Orbital Observations of Dust Lofted by Daytime Convective Turbulence

Over the past several decades, orbital observations of lofted dust have revealed the importance of mineral aerosols as a climate forcing mechanism on both Earth and Mars. Increasingly detailed and diverse data sets have provided an ever-improving understanding of dust sources, transport pathways, and sinks on both planets, but the role of dust in modulating atmospheric processes is complex and not always well understood. We present a review of orbital observations of entrained dust on Earth and Mars, particularly that produced by the dust-laden structures produced by daytime convective turbulence called “dust devils”. On Earth, dust devils are thought to contribute only a small fraction of the atmospheric dust budget; accordingly, there are not yet any published accounts of their occurrence from orbit. In contrast, dust devils on Mars are thought to account for several tens of percent of the planet’s atmospheric dust budget; the literature regarding martian dust devils is quite rich. Because terrestrial dust devils may temporarily contribute significantly to local dust loading and lowered air quality, we suggest that martian dust devil studies may inform future studies of convectively-lofted dust on Earth

Loess as a Quaternary paleoenvironmental indicator

Loess (aeolian silt) is widespread in Eurasia and the Americas. Paleowind direction and wind strength can be reconstructed from spatial and temporal trends of loess thickness and particle size. Fossil land snails in loess can reveal much about past climate and vegetation. Loess is aeolian sediment that is dominated by silt-sized particles. Unlike either coarser dune sand or finer-grained, long-rangetransported dust, loess is relatively poorly sorted, reflecting a combination of transport processes, including saltation, low suspension, and high suspension. Loess can be readily identified in the field; deposits range in thickness from a few centimeters to many tens of meters, and are found over large areas of Eurasia, South and North America (Fig. 1), and smaller areas of New Zealand, Australia, Africa and the Middle East. Loess covers approximately 10% of the Earth’s land surface and is therefore one of the most important terrestrial archives of paleoenvironmental change during the Quaternary. In many regions, loess sections consist of deposits of mostly unaltered sediment with intercalated paleosols. Paleosols represent periods of landscape stability when loess deposition ceased altogether, or at least slowed significantly. Loess can be dated directly using luminescence, radiocarbon, and amino acid geochronology methods

Carbonate drifts as marine archives of aeolian dust (Santaren Channel, Bahamas)

Sediment data from the Bahamian Santaren carbonate drift reveal the variability of trans-Atlantic Saharan dust transport back to about 100 ka BP (Marine Isotope Stage 5-3) and demonstrate that carbonate drifts are a valuable pelagic archive of aeolian dust flux. Carbonate drift bodies are common around tropical carbonate platforms; they represent large-scale accumulations of ocean-current transported material, which originates from the adjacent shallow-water carbonate factory as well as from pelagic production, i.e. periplatform ooze. Subordinately, there is a clay-size to silt-size non-carbonate fraction, which typically amounts to less than 10% of the sediment volume and originates from aeolian and fluvial input. Sedimentation rates in the 5.42 m long core GeoHH-M95-524 recovered 25 km west of Great Bahama Bank in the Santaren Channel ranges from 1-5 to 24.5 cm ka-1 with lowest values during the last glacial lowstand and highest values following platform re-flooding around 8 ka BP. These sedimentation rates imply that carbonate drifts have not only the potential to resolve long-term environmental changes on orbital timescales, but also millennial to centennial fluctuations during interglacials. The sediment core has been investigated with the aim of characterizing the lithogenic dust fraction. Laboratory analyses included X-ray fluorescence core scanning, determination of carbonate content and grain-size analyses (of bulk and terrigenous fraction), as well as visual inspections of the lithogenic residue; the age model is based on oxygen isotopes and radiocarbon ages. Data show that the input of aeolian dust in the periplatform ooze as indicated by Ti/Al and Fe/Al element ratios abruptly increases at 57 ka BP, stays elevated during glacial times, and reaches a Holocene minimum around 6.5 ka BP, contemporary to the African Humid Period. Subsequently, there is a gradual increase in dust flux which almost reaches glacial levels during the last centuries. Grain-size data show that the majority of dust particles fall into the fine silt range (below 10 µm); however, there is a pronounced coarse dust fraction in the size range up to 63 µm and individual ‘giant’ dust particles are up to 515 µm in size. Total dust flux and the relative amounts of fine and coarse dust are decoupled. The time-variable composition of the grain-size spectrum is interpreted to reflect different dust transport mechanisms: fine dust particles are delivered by the trade winds and the geostrophic winds of the Saharan Air Layer, whereas coarse dust particles travel with convective storm systems. This mode of transport ensures continuous re-suspension of large particles and results in a prolonged transport. In this context, grain-size data from the terrigenous fraction of carbonate drifts provide a measure for past coarse dust transport, and consequently for the frequency of convective storm systems over the dust source areas and the tropical Atlantic

Interglacial History of a Palaeo-lake and Regional Environment: A Multi-proxy Study of a Permafrost Deposit from Bolshoy Lyakhovsky Island, Arctic Siberia

Chironomid, pollen, and rhizopod records from a permafrost sequence at the Bolshoy Lyakhovsky Island (New Siberian Archipelago) document the evolution of a thermokarst palaeo-lake and environmental conditions in the region during the Last Interglacial (MIS 5e, ca. 130120 ka). Open Poaceae and Artemisia associations dominated vegetation at the beginning of the interglacial period, ca. 130 ka. Rare shrub thickets (Salix, Betula nana, Alnus fruticosa) grew in more protected and wetter places as well. Saalian ice wedges started to melt during this time, resulting in the formation of an initial thermokarst water body. The high percentage of semi-aquatic chironomids suggests that a peatland-pool palaeo-biotope existed at the site, when initial water body started to form. A distinct decrease in semi-aquatic chironomid taxa and an increase in lacustrine ones point to a gradual pooling of water in basin, which could in turn create thaw a permanent pond during the subsequent period. The highest relative abundance of Chironomus and Procladius reflects an existence of unfrozen water remaining under the ice throughout the ice-covered period during the later stage of palaeo-lake development. Chironomid record points to three successive stages during the water body evolution: (1) a peatland pool; (2) a pond (i.e., less deep than the maximum ice-cover thickness); and (3) a shallow lake (i.e., more deep than the maximum ice-cover thickness). The evolutionary trend of palaeo-lake points to intensive thermokarst processes occurring in the region during the Last Interglacial. Shrub tundra communities with Alnus fruticosa, Betula nana dominated the vegetation during the interglacial optimum that is evidenced by pollen record. The climate was relatively moist and warm. The results of this study suggest that quantitative chironomid-based temperature reconstructions from the Arctic thermokarst ponds/lakes may be problematic owing to other key environmental factors, such as prolonged periods of winter anoxia and local hydrological/geomorphological processes, controlling the chironomid assemblage