Episodes Of Deposition Research Articles

The role of climate, marine influence and sedimentation rates in late-Holocene estuarine evolution (SW Portugal)

Estuaries are sensitive to changes in global to regional sea level, to climate-driven variation in rainfall and to fluvial discharge. In this study, we use source and environmentally sensitive proxies together with radiocarbon dating to examine a 7-m-thick sedimentary record from the Sado estuary accumulated throughout the last 3.6 kyr. The lithofacies, geochemistry and diatom assemblages in the sediments accumulated between 3570 and 3240 cal. BP indicate a mixture between terrestrial and marine sources. The relative contribution of each source varied through time as sedimentation progressed in a low intertidal to high subtidal and low-energy accreting tidal flat. The sedimentation proceeded under a general pattern of drier and higher aridity conditions, punctuated by century-long changes of the rainfall regime that mirror an increase in storminess that affected SW Portugal and Europe. The sediment sequence contains evidence of two periods characterized by downstream displacement of the estuarine/freshwater transitional boundary, dated to 3570–3400 cal. BP and 3300–3240 cal. BP. These are intercalated by one episode where marine influence shifted upstream. All sedimentation episodes developed under high terrestrial sediment delivery to this transitional region, leading to exceptionally high sedimentation rates, independently of the relative expression of terrestrial/marine influences in sediment facies. Our data show that these disturbances are mainly climate-driven and related to variations in rainfall and only secondarily with regional sea-level oscillations. From 3240 cal. BP onwards, an abrupt change in sediment facies is noted, in which the silting estuarine bottom reaches mean sea level and continued accreting until present under prevailing freshwater conditions, the tidal flat changing to an alluvial plain. The environmental modification is accompanied by a pronounced change in sedimentation rate that decreased by two orders of magnitude, reflecting the loss of accommodation space rather than the influence of climate or regional sea-level drivers.

Diatom ecological response to deposition of the 833-850 CE White River Ash (east lobe) ashfall in a small subarctic Canadian lake.

A <5 mm thick volcanic ashfall layer associated with the White River Ash (east lobe [WRAe]) originating from the eruption of Mount Churchill, Alaska (833-850 CE; 1,117–1,100 cal BP) was observed in two freeze cores obtained from Pocket Lake (62.5090°N, −114.3719°W), a small subarctic lake located within the city limits of Yellowknife, Northwest Territories, Canada. Here we analyze changes in diatom assemblages to assess impact of tephra deposition on the aquatic biota of a subarctic lake. In a well-dated core constrained by 8 radiocarbon dates, diatom counts were carried out at 1-mm intervals through an interval spanning 1 cm above and below the tephra layer with each 1 mm sub-sample represented about 2 years of deposition. Non-metric Multidimensional Scaling (NMDS) and Stratigraphically Constrained Incremental Sum of Squares (CONISS) analyses were carried out and three distinct diatom assemblages were identified throughout the interval. The lowermost “Pre-WRAe Assemblage (Pre-WRAeA)” was indicative of slightly acidic and eutrophic lacustrine conditions. Winter deposition of the tephra layer drove a subsequent diatom flora shift to the “WRAe Assemblage (WRAeA)” the following spring. The WRAeA contained elevated abundances of taxa associated with oligotrophic, nutrient depleted and slightly more alkaline lake waters. These changes were only apparent in samples within the WRAe containing interval indicating that they were short lived and only sustained for a single year of deposition. Immediately above the WRAe horizon, a third, “Post-WRAe Assemblage (Post-WRAeA)” was observed. This assemblage was initially similar to that of the Pre-WRAeA but gradually became more distinct upwards, likely due to climatic patterns independent of the WRAe event. These results suggest that lacustrine environments are sensitive to perturbations such as deposition of ash fall, but that ecological communities in subarctic systems can also have high resilience and can recover rapidly. If subsampling of the freeze cores was carried out at a more standard resolution (0.5–1 cm) these subtle diatom ecological responses to perturbation associated with the WRAe depositional event would not have been observed. This research illustrates the importance of high-resolution subsampling when studying the environmental impact of geologically “near instantaneous” events such as episodic deposition of ashfalls.

