Spatial patterns of ice storm disturbance on a forested landscape in the Appalachian Mountains, Virginia

Ice storms (major freezing rain events) periodically disturb forests in eastern North America. The damage may vary spatially, especially in complex terrain. This study uses satellite imagery to investigate spatial heterogeneity of forest damage caused by ice storms that affected the Appalachian Mountains, Virginia during 1994. The results display a region-scale (southwest-to-northeast) gradient in damage that apparently corresponds to a gradient in the depth of ice that accumulated during the storms. Damage also varied topographically, particularly by aspect. Damage was most extensive on east-, southeast- and south-facing slopes; at middle elevations; and on slopes of moderate steepness.     

Nature and Estimated Human Toxicity of Polar Metabolite Mixtures in Groundwater Quantified as TPHd/DRO at Biodegrading Fuel Release Sites

Polar metabolites resulting from petroleum biodegradation are measured in groundwater samples as TPHd unless a silica gel cleanup (SGC) is used on the sample extract to isolate hydrocarbons. Even though the metabolites can be the vast majority of the dissolved organics present in groundwater, SGC has been inconsistently applied because of regulatory concern about the nature and toxicity of the metabolites. A two‐step approach was used to identify polar compounds that were measured as TPHd in groundwater extracts at five sites with biodegrading fuel sources. First, gas chromatography with mass spectrometry (GC‐MS) was used to identify and quantify 57 individual target polar metabolites. Only one of these compounds—dodecanoic acid, which has low potential human toxicity—was detected. Second, nontargeted analysis was used to identify as many polar metabolites as possible using both GC‐MS and GC×GC‐MS. The nontargeted analysis revealed that the mixture of polar metabolites identified in groundwater source areas at these five sites is composed of approximately equal average percentages of organic acids, alcohols and ketones, with few phenols and aldehydes. The mixture identified in downgradient areas at these five sites is dominated by acids, with fewer alcohols, far fewer ketones, and very few aldehydes and phenols. A ranking system consistent with systems used by USEPA and the United Nations was developed for evaluating the potential chronic oral toxicity to humans of the different classes of identified polar metabolites. The vast majority of the identified polar metabolites have a “Low” toxicity profile, and the mixture of identified polar metabolites present in groundwater extracts at these five sites is unlikely to present a significant risk to human health.     

A late Holocene paleo-productivity record in the western Gulf of Maine,USA, inferred from growth histories of the long-lived ocean quahog (<Emphasis Type="Italic">Arctica islandica</Emphasis>)

To investigate environmental variability during the late Holocene in the western Gulf of Maine, USA, we collected a 142-year-old living bivalve (Arctica islandica) in 2004, and three fossil A. islandica shells of the Medieval Warm Period (MWP) and late MWP / Little Ice Age (LIA) period (corrected ¹⁴C_AMS = 1030 ± 78 ad; 1320 ± 45 ad; 1357 ± 40 ad) in 1996. We compared the growth record of the modern shell with continuous plankton recorder (CPR) time-series (1961–2003) from the Gulf of Maine. A significant correlation (r ² = 0.55; p < 0.0001) exists between the standardized annual growth index (SGI) of the modern shell and the relative abundance of zooplankton species Calanus finmarchicus. We therefore propose that SGI data from A. islandica is a valid proxy for paleo-productivity of at least one major zooplankton taxa. SGIs from these shells reveal significant periods of 2–6 years (NAO-like) based on wavelet analysis, multitaper method (MTM) analysis and singular spectrum analysis (SSA) during the late Holocene. Based on established physical oceanographic observation in the Gulf of Maine, we suggest that slope water variability coupled with North Atlantic Oscillation (NAO) dynamics is primarily responsible for the observed SGI variability. Special Issue: AGU OS06 special issue “Ocean’s role in climate change—a paleo perspective”.     

Small-scale spatial variations of natural radionuclide and trace metal distributions in sediments from the Hudson River estuary

Multiple sediment cores were collected in June 1994 in the turbidity maximum zone of the Hudson River estuary off Manhattan, New York. Results from X-radiography of the sediments and measurements of natural radionuclides (²³⁴Th,⁷Be, and²¹⁰Pb) and trace metals (Ag, Cd, Cu, Pb, and Zn) show significant spatial variability of sediment composition and structure and patchy distributions of radionuclides activities and trace metal concentrations in this small area (0.6 km × 0.5 km). Radionuclide and trace metal analyses confirm prior work (Olsen et al. 1978; Olsen et al. 1981; Hirschberg et al. 1996) that show the western margin area of the river acts as a repository of these chemical constituents at least for the short-term period (0.5–1 yr), and the mid-channel area is not a depositional area for sediments and associated chemical constituents.⁷Be profiles reveal short-term sediment deposition rates ranging from 6 cm yr^?1 to 26 cm yr^?1 in the western margin area. Significant spatial variations in excess²³⁴Th and⁷Be inventories (up to a factor of 10 and 5 for²³⁴Th and⁷Be, respectively) are found in the western margin depositional area, although the inventories are balanced, on average, with in situ production in water column and atmospheric supply. The spatial variation of surficial excess²¹⁰Pb and trace metal concentrations in depositional areas of the western margin are ≤10% for Ag, Cu, Pb, and Zn and 29% for Cd. However, the variations in the transition zone range from 28% to 93%. This variability is likely related to variations in tidal current velocity, bottom shear stress, and river channel morphology.     

High-resolution records of the response of coastal wetland systems to long-term and short-term sea-level variability

While it is well known that coastal systems respond to long-term sea-level changes, the importance of short-term sea-level dynamics is often overlooked. Year-to-year variability in annual mean sea level along the North American Atlantic coast is part of a regionally consistent pattern that is coupled to low atmospheric pressure and high wind field anomalies persisting over 100s to 1000s of km. These short-term sea-level dynamics, along with long-term sea-level changes are shown to be closely coupled to a set of high resolution excess ²¹⁰Pb geochronologies from four physiographically distinct salt marsh estuaries surrounding Long Island, NY, USA. However, the degree to which a marsh responds to either forcing depends on its physiographic setting. Accretion and mineral deposition rates in marshes situated in embayments with long fetches and low-tidal ranges are shown to respond most to the short-term dynamically driven changes in sea level. On the other hand, accretion and mineral deposition in a marsh in an embayment with a high-tidal range and reduced fetch best track the long-term changes in mean sea level, presumably because the physiography limited the meteorological drivers of short-term sea-level change. The close coupling between marsh accretion, physiographic setting and indices of sea-level change indicates that these coastal system respond both differently and rapidly (2–5 yr) to climate variability.     

Computing the steady oceanic circulation using an optimization approach

The traditional method for computing the steady oceanic circulation has been by stepping an oceanic model forward in time until transients are damped by friction. An alternative method, which has the potential for being more economical is to minimize the sum of the squares of the residuals of the steady model equations. A variety of algorithms might be considered for computing the minimum; attention here is focused on preconditioned conjugate-gradient descent with the gradient computed using an adjoint model. The choice of varibles, i.e. the preconditioning transformation used in the optimization process, is found to be critical to the efficiency of the method. An appropriate preconditioning transformation can be suggested by a heuristic analysis similar to that commonly used to test the stability of numerical models. The method is demonstrated within the context of the barotropic vorticity equation.