Description: Over the last 60 years, our understanding of how cosmic rays produce cosmogenic nuclides has grown from basic physical considerations. We introduce the different types of cosmic ray particles and how their flux varies with altitude, latitude, and time. Accurately describing these variations remains a challenge for some regions when calculating production rates. We describe current and emerging computational methods for calculating production rates that address this challenge. Continuing developments in our understanding of modern and prehistoric cosmic ray fluxes and energy spectra in Earth's atmosphere and at its surface are bound to contribute in the future to more robust applications.

Description: CologneAMS is a new centre for accelerator mass spectrometry (AMS) at the University of Cologne. It has been funded by the German Research Foundation (DFG) to improve the experimental conditions espe- cially for those German scientists that apply the AMS technique for their geologic, environmental, nuclear chemical, and nuclear astrophysical research. The new AMS-device has been built by High Voltage Engineering Europe (HVEE) and has been installed in the existing accelerator area of the Institute of Nuclear Physics. The AMS-facility is designed for the spectrometry of 10Be, 14C, 26Al, 36Cl, 41Ca, 129I in and heavy ions up to 236U and 244Pu. The central part of the AMS-facility is a 6 MV Tandetron™ acceler- ator. Downstream of the high energy mass spectrometer an additional switching magnet is used as a further filter element which supplies also additional ports for future extensions of the detector systems. The current status of CologneAMS and the results of the first test measurements will be presented.

Description: The thermal and erosional history of convergent plate boundaries is important for understanding the links between subduction, arc magmatism, genesis of ore deposits, topography and climate of orogenic belts. Unlike the continent–continent collision that formed many of the largest orogenic belts known today, the Central Andes of South America is a unique case where an oceanic-continent collision has given rise to the Earth's longest and second tallest orogenic belt. Over the last thirty years a plethora of models have been suggested in an attempt to explain how a plateau-type orogen formed at the leading edge of western South America. In the Central Andes most research have focussed attention on the study of the evolution of the arc and backarc, since continuous subduction erosion of the forearc has left little trace of the interplate dynamics that initiated the orogenic belt. In this article, we present a new insight into the thermal and exhumation history of the forearc along the Coastal Cordillera of northern Chile based on biotite K–Ar, apatite fission-track, and apatite/zircon (U–Th)/He dating. We collected diorite samples in a 2 km thick crustal section at the coastal cliff (~ 22°S), and a sea level isoelevation profile between 21 and 27°S. Results from all three dating methods show that the cooling of Coastal Cordillera took place shortly after emplacement during a period of rifting in Jurassic times. Cooling took place in two episodes, mainly in Late Jurassic–Early Cretaceous (~ 118–152 Ma) but also during Late Cretaceous (60–80 Ma) due to the resumption of compression, rift closure, arc uplift, exhumation, eastward migration of magmatic arc activity, and thermal relaxation. The youngest apatite (U–Th)/He ages reveal a cooling event, never reported previously, between 40 and 50 Ma (Eocene). This thermal event affected a 〉 500 km long and 〉 1 km thick section of the Coastal Cordillera in northern Chile. Rock cooling recorded in the Eocene cannot be explained by the thermal effect of the magmatic arc. We explore two scenarios that can provide an explanation for the observed cooling; 1) forearc uplift and exhumation, and 2) changes in plate subduction dynamics. Erosion rates of 0.24 to 0.36 km/Myr, for a period of 10 Myr, are necessary to explain the cooling event. Alternatively, the subduction of a ridge could explain significant cooling by flat slab dewatering. Based on recent sea floor spreading reconstruction, we suggest the subduction of the Farallon–Phoenix ridge as a possible candidate. The subduction of this ridge can account for the cooling event at 40–50 Ma, but also for some important aspects of the Eocene development of the Central Andes; notably the onset of uplift, an abrupt decrease in magmatic activity, and porphyry copper mineralization.

