Physiographic Controls Research Articles

Streamflow drought onset and severity explained by non‐linear responses between climate‐catchment and land surface processes

AbstractKnowledge of drought onset and its relationship with drought severity (deficit volume) is crucial for providing timely information for reservoir operations, irrigation scheduling, devising cropping choices and patterns and managing surface and groundwater water resources. An analysis of the relationship between drought onset timing and deficit volume can help in drought hazard assessments and associated risks. Despite its importance, little attention has been paid to understand the drought onset timing and its potential linkage with deficit volume for effective drought monitoring and its impact assessment. Further, only a few studies have explored the role of environmental controls, encompassing the interaction between climate, catchment and land‐surface processes in influencing streamflow droughts and associated characteristics such as onset time and severity. This study leverages quality‐controlled streamflow observations from 1965 to 2018 to unveil regional patterns of streamflow drought onset, at‐site trends in deficit volume and detect non‐linear relationships between onset timing and deficit volume across 82 rain‐fed catchments in peninsular India (8°–24° N, 72°–87° E). We show that around 12% of catchments show an earlier onset of streamflow droughts in conjunction with a decreasing trend in deficit volume. Further, approximately one‐third of the catchments show a significant non‐linear dependency between drought deficit volume and onset time. Among catchment controls, such as soil and topographic properties, we found soil organic carbon stock and stock as dominant drivers controlling the streamflow drought onset time. Likewise, sand content and vertical distance to channel network control the streamflow deficit volume. Finally, the linkages between inferred dominant low‐flow generation mechanisms and the specific combinations of environmental controls are synthesized in a conceptual diagram that might assist in developing appropriate models for low‐flow simulations and predictions, especially across ungauged sites. The new insights add value to understanding the chain of physical processes linking climatic and physiographic controls on streamflow droughts, which can support drought forecasting and climate impact assessment efforts.

Vegetation communities and summer net ecosystem CO2 exchange on western Axel Heiberg Island, Canadian High Arctic

Climate change is expected to result in the Arctic transitioning from a carbon sink to a carbon source environment, with models predicting half of the carbon stock of the upper 3 m soil layer to be released by the year 2300 (van Huissteden and Dolman 2012). However, uncertainty in latitudinal warming and changes in Arctic ecosystem functions, such as gross carbon ecosystem exchange (GEE), are poorly understood, in part a reflection of a high variability in vascular plant community diversity that is dependent upon and sensitive to physiographic controls, such as soil moisture, topography, and seasonal active layer depth (Walker et al. 2005). This heterogeneity complicates assessments of carbon fluxes on a landscape scale and how they will change in the future (Shaver et al. 2007), especially given their sensitivity to local changes in climate, such as warming and higher rates of rainfall (Bintanja 2018, Bintanja and Andry 2017). As part of the creation of a long-term ecological and environmental monitoring program at the McGill Arctic Research Station at Expedition Fiord, western Axel Heiberg Island, field-based studies in 2021-2022 of plant surveys and summer net ecosystem CO2 exchange monitoring were undertaken to: define the major vegetation communities; quantify and investigate CO2 fluxes with chambers and their analogous biophysical variables; and upscale plot level CO2 measurements to the landscape scale using high spatial resolution remote sensing data. define the major vegetation communities; quantify and investigate CO2 fluxes with chambers and their analogous biophysical variables; and upscale plot level CO2 measurements to the landscape scale using high spatial resolution remote sensing data. The Expedition Fiord area is recognized as a polar oasis, with high plant species richness existing within an environment of heterogeneous physiography. At the moment, five vegetation communities have been identified (xeric dwarf shrub barren, xeric-mesic dwarf shub barren, mesic dwarf shrub tundra, cassiope heath, and sedge meadow) that varied as a function of species diversity, percent cover, soil moisture, and net ecosystem carbon exchange. Barren vegetation communities having stronger respiration fluxes (i.e., carbon source environments) while more vegetated communities have stronger photosynthesis fluxes (i.e., carbon sink environments). Landcover classification revealed with high accuracy (79.3%) that barren ground and barren vegetation communities cover a much larger area compared to wetter habitats. Upscaling summer season measured carbon fluxes based on the landcover map revealed that Expedition Fiord is a carbon source environment, with an average efflux of +94.6 g CO2/day. Ongoing work focuses on the expansion of carbon flux and subsurface monitoring locations, as well as studies of soil carbon and microbial diversity across the different land cover classifications, which will help to better resolve how soil microorganisms, plant detritus, labile organic carbon, soil moisture, slope, aspect, and bedrock geology influence CO2 fluxes throughout the summer season in this high Arctic setting.

