Sediment geochemistry of ODP Leg 127 sites

The relative effects of paleoceanographic and paleogeographic variations, sediment lithology, and diagenetic processes on the final preserved chemistry of Japan Sea sediments are evaluated by investigating the rare earth element (REE), major element, and trace element concentrations in 59 squeeze-cake whole-round and 27 physical-property sample residues from Sites 794, 795, and 797, cored during ODP Leg 127. The most important variation in sedimentary chemical composition is the increase in SiO2 concentration through the Pliocene diatomaceous sequences, which dilutes most other major and trace element components by various degrees. This biogenic input is largest at Site 794 (Yamato Basin), moderately developed at Site 797 (Yamato Basin), and of only minor importance at Site 795 (Japan Basin), potentially reflecting basinal contrasts in productivity with the Yamato Basin recording greater biogenic input than the Japan Basin and with the easternmost sequence of Site 794 lying beneath the most productive waters. There are few systematic changes in solid-phase chemistry resulting from the opal-A/opal-CT or opal-CT/quartz silica phase transformations. Most major and trace element concentrations are controlled by the aluminosilicate fraction of the sediment, although the effects of diagenetic silica phases and manganese carbonates are of localized importance. REE total abundances (Sum REE) in the Japan Sea are strongly dependent upon the paleoceanographic position of a given site with respect to terrigenous and biogenic sources. REE concentrations at Site 794 overall correspond well to aluminosilicate chemical indices and are strongly diluted by SiO2 within the upper Miocene-Pliocene diatomaceous sequence. Eu/Eu* values at Site 794 reach a maximum through the diatomaceous interval as well, most likely suggesting an association of Eu/Eu* with the siliceous component, or reflecting slight incorporation of a detrital feldspar phase. SumREE at Site 795 also is affiliated strongly with aluminosilicate phases and yet is diluted only slightly by siliceous input. At Site 797, SumREE is not as clearly associated with the aluminosilicate fraction, is correlated moderately to siliceous input, and may be sporadically influenced by detrital heavy minerals originating from the nearby rifted continental fragment composing the Yamato Rise. Ce/Ce* profiles at all three sites increase essentially monotonically with depth and record progressive diagenetic LREE fractionation. The observed Ce/Ce* increases are not responding to changes in the paleoceanographic oxygenation state of the overlying water, as there is no independent evidence to suggest the proper oceanographic conditions. Ce/Ce* correlates slightly better with depth than with age at the two Yamato Basin sites. The downhole increase in Ce/Ce* at Sites 794 and 797 is a passive response to the diagenetic transfer of LREE (except Ce) from sediment to interstitial water. At Site 795, the overall lack of correlation between Ce/Ce* and Lan/Ybn suggests that other processes mask the diagenetic behavior of all LREEs. First-order calculations of the Ce budget in Japan Sea waters and sediment indicate that ~20% of the excess Ce adsorbed by settling particles is recycled within the water column and that an additional ~38% is recycled at or near the seafloor. Thus, because the remaining excess Ce is only ~10% of the total Ce, there is not a large source of Ce to the deeply buried sediment, further suggesting that the downhole increase in Ce/Ce* is a passive response to diagenetic behavior of the other LREEs. The REE chemistry of Japan Sea sediment therefore predicts successive downhole addition of LREEs to deeply buried interstitial waters.

Banzato, Guillermo (ed.) (2013), Tierras rurales. Políticas, transacciones y mercados en Argentina, 1780-1914, Rosario, Prohistoria ediciones, 242 p. ISBN 978-987-1855-38-4

Fil: Artaza, Sebastián Ignacio. Universidad Nacional Tres de Febrero

Philippe Ariés, Morir en occidente: desde la Edad Media hasta nuestros días, trad. Víctor Goldstein, Adriana Hidalgo editora, Buenos Aires, 2007. ISBN-13: 978-987-9396-38-4. 270 pp.

Fil: Mendoza, José María Felipe.

Nd and Sr isotopic composition of Cretaceous MORB

Chemical and isotopic (Nd and Sr) compositions have been determined for 12 Cretaceous basaltic samples (108 Ma old) from Holes 417D and 418A of Legs 51,52 and 53. We have found that: (1) The chemical compositions are typical of MORB. They do not vary systematically with the stratigraphic positions of the analyzed samples; thus, the chemical evolution is independent of the eruption sequence that occurred at this Cretaceous ridge. (2) REE patterns for all rocks are characterized by a strong LREE depletion with (La/Sm)N = 0.38-0.50; no significant Eu anomalies are found; HREE are nearly flat or slightly depleted towards Yb-Lu and have 12-18 * chondritic abundances. Combining the results of previous studies, it suggests that no significant temporal and spatial variation in magma chemistry (especially for LIL elements) has occurred in the 'normal' ridge segments over the last 150 Ma. (3) lsotopically, 143Nd/144Nd ratios vary from 0.513026 to 0.513154, corresponding to epsilon-Nd(0) = +7.5 to +10, and they fall in the typical range of MORB. However, these rocks have unexpectedly high 87Sr/86Sr ratios (0.70355-0.70470) which are attributed to the result of seawater-rock interaction. (4) The Nd model ages (Tin), ranging from 1.53 to 2.47 (average 2.06) AE, suggest that the upper mantle source(s) underwent a large scale chemical differentiation leading to LREE and other LIL element depletion about 2 AE ago, assuming a simple two-stage model. More realistically, the variation in Tm(Nd) or epsilon-Nd could be derived from mixing of heterogeneous mantle sources that were a consequence of continuous mantle differentiation and continental formation. (5) Because of the low mg values (0.52-0.63), the analyzed basaltic rocks do not represent primary liquids of mantle melting. The variation in La/Sm ratios and TiO2 are not compatible with a model in which all rocks are genetically related by a simple fractional crystallization. Rather, it is proposed that the basaltic rocks might have been derived from some heterogeneous upper mantle source with or without later magmatic mixing, and followed by some shallow-level fraetionations.