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11 results on '"Algeo, T.J."'

6. Carbonate-to-biosilica transition at the Norian–Rhaetian boundary controlled by rift-related subsidence in the western Tethyan Lagonegro Basin (southern Italy)

Author

Casacci M.[1], Bertinelli A.[1], Algeo T.J.[2, and Rigo M. [4

Subjects
010504 meteorology & atmospheric sciences, Evolution, Weathering, Structural basin, 010502 geochemistry & geophysics, Oceanography, 01 natural sciences, UPPER Triassic, Paleontology, chemistry.chemical_compound, Chemical index of alteration, Elemental geochemistry, Rare earth elements, Redox conditions, Ecology, Evolution, Behavior and Systematics, Earth-Surface Processes, Behavior and Systematics, UpperTriassic, 0105 earth and related environmental sciences, Rift, Ecology, Elemental geochemistry, Rare earth elements, Chemical index of alteration, Redox conditions, UpperTriassic, Tethys Ocean, Debris, Seafloor spreading, Secular variation, chemistry, Carbonate, Geology
Abstract

The Lagonegro Basin, located in the Southern Apennines, Italy, records a transition from carbonate sediments of the Calcari con Selce Fm. (“Cherty Limestone”) to biosiliceous sediments of the Scisti Silicei Fm. (“Siliceous Shale”) around the Norian–Rhaetian boundary (NRB), Upper Triassic. Sedimentologic and chemostratigraphic analysis of three sections (Pignola-Abriola, Monte Volturino, and Madonna del Sirino) representing a proximal-to-distal transect across the basin document marine productivity, redox, and chemical weathering conditions. The basin was characterized by low to moderate productivity and mainly oxic deep waters, with no systematic secular variation throughout the ~ 10-Myr-long study interval. A weathering proxy, the chemical index of alteration (CIA*), shifts toward higher values (from ~ 0.70 to ~ 0.80) in all three study sections, suggesting development of warmer and/or more humid conditions just before the NRB. The lack of evidence for changes in marine productivity or redox conditions and the time-transgressive nature of the carbonate-biosilica transition suggests that the latter was caused not by changes in planktonic communities but, rather, by a rise in the CCD, the most likely cause of which was seafloor subsidence within the actively rifting Ionian Ocean (i.e., the westernmost branch of the Tethys Ocean). The tectonically active character of the Lagonegro Basin is also reflected in evidence of synsedimentary paleoseismic activity, e.g., numerous debris flows around the basin margins. The present study thus documents sedimentologic changes within a marginal basin in a geologically young ocean.

Published
2016

7. Discussion on modern and ancient continental hypsometries

Author

Algeo, T.J.

Subjects
Altitudes -- Measurement, Continents -- Research, Earth sciences
Abstract

A.R. Wyatt's comment that hypsometric slopes for individual continents have large variations over the -50 to +250 m range with respect to the current sea level is contentious. Taking continental area as constant results in linear hypsometric slope changes with increments in flooded areas for the above elevations. The this assumption is valid given that the physical dimensions of the blocks do not change with flooding. Aside from the difficulties in using the normalized hypsographic curve, it represents data fora single continent, whereas the hypsometric data used involved a group of continents.

Published
1992

9. Modern and ancient continental hypsometries

Author

Algeo, T.J. and Wilkinson, B.H.

Abstract

Controls on coastal hypsometries of modern continents and oceanic islands may be used to estimate palaco-continental area-elevation distributions. The most important controls on coastal hypsometry are landmass area and coastal gradient, increases in which result in steeper area-elevation distributions. Thus, any change in sea level will flood or expose a smaller fractional area of continents which are large or have steep coastal gradients than of those which are small or have gentle coastal gradients. Coastal gradients are dictated primarily by the tectonic setting and age of continental margins. While active and transform margins generally have steep coasts, mean coastal gradients for passive margins decrease exponentially with increasing margin age. Consequently, continents comprised mostly of mature passive margins are generally more 'floodable'.
These observations are applicable to a variety of palaeo-eustatic problems. Hypsometries corrected for changes in landmass area reduce variance in flooding records between different Palaeozoic continents by more than 50%. Assuming minimum-variance superimposition of flooding curves for different Palaeozoic continents permits identification of possible hypsometric anomalies. Calculated Palaeozoic sea level maxima are between +200 m and +400 m with respect to present sea level, substantially lower than previous estimates to +600 m.
Modelling evolution of coastal hypsometries due to changes in length and age of passive margins suggests that hypsometry varies cyclically in response to continental rifting and collision. Coastal hypsometries are steepest during rifting events and decrease by a factor of two to three as passive margins age. Coastal hypsometries in a 'sutured' world are steeper than those of a 'rifted' world due to increased continent size and decreased passive margin length. Thus, sea level highstands associated with times of continent dispersal may result in part from the enhanced 'floodability' of rifted continents.

Published
1991

11. Natural fractionation of 238U/235U

Author

Weyer, S., Anbar, A.D., Gerdes, A., Gordon, G.W., Algeo, T.J., and Boyle, E.A.

Subjects
*SEPARATION (Technology), *ANALYTICAL chemistry, *ADSORPTION (Chemistry), *SURFACE chemistry
Abstract

Abstract: The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U–233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41±0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U=−0.29‰). Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U–Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3Ma, and up to 2Ma for minerals that are 3 billion years old. [Copyright &y& Elsevier]

Published
2008
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