Insights into the influence of morphology on the hydrological processes of river catchments using stable isotopes

ABSTRACT Water isotopes (δ18O, δ2H) were systematically monitored in two river catchments to investigate the isotopic spatiotemporal variation and the differences between them. The Pinios River Basin (PRB) exhibited lower average δ 18O and δ 2H values (−7.9‰ and −50.8‰, respectively) compared to the Evrotas River Basin (ERB) (−6.5‰ and −38.2‰, respectively) but higher in range (3.3‰ vs 1.2‰ for δ18O, respectively). The Bayesian modelling results showed higher groundwater contribution in the PRB (25–50%) than in the ERB (15–35%) relative to precipitation during the wet period. The isotopic spatial variability was attributed to the influence of local precipitation, evaporation and additional flow pathways (e.g. soil water). The correlation analysis showed that the isotopic composition is controlled by the catchment altitude, slope and discharge. This study highlights the catchment physiographic control on the isotopic composition of rivers, which can support strategies for better water resources management.

Physiographic Controls on Landfast Ice Variability from 20 Years of Maximum Extents across the Northwest Canadian Arctic

Landfast ice is a defining feature among Arctic coasts, providing a critical transport route for communities and exerting control over the exposure of Arctic coasts to marine erosion processes. Despite its significance, there remains a paucity of data on the spatial variability of landfast ice and limited understanding of the environmental processes’ controls since the beginning of the 21st century. We present a new high spatiotemporal record (2000–2019) across the Northwest Canadian Arctic, using MODIS Terra satellite imagery to determine maximum landfast ice extent (MLIE) at the start of each melt season. Average MLIE across the Northwest Canadian Arctic declined by 73% in a direct comparison between the first and last year of the study period, but this was highly variable across regional to community scales, ranging from 14% around North Banks Island to 81% in the Amundsen Gulf. The variability was largely a reflection of 5–8-year cycles between landfast ice rich and poor periods with no discernible trend in MLIE. Interannual variability over the 20-year record of MLIE extent was more constrained across open, relatively uniform, and shallower sloping coastlines such as West Banks Island, in contrast with a more varied pattern across the numerous bays, headlands, and straits enclosed within the deep Amundsen Gulf. Static physiographic controls (namely, topography and bathymetry) were found to influence MLIE change across regional sites, but no association was found with dynamic environmental controls (storm duration, mean air temperature, and freezing and thawing degree day occurrence). For example, despite an exponential increase in storm duration from 2014 to 2019 (from 30 h to 140 h or a 350% increase) across the Mackenzie Delta, MLIE extents remained relatively consistent. Mean air temperatures and freezing and thawing degree day occurrences (over 1, 3, and 12-month periods) also reflected progressive northwards warming influences over the last two decades, but none showed a statistically significant relationship with MLIE interannual variability. These results indicate inferences of landfast ice variations commonly taken from wider sea ice trends may misrepresent more complex and variable sensitivity to process controls. The influences of different physiographic coastal settings need to be considered at process level scales to adequately account for community impacts and decision making or coastal erosion exposure.

