M. de Angelis

Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles

Sea ice and dust flux increased greatly in the Southern Ocean during the last glacial period. Palaeorecords provide contradictory evidence about marine productivity in this region, but beyond one glacial cycle, data were sparse. Here we present continuous chemical proxy data spanning the last eight glacial cycles (740,000 years) from the Dome C Antarctic ice core. These data constrain winter sea-ice extent in the Indian Ocean, Southern Ocean biogenic productivity and Patagonian climatic conditions. We found that maximum sea-ice extent is closely tied to Antarctic temperature on multi-millennial timescales, but less so on shorter timescales. Biological dimethylsulphide emissions south of the polar front seem to have changed little with climate, suggesting that sulphur compounds were not active in climate regulation. We observe large glacial–interglacial contrasts in iron deposition, which we infer reflects strongly changing Patagonian conditions. During glacial terminations, changes in Patagonia apparently preceded sea-ice reduction, indicating that multiple mechanisms may be responsible for different phases of CO2 increase during glacial terminations. We observe no changes in internal climatic feedbacks that could have caused the change in amplitude of Antarctic temperature variations observed 440,000 years ago.Sea ice and dust flux increased greatly in the Southern Ocean during the last glacial period. Palaeorecords provide contradictory evidence about marine productivity in this region, but beyond one glacial cycle, data were sparse. Here we present continuous chemical proxy data spanning the last eight glacial cycles (740,000 years) from the Dome C Antarctic ice core. These data constrain winter sea-ice extent in the Indian Ocean, Southern Ocean biogenic productivity and Patagonian climatic conditions. We found that maximum sea-ice extent is closely tied to Antarctic temperature on multi-millennial timescales, but less so on shorter timescales. Biological dimethylsulphide emissions south of the polar front seem to have changed little with climate, suggesting that sulphur compounds were not active in climate regulation. We observe large glacial–interglacial contrasts in iron deposition, which we infer reflects strongly changing Patagonian conditions. During glacial terminations, changes in Patagonia apparently preceded sea-ice reduction, indicating that multiple mechanisms may be responsible for different phases of CO2 increase during glacial terminations. We observe no changes in internal climatic feedbacks that could have caused the change in amplitude of Antarctic temperature variations observed 440,000 years ago.Its a long story...At over 3 km long, the ice core drilled at Dome C in Antarctica represents a record of 740,000 years, or eight glacial cycles. This will be the longest climate record available for years to come, so information gleaned from it will become a benchmark for Antarctic climate research. An examination of the core shows that sea ice around Antarctica waxed and waned in line with temperature over multimillennial timescales, but less so over shorter periods. During cold periods, larger amounts of dust were produced from a drier Patagonia, landing in the Southern Ocean where they probably affected marine productivity. Oceanic production of sulphur compounds, which might affect cloud nucleation, was remarkably constant throughout the period.Data from the Southern Ocean sea-ice extent, the biological productivity of the ocean, and atmospheric iron flux over the past eight glacial cycles indicate that during glacial terminations, changes in Patagonia apparently preceded Antarctic sea-ice reduction — showing that multiple mechanisms may be responsible for different phases of CO2 increase during glacial terminations.

Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I. Hydrological provinces

Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. The ridge has a northern peak at a depth of about 600 m, which is covered by extensive carbonate deposits, and an 800 m deep southern peak that is predominantly sediment covered. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes from the seafloor vary by several orders of magnitude at sites separated by distances of only a few meters. We identified three distinct active fluid regimes at Hydrate Ridge. The first province is represented by discrete sites of methane gas ebullition, where the bulk of the flow occurs through channels in which gas velocities reach 1 m s−1. At the northern summit of the ridge the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ∼6×104 mol day−1 following tidal periodicity. Qualitative observations at the southern peak suggest that the gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor. The second province is characterized by the presence of extensive bacterial mats that overlay sediments capped with methane hydrate crusts, both at the northern and southern summits. Here fluid typically flows out of the sediments at rates ranging from 30 to 100 cm yr−1. The third province is represented by sites colonized by vesicomyid clams, where bottom seawater flows into the sediments for at least some fraction of the time. Away from the active gas release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations. Methane fluxes out of mat-covered sites range from 30 to 90 mmol m−2 day−1, whereas at clam sites the methane flux is less than 1 mmol m−2 day−1.

