Coccolith Ooze Research Articles

Assessment of Chalk as an Archive for the Lithium Isotope Composition of Seawater

AbstractThe understanding of silicate weathering and its role as a sink for atmospheric CO2 is important to get a better insight into how the Earth shifts from warm to cool climates. The lithium isotope composition (δ7Li) of marine carbonates can be used as a proxy to track the past chemical weathering of silicates. A high‐resolution δ7Li record would be helpful to evaluate the role of silicate weathering during the late Cretaceous climate cooling. Here, we assess chalk as a potential archive for reconstructing Late Cretaceous seawater Li isotope composition by comparing Maastrichtian chalk from Northern Germany (Hemmoor, Kronsmoor) to a Quaternary coccolith ooze from the Manihiki Plateau (Pacific Ocean) as a lithological analog to modern conditions. We observe a negative offset of 3.9 ± 0.6‰ for the coccolith ooze relative to the modern seawater Li isotope composition (+31.1 ± 0.3‰; 2SE; n = 54), a value that falls in the range of published offsets for modern core‐top samples and for brachiopod calcite. Further, the negative offset between the Li isotope compositions of Manihiki coccolith ooze and modern planktonic foraminifera is 2.3 ± 0.6‰. Although chalk represents a diagenetically altered modification of pelagic nannofossil ooze, manifested by changes in the composition of trace elements, we observe a consistent offset of Li isotope data between Maastrichtian chalk and Maastrichtian planktonic foraminiferal data (−1.4 ± 0. 5‰) that lies within the uncertainty of modern values. We therefore suggest that chalk can be used as a reliable archive for δ7Li reconstructions.

Diagenetic transformation of organic matter in the Holocene sediments of the Black Sea: pyrolysis data

The article presents the results of pyrolysis (Rock-Eval) and pyrolysis-gas chromatography-mass spectrometry analysis of organic matter (OM) in the Holocene sediments of the Black Sea. The studied samples represent two parts of the sedimentary column: coccolith ooze with 2.8% Corg at the top and sapropels oozes with 8.5% Corg at the bottom. The deposition of Holocene sediments was inferred to take place under euxinic conditions, which are more favorable for organic-matter preservation and sapropel deposition during the Old Black Sea stage. The results show that the modern processes of microbially mediated methane oxidation in the Old Black Sea oozes are interpreted as the main cause of their apparently lower degree of diagenetic transformation as compared to that of coccolith oozes.

Explaining the Phanerozoic Ca isotope history of seawater

A new geochemical budget for the modern marine carbonate sink helps to explain the major features of the Phanerozoic Ca isotope record. A large compilation of Ca isotope ratios for modern carbonates, incorporating more than 50 new measurements, represents the quantitatively important components of the system. With this data set, distinct Ca isotope ratios are identified for different types of marine carbonate, the balance of which has changed over time with shifts between calcite and aragonite seas and with the development of pelagic calcification during the Mesozoic. It is suggested that large-scale changes in the Ca isotope ratio of seawater, as exemplified by that in the Carboniferous, were no longer possible after Jurassic time because of the generation of a deep-sea calcite sink expressed by deposition of foraminiferal–coccolith ooze across the world ocean. This work demonstrates the close connection between isotopic cycling, carbonate sedimentation, seawater chemistry, and evolutionary trends.

