Silt Carbon Research Articles

Torosites: A New Microbially Mediated Speleothem from Sistema los Toros, Nuevo Léon, Mexico

Although several kinds of speleothems are known to grow with pendant morphology (i.e., helictites, pool fingers, and stalactites), we describe here a new form found in Sistema Los Toros, Nuevo Léon, Mexico, which we propose to call torosites. A laminated, bladed, spar crust grew beneath remnants of a stromatolitic micritic crust and extended mostly downward as irregular finger-like protrusions (torosites) that are 2–3 mm wide and 2–3 cm long. Although mainly pendants, the individual torosites are quite irregular with growth sideways and even upward in places. The torosites thicken and thin, like knuckles on a finger, but maintain a relatively uniform diameter. Laminations in the bladed spar and protrusions are defined by micritic intervals with minor intermixed detrital grains (clay, quartz, and carbonate silt) and rare, ropy biofilm. Bladed-spar crystals nucleate off thicker detrital layers, but they extend through thinner lamina. The torosites lack the central canal of helictites and most stalactites. Unlike pool fingers and stalactites, torosites only grow on the end of the speleothem, not on the sides. We hypothesize that the torosites grew from dripping water at their ends. Microbial biofilm communities colonized the growth surface, probably during drier conditions, forming the micritic layers. We speculate that the microbial mat likely caused the irregular sideways growth, but we cannot identify how on these fossil forms. Torosites are unique and add to the growing list of known microbially mediated speleothems.

A methodology to calibrate the PICSI cyclic p-y model using experimental results and optimisation

Fatigue life estimation of the components of a drilling conductor system is often a critical design consideration. The analysis is usually performed using a coupled system, with the soil-conductor interaction modelled using p-y springs that remain constant throughout the analysis. This means that shifts in bending moment profile that occur due to degradation and recovery of the soil during or after cycling are not accurately modelled. While the PICSI framework (White et al., 2022) can model changes in stiffness and strength of p-y curves due to cycling and pore-pressure dissipation, guidance has not yet been provided on how to calibrate its parameters. This paper presents an experimental methodology for this calibration process based on centrifuge and p-y model testing in reconstituted carbonate silt and kaolin clay. The procedure uses numerical optimisation, and the calibrated parameters are validated against results from an independent set of centrifuge tests on carbonate silt using a flexible pile of similar dimensions to a conductor. It is found that the calibrated model is able to match the changes in cyclic bending moment through a sequence of different packets of cyclic loading. This calibration procedure provides an objective approach for more accurate modelling of conductor fatigue.

Theoretical framework for predicting accumulation of soil berms and peak sliding resistance for tolerably mobile foundations

Tolerably mobile subsea foundations are designed to slide on the seabed to accommodate flowline thermal expansion and contraction, and are a potential alternative to conventional (fixed) foundations. During the periodic sliding events that occur during operation, soil berms form at the extremities of the foundation footprint. The size of the berm increases throughout the life-cycle of the foundation, leading to increasing peak sliding resistance. This may hinder mobility of the foundation and overstress the pipeline connections that the foundation is designed to support. Equally, the berms may be relied on to reduce sliding and thus minimize settlement of the foundation, which can also overstress pipeline connections. This paper analyses the mechanism leading to berm accumulation and its mobilisation, also addressing periodic remoulding and reconsolidation of the sediment in the berm. A framework is proposed to predict the accumulation of soil berms and the resulting peak sliding resistance, and is validated by eight centrifuge model tests performed on a kaolin clay and a calcareous silt.

