Depositional Events Research Articles

Geochemical characteristics and sedimentary setting of chang 9 shale in the Upper Triassic Yanchang Formation of southeastern Ordos Basin (NW China)

In addition to the organic-rich shale in the Chang 7 Member (Ch7 shale) of the Ordos Basin, the shale hosted in the Chang 9 Member (Ch9 shale) is another source rock and potential unconventional target. However, there is a general lack of consensus on the features and potential of the Ch9 shale. In order to determine the potential of the Ch9 shale, this study analyzes its paleoenvironment and sedimentary setting based on the integrated analysis of geochemistry and sedimentary facies. A comparison of geochemical features between the Ch9 shale and the organic-rich Ch7 shale is made to evaluate the factors controlling organic matter (OM) enrichment of the Ch9 shale. The results show that the Ch9 shale has good hydrocarbon potential, and terrestrial OM is important component of the OM in the Ch9 shale. The Ch9 shale was deposited in climatic transition period from cold and dry to warm and humid, when the water body was suboxic-anoxic and freshwater. The relatively cold and dry climate and suboxic-anoxic water condition in the Ch9 period could not provide the same excellent conditions for OM enrichment as that in the Ch7 period. The Ch9 shale is composed of sediments mixed with felsic rocks that originated from continental island arcs and active continental margins. The active tectonic setting resulted in event deposition (e.g. turbidity flows and hydrothermal fluid activity) during the deposition of Ch9 shale. The input of terrestrial OM carried by turbidity flows also promoted OM enrichment, however, the very high depositional rates caused by turbidity flows diluted the OM, which probably offset the effect of hydrothermal on OM enrichment. Our results are helpful to revealing the source potential of the Ch9 shale, providing a theoretical basis for the shale oil and gas exploration in the Ordos Basin.

Ichnological analysis as a tool for assessing deep-sea circulation in the westernmost Mediterranean over the last Glacial Cycle

During the Last Glacial Cycle (last ~130 kyr) climatically induced changes in the ocean dynamics affected the tracemaker habitat in the Alboran Sea Basin (westernmost Mediterranean), as observed in sediment records from ODP Leg 161 Sites 976 and 977. The trace fossil assemblage present is assigned to the Zoophycos ichnofacies and is of low/moderate diversity and comprises common Planolites, Chondrites and Thalassinoides, with occasional Scolicia and Zoophycos. Ichnodiversity, size of biogenic structures and percentage of bioturbational sedimentary structures clearly correlate. Fluctuations in ichnological features evidence a well-developed short-term cyclic pattern that could be related to environmental changes such as export production and oxygenation at the seafloor. The percentage of bioturbational sedimentary structures correlates well with sea-surface temperature (SST) records obtained for the Alboran Sea and Gulf of Lions, as well as with the δ18O profiles of Greenland ice cores. Correlation is seen for both the long-term (over the Last Glacial Cycle) and short-term changes, the latter comprising climate oscillations such as Heinrich Events, Younger Dryas, and periods of organic-rich layer deposition. Ichnological data also allow for a reconstruction of climatically induced changes in the ocean dynamics, which have a major incidence in the Western Mediterranean Deep Water that, in turn, affects deep-sea environmental conditions.

Age, provenance and tectonic setting of metasedimentary sequences of the Gurupi Belt and São Luís cratonic fragment, northern Brazil: Broadening the understanding of the Proterozoic-Early Cambrian tectonic evolution