Spatiotemporal shifts in key hydrological variables and dominant factors over China

AbstractQuantitatively and physically understanding changes in land surface hydrology is a hot topic in the hydro‐meteorological research, especially over China with high population density but uneven distribution of water resources. Therefore, the spatiotemporal dynamics (i.e., the 1980s [1981–1990] and 1990s [1991–2000] relative to the baseline of 1961–1980) in evapotranspiration (ET) and streamflow/runoff (Q) were examined across 426 hydrological divisions (HDs) of China. Both the 1980s and 1990s ET decreased over roughly 50% HDs mainly in the central and southeast parts of China, while the 1980s (1990s) Q decreased over slightly higher than 50% (63%) HDs generally in north and southwest China (the central and north parts of China). Relative to the 1980s, more HDs had strong changes in ET and Q in 1990s. Based on the separated contributions of precipitation (P), reference ET and Budyko‐type equation parameter n (an integrated variable of catchment property) to ET and Q changes, we found that in 1980s and 1990s, n dominated ET changes over about 60% HDs mainly in the south, with the dominant of P around 35% HDs generally in the north and northeast, while for Q changes, nearly 60% (slightly less than 40%) HDs were dominated by n (P). Moreover, the dominants for ET (Q) changes have shifted from 1980s to 1990s over 36% (45%) HDs, mainly changing from P to n. This study provides a framework for quantitatively understanding land surface hydrological dynamics from the perspectives of climatic and physiographic controls over regions.

Regional Patterns and Physical Controls of Streamflow Generation Across the Conterminous United States

AbstractThis study unveils regional patterns of dominant streamflow generation mechanisms across the conterminous United States and identifies their climatic and physiographic controls. Six signatures describing the characteristics of rainfall‐stormflow responses and the characteristics of base flow response are extracted from continuous rainfall‐streamflow time series for 432 US catchments. These study catchments are classified into eight classes based on similarities of the six signatures, and dominant streamflow generation mechanisms for each class are inferred based on the magnitudes of the six signatures. This catchment classification reveals geographically coherent regional patterns across the conterminous United States. Catchment climatic and physiographic characteristics most relevant to this catchment classification are identified using techniques such as correlation analysis and random forest modeling. The most relevant characteristics are climate aridity, median event rainfall volume, topographic slope, soil permeability, rock type, and vegetation density, which are found to be the main controlling factors governing the dominant streamflow generation mechanisms. The linkages between the inferred dominant streamflow generation mechanisms and specific combinations of the climatic and catchment physiographic characteristics identified above are synthesized in a conceptual diagram, as an extension of the classical Dunne diagram. The new findings, while largely consistent with the results of previous empirical and modeling studies, add rich process detail to the transferable understanding of regional patterns of dominant streamflow generation mechanisms and their climatic and physiographic controls.

Pathways of tundra encroachment by trees and tall shrubs in the western Brooks Range of Alaska

Climate change is expected to increase woody vegetation abundance in the Arctic, yet the magnitude, spatial pattern and pathways of change remain uncertain. We compared historical orthophotos photos (1952 and 1979) with high‐resolution satellite imagery (2015) to examine six decades of change in abundance of white spruce Picea glauca and tall shrubs (Salix spp., Alnus spp.) near the Agashashok River in northwest Alaska. We established ~3000 random points within our ~5500 ha study area for classification into nine cover types. To examine physiographic controls on tree abundance, we fit multinomial log‐linear models with predictors derived from a digital elevation model and with arctic tundra, alpine tundra and ‘tree’ as levels of a categorical response variable. Between 1952 and 2015, points classified as arctic and alpine tundra decreased by 31% and 15%, respectively. Meanwhile, tall shrubs increased by 86%, trees mixed with tall shrubs increased by 385% and forest increased by 84%. Tundra with tall shrubs rarely transitioned to forest. The best multinomial model explained 71% of variation in cover and included elevation, slope and an interaction between slope and ‘northness’. Treeline was defined as the elevation where the probability of tree presence equaled that of tundra. Mean treeline elevation in 2015 was 202 m, corresponding with a June–August mean air temperature > 11°C, which is > 4°C warmer than the 6–7°C isotherm associated with global treeline elevations. Our results show dramatic increases in the abundance of trees and tall shrubs, question the universality of air temperature as a predictor of treeline elevation and suggest two mutually exclusive pathways of vegetation change, because tundra that gained tall shrubs rarely transitioned to forest. Conversion of tundra to tall shrubs and forest has important and potentially contrasting implications for carbon cycling, surface energy exchange and wildlife habitat in the Arctic.

