Mario Parra

Sr-Nd isotopes as tracers of fine-grained detrital sediments: the South-Barbados accretionary prism during the last 150 kyr

We studied a core retrieved from the deformation area of the South-Barbados accretionary prism. Sr and Nd isotopic ratios were used in conjunction with the clay mineral assemblage to determine the origin and fluxes of fined-grained detrital material and the sedimentological processes in this area during the last 150,000 years. During the high sea-level stands of events 1.1, 3.3 and stage 5, sedimentation was mainly fed by Amazon material conveyed by the Guiana Surface Current and by volcanogenic material transported by the southward flowing North Atlantic Deep Water (NADW); the flux of clays were very low (< 0.2 g cm−2 10−3 yr), except during stage 5 (0.7 g cm−2 10−3 yr). During the low sea-level stands of events 2.2, 4.2, 5.2, 5.4 and 6.4 and stage 5, the appearance of Orinoco material (1.5 g cm−2 10−3 yr at event 2.2), primarily derived from granitic batholiths of the Guiana Shield and transported directly by turbidity currents, indicate an important change in sedimentary processes. The high flux of volcanogenic materials (0.9 g cm−2 10−3 yr at event 2.2) indicates the importance of air-fall ashes from Dominica, Guadeloupe and/or Martinique islands and of the NADW as the transport agent. The importance of allochthonous fine-grained material originating from the South-American continent (Andes or Guiana Shield) and the northern part of the Lesser Antilles arc clearly stresses the major role that climate and eustacy had on detrital sedimentation during the last climatic cycle.

Mineralogie et geochimie des sediments quaternaires de l'Ocean atlantique nord-oriental (Mer de Novege — Golfe de Cascogne) — Essais d'interpretations sedimentologiques

Abstract The mineralogical and geochemical study of sediments of 14 cores (12 Kullenberg — 2 Reineck) allows us to determine the origin and the process of transport and deposition of detritic fine-grained materials and biogenetic carbonates of the North Atlantic Ocean between the Norwegian Sea and the Bay of Biscay during the different climatic periods of the recent Quaternary. During the cold periods (Wurm and Riss), we can notice a mineralogical and geochemical individualization between two types of sediments: 1. (1) One comes from erosion of acidic eruptive rocks, metamorphic rocks and silicic sedimentary rocks (all rich in quartz and illite, presence of correlation between clay minerals/trace elements and calcite/Sr). 2. (2) The other comes from the dismantling of volcanic basic rocks (poor in quartz, rich in montmorillonite, absence of correlation between clay minerals/trace elements, calcite/Sr, and association plagioclase feldspars/Fe, Ti, Ba). The geographic distribution of these two types of material shows that the fine-grained detritic deposits come directly from close-by springs. This explains the blockage of the north—south circulation of Norwegian Sea water, especially north of the Wyville-Thomson ridge. As for the light or non-existent carbonate sedimentation north of the Wyville-Thomson ridge, it develops progressively towards the south, thus permitting the localization of the southern limit of the polar front near the Wyville-Thomson ridge. During the warm periods (Holocene and Riss—Wurm Interglacial) on the contrary, the fading of sedimentary differentiation appears to indicate the existence of the north—south circulation of Norwegian Sea water to the south of Rockall Bank. These currents transport the fine-grained materials to the other side of the Wyvile-Thomson ridge and deposit them preferentially south of Rockall Bank, after having taken up coarse quartz elements while crossing the Wyville-Thomson ridge north-west of the Bay of Biscay, the nordic deposits are sparse and the carbonate pelagic sedimentation predominates.

Late Quaternary sedimentation between the Gibbs Fracture and the Greenland Basin: Mineralogical and geochemical data

The geochemical study of core samples taken between the Gibbs Fracture zone and the south of the Greenland Basin leads to an understanding of the sedimentogenesis of this region since the last interglacial stage. By demonstrating well-defined mineralogical and chemical assemblages, it is possible to suggest an origin for sediments and the direction of flow. Two major families of influx, one of a petrographically acidic nature, the other basic, observed in the neighbouring basins, are distinguished; variations in their relative importance are linked to climate and hydrology. The basic phase is represented chiefly by an inflow of volcanic detritus, derived from the erosion of Iceland and the Faeroes. Iron, titanium and smectite distribution patterns indicate that Iceland is their probable source, while nickel and copper would come from the Faeroe islands. In the south of the basin (Gibbs Fracture), the basic deposition reaches 10 cm103yrs. The acidic phase is brought by ice-rafting during the colder period and by bottom currents during the warmer period. In the post-glacial its contribution reaches 30 cm103yrs as far away as the Gibbs Fracture Zone.

