Authigenic zeolites and their relation to silica diagenesis in ODP Site 661 sediments (Leg 108, Eastern Equatorial Atlantic)

At ODP Site 661 from the continental margin of Northwest Africa, authigenic clinoptilolites were investigated in detail. Based on chemical data, crystal size and shape and their intergrowth with associated mineral phases, two types of clinoptilolite (type A and B) occur representing different diagenetic conditions under which the formation of zeolites took place. Clinoptilolite type A was found in a sediment section comprises numerous hiatuses and stratigraphically condensed sediments and could have been formed during early diagenesis. In contrast, clinoptilolite type B was precipitated in Campanian sediments during later diagenesis, after the transformation from opal-A to opal-CT had taken place. At Site 661, the silica source for the authigenesis of both clinoptilolite types is biogenic opal. Volcanic ash material occurs, but in low amounts and thus is not a prerequisite for the formation of clinoptilolite at Site 661.

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Zusammenfassung In der ODP-Bohrung 661 (Leg 108) vom Kontinentalrand vor Nordwest-Afrika wurden authigene Klinoptilolith-Kristalle mit Hilfe der Rasterelektronenmikroskopie, Röntgendiffraktometrie und Mikrosonde im Detail untersucht. Anhand der Kristallgrößen und -formen, ihren Verwachsungen mit anderen Mineralphasen und ihren chemischen Zusammensetzungen wurden 2 Klinoptilolithtypen (Typ A und B) unterschieden. Typ A wurde in stratigraphisch kondensierten Sedimenten nachgewiesen und könnte im Zuge von frühdiagenetischen Porenwasserreaktionen entstanden sein. Klinoptilolithtyp B, welcher vorwiegend in den campanen Sedimenten der ODP-Bohrung 661 vorkommt, wurde während der späteren Silikatdiagenese nach der Transformation von Opal-A in Opal-CT gebildet. Als Silikatquelle für die Kristallisation beider Klinoptilolithtypen wurden überwiegend biogene Opalskelette benutzt. Vulkanisches Aschenmaterial, welches ebenfalls ein Potential für die Klinoptilolithentstehung bildet, wurde nur in einem Horizont nachgewiesen und hatte keinen wesentlichen Einfluß auf die Klinoptilolithentstehung.

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Résumé Des cristaux de clinoptilolite authigène, provenant du site ODP 661 (marge continentale nord-ouest-africaine), ont fait l'objet d'une étude détaillée par microscopie électronique, diffractométrie X et microsonde. Les données chimiques, la taille et la forme des cristaux ainsi que leur intercroissance avec d'autres phases minérales permettent de distinguer 2 types (A et B) de clinoptinolite, qui correspondent aux conditions diagénétiques différentes dans lesquelles les zéolites se sont formées. La clinoptilolite de type A a été rencontrée dans une succession de sédiments stratigraphiquement condensés; elle pourrait s'être formée au début de la diagenèse. La clinoptilolite de type B, par contre, rencontrée surtout dans les sédiments campaniens a précipité au cours des stades tardifs de la diagenèse, après la transformation de l'opale A en opale CT. Au site 661, la source de silice nécessaire à la formation authigène des deux types de clinoptilolite est constituée par l'opale biogénique. Les cendres volcaniques qui pourraient éventuellement être à l'origine de la formation de clinoptilolite, n'ont été rencontrées que dans un seul horizon et n'ont joué aucun rôle en l'occurence.

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Formation and alteration of plagiogranites in an ultramafic-hosted detachment fault at the Mid-Atlantic Ridge (ODP Leg 209)

We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low δ¹⁸O of altered plagiogranite veins (+3.0–4.2‰) and adjacent serpentinites (+ 2.6–3.7‰). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Clay mineral sedimentation in high northern latitude deep-sea basins since the Middle Miocene (ODP Leg 151, NAAG)