Sediment storage, partial transport, and the evolution of an experimental gravel bed under changing sediment supply regimes

The adjustment of a gravel bed during cycles of sediment storage was analyzed for a flume experiment, designed to study the effects of episodic sediment supply regimes. As the bed aggraded and degraded due to changes in sediment feed, it developed a complex topography with bedforms at different scales (e.g., pebble-clusters, bars). Sediment inputs increased bedload rates and promoted large changes in bed morphology. Without an external supply, the bed armored, which increased bed stability, counteracted bed degradation, and decreased bedload rates. Fully-mobile gravel (2–8 mm) was the only fraction that exhibited a deficit over the experiment. Preferential storage of coarse grains caused the development of a sediment wedge upstream and downstream fining on the bed surface. The storage of sand was also significant (60% of feed) probably because of infiltration (e.g., in the upstream wedge) and hiding effects promoted by bed armor and structures. Size-selective transport caused a cumulative increase in bed slope, which was likely to affect sediment transport capacity. The large variability in bedload rates for same levels of bed storage indicated that sediment availability and bed state were the main controls on sediment transport regime. We provide empirical evidence that cycles of aggradation and degradation, caused by changes in sediment feed upstream, condition bed composition and topography, and the rate and texture of sediment output.

El registro paleoambiental del Pleistoceno tardío al presente en la Laguna de Lobos, Buenos Aires, Argentina

Se caracterizó el medio físico del ambiente, la fisiografía, régimen hidrológico y la historia geológica de la cubeta lagunar. La geología superficial está representada por seis unidades litoestratigráficas y dos pedoestratigráficas, que abarcan del Pleistoceno temprano al tardío y del Holoceno medio al presente. La más antigua, la Formación Ensenada, constituye el sustrato regional, sobre el cual se ubican cuatro unidades eólicas, las Formaciones Buenos Aires, La Postrera I, III y IV, los Geosuelos Sin Nombre y Puesto Berrondo y el Aluvio reciente. Los sedimentos colmatantes fueron obtenidos mediante perforaciones, reconociéndose siete unidades litoestratigráficas (cinco subácueas y dos subaéreas). Las subácueas corresponden a la Formación Luján, Miembros La Chumbiada, Lobos, Río Salado y Monte (Pleistoceno tardío al Holoceno tardío) y al Aluvio reciente. Los depósitos subaéreos a las Formaciones La Postrera I y IV del Pleistoceno tardío y Holoceno tardío-tardío, respectivamente. La excavación de la cubeta se inició en la etapa seca del ?Bonaerense?, la posterior alternancia climática: seco-húmedo facilitó la evolución. Las etapas secas se caracterizaron por deflación, generación de cubetas, sabkhas o lagos salados, precipitación de sales y acumulación de depósitos eólicos. En las húmedas predominó la sedimentación y la pedogénesis. De esta manera, el conjunto de depósitos posteriores a la Formación Ensenada está representado por cinco episodios de clima seco, que abarcan del Glacial Temprano a la Pequeña Edad de Hielo y cinco húmedos, cuyas acumulaciones límnicas conforman los cuatro miembros de la Formación Luján, el aluvio actual y los suelos sincrónicos S4, S1 y S0.

Automated detection of thermoerosion in permafrost ecosystems using temporally dense Landsat image stacks

Anthropogenic climate change has been linked to the degradation of permafrost across northern ecosystems, with notable implications for regional to global carbon dynamics. However, our understanding of the spatial distribution, temporal trends, and seasonal timing of episodic landscape deformation events triggered by permafrost degradation is hampered by the limited spatial and temporal coverage of high-resolution optical, RADAR, LIDAR, and hyperspectral remote sensing products. Here we present an automated approach for detecting permafrost degradation (thermoerosion), using meso-scale high-frequency remote sensing products (i.e., Landsat image archive). This approach was developed, tested, and applied in the ice-rich lowlands of the Noatak National Preserve (NOAT; 12,369 km2) in northwestern Alaska. We identified thermoerosion (TE) by capturing the spectral signal associated with episodic sediment plumes in adjacent water bodies following TE. We characterized and extracted this episodic turbidity signal within lakes during the snow-free period (June 15–October 1) for 1986–2016 (continuous data limited to 1999–2016), using the cloud-based geospatial parallel processing platform, Google Earth Engine™. Thermoerosional detection accuracy was calculated using seven consecutive years of sub-meter high-resolution imagery (2009–2015) covering 798 (~33%) of the 2456 lakes in the NOAT lowlands. Our automated TE detection algorithm had an overall accuracy and kappa coefficient of 86% and 0.47 ± 0.043, indicating that episodic sediment pulses had a “moderate agreement” with landscape deformation associated with permafrost degradation. We estimate that lake shoreline erosion, thaw slumps, catastrophic lake drainage, and gully formation accounted for 62, 23, 13, and 2%, respectively, of active TE across the NOAT lowlands. TE was identified in ~5% of all lakes annually in the lowlands between 1999 and 2016, with a wide range of inter-annual variation (ranging from 0.2% in 2001 to 22% in 2004). Inter-annual variability in TE occurrence and spatial patterns of TE probability were correlated with annual snow cover duration and snow persistence, respectively, suggesting that earlier snowmelt accelerates permafrost degradation (e.g. TE) in this region. This work improves our ability to detect and attribute change in permafrost degradation across space and time.

