Quaternary alluvial-fan development, climate and morphologic dating of fault scarps in Laguna Salada, Baja California, Mexico

Late Quaternary slip across the Cañada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~ 60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q₁ [youngest] to Q₈ [oldest]) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide ¹⁰Be concentrations from individual boulders on alluvial-fan surfaces Q₄ and Q₇ yield surface exposure ages of 15.5 ± 2.2 ka and 204 ± 11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial-fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets < 4 m is 0.051 and 0.066 m²/ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada.     

Processes and rates of rock fragment displacement on cliffs and scree slopes in an amba landscape, Ethiopia

Distinct rock fragment displacements occur on the ambas, or structurally determined stepped mountains of the Northern Ethiopian Highlands. This paper describes the rock fragment detachment from cliffs by rockfall, quantifies its annual rate, and identifies factors controlling rock fragment movement on the scree slopes. It further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes. In the May Zegzeg catchment (Dogu'a Tembien district, Tigray), rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4-year period (1998–2001). Rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes. Along a 1500-m long section of the Amba Aradam sandstone cliff, at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0.37 mm y^− 1. Yearly unit rock fragment transport rates on scree slopes ranged between 23.1 and 37.9 kg m^− 1 y^− 1. This process is virtually stopped when exclosures are established. Corresponding mean rock fragment transport coefficients K are 32–69 kg m^− 1 y^− 1 on rangeland but only 3.9 kg m^− 1 y^− 1 in densely vegetated exclosures. A conceptual model indicates that besides rockfall from cliffs and argillipedoturbation, all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin.     

Response of the St. Joseph River to lake level changes during the last 12,000 years in the Lake Michigan basin

The water level of the Lake Michigan basin is currently 177 m above sea level. Around 9,800 ¹⁴C years B.P., the lake level in the Lake Michigan basin had dropped to its lowest level in prehistory, about 70 m above sea level. This low level (Lake Chippewa) had profound effects on the rivers flowing directly into the basin. Recent studies of the St. Joseph River indicate that the extreme low lake level rejuvenated the river, causing massive incision of up to 43 m in a valley no more than 1.6 km wide. The incision is seen 25 km upstream of the present shoreline. As lake level rose from the Chippewa low, the St. Joseph River lost competence and its estuary migrated back upstream. Floodplain and channel sediments partially refilled the recently excavated valley leaving a distinctly non-classical morphology of steep sides with a broad, flat bottom. The valley walls of the lower St. Joseph River are 12–18 m tall and borings reveal up to 30 m of infill sediment below the modern floodplain. About 3 × 10⁸ m³ of sediment was removed from the St. Joseph River valley during the Chippewa phase lowstand, a massive volume, some of which likely resides in a lowstand delta approximately 30 km off-shore in Lake Michigan. The active floodplain below Niles, Michigan, is inset into an upper terrace and delta graded to the Calumet level (189 m) of Lake Chicago. In the lower portion of the terrace stratigraphy a 1.5–2.0 m thick section of clast-supported gravel marks the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin. This gravel layer represents the consolidation of drainage that probably occurred during final melting out of ice-marginal kettle chains allowing stream piracy to proceed between Niles and South Bend. It is unlikely that the St. Joseph River is palimpsest upon a bedrock valley. The landform it cuts across is a glaciofluvial-deltaic feature rather than a classic unsorted moraine that would drape over pre-glacial topography. This is the fifth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.     

