Turbidite Frequency Research Articles

Sediment dynamics and geomorphology of a submarine carbonate platform canyon system situated in an arid climate setting

ABSTRACTChanges in neritic carbonate production and sediment transport off platforms are related to climate variations, sea‐level fluctuations and tectonic processes. Canyon systems marking the platform slopes represent critical source‐to‐sink pathways transporting shallow‐water sediments basinward. However, these export systems and related processes are primarily studied on platform slopes in humid to tropical climate settings. A newly discovered canyon system on the leeward margin of the Al Wajh platform (north‐east Red Sea) represents the ideal laboratory to investigate source‐to‐sink pathway dynamics in an arid climate that prevailed since the Late Pleistocene. A high‐resolution bathymetry map was established to characterize the slope morphology. The system displays a U‐shaped, 10 km long main channel dominantly sourced by the north‐west/south‐east running outer channel and two smaller 2 to 3 km long canyons. The latter are positioned perpendicular to the main canyon. A 4 km wide head scarp at the reef edge and dozens of amphitheatre‐shaped scarps along the mid to lower slope suggest significant slope failures over time. The analysis of four sediment cores collected on a profile down the canyon revealed sedimentation rates of 26 cm/ka at the mid‐slope to 9.4 cm/ka in the basin. Three main sediment‐export processes were identified: (i) sandy and neritic component‐poor turbidites; (ii) winnowing of strontium‐rich carbonate fines through surface currents; and (iii) remobilized carbonate fines on the upper slope. As of the Last Glacial, turbidites are predominantly deposited during times of significant sea‐level instability, both rises and falls, whereas their flat‐topped‐tropical counterparts show a higher turbidite frequency during highstands. Strontium‐rich carbonate fines are exported similarly through time in both climate settings. Overall, sediment export is controlled by: (i) the platform morphology (flat‐topped versus rimmed lagoon); (ii) variations in sediment production; (iii) sea‐level variations (exposure or flooding of sediment production areas); and (iv) the interaction between the sedimentary system and atmospheric changes (sediment production and delivery).

Clay mineral assemblages of late Holocene turbidites in the Gaoping Submarine Canyon of the northeastern South China Sea and their responses to typhoon activities

Submarine canyons are the principal conduits for turbidity currents transporting sediments to the deep sea. Thus, turbidites, as sedimentary products of turbidity currents, can be used to trace sediment sources and transport processes and to reconstruct mechanisms of turbidity currents in submarine canyons during the geological past. Herein, clay mineral assemblage of late Holocene turbidites at Core MD18–3565 in the Gaoping Submarine Canyon is investigated to reveal the triggering mechanisms of turbidity currents. High-resolution grain-size analysis of typical coarse-grained layers characterized by basal erosion surface and fining-upward sequence in the core verifies their turbidity current origin. A total number of 318 such turbidite layers, covering ∼30% of the total thickness (19.68 m) of the core, are recognized in the late Holocene sediments (33–3954 cal yr BP). Clay mineral assemblage of the turbidites is composed mainly of illite (52–60%) and chlorite (35–45%), with small quantities of smectite (0–5%) and kaolinite (0–4%), while clay mineral assemblage of the non-turbidite sediments (hemipelagites) is composed of approximately the same contents of illite (51–62%) and chlorite (34–43%), but with slightly higher smectite (0–10%) and kaolinite (1–6%). Despite the small differences in smectite and kaolinite contents, the provenance analysis identifies the Gaoping river as the predominant source of sediments in the turbidite layers, while the hemipelagite layers receive small amounts of sediments from the neighboring Erhjen, Tsengwen, and Pachang rivers besides the Gaoping river in southwest Taiwan. In combination with clay mineral compositions of surface sediments in the adjacent continental shelf and slope, as well as in-situ observational results by subaqueous moorings, we believe that the turbidity currents, in which the Gaoping river directly supplied clay minerals to the submarine canyon to form the turbidites, have been triggered mainly by typhoons passing through Taiwan. The frequency of turbidites generally keeps pace with long-term variations of El Niño-Southern Oscillation (ENSO) events, implying the potential modulation of ENSO on occurrences of late Holocene typhoon activities in the Western Pacific.

