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The natural archives of paleoclimate history
Natural materials accumulating progressively over time and responding to environmental and climatic conditions constitute natural archives of paleoclimate history (Orombelli, 1996).
Paleoclimate proxies of different typologies and characteristics exist on Earth, and each of them is useful for specific purposes, and for a particular area and time interval.
The sequences of interest should be continuous in time, unaltered, and the “real” climatic signal must be separable from non-climatic “noises”. Moreover, the climatic dependency of each proxy has to be calibrated in order to estimate as precisely as possible the magnitude of climatic variations associated to each response.
A synthetic overview of the most important types of proxies is reported in Tab. 1.2. From each of them several variables can be measured, each one responding to one or more specific parameters of the Earth’s climate system. Ice cores are extremely valuable paleo-record, and are the unique proxy preserving the past atmospheric compositions, recorded in the entrapped air inclusions of the ice (Petit et al., 1999; see § 1.5).
The sample resolution and the length of the record that can be obtained from each proxy spans different orders of magnitude. Instrumental and historical records provide the finest sample resolution but are unable to go back in the past over a few hundred or thousand years respectively. Tree rings provide sequences up to ~104 years, pollen records can overstep ~105 years and marine records potentially provide the longest climatic sequences – in the order of ~106 years or more – but with the lowest temporal resolution.
Polar ice cores provide climatic records of variable length (until some hundred thousand years) and time resolution (from seasonal to annual). Ice core records are comparable to some rare highly-resolved marine records.
Climatic tales from the Vostok ice core
The first 3310 m of the 3623 m deep Vostok ice core preserve the climatic memory of the last 420 kyrs. The climatic and atmospheric records provided by this ice core (Petit et al., 1999) have a spatial significance spanning from regional to hemispheric or even global scale.
The stable isotope record (Fig. 1.8a), proxy for temperature variations (Fig. 1.9b), shows four glacial-interglacial cycles, characterized by a rapid “sawtooth” sequence of warm interglacials (Stages 11.3, 9.3, 7.5 and 5.5) followed by glacial periods increasingly colder and punctuated by cool interstadials. The end of each cycle is marked by a rapid (5-10 kyrs) return to warmer interglacial conditions.
The first-order variability of the marine and terrestrial aerosols is anti-correlated with the temperature record (Fig. 1.7d and 1.7e). In particular, the input of marine Na and dust is highest in glacial periods, lower in interstadials and mimimum in interglacials. The enhanced dust input during cold periods was primarily associated to the widespread aridity on the continents, to the high wind speeds and the reduced hydrological cycle. In such cold periods the mineral particles reaching Vostok likely had a southern South American provenance (Basile et al., 1997).
Southern Hemisphere atmospheric circulation
To understand variations in climate and atmospheric circulation in time and space, some of the salient features of the Southern Hemisphere (SH) circulation have to be taken into consideration. The SH is characterized by relatively small continental land masses over a large oceanic extent. These continental regions are mostly located at tropical and subtropical latitudes, and except the Antarctic continent only few sizable landmasses occur in the Southern Ocean south of 35°S. The atmospheric circulation in the SH follows the general scheme of Hadley and Ferrel cells, the first thermally-direct with ascending motion in equatorward regions and subsidence on its poleward extremity, the second with its ascending branch in the poleward side and the subsiding one in tsi equatorward side (Tyson, 1986).
The maximum subsidence and surface divergence is situated at about 30 °S, where the surface pressure field is characterised by large anticyclonic cells (South Atlantic, South Indian, and East Pacific anticyclones). These semi-permanent oceanic anticyclones control the weather between ~15°S and ~40°S, generating areas of reduced cloudiness and anti-clockwise direction of winds. Anticyclones occurring on the continental landmasses instead, are weaker, smaller and much more sensitive to climatic changes. Today the strongest continental high pressure cell is the Australian anticyclone, covering two thirds of the continent; the South African and south American ones are smaller and less stable.
South of the latitudes dominated by the oceanic and the continental ridges, the pressure drops to a minimum (60-70°S), forming a belt of cyclonic cells – the circumpolar trough – whose axis oscillates around 65°S (Schwerdtfeger, 1984), and probably around 62°S between 30°E and 150°E. Antarctica is characterized by a surface anticyclone and a permanent polar vortex8 at high altitude (Fig. 1.11).
The role of mineral dust on climate
It is now well established that the windblown mineral aerosol (dust) has a considerable influence on the climate system. However, there are still important uncertainties in the quantitative estimation of direct and indirect effects of dust on the global energy balance (IPCC, 2001) both in magnitude and sign.