Post-wildfire erosion response in two geologic terrains in the western USA

Volumes of eroded sediment after wildfires vary substantially throughout different geologic terrains across the western United States. These volumes are difficult to compare because they represent the response to rainstorms and runoff with different characteristics. However, by measuring the erosion response as the erodibility efficiency of water to detach and transport sediment on hillslopes and in channels, the erosion response from different geologic terrains can be compared. Specifically, the erodibility efficiency is the percentage of the total available stream power expended to detach, remobilize, or transport a mass of sediment. Erodibility efficiencies were calculated for the (i) initial detachment, and for the (ii) remobilization and transport of sediment on the hillslopes and in the channels after wildfire in two different geological terrains.The initial detachment efficiencies for the main channel and tributary channel in the granitic terrain were 10 ± 9% and 5 ± 4% and were similar to those for the volcanic terrain, which were 5 ± 5% and 1 ± 1%. No initial detachment efficiency could be measured for the hillslopes in the granitic terrain because hillslope measurements were started after the first major rainstorm. The initial detachment efficiency in the volcanic terrain was 1.3 ± 0.41%. The average remobilization and transport efficiencies associated with flash floods in the channels also were similar in the granitic (0.18 ± 0.57%) and volcanic (0.11 ± 0.41%) terrains. On the hillslope the remobilization and transport efficiency was greater in the volcanic terrain (2.4%) than in the granitic terrain (0.65%). However, this may reflect the reduced sediment availability after the first major rainstorm (30-min maximum rainfall intensity  90 mm h^− 1) in the granitic terrain, while easily erodible fine colluvium remained on the hillslope after the first rainstorm (30-min maximum rainfall intensity = 7.2 mm h^− 1) in the volcanic terrain. The erosion response in channels and on hillslopes of the granitic and volcanic terrains was similar when compared using erodibility efficiencies.     

Stratigraphy and 40Ar/39Ar geochronology of the Santa Rosa basin,Baja California: Dynamic evolution of a constrictional rift basin during oblique extension in the Gulf of California

The Santa Rosa basin of northeastern Baja California is one of several transtensional basins that formed during Neogene oblique opening of the Gulf of California. The basin comprises Late Miocene to Pleistocene sedimentary and volcanic strata that define an asymmetric half‐graben above the Santa Rosa detachment, a low‐angle normal fault with ca. 4–5 km of SE‐directed displacement. Stratigraphic analysis reveals systematic basin‐scale facies variations both parallel and across the basin. The basin‐fill exhibits an overall fining‐upward cycle, from conglomerate and breccia at the base to alternating sandstone‐mudstone in the depocentre, which interfingers with the fault‐scarp facies of the detachment. Sediment dispersal was transverse‐dominated and occurred through coalescing alluvial fans from the immediate hanging wall and/or footwall of the detachment. Different stratigraphic sections reveal important lateral facies variations that correlate with major corrugations of the detachment fault. The latter represent extension‐parallel folds that formed largely in response to the ca. N‐S constrictional strain regime of the transtensional plate boundary. The upward vertical deflection associated with antiformal folding dampened subsidence in the northeastern Santa Rosa basin, and resulted in steep topographic gradients with a high influx of coarse conglomerate here. By contrast, the downward motion in the synform hinge resulted in increased subsidence, and led to a southwestward migration of the depocentre with time. Thus, the Santa Rosa basin represents a new type of transtensional rift basin in which oblique extension is partitioned between diffuse constriction and discrete normal faulting. ⁴⁰Ar/³⁹Ar geochronology of intercalated volcanic rocks suggests that transtensional deformation began during the Late Miocene, between 9.36 ± 0.14 Ma and 6.78 ± 0.12 Ma, and confirms previous results from low‐temperature thermochronology (Seiler et al., 2011). Two other volcanic units that appear to be part of a conformable syn‐rift sequence are, in fact, duplicates of pre‐rift volcanics and represent allochthonous, gravity‐driven slide blocks that originated from the hanging wall.     

Late Quaternary (24–10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala

We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g⁻¹), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm⁻¹, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g⁻¹, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm⁻¹, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g⁻¹, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.     

The use of diatom records to establish reference conditions for UK lakes subject to eutrophication

