Distinctive C isotopic signature distinguishes a novel sea ice biomarker in Arctic sediments and sediment traps

A C₂₅ highly branched isoprenoid (HBI) monoene hydrocarbon, designated IP₂₅, has been proposed previously to originate from diatoms living in Arctic sea ice, while the presence of IP₂₅ in sediments has been suggested to be a proxy for the occurrence of former Arctic sea ice. Here, we show that the ¹³C isotopic composition of IP₂₅ in sea ice, in sediment trap material collected under sea ice, and in high latitude northern sediments, is distinctive (isotopically ‘heavy’) and distinguishable from that of organic matter of planktonic or terrigenous origin. Mean δ¹³C values for IP₂₅ were − 22.3 ± 0.4‰ (sea ice), − 19.6 ± 1.1‰ (sediment traps) and − 19.3 ± 2.3‰ (sediments). These measurements, therefore, support further the proposed use of IP₂₅ as an Arctic sea ice proxy.     

Proglacial sediment dynamics from daily to seasonal scales in a glaciated Alpine catchment (Bossons glacier,Mont Blanc massif,France)

The sediment yields of Alpine catchments are commonly determined from streamload measurements made some distance downstream from glaciers. However, this approach indiscriminately integrates erosion processes occurring in both the glacial and proglacial areas. A specific method is required to ascertain the respective inputs from (i) subglacial and supraglacial sediments, (ii) proglacial hillslopes and (iii) proglacial alluvial areas or sandurs. This issue is addressed here by combining high‐resolution monitoring (2 min) of suspended sediment concentrations at different locations within a catchment with discharge gauging and precipitation data. This methodological framework is applied to two proglacial streams draining the Bossons glacier (Mont Blanc massif, France): the Bossons and Crosette streams. For the Bossons stream, discharge and suspended load data were acquired from June to October 2013 at 1.15 and 1.5 km from the glacial terminus, respectively upstream and downstream from a small valley sandur. These hydro‐sedimentary data are compared with the Crosette stream dataset acquired at the outlet of the Bossons glacier subglacial drainage system. A fourfold analysis focusing on seasonal changes in streamload and discharge, multilinear regression modelling, evaluation of the sandur flux balance and probabilistic uncertainty assessment is used to determine the catchment sediment budget and to explain the proglacial sediment dynamics. The seasonal fluctuation of the sediment signal observed is related to the gradual closing of the subglacial drainage network and to the role of the proglacial area in the sediment cascade: the proglacial hillslopes appear to be disconnected from the main channel and the valley sandur acts as a hydrodynamic sediment buffer both daily and seasonally. Our findings show that an understanding of proglacial sediment dynamics can help in evaluating paraglacial adjustment and subglacial erosion processes. Copyright © 2017 John Wiley & Sons, Ltd.     

Active tectonics,seismicity and geomorphology with special reference to Normandy (France)

Integrated studies based on tectonic, seismotectonic and geomorphological analyses indicate that Normandy (northwest France) has been an active area during the Quaternary. Topography and landform discontinuities reflect the dislocation and differential uplift of a late Cenozoic platform. The tectonic activity is represented by (i) active faults, indicated by linear scarps and seismic activity, (ii) offsetting of pre‐existing surfaces, (iii) Plio‐Pleistocene sedimentation restricted within narrow subsiding zones, and (iv) morphometric properties of drainage basins that indicate zones of differential uplift. The inferred strain pattern involves (i) a shortening direction that strikes NW–SE as expected in the European context of Alpine compression, and (ii) a NE–SW trending extension accommodated by NW–SE normal faults. The geomorphological systems encountered in Normandy preferentially record differential vertical displacements. Copyright © 2000 John Wiley & Sons, Ltd.     

Models for convectively driven hydromagnetic dynamos

Abstract

Models of a convectively driven hydromagnetic dynamo are constructed using a truncated modal expansion. The resulting nonlinear partial differential equations are integrated numerically. The results confirm that rotation is a necessary condition for effective dynamo action, and suggest that equipartition of kinetic and magnetic energies is qualitatively valid, and that toroidal field energies can be much larger than poloidal.     

