Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

The period in the Upper Mississippi Valley (UMV) from about 25 000 years B.P. until the time of strong human influence on the landscape beginning about 150–200 years ago can be characterized by three distinctly different alluvial episodes. The first episode is dominated by the direct and indirect effects of Late Wisconsin glacial ice in the basin headwaters. This period, which lasted until about 14 000 years B.P., was generally a time of progressive valley aggradation by a braided river system transporting large quantities of bedload sediment. An island braided system evolved during the second episode, which extended from about 14 000 to 9000 years B.P. The second episode is associated with major environmental changes of deglaciation when occurrences of major floods and sustained flows of low sediment concentration from drainage of proglacial lakes produced major downcutting. By the time of the beginning of the third episode about 9000 years B.P., most vegetation communities had established their approximate average Holocene locations. The change of climate and establishment of good vegetation cover caused upland landscapes of the UMV to become relatively stable during the Holocene in comparison to their relative instability during the Late Wisconsin. However, Holocene remobilization of Late Wisconsin age sediment stored in tributary valleys resulted in a return to long-term upper Mississippi River aggradation. The dominance of Holocene deposition over transportation reflects the abundance of sandy bedload sediment introduced from tributaries and the situation that energy conditions for floods and the hydraulic gradient of the upper Mississippi River are much less for the Holocene than they were for the Late Wisconsin and deglaciation periods.Outburst floods from glacial lakes appear to have been common in the UMV during the Late Wisconsin and especially during deglaciation. Magnitudes for the Late Wisconsin floods are generally poorly understood, but an estimate of 10 000–15 000 m³ s⁻¹ was determined for one of the largest events in the northern UMV based on heights of paleo-foreset beds in a flood unit deposited in the Savanna Terrace. For comparison, the great flood of 1993 on the upper Mississippi River was about 12 000 m³ s⁻¹ at Keokuk, Iowa, near the Des Moines River confluence where it represented the 500-year event in relation to modem flood series. Exceptionally large outburst floods derived from the rapid drainage of pro-glacial Lake Michigan and adjacent smaller proglacial lakes between about 16 000 and 15 500 years B.P. are a likely cause of the final diversion of the Mississippi River through the Bell City-Oran Gap at the upstream end of the Lower Mississippi Valley (LMV). The largest outburst flood from northern extremities of the UMV appears to have occurred between about 11700 and 10 800 years B.P. when the southern outlet of Lake Agassiz was incised. Based on the probable maximum capacity of the Agassiz flood channel 600 km downstream near the junction of the Wisconsin and Mississippi Rivers, the Agassiz flood discharge apparently did not exceed 30 000 m³ s⁻¹. However, if the Agassiz flood channel here is expanded to include an incised component, then the flood discharge maximum could have been as large as 100,000 to 125 000 m³ s⁻¹. The larger flood is presently viewed as unlikely, however, because field evidence suggests that the incised component of the cross-section probably developed after the main Agassiz flood event. Nevertheless, the large Agassiz flood between about 11 700 and 10 800 years B.P. produced major erosional downcutting and removal of Late Wisconsin sediment in the UMV. This flood also appears to be mainly responsible for the final diversion of the Mississippi River through Thebes Gap in extreme southwestern Illinois and the formation of the Charleston alluvial fan at the head of the LMV.After about 9000 years B.P. prairie-forest ecotones with associated steep seasonal climatic boundaries were established across the northern and southern regions of the UMV. The general presence of these steep climatically sensitive boundaries throughout the Holocene, in concert with the natural tendency for grasslands to be especially sensitive to climatic change, may partially explain why widespread synchroneity of Holocene alluvial episodes is recognized across the upper Mississippi River and Missouri River drainage systems. Comparison of estimated beginning ages of Holocene flood episodes and alluvial chronologies for upper Mississippi River and Missouri River systems with beginning ages for LMV meander belts and delta lobes shows a relatively strong correlation. At present, dating controls are not sufficiently adequate and confidence intervals associated with the identified ages representing system changes are too large to establish firm causal connections. Although the limitations of the existing data are numerous, the implicit causal connections suggested from existing information suggest that further exploration would be beneficial to improving the understanding of how upper valley hydrological and geomorphic events are influencing hydrological and geomorphic activity in the LMV. Since nearly 80% of the Mississippi River drainage system lies upstream of the confluence of the Mississippi and Ohio Rivers, there is a strong basis for supporting the idea that UMV fluvial activity should be having a strong influence on LMV fluvial activity. If this assertion is correct, then the traditional assignment of strong to dominant control by eustatic sea level variations for explaining channel avulsions, delta lobes, and meander belts in the LMV needs re-examination. A stronger role for upper valley fluvial activity as a factor influencing lower valley fluvial activity does not disregard the role of eustatic sea level, tectonic processes or other factors. Rather, upper valley fluvial episodes or specific events such as extreme floods may commonly serve as a “triggering mechanism” that causes a threshold of instability to be exceeded in a system that was poised for change due to sea level rise, tectonic uplift, or other environmental factors. In other situations, the upper valley fluvial activity may exert a more dominant control over many LMV fluvial processes and landforms as frequently was the case during times of glacial climatic conditions.     

