Fresh ground water entering an estuary can be mixed with salt water in the upper few decimeters of the sediment. As a result, net measured discharge rates at the sediment‐water interface can be equal to the volume discharge of fresh ground water, although the salinity of the escaping water is high. Benthic chambers, vented to a collection bag, were used to measure specific volume discharge rates over ∼5 cm/day near the shoreline of a wide, shallow lagoon—Great South Bay, New York—situated at the surface of a coastal plain aquifer. These rates decreased to 1.5 cm/day at a distance of 100 m from shore. Although the flow rate is expected to be inversely correlated to the tidal water level elevation, and has been shown to be so modulated at other sites, no consistent variation in discharge with tidal phase was found; the tidal range (25 cm) may have been too small to have a measurable effect. Water collected at a particular location freshened over time from 30 to 23 ppt in 12 hours. Piezometers recorded vertical hydraulic gradients (at ambient salinity) between 0.08 and 0.02 in the upper meter of the sediment and the vertical hydraulic conductivity was measured by a falling head test to be between 1 and 20 m/day. The discharge rates calculated by Darcy's law decreased offshore from dozens of cm/day within 10 m of shore to 1.8 cm/day at a distance of 100 m, consistent with measured seepage rates. Conductivity measurements imply that pore water salinity decreases from ambient bay values at the sea floor to near fresh water values at a depth of 0.6 m into the sediment. The vertical downward dispersion coefficient for salt was estimated to be 0.02 m2/day. This could be due to the superposition of water wave‐induced dispersion in the sediment with gravitational convection (salt fingering).

No abstract

[1] The northern scarp of the Western Blanco Transform (BT) Fault Zone provides a ''tectonic window'' into crust generated at an intermediate-rate spreading center, exposing a 2000 m vertical section of lavas and dikes. The lava unit was sampled by submersible during the Blancovin dive program in 1995, recovering a total of 61 samples over vertical distances of 1000 m and a lateral extent of 13 km. Major element analyses of 40 whole rock samples exhibit typical tholeiitic fractionation trends of increasing FeO*, Na 2 O, and TiO 2 and decreasing Al 2 O 3 and CaO with decreasing MgO. The lava suite shows a considerable range in extent of crystallization, including primitive samples (Mg# 64) and evolved FeTi basalts (FeO > 12%; TiO 2 > 2%). On the basis of rare earth element and trace element data, all of the lavas are incompatible-element depleted normal mid-ocean ridge basalts (N-MORB; La/Sm N < 1). The geochemical systematics suggest that the lavas were derived from a slightly heterogeneous mantle source, and crystallization occurred in a magmatic regime of relatively low magma flux and/or high cooling rate, consistent with magmatic processes occurring along the present-day southern Cleft Segment. The BT scarp reveals the oceanic crust in two-dimensional space, allowing us to explore temporal and spatial relationships in the horizontal and vertical directions. As a whole, the data do not appear to form regular spatial trends; rather, primitive lavas tend to cluster shallower and toward the center of the study area, while more evolved lavas are present deeper and toward the west and east. Considered within a model for construction of the upper crust, these findings suggest that the upper lavas along the BT scarp may have been emplaced off-axis, either by extensive off-axis flow or off-axis eruption, while the lower lavas represent axial flows that have subsided with time. A calculation based on an isochron model for construction of the upper crust suggests that the Cleft Segment requires at least 50 kyr to build the lower extrusive section, consistent to first order with independent estimates for the construction of intermediatespreading rate crust.

[1] The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated $3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two $8 km 2 study areas. Most of the PDR samples are incompatible-element-depleted normal mid-ocean ridge basalts (NMORB; (La/Sm) N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb) N ) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207 Pb/ 204 Pb, suggesting that an enriched component of the mantle was present in this region in the past $3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206 Pb/ 204 Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of $1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers.

There is little argument about the merits of undergraduate research, but it can seem like a complex, resource‐intensive endeavor [e.g., Laursen et al., 2010; Lopatto, 2009; Hunter et al., 2006]. Although mentored undergraduate research can be challenging, the authors of this feature have found that research programs are strengthened when students and faculty collaborate to build new knowledge. Faculty members in the geology department at The College of Wooster have conducted mentored undergraduate research with their students for more than 60 years and have developed a highly effective program that enhances the teaching, scholarship, and research of our faculty and provides life‐changing experiences for our students. Other colleges and universities have also implemented successful mentored undergraduate research programs in the geosciences. For instance, the 18 Keck Geology Consortium schools (http://keckgeology.org/), Princeton University, and other institutions have been recognized for their senior capstone experiences by U.S. News & World Report.