La sedimentation au Quaternaire recent dans le “Northwest Atlantic Mid-Ocean Canyon” - Apport des donnees mineralogiques et geochimiques☆

A mineralogical (clay minerals and other minerals) and geochemical (Fe, Ti, P, Mn, Ba, Sr, Zn, Cu, Ni, Pb, Rb, Zr, S) study was made of four cores of Quaternary sediments from the “Northwest Atlantic Mid-Ocean Canyon”. A group of common characteristics has been noted for each climatic episode. Some more specific characteristics have also been identified. During the cold period (Wurm), the materials are mainly terrigenous detrital (rich in calcite, dolomite, illite, chlorite, feldspars, poor in trace elements and marked by illite/Fe, Ti and quartz/Zr correlations) carried from the nearby continental platform by turbidity currents. The almost complete lack of other inputs can be explained by a blockage of the main circulation of North Atlantic waters masses during the glacial period. During the warm period (Riss-Wurm and Holocene interglacials), the sedimentary material inputs were more complex: the terrigenous detrital deposits (similar to the Wurm ones), are mixed with new materials (rich in montmorillonite, plagioclase feldspars, augite, epidote and characterized by montmorillonite and plagioclase feldsparts/trace elements correlations). They could have come from volcanic areas such as Iceland, Reykjanes Ridge or the Gibbs fracture and could have been carried by “Norwegian deepsea waters”. Biogenetic carbonate fine-grained materials brought in surface water by the Gulf Stream are also abundant. Furthermore, the Gulf Stream seems to be responsible for montmorillonitic clay minerals input from southern areas toward northern areas. Finally, the coarser-grained materials, ice-rafted by icebergs, can be important in high latitudes (especially southeast of Cape Farewell). Thus, the sedimentary evolution of “the Northwest Atlantic Mid-Ocean Canyon” during the recent Quaternary can be explained both by climatic changes and the nature of the sedimentary sources which successively contributed to the deposits. It gives information concerning the hydrological characteristics of the studied area since 120,000 years. During the glacial period, the mainly autochthonous characteristics of the deposits show that oceanic circulations were reduced; the Polar Front was probably located near 40°N which led to a blockage of the “Norwegian deep-sea water” circulation and to a displacement of the surface currents towards the south of the studied area. During the interglacial periods on the contrary, the diversification of sedimentary materials shows the establishment of circulations similar to those of the present.

Quaternary deposits on the South Barbados accretionary prism

Abstract A study of recent Quaternary deposits of the South Barbados prism has been carried out over a limited area close to the Venezuela margin. 3.5 kHz lines, submarine photographs and many piston and gravity cores have been collected throughout this area during the cruise of the Caracolante II . The detailed morphology of this area comprises a jumble of microbasins and mud diapiric highs: In the basins, the predominant deposits are sandy and muddy turbidites. On the tops of the highs, the diapiric mud is a mixture of Late Miocene, Pliocene and Quaternary sediments. These muds are clearly identified by their high content of smectite and sometimes by the presence of muddy gravels with a diagenetic calcareous cement. On the seafloor, the muds are often covered by an indurated crust on which deep-sea communities are frequently observed. The three major factors controlling sedimentation on the prism are tectonics, sea-level variations and sediment sources. Tectonics play the major role, being responsible for the mud diapiric processes, the fluid circulation throughout the deposits and the triggering of gravity flows on the steep slopes of the diapiric highs. As a consequence, significant inputs of diapir mud supply the deposits of the adjacent basins, and local calcareous crusts, linked to the upward fluid migration, occur on the seafloor at the tops of the diapirs. Sea-level variations and continental sediment sources partly control the character of the deposits in the ponded basins. During the Holocene, the sediment flux was low (1.7–3.7 g cm −2 ∗ 10 −3 yrs) and the deposits were composed of fine-grained particles comprising equally biogenic calcareous and continental material. The continental material is provided by the Orinoco and the Amazon rivers. During the last glacial period, the flux was much higher (8.5–19 g cm −2 ∗ 10 −3 yrs) as a consequence of a huge increase in coarse-grained terrigenous material introduced by the Orinoco river.

ND-SR ISOTOPIC COMPOSITION OF PRESENT-DAY SEDIMENTS FROM THE GIRONDE ESTUARY, ITS DRAINING BASINS AND THE WESTGIRONDE MUD PATCH (SW FRANCE)

Abstract The isotopic composition of silty clay sediments in the Gironde Estuary, which were sampled between 1986 and 1995, shows homogeneity. The sediments from the Garonne and the Dordogne Rivers, including the tributary Isle River, are well mixed within the turbidity maximum zone. The composition of the clay solid load of the upstream Garonne River was also homogeneous between 1985 and 1990, even though there is a large diversity and temporal variabilities in the floods of the many tributaries of this river. Isotopic data were used to calculate the silty clay contribution of the two main rivers to the total estuarine stock: 70% of the total material is brought in by the Garonne River and 30% by the Dordogne River, including its tributary (the Isle River). The same isotopic signature can be observed in the present-day sediments and suspended materials of the WestGironde mud patch, on the adjacent Aquitaine continental plateau.