Erosion rates of alpine bedrock summit surfaces deduced from in situ Be and Al

We have measured the concentration of in situ produced cosmogenic ¹⁰Be and ²⁶Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My⁻¹. Individual measurements vary between 2 and 19 m My⁻¹. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My⁻¹). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.

We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.     

Privatizing conditions of production: trade agreements as neoliberal environmental governance

Recent multilateral trade agreements are among the major manifestations of neoliberalism. They are also emerging as some of the most important sites of environmental governance in the 21st century. I argue here that these trade agreements, particularly the sweeping new protections they provide for investors, are redefining property rights and environmental governance in fundamental ways. I suggest that in addition to furthering the centuries-long process of the enclosure of nature under capitalism, the neoliberal agenda of NAFTA and similar trade agreements also involves something new: the privatization, or primitive accumulation, of conditions of production as an accumulation strategy. I explore these dynamics through examination of two cases, one in the United States and one in Mexico. I also explore the roles of social movements in these dynamics.     

Barite deposition resulting from phototrophic sulfide-oxidizing bacterial activity

Barite (BaSO₄) deposits generally arise from mixing of soluble barium-containing fluids with sulfate-rich fluids. While the role of biological processes in modulating barium solubility has been shown, no studies have shown that the biological oxidation of sulfide to sulfate leads to barite deposition. Here we present an example of microbially mediated barite deposition in a continental setting. A spring in the Anadarko Basin of southwestern Oklahoma produces water containing abundant barium and sulfide. As emergent water travels down a stream to a nearby creek, sulfate concentration increases from 0.06 mM to 2.2 mM while Ba²⁺ concentration drops from 0.4 mM to less than 7 μM. Stable isotope analysis, microbial activity studies, and in situ experiments provide evidence that as sulfide-rich water flows down the stream, anaerobic, anoxygenic, phototrophic bacteria play a dominant role in oxidizing sulfide to sulfate. Sulfate then precipitates with Ba²⁺ producing barite as travertine, cements, crusts, and accumulations on microbial mats. Our studies suggest that phototrophic sulfide oxidation and concomitant sulfur cycling could prove to be important processes regulating the cycling of barium in continental sulfur-containing systems.     

Asia–Europe–United States: the geoeconomics of uncertainty

This paper examines a key aspect of how the global geometries of finance and trade articulate the euro–dollar relationship. Here, European integration extends into a global financial and trading system replete with contradictions. The paper examines these, drawing upon a literature from international political economy, foregrounding the spatialities that they signal.     

Continental rift systems and anorogenic magmatism

Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncated-icosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Long-lived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes.     

Representing and communicating deep uncertainty in climate-change assessments

IPCC reports provide a synthesis of the state of the science in order to inform the international policy process. This task is made difficult by the presence of deep uncertainty in the climate problem that results from long time scales and complexity. This paper focuses on how deep uncertainty can be effectively communicated. We argue that existing schemes do an inadequate job of communicating deep uncertainty and propose a simple approach that distinguishes between various levels of subjective understanding in a systematic manner. We illustrate our approach with two examples. To cite this article: M. Kandlikar et al., C. R. Geoscience 337 (2005).     

The influence of edge sites on the development of surface charge on goethite nanoparticles: A molecular dynamics investigation

Large-scale molecular simulation of proton accumulations were carried out on (i) (110) and (021) slabs immersed in aqueous solution and (ii) a series of model goethite nanoparticles of dimension 2 to 8 nm with systematically varying acicularity and (110)/(021) surface areas. In the slab systems, the (021) surface exhibits 15% more proton charge per unit area than the (110) surface. In the particulate systems, the acicular particles having the highest (110)/(021) ratio accumulate the most charge, opposite to the trend expected from the slab simulations, indicating that, at length scales on the order of 10 nm, the slab results are not a good indicator of the overall charging behavior of the particles. The primary reason for the discrepancy between the particulate systems and slab systems is the preferential accumulation of protons at acute (110)-(110) intersections. Charge accumulates preferentially in this region because excess proton charge at an asperity is more effectively solvated than at a flat interface.