Holocene litho- and biostratigraphy of the NW Black Sea (Romanian shelf)

Detailed lithological and micropaleontological studies were performed on several cores from the Romanian Black Sea shelf, located in front of the Danube Delta, in front of the Razelm–Sinoe lagoonal complex, close to the Eforie beach, as well as on the Romanian inner shelf. All the studied cores are situated at a water depth varying between 12m and 67m. The purpose of the investigations was to reveal Upper Holocene lithological changes and the associated calcareous nannoplankton fluctuations in a very shallow marine setting such as the Romanian Black Sea inner shelf. The youngest Holocene unit or “Shallow Unit” is composed of alternating sequences of mud, silt, clay and coquina layers, containing marine mollusc faunas. The upper part of the Shallow Unit contains blooms of the nannoplankton species Emiliania huxleyi, while its lower part yields, besides common E. huxleyi, rare specimens of Braarudosphaera bigelowii. The Shallow Unit overlies a “shell hash layer”, mainly made by coquinas, containing mixing marine and brackish mollusc faunas. Towards the upper part of this level, monospecific assemblages with B. bigelowii were observed in a layer rich in carbonate acicular crystals. The Shallow Unit partly corresponds to the Unit 1 (finely laminated coccolith ooze) and Unit 2 (sapropelic sediments) of slope and basinal settings of the Black Sea. The layer rich in carbonate acicular crystals could be a shallower equivalent of the oldest layer of aragonite bands placed at the lower part of the Unit 2 in deep parts of the Black Sea. In the analyzed cores, the oldest lithological unit (Unit 3, the muddy clay sediments deposited in a brackish to freshwater environment) is characterized by the deposition of green-yellowish clay devoid of any Holocene calcareous nannoplankton species and containing brackish mollusc faunas. Based on nannofloral fluctuations, an increasing salinity from the base towards the upper part of the Shallow Unit may be supposed, together with the progressive establishment of a stable marine setting, similar to today.

DNA and lipid molecular stratigraphic records of haptophyte succession in the Black Sea during the Holocene

Previous studies suggest that the coccolithophorid haptophyte Emiliania huxleyi entered the Black Sea ~ 3400 yrs ago and since then a coccolith ooze defined as Unit I has developed. Unit I sediments contain long-chain alkenones derived from E. huxleyi whereas the alkenone distribution of the deeper coccolith-free sapropel (Unit II) is rather unusual. Alkenone-derived past sea surface temperature (SST) estimates suggest a large difference between Unit II and Unit I, which is likely a result of unusual biological precursors of the alkenones in Unit II. Here, we report a high-resolution stratigraphic analysis of ancient haptophyte DNA to establish the Holocene succession of haptophytes as sources of the alkenones in the Black Sea. Haptophytes related to brackish Isochrysis spp. were the initial sources of alkenones, and appeared immediately after the onset of sapropel deposition (~ 7550 yrs before present [a BP]). As salinity increased, Isochrysis-related haptophytes were slowly replaced by a complex suite of E. huxleyi strains as sources of alkenones. Our paleogenetic data showed that E. huxleyi colonized the Black Sea shortly after the onset of sapropel deposition, ~ 4000 yrs earlier than previously recognized based on their preserved coccoliths. E. huxleyi strains were the most likely source of the previously reported abundant and unusual C 36 di-unsaturated “Black Sea alkenone”. Strong haptophyte species and strain-specific effects were observed on the level of unsaturation of alkenones which resulted in spurious alkenone-derived SST estimates before 5250 a BP. In contrast, from ~ 5250 a BP onwards a relatively stable haptophyte assemblage dominated by a different suite of E. huxleyi strains yielded robust alkenone-SST values and indicated a gradual cooling from 19 °C to ~ 15 °C at the top of the record (~ 450 a BP).

Authigenic carbonate precipitates from the NE Black Sea: a mineralogical, geochemical, and lipid biomarker study

