John W. Jenson

DEGLACIATION OF A SOFT-BEDDED LAURENTIDE ICE SHEET

We present a series of numerical reconstructions of the Laurentide Ice Sheet during the last deglaciation (18–7 14C ka) that evaluates the sensitivity of ice-sheet geometry to subglacial sediment deformation. These reconstructions assume that the Laurentide Ice Sheet flowed over extensive areas of water-saturated, deforming sediment (soft beds) corresponding to the St. Lawrence lowland, the Great Lakes region, the western prairies of the U.S. and Canada, and the Hudson Bay and Hudson Strait regions. Sediment rheology is based on a constitutive law that incorporates experimental results from late Wisconsin till deposited by the Laurentide Ice Sheet which suggest only mildly nonlinear viscoplastic behavior. By varying the effective viscosity of till, we produced four reconstructions for the ice sheet during the last glacial maximum 18 14C ka, and two reconstructions each of the ice sheet at 14, 13, 12, 11 and 10 14C ka. We also produced one reconstruction for 9, 8.4, 8, and 7 14C ka. Reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl-shaped depression over Hudson Bay and thin ice (<1000 m above modern sea-level) over the western and southern margins. Those reconstructions that assume a higher effective viscosity of till in the Hudson Bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over Hudson Bay and a more northerly position of the central ice divide. These two different geometries may represent ice-sheet orographic changes associated with a Heinrich event. Further increases in effective viscosity of till, approaching the effective viscosity of ice, would result in a high, monolithic ice dome centered over Hudson Bay, reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology. Modeled ice-surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds. Our reconstructions with higher effective till viscosities in Hudson Bay also agree with the ICE-4G reconstructions (Peltier, 1994), which are based on inversion of relative sea-level data, for the early part of the last deglaciation (18–13 14C ka), but then depart significantly from ICE-4G beginning at about 12 14C ka due to differing assumptions of the history of deglaciation. Modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50–55 m by a soft-bedded Laurentide Ice Sheet; this would increase as the effective viscosity of till increases. Subsequent ice-volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at Barbados, New Guinea, and Tahiti, but suggest that the Laurentide Ice Sheet was not the source of meltwater pulse 1A.

Numerical reconstruction of a soft-bedded Laurentide Ice Sheet during the last glacial maximum

We used a numerical ice-sheet model to reconstruct the North American Laurentide Ice Sheet during the last glacial maximum. Our model simulates ice-sheet conditions that can be specified experimentally as either a rigid substrate (hard bed) or a wet, deformable till (soft bed); basal sliding is excluded. We use geologic records of former basal ice-sheet processes to prescribe the distribution of hard and soft beds. Our reconstruction of the Laurentide Ice Sheet is significantly lower in ice-surface height and contains less ice volume than the CLIMAP (maximum) reconstruction. In contrast, our reconstruction agrees well with the ICE-4G reconstruction, both in height and volume. Because the ICE-4G reconstruction is based on the inversion of relative sea-level data, whereas our reconstruction is based on glacial geology and ice mechanics, this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the Laurentide Ice Sheet that is most consistent with observations of relative sea-level change and other geodynamic considerations.

Numerical modeling of subglacial sediment deformation: Implications for the behavior of the Lake Michigan Lobe, Laurentide Ice Sheet

We apply a numerical model of the late Wisconsin (circa 20,000 years B.P.) Lake Michigan Lobe (LML), Laurentide Ice Sheet, to investigate how fine-grained subglacial sediment might influence lobe behavior, particularly rapid millennial-scale marginal oscillations observed in the geologic record. Over the Canadian Shield, we assume a rigid bed (“hard bedded”) basal boundary condition. In areas overlain by fine-grained sediment (“soft bedded”), the base of the ice is coupled to a deformable sediment layer, using a rate-dependent stress-strain law. Geotechnical tests of clay-rich till deposited by the LML provide control for sediment rheologic parameters. Simulated cross-sectional profiles are consistent with reconstructions from geologic evidence. Time-dependent simulations suggest that a soft-bedded lobe could have reached steady state in about 20,000 years or less, in contrast to 50,000 to 60,000 years for an otherwise identical hard-bedded lobe. A soft-bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial-scale shifts in accumulation or ablation as a nonsliding hard-bedded lobe, but in both cases the response is slower than that indicated by the geologic record. Results suggest that while strong millennial-scale changes in accumulation and ablation can produce responses in hard-bedded or soft-bedded ice that are consistent with the geologic record, changes in subglacial sediment viscosity, even relatively modest changes (whether independent or in conjunction with climate change), might more readily account for millennial- and submillennial-scale fluctuations of the lobe margin. These observations do not exclude a role for sliding, but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior.

