05-03 "Securing Social Security: Sensitivity to Economic Assumptions and Analysis of Policy Options"

Revamping the Social Security program has become a domestic policy priority of the Bush administration. The President has stated that the system is facing a “crisis” and will be “bankrupt” in 2041. His proposal to change Social Security is centered on the introduction of private accounts that would allow workers to direct a share of their Social Security taxes into investments such as stocks and bonds. In this paper we consider whether Social Security is really facing a crisis and whether any potential future shortfalls could be remedied without changing the basic structure of the existing program.

Late Quaternary Mediterranean Outflow Water: implications from radiogenic Nd, Sr, Pb isotopes and clay minerals

Mediterranean Outflow Water (MOW) is characterised by higher temperatures and salinities than other ambient water masses. MOW spreads at water depths between 500 and 1500 m into the eastern North Atlantic and has been a source of salinity for the Atlantic Meridional Overturning Circulation. We used high-resolution Nd and Pb isotope records of past ambient seawater obtained from authigenic ferromanganese coatings of sediments in three gravity cores at 577, 1745 and 1974 m water depths in the Gulf of Cadiz and along the Portuguese margin complemented by a selection of surface sediments to reconstruct the extent and pathways of MOW over the past 23 000 years. In addition, radiogenic Nd, Pb and Sr isotope ratios obtained from total digestion of the residual clay fraction of the leached samples were used to evaluate any changes in the endmember compositions. The surface and downcore seawater Nd isotope data from all water depths exhibit only a very small variability close to the present day composition of MOW but do not reflect the present day Nd isotopic stratification of the water column as determined from a nearby open ocean hydrographic station, which is most likely the consequence of downslope sediment transport in the nepheloid boundary layer as well as the small variations in the Nd endmember compositions. In contrast, the seawater Pb isotope records show significant and systematic variations, which provide evidence for a significantly different pattern of the MOW pathways between 20 000 and 12 000 years ago compared with the subsequent period of time. A deeper situated MOW during the Last Glacial Maximum (LGM) raised during the early deglaciation with its shallowest position around Heinrich event H1, followed by a moderate deepening and the establishment of present-day MOW hydrography. The radiogenic isotope signatures of the residual clay fractions document a pulse of sediment input from the north during Heinrich event H1 around 14.8 ka, but other than that exhibit little varibility over time suggesting surprisingly constant sedimentary endmember compositions and mixing ratios since the LGM

A Survey of Agent-Based Modeling Practices (January 1998 to July 2008)

In the 1990s, Agent-Based Modeling (ABM) began gaining popularity and represents a departure from the more classical simulation approaches. This departure, its recent development and its increasing application by non-traditional simulation disciplines indicates the need to continuously assess the current state of ABM and identify opportunities for improvement. To begin to satisfy this need, we surveyed and collected data from 279 articles from 92 unique publication outlets in which the authors had constructed and analyzed an agent-based model. From this large data set we establish the current practice of ABM in terms of year of publication, field of study, simulation software used, purpose of the simulation, acceptable validation criteria, validation techniques and complete description of the simulation. Based on the current practice we discuss six improvements needed to advance ABM as an analysis tool. These improvements include the development of ABM specific tools that are independent of software, the development of ABM as an independent discipline with a common language that extends across domains, the establishment of expectations for ABM that match their intended purposes, the requirement of complete descriptions of the simulation so others can independently replicate the results, the requirement that all models be completely validated and the development and application of statistical and non-statistical validation techniques specifically for ABM.Agent-Based Modeling, Survey, Current Practices, Simulation Validation, Simulation Purpose

The Case of Canoe Brook Wells in Glacial Sand and Gravel, Livingston, New Jersey : Establishing the Hydraulic Signs of Well Contamination Risk Using Topography

