Reconstructing air temperature at eleven remote alpine and arctic lakes in Europe from 1781 to 1997 AD

Pristine and sensitive environments, such as remote alpine and arctic lakes, are particularly susceptible to the effects of climate change. However, these remote environments do not have sufficiently long instrumental climate records to support studies on contemporary climate change. The issue of the scarcity of instrumental climate data at remote regions is addressed by reconstructing monthly mean air temperatures from 1781 to 1997 AD at eleven remote alpine and arctic lakes in Europe, as part of the MOuntain LAke Research (MOLAR) project. Stepwise multiple regression is applied to establish linear transfer functions of temperatures between each of eleven upland records and twenty homogenised long lowland records. Twelve monthly transfer functions are obtained for each lake. The skill of these transfer functions is found to range typically between 60 and 99%. The lower skill values generally correspond to winter months. The temperature reconstructions obtained using the transfer functions need to be corrected with vertical temperature gradients. Air-temperature lapse rates were obtained for each lake region by spatial interpolation of radiosonde air-temperature data (1990–1997). The resulting reconstructions at each lake were checked using air-temperature data (1996–1997) from automatic weather stations installed at the lakes during the MOLAR project. We estimate the typical reconstruction errors to be about 1.3 °C for low-sun months and about 0.98 °C for high-sun months. Trend analyses on the reconstructed annual mean air temperatures at the lakes show two distinct types of trends for the 19th and 20th centuries. During the period 1801–1900, the western European lakes show no significant trend whereas annual mean air temperatures at the eastern European lakes decrease significantly. The period 1901–1997 presents a warming trend at all but the Fennoscandian lakes. Our results are in good agreement with previous studies on the spatial distribution and magnitude of temperature change in Europe. Principal component analysis performed on the reconstructed annual mean air temperature reveals two different regimes of trends for the past two centuries. It also allows a regional clustering of the inter-annual variability of air temperature at the lakes to be identified.     

Lake Redó ecosystem response to an increasing warming the Pyrenees during the twentieth century

The ecosystem response of Lake Redó (Central Pyrenees) to fluctuations in seasonal air temperature during the last two centuries was investigated by comparison of reconstructed air temperatures with the sediment record. Fine slicing allowed a resolution of 3–6 years according to the ²¹⁰Pb dating, although it was still difficult to easily investigate the response to air temperature forcing, since extreme fluctuations in temperature occur on interannual time-scales. However, the resolution was sufficient to show responses on decadal and century scales. An overall tendency to warming in mean annual temperature in the Central Pyrenees has been caused by summer and in particular by autumn increases. Many of the measured sediment variables apparently responded to these long term trends, but the significance of the relationships was highly conditioned by the structure of the data. The variables responding most on the finer time scales were the microfossils. For diatoms, chironomids and chrysophytes the main variability correlated to summer and to autumn temperatures. For two planktonic species, Fragilaria nanana and Cyclotella pseudostelligera, we found a link of their variability with temperature fluctuations in their growing months (September and October, respectively). This relationship appeared at a certain point during a general warming trend, indicating a threshold in the response. On the other hand, no significant changes in the dominant species could be linked to temperature, nor in any significant subgroup of the 180 diatom species present in the core. In contrast, for most chironomids (particularly Paratanytarsus austriacus, Heterotrissocladius marcidus and Micropsectra radialis) a negative relationship with summer temperature extended throughout the studied period. This response of the whole group gives chironomids a more robust role as indicators for recording temperature changes on long time-scales (e.g., through the Holocene) and for lake signal inter-comparison. Finally, our results indicated that, in all cases, there was a significant resilience to high frequency changes and hysteresis despite extreme fluctuations. Although we were dealing with organisms with one or many generations per year, their populations seemed to follow the decadal trends in air temperature.     

