Barbara Mauz

Rapid and widespread response of the Lower Mississippi River to eustatic forcing during the last glacial-interglacial cycle

The Lower Mississippi Valley provides an exceptional fi eld example for studying the response of a continental-scale alluvial system to upstream and downstream forcing associated with the large, orbitally controlled glacialinter glacial cycles of the late Quaternary. However, the lack of a numerical chronology for the widespread Pleistocene strata assemblage known as the Prairie Complex, which borders the Holocene fl oodplain of the Lower Mississippi River, has so far precluded such an analysis. Here, we apply optically stimulated luminescence (OSL) dating, mainly on silt-sized quartz from Prairie Complex strata. In total, 27 OSL ages indicate that the Prairie Complex consists of multiple allostratigraphic units that formed mainly during marine isotope stages 7, 5e, and 5a. Thus, the aggradation of the Prairie Complex is strongly correlated with the sea-level highstands of the last two glacialinterglacial cycles. Fluvial incision during the sea-level fall associated with the MIS 5a–MIS 4 transition extended as far inland as ~600 km from the present-day shoreline, testifying to the dominant downstream control of fl stratigraphic architecture in the Lower Mississippi Valley. In addition, the short reaction time of the Lower Mississippi River suggests that large fl uvial systems can respond much more rapidly to allogenic forcing than is commonly believed.

Episodic overbank deposition as a dominant mechanism of floodplain and delta-plain aggradation

The common view that frequent overbank flooding leads to gradual aggradation of alluvial strata on floodplains and delta plains has been challenged by a variety of studies that suggest that overbank aggradation occurs in a strongly episodic fashion. However, this remains a largely untested hypothesis due to the difficulty in establishing age models with sufficiently high resolution. Here we use 39 optically stimulated luminescence (OSL) ages from proximal overbank deposits in the Mississippi Delta to demonstrate for the first time that alluvial aggradation over centennial to millennial time scales is predominantly episodic, with aggradation rates of 1–4 cm/yr that can persist for centuries. OSL ages from three separate study areas produce age clusters that are distinctly different yet complement each other. These findings suggest that a substantial portion of the continental stratigraphic record consists of patchworks of relatively discrete, centennial- to millennial-scale sediment bodies assembled by autogenic processes.

Reconstruction of late-Holocene slope and dry valley sediment dynamics in a Belgian loess environment

To unravel the evolution of a dry valley in the Belgian loess area soils and sediments along a slope catena were studied in detail. A 67 m long trench was opened from the upper slope to the centre of the valley bottom. The exposed soils and sediments showed evidence of severe soil erosion and other human disturbances that significantly changed the valley topography. The early-Holocene slope gradient was up to 25%, whereas now it is less than 10%. In the thalweg, a remnant of the early-Holocene soil was found underneath colluvial deposits, which were more than 3 m thick. A chronology for the valley evolution was established based on AMS 14C dating of charcoal fragments and optical dating of colluvial sediments. The first sediment deposition occurred in the early Iron Age, with an average sedimentation rate of approximately 3.4 ± 1.3 t/ha per yr. This increased to c. 5.4 ± 2.2 t/ha per yr during the Roman Period and further to 18.0 ± 2.2 t/ha per yr in the Middle Ages. Although sediment accumulation in the valley was substantial, soil-erosion processes were mainly low-magnitude and signs of gullying are absent in the thalweg until the last few centuries.

Source-trap characterization of thermally transferred OSL in quartz

Thermally transferred optically stimulated luminescence (TT-OSL) of quartz is the low intensity OSL measured after heating a previously optically zeroed quartz to temperatures below that erasing the OSL electron trap. We identify the source traps contributing to TT-OSL by studying the changes in TT-OSL and thermoluminescence (TL) caused by optical bleaching at various temperatures and by repeated TT-OSL measurements, and quantify source-trap parameters using the Hoogenstraaten method. We find that both the single transfer mechanism and the double transfer mechanism are contributing to TT-OSL production. Three source traps are identified when the samples are heated to 300 °C for 10 s to induce the thermal transfer. The first one corresponds to a TL peak at ~200 °C. It captures electrons during the optical bleaching and releases these charges during subsequent heating. It provides ~10% of the electrons that give rise to TT-OSL. The second trap corresponds to a TL peak at 290–300 °C and provides ~80% of the electrons for TT-OSL through the single transfer mechanism. This electron trap has a depth of 1.34 ± 0.05 eV and a frequency factor in the order of 1011 s−1. It has a mean lifetime of 0.24 Ma at 10 °C. The third trap corresponds to a TL peak at ~380 °C and provides ~10% electrons for TT-OSL through the single transfer mechanism. It has a depth of 1.66 ± 0.07 eV and a frequency factor in the order of 1012 s−1. To validate the techniques used we also determined the parameters of the fast component OSL trap and received results consistent with published values. Most importantly, our results show that the relatively short lifetime of the main TT-OSL source trap limits possibilities of using TT-OSL to extend the age range of quartz OSL dating, as has been suggested by various authors.