A viscoplastic recoverable sensitivity model for fine-grained soils

Full-flow penetrometer testing in fine-grained soils indicates that undrained shear strength decreases due to remoulding induced generation of excess pore pressures and increases due to strain rate effects and dissipation of excess pore pressures. This paper presents a viscoplastic strain-softening–hardening constitutive model that captures this duality of behaviour via non-local regularisation, strain rate dependency, and consolidation induced recovery of sensitivity. The model is based on Structured Modified Cam Clay and Perzyna’s overstress framework. Validation of the implementation is first demonstrated through simulation of constant rate of strain triaxial and oedometer compression tests performed on different clays. The model is then applied in large deformation finite element analyses of monotonic and variable rate T-bar penetration to demonstrate successful numerical implementation and its ability to simulate strain rate effects. Finally, the model is applied in the simulation of undrained cycles of penetration of a T-bar penetrometer in kaolin clay and a carbonate silt and compared with data from geotechnical centrifuge experiments. The results show that the model without sensitivity recovery significantly underestimates the consolidated-undrained resistance in both soils, suggesting that recoverable sensitivity is an aspect of behaviour that ought to be considered in the development of constitutive models for soft sensitive soils.

New data on geology of the Rybalsky Quarry, unique object of geological heritage of Global Significance


 
 
 Among the great variety of geological relics of Ukraine, one of the most attractive is the Rybalsky Quarry, located in the city of Dnipro, and well known outside Ukraine. First of all, it is famous for the Mandrykivski Layers exposed in one of the ridges of the Quarry back in the 1970s, although there are many other important peculiarities of the geological structure of the Quarry which attract scientists from Ukraine, Germany, France, Netherlands, Russia and other countries. There are full data on the history of discovery and survey of the Mandrykivski Layers from their discovery by Valerian Domger in 1882 to current studies that reveal various aspects of geological structure of the Quarry, the results of study of different groups of fossil fauna, compare them with the fauna of the Paris Upper Eocene basin and other well- known locations. Particularly in the Rybalsky Quarry, thanks to the author`s efforts, the Layers received the status of “layers with geographic name”, as confirmed by the decision of Cenozoic Commission of the National Stratigraphic Committee of Ukraine in 2001. New fragments of the section of subaerial and subaqueous deposits of the Quaternary deposits were found and their composition, structure and complete stratigraphic content were studied. The research allows us to consider it typical for the Middle Prydniprovia. Fluvioglacial and lake-glacial deposits of the Dnipro glaciations in the Quarry exposed for the second time in 25 years, but this is the first time when their genetic origin, position in the section and lithological-facies peculiarities were determined. Thick layer of sands embedded on the roof of the Mandrykivski Layers were identified to the fifth or Hadzhybeiska terrace of the Dnipro, in the upper part of which there were notable siliceous-clayey-ferruginous formations or lateral crust (ferruginous crusts). The studies of the layer of brown-green and red-brown clays in the roof of the Mandrykivski layers allows us to presume their marine origin. In this case, they are underwater weathering crust (terra rossa) developed in the process of halmyrolysis or are the product of dissolution of carbonate silt enriched with detritus of mollusks, corals and other inhabitants of the warm Mandrykivske Sea.
 
 

Mixed carbonate–siliciclastic tidal sedimentation in the Miocene to Pliocene Bouse Formation, palaeo‐Gulf of California

