High-latitude Basins Research Articles

Valles Marineris dune fields as compared with other martian populations: Diversity of dune compositions, morphologies, and thermophysical properties

Planetary dune field properties and their bulk bedform morphologies relate to regional wind patterns, sediment supply, climate, and topography. On Mars, major occurrences of spatially contiguous low-albedo sand dunes are primarily found in three major topographic settings: impact craters, high-latitude basins, and linear troughs or valleys, the largest being the Valles Marineris (VM) rift system. As one of the primary present day martian sediment sinks, VM holds nearly a third of the non-polar dune area on Mars. Moreover, VM differs from other regions due to its unusual geologic, topographic, and atmospheric setting. Herein, we test the overarching hypothesis that VM dune fields are compositionally, morphologically, and thermophysically distinct from other low- and mid-latitude (50°N–50°S latitude) dune fields. Topographic measurements of dune fields and their underlying terrains indicate slopes, roughnesses, and reliefs to be notably greater for those in VM. Variable VM dune morphologies are shown with topographically-related duneforms (climbing, falling, and echo dunes) located among spur-and-gully wall, landslide, and chaotic terrains, contrasting most martian dunes found in more topographically benign locations (e.g., craters, basins). VM dune fields superposed on Late Amazonian landslides are constrained to have formed and/or migrated over >10s of kilometers in the last 50My to 1Gy. Diversity of detected dune sand compositions, including unaltered ultramafic minerals and glasses (e.g., high and low-calcium pyroxene, olivine, Fe-bearing glass), and alteration products (hydrated sulfates, weathered Fe-bearing glass), is more pronounced in VM. Observations show heterogeneous sand compositions exist at the regional-, basinal-, dune field-, and dune-scales. Although not substantially greater than elsewhere, unambiguous evidence for recent dune activity in VM is indicated from pairs of high-resolution images that include: dune deflation, dune migration, slip face modification (e.g., alcoves), and ripple modification or migration, at varying scales (10s–100sm2). We conclude that VM dune fields are qualitatively and quantitatively distinct from other low- and mid-latitude dune fields, most readily attributable to the rift’s unusual setting. Moreover, results imply dune field properties and aeolian processes on Mars can be largely influenced by regional environment, which may have their own distinctive set of boundary conditions, rather than a globally homogenous collection of aeolian sediment and bedforms.

Assessment of evolving TRMM‐based multisatellite real‐time precipitation estimation methods and their impacts on hydrologic prediction in a high latitude basin

The real‐time availability of satellite‐derived precipitation estimates provides hydrologists an opportunity to improve current hydrologic prediction capability for medium to large river basins. Due to the availability of new satellite data and upgrades to the precipitation algorithms, the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis real‐time estimates (TMPA‐RT) have been undergoing several important revisions over the past ten years. In this study, the changes of the relative accuracy and hydrologic potential of TMPA‐RT estimates over its three major evolving periods were evaluated and inter‐compared at daily, monthly and seasonal scales in the high‐latitude Laohahe basin in China. Assessment results show that the performance of TMPA‐RT in terms of precipitation estimation and streamflow simulation was significantly improved after 3 February 2005. Overestimation during winter months was noteworthy and consistent, which is suggested to be a consequence from interference of snow cover to the passive microwave retrievals. Rainfall estimated by the new version 6 of TMPA‐RT starting from 1 October 2008 to present has higher correlations with independent gauge observations and tends to perform better in detecting rain compared to the prior periods, although it suffers larger mean error and relative bias. After a simple bias correction, this latest data set of TMPA‐RT exhibited the best capability in capturing hydrologic response among the three tested periods. In summary, this study demonstrated that there is an increasing potential in the use of TMPA‐RT in hydrologic streamflow simulations over its three algorithm upgrade periods, but still with significant challenges during the winter snowing events.