Carbonate precipitates recovered from 2,000 m water depth at the Dolgovskoy Mound (Shatsky Ridge, north eastern Black Sea) were studied using mineralogical, geochemical and lipid biomarker analyses. The carbonates differ in shape from simple pavements to cavernous structures with thick microbial mats attached to their lower side and within cavities. Low δ13C values measured on carbonates (−41 to −32‰ V-PDB) and extracted lipid biomarkers indicate that anaerobic oxidation of methane (AOM) played a crucial role in precipitating these carbonates. The internal structure of the carbonates is dominated by finely laminated coccolith ooze and homogeneous clay layers, both cemented by micritic high-magnesium calcite (HMC), and pure, botryoidal, yellowish low-magnesium calcite (LMC) grown in direct contact to microbial mats. δ18O measurements suggest that the authigenic HMC precipitated in equilibrium with the Black Sea bottom water while the yellowish LMC rims have been growing in slightly 18O-depleted interstitial water. Although precipitated under significantly different environmental conditions, especially with respect to methane availability, all analysed carbonate samples show lipid patterns that are typical for ANME-1 dominated AOM consortia, in the case of the HMC samples with significant contributions of allochthonous components of marine and terrestrial origin, reflecting the hemipelagic nature of the primary sediment.

Anaerobic methane oxidation and a deep H 2S sink generate isotopically heavy sulfides in Black Sea sediments

The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2–4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO 4 2−-CH 4 transition where H 2S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H 2S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe 2+ diffusing up from below. The H 2S-Fe 2+ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO 4 2− and 10 yr for H 2S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO 4 2−-CH 4 and H 2S-Fe 2+ cause the trapping of isotopically heavy iron sulfide with δ 34S = +15 to +33‰ at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO 4 2− diffusing downward into the SO 4 2−-CH 4 transition has an isotopic composition of +19‰, close to the +23‰ of H 2S diffusing upward. These isotopic compositions are, however, very different from the porewater SO 4 2− (+43‰) and H 2S (−15‰) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H 2S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth’s history.

99/00162 Effect of the mineral matrix in the reactions of oil shales: 1. Pyrolysis reactions of Turkish Göynuk and US green river oil shales

Within the Norwegian 15th Round deep-water production Licence No. 217 on the Vøring Plateau on the Norwegian continental slope, there is an unusually rugged seafloor topography. Our study addresses new 2D-, 3D-, and bathymetric data acquired in the Vema Dome area, where evidence for active piercement shale diapirism is found. The cores of the shale diapirs consist of coccolith oozes deposited during the Paleocene and Miocene. Individual shale diapirs and diapiric escarpments rise up to 70 m above the general sea floor (at 1200 m water depth). One of the key questions in our study has been to find out why diapirism is triggered over the Vema Dome and not over the nearby Nyk High, which has similar buried deposits of coccolith oozes. On the basis of interpretations of the new data, we conclude that the triggering is caused by a combination of at least three factors: (a) a buried low-density, high-porosity layer of deformable material (the coccolith ooze) ; (b) a doming substratum which causes extension and faulting; (c) migration of light hydrocarbons through some of the faults and fault zones. A fourth factor which probably contributes to triggering is tectonic instability. The one factor that discriminates the Vema Dome area from the neighbouring Nyk High area is the dome itself. We therefore suspect that this in some way is the main factor responsible for the triggering. Furthermore, we conclude that the process of piercement shale diapirism in the Vema Dome area is probably still active, but, because the movement of the sediments is suspected to be very slow, it should be of only minor concern to future oil field development.

Piercement shale diapirism in the deep-water Vema Dome area, Vøring basin, offshore Norway

Within the Norwegian 15th Round deep-water production Licence No. 217 on the Vøring Plateau on the Norwegian continental slope, there is an unusually rugged seafloor topography. Our study addresses new 2D-, 3D-, and bathymetric data acquired in the Vema Dome area, where evidence for active piercement shale diapirism is found. The cores of the shale diapirs consist of coccolith oozes deposited during the Paleocene and Miocene. Individual shale diapirs and diapiric escarpments rise up to 70 m above the general sea floor (at 1200 m water depth). One of the key questions in our study has been to find out why diapirism is triggered over the Vema Dome and not over the nearby Nyk High, which has similar buried deposits of coccolith oozes. On the basis of interpretations of the new data, we conclude that the triggering is caused by a combination of at least three factors: (a) a buried low-density, high-porosity layer of deformable material (the coccolith ooze) ; (b) a doming substratum which causes extension and faulting; (c) migration of light hydrocarbons through some of the faults and fault zones. A fourth factor which probably contributes to triggering is tectonic instability. The one factor that discriminates the Vema Dome area from the neighbouring Nyk High area is the dome itself. We therefore suspect that this in some way is the main factor responsible for the triggering. Furthermore, we conclude that the process of piercement shale diapirism in the Vema Dome area is probably still active, but, because the movement of the sediments is suspected to be very slow, it should be of only minor concern to future oil field development.