Regional-scale paleobathymetry controlled location, but not magnitude, of tidal dynamics in the Late Cretaceous Western Interior Seaway, USA

Abstract Despite extensive outcrop and previous sedimentologic study, the role of tidal processes along sandy, wave- and river-dominated shorelines of the North American Cretaceous Western Interior Seaway remains uncertain, particularly for the extensive mid-Campanian (ca. 75–77.5 Ma) tidal deposits of Utah and Colorado, USA. Herein, paleotidal modeling, paleogeographic reconstructions, and interpretations of depositional process regimes are combined to evaluate the regional-scale (hundreds to thousands of kilometers) basin physiographic controls on tidal range and currents along these regressive shorelines in the “Utah Bight”, southwestern Western Interior Seaway. Paleotidal modeling using a global and astronomically forced tidal model, combined with paleobathymetric sensitivity tests, indicates the location of stratigraphic units preserving pronounced tidal influence only when the seaway had a deep center (∼400 m) and southern entrance (>100 m). Maximum tidal velocity vectors under these conditions suggest a dominant southeasterly ebb tide within the Utah Bight, consistent with the location and orientation of paleocurrent measurements in regressive, tide-influenced deltaic units. The modeled deep paleobathymetry increased tidal inflow into the basin and enhanced local-scale (tens to hundreds of kilometers) resonance effects in the Utah Bight, where an amphidromic cell was located. However, the preservation of bidirectional, mudstone-draped cross-stratification in fine- to medium-grained sandstones requires tides in combination with fluvial currents and/or local tidal amplification below the maximum resolution of model meshes (∼10 km). These findings suggest that while regional-scale controls govern tidal potential within basins, localized physiography exerts an important control on the preservation of tidal signatures in the geologic record.

Geology controls streamflow dynamics

Relating stream dynamics to catchment properties is essential to anticipate the influence of changing environmental conditions and to predict flows of ungauged rivers. Although the importance of subsurface processes in catchment hydrology is widely acknowledged, geological characteristics are rarely explicitly included in studies assessing physiographic controls on catchment dynamics. In this investigation of 22 catchments of the Swiss Plateau and Prealpes, we use a simple linear regression approach to analyze the relationship between streamflow indicators and various geological and hydrogeological properties of the bedrock and quaternary deposits, along with meteorological, soil, land use and topographical characteristics. We use long-term discharge percentiles, as well as dimensionless flow duration curves (FDC, standardized by long-term mean discharge) that allow to evaluate the catchment response to climate forcing. While climate conditions dominate the high to medium discharge percentiles (Q5–Q50), the capacity of the catchments to buffer the meteorological forcing can only be attributed to geological characteristics. The sandstone proportion in the catchments explains 54% of the variance of both extremities of the dimensionless FDC (Q5/Qmean and Q95/Qmean) and productive quaternary deposits are responsible of 55% resp. 58% of the variance of the two ratios. Examining the hydrogeological characteristics of both bedrock and quaternary lithologies considerably improves the understanding of catchment dynamics.

