The Search for Supernova-produced Radionuclides in Terrestrial Deep-sea Archives

An enhanced concentration of 60Fe was found in a deep ocean's crust in 2004 in a layer corresponding to an age of ~2 Myr. The confirmation of this signal in terrestrial archives as supernova-induced and detection of other supernova-produced radionuclides is of great interest. We have identified two suitable marine sediment cores from the South Australian Basin and estimated the intensity of a possible signal of the supernova-produced radionuclides 26Al, 53Mn, 60Fe and the pure r-process element 244Pu in these cores. A finding of these radionuclides in a sediment core might allow to improve the time resolution of the signal and thus to link the signal to a supernova event in the solar vicinity ~2 Myr ago. Furthermore, it gives an insight on nucleosynthesis scenarios in massive stars, the condensation into dust grains and transport mechanisms from the supernova shell into the solar system

Copper-nickel-rich, amalgamated ferromanganese crust-nodule deposits from Shatsky Rise, NW Pacific

A unique set of ferromanganese crusts and nodules collected from Shatsky Rise (SR), NW Pacific, were analyzed for mineralogical and chemical compositions, and dated using Be isotopes and cobalt chronometry. The composition of these midlatitude, deep-water deposits is markedly different from northwest-equatorial Pacific (PCZ) crusts, where most studies have been conducted. Crusts and nodules on SR formed in close proximity and some nodule deposits were cemented and overgrown by crusts, forming amalgamated deposits. The deep-water SR crusts are high in Cu, Li, and Th and low in Co, Te, and Tl concentrations compared to PCZ crusts. Thorium concentrations (ppm) are especially striking with a high of 152 (mean 56), compared to PCZ crusts (mean 11). The deep-water SR crusts show a diagenetic chemical signal, but not a diagenetic mineralogy, which together constrain the redox conditions to early oxic diagenesis. Diagenetic input to crusts is rare, but unequivocal in these deep-water crusts. Copper, Ni, and Li are strongly enriched in SR deep-water deposits, but only in layers older than about 3.4 Ma. Diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals. The highest concentrations of Li are in crust layers that formed near the calcite compensation depth. The onset of Ni, Cu, and Li enrichment in the middle Miocene and cessation at about 3.4 Ma were accompanied by changes in the deep-water environment, especially composition and flow rates of water masses, and location of the carbonate compensation depth. Key Points - Fe-Mn crusts can have a diagenetic component - Mid-latitude N. Pacific deep-water Fe-Mn crusts are uniquely enriched in Cu, Th, Li - Temporal changes in deep-ocean geochemical processes are recorde

Deglaciation of coastal south-western Spitsbergen dated with in situ cosmogenic 10Be and 14C measurements

The Svalbard-Barents ice sheet was predominantly a marine-based ice sheet and reconstructing the timing and rate of its decay during the last deglaciation can inform predictions of future decay of marine-based ice sheets (e.g. West Antarctica). Records of ice-sheet change are now routinely built with cosmogenic surface exposure ages, but in some regions, this method is complicated by the presence of isotopic inheritance yielding artificially old and erroneous exposure ages. Here, we present forty-six 1025 Be ages from bedrock (n = 38) and erratic boulders (n = 8) in southwestern Spitsbergen that, when paired with in situ 1427 C measurements (n = 5), constrain the timing of coastal deglaciation following the last glacial maximum. 10Be and 1428 C measurements from bedrock along a ~400 m elevation transect reveal inheritance-skewed 10Be ages, whereas 1429 C measurements constrain 400 m of ice-sheet thinning and coastal deglaciation at 17.4 ± 1.5 ka. Combined with three additional 10Be-dated coastal sites, we show that the southwestern margin of the Svalbard-Barents ice sheet retreated out of Norwegian Sea between ~18-16 ka. In situ 1432 C measurements can provide key chronological information on ice-sheet response to the last termination in cases where measurements of long-lived nuclides are compromised by isotopic inheritance

Late Devensian deglaciation of south‐west Wales from luminescence and cosmogenic isotope dating

