Regional seismic observations of the Ontong Java plateau and East Mariana Basin

P-waves recorded on Ponape Island at the northern end of the Ontong Java plateau have been investigated. Different modes of propagation in the distance range 12° to 17° Δ between paths from Melanesian earthquakes across the Ontong Java plateau and paths from Mariana earthquakes across the eastern Mariana basin suggest that the mantle underlying the regions is not homogeneous. Travel-times of P-waves beneath the Ontong Java plateau are slower than for paths beneath the east Mariana basin. The frequency content of the first-arriving P-waves is lower for paths across the Ontong Java plateau than for the east Mariana basin. The disparate crustal thicknesses of the Ontong Java plateau and east Mariana basin may influence the relative amplitude of oceanic Pn, with smaller amplitudes corresponding to a thicker crust. Pn-P differential times for the Ontong Java plateau show a general decrease with the depth of earthquake focus but no comparable trend is seen in the Mariana data, possibly because of the scatter. The observation of high-frequency oceanic Pn propagation across the Ontong Java Plateau is suggestive that the plateau is not of continental origin.     

Medium-term sedimentation on high intertidal mudflats and salt marshes in the Severn Estuary, SW Britain: the role of wind and tide

Six sites monitored monthly on salt marshes and mudflats in the middle and outer Severn Estuary gave potentially predictive–retrodictive relationships between the vertical response of the sedimentary surface (erosion/accretion), a factor reflecting tidal heights, and the wind-wave power-supply. The continuing rise of relative sea level in the area, at the rate of a few millimetres annually, is providing accommodation space at a pace low enough to permit the continuing vertical build-up of the salt marshes throughout the estuary. On the mudflats, wind and tidal conditions during the survey maintained an accretionary regime in the middle estuary but an erosional one in the outer part. The Severn Estuary is a system in delicate balance which is likely to respond dramatically to modest changes in the tidal and/or wind regimes.     

西沙群岛生物碳酸盐的成因和沉积:来自活体仙掌藻的证据

A recent island survey reveals that the Xuande Atoll and the Yongle Atoll in the Xisha Islands can be classified into one of two systems: the depleted atoll system and growth atoll system; the survey also indicates that the decreased area of several shoals is an unbearable burden for the Xisha Islands, of which the largest island area is 2.13 km2 and the minimum elevation is 1.4 m. According to a survey on the ecological characteristics of Halimeda in the Laolongtou breaker zone of Shidao Island in the Xisha Islands, the green and white living Halimeda are collected,the isotopic ages of 14 C contained in the Halimeda are shown to be 27 years and 55 years, respectively, and carbonate mainly occurs in five types, i.e., luster, segment, sand, sand grain, and marl in the formation. The Halimeda segments mainly provide the carbonate sediments of long-term biogenic deposits in the reef environment and the annual productivity per area is 60–100 g/m2; the characteristics of the microstructure of the Halimeda are analyzed, the aragonite raphide carbonate is deposited and enriched in the cortexes, medullas and cysts, and the Halimeda generally contain major elements such as C, O, Ca, Cl, Mg, K, Na, S and Al, and are rich in trace elements such as tellurium(Te), rhodium(Rh) and strontium. It is believed that the Halimeda grow slowly,including the biotic community of reef corals in the reef areas, thus they possess an environmental remediation capacity, but it takes much time to remedy the environment, and it is necessary to make the law to protect the diversity and vulnerability of the Xisha marine ecology, the ecology of the reef community and the island environment in a scientific way. As indicated in the survey, under the background of global warming and sea-level rise, the discovery of large amounts of Halimeda in the Laolongtou sea area is significant for the natural increase of the depleted atoll system of the Xuande Atoll, while the Halimeda segments represent the primary form of the fossil Halimeda, of which the species can be identified and preserved in great numbers under geological conditions. The Miocene was discovered in large amounts in the Xichen-1 well, therefore the study on the characteristics and mechanism of Halimeda carbonate sediments plays a pivotal role in the formation and construction of organic reefs in the South China Sea as well as oil and gas exploration.     

Origins and maturity of organic matter in mid-Cretaceous black shales from ODP Site 1138 on the Kerguelen Plateau

We have conducted elemental, isotopic, and Rock-Eval analyses of Cenomanian–Santonian sediment samples from ODP Site 1138 in the southern Indian Ocean to assess the origin and thermal maturity of organic matter in mid-Cretaceous black shales found at this high-latitude location. Total organic carbon (TOC) concentrations range between 1 and 20 wt% in black to medium-gray sediments deposited around the Cenomanian–Turonian boundary. Results of Rock-Eval pyrolysis indicate that the organic matter is algal Type II material that has experienced modest alteration. Important contributions of nitrogen-fixing bacteria to the amplified production of organic matter implied by the high TOC concentrations is recorded in δ¹⁵N values between −5 and 1‰, and the existence of a near-surface intensified oxygen minimum zone that favored organic carbon preservation is implied by TOC/TN ratios between 20 and 40. In contrast to the marine nature of the organic matter in the Cenomanian–Turonian boundary section, deeper sediments at Site 1138 contain evidence of contributions land-derived organic matter that implies the former presence of forests on the Kerguelen Plateau until the earliest Cenomanian.     

