The magnitude of error in conventional bulk-sediment radiocarbon dates from central North America

Although the carbon-reservoir problem with bulk-sediment radiocarbon dates from lakes has long been recognized, many synoptic studies continue to use chronologies derived from such dates. For four sites in central North America, we evaluate chronologies based on conventional radiocarbon dates from bulk sediment versus chronologies based on accelerator mass spectrometry (AMS) radiocarbon dates from terrestrial plant macrofossils. The carbon-reservoir error varies among sites and temporally at individual sites from 0 to 8000 yr. An error of 500–2000 yr is common. This error has important implications for the resolution of precise event chronologies.     

Mississippi Valley regional source of loess on the southern Green Bay Lobe land surface, Wisconsin

Loess has been recognized on the glacial land surface of the Green Bay Lobe for over 100 yr, but no systematic explanation of the source of the loess has been advanced. Intriguingly, the loess on the Green Bay Lobe land surface is thicker than predicted by regional thinning trends from the Mississippi Valley and is geographically separated from much loess of southwest Wisconsin by a sandy region devoid of loess. Mapping based on soil survey interpretation indicates that the loess occurs above an escarpment marking the eastern end of the sandy loess-free region. Particle size fining trends demonstrate that the loess was transported by northwesterly winds. Clay mineralogy of the Green Bay Lobe loess is distinctly different than glaciogenic sediments and matches loess of the Mississippi Valley, indicating a regional source and long distance transport of the loess. We propose the loess was transported from the regional source along a surface of transport produced by migration of eolian sand through low-relief landscapes including the glacial Lake Wisconsin basin. Eolian sand migration caused repeated entrainment of dust leading to east-southeastward transport. The loess accumulated above an escarpment that limited sand mobility and re-entrainment of loess beyond this topographic barrier.     

Use of paleolimnology to document the effect of lake shoreland development on water quality

Four Wisconsin, USA lakes were examined with paleolimnological techniques to determine the effects of shoreland development on water quality. Geochemical parameters such as aluminum, iron, manganese, phosphorus, nitrogen, and carbon were used to document watershed inputs while redox sensitive elements provided information on changes in hypolimnetic oxygen levels. Changes in the diatom community were used to assess the impacts of development upon the lakes' trophic status as well as changes in the littoral community. We assessed the relative sensitivity of high vs. low ANC lakes.The initial shoreland development, late in the 19^th or early in the 20^th century, involved seasonal cabins and minimal disturbance of the shoreland. This development phase had minimal impact upon the lakes compared to that during the mid- and late-twentieth century. Increased levels of iron and aluminum indicated the highest input of sediment occurred during the construction phase of converting shoreland homes from seasonal to year-round usage. Phosphorus deposition increased moderately in the high alkalinity lakes but more so in the low alkalinity lakes. In the low alkalinity stratified lake, increased levels of iron and manganese in the last decade indicated more anoxia in the hypolimnion. Phosphorus levels have also increased during this time period most likely as a result of elevated phosphorus release from the sediments.In the high alkalinity lakes, as the nutrient levels increased, diatom production initially shifted from benthic taxa of the family Fragilariaceae to metalimnetic taxa and as nutrients increased further, epilimnetic species. In the low alkalinity lakes, prior to settlement the major site of diatom prodution was the open water. Coincidental with the early shoreland development was an increase of macrophyte density as indicated by the epiphytic diatom Achnanthidium minutissimum ((Kützing) Czarnecki (= Achnanthes minutissima Kützing). The water quality in the high alkalinity lakes showed some improvement following completion of the home construction phase, especially in the lake with lower development density. The low alkalinity lakes did not show improved water quality and appear more sensitive to shoreland development.     

Late Quaternary climates and limnology of the Lake Winnebago basin, Wisconsin, based on ostracodes

Ostracodes document a series of late Quaternary climatic and limnologic changes within the Lake Winnebago basin of east-central Wisconsin. Using a ¹⁴ C, ¹³⁷Cs, and ²¹⁰Pb-based geochronology, Lake Winnebago ostracode abundances were compared to regional patterns of ostracode biogeography and the paleontological, sedimentological, and geochemical records of Elk Lake (Clearwater County), Minnesota, in order to interpret past temperature and hydrochemical changes in Lake Winnebago. Lake Winnebago sediments contain five major ostracode species, Candona ohioensis, Candona rawsoni, Cytherissa lacustris, Limnocythere verrucosa, and Physocypria pustulosa. In combination with sedimentology and geochronology, variations in the abundances of these species allow the late Quaternary record of the Lake Winnebago basin to be subdivided into five major climatic intervals: (1) glacial to post-glacial (15.5–11.0 ka), (2) cold and variable immediate post-glacial (11.0–10.4 ka), (3) warmer and wet early Holocene (10.4–7.8 ka), (4) warm but not particularly dry middle Holocene (7.8–4.2 ka), and (5) warm and moist late Holocene (4.2 ka-present).     

The genesis of the northern Kettle Moraine, Wisconsin

Interpreting past glacial dynamics from the glacial record requires that the depositional environments of glacial sediments and landforms be understood. In the case of interlobate deposits, models that incorporate various components of pro, supra and subglacial deposition have been developed and tested in the northern Kettle Moraine (nKM), Wisconsin; a large interlobate deposit that formed between the Green Bay and Lake Michigan lobes of the Laurentide Ice Sheet during the last deglaciation. In this paper, we interpret a new genesis for the nKM using sediment analysis and distribution along with landform distribution. In Sheboygan County, the nKM consists of two steep-sided, high-relief, hummocky ridges separated by a low elevation and low-relief central axis. Gravel in the bounding hummocky ridges is well-sorted and well-rounded. Some bedding is collapsed. Large, isolated moulin kames are restricted to the axis area and composed of relatively poorly sorted, more angular gravel and diamicton. The distribution of these different sediments and landforms are explained by the accumulation of supraglacial debris that insulated the ice below the axis of the nKM, while the melting of cleaner ice on either side formed channels on the ice surface. As deglaciation proceeded, a substantial thickness of well-rounded, stream-deposited sand and gravel accumulated on ice in the bounding channels. Eventual collapse of this sediment formed the two hummocky ridges. Poorly sorted debris along the axis fell and slid into moulins and larger collapse areas in the ice. Thus, differential debris insulation and ice ablation controlled the mainly supraglacial deposition of this part of the nKM.     

Karstic problems in the construction of Milwaukee’s Deep Tunnels

A critical component of Milwaukees $ 2.8 billion Water Pollution Abatement Program is a 31.2-km inline storage system comprising three Deep Tunnel sections that were bored between 1984 and 1993 at a depth of 80–100 m within Silurian-aged Niagara dolostone. Construction of these Deep Tunnels proved more difficult and expensive than estimated because the karstic nature of the dolostone, particularly its hydrology, had not been fully appreciated. Rock collapse, subsidence and groundwater intrusion necessitated remedial grouting and lining of about 45% of the tunnels, costing some $ 50 million above estimates and delaying completion by 9 months. Tunnel performance since completion continues to be controversial.