Balearic Abyssal Plain: An example of modern basin plain deformation by salt tectonism

High-resolution sub-bottom (3.5 kHz) traverses of the southwestern portion of the Balearic Abyssal Plain between North Africa and the Balearic block show an irregular topography where rates of deformation appear to locally exceed those of sedimentation. The basin is underlain by salt of Messinian age, and diapirism has disturbed post-Messinian sediments. The 3.5 kHz records, augmented by 30,000 Joule seismic reflection profiles, indicate that this diapirism is continuing at present and is even now significantly deforming the uppermost sediment sequences.A high concentration of structural features including doming and the development of rim synclines within the 35–60 m of section penetrated are associated with diapirism. Over 70% of the domes have penetrated the surface in the studied area; of these, most display a relief of 10 m or more. In some instances, domes apparently act as dams behind which sediment is ponded. Non-faulted relief averages 3–4 m/km, notable for an abyssal plain surface. Frequently, the uppermost Holocene strata are offset several meters as a result of normal faults and grabens. This concentration of sea-floor irregularities necessarily modifies the present-day sediment dispersal pattern and rules out the uniform flow of turbidity currents and lateral continuity of sedimentation units across this basin plain.     

Holocene climate in the Persian Gulf as deduced from grain-size and pteropod distribution

Detailed coarse-fraction analyses have been made of twelve sediment cores from the Persian Gulf (40–100 m water depth) in order to detect Holocene climatic changes.The only indication of such a change was provided by the terrigenous material (i.e., the < 63 μ fraction) carried into the Gulf by rivers. All cores show characteristic variations in the quantity of terrigenous material and alternating fine- and coarse-grained layers.There was no other parameter which could point to variations in climate. This might be due to the fact that the sediments are sorted as a result of reworking by tidal currents (velocities 25–50 cm/sec): fine and light particles are removed. Any climatically-produced variations in amount or size of benthonic and planktonic organisms have probably been obliterated by tidal currents. The importance of this factor varied with time: during increased terrigenous supply, tidal currents modified the sediment surface for only short periods and a fine-grained layer was, therefore, deposited. A lesser supply of terrigenous material, on the other hand, resulted in the deposition of a coarse-grained layer.Contemporaneous sedimentation of layers with much terrigenous material (fine-grained layer) and with little terrigenous material (coarse-grained layer) is suggested. Observed petrographic variations are presumably due to large-scale changes of climate and not to local sedimentary variations (including shifts of river-mouth processes).Accordingly, the cores were correlated by examining the percentage of coarse fraction and the median diameter of the sand fraction. The assumption is that the higher the percentage of coarsegrained material, the lower the amount of terrigenous material transported into the Persian Gulf.In support of this correlation, another parameter - varying independently from grain size within one core - was examined: the species composition of pteropod shells. These quantitative changes could also be correlated, because there were different core sections with different species compositions.Correlation of the cores by grain size and by pteropod species provides the same results.Four different climatic zones could be distinguished between the lower and the upper part of the Holocene: a relatively arid period at about 9000 years B.P. was succeeded by a more humid period; this then was followed by a period of less rainfall and finally a period during the Late Holocene when rainfall increased again. This sequence is comparable to the established European climate chronology.     

The late Miocene onset of high productivity in the Benguela Current upwelling system as part of a global pattern

We have examined the history of the elevated primary productivity associated with the Benguela Current upwelling system off southwest Africa using sediments from 7.5 to 4.8 Ma at Ocean Drilling Program Site 1085 in the middle Cape Basin. Sedimentation rates are low until 6.9 Ma. Low accumulation rates of benthic foraminifers and organic carbon indicate that biological productivity was also low. Paleoproductivity dramatically increased at 6.7–6.5 Ma and was highly variable until 4.8 Ma with productivity maxima during cooler periods. The presence of radiolarian opal only between 5.8 and 5.2 Ma suggests an interlude of silica-rich intermediate water in the Cape Basin. The onset of heightened productivity under the Benguela Current is mirrored by similar increases reported between 6.9 and 6.7 Ma in the tropical eastern Pacific, the western and northern Pacific, and the Indian Ocean. The similarity between the patterns at Site 1085 and in the Pacific and Indian Oceans suggests that the dramatic productivity increase off southwest Africa is part of a global response to paleoceanographic changes.     

