The cosmic ray flux increases at higher altitude as air pressure and the shielding effect of the atmosphere decrease. Altitude-dependent scaling factors are required to compensate for this effect in calculating cosmic ray exposure ages. Scaling factors in current use assume a uniform relationship between altitude and atmospheric pressure over the Earth's surface. This masks regional differences in mean annual pressure and spatial variation in cosmogenic isotope production rates. Outside Antarctica, air pressures over land depart from the standard atmosphere by ±4.4 hPa (1σ) near sea level, corresponding to offsets of ±3–4% in isotope production rates. Greater offsets occur in regions of persistent high and low pressure such as Siberia and Iceland, where conventional scaling factors predict production rates in error by ±10%. The largest deviations occur over Antarctica where ground level pressures are 20–40 hPa lower than the standard atmosphere at all altitudes. Isotope production rates in Antarctica are therefore 25–30% higher than values calculated by scaling Northern Hemisphere production rates with conventional scaling factors. Exposure ages of old Antarctic surfaces, especially those based on cosmogenic radionuclides at levels close to saturation, may be millions of years younger than published estimates.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900181

Air pressure and cosmogenic isotope production

The continental bio-cycling of silicon (Si) plays a key role in global Si cycle and as such partly controls global carbon (C) budget through nutrition of marine and terrestrial biota, accumulation of phytolith-occluded organic carbon (PhytOC) and weathering of silicate minerals. Despite the key role of elemental composition of phytoliths on their solubility in soils, the impact of plant cultivar and organ on the elemental composition of phytoliths in Si high-accumulator plants, such as rice (Oryza sativa) is not yet fully understood. Here we show that rice cultivar significantly impacts the elemental composition of phytoliths (Si, Al, Fe and C) in different organs of the shoot system (grains, sheath, leaf and stem). The amount of occluded OC within phytoliths is affected by contents of Si, Al and Fe in plants, while independent of the element composition of phytoliths. Our data document, for different cultivars, higher bio-available Si release from phytoliths of leaves and sheaths, which are characterized by higher enrichment with Al and Fe (i.e., lower Si/Al and Si/Fe ratios), compared to grains and stems. We indicate that phytolith solubility in soils may be controlled by rice cultivar and type of organs. Our results highlight that the role of the morphology, the hydration rate and the chemical composition in the solubility of phytoliths and the kinetic release of Si in soil solution needs to be studied further. This is central to a better understanding of the impact of soil amendment with different plant organs and cultivars on soil OC stock and on the delivery of dissolved Si as we show that sheath and leaf rice organs are both characterized by higher content of OC occluded in phytolith and higher phytolith solubility compared to grains and stems. Our study shows the importance of studying the impact of the agro-management on the evolution of sinks and sources of Si and C in soils used for Si-high accumulator plants.

/pdf/impact-of-rice-cultivar-and-organ-on-elemental-composition-4oez9nvtmp.pdf

Impact of rice cultivar and organ on elemental composition of phytoliths and the release of bio-available silicon.

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

http://quebec.hwr.arizona.edu/classes/hwr696t/gosse01-cosmogenic-nuclide-review.pdf

Terrestrial in situ cosmogenic nuclides: theory and application

The community structure and ecological function of contemporary marine ecosystems are critically dependent on eukaryotic phytoplankton. Although numerically inferior to cyanobacteria, these organisms are responsible for the majority of the flux of organic matter to higher trophic levels and the ocean interior. Photosynthetic eukaryotes evolved more than 1.5 billion years ago in the Proterozoic oceans. However, it was not until the Mesozoic Era (251 to 65 million years ago) that the three principal phytoplankton clades that would come to dominate the modern seas rose to ecological prominence. In contrast to their pioneering predecessors, the dinoflagellates, coccolithophores, and diatoms all contain plastids derived from an ancestral red alga by secondary symbiosis. Here we examine the geological, geochemical, and biological processes that contributed to the rise of these three, distantly related, phytoplankton groups.

/pdf/the-evolution-of-modern-eukaryotic-phytoplankton-30et7k2jol.pdf

The Evolution of Modern Eukaryotic Phytoplankton

https://hal.archives-ouvertes.fr/hal-01909699/document

Plant impact on the biogeochemical cycle of silicon and related weathering processes

The role of plants in soil development has become of crucial interest in quantifying global weathering processes. On the volcanic Reunion Island (Indian Ocean), a 15-cm-thick phytolith-rich horizon (biogenic opal-A) developed at the expense of trachytic ashes between 3820 ± 85 and 335 ± 90 yr B.P. Then, 97–138 t/km 2 /yr of SiO 2 were biogeochemically recycled from the weathering of parent rocks through a past forest of bamboos ( Nastus borbonicus ). This rate is in the same range as the present chemical weathering rates of silica in the area, considered to be among the highest in the world. These results demonstrate that the storage of biogenic silica in soils may be significant and may retard the output of silica to rivers and ocean.

/pdf/biogenic-silica-storage-in-soils-1ygatzy9qt.pdf

Biogenic silica storage in soils

In situ produced 10Be measurements at great depths: implications for production rates by fast muons

Preface Review Paper 'A Crumudgeon's View of Grasss Phytolithology' * Phytoliths in palaeoclimatology and palaeoecology * Phytoliths, diet and health * Archaeological structures and ancient agriculture * Methodology, taxonomy and taphonomy * Soil-plant interactions

Phytoliths : applications in earth sciences and human history

Chemical weathering and mechanical erosion are first-order processes of long-term tropical morphogenesis, which is still poorly deciphered for lack of time constraints. We address this issue by laser probe 39Ar-40Ar dating of generations of cryptomelane [K1-2Mn8O16, nH2O] from the manganese ore deposit of Tambao in northern Burkina Faso. This Mn deposit results from the supergene weathering of carbonate and silicate Mn protores underneath lateritic palaeolandsurfaces. It consists of an upper cryptomelane-rich domain and a lower domain where pyrolusite (b-MnO2) is the dominant Mn oxide. The oldest 39Ar-40Ar ages (59-45 Ma) are obtained on surface outcrops while the youngest ones characterize deep oxidation fronts (3.4-2.9 Ma). Apparent correlations of 39Ar-40Ar age groups with d18O and eustatic curves allow definition of the different stages of morphogenesis. Paleocene-Eocene ages (59-45 Ma) bracket a greenhouse period propitious to bauxitic weathering. The lack of significant ages between 45 and 29 Ma characterizes a period dominated by mechanical erosion, during which detrital sediments, including lateritic materials, were accumulated in intracratonic basins allowing the exhumation of a new lateritic landsurface. Two major weathering periods separated by a second erosion episode (24-18 Ma) are also depicted at the end of Oligocene (29-24 Ma) and lower to mid-Miocene (18-11.5 Ma) in the upper domain, during which newly shaped land surfaces conspicuously weathered. The shorter-weathering and erosion episodes recorded in the lower domain from 18 to 2.9 Ma led to the final geomorphic changes that were conducive to the formation of glacis. The preservation of old cryptomelane (59-45 Ma) in the upper part of the ore deposit indicates a Cenozoic denudation limited to the erosion of previous bauxites, and partly, of ferricretes.

/pdf/chemical-and-physical-erosion-rhythms-of-the-west-african-e9stznkupu.pdf

Fabrice Colin

Papers

Plant impact on the biogeochemical cycle of silicon and related weathering processes

Biogenic silica storage in soils

In situ produced 10Be measurements at great depths: implications for production rates by fast muons

Phytoliths : applications in earth sciences and human history

Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar- 40 Ar dating of supergene K-Mn oxides