A knowledge of pre-disturbance conditions is important for setting realistic restoration targets for lakes. For European waters this is now a requirement of the European Council Water Framework Directive where ecological status must be assessed based on the degree to which present day conditions deviate from reference conditions. Here, we employ palaeolimnological techniques, principally inferences of total phosphorus from diatom assemblages (DI-TP) and classification of diatom composition data from the time slice in sediment cores dated to ~1850 AD, to define chemical and ecological reference conditions, respectively, for a range of UK lake types. The DI-TP results from 169 sites indicate that reference TP values for low alkalinity lakes are typically <10 μg L⁻¹ and in many cases <5 μg L⁻¹, whilst those for medium and high alkalinity lakes are in the range 10–30 and 20–40 μg L⁻¹, respectively. Within the latter two alkalinity types, the deeper waters (>3 m mean depth) generally had lower reference TP concentrations than the shallow sites. A small group of shallow marl lakes had concentrations of ~30 μg L⁻¹. Cluster analysis of diatom composition data from 106 lakes where the key pressure of interest was eutrophication identified three clusters, each associated with particular lake types, suggesting that the typology has ecological relevance, although poor cross matching of the diatom groups and the lake typology at type boundaries highlights the value of a site-specific approach to defining reference conditions. Finally the floristic difference between the reference and present day (surface sample) diatom assemblages of each site was estimated using the squared chord distance dissimilarity coefficient. Only 25 of the 106 lakes experienced insignificant change and the findings indicate that eutrophication has impacted all lake types with >50% of sites exhibiting significant floristic change. The study illustrates the role of the sediment record in determining both chemical and ecological reference conditions, and assessing deviation from the latter. Whilst restoration targets may require modification in the future to account for climate induced alterations, the long temporal perspective offered by palaeolimnology ensures that such changes are assessed against a sound baseline.     

Beach–dune interactions on the dry–temperate Danube delta coast

Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolution assessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube delta coast, from 1997 to 2004. Dune volume increased consistently (1.96 m³ m^− 1 y^− 1 to 5.1 m³ m^− 1 y^− 1) over this 7-year period with higher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a new evolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during a year, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency is present. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transport measurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach. This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on the seaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s^− 1) and erodes during high winds (> 12 m s^− 1).     

Wind control on the accumulation of heavy metals in sediment of Lake Ulubat,Anatolia, Turkey

Freshwater Lake Ulubat (z _mean = 1.5–2.0 m and Area = ~138 km²), NW Anatolia, Turkey was filled in by fine-to-medium-grain silts during the late Holocene. Deposition in Lake Ulubat has been 1.6 cm year⁻¹ for the last 50 years, but the sedimentation rate over the last ~1,600 years was lower (0.37 mm year⁻¹). The organic matter and carbonate contents of the infill show cyclic changes that reflect environmental fluctuations. The silt-dominated lithology and the vertically uniform heavy metal distributions are probably due to wind-controlled sedimentation in the lake. Heterogeneous mud, derived from a large, mountainous drainage basin, is deposited in the lake mostly during summer, June to October, when conditions are hot and calm. Winter months are stormier and sediments are re-suspended due to the shallow water depth and the effect of waves on the lake bottom. It is likely that re-suspended sediments, particularly fine-grained particles, together with the heavy metals, are transported out of the lake via the outlet, especially during periods of high lake level. This resuspension and removal process probably caused the lake sediments to become silt-dominated and depleted in heavy metals. The role of broad shallow lakes in sequestering sediments and heavy metals can be described more accurately when wind data are considered. Such information may also be helpful for land-use planning in downstream areas.     

Late Pleistocene outburst flooding from pluvial Lake Alvord into the Owyhee River, Oregon

At least one large, late Pleistocene flood traveled into the Owyhee River as a result of a rise and subsequent outburst from pluvial Lake Alvord in southeastern Oregon. Lake Alvord breached Big Sand Gap in its eastern rim after reaching an elevation of 1292 m, releasing 11.3 km³ of water into the adjacent Coyote Basin as it eroded the Big Sand Gap outlet channel to an elevation of about 1280 m. The outflow filled and then spilled out of Coyote Basin through two outlets at 1278 m and into Crooked Creek drainage, ultimately flowing into the Owyhee and Snake Rivers. Along Crooked Creek, the resulting flood eroded canyons, stripped bedrock surfaces, and deposited numerous boulder bars containing imbricated clasts up to 4.1 m in diameter, some of which are located over 30 m above the present-day channel.Critical depth calculations at Big Sand Gap show that maximum outflow from a 1292- to 1280-m drop in Lake Alvord was  10,000 m³ s^− 1. Flooding became confined to a single channel approximately 40 km downstream of Big Sand Gap, where step-backwater calculations show that a much larger peak discharge of 40,000 m³ s^− 1 is required to match the highest geologic evidence of the flood in this channel. This inconsistency can be explained by (1) a single 10,000 m³ s^− 1 flood that caused at least 13 m of vertical incision in the channel (hence enlarging the channel cross-section); (2) multiple floods of 10,000 m³ s^− 1 or less, each producing some incision of the channel; or (3) an earlier flood of 40,000 m³ s^− 1 creating the highest flood deposits and crossed drainage divides observed along Crooked Creek drainage, followed by a later 10,000 m³ s^− 1 flood associated with the most recent shorelines in Alvord and Coyote Basins.Well-developed shorelines of Lake Alvord at 1280 m and in Coyote Basin at 1278 m suggest that after the initial flood, postflood overflow persisted for an extended period, connecting Alvord and Coyote Basins with the Owyhee River of the Columbia River drainage. Surficial weathering characteristics and planktonic freshwater diatoms in Lake Alvord sediment stratigraphically below Mt. St. Helens set Sg tephra, suggest deep open-basin conditions at  13–14 ka (¹⁴C yr) and that the flood and prominent shorelines date to about this time. But geomorphic and sedimentological evidence also show that Alvord and Coyote Basins held older, higher-elevation lakes that may have released earlier floods down Crooked Creek.     