Fossilisation exceptionnelle d'une écaille de Synura (Chrysophytes) dans une boue à coccolithes miocènes de l'Atlantique central

Fossilisation exceptionnelle d'une écaille de Synura (Chrysophytes) dans une boue à coccolithes miocènes de l'Atlantique central

Oligocene laminated limestones as a high-resolution correlator of palaeoseismicity, Polish Carpathians

Abstract Thin intercalations of laminated coccolith limestones in a series of flysch and black shales define a regional marker horizon in the Oligocene strata of the Outer Carpathians. Distinctive sequences of laminae in the coccolith limestones have been correlated over distances of up to 240 km and across major facies zones. The correlated succession of laminae provided the basis for correlation of intercalated turbidites. The successions of laminae are disturbed at all studied localities by erosional gaps, intercalations of non-laminated limestone and deformation of some intervals of the laminated sequence. The disturbances themselves have been precisely correlated between three major nappes of the Outer Carpathians. The characteristics of the disturbances and their long-distance continuity indicate their seismic origin. The coccolith ooze at the top of the sediment column was firm and its simultaneous disturbance over large areas required earthquakes of magnitudes above 6.5. Laminated pelagic limestones provide a precise chronostratigraphic framework for long-distance correlation of seismic and other events recorded within their sequence.

Dynamic Depositional and Early Diagenetic Processes in a Deep-Water Shelf Setting, Upper Cretaceous Austin Chalk, North Texas: ABSTRACT

ABSTRACT The Austin Chalk of north Texas was deposited on a deep-water shelf north of the San Marcos Platform during a worldwide Coniacian and Santonian sea-level highstand. Transgressive (lowermost lower Austin Chalk), highstand (uppermost lower Austin Chalk), and regressive (middle and upper Austin Chalk) phases of cyclic chalk and marl sedimentation are recognized in core and outcrop and on log cross sections. Sedimentary structures in excellent exposures in excavations and tunnels created in Ellis County for the Superconducting Super Collider provide new evidence of sediment transport during Austin Chalk deposition. During transgression, bottom currents syndepositionally reworked nannoplankton oozes, incising channels as much as 120 ft across and 8 ft deep. Weakly burrowed channel fills having preservation of fine lamination document rapid infilling. Channel fills are composed of pyritized and carbonized wood and Inoceramus lag deposits, pellets, echinoderm fragments, and globigerinid grainstones, and coccolith ooze. During maximum highstand, bottom reworking was suppressed. Detrital content of highstand marls is low (<20 percent); organic content is high (1.4 to 3.5 percent). Coccolith preservation is excellent because of minimal diagenetic alteration. Regression is marked by resumed channel cutting and storm-bed winnowing in the middle and upper Austin Chalk. Suppressed resistivity log response and recessive weathering characteristics of the middle Austin Chalk are not primarily related to depositional environment but rather to increased input of volcanic ash during the accumulation of this interval. Early stabilization of ash produced clay-coated microfabrics in sediments that are otherwise similar to the transgressive deposits.