Sedimentological and geochemical evidence for seismoturbidite generation in the Kumburgaz Basin, Sea of Marmara: Implications for earthquake recurrence along the Central High Segment of the North Anatolian Fault

Holocene earthquake history of the Central High Segment of the North Anatolian Fault is examined here for the first time based on analysis of seismoturbidites within a 21-m-long piston core recovered from the Kumburgaz Basin in the Sea of Marmara. The visual lithological description combined with detailed grainsize analyses indicate that the deep basin hemipelagic sediments are interrupted by 28 turbidite units during the last 6.1 cal kyrs BP. The turbidites show strong segregation and a sharp boundary between a coarse basal part and overlying homogenite as inferred from detailed sedimentological and geochemical data. Several amalgamated turbidites are recognized by repeated fining upward sequences with no intervening homogenite indicating multiple episodes of traction and deposition as a result of various slope failures and turbidity currents. Each amalgamated unit was possibly triggered by the same earthquake event rupturing in the Sea of Marmara. The most common sedimentary feature is the continuous parallel lamination that was presumably introduced by long lasting water oscillations on suspended sediments due to the seiche effect. The establishment of geochemical criteria and exclusive sedimentary processes distinguish earthquake triggered turbidites (seismoturbidites) from other trigger factors. Moreover, such distinction allows us to evaluate hydrodynamic sedimentary conditions and processes in the Kumburgaz Basin. The base of most seismoturbidites are associated with a sharp increase in Mn concentration that can be explained by a diagenetic enrichment of Mn at the oxic/anoxic interface of the sediments near the seafloor prior to the deposition of the turbidite. An age-depth model of the studied core based on seven AMS 14C ages allows correlation between historical earthquakes and seismoturbidites in the Kumburgaz Basin. At least the latest nine of them fit well with the previously recorded major earthquake events between ca. ~500 cal yrs BP and 2.5 cal kyrs BP.

Stable deep-sea macrobenthic trace maker associations in disturbed environments from the Eocene Lefkara Formation, Cyprus

Ichnological analysis of Eocene deep-pelagic whitish chalky calcilutites interstratified with high-energy calcarenite beds in the lower part of the Petra Tou Romiou section (southern Cyprus) was conducted to interpret the development and evolution of the trace maker associations during calcilutite deposition after high-energy episodes. The trace fossil assemblage from the chalky calcilutites consists of Chondrites isp. (Chondrites intricatus and Chondrites targionii), Planolites isp., Taenidium isp., Thalassinoides isp., and Zoophycos isp., typical of the Zoophycos ichnofacies. A composite chalky ichnofabric reveals a multi-tiered association of burrowing animals: the uppermost tier determines a mottled background, the upper tier shows the highest trace fossil abundance and diversity (Planolites, Taenidium and Thalassinoides), the middle tier features Zoophycos and large Chondrites, and the deepest tier consists mainly/exclusively of small Chondrites. This ichnofabric has an autocomposite character, associated with bioturbation by a single ichnocoenosis and gradual upward migration of the tiered macrobenthic community as the pelagic calcilutite sedimentation slowly progresses. There are no changes in the trace fossil assemblage between or within calcilutite intervals, regardless of the associated calcarenite beds. This supports a stable, mature, background calcilutite trace maker association, which recovers shortly after the deposition of high-energy calcarenites. In turn, there would have been a rapid re-establishment of paleoenvironmental conditions during pelagic calcilutite accumulation after episodic deposition of any calcarenitic material.

