Permafrost thaw couples slopes with downstream systems and effects propagate through Arctic drainage networks

The intensification of thaw-driven mass wasting is transforming glacially-conditioned permafrost terrain, coupling slopes with aquatic systems, and triggering a cascade of downstream effects. Within the context of recent, rapidly evolving climate controls on the geomorphology of permafrost terrain we: A) quantify three-dimensional slump enlargement and described the processes and thresholds coupling slopes to downstream systems; B) investigate catchment-scale patterns of slope thermokarst (thaw slumps and slides) impacts and the geomorphic implications; and C) project the propagation of effects through hydrological networks draining continuous permafrost of northwestern Canada. Power-law relationships between thaw-slump area and volume (R2 = 0.90), and thickness of permafrost thawed (R2 = 0.63), combined with the multi-decadal (1985–2018) increase in areal extent of thaw-slump disturbance show a two-order of magnitude increase in catchment-scale geomorphic activity and the coupling of slope and hydrological systems. Predominant catchment effects are to first- and second-order streams where sediment delivery commonly exceeds stream transport capacity by orders of magnitude indicating millennial-scale perturbation of downstream systems. Assessment of hydrological networks indicates thaw-driven mass wasting directly affects over 6,760 km of stream segments, 890 km of coastline, and 1,370 lakes in the 994,860 km2 study area. Downstream propagation of slope thermokarst indicates a potential increase in the number of affected lakes by at least a factor of 4 (n > 5,600), impacted stream length by a factor of 7 (> 48,000 km) and defines several major impact zones to lakes, deltas, and coastal areas. Prince of Wales Strait is the receiving marine environment for greatly increased sediment and geochemical fluxes from numerous slump impacted hydrological networks draining the landmasses of Banks and Victoria Islands. Peel and Mackenzie Rivers are globally significant conveyors of the slope thermokarst cascade delivering effects to North America’s largest Delta and the Beaufort Sea. Climate-driven erosion of ice-rich slopes in permafrost preserved glaciated terrain has triggered a time-transient cascade of downstream effects that signal the renewal of post-glacial landscape evolution. Glacial legacy and the patterns of continental drainage dictate that terrestrial, freshwater and marine environments of western Arctic Canada will be an interconnected hotspot of thaw-driven change through the coming millennia.

Along-Stream Evolution of Gulf Stream Volume Transport

The Gulf Stream affects global climate by transporting water and heat poleward. The current’s volume transport increases markedly along the U.S. East Coast. An extensive observing program using autonomous underwater gliders provides finescale, subsurface observations of hydrography and velocity spanning more than 15° of latitude along the path of the Gulf Stream, thereby filling a 1500-km-long gap between long-term transport measurements in the Florida Strait and downstream of Cape Hatteras. Here, the glider-based observations are combined with shipboard measurements along Line W near 68°W to provide a detailed picture of the along-stream transport increase. To account for the influences of Gulf Stream curvature and adjacent circulation (e.g., corotating eddies) on transport estimates, upper- and lower-bound transports are constructed for each cross–Gulf Stream transect. The upper-bound estimate for time-averaged volume transport above 1000 m is 32.9 ± 1.2 Sv (1 Sv ≡ 106 m3 s−1) in the Florida Strait, 57.3 ± 1.9 Sv at Cape Hatteras, and 75.6 ± 4.7 Sv at Line W. Corresponding lower-bound estimates are 32.3 ± 1.1 Sv in the Florida Strait, 54.5 ± 1.7 Sv at Cape Hatteras, and 69.9 ± 4.2 Sv at Line W. Using the temperature and salinity observations from gliders and Line W, waters are divided into seven classes to investigate the properties of waters that are transported by and entrained into the Gulf Stream. Most of the increase in overall Gulf Stream volume transport above 1000 m stems from the entrainment of subthermocline waters, including upper Labrador Sea Water and Eighteen Degree Water.

Neogene Tree Trunk Fossils from the Meshgin Shahr Area, Northwest Iran

In 2016, an extensive fossil forest was discovered near Meshgin Shahr, northwest Iran. Silicified tree trunks occur in Miocene fluvial sediments and at multiple stratigraphic levels within a 27-m thick sequence of Pleistocene volcaniclastics. The Miocene trunks likely represent stream transport. Pleistocene examples originated during repeated eruptive events when volcaniclastic sediments buried a standing forest. The site, informally named Meshgin Shahr Fossil Forest, was registered in 2017 as a national natural monument by the Iranian Cultural, Handicraft and Tourism Organization. To date, 16 fossilized trunks have been found, all but one of them representing gymnosperms. The ancient coniferous forest was very different from modern forests in Iran and adjacent Azerbaijan, a result of climatic changes that were principally caused by the demise of the Paratethys Sea and by rain shadow effects caused by the uplift of the Alborz and Zagros mountain ranges. X-ray diffraction patterns reveal that woods from the fossil forest contain three types of silica: opal-CT, pure quartz, and a mixture of opal-CT and quartz. In addition, optical photomicrographs show the abundant presence of amorphous opal-A. Mineralogic variations occur among different fossil trees and within a single trunk. These silica polymorphs resulted from a combination of processes: silica minerals precipitated in multiple episodes under differing geochemical conditions and the diagenetic transformation of an opaline parent material.

Analyzing Particle-Associated Pollutant Transport to Identify In-Stream Sediment Processes during a High Flow Event

Urban areas are a leading source of polycyclic aromatic hydrocarbons (PAHs) that result from combustion processes and are emitted into rivers, especially during rain events and with particle wash-off from urban surfaces. In-stream transport of suspended particles and attached PAHs is linked strongly to sediment turnover processes. This study aimed to identify particle exchange processes that contribute to the transport of suspended particles during flood events. An urban high-flow signal was tracked in high temporal resolution at two sampling sites in the Ammer River (South-western Germany). Samples were analyzed for turbidity, total suspended solids concentrations (TSS), particle-size distribution, organic carbon, and PAH. Maximum discharge and the highest TSS occurred nearly simultaneously at the upstream sampling site, whereas a temporally shifted course was observed for downstream. The total load of particles was similar, yet a decrease of PAH mass (~28%) and an increase of the particulate organic carbon (POC) content (~3.5%-points) occurred. Coarser particles (≥26 µm) dominated at the beginning of the event at both sampling sites. The signal of remobilized riverbed sediment increases downstream and leads to well-established, robust linear correlations between TSS and PAHs. This study highlights that riverbed sediment acts as intermediate storage for contaminated particles from upstream sources that shape, together with the fresh urban input, the “particle signature” of suspensions moving through catchments during high discharge conditions.