Controls on tidal sedimentation and preservation: Insights from numerical tidal modelling in the Late Oligocene–Miocene South China Sea, Southeast Asia

AbstractNumerical tidal modelling, when integrated with other geological datasets, can significantly inform the analysis of physical sedimentation processes and the depositional and preservational record of ancient tide‐influenced shoreline–shelf systems. This is illustrated in the Oligo–Miocene of the South China Sea, which experienced significant changes in basin physiography and where tide‐influenced, shoreline–shelf deposition is preserved in ca 10 sub‐basins. Palaeogeographic reconstructions, palaeotidal modelling and regional sedimentary facies analysis have been integrated in order to evaluate the spatial–temporal evolution and physiographic controls on tidal sedimentation and preservation during the ca 25 Myr Oligo–Miocene record in the South China Sea. Palaeotidal modelling, using an astronomically‐forced and global tidal model (Fluidity) at a maximum 10 km resolution, indicates that spring tides along Late Oligocene to Middle Miocene coastlines were predominantly mesotidal–macrotidal and capable of transporting sand, which reflects two main conditions: (i) increased tidal inflow through wider ocean connections to the Pacific Ocean; and (ii) tidal amplification resulting from constriction of the tidal wave in a ‘blind gulf’ type of basin morphology. Since the Middle to Late Miocene, a reduction in the amplitude and strength of tides in the South China Sea was mainly due to diminishing tidal inflow from the Pacific Ocean caused by the northward movement of the Philippines and Izu–Bonin–Mariana arc. Sensitivity tests to palaeogeographic and palaeobathymetric uncertainty indicate that regional‐scale (hundreds to thousands of kilometres) palaeogeographic changes influencing tidal inflow versus outflow can override local‐scale (one to hundreds of kilometres) changes to tidal resonance and convergence effects (funnelling and shoaling), such as shelf width and shoreline geometry. Palaeotidal model results compare favourably to the distribution and sedimentary fabric of Oligo–Miocene, tide‐influenced, shoreline–shelf successions in peripheral South China Sea basins. However, the preservation potential of tidal deposits is lower in open coastline environments, probably due to enhanced reworking during storms and river floods.

Hydroclimatic influences and physiographic controls on phosphorus dynamics in prairie pothole wetlands

While wetlands are known as long-term storages or sinks for contaminants, not all are equally effective at trapping phosphorus (P). The prevalence of P-sink behavior in prairie pothole wetlands remains unclear, especially across gradients of human disturbance. The objectives of the current study were three-fold: (1) characterize the spatiotemporal variability of wetland hydrology and wetland water P concentration across a range of prairie potholes; (2) establish the propensity of different pothole wetlands to act as sources or sinks of P; and (3) assess the potential controls of climatic conditions, landscape characteristics, wetland soil physiochemical properties and local hydrology on source versus sink dynamics. Ten intact and three consolidated (i.e., drained) wetlands located in southwestern Manitoba, Canada, were monitored for water level fluctuations and water soluble reactive P (SRP) concentration over two years with contrasting antecedent wetness conditions. Soil cores were also collected to measure soil physiochemical properties such as the equilibrium phosphorus concentration (EPC). Water column SRP concentrations were compared to EPC values to infer the time-variable source versus sink behavior of each of wetland. Statistical analyses were then performed to assess whether the source versus sink behavior of individual wetlands could be linked to their physiographic or hydrologic characteristics. Results show that some wetlands persistently acted as P sinks while others switched between source and sink behavior. Persistent P-sink behavior was more common with intact wetlands, as opposed to consolidated wetlands. Wetland soil texture, storage volume and short-term water level fluctuations appeared to control the source versus sink behavior of individual wetlands. The dominant controls on P-sink behavior identified under dry conditions were, however, different from those identified under wetter conditions. This study therefore highlights the importance of considering the non-stationary nature of P-sorption dynamics and their controls, even at sub-annual timescales, in the prairie pothole region.