Postglacial sedimentary record and history of West Hawk Lake crater,Manitoba

West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter. Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal, diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence; possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 ¹⁴C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ¹⁸O and δ¹³C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 ¹⁴C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 ¹⁴C years BP), plus a decrease in δ¹³C_cell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to the basin and possibly an increase in productivity.     

Basin modelling of a lignite‐bearing salt rim syncline: insights into rim syncline evolution and salt diapirism in NW Germany

The Helmstedt‐Staßfurt salt wall is 70 km long, 6–8 km wide and one of the most important diapiric structures in northern Germany, based on the economically significant lignite‐bearing rim synclines. The analysed Schöningen rim syncline, located on the southwestern side of the Helmstedt‐Staßfurt structure, is 8 km long and 3 km wide. The basin‐fill is up to 366 m thick and characterized by 13 major lignite seams with thicknesses between 0.1 and 30 m. The key objectives of this article were to expand on the classical cross‐section based rim syncline analysis by the use of 3D models and basin simulations. Cross‐sections perpendicular to the basin axis indicate that the basin‐fill has a pronounced lenticular shape. This shape varies from more symmetric in the NW to clearly asymmetric in the SE. Isopach maps imply a two‐fold depocentre evolution. The depocentre migrated over time towards the salt wall and also showed some distinct shifts parallel to the salt wall. The basin modelling part of the study was carried out with the software PetroMod^®, which focused on the burial history of the rim syncline. Modelling results also show the progressive migration of the rim syncline depocentre towards the salt wall. The present‐day asymmetry of the basin‐fill was already developed in the early phases of rim syncline evolution. The extracted geohistory curve shows initial rapid subsidence between 57 and 50 Ma and more moderate subsidence from 50 to 34 Ma. This pattern is interpreted to reflect salt evacuation from the source layer into the salt wall. The initial salt‐withdrawal rate was rapid, but later decreased probably due to depletion of the source layer.     

Diatom habitats,species diversity and water-depth inference models across surface-sediment transects in Worth Lake,northwest Ontario,Canada

We analyzed surface-sediment samples collected along transects from three sub-basins of a relatively large (~115 ha), bathymetrically complex lake, in northwest Ontario, Canada, to assess the reproducibility of diatom species habitats and diversity along a water-depth gradient. Transects displayed different orientations with respect to prevailing wind direction and varied in complexity and degree of slope along the lake bottom. Each transect consisted of three replicate samples at a resolution of ~1 m water depth from ~1 to 30 m for the two deep-basin transects and from ~1 to 18 m in the shallower basin. Distinct diatom assemblages were identified in all transects: (1) a near-shore community composed largely of attached life-forms and some motile benthic taxa, (2) a mid-depth community composed largely of motile life-forms and other benthic taxa that are adapted to lower light conditions (e.g. Staurosirella pinnata), and (3) a deep-water community dominated by planktonic taxa. Species richness was highest in the benthic zones (<9 m), with greatest species evenness in the mid-depth zone (~3–9 m). Species richness and evenness were highly correlated across the three transects (r = 0.89–0.93, p < 0.01). Diatom-inferred depth models were developed from the individual transects to assess reproducibility and applicability for down-core analyses using modern analog (MAT) and weighted-averaging (WA-PLS) approaches. Coefficients of determination (r ²) for these models ranged from 0.80 to 0.98, and RMSEP ranged from 1.2 to 4.2 m. The models developed from the transect with the highest resolution sampling, gentlest non-complex slope and shallowest maximum depth were the strongest ( r_\textMAT² = 0.97 r_{\text{MAT}}^{2} = 0.97 ; r_{\textWA - PLS}² = 0.98 r_{\text{WA - PLS}}^{2} = 0.98 ) and had the lowest RMSEP (MAT = 1.2 m, WA-PLS = 1.3 m). These inference models can be used to infer past fluctuations in the depth of the benthic/planktonic boundary from cores retrieved near this ecotone and provide a sensitive record of the past change in location of the benthic zone. These types of data can be used to assess past variability in droughts and lake levels to better plan for potential future extremes. Such records incorporate more realistic estimates of natural variability than the ~100-year instrumental records currently used by water resource managers.     