Validation of River Flows in HadGEM1 and HadCM3 with the TRIP River Flow Model

AbstractThe Total Runoff Integrating Pathways (TRIP) global river-routing scheme in the third climate configuration of the Met Office Unified Model (HadCM3) and the newer Hadley Centre Global Environmental Model version 1 (HadGEM1) general circulation models (GCMs) have been validated against long-term average measured river discharge data from 40 stations on 24 major river basins from the Global Runoff Data Centre (GRDC). TRIP was driven by runoff produced directly by the two GCMs in order to assess both the skill of river flows produced within GCMs in general and to test this as a method for validating large-scale hydrology in GCMs. TRIP predictions of long-term-averaged annual discharge were improved at 28 out of 40 gauging stations on 24 of the world’s major rivers in HadGEM1 compared to HadCM3, particularly for low- and high-latitude basins, with predictions ranging from “good” (within 20% of observed values) to “poor” (biases exceeding 50%). For most regions, the modeled annual average river flows tended to be exaggerated in both models, largely reflecting inflated estimates of precipitation, although lack of human interventions in this modeling setup may have been an additional source of error. Within individual river basins, there were no clear trends in the accuracy of HadGEM1 versus HadCM3 predictions at up- or downstream gauging stations. Relative root-mean-square error (RRMSE) scores for the annual cycle of river flow ranged from poor (>50%) to “fair” (20%–50%) with an overall range of 20.7%–1023.5%, comparable to that found in similar global-scale studies. In both models, simulations of the annual cycle of river flow were generally better for high-latitude basins than in low or midlatitudes. There was a relatively small improvement in the annual cycle of river flow in HadGEM1 compared to HadCM3, mostly in the low-latitude rivers. The findings suggest that there is still substantial work to be done to enable GCMs to simulate monthly discharge consistently well over the majority of basins, including improvements to both (i) GCM simulation of basin-scale precipitation and evaporation and (ii) hydrological processes (e.g., representation of dry land hydrology, floodplain inundation, lakes, snowmelt, and human intervention).

Permian higher latitude bryozoan biogeography

Bryozoans from Gondwana and northern Eurasia are within the higher latitude Gondwana and Boreal Realms respectively and display a clear antitropical faunal distribution, with lower diversity than tropical realms. Within these realms however, faunas at highest latitudes are the least diverse and dominated by cosmopolitan genera. Further, separate geological and oceanographic developmental histories of the boreal and Gondwanan regions have resulted in the Boreal Realm showing much higher diversity than its southern counterpart. Gondwana was in a high-latitude position during the Permian experiencing the polar climatic conditions of the Late Palaeozoic Ice Age, and the distribution of bryozoans show a distinct diversity decrease towards cold-water high latitudes where the eurytopic fenestrates and trepostomes were the dominant groups. The eastern Australian margin, with the least diverse faunas of the Gondwanan realm, is oriented north–south as a narrow shelf, hindering the migration of taxa into the southern coldest environments. In contrast, northern hemisphere mid to high-latitude basins experienced cooling through the Permian as Pangaea moved northward and tectonics altered oceanographic circulation; however glaciomarine deposits were not widespread even at highest latitudes. The marine shelf of the northern Eurasian margin was broad and oriented west–east allowing establishment of widespread stable faunas in waters that became increasingly nutrient-fed through the Permian from cool-water upwelling from the Panthalassa Ocean in the west. This cool-water mid-latitude setting, west–east shelf, and nutrient enrichment resulted in a Boreal Realm bryozoan fauna of over 100 genera; more diverse than the Gondwanan Realm with only 61 genera. Antitropical taxa restricted to Boreal and Gondwanan Realms only include Diploporaria, Lyropora, and Metelipora, however, a number of other genera are more abundant outside the tropics even if they occur in all realms. These genera include Alternifenestella, Rectifenestella, Polypora, Polyporella, Shulgapora, Dyscritella, Stenopora, Streblascopora, Pinegopora, Maychella, Maychellina, Ramipora, Tabulipora and Hexagonella. The wide nutrient-fed mid-latitude shelf of the Boreal Realm also allowed development of a number of endemic taxa, or taxa that are rare elsewhere, including Permoheloclema, Timanodictya, Permofenestella, Lyrocladia, Wjatkella, Parafenestralia, Triznella and Gilmouropora. In southern Gondwana endemic taxa are limited to Crockfordia and Paramaychella, however, there is an abundance of Evactinostella, Etherella and Liguloclema that are distinctive of Western Australia.