High-resolution clay mineralogical investigations and their comparison with other sedimentological data from ODP Sites 908 and 909 (central Fram Strait) were used to reconstruct the paleoclimate and paleoceanography in the high northern latitudes since the Middle Miocene. Ice rafting has probably occurred since 15 Ma. The comparison of sand-sized components and clay mineral distribution demonstrate that both were not delivered by the same transport process. The input of the clay fraction is related to transport through sea ice and/or oceanic currents. A provenance change at 11.2 Ma is indicated by variations within clay mineral distribution and increased accumulation rates. This is interpreted as a result of an increase in water mass exchange through the Fram Strait. Decreases of the smectite to illite and chlorite ratio at Site 909 suggest a Middle Miocene cooling phase between 14.8 and 14.6 Ma, and a further cooling phase between 10 and 9 Ma. The intensification of glaciation in the Northern Hemisphere is documented by an increase of illite and chlorite from 3.4 to 3.3 Ma, which is synchronous to the onset of major Northern Hemisphere glaciation as indicated by oxygen isotope data.     

沉积物成岩蚀变过程中的Mn、Cd和Mo元素活动特征:以ODP 1148站钻孔沉积物记录为例

通过对南海北部的ODP 1148站岩芯600 mcd以上(约30 Ma以来)的沉积物中自生富集Mn、Cd和Mo等过渡金属元素的含量变化的研究,并结合相关的化学组成结果,探讨了岩芯内部氧化-还原条件的变化以及相关元素的活动特征,反演了相应沉积时期的环境演变.结果显示,岩芯387 mcd以上,自生Mn富集明显,代表氧化的环境;387～485 mcd之间,自生Cd含量明显富集,Mn含量显著降低,代表少氧的环境;485 mcd以下,Mn和Cd含量极低,自生Mo明显富集,代表缺氧的环境.随氧化-还原条件的变化,Mo存在明显的向下迁移并在缺氧界面的缺氧一方达到最大值的趋势,而Cd在少氧环境形成的固相态则可能在缺氧环境下不稳定,溶解态的Cd有向上迁移的趋势,并且在少氧/缺氧界面的少氧一方富集.这些过渡金属元素记录的氧化-还原条件的变化,反映出ODP 1148站所在海区的沉积环境变化:早期有较丰富的陆源输入,表层海水生产力较高,随着南海不断扩张以及全球海平面上升,该海区表层海水生产力逐渐降低.     

Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin)

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra‐salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self‐charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post‐dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re‐precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack‐seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins.     

Composition of organic matter in subducted and unsubducted sediments off the Nicoya peninsula, Costa Rica (ODP Leg 170, Sites 1039 and 1040)

Investigation of sediment samples from Sites 1039 and 1040 (ODP Leg 170) drilled off the Nicoya peninsula (Costa Rica) by organic geochemical and organic petrological methods has revealed that subduction has only little influence on the composition of the sedimentary organic matter. Organic carbon contents reached 1.5% in the Pleistocene samples but Miocene and Pliocene sediments had an average organic carbon content of less than 0.5%. Organic carbon/sulfur ratios are generally below 2.8, reflecting an intense sulfate reduction in the uppermost sediments which was enhanced by sulfate supply both from sea water and deeper strata. Microscopical examinations indicate that the organic matter is mainly derived from marine sources. A small amount of terrigenous organic matter is, however, present as well according to n-alkane and fatty acid distributions. The alkenone unsaturation index U₃₇^k′ shows only a slight decrease during the Miocene and Pliocene, and stronger variations in the Pleistocene, probably indicating more stable sea surface temperatures during the Miocene and Pliocene. Variations in the Pleistocene can possibly be related to glacial/interglacial changes.     