Factors affecting thickness and frequency of turbidites triggered by earthquakes in Kumburgaz Basin, Sea of Marmara

Submerged fault ruptures generate earthquake-triggered mass flow deposits, which are extensively used as a tool in subaqueous paleoseismology. In tectonically active deep sedimentary basins, such as the Sea of Marmara (SoM), these mass flow deposits are defined as turbidite-homogenite units (THUs), consisting essentially of a coarse basal part and an overlying homogeneous mud (homogenite). Detailed characterization of THUs is crucial in order to establish meaningful criteria to link these units with earthquakes events and to identify their transport routes and depositional mechanisms. Here, we combine μ-X-ray Fluorescence (μ-XRF), Anisotropy of Magnetic Susceptibility (AMS) and additional rock magnetism analyses of a 21-m long piston core from the Kumburgaz Basin of SoM to define the upper stratigraphic boundary of THUs with hemipelagic sediments and investigate the controls of hydrological changes on turbidite frequency and thickness over the last 15 kyrs BP. The sedimentary succession of this period includes a lower lacustrine and an upper marine unit with two Holocene sapropel intervals. The sequence is interrupted by a total of 70 THUs, characterized by a significant magnetic foliation related to the depositional setting rather than the magnetic signature. Magnetic mineralogy of the coarse basal parts of THUs have more ferromagnetic particles than the overlying homogenites and background sediments. While the homogenite parts have a more constant mineralogy than the basal parts, they do not differ the background sediments.Based on an event-free chronostratigraphic model derived from radiocarbon ages and the published age of lacustrine-marine (L-M) transition, the average THU occurrence intervals in the lacustrine (14.8–12.6 kyrs BP), lower sapropel (11.2–5.7 kyrs BP), upper sapropel (5.4–2.7 kyrs BP) and non-sapropelic part of the marine unit (2.7 kyrs BP-present) are 235 yrs., 287 yrs., 114 yrs. and 160 yrs. respectively. The average thickness of the THUs in the same units are 20.8 cm, 15.7 cm, 6.1 cm and 6.1 cm. The variability of average THU occurrence intervals and THU thicknesses are controlled by the sea level rise and salinity increase following the full marine connection of the SoM at 12.6 kyrs BP, which caused changes in slope stability, sediment composition and sediment deposition in different parts of the basin. Geomechanical properties of the lower sapropel appears to have been important in resulting long THU recurrence intervals and relatively high THU thicknesses.

Linking carbonate sediment transfer to seafloor morphology: Insights from Exuma Valley, the Bahamas

AbstractThe depositional record of carbonate slopes provides a valuable archive of past environmental and climatic changes. Modern carbonate slopes reveal morphological variabilities (for example, gullies and canyons) shaped by episodic slope collapses and turbidity currents. Furthermore, climate‐induced fluctuations in sea level regulate sediment availability and delivery to the deep‐sea. Morphological and climatic controls on calciclastic sediment transfer are often complex to decipher. The aim of this study is to link seafloor morphology and depositional processes in an active carbonate submarine channel (Exuma Valley, the Bahamas) over the last 40 kyr. The dataset includes multibeam and seismic surveys, and two sediment cores retrieved from the valley axis. A series of abrupt slope‐breaks, called knickpoints, occurs along Exuma Valley, and plays a key role in sediment transport and accumulation. Initiation processes proposed for knickpoint formation include bank‐collapse, side gully erosion and loss of confinement. Slope collapses detected on the bathymetry prevail in the upstream muddy section of the submarine valley, as attested to by a planktic‐rich debrite–turbidite couplet in the first core. In contrast, the second core collected downstream of the knickpoints train, includes 32 bioclastic sandy event‐beds (i.e. turbidites). Hydrodynamic sorting generates grain segregation (for example, Halimeda‐rich base versus planktic‐rich top) and geochemical contrasts (Sr/Ca) in turbidites. Turbidite frequency and grain composition within beds reflect the variation of carbonate sources during glacial–interglacial periods. This research allows to link slope morphology with deposits of a modern large‐scale carbonate factory, and to deduce sea‐level changes over that last 40 kyr in the Bahamas. These results can provide new perspectives on the understanding of ‘source to sink’ mechanisms in carbonate systems.