Plio-Pleistocene sedimentation regimes leading to chalk-marl-cycles in the North Atlantic (DSDP Site 552 — Hole 552A)

Pliocene sediments from the Rockall Plateau (Northeast Atlantic) older than 2.4 my, the onset of northern Hemisphere glaciation, consist of calcareous oozes and thereafter Plio-Pleistocene sediments consist of ooze-marl cycles. The marls are formed through the addition of icerafted detritus to marine carbonates. The two-to threefold increase in glacial terrigenous matter accumulation is balanced by a concomitant reduction in carbonate accumulation. Carbonate dissolution is strongest in Quaternary interglacial oozes and weak in glacial marls, while the reverse is found in the late Pliocene: strong dissolution in glacial marls and weak dissolution in interglacial oozes. Carbonate dissolution is tied to the introduction of organic matter from surface water productivity. Pliocene preglacial calcareous oozes show carbonate dissolution cycles, which did not however lead to the formation of marly sediments.The composition of the benthic foraminiferal faunas indicates that during interglacial times of the late Quaternary Site 552 was occupied by North Atlantic Deep Water into which greater amounts of Labrador Sea water are entrained during glacial-interglacial transitions while during glacial periods the entrainment of Antarctic Bottom Water increases. During the late Pliocene and early Pleistocene maximal e ntrainment of AABW occurred during interglacial times.Dissolution cycles are out-of-phase with the peak occurrences of AABW and are thus primarily controlled by surface productivity, rather than by bottom water circulation.Volcanic matter is found as dark basaltic grains and as light siliceous ash and glass. Dark volcanics are present in all glacial sediments. Light ash occurs also in some interglacial and preglacial sediments which are free of ice-rafted debris. Eolian transport is suggested for such ashes.

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Zusammenfassung Plio-Pleistozäne Sedimentation im Nord-Atlantik (DSDP Site 552-Hole 552 A): Entstehung von Kalk-Mergel-Zyklen.Pliozäne Sedimente vom Rockall Plateau (NE Atlantik), die vor Beginn der nordhemisphärischen Vereisung (2.4 Ma) abgelagert wurden, bestehen aus Kalkschlämmen, die jüngeren Ablagerungen hingegen aus Kalk-Mergel-Wechselfolgen. Während der jungpliozänen-quartären Kaltzeiten nahm die Akkumulationsrate von pelagischem Karbonat um den Faktor 2–3 ab, während die Akkumulationsrate von klastischem, i. W. eisverdriftetem Material um den gleichen Faktor zunahm.Karbonatlösung ist in quartären, interglazialen Kalkschlämmen am stärksten und gering in glazialen Mergeln. In jungpliozänen Kalk-Mergel-Wechselfolgen hingegen wurde das Gegenteil beobachtet: stärkere Lösung in Glazial- als in Interglazialzeiten. Die Karbonatlösung wird durch Zufuhr organischer Substanz aus dem Oberflächenwasser hervorgerufen.Die präglazialen pliozänen Kalkschlämme zeigen Lösungszyklen, die aber nicht zur Bildung von Mergeln führten.Die Benthos-Foraminiferen-Fauna läßt den Schluß zu, daß in spätquartären Interglazialzeiten das Bodenwasser im Gebiet von Site 552 von nordatlantischem Tiefenwasser geprägt war. Während des Überganges von Glazial- zu Interglazialzeiten wurden größere Mengen von Wasser aus der Labradorsee beigemischt. In spätquartären Glazialzeiten nahm die Zufuhr antarktischen Bodenwassers deutlich zu. Im Jungpoliozän und Altquartär hingegen war der maximale Einfluß antarktischen Bodenwassers in Interglazialzeiten.Karbonatlösungszyklen und der maximale Einfluß von antarktischem Bodenwasser korrelieren nicht. Lösungszyklen sind durch die Produktion im Oberflächenwasser, nicht durch Bodenwasserzirkulation gesteuert.Vulkanisches Material kommt als dunkle, basaltische und helle, silikatreiche Asche und Glas vor. Basaltisches Material kommt in allen glazialen Sedimenten vor, während helles vulkanisches Material auch in einigen interglazialen und vorglazialen Sedimenten auftritt, die kein eisverdriftetes Material enthalten. Äolische Zufuhr wird für diese Aschen angenommen.