Evidence for textural and alteration changes in basaltic lava flows using variations in rock magnetic properties (ODP Leg 183)

The different effects of cooling and alteration on magnetic properties, in single thick flows from subaerial and submarine eruptions, cored and logged during Ocean Drilling Program (ODP) Leg 183 at Sites 1137 and 1140 (Kerguelen Plateau) are examined. Downhole logging data from both sites is supplemented by petrology and geochemistry of 32 samples from three subaerial lava flows at Site 1137 and two flows units at Site 1140, covering transects from fresh to highly altered basalts.Changes in magnetic properties have previously been observed in several ODP drill holes, which penetrate basaltic basement. In subaerial basalts, a typical trend is that of high magnetic susceptibility and natural remanent magnetization (NRM) values in the altered flow top, and lower values in the less-altered massive flow interior. In contrast, submarine lava flows display the opposite behavior in their magnetic properties. Altered pillow rims have lower susceptibility and NRM values than the fresh pillow interiors. It is concluded that rate of cooling and degree of alteration are the main factors influencing the magnetization and, hence, the distribution of iron oxides. The effects of low-temperature alteration are most noticeable in the distribution of more mobile elements, such as K. Consequently, the spectral gamma ray (SGR) log, which in basaltic basement is largely controlled by K concentration, is an excellent proxy to the downhole identification of alteration. The strong positive correlation observed for the subaerial basalts between the downhole total magnetic field (F_tot) and SGR, suggest a potential link with alteration in the drilled sections. The alteration of the submarine basalts is not as pronounced and therefore no correlation is evident.     

Toe-of-slope of a Cretaceous carbonate platform in outcrop,seismic model and offshore seismic data (Apulia,Italy)

Synthetic seismic models of outcrops in the Early Cretaceous slope of a carbonate platform on the Gargano Promontory (southern Italy) were compared to an offshore seismic section south of the Promontory. Outcrops of the same age on the promontory have the same sequence stratigraphic characteristics as their offshore equivalent, and are the only areas where the transition from platform to basin of Early Cretaceous is exposed on land. Two adjacent outcrop areas were combined into one seismic-scale lithologic model with the aid of photo mosaics, measured sections, and biostratigraphic data. Velocity, density, and porosity measurements on spot samples were used to construct the impedance model. Seismic models were generated by vertical incidence and finite difference programs. The results indicate that the reflections in the seismic model are controlled by the impedance contrast between low porous intervals rich in debris from the platform and highly porous intervals of pelagic lime mudstone, nearly devoid of debris. Finite difference seismic display showed best resemblance with the real seismic data, especially by mapping a drowning unconformity.     

Biogenic and abiogenic low-Mg calcite (bLMC and aLMC): Evaluation of seawater-REE composition,water masses and carbonate diagenesis

The ΣREE and shale-normalized (PAAS) REE_SN values of modern brachiopods (biogenic low-Mg calcite: bLMC) represented by several species from high- to low latitudes, from shallow- to deep waters and from warm- and cold-water environments, define three distinct average ‘seawater’ trends. The warm- and cold-water brachiopods define two indistinguishable (p < 0.050) groups that mimic open-ocean seawater REE chemistry, exhibiting the typical LREE enrichment with a slightly positive to negative Ce anomaly followed by an otherwise invariant series. Other recent brachiopods from an essentially siliciclastic seabed environment are distinct in both ΣREE and REE_SN trends from the previous two populations, showing a slight enrichment in the MREEs and an increasing trend in the HREEs. Other groups of modern brachiopods are characterized by elevated REE_SN trends relative to the ‘normal’ groups as well as by complexity of the series trends. The most characteristic feature is the decrease in the HREEs in these brachiopods from areas of unusual productivity (i.e., such as upwelling currents, fluvial input and aerosol dust deposition). Preserved brachiopods from the Eocene and Silurian exhibit REE_SN trends and Ce anomalies similar to that of the ‘open-ocean’ modern brachiopods, although, their enriched ΣREE concentrations suggest precipitation of bLMC influenced by extrinsic environmental conditions.Preservation of the bLMC was tested by comparing the ΣREE and REE_SN trends of preserved Eocene brachiopods to those of Oligocene brachiopods that were altered in an open diagenetic system in the presence of phreatic meteoric-water. The altered bLMC is enriched by approximately one order of magnitude in both ΣREE and REE_SN trends relative to that in bLMC of their preserved counterparts. Similarly, the ΣREE and REE_SN of preserved Silurian brachiopod bLMC were compared to those of their enclosing altered lime mudstone, which exhibits features of partly closed system, phreatic meteoric-water diagenesis. Despite these differences in the diagenetic alteration systems and processes, the ΣREEs and REE_SN trends of the bLMC of altered brachiopods and of originally mixed mineralogy lime mudstones (now diagenetic low-Mg calcite) are enriched by about one order of magnitude relative to those observed in the coeval and preserved bLMC.In contrast to the changes in ΣREE and REE_SN of carbonates exposed to phreatic meteoric-water diagenesis, are the REE compositions of late burial calcite cements precipitated in diagenetically open systems from burial fluids. The ΣREE and REE_SN trends of the burial cements mimic those of their host lime mudstone, with all showing slight LREE enrichment and slight HREE depletion, exhibiting a ‘chevron’ pattern of the REE_SN trends. The overall enrichment or depletion of the cement REE_SN trends relative to that of their respective host rock material reflects not only the openness of the diagenetic system, but also strong differences in the elemental and REE compositions of the burial fluids. Evaluation of the (Ce/Ce*)_SN and La = (Pr/Pr*)_SN anomalies suggests precipitation of the burial calcite cements essentially in equilibrium with their source fluids.     