Controls on Sediment Suspension, Flux, and Marsh Deposition near a Bay-Marsh Boundary

The sustainability of marshes adjacent to coastal bays is driven by the exchange of sediment across the marsh-bay boundary, where edge erosion commonly leads to lateral marsh loss and enhanced vertical accretion. The timing and patterns of sediment deposition on salt marshes adjacent to larger bodies of water such as coastal bays, however, differ from those on better-studied tidal creek marshes primarily owing to the importance of wind-waves. We combined field measurements and modeling to examine controls on suspended sediment concentrations and fluxes on a tidal flat (tidal range of 1.2 m) and rates of sediment deposition on the adjacent marsh at a site on the Eastern Shore of Virginia. Suspended sediment concentrations over tidal flats were strongly controlled by waves. Yet, storm winds sufficient to drive large resuspension events often coincided with peak tidal elevations that were too low to flood the marsh, which was oriented away from the wind directions most favorable for storm surge, thereby restricting storm-driven, episodic sediment delivery to the marsh. Winds also drove wide variability in the direction of surface currents near the marsh edge when water depths were high enough to flood the marsh. Nevertheless, our results show that sediment in the upper water column over the tidal flat was effectively transported across the marsh edge during flooding tides. A sediment deposition model developed to investigate the combined effects of vegetation and wave action on depositional patterns predicted that waves displace maximum deposition inland from the marsh edge, consistent with measured deposition at the study site. Marsh deposition was sensitive to inundation frequency as well as the concentration of sediment in water flooding the marsh, underscoring the importance of nontidal controls on water surface elevation, such as meteorological effects (e.g., storm surge) and sea level rise. Whereas short-term increases in marsh inundation enhance deposition, sea level rise that results in deeper average water depths over the tidal flats decreases deposition if marsh elevation is rising in step with sea level.

Probing supraglacial debris on Mars 2: Crater morphology

Lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF) on Mars, interpreted to be debris-covered glaciers, possess craters with a suite of distinct interior landforms (called “ring-mold craters”) that have been attributed to the presence of glacial ice at depth or surface modification processes. We tested current hypotheses for the formation of ring-mold craters by conducting a comprehensive analysis of the size and morphology of 16,457 impact craters ≥125 m in diameter formed within glacial deposits in Deuteronilus Mensae. Two major groups, bowl-shaped craters and ring-mold craters, are found, with at least nine distinct crater types. While there is statistical difference in median diameters between these crater types, this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring-mold craters. Clear degradation sequences are observed, supporting a role for post-impact modification in producing at least some of the diversity in crater landforms. The spatial density of ring-mold craters is also directly correlated with the development of LDA, LVF, and CCF surface textures. Flow lineations cross-cut two ring-mold crater types but they maintain their circular planforms in some cases, suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased. We also find analogous craters in non-glacial units; glacial ice is therefore not required to form the observed morphologic diversity. Our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust (i.e., “mantle”). This mantle was initially tens of meters in thickness to support crater formation, and has experienced much downwasting and erosion since emplacement. Derived crater retention ages of 460 Ma for LDA, LVF, and CCF features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of LDA, LVF, and CCF.

The Effect of Episodic Sediment Supply on Bedload Variability and Sediment Mobility

AbstractThe effect of sediment supply on sediment mobility is analyzed for a poorly sorted (0.5–64 mm) experimental bed. Water discharge was held constant over a sequence of seven runs, and 300 kg of sediment was supplied during each run in different magnitudes and frequencies. In runs with constant feed bedload transport rate increased gradually. In contrast, runs that received large sediment pulses showed pronounced increases in bedload rate as the bed surface got finer, followed by monotonic declines as the bed surface coarsened. We studied the temporal scales of bedload fluctuations by means of sample autocorrelation coefficients and the rates of decrease in bedload fluctuation with sampling time scale. The significant trends caused in bedload rate by large occasional sediment pulses increased long‐term autocorrelation in bedload rate time series relative to runs with constant feed. Bed evolution and local changes in sediment storage caused multiple scales of variability in bedload rate, which increased autocorrelation and caused long‐term persistence in bedload series over periods with a nearly constant mean. The scaling statistics of bedload transport fluctuation depended on grain size, and those for total bedload were similar to those for fine gravel (2–8 mm), which was fully mobile and dominated bedload transport. Grain size dependence of bedload fluctuation was not affected by changes in sediment feed because water discharge and sediment texture were held constant.