Geological, U-Pb (detrital zircon), whole-rock Nd isotopes, and geophysical information constrain the timing, provenance, and depositional and tectonic settings of sedimentary successions from the Gurupi Belt (GB) and São Luís cratonic fragment (SLCF) in northern Brazil. The results, along with published information about the magmatic events are used to discuss the crustal framework and regional correlations. We defined the following stages of sedimentation related to the Rhyacian and Neoproterozoic-Early Cambrian evolution. (1) 2240–2160 Ma –metavolcano-sedimentary sequences formed in arc- (SLCF) and back-arc (GB) basins. (2) 2165–2120 Ma – two syn-orogenic, siliciclastic (turbidite) sequences; the oldest (>2149 Ma) formed in a forearc or intra-arc setting, the youngest (>2116 Ma) formed in forearc or distal intra-arc system. (3) 2120–2088 Ma – post-collisional, continental, siliciclastic basin(s) formed in fluviatile setting, and representing a foreland system, which was split into two sub-basins in more recent times. These Rhyacian sequences correlate with similar successions in the West-African and Amazonian cratons, and formed during the agglutination of a continental block in pre-Columbia times (>2000 Ma), which remained joined through the assembly (~1800 Ma) and breakup (~1300 Ma) of Columbia, being only fragmented during Rodinia breakup (~750 Ma). This breakup produced, probably distal, intracontinental effects that led to the deposition of (4) 1130–740 Ma – siliciclastic sequences formed in rift and passive margin basins; no evidence of oceanization is observed, and the sequences correlate in time with sedimentary formations of the Rokelide Belt in West Africa. The closure of the epicontinental sea took place during intracontinental orogeny, which represents a distal effect of the collisional orogenies that assembly West-Gondwana at the Neoproterozoic-Early Cambrian boundary. The last sedimentation event comprises (5) < 591 Ma – a post-orogenic/post-collisional siliciclastic basin formed in a rift that might also have been acted as a precursor rift of the Phanerozoic intracratonic basins.

Sedimentary Environment Division by Clayey Water Electrical Conductivity and Grain-Size Analysis: A Case Study of an xz-Borehole in the Western Plain of Bohai Bay, China

Fang, J.; Zhang, L.; Liu, Y.; Chen, G.; Yang, S.; Wang, T., and Wang, F., 2020. Sedimentary environment division by clayey water electrical conductivity and grain-size analysis: A case study of an xz-borehole in the western plain of Bohai Bay, China. In: Yang, D.F. and Wang, H. (eds.), Recent Advances in Marine Geology and Environmental Oceanography. Journal of Coastal Research, Special Issue No. 108, pp. 99–103. Coconut Creek (Florida), ISSN 0749-0208.According to the measured value of clayey water electrical conductivity (EC) of xz-borehole core samples from the south bank plain of the Haihe River in Tianjin, the marine and continental strata of the xz-borehole core were divided. From bottom to top, the core represents the regression process of a neritic facies deposit (Zone I), intertidal deposit (Zone II), and continental deposit (Zone III). The hydrodynamic conditions of xz-borehole marine facies, sea–land transitional facies, and terrestrial strata were analyzed by grain-size analysis methods such as probability accumulation curve and CM diagram, and the sedimentary facies were further divided according to sediment grain size and sorting characteristics. Zone I is subdivided into a neritic facies deposit with a finer grain size and hydrodynamic stability (I-1 and I-3), a neritic facies deposit with a coarser grain size and stronger hydrodynamic force (I-2), and a delta front deposit with rapid EC value decline and a coarse grain size (I-4). Zone III, with a low EC value, is subdivided into fine-grained floodplain–flooding plain sediments (III-2) and coarse-grained crevasse splay sediments (III-1). High-energy hydrodynamic events in the deposition process can be accurately identified with grain-size analysis and EC indicators. Therefore, it can make up for the defect that EC cannot further refine the internal sedimentary facies for the marine facies, transitional facies, and continental facies. It has a good effect on the detailed differentiation of sediments and sedimentary facies in the coastal plain, and the identification of sedimentary events.

Sediment dynamics of estuarine Holocene incised-valley fill deposits recorded by Siphonichnus (ancient Red River, Gulf of Tonkin)

The Holocene muddy infill of the valley incised by the ancient Red River during Pleistocene sea-level lowstand records a change from fluvial to estuarine and finally marine depositional conditions. Siphonichnus, a mainly vertical trace fossil produced by burrowing bivalves, documents otherwise unrecorded episodes of enhanced erosion, bypass and deposition of, on average, rapidly aggrading (> 1 m kyr−1), mainly completely bioturbated sediments within the fluvial-marine transition zone (FMTZ), supposedly within the polyhaline domain. The producers of Siphonichnus moved nearly 1 m downward or upward in response to erosional or depositional phases, respectively. The fill of Siphonichnus burrows, in addition, records erosion and/or bypass of sediment: The traces were produced in mud, but many of them are filled with coarse silt and sand that is otherwise not present in the studied intervals in distinct layers. Obviously, this material was available when Siphonichnus was produced and connected to the sediment surface. In addition to seasonal erosion and deposition during freshet and its waning stage, Siphonichnus records exceptional events that displace the sediment surface by >0.5 m. Such events occurred episodically at an estimated frequency of 1 event per several hundreds of years. The complete bioturbation of the deposits suggests apparent continuous accumulation, but in fact they represent a pile of stacked event deposits resulting from depositional phases and intermittent severe erosion.