The sedimentary and palynological records of Serpent River Bog,and revised early Holocene lake-level changes in the Lake Huron and Georgian Bay region

Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve ¹⁴C dates on peat, wood and plant macrofossils combined with previously published ¹⁴C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which had been previously identified as seismo-stratigraphic sequence boundaries.     

Relationships between mesoscale morphological units, stream hydraulics and Chinook salmon (Oncorhynchus tshawytscha) spawning habitat on the Lower Yuba River, California

An expert-based approach was used to identify 10 morphological unit types within a reach of the gravel bed, regulated Yuba River, California, that is heavily utilized by spawning Chinook salmon (Oncorhynchus tshawytscha). Analysis of these units was carried out using two-dimensional hydrodynamic modeling, field-based geomorphic assessment, and detailed spawning surveying. Differently classified morphological units tended to exhibit discrete hydraulic signatures. In most cases, the Froude number adequately differentiated morphological units, but joint depth–velocity distributions proved the most effective hydraulic classification approach. Spawning activity was statistically differentiated at the mesoscale of the morphological unit. Salmon preferred lateral bar, riffle, and riffle entrance units. These units had moderately high velocity (unit median > 0.45 m s^− 1) and low depth (unit median < 0.6 m), but each exhibited a unique joint depth–velocity distribution. A large proportion of redds (79%) were associated with conditions of convective flow acceleration at riffle and riffle entrance locations. In addition to reflecting microhabitat requirements of fish, it was proposed that the hydraulic segregation of preferred from avoided or tolerated morphological units was linked to the mutual association of specific hydraulic conditions with suitable caliber sediment that promotes the provision and maintenance of spawning habitat.     

Sediment accumulation rates in European lakes since AD 1850: trends,reference conditions and exceedence

Sediment accumulation rate (SAR) is an important physical parameter in all lakes and increases have been observed in many over the last c.100 years. This has been ascribed to changes in land-use and land-management causing accelerated catchment soil erosion and an increase in autochthonous organic matter production. The EU Water Framework Directive requires that assessment of biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions. Currently however, the reference condition for sediment accumulation rate for lakes of different types is undefined. To improve our understanding of the controls on SARs we compiled SAR and lake typology data for 207 European lakes derived from ²¹⁰Pb dated cores to assess how rates have changed through time (in 25 year classes) both overall and for lakes of different types. Seventy-one percent of these sediment cores showed surface SARs higher than “basal” (mainly nineteenth century) rates, 11% showed no change while 18% showed a decline. Lakes were then classified into lake-types using four variables: alkalinity (3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake classes of which 25 were represented in the dataset. Nine lake-types contained >10 lakes. Little change in SAR occurred prior to 1900 and most increases occurred in more recent periods, in particular 1950–1975 and post-1975. This indicates a general acceleration in SAR in European lakes during the second half of the twentieth century. Reference SARs were estimated for six lake-types with the highest number of sites. European mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm⁻² yr⁻¹) while lowland, high alkalinity sites had the highest (0.03–0.04 g cm⁻² yr⁻¹). SARs for other lake-types ranged between 0.012 and 0.024 g cm⁻² yr⁻¹. Using the mountain lake-type as an example, the 1850 reference SAR appears to show good agreement with available data for lakes beyond Europe indicating these values may be more broadly applicable. Contemporary SARs in lakes of all classes showed exceedence over their defined reference SAR. This may be partly due to diagenetic processes. Greatest exceedences were found in shallow, low altitude lakes and these are considered to be the ones under the greatest threat from continued elevation of SAR. It is considered that climate change may play a progressively more important role in driving SAR in the future.     