Middle Triassic SW Gondwana paleogeography and sedimentary dispersal revealed by integration of stratigraphy and U-Pb zircon analysis: The Santa Cruz Sequence, Paraná Basin, Brazil

The Paraná Basin represents an important sedimentation episode that occurred in the south margin of Gondwana during the Paleozoic and Mesozoic. The evolution of the basin extended mainly from the Ordovician to the Lower Cretaceous, with the continental break-up and the formation of the rift that originated the South Atlantic Ocean. The stratigraphy and depositional architecture of the Triassic strata in the southern portion of the basin are characterized by significant variations in depositional styles, interpreted as a response to tectonic and climatic changes. The sedimentary succession was grouped into three major chronostratigraphic intervals based on vertebrate ages (Late Permian-Early Triassic, Middle-Late Triassic and Early Cretaceous). The integration between stratigraphic and structural data with U-Pb ages of detrital zircon was used to investigate the origin and maximum depositional age of the Santa Maria Formation (Santa Cruz Sequence) which marks the Middle Triassic in the basin. This sequence is composed by conglomeratic sandstones with sandstones at the base, covered by a thick package of reddish mudstones containing vertebrate fossils. Paleocurrent data show N-NE sense of sedimentary transport, indicating that the sedimentation sources came from southwest Gondwana. The transition of the alluvial braidplain deposits of Sanga do Cabral Formation (Early Triassic) to the fluvial braided, floodplains and sinuous/anastomosing deposits, rich vertebrate fauna of the Santa Maria Formation (Middle-Late Triassic) indicates a sedimentation controlled by tectonic processes that uplifted the basement. The elongated positive feature of NW-SE strike named the Rio Grande Arch represented a topographic high and limited the deposition of the Triassic strata to south of the basin. The analyzed detrital zircon grains record the basement units located in southern Brazil and Uruguay. The main sources are the Tonian to Ediacaran rocks of the Dom Feliciano Belt and the sedimentary layers of the Paraná Basin deposited between the Carboniferous and the Permian that occur in the Sul-rio-grandense Shield, the source of the Paleozoic and Mesozoic age zircons. Archean to Mesoproterozoic ages are subordinate, between 2.7 and 2.0 Ga and 1.5–1.0 Ga. The siltstones of the upper portion of the sequence show elongated prismatic and euhedral zircon, aged 237 ± 1.5 Ma, defining the maximum deposition to the top of the Santa Cruz Sequence. These crystals zircon were deposited from ashes, probably originated from volcanic rocks of central and southern Argentina and associated with an extensional environment after the magmatism of the Choiyoi Province.

Ice in Micro Cold Traps on Mercury: Implications for Age and Origin

AbstractEvidence in radar, reflectance, and visible imagery indicates that surface and subsurface water ice is present inside permanently shadowed regions in the north polar region of Mercury. The origin of this ice and the time at which it was delivered to the planet are both unknown. Finding the smallest, most easily eroded ice deposits on Mercury can help answer these questions. Here we present evidence for volatiles trapped in cold traps of scales ∼ 1–10 m. We consider two possible delivery methods for these deposits: a gradual, slow accumulation by micrometeorites or solar wind implantation and an episodic deposition, either primordial or by a recent comet impact. We conclude that the mechanism that best explains the presence of volatiles in these micro cold traps is a comet impact that most likely occurred in the last ∼ 100 Ma.

Assessment of Radiative Forcing by Light-Absorbing Particles in Snow from In Situ Observations with Radiative Transfer Modeling

Abstract It is well established that episodic deposition of dust on mountain snow reduces snow albedo and impacts snow hydrology in the western United States, particularly in the Colorado Rockies, which are headwaters for the Colorado River. Until recently the snow observations needed to physically quantify radiative forcing (RF) by dust on snow were lacking, and analysis of impacts used a semiempirical relationship between snow optical properties and observed surface reflectance. Here, we present a physically based daily time series of RF by dust and black carbon (BC) in snow at Senator Beck Basin Study Area, Colorado. Over the 2013 ablation season (March–May), a snow–aerosol radiative transfer model was forced with near daily measured snow property inputs (density, effective grain size, and dust/BC concentrations) and validated with coincidentally measured spectral albedo. Over the measurement period, instantaneous RF by dust and BC in snow ranged from 0.25 to 525 W m−2, with daily averages ranging from 0 to 347 W m−2. Dust dominated particulate mass, accounting for more than 90% of RF. The semiempirical RF values, which constitute the continuous long-term record, compared well to the physically based RF values; over the full time series, daily reported semiempirical RF values were 8 W m−2 higher on average, with a root-mean-square difference of 16 W m−2.

