Description: Mineral dust plays an important role in the atmospheric radiation budget as well as in the ocean carbon cycle through fertilization and by ballasting of settling organic matter. However, observational records of open‐ocean dust deposition are sparse. Here, we present the spatial and temporal evolution of Saharan dust deposition over 2 years from marine sediment traps across the North Atlantic, directly below the core of the Saharan dust plume, with highest dust fluxes observed in summer. We combined the observed deposition fluxes with model simulations and satellite observations and argue that dust deposition in the Atlantic is predominantly controlled by summer rains. The dominant depositional pathway changes from wet deposition in summer to dry deposition in winter. Wet deposition has previously been suggested to increase the release of dust‐derived nutrients and their bioavailability, which may be a key contributor to surface‐ocean productivity in remote and oligotrophic parts of the oceans.

Description: Mineral dust aerosol constitutes an important component of the Earth's climate system, not only on short timescales due to direct and indirect influences on the radiation budget but also on long timescales by acting as a fertilizer for the biosphere and thus affecting the global carbon cycle. For a quantitative assessment of its impact on the global climate, state-of-the-art atmospheric and aerosol models can be utilized. In this study, we use the ECHAM6.3-HAM2.3 model to perform global simulations of the mineral dust cycle for present-day (PD), pre-industrial (PI), and last glacial maximum (LGM) climate conditions. The intercomparison with marine sediment and ice core data, as well as other modeling studies, shows that the obtained annual dust emissions of 1221, 923, and 5159 Tg for PD, PI, and LGM, respectively, generally agree well with previous findings. Our analyses focusing on the Southern Hemisphere suggest that over 90 % of the mineral dust deposited over Antarctica are of Australian or South American origin during both PI and LGM. However, contrary to previous studies, we find that Australia contributes a higher proportion during the LGM, which is mainly caused by changes in the precipitation patterns. Obtained increased particle radii during the LGM can be traced back to increased sulfate condensation on the particle surfaces as a consequence of longer particle lifetimes. The meridional transport of mineral dust from its source regions to the South Pole takes place at different altitudes depending on the grain size of the dust particles. We find a trend of generally lower transport heights during the LGM compared to PI as a consequence of reduced convection due to colder surfaces, indicating a vertically less extensive Polar cell.

Description: Mineral dust plays an important role in the atmospheric radiation budget as well as in the ocean carbon cycle through fertilization and by ballasting of settling organic matter. However, observational records of open‐ocean dust deposition are sparse. Here, we present the spatial and temporal evolution of Saharan dust deposition over 2 years from marine sediment traps across the North Atlantic, directly below the core of the Saharan dust plume, with highest dust fluxes observed in summer. We combined the observed deposition fluxes with model simulations and satellite observations and argue that dust deposition in the Atlantic is predominantly controlled by summer rains. The dominant depositional pathway changes from wet deposition in summer to dry deposition in winter. Wet deposition has previously been suggested to increase the release of dust‐derived nutrients and their bioavailability, which may be a key contributor to surface‐ocean productivity in remote and oligotrophic parts of the oceans.

Description: The Southern Ocean (SO) has long been recognised as a key player in regulating atmospheric CO2 variations and hence global climate. Here, the biological utilisation of nutrients regulates the preformed nutrient inventory for most of the deep ocean and, therefore, the global average efficiency of the biological pump. Marine sediment records from the Subantarctic Atlantic and Pacific document that higher mineral dust flux, increased bioproductivity, and lower atmospheric CO2 co-varied on glacial-interglacial time scales, which has been associated with iron fertilisation. It has been suggested that up to 40-50 ppmv of past atmospheric CO2 changes are related to iron fertilisation in the Subantarctic Ocean. The main objective of DustIron is an improved characterisation of the modern and past dust cycle and its link to SO iron fertilisation and atmospheric CO2 through a closely coupled novel datamodel approach. Within this project we want to extend the available geographic coverage of modern dust deposition, provenance and marine productivity records as well as the spatial and temporal variability during past glacial-interglacial cycles across all SO sectors. For the modern ocean we will explore dust fluxes, grain-size, and geochemical source area fingerprints (iron mineralogy, isotopy). Iron fertilisation and productivity will be assessed with a variety of both traditional (e.g., fluxes of biogenic barium or opal) and novel proxies for nutrient utilisation (δ15N in foraminifera). Our paleostudies will focus on the last glacial-interglacial climate transitions from the western Atlantic proximal to the Patagonian sources and from the central Indian SO (Kerguelen) to the eastern Indian Ocean sector, in order to obtain a circum-Antarctic view of dust-productivity changes. This will be complemented by a modelling study, simulating glacial-interglacial changes of atmospheric dust concentrations, deposition fluxes and linked SO bioproductivity.