On the origin of rock fragment mulches on Vertisols: A case study from the Ethiopian highlands

Many Vertisols in Tigray, Ethiopia, typically carry a discontinuous rock fragment (RF, size 0.5–> 40 · 10^− 2 m) cover with 10 to 100 RFs m^− 2. Such RF mulches are of agricultural and environmental significance because they influence the water balance in the underlying soils and the crop yield. Natural RF concentrations are mostly considered as eolian or hydraulic lag deposits, or as the result of lateral transport over the soil surface from a rock outcrop, upslope. In cultivated areas RF mulches can develop by tillage.This paper presents the case of a natural RF mulch whose lithology indicates that the RFs are up-squeezed by the local Vertisol. The study site is located in the pass of Enda Maryam, Tigray, Northern Ethiopia (39°8′ E and 13°36′ N). A circular area of 10 m diameter, about 200 m away from the water divide in the valley has been cleared annually between 01/1999 and 05/2003. During this period, 625 RFs, 17 being > 7.5 · 10^− 2 m in size, totalling a mass of nearly 62 kg, have been collected. After correction for measurement procedures, the rate of RF up-warping by the Vertisol at Enda Maryam is assessed at 5 RFs m^− 2 in 3 years. At this rate of appearance, the formation of current RF concentrations on top of active valley Vertisols is only a matter of 10^1–2 years, provided the availability of RFs below the soil surface.Although important underground displacements were measured in the Vertisol between 01/1999 and 05/2002, the supposed link between up-squeezing of RFs and plastic deformations of ‘chimney’, ‘diapir’ or ‘intrusion’-like type in the Vertisol could not be evidenced. Instead, RFs are clearly concentrated on the soil surface as well as in depth, along the existing vertical desiccation cracks, often > 1 m deep which display polygonal configurations at the soil surface. Further, bundles of slickensides containing some RFs, have been mapped at the base of the Vertisol. The slickenside configuration suggests that the RF-bearing substrate is being scraped off.While the underground displacement of RFs along active slickensides seems normal, the process of RFs ascending in ‘upright’ position in the edge of desiccation cracks needs explanation. The closure of a desiccation crack is a peristaltic-like movement, following ascent or descent of the capillary fringe. It is hypothesized that this movement gradually pushes the RF to the surface or to another place or level in the soil profile where the crack closes in last instance.The apparent young age of the valley Vertisol mulches in Ethiopia might indicate the very recent formation of yearly recurrent desiccation cracks of Vertisols in the area. Available information confirms that most valleys in the study area used to be perennially marshy. Under these conditions no movements of RFs in the soil profile are expected to occur. Gullying, leading to pronounced seasonal desiccation of the Vertisols, started in several cases not more than 50 years ago.     

Nipa (Distichlis palmeri): A perennial grain crop for saltwater irrigation

The perennial saltgrass nipa (Distichlis palmeri, Poaceae) is endemic to northern Gulf of California tidal marshes flooded with hypersaline (38–42 g L⁻¹) seawater. Nipa was a wild harvest staple of the Cocopah people of the Río Colorado delta. We investigated the physiology, anatomy, chromosome number, and agronomic potential of nipa as a global food crop. Nipa seeds had 60–93% germination on salinities ranging from 0 to 30 g L⁻¹. Relative Growth Rates (RGR) on both flooded and aerobic conditions remained above 4% d⁻¹ up to 30 g L⁻¹, about half the RGR on freshwater. Nipa grain (caryopses) had 7–8% protein, 8% sugar and 79% total digestible carbohydrates (mostly starch) and only 2% ash and 8% fiber, equal to conventional grains in apparent nutritional value. Shoots were low in ash and sodium, and compared favorably to alfalfa forage in protein, digestible carbohydrates and energy contents. Mature female stands in the Colorado River delta produced an estimated 1.25 t ha⁻¹ of grain, but over two years in the greenhouse only partial flowering was observed. Nevertheless, D. palmeri appears to be worth developing as a perennial grain and forage crop, especially for salinized, flooded soils.     