Hydrological and sedimentological changes following the 2010-forest fire in the Nahal Oren Basin, Mt. Carmel Israel–a comparison to pre-fire natural rates

The present study compares pre-fire and post-fire hydrological and sedimentological data at the mountainous, Upper Nahal Oren basin (18 km2), in Mt. Carmel, northwestern Israel, and traces its recovery after 4–6 years. The study summarizes 15 years − 2001–2016 of monitoring rainfall, streamflow and sediment transport during which, the 2010-fire consumed about a third of the basin vegetation cover. Sediments were monitored in a reservoir and at a gaging station. The results indicate that: (a) The threshold rainfall amounts for flow generation during the period 2010–2013 decreased by about < 50% indicating higher hydrological sensitivity. Since 2014 – 4 years after the fire, these values almost returned to their pre-fire background rates. (b) The annual average number of flow events significantly increased from 4.4 flows per year pre-fire to 8 flows per year post-fire (an increase of 180%). (c) No changes in peak discharges between pre- and post-fire, were documented. (d) Flow duration significantly increased with increasing storm rainfall; the slope of the regression line for the post-fire periods is significantly steeper indicating stronger basin response. (e) Post-streamflow events base flows significantly shortened from 7.2 days to 1.8 days (in average), due to the decrease in deep infiltration and recharge to the shallow perched aquifers along the channel resulting in decrease of spring discharge. During 2010–12, almost no base flows were documented. (f) Post-fire suspended sediment concentrations (SSC) significantly increased 2–3 orders of magnitude. The slope of regression of the post-fire period is significantly steeper than the pre-fire period. These, together with the increase in number of sedimentary events explains the increase in annual sediment weight by about 35%. Since 4th-6th year, post-fire SSC values decreased to almost pre-fire rates and annual sediment weight to even lower (73%) than pre-fire rates. Event sediment weight significantly increase with storm rainfall. The various, basin scale hydrological and sedimentological parameters documented at the present study indicate that the major impact of the fire continues 2–3 years, thereafter (4–6 years) they almost return to the pre-fire values, probably due to the recovery of the vegetation cover.

Loess-derived polygenetic soils of North-Western Italy: A deep characterization of particle size, shape and color to draw insights about the past

The multiplicity of units present in a single loess bed greatly complicates the reconstruction of the history of polycyclic loess-derived soils. The present study implemented a combination of techniques to characterize different depositional events in a pedogenized loess formation of North-Western Italy. Field morphological observations, laboratory Particle Size Distribution (PSD) analyses, and single-grain image processing techniques were applied.Five different loess depositions, or cycles, were distinguished in the field. Laser PSD curves highlighted the fine nature of the studied loess, which stood out even in comparison to similar Italian loess-derived soils. All PSD functions showed a tri/bimodal distribution with a dominant peak at 8–14 μm, wider in younger depositions. The second most relevant peak, at 0.5–4 μm, displayed a greater intensity in older cycles where it also fell in correspondence of slightly bigger particles. The fine-grained nature of the loess was read as a sign of its alluvial origin, therefore the Po plain has been addressed as the source of the material. This hypothesis was supported by evidence of a dominant SE-to-NW paleo-wind direction, proved by the spatial trends of the coarse and fine silt fractions. The typical aeolian size-dependant depositional trait of one of the cycles (IV) was however opposite to that of the other four loess depositions. Therefore, the source of this material was attributed to a site North of the study area where, right behind the Lanzo ultramafic massif, an ice lobe stretched during the Last Glacial Maximum (LGM). Image analysis supported this hypothesis: as the loess grains of cycle IV were significantly darker colored than the others, this would agree with the provenance from an ultramafic source. In conclusion, the variety of applied techniques allowed us to combine different pieces of information in order to assess the physical and morphological properties of the studied loess-derived soils, trying to determine the most probable sources of the material and the weathering pathways.