Factors controlling frequency of turbidites in the Bengal fan during the last 248 kyr cal BP: Clues from a presently inactive channel

The seafloor of the Bay of Bengal is covered by thick sediment deposits that constitute the largest turbiditic system in the world. This system is fed primarily by the Ganges and Brahmaputra rivers, which drain the high Himalayan ranges. Sediment transfers from the delta to the deep-sea fan take place as turbidity currents in channel-levee systems. Previous studies have shown that, during high sea-level stand periods, the sediments were being mainly stored in the Ganges-Brahmaputra delta, and turbiditic transfer was occurring through active channels. Most of these channels are now inactive and sealed by hemipelagic deposits. However, the evolution of the inactive channels during the last sea-level variations has never been described in detail. Sedimentation in the currently active channel, the Active Valley, was particularly important during the last sea-level rise, which suggests a very good connection between the fluvial systems and the deep turbidite system at this time. During the MONOPOL cruise (2012), we retrieved a giant piston core (MD12-3412) near the currently inactive E4 channel. Previous studies have hypothesized that this channel is connected to the Swatch of No Ground canyon on the upper fan. The upper part of the core covers the last ~250 kyr. It reveals that, contrary to what is known about the Active Valley, the turbidite activity in E4 took place mainly during low sea-level phases (glacial stages), and stopped around 11.8 kyr cal BP. This different mode of activity suggests that (i) E4 was not abandoned but served as a secondary channel, and (ii) that the supply of turbidite material at the site of core MD12-3412 was not related to past changes in summer monsoon strength. Periods of activation of the E4 channel observed on core MD12-3412 were previously identified on the shelf area, by Hubscher and Spiess (2005), as thick Forced Regression System Tracts (FRST) after a displacement of deltaic edifices. High turbidites record on the deep basin are mainly synchronous with sea-level fall and rise conditions, but mostly during low sea-level periods. This could be explained by a residual connection between the coastal system and the E4 channel during sea level low stands.

Sea level-controlled sediment transport to the eastern Arabian Sea over the past 600 kyr: Clay minerals and Sr[sbnd]Nd isotopic evidence from IODP site U1457

Long marine sequences deposited in the Arabian Sea are suitable to study the orbital sensitivity of sediment dynamics from a source-to-sink perspective. The clay mineralogy and SrNd isotopic compositions have been analyzed for the detrital fraction of sediments from IODP Site U1457 located in the Laxmi basin, eastern Arabian Sea to constrain the sediment sources and reconstruct a high-resolution record of sediment export to the Indus deep-sea fan over the past 600 kyr. The 87Sr/86Sr ratio (varying from 0.7154 to 0.7255) and εNd (varying from −12.9 to −9.5) imply a mixing of sediments derived from not only the Indus River but also the rivers draining the Deccan Traps. The variations in the smectite/(illite + chlorite) ratio combined with SrNd isotopes permit the reconstruction of past changes in the relative proportions of sediments derived from the Indus River (mainly illite, chlorite and kaolinite) and the Deccan Traps (smectite). The wavelet and cross-spectral analyses further reveal that sediment transport to the eastern Arabian Sea since 600 kyr is mainly dominated by sea-level variations in the eccentricity (100-kyr) band, while the Indian summer monsoon might contribute moderately to the precession (23-kyr) band. The εNd and turbidite frequency indicate that more turbidite events are associated with Indus River-sourced sediments at IODP Site U1457 during sea-level lowstands, which might reflect a reconnection of deep-sea channels to the Indus River mouth and/or reworking of previously deposited Indus River sediments located northward.

Paleoenvironmental implications of the calcium isotope characteristics in the MD81349 from the Nintyeast Ridge in the Indian Ocean

A δ44Ca curve from shells of the planktonic foraminifera Globigerinoides sacculifer in calcareous biogenic oozes has been extracted from the Nintyeast Ridge in the Indian Ocean since 300 ka. By combining terrigenous inputs (e.g., grain size, magnetic susceptibility, and turbidite frequency) with the oceanic productivity (e.g., biogenic content and Neogloboquadrina dutertrei content), it is found that the curve’s variations are closely related to the historical evolution of the oceanic calcium cycle. The δ44Ca value is in lower tendency and has small oscillation during Marine Isotope Stage (MIS) 6, when the supply of terrigenous detrital is highest. In contrast, during MIS 3, 5 and 7, the δ44Ca values are in higher tendency, and their fluctuations are consistent with the variations of the productivity proxies. These results suggest that the calcium isotopes are mainly influenced by the input of the Himalayan erosion products to the northern Indian Ocean. In addition, the developmental stages of calcareous planktons may have a secondary impact on the fluctuations of the calcium isotope ratio of sea water.

