Episodes Of Erosion Research Articles

Middle–Late Pleistocene landscape evolution of the Dover Strait inferred from buried and submerged erosional landforms

Prominent landforms, either buried or preserved at the seafloor, provide important constraints on the processes that led to the opening and present-day configuration of the Dover Strait. Here, we extend previous investigations on two distinct landform features, the Fosse Dangeard and Lobourg Channel, to better understand the poly-phase history of their formation and inferences for the opening and Pleistocene evolution of the Dover Strait.The Fosse Dangeard consist of several interconnected palaeo-depressions. Their morphology and spatial distribution are interpreted to be the result of plunge-pool erosion generated at the base of north-eastward retreating waterfalls. Their infills comprise internal erosional surfaces that provide evidence for the occurrence of several erosional episodes following their initial incision.The Lobourg Channel comprises various sets of erosional features, attesting to the occurrence of several phases of intense fluvial and/or flood erosion. The last one of these carved a prominent inner channel, which truncates the uppermost infill of the Fosse Dangeard. The morphology of the Lobourg inner channel and the erosional features associated with its incision strongly resemble landforms found in megaflood-eroded terrains, indicating that this valley was likely eroded by one or several megafloods.Our study therefore corroborates the existence of waterfalls in the Dover Strait at least once during the Pleistocene Epoch. It also provides evidence of the occurrence of multiple episodes of fluvial and flood erosion, including megafloods. Finally, this study allows us to establish a relative chronology of the erosional/depositional episodes that resulted in the present-day morphology of this region.

Episodic erosion in West Antarctica inferred from cosmogenic 3He and 10Be in olivine from Mount Hampton

The polar climate of Antarctica results in the lowest erosion rates on Earth. The low long-term erosion history of high elevation mountain tops that are exposed above the ice preserve a record of climate change that can be accessed using cosmogenic nuclides. However, unravelling the complexity of the long-term denudation histories of Antarctic summits is frequently hampered by intermittent ice cover. The aim of this work is to identify denudation rate changes in a surface that has been continuously exposed since the middle Miocene. We have measured stable (3He) and radioactive (10Be) cosmogenic nuclides in olivine from lherzolite xenoliths from the summit of the Mount Hampton shield volcano within the West Antarctic Ice Sheet. The peak (3200 m) has never been covered by the current ice sheet and local ice caps, consequently the data record the subaerial erosion history of a mountain top within the Antarctic interior. The 10Be concentrations in the olivines yield minimum exposure ages (33 to 501 ka) that are significantly younger than those derived from the cosmogenic 3He (90 to 1101 ka). The data reveal a complex exposure history that provide an integrated long-term erosion rate of between 0.2 and 0.7 m/My that is most likely caused by mechanical weathering. Inverse modelling shows that the data are readily explained by episodic erosion, consisting of one to five erosion pulses that may record major regional climatic changes.

A honeycomb seafloor morphology in carbonate sediment of the Carnegie Ridge (offshore Ecuador): Formation and potential geodynamic significance

Based on swath bathymetry, two-dimensional, high-resolution seismic reflection profiles, and Ocean Drilling Program/Deep Sea Drilling Project (ODP/DSDP) data, we describe a seafloor honeycomb pattern and propose a model for its formation in Pliocene–Miocene carbonate deposited on the uneven oceanic basement of the Carnegie Ridge (offshore Ecuador). Hydrothermal fluids derived from the basement aquifer fractured and dissolved carbonate sediment, creating seafloor pits above basements highs. Fluids expelled along polygonal faults may have assisted the nucleation of seafloor depressions. At the Pliocene-Pleistocene boundary, strong bottom currents scoured previously damaged sediments, enlarging the initial depressions and producing the seafloor honeycomb pattern. This regional erosive episode was contemporaneous with the final closing of the Isthmus of Panama and the clogging of the Ecuador Trench by the subduction of the Carnegie Ridge, so that the honeycomb pattern may be viewed as a regional marker of these two geodynamic events.