Primary aerosol (sea salt and soil dust) deposited in Greenland ice during the last climatic cycle: Comparison with east Antarctic records

Ion chromatography data of Ca, Mg, Cl and Ca and Coulter® counter particle measurements are used to study the cycle of marine and continental primary aerosol reaching Greenland in relation to climatic changes over the last 150 kyr. A detailed comparison between Greenland (Dome Summit) and Antarctic (Vostok) records provides new insight on a potential link between northern and southern patterns. Ca is a good indicator of continental input and is mainly emitted as CaCO3. An attempt is made to estimate the contribution of aluminosilicates using the concentration of insoluble particles greater than 0.5 μm in diameter. The relative abundance of non-sea-salt Mg and Ca and of aluminosilicate shows that the calcium content of continental background aerosol over Greenland was much higher during the glacial age. The neutralization capacity of carbonaceous aerosol is estimated. The inverse relationship between δ18O and continental input as well as the response of this input to the rapid climatic variations that have occurred during the second part of the glacial age are discussed in terms of source and transport modification in relation to the presence and the extent of the great Laurentide ice cap. The corresponding Vostok profiles strongly suggest that some of the phenomena observed at high northern latitudes are of global concern. The marine component of Na (Nam) is a good tracer of sea-salt aerosol. Similarly to continental input, it shows an inverse but more linear relationship with δ18O. The sensitivity of Greenland and Antarctic marine input to climate variations of small and large amplitude is compared, and a corresponding estimation is made for the aeolian contribution. The respective influence of atmospheric circulation and the water vapor cycle is discussed. The chlorine to marine sodium weight ratio increases with temperature from values very close to the bulk seawater ratio during the last glacial maximum (18–20 kyr B.P.) to values significantly higher during the Holocene and warm Eemian. The corresponding excess of chloride (HCl) is discussed in terms of atmospheric transport, taking into account the role of atmospheric acidity on sea-salt fractionation processes. Owing to postdepositional phenomena, similar Vostok data must be considered cautiously. Nevertheless, aerosol fractionation seems to have been much more important over the Vostok site, except during glacial extrema.

Is methane venting at the seafloor recorded by δ13C of benthic foraminifera shells

The research was supported by WCNURP grant PF806880 and by NSF grants OCE-9731157, OCE-9815186, and OCE-9906944. Curation of sediment cores at the OSU/NORCOR repository is supported by NSF.

Field investigation of major and minor ions along Summit (Central Greenland) ice cores by ion chromatography

As a part of the European EUROCORE and GRIP (Greenland Ice Core Project) operations aimed at recovering deep ice cores at Summit (Central Greenland), we have for the first time successfully performed ion chromatography measurements in the field and investigated in detail the soluble impurities, including Na+, NH+4, K+, Mg2+, Ca2+, F−, CH3COO−, CH2 OHCOO−, HCOO−, CH3SO3−, Cl−, NO−2, SO42− and C2O42−, trapped in ice deposited over some 200 000 years in Greenland.

Sulfur‐containing species (methanesulfonate and SO4) over the last climatic cycle in the Greenland Ice Core Project (central Greenland) ice core

A high-resolution profile covering the last two centuries and a discontinuous study spanning the complete last glacial-interglacial cycle of methanesulfonate (MSA) (CH3SO3−) and sulfate were obtained along Summit (central Greenland) ice cores. MSA concentrations were close to 4±1.4 ng g−1 from 1770 to 1870 A.D. and 3 ng g−1 in 1900, and exhibited a well-marked decreasing trend from 1945 to the present. These changes of Summit snow MSA concentrations between 1770 and 1945 are discussed in terms of possible modulation of dimethylsulfide (DMS) marine emissions influencing the Greenland Ice Sheet by past climatic fluctuations in these regions. The decrease of MSA levels in Summit snow layers deposited since 1945 suggests either a decline in marine biota at high northern latitudes or a changing yield of MSA from DMS oxidation driven by modification of the oxidative capacity of the atmosphere in response to increasing anthropogenic NOx, and hydrocarbon emissions. While interglacial ice concentrations of MSA and sulfate are close to 2.9±1.9 ng g−1 and 27±10 ng g−1, respectively, reduced MSA (1.2±0.7 ng g−1) and enhanced sulfate (55±19 ng g−1) levels characterized the early Holocene stage (9000 to 11,000 years B.P.). MSA concentrations in glacial ice remain similar to the ones observed during interglacial stages. In contrast, sulfate levels are strongly enhanced (243±84 ng g−1) during the last glacial maximum (14,400 to 15,700 B.P.) compared with the interglacial ones. These variations of sulfur-containing species in response to past climatic conditions are similar to those found in other Greenland cores. In contrast, they are different from those revealed in the Antarctic Vostok ice core, where colder climates were associated with an increase by a factor of 5 and 2 in MSA and sulfate concentrations, respectively. These glacial-interglacial changes are discussed in terms of present and past contributions of marine DMS emissions versus other sulfate sources such as volcanic emissions and continental dust to the Greenland precipitation.