Combined tectonic and paleogeographic controls on the genesis of bauxite in the Early Carboniferous to Permian Central Yangtze Island

Two bauxite horizons, with a total reserve of 1.29 Bt, occur in the Lower Carboniferous and Lower Permian in the central South China Block. Different from other intracontinental bauxites in China, which are genetically linked to the orogenesis along the continental margin, these bauxites were generated in the absence of marginal orogenesis. The genesis of these bauxites were studied based on tectonic, paleogeographic, and paleoclimatic controls. The orebody in Huofengchong (HFC) region is hosted within the Lower Carboniferous, which overlies upon the Cambrian dolomite unconformably. The orebody in the regions of Yanfengba (YFB) and Baiyantou (BYT) occurs in Lower Permian, which overlies upon the Silurian shale or Devonian dolomite unconformably. The mineral assemblages in the Carboniferous HFC bauxite consist of diaspore, illite, anatase, and in Permian YFB and BYT bauxite, the minerals are diaspore, anatase, illite, clinochlore, and kaolinite. The data from the U-Pb isotope measurements of detrital zircons from these bauxites provide two main ages: (1) a maximum U-Pb age peak of ca. 800 Ma from the Carboniferous HFC and (2) a maximum age peak of ca. 1100 Ma for the Permian BYT and YFB. The ca. 800 Ma detrital zircons from the Carboniferous bauxite are dominating in the Paleozoic sedimentary units, which likely derive from the erosion of the Mid- to Neo-proterozoic felsic volcanic and intrusive rocks (Panxi-Hannan arc) occurring in the western margin of the South China Block. By contrast, the zircons with ages of ca. 1100 Ma are prevalent in the Paleozoic sediments from the center of the block. The above results indicate that the Lower Carboniferous and Lower Permian bauxites are sourced from the sedimentary rocks in the western and central parts of the South China Block, respectively. Despite the difference in the provenance, these two episodes of deposits were both deposited in a tidal flat-restricted platform attached to an EW-trending Yangtze island, occupies the western to central parts of the South China block. This indicates that the shift from a marginal to central uplift within the paleo-island from Early Carboniferous to Early Permian has induced the transition in the provenance of the source material. In addition, the weathered materials from the two provenances both accumulated in the coastal region receiving further bauxitization. The bauxitization was boosted by the persistent preferable tepid and humid paleoclimate, defined by the equatorial position of South China block and the drop of global temperature during the metallogenic period.

Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events

Floods carry sediments to river deltas and the coastal zone, but little is known about the geochemical evolution of this particulate material deposited over a short period of time. Here, we studied two recent contrasting flood deposits in the Rhône River prodelta area (northwestern Mediterranean Sea). We monitored the porewater and solid-phase chemistry over periods ranging from a few days to 6 months after deposition. Non-steady state diagenetic processes associated with episodic deposition promote a wide spectrum of transient redox conditions in the shallow prodelta region of the Rhône. Specific attributes of diagenetic responses depend on the sources of flood material and scale (thickness) of deposition.The first flood unit of 20–30 cm was composed of light gray mud, poor in organic carbon and rich in reactive manganese oxides. The short-term responses of early diagenetic processes contrasted with a rapid consumption of O2 and NO3- over a few hours just after the deposition event, accompanied by a slower build-up of Mn2+ concentration, and a lagged response in Fe2+ concentration over a few days or weeks. This difference was due to the redox capacity of the sediment, evolving from oxidized, during the flood layer deposition, to more reducing conditions, after a few days or weeks, allowing Fe2+ to build up and remain in solution. Sulfate reduction may have started within a few days within the flood deposit and was greatly enhanced just below the former redox front due to a fresh input of organic matter (OM). This large production of H2S probably led to the precipitation of sulfide minerals in close vicinity to the former redox front, limiting the accumulation of Fe2+ and H2S. The unit was sampled repeatedly three times during the six months following the flood event, and showed that manganese oxides were reduced at a rate of 1.8 mmol m−2 d−1, whereas the iron oxide concentration did not vary substantially.The second flood unit was composed of darker sediment, rich in organic carbon and reactive manganese oxides. The first step of OM degradation was enhanced within this dark deposit with a high release of dissolved organic carbon (DOC). A peak in Mn2+ concentration was also measured in association with the peak in reactive manganese oxides, also showing the rapid reduction of manganese after an input of fresh reactive oxides, and its potential increased release into the water column. Finally, a peak in nitrate concentration appeared in the sediment porewater within this anoxic organic/manganese-rich layer, likely resulting from the anaerobic oxidation of ammonium by manganese oxides.

