15 research outputs found
Aeolian particles in marine cores as a tool for quantitative high-resolution reconstruction of upwelling favorable winds along coastal Atacama Desert, Northern Chile
Upwelling areas play a major role in ocean biogeochemical cycles and ultimately in global climate, especially in higly productive regions as the South Eastern Pacific. This work is based on the analysis of the aeolian lithic particles accumulated in laminated sediments off Mejillones (23 degrees S) in the eastern boundary Humboldt Current System. It proposes a high-resolution quantitative reconstruction of the upwelling-favorable southerly wind strength in the past similar to 250 years, comparing its variability with changes in organic carbon export/preserved changes to the sea bottom. The increase of the intensity and variability in fluxes of particles larger than 35 mu m and 100 mu m since the second half of the 19th century and during the 20th century confirms a general strengthening of southerly winds in the region. Spectral analysis on the complete time-series of yearly depositional fluxes indicates that sedimentary variability can be explained by a combination of interannual (ENSO) to decadal (PDO) oscillations similar to the ones yielded by the analysis of the Interdecadal Pacific Oscillation index. However, when applied separately to the lithic fluxes of the first and last centuries of the time-series, the method shows that relative to the one of the interannual mode of variability, the influence of the decadal mode has increased in the recent period. Based on the presence/absence of particles with sizes larger than 35/100 mu m, each year of the time series is classified as a 'Low wind' (12 m/s) year. From the AD 1754-1820 period to the AD 1878-1998 one, the proportion of Low and Intermediate wind years decreased from 12% and 74% to 3% and 68%, respectively, whereas the proportion of strong wind years increased from 14% to 29%. For these periods the mean organic carbon also increased 22%, stating the strong relation between export/preservation productivity rate and southerly wind intensity. In the recent period (from AD 1950 on) for which the Oceanic Nino Index is available, the strong wind years (AD 1982, 1983, 1994, and 1997) correspond to large values of this index, suggesting that constructive interferences that result from the interplay between interannual and decadal oscillations modes might explain in part the reinforcement of the winds along the North Chilean coast
The South American monsoon variability over the last millennium in climate models
In this paper we assess South American monsoon system (SAMS) variability in
the last millennium as depicted by global coupled climate model simulations.
High-resolution proxy records for the South American monsoon over this
period show a coherent regional picture of a weak monsoon during the
Medieval Climate Anomaly and a stronger monsoon during the Little Ice Age
(LIA). Due to the small external forcing during the past 1000Â years, model
simulations do not show very strong temperature anomalies over these two
specific periods, which in turn do not translate into clear precipitation
anomalies, in contrast with the rainfall reconstructions in South America.
Therefore, we used an ad hoc definition of these two periods for each model
simulation in order to account for model-specific signals. Thereby, several
coherent large-scale atmospheric circulation anomalies are identified. The
models feature a stronger monsoon during the LIA associated with (i)Â an
enhancement of the rising motion in the SAMS domain in austral summer;
(ii)Â a stronger monsoon-related upper-tropospheric anticyclone; (iii)Â activation
of the South American dipole, which results in a poleward shift of the South
Atlantic Convergence Zone; and (iv)Â a weaker upper-level subtropical jet
over South America. The diagnosed changes provide important insights into
the mechanisms of these climate anomalies over South America during the past
millennium
A simple model accounting for the uptake, transport, and deposition of wind-eroded mineral particles in the hyperarid coastal Atacama Desert of northern Chile
As previously observed in marine sediments collected downwind of African or South American continental sources, recent studies of sediment cores collected at the bottom of Mejillones Bay in north Chile (23 degrees S) show a laminated structure in which the amount of particles of aeolian origin and their size create significant differences between the layers. This suggests inter-annual to inter-decadal variations in the strength of the local southerly winds responsible for (1) the erosion of the adjacent hyperarid surface of the Mejillones Pampa, and (2) the subsequent transport of the eroded particles towards the bay. A simple model accounting for the vertical uptake, transport, and deposition of the particles initially set into motion by wind at the surface of the pampa is proposed. This model, which could be adapted to other locations, assumes that the initial rate of (vertical) uptake is proportional to the (horizontal) saltation flux quantified by means of White's equation, that particles are lifted to a height (H), increasing with the magnitude of turbulence, and that sedimentation progressively removes the coarsest particles from the air column as it moves towards the bay. In this model, the proportionality constant (A) linking the vertical flux of particles with the horizontal flux, and the injection height (H) control the magnitude and size distribution of the deposition flux in the bay. Their values are determined using the wind speed measured over the pampa and the size distribution of particles collected in sediment traps deployed in the bay as constraints. After calibration, the model is used to assess the sensitivity of the deposition flux to the wind intensity variations. The possibility of performing such quantitative studies is necessary for interpreting precisely the variability of the aeolian material in the sediment cores collected at the bottom of Mejillones Bay