Lake level oscillations recorded in Buracica Stage (Barremian, Early Cretaceous) of Recôncavo Basin, Bahia State (N.E. Brazil)

This work studies the Buracica Stage (Barremian, Early Cretaceous) in the Reconcavo Basin, Bahia State (N.E. Brazil), comprising the Sao Sebastiao and Pojuca formations, from the analysis of 143 well logs and outcrop records. The Reconcavo Basin is a rift basin associated with the beginning of the South Atlantic Ocean. The basin started with very slow subsidence rates, leading to the formation of shallow lakes and the deposition of shales and sandstones, which compose the Brotas Group; later, subsidence accelerated and gave rise to the formation of a deep lake. Then the deep lake basin was filled throughout six depositional episodes making up the so-called Reconcavo tectonic sequence. Previous studies divided the last depositional event of Reconcavo Basin into seven fourth-order sequences concerning the Marker Bed “1” to the Marker Bed “F” stratigraphic interval. Sequence boundaries were marked by sharp flooding surfaces related to the transgression of lacustrine system due to rapid tectonic subsidence and generation of accommodation. In turn, the sequences are divided into three systems tracts: the lake lowstand systems tract, the transgressive systems tract and the lake highstand systems tract. The maximum flood surface of the sequences generally coincides with the stratigraphic marker “B” to “F”. Depositional systems like braided-fluvial plus eolian dunes, meandering-fluvial, deltaic, or moving-lake and near-shore deposits were identified in this work. Isopach, isolith and sand percentage maps indicate depocentre and source area distribution like the previous Aratu Stage. The primary sources of the sediments are located in the west and north areas; only from the third sequence, an eastern source could be described conspicuously. The fourth-order cyclicity is controlled by lake level changes, related to climatic factors (Milankovitch cycles). The results from the studies of cyclicity confirm the proposed stratigraphic model. They indicate 43,000 years for each fourth-order sequence and 150,000 years for the third-order sequences, which provide an accumulation rate of 1200 mm per 1000 years for the studied interval.

Preservation of the last aggradation phase in climate‐driven terraces: Evidence from Late Quaternary reach‐specific fluvial dynamics of the Allier River (France)

AbstractThere is limited knowledge about the preservation of aggradation phases in Quaternary fluvial records. Previous numerical modelling of erosion and deposition in Late Quaternary Allier River (France) generated the prediction that this river has reach‐specific fluvial dynamics related to climate‐driven tributary sediment‐flux dynamics. To test this prediction, new optically stimulated luminescence (OSL) samples were collected of the Late Quaternary Fx terrace at five locations along a stretch of 60 km. OSL dates of both quartz and feldspar sand grains indicate that all relatively basalt‐poor sediments display significantly different ages for each reach (ranging from 36.3 ± 2.0 to 21.1 ± 2.3 ka). The more basalt‐rich terrace body consistently yields ages in the range 21.1 ± 1.7 to 16.1 ± 1.5 ka, suggesting contemporaneous aggradation along the whole studied Allier reach during this interval. Our own new OSL date of a Tartaret eruption around 16.8 ± 2.5 ka also fits this time window, suggesting a direct link with volcanic activity. However, there are many more dated volcanic events that coincide with the older basalt‐poor units, making it less likely that a direct link between terrace‐sediment basalt content and volcanic activity exists. The timings of the dated depositional events in MIS 3 and 2 all match with simulated climate drivers and published landscape erosion rates. Counterintuitively, the volcanic Chaîne des Puys area supplied more sediment during the cold and dry Last Glacial Maximum. Basalt content in the Allier terrace sediments reflects climate‐related sediment‐flux dynamics upstream. The scarcity of older basalt‐poor sediment bodies from MIS 4 and 3 in the Fx terrace suggests that less sediment was supplied and/or the intermittent erosional phases in the Allier were very effective at removing them. We hypothesize that this observation of predominant preservation of the last aggradation phase could be a common phenomenon in most climate‐driven terraces. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd

Chronostratigraphic constraints and paleoenvironmental interpretation of the Romualdo Formation (Santana Group, Araripe Basin, Northeastern Brazil) based on palynology

The Santana Group of the Araripe Basin is a major paleontological unit in Brazil, incorporating two important Cretaceous Konservat-Lagerstätten. The lower formations are Aptian, but the age of the uppermost Romualdo Formation has been extended into the Albian. The precise chronostratigraphic position of this upper unit is of considerable significance because the Romualdo Formation records the Cretaceous marine ingression into the interior of Northeastern Brazil, thus playing a significant role in understanding the paleogeography of Gondwana. The main objective of this paper is to establish the chronostratigraphy of the Romualdo Formation based on its most complete section in the Araripe Basin, by determining the occurrence of palynomorphs, in particular establishing the vertical range of the species Sergipea variverrucata, a guide fossil of the Aptian in Brazil. Based on the presence of S. variverrucata, the Romualdo Formation is almost entirely Aptian in age. The palynological assemblage confirms the presence of at least three intervals with marine palynomorphs (dinoflagellate cysts and foraminiferal linings); and a typical regressive pattern towards the top of the Romualdo Formation, evidenced by progressive increase in inaperturate pollen grains (Araucariacites, Uesuguipollçenites and Inaperturopollenites). The palynological data presented herein indicate that the entire Santana Group is of Aptian age because of the conspicuous presence of Sergipea variverrucata and the absence of indisputably Albian forms. Precise dating of the Romualdo Formation signifies that the marine transgression into the interior of northeastern Brazil occurred in the late Aptian, thereby enabling correlation with the sedimentary events recorded in the Brazilian marginal basins.

Chemical, physical and mineralogical attributes as markers in the identification of depositional events in the Eastern Northeast of Brazil

Geochemical discontinuities have been used to identify the provenience and degree of sediment alteration from easily mobilized elements during weathering, as well as the most resistant ones such as zircon (Zr) and titanium. Therefore, the aim of this article is to contribute to the interpretation of the origins of depositional landforms in a sector of the Borborema Highlands Piedmont, Eastern Northeast of Brazil, by means of assessing the geochemical signature of sediments. For this purpose, samples were collected in morphostratigraphical sections, and subjected to qualitative and quantitative analysis of the major and minor elements by x-ray fluorescence spectrometry. Morphological and geochemical evidences confirmed the allochthonous character of the sediments from the correlation with previously known autochthonous material. The results suggest the sediments that underlie the geomorphic units were deposited in hillslope and fluvial environments, being later disarticulated of their respective base levels and remobilized.

Repeated occurrence of surface-sediment remobilization along the landward slope of the Japan Trench by great earthquakes

Deep-sea turbidites have been utilized to understand the history of past large earthquakes. Surface-sediment remobilization is considered to be a mechanism for the initiation of earthquake-induced turbidity currents, based on the studies on the event deposits formed by recent great earthquakes, such as the 2011 Tohoku-oki earthquake, although submarine slope failure has been considered to be a major contributor. However, it is still unclear that the surface-sediment remobilization has actually occurred in past great earthquakes. We examined a sediment core recovered from the mid-slope terrace (MST) along the Japan Trench to find evidence of past earthquake-induced surface-sediment remobilization. Coupled radiocarbon dates for turbidite and hemipelagic muds in the core show small age differences (less than a few 100 years) and suggest that initiation of turbidity currents caused by the earthquake-induced surface-sediment remobilization has occurred repeatedly during the last 2300 years. On the other hand, two turbidites among the examined 11 turbidites show relatively large age differences (~ 5000 years) that indicate the occurrence of large sea-floor disturbances such as submarine slope failures. The sedimentological (i.e., of diatomaceous nature and high sedimentation rates) and tectonic (i.e., continuous subsidence and isolated small basins) settings of the MST sedimentary basins provide favorable conditions for the repeated initiation of turbidity currents and for deposition and preservation of fine-grained turbidites. The MST small basin is a suitable site for examining deep-sea turbidite paleoseismology.