The paleolimnological analysis of sediments from high mountain lake Nižné Terianske pleso in the High Tatras (Slovakia)

Sedimentological climate proxies and a 200-year long climate record, reconstructed using a data-set of European-wide meteorological data, have been compared at the high mountain lake Niné Terianske pleso in the High Tatras, Slovakia. Diatoms, chrysophyte stomatocysts, chironomids, plant pigments and spheroidal carbonaceous particles (SCPs) were analysed as well as sediment lithostratigraphic parameters. Using a radiometric approach the sediment core was dated and a depth of 4.6 cm was found to correspond to 1852 AD. The sediment accumulation rate (0.0034 g·cm^–2·yr^–1) was one of the lowest identified in the European mountain lake project, MOLAR. Despite this slow accumulation rate a remarkably coherent lithological and stratigraphic record has been recovered. The sediments of this remote mountain site, largely free from the effects of direct human impact, have been found to display a wealth of variability over the last 200 years.The record of spheroidal carbonaceous particles, indicators of anthropogenic pollution deposition, begins around 4.5–5.0 cm in depth (1833–1857). Temporal patterns are typical of European lake sites with the concentration peaking in the late 1970's. The SCP/²¹⁰Pb inventory ratio for the site is also in good agreement with the European latitudinal pattern. A strong influence of sample age on the chrysophyte assemblage composition in the upper-most 4–6 cm indicates that the main changes in the cysts have been related to long term environmental changes, probably pH. Analysis of chironomid remains revealed a stable profundal community. Chironomids as a whole showed no correlations to temperature fluctuations in the last 200 years. Relatively abundant remains of Diamesa sp. head capsules and other taxa closely associated with stream conditions in the older layers contrast with the absence of Diamesa sp. in the recent sediments. This change is considered to be evidence for the existence of a stronger, more stable inlet supplied from permanent granular snow fields in the lake basin. The most important changes in diatom assemblages were observed at 3cm. Many species of the genus Achnanthes spp. together with Navicula schmassmannii and Orthoseira roeseana made up the greatest part of the diatom community above 3 cm, being absent or rare lower in the record. A positive correlation between diatoms and mean summer temperature was found.     

Structural analysis of a transform fault-rift zone junction in North Iceland

On the north coast of Iceland, the rift zone in North Iceland is shifted about 120 km to the west where it meets with, and joins, the mid-ocean Kolbeinsey ridge. This shift occurs along the Tjörnes fracture zone, an 80-km-wide zone of high seismicity, which is an oblique (non-perpendicular) transform fault. There are two main seismic lineaments within the Tjörnes fracture zone, one of which continues on land as a 25-km-long WNW-trending strike-slip fault. This fault, referred to as the Husavik fault, meets with, and joins, north-trending normal faults of the Theistareykir fissure swarm in the axial rift zone. The most clear-cut of these junctions occurs in a basaltic pahoehoe lava flow, of Holocene age, where the Husavik fault joins a large normal fault called Gudfinnugja. At this junction, the Husavik fault strikes N55°W, whereas Gudfinnugja strikes N5°E, so that they meet at an angle of 60°. The direction of the spreading vector in North Iceland is about N73°W, which is neither parallel with the strike of the Husavik fault nor perpendicular to the strike of the Gudfinnugja fault. During rifting episodes there is thus a slight opening on the Husavik fault as well as a considerable dextral strike-slip movement along the Gudfinnugja fault. Consequently, in the Holocene lava flow, there are tension fractures, collapse structures and pressure ridges along the Husavik fault, and pressure ridges and dextral pull-apart structures subparallel with the Gudfinnugja fault. The 60° angle between the Husavik strike-slip fault and the Gudfinnugja normal fault is the same as the angle between the Tjörnes fracture zone transform fault and the adjacent axial rift zones of North Iceland and the Kolbeinsey ridge. The junction between the faults of Husavik and Gudfinnugja may thus be viewed as a smaller-scale analogy to the junction between this transform fault and the nearby ridge segments. Using the results of photoelastic and finite-element studies, a model is provided for the tectonic development of these junctions. The model is based on an analogy between two offset cuts (mode I fractures) loaded in tension and segments of the axial rift zones (or parts thereof in the case of the Husavik fault). The results indicate that the Tjörnes fracture zone in general and the Husavik fault in particular, developed along zones of maximum shear stress. Furthermore, the model suggests that, as the ridge-segments propagate towards a zero-underlapping configuration, the angle between them and the associated major strike-slip faults gradually increases. This conclusion is supported by the trends of the main seismic lineaments of the Tjörnes fracture zone.     