Paleogeographic regionalization of Triassic seas based on conodontophorids

Geographic differentiation of conodontophorids between northern and southern latitudes commenced in the Triassic since the early Induan. Cosmopolitan long-lived genera of predominantly smooth morphotypes without sculpturing were characteristic of high-latitude basins of the Panboreal Superrealm. Since the early Olenekian until the Carnian inclusive, this superrealm consisted of the Siberian Realm that extended over Northeast Asia and the Canada-Svalbard Realm that included the Svalbard Archipelago and northern regions of Canada. Throughout the Triassic period, conodontophorids characteristic of the Tethys-Panthalassa Superrealm spanning the Tethys and low-latitude zones of the Pacific were highly endemic, very diverse in taxonomic aspect, having well-developed sculpturing and tempos of morphological transformations. Distinctions between the Early-Middle Triassic conodontophorids from northern and southern zones were not as great as afterward, and their impoverished assemblages from southern Tethyan basins were close in some respects to the Boreal ones. Their habitat basins of that time can be grouped into the Mediterranean-Pacific and India-Pakistan realms. Hence, the extent of geographic differentiation of conodontophorids was not constant and gradually grew, as their taxonomic diversity was reducing in northern basins but relatively increasing in southern ones. The Panboreal e Tethys-Panthalassa superrealms of conodontophorids, which are most clearly recognizable, are close to first-rank paleobiochores (superrealms) established earlier for ammonoids and bivalve mollusks. Main factor that controlled geographic differentiation of Triassic conodontophorids was climatic zoning. Initially lower diversity of southern Tethyan assemblages points probably to relatively cooler water regime in the peri-Gondwanan part of the Tethys. The established patterns in geographic distribution of conodontophorids characterize most likely the real trend of their differentiation and evolution, i.e., the distribution area contraction prior to complete extinction at the end of the Triassic

Influence of the sea surface temperature conditions in the Pacific subarctic frontal zone on the pink salmon’s winter survival ability

Similar to other high-latitude ocean basins, the subarctic Pacific exhibits strong seasonal and inter-annual variations in the abiotic factors of the environment, which, in turn, strongly influence biological objects. One of the principal factors of this kind is the temperature regime. For our research, we chose pink salmon because more than 90% of its natural mortality occurs precisely during the wintering period. The lifetime of pink salmon is only one year, and the conditions of their populations reflect the thermal regime of the given year. The main wintering area of Asian pink salmon is the part of the subarctic frontal zone located south of the Aleutian Islands (43°–46°N). This region features sufficiently high wintertime concentrations of chlorophyll a and temperature conditions favorable for pink salmon wintering. The interannual temperature variability in the frontal zone is close to zero, and the width of the frontal zone may significantly change depending on the winter severity. In “milder” winters, the area of wintering extends, while, in “severe” winters, it is rather narrow, the forage base for fish decreases, they become more accessible for predators, and their survival rates sharply drop.

Isotopic signals from late Jurassic–early Cretaceous (Volgian–Valanginian) sub-Arctic belemnites, Yatria River, Western Siberia