Cosmogenic nuclide-based investigation of spatial erosion and hillslope channel coupling in the transient foreland of the Swiss Alps

Transient landscape disequilibrium is a common response to climatic fluctuations between glacial and interglacial conditions. Such landscapes are best suited to the investigation of catchment-wide response to changes in incision. The geomorphology of the Trub and Grosse Fontanne, adjacent stream systems in the Napf region of the Swiss Molasse, was analyzed using a 2-m LIDAR DEM. The two catchments were impacted by the Last Glacial Maximum, LGM, even though the glaciers never overrode this region. They did, however, cause base levels to drop by as much as 80 m. Despite their similar tectonic, lithologic and climatic settings, these two basins show very different responses to the changing boundary conditions. Stream profiles in the Trub tend to be smooth, while in the Fontanne, numerous knickzones are visible. Similarly, cut-and-fill terraces are abundant in the Trub watershed, but absent in the Fontanne, where deep valleys have been incised. The Trub appears to be a coupled hillslope–channel system because the morphometrics throughout the basin are uniform. The morphology of hillslopes upstream of the knickzones in the Fontanne is identical to that of the Trub basin, but different downstream of the knickzones, suggesting that the lower reaches of the Fontanne have been decoupled from the hillslopes. However, the rapid incision of the Fontanne is having little effect on the adjacent upper hillslopes.We tested this interpretation using cosmogenic ¹⁰Be-derived basin-averaged denudation rates and terrace dating. The coupled nature of the Trub basin is supported by the similarity of denudation rates, 350 ± 50 mm ky^− 1, at a variety of spatial scales. Upstream of the knickzones, rates in the Fontanne, 380 ± 50 mm ky^− 1, match those of the Trub. Downstream of the knickzones, denudation rates increase to 540 ± 100 mm ky^− 1. The elevated rates in the downstream areas of the Fontanne are due to rapid incision causing a decoupling of the hillslope from the channel. Basin response time and the magnitude of base level drop exert the principal control over the difference in geomorphic response between the two basins. The timing of the filling of the Trub valley, 17 ± 2 ka, and the initial incision of the Fontanne, 16 ± 3 ka, were calculated, verifying that these are responses to late glacial perturbations. Unique lithologic controls allow for one of the fastest regolith production rates yet to be reported,  380 mm ky^− 1.     

Mineral magnetism and other characteristics of sediments from an alpine lake (3,410 m a.s.l.) in central China and implications for late Holocene climate and environment

The Qinling Mountain Range (33°–34°30′N, 107°–111°E; 3,767 m a.s.l.) lies south of the Chinese Loess Plateau and functions as the boundary between ‘north’ and ‘south’ China. Taibai Mountain (33°41′–34°10′N, 107°19′–107°58′E; 3,767 m a.s.l.) is the central massif and highest part of the range and is the highest mountain in eastern and central China, east of 105°E. It is also one of two mountains higher than the modern climatic timberline and the only one where high alpine lakes (>2,500 m a.s.l.) exist in eastern and central China. Sediments were recovered from Foye Chi (33°57′N, 107°44′E; 3,410 m a.s.l.), a small lake on the southern slope of the mountain, and measured for magnetic properties. Chronological control was achieved with AMS ¹⁴C dating. Combined with analyses of particle-size, TOC, C/N, δ¹³C_org and pollen in these sediments, and magnetic properties of catchment soils, the mineral-magnetic data reveal late Holocene palaeoenvironmental changes on the high-altitude southern slope of Taibai Mountain. Climate gradually ameliorated about 2,300 cal yr BP and warm and wet conditions occurred afterwards, culminating from 1,700 to 1,510 cal yr BP. The climate began to deteriorate at 1,510 cal yr BP, but was still warmer and wetter than present until ~663 cal yr BP. Cool, arid conditions peaked and were cooler and drier than the present at 663–290 cal yr BP, coincident with the Little Ice Age. Climate became warmer and more humid again after 290 cal yr BP. Data from these less anthropologically-disturbed alpine-lake sediments provide a record of late Holocene palaeoenvironmental change that supplements information from historical documents and literature for eastern and central China.