6600 years of earthquake record in the Julian Alps (Lake Bohinj, Slovenia)

AbstractSequences of lake sediments often form long and continuous records that may be sensitive recorders of seismic shaking. A multi‐proxy analysis of Lake Bohinj sediments associated with a well‐constrained chronology was conducted to reconstruct Holocene seismic activity in the Julian Alps (Slovenia). A seismic reflection survey and sedimentological analyses identified 29 homogenite‐type deposits related to mass‐wasting deposits. The most recent homogenites can be linked to historical regional earthquakes (i.e. 1348 ad, 1511 adand 1690 ad) with strong epicentral intensity [greater than ‘damaging’ (VIII) on the Medvedev–Sponheuer–Karnik scale]. The correlation between the historical earthquake data set and the homogenites identified in a core isolated from local stream inputs, allows interpretation of all similar deposits as earthquake related. This work extends the earthquake chronicle of the last 6600 years in this area with a total of 29 events recorded. The early Holocene sedimentary record is disturbed by a seismic event (6617 ± 94 cal yr bp) that reworked previously deposited sediment and led to a thick sediment deposit identified in the seismic survey. The period between 3500 cal yr bpand 2000 cal yr bpis characterized by a major destabilization in the watershed by human activities that led to increases in erosion and sedimentation rates. This change increased the lake's sensitivity to recording an earthquake (earthquake‐sensitivity threshold index) with the occurrence of 72 turbidite‐type deposits over this period. The high turbidite frequency identified could be the consequence of this change in lake earthquake sensitivity and thus these turbidites could be triggered by earthquake shaking, as other origins are discarded. This study illustrates why it is not acceptable to propose a return period for seismic activity recorded in lake sediment if the sedimentation rate varies significantly.

Sea level and climatic controls on turbidite occurrence for the past 26 kyr on the flank of the Gaoping Canyon off SW Taiwan

Submarine canyons are major conduits for delivery of sediments originating from the continental margin into the deep sea. Here we analyze the effects of changing sea levels and climate (e.g. sediment supply) on the frequency of turbidites over the last 26kyr. Our study is based on high resolution age controls for turbidites in core MD178-10-3291 from a water depth of 2070m on the flank of the Gaoping Canyon offshore in SW Taiwan. Unlike most other submarine canyons worldwide, the head of the Gaoping Canyon has remained connected to the river mouth during the recent flooding of the continental shelf. Our results reveal that turbidity currents are less frequent during periods of sea-level lowstand. In contrast, turbidity currents have been more frequent since ~12kyrBP, during the final stage of rising sea level and sea-level highstands. This may be the result of increased terrestrial sediment delivery due to enhanced rainfall intensity. Moreover, comparing to other source-to-sink systems, the turbidite occurrence in the flank of Gaoping Canyon through the last glacial cycle might have been more strongly influenced by climatic changes due to short sediment storage/response time.

Shelfal sediment transport by an undercurrent forces turbidity-current activity during high sea level along the Chile continental margin

Terrigenous sediment supply, marine transport, and depositional processes along tectonically active margins are key to decoding turbidite successions as potential archives of climatic and seismic forcings. Sequence stratigraphic models predict coarse-grained sediment delivery to deep-marine sites mainly during sea-level fall and lowstand. Marine siliciclastic deposition during transgressions and highstands has been attributed to sustained connectivity between terrigenous sources and marine sinks facilitated by narrow shelves. To decipher the controls on Holocene highstand turbidite deposition, we analyzed 12 sediment cores from spatially discrete, coeval turbidite systems along the Chile margin (29°–40°S) with changing climatic and geomorphic characteristics but uniform changes in sea level. Sediment cores from intraslope basins in north-central Chile (29°–33°S) offshore a narrow to absent shelf record a shut-off of turbidite deposition during the Holocene due to postglacial aridification. In contrast, core sites in south-central Chile (36°–40°S) offshore a wide shelf record frequent turbidite deposition during highstand conditions. Two core sites are linked to the Biobio river-canyon system and receive sediment directly from the river mouth. However, intraslope basins are not connected via canyons to fluvial systems but yield even higher turbidite frequencies. High sediment supply combined with a wide shelf and an undercurrent moving sediment toward the shelf edge appear to control Holocene turbidite sedimentation and distribution. Shelf undercurrents may play an important role in lateral sediment transport and supply to the deep sea and need to be accounted for in sediment-mass balances.