Modelling high‐latitude summer temperature patterns using physiographic variables

Recent investigations of high‐resolution temperature patterns employing dense observational networks have improved our understanding of fine‐scale climatic influences on hydrological and ecological processes. Few of these investigations, however, have focused on high‐latitude environments. In this study—one of the first to evaluate drivers of fine‐scale temperature patterns in a non‐glaciated high‐latitude environment—we evaluated physiographic controls on summer temperature while accounting for free‐air temperature conditions along a north–south transect of temperature sensors on Mt. Healy, at the eastern edge of Denali National Park and Preserve in Alaska, United States. Although the study was limited to summer, when near‐surface temperature inversions are rare, we identified average daily minimum temperature (Tmin) lapse rates shallower than −2 °C/km over five summers. Lapse rates of average temperature (Tavg) were steeper (−5.3 °C/km), but still did not approach the global average lapse rate of −6.5 °C/km. We also identified positive effects of solar radiation on both Tmin and Tavg, variation in Tmin and Tavg lapse rates across a range of synoptic temperature conditions, and variation in Tmin lapse rates across terrain convergence values. Finally, we compared relationships between elevation and temperature from transect sensors and a gridded product. Over 15 months of comparison, sensor Tavg was 0.6 °C cooler than gridded data at similar elevations. Sensor lapse rates were on average 0.5 °C steeper than those from gridded data but only significantly so in the cool month of June 2014. These differences highlight the need for additional instrumentation across this remote and topographically variable region to improve accuracy and reduce biases in gridded climate products.

Climatic and physiographic controls of spatial variability in surface water balance over the contiguous United States using the Budyko relationship

AbstractThe geographic variability in the partitioning of precipitation into surface runoff (Q) and evapotranspiration (ET) is fundamental to understanding regional water availability. The Budyko equation suggests this partitioning is strictly a function of aridity, yet observed deviations from this relationship for individual watersheds impede using the framework to model surface water balance in ungauged catchments and under future climate and land use scenarios. A set of climatic, physiographic, and vegetation metrics were used to model the spatial variability in the partitioning of precipitation for 211 watersheds across the contiguous United States (CONUS) within Budyko's framework through the free parameter ω. A generalized additive model found that four widely available variables, precipitation seasonality, the ratio of soil water holding capacity to precipitation, topographic slope, and the fraction of precipitation falling as snow, explained 81.2% of the variability in ω. The ω model applied to the Budyko equation explained 97% of the spatial variability in long‐term Q for an independent set of watersheds. The ω model was also applied to estimate the long‐term water balance across the CONUS for both contemporary and mid‐21st century conditions. The modeled partitioning of observed precipitation to Q and ET compared favorably across the CONUS with estimates from more sophisticated land‐surface modeling efforts. For mid‐21st century conditions, the model simulated an increase in the fraction of precipitation used by ET across the CONUS with declines in Q for much of the eastern CONUS and mountainous watersheds across the western United States.

Physiographic factors underlie rates of biomass production during succession in Great Lakes forest landscapes

Biomass production in living trees is the basis of numerous forest ecosystem functions and services. However, rates of and controls on biomass production vary widely across temperate forests, particularly over successional timescales of decades and centuries. Biomass production in temperate forests is most often interpreted within the context of biotic or top-down controls, such as species composition or disturbance. However, there is need to investigate how bottom-up physiographic factors, such as landform attributes, drainage, and soil properties mediate biomass production. In order to investigate patterns, controls, and potentials for biomass production across spatial levels ranging from individual ecosystems, to landscapes, to entire regions, we synthesized long-term forest inventory datasets from the United States Great Lakes region, placed them in the context of a hierarchical ecological unit classification, and tested the influence of physiographic factors on biomass production rates and temporal trajectories across ecological levels. Key findings include: 1) At nearly all ecological levels, physiographic controls (e.g., soil texture, drainage class, water table depth) on soil moisture status are significant predictors of variation in biomass production rates, with mesic sites accumulating biomass more rapidly than xeric sites, which, in turn accumulate biomass more rapidly than hydric sites. 2) Aboveground live biomass can apparently continue to accumulate through 2–3 centuries of succession, exceeding 300–400Mgha−1 on mesic sites throughout the region. 3) Stand age distributions indicate that hydric sites are harvested least often, while the high production rates of mesic sites suggest they are most appropriate for frequent harvesting. 4) Median, 1st-quartile, and 3rd-quartile growth rates of individual ecosystems, landscapes, and ecoregional subsections and sections reveal ecological units in which forests may vary in their potential for increases or decreases in biomass production, e.g., due to management interventions, climate change, or disturbances. Specifically, some units have tightly constrained distributions, suggesting little capacity for change in production rates relative to observed medians, while other units have wide variation in biomass production rates, indicating the potential for relatively large increases or decreases in production. Altogether, the results of this analysis show that physiography exerts widespread, bottom-up controls on biomass production across the region of study, and can be used in spatially explicit frameworks to understand ecosystem functioning and inform scientific forest management.