AbstractMixed carbonate–siliciclastic deposits provide unique insights into hydrodynamic processes that control sedimentation in tidal systems. This study presents sedimentological and ichnological data from the upper Miocene to lower Pliocene Bouse Formation, which accumulated during regional transgression at the margin of a tidal strait near the north end of the ancestral Gulf of California. The basal carbonate member of the Bouse Formation records deposition in a tide‐influenced, compositionally mixed carbonate–siliciclastic system dominated by salt marsh, tidal flat and channel environments. The basal carbonate member is an overall deepening up succession of facies associations comprising: Facies Association 1 – siliciclastic‐rich heterolithic facies, lime mudstone with desiccation cracks, and plant debris rich carbonate silt interpreted as siliciclastic‐rich tidal flats; Facies Association 2 – well‐sorted gravels, siliciclastic‐rich sandy strata, lime mudstone with desiccation cracks, and sandy microbial micrite interpreted as tidal‐channel deposits; Facies Association 3 – carbonate‐rich heterolithic lime mudstone to well‐sorted, cross‐bedded bioclastic grainstone interpreted as intertidal to shallow subtidal deposits; and Facies Association 4 – lime mudstone interpreted as shallow to deep subtidal low‐energy deposits that record the end of tidal conditions in the basin. Trace fossils include marine forms Gyrolithes, Teichichnus and Thalassinoides, and non‐diagnostic forms Arenicolites, Cochlichnus, Conichnus, Lockeia, Planolites, Skolithos and Treptichnus (known from marine, brackish and freshwater environments). The diminutive size of trace fossils reflects brackish conditions created by mixing of freshwater and seawater. This study provides evidence for a late Miocene to early Pliocene humid climate in south‐western North America, in stark contrast to the modern hyperarid climate. Factors that controlled the relative percentage of mixed carbonate and siliciclastic sediment include siliciclastic input from local rivers, in situ carbonate production, current energy, degree of tidal mixing and relative sea level. Pronounced facies variability at bedform, outcrop and basin scale documented in this study appears to be an important characteristic of mixed carbonate–siliciclastic deposits in tidal depositional systems.

The changing strength of carbonate silt: parallel penetrometer and foundation tests with cyclic loading and reconsolidation periods

This paper describes a centrifuge study using novel penetrometer tests (T-bar and piezoball) and model foundation tests to explore through-life changes in the strength of a reconstituted natural carbonate silt. The test procedures include episodic cyclic loading, which involves intervals of pore pressure dissipation between cyclic packets. These loads and the associated remoulding and reconsolidation cause significant changes in the soil strength and foundation capacity. Soil strength changes from penetrometer tests differed by a factor of 15 from the fully remoulded strength to a limiting upper value after long-term cyclic loading and reconsolidation. For the model foundation tests, the foundation capacity of a surface foundation and a deep-embedded plate were studied. The soil strength interpreted from the measured foundation capacity varied by a factor of up to three due to episodes of loading and consolidation, with an associated order of magnitude increase in the coefficient of consolidation. The results show a remarkable rise in soil strength over the loading events and provide a potential link between changes in soil strength observed in penetrometer tests and the capacity of foundations, allowing the effects of cyclic loading and consolidation to be predicted.

Improvements in plate anchor capacity due to cyclic and maintained loads combined with consolidation

Plate anchor technology is an efficient solution for mooring offshore floating facilities for oil and gas or renewable energy projects. When used with a taut mooring, the anchor is typically subjected to a maintained load component and intermittent episodes of cyclic loading throughout the design life. These loads, and the associated shearing, remoulding and consolidation processes, cause changes in the anchor capacity, particularly in soft, fine-grained soils. The changing anchor capacity affects the mooring performance by changing the safety margin and also the overall system reliability. In this paper the changing anchor capacity in reconstituted, normally consolidated natural carbonate silt was assessed through a series of beam centrifuge tests on horizontally loaded circular plate anchors. The results demonstrate that full consolidation under a typical maintained load leads to a 50% gain in the anchor capacity, and subsequent cyclic loading and reconsolidation can triple this increase. An effective stress framework based on critical state concepts is employed to explain and support the experimental observations. This study shows that, when viewed from a whole-life reliability perspective, maintained and cyclic loading provide a long-term enhancement of anchor capacity in soft, fine-grained soils. This beneficial effect is currently overlooked in design practice, but can be predicted using the framework shown here, which can form the basis for a digital twin that monitors the through-life integrity of a plate anchor.