Late quaternary fluvial incision and aggradation in the Lesser Himalaya, India

Reconstructing how rivers respond to changes in runoff or sediment supply by incising or aggrading has been pivotal in gauging the role of the Indian Summer Monsoon (ISM) as a geomorphic driver in the Himalayas. Here we present new chronological data for fluvial aggradation and incision from the Donga alluvial fan and the upper Alaknanda River, as well as a compilation of previous work. In addition to conventional OSL-SAR (Single-Aliquot Regenerative-Dose) dating method, we have tested and applied pulsed OSL (POSL) dating for quartz samples that include K-rich feldspar inclusions, which is expected to improve the applicability and validity of OSL ages in the Lesser Himalaya. For previously dated deposits, our OSL ages are shown to be systematically older than previously reported ages. These results suggest periods of aggradation in the Alaknanda and Dehradun Valleys mainly between ∼25 and 35 ka. This most likely reflects decreased stream power during periods of weakened monsoon. In addition, in-situ cosmogenic beryllium-10 was used to infer bedrock surface exposure ages, which are interpreted as episodes of active fluvial erosion. Resulting exposure ages span from 3 to 6 ka, suggesting that strath terraces were exposed relatively recently, and incision was dominant through most of the Holocene. In combination, our results support precipitation-driven fluvial dynamics, which regulates the balance between stream power and sediment supply. On a larger spatial scale, however, fluvial dynamics are probably not spatially homogeneous as aggradation could have been taking place in adjacent catchments while incision dominated in the study area.

Xrf-based Chemostratigraphy Between and Across Two Disconformities in the Ordovician Trenton Group and Utica Shale of Central New York, U.s.a.

Abstract Thick mudstone successions contain a wealth of elemental proxy information than can be readily accessed by hand-held X-ray fluorescence (XRF) spectroscopy. Mudstones of the Upper Ordovician Trenton Group and Utica Shale record drowning of a carbonate-shelf in a peripheral foreland basin during the Taconic Orogeny. Two discrete episodes of shelf erosion, the Thruway and Honey Hill disconformities, followed by deepening occurred during this period. XRF-based bulk-rock elemental data, coupled with detailed lithologic descriptions and XRD analysis, highlight previously unidentified changes in the composition of siliciclastic detritus across these disconformities. This likely relates to variation in the relative contribution of sediment derived from the cratonic interior, eroded carbonate-shelf, and rising orogenic wedge, as well as possible exhumation of the Adirondack Massif. The data also indicate that mudstones of the lower Indian Castle Shale, the interval of the Utica Shale deposited between the Thruway and Honey Hill disconformities, are more carbonate-rich than the underlying mudstones of the Trenton Group. This is surprising given the large volume of limestone in the Trenton Group, and attests to the considerable amount of intrabasinal carbonate winnowed from the shelf during formation of the Thruway Disconformity. Drowning, following erosion along the Honey Hill Disconformity, diminished this sediment source and led to a significant reduction in the amount of carbonate mudstone present in the upper Indian Castle Shale. The concentrations of redox-sensitive trace elements normalized to Al increase sharply across the Thruway Disconformity, suggesting increased bottom-water anoxia and organic-matter preservation in a deepening basin. Increases in the S and Fe concentrations above the Honey Hill Disconformity suggest a shift to even more strongly reducing conditions. Results of this study are consistent with previous interpretations, indicating that higher-resolution XRF-based elemental concentration data may reveal more cryptic changes in sediment provenance and bottom-water redox conditions in the Taconic foreland basin, and in other thick mudstone successions.

OSL dating of a carbonate island in the Chobe Enclave, NW Botswana

Abstract Carbonate platform islands are important landscape features in the northeastern part of the Okavango Delta region of Botswana, the Chobe Enclave. However, the formation processes and the timing of these “Chobe Islands” remain unclear. They are assumed to be the result of late Quaternary hydrological changes. Records of such changes are poorly preserved, though the occurrence of beach ridges in the Middle Kalahari Basin attests to the existence of large paleo-lakes in the past. Carbonate rocks from the Chobe Island appear to be relics of palustrine environments, but their relationship with the other hydrological archives is still unclear. Here, we report optically stimulated luminescence dating of key sedimentary beds in and around a single Chobe Island. It was necessary to model dose rate evolution for each sample individually, taking into account post-depositional changes in the sediment chemistry and its burial depth. The resulting ages suggest that the dated units were deposited between MIS6 to MIS1. The carbonate platform itself appears to have been deposited in two phases, separated by either an unexpectedly long (∼40 ka) depositional hiatus or an episode of erosion. This study demonstrates the potential of using luminescence dating in such settings, and offers the possibility of linking sedimentary processes within the Chobe Enclave to regional paleo-hydrological records.