This contribution presents the first detailed oxygen and carbon isotope record from the latest Jurassic–early Cretaceous interval of the Yatria River, subpolar Urals, Siberia. Isotopic compositions have been determined on well-preserved belemnite samples from the genera Lagonibelus , Cylindroteuthis and Acroteuthis . These new data indicate a shift to lower temperatures from the late Volgian into the late Valanginian, with some warmer phases recognized within the Ryazanian and earliest Valanginian. The lowest temperatures of the late Valanginian, consistent with subfreezing polar temperatures, are coincident with an inferred eustatic sea-level fall. A late Valanginian positive shift in carbon isotopes correlates with the carbon isotope excursion recorded from Tethyan successions. The most positive carbon isotope values correspond to the most positive oxygen isotope values (and hence lowest palaeotemperatures). In the absence of widespread Valanginian organic-rich black shale deposition, the carbon isotope excursion may point to increased storage of organic carbon in coastal areas and/or enhanced preservation within stratified waters in high-latitude basins. At these higher latitudes, where rates of weathering were presumably much lower because of the prevalent cold climate, the isotopic data may point to pulses of productivity being brought about by increased riverine nutrient transfer and also by nutrients being released by the melting of ice. The correlation between positive carbon isotopes and cool climates may indicate the effectiveness of these high-latitude carbon sinks and their ability to draw down atmospheric CO 2 , resulting in an ‘inverse greenhouse’ effect.

Aggregated and distributed modelling of snow cover for a high-latitude basin

A distributed land-surface model is used to simulate the seasonal cycle of snow cover for a high-latitude basin. In comparison with the distributed model, a simulation using surface parameters and meteorological data averaged over the entire basin overestimates the peak snow accumulation and underestimates the duration of snow cover. Dividing the basin into a small number of elevation bands and performing separate simulations for each band greatly improves the results. The improvement is less marked if information on the elevation dependence of vegetation cover and meteorological conditions is not used. Distributed simulations with different models produce a wider range of results than distributed and aggregated simulations with the same model.

Hydrologic sensitivity of global rivers to climate change

Climate predictions from four state-of-the-art general circulation models (GCMs) were used to assess the hydrologic sensitivity to climate change of nine large, continental river basins (Amazon, Amur, Mackenzie, Mekong, Mississippi, Severnaya Dvina, Xi, Yellow, Yenisei). The four climate models (HCCPR-CM2, HCCPR-CM3, MPI-ECHAM4, and DOE-PCM3) all predicted transient climate response to changing greenhouse gas concentrations, and incorporated modern land surface parameterizations. Model-predicted monthly average precipitation and temperature changes were downscaled to the river basin level using model increments (transient minus control) to adjust for GCM bias. The variable infiltration capacity (VIC) macroscale hydrological model (MHM) was used to calculate the corresponding changes in hydrologic fluxes (especially streamflow and evapotranspiration) and moisture storages. Hydrologic model simulations were performed for decades centered on 2025 and 2045. In addition, a sensitivity study was performed in which temperature and precipitation were increased independently by 2 °C and 10%, respectively, during each of four seasons. All GCMs predict a warming for all nine basins, with the greatest warming predicted to occur during the winter months in the highest latitudes. Precipitation generally increases, but the monthly precipitation signal varies more between the models than does temperature. The largest changes in the hydrological cycle are predicted for the snow-dominated basins of mid to higher latitudes. This results in part from the greater amount of warming predicted for these regions, but more importantly, because of the important role of snow in the water balance. Because the snow pack integrates the effects of climate change over a period of months, the largest changes occur in early to mid spring when snow melt occurs. The climate change responses are somewhat different for the coldest snow dominated basins than for those with more transitional snow regimes. In the coldest basins, the response to warming is an increase of the spring streamflow peak, whereas for the transitional basins spring runoff decreases. Instead, the transitional basins have large increases in winter streamflows. The hydrological response of most tropical and mid-latitude basins to the warmer and somewhat wetter conditions predicted by the GCMs is a reduction in annual streamflow, although again, considerable disagreement exists among the different GCMs. In contrast, for the high-latitude basins increases in annual flow volume are predicted in most cases.

