At the edge of the sea: sediments, geomorphology, tectonics, and stratigraphy in Quaternary studies, 85-86, Sep 2012
Central East Sardinia is characterised by the outcropping of Mesozoic carbonates that overly a Pa... more Central East Sardinia is characterised by the outcropping of Mesozoic carbonates that overly a Palaeozoic Variscan basement composed of granites and phyllites. These Mesozoic rocks are often covered with Plio-Pleistocene volcanic rocks and alluvial and periglacial deposits. This area is a typical fluviokarst, which hosts some of the most important cave systems of the island constituted by abandoned upper conduits and an active lower level with various phases of entrenchment in response to river erosion, over some millions of years. The main surface features are major canyons, deeply cut into the dolostones and limestones, with allogenic recharge, characterised by low or absent water flow during most of the year and sporadic flood pulses during which discharge can increase two orders of magnitude: Flumineddu Gorge and Cedrino valley in the internal part of the Supramonte karst massif and Codula Fuili, Codula Ilune and Codula Sisine in the coastal sector of Gulf of Orosei are the most important of those canyons. This almost dry hydrographic network is a relict of the ancient drainage pattern related to wetter (thus warmer) periods and continues also on the continental shelf for several kilometres up to a water depth of at least 120 metres. It is debated if these canyons are inherited from the Messinian period (" Salinity crisis ") when Mediterranean sea level dropped dramatically due to the closure of Gibraltar Strait, or if they were entrenched during Quaternary sea level low stands, such as the LGM around 22,000 years ago. The pre-Quaternary age of the beginning of cave formation is indicated by quartz pebbles sampled in the uppermost phreatic passages of Codula Ilune cave system that gave a cosmogenic burial date of around 2 million years. From a morphological point of view, the landscape is enriched not only by typical karst landforms, but also by volcanic morphologies such as flat basaltic plateaux and by strong contrasting relieves between volcanic rocks and fluviokarst. In Central East Sardinia, Plio-Quaternary volcanic rocks outcrop on a surface of about 150 km 2 , one of the volumetrically most important areas of the island. This magmatic cycle has recently been interpreted as a product of deep mantle melt, with magmas rising during extensional tectonic phases related to the formation of the southern Tyrrhenian basin within a continental plate context. The effusive products of this sector are mainly represented by alkaline affinity lava flows (about 75%), emitted from fissures, aligned along two main opposite tectonic directions, NE-SW and NW-SE respectively. Basaltic flows lie unconformably on Mesozoic marine successions and sometimes directly on the Paleozoic basement. These basalts are massif and tabular lava bodies, which fossilize palaeo-valleys. Selective erosion of the limestone has caused a relief inversion, and these basalts are currently located at the highest altitudes. In the northwestern part of the karst area, the Gollei plateau, with a mean altitude of about 200 m asl, represents the widest continuous basaltic outcrop, constituted by very fluid lavas repeatedly overflown in different times on the palaeo, alluvial plain of Cedrino River, that nowadays flows at 100 m asl on carbonate bedrock. In the central part of Supramonte, another extensive basic lava flow, mostly erupted from the S. Elene volcano, rests in the Oddoene valley at 230 m asl on the palaeo, alluvial sediments of the Flumineddu River (125 m asl). The NE elongate Fruncu 'e Pala lava flow (300 m asl) in the eastern part of the karst area overlooks Codula Fuili that flows at an altitude of about 150 m asl. Also the Codula Ilune canyon crosses some volcanic outcrops downstream: Bidunie on the right side of the valley at 225 m asl and Fruncu Nieddu on the left side (120 m asl), while the talweg elevation is 50 m and 0 m asl respectively. On the southern coast of Orosei Gulf, alkaline basalt flows of about 5 km in length outcrop at a average elevation of about 350 m asl near S. Pietro – Baunei, entrenched by Codula Sisine river that flows at 240 m asl along a NW-SE fault.
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the Regional Natural Park of Porto Conte. The area is also part of the Fluxnet project, a global network that coordinates
the analysis of the net ecosystem carbon balance by direct measurement of gas exchanges across canopy-atmosphere
interface using micrometeorological flux tower.
The carbonate terrains of this sector host well-developed cave systems. The karst aquifer has a high dissolved
carbon and the underground atmosphere is often characterised by anomalous level of carbon dioxide that has never been
investigated before. As geological diffusive background emission over the carbonate outcrop was previously neglected,
this study is aimed to assess the magnitude of the CO2 outgassing from caves to outside atmosphere.