Climate and sea-level controls on turbidity current activity on the Tanzanian upper slope during the last deglaciation and the Holocene

Turbidity current is one of the most important mechanisms for rapid transportation of terrigenous sediments from the continental margin to the deep sea. Although extensive work on turbidite systems has been carried out globally, the Tanzanian margin, off East Africa, is poorly understood. This paper will therefore use a well-dated high-resolution marine core (GeoB12624-1), obtained during RV Meteor Cruise M75/2, from the upper Tanzanian continental slope, offshore the Rufiji River delta, to demonstrate which environmental parameters, e.g., climate versus sea level, control the temporal distribution of the turbidite deposits during the last deglaciation and the Holocene. Results show that the turbidite deposits are composed of coarser-grained sediments with normally graded bedding. The thickness, mean grain size and frequency of the turbidite beds were estimated to detect turbidity current activity since the Last Glacial Maximum (LGM). A total of 12 turbidite beds (TI-T12) was recognized and divided into three intervals, based on the presence and intensity of the turbidite deposits. (I) a glacial sea-level lowstand (LGM and Heinrich Stadial 1; 19.3–14.6 ka): suppressed turbidity current activity due to arid conditions in the hinterland and sea-level lowstand. (II) A deglacial sea-level rise period (Bølling-Allerød interval to the early Holocene; 14.6–8 ka): turbidity current activity started to be strengthened during the Bølling-Allerød warming interval (T1), followed by a first increase in turbidite frequency, thickness and mean grain-size during the Younger Dryas (T2–T4) and a second increase during the early Holocene (T5–T10). (III) An interglacial sea-level highstand period (mid-to late Holocene; 8–2.5 ka): turbidity current activity diminished from the mid- (T11, T12) to late Holocene. This temporal distribution of turbidite deposits indicates that turbidity currents were most active during the last sea-level rise, synchronous with more humid climate in the hinterland, than during a sea-level lowstand (e.g., LGM) when climate was more arid. Thus, we propose that it is the climate conditions in the hinterland that form the primary controlling factor for turbidity current activity of Tanzanian slope. The remobilization of lowstand sediments on the Tanzanian continental margin during a sea-level rise, combined with shedding sediments towards the study area from the main canyon/channel, also plays a role in supplying new sediments to the turbidite system. These new data demonstrate that, in specific continental margin, high fluvial discharge will promote sediment transfer along the shelf and slope even during sea-level rise and highstand.

Sub-decadal turbidite frequency during the early Holocene: Eel Fan, offshore northern California

Remotely operated and autonomous underwater vehicle technologies were used to image and sample exceptional deep sea outcrops where an ~100-m-thick section of turbidite beds is exposed on the headwalls of two giant submarine scours on Eel submarine fan, offshore northern California (USA). These outcrops provide a rare opportunity to connect young deep-sea turbidites with their feeder system. 14 C measurements reveal that from 12.8 ka to 7.9 ka, one turbidite was being emplaced on average every 7 yr. This emplacement rate is two to three orders of magnitude higher than observed for turbidites elsewhere along the Pacific margin of North America. The turbidites contain abundant wood and shallow-dwelling foraminifera, demonstrating an efficient connection between the Eel River source and the Eel Fan sink. Tur bidite recurrence intervals diminish fivefold to ~36 yr from 7.9 ka onward, reflecting sea-level rise and re-routing of Eel River sediments.

Rethinking turbidite paleoseismology along the Cascadia subduction zone

A stratigraphic synthesis of dozens of deep-sea cores, most of them overlooked in recent decades, provides new insights into deepsea turbidites as guides to earthquake and tsunami hazards along the Cascadia subduction zone, which extends 1100 km along the Pacific coast of North America. The synthesis shows greater variability in Holocene stratigraphy and facies off the Washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows. That test compared counts of Holocene turbidites upstream and downstream of a deep-sea channel junction. Similarity in the turbidite counts among seven core sites provided evidence that turbidity currents from different submarine canyons usually reached the junction around the same time, as expected of widespread seismic triggering. The fuller synthesis, however, shows distinct differences between tributaries, and these differences suggest sediment routing for which the confluence test was not designed. The synthesis also bears on recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies. The recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply. These concerns underscore the complexity of extracting earthquake history from deep-sea turbidites at Cascadia.

