ABSTRACT
An accurate record of preindustrial (pre-1900 CE) sea level is necessary to contextualize modern global mean sea level (GMSL) rise with respect to natural variability. Precisely dated phreatic overgrowths on speleothems (POS) provide detailed rates of Late Holocene sea-level rise in Mallorca. Statistical analysis indicates that sea level rose locally by 0.12 to 0.31 m (95% confidence) from 3.26 to 2.84 thousand years (ka) ago (2σ) and remained within 0.08 m (95% confidence) of preindustrial levels from 2.84 ka to 1900 CE. This sea-level history is consistent with glacial isostatic adjustment models adopting relatively weak upper mantle viscosities of ~1020 Pa s. There is virtual certainty (>0.999 probability) that the average GMSL rise since 1900 CE has exceeded even the high average rate of sea-level rise between 3.26 and 2.84 ka inferred from the POS record. We conclude that modern GMSL rise is anomalous relative to any natural variability in ice volumes over the past 4000 years.
ABSTRACT
Rock substrates beneath active volcanoes are frequently subjected to temperature changes caused by the input of new magma from the depth and/or the intrusion of magma bodies of variable thickness within the subvolcanic rocks. The primary effect of the influx of hot magma is the heating of surrounding host rocks with the consequent modification of their physical and chemical properties. To assess mobilization in subvolcanic thermal regimes, we have performed radon (220Rn) thermal experiments on a phonolitic lava exposed to temperatures in the range of 100-900°C. Results from these experiments indicate that transient Rn signals are not unequivocally related to substrate deformation caused by tectonic stresses, but rather to the temperature-dependent diffusion of radionuclides through the structural discontinuities of rocks which serve as preferential pathways for gas release. Intense heating/cooling cycles are accompanied by rapid expansion and contraction of minerals. Rapid thermal cycling produced both inter- and intra-crystal microfracturing, as well as the formation of macroscopic faults. The increased number of diffusion paths dramatically intensified Rn migration, leading to much higher emissions than temperature-dependent transient changes. This geochemical behaviour is analogous to positive anomalies recorded on active volcanoes where dyke injections produce thermal stress and deformation in the host rocks. An increased Rn signal far away from the location of a magmatic intrusion is also consistent with microfracturing of subsurface rocks over long distances via thermal stress propagation and the opening of new pathways.
ABSTRACT
An integration of laboratory radon and thoron exhalation data with gamma radiation mapping is applied to assess the geogenic radon and the exposure of people to natural radiation in a highly-urbanized city (Roma, Italy). The study area is a protected territory where ignimbrites from Colli Albani volcano and alluvial sediments largely crop out. A map of total gamma radiation, a gamma transect across Caffarella valley and 9 vertical gamma profiles have been carried out, showing that the main control of gamma levels is, of course, the lithological nature, without neglecting the simultaneous effect of other parameters such as slope morphology, erosion/weathering processes, occurrence of sinkholes or underground tunnels. The surveys allowed to distinguish the medians of ignimbrites (from 816⯱â¯16 cps to 936⯱â¯19 cps) from that of alluvial materials (611⯱â¯14) cps), but showed also that alluvial sediments with anomalously high radioactivity (769⯱â¯14 cps) can be locally recognized, providing valuable information on the interaction between sedimentation and erosion in fluvial valleys. Total gamma activity was converted into absorbed gamma dose rate ranging from 0.33 to 0.38 µSv/hr. Outdoor Annual Effective Dose Equivalents were also estimated between 0.58 and 0.67â¯mSv y-1. Laboratory radon and thoron exhalation rates of collected material are positively correlated with gamma radiation. Volcanic and alluvial sediments are well-discriminated. The correlation between the two variables is evident, but not robust because of the variable concentration of 40â¯K, which is not contributing to radon and thoron exhalation rates. Anomalous data of soil samples located at the foot of a slope can be interpreted as due to reworking and accumulation processes. Similar gamma radiation data documents analogous concentration of radon and thoron parent-nuclides, but coexisting different radon and thoron exhalation rates provides an additional information on different grain size distributions which can be considered as a proxy for soil gas permeability. The integration of gamma mapping and radon and thoron exhalation measurements is a very useful tool to assess people exposure to natural radiation, in terms of dose rates and potential indoor radon. Gamma mapping, which provides data on the radiation source (the bedrock) is fast and not expensive. It allows to obtain very detailed pictures of a study area, but it needs to be combined with laboratory determination of radon and thoron release in order to definitely and correctly interpret variations of gamma signal. Furthermore, laboratory determination of soil radon exhalation gives information on the release of radon and is a good proxy for soil gas permeability. It has the great advantage over in-situ measurements of gas flow not to be influenced by seasonal pedoclimatic parameters and is affected by lower analytical uncertainties. These data are thus reproducible and precise and can be used to estimate potential radon hazard, which is the main source of exposure and thus the most important parameter for human protection from environmental radioactivity.