Upper Pleistocene trace fossils from Ponta das Bicudas, Santiago, Cape Verde Islands: Systematics, taphonomy and palaeoenvironmetal evolution

Bioturbation structures from Ponta das Bicudas in Santiago Island, Cape Verde Archipelagoare located in coarse-grained biocalcirudite sandwiched between basaltic lavas of the Upper Pleistocene Assomada Formation. Four ichnospecies have been identified: Rhizocorallium commune irregulare, Rhizocorallium jenense versum, nov ichnosubsp., Alaichnus kabuverdiensis nov. ichnogen. nov. ichnosp. and cf. Dactyloidites. From a taphonomic point of view, most trace fossil specimens are undergoing throughout intense erosion. Nevertheless, the observations made on their taphonomy and state of preservation allowed i) to reconstruct the original, complete structures of the trace fossils, ii) to evaluate the existence and magnitude of erosional and depositional episodes, and iii) to quantify the reach of erosion on the seafloor sediment. In this context, it was possible to quantify the thickness of sediment being eroded at the final erosional episode, which happened right before the second extrusion episode of volcanic pillow lava, which buried all the traces previously formed: the total interval of sediment eroded was estimated in ca. 1 m. From a palaeoenvironmental point of view, in a synthesis, the traces were produced in a soft to stiffground substrate composed of very coarse-grained biocalcirudite with common rhodolith debris, in a shallow setting (4 to10 m of depth) with moderate wave energy, nearby rocky basalt knobs where corals and red coralline algae crusts were growing. Organisms producing Rhizocorallium commune irregular settled in the seafloor during a first phase of sedimentation, which was followed by an erosional episode. Once sedimentation resumed, specimens of R. jenense versum nov. ichnosubsp. were produced and overlapped with the former ones. Later, a new erosional event erased most of the previous traces, being preserved only the basal imprint of the specimens. This was followed by a rapid sedimentation episode where Alaichnus kabuverdiensis nov. ichnogen. nov. ichnosp. recorded intense bioturbation activity of the bivalve Panopea. Subsequently, under low-energy sedimentation, a new episode of intense bioturbation of the seafloor occurred, resulting in the development of abundant cf. Dactyloidites. Finally, a strong erosional event – probably related to the onset of the second volcanic episode – cut all previous structures before being buried by the overlying, well preserved pillow lava.

A 2500-year climate and environmental record inferred from subfossil chironomids from Lugu Lake, southwestern China

We present a sediment record from Lugu Lake, a large and deep alpine lake in southwestern China, spanning the last c.2500 cal yr BP. This multi-proxy study focussed on subfossil chironomids but also included analyses of chemical elements using inductively coupled plasma-atomic emission spectrometry (ICP-AES), magnetic susceptibility, total organic carbon (TOC), total nitrogen (TN) and grain size. The chironomid assemblage is dominated by Procladius, Tanytarsus gracilentus-type and Polypedilum nubeculosum-type throughout the core. The record reveals that these chironomid taxa responded to two periods of catchment erosion between c.1700-1100 cal yr BP and c.1920 A.D. to the present. We provide evidence which suggests that the recent erosion episode is caused by human activities; however, the earlier event (c.1700-1100 cal yr BP) is likely related to increased regional precipitation, possibly linked to the strengthening of the Indian summer monsoon in the late Holocene. It is notable that the chironomids, through their varied ecologies, are able to detect the human-induced changes as well as natural climate changes, for instance, enhanced precipitation. Coupling palaeoecological studies using chironomids with more traditional catchment erosion indicators is thus a powerful tool for reconstructing past environmental and climate change.

Extreme geomorphic events in the contemporary evolution of forested slopes in a Central European mountain range, the Sudetes

Forested slopes of the Sudetes have long been implicitly considered as stable under contemporary environmental conditions, with little geomorphic change throughout the Holocene. This view is difficult to sustain and this review-type paper brings together evidence that infrequent but potent surface processes locally cause significant remodelling of slopes and regolith removal or redistribution. These processes are debris flows, shallow and deepseated landslides, episodes of efficient linear water erosion and windthrows. All are triggered by exceptional meteorological events such as heavy rain or strong wind, but additional factors of slope steepness and suitable lithology play a role. Scarce database does not permit to firmly establish recurrence intervals of such events but they seem to occur at least once per decade (within the entire mountain range) for rain-induced phenomena and two-three times per century for wind-induced phenomena.