Seasonality and growth patterns using isotope sclerochronology in shells of the Pliocene scallop Chesapecten madisonius

Growth lines and variation in oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) in shells of the Pliocene scallop Chesapecten madisonius preserve seasonal chronologies of biological and environmental change. This study evaluated whether (1) prominent growth lines were formed annually, and (2) growth rates estimated using isotope sclerochronology were comparable to rates estimated using visual inspection (measuring the width between external growth lines). We compared both techniques for estimating growth rates and age on three late to mid-Pliocene C. madisonius shells. The first approach located prominent growth lines on the δ¹⁸O time series, and differentiated between annual and non-annual (disturbance) growth lines. The second approach assumed all prominent lines were annual. This comparison showed that visual inspection underestimated growth rates and overestimated age. Seasonal timing of annual growth line formation using isotope sclerochronology provided unexpected results. Because this region fell within the warm-temperate paleobiogeographic province, we predicted annual lines formed during summers (most negative δ¹⁸O values). Instead, annual growth lines coincided with the most positive δ¹⁸O values (winter), typical of bivalves from cold-temperate regions. Moreover, shells recorded seasonal temperatures ranging from 3.2–20.8°C, a range lower than the thermal regime defined for warm-temperate environments (8–25°C). Possibly, the Sea Slope Gyre, which mixed eddies and cold filaments of the Labrador Current and warm waters of the Gulf Stream, penetrated the warm-temperate environment in this region. Alternatively, warm-water fauna from the zoogeographic Carolinian subprovince migrated northward and endured by virtue of warm summer temperatures. Regardless of the explanation, our findings provide a glimpse of mid-latitude seasonal temperature range for a warm climate episode during the mid-Pliocene.     

Sedimentary and structural patterns on the Iberian continental margin: an alternative view of continental margin sedimentation

A long-range side-scan sonar (GLORIA) survey of the entire West Iberian slope and rise has provided the first overview of the interrelationship between structure and sedimentation patterns on a continental margin. The results emphasize the importance of slope-following contour currents as a depositional mechanism in fashioning this continental rise. Terrigenous sediments transported down-canyon by-pass the rise which does not consist of a series of coalescing fans. The sedimentation patterns identified on the sonographs can be interpreted in terms of facies models and caution must be exercised against over-emphasis of downslope processes in models for the construction of lower slopes and rises.     

Influence of magnesium on the ion-activity product of calcium and carbonate dissolved in seawater: A new approach

The influence of Mg²⁺ ions on the ion-activity product of calcium and carbonate dissolved in seawater has been investigated. The present approach is based mainly on: (1) the preponderance of this element in the marine system; (2) the observations by Weyl (1965), Jansen and Kitano (1963), and more recently by Plummer and Mackenzie (1974) on the higher solubility of magnesian calcite relative to pure calcite; and (3) the inferences drawn from our laboratory studies on the Ca²⁺—Mg²⁺ ion-exchange behaviour on surfaces of pure calcite. From these considerations, the pK value corresponding to a calcite containing 12 mole% of MgCO, has been estimated to be 7.7± 0.1 at 20°C. This value has serious reflections on the extent of saturation of seawater with respect to calcium-carbonate polymorphs. The reassessed degree of carbonate saturation for an average warm-surface ocean turns out to be only between 1.0 and 2.2.     

Sedimentation rates off SW Africa since the late Miocene deciphered from spectral analyses of borehole and GRA bulk density profiles: ODP Sites 1081–1084

Sedimentation rates (SRs) off SW Africa were calculated by performing spectral analyses in the depth domain on borehole and gamma-ray attenuation (GRA) bulk density data from ODP Sites 1081–1084. Inversion and integration of SRs versus depth from spectral analysis yielded detailed SR profiles in the time domain. Our technique allowed the detection of excursions in calculated SRs that not only often differed from those established through coarse-scaled biostratigraphic data, but also revealed a greater regional variability in the sediment accumulation over time. High-resolution bulk density data exhibited distinct periodicity in the waveband of Milankovitch cycles (precession at 19–23 kyr; obliquity at 41 kyr; eccentricity at 100 kyr). The pronounced Milankovitch cyclicity suggests that climate variability and trends in SRs along the Benguela Current System (BCS) were responding to insolation patterns during the past 6 Myr. We find relatively low SRs when evolutive amplitude spectra are dominated by obliquity and eccentricity periods. In contrast, significant SR peaks at all sites often occur when strong precessional amplitudes coexist with obliquity and eccentricity cycles. Episodes of high SRs often coincide with peaks in organic carbon mass accumulation rates (MAR C_org) and reduced sea surface temperature (SST) in the southern Walvis Basin, which have been associated with increased regional upwelling. This suggests that the high SRs reflect high productivity (high MAR C_org) attributed to strong wind and upwelling intensity during cool climate periods (low SSTs) in accordance with orbital forcing patterns observed in our spectra.     