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Résumé Sur le plateau de Rockall (NE de l'Atlantique), les sédiments pliocénes qui se sont déposés avant le début de la glaciation de l'hémisphère nord (2,4 MA) consistent en boues calcaires, tandis que les dépôts plus jeunes montrent une alternance de boues calcaires et de marnes. Les marnes résultent de l'addition aux carbonates marins de débris transportés par les glaces. L'augmentation (d'un facteur 2 à 3) de l'accumulation des matériaux terrigènes est contrebalancée par une réduction correspondante de l'accumulation du carbonate. Dans les dépôts quaternaires la dissolution des carbonates est marquée dans les boues interglaciaires et faible dans les marnes glaciaires; la situation est inverse dans les dépôts du Pliocène supérieur. La dissolution du carbonate est en relation avec l'introduction de matières organiques provenant des eaux de surface. Les boues calcaires pliocénes pré-glaciaires montrent des cycles de dissolution du carbonate, qui n'aboutissent cependant pas à la formation de sédiments marneux.La composition de la faune de foraminifères benthiques indique que, pendant les périodes interglaciaires du Quaternaire récent, le site 552 était occupé par de l'eau profonde nord-atlantique, qu'au cours des transitions glaciaires-interglaciaires il était envahi par de grandes quantités d'eau de la Mer du Labrador et que les épisodes glaciaires étaient marqués par une arrivée accrue d'eau des fonds antarctiques. Par contre, au Pliocène supérieur et au Quaternaire ancien, l'afflux maximal des eaux des fonds antarctiques se produisait pendant les épisodes interglaciaires.Il n'y a pas de corrélation entre les cycles de dissolution et les maxima d'afflux d'eau profonde antarctique, de sorte que ces cycles sont régis par la productivité des eaux de surface et non par la circulation des eaux de fond.Des matériaux volcaniques sont présents, sous la forme de grains basaltiques et de cendres et fragments siliceux. Les grains basaltiques existent dans tous les sédiments glaciaires; les cendres légères existent également dans certains sédiments interet préglaciaires dépourvus de débris transportés par la glace. Ces cendres peuvent avoir fait l'objet d'un transport éolien.

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Late quaternary climatic variations in Northwest Africa deduced from East Atlantic sediment cores

Characteristic variations in glacial and interglacial periods are shown by the CaCO₃ contents, amounts and grain size of terrigenous material (>40 μm), quartz/mica ratios, and desert quartz numbers in East Atlantic sediment cores, collected during Meteor cruise 25 off Sahara and Senegal, 15–27°N. The following results were obtained. During Holocene an arid climate with eolian supply from the continent prevailed throughout the region (fine grained, slight terrigenous input; high CaCO₃ values; high desert quartz numbers) except in the Senegal area. Here terrigenous muds indicate river supply and hence humid climate. During upper and lower Würn the climate was humid in the present day Sahara north of 20°N (low desert quartz numbers: input of large quantities of coarse terrigenous material; low CaCO₃ values). South of 20°N the climate was arid, the Senegal river input disappeared during upper Würm, desert quartz numbers are as high as in Sahara dune samples, and wind strength was stronger than in the Holocene. Climate during middle Würm resembled Holocene climatic conditions. Climatic conditions during Eem (Riss) were probably similar to those during Holocene (Würm).