Precipitation as the main driver of Neoglacial fluctuations of Gualas glacier, Northern Patagonian Icefield

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Climate of the Past 8 (2012): 519-534, doi:10.5194/cp-8-519-2012.Glaciers are frequently used as indicators of climate change. However, the link between past glacier fluctuations and climate variability is still highly debated. Here, we investigate the mid- to late-Holocene fluctuations of Gualas Glacier, one of the northernmost outlet glaciers of the Northern Patagonian Icefield, using a multi-proxy sedimentological and geochemical analysis of a 15 m long fjord sediment core from Golfo Elefantes, Chile, and historical documents from early Spanish explorers. Our results show that the core can be sub-divided into three main lithological units that were deposited under very different hydrodynamic conditions. Between 5400 and 4180 cal yr BP and after 750 cal yr BP, sedimentation in Golfo Elefantes was characterized by the rapid deposition of fine silt, most likely transported by fluvio-glacial processes. By contrast, the sediment deposited between 4130 and 850 cal yr BP is composed of poorly sorted sand that is free of shells. This interval is particularly marked by high magnetic susceptibility values and Zr concentrations, and likely reflects a major advance of Gualas glacier towards Golfo Elefantes during the Neoglaciation. Several thin silt layers observed in the upper part of the core are interpreted as secondary fluctuations of Gualas glacier during the Little Ice Age, in agreement with historical and dendrochronological data. Our interpretation of the Golfo Elefantes glaciomarine sediment record in terms of fluctuations of Gualas glacier is in excellent agreement with the glacier chronology proposed for the Southern Patagonian Icefield, which is based on terrestrial (moraine) deposits. By comparing our results with independent proxy records of precipitation and sea surface temperature, we suggest that the fluctuations of Gualas glacier during the last 5400 yr were mainly driven by changes in precipitation in the North Patagonian Andes.This research was supported by an EU FP6 Marie Curie Outgoing Fellowship to S.B. Cruise NBP0505 was funded by the US National Science Foundation, Office of Polar Programs grant number NSF/OPP 03-38137 to J. Anderson (Rice University) and J. Smith Wellner (University of Houston). The Cimar-7 Program was supported by the Chilean National Oceanographic Committee (CONA, Grant C7F 01-10 to S. Pantoja)

Microscale spatial distributions of microbes and viruses in intertidal photosynthetic microbial mats

Intertidal photosynthetic microbial mats from the Wadden Sea island Schiermonnikoog were examined for microscale (millimetre) spatial distributions of viruses, prokaryotes and oxygenic photoautotrophs (filamentous cyanobacteria and benthic diatoms) at different times of the year. Abundances of viruses and prokaryotes were among the highest found in benthic systems (0.05–5.43 × 1010 viruses g−1 and 0.05–2.14 × 1010 prokaryotes g−1). The spatial distribution of viruses, prokaryotes and oxygenic photoautotrophs were highly heterogeneous at mm scales. The vertical distributions of both prokaryotic and viral abundances were related to the depth of the oxygenic photoautotrophic layer, implying that the photosynthetic mat fuelled the microbial processes in the underlying layer. Our data suggest that viruses could make an important component in these productive environments potentially affecting the biodiversity and nutrient cycling within the mat

Atmospheric deposition of iron from mineral aerosols to the ocean

Atmospheric deposition of iron (Fe) from mineral aerosols to the ocean has been suggested to increase the amount of ocean uptake of CO2 and emissions of marine biogenic aerosols. The marine organic material may be an important source of ice-nucleating particles in remote marine environment such as the Southern Ocean, North Pacific Ocean, and North Atlantic Ocean. Significant progress has been made in our understanding of atmospheric inputs of labile Fe from natural and anthropogenic sources to the surface oceans. Different emission sources and transformation processes affect aerosol Fe solubility. Mineral dust contains a small amount of labile Fe (e.g., ferrihydrite) on the surface (about 1% of Fe solubility) and thus may deliver insignificant labile Fe fluxes to the polar oceans in present days. However, about 10% of mean Fe solubility is measured for the Last Glacial Maximum (LGM) aerosols in Antarctica. If this value is applied to mineral dust during the LGM, the atmospheric input of labile Fe could be comparable to that provided by upwelling in present days. However, there are still large uncertainties regarding the relative importance of different sources of Fe and the effects of atmospheric aerosols on dissolved Fe in the ocean. Here, we use atmospheric chemistry transport model and ocean ecosystem model to investigate the effects of atmospheric deposition of Fe from mineral aerosols to dissolved Fe in the ocean, based on measurements in the North Atlantic. When a constant Fe solubility of 2% was used in the ocean model, the model overestimated the dissolved Fe concentration in the surface ocean downwind from the North African dust plume. Considering different degrees of atmospheric Fe processing reduced the overestimates. However, the atmospheric model underestimated labile Fe concentration over the Southern Ocean. Further investigation of the mechanisms of emissions, transport, and deposition of Fe-containing particles over the oceans is needed to improve our understanding of labile Fe supply to open ocean.Abstract A14B-08 presented at 2018 Fall Meeting, AGU, Washington D. C., 10-14 Dec

Photocatalytic chlorine atom production on mineral dust–sea spray aerosols over the North Atlantic