Numerical modeling of advective transport of saturated deforming sediment beneath the Lake Michigan Lobe, Laurentide Ice Sheet

Abstract We investigated the dynamics of advective transport of saturated, fine-grained deforming sediment beneath the Late Wisconsinan Lake Michigan Lobe (LML) of the Laurentide Ice Sheet using a numerical flowline model. Sediment rheology is modeled using a general constitutive law for mass movement of fine-grained material. Geotechnical tests on selected tills deposited by the LML in northern Illinois suggest an effective viscosity of about 5 × 109 Pa-s for fully saturated sediment and mild nonlinearity in the stress-strain relation. Numerical calculations of sediment flux, based on conservative parameters and assumptions, yield estimates on the order of 100 m3/y/meter width of the lobe, suggesting that advective transport could account directly for at least 25% of the total sediment flux estimated on the basis of field evidence. Less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice-velocity-dependent sediment transport processes. Modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment. These results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine-grained glacial till, and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity.

Simulated effect of vadose infiltration on water levels in the Northern Guam Lens Aquifer.

Regional-scale hydrology of the fresh water lens in the Northern Guam Lens Aquifer has been simulated in the past using a finite element, sharp interface computer model, SWIG2D. Systematic differences exist between observed and computed water levels. Computed seasonal peak water levels are higher, and the computed seasonal lows are lower than the respective observed levels. It is hypothesized that vadose storage must store a substantial amount of water during the wet season and release it gradually into the lens during the dry season. Flow through the vadose zone was simulated with a one-dimensional finite element, unsaturated flow program, UNSAT1D, in which the van Genuchten model is used to characterize unsaturated diffuse flow through the matrix of the vadose zone. An additional parameter (SINK) was added to the van Genuchten set to account for rapid infiltration down open pathways (fractures) associated with the closed depressions of the karst terrain. A global-optimization technique (Shuffled Complex Evolution or SCE-UA Method) was used to obtain the parameters that minimized the difference between simulated and observed water levels. Simulations incorporating the van Genuchten model were accomplished by combining the two programs, UNSAT1D and SWIG2D, into a single program. The sum-of-squared-errors (SSE) between computed and observed water levels in four observation wells was minimized using SCE-UA, reducing the arithmetically averaged SSE of the four wells by 30% compared with the SSE obtained when the vadose zone was not modeled. These results suggest that vadose storage is significant. On the other hand, the fact that the best fit obtained with an optimum parameter set was able to reduce the SSE by no more than 30% suggests that additional phenomena have yet to be accounted for to more fully explain differences between simulated and observed well water levels.

Estimating the freshwater-lens thickness of atoll islands in the Federated States of Micronesia