North Pacific freshwater events linked to changes in glacial ocean circulation.

There is compelling evidence that episodic deposition of large volumes of freshwater into the oceans strongly influenced global ocean circulation and climate variability during glacial periods1,2. In the North Atlantic region, episodes of massive freshwater discharge to the North Atlantic Ocean were related to distinct cold periods known as Heinrich Stadials1-3. By contrast, the freshwater history of the North Pacific region remains unclear, giving rise to persistent debates about the existence and possible magnitude of climate links between the North Pacific and North Atlantic oceans during Heinrich Stadials4,5. Here we find that there was a strong connection between changes in North Atlantic circulation during Heinrich Stadials and injections of freshwater from the North American Cordilleran Ice Sheet to the northeastern North Pacific. Our record of diatom δ18O (a measure of the ratio of the stable oxygen isotopes 18O and 16O) over the past 50,000 years shows a decrease in surface seawater δ18O of two to three per thousand, corresponding to a decline in salinity of roughly two to four practical salinity units. This coincided with enhanced deposition of ice-rafted debris and a slight cooling of the sea surface in the northeastern North Pacific during Heinrich Stadials 1 and 4, but not during Heinrich Stadial 3. Furthermore, results from our isotope-enabled model6 suggest that warming of the eastern Equatorial Pacific during Heinrich Stadials was crucial for transmitting the North Atlantic signal to the northeastern North Pacific, where the associated subsurface warming resulted in a discernible freshwater discharge from the Cordilleran Ice Sheet during Heinrich Stadials 1 and 4. However, enhanced background cooling across the northern high latitudes during Heinrich Stadial 3-the coldest period in the past 50,000 years7-prevented subsurface warming of the northeastern North Pacific and thus increased freshwater discharge from the Cordilleran Ice Sheet. In combination, our results show that nonlinear ocean-atmosphere background interactions played a complex role in the dynamics linking the freshwater discharge responses of the North Atlantic and North Pacific during glacial periods.

Neogene siliciclastic deposition and climate variability on a carbonate margin: Australian Northwest Shelf

The Bare Formation represents a unique episode of Neogene siliciclastic deposition on the carbonate-dominated Australian Northwest Shelf (NWS). Seismic interpretation coupled with age control from International Ocean Discovery Program (IODP) Expedition 356 Sites U1462, U1463 and U1464 allow us to constrain the timing of siliciclastic deposition and the associated sedimentary processes. The Bare Formation is preceded by middle to late Miocene shelf exposure and karstification (~12 Ma). Elongate sandbars deposits with small lobate deltas developed during the late Miocene. Fluvial deposition increased markedly in the Zanclean (5.3–3.6 Ma), resulting in development of a large wave dominated delta, with evidence of channels, comprising the thickest component of the Bare Formation. Siliciclastic input decreased in the Piacenzian (3.6–2.58 Ma), leading to margin retreat and final termination in the early Pleistocene (~2.39 Ma). The results correlate with regional climate and sedimentary records from Sites U1463 and U1464, which indicate an arid middle to late Miocene on the NWS, followed by a humid interval in the Zanclean and a return to arid conditions during the Piacenzian. We suggest that fluctuation of surface runoff patterns in the continental hinterland was the primary control on Bare Formation evolution. Sedimentary and climate transitions are linked to reorganization of Indian Ocean paleoceanography, accompanying northward migration of the Australian continent and progressive restriction of the Indonesian Throughflow.

Geología y paleolimnología del complejo lagunar ?Santa Rosa del Monte?, Provincia de Buenos Aires, Argentina