An environmental monitoring programme has been carried out in the Monte Doglia cave, a 100 m-deep vertical
shaft located at 320 m asl and 2 km far from the coast. It opens in an area where a significant part of the surface is bare
rock. The soil cover, where it exists, rarely exceeds 20 cm in thickness and consists of fine red, sandy material. The
natural vegetation consists mainly of shrubs.
The cave is monitored with multiple meteorological sensors that record continuously air CO2 and microclimatic
parameters. Carbon dioxide concentration is measured at -10 m depth from the entrance by an infrared spectrometer
(NDIR technology, range 0-10,000 ppm).
Preliminary data indicate periodic oscillations of cave air CO2 levels ranging from 500 ppm to 1600 ppm,
punctuated by events that provide clues to ventilation and degassing mechanisms. These gas plumes reach concentration
>10,000 ppm. This huge CO2 content is hidden when a prevailing advective-renewal of cave air is established.
Ventilation occurs via density driven flow and by wind across the entrance. Subsurface airflow in unsatured zones can
be also induced by atmospheric pressure fluctuations, topographic effects, water table oscillations and meteoric water
infiltration. About the origin of this CO2-rich gas, examples of volatiles escapes along faults have been documented in
Sardinia. They have been related to mantle-derived fluxes associated to Quaternary volcanism. Other non-volcanic
degassing involved on natural CO2 emissions from Nurra karst terrain might derive from carbonate rock weathering,
oxidation of sulphur deposits and surface processes. Future stable isotopes analyses will be devoted to clarify these
points.
Finally, by considering the density of cave entrances and the contribution of numerous fractures widespread in the
karst terrain, the temporal CO2 pattern of Nurra region provides evidence that the amounts of carbon that might be
released from subsurface atmosphere in the vadose zone could be noticeable at both local and regional scale.
Although both streams have confirmed a calcium bicarbonate character, the results show a good dissimilarity between the waters which supply the main collector, with a higher content of calcium and bicarbonate for the Blue Nile stream and a highest concentration values for other mayor ions for the White Nile. Even the monitoring of physicochemical elements shows clearly heterogeneity in the hydro-chemical feature of the drainage network. It is characterized by a high stability for the parameters of the water coming from the innermost area of the aquifer, with temperature and conductivity practically constant during the whole year, while the portion directly feed by the sub-riverbed Codula Elune suffers the seasonality of the external input, which mitigates its effects gradually penetrating towards the zone of confluence. In general, the data show that the White Nile underground water has a composition influenced by weathering of feldspars on which overflows before entering the karst system and by rainfall while the Blue Nile stream extensively interacts with the limestone rock, canceling its original chemical signal.
In this area, two micro-erosion meter stations have been placed at different environmental conditions along the 50 m high gypsum cliff. One of the MEM stations is located on dipping homogeneous gypsum bedrock at around 4 metres above sea level and exposed to rainfall and runoff. The second one is sited closely to the previous one, on the same sedimentary facies, in an overhanging rock wall at 2 metres above sea level.
The one-year monitoring data show an erosion rate different for the two settings with a negligible value for the MEM station on the oblique bedrock and with a lowering rate of -0.51 mm a-1 on the vertical coast. The mean gypsum degradation value calculated in this last point is -0.93 mm mm-1 10-3 for an amount of 1000 mm rainfall.
Excluding the low values recorded on the slopping surface that seem to have been affected by flood events, the erosion rate obtained for the MEM station on the very steep side is in complete agreement with those reported in other Mediterranean area.
In the last 15 years, the monitoring of the position of several endoglacial caves have been performed in the Gornergletscher, the most interesting glacier in the Alps (South West of Switzerland), with the aim to distinguish local factors from global processes in the annual evolution of its moulins. Since 1999 the study took place with multiple field campaigns for morphological and location data compilation, rendered by maps. All field items have been inventoried with corresponding spatial position and compared with the previously existing information.
The preliminary data elaboration shows that, even though the Gorner glacier has not yet adapted its size to the new climate and are still far away from a steady-state, moulins still form at the same spot year after year, surviving at the surface ice movement toward the margins as observed during events triggered by intensive surface melt.
