Potential paleocirculation implications by coupling Pb and Nd isotope analyses on different grain-size fractions from Labrador Sea sediments

The production of deep water in the North Atlantic is one of the puzzle in understanding oceanic influence in climate changes. In this work, Nd and Pb isotopes were measured on the fine fraction of Labrador Sea sediments. Our aim is to record the relative contribution of fine particle supplies carried by the North Atlantic deep components into Labrador Sea. Based on characterization of potential geographical sources of particles, three main sources contribute to sediment mixture at core location: an old Precambrian crustal material from Canada, Greenland and Scandinavia (NAS), a Paleozoic or younger crustal material from East Greenland, Europa, and Scandinavia (YC) and a volcanic source from Iceland, Faeroe and Reykjanes Ridge (MAR). For the last 12 kyr, clay isotope signatures indicate two mixtures of sediment sources. The first mixture is composed of proximal material from Labrador Sea margins and distal deep current-driven crustal source. From 6.5 kyr onward, the mixture is characterized by the crustal and volcanic components. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of sediment sources suggests major changes in relative contributions of the deep water masses carried by the Western Boundary Undercurrent (WBUC) over the past 8.4 kyr. The progressive intensification of WBUC was associated mainly with the transport of North East Atlantic Deep Water mass until 6.5 kyr and with Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 kyr suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Sea Water mass. Our isotopic dataset emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene. However, if the clay-size fraction gives informations about the inception and presence of a deep current, it does tell nothing about the strength of this current. Moreover, clays are not likely to be deposited in case of high-strength deep current, resulting in an uncomplete and/or biased reconstruction of deep current evolution. To overcome this problem, it is necessary to look also at coarser fractions. Pb and Nd isotope compositions were thus analysed by MC-ICP-MS on different grain-size fractions (clay-size < 2 `µm, cohesive silt 2-10 µm, fine silt 10-30 µm and coarse silt 30-60 µm) on Late Glacial and Holocene sediments. Four grain-size fractions were investigated for a set of 12 samples from core MD99-2227:. Our results show a clear variation of Pb concentration and isotopic signatures according to grain-size (Nd data still in progress). The observed shifts are interpreted in terms of changes in deep current strength. This approach allows to monitor deep current changes through time, whatever the strength of paleocurrent

Climate variability of Southern Chile since the Last Glacial Maximum: a continuous sedimentological record from Lago Puyehue (40°S)

A key region to study high resolution climate changes of the Southern Hemisphere is undoubtedly the southern part of Chile because it has the advantage to be far removed from the Northern Hemisphere ice sheets and thermohaline circulation influences. In order to reconstruct the regional climate evolution since the Last Glacial Maximum, we investigated the sedimentary infilling of Lago Puyehue (40°S, 164 km2, elevation 185 m) by a multi-proxy analyse of a 11 m long core. Sediments from this core are transported by interflow currents and are made of finely laminated silts, with only small disturbances due to volcanic and seismic activities. Several proxies were measured: grain-size, mineralogy, magnetic susceptibility, major elements geochemistry and biogenic silica concentration. These are used to reconstruct paleo-precipitation and paleo-productivity changes around 40°S. Results evidence that sediment grainsize is highly correlated with the biogenic sediment content and can be used as a proxy for lake paleoproductivity. On the other hand, the magnetic susceptibility signal is highly correlated with the aluminium and titanium concentrations and can be used as a proxy for the terrigenous supply. Temporal variations of sediment composition demonstrate that, since the Last Glacial Maximum, the Chilean Lake District was characterized by 3 abrupt climate changes superimposed on a long term climate evolution. These rapid climate changes are: (1) the end of the Last Glacial Maximum at 17,300 cal. yr. BP; (2) a 13,100-12,300 cal. yr. BP cold event, ending rapidly and interpreted as the local counter part of the European Younger Dryas event, and (3) a 3400-2900 cal. yr. BP climatic instability related to low solar activity. The timing of the 13,100-12,300 cold event is compared with similar records in both hemispheres and demonstrates that this Southern Hemisphere climate change lags behind the Northern Hemisphere Younger Dryas cold period by 500 to 1000 years