Soluble and insoluble impurities along the 950 m deep Vostok ice core (Antarctica) ― Climatic implications

Simultaneous measurements of soluble and insoluble impurities were made on the 950 m deep Vostok (78°30′S, 106°54′E, 3420 m a.s.l.) ice core, spanning roughly 50000 yr, using various analytical techniques. We observed higher continental (×37) and marine (×5.1) inputs during the last glacial age than during the Holocene stage. A study of microparticle compositions and of volcanic indicators (Zn, H2SO4), shows that the high observed crustal input is not due to enhanced volcanism, but is rather of continental eolian origin. For the first time, the ionic balance along a deep ice core is established, mainly used in discussing the evolution of the Cl to Na ratio over central East Antarctica with changing climatic conditions: the presence of relatively high amounts of Na2SO4 in the marine aerosol at the Vostok site during the Holocene is demonstrated. Comparison with the Dome C (74°39′S, 124°10′E, 3040 m a.s.l.) results confirms the chronology of the major events: (i) maximum terrestrial input around the last glacial maximum (∼18 ka BP); (ii) end of the high continental flux over Antarctica near 13 ka BP; (iii) marine input varying in an opposing manner to isotopic fluctuations with rather high concentrations beginning to decrease when isotopic values increase and reaching Holocene values at the end of the transition between cold and warmer climate conditions. Detailed comparison with results provided by deep ice cores from other sites which are probably more influenced by oceanic air masses seems to indicate that most of the aerosol reaching central East Antarctica travel over large distance probably at rather high altitude through the troposphere. We can consider that central East Antarctica is well representative of the upper part of the troposphere (higher than i.e., 3000 m) and should, therefore, provide valuable data for global and Antarctic paleoclimatological models.

Sources of continental dust over Antarctica during the last glacial cycle

The soluble and insoluble parts of 4 major components (Al, Ca, K and Mg) of the continental dust input over East Antarctica, as well as size, distribution parameters of the insoluble part of this dust, have been studied along an ice core which spanns the last climatic cycle (160 kyr). These results provide a better understanding of the respective impact of the different potential dust sources. While Al and K were probably entrapped in illite originating from arid areas and in a lesser extent from shallow marine sediments, Ca and Mg inputs were dominated by marine carbonate of exposed continental shelves emissions.

Comments on the origin of dust in East Antarctica for present and ice age conditions

We have studied the distribution of 327 clay mineral particles retrieved from four Antaretic ice smaples corresponding to present and Last Glacial Maximum (LGM) climate conditions. Illite, chlorite, smectite and kaolinite were identified in all samples. Focusing on kaolinite, because of its use as a possible tracer of low latitude soils, we find a significantly smaller amount for LGM samples while the dust concentration in snow during the LGM was about 30 times higher than for present climate conditions. This can be interpreted as change in the contribution of the Australian source with climate.A second approach was based on the modeling of the desert dust cycle using an Atmospheric General Circulation Model (AGCM) under both present-day and ice age conditions. Unlike mineralogical results, the model suggests the prevalence of the Australian dust source in the deposits over East Antarctica under both present-day and LGM climate conditions. However the model fails to reproduce the strong increase in dust deposits during the LGM. This discrepancy could be partly due to the lack of a higher latitude dust source in the model.The stronger dust input recorded in ice cores for the LGM could be related to an additional active high latitude source (possibly close to South America) overlapping the atmospheric background coming from low latitude areas.

Reassessing Lake Vostok's behaviour from existing and new ice core data

Abstract Interpretation of new ice core data and reappraisal of existing data, both from the basal part of the Vostok ice core, give strong support to a kind of thermohaline circulation in Lake Vostok. Although the salinity of the lake is considered as weak (less than 1‰), the prominent influence of salinity at high pressure and low temperature on water density makes such a circulation possible. As a consequence, subglacial melting along the northern shores of the lake is balanced, further south, by frazil ice production in the upper water column, its accretion and consolidation at the ice–water interface followed by accreted ice export out of the system together with the southeasterly glacier flow. The dynamics of the system is documented by a stable water isotope budget estimate, by inferences concerning accreted ice formation and by an investigation of ice properties at the transition between meteoric ice and accreted ice. This complex behaviour is the controlling factor on water, biota and sediment fluxes in the lake environment.