Tracking the sources and fate of nitrate pollution by combining hydrochemical and isotopic data with a statistical approach

This study contributes to identifying and spatializing the different types of nitrate sources by combining hydrogeochemical and isotopic data with principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE) multicriteria statistical methods. The methodology is applied to the strategic Mons Basin chalk aquifer (Belgium). The results are based on a whole dataset containing 72 water samples with analyses of the hydrogeochemical parameters (temperature, pH, electrical conductivity (EC), redox potential, dissolved O₂), alkalinity, total organic carbon (TOC), silica (SiO₂), major and minor ions (NO₃^–, NH⁴⁺, Ca²⁺, dissolved Fe and Mn, K⁺, Mg²⁺, Na⁺, Sr²⁺, Cl^–, F^–, SO₄^–_, B) and multiple stable isotope ratios (δ¹¹B, δ¹⁵N–NO₃^–, δ¹⁸O–NO₃^–). Compared to classical PCA, the recently developed t-SNE method, which considers nonlinear relationships between variables and preserves local-scale similarities in a low-dimensional space, showed much better performance in discriminating different groups of samples and related zones in the aquifer. t-SNE results combined with isotope ratios highlighted four zones in the aquifer (grouped as A–D) and the presence of denitrification fronts. Group A presents a manure signature (δ¹⁵N–NO₃^– – mean (μ) +12.78‰, standard deviation (σ) 6.48‰; δ¹¹B – μ 29.96‰, σ 6.91‰). Group B exhibits both manure and inorganic fertilizer signatures (δ¹⁵N–NO₃^– – μ 6.27‰, σ 2.55‰; δ¹¹B – μ 15.86‰, σ 9.69‰). Group C shows a contamination by sewage (δ¹⁵N–NO₃^– – μ 12.67‰, σ 5.60‰; δ¹¹B – μ 9.97‰, σ 7.08‰). Group D presents a mixed signature (δ¹⁵N–NO₃^– – μ 9.25‰, σ 2.94‰; δ¹¹B – μ 20.00‰, σ 6.70‰).

     

Optimization of Subsurface Flow Operations Using a Dynamic Proxy Strategy

Mathematical Geosciences - Machine-learning-based proxy models are often used to replace many of the flow simulations required in optimizations of subsurface flow processes. Because the optimizer...     

Sedimentary Processes and Depositional Characteristics of Coarse-grained Subaqueous Fans along Steep Slopes in a Lacustrine Rift Basin: A Case Study from the Dongying Depression, Bohai Bay Basin, China

Coarse-grained subaqueous fans are vital oil and gas exploration targets in the Bohai Bay basin, China. The insufficient understanding of their sedimentary processes, depositional patterns, and controlling factors restricts efficient exploration and development. Coarse-grained subaqueous fans in the Yong′an area, Dongying Depression, are investigated in this study. These fans include nearshore subaqueous fans, and sublacustrine fans, and their sedimentary processes, depositional patterns and distribution characteristics are mainly controlled by tectonic activity and paleogeomorphology. Nearshore subaqueous fans developed near the boundary fault during the early–middle deposition stage due to strong tectonic activity and large topographic subsidence. Early sublacustrine fans developed at the front of the nearshore subaqueous fans in the area where the topography changed from gentle to steep along the source direction. While the topography was gentle, sublacustrine fans did not develop. During the late weak tectonic activity stage, late sublacustrine fans developed with multiple stages superimposed. Frequent fault activity and related earthquakes steepened the basin margin, and the boundary fault slopes were 25.9°–34°. During the early–middle deposition stage, hyperpycnal flows triggered by outburst floods developed. During the late deposition stage, with weak tectonic activity, seasonal floods triggered hyperpycnal flows, and hybrid event beds developed distally.     

A case history of field pumping tests in a deep gravel formation in the Taipei Basin,Taiwan

33 large-diameter wells embedded in 2-m thick, 63-m deep diaphragm walls were constructed to reduce both the uplift pressures and the groundwater inflow during the excavations. As the actual thickness of the pumped aquifer is unknown, the installed wells are regarded as partial penetration wells. Single-well and multi-well pumping tests were conducted in the deep gravel formation of Taipei Basin to derive the hydraulic parameters and to investigate the drawdown characteristics at both the construction and remote sites. However, the tidal effect on the drawdown of both the pumping well and nearby observation wells was found significant. Additionally, wellbore storage, skin, and leakage need to be taken into account for deriving the hydraulic parameters. Hence, a method to remove these five factors influencing the drawdown curve is developed, which takes advantage from the late-time characteristics of drawdown data and the early-time behavior of drawdown. Some currently available semi-log graphic techniques are therefore proven applicable for parameter determination. Validity of the proposed method is verified by the good agreement between the calculated and the measured drawdown of both the pumping well and observation well.