EVALUATION DES INDICES D’ALTERATION CHIMIQUE SUR UN PROFIL KAOLINISE CAS : TAMAZERT (NE ALGERIE)

Chemical weathering indices are commonly used for characterizing weathering profiles developed on heterogeneous or homogeneous parent rock by incorporating bulk major element oxide chemistry into single metric of each sample. Chemical weathering indices evaluated in this study are: weathering index of Parker (WIP), the chemical index of alteration (CIA), chemical index of weathering (CIW), plagioclase index of alteration (PIA), Ruxton ratio (R). The main purpose of this study is to review the scope of chemical weathering affected different gneissic rocks especially in Tamazert area and test the ability of application of chemical weathering index for heterogeneous alteration profiles. Petrology and geochemistry of the major elements of rock and alteration product have been discussed in this work.Les indices d’altération chimique sont couramment utilisés pour caractériser les profils d’altération développés sur des roches mères hétérogènes ou homogènes. Parmi les indices d’altération évalués dans cette étude, l’indice de Parker (WIP), l’indice chimique d’altération (CIA), l’indice de météorisation chimique (CIW), l’indice d’altération des plagioclases (PIA), Ruxton ratio (R). L’objectif principal de cette étude est d'examiné l'étendue de l'altération chimique affectée les différentes roches surtout gneissiques dans la zone de Tamazert et de tester l’aptitude d’application des indices d’altération chimique pour des profils d’altération hétérogènes. La pétrographie et la géochimie des éléments majeurs de la roche mère et du produit d’altération ont été discuté dans ce travail

Paleoproductivity of Puyehue Lake (Southern Chile) during the last millenium: climatic significance

Southern Chile is a key site for the understanding of past climatic variations since it is influenced by the El Nino Southern Oscillation (ENSO). We investigated high resolution climate changes during the last millennium by a multi-proxy analyse of short cores (60 cm long) collected in Puyehue lake (40°S): magnetic susceptibility, grain-size, mineralogy, density, gamma-density, LOI, biogenic silica content and bulk XRF geochemistry. According to age-depth model (210Pb, 137Cs and varve counting - Boës et al., this session), the cores cover the last 600 yr. The sediment is characterized by volcanic minerals and a high diatom content, due to the important lacustrine silica supply characteristic of volcanic environments. Moreover, the active volcanism of the Chilean Lake District is responsible of a high number of tephra deposits. Our main aim is to quantify biogenic particles fluxes throughout the last millennium by Na2CO3 dissolution and by normative calculation based on bulk XRF analyses. The result shows that volcanic eruptions do not influence the biogenic productivity of the lake. From 1400 to 1880 yr. AD, paleoproductivity shows a global trend from low to high biogenic production. Important paleoproductivity changes are observed over the last 120 yr. Results are compared with historical data and discussed in terms of climate changes and/or anthropic influence

Inter-hemispheric comparison of mid-latitude lacustrine archives and high-latitude ice cores over the Younger Dryas and the Little Ice Age

The ice-core records from Greenland and Antarctica provide some of the most suitable sequences to detect the onset of significant climate changes like the Younger Dryas (YD). In terms of inter-hemispheric comparison, the calibration of the age models is probably the most determinant parameter. In this context, the annual ice layers like in e.g. the GISP2 or Taylor Dome ice cores constitute important calibrated climate data for the assessment of the time responses between the northern and southern hemisphere. However, in order to fully understand the origin and the propagation of significant cold or warm events, it is necessary to include results from paleoclimate research from the mid-latitudes free of ice sheets. In this context the laminated lake sediments can provide long and continuous climate records that can be compared with the polar ice cores. Here, we compare calibrated lake sediment records from the mid-latitudes, i.e. in South America and Eurasia, to calibrated ice core records from Greenland and Antarctica. The age models are established by varve counting, 210Pb, 137Cs or 14C data. Two time periods are investigated: the Termination I and the Last Millennium. The first time window includes the last cold event of the Pleistocene (i.e. the YD) and the second one, the most recent historic cold period (i.e. the Little Ice Age or LIA). A new chronology of the different inter-hemispheric climate responses is proposed over the two selected periods. Our new data show that the last cold event of the Pleistocene occurs 500 years earlier in the Southern Hemisphere. For the Last Millennium, the equivalent cold period of the LIA occurs ˜80 years later in the Southern Hemisphere than in the northern one. In the Southern Hemisphere, the LIA is characterised by an increase of the precipitation and the influence of El Nino. The time lag observed between the Northern and the Southern Hemisphere could be due to the occurrence of a local Late Medieval Warm Period (LMWP)