Cretaceous

Abstract Early Cretaceous The Cretaceous Period lasted for about 70 million years. During this time there was a major change in the sedimentary history of the area as tectonism died down and deposition started of an extensive blanket of coccolith ooze: the Chalk. The change took place mainly over a brief interval across the Albian/Cenomanian (Lower/Upper Cretaceous) boundary, at about 95 Ma. Until that time crustal extension along the Arctic-North Atlantic megarifts continued to influence the tectonic evolution of northwest Europe (Ziegler 1982, 1988). This tensional régime caused rifting and block faulting, particularly across the Jurassic-Cretaceous boundary (Late Cimmerian movements) and in the mid Aptian (Austrian phase). During the latter phase, sea-floor spreading commenced in the Biscay and central Rockall Rifts. The northern part of the Rockall Rift began to widen too, possibly by crustal stretching rather than sea-floor spreading (Ziegler 1988, p. 75). During the Albian the regional pattern began to change and by the beginning of the Cenomanian rifting had effectively ceased away from the Rockall/Faeroe area. Most of the Jurassic sedimentary basins continued as depositional areas during the Early Cretaceous, but the more extensive preservation of Lower Cretaceous sediments provides firmer constraints on some of the geographical reconstructions. The marked sea-level fall across the Jurassic-Cretaceous boundary isolated the more southerly basins as areas of non-marine sedimentation, and it was not until the beginning of the Aptian that they became substantially marine. The extent of emergence of highs in the North Sea area is difficult to assess, especially where

Porometry and fabric of marine clay and carbonate sediments: Determinants of permeability

The porometry of a marine sediment is determined by the fabric, i.e., the shape, orientation, arrangement (spatial distribution) and associations of the solid particles. Fabric ultimately determines the permeability of the sediment by controlling the size, shape and continuity of the pore space. Electron microscopy studies have revealed complex fabrics and porometries of surficial and deeply buried sediments from various geological environments. When viewed in profile using thin sections, the pores of unconsolidated, high-porosity marine clays are often very irregular in shape and span a broad size range. In general, however, the pore profiles of these sediments can be described as having aspect ratios (length-to-width ratios) which approach 1.0. Pore profiles of consolidated, low-porosity clays are characterized by aspect ratios which approach infinity. Coefficients of permeability for marine clays typically range from 10 −5 cm/s (porosities of 70–75%) to 10 −8 cm/s (porosities of about 50%). The permeability coefficient was 10 −13 cm/s and the porosity was 27% for bentonite (a smectite) consolidated in the laboratory to a maximum load of 68.9 × 10 3 kPa ∗ ∗ Enderby and Thompson (1981). . The pore space characteristics in carbonate sediments vary greatly depending on the constituent particles. Aspect ratios for pore profiles can range from 1.0 in a coccolith ooze to 2.0 in an aragonitic needle matrix consolidated uniaxially at pressures of 4.32 × 10 3 kPa. The permeability of shallow-water (3–4 m) oolitic carbonates is largely controlled by the permeability of the aragonite needle matrix. Despite the large quantity (in one case 94% by weight) of fine-sand-size ooids in these deposits, the comparatively impermeable ooids are matrix supported. This results in a sediment with relatively high porosity and medium permeability, similar to fine sands that are tightly packed and, therefore, grain supported. Permeability coefficients of these oolitic carbonate sediments range from 10 −2 to 10 −4 cm/s. Carbonate sediments recovered from intermediate water depths (970–1980 m) are composed of grains (mostly large shell fragments) embedded in either a coccolith matrix or an aragonite needle matrix. The permeability coefficients for these carbonates range from 10 −5 to 10 −6 cm/s. The sediments studied range in size from sands to clays, and the differences in porosity and permeability observed for these sediments reflect differences both in the fabrics of the fine-grained matrices and in the grain-size distributions. The data presented suggest that the sediment fabric is a function of the characteristics of the constituent particles and the physical and chemical environments of deposition and that the fabric plays a major role in determining the permeability of the deposit. Limited but revealing computerized image analyses have been carried out on scanning and transmission electron micrographs of these sediments. Initial results indicate that this approach may prove useful for quantifying fabric parameters and providing a statistical basis for fabric descriptions.