On the potential for terrestrial diatom communities and diatom indices to identify anthropic disturbance in soils

A large amount of studies focuses on aquatic diatoms’ ecology and their use in the assessment of water quality. Little is known about terrestrial diatoms’ ecological behaviour and sensitivity to environmental factors. We hypothesise that terrestrial diatom communities can serve as a proxy of anthropic disturbance levels in terrestrial sites. To test our hypothesis, we apply an aquatic index to soil communities that is to deliver new information on the physiographic controls on soil diatoms. Diatom and soil samples were collected in the Attert River basin in Luxembourg during three seasons, in sites characterised by different combinations of geological, soil (schist, marl and sandstone) and land use (forest, grassland and agriculture) features. We found an effect of seasonality on soil diatom communities, reflected by different species dominance and abundances in samples during the three seasons. Soil pH and land use (which translates in a different amount of total carbon and nitrogen in soil) were identified as the variables having the largest impact in structuring the communities and as among the features with the highest importance in defining the ecological status of the sites (i.e. disturbed farmlands having higher pH and lower carbon and nitrogen content). However, the lack of information about the sensitivity of some of the most abundant terrestrial species in our study area caused some discrepancies between the expected (i.e. forested areas with low anthropic disturbance) and the obtained results, with several forested sites classified as having high anthropic disturbance. These results suggest that soil communities are likely to contain information about soil ecological status and highlight the importance of a better characterisation of terrestrial diatom species for developing a quality index based on soil communities.

Isoscapes of δ18O and δ2H reveal climatic forcings on Alaska and Yukon precipitation

AbstractSpatially extensive Arctic stable isotope data are sparse, inhibiting the climatic understanding required to interpret paleoclimate proxy records. To fill this need, we constrained the climatic and physiographic controls on δ18O and δD values of stream waters across Alaska and the Yukon to derive interpolated isoscape maps. δ18O is strongly correlated to winter temperature parameters and similarity of the surface water line (δ2H = 8.0 × δ18O + 6.4) to the Global Meteoric Water Line suggests stream waters are a proxy for meteoric precipitation. We observe extreme orographic δ18O decreases and a trans‐Alaskan continental gradient of −8.3‰ 1000 km−1. Continental gradients are high in coastal zones and low in the interior. Localized δ18O increases indicate inland air mass penetration via topographic lows. Using observed δ18O/temperature gradients, we show that δ18O decreases in a ∼24 ka permafrost ice wedge relative to the late Holocene indicate mean annual and coldest quarter temperature reductions of 8.9 ± 1.7°C and 17.2 ± 3.2°C, respectively.

Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica

Abstract. Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

Geologic and physiographic controls on bed-material yield, transport, and channel morphology for alluvial and bedrock rivers, western Oregon