The radiative impact of aerosols depends on its concentration within the atmosphere, but is also strongly dependent on in particle size distribution, shape and chemical composition. Actually, only particles smaller than 20 µm (in diameter) are considered for calculations of radiative effects since the atmospheric lifetime of larger particles is too short (Tegen and Lacis, 1996).
Dust scatters and partially absorbs incoming sunlight, playing a “greenhouse effect” in absorbing thermal radiation outgoing from the Earth. Changes in the amount of dust in the atmosphere therefore are capable to induce changes in radiation balance and consequently surface temperatures.
However, the sign and magnitude of radiative changes induced by mineral dust depend on many factors. Beside the vertical distribution of dust in the atmosphere, the presence of clouds and the brightness of the surface underlying the dust plume, a crucial role is exerted by the optical properties of the dust, depending in turn on the size and mineral composition (e.g. Sokolik and Toon, 1996; Claquin et al., 1998). Moreover, it has been previously observed (§ 2.2) that the shape and size (hence a different mineralogical nature) of the minerals composing the dust plume can vary significantly, and therefore the spatial and temporal forcing estimates are highly uncertain. In order to translate aerosol burdens into aerosol optical depths, and subsequently in radiative perturbation, four quantities have to be accurately known as function of wavelength (IPCC 2001): the mass light-scattering efficiency, the functional dependence of light-scattering on relative humidity, the single-scattering albedo and the asymmetry parameter.
There are also several indirect influences of mineral dust, on climate and atmospheric chemistry. The large specific surface areas provided by airborne minerals result in abundant crystallographic sites for heterogeneous condensation and subsequent reaction of gaseous species (e.g. Dentener et al., 1996). Moreover, dust can influence cloud droplet formation, with subsequent effects on cloud brightness and rainfall (e.g. Zhang and Carmicael, 1999). Dust can also moderate the photochemical processes (Dickerson et al., 1997). One important indirect radiative forcing is associated with the role of dust particles, and clays in particular, as ice forming nuclei (e.g. Rogers and Yau, 1989).
Mineral dust has also an important role on terrestrial (e.g. Swap et al., 1992) and marine (e.g. Hutchins and Brunland, 1998) ecosystems by providing nutrients. Some trace metals on dust are of crucial importance for some marine biological communities. Iron for example represents the limiting nutrient for phytoplankton communities in some ocean regions (e.g. Fung et al., 2000; Falkowski et al., 1998). The dust input consequently can potentially influence the global carbon cycle and the atmospheric greenhouse gas content. However, the magnitude of such a possible impact of dust is still highly uncertain.
The knowledge for present time
The importance of mineral aerosol on climate has led scientists to improve the study of physical and radiative characteristics of dust, and to improve dust-cycle models in order to assess the dust impacts on climate. The major progresses in the matter have been made in the last four-five years, but actually the available data sets used for model input parameters and for validating the simulations are still very limited1.
Atmospheric dust load
Global simulations of the modern dust cycle are capable to reproduce the first-order pattern of dust transport and deposition under modern climate conditions.
The estimates of the global dust emissions for present climate published in the literature span a very large range, from about 100 Mt/yr to about 3000 or even 5000 Mt/yr on a global scale (IPCC 2001, Tegen et al., 2002a), and 800-1700 Mt/yr according to one of the most recent estimates assembling the largest compilation of quantitative observations of the modern global dust cycle (Tegen et al., 2002a).
The IPCC 2001 estimates the mineral (soil) dust emission in ~2150 Mt/yr on global scale, with very high spatial and temporal variability. The estimate for the dust emission in the Southern Hemisphere is less than 1/5 the emission estimated for the Northern Hemisphere (about 350 and 1800 Mt/yr respectively).
These uncertainties are linked to the scarcity of global datasets used to determine the model input parameters and to validate model simulations: observations around the globe are very scarce and they are often representative of a limited period of time. Local observations are often extrapolated to yield a global estimate even if they come from specific regions having conditions that are not necessarily typical of all dust-source regions. Moreover, there are many desert areas in the world that are still too poorly studied.
Common characteristics of source terrains identified by satellite observations
In general terms, the regions providing the bigger dust fluxes at present time are primarily those with little or no ground cover, easily wind-erodible soils and associated to seasonal wetness (Mahowald et al., 1999).