ZOOPLANKTON FECAL PELLETS AS A SOURCE OF HYDROCARBONS IN CHALK

Evidence from biochemistry, marine geology and sedimentology shows that chalk could be a source of hydrocarbons, and that some hydrocarbons in some chalk reservoirs could have originated within the chalk. Coccolith ooze, the precursor of chalk, is deposited as the fecal (faecal) pellets of zooplankton, most probably copepods. These fecal pellets contain dihydrophytol which is a precursor of pristane; the latter is a precursor of some crude oil deposits. Zooplankton fecal pellets probably provided the organic source for some of the hydrocarbons in chalk. Some hydrocarbons in marine‐ and lacustrine‐shale source rocks could have originated in much the same way.

Sediment waves on the monterey fan levee: A preliminary physical interpretation

Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km 2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.

Features and Origin of Italian Jurassic Radiolarites Deposited on Continental Crust

ABSTRACT Jurassic radiolarites that were deposited on continental crust and which are overlain and underlain by pelagic limestones were examined in four basins. Radiolarites studied range from bedded chert, where chert occurs in even, continuous beds separated by conscpicuous rhythmic shale interbeds, to nodular-lumpy chert, where chert occurs as isolated nodules in limestone at one extreme to lumpy beds of irregular thickness at the other extreme. Bedded chert formations are composed of microcrystalline quartz, clay minerals, and minor amounts of either hematite or organic matter pigment; nodular cherts contain in addition considerable micrite (altered coccolith ooze). Chert nodules, lenses, and beds all formed by the diagenetic reorganization of silica almost entirely of biogenic origin, chiefly from Radiolaria. Nodular-lumpy chert beds and even some bedded chert formed by partial to complete replacement of limestone. Lumpy beds formed because compaction, dissolution of calcite, and replacement of calcite were not uniform in either time or place within a bed. Many even-bedded chert layers were originally silts or sands composed almost entirely of current-deposited Radiolaria. The rhythmic bedding of radiolarite is the product of 1) episodes of rapid current-deposition (e.g., turbidity currents) of radiolarian sediment alternating with slow deposition of hemipelagic mud and 2) episodic growth of Radiolaria. Bioturbation blurs the evidence of these two different origins, but the following features indicate that many chert-precursor beds were current-deposited: sharp chert-shale bedding contacts, flute casts, graded beds, clay clasts, laminations and rapidly buried animal tracks and trails. Water depths during radiolarite deposition are uncertain. Regional stratigraphic data, evidence of redeposited radiolarian sand, and acceptance of the Bosellini-Winterer model of carbonate dissolution surfaces during Late Jurassic time leads EFM to conclude that well-bedded radiolarite was deposited at depths close to the CCD (2500 m) and that lumpy-bedded chert was deposited between the ACD and the CCD (1500 to 2000 m). Features indicating the presence of evaporites (evaporite pseudomorphs, breccias formed by crystal growth, quartzine, lutecite), paleosoil fabrics, stalactite-stalagmite fracture fillings, and disbelief in the validity of the CCD in Late Jurassic time in the Tethys leads RLF to conclude that the Lombardy radiolarites were deposited in environments that were, in part, shallow intertidal mudflats where local precipitation and solution of evaporite minerals and occasional subaerial exposure took place.