Description: The SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt with an 40Ar/39Ar age of 72 ± 4 ka (2σ). The flow is preserved in the desert climate of northern Arizona, USA. Its unweathered appearance and the lack of soil development indicate it has undergone negligible erosion and/or burial, making it an ideal site for direct calibration of cosmogenic nuclide production rates. Cross-calibrated production rates and production rate ratios for cosmogenic 21Ne, 10Be, and 14C have been determined from SP flow quartz. Production rate ratios for 21Ne/10Be, 21Ne/14C, and 14C/10Be are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2σ uncertainty) of 4.44 ± 0.32, 1.43 ± 0.10, and 2.85 ± 0.21, respectively. Error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of 21Ne, 10Be, and 14C are 17.0 ± 1.1, 3.84 ± 0.27, and 11.2 ± 0.6 at/g/yr (2σ), respectively, using time-independent Lal (1991) / Stone (2000) (St) scaling factors. St scaled spallogenic 10Be and 14C rates are 3.73 ± 0.26 and 9.2 ± 0.6 at/g/yr, respectively. 21Ne and 10Be production rates are integrated over the past 72 ka, whereas 14C production rates are integrated over 25 ka, the time at which SP flow quartz has reached saturation with respect to 14C. These rates overlap within 2σ uncertainty with other St-scaled production rates in the literature, including the total reference SLHL 21Ne production rate of Niedermann (2000), which is revised in this paper to 16.8 ± 3.3 at/g/yr (2σ; St scaling) to reflect a recent change in age control at the Sierra Nevada sites. All SLHL production rates are lower if time- dependent Sf, Sa, and Lm scaling factors are used. For example, error-weighted mean, sea-level, high latitude (SLHL) total reference production rates for 10Be as calculated in the CREp online calculator range from 3.49 ± 0.23 to 3.74 ± 0.25 at/g/yr (2σ), using time-dependent Lm scaling factors. Commonly used SLHL 10Be and 14C production rates in the literature were calibrated on surfaces that have been exposed to cosmic rays for less than 20 ka. Between 20 and 50 ka, the geomagnetic field is proposed to have been weaker than it is today. Production rates of cosmogenic nuclides increase during periods of weaker geomagnetic field strength. However, our study finds no measureable difference between St-scaled production rates of cosmogenic 21Ne and 10Be over the past 20 ka and St-scaled 21Ne and 10Be production rates over the past 72 ka. As such, the study suggests that 21Ne and 10Be production rates in quartz were not significantly greater during the proposed period of decreased magnetic strength from 20 to 50 ka.

Description: Preserved remnants of fluvial activity in deserts constitute evidence for changing boundary conditions. The Atacama Desert of northern Chile is the global end-member for aridity, so the history of relict stream networks in this region is a record of how landscapes develop under extreme conditions. On Pampa de Tana in northern Chile (19.4°S), a series of channel forms that are presently inactive but in the past flowed westward are incised into the surface of a fault bounded, topographically elevated portion of the El Diablo Formation, a regionally extensive, relict pediment. We measure cosmic-ray produced 10Be, 26Al and 21Ne in fluvial deposits to date the timing of abandonment of three channels and couple this with topographic profile information from a SPOT-6 derived, 2 m resolution digital elevation model. We find two of the channels were abandoned approximately 〉5.6 Myr and 2.0 Myr ago. One channel is still capable of flow and has ages suggesting it was fluvially active within the last few hundred thousand years. Using the paleochannel ages measured here and published ages for the end of aggradation of the El Diablo Formation we estimate the rates of fluvial channel incision before channel abandonment, and uplift rates on the faults after channel abandonment. Maximum uplift rates of ~12 m/Myr over the last 2 Myr are found. In general, while rates of uplift are relatively low they are several-fold more rapid than the rates of fluvial incision prior to channel abandonment. This implies that westward channel flow was interrupted by uplift of topography above a blind NW-SE striking reverse fault that affects the Central Depression, an alluvial forearc basin. We consider also that shrinkage of the upstream catchment area by stream capture, promoted via headward erosion and lateral expansion of adjacent canyons (quebradas) could be a factor in the abandonment of the channels on Pampa de Tana. Our results highlight the polygenetic nature of this landscape and show that relatively minor amounts of fault displacement in hyperarid regions can have implications for stream network evolution. Even subtle topographic uplift upstream should be taken into account when fluvial deposits are used as proxies for long-term environmental conditions.