Trace-fossil suites and composite ichnofabrics from meandering fluvial systems: The Oligocene Lower Freshwater Molasse of Switzerland

The Oligocene Lower Freshwater Molasse of Switzerland is host to spectacular outcrops of fluvial deposits. In distal areas of the Swiss Molasse Basin, deposits of a north-eastward draining meandering system are exposed. Combined sedimentologic and ichnologic observations in the Chli Sunnhalde and Roggwil quarries allow an improved understanding of animal-substrate interactions in deposits of meandering fluvial systems. Strata include meander-belt sandstone, levee sandstone and mudstone, crevasse-channel and splay sandstone, floodplain mudstone and sandstone, and paleosols. While the meander belt and the more proximal crevasse-channel deposits are typically unbioturbated, the more distal deposits show a clear biogenic overprint, with distal crevasse-channel, crevasse-splay, and floodplain deposits being intensely bioturbated.Simple grazing and locomotion trails (included in the Cochlichnus anguineus-Helminthoidichnites tenuis suite) were formed subaqueously in ponded areas of the floodplain. Meniscate burrows (Taenidium isp. suite) and root trace fossils occur along the margin of these floodplain water-bodies and at the top of crevasse-splay deposits. Some of the floodplain, crevasse-splay, and crevasse-channel deposits are pervasively bioturbated by crayfish burrows (Camborygma litonomos suite). Crevasse-splay deposits locally host escape trace fossils.Where cross-cutting relationships are apparent, a colonization sequence can be established as follows: the Cochlichnus anguineus-Helminthoidichnites tenuis suite is cross-cut by the Taenidium isp. suite, which in turn is cross-cut by the Camborygma litonomos suite and root trace fossils. Composite ichnofabrics are common in these meandering river deposits, recording progressive lowering of the water table in overbank subenvironments. Frequency of depositional events was a first-order control on degree of bioturbation. Position of the water table and degree of substrate consolidation exerted considerable influence over the types of trace fossils produced. Distinct bioturbation phases may result in the formation of different trace-fossil suites - representative of disparate ichnofacies - preserved on a single bedding plane. This study further supports the notion that meandering fluvial systems are characterized by trace-fossil suites belonging to the Scoyenia, Camborygma and/or Mermia ichnofacies.

Stratigraphy and periodic variations of grain-size distribution recognized in flood event deposits : a case study of the sediment cores from the Kazusa Group, central eastern part of the Boso Peninsula, Japan

Stratigraphy and periodic variations of grain-size distribution recognized in flood event deposits : a case study of the sediment cores from the Kazusa Group, central eastern part of the Boso Peninsula, Japan

The Sveconorwegian orogeny – Reamalgamation of the fragmented southwestern margin of Fennoscandia

The Sveconorwegian orogeny encompasses magmatic, metamorphic and deformational events between ca. 1140 and 920 Ma at the southwestern margin of Fennoscandia. In recent years, the tectonic setting of this nearly 200 Myr-long evolution has been debated, with some workers arguing for collision with an unknown continent off the present-day southwest coast of Norway, and others advocating accretionary processes inboard of an active margin. Recently, it has been suggested that orogeny may have been gravity-driven by delamination and foundering of heavy subcontinental lithospheric mantle in an intraplate setting, in some ways similar to proposed sagduction processes in the Archaean. Resolving the tectonic setting of the Sveconorwegian orogen has implications for correlation with other orogens and Rodinia supercontinent reconstructions and for assessments of the evolution of plate tectonics on Earth, from the Archaean to the present. Here, we present new mapping and geochronological data from the Bamble and Telemark lithotectonic units in the central and western Sveconorwegian orogen – the former representing a critical region separating western parts of the orogen that underwent long-lived high- to ultrahigh-temperature metamorphism and magmatism from parts closer to the orogenic foreland that underwent episodic high-pressure events. The data show that the units constituting the Sveconorwegian orogen most likely formed at the southwestern margin of Fennoscandia between ca. 1800 and 1480 Ma, followed by fragmentation during widespread extension between ca. 1340 and 1100 Ma marked by bimodal magmatism and sedimentation. A summary of Sveconorwegian magmatic, metamorphic and depositional events in the different units shows disparate histories prior to their assembly with adjacent units. The most likely interpretation of this record seems to be that episodic, Sveconorwegian metamorphic and deformational events in the central and eastern parts of the orogen represent accretion and assembly of these units. This process most likely took place behind an active margin to the southwest that sustained mafic underplating in the proximal back-arc, resulting in high- to ultrahigh-temperature metamorphism in the western parts. In this interpretation, all features of the Sveconorwegian orogen are readily explained by modern-style plate tectonic processes and hypotheses involving some form of vertical, intraplate tectonics are not supported.