Geologic history of the Maurice Ewing Bank of the Falkland Plateau (southwest Atlantic sector of the Southern Ocean) based on piston and drill cores

Paleontologic and sedimentologic analyses of fifty-five piston cores and three drill-core sequences, combined with seismic reflection profiler data, provide the basis for a geologic study of the intermediate-depth Maurice Ewing Bank located at the eastern end of the Falkland (Malvinas) Plateau, southwest Atlantic sector of the Southern Ocean. Presented is a geologic map and cross-section of older (pre-Pliocene) units sampled beneath a thin veneer (1–2 m) of Plio-Pleistocene siliceous ooze and glacial marine clastics which mantle the plateau. Thirty-four piston cores penetrated pre-Pliocene sediments, the oldest being Campanian in age. A comparison of faunal and floral assemblage characteristics of Cretaceous sediment from opposite sides of the Maurice Ewing Bank shows significant paleoecological differences which suggest that the Falkland Plateau served as an important barrier between water masses in the South Atlantic during this time. During the Tertiary, a relatively high carbonate compensation depth coupled with a significant variation in topography across the bank contributed to marked changes in lithologic facies ranging from coccolith ooze near the apex of the bank to diatom ooze and zeolitic clay on its flanks. Miocene coccolith ooze deposition ceased when the center of high carbonate productivity (and the Miocene Polar Front?) migrated north of the bank in response to deteriorating climatic conditions associated with a severe Late Miocene continental glaciation on the Antarctic continent. Strong bottom currents associated with the Cretaceous boundary event, the Early Miocene opening of the Drake Passage, and the Late Miocene Antarctic glaciation caused scouring and removal of significant volumes of sediment, thus contributing to the complex erosional history of the bank. During the Late Miocene the last and most severe of these events truncated the previously deposited Upper Cretaceous—Miocene sequence of sediments of the bank. The primary erosional agent during the Late Miocene was probably a northerly component of the circum-polar deep water which impinged on the bank from the southwest. Erosion of older pelagic oozes ceased with the deposition of Plio-Pleistocene glacial marine clastics which essentially “armoured” the sediment surface with a protective, erosion-resistant cover. Carbonate deposition in the study area resumed during the latest Pleistocene when the polar front migrated to a mean position near the southern margin of the bank.

Coccoliths: Production, transportation and sedimentation

The presence of coccolith ooze on the deep-sea floor and the well preserved suspended coccoliths in the undersaturated water column is explained by accelerated and communal sinking of coccoliths and coccospheres in small zooplankton's fecal pellets. The community structure in the euphotic layer will be replicated in the underlying thanatocoenosis with high resolution without significant time-lag between the production and the deposition. The euphotic biocoenosis of coccoliths drift only a few hundred kilometers at the maximum while they descend at the rate of more than 150 m per day through 5,000 m of water column where the variance of advection is approximately 5 km a day. At an Equatorial Pacific station it was estimated that 92% of coccoliths produced in the euphotic layer were thus being transported to the deep-sea bottom. Coccolithophore production provides several grams of calcite to a square meter of sea floor per year. Coccoliths do not undergo significant change during passage through copepods and other planktonic grazers. The membrane which covers a copepod's fecal pellet is coherent particularly in cold water. It protects contents from spilling and from dissolution and accelerates sinking rates by providing a smooth surface. The membrane is biodegraded rapidly in warm water and its contents exposed. As such naked pellets sink, coccoliths are shed and suspended in aphotic water. The majority of freed coccoliths will be dissolved in the undersaturated water column before arriving at the bottom. Approximately 8% of coccoliths produced were estimated to be remineralized in the water column at the above-mentioned station if Peterson's dissolution rate of calcite is applicable.

Franciscan Limestones and Their Environments of Deposition

Detailed study of the distribution, petrography, and microfossils of the Calera and Laytonville Franciscan limestones allows us to describe their most probable areas of deposition. The Calera Limestone Member was deposited in shallow submerged basins situated on a ridge. Coccolith ooze and rarer radio-larian limestone were deposited on basin floors which were generally below the lysocline. Planktonic foraminifera deposited above the lysocline on basin walls were sporadically swept down into the basins as turbidity currents and, because of this rapid deposition, were preserved. The ridge along which the basins were located may have been located behind an island arc of Mesozoic age. The Laytonville limestone was deposited in deep water, possibly in the arc-trench gap, or as abyssal plain deposits that were subsequently subducted or accreted to the continent.