Description: Downcore sediment grain-size records of mineral dust (2–10 μm) can provide key insights into changes in wind strength and source-area characteristics over glacial-interglacial timescales. However, so far, little is known about glacial-interglacial changes of dust grain size in the open Southern Ocean, which are potentially associated with changes in the strength and position of the southern westerly winds. Here, we analyzed the grain-size distributions of subantarctic deep-sea sediments from the Pacific (PS75/056–1) and Atlantic (ODP Site 1090) sectors of the Southern Ocean, downwind of the major Southern Hemisphere dust source regions. Dust mean grain sizes show opposite trends in the two Southern Ocean sectors. Larger glacial grain sizes are observed in the Pacific sector, while finer glacial grain sizes are observed in the Atlantic sector. In the South Pacific, larger mean dust grain sizes parallel higher Fe fluxes during glacials. In contrast, in the South Atlantic record increased glacial Fe fluxes coincide with a decrease in glacial mean dust grain sizes consistent with some Antarctic ice core records. Our results suggest that the opposing grain-size trends are the result of different responses to glacial conditions in the sources and of changing wind and transport patterns. For the South Pacific, a possible explanation of our results could be an intensification of wind strength over Australia enabling emission of larger dust particles. This strengthening would imply a northward shift of the westerlies which facilitated the transport of dust from enhanced and/or more Australian and New Zealand sources. For the Atlantic, the decreased glacial dust grain size could be the consequence of increased glacial activity in the Patagonian Andes, generating and supplying more and finer-grained dust from the exposed continental shelf to the South Atlantic. These findings indicate that more extensive studies of wind-blown sediment properties in the Southern Ocean can provide important insights on the timing and latitudinal extent of climatic changes in the sources and variations of transport to the Southern Ocean by the westerly winds.

Description: Southern Ocean westerly wind intensity and position are thought to play a crucial role in controlling glacial/interglacial CO2 changes through their impact on Antarctic upwelling intensity and the delivery of iron-rich dust that stimulates biological production during glacial periods. Sediment-core grain size records can provide key insights into changes in wind strength and source-area characteristics over glacial-interglacial timescales. However, so far, little is known about G/IG grain size changes in Southern Ocean sediments. For this study, we analyzed the grain-size distributions of two subantarctic deep sea sediments cores from the Pacific (PS75/056-1) and Atlantic (ODP Site 1090) sectors of the Southern Ocean. Dust mean grain size shows opposing trends in the two Southern Ocean sectors. Coarser glacial grain sizes are observed in the Pacific sector, while finer glacial grain-sizes are observed in the Atlantic. Our results suggest that changes in the latitudinal position of the SWW had distinct impacts on grain size distribution in the Atlantic and Pacific sectors, also likely associated with shifts in the dust source areas. These findings indicate that more extensive studies of grain-size distribution in the Southern Ocean can provide important insights on the timing and latitudinal extent of the westerly winds changes during ice ages.

Description: Saharan dust is transported in great quantities from the North African continent every year, most of which is deposited across the North Atlantic Ocean. This dust impacts regional and global climate by affecting the atmospheric radiation balance and altering ocean carbon budgets. However, little research has been carried out on time series of Saharan dust collected in situ across the open Atlantic. Here, we present a unique three-year time series of Saharan dust along a trans-Atlantic transect, sampled by moored surface buoys and subsurface sediment traps. Results show a good correlation between the particle-size distributions of atmospheric dust and the lithogenic particles settling to the deep ocean floor, confirming the aeolian origin of the lithogenic particles intercepted by the subsurface sediment traps, even in the distal western part of the Atlantic Ocean. Dust from both dry and wet deposition as collected by the sediment traps, shows increased deposition fluxes and coarser grain size in summer and/or autumn that coincides with increased precipitation at the sampling sites as derived from satellite data. In contrast, both buoys that sampled dust during transport at sea level show little seasonal variation in both concentration and particle size, as the large amounts of dust transported in summer and early autumn at high altitudes are far above their sampling range. This implies that wet deposition in summer and autumn defines the typical seasonal trends of both the dust deposition flux and its particle-size distribution observed in the sediment traps.

Description: The particle sizes of Saharan dust in marine sediment core records have been used frequently as a proxy for trade-wind speed. However, there are still large uncertainties with respect to the seasonality of the particle sizes of deposited Saharan dust off northwestern Africa and the factors influencing this seasonality. We investigated a three-year time-series of grain-size data from two sediment-trap moorings off Cape Blanc, Mauritania and compared them to observed wind-speed and precipitation as well as satellite images. Our results indicate a clear seasonality in the grain-size distributions: during summer the modal grain sizes were generally larger and the sorting was generally less pronounced compared to the winter season. Gravitational settling was the major deposition process during winter. We conclude that the following two mechanisms control the modal grain size of the collected dust during summer: (1) wet deposition causes increased deposition fluxes resulting in coarser modal grain sizes and (2) the development of cold fronts favors the emission and transport of coarse particles off Cape Blanc. Individual dust-storm events throughout the year could be recognized in the traps as anomalously coarse-grained samples. During winter and spring, intense cyclonic dust-storm events in the dust-source region explained the enhanced emission and transport of a larger component of coarse particles off Cape Blanc. The outcome of our study provides important implications for climate modellers and paleo-climatologists.