Origin of fine-grained carbonate and siliciclastic sediments in an Early Palaeozoic slope sequence, Cow Head Group, western Newfoundland

The Cow Head Group is an Early Palaeozoic base-of-slope sediment apron composed of carbonate and shale. Whereas coarse-grained conglomerate and calcarenite are readily interpreted as debris-flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre- to centimetre-thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate-shale sequences (GCSS) deposited from waning carbonate turbidites and fall-out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin. The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments. These attributes characterize not only fine-grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has wider application.     

塔里木盆地塔中上奥陶统碳酸盐台地高频层序控制的早期成岩作用及其对储层分布的影响

上奥陶统良里塔格组碳酸盐岩是塔里木盆地塔中地区重要的油气储层。在碳酸盐岩岩石学、微相和测井曲线分析基础上,将塔中良里塔格组划分为5个四级层序、15个五级层序,建立了高频层序地层格架。显著的选择性溶蚀,悬垂型、新月型等大气水胶结物的发育,溶蚀孔壁较弱的阴极发光特征,粒内与粒间胶结物较低的Fe、Mn含量,亮晶颗粒灰岩的δ13C、δ18O与泥晶灰岩接近等特征表明,塔中地区Ⅰ号断裂带附近准同生期大气水溶蚀作用以及早期海水胶结作用普遍发育。对比分析显示大气淡水透镜体均发育于高频层序向上变浅旋回的顶部,即高频层序格架制约了早期成岩作用的形成分布。现在保存的早期成岩溶蚀孔面孔率可达到4%~5%,因此早期溶蚀孔的发育为晚期溶蚀改造提供了流体活动空间和条件,对碳酸盐岩有效储层的产出具有重要控制。综合分析提出,塔中Ⅰ号断裂坡折带TZ54-TZ826和TZ72-TZ62-TZ24井区等高陡型台缘是同生-准同生溶蚀孔发育的有利储层区。     

Early diagenesis of carbohydrates and lignin in mangrove sediments subject to variable redox conditions (French Guiana)

A comparative study of lignin and neutral carbohydrate compositions, combined with C, N and δ¹³C analyses, was carried out on sedimentary cores, and on various vascular plant species collected in mangrove swamps of French Guiana. The main purpose of this study was to assess the diagenesis of carbohydrates and lignin in brackish to hypersaline fine-grained mangrove sediments characterized by great changes in redox conditions. Distribution of carbohydrates in sediments reflects both the lability of these compounds and their efficient recycling. They are subject to selective degradation, cellulosic glucose and xylose appearing to be the two most labile neutral sugars. In contrast a relative increase in arabinose, rhamnose, fucose and hemicellulosic glucose between plants and sediments, suggests that they may be more refractory and/or that they also derive from microbial synthesis. The total carbon from lignin-derived phenols is higher in sediments than in mangrove plants as a consequence of their rather refractory character. Nevertheless, evidence of lignin decomposition was found to be independent of local environmental conditions. The various redox processes that occur in mangrove sediments depend on plant species, stages in forest development and season. Different redox conditions induce different mechanisms for the decomposition of lignin and thus induce changes in phenol distributions. At depth, in most mangroves, an increase in (Ad/Al)_v ratios and in deoxy sugars (fucose and rhamnose) content was significantly correlated with increased proportions of oxidized allochthonous organic debris deriving from the Amazonian detrital discharge, thus suggesting a specific source effect rather than a diagenesis induced change. Therefore, this study illustrates that both lignin and cellulose, derived from vascular plant debris, can be degraded in waterlogged mangrove sediments, and that their distribution depends on environmental conditions.     

