University of Maine

About: University of Maine is a education organization based out in Orono, Maine, United States. It is known for research contribution in the topics: Population & Ice sheet. The organization has 8637 authors who have published 16932 publications receiving 590124 citations. The organization is also known as: University of Maine at Orono.

Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica

Abstract: A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice. Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patterned-ground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feedback associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondary-ice lenses contrast markedly with glacial ice by lying on a δD versus δ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs. The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains >25% striated clasts in the >16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarse-grained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ± 0.08 Ma and 0.18 ± 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. Although Granite drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates. Nevertheless, these rates are orders of magnitude lower than those computed on theoretical grounds. During eruptions of the nearby McMurdo Group volcanic centers, ash-fall debris collects at the surface of Granite drift, either in open thermal-contraction cracks or in deep troughs that lie above contraction cracks; these deposits subsequently lower passively as the underlying glacier ice sublimes. The fact that some regions of Granite drift have escaped modification by patterned ground for at least 8.1 Ma indicates long-term geomorphic stability of individual polygons. Once established, polygon toughs likely persist for as long as 10 5 –10 6 yr. Our model of patterned-ground formation, which applies to the hyperarid, cold-desert, polar climate of Antarctica, may also apply to similar-sized polygons on Mars that occur over buried ice in Utopia Planitia.

Dissolved Organic Matter Characterization Using Multiway Spectral Decomposition of Fluorescence Landscapes

Abstract: Dissolved organic matter (DOM) plays an important role in many soil ecosystem functions. Multidimensional fluorescence spectroscopy of DOM with parallel factor analysis (PARAFAC) of the resulting spectral landscape has been successful in characterizing DOM from a variety of aquatic sources. This study was conducted to assess the multiway PARAFAC approach for quantitatively characterizing the fluorescent landscapes of DOM from aqueous extracts of soils and soil amendments. The DOM was extracted from plant biomass representative of crop, wetlands, and tree species; animal manures; and soils from controlled studies of cropping systems with known histories of organic amendments. The fluorescence landscape spectra were collected in the excitation range from 240 to 400 nm and emission range from 300 to 500 nm in 3-nm increments. The excitation and emission spectra modeled from the PARAFAC analysis showed that the plant biomass, animal manure, and soil DOM contained five fluoresdng components: tryptophan-like (peak location at excitation 270 nm, emission 354 nm), tyrosine-like (273/309 nm), and three humic-substance-like components (>240/465 nm, 306/405 nm, and 315/447 nm). Principal component analysis of the concentration loading showed that the soil-derived DOM was very similar despite the different types and quantities of organic amendments incorporated in the different cropping systems. This study shows that PARAFAC analysis of multidimensional fluorescence spectra can model the chemical profile of terrestrial DOM in a chemically meaningful way. This represents a significant advance over current approaches to interpreting the complex DOM fluorescence spectra.

How severe is sperm limitation in natural populations of marine free-spawners?

Abstract: Successful fertilization in marine organisms that release sperm into seawater is potentially limited by the rapid dilution of gametes; cases of severe sperm limitation have been demonstrated in nature. However, recent surveys of naturally spawning populations indicate fairly high fertilization levels in many taxa. The extreme selection exerted by sperm limitation has resulted in numerous adaptations to reduce sperm limitation and enhance fertilization. Thus, most taxa show indications of the evolutionary consequences of sperm limitation even when population level, ecological effects are minimal.

Assessment of skill and portability in regional marine biogeochemical models : Role of multiple planktonic groups

Abstract: [1] Application of biogeochemical models to the study of marine ecosystems is pervasive, yet objective quantification of these models’ performance is rare. Here, 12 lower trophic level models of varying complexity are objectively assessed in two distinct regions (equatorial Pacific and Arabian Sea). Each model was run within an identical onedimensional physical framework. A consistent variational adjoint implementation assimilating chlorophyll-a, nitrate, export, and primary productivity was applied and the same metrics were used to assess model skill. Experiments were performed in which data were assimilated from each site individually and from both sites simultaneously. A cross-validation experiment was also conducted whereby data were assimilated from one site and the resulting optimal parameters were used to generate a simulation for the second site. When a single pelagic regime is considered, the simplest models fit the data as well as those with multiple phytoplankton functional groups. However, those with multiple phytoplankton functional groups produced lower misfits when the models are required to simulate both regimes using identical parameter values. The cross-validation experiments revealed that as long as only a few key biogeochemical parameters were optimized, the models with greater phytoplankton complexity were generally more portable. Furthermore, models with multiple zooplankton compartments did not necessarily outperform models with single zooplankton compartments, even when zooplankton biomass data are assimilated. Finally, even when different models produced similar least squares model-data misfits, they often did so via very different element flow pathways, highlighting the need for more comprehensive data sets that uniquely constrain these pathways.