A worldwide geographical mapping of major atmospheric dust sources has been recently provided by Prospero et al. (2002) on the basis of data from TOMS (Total Ozone Mapping Spectrometer) sensor on NIMBUS-7 satellite for a period of 13 years (1980-1992). The authors evidenced that the major sources for long-range transported dust are located in arid regions and are centered over topographical lows or on lands adjacent to topographical relief. Dry lake beds, relics of extensive lakes in the past, are a good example since lacustrine sediments are characteristically fine-grained, but also glacial outwash plains, riverine floodplains, alluvial fans and all areas where the recent geomorphological history has favoured the concentration of fine-grained material and the creation of large areas with low surface roughness are preferential sources (Tegen et al., 2002a).
Despite the arid (from semi-arid to hyper-arid) conditions of these environments at present time, these sources had either a relatively recent (Pleistocene) pluvial history or are associated with water features, such as ephemeral rivers and streams, alluvial fans, playas and saline lakes. Chemical weathering is enhanced by the abundance of water, and liquid transport is an efficient mechanism for production of small particles, separated from the soil or from the primary rock and carried to a depositional basins or an alluvial plain where, after drying, become mobilizable by wind (Prospero et al., 2002). Vegetation cover and soil crusting are also two important factors that can sensibly lower or suppress dust emission.
Sand dune systems do not appear good sources for long range transported dust (Prospero et al., 2002). This is not paradoxical, since they are relatively coarse grained, from tens to several hundred micrometers (Lancaster, 1995), and are already impoverished in the fine fraction. The coarse sandy particles have a high settling velocity in air and therefore are not carried more than few hundred kilometers away by winds; however, the role of sandy particles in generation of dust particles by saltation is crucial (Gillette et al., 1982).
Terrains with a recent history of aridity therefore, appear much more active sources than old arid sandy areas.
Source regions at global scale
The major global dust sources have been identified by Prospero et al. (2002) through TOMS Absorbing Aerosol Index (AAI). The authors constructed a world map, reported in Fig. 2.2, on the basis of the long term frequency of occurrence (FoO) distribution2 for each source, and therefore is not representative of a particular period of the year. The sources, in fact, show a large variability and characteristic geometries in function of the seasons.
The principal sources for dust that can potentially be transported long-distance are located in the Northern Hemisphere, in particular in North Africa, in the Middle East, in central Asia and in the Indian subcontinent.
For the Southern Hemisphere, observations evidence that it is devoid of major dust sources impacting on large areas. This observations are consistent with concentration of dust-derived Al element in southern-ocean waters, that is much lower than in northern oceans (Measures and Vink, 2000) and with the flux of aeolian materials in deep sea sediments (Rea, 1994).
The LGM atmospheric dust load: evidence from paleoclimate proxies
There is unequivocal and widespread evidence allover the Earth, from terrestrial (e.g. Ding et al., 1994; Kukla, 1989), marine (e.g. Rea, 1994) and polar (e.g. Petit et al., 1999) paleoclimate records for the Quaternary period, that the atmospheric dust load was considerably higher in glacial periods than during interglacials. Such increases however were neither globally uniform nor ubiquitous. Among the cold periods of the Pleistocene, the Last Glacial Maximum (onward LGM ~21 kyrs B.P.) has become a major focus for dust cycle modeling, largely because of the considerable amount of evidences documenting deposition rates and transport paths of aeolian lithogenic material, that are necessary for evaluation of model results.
The most recent compilation of literature data on dust accumulation rates for the LGM and the Holocene from ice cores, loess sediments, marine sediment traps and marine sediments is assembled in the DIRTMAP database4 (Kohfeld et al., 2001). Marine sediment records from low- and mid- latitudes show a smaller increase (maximum 5 times that of present time), and some equatorial regions actually show a decreased dust flux. Sediments from the North Pacific show an increase of 1-3 times during LGM, while those from the North Atlantic 2-5 times. A 2 to 9 fold increase in dust is found in LGM records downwind of Australian dust sources.
Table of contents :
