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Methane seepage from the upper continental slopes of Western Svalbard has previously been attributed to gas hydrate dissociation induced by anthropogenic warming of ambient bottom waters. Here we show that sediment cores drilled off Prins Karls Foreland contain freshwater from dissociating hydrates. However, our modeling indicates that the observed pore water freshening began around 8 ka BP when the rate of isostatic uplift outpaced eustatic sea-level rise. The resultant local shallowing and lowering of hydrostatic pressure forced gas hydrate dissociation and dissolved chloride depletions consistent with our geochemical analysis. Hence, we propose that hydrate dissociation was triggered by postglacial isostatic rebound rather than anthropogenic warming. Furthermore, we show that methane fluxes from dissociating hydrates were considerably smaller than present methane seepage rates implying that gas hydrates were not a major source of methane to the oceans, but rather acted as a dynamic seal, regulating methane release from deep geological reservoirs.

We linked hydraulic vulnerability in Norway spruce (Picea abies (L.) Karst.) trunkwood with different cambial age to wood density and applied the findings on annual density variations in healthy and declining trees from southern Norway during 1990 to 2010. We hypothesized that drought stress due to the 2003 or 2006 European heat waves were the triggers for tree decline and focused analyses on the structure of wood that was produced after, as well as before, signs of stress, i.e., when decreases in height or diameter growth were visible. In the data set comprising previously published and new measurements, [P.sub.50], i.e., the pressure potential necessary to induce a 50% loss in hydraulic conductivity, was negatively related to wood density. Declining trees produced wider annual rings with lower density than vigorous trees before their radial and height increment started to decline in 2003 or 2006. Trees that produced low-density wood under favorable water availability were more stressed by a sudden drought event because of higher [P.sub.50] values in their sapwood. Due to the strong genotypic relationship between wood density and growth, we suggest that spruce trees selected for fast growth might experience limited hydraulic performance under the impact of extreme heat waves. Resume: Nous avons relie la vulnerabilite hydraulique dans le bois de tronc d'age cambial variable a la densite du bois chez l'epicea commun (Picea abies (L.) Karst.) et nous avons applique les resultats aux variations annuelles de densite dans des arbres sains et deperissants croissant dans le sud de la Norvege, de 1990 a 2010. Nous avons fait l'hypothese que le stress du a la secheresse causee par les vagues de chaleur de 2003 et 2006 en Europe avait declenche le deperissement des arbres et nous avons axe les analyses sur la structure du bois produit apres aussi bien qu'avant l'apparition des signes de stress, c.-a-d. lorsque la diminution de croissance en hauteur et en diametre est devenue visible. Dans le jeu de donnees compose de mesures deja publiees et de nouvelles mesures, [P.sub.50], c.-a-d. le potentiel hydrique qui entraine une perte de 50 % de la conductivite hydraulique, etait negativement relie a la densite du bois. Les arbres deperissants produisaient des cernes annuels plus larges avec une densite plus faible que les arbres vigoureux avant que leur accroissement radial et en hauteur commence a diminuer en 2003 ou 2006. Les arbres qui produisaient du bois de faible densite dans des conditions de disponibilite hydrique favorables etaient plus stresses par un episode soudain de secheresse parce que la valeur de [P.sub.50] dans leur bois d'aubier etait plus elevee. Etant donne l'etroite relation genotypique entre la densite du bois et la croissance, nous croyons que les epiceas selectionnes pour leur croissance rapide pourraient avoir une moins bonne performance hydraulique sous l'impact de vagues de chaleur extreme. [Traduit par la Redaction], Introduction Frequently occurring extreme weather events in the northern hemisphere such as the European heat waves of 2003 and 2006 (Intergovernmental Panel on Climate Change (IPCC) 2012) are a challenge [...]

At high-latitude continental margins, large-scale submarine sliding has been an important process for deep-sea sediment transfer during glacial and interglacial periods. Little is, however, known about the importance of this process prior to the arrival of the ice sheet on the continental shelf. Based on new two-dimensional seismic data from the NW Barents Sea continental margin, this study documents the presence of thick and regionally extensive submarine slides formed between 2.7 and 2.1 Ma, before shelf-edge glaciation. The largest submarine slide, located in the northern part of the Storfjorden Trough Mouth Fan ( TMF), left a scar and is characterized by an at least 870-m-thick interval of chaotic to reflection-free seismic facies interpreted as debrites. The full extent of this slide debrite 1 is yet unknown but it has a mapped areal distribution of at least 10.7 × 103 km2 and it involved >4.1 × 103 km3 of sediments. It remobilized a larger sediment volume than one of the largest exposed submarine slides in the world - the Storegga Slide in the Norwegian Sea. In the southern part of the Storfjorden TMF and along the Kveithola TMF, the seismic data reveal at least four large-scale slide debrites, characterized by seismic facies similar to the slide debrite 1. Each of them is ca. 295-m thick, covers an area of at least 7.04 × 103 km2 and involved 1.1 × 103 km3 of sediments. These five submarine slide debrites represent approximately one quarter of the total volume of sediments deposited during the time 2.7-1.5 Ma along the NW Barents Sea. The preconditioning factors for submarine sliding in this area probably included deposition at high sedimentation rate, some of which may have occurred in periods of low eustatic sea-level. Intervals of weak contouritic sediments might also have contributed to the instability of part of the slope succession as these deposits are known from other parts of the Norwegian margin and elsewhere to have the potential to act as weak layers. Triggering was probably caused by seismicity associated with the nearby and active Knipovich spreading ridge and/or the old tectonic lineaments within the Spitsbergen Shear Zone. This seismicity is inferred to be the main influence of the large-scale sliding in this area as this and previous studies have documented that sliding have occurred independently of climatic variations, i.e. both before and during the period of ice sheets repeatedly covering the continental shelf. [ABSTRACT FROM AUTHOR]