Inter-annual variability of southerly winds in a coastal area of the Atacama Desert : implications for the export of aeolian sediments to the adjacent marine environment
The analysis of the aeolian content of marine cores collected off the coast of the Atacama Desert (Mejillones Bay, Chile) suggests that marine sediments can record inter-annual to inter-decadal variations in the regional southerly winds responsible for particle entrainment at the surface of the nearby desert. However, the establishment of a simple and direct correlation between the sediment and wind records is complicated by the difference of time scales between the erosion and accumulation processes. The aim of this work is to: (i) assess the inter-annual variability of the surface winds responsible for the sand movements; and (ii) determine whether the integration over periods of several months completely smoothes the rapid changes in characteristics of the transported and deposited aeolian material. To accomplish this aim, 14 years of 10 m hourly wind speed, measured at the Cerro Moreno (Antofagasta) Airport between 1991 and 2003 and at the Orica Station between 2000 and 2004, were analyzed. For each year, the wind speed statistical distribution can be represented by a combination of two to three Weibull functions. Winds of the lowest Weibull mode are too weak to move the sand grains at the surface of the pampa; this is not the case for the intermediate mode and especially for the highest speed mode which are able to erode the arid surface and transport particles to the bay. In each individual year of the period of study, the highest speed mode only accounted for a limited number of strong erosion events. Quantitative analysis of the distribution of the friction velocities and of their impact on erosion using a saltation model suggests that, although all wind speeds above threshold produce erosion events, values around 0.45 m sec-1 contribute less to the erosion flux. This gap allows separation of the erosion events into low and high saltation modes. The correlation (r = 0.997) between the importance of the third Weibull mode and the extent of higher rate saltation indicates that the inter-annual variability of the erosion at the surface of the pampa, as well as the transport of coarse particles (>100 mu m), are directly related to inter-annual variations in the prevalence of the strongest winds. Finally, a transport and deposition model is used to assess the possible impact of the wind inter-annual variability on the deposition flux of mineral particles in the bay. The results suggest that inter-annual differences in the wind speed distributions have a quantifiable effect on the intensity and size-distribution of this deposition flux. This observation suggests that a detailed analysis of the sediment cores collected from the bay could be used for reconstructing the inter-annual variability of past winds
Aeolian erosion and sand transport over the Mejillones Pampa in the coastal Atacama Desert of northern Chile
The Mejillones Peninsula in the coastal Atacama Desert of northern Chile is a region in which ocean-atmosphere-land interactions are particularly strong, resulting in enhanced alongshore winds that erode the surface and transport sand particles to the sea. Because the aeolian particles in the laminated sediments at the bottom of Mejillones Bay record long-term changes in the intensity of prevailing southerly winds, it is fundamental to understand aeolian processes such as wind erosion and sand transport to improve paleoceanographic reconstructions. The aim of the present study is to characterize the wind erosion process over the flat geomorphology of the northern portion of the Mejillones Peninsula, the Mejillones Pampa, including the influence of wind erosion on the initial particle size distribution and the associated fractionation processes of the mineralogical composition of moving particles, through field measurements. In addition, we test the ability of an existing saltation model (MB95) to reproduce the variability of the erosion process during the field experiment. Soil samples from 17 locations on this flat surface contain significant amounts of highly erodible particles with diameters in the 200-300 mu m and 100-150 mu m size ranges. Aeolian particles collected in BSNE sand traps located at different heights near the surface, exhibit a bimodal size distribution similar to that of the erodible fraction of the soils: the abundance of the fine class increasing with height. Small stones that have a spatially variable distribution can locally reduce the intensity of wind erosion. The mineralogical composition of moving particles is similar to that of the soils, with quartz, feldspar and calcite as the most important minerals, followed by clay minerals, gypsum and amphibole. A value of u(t)* is calculated for each soil particle size class. Subsequently, the elementary contribution of each size class to the horizontal flux is calculated using White (1979)'s equation and the total flux is finally obtained by integration. The saltation model successfully reproduces the variability of the wind erosion process during the field experiment, but over-estimates the vertically integrated mass fluxes measured in situ by two orders of magnitude
