Seasonal Alternation Between Groundwater Discharge and Benthic Coupling as Nutrient Sources in a Shallow Coastal Lagoon

Submarine groundwater discharge (SGD) is now recognized as an important source of nutrients and freshwater to some coastal environments. We studied a shallow coastal lagoon (Little Lagoon, AL, USA) in the northern Gulf of Mexico that lacks riverine inputs but has been suspected to receive significant SGD. We observed persistent salinity gradients between the east and west ends of the lagoon and the pass connecting it to the Gulf of Mexico. Covariance between salinity in the lagoon and the groundwater tracer ²²²Rn indicated that SGD was responsible for the salinity gradients and is the primary source of freshwater to the lagoon. Cluster analysis of 246 biweekly samples based on temperature, salinity, and two proxies of SGD revealed two hydrographic regimes with different drivers for nutrient inputs. In samples characterized by high discharge and low temperatures (generally December–April), total nitrogen (TN) was negatively correlated with salinity, while total phosphorus (TP) was positively correlated with temperature. Total nitrogen in the groundwater was very high (0.36–4.80 mM) while total phosphorus was relatively low (0.3–2.3 μM), consistent with SGD as the source of TN during the high-discharge periods. In periods with low discharge and higher temperatures (approx. May–November), TN and TP had strong positive correlations with temperature and are inferred to originate from benthic efflux. Seasonal changes in nutrient stoichiometry in the lagoon water column also indicate an alternation between low TN/TP sediments and high TN/TP groundwater as the primary sources of nitrogen in this system.     

Experimental characterization,design and modelling of the RBRL seismic‐isolation system for lightweight structures

The Rolling‐Ball Rubber‐Layer (RBRL) system was developed to enable seismic isolation of lightweight structures, such as special equipment or works of art, and is very versatile, a great range of equivalent natural frequencies and coefficients of damping being achievable through choice of the system parameters. The necessity to have a simple and effective design procedure has led to a new parametric experimentation at Tun Abdul Razak Research Centre (TARRC) on the rolling behaviour of the RBRL system and load–deflection behaviour of the recentering springs. The experimental results, together with theories for the rolling resistance of a loaded steel ball on a thin rubber layer and the lateral load–deflection behaviour of cylindrical rubber springs, are used to develop a general design method for the RBRL system, which allows the system to be tailored to the specific application. Sinusoidal test results are presented for the small‐deflection behaviour of the system, influenced by the presence of a viscoelastic depression on the rubber tracks beneath each ball, and an amplitude‐dependent time‐domain model is proposed, based on these results and on the steady‐state behaviour of the system. The model is validated through comparison with previously performed shaking‐table tests. Attention is here restricted to uniaxial behaviour. Copyright © 2016 John Wiley & Sons, Ltd.     

Accelerated sediment delivery to continental margins during post-orogenic rebound of mountain ranges

Changes in sediment flux to continental margins are commonly interpreted in terms of tectonic growth of topography or climatic change. Here, we show that variations in sediment yield from orogenic systems, previously considered as resulting from climate change, drainage reorganisation or mantle processes can be explained by intrinsic mechanisms of mountain belt/foreland basin systems naturally evolving during post-orogenic decay. Numerical modelling indicates an increase of sediment flux leaving the orogenic system synchronous with the cessation of deposition in the foreland basin and the transition from late syn- to post-orogenesis. Experiments highlight the importance of lithospheric flexure that causes the post-orogenic isostatic rebound of the foreland basin. Erosion of the rebounding foreland basin combined with continued sediment flux from the thrust wedge drives an acceleration in sediment outflux towards continental margins. Sediment budget records in natural settings such as the Northern Pyrenees or Western European Alps also indicate accelerated post-orogenic sediment delivery to the Bay of Biscay and Rhône Delta respectively. These intrinsic processes that determine sediment yield to continental margins must be accounted for prior to consideration of additional external tectonic or climatic controls.