The rivers of western Oregon have diverse forms and characteristics, with channel substrates ranging from continuous alluvial gravel to bare bedrock. Analysis of several measurable morphologic attributes of 24 valley reaches on 17 rivers provides a basis for comparing nonalluvial and alluvial channels. Key differences are that alluvial reaches have greater bar area, greater migration rates, and show systematic correlation among variables relating grain size to bed-material transport capacity. We relate these differences between channel types to bed-material transport rates as derived from a coupled regional analysis of empirical sediment yield measurements and physical experiments of clast attrition during transport. This sediment supply analysis shows that overall bed-material transport rates for western Oregon are chiefly controlled by (1) lithology and basin slope, which are the key factors for bed-material supply into the stream network, and (2) lithologic control of bed-material attrition from in-transport abrasion and disintegration. This bed-material comminution strongly affects bed-material transport in the study area, reducing transport rates by 50%–90% along the length of the larger rivers in the study area. A comparison of the bed-material transport estimates with the morphologic analyses shows that alluvial gravel-bed channels have systematic and bounding relations between bed-material transport rate and attributes such as bar area and local transport capacity. By contrast, few such relations are evident for nonalluvial rivers with bedrock or mixed-bed substrates, which are apparently more influenced by local controls on channel geometry and sediment supply. At the scale of western Oregon, the physiographic and lithologic controls on the balance between bed-material supply and transport capacity exert far-reaching influence on the distribution of alluvial and nonalluvial channels and their consequently distinctive morphologies and behaviors—differences germane for understanding river response to tectonics and environmental perturbations, as well as for implementing effective restoration and monitoring strategies.

Cold-seep habitat mapping: High-resolution spatial characterization of the Blake Ridge Diapir seep field

Relationships among seep community biomass, diversity, and physiographic controls such as underlying geology are not well understood. Previous efforts to constrain these relationships at the Blake Ridge Diapir were limited to observations from piloted deep-submergence vehicles. In August 2012, the autonomous underwater vehicle (AUV) Sentry collected geophysical and photographic data over a 0.131km2 area at the Blake Ridge Diapir seeps. A nested survey approach was used that began with a regional or reconnaissance-style survey using sub-bottom mapping systems to locate and identify seeps and underlying conduits. This survey was followed by AUV-mounted sidescan sonar and multibeam echosounder systems mapping on a mesoscale to characterize the seabed physiography. At the most detailed survey level, digital photographic imaging was used to resolve sub-meter characteristics of the biology. Four pockmarks (25–70m diameter) were documented, each supporting chemosynthetic communities. Concentric zonation of mussels and clams suggests the influence of chemical gradients on megafaunal distribution. Data collection and analytical techniques used here yield high-resolution habitat maps that can serve as baselines to constrain temporal evolution of seafloor seeps, and to inform ecological niche modeling and resource management.

Early to Middle Holocene transition in the Pastos Chicos record, dry Puna of Argentina

This paper presents a multi-proxy record and its paleoenvironmental interpretation from Pastos Chicos (23° 40′ 29″ S; 66° 25′ 32″ W; 3781 m asl), Susques, Jujuy province, Argentina. The analysis includes a study of fluvial sediments and geomorphology, and the contained diatoms and pollen record at centenary resolution. Two main environmental phases characterize the ∼9000–4200 BP interval. The first phase was a humid period between ∼9000 and 7300 BP (circa 10,000–8100 cal. BP) which showed organic soils formation in a floodplain broadly vegetated by grasses with a high and stable water table level. The second phase developed between 7300 and 6000 BP (∼8100–6800 cal. BP) and showed a moderately drier environment interrupted by punctuated humid events such as those around 7000 and 6300 BP. This environment could be interpreted as a permanently existing meandering river with a floodplain with shallow oxbow lakes, bordered by a diverse shrub steppe. After 6000 BP, the fluvial system turned into a braided river situated in a sandy floodplain with dunes and ephemeral ponds. At the end of the sequence (i.e. post ∼4200 BP), salt crusts developed and the river lowered its base level by 8 m. These results seem to show that the Pastos Chicos river basin evolved from low energy and more humid conditions established during the Early and the first part of the Middle Holocene, to drier ones in a system with more energy during some events of the second part of the latter period. The start of this aridization process at ∼6000 BP in the basin, seems to be substantially later than other observations in local records of the Andean area. This could be due to physiographic control of the moisture by the catchment area (∼1000 km2) that generates a catchment-averaged regional scale signal of smaller amplitude, which is delayed for individual tributaries. These studies, with other ongoing analysis, will improve the accuracy of the models of resource structure in the area in several space–time scales and, consequently, advance knowledge concerning the organizational pattern of human societies in the past.