Stress estimates from the length/width ratios of fractures

The hypothesis is advanced that, provided Young's modulus and Poisson's ratio of the rock are known, the length/width ratios of tension fractures can be used to estimate the tensile stress (assumed constant along the length of each fracture) at the time of fracture formation. The hypothesis is tested on a fissure swarm in a 10,000 year-old basaltic lava in Iceland. The length/width ratios of the fissures give the average tensile stress as of the order of a few MPa.     

Tectonic framework and extensional pattern of the Malaguide Complex from Sierra Espuña (Internal Betic Zone) during Jurassic–Cretaceous: implications for the Westernmost Tethys geodynamic evolution

Mapping, lithostratigraphic, biostratigraphic and structural detailed analyses in Sierra Espuña area (Internal Betic Zone, SE Spain) have allowed us to reconstruct the Jurassic–Cretaceous evolution of the Westernmost Mesomediterranean Microplate palaeomargin and, by correlation with other sectors (Northern Rift, central and western Internal Betic Zone), to propose a geodynamic evolution for the Westernmost Tethys. Extension began from Late Toarcian, when listric normal faults activated; these faults are arranged in three categories: large-scale faults, separating hectometric cortical blocks; main faults, dividing the former blocks into some kilometre-length blocks; and secondary faults, affecting the kilometric blocks. This fault ensemble, actually outcropping, in the Sierra Espuña area, broke the palaeomargin allowing the westerly Tethyan Oceanic aperture with an extension at about 17.2%. Extension was not homogeneous in time, being the Late Toarcian to the Dogger–Malm boundary the period when blocks underwent the greatest movement (rifting phase), leading to the drowning of the area (8.2% extension). During the Malm (drifting phase) extension followed (5.7%), while during the Cretaceous a change to pelagic facies is recorded with an extension of about 3.3% (post-drift stage). This evolution in the Westernmost Tethys seems to be related to areas out of the limit of significant crustal extension in the hanging wall block of the main cortical low-angle fault of the rifting.     

A ~565 Ma old glaciation in the Ediacaran of peri-Gondwanan West Africa

International Journal of Earth Sciences - In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia, a post-Gaskiers glacial event dated at c. 565 Ma has been detected. Such...     

Paleomagnetic and gravimetrical reconnaissance of Cretaceous volcanic rocks from the Western Colombian Andes: paleogeographic connections with the Caribbean Plate

The reconstruction of the tectonic evolution of the oceanic crust, including the recognition of ancient oceanic plumes and the differentiation between multiple and single oceanic arcs, relies on the paleogeographic analysis of accreted oceanic fragments found in orogenic belts. Here we present paleomagnetic and gravity data from Cretaceous oceanic basaltic and gabbroic rocks, the continental metamorphic basement, and their associated cover from northwestern Colombia. Based on regional scale tectonic reconstructions and geochemical constraints, such rocks have been interpreted as remnants of an oceanic large igneous province formed in southern latitudes, which was accreted to the sialic continental margin during the Late Cretaceous. Gravity analyses suggest the existence of a coherent high density segment separated by major suture zones from a lower density material related to the continental crust and/or thick sedimentary sequences trapped during collision. A characteristic paleomagnetic direction in Early and Late Cretaceous oceanic volcano-plutonic rocks, revealing a southeastern declination (D) and a negative inclination (I), may be interpreted in two different ways: (1a primary magnetization (tilt-corrected direction D = 130.3°, I = -23.3°, k = 23.4, α₉₅ = 26.4°), suggesting clockwise rotation around 130°, and magnetization acquired in southern latitudes (range of 4°S to 21°S); or (2) a remagnetization event during a reverse interval of the Earth’s magnetic field in the Cenozoic (in situ direction D = 128.7°, I = -6.2°, k = 23.1, α₉₅ = 26.1°), suggesting a counter-clockwise rotation around 50°. The first scenario seems more plausible, as it is consistent with previous paleomagnetic studies at other localities; it is compatible with a southern paleogeography for this block, and when integrated with other regional geological and paleomagnetic studies, supports a southern Pacific origin of a major oceanic block, formed as a part of a broader Cretaceous plateau that may have extended south or southwest of Galapagos. After its initial accretion, this block was subsequently fragmented due to the oblique SW-NE approach to the continental margin during the Late Cretaceous.