Active chlorine in the atmosphere is poorly constrained and so is its role in the oxidation of the potent greenhouse gas methane, causing uncertainty in global methane budgets. We propose a photocatalytic mechanism for chlorine atom production that occurs when Sahara dust mixes with sea spray aerosol. The mechanism is validated by implementation in a global atmospheric model and thereby explaining the episodic, seasonal, and location-dependent 13C depletion in CO in air samples from Barbados [J.E. Mak, G. Kra, T. Sandomenico, P. Bergamaschi, J. Geophys. Res. Atmos. 108 (2003)], which remained unexplained for decades. The production of Cl can also explain the anomaly in the CO:ethane ratio found at Cape Verde [K. A. Read et al., J. Geophys. Res. Atmos. 114 (2009)], in addition to explaining the observation of elevated HOCl [M. J. Lawler et al., Atmos. Chem. Phys. 11, 7617–7628 (2011)]. Our model finds that 3.8 Tg(Cl) y−1 is produced over the North Atlantic, making it the dominant source of chlorine in the region; globally, chlorine production increases by 41%. The shift in the methane sink budget due to the increased role of Cl means that isotope-constrained top–down models fail to allocate 12 Tg y−1 (2% of total methane emissions) to 13C-depleted biological sources such as agriculture and wetlands. Since 2014, an increase in North African dust emissions has increased the 13C isotope of atmospheric CH4, thereby partially masking a much greater decline in this isotope, which has implications for the interpretation of the drivers behind the recent increase of methane in the atmosphere

A review of coarse mineral dust in the Earth system

Mineral dust particles suspended in the atmosphere span more than three orders of magnitude in diameter, from <0.1 µm to more than 100 µm. This wide size range makes dust a unique aerosol species with the ability to interact with many aspects of the Earth system, including radiation, clouds, hydrology, atmospheric chemistry, and biogeochemistry. This review focuses on coarse and super-coarse dust aerosols, which we respectively define as dust particles with a diameter of 2.5–10 µm and 10–62.5 µm. We review several lines of observational evidence indicating that coarse and super-coarse dust particles are transported farther than previously expected and that the abundance of these particles is substantially underestimated in current global models. We synthesize previous studies that used observations, theories, and model simulations to highlight the impacts of coarse and super-coarse dust aerosols on the Earth system, including their effects on dust-radiation interactions, dust-cloud interactions, atmospheric chemistry, and biogeochemistry. Specifically, coarse and super-coarse dust aerosols produce a net positive direct radiative effect (warming) at the top of the atmosphere and can modify temperature and water vapor profiles, influencing the distribution of clouds and precipitation. In addition, coarse and super-coarse dust aerosols contribute a substantial fraction of ice-nucleating particles, especially at temperatures above –23 °C. They also contribute a substantial fraction to the available reactive surfaces for atmospheric processing and the dust deposition flux that impacts land and ocean biogeochemistry by supplying important nutrients such as iron and phosphorus. Furthermore, we examine several limitations in the representation of coarse and super-coarse dust aerosols in current model simulations and remote-sensing retrievals. Because these limitations substantially contribute to the uncertainties in simulating the abundance and impacts of coarse and super-coarse dust aerosols, we offer some recommendations to facilitate future studies. Overall, we conclude that an accurate representation of coarse and super-coarse properties is critical in understanding the impacts of dust aerosols on the Earth system

Glacial to Holocene swings of the Australian–Indonesian monsoon

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 4 (2011): 540–544, doi:10.1038/ngeo1209.The Australian-Indonesian monsoon is an important component of the climate system in the tropical Indo-Pacific region. However, its past variability, relation with northern and southern high latitude climate and connection to the other Asian monsoon systems are poorly understood. Here we present high-resolution records of monsoon-controlled austral winter upwelling during the past 22,000 years, based on planktic foraminiferal oxygen isotope and faunal composition in a sedimentary archive collected offshore southern Java. We show that glacial-interglacial variations in the Australian-Indonesian winter monsoon were in phase with the Indian summer monsoon system, consistent with their modern linkage through cross-equatorial surface winds. Likewise, millennial-scale variability of upwelling shares similar sign and timing with upwelling variability in the Arabian Sea. On the basis of element composition and grain-size distribution as precipitation-sensitive proxies in the same archive, we infer that (austral) summer monsoon rainfall was highest during the Bølling-Allerød period and the past 2,500 years. Our results indicate drier conditions during Heinrich Stadial 1 due to a southward shift of summer rainfall and a relatively weak Hadley Cell south of the Equator. We suggest that the Australian-Indonesian summer and winter monsoon variability were closely linked to summer insolation and abrupt climate changes in the northern hemisphere.This study was funded by the German Bundesministerium für Bildung und Forschung (PABESIA) and the Deutsche Forschungsgemeinschaft (DFG, HE 3412/15-1). DWO’s participation was funded by the U.S. National Science Foundation

Grain-size distributions of the terrigenous fraction of two deep-sea sediment cores GeoB3375-1 and MD96-2094

In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we 'unmix' into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental aridity in southwestern Africa and northern Chile to changes in the latitudinal position of the moisture-bearing Southern Westerlies, potentially driven by the sea-ice extent around Antarctica and overprinted by tropical forcing in the equatorial Pacific Ocean