterized by high turbulence due to rugged bottom topography. Our data together with literature data reveal an exponential decrease of dissolved Fe (DFe) concentrations with increasing distance from the continental shelves of the Antarctic Peninsula and the western Weddell Sea. This decrease can be observed 3500 km eastward of the Antarctic Peninsula area, downstream the ACC. We estimated DFe summer fluxes into the upper mixed layer of the Atlantic sector of the Southern Ocean and found that horizontal advection dominates DFe supply, representing 54 ± 15% of the total flux, with significant vertical advection second most important at 29 ± 13%. Horizontal and vertical diffusion are weak with 1 ± 2% and 1 ± 1%, respectively. The atmospheric contribution is insignificant close to the Antarctic continent but increases to 15 ± 10% in the remotest waters (>1500 km offshore) of the ACC. Translating Southern Ocean carbon fixation by primary producers into biogenic Fe fixation shows a twofold excess of new DFe input close to the Antarctic continent and a one-third shortage in the open ocean. Fe recycling, with an estimated “fe” ratio of 0.59, is the likely pathway to balance new DFe supply and Fe fixation.

Sea ice contribution to the air-sea CO2 exchange in the Arctic and Southern oceans

Although salt rejection from sea ice is a key process in deep-water formation in ice-covered seas, the concurrent rejection of CO2 and the subsequent effect on air–sea CO2 exchange have received little attention. We review the mechanisms by which sea ice directly and indirectly controls the air–sea CO2 exchange and use recent measurements of inorganic carbon compounds in bulk sea ice to estimate that oceanic CO2 uptake during the seasonal cycle of sea-ice growth and decay in ice-covered oceanic regions equals almost half of the net atmospheric CO2 uptake in ice-free polar seas. This sea-ice driven CO2 uptake has not been considered so far in estimates of global oceanic CO2 uptake. Net CO2 uptake in sea-ice–covered oceans can be driven by; (1) rejection during sea–ice formation and sinking of CO2-rich brine into intermediate and abyssal oceanic water masses, (2) blocking of air–sea CO2 exchange during winter, and (3) release of CO2-depleted melt water with excess total alkalinity during sea-ice decay and (4) biological CO2 drawdown during primary production in sea ice and surface oceanic waters.

Dynamics of pCO2 and related air-ice CO2 fluxes in the Arctic coastal zone (Amundsen Gulf, Beaufort Sea)

We present an Arctic seasonal survey of carbon dioxide partial pressure (pCO 2 ) dynamics within sea ice brine and related air-ice CO 2 fluxes. The survey was carried out from early spring to the beginning of summer in the Arctic coastal waters of the Amundsen Gulf. High concentrations of pCO 2 (up to 1834 matm) were observed in the sea ice in early April as a consequence of concentration of solutes in brines, CaCO 3 precipitation and microbial respiration. CaCO 3 precipitation was detected through anomalies in total alkalinity (TA) and dissolved inorganic carbon (DIC). This precipitation seems to have occurred in highly saline brine in the upper part of the ice cover and in bulk ice. As summer draws near, the ice temperature increases and brine pCO 3 shifts from a large supersaturation (1834 matm) to a marked undersaturation (down to almost 0 matm). This decrease was ascribed to brine dilution by ice meltwater, dissolution of CaCO 3 and photosynthesis during the sympagic algal bloom. The magnitude of the CO 2 fluxes was controlled by ice temperature (through its control on brine volume and brine channels connectivity) and the concentration gradient between brine and the atmosphere. However, the state of the ice-interface clearly affects air-ice CO 2 fluxes.

Chlorophyll a in Antarctic sea ice from historical ice core data

Sea ice core chlorophyll a data are used to describe the seasonal, regional and vertical distribution of algal biomass in Southern Ocean pack ice. The Antarctic Sea Ice Processes and Climate – Biology (ASPeCt – Bio) circumpolar dataset consists of 1300 ice cores collected during 32 cruises over a period of 25 years. The analyses show that integrated sea ice chlorophyll a peaks in early spring and late austral summer, which is consistent with theories on light and nutrient limitation. The results indicate that on a circum-Antarctic scale, surface, internal and bottom sea ice layers contribute equally to integrated biomass, but vertical distribution shows distinct differences among six regions around the continent. The vertical distribution of sea ice algal biomass depends on sea ice thickness, with surface communities most commonly associatedwith thin ice (<0.4m), and ice ofmoderate thickness (0.4– 1.0 m) having the highest probability of forming bottom communities.

Gas diffusion through columnar laboratory sea ice: implications for mixed‐layer ventilation of CO2 in the seasonal ice zone

Gas diffusion through the porous microstructure of sea ice represents a pathway for ocean.atmosphere exchange and for transport of biogenic gases produced within sea ice. We report on the experimental determination of the bulk gas diffusion coefficients, D, for oxygen (O2) and sulphur hexafluoride (SF6) through columnar sea ice under constant ice thickness conditions for ice surface temperatures between -4 and -12°C. Profiles of SF6 through the ice indicate decreasing gas concentration from the ice/water interface to the ice/air interface, with evidence for solubility partitioning between gas-filled and liquid-filled pore spaces. On average, DSF6 was 1.3 × 10-4 cm2 s-1 (±40%) and DO2 was 3.9 × 10.5 cm2 s-1 (±41%). The preferential partitioning of SF6 to the gas phase, which is the dominant diffusion pathway produced the greater rate of SF6 diffusion. Comparing these estimates of D with an existing estimate of the air.sea gas transfer through leads indicates that ventilation of the mixed layer by diffusion through sea ice may be negligible, compared to air.sea gas exchange through fractures in the ice pack, even when the fraction of open water is less than 1%.

Tank study of physico-chemical controls on gas content and composition during growth of young sea ice

During an ice tank experiment, samples were taken to study the processes of acquisition and alteration of the gas properties in young first-year sea ice during a complete growth-warming-cooling cycle. The goal was to obtain reference levels for total gas content and concentrations of atmospheric gases (O2, N2, CO2) in the absence of significant biological activity.The range of total gas content values obtained (3.5 to 18 ml of gas per kilo of ice) was similar to previous measurements or estimates. However, major differences occurred between the current and quiet basins, showing the role of water dynamics at the ice-water interface in controlling bubble nucleation processes.Extremely high CO2 concentrations were observed in all the experiments (up to 57% in volume parts). It is argued that these could have resulted from two unexpected biases in the experimental settings.Concentrations of bubbles nucleated at the interface are controlled by diffusion both from the ice-water interface towards the well-mixed reservoir, and between the interface water and the bubble itself. This double kinetic effect results in a transition of the gas composition in the bubbles from values close to solubility in sea water towards values close to atmospheric, as the ice cover builds up.