The water resources of the 32 atolls of the Federated States of Micronesia (FSM) are under continual threat from El Niño-induced droughts and other natural hazards. With government policies emphasizing sustainable development of atoll-island communities, local managers are in need of tools for predicting changes in the availability of fresh groundwater, which communities depend upon during droughts that incapacitate rain-catchment systems. An application of a recently developed, readily portable algebraic model is demonstrated here, to estimate the freshwater-lens thickness of atoll islands in the FSM, a key component of FSM groundwater resource assessment. Specifically, the model provides estimates of the lens thickness of atoll islands in the FSM during normal and drought conditions. The model was tested for use in the FSM through comparison with available lens data under both average rainfall conditions and intense drought conditions, and then applied to major islands of each atoll within the FSM. Results indicate that out of 105 major islands on FSM atolls, only six would likely retain sufficient groundwater to sustain the local community during an intense drought.RésuméLes ressources en eau des 32 atolls des États Fédérés de Micronésie (EFM) sont sous la menace continue des sécheresses et autres catastrophes naturelles dues à El Niño. Avec les politiques gouvernementales insistant sur le développement durable des communautés d’îles-atoll, les gestionnaires locaux ont besoin d’outils pour prévoir les changements de disponibilité de l’eau douce souterraine dont les communautés dépendent en période de sécheresse qui rend inutiles les systèmes de captage de pluie. Pour estimer l’épaisseur des lentilles d’eau d’îles-atoll du FSM, composante clef de l’évaluation de la ressource en eau des EFM, un modèle numérique transférable facile à mettre en œuvre a été récemment développé, il est présentée ici. Le modèle fournit en particulier l’estimations de l’épaisseur des lentilles d’eu douce d’atolls des EFM dans des conditions normales et de sécheresse. Pour une utilisation dans les EFM, le modèle a été testé par comparaison avec les données disponibles sur des lentilles, à la fois dans des conditions pluviométriques moyennes et de sécheresses intenses, puis appliqué aux îles principales de chaque atoll des EFM. Les résultats indiquent que, sur 105 des principales îles des atolls des EFM, vraisemblablement six seulement retiendraient suffisamment d’eau souterraine pour alimenter eau la communauté locale durant une sécheresse intense.ResumenLos recursos de agua de 32 atolones de los Estados Federados de Micronesia (FSM) están bajo una continua amenaza de sequías inducidas por El Niño y otros riesgos naturales. Con las políticas del gobierno que enfatizan el desarrollo sostenible de las comunidades de las islas atolones, los gestores locales necesitan herramientas para predecir cambios en la disponibilidad de agua dulce, de las cuales dependen las comunidades durante las sequías que se inhabilitan los sistema de captación de la lluvia. Se demuestra que una aplicación recientemente desarrollada de un modelo algebraico fácilmente transportable para estimar los espesores de las lentes de agua dulce de las islas atolones en los FSM, es una componente clave de la evaluación de los recursos de agua subterránea de los FSM. Específicamente, el modelo provee estimaciones del espesor de las lentes de las islas atolones en los FSM durante condiciones normales y de sequía. El modelo fue probado para el uso en los FSM a través de la comparación con datos disponibles de lentes tanto en condiciones de precipitaciones promedio como de sequías intensas, y luego aplicado a las islas principales de cada atolón dentro de los FSM. Los resultados indican que de las 105 islas principales en las islas atolones de los FSM, solo en seis casos probablemente podrían retener suficiente agua subterránea para sostener a la comunidad local durante una intensa sequía.摘要密克罗尼西亚联邦(FSM)的32个环礁岛的水资源持续受到厄尔尼诺现象导致的干旱和其它自然灾害的威胁。在强调环礁岛社区可持续发展的政府政策的指导之下,当地的管理人员正在寻求一种可以预测地下淡水可用性的工具,使环礁岛社区可以在降雨汇流系统失效的旱季对其加以利用。本文应用了一种近期建立起来的便携的代数模型,用来估计FSM的环礁岛中淡水透镜体的厚度,这是FSM地下水资源评价的重要组成部分。具体来说,这个模型估计了正常与干旱条件下FSM环礁岛的透镜体厚度。此模型通过在FSM应用而进行检验测试,对比了在平均降雨条件和严重的干旱条件下可用的透镜体数据,然后应用到了FSM的每个环礁岛的主要岛屿。结果表明,在FSM的105个主要岛屿中,只有6个可以在严重的干旱条件下为当地的社区提供足够的地下水源。ResumoOs recursos hídricos dos 32 atóis dos Estados Federados da Micronésia (EFM) estão sob ameaça contínua das secas induzidas pelo El Niño e outros desastres naturais. Com a ênfase das políticas públicas no desenvolvimento sustentável das comunidades de ilhas-atol os gestores locais têm necessidade de possuir ferramentas para prever as alterações na disponibilidade de água doce subterrânea de que dependem as comunidades durante os períodos de seca que provocam o colapso dos sistemas de captação de água da chuva. Este trabalho apresenta a aplicação de um modelo algébrico recém-desenvolvido, de fácil manuseamento, que permite estimar a espessura da lente da de água doce nas ilhas-atol dos EFM, uma componente crucial para a avaliação dos recursos hídricos subterrâneos dos EFM. Mais especificamente, o modelo fornece estimativas da espessura da lente de água doce nas ilhas-atol no FSM durante condições normais e em eventos de seca. O modelo foi testado para uso nos EFM através de comparação com os dados de lentes de água doce disponíveis para ambas condições médias de precipitação e de seca intensa, sendo posteriormente aplicado nas ilhas principais de cada atol dentro dos EFM. Os resultados indicam que, das 105 principais ilhas em atóis dos EFM apenas seis poderão provavelmente reter água subterrânea suficiente para sustentar a comunidade local durante uma seca intensa.

Effects of Marine Overwash for Atoll Aquifers: Environmental and Human Factors

Inundation of atoll islands by marine overwash is a serious threat to fresh groundwater, which can be a critical emergency water resource after artificial storage or other water resource infrastructure has been exhausted or destroyed. In contrast to drought, which slowly exhausts water supplies and often can be forecasted in time, overwash can occur with little warning and can ruin both rain catchment storage and groundwater reserves. In this study, a SUTRA-based model is applied to estimate how groundwater contamination by overwash and subsequent recovery of fresh groundwater are influenced by geologic factors (aquifer hydraulic conductivity, dispersivity, and the presence or absence of a reef flat plate), the seasonal timing of the event (wet vs. dry), and the presence of hand-dug wells that penetrate the reef flat plate. Actual tidal and rainfall data from regions in the western Pacific are applied to simulated 30-month recovery periods for hypothetical islands with properties and conditions characteristic of the western Pacific. For all scenarios, results indicate that 12 to 16 months are required to achieve 60% recovery of fresh groundwater. However, the time required to restore useful quantities of groundwater to acceptable salt concentration at depths typical of hand-dug wells is only 3 to 6 months. Of particular interest is the influence of the reef flat plate, which acts as a barrier to infiltrating seawater, thus preserving a pocket of confined freshwater during an overwash event and the recovery, which could probably be utilized if the necessary tools and equipment are on hand.

