Depth and controls of Ca-rhodochrosite precipitation in bioturbated sediments of the Eastern Equatorial Pacific, ODP Leg 201, Site 1226 and DSDP Leg 68, Site 503

The occurrence of early diagenetic Ca‐rhodochrosite [(Mn,Ca)CO₃] is reported in association with ‘griotte’‐type nodular limestones from basinal settings in the geological record; however, without the comparison of analogous modern examples, the controls on precipitation remain speculative. Here the findings of four layers of primary Ca‐rich rhodochrosite recovered from a modern deep‐sea setting in the Eastern Equatorial Pacific, from bioturbated sediments 300 m below sea floor, are reported (Ocean Drilling Program, Leg 201, Site 1226). The mineralogy is similar to cements in burrows recovered during Deep Sea Drilling Project Leg 68 at Eastern Equatorial Pacific Site 503 and from Ca‐rhodochrosite laminae in sediments of the central Baltic Sea. Petrographic relationships and constant oxygen isotopic compositions in the Ca‐rhodochrosite around 5‰ at all depths indicate a shallow burial depth of formation. The onset of 1‰ heavier oxygen isotope composition of Ca‐rhodochrosite at Site 503, about 30 m below the Pliocene/Pleistocene boundary, further suggests that precipitation occurs in the range of 30 m below sea floor. The approximate depth of formation allowed an approximate empirical fractionation factor for marine Ca‐rhodochrosite to be constrained that strongly differs from previously published theoretical values. Based on the approximate precipitation depth, authigenic Ca‐rhodochrosite forms within the SO₄^2?‐reduction zone. Moderately negative δ¹³C values (around ?3‰) and total organic carbon lower than 2 wt% indicate a relatively low contribution of CO₃^2? from organic C mineralization within the expanded redox zonation in the Eastern Equatorial Pacific. It is suggested that the alkalinity is increased by a rise in pH at focused sites of Mn‐reduction coupled with S^2? oxidation. High concentrations of Mn‐oxide can accumulate in layers or burrows because of Mn‐cycling in suboxic sediments as suggested for the Baltic Sea Ca‐rhodochrosites. This study demonstrates how early diagenetic precipitates document biogeochemical processes from past diagenetic systems.     

Depositional systems,composition and geochemistry of Triassic rifted‐continental margin redbeds of the Internal Rif Chain,Morocco

The Middle to Upper Triassic redbeds at the base of the Ghomaride and Internal ‘Dorsale Calcaire’ Nappes in the Rifian sector of the Maghrebian Chain have been studied for their sedimentological, petrographic, mineralogical and chemical features. Redbeds lie unconformably on a Variscan low‐grade metamorphic basement in a 300 m thick, upward fining and thinning megasequence. Successions are composed of predominantly fluvial red sandstones, with many intercalations of quartzose conglomerates in the lower part that pass upwards into fine‐grained micaceous siltstones and massive mudstones, with some carbonate and evaporite beds. This suite of sediments suggests that palaeoenvironments evolved from mostly arenaceous alluvial systems (Middle Triassic) to muddy flood and coastal plain deposits. The successions are characterized by local carbonate and evaporite episodes in the Late Triassic. The growth of carbonate platforms is related to the increasing subsidence (Norian‐Rhaetian) during the break‐up of Pangea and the earliest stages of the Western Tethys opening. Carbonate platforms became widespread in the Sinemurian. Sandstones are quartzose to quartzolithic in composition, testifying a recycled orogenic provenance from low‐grade Palaeozoic metasedimentary rocks. Palaeoweathering indices (Chemical Index of Alteration, Chemical Index of Weathering and Plagioclase Index of Alteration) suggest both a K‐enrichment during the burial history and a source area that experienced intense weathering and recycling processes. These processes were favoured by seasonal climatic alternations, characterized by hot, episodically humid conditions with a prolonged dry season. These climatic alternations produced illitization of silicate minerals, iron oxidation and quartz‐rich red sediments in alluvial systems. The estimated burial temperature for the continental redbeds is in the range of 100 to 160 °C with lithostatic/tectonic loading of ca 4 to 6 km. These redbeds can be considered as regional petrofacies that mark the onset of the continental rift valley stage in the Western Pangea (Middle Triassic) before the opening of the western part of Tethys in the Middle Jurassic. The studied redbeds and the coeval redbeds of many Alpine successions (Betic, Tellian and Apenninic orogens) show a quite similar history; they identify a Mesomediterranean continental block originating from the break‐up of Pangea, which then played an important role in the post‐Triassic evolution of the Western Mediterranean region.     

The tail of the Storegga Slide: insights from the geochemistry and sedimentology of the Norwegian Basin deposits

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene‐aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable‐coloured, finely layered Holocene sediment caps a homogeneous, extremely well‐sorted, poorly consolidated, very fine‐grained, grey‐coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re‐deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub‐bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re‐deposited sediments.