Application of 137Cs and 210Pb radionuclides to determine sedimentation rates of recent sediments from admiraty Bay, Antarctica Peninsula

Studies about natural and artificial radionuclides in areas such as the Antarctic are key to understand natural and dynamic processes in marine environments. These studies are important to determine levels of radioactive elements and local sedimentation rates. Five marine sediment cores were collected in different points of Admiralty Bay, in the Antarctic Peninsula. The purpose of this study was to determine 137Cs, 226Ra and 210Pb and sedimentation rates at each site. 137Cs, 210Pb and 226Ra were assayed by gamma-counting through direct measurement of the peak at 661 keV, 47 keV and 609 keV, respectively. Sedimentation rates were obtained by 137Cs and 210Pb (CIC and CRS). The activities for 137Cs ranged from 0.84 to 7.09 Bq kg-1; to 226Ra from 6.77 to 31.07 Bq kg-1 and for 210Pb ranged from 1.10 to 36.90 Bq kg-1. The sedimentation rates obtained by the three models ranged from 0.11±0.01 cm y-1 to 0.46±0.05 cm y-1. The levels of 137Cs registered in this study, as well as in other studies in the Antarctic region indicate that global fallout is the main cause of artificial radionuclides present in this environment, since the Antarctic has not suffered a direct action of human activities that released radioactive elements. The possible grain size variations that occur in the studied points of Admiralty Bay may explain the differences found in the vertical distribution of radionuclides, because of the different values of sedimentation rates and respective dating determined in their profiles

Upper crustal earthquake swarms in São Caetano: reactivation of the Pernambuco shear zone and trending branches in intraplate Brazil

Seismogenic fault reactivation of continental-scale structures has been observed in a few intraplate areas, but its cause is still amatter of debate. The objective of the present study is to analyze two seismic swarms that occurred along the EW-trending Pernambuco ductile shear zone and in a NE-trending branch, in 2007 and 2010 in São Caetano County, Northeastern Brazil.We studied both epicentral areas using a nine- and a seven-station network during 180 and 54 days, respectively. The results indicate that the 2007 swarm correspond to a right-lateral, strike–slip fault with a normal component of slip (strike 74°, dip 60°, and rake−145°) and the 2010 swarmcorresponds to a normal fault (strike 265°, dip 79°, and rake −91°). The former reactivated a NE-trending branch, whereas the latter reactivated the main E-W-trending mylonitic belt of the Pernambuco shear zone. These results are consistent with seismogenic reactivation of this major structure, generated by the present-day EW-trending compression and NS-trending extension, as observed by previous studies. This shear zone was reactivated as rift faults in the Cretaceous during the South America–Africa breakup. However, our study confirms that the basement fabric such as continental-scale ductile shear zones, show evidence of crustal weakness outside areas of previous rifting, and it reveals the potential for large earthquakes along dormant rift segments associated with major basement shear belts.