Isotopic and sedimentary signature of megathrust ruptures along the Japan subduction margin

The M9.3 2004 Sumatra and M9.0 2011 Tohoku earthquakes and tsunamis had devastating consequences for the adjacent heavily populated coasts. They had large shallow coseismic slip that reached the trench, which is thought to be a key factor in generating the huge tsunamis. The sedimentary record contains unique spatial and temporal information on the effects of megathrust earthquakes. It can provide a better understanding of earthquake-related tectono-sedimentary processes and of the earthquakes themselves. We present new results obtained from high-resolution multibeam bathymetry, subbottom profiles acquired with Parasound and ten, 10 m long piston cores recently collected from the Japan Trench margin during R/V Sonne SO251 expedition. We identify the 2011 Tohoku event-deposit extending for ~250 km along the Japan Trench basins from the enrichment and concentrations of short-lived radioisotopes (excess) xs210Pb, 137Cs and 134Cs in all ten cores. The high enrichment of xs210Pb points to surficial sediment remobilization as the main source. In three cores from the northern, central and southern segments of the Japan Trench, for the first time, we identify sediment remobilization by its 87/86Sr, εNd and Pb isotope signatures and the composition and texture of the sediment. With this new approach, we fingerprint older sedimentation events, potentially linked to earthquakes and infer processes responsible for their deposition. Thick, acoustically transparent and homogeneous units we name “homogenites”, characterize these event deposits. Physical, geochemical and acoustic properties link homogenites to the 2011 Tohoku and previous earthquakes. We propose a multi-stage entrainment and depositional process related with both the high frequency and the exceptionally large low frequency and long duration motions expected from ruptures of the shallow megathrust below the compliant outer upper-plate. These large long-period motions could propagate along the outer upper plate beyond the rupture zone and could generate widespread entrainment of surficial sediment.

The year after an extraordinary sedimentation event in a regulated Alpine river: The impact on benthic macroinvertebrate communities

AbstractOn August 2017, a massive rock slope failure triggered an extreme sedimentation event in a regulated Alpine river. Consequently, high sediment load affected the river for about 1 month, with sediment concentration peaking up to several hundred g/L, and overall sediment yield of 0.4–0.8 Mm3. The availability of sound pre‐event data concerning benthic macroinvertebrate assemblages gave us the opportunity to assess the impact of this unusually severe sediment disturbance through before/after comparison. Within 1 month from the event, all the monitored communities displayed low density, richness and diversity, with minimum values never recorded before, particularly at the most upstream investigated reach, and where riverbed aggradation was comparatively larger. Persisting riverbed alteration was also identified as a major determinant justifying the slow recovery observed during the year after the event. Overall, the high resilience of some taxa, suited to a baseline unnatural condition, allowed the fast recolonization of the investigated river reaches. However, the recovery of other taxa, more sensitive to sediment pressure, was slow or did not occur during the monitored year.

The Impacts of Flood, Drought, and Turbidites on Organic Carbon Burial Over the Past 2,000 years in the Santa Barbara Basin, California