Mid- to Late Holocene elemental record and isotopic composition of lead in a peat core from Wolbrom (S Poland)

Peat Core W3 was taken from the fen in Wolbrom (Silesian-Cracovian Upland, Southern Poland) in September 2015. Previous analyses of Core W3 showed a significant increase in lead concentration during the time of the Roman Empire as well as some changes in peat accumulation conditions. The work reported here investigates its geochemical composition in terms of major and trace elements (Pb, Zn, Na, K, Ca, Fe, Mg, Mn, Cu, Ni, Cr by AAS) as well as Pb isotopic composition, to identify the sources of metal pollution in the Wolbrom peat deposit. The geochemical record spans the period from 4900 BC to modern times, with a likely hiatus corresponding to the period from the Middle Ages to the beginning of the contemporary metallurgical industry. The Pb isotopic composition combined with a cluster analysis allows identification of the primary sources of Pb. In addition, Principal Component Analysis (PCA) helps to decipher the most important factors that affected the chemical composition of sediments at Wolbrom. These factors were linked to chemical denudation and human activity

Thickness variation of sediment lamination in Puyehue Lake (Lake District, Southern Chile) during the last millennium: a regional southern hemisphere record of El Niño?

Lake District (Southern Chile) is investigated as a new regional record of past climate changes in Southern Hemisphere, in particular in order to evidence any regional impact of ENSO in South-America. We analyzed three short cores (60 cm) from the key-site of Puyehue Lake (40°S) which has been selected for multiproxy analyses (Bertrand et al., this session). Sedimentation model is related by a laminated mud increment mainly controlled by the biogenic activity and by the annual thermal lake cycles (turn-over of the nutrients during autumn and winter-time). We analysed lamination occurrence and thickness from enlarged images of thin-sections preparation (magnitude 5x) in order to increase sediment resolution. The age-model of the cores is based on counting laminations, assuming that sedimentation is varved. Indeed, this varve sedimentation model is in accordance with chronology based on the decrease of 210Pb rates and peaks of 137Cs. Variation of the lamination thickness shows four different phases of sedimentation. (1) Since c.a. 1350 A.D. (base of the cores) to 1460 A.D., varve-thickness ranges around 400 µm and sedimentation rates are 0,5 mm/yr. (2) From 1460 A.D. to 1890 A.D., varve-thickness is about 600 µm with a minimum at 1730 A.D., and sedimentation rates increases from 0,7 to 1,2 mm/yr. (3) From 1890 A.D. to c.a 1930 A.D., varve-thickness increases up to 2000 µm, and sedimentation rates vary between 1,2 to 2,3 mm/yr. (4) From c.a. 1930 A.D. to Actual, varves are about 500 µm with a destratified layer coincident with the 1960 seismic event of Valdivia; sedimentation rates are between 0,6 to 1,2 mm/yr. The four phases are discussed according to variations of the lake palaeoproductivity by respect with river run-off detrital supplies; the influence of the westerlies on the variations of the lamination thickness is discussed in term of possible regional impact of ENSO

Geophysical characterization of the sedimentary environments in Lago Puyehue and Lago Icalma (Chilean Lake District, SW Andes)