Meso-Cenozoic exhumation and relevant isostatic process: The Barents and Kara shelves

Fennoscandia and Arctic shelves are known to have experienced several periods of Meso-Cenozoic uplift and erosion, but their magnitude and timing are still debated. The aim of our study is to estimate erosion volumes and distinguish between different erosional episodes, including glacial and pre-glacial ones. Our estimation approach is based on a combination of geological-geomorphological analysis, computer simulations of different erosion processes in time and space, mass-balance, burial control, and other methods Isostatic modelling is an obligatory component in the geomorphological reconstructions, since the erosion and subsequent isostatic uplift change the inclinations of various surfaces. Isostatic modelling is especially of significant importance in cases of poor or lacking age control.We find that the role of Neogene erosion might previously have been strongly exaggerated in some areas. Intensive Triassic-Jurassic deformations mark exhumation of positive structures (i.e. Novaya Zemlya, Pay-Khoy, Fedynsky). Novaya Zemlya − Pay-Khoy − Polar Ural belt was likely strongly exhumed during Neocomian (Berriasian). Upper Cretaceous erosion was noticeable in some structural zones, and strong Rupelian erosion is evident in Cenozoic. Changes of depositional provinces complicated the pattern, especially in Eocene.The role of climatic changes with global cooling could be discussed for Oligocene. Neogene erosion, with final glacial substages completed the erosional history, creating several prominent troughs and basins.We present a high-resolution model of Quaternary glacial and fluvioglacal erosion. Many landforms in the area were modified by glacial and periglacial processes, in particular by variable glacial erosion due to lithological, structural and topographic diversity. Glacial erosion of relative highs was often of minor importance, but other erosional agents could be active locally. We suggest a scenario of typical ice sheet development in the Barents Region, for the use in erosion computations. Radial glacial erosion pattern and significant basal ice velocities seem to be expected at the beginning of the early ice-age stage, resulting in widening of pre-glacial drainage elements. Pre-glacial lowlands and river pattern could provoke early stage onset of topographic ice-streams, especially in permafrost condition.Major ice streams were predefined by geological structures and tectonics.Different values of the elastic properties of the lithosphere dramatically impact isostatic movements and influence mass-balance computations, landscape features and delineation of watersheds.

Quantifying episodic erosion and transient storage on the western margin of the Tibetan Plateau, upper Indus River

AbstractTransient storage and erosion of valley fills, or sediment buffering, is a fundamental but poorly quantified process that may significantly bias fluvial sediment budgets and marine archives used for paleoclimatic and tectonic reconstructions. Prolific sediment buffering is now recognized to occur within the mountainous upper Indus River headwaters and is quantified here for the first time using optically stimulated luminescence dating, petrography, detrital zircon U-Pb geochronology, and morphometric analysis to define the timing, provenance, and volumes of prominent valley fills. This study finds that climatically modulated sediment buffering occurs over 103–104yr time scales and results in biases in sediment compositions and volumes. Increased sediment storage coincides with strong phases of summer monsoon and winter westerlies precipitation over the late Pleistocene (32–25 ka) and mid-Holocene (~8–6 ka), followed by incision and erosion with monsoon weakening. Glacial erosion and periglacial frost-cracking drive sediment production, and monsoonal precipitation mediates sediment evacuation, in contrast to the arid Transhimalaya and monsoonal frontal Himalaya. Plateau interior basins, although volumetrically large, lack transport capacity and are consequently isolated from the modern Indus River drainage. Marginal plateau catchments that both efficiently produce and evacuate sediment may regulate the overall compositions and volumes of exported sediment from the Himalayan rain shadow.