Facies architecture, seismic stratigraphy and development of a high-latitude basin: the Powell Basin (Antarctica)

Ten seismic facies assemblages organized into architectural elements have been identified on the basis of multichannel seismic profiles in the Powell Basin. These facies include: sheet drapes with parallel high-amplitude reflectors, sheet drapes with cyclic reflectors, sheet drapes with chaotic and moderate-amplitude reflectors, broad lenses with central channel, slope front fills with hummocky reflectors, sheet drapes with wavy reflectors, sigmoid wedges, irregular bodies with contorted reflectors, lenses with chaotic reflections and trough fills with divergent reflectors. The vertical and horizontal distribution of these architectural elements define the syn-rift, syn-drift and three post-drift sequences, whose boundaries are characterized by unconformities indicating significant changes in the factors controlling sedimentation on the margins and basin plain. The allogenic factors that appear to be more influential on the margin growth patterns are: the evolution of subsidence mechanisms, the tectonic regime on each of the margins, and the distribution of bottom currents related to the Weddell Sea Gyre. Subsidence controls the evolution of the accommodation space, thus causing a trend in the sequence external geometry from wedges during the syn-rift phase, to sigmoids during the drifting, followed by flat-downward convex lenses at the beginning of the post-drift phase and finally sheet-like geometries. Tectonics has a considerable influence on the extension of the source areas and margin geometry, and also modifies the depositional cycles associated to climatic changes, processes of ice sheet advance and retreat, sea-level fluctuations and isostatic adjustments. Bottom currents influence the distribution of depositional bodies, controlling the development of sediment drifts that constitute an important proportion of the sequences on the northern margin and in the basin plain. The evolution of the depositional sequences of the Powell Basin clearly depicts the influence of global processes linked to glacio-eustatic changes and the tectonics controlling the different phases in the development of a rifted basin. This high-latitude basin may be considered as a good example of a semi-isolated basin, which serves as a model for the stratigraphic reconstruction of margins and basins of high latitudes.

High-resolution record of foraminiferal response to late Quaternary sea-ice retreat in the Norwegian-Greenland Sea

Dramatic oceanic changes during the transition from glacial to interglacial conditions had significant effects on pelagic and benthic environments in the Norwegian-Greenland Sea. Fossil marine biota in deep-sea sediments provide the means to reconstruct past oceanographic conditions and climatic fluctuations. Here we present the results of an investigation with high temporal resolution (±200 yr) of four sites distributed along a north-south transect across this high-latitude basin with the aim to decipher timing and regional relocation of water-mass boundaries. Results show that termination I in the Norwegian-Greenland Sea is characterized by a prominent maximum of benthic foraminiferal abundance, which progressively moved northward from the eastern North Atlantic Ocean to Fram Strait at a mean velocity of 0.77 km ṁ yr−1. Benthic foraminiferal accumulation rates during this abundance peak increase from south to north from 184 to 5863 specimens ṁ cm−2 ṁ k.y.−1. We interpret this abundance maximum to be a result of high organic carbon fluxes under a moving high productivity area, on the basis of the progression of climatic amelioration and retreat of sea-ice cover during the gradual deglaciation. The benthic foraminiferal record mirrors this time-transgressive belt directly.

Dinoflagellate cyst distribution in high-latitude marine environments and quantitative reconstruction of sea-surface salinity, temperature, and seasonality

A data base of 179 reference sites documents the relations between the assemblages of organic-walled dinoflagellate cysts and sea-surface temperature, salinity, and seasonality throughout the North Atlantic, adjacent subpolar basins (Labrador Sea, Baffin Bay, Irminger and Iceland basins) and epicontinental environments off eastern Canada (estuary and Gulf of St. Lawrence, Hudson Bay). Principal-component analyses show close relationships between dinoflagellate cyst data and sea-surface conditions: the first component (71.1% of the variance) correlates with the winter temperature, salinity, and seasonal duration of sea-ice cover, whereas the second component (11.3% of the variance) appears mainly related to summer temperature. Transfer functions using the best analogue method were tested by reconstructing modern sea-surface conditions on the basis of the reference dinoflagellate cyst assemblages. The correlation coefficient between instrumental averages and reconstructed values ranges from 0.87 (August temperature) to 0.97 (annual duration of sea-ice cover). These transfer functions appear most accurate for the reconstruction of sea-surface conditions in marginal marine environments of high-latitude basins. The only reservation concerns the validity of reconstruction in offshore regions characterized by low productivity where sparse cyst fluxes may result from long-distance transport through currents. The transfer functions that were applied in, as an example, a late Quaternary sequence of the Davis Strait in the northern Labrador Sea, notably suggest seasonal sea-ice cover extent of 6–10 months/year and August temperature and salinity of 1–4 °C and 31–33‰, respectively, during the last glacial optimum (isotopic stage 2).

