Broken Beds,but better science; using multiple hypotheses to interpret geological data

In this study, we propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi-quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi-quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down-played to favour a single hypothesis. Furthermore, by integrating expertise from different sub-disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi-process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.     

40Ar–39Ar geochronology across Archean and Paleoproterozoic terranes from southeastern Guiana Shield (north of Amazonian Craton,Brazil): Evidence for contrasting cooling histories

A ⁴⁰Ar/³⁹Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma.These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new ⁴⁰Ar/³⁹Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T–t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.     

Lake‐level fluctuations at Lake Bourget (eastern France) around 4500–3500 cal. a BP and their palaeoclimatic and archaeological implications

On the basis of sedimentological analysis of two cores taken at Chatillon, Lake Le Bourget (northern French Pre‐Alps), and well dated by radiocarbon dates in addition to tree ring dates obtained from an archaeological layer, this paper presents a high‐resolution lake‐level record for the period 4500–3500 cal. a BP. The collected data provide evidence of a complex palaeohydrological (climatic) oscillation spanning the ca. 4300–3850 cal. BP time interval, with major lake‐level maxima at ca. 4200 and 4050–3850 cal. a BP separated by a lowering episode around 4100 cal. a BP. The lake‐level highstands observed at Chatillon between 4300 and 3850 cal. BP appear to be synchronous with (i) a major flooding period recorded in deep cores from the large lakes Le Bourget and Bodensee, and (ii) glacier advance and tree line decline in the Alps. Such wetter and cooler climatic conditions in west‐central Europe around 4000 cal. a BP may have been a nonlinear response to decrease and seasonal changes in insolation. They may also provide a possible explanation for the general abandonment of prehistoric lake dwellings north of the Alps between 4360 and 3750 cal. a BP. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydropyrolysis of algae, bacteria, archaea and lake sediments; insights into the origin of nitrogen compounds in petroleum

Nitrogen compounds are ubiquitous in fossil fuels and yet our understanding of their origins in the geosphere is limited. In this study, high hydrogen pressure pyrolysis was performed on sample material representing potential contributors to sedimentary organic matter (algae, bacteria and archaea) and sediments representing early diagenetic accumulations from Lake Pollen (Norway) and Priest Pot (UK). Previous workers demonstrated the structurally conservative nature of high hydrogen pressure pyrolysis in that the technique maximizes the yields of covalently bound hydrocarbon biomarkers from organic matter without adversely affecting their stereochemistry (Love et al., 1995). Release of covalently bound biomarkers in high yields from kerogen via catalytic hydropyrolysis. In this study, the types and distributions of organic nitrogen compounds in the hydropyrolysates were characterised under similar conditions to such experiments where biomarker hydrocarbons undergo minimal rearrangement. Compounds identified by gas chromatography–mass spectrometry included alkyl-substituted indoles, carbazoles, benzocarbazoles, quinolines and benzoquinolines.Indoles are present in all hydropyrolysates, suggesting a common origin. A potential source of indoles is represented by tryptophan which was shown to degrade through a series of alkylated intermediates to indole. Carbazole, quinoline and benzoquinoline were also found in the hydropyrolysates of algae, bacteria and archaea. The presence of these petroleum-related nitrogen compounds in hydropyrolysates generated from biomass suggests an early origin for petroleum nitrogen compounds. A potential source of naturally occurring nitrogen compounds such as that in the alkaloids has yet to be realised.Benzocarbazoles were absent from hydropyrolysates of algae, bacteria and archeae, but present in those from recent sediments, suggesting their presence may be related to processes occurring during early diagenesis at, or immediately below, the sediment–water interface. In sediments from Lake Pollen, changes in the benzocarbazole ratio [a]/([a] + [c]) ratio coincides with the interval described as a transition from fjord to lake environment, suggesting that benzocarbazoles are sensitive to changes in depositional environment and may have potential to act as a marker for environmental conditions.     

Sr and Pb isotopic composition of five USGS glasses (BHVO-2G, BIR-1G, BCR-2G, TB-1G, NKT-1G)

Sr isotopic compositions and Rb / Sr ratios of three USGS glasses (BHVO-2G, BIR-1G, BCR-2G) are identical to those of the original USGS reference materials. NKT-1G and TB-1G give values of 0.70351 and 0.70558, respectively. Pb isotopic ratios were measured by the standard-sample bracketing technique on an MC-ICP-MS, which give results that are comparable in accuracy and reproducibility to double spike analyses. However, assessment of the reproducibility of the technique is hampered by inhomogeneous contamination of all USGS reference materials analysed. This contamination is likely to be the reason why the USGS glasses do not all have the same Pb isotopic composition as their unfused originals. Powdered glasses, distributed for characterisation of the glasses by bulk analytical techniques, do not all have the same Pb isotopic compositions as the solid glass material, and can therefore not be used for this purpose.     

Tracing the temporal evolution of soil redistribution rates in an agricultural landscape using 239+240Pu and 10Be

