Peter W. Kubik

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons

We have determined the production yields for radionuclides in Al2O3, SiO2, S, Ar, K2SO4, CaCO3, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides 10Be, 14C and 26Al produced in quartz targets, 26Al in Al2O3 and S targets, 36Cl in K2SO4 and CaCO3 targets, and 53Mn in Fe2O3 targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ− capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ− capture, by fast muons and by neutron capture.

Production of selected cosmogenic radionuclides by muons 1. Fast muons

To investigate muon-induced nuclear reactions leading to the production of radionuclides, targets made of C9H12, SiO2 ,A l 2O3, Al, S, CaCO3, Fe, Ni, Cu, Gd, Yb and Tl were irradiated with 100 and 190 GeV muons in the NA54 experimental setup at CERN. The radionuclide concentrations were measured with accelerator mass spectrometry and Q-spectroscopy. Results are presented for the corresponding partial formation cross-sections. Several of the long-lived and short-lived radionuclides studied are also produced by fast cosmic ray muons in the atmosphere and at depths underground. Because of their importance to earth sciences investigations, calculations of the depth dependence of production rates by fast cosmic ray muons have been made. : 2002 Elsevier Science B.V. All rights reserved.

9,400 years of cosmic radiation and solar activity from ice cores and tree rings

Understanding the temporal variation of cosmic radiation and solar activity during the Holocene is essential for studies of the solar-terrestrial relationship. Cosmic-ray produced radionuclides, such as 10Be and 14C which are stored in polar ice cores and tree rings, offer the unique opportunity to reconstruct the history of cosmic radiation and solar activity over many millennia. Although records from different archives basically agree, they also show some deviations during certain periods. So far most reconstructions were based on only one single radionuclide record, which makes detection and correction of these deviations impossible. Here we combine different 10Be ice core records from Greenland and Antarctica with the global 14C tree ring record using principal component analysis. This approach is only possible due to a new high-resolution 10Be record from Dronning Maud Land obtained within the European Project for Ice Coring in Antarctica in Antarctica. The new cosmic radiation record enables us to derive total solar irradiance, which is then used as a proxy of solar activity to identify the solar imprint in an Asian climate record. Though generally the agreement between solar forcing and Asian climate is good, there are also periods without any coherence, pointing to other forcings like volcanoes and greenhouse gases and their corresponding feedbacks. The newly derived records have the potential to improve our understanding of the solar dynamics and to quantify the solar influence on climate.

Large-scale erosion rates from in situ-produced cosmogenic nuclides in European river sediments

Abstract We have calculated long-term erosion rates of 20–100 mm/kyr from quartz-contained 10Be in the bedload of middle European rivers for catchments ranging from 102 to 105 km2. These rates average over 10–40 kyr and agree broadly with rock uplift, incision and exhumation rates, historic soil erosion rates, and erosion rates calculated from the measured sediment loads of the same rivers. Moreover, our new erosion rate estimates correlate well with lithology and relief. However, in the Regen, Neckar, Loire, and Meuse catchments, cosmogenic nuclide-derived erosion rates are consistently 1.5–4 times greater than the equivalent rates derived from measured river loads. This may be due to the systematic under-representation of high-magnitude, low-frequency transport events in the gauging records which cover less than a century. Alternatively the discrepancy may derive from spatially non-uniform erosion and preferential tapping of deeper sections of the irradiation profile. A third explanation relates the high cosmogenic nuclide-derived erosion rates to inheritance of an elevated Pleistocene erosion signal. Uncertainties associated with the cosmogenic nuclide-derived erosion rate estimates are not greater than the potential errors in conventional estimates. Therefore, the cosmogenic nuclide approach is an effective tool for rapid, catchment-wide assessment of time-integrated rates of bedrock weathering and erosion, and we anticipate its fruitful application to the Quaternary sedimentary record.

Beryllium 10 in the Greenland Ice core Project ice core at Summit, Greenland

Concentrations of the cosmogenic isotope 10Be have been measured in more than 1350 samples from the Greenland Ice Core Project (GRIP) ice core drilled at Summit, Greenland. Although a dust-associated component of 10Be retained by 0.45 μm filters in some of the samples complicates the interpretations, the results confirm that the first-order origin of 10Be concentration variations is changes in precipitation rate associated with different climate regimes. This effect is seen not only between glacial and interglacial periods, but also during the shorter “Dansgaard-Oeschger” interstadials. By contrast, the 10Be data do not support the interpretation of rapidly varying accumulation (i.e., climate) during the last interglacial. They can, however, be used to help place limits on the origin of the ice in these events. After taking into account variable snow accumulation effects, variations in the 10Be flux are observed, probably caused by solar and geomagnetic modulation, but possibly also by primary cosmic ray variations. The most dramatic is a 10Be peak ∼40,000 years ago, similar to that found in the Vostok ice core, thus permitting a very precise correlation between climate records from Arctic and Antarctic ice cores. The 36Cl/10Be ratio (considering either “total” or only ice-associated 10Be) shows significant variability over the whole core depth, thus confirming the difficulty in using this parameter for “dating” ice cores.