Sediment dynamics in the subaquatic channel of the Rhone delta (Lake Geneva, France/Switzerland)

With its smaller size, well-known boundary conditions, and the availability of detailed bathymetric data, Lake Geneva’s subaquatic canyon in the Rhone Delta is an excellent analogue to understand sedimentary processes in deep-water submarine channels. A multidisciplinary research effort was undertaken to unravel the sediment dynamics in the active canyon. This approach included innovative coring using the Russian MIR submersibles, in situ geotechnical tests, and geophysical, sedimentological, geochemical and radiometric analysis techniques. The canyon floor/levee complex is characterized by a classic turbiditic system with frequent spillover events. Sedimentary evolution in the active canyon is controlled by a complex interplay between erosion and sedimentation processes. In situ profiling of sediment strength in the upper layer was tested using a dynamic penetrometer and suggests that erosion is the governing mechanism in the proximal canyon floor while sedimentation dominates in the levee structure. Sedimentation rates progressively decrease down-channel along the levee structure, with accumulation exceeding 2.6 cm/year in the proximal levee. A decrease in the frequency of turbidites upwards along the canyon wall suggests a progressive confinement of the flow through time. The multi-proxy methodology has also enabled a qualitative slope-stability assessment in the levee structure. The rapid sediment loading, slope undercutting and over-steepening, and increased pore pressure due to high methane concentrations hint at a potential instability of the proximal levees. Furthermore, discrete sandy intervals show very high methane concentrations and low shear strength and thus could correspond to potentially weak layers prone to scarp failures.

Frequency and timing of landslide-triggered turbidity currents within the Agadir Basin, offshore NW Africa: Are there associations with climate change, sea level change and slope sedimentation rates?

Abstract Older sequence stratigraphic models suggested that submarine landslide and turbidite activities are greatest during sea-level lowstands. However, growing evidence indicates that many turbidite systems are also active during sea-level transgressions and highstands. The Moroccan Turbidite System comprises three depocentres, of which Agadir Basin is closest to the Moroccan slope and Canary archipelago. The very large volumes of sediment transported by individual sediment flows in this system suggest that they are triggered by landslides. Extensive core coverage and dating control for the Agadir Basin deposits have provided an excellent opportunity to derive accurate records of turbidite (and associated landslide) frequency for the last 600 ka. Previous studies in the more distal Madeira Abyssal Plain depocentre have indicated that large volume (> 50 km 3 ) turbidites occurred at oxygen isotope stage (OIS) boundaries. This study of Agadir Basin confirms that two major turbidites (beds A5 and A12) occurred during glacial–interglacial transitions associated with OIS4 and OIS6. However, this association is based on just two examples, and two other large-volume turbidites (beds A7 and A11), did not occur at a stage boundary. The main conclusion of this study is that 90% of turbidites and landslides occurred during rising and high sea level, which represents 40% of the total time during the last 600 ka. Only 10% of the turbidites and landslides occurred during glacials (40% of the time), with a paucity of turbidites and landslides at peak glacial lowstands. A comparison to sediment accumulation rates in the source area of the turbidite suggests that landslides did not occur preferentially during periods of more rapid sedimentation rate, although sedimentation rates in this area only varied from 4 to 6 g cm − 2 ka − 1 .

Sediment accumulation rates and turbidite frequency in the eastern Algerian margin. An attempt to examine the triggering mechanisms

From the analysis of seven new sediment piston-cores sampled in 2005 (MARADJA-2 French–Algerian cruise), this study aims to identify for the first time possible late Pleistocene to recent sedimentary instabilities controlled by seismicity off or close to the eastern coast of Algeria. The detailed lithologic study allows us to identify the frequency of the gravity events (turbidites, debrites) and to discuss their geographical sources and triggering mechanisms. Based on a chronostratigraphy of 24 14C AMS datings, sediment accumulation rates in zones extending off Bejaia and Annaba and, in particular, semi-quantitative analysis of the microfossils and lithogenic tracers of the origin of gravity flows was discussed. Two sediment cores, here considered as reference cores, enabled the estimation of palaeoenvironmental parameters that controlled sedimentation: (1) in the prodelta of Soummam Oued, after 2215calyrBP, floods were less frequent and sediment accumulation rates decreased because of a drier climate; (2) in the middle slope to the NE of Annaba, a location shielded from gravity flows, an increased sedimentation rate coincided with the passage of warmer waters leading to maxima of carbonate biogenic fluxes (particularly pteropods). Off Bejaia, two deep sediment cores show a spectacular increase in sediment accumulation rate between 2200 and 1000calyrBP while turbidites become more frequent. According to the eustatic and climatic stability of this interval, an episode of strong slope instability of the slope is suggested. Both sediment cores on the slope of Annaba indicate an increase in gravity flows during the same last thousand years, which is tentatively related to a regional increase of seismicity during this interval. This spatial distribution of gravity events is clearly different to that of the western margin where the sedimentation is less perturbed.

Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology

Abstract. We summarize the importance of great earthquakes (Mw ≳ 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which results in a margin stratigraphy of minor MTDs compared to the turbidite-system deposits. In contrast, the MTDs and turbidites are equally intermixed on basin floors along passive margins with a mud-rich continental slope, such as the northern Gulf of Mexico. Great earthquakes also result in characteristic seismo-turbidite lithology. Along the Cascadia margin, the number and character of multiple coarse pulses for correlative individual turbidites generally remain constant both upstream and downstream in different channel systems for 600 km along the margin. This suggests that the earthquake shaking or aftershock signature is normally preserved, for the stronger (Mw ≥ 9) Cascadia earthquakes. In contrast, the generally weaker (Mw = or <8) California earthquakes result in upstream simple fining-up turbidites in single tributary canyons and channels; however, downstream mainly stacked turbidites result from synchronously triggered multiple turbidity currents that deposit in channels below confluences of the tributaries. Consequently, both downstream channel confluences and the strongest (Mw ≥ 9) great earthquakes contribute to multi-pulsed and stacked turbidites that are typical for seismo-turbidites generated by a single great earthquake. Earthquake triggering and multi-pulsed or stacked turbidites also become an alternative explanation for amalgamated turbidite beds in active tectonic margins, in addition to other classic explanations. The sedimentologic characteristics of turbidites triggered by great earthquakes along the Cascadia and northern California margins provide criteria to help distinguish seismo-turbidites in other active tectonic margins.

Sedimentological imprint on subseafloor microbial communities in Western Mediterranean Sea Quaternary sediments

Abstract. An interdisciplinary study was conducted to evaluate the relationship between geological and paleoenvironmental parameters and the bacterial and archaeal community structure of two contrasting subseafloor sites in the Western Mediterranean Sea (Ligurian Sea and Gulf of Lion). Both depositional environments in this area are well-documented from paleoclimatic and paleooceanographic point of views. Available data sets allowed us to calibrate the investigated cores with reference and dated cores previously collected in the same area, and notably correlated to Quaternary climate variations. DNA-based fingerprints showed that the archaeal diversity was composed by one group, Miscellaneous Crenarchaeotic Group (MCG), within the Gulf of Lion sediments and of nine different lineages (dominated by MCG, South African Gold Mine Euryarchaeotal Group (SAGMEG) and Halobacteria) within the Ligurian Sea sediments. Bacterial molecular diversity at both sites revealed mostly the presence of the classes Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria within Proteobacteria phylum, and also members of Bacteroidetes phylum. The second most abundant lineages were Actinobacteria and Firmicutes at the Gulf of Lion site and Chloroflexi at the Ligurian Sea site. Various substrates and cultivation conditions allowed us to isolate 75 strains belonging to four lineages: Alpha-, Gammaproteobacteria, Firmicutes and Actinobacteria. In molecular surveys, the Betaproteobacteria group was consistently detected in the Ligurian Sea sediments, characterized by a heterolithic facies with numerous turbidites from a deep-sea levee. Analysis of relative betaproteobacterial abundances and turbidite frequency suggested that the microbial diversity was a result of main climatic changes occurring during the last 20 ka. Statistical direct multivariate canonical correspondence analyses (CCA) showed that the availability of electron acceptors and the quality of electron donors (indicated by age) strongly influenced the community structure. In contrast, within the Gulf of Lion core, characterized by a homogeneous lithological structure of upper-slope environment, most detected groups were Bacteroidetes and, to a lesser extent, Betaproteobacteria. At both site, the detection of Betaproteobacteria coincided with increased terrestrial inputs, as confirmed by the geochemical measurements (Si, Sr, Ti and Ca). In the Gulf of Lion, geochemical parameters were also found to drive microbial community composition. Taken together, our data suggest that the palaeoenvironmental history of erosion and deposition recorded in the Western Mediterranean Sea sediments has left its imprint on the sedimentological context for microbial habitability, and then indirectly on structure and composition of the microbial communities during the late Quaternary.

