Bacterial reworking of terrigenous and marine organic matter in estuarine water columns and sediments

Amino acids and the bacterial biomarkers muramic acid and d-amino acids were quantified in the ultrafiltered dissolved, particulate and sedimentary organic matter (UDOM, POM and SOM) of the St. Lawrence system (Canada). The main objectives were to better describe the fate of terrigenous and marine organic matter (OM) in coastal zones and to quantify the bacterial contributions to OM composition and diagenesis. Regardless of their origin, the carbon (C) content of the particles substantially decreased with depth, especially near the water-sediment interface. Major diagenetic transformations of organic nitrogen (N) were revealed and important differences were observed between terrigenous and marine OM. Amino acid contents of particles decreased by 66-93% with depth and accounted for 12-30% of the particulate C losses in marine locations. These percentages were respectively 18-56% and 7-11% in the Saguenay Fjord where terrigenous input is important. A preferential removal of particulate N and amino acids with depth or during transport was measured, but only in marine locations and for N-rich particles. This leads to very low amino acid yields in deep marine POM. However, these yields then increased to a level up to three times higher after deposition on sediments, where SOM showed lower C:N ratios than deep POM. The associated increase of bacterial biomarker yields suggests an active in situ resynthesis of amino acids by benthic bacteria. The N content of the substrate most likely determines whether a preferential degradation or an enrichment of N and amino acid are observed. For N-poor OM, such as terrigenous or deep marine POM, the incorporation of exogenous N by attached bacteria can be measured, while the organic N is preferentially used or degraded in N-rich OM. Compared to the POM from the same water samples, the extracted UDOM was poor in N and amino acids and appeared to be mostly made of altered plant and bacterial fragments. Signs of in situ marine production of UDOM were observed in the most marine location. The POM entering the St. Lawrence Upper Estuary and the Saguenay Fjord appeared made of relatively fresh vascular plant OM mixed with highly altered bacterial debris from soils. In contrast, the POM samples from the more marine sites appeared mostly made of fresh planktonic and bacterial OM, although they were rapidly degraded during sinking. Based on biomarker yields, bacterial OM represented on average ∼20% of bulk C and approximately 40-70% of bulk N in POM and SOM, with the exception of deep marine POM exhibiting approximately two times lower bacterial contributions.     

Use of Large-Eddy Simulation for the bed shear stress estimation over a dune

Environmental flows are generally characterized by complex bed morphology and high current speeds. Such configurations favor the formation of vortex structures that strongly affect hydrody-namics and sediment transport. Large-Eddy Simulation (LES) enables investigation of the dynam-ics of the largest turbulence scales and, thanks to enhanced calculation resources, has now become applicable for simulating environmental flows. In this paper, a LES approach is developed in a CFD code (TELEMAC-3D), which was originally developed to simulate free surface flows using RANS methods. The present developments involve implementing subgrid models, boundary con-ditions and numerical schemes suitable for LES. The LES version of TELEMAC-3D was validated by comparing results on the model with experimental data for flow past a cylinder. Then, the model was applied to a test case representing flow over dunes. After validating the hydrodynamics, the model was used to assess the bottom shear stress, using both a RANS and a LES approach. Com-parison highlighted the potential contribution of LES to investigating the hydrodynamic forces acting on the bottom.     

Environmental Instability and Demographic Variability in Acrocnida brachiata (Echinodermata: Ophiuroidea) in Douarnenez Bay (Brittany: France)

Abstract. The recruitment and growth modalities of two populations of Acrocnida brachiata (Echinodermata: Ophiuroidea) from the intertidal and from a depth of 20 m in Douarnenez Bay (Brittany) were compared. This study was conducted in order to establish whether the populations developed differently according to environmental stability. The results were compared with those found in another amphiurid, Amphiura filiformis , distributed along a similar gradient of disturbance in the subtidal zone. The purpose was to determine whether the biological parameters from different species vary in a similar manner along such a gradient and. if not, what is the role of biotic and physical factors in the control of demographic structure in intertidal and subtidal populations.     

Cross dating (Th/U-C) of calcite covering prehistoric paintings in Borneo

We present the first application of cross-dating (Th/U measured by thermo-ionization mass spectrometry (TIMS) and ¹⁴C measured by accelerator mass spectrometry (AMS)) of calcite covering prehistoric paintings. Th/U age estimates of cave drapery range from 9800 to 27,300 yr B.P. while conventional ¹⁴C age is estimated between 9900 and 7610 yr B.P. depending on the dead carbon correction. The age discrepancy is attributed to a disturbance of Th/U and/or ¹⁴C geochemical systems, showing the limits of the geochronological approach applied to this kind of material. For the Th/U system, the poor consistency of U data (U content, ²³⁴U/²³⁸U activity ratios) and apparent ages argue for open system conditions. For ¹⁴C system, variation of the dead carbon fraction (dcf) and a possible mixing of successive generations of calcite could account for age discrepancy. Nevertheless, one sample shows concordant ages for the two methods. Compatible ages through corrections for open system conditions are assumed for other samples. Then, the cross-dating suggests 9900 yr as the minimum age of the piece of drapery; the underlying painting must be older. This study of rock art demonstrates the presence of a Pleistocene population before 9900 yr in the southeast of Borneo, whereas previously the only population in evidence in this area was of Austronesian type from ∼5000 to 6000 yrs ago.