When little groundwater level data is available, the potential energy gradients reflected in the topography, assumed to be saturated to the surface, can be used to estimate the directions and relative rates of groundwater flow (flow systems). Darcy’s law conveniently relates groundwater water levels to the rate of groundwater flow. Toth (1963) related topography to groundwater flow systems. Flow systems can transport surface contamination, if present, to wells. The topography and likely contamination point source surface locations were used to create and contrast the flow systems impacting two wells in order to assess contamination risk. The two wells, Canoe Brook well no. 1 (CB-1) and Canoe Brook no. 3 (CB-3) are in the East Orange Water Reserve (EOWR). Only well CB-3 has high chloride ion concentrations. Both wells are completed in the EOWR’s sand and gravel aquifer, which is overlain by low permeability clay-rich till and underlain by a fractured bedrock aquifer (Towaco and Preakness Basalt Formations). The distribution of groundwater flow (flow systems), well capture zones and recharge zones were characterized using MODFLOW. Features of the topography, geology, flow systems, capture zones and recharge zones which might relate to contamination risk for each well were compared. The Canoe Brook well field was conceptualized as a 10,667 ft. by 8,888 ft. by 400 ft. deep drainage basin- with the Livingston half-basin on Canoe Brook’s west bank and the Millbum-South Mountain half-basin on its east bank. Maximum elevations in the Millbum-South Mountain halfbasin were double those in the Livingston half-basin. The domain’s three layers were represented using hydraulic conductivities typical for silty clay, sandy gravel and fractured bedrock, respectively. Consistent with Tôth s (1963) findings, high magnitude relief generated deeper surface-influenced groundwater flow (local flow systems). Well CB-3 was in high relief topography with low overall basin slope. The local flow systems were the dominant flow systems at CB-3. More relief features and lower basin slope meant more recharge starting points on the surface area, which caused more drainage over a wider area. There was no deep groundwater flow at CB-3. Well capture zones show the recharge starting points and flow path of the majority of the groundwater supplying a well. The well CB-3 capture zone directly received bulk transport from scattered recharges resulting in a capture zone with less integrity. Many recharges are overlain by roads. In addition, chloride dispersion downward could occur most easily into the CB-3 capture zone based on its shallow subsurface position, long length and flat shape. Simulated aquifer recharge was occurring directly above CB-3; chloride could disperse into the well itself. Geology also predisposed well CB-3 to contamination. The protective clay-rich till layer at CB-3 was the thinnest. The hydraulic and geological features of the well CB-1 were opposite to CB-3. Topographically, the basin slope was high and relief was low. The groundwater flow at CB-1 was marked by a deep intermediate flow system. The CB-1 capture zone had no recharges under roads. Its structure was deep, squat and fast flowing. No aquifer recharge occurred above the well. The protective clay layer at CB-1 was the thickest. In light of Toth’s topography-flow system relationships and Darcy’s law, the results of the simulation predict the long term impact of the development over regional recharges. With the loss of regional recharges, regional flow systems are replaced by local flow systems. High relief areas become more strongly influenced by surface-associated local flow systems. As a result, with regional development, high relief areas may become more prone to contamination from the surface

How does insect resistance to phosphine affect insect control costs of stored-grain?

Crop Production/Industries,

Plio-Pleistocene changes in water mass exchange and erosional inputs in the Fram Strait

We determined the isotopic composition of neodymium (Nd) and lead (Pb) of past seawater to reconstruct water mass exchange and erosional input between the Arctic Ocean and the Norwegian-Greenland Seas over the past 5 Ma. For this purpose, sediments of ODP site 911 (leg 151) located at 900 m water depth on the Yermak Plateau in the Fram Strait were used. The paleo-seawater variability of Nd and Pb isotopes was extracted from the sea water-derived metal oxide coatings on the sediment particles following the leaching method of Gutjahr et al. (2007). All radiogenic isotope data were acquired by Multi-Collector (MC) ICP-MS. The site 911 stratigraphy of Knies et al. (2009) was applied. Surface sediment Sr and Nd isotope data, as well as downcore Sr isotope data obtained on the same leaches are close to seawater and confirm the seawater origin of the Nd and Pb isotope signatures. The deep water Nd isotope time series extracted from site 911 was in general more radiogenic ("Nd = -7.5 to -10) than present day deep water ("Nd = -9.8 to -11.8) in the area of the Fram Strait (Andersson et al., 2008) and does not show a systematic trend with time. In contrast, the radiogenic isotope composition of Pb evolved from 206Pb/204Pb ratios around 18.7 to more radiogenic values around 19.2 between 2 Ma and today. The data indicate that mixing of water masses from the Arctic Ocean and the Norwegian-Greenland Seas has controlled the Nd isotope signatures of deep waters on the Yermak Plateau over the past 5 Ma. Prior to 1.7 Ma the Nd isotope signatures on the Yermak Plateau were less radiogenic than waters from the same depth in the central Arctic Ocean (Haley et al., 2008) pointing to a greater influence from the Norwegian-Greenland Seas. After 1.7 Ma the central Arctic and Yermak Plateau data have varied around similar values indicating water mass mixing overall similar to today. In contrast, the Pb isotope composition of deep waters in the Fram Strait appears to have been dominated by weathering inputs from glacially weathering old continental landmasses, such as Greenland or parts of Svalbard since 2 Ma. A similar control over the Pb isotope evolution of seawater since the onset of Northern Hemisphere Glaciation was recorded by ferromanganese crusts that grew from North Atlantic DeepWater in the western North Atlantic. References: Gutjahr, M., Frank, M., Stirling, C.H., Klemm, V., van de Flierdt, T. and Halliday, A.N. (2007): Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments.- Chemical Geology 242, 351-370 Haley B. A., M. Frank, R.F. Spielhagen and A. Eisenhauer (2008): Influence of brine formation on Arctic Ocean circulation over the past 15 million years. Nature Geoscience 1, 68–72 Andersson, P.S., Porcelli, D., Frank, M., Björk, G., Dahlqvist, R. and Gustafsson, Ö. (2008): Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic- Atlantic gateways.- Geochim. Cosmochim. Acta 72, 2854-2867 Knies, J., J. Matthiessen, C. Vogt, J.S. Laberg, B.O. Hjelstuen, M.Smelror, E. Larsen, K. Andreassen, T. Eidvin and T.O. Vorren (2009): The Plio-Pleistocene glaciation of the Barents Sea–Svalbard region: a new model based on revised chronostratigraphy - Quaternary Science Reviews 28, 9-10, 812-82