Multisequal aeolian deposition during the Holocene in southwestern Patagonia (51°S) was modulated by southern westerly wind intensity and vegetation type
International audienceWe studied a multisequal soil succession (MSS) just south of Torres del Paine National Park (51°S), at the present-day core of the Southern Westerly Winds (SWW). The RĂo Serrano Section comprises paleosol horizons with associated intervening loess and sandy loess beds formed during the Holocene. Our record suggests strong and stable aeolian activity between âŒ9.3-7.2 ka followed by a decline with centennial-scale variations until âŒ5 ka. A strengthening commenced at âŒ5 ka and culminated in a maximum between âŒ2.2-0.5 ka with millennial-scale variations. Subsequent weakening of aeolian activity between âŒ0.5 and 0 ka was coeval with the deposition of a âŒ40 cm-thick paleosol, after which aeolian activity increased abruptly and reached an unprecedented maximum starting in the mid-20th century. The inferred wind intensity variations from our data bear partial agreement with competing hypotheses of SWW evolution, which postulate minimum SWW influence in SW Patagonia during the early Holocene and maximum influence during the Late Holocene, or vice versa. When analyzed through the lens of vegetation physiognomy/distribution and associated hydrological balance inferences from neighboring sites, our results suggest a primary control by precipitation and wind speeds associated to SWW strength at regional scale, modulated by the position of the forest/steppe ecotone east of the austral Andes. Human activities during the mid-20th century (deforestation, fire-regime shifts, livestock grazing, land use changes) caused an unprecedented increase in aeolian activity through decreased vegetation cover that increased sediment availability for aeolian transport, marking a striking difference with the magnitude of natural processes before the Anthropocene. Our results highlight the importance of climate change and natural/human-driven changes in vegetation cover for deciphering wind intensity histories, particularly in the transition from humid to semiarid environments along the eastern slope of the southern Patagonian Andes
Linnéfest och doktorspromotion 23 och 26 maj 2007
Dust storms that develop along the Pisco-Ica desert in Southern Peru, locally known as "Paracas" winds have ecological, health and economic repercussions. Here we identify dust sources through MODIS (Moderate Resolution Imaging Spectroradiometer) imagery and analyze HYSPLIT (Hybrid Single Particles Lagrangian Integrated Trajectory) model trajectories and dispersion patterns, along with concomitant synoptic-scale meteorological conditions from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis (NCEP/NCAR). Additionally, surface pressure data from the hourly METeorological Aerodrome Report (METAR) at Arica (18.5 degrees S, 70.3 degrees W) and Pisco (13.7 degrees S, 76.2 degrees W) were used to calculate Alongshore (sea-level) Pressure Gradient (APG) anomalies during Paracas dust storms, their duration and associated wind-speeds and wind directions. This study provides a review on the occurrence and strength of the Paracas dust storms as reported in the Pisco airfield for five-year period and their correspondence with MODIS true-color imagery in terms of dust-emission source areas. Our results show that most of the particle fluxes moving into the Ica-Pisco desert area during Paracas wind events originate over the coastal zone, where strong winds forced by steep APGs develop as the axis of a deep mid-troposphere trough sets in along north-central Chile. Direct relationships between Paracas wind intensity, number of active dust-emission sources and APGs are also documented, although the scarcity of simultaneous METAR/MODIS data for clearly observed MODIS dust plumes prevents any significant statistical inference. Synoptic-scale meteorological composites from NCEP/NCAR reanalysis data show that Paracas wind events (steep APGs) are mostly associated with the strengthening of anticyclonic conditions in northern Chile, that can be attributed to cold air advection associated with the incoming trough. Compared to the MODIS images, HYSPLIT outputs were able to spatially reproduce trajectories and dust dispersion plumes during the Paracas wind storms. HYSPLIT trajectories revealed that part of the wind-eroded lithological material can be transported downwind several kilometers along the Peruvian coast and also deposited over the nearby coastal ocean, giving support to the presence of an aeolian signal in continental shelf sediments, of great importance for paleoenvironmental studies
ANTIFUNGAL AND ANTIBACTERIAL ACTIVITY OF DITERPENES ISOLATED FROM WOOD EXTRACTABLES OF CHILEAN PODOCARPACEAE
Surface wind strength and sea surface temperature connections along the south peruvian coast during the last 150Â years
International audienc