Asynchronous Holocene colluvial and alluvial aggradation: A matter of hydrosedimentary connectivity

Based on Optically Stimulated Luminescence (OSL) and radiocarbon dating we establish chronologies of colluviation and alluviation in different floodplain sections of the northwestern Wetterau loess basin (Germany). Similar to some other European valley floors, Holocene floodplain aggradation is marked by two important breaks: (1) a millennial-scale delay between the Neolithic agricultural colluviation and floodplain aggradation. In loess catchments agricultural colluviation started at about 7000 cal. BP and anthropogenic floodplain aggradation only at about 2200 ± 200 cal. BP; (2) a centennial-scale variability in a temporary rise in rates of anthropogenic floodplain aggradation (up to 3.6 ± 1.7 mm/yr) during the High Middle Ages in directly neighbouring reaches. Independent archaeologic, historic, and vegetation records document distinct agricultural histories of hillsides and floodplains and highlight the importance of hydrosedimentary connectivity as compared with land use intensity. The late Iron Age start of alluviation can be linked to the introduction of an integrated land use system with intense cultivation on hillsides and immediate neighbouring floodplains. The centennial-scale variability of medieval peak aggradation is a result of the successive introduction (or temporal failure) of hydraulic water milling infrastructure. Using palaeoecological and geomorphological information for reconstructing cause and consequence of sediment redistribution in coupled human–natural systems requires firm information about the spatial organisation and technological abilities that are associated with socio-agricultural transformations.

Palaeosols in Saharan and Sahelian dunes of Chad: archives of Holocene North African climate changes

Limnic sediments and palaeosols are frequently present in dunes of the Sahara and Sahel deserts of Chad. This study focused on genesis and degree of development of the palaeosols and the timing of soil formation. The soils are Cambic Arenosols formed by precipitation-induced silicate weathering and leaching. The upper parts of the Cambic Arenosols are indurated due to accumulation of salt and amorphous silica during a drier climate following soil formation. The optical age data reveal the occurrence of one or two distinct short-lived humid climate intervals shortly after-5 ka and before-2.5 ka and the formation of local desert lakes shortly after -5 ka. The study further suggests that rapid climate changes occurred mainly in the mid-Holocene whereas the early-Holocene humid period lasted longer.

Anatomy of Mississippi Delta growth and its implications for coastal restoration

Prehistoric rates of land gain in a large portion of the Mississippi Delta are significantly outpaced by present-day rates of land loss. The decline of several of the world’s largest deltas has spurred interest in expensive coastal restoration projects to make these economically and ecologically vital regions more sustainable. The success of these projects depends, in part, on our understanding of how delta plains evolve over time scales longer than the instrumental record. Building on a new set of optically stimulated luminescence ages, we demonstrate that a large portion (~10,000 km2) of the late Holocene river–dominated Mississippi Delta grew in a radially symmetric fashion for almost a millennium before abandonment. Sediment was dispersed by deltaic distributaries that formed by means of bifurcations at the coeval shoreline and remained active throughout the life span of this landform. Progradation rates (100 to 150 m/year) were surprisingly constant, producing 6 to 8 km2 of new land per year. This shows that robust rates of land building were sustained under preindustrial conditions. However, these rates are several times lower than rates of land loss over the past century, indicating that only a small portion of the Mississippi Delta may be sustainable in a future world with accelerated sea-level rise.

The last interglacial shoreline in northern Brittany, western France

Abstract Shorelines of the last interglacial highstand in western France are mainly represented by rocky abrasion platforms on which fossil gravel barriers are located. These coastal accumulations are scattered around islands (Ouessant, Belle-Ile) and on the mainland. They are buried by periglacial deposits, consisting of successions of sandy/gravelly units, then heads, loess and sands. OSL dating has allowed the first sandy/gravelly layers to be dated to 90 ka and the loess and sands to 6.4 ka and 0.47 ka. The comparison of the precise stratigraphy of several cross-sections enables the reconstruction of the evolution of the landscape and the description of the configuration of the coastline at the end of the last interglacial.