Tres lagunas integran el complejo, alojadas en cinco cubetas eólicas de diferente edad, que a nivel hídrico normal forman un ambiente único, donde se caracterizaron los aspectos fisiográficos, hidrológicos y geolimnológicos. En los afloramientos se reconocieron cinco unidades litoestratigráficas y tres pedoestratigráficas, que abarcan del Pleistoceno temprano/medio al tardío y del Pleistoceno final al presente. La Formación Ensenada, constituye el sustrato regional, sobre el cual se disponen cuatro unidades eólicas, las Formaciones Buenos Aires, La Postrera II, III, IV, los Geosuelos Sin Nombre, Puesto Callejón Viejo y Puesto Berrondo y, el aluvio reciente. En los lechos se registraron ocho unidades litoestratigráficas: cinco límnicas y tres eólicas. Las límnicas corresponden a los cuatro paleoambientes de la Formación Luján: Miembros La Chumbiada, Lobos, Río Salado y Monte, del Pleistoceno tardío al Holoceno tardío-tardío y el aluvio reciente; las unidades eólicas a las Formaciones La Postrera I y II del Pleistoceno tardío al Holoceno temprano y la IV del Holoceno tardío-tardío. El origen de las primeras cubetas se inició en la etapa seca del ?Bonaerense?, la posterior alternancia climática seco/húmedo generó las otras cubetas. Las etapas secas se caracterizaron por deflación, sabkhas, precipitación de sales y depósitos eólicos. En las húmedas predominó el aluvionamiento y la pedogénesis. Así, el conjunto de depósitos ?posensenadenses? está representado por cinco episodios secos (Glacial Temprano a la Pequeña Edad de Hielo) y cinco húmedos, con acumulaciones límnicas, desde el Miembro La Chumbiada al aluvio reciente y en los afloramientos los suelos sincrónicos S4, S2, S1 y S0.

PRELIMINARY SEDIMENTOLOGICAL RESULTS INDICATE A NEW DETAILED STRATIGRAPHY FOR THE FLORINA SEDIMENTARY BASIN AND RELATE THEM WITH C02 PRESENCE

Drill core analysis showed 41 sedimentary cycles in the sedimentary association of the Neogene to Quaternary Fiorina Basin, with a total thickness up to 560m. The lower part is the Base formation with an up to 297m thick, whereas middle part, Vevi formation is up to 127m thick and the upper part which corresponds to Lophon formation is up to 124m thick. The beginning of sedimentation in the basin, with alternated sand, clay and conglomeratic beds, with presence of xylite, at the lowermost part of Base formation, indicates a weak tectonic activity and low sedimentation rate. Tectonic activity increased and alluvial fans were formed in the rest part of Base formation. The 21 cycles within alluvial fans could be related mostly with 21 episodes of tectonic activity and sedimentation and less with climatic changes or autocyclic events. Depositional environment changed in the Vevi formation from terrestrial to lacustrine, where lacustrine and fluvio-lacustrine deposits were accumulated. Finally, the basin filled up with terrestrial deposits in the Lophon formation. So, the ratio of subsidence rate/sedimentation rate starts with &lt;1, in Base formation, becomes =1 in the case of Vevi's formation and finally becomes again &lt;1 in Lophon formation. Fluvial and alluvial fan environments can be used as CO2 storage sites, whereas the internal change of stratigraphy in the same environment of sedimentation does not allow the escape of CO2.

Are early plants significant as paleogeographic indicators of past coastlines? Insights from the taphonomy and sedimentology of a Devonian taphoflora of Paraná Basin, Brazil

The continental colonization process by plants is an underexplored theme in Brazil, especially concerning taphonomic studies of plants that existed during this age. This study aims to interpret the taphonomic and paleoenvironmental settings that best represent the fossil plant assemblages preserved in the upper strata of the Furnas Formation (Lower Devonian of Paraná Basin), at Jackson de Figueiredo county, municipality of Jaguariaíva (Paraná), clarifying the potential of these fossil plant assemblages for paleogeographic reconstructions. Taphonomic and sedimentological signatures were analyzed and submitted to multivariate statistical analyses (cluster and correspondence analyses). Two taphonomic groups were identified (Taphofacies A and B). The interpretation of the two groups allowed the visualization of: Taphofacies A, which consists of samples with a dispersed degree of packing, fragmented plant fossils, 40% mica with sizes between 0.5 and 1 mm and no trace fossils, is representative of relatively high-energy conditions; and Taphofacies B, which consists of densely packed plants with partial degree of fragmentation, 10% mica with sizes under 0.5 mm and both horizontal and vertical trace fossils, is representative of relatively low-energy conditions. Our taphonomic data are consistent with previous facies interpretations of a lagoon/interdistributary bay environment. In this context, taphofacies A represents episodic sedimentation events influenced by fluvial dynamics, yielding assemblages that are not suitable for paleogeographic interpretations, while Taphofacies B represents fossil assemblages originated in a low-energy sedimentary regime, with utility for paleogeographic reconstructions.