Investigating the internal lithological structure and rock magnetic signature of Heinrich Event layers at SE Grand Banks Slope, Newfoundland

<p>Periodic mass discharges of icebergs from the Laurentide ice-sheet into the North Atlantic Ocean during the last glacial period deposited abundant ice-rafted detritus (IRD) accumulated in sequences of typically six major Heinrich Event layers, each with some tens of cm thickness, at all eastern slopes of the Grand Banks submarine platform of Newfoundland. Compositionally, it is well established that these IRD layers consist of varied rock contents emanating from distinct, but not yet clearly defined bedrock provinces of the Canadian Shield. The, most prominently reported constituent is detrital dolomite, but the entire lithological range of the IRD is much broader. Rock magnetic records, e.g. magnetic susceptibility logs of SE Grand Banks cores, therefore depict complex and partly repeating internal substructures across the Heinrich Event layers owing to distinct successions in IRD lithology over the course of every mass calving event.</p><p>We investigated IRD sieve fractions (1mm – 4cm) of the entire glacial section (550–1054 cm) of SE Grand Banks slope gravity core GeoB 18530-1, sampled in 2.3 cm steps. Therefrom, we identified and classified distinct IRD rock types as well as monocrystalline rock-forming mineral particles, for which we established so far 24 well-defined lithological categories of sedimentary, igneous and metamorphic origin. This initial identification of IRD lithology was performed based on all available visual criteria including texture (crystallinity, grain-size), color and translucency (mineralogy), hardness and surface structures (e.g., cleavage) using a binocular microscope. This rock type classification is now being substantiated by polarized light microscopy of exemplary thin sections created from larger IRD clasts.</p><p>To established cumulative rock magnetic fingerprints of all IRD magnetic mineral assemblages, isothermal remanent magnetization acquisition curves of all sieve fractions as well as individual specimens of all the classified rock types have been measured. These records systematically revealed higher concentrations of magnetic minerals at the tops and bottoms of most Heinrich Event layers and also clear variations in coercivity spectra. This finding is mirrored by the IRD rock count records, where magmatic rock types predominate mostly at Heinrich Event layer boundaries. Preferred deposition of these IRD rock types during the initiation and ending of events and their variation from older to younger events,- highlight repetitive patterns in the cyclic Laurentide ice-sheet collapses to be further explored.</p>

A study on the interaction between extreme waves and coastal development processes for identification of tsunami and storm deposits in the tsunami far-field

<p>In order to estimate the size and the origin of giant tsunamis, it is useful to investigate “tsunami far-field” as the coastal area far from the source. However, it is challenging to distinguish a tsunami deposit from an extreme storm deposit in these areas. In this study, we report sand layers induced by extreme waves on the coast of Hokkaido, Japan, facing the southern Kuril Trench. In the study area (central part of the Hidaka coast), it is said that the tsunamis caused by observed earthquakes have never exceeded the dune or beach. However, geological evidence indicates that giant earthquakes and tsunamis occurred at intervals of several hundred years in the Kuril Trench, and the traces of these tsunamis are still unclear in the Hidaka region.</p><p>The study area can be classified into the inland zone consisting of peatland and the seaward zone consisting of floodplain muds by the paleo beach ridge. We identified three volcanic ash layers and three to four sand layers with clear boundaries to the ordinary mud layer in each zone. However, there are large gaps in the ages of the sand layers discovered in both inland and seaward zones, and their distributions are limited (ranging from a few tens to 100 m from the ridge at that time) and do not overlap. To understand the peculiarities of the depositional age and distribution of the sand layers, we clarified the sedimentary environmental changes and sea-level index in the late Holocene by analyzing the diatom assemblage and CNS of the mud layer. The inland zone showed the paleoenvironments from the sandy tidal flat formed by the transgression in the mid-Holocene to the beach ridge formed by the regression, and the sheet sand layers were formed only during the period of the beach ridge development. On the other hand, the seaward zone showed various changes due to the formation of meandering rivers and beach ridges associated with the regression, and the formation of recognizable event layers is accompanied by changes in the depositional environment, such as the opening of lagoons and rapid changes to upland. Thus, especially in the tsunami far-field, the preservation potential of the event layers is strongly influenced by the coastal development and relative sea level, and such geological information will provide clues to identify the origin of the sand layer. In the presentation, the numerical simulation of the paleo-tsunami considering the reconstructed relative sea-level change and topographic development will be reported.</p>

Did the Late Glacial to Holocene climatic transition trigger large earthquakes in the Western Alps?