Load-controlled cyclic T-bar tests: a new method to assess effects of cyclic loading and consolidation

Full-flow T-bar and ball penetrometer tests are often used to measure intact and remoulded soil strengths, with the latter determined after several large-amplitude displacement cycles. In offshore design, the remoulded soil strength is often the governing design parameter during installation of subsea infrastructure, while a ‘cyclic strength’ applies for the less severe operational cyclic loading. This paper utilises T-bar penetrometer tests to measure both remoulded and cyclic strengths, where the latter is determined by way of a new test protocol involving cycles between load rather than displacement limits. The tests use kaolin clay and a reconstituted carbonate silt and involve three cyclic phases with intervening consolidation periods. The results demonstrate the important and beneficial role of consolidation, with the loss in strength due to remoulding sometimes surpassed by the strength recovery from consolidation. The most significant gains in strength, to 2·5 times the initial value, were measured in the load-controlled cyclic tests. These data demonstrate a novel way to characterise undrained cyclic strength, taking advantage of consolidation to reduce conservatism.

Diversity and Distribution of Benthic Polychaetes of Nizampatnam Bay, Bay of Bengal

Diversity and Distribution of Benthic Polychaetes of Nizampatnam Bay, Bay of Bengal A study was carried out to observe the diversity and distribution of soft bottom macrobenthic polychaetes in Nizampatnam bay, Bay of Bengal. The aim of this study is to determine patterns of abundance, species richness and community structure along with the influence of environmental parameters on the diversity of polychaetes collected from the Nizampatnam Bay. Four cruises were conducted during 2006 to 2008 covering 20 stations along the Bay having a maximum depth of 40 m. Altogether 128 infaunal samples (van Veen grab 0.1 m2) were collected from the bay along with the hydrographical and sediment samples. Polychaete diversity indices S, N, d and H' were higher in the stations located nearer to the shoreline (15 m). Temporally post-monsoon seasons supported high polychaete diversity than pre-monsoon season. Multivariate analyses were used to define assemblages named after the most important (determining) taxon. Occurrence of Prionospio pinnata and Capitella capitata, the deposit feeders and indicators of organic pollution suggested that the stations that lie nearer the shore are organically rich. Canonical correspondence analysis (CCA) showed that depth, sediment organic matter and sediment mean size influenced polychaete distribution. Canonical correspondence analysis (CCA) showed that majority of polychaete species were deposit feeders and they preferred high organic carbon and clayey silt substratum.

Glacial origin for cave rhythmite during MIS 5d-c in a glaciokarst landscape, Picos de Europa (Spain)

Laminated slackwater deposits have been identified in many karst caves related to fluvial and lacustrine sedimentation. However, sedimentological evidence rarely supports a glacial origin for these deposits, which was proposed by previous studies. The Torca La Texa shaft is located in a glaciokarst area that comprises numerous slackwater-type deposits, piled up in fining-upward sequences. A basal sandy erosive layer and millimeter-sized laminated rhythmite with interbedded flowstone characterize these sequences. Fining-upward layers of carbonate silt, clay, and minor quartz sand deposited in flooded conduits define the rhythmite lamination. The presence of allochthonous minerals indicates that the rhythmite sediment comes from the glacial erosion of nearby carbonate mountains. Two 234U/230Th radiometric ages dated the rhythmite deposits around 109 and 95ka, coinciding with relative cold periods included in the MIS 5d-c. These cold periods were marked by a high annual seasonality, immediately after the glacial local maximum extension, in agreement with a varve-type deposit. The combination of these sedimentological mineralogical, geomorphological and paleoclimate information indicates that the rhythmite should be introduced into the studied cave during the summer melting of the glaciers, which produced the recharge of the karst aquifer, triggering cave floods. In addition, punctual glacier collapses would also have their imprint in the slackwater sequences with thicker, coarser and erosive sand deposits and the spring blocking by glaciers may have promoted floods inside the cave. Therefore, the studied rhythmite can be interpreted as glacial varves decanted during the relatively cold climate conditions.

