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)

Development of paleoseismic trench logging and dating techniques: a case study on the Central North Anatolian Fault

The North Anatolian Fault (NAF) is a dextral strike slip fault zone extending ~1400km in an arc across northern Turkey. This study seeks to further constrain the timing of ground rupturing earthquakes of the NAF while developing the techniques used in paleoseismology. A paleoseismic trench was opened ~2.7km NW of Destek on a segment which ruptured (for ~280km) in the 1943 Tosya Earthquake (Mw:7.7). The trench site comprises a pop-up structure formed by a small releasing step-over at a restraining bend which has caused progressive growth of an upslope facing scarp. The trench is situated across the main fault trace and a trapped sedimentary sequence that includes several paleosoils. The stratigraphy is expected to be Late Holocene and historic in age due to the high level of activity on the NAF, although this has yet to be confirmed by radiometric dating. Preliminary interpretation of the trench stratigraphy indicates a record of up to 6 paleoearthquake events, the presence of an angular unconformity suggests the record may be incomplete beyond the 3 most recent events on this strand.Subtle contrasts in stratigraphy made conventional face logging difficult and was therefore augmented by mapping the magnetic susceptibility (MS) of the west wall. Approximately 6000 measurements were made using a Bartington MS2 Magnetic Susceptibility Meter with a MS2E (point) Sensor with a 5cm vertical spacing and a 20cm horizontal spacing predominantly on one side of the trench. A pilot test led to development of a strategy of moving the sensor to the nearest exposure of coarse sand or finer grained material where possible to minimize the noise generated by individual clasts. To negate the sensitivity of the MS logging method to variations in temperature the survey was conducted at night. Plotted data clearly shows the contact between rock units, the rock-soil interface (reflecting fault juxtaposition), anthropogenic influence and some soil stratigraphy. Other paleoseismic investigations on this section of the NAF (Hartleb R. et al 2003 and Yoshioka T. et al 2000) have encountered out-of-stratigraphic-order ranges in 14C ages. They attributed this to reworking, in addition to which the effects of long term human occupation are likely to be similar. The trench yielded a large amount of datable material including 158 charcoal and 140 minute gastropod samples, and some ceramic, bone and slag samples. Unlike charcoal and bone fragments, fragile minute gastropods are unlikely to have been transported, reworked or used by humans, ultimately providing improved accuracy of temporal constraints on paleoearthquakes. Using both charcoal and gastropod samples, the trench chronology can be established and the use of minute gastropods for dating paleoearthquakes can be critiqued

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

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

Radionuclide profiles and recent earthquakes history of Lake Hazar Pull-apart basin (East Anatolian Fault, Turkey)

In Turkey, the continuous Pull-apart sediment records constitute powerful chronometers for tracking environmental perturbations such as earthquakes. In South-east Turkey, the East Anatolian Fault (EAF) is a major strike-slip fault along which large earthquakes (Ms > 7) occurred in the 19e century. According to chronicles, the seismicity of this area has been minimal for most of the last century; the latest surface rupturing earthquakes may be the Ms = 7.1 in AD 1874 and the Ms = 6.7 in AD 1875. The EAF consists of two large surface rupturing segments interrupted by a pull-apart basin at Lake Hazar (the Sincik/Lake Hazar and the Lake Hazar/Palu segments). In this geological context, the present project seeks to assess: 1) the recent sedimentation rates of Lake Hazar main Pull-apart system located on the EAF; 2) the occurrence of recent past earthquakes along the EAF. For these purposes, we use a diverse array of complementary techniques involving sediment coring, and radionuclide profiles of sediment cores. Here, we present the first results obtained within the framework of a EU-project focusing on the “seismic cycles” in Turkey (“Understanding the irregularity of seismic cycles: A case study in Turkey”). We present 210Pb and 137 Cs age models obtained from a series of short sediment cores. The radionuclide profiles are utilized for both, annual sediment rates estimates, and for tracking the historic earthquakes. The correlation between several cores and the comparison between radionuclide profiles and preliminary sedimentological data shows that sedimentary structures induced by the last AD 1874 and 1875 earthquakes can be detected by ultra-high resolution X-ray radiographies. However, our results show the presence of an additional hypothetic event in the early 20e century. These first results will be further utilized for tracking past earthquakes in longer Lake Hazar sediment time series

Large earthquakes Cycles in Lake Sediments along the North Anatolian Fault, Turkey

In 1999, the large surface-rupturing earth- quakes of Izmit and Duzce completed a 60-year cycle that included a westward migration of nine consecutive large earthquake failures ([50 km surface rupture), which started with the 1939 Erzincan earthquake in eastern Turkey. In this study, we focused on seismic cycles and seismic risk predictability along the North Anatolian Fault (NAF). Toward the west end of the NAF (26°E–32°E, i.e. Bolu), large earthquake fre- quency is measured from either historic earthquake catalogs, or geologic records from isolated outcrops and marine sediment cores from the Marmara Sea. In comparison, the eastern part of the NAF zone (32°E– 42°E) is less well documented by palaeo-seismologic archives. Thus, the sediment records of lake basins located on the eastern NAF zone constitute a unique opportunity for testing a new palaeo-seismologic approach. To this end, we used a diverse array of complementary methods involving: (1) a 600-km transect of fault-related lakes, (2) sedimentologic observations on cores from six lakes, and (3) a comparison between records of catastrophic sediment transfers in lakes (i.e. radionuclide chronomarkers and erosion tracers) and historic earthquake reports. Our study indicates that lakes along the NAF are sensitive geologic recorders of large surface-rupturing earth- quakes (surface-wave magnitude (Ms) C 6.9); smaller intensities are not recorded. The most responsive lake systems exhibit increases in sediment accumulation by a factor of [40 for a [3-m strike-slip displacement (Ms C 7). However, based on results from the 1939 Erzincan earthquake (Ms = 7.8) chronostratigraphic marker, large surface-rupturing earthquakes are detected only by certain lake records and not by others. Matching multiple lake records along the NAF pro- vides information both on the location of a surface rupture of a paleo-earthquake as well as its magnitude. Finally, the shallow lake basins along the NAF could potentially document cycles of large seismic events for at least the late Holocene.Understanding the Irregularity of Seismic Cycles: A Case Study in Turke

Radionuclide dating (²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am) of recent lake sediments in a highly active geodynamic setting (Lakes Puyehue and Icalma—Chilean Lake District)

This study presents an attempt to use radionuclide profiles to date four short sediment cores taken from two Chilean lakes located in a highly active geodynamic setting. In such settings, sediment series commonly contain earthquake-triggered reworked layers and/or volcanic ash layers. All of these layers affect the vertical distribution of radionuclides. The drawing up of accurate chronologies is made even more problematic by the low fallout rates of both natural (210Pb) and artificial (137Cs, 241Am) radionuclides. However, radionuclide profiles can be “corrected” by subtracting the influence of instantaneous deposits that have been identified from detailed sedimentological studies. Thus, radionuclides can be used to provide approximate dates for sediment. Independent confirmation of these dates can be provided by varve counting and/or the recognition of historical events. For Lake Puyehue, this approach has allowed particular sediment features to be related to the effects of the 1960 Chilean earthquake (Mw 9.5) on the lake basin and its catchment area. For Lake Icalma, there is a good agreement between radionuclide dates and the dates of the three tephra layers formed during large eruptions of the Llaima volcano in 1946, 1917 and 1883. For both lakes, artificial radionuclide fallout, which culminated in 1965, provides more robust chronological information than 210Pb dating

Entrechaux : Grand abri aux Puces

National audienc