Chap. 1 – QUATERNARY CLIMATE AND ENVIRONMENTAL CHANGES
1.1 Quaternary climate variability
1.2 The timescales of climate changes
1.3 Quaternary environmental changes: a short overview
1.4 The natural archives of paleoclimate history
1.4.1 Loess deposits
1.5 Climatic tales from the Vostok ice core
1.6 Southern Hemisphere atmospheric circulation
Chap. 2 – MINERAL DUST IN THE CLIMATE SYSTEM, TODAY AND IN THE PAST
2.1 Aerosol and dust
2.2 Mineralogical nature of aeolian dust
2.3 The role of mineral dust on climate
2.4 The knowledge for present time
2.4.1 Atmospheric dust load
2.4.2 Common characteristics of source terrains identified by satellite observations
2.4.3 Source regions at global scale
2.4.4 Principal dust “hot spots” in the Southern Hemisphere
2.5 The Last Glacial Maximum
2.5.1 The LGM atmospheric dust load: evidence from paleoclimate proxies
2.5.2 The climatic and environmental conditions during the LGM
2.5.3 The atmospheric dust load and the potential source regions: evidence from model simulations.
Chap. 3 – MINERAL DUST CYCLE FROM THE SOURCE TO THE SINK: CONCENTRATION AND SIZE DISTRIBUTION CHANGES
3.1 Dust mobilization at the source, “source strength”
3.2 The long-range transport
3.2.1. The horizontal dimension
3.2.2 The vertical dimension
3.3 The sink: dust in polar ice cores
3.3.1 Dust concentration: what information?
3.3.2 Dust size distribution: what information?
Chap. 4 – IDENTIFICATION OF DUST ORIGIN THROUGH THE Sr-Nd ISOTOPIC SIGNATURE
4.1 An introduction to the Rb-Sr isotopic systems
4.1.1 Geochemistry of Rb and Sr
4.2 An introduction to the Samarium and Neodymium isotopic systems
4.2.1 Geochemistry of Sm and Nd
4.3 The 87Sr/86Sr versus 143Nd/144Nd isotopic ratios
4.3.1 The Sr-Nd correlation diagram
4.4 87Sr/86Sr – 143Nd/144Nd isotopic signature as tracer for sediment provenance
4.4.1 The conditions of applicability and limits of the method
Chap. 5 – ANALYTICAL TECHNIQUES
5.1 Dust concentration and size distribution measurements
5.1.1 Decontamination and sample preparation
5.1.2 The measurement: principle, handling operations, limits of the technique
5.2 Analytical procedures and Sr-Nd measurements of PSA samples
5.2.1 Size selection
5.2.2 Leaching procedure
5.2.3 Sr and Nd extraction
5.2.4 Isotopic analysis of PSA samples
5.3 Analytical procedures and Sr-Nd measurements of Ice Core Dust (ICD)
5.3.1 Dust extraction from ice cores
5.3.2 Sr and Nd extraction from ice core dust
5.3.3 Isotopic analysis
Chap. 6 – THE SAMPLES ANALYZED IN THIS STUDY
6.1 Samples measured for dust concentration and size distribution
6.1.1 EPICA-Dome C ice core
6.1.2 The VOSTOK BH7 (VK-BH7) ice core
6.1.3 The Dome B (DB) ice core
6.1.4 The Komsomolskaia (KMS) ice core
6.2 Samples selected for Sr -Nd geochemical analysis
6.2.1 The PSA samples
6.2.2 Ice Core Dust samples
Chap. 7 – RESULTS AND DISCUSSION ABOUT DUST PROVENANCE
7.1 The isotopic signature of ice core dust
7.1.1 Glacial dust
7.1.2 interglacial dust
7.2 The signature of the PSAs of the Southern Hemisphere
7.2.1 Southern South America
7.2.2 New Zealand
7.2.3 The non-glaciated areas of Antarctica
7.2.4 South Africa
7.3 Discussion
7.3.1 The dust provenance in glacial periods
7.3.2 The interglacials
7.3.3 The Sr contribution from carbonates
Chap. 8 – RESULTS AND DISCUSSION ABOUT DUST VARIABILITY
8A: THE LAST CLIMATIC TRANSITION
8A.1 The first dust record from EPICA-Dome C (EDC) ice core
8A.2 High resolution EDC dust record: the last deglaciation and the Holocene
8A.3 Dome B, EDC and Komsomolskaya: two poles apart inside East Antarctica.
8B: DUST VARIABILITY IN THE HOLOCENE
8B.1 Holocene dust records from Vostok-BH7 and EPICA Dome C ice cores
8B.2 Holocene dust variability in the Dome B ice core
8B.3 Comparing the different sites
8B.3.1 Dome B and EDC ice cores
8B.3.2 Dome B, EDC and Vostok dust size spectra solar variability
8B.3.3 Periodicities nested in the dust concentration records
8C: A TENTATIVE SCENARIO FOR LGM AND HOLOCENE DUST TRANSPORT
8C.1 Factors influencing dust advection
8C.2 Dust variability over East Antarctica: a response to the atmospheric circulation patterns
8D: LATE QUATERNARY EDC DUST RECORD
Appendix I : Sm-Nd dating and Sr-Nd isotopic signature of a bedrock inclusion from Lake Vostok accretion ice
Appendix II : Supplementary tables
Appendix III : List of publication outcome from this thesis
References