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps,Italy)

The Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ¹⁸O values fall between ?8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ¹³C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.     

Mississippian (Early Carboniferous) stromatolite mounds in a fore-reef slope setting, Laibin, Guangxi, South China

The Mississippian (Early Carboniferous) is generally a period of scarce carbonate buildups in South China. This study documents outcrops of stromatolite mounds at Mengcun and Helv villages, in Laibin City, Guangxi Province, South China. The stromatolite mounds contain various stromatolite morphologies including laminar, wavy-laminar, domal or hemispheroidal, bulbous, and flabellate-growth columns. Intramound rocks are brachiopod floatstone and dark thin-bedded laminated micrite limestone. Individual stromatolites at Mengcun village are generally 3–6 cm thick and morphologically represent relatively shallow-water laminar (planar and wavy-undulated stromatolites) and deeper-water domal, bulbous and columnar forms. Where mounds were formed, the stromatolites continued growing upward up to 60 cm thick. Thrombolitic fabrics also occur but are not common. Stromatolite microscopic structure shows the bulk of the lamination to consist of wavy microbialite and discrete thin micritic laminae. These mounds are intercalated in deep-water fore-reef talus breccia, packstone formed as a bioclastic debris flow and thin-bedded limestone containing common chert layers of the Tatang Formation (late Viséan). Further evidence supporting the deep-water setting of the stromatolite mounds are: (1) a laterally thinning horizon of brachiopod floatstone containing deep-water, small, thin-shelled brachiopods, peloidal micritic sediments and low-diversity, mixed fauna (e.g., thin-shelled brachiopods, tube-like worms and algae) that have been interpreted as storm deposits, (2) common fore-reef talus breccias, (3) lack of sedimentary structures indicating current action, (4) preservation of lamination with sponge spicules, and (5) lack of bioturbation suggesting that the stromatolites grew in a relatively low energy, deep-water setting. The stromatolite mounds are the first described stromatolite mounds in Mississippian strata of South China and contain evidence that supports interpretations of (1) growth history of Mississippian microbial buildups and (2) environmental controls on stromatolite growth and lithification.     

A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic)

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement ( <6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1?N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24?h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe²⁺. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.     

Differential response at the seafloor during Palaeocene and Eocene ocean warming events at Walvis Ridge,Atlantic Ocean (ODP Site 1262)

The Latest Danian Event (LDE, c. 62.1 Ma) is an early Palaeogene hyperthermal or transient (<200 ka) ocean warming event. We present the first deep‐sea benthic foraminiferal faunal record to study deep‐sea biotic changes together with new benthic (Nuttallides truempyi) stable isotope data from Walvis Ridge Site 1262 (Atlantic Ocean) to evaluate whether the LDE was controlled by similar processes as the minor early Eocene hyperthermals. The spacing of the double negative δ¹³C and δ¹⁸O excursion and the slope of the δ¹⁸O–δ¹³C regression are comparable, strongly suggesting a similar orbital control and pacing of eccentricity maxima as well as a rather homogeneous carbon pool. However, in contrast to early Eocene hyperthermals, the LDE exhibits a remarkable stability of the benthic foraminiferal fauna. This lack of benthic response could be related to the absence of threshold‐related circulation changes or better pre‐adaptation to elevated deep‐sea temperatures, as the LDE was superimposed on a cooling trend, in contrast to early Eocene warming.