Rates of calcium carbonate dissolution and organic carbon decomposition in the North Pacific ocean

Recent carbonate data collected in the North Pacific were combined with the data in the literature in order to understand more clearly the carbonate chemistry in the North Pacific. Our analyses show that inorganic CaCO₃ dissolution contributes about 26% of the total inorganic CO₂ increase of deep water, after leaving the Southern Ocean. The calcium and alkalinity data indicate a CaCO₃ dissolution rate of 0.060±0.010 and 0.053±0.005 µ mol kg^–1 yr^–1 respectively, for waters deeper than 2,000 m in reference to the Weddell Sea Deep Water. The organic carbon decomposition rate is 0.107±0.012 µ mol kg^–1 yr^–1 while the oxygen consumption rate is 0.13±0.002 µ mol kg^–1 yr^–1. These results which are based on the direct comparison of two water masses agree well with other estimates which are based on methods such as the one-dimensional-diffusion-advection model. Comparison of data along the two sections at 165°E and 150°W shows no significant difference in the ratio of the CaCO₃ dissolution rate and the organic carbon decomposition rate. The eastern section, however, has a higher TCO₂ input than the western section because of the older age of the deep water along the eastern section.     

In Situ Correction of Porosity and Velocity of Pelagic Carbonate Sediments in the Eastern Equatorial Pacific (Sites 846 and 850, ODP Leg 138)

During Ocean Drilling Program Leg 138, eleven sites were drilled. The sediments recovered are mainly composed of carbonate-rich pelagic ooze including minor diatom and radiolarian ooze. Average carbonate contents of Sites 846 and 850 are about 59% and 75%, respectively. The porosity and velocity measured in laboratory were corrected to in situ condition using empirical equation (Hamilton, 1976) and delta velocity (log value-laboratory value). The corrected laboratory porosity matches well with logging data, but it is deviating in the intervals of low carbonate content. The correlation between log porosity and the corrected porosity shows a similar trend. Depth profiles between the corrected velocity and logging data do not agree in the intervals of low carbonate content, but the intervals characterized by high carbonate content are relatively matched. This suggests that Hamilton's equation is not appropriate for diatom-rich sediments. The correlation between log velocity and the corrected velocity is also a much better fit than that of laboratory velocity. In addition, the relationship between velocity and porosity is a better fit using corrected data than uncorrected data. Thus, mechanical rebound correction needs to interpret laboratory data to in situ condition. However, the magnitude of rebound by the removal of overburden pressure should be carefully considered because the rebound depends on sediment types.     

The steady-state calcium carbonate ion activity product of recent shallow water carbonate sediments in seawater: a comparison of field observations and laboratory experiments

Closed system equilibration experiments between natural seawater and shallow water calcium carbonate-rich sediments from the Bahamas yielded steady-state calcium carbonate ion activity products (CCIAP). Results obtained from initially supersaturated and undersaturated solutions were in good agreement. Experiments conducted with the addition of a biocide and/or the destruction of sediment organic matter gave results similar to those obtained in systems where these treatments were not used. Excellent agreement was also found between CCIAP values for 8 day and more than 50 day equilibration times. Our results, therefore, meet the major criteria for at least metastable equilibrium between the solution and carbonate sediment.Fine-grained samples produced a CCIAP close to the value predicted for aragonite, which is the major carbonate phase in all samples. Coarse-grained sediments produced larger CCIAP values of up to 2.8 times that predicted for aragonite equilibrium. The CCIAP for the coarse-grained sediments is probably produced by high-Mg calcite which is a significant component of these sediments. Oolite samples were among the coarse-grained sediment samples studied. They also produced results much greater than expected for aragonite equilibrium. This brings into question their use as material for measuring aragonite solubility as has been done in the past.The CCIAP measured in the laboratory experiments are in good agreement with field observations of pore-water CCIAP values from the fine-grained sediments. Coarse-grained sediments showed greater variability, with higher CCIAP values generally occurring in the pore waters than in the laboratory experiments. Since the overlying waters were always at a higher CCIAP