<p>Seismic hazard calculations are based on the assumption that seismicity rates are stable over time. In a given area, the seismicity recorded through historical archives and seismometers is considered a reliable indicator to model the occurrence of future high magnitude seismic events. But, to discuss this hypothesis regionally, it is essential to reconstruct long term seismicity.</p><p>The junction between the Jura mountains and the Alps is seismically active, as shown by the occurrence of numerous seismic events and the presence of several active faults (De La Taille, 2015). Since the 15<sup>th </sup>century, more than twenty earthquakes of epicentral intensity greater than VII have been identified in this area. In addition, sedimentary sequences from Lake Annecy and Lake du Bourget have highlighted the capacity of these "natural archives" to record recurrent seismic activity (Beck 2009), with a potential major seismic event identified around 9900 cal. BP (Arnaud et al., 2012). Such lacustrine archives are key to better understand 1) the occurrence of major seismic events and 2) the evolution of seismicity rates through time, prior to historical and instrumental records.</p><p>Here, we present two sedimentary sequences of 11 and 16 metres long respectively, sampled in the shallowest and deepest basins of Lake Aiguebelette (altitude: 374 m). We performed sedimentological, geochemical and paleomagnetic analyses combined with seismic profile analyses and radiocarbon dating to study processes of event layer deposition in this lake. Multi-proxy analyses allow a quantitative identification of event layers, contrasting with varved-sedimentation. In the deepest basin sequence, 33 homogenites are identified through variations of the laboratory induced isothermal remanent magnetization of sediments measured with a high-resolution fluxgate scanner (Demory et al., 2019) and high foliation (>2%) of the Anisotropy of Magnetic Susceptibility. These parameters are usually associated with seiche effect induced by seismic activity (Campos et al., 2013). Among these event layers, archived in the deep basin sequence, three of them occured synchronously in the shallow basin (at 3000 ± 100, 6900 ± 100 and 11400 ± 300 cal. BP, respectively).</p><p>The oldest and thickest event layer recorded in Lake Aiguebelette was deposited at the transition between the Late Glacial and Holocene stages. In the deepest basin, this 1.15-meter-thick deposit is composed of an upward-graded base and a 0.84 meter-thick homogenite, which was also identified as a transparent facies on seismic profiles. In Lake Le Bourget, Lake Annecy, and central Swiss perialpine lakes, several seismic profiles analyses show transparent seismic facies interpreted as mass movement deposits occurring at the same period of time: the Late glacial-Holocene transition.</p><p>Did this climatic transition influence the seismic activity in the Alps? If so, the impact of such climatic forcing on seismic hazard assessment should be evaluated.</p><p> </p><p>Arnaud et al (2012). Lake Bourget regional erosion patterns... QSR., 51, 81-92.</p><p>Beck (2009). Late Quaternary lacustrine paleo-seismic... EarthSciRev., 96(4), 327-344.</p><p>Campos et al (2013). Deciphering hemipelagites from homogenites... SedGeol., 292, 1-14.</p><p>De La Taille et al (2015). Impact of active faulting... Tectonophysics, 664, 31-49.</p><p>Demory et al (2019). A new high‐resolution magnetic scanner... Geochem,Geophy,Geosys., 20(7), 3186-3200.</p><p> </p><p><strong>Keywords: </strong>Lake sediment, homogenites, paleo-earthquakes, seismic hazard, French Alps</p><p> </p><p> </p>

Extreme flood events and their frequency variations during the middle to late-Holocene recorded in the sediment of Lake Suigetsu, central Japan

Many studies are reconstructing flood records in the continental margins during the middle to late-Holocene. However, distinguishing the frequency and magnitude of flood events was difficult. Light gray event layers (GELs) in the sediment of Lake Suigetsu in Central Japan can solve this problem because they are recording the occurrence and magnitude of flood events during the last 80 years. Using these GELs, we aimed to reconstruct the frequency and magnitude of flood events during the last 8000 years. First, we verified whether GELs maintained the recording of flood events. We found that thin GELs (less than 40 mg/cm2 in mass) were rare, probably because of bioturbation, whereas thick GELs (larger than 40 mg/cm2 in mass) were rarely erased. We also found that GELs formed by the same amount of rainfall could have been thinner before 100 years ago. We revealed that the occurrence of extreme flood-origin event layers (EFELs: GELs thicker than 40 mg/cm2) during the last 8000 years may indicate the occurrence of extreme flood events (EFEs). Mass accumulation of EFELs may indicate the minimum estimation of rainfall on EFEs. There are several periods with higher frequency and magnitude of EFEs than those of the present level, agreeing with the periods of high flood activity in western Japan and East China. The frequency and magnitude of EFEs show different temporal variations. Northward and southward shifts in the westerly jet and monsoon fronts can partially explain such patterns.