The use of karst cavities as "laboratories" for studying underground environmental parameters (temperature, relative humidity, CO2 concentration, etc.) is increasingly common in researches on climate change. This is due to the high thermal inertia of karst systems that eliminates the high frequency meteorological signals ("background noise") that often distort the original records when making measurements at the surface.
In this sense, one of the main topics of the GLOCHARID Project is to focus on Covadura Cave System, a cave complex in the gypsum karst of Sorbas (Almeria, South-East Spain). This network of underground passages is developed within gypsum strata of Messinian age, with up to 7 levels of galleries connected by shafts, reaching a maximum depth of 120 metres. This cave system has several entrances that connect underground and external atmospheres, thus enabling a strong air flow.
Temperature and relative humidity in the cave have been monitored from September 2011 by 25 dataloggers distributed in all cave levels at different depths and distances from the entrances. Data acquisition has hour-frequency. Furthermore, there are similar sensors at the surface, in order to compare the underground record with the external one.
The preliminary results have shown that the relative humidity and air temperature recorded at each cave microclimatic station vary considerably both in time and space. The trend in the relative humidity reveals an increase with the depth and the distance from the main entrances of the system. Furthermore, it was found that the uppermost cave conduits (Level 1, 2 and 3) have less inertia to changes respect to the deeper levels and are more subject to external weather variations with only a slight delay. Even daily cycles have been observed. As expected, the atmosphere of the deepest part of the cave system has been more stable and remains saturated in water vapour (relative humidity of 100%), except when external relative humidity drastically decreases and temperature drops. This effect propagates from the surface across the galleries within hours.
The air temperature spatial pattern is characterized by a decreasing trend from the entrance to the deepest parts of the cavity, with some exceptions. The behaviour of the climatic series obtained in Level 5 is worth to note (only recorded during 3 months), which has a temperature lower than the deeper levels, behaving as a "cold air trap" capable of keeping inside for a long period of time, masses of cold, dense air flowing through the other galleries. Furthermore, the temperature in the deepest level (Level 7) is largely unaffected by external climatic variables and remains almost unchanged, with values close to the average annual of Sorbas (15 °C). This level, where the high frequency fluctuations are minimized, is the ideal place to install a microclimatic monitoring system to conduct a long-term study on major shifts in the global climate system. Both Level 7 and 5 show thermal inversion phenomena (cave air warmer than outside in winter), which occurred since mid-November at the deepest level and since mid-December at Level 5. Finally, the areas where the cave temperature varies more quickly are located at the interconnections among the various levels, particularly in correspondence of the vertical shafts. Clear evidence is represented by the Level 6 where, despite its depth, cave atmosphere shows the influence of the strong airflow caused by differences between cave and surface temperature.
According to these preliminary results, it seems evident that the effects of Global Warming will affect irreversibly the stability of the deepest part of the cave system, where microclimatic variables have a high inertia and are altered only in exceptional circumstances, generally arid. Taking into account that the general observations point to a high frequency of extreme events, including the continued droughts and the progressive increase in the average temperature in south-eastern Iberia up to +5.8 °C in 2080 (IPCC, 1999), cave micrometeorology may play a key role in monitoring and controlling the effects of climate change in semiarid environments.
A 6.7 cm long white and well laminated calcite stalagmite has been sampled at 10 m depth. U/Th dating has given ages of 11.8 and 11.9 ky, at 66 and 10 mm from the bottom of the stalagmite respectively, placing the speleothem’s growth right before the Younger Dryas.
The speleothem shows a very rapid growth rate, around 560 mm in 1000 years, one of the fastest ever recorded in speleothems. Although this growth rate is only indicative, it records an extremely rapid deposition of calcite, as the one occurred during the early to middle Holocene in the East Mediterranean. The stable isotope signal, moreover, shows an increase in both δ18O and δ13C, suggesting a rapid climate change towards colder and more arid conditions. The oxygen isotopic composition directly reflects the temperature of deposition with a 18O value around -4 ‰ relative to V-PDB rising to a 18O value of about -1 ‰.
Also the carbon isotopic composition with values that fall in the range of -9 ‰ to -7 ‰, reflects changes of the vegetation type reflecting a low activity in the soil during a drier and colder period (-1 ‰ V-PDB).
This would be in agreement with an onset of the Younger Dryas in N-Sardinia around 11.8 ky BP, some centuries later than what happened in areas located a little more to the North.