Molar tooth structures and origin of peloids in proterozoic carbonate platforms (Middle Riphean of the Turukhansk Uplift, Siberia)

Structures and textures of the peloidal wackestones, as well as size, shape, and composition of peloidal grains, from the Mesoproterozoic (Middle Riphean) Sukhaya Tunguska carbonate platform in the Turukhansk Uplift (Siberia) are considered. It is shown that these grains formed in the course of diagenesis were closely associated with the microsparitic replacement and the formation of molar tooth (MT) structures. Diagenetic transformations of rocks were related to the activity of anaerobic microbial communities inside the buried carbonate silt layers. The microbial activity during diagenesis was governed by the carbonate sediment composition and conservation mechanism of the high-molecular organic matter of primary producers therein, since this organic matter was the nutritious substrate for the primary anaerobe communities.

Evidences for Bioprecipitation of Pedogenic Calcite by Calcifying Bacteria from Three Different Soils of L'Aquila Basin (Abruzzi, Central Italy)

To provide further evidences on the role of bacterial soil species in the development of calcium carbonate deposits in soil, we isolated 36 heterotrophic bacterial strains from three soils of L'Aquila basin characterized by different CaCO3 content and tested their ability to precipitate CaCO3 when cultured on a Ca-rich medium. We found that the majority (63.89%) of these isolates could precipitate CaCO3 minerals at 27°C. The aptitude to calcification (time and crystal amount) of each calcifying strains, morphology (SEM) and mineralogy of the formed bioliths were also investigated. X-ray diffraction confirmed the production of calcite. Crystal formation was not observed in the controls. Organic matter, total N and assimilable P, cation exchange capacity and exchangeable Ca2+, Mg2+, K+, Na+, pH, total and active calcium carbonate content, electric conductivity, skeleton, sand, silt and clay fractions of each soil sample were determined and related with its microbiological parameters. We found that the CaCO3 content of soil was significatively related, in particular, to the percentage of calcifying bacterial strains (r = 0.95) and to the heterotrophic bacterial density (r = 0.98), which was found significatively related also with Ca2+ content of soil (r = −0.97) and its CEC (r = −0.97).

Sedimentology and magnetic susceptibility of recent sediments from New Caledonia

Abstract The interpretation of the primary origin of the minerals carrying the magnetic susceptibility (MS) signal from ancient rocks suffers notably from the scarcity of studies on Recent sediments. To bring new data, a study of tropical coastal sediments of New Caledonia was undertaken. This island is surrounded by a nearly uninterrupted reef barrier, isolating a wide lagoon from the open ocean. The erosion of extremely varied rocks (from mantle rocks to laterites) produces different types of detrital sediments, which are mixed with the indigenous precipitated carbonates. This generates different types of coastal sediments, detrital- or carbonate-dominated or mixed. More than 300 samples were analysed for grain size, nature of sediment, MS and geochemistry (major elements). The first results show that: (a) carbonate sands and carbonate silts are characterized by lower MS than detrital sediments; (b) the MS signal of mixed sediments is mostly influenced by the proportion of detrital sediments; (c) MS is directly correlated with Mn and Fe content; (d) beachrocks are characterized by lower MS than equivalent loose sediment; (e) the MS signal of carbonate sediments is locally positively correlated with granulometry; (f) there is no MS change between surface and 20 cm deep samples; and (g) when the subsurface sediment is reducing, the MS is higher than that from surface sediment.

Glacial crushing of limestone and the production of carbonate‐rich silts in a pleistocene glaciofluvial system: a potential source of loess in southern europe

This paper explores the modification of limestone bedrock and fine‐grained sediment within the Pleistocene glacial and proglacial environments of the rjen massif in western Montenegro. We have characterised the fine‐grained components of till and outwash using lithological and particle size data. Two main types of fine‐grained outwash sediment have been recognised and each is associated with a distinctive meltwater route. Fine‐grained sediments deposited downstream of bedrock gorges (type 1 meltwater route) typically display a bimodal particle size distribution, where the carbonate silt fraction (10–60 μm) has been depleted and non‐carbonate silts become dominant. This probably reflects both physical sorting and corrosion of the fine‐grained limestone sediment within the proglacial fluvial environment. The 10–60 μm component matches the particle size characteristics of typical loess. Fine‐grained outwash deposits from directly in front of the former ice margin (type 2 meltwater route) show unimodal particle size distributions which more closely resemble the grain size characteristics of the glacial till, but they also show evidence of silt depletion. We argue that the fine‐grained sediment exported from glaciated limestone catchments during the cold stages of the Pleistocene formed an important source of the carbonate‐rich loess in this region.