Response of Th/U in deep Labrador Sea sediments (ODP Site 646 to changes in sedimentation rates and paleoproductivities

Research Article| February 01, 1990 Response of Th/U in deep Labrador Sea sediments (ODP Site 646 to changes in sedimentation rates and paleoproductivities C. Hillaire-Marcel; C. Hillaire-Marcel 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar A. Aksu; A. Aksu 2Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1 B 3X5, Canada Search for other works by this author on: GSW Google Scholar C. Causse; C. Causse 3Laboratoire de Géologie du Quaternaire, CNRS Luminy, case 907, 13288 Marseille Cedex 9, France Search for other works by this author on: GSW Google Scholar A.de Vernal; A.de Vernal 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar B. Ghaleb B. Ghaleb 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information C. Hillaire-Marcel 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada A. Aksu 2Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1 B 3X5, Canada C. Causse 3Laboratoire de Géologie du Quaternaire, CNRS Luminy, case 907, 13288 Marseille Cedex 9, France A.de Vernal 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada B. Ghaleb 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (2): 162–165. https://doi.org/10.1130/0091-7613(1990)018<0162:ROTUID>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. Hillaire-Marcel, A. Aksu, C. Causse, A.de Vernal, B. Ghaleb; Response of Th/U in deep Labrador Sea sediments (ODP Site 646 to changes in sedimentation rates and paleoproductivities. Geology 1990;; 18 (2): 162–165. doi: https://doi.org/10.1130/0091-7613(1990)018<0162:ROTUID>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Labrador Sea is a particularly suitable high-latitude basin for investigating U and Th behavior in deep-sea sediments. During the late Quaternary, the cyclic development and decay of huge ice sheets on adjacent land masses resulted in large-amplitude changes in sedimentation rates and organic paleoproductivities. The resulting magnification of U and Th response is well illustrated by high-resolution studies on piston-cored sediments from the Greenland continental rise at Ocean Drilling Program Leg 105 Site 646 spanning isotopic stages 8 to 1. Our results show a clear positive correlation of 238U/232Th ratios with organic paleoproductivity indicators (e.g., dinocyst) due to U uptake in the water column and/or during the early early diagenesis of organic matter responding to carbon fluxes and to their climate forcing. 230Th excesses over 234U exceed the theoretical value of the 230Th rain from the overlying water column, indicating lateral input possibly from the Greenland slope and shelf. Because these horizontal fluxes of 230Th may be partly controlled by physical parameters, 230Th excesses cannot be unequivocally correlated with sedimentation rates and/or productivity as reported elsewhere. In this subarctic basin characterized by low overall organic carbon burial, the 238U/232Th ratio appears to be a sensible geochemical indicator of organic activity and paleoproductivity. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Continental Drift, II: High-Latitude Evaporite Deposits and Geologic History of Arctic and North Atlantic Oceans

High-latitude evaporites-late Proterozoic through Early Permian-are widespread in Canada and Eurasia. Their distribution patterns show that they precipitated from marine waters entering the continents from the Eurasian-Arctic basin and from the North Atlantic, but not from the Canadian-Arctic basin, which is separated from the former by the Proterozoic Lomonosov sill and from the North Pacific by the Archean Bering-Chukotsk Shelf. Late Proterozoic through Devonian evaporites which precipitated from Arctic waters do not extend (except locally) west of the present Rocky Mountains or east of the present Chukotsk-Koryak Ranges. Marine connections between the high-latitude evaporite basins and those of Tethys were minimal. After Devonian time, evaporite depocenters shifted systematically Atlanticward with the formation of the Franz Josef and Faeroes-Greenland sills. High-latitude evaporite deposition was scarce after the Faeroes-Greenland sill formed. Thus the requisite temperature and salinity for late Proter...