Late Holocene Baltic Sea outflow changes reconstructed using C37:4 content from marine cores

The Baltic Sea is an intra‐continental brackish water body. Low saline surface water, the so‐called Baltic outflow current, exits the Baltic Sea through the Kattegat into the Skagerrak. Ingressions of saline oxygen‐rich bottom water enter the Baltic Sea basins via the narrow and shallow Kattegat and are of great importance for the ecological and ventilation state of the Baltic Sea. Over recent decades, progress has been made in studying Holocene changes in saline water inflow. However, reconstructions of past variations in Baltic Sea outflow changes are sparse and hampered because of the lack of suitable proxies. Here, we used the relative proportion of tetra‐unsaturated C₃₇ ketones (C_37:4 %) in long‐chain alkenones produced by coccolithophorids as a proxy for outflowing Baltic Sea water in the Skagerrak. To evaluate the applicability of the proxy, we compared the biomarker results with grain‐size records from the Kattegat and Mecklenburg Bay in addition to previously published salinity reconstructions from the Kattegat over the last 5000 years. All Skagerrak records showed an increase in C_37:4 % that is accompanied by enhanced bottom water currents in the Kattegat and western Baltic Sea over the past 3500 cal. a BP, indicating an increase in Baltic Sea outflow. This probably reflects higher precipitation in the Baltic Sea catchment area owing to a re‐organization of North Atlantic atmospheric circulation with an increased influence of wintertime Westerlies over the Baltic catchment from the mid‐ to the late Holocene.     

210Po in Nevada Groundwater and Its Relation to Gross Alpha Radioactivity

Polonium‐210 (²¹⁰Po) is a highly toxic alpha emitter that is rarely found in groundwater at activities exceeding 1 pCi/L. ²¹⁰Po activities in 63 domestic and public‐supply wells in Lahontan Valley in Churchill County in northern Nevada, United States, ranged from 0.01 ± 0.005 to 178 ± 16 pCi/L with a median activity of 2.88 pCi/L. Wells with high ²¹⁰Po activities had low dissolved oxygen concentrations (less than 0.1 mg/L) and commonly had pH greater than 9. Lead‐210 activities are low and aqueous ²¹⁰Po is unsupported by ²¹⁰Pb, indicating that the ²¹⁰Po is mobilized from aquifer sediments. The only significant contributors to alpha particle activity in Lahontan Valley groundwater are ^234/238U, ²²²Rn, and ²¹⁰Po. Radon‐222 activities were below 1000 pCi/L and were uncorrelated with ²¹⁰Po activity. The only applicable drinking water standard for ²¹⁰Po in the United States is the adjusted gross alpha radioactivity (GAR) standard of 15 pCi/L. ²¹⁰Po was not volatile in a Nevada well, but volatile ²¹⁰Po has been reported in a Florida well. Additional information on the volatility of ²¹⁰Po is needed because GAR is an inappropriate method to screen for volatile radionuclides. About 25% of the samples had ²¹⁰Po activities that exceed the level associated with a lifetime total cancer risk of 1× 10^?4 (1.1 pCi/L) without exceeding the GAR standard. In cases where the 72‐h GAR exceeds the uranium activity by more than 5 to 10 pCi/L, an analysis to rule out the presence of ²¹⁰Po may be justified to protect human health even though the maximum contaminant level for adjusted GAR is not exceeded.     

A Barometric Survey of Dust-Devil Vortices on a Desert Playa

Dust devils, and other columnar vortices, are associated with local surface pressure drops that can be observed in time-series data on both Earth and Mars. High cadence measurements are needed to resolve these small structures, and we report a month-long survey (June/July 2012) on a Nevada desert playa using microbarographs sampled multiple times per second. Candidate dust-devil signatures are classified, with detections being robust at about one per day for pressure drops exceeding 0.3 hPa (roughly a 5:1 signal-to-noise threshold, where the observed noise level corresponds reasonably well with the dynamic pressure associated with the estimate convective velocity scale). The vortex population is evaluated and compared with those observed on Mars: a broken power law or a more convex distribution describes the terrestrial data. A single station observes about three events per week (for normalized pressure drops of 0.06 %), about three times fewer than Mars observations for the same normalized drop. We find evidence for clustering of vortex events in a pseudo-periodic manner with a 20-min period, consistent with the size of boundary-layer convection cells.     

The DRASTIC-Sg model: an extension to the DRASTIC approach for mapping groundwater vulnerability in aquifers subject to differential land subsidence,with application to Mexico City

Mexico City relies on groundwater for most of its domestic supply. Over the years, intensive pumping has led to significant drawdowns and, subsequently, to severe land subsidence. Tensile cracks have also developed or reactivated as a result. All such processes cause damage to urban infrastructure, increasing the risk of spills and favoring contaminant propagation into the aquifer. The effects of ground deformation are frequently ignored in groundwater vulnerability studies, but can be relevant in subsiding cities. This report presents an extension to the DRASTIC methodology, named DRASTIC-Sg, which focuses on evaluating groundwater vulnerability in urban aquifers affected by differential subsidence. A subsidence parameter is developed to represent the ground deformation gradient (Sg), and then used to depict areas where damage risk to urban infrastructure is higher due to fracture propagation. Space-geodetic SqueeSAR data and global positioning system (GPS) validation were used to evaluate subsidence rates and gradients, integrating hydrogeological and geomechanical variables into a GIS environment. Results show that classic DRASTIC approaches may underestimate groundwater vulnerability in settings such as the one at hand. Hence, it is concluded that the Sg parameter is a welcome contribution to develop reliable vulnerability assessments in subsiding basins.     

What controls the star formation rate in galaxies?

Summary of the third discussion of the conference. Topics covered include the mechanisms controlling the star formation rate ingalaxies, the source of the driving energy for interstellar turbulence, and the reasons for different star formation rates ingalaxies with similar masses. This revised version was published online in September 2006 with corrections to the Cover Date.