The Chilean Lake District, located in Southern Chile, comprises 17 lakes at the foothill of the Cordillera de los Andes. These lakes, dammed by frontal moraines, were formed during the last deglaciation (12500-12000 BP). Their sedimentary infilling has the potential to contain a complete and continuous Holocene sedimentary record of environmental and climatic changes having affected the area.High-resolution reflection seismic data (sparker and pinger) collected during the 2001-2002 expedition in the framework of the Belgian ENSO-CHILE project have allowed us to select two lakes for the collection of long and short sediment cores:Lago Icalma (38°50’S, alt. 1150 m) is located in the Cordillera de los Andes, in the upper part of the Bio-Bio River. Its watershed (148 km2) is dominated by a soft post-glacial sediment cover, interrupted by two important pumice layers. According to the high-resolution seismic survey, the 70m-thick sedimentary infilling consists of morainic deposits, under- and interflows and laminated lacustrine deposits. The western part of the main basin represents an elevated platform, free of the influence of bottom-currents and turbidites and possibly consisting of interflow deposits. Core descriptions and physical property analyses of sediments (gamma-density, low and high-resolution magnetic susceptibility) suggest that the deposits consist of an alternation of volcanic deposits and terrigenous sediments correlated on pinger profiles, showing the presence of several low-amplitude layers.Lago Puyehue (40°40’S, alt. 185 m) is located at the foothill of the Cordillera de los Andes and presents a glacial morphology much more complicated than Lago Icalma. Its watershed is larger (1267 km2) and dominated by Quaternary and Tertiary volcanic rocks. The lake is composed, in its western part, by a large basin, filled by 250 m of sediments, as can be deduced from sparker profiles. The eastern part of the lake presents a complex substratum morphology. However, it was possible to find a suitable location in underflow and interflow deposit areas for the collection of two long cores. Core description and physical property analyses of sediments of the interflow area suggest a good and continuous sedimentary record.With this contribution, we wish to illustrate the potential of high resolution geophysical site-survey data for interpreting core descriptions and physical property analyses

Climate oscillations recorded in Chilean lacustrine sediments (Lago Puyehue)

The Chilean Lake District is located in Southern Chile, between 38° and 42°S. Several long sediment cores were collected in these lakes. Their analysis aims at a better understanding of the climate mechanisms related to ENSO in this part of the world. The recognition of ENSO related periodicities and their stability is studied through the analysis of two cores collected in Lago Puyehue. Several methods of spectral analysis were applied to identify potential periodicities in the signal. Blackman-Tuckey, Maximum Entropy, Multi-Taper Methods (MTM) and singular spectrum analysis were applied on the whole record. In addition evolutive MTM and wavelet analyses allow identifying temporary influence of some periodicities. First, annual varve thickness was analysed for two pilot cores. The first core is rather short, i.e. 282 years. A period at ˜3.0 year appears in a large part of the interval, mostly in the most recent part. Periods at ˜5.2 year and ˜23 years also show up. The second one (longer than 550 years) displays the most robust periodicities at around 15, 9, 4.4, 3.2 and 2.4 years. These periodicities are in good agreement with the sub-decadal periods identified by Dean and Kemp (2004) and linked to the Quasi-Biennal Osciallation, El Niño Southern Oscillation and the Pacific Decadal Oscillation. Moreover, the evolutive MTM analysis and the wavelet analysis suggest a striking break in the periodicities at around 1820 A.D. This could be coherent with the end of the Little Ice Age. In fact the sedimentation process is slightly different for the two cores. Varves in the first one can be related to the flood of the Rio Golgol and in all the drainage basin, while in the second core they are more directly related to local precipitation. This could explain the difference in the recorded periodicities in the two sites. Second, magnetic susceptibility of a longer core, covering the last 18 kyr, was also analysed. It suggests several periods, i.e. 950, 750, 380, 280, 220, 208, 180 years, although their significance is questionable

Geochemical and Pb isotopic investigations in peat bogs from Southern Chile: Identification of particles supplies and possible paleoclimate record

peer reviewe