Two principal groups of processes shape mass fluxes from and into a soil: vertical profile development and lateral soil redistribution. Periods having predominantly progressive soil forming processes (soil profile development) alternate with periods having predominantly regressive processes (erosion). As a result, short-term soil redistribution – years to decades – can differ substantially from long-term soil redistribution; i.e. centuries to millennia. However, the quantification of these processes is difficult and consequently their rates are poorly understood. To assess the competing roles of erosion and deposition we determined short- and long-term soil redistribution rates in a formerly glaciated area of the Uckermark, northeast Germany. We compared short-term erosion or accumulation rates using plutonium-239 and -240 (^239+240Pu) and long-term rates using both in situ and meteoric cosmogenic beryllium-10 (¹⁰Be). Three characteristic process domains have been analysed in detail: a flat landscape position having no erosion/deposition, an erosion-dominated mid-slope, and a deposition-dominated lower-slope site. We show that the short-term mass erosion and accumulation rates are about one order of magnitude higher than long-term redistribution rates. Both, in situ and meteoric ¹⁰Be provide comparable results. Depth functions, and therefore not only an average value of the topsoil, give the most meaningful rates. The long-term soil redistribution rates were in the range of −2.1 t ha^-1 yr^-1 (erosion) and +0.26 t ha^-1 yr^-1 (accumulation) whereas the short-term erosion rates indicated strong erosion of up to 25 t ha^-1 yr^-1 and accumulation of 7.6 t ha^-1 yr^-1. Our multi-isotope method identifies periods of erosion and deposition, confirming the ‘time-split approach’ of distinct different phases (progressive/regressive) in soil evolution. With such an approach, temporally-changing processes can be disentangled, which allows the identification of both the dimensions of and the increase in soil erosion due to human influence. © 2019 John Wiley & Sons, Ltd.     

Impacts of Kelvin wave forcing in the Peru Humboldt Current system: Scenarios of spatial reorganizations from physics to fishers

Because climate change challenges the sustainability of important fish populations and the fisheries they support, we need to understand how large scale climatic forcing affects the functioning of marine ecosystems. In the Humboldt Current system (HCS), a main driver of climatic variability is coastally-trapped Kelvin waves (KWs), themselves originating as oceanic equatorial KWs. Here we (i) describe the spatial reorganizations of living organisms in the Humboldt coastal system as affected by oceanic KWs forcing, (ii) quantify the strength of the interactions between the physical and biological component dynamics of the system, (iii) formulate hypotheses on the processes which drive the redistributions of the organisms, and (iv) build scenarios of space occupation in the HCS under varying KW forcing. To address these questions we explore, through bivariate lagged correlations and multivariate statistics, the relationships between time series of oceanic KW amplitude (TAO mooring data and model-resolved baroclinic modes) and coastal Peruvian oceanographic data (SST, coastal upwelled waters extent), anchoveta spatial distribution (mean distance to the coast, spatial concentration of the biomass, mean depth of the schools), and fishing fleet statistics (trip duration, searching duration, number of fishing sets and catch per trip, features of the foraging trajectory as observed by satellite vessel monitoring system). Data sets span all or part of January 1983 to September 2006. The results show that the effects of oceanic KW forcing are significant in all the components of the coastal ecosystem, from oceanography to the behaviour of the top predators – fishers. This result provides evidence for a bottom-up transfer of the behaviours and spatial stucturing through the ecosystem. We propose that contrasting scenarios develop during the passage of upwelling versus downwelling KWs. From a predictive point of view, we show that KW amplitudes observed in the mid-Pacific can be used to forecast which system state will dominate the HCS over the next 2–6 months. Such predictions should be integrated in the Peruvian adaptive fishery management.     

The grain-size characterisation of coastal sand from the Somme estuary to Belgium: Sediment sorting processes and mixing in a tide- and storm-dominated setting

Sand-rich Holocene to modern clastic deposits in the eastern English Channel and the southern North Sea coasts of France and Belgium occur extensively as nearshore-sand bank, estuarine-tidal flat, aeolian dune and beach sub-environments. Sand samples (n = 665) collected from these deposits suggest the presence of three different populations: a largely dominant (83%) medium to fine quartz sand population (“b”), and finer- (14%) and coarser-grained (4%) populations (respectively “c” and “a”). The distribution of these populations among the four sub-environments reflects tide- and storm-dominated sorting and transport processes and a variable degree of mixing. These populations are derived from a mixture of very fine- to very coarse-grained fluvial, outwash and paraglacial sediments deposited on the beds of the eastern English Channel and southern North Sea during the late Pleistocene lowstand. The nearshore-sand bank environment, which also corresponds to the main offshore source area of the coastal deposits, exhibits population heterogeneity reflecting the variability of hydrodynamic conditions and sediment sorting in this zone. The nearshore topography of tidal ridges, banks and troughs in these tidal seas leads to variable bed and tide- and storm-induced shear stress conditions. These conditions only allow for the mobilisation and onshore transport of some of the finer fractions (populations “b” and “c”), leaving an offshore mixture of these finer populations with coarser, less mobilisable sediments (population “a”). Once in the coastal zone, these two finer populations undergo further hydrodynamic sorting and segregation. Variably sorted very fine sands to silts (population “c”) are trapped in the low-energy estuarine-tidal flat sub-environment, while the highly homogeneous population “b” is further sorted in aeolian dune and beach sub-environments. This sorting occurs via a coastal sand transport pathway linking the Somme estuary mouth to the southern North Sea bight where tidal range and wave energy decrease relative to the English Channel. Since this sand transport pathway enables longshore transport of hydrodynamically sorted medium to fine sand derived directly from the immediate nearshore zone, it has further contributed to a net flux of this sand population from the eastern English Channel sea bed to the southern North Sea.     

Pollution au plomb dans la Savoie antique (II–IIIe s. apr. J.-C.) en relation avec une installation métallurgique de la cité de Vienne

In Lake Anterne (NW French Alps), the Roman period is characterised by a lead contamination more important than that of the 1970s, due to the use of leaded gasoline. The maximum of this contamination period has been varve-dated 220 AD, i.e., during the optimum of Roman civilization in the Alps. Crossing palaeoenvironmental and archaeological evidences points out the economic importance of the town of Passy for the City of Vienne and yields a new example of lead contamination due to local metallurgical activity rather than distant impact of the exploitation of the Spanish Rio Tinto mine. To cite this article: F. Arnaud et al., C. R. Geoscience 338 (2005).