Historic impact of watershed change and sedimentation to reefs along west-central Guam

Using coral growth parameters (extension, density, calcification rates, and luminescence) and geochemical measurements (barium to calcium rations; Ba/Ca) from coral cores collected in west-central Guam, we provide a historic perspective on sediment input to coral reefs adjacent to the Piti-Asan watershed. The months of August through December are dominated by increased coral Ba/Ca values, corresponding to the rainy season. With river water enriched in barium related to nearshore seawater, coral Ba/Ca ratios are presented as a proxy for input of fine-grained terrigenous sediment to the nearshore environment. The century-long Ba/Ca coral record indicates that the Asan fore reef is within the zone of impact from discharged sediments transported from the Piti-Asan watershed and has experienced increased terrestrial sedimentation since the 1940s. This abrupt shift in sedimentation occurred at the same time as both the sudden denudation of the landscape by military ordinance and the immediate subsequent development of the Asan area through the end of the war, from 1944 through 1945. In response to rapid input of sediment, as determined from coral Ba/Ca values, coral growth rates were reduced for almost two decades, while calcification rates recovered much more quickly. Furthermore, coral luminescence is decoupled from the Ba/Ca record, which is consistent with degradation of soil organic matter through disturbance by forest fires, suggesting a potential index of fire history and degradation of soil organic matter. These patterns were not seen in the cores from nearby reefs associated with watersheds that have not undergone the same degree of landscape denudation. Taken together, these records provide a valuable tool for understanding the compounding effects of land-use change on coral reef health.

Field Observations of Coastal Discharge from an Uplifted Carbonate Island Aquifer, Northern Guam, Mariana Islands: A Descriptive Geomorphic and Hydrogeologic Perspective

ABSTRACT Taboroši, D.; Jenson, J.W., and Mylroie, J.E., 2013. Field observations of coastal discharge from an uplifted carbonate island aquifer, northern Guam, Mariana Islands: a descriptive geomorphic and hydrogeologic perspective. Understanding the hydrogeologic properties of carbonate island aquifers requires a conceptual framework within which specific and local properties, especially quantitative phenomena, can be placed and evaluated. In this paper, we present observations and hypotheses for such a qualitative conceptual model based on more than a decade of field studies in Guam. Fieldwork has shown that coastal groundwater discharge features can be usefully classified in terms of their geomorphic context and distinctive, readily observable characteristics: beach seeps and springs, reef seeps and springs, fracture springs, and cave springs. Seeps and springs on beaches and reefs are associated with matrix porosity and diffuse flow or are fed by fissures buried under sediment. Concentrated discharges from fractures and conduits are directly visible on rocky coasts, where springs emerge from dissolution-widened fractures and karst cavities. Although coastal zone conditions make it very difficult to accurately quantify the discharges from each type of feature, the fact that discharge is visibly distributed among these four categories suggests that aquifers of islands such as Guam—composed of uplifted geologically young limestones—can generally be expected to be triple-porosity aquifers, in which matrix, fracture, and conduit porosity each make a substantial contribution to discharge, and presumably to internal flow as well. This is in contrast to noncarbonate aquifers, from which coastal discharge is generally dominated by diffuse flow, and to continental karst aquifers in diagenetically mature limestones, in which flow and discharge are dominated by conduit flow. We propose a conceptual framework that relates these four geomorphic categories of coastal discharge to the evolution and reorganization of matrix, fracture, and conduit porosities in the aquifer, and offer a hypothesis for the general distribution of hydraulic conductivity on uplifted carbonate island aquifers, in particular the juxtaposition of high conductivity in the interior against lower conductivity in the periphery.

Trade winds drive pronounced seasonality in carbonate chemistry in a tropical Western Pacific island cave—Implications for speleothem paleoclimatology

Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in mid-latitude caves have observed seasonal variability in cave pCO2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it is has been proposed caves will ventilate on daily timescales, preventing cave pCO2 from increasing substantially above atmospheric pCO2. By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8-year cave monitoring study, including observations of cave pCO2 and carbonate chemistry, at Jinapsan Cave, Guam (13.4°N, 144.5°E). We find that cave pCO2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO2 ranging from 500 - 5000 ppm. The seasonality of cave pCO2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO2. Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO2. These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave effect the interpretation of speleothem-based paleoclimate records. This article is protected by copyright. All rights reserved.