AbstractClimate conditions and instantaneous depositional events can influence the relative contribution of sediments from terrestrial and marine environments and ultimately the quantity and composition of carbon buried in the sediment record. Here, we analyze the elemental, isotopic, and organic geochemical composition of marine sediments to identify terrestrial and marine sources in sediment horizons associated with droughts, turbidites, and floods in the Santa Barbara Basin (SBB), California, during the last 2,000 years. Stable isotopes (δ13C and δ15N) indicate that more terrestrial organic carbon (OC) was deposited during floods relative to background sediment, while bulk C to nitrogen (C/N) ratios remained relatively constant (~10). Long‐chain n‐alkanes (C27, C29, C31, and C33), characteristic of terrestrial OC, dominated all types of sediment deposition but were 4 times more abundant in flood layers. Marine algae (C15, C17, and C19) and macrophytes (C21 and C23) were also 2 times higher in flood versus background sediments. Turbidites contained twice the terrestrial n‐alkanes relative to background sediment. Conversely, drought intervals were only distinguishable from background sediment by their higher proportion of marine algal n‐alkanes. Combined, our data indicate that 15% of the total OC buried in SBB over the past 2,000 years was deposited during 11 flood events where the sediment was mostly terrestrially derived, and another 12% of deep sediment OC burial was derived from shelf remobilization during six turbidite events. Relative to twentieth century river runoff, our data suggest that floods result in considerable terrestrial OC burial on the continental margins of California.

Lake diatom response to climate change and sedimentary events on the southeastern Tibetan Plateau during the last millennium

Climate change on the southeastern Tibetan Plateau (TP) is assumed to impact lake ecosystems. However, parallel changes in climate and non-climate factors and mediation of signals by catchment processes complicate the assessment of climate-change effects on lake ecosystems requiring long-term and high resolution multi-variable time-series. Here we present a diatom record from sediments in the lake Basomtso, southeastern TP, covering the period 1083–2012 CE with sub-decadal resolution which we relate to available proxies for potential climate and non-climate factors. The record is dominated by planktonic species and the compositional change is mainly characterized by alternations between big- and small-celled Pantocsekiella ocellata and between planktonic and benthic diatom species. The diatom assemblages show a decrease of P. ocellata (big form) since the end of 17th century and an increase of P. ocellata (small form) as well as an increase in diatom productivity, reflecting changes on a centennial timescale which mainly follow temperature increase. We recorded several changes in the ratio of planktonic to benthic species abundance (P: B) on a sub-decadal or decadal timescale. Noticeable decreases in P: B were a result of rapid sedimentary events caused by high energy flows. Cross-correlation analyses reveal synchronous responses of diatoms and asynchronous responses of terrestrial vegetation to regional temperature change, indicating a higher potential of lake diatoms to detect the timing and magnitude of climate shifts on the southeastern TP. The results imply that aquatic ecosystems in Basomtso, as well as in other mountain lakes on the southeastern TP, are probably subject to a larger risk of regime shift than terrestrial ecosystems under future climate scenarios.

Deciphering the geodynamic evolution of the Dinaric orogen through the study of the ‘overstepping’ Cretaceous successions

AbstractAlong the Dinaric–Hellenic orogen, the Late Jurassic – Early Cretaceous ophiolite obduction over the Adria continental margin was sealed by sedimentation of clastic terrestrial deposits rapidly followed by a widespread carbonate platform system since the Early Cretaceous period. These Cretaceous sediments presently crop out over areas of varying extension, from several hundred kilometre wide undeformed continuous covers to small-scale tectonic slivers involved in the tectonic stack following the latest Cretaceous–Palaeogene collision. These deposits are unconformably sedimented above the units formed by the Late Jurassic to Early Cretaceous nappe stacking above the eastern Adria continental margin. We studied these deposits in a large area between western Serbia and eastern Bosnia. In the studied area, these deposits are divided into three lithostratigraphic groups according to their age, depositional environment and type of underlying basement. The Mokra Gora Group sediments (upper Aptian–Maastrichtian) were deposited on top of previously obducted and weathered ophiolites, the Kosjerić Group (Cenomanian–Campanian) overlies composite tectonic units comprising obducted ophiolites and their underlying continental basement portions, while the Guča Group (Campanian–Maastrichtian) exclusively rests on top of continental basement. The reconstructed sedimentary evolution of these groups, together with the comparison with the syn- and post-obduction deposits at the front of the ophiolitic nappe(s) in a wider area of the internal Dinarides (e.g. Pogari Group and Bosnian flysch), allowed us to clarify the obduction mechanisms, including their tectonic context, the changes in depositional environments and the timing of depositional and tectonic events, and, in a wider view, shed light on the geodynamic evolution of the Dinaric belt.