Long term records of erosional change from marine ferromanganese crusts

Ferromanganese crusts from the Atlantic, Indian and Pacific Oceans record the Nd and Pb isotope compositions of the water masses from which they form as hydrogenous precipitates. The10Be/9Be-calibrated time series for crusts are compared to estimates based on Co-contents, from which the equatorial Pacific crusts studied are inferred to have recorded ca. 60 Ma of Pacific deep water history. Time series of ɛNd show that the oceans have maintained a strong provinciality in Nd isotopic composition, determined by terrigenous inputs, over periods of up to 60 Ma. Superimposed on the distinct basin-specific signatures are variations in Nd and Pb isotope time series which have been particularly marked over the last 5 Ma. It is shown that changes in erosional inputs, particularly associated with Himalayan uplift and the northern hemisphere glaciation have influenced Indian and Atlantic Ocean deep water isotopic compositions respectively. There is no evidence so far for an imprint of the final closure of the Panama Isthmus on the Pb and Nd isotopic composition in either Atlantic or Pacific deep water masses

Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory

Lithologic differences give rise to the differential weatherability of the Earth’s surface and globally variable silicate weathering fluxes, which provide an important negative feedback on climate over geologic timescales. To isolate the influence of lithology on weathering rates and mechanisms, we compare two nearby catchments in the Luquillo Critical Zone Observatory in Puerto Rico, which have similar climate history, relief and vegetation, but differ in bedrock lithology. Regolith and pore water samples with depth were collected from two ridgetops and at three sites along a slope transect in the volcaniclastic Bisley catchment and compared to existing data from the granitic Río Icacos catchment. The depth variations of solid-state and pore water chemistry and quantitative mineralogy were used to calculate mass transfer (tau) and weathering solute profiles, which in turn were used to determine weathering mechanisms and to estimate weathering rates. Regolith formed on both lithologies is highly leached of most labile elements, although Mg and K are less depleted in the granitic than in the volcaniclastic profiles, reflecting residual biotite in the granitic regolith not present in the volcaniclastics. Profiles of both lithologies that terminate at bedrock corestones are less weathered at depth, near the rock-regolith interfaces. Mg fluxes in the volcaniclastics derive primarily from dissolution of chlorite near the rock-regolith interface and from dissolution of illite and secondary phases in the upper regolith, whereas in the granitic profile, Mg and K fluxes derive from biotite dissolution. Long-term mineral dissolution rates and weathering fluxes were determined by integrating mass losses over the thickness of solid-state weathering fronts, and are therefore averages over the timescale of regolith development. Resulting long-term dissolution rates for minerals in the volcaniclastic regolith include chlorite: 8.9 × 10‾¹⁴ mol m‾² s‾¹, illite: 2.1 × 10‾¹⁴ mol m‾² s‾¹ and kaolinite: 4.0 × 10‾¹⁴ mol m‾² s‾¹. Long-term weathering fluxes are several orders of magnitude lower in the granitic regolith than in the volcaniclastic, despite higher abundances of several elements in the granitic regolith. Contemporary weathering fluxes were determined from net (rain-corrected) solute profiles and thus represent rates over the residence time of water in the regolith. Contemporary weathering fluxes within the granitic regolith are similar to the long-term fluxes. In contrast, the long-term fluxes are faster than the contemporary fluxes in the volcaniclastic regolith. Contemporary fluxes in the granitic regolith are generally also slightly faster than in the volcaniclastic. The differences in weathering fluxes over space and time between these two watersheds indicate significant lithologic control of chemical weathering mechanisms and rates

A record of Late Quaternary continental weathering in the sediment of the Caspian Sea: evidence from U–Th, Sr isotopes, trace element and palynological data

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Origin of calcium isotope fractionation in river waters: evidence from the Strengbach catchment, France

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Weathered profiles in tropical volcanic islands by combined geochemical and geophysical approaches

International audienceTropical volcanic islands commonly have pronounced relief and high runoff rates, and consist of easily weathered volcanic material. Intense mechanical and chemical weathering in volcanic terrains has been recognized as being an important component in the transport of the global dissolved load to the oceans [1]. High chemical weathering rates are mainly due to the impact of hydrothermalism inputs with subsurface water circulations [2]. A helicopter-borne TDEM (Time Domain ElectroMagnestism) and magnetic survey was conducted by BRGM (French Geological Survey) over 3 islands of Guadeloupe, Martinique and Reunion in 2012 and 2013 for a total of 20,000 km of flightlines. TDEM method uses the diffusion of a transient EM field to determine the electrical resistivity versus depth. Erosion timescales were calculated from U-series analyses of river sediments. Our results show a broad range: 0 to150 ka in Martinique, 0 to 60 ka in Guadeloupe and 55 to 90 ky for Piton de la Fournaise (Réunion). At watershed scale, the estimated weathered profile (WP) depth obtain by using U-series method are consistent with TDEM helicopter-borne geophysical imagery method, ranging from 0 to 70 m. WP are locally impacted by hydrothermal circulations with associated secondary minerals (halloysite, tridymite…). Among the combined impact of all parameters (climate, runoff, slopes, hydrothermalism inputs, vegetation etc.), basin age seems to be the key control parameter: the younger the basin, the higher the weathering rate is

Time-scales of sedimentary transfer and weathering processes from U-series nuclides : Clues from the Himalayan rivers

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