Two potential sources for Holocene clay sedimentation in the Caribbean Basin: The Lesser Antilles Arc and the South American continent

Abstract Clay sedimentation in the Caribbean Basin is the result of the mixing of two types of materials of different origin. (1) Those derived from weathered volcanic products of the Lesser Antilles (Martinique) Island Arc and identified in the Bay of Fort-de-France are made up of ferriferous montmorillonites and beidellites rich in Mg and poor in Si, K, trace elements and halloysite. Materials derived from the meteoric weathering of volcanic rocks (andesites-dacites) are transported to deeper facies by turbidity and bottom currents. The contribution of these materials to the sedimentation of the Grenada Basin and the eastern flank of the Aves Ridge is relatively important. (2) South American terrigenous materials identified in the Venezuela Basin consist of illites, kaolinites and ferriferous beidellites; they are poor in Mg and relatively rich in Si, K and trace elements. These materials are formed of weathered metamorphic and magmatic acid rocks from the northern provinces of the South American continent, brought into the Grenada Basin by Guyana and Equatorial surface currents. The contribution of these materials to sedimentation on the Island Arc platform of the Lesser Antilles is relatively significant. This work shows that during the transport of fine-grained materials variations in the composition of clays took place only on the insular platform.

Les minéraux lourds des sables du littoral de la Guyane française : bilan sur l’origine des dépôts de la plate-forme des Guyanes

Resume Les mineraux lourds des sables de plage et des cordons littoraux de la Guyane francaise sont assembles en trois corteges: amphiboles dominantes, staurotides, epidotes et pyroxenes dans la presqu’ile de Cayenne; staurotides cardinales, tourmalines et amphiboles jusqu’a la Mana; grenats, staurotides et tourmalines de la Mana au Maroni. En mer, les corteges mineralogiques des sables offrent une grande similitude et une grande continuite avec les precedents de par leur composition mineralogique et leur repartition geographique. Il existe une parfaite coherence entre ces corteges de mineraux lourds et ceux decrits dans les fleuves et les roches meres qui constituent le bouclier guyanais. Les arguments mis en exergue montrent que les mineraux lourds et les sables qui les renferment sont issus d’apports locaux.

Caractéristiques des argiles hydrothermales de la Martinique — Petites Antilles Françaises

Abstract Martinique constitutes an island of the active volcanic arc of the “Petites Antilles” and is situated at the divergent point between the old and recent arcs. The paleogeographical context of the old arc, partially elaborated in a submarine to subaerial environment, is favorable to the development of hydrothermal formations. The mineralogical and chemical characteristics of three types of hydrothermal clay, derived from dacitic rocks and situated respectively in Bellefontaine, Trois-Ilets and on Caravelle Peninsula were determined. These formations do not show the same assemblage of clay minerals: very well crystallized smectites, can be largely dominant (80%) as in Bellefontaine, or illites as in Trois-Ilets, or celadonites as on the Caravelle Peninsula; illites and celadonites are never associated within climatic alterites from the island. Trace element assemblages point out strong concentrations of Cl, S, Cu, Zn, As, Pb etc., which are evidences for hydrothermal enrichments. Every type of weathering is characterized by a particular association that shows variable chemical compositions of the hydrothermal fluids circulating within the deep fractures, as well as the physical conditions in which these recrystallizations occurred. The 87 Sr 86 Sr ratios show that the formation of these clay minerals is accompanied by an enrichment in radiogenic Sr. Clay formations of the Trois-Ilets could prove Upper Miocene deep hydrothermal activity.

Micropaleontological, mineralogical, and geochemical record in late quaternary cores from abyssal Grenada Basin

Clay mineralogy and trace-element geochemistry of two abyssal cores indicate that the primary source of surface-current-transported detrital material in the southern Grenada Basin changed from a continental, South American terrane to a volcanic, Lesser Antillean terrane at the end of the Pleistocene. The record of benthic foraminiferal assemblages demonstrates that the Caribbean Bottom Water (CBW) was relatively oxygen poor and less corrosive in late Pleistocene glacial times than in interglacial times. The change in the properties of CBW in the Holocene was related to a renewed influx of North Atlantic Deep Water in the Caribbean.