Editorial Comment

This is a well presented and researched manuscript describing a multi-institutional retrospective review of 31 men with bulbar urethral strictures that occurred after artificial urinary sphincter (AUS) erosion who underwent delayed urethroplasty (after the erosion episode) with or without subsequent AUS replacement during 2009-2016. These patients represent a rather uncommon but extremely clinically challenging group even for experienced reconstructive urologists and AUS implant surgeons. Theirs is a very complex situation involving prior prostate surgery and/or radiation therapy, potential hormonal therapy, prior bladder neck and/or penile surgeries, multiple urethral surgeries, as well as other medical comorbidities such as metabolic syndrome, etc. The findings reported by the authors are certainly limited by the short clinical follow-up of the small number of patients who underwent a variety of pre- and post-AUS surgical procedures and/or other treatments. Taking these limitations into consideration, this collaborative group's report advances our understanding of this complex heterogeneous population of men and adds value to the existing reconstructive urology/implant literature and cancer survivorship literature. The authors' conclusion that urethroplasty appears to be very effective for these typically short densely spongiofibrotic urethral strictures is a welcome addition to the literature supporting this concept. Urologists familiar with effectively treating physiologically similar urethral strictures (such as perineal straddle injuries) with urethroplasty may find this conclusion fairly evident. In this small series, despite urethroplasty for AUS erosion related stricture being successful, the subsequent AUS replacement was found to have a high erosion rate in the short-term, perhaps greater than other series involving high risk patients. It is not possible in this small series to determine to what degree any prior prostate surgery and/or radiation play a main role; to what degree the urethroplasty and/or prior bladder neck/urethral/penile surgery play a part; to what degree their urethral vascularity is compromised or not; to what degree metabolic and hormonal factors are involved; etc. We recognize that all of this is multifactorial. Longer term follow-up with larger patient numbers will be helpful in terms of determining which patient characteristics and populations are at even higher risk for erosion than those men we already consider “high risk” going forward. This manuscript provides tangible value for surgeons when we are counseling our patients in the midst of them dealing withwhat is typically a devastating AUS erosion that may be followed by a consequent urethral stricture requiring additional surgical management in the hope of being able to potentially replace the AUS at a later date. This informed and shared decision making process requires a significant collaborative effort between us as urologic surgeons and our patients in an effort to provide them with best possible durable outcomes even in these most challenging clinical situations.

The “Lagoa do Camargo 1” Paleoindian site: some implications for tropical geomorphology, pedology, and paleoenvironments in southeastern Brazil

AbstractLagoa do Camargo 1, a Paleoindian archaeological site located in southeast Brazil dated between 10.5 and 8.3 ka, was studied using a variety of analyses: artifact and charcoal frequencies, soil morphology and micromorphology, magnetic analysis, optically stimulated luminescence, and radiocarbon dating. Our results show that the sandy tropical Oxisol present at the site was accumulated by episodic erosion and accumulation of materials along the slope, by means of diffuse runoff (sheet wash), and that bioturbation, albeit present, played a minor role in the soil formation. From a geoarchaeological point of view, the site poses an important problem, which is the presence of a deeply buried archaeological site almost at the top of a plateau. Our conclusions are relevant for understanding the genesis of sandy Oxisols that cover a large portion of southeast Brazil, where charcoal fragments are abundant, and whose genesis is considered by some authors to be strongly related to pedoturbation.

Integrative 1D-2D Basin Modeling of the Cretaceous Beni Suef basin, Western Desert, Egypt

An integrated seismic, borehole petrophysical and geochemical data constrained 1D and 2D basin model study is conducted for the Beni Suef basin (Western Desert, Egypt) to enhance the understanding of the Cretaceous plays while demonstrating how basin modeling can be used to reduce exploration risk and improve exploration success. The Cretaceous petroleum systems consist of two major source rocks (the Upper Cretaceous limestone of Abu Roash “F’’ member, and Lower Cretaceous shales of Kharita and Alam El-Bueib formations), five reservoirs (Cretaceous sandstones of Abu Roash “A, E & G’’, Baharyia and Kharita formations), three seals (Lower Cretaceous and Upper Cretaceous shales of Kharita and Bahariya formations and fine grained limestones of the Abu Roash Formation), and vertical source mixing migration pathways owing to vertical faults resulting from four episodes of elastic crustal extension and above trap lateral migration. The entrapment style of the generated hydrocarbons and its roles on the charge and migration process in the Cretaceous reservoir rocks were determined from the seismic interpretation. Three episodes of uplift erosion (Early Cretaceous, Late Cretaceous-Paleocene, and Paleogene-Neogene) have occurred in the basin and have increased exploration risk in some areas by breaching the integrity of the seal. Migration from the source rock into the trap is mainly vertical migration. However, in areas where the hydrocarbons have escaped through the faults at the boundaries of the basin, migration in the layers above the trap is mainly lateral. The hydrocarbons generated are predominantly oil. The Lower Cretaceous shales of the Kharita and Alam El-Bueib formations exhibit good thickness, total organic content (TOC), kerogen type (mixed type II and III), and maturation.