The Welsh Ice Cap was a dynamic component of the last British–Irish Ice Sheet at the Last Glacial Maximum, but there are few chronological constraints on the pace and timing of deglaciation. This paper presents new geomorphological and geochronological evidence that constrains the timing of the separation of the Welsh Ice Cap from the Irish Sea Ice Stream and the subsequent deglaciation of south-west Wales; and allow these to be assessed in the context of late Pleistocene climatic events. Luminescence ages from glacial outwash sediments south of Cardigan demonstrate that the Irish Sea Ice Stream was receding by ∼26.7 ka. The subsequent recession of the Welsh Ice Cap is documented by cosmogenic ages from landforms and sediments in the Aeron and Teifi valleys and upland areas. Deglaciation of the Cambrian Mountains was underway by ∼19.6 ka. Cross-valley moraines and associated deglaciation deposits show that minor re-advances interrupted the recession of the Aeron Glacier twice, and the Teifi Glacier on at least 12 occasions. By ∼14.9 ka, the Aeron valley was probably ice-free, but the northwards withdrawal of the Teifi glacier had halted in the Tregaron area. The final rapid recession of this glacier into the uplands of central Wales was completed during the Windermere Interstadial (13.5 cal ka BP) when, in common with much of the UK, the whole of Wales is believed to have become ice-free. There is no evidence that the Cambrian Mountains contained ice-free enclaves at the Last Glacial Maximum, as has been previously suggested. The new ages presented here support suggestions that there was rapid change in the configuration of the Welsh Ice Cap between 20 and 17 ka as upland areas became exposed and there was increasing topographic control on patterns of ice discharge

Geological calibration of spallation production rates in the CRONUS-Earth project

Models of the production of cosmogenic nuclides typically incorporate an adjustable production rate parameter that is scaled for variations in production with latitude and altitude. In practice, this production rate parameter is set by calibration of the model using cosmogenic nuclide data from sites with independent age constraints. In this paper, we describe a calibration procedure developed during the Cosmic-Ray Produced Nuclide Systematics on Earth (CRONUS-Earth) project and its application to an extensive data set that included both new CRONUS-Earth samples and samples from previously published studies. We considered seven frameworks for elevation and latitude scaling and five commonly used cosmogenic nuclides, 3He, 10Be, 14C, 26Al, and 36Cl. In general, the results show that the calibrated production rates fail statistical tests of goodness-of-fit. One conclusion from the calibration results is that two newly developed scaling frameworks and the widely used Lal scaling framework provide qualitatively similar fits to the data, while neutron-monitor based scaling frameworks have much poorer fit to the data. To further test the fitted models, we computed site ages for a number of secondary sites not included in the primary calibration data set. The root-mean-square percent differences between the median computed ages for these secondary sites and independent ages range from 7.1% to 27.1%, differences that are much larger than the typical uncertainties in the site ages. The results indicate that there are substantial unresolved difficulties in modeling cosmogenic nuclide production and the calibration of production rates

Denudation outpaced by crustal thickening in the eastern Tianshan

The modern high topography of the Tianshan resulted from the reactivation of a Paleozoic orogenic belt by the India/Asia collision. Today, the range exhibits tectonically active forelands and intermontane basins. Based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10Be dating, single-grain post-infrared infrared stimulated luminescence (p-IR IRSL) dating and modeling of fault scarp degradation, we quantify the deformation in the Nalati and Bayanbulak intermontane basins in the central Eastern Tianshan. Our results indicate that at least 1.4 mm/yr of horizontal crustal shortening is accommodated within these two basins. This shortening represents over 15% of the 8.5 ± 0.5 mm/yr total shortening rate across the entire range at this longitude. This shortening rate implies that the Eastern Central Tianshan is thickening at a mean rate of ∼1.4 mm/yr, a rate that is significantly higher than the average denudation rate of 0.14 mm/yr derived from our cosmogenic analysis. This discrepancy suggests that the Tianshan range has not yet reached a steady-state topography and remains in a transient state of topographic growth, most likely due to limited denudation rates driven by the arid climate of Central Asia

Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial–interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present

Mid-Holocene pulse of thinning in the Weddell Sea sector of the West Antarctic ice sheet

Establishing the trajectory of thinning of the West Antarctic ice sheet (WAIS) since the last glacial maximum (LGM) is important for addressing questions concerning ice sheet (in)stability and changes in global sea level. Here we present detailed geomorphological and cosmogenic nuclide data from the southern Ellsworth Mountains in the heart of the Weddell Sea embayment that suggest the ice sheet, nourished by increased snowfall until the early Holocene, was close to its LGM thickness at 10 ka. A pulse of rapid thinning caused the ice elevation to fall ~400 m to the present level at 6.5–3.5 ka, and could have contributed 1.4–2 m to global sea-level rise. These results imply that the Weddell Sea sector of the WAIS contributed little to late-glacial pulses in sea-level rise but was involved in mid-Holocene rises. The stepped decline is argued to reflect marine downdraw triggered by grounding line retreat into Hercules Inlet