How to interpret Holocene palaeoenvironmental and cultural changes in SW Iberia based on the palynological record from the GeoB23519-01 core (RV METEOR cruise M152)

<p>The southwest of the Iberian peninsula is, due to its border position between Africa and Europe, a key territory of major geoarchaeological interest, as well as a reservoir of biodiversity and a wildlife refuge area during the Holocene. Bioclimatic conditions have been significantly unstable during this period in the Western Mediterranean. Therefore, further studies are still required to understand how abrupt climate changes such as the 8.2 and 4.2 ka cal BP events impacted societies and environment.</p><p> </p><p>In November 2018 the RV Meteor cruise M-152 retrieved 19 vibracores and 4 gravity cores along the Algarve coast after mapping the bathymetry. One of these cores, GeoB23519-01, was taken 65 m below present sea level and recovered 365 cm of sediment. Four potential event layers were identified over the last 11 ka cal BP and, at least two of them, are related to tsunami deposits (<em>ca</em>. 4370 cal BP and AD 1755).</p><p> </p><p>This sedimentary archive was analysed in a multi-proxy approach, including palynological and micropalaeontological analyses, which allow characterizing palaeoenvironmental changes along the core. However, considering the characteristics of these deposits, we raise questions about how complex this palynological record is and how it mirrors some short-term events, climate dynamics, and cultural disruptions.</p>

A 4000-year record of hydrologic variability from the Olympic Mountains, Washington, USA

Sedimentological and geochemical analyses of gravity and piston cores retrieved from Lake Quinault, Washington, reveal an ~4000-year flood-dominated depositional record. Individual flood event layers are identified by combining core stratigraphy, sedimentology, and the ratio of incoherent to coherently scattered x-ray radiation ( inc/coh) from µXRF (x-ray fluorescence) core scans. The inc/coh time series is used as a proxy for sediment grain size and, in combination with radiocarbon-anchored core age–depth models, enables the reconstruction of late-Holocene hydrologic variability for the Quinault River catchment. Decadal to centennial variability in inc/coh is interpreted to reflect trends in ocean-atmosphere teleconnections favorable for the formation of land-falling atmospheric rivers along the Pacific Ocean flank of the Olympic Mountains. Such processes likely modulate the rate of flooding and may explain notable increases in the frequency of flood event layers observed during the periods 2350–2450 cal. yr BP and the most recent century (AD 1910–2010). Understanding past hydrologic variability has important implications for the landscape and ecosystem response of Olympic Mountain catchments to future climate warming.

Sources of laminated sediments in the northeastern Arabian Sea off Pakistan and implications for sediment transport mechanisms during the late Holocene

Laminated sediments of the continental slope off the Makran coast in the northern Arabian Sea are well-known climate archives and record productivity, as well as supply of material from land. Here, we studied sediment core 275KL off Pakistan in concert with sediment trap, dust and river samples in order to characterize and quantify land-derived material deposited in varves and event layers. We analysed grain sizes, mineral assemblages, bulk components and stable isotopes (δ13C, δ18O) of carbonates. In winter, enhanced river discharge is the main source of lithogenic matter contributing the major amounts to the total annual sedimentation of the northern Arabian Sea. During the late summer season, lithogenic matter accumulation is slightly enhanced, probably carried along with the south-eastward blowing Levar winds from the Balochistan and the Sistan Basins and the summer monsoon discharge maximum of perennial streams. C/N ratios and stable carbon and oxygen isotopes could not be used to distinguish between organic matter produced on land and in the ocean, whereas stable carbon and oxygen isotope ratios of carbonates suggest that sedimentation of event layers is dominated by direct inputs from land. Catastrophic denudation and storm events occur on average once every 50 years and lead to sedimentation rates that exceed the mean annual sedimentations of 983 g m−2 yr−1 by 6 to 10 times. Nevertheless, due to their rare occurrence, they contributed only 7% to the total sedimentation during the last ca. 5000 years. End-member modelling of grain sizes in accordance with lithogenic matter accumulation rates and event layer frequencies showed that arid conditions prevailed between 4000 and 5000 a BP while more humid conditions commenced around 2000 ka BP in accordance with the Pacific ENSO record.