Fault and basin depocentre migration over the last 2 Ma in the L'Aquila 2009 earthquake region, central Italian Apennines

Morphological, stratigraphical and structural investigations integrated with palaeomagnetic and tephrostratigraphic studies and 40Ar/39Ar measurements allowed us to define the Quaternary tectonic–sedimentary evolution of the epicentral area of the L'Aquila 2009 earthquake (Mw 6.3). This area roughly matches the Paganica–San Demetrio–Castelnuovo (PSC) intermountain basin filled by a fluvio-lacustrine succession that is variously deformed by normal faults. At its early stage (Early Pleistocene; ca 2 Ma), the basin hosted a lake that was characterised by whitish carbonate silt and mud deposition. Towards the end of the Early Pleistocene, this was partly filled by a south-eastward prograding deltaic system. The south-easternmost portion of the deltaic system was successively abandoned and eroded, while sedimentation of a braided river system developed in a smaller area westwards. Before 780 ka, this fluvio-lacustrine system (the Lower PSC syntheme) ended, and the area underwent erosional and pedogenic processes. These processes continued up to >460 ka, with the onset of fluvial gravel and volcanic-rich silty sand deposition over a narrow area of the south-western PSC basin (an earlier unit of the Upper PSC syntheme, early Middle Pleistocene). Around 460 ka, when most of this area was again subjected to denudational processes, the sedimentation migrated into the previously exposed north-western area of the PSC basin, where fluvial/alluvial sediments were deposited, between ca 460 ka and ca 350 ka (a later unit of the Upper PSC syntheme). Between ca 350 ka and the present, the sedimentation continued uninterrupted in a slight westwards restricted area of the PSC basin (Upper–Late PSC syntheme). The significant contraction of the basin, the shifting of its depocentre, the variation in its geometry and in the spatial distributions and thicknesses of the units constituting the Lower, Upper and Late PSC synthemes, provide evidence for strong tectonic control. This resulted in progressive reduction of the activity of the south-easternmost tectonic structures and parallel migration of the fault activity on the westernmost structures. In this perspective, the fault system responsible for the L'Aquila 2009 earthquake can be seen as the most recent expression of a long-lasting westwards polarised process of tectonic activity transfer.

Phosphorus Fractionation in River Sediments, Hamadan, Western Iran

Phosphorus (P) in sediments was fractionated by sequential extraction procedure, in which the P fractions were experimentally defined as exchangeable (KCl-P), Fe- and Al-bound (NaOH-P), Ca-bound (HCl-P), and residual P (Res-P) fractions. In all sediment samples the rank order of P-fractions was HCl-P > Res-P > NaOH-P > KCl-P. The loosely sorbed P (KCl-P) represented 6.7% of sedimentary inorganic P, while Res-P ranged from 27.1–61.2%. The Ca-bound P showed considerable contribution (32.8–52.0%) to the sedimentary inorganic P. Relative NaOH-P in sediments ranged from 6.6 to 13.5%. The dominant P fraction in the sediments generally changed from Res-P at upstream to HCl-P at downstream, suggesting that more P and Ca are added by point and diffuse sources along the river. Multivariate analysis was used to identify the factors which influence the P fractions. The principal component analysis showed that the first four factors explained 82.8% of the overall variation. Carbonate calcium and silt seem to play a significant role in regulating the NaOH-P fraction, while KCl-P and HCl-P are affected by EC.