External controls on turbidite sedimentation on the glacially-influenced Armorican margin (Bay of Biscay, western European margin)

Sequence stratigraphic models predict increased sediment delivery to deep-water areas during sea-level lowstand. The Armorican margin (Bay of Biscay, western European margin) is an interesting area to test this hypothesis because the margin has a wide continental shelf, still partly flooded during sea-level lowstand, and the Armorican turbidite system has experienced fluctuating sediment fluxes since the last glacial period. The stratigraphic response of the Armorican turbidite system to sea-level oscillations and climate changes was assessed for the last 35,000years through the study of the Guilcher, Crozon and Audierne levees. Millennial-timescale resolution chronostratigraphy allowed us to reconstruct the sediment accumulation and turbidite frequency, thickness and grain-size over this period of time.We found that the Armorican turbidite system was sediment-starved during highstand conditions (ca. 8–0ka) and that glacial conditions favoured sediment delivery to the deep Bay of Biscay. However, contrary to what would be expected from sequence stratigraphic models, the turbidite flux did not reach a maximum during the LGM lowstand (ca. 26–20ka) but at the onset of Termination I (between ca. 20 and 17ka). This makes the Armorican turbidite system a transgression-dominated one. This sediment pulse can be interpreted as a huge increase in the meltwater discharge of the Fleuve Manche palaeoriver in response to the decay of the British and Fennoscandian ice-sheets. At that time, despite the rising sea-level, a large deltaic system had to have developed on the outer shelf, leading to the delivery of the Fleuve Manche sediment load into the canyon heads. On the other hand, our dataset suggest that the delivery of sediment into canyons was mainly forced by the winnowing and reworking of the sediment stored on the wide, drowned shelf during the last glacial period (between ca. 35 and 20ka, and between 17 and 8ka). These findings illustrate the competing influences of accommodation and sediment supply on the Armorican margin over the last 35,000years, with a shelf acting as a buffer for the sediment supply signal for most of the period, except during the last deglaciation. At that time, the western European sediment-routing system was reactive, the climatic signal rapidly propagated from the southern limb of the European ice-sheet to the Armorican turbidite system. Finally, our study demonstrates that precise reconstruction of turbidite flux in deep-water areas, added to knowledge about the morphology of the margin and the palaeoenvironmental changes (fluvial system, shoreline position, etc.), are crucial for determining the response of turbidite systems to external forcing.

Reconstructions of late Holocene paleofloods and glacier length changes in the Upper Engadine, Switzerland (ca. 1450 BC–AD 420)

The relationship between summer-autumn floods in Central Europe and climate warming is poorly constrained by available instrumental, historical, proxy and model data. To investigate this relationship, a complete record of paleofloods, regional glacier length changes (and associated climate phases) and regional glacier advances and retreats (and associated climate transitions) are derived from the varved sediments of Lake Silvaplana (ca. 1450 BC–AD 420; Upper Engadine, Switzerland). In combination, these records provide insight into the behavior of floods (i.e. frequency) under a wide range of climate conditions. Eighty-five paleofloods are identified from turbidites in the sediments of Lake Silvaplana. Regional glacier length changes (and associated cool and/or wet and warm and/or dry climate phases) are inferred from centennial anomalies in the square root of low-pass (LP) filtered Mass Accumulation Rates (MAR LP 1/2). Regional glacier advances and retreats (and associated cooling and/or wetting and warming and/or drying climate transitions) are inferred from centennial trends in MAR LP. This is the first continuous record of glacier length changes in the Lake Silvaplana catchment for this time period. These data agree with regional records of land-use, glacier activity and lake levels. More frequent turbidites are found during cool and/or wet phases of ca. 1450 BC to AD 420. However, no relationship to climate transitions is discerned. Consistently, June–July–August (JJA) temperatures dating ca. 570 BC–AD 120 are inversely correlated to the frequency of turbidites. The rate that turbidite frequency increases with cooler JJA temperatures is not linear. Finally, 130 analogues for a 21st century climate in the Alps between ca. 570 BC–AD 120 (i.e. 50 year windows with a warming trend and average JJA temperature exceeding AD 1950–AD 2000 values from nearby meteo station Sils Maria) are considered. These reveal that turbidites are less frequent than between ca. 1450 BC–AD 420.