Impacts of Past Climate and Environmental Change: The Effects on Prehistoric and Roman Coastal Saltmaking in Lincolnshire, UK

As an example of the effect of climate and environmental and, therefore, landscape change on archaeological sites this paper sets out the positions of known prehistoric and Roman salterns in Lincolnshire (Eastern England) and the present condition and archaeological potential of such sites. Their current position – some recorded as surface finds and some sealed by later sedimentation – demonstrates the dynamic nature of wetland landscapes in various parts of Lincolnshire. Both freshwater and marine deposits have at times affected the salterns, sometimes in restricted zones and sometimes over extensive areas. Also considered are methodologies for discovering such sites. The paper is a case study of one type of coastal site in Lincolnshire and serves as an example of the localised and complex chronology of sedimentation and/or erosion episodes in just one area. It may reflect wider changes around the North Sea coast and raises issues of generic importance.

Two-stage opening of the Dover Strait and the origin of island Britain

Late Quaternary separation of Britain from mainland Europe is considered to be a consequence of spillover of a large proglacial lake in the Southern North Sea basin. Lake spillover is inferred to have caused breaching of a rock ridge at the Dover Strait, although this hypothesis remains untested. Here we show that opening of the Strait involved at least two major episodes of erosion. Sub-bottom records reveal a remarkable set of sediment-infilled depressions that are deeply incised into bedrock that we interpret as giant plunge pools. These support a model of initial erosion of the Dover Strait by lake overspill, plunge pool erosion by waterfalls and subsequent dam breaching. Cross-cutting of these landforms by a prominent bedrock-eroded valley that is characterized by features associated with catastrophic flooding indicates final breaching of the Strait by high-magnitude flows. These events set-up conditions for island Britain during sea-level highstands and caused large-scale re-routing of NW European drainage.

Lacustrine clay mineral assemblages as a proxy for land-use and climate changes over the last 4 kyr: The Amik Lake case study, Southern Turkey

Abstract Lake sediments are sensitive to landscape changes and most of these changes seem to be modulated by land-use (anthropogenic factors) coupled to palaeoenvironmental/palaeoclimatic changes. In its detrital fraction, the lacustrine sediments record the history of soil erosion within its catchment via the inputs of clays and others detrital products. Within a Mediterranean context, the study investigates the upper sediments infilling the central part of the Amik basin in southern Turkey. This tectonic basin was occupied and exploited by modern human at least since 6000–7000 BCE. We focus on the clay mineralogy (x-ray diffraction on oriented aggregates) and magnetic susceptibility measurements (Bartington) of the sedimentary record in the area over the last 4000 years, to assess environmental changes in relation with the different land uses and/or weathering during the successive Bronze, Iron, Roman, Islamic/Ottoman and Modern civilizations. The clay fraction of Amik Lake sediments comprises smectite, kaolinite, illite, chlorite and chlorite/smectite mixed layers that are the inherited clay phases. A relative change in abundance and crystallinity and chemistry of illite attests that environmental conditions evolved in the Amik Plain from the Bronze to Modern Age in relation with climates and/or land-use changes. The history of the Amik Lake reveals different soil erosion episode. The most intense erosion phase occurred during the Bronze/Iron Ages as indicated by the clay and magnetic susceptibility proxies. The Roman period was an exceptional period with soil erosion products arriving from the watershed, probably due the water channelization. A reduction of soil erosion occurred during the post Roman period until nowadays. Significant pedogenesis transformations are evidenced, especially during the Islamic/Ottoman periods suggesting intense chemical weathering conditions related to climate change.

Cenozoic North Atlantic deep circulation history recorded in contourite drifts, offshore Newfoundland, Canada