Effects of fluorination modification on pore size controlled electrospun activated carbon fibers for high capacity methane storage

Electrospun carbon fibers were prepared as a methane storage medium. Chemical activation was carried out using potassium carbonate to develop the pore structure, which can provide sites for the uptake of methane, and then fluorination surface modification was conducted to enhance the capacity of storage. Chemical activation provided a highly microporous structure, which is beneficial for methane storage, with a high specific surface area greater than 2500 m 2/g. The pore size distribution showed that the prepared samples have pore sizes in the range of 0.7–1.6 nm. The effect of fluorination surface modification was also investigated. The functional groups, which were confirmed by XPS analysis, played an important role in guiding methane gas into the carbon silt pores via the attractive force felt by the electrons in the methane molecules due to the high electronegativity of fluorine. Eventually, the methane uptake increased up to 18.1 wt.% by the synergetic effects of the highly developed micropore structure and the guiding of methane to carbon pores by fluorine.

Carbonate dissolution revealed by silt grain‐size distribution: comparison of Holocene and Last Glacial Maximum sediments from the pelagic South Atlantic

AbstractThe current issue of global warming and the role of the ocean in global exchange of CO2 increases the interest in solid budgets of marine carbonate production and dissolution. The present study utilizes grain‐size composition of pelagic sediments in order to trace spatial and temporal variability of carbonate sedimentation in the South Atlantic for the Holocene and Last Glacial Maximum (LGM, 19–23 cal kyr BP). A decrease in grain size (e.g. sand content, mean grain size of coarse carbonate silt) indicates increased carbonate dissolution as a result of increased fragmentation of calcareous microfossils. The spatial grain‐size pattern suggests a threshold water depth below which a gradual grain‐size decrease becomes increasingly rapid. This water depth is considered as the sedimentary lysocline. For the Holocene time slice, a constant, gradual decrease of foraminifer carbonate of about 5–10% per 1000 m water depth above the lysocline gives evidence for supra‐lysoclinal dissolution. The water depth of the lysocline for the Holocene is tied to the interface of North Atlantic Deep Water and Antarctic Bottom Water (AABW) (ca 4100 m). Submarine ridges which restrict intrusion of AABW into the Angola Basin cause an asymmetry in carbonate preservation across the Mid‐Atlantic Ridge. The lysocline was reconstructed at ca 3100 m for the LGM. These data suggest that the ca 1000 m rise of the lysocline eradicated the Holocene east–west asymmetry.

Carbonate preservation patterns at the Ceará Rise – Evidence for the Pliocene super conveyor

Enhanced Atlantic overturning during the Pliocene was first proposed almost 10 yrs ago. Evidence for this Pliocene super conveyor scenario has been collected using a number of proxies (e.g., benthic δ 13C, Nd isotopic composition of manganese crusts). The present study contributes to the existing evidences by using carbonate dissolution and current vigour history of early Pliocene sediments from the Ceará Rise (ODP Sites 927 and 929). In order to reveal carbonate dissolution history, a number of commonly used and newly established proxies were applied, i.e., sand and carbonate contents, foraminifer fragmentation index, Bulloides Dissolution Index and carbonate silt grain-size distributions. Terrigenous silt grain-size distributions were used to unravel variations in relative current strength and sediment input to the two sites. Overall good carbonate preservation at the shallow Site 927 (3314 m water depth) shows that this level was bathed in North Atlantic Deep Water throughout the early Pliocene. The contrastingly poor carbonate preservation record of the deeper Site 929 (4358 m water depth, at present exposed to Antarctic Bottom Water) is frequently interrupted by phases of good carbonate preservation. These results indicate that the depth of the calcite lysocline was mainly tied to present level (∼ 4200 m water depth), and sometimes even dropped to water depths greater than 4360 m due to even more enhanced circulation. Surprisingly the expansion of NADW is not clearly reflected by an increase in current speed as shown by continuously fine terrigenous grain size.