The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean

We have examined the relative affinity of Th, Pa and Be for sorption from seawater onto particles of variable composition (opal, carbonate, lithogenic particles and organic carbon). Nuclide concentrations in particles collected from time-series sediment traps were normalized by the dissolved nuclide concentration in the overlying water column in order to compute partition coefficients under conditions spanning a wide range of particle flux and particle composition. Our results suggest that the affinity of particles for Pa and Be increases with their increasing opal content and decreasing carbonate content, while the affinity of particles for Th increases with increasing carbonate content, and decreases with increasing opal content. We find no correlation between the aluminosilicate content of particles and their affinity for scavenging of any of these elements. Extrapolating to a pure CaCO3 end member, the partition coefficient for Th (9.0×106 g g−1) is ∼40 times greater than for Pa, and roughly 100 times greater than for Be, whereas for a pure opal end member, the partition coefficient for Th (3.9×105 g g−1) is slightly less than that for Pa and Be. Partition coefficients decrease with increasing particle flux in open-ocean settings, but not in an ocean-margin region. This kinetic effect reflects the increasing contribution of unaltered surface material reaching the sediment traps as particle flux increases. The degree of fractionation between Pa and Th and between Be and Th depends on the opal:carbonate rain ratio. These results challenge the use of sedimentary 231Pa/230Th and 10Be/230Th ratios as simple proxies of particle flux. However, the strong dependence of nuclide scavenging on the opal:carbonate rain ratio may provide a needed tool for reconstructing past changes in planktonic community composition.

Near-Synchronous Interhemispheric Termination of the Last Glacial Maximum in Mid-Latitudes

Isotopic records from polar ice cores imply globally asynchronous warming at the end of the last glaciation. However, 10Be exposure dates show that large-scale retreat of mid-latitude Last Glacial Maximum glaciers commenced at about the same time in both hemispheres. The timing of retreat is consistent with the onset of temperature and atmospheric CO2 increases in Antarctic ice cores. We suggest that a global trend of rising summer temperatures at the end of the Last Glacial Maximum was obscured in North Atlantic regions by hypercold winters associated with unusually extensive winter sea ice.

Erosion and Exhumation in the Himalaya from cosmogenic isotope inventories of river sediments

The outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics, erosion and climate. Here we use cosmogenic nuclides (10Be and 26Al) analysed in quartz from river sediments from the Upper Ganges catchment to make the first direct measurements of large-scale erosion rates in a rapidly uplifting mountain belt. The erosion rates are highest in the High Himalaya at 2.7±0.3 mm/yr (1σ errors), fall to 1.2±0.1 mm/yr on the southern edge of the Tibetan Plateau and are 0.8±0.3 to <0.6 mm/yr in the foothills to the south of the high mountains. These relative estimates are corroborated by the Nd isotopic mass balance of the river sediment. Analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years. The data presented here, along with data recently published for European river catchments, demonstrate that a log–linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes. The erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the Indian Himalaya. This confirms that much of the exhumation in the Himalayan mountain chain is now balanced by erosion. However, exhumation rates calculated from high blocking temperature systems, such as 40Ar/39Ar in muscovite, imply lower exhumation rates. Rocks presently at the surface must have undergone a three- to six-fold increase in exhumation rate within the last few million years. We show how this could be explained either by climatic forcing of erosion rate changes or by tectonics. Published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates.

Cross sections for the production of residual nuclides by low- and medium-energy protons from the target elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb, Ba and Au

Abstract Cross sections for residual nuclide production by p-induced reactions were measured from thresholds up to 2.6 GeV using accelerators at CERN/Geneve, IPN/Orsay, KFA/Julich, LANL/Los Alamos, LNS/Saclay, PSI/Villigen, TSL/Uppsala, LUC/Louvain La Neuve. The target elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb, Ba and Au were investigated. Residual nuclides were measured by X- and γ-spectrometry and by Accelerator Mass Spectrometry (AMS). The measured cross sections were corrected for interfering secondary particles in experiments with primary proton energies above 200 MeV. Our consistent database covers presently ca 550 nuclear reactions and contains nearly 15000 individual cross sections of which about 10000 are reported here for the first time. They provide a basis for model calculations of the production of cosmogenic nuclides in extraterrestrial matter by solar and galactic cosmic ray protons. They are of importance for many other applications in which medium energy nuclear reactions have to be considered ranging from astrophysics over space and environmental sciences to accelerator technology and accelerator-based nuclear waste transmutation and energy amplification. The experimental data are compared with theoretical ones based on calculations using an INC/E model in form of the HETC/KFA2 code and on the hybrid model of preequilibrium reactions in form of the AREL code.