In the North Atlantic Ocean, contour-following deep currents have created regional erosional unconformities and deposited contourite drifts that exceed 2km in thickness and extend for 100s of km. The stratigraphic records in the drifts have been used to reconstruct variations in North Atlantic deep-water circulation throughout the Cenozoic; however, uncertainties remain about certain aspects of the timing, intensity, depth distribution, and regional impact of these currents. Here, we use an integrated dataset of seismic-reflection profiles and IODP core data (lithology, biostratigraphy, and magnetostratigraphy) to document sedimentation history and the development of current effects in the Cretaceous to present sedimentary record on the J-Anomaly Ridge and Southeast Newfoundland Ridge, offshore Newfoundland, Canada. The Newfoundland ridges are in a key location, lying between well-studied areas in the northern and western North Atlantic and under the path of both the modern Deep Western Boundary Current and the Gulf Stream. Late Cretaceous through Early Eocene sedimentation on the ridges was dominated by biogenic pelagic sedimentation, but at ~47Ma, near the Early-Middle Eocene boundary, well developed contourite drifts began to accrete in paleo-water depths of ~4000–4500m, accompanied by an order-of-magnitude increase in terrigenous sediment mass accumulation rates. From this time forward, drift deposition, interrupted by brief episodes of erosion, continued unabated. This timing for the onset of persistent deep currents is coincident with reorganization of Atlantic circulation inferred from a change from biosiliceous to non-biosiliceous sedimentation in the western North Atlantic (Horizon AC) and with the current-eroded Intra-Eocene Unconformity (IEU) in the northern North Atlantic. A change in sedimentation style occurred within the Middle Eocene to upper Oligocene drift sequence, and it likely was related to a shift to deeper, more intense currents that eroded the widespread Horizon AU along the margin of eastern North America about Early Oligocene time. Beginning in the Late Oligocene (~25Ma) a thick drift exhibiting seismically laminated mudwaves was deposited in a distinct belt at ~3500–4500m paleodepth on the Southeast Newfoundland Ridge. This development correlates with widespread Late Oligocene through Miocene-Pliocene drift accumulation throughout the North Atlantic. The most recent phase of drift deposition, since Late Pliocene time (~3Ma), occurred after a shift to the ‘modern’ circulation system of deeper, swifter currents, and it includes mixed pelagic-hemipelagic sediments and ice-rafted debris that reflect glacial-interglacial influences on sedimentation.

The Don formation, Toronto, Canada: a record of the sangamonian interglacial and early wisconsinan (warm part of MIS 5e to a MIS 5 cold substage)

Regarded as a reference unit of the Sangamonian Interglacial in North America, the Don Formation was deposited however during a longer interval of time, including the interglacial climatic optimum of MIS 5e followed by an erosive and cool phase (intra-MIS 5e or MIS 5d), a boreal phase with a fir forest (late MIS 5e transition or MIS 5c), erosive episodes and the early part of a cold stadial (MIS 5d or MIS 5b). From the planar geometry of the beds exposed at the former Don Valley Brickyard in Toronto, the formation comprises five continuous allozones with lateral variations of thickness. Each allozone corresponds to dominant conditions of sedimentation in a shoreface of Lake Coleman within a limited accommodation space. Allozone D2 is correlated to other interglacial units of Eastern Canada, with ages between 128 and 115 ka. The interglacial climatic optimum is interrupted in the area by two cool events, the first one similar to the Tunturi event of Finland, the second one probably related to a Greenland Stadial. The heterogeneous upper Allozone D5 was deposited in shallow waters when a Northern Boreal Forest prevailed in the watershed. This zone is correlated to the Deschaillons Varves of the central St. Lawrence River Valley, deposited prior to the first regional Wisconsinan glacial advance in the central St. Lawrence River Valley and the resulting glacial Lake Scarborough Formation. From the 80 ka minimalU/Th age of the Deschaillons Varves and the 98 ± 8 ka TL age of the later marine La Perade Clay, Allozone D5 was deposited during the beginning of one of the cold episodes of MIS 5.

Site Formation Processes and Depositional Environment of a Fine‐Grained Alluvial Floodplain at La Playa Archaeology Site, Sonora, Mexico

Located on a floodplain of the Rio Boquillas, the site of La Playa contains a principal occupation during the Early Agricultural period, 1200 B.C.–A.D. 50 (3150–1900 cal. yr B.P.). Local complexity in geomorphic features and extensive historic dissection of the floodplain have resulted in limited understanding of the site's alluvial history, which hampers understanding of archaeological features and an extensive canal system. This study reconstructs the site formation processes of the floodplain before, during, and after occupation by comparing soils and sediments from the Los Montículos area near the channel with those of more distant areas of the site. Sediments on the floodplain were deposited by low‐energy overbank deposition from ca. 4300 to 1600 cal. yr B.P., resulting in stratified deposits with weak soil development nearer to the channel, and more strongly developed soils farther from the channel. Low‐energy deposition of fine‐grained sediments was possible due to the stable location of the river channel during the Holocene, creating an attractive place for canal agriculture. Sometime after the Early Agricultural period, higher‐energy flows resulted in deposition of sandier sediments and multiple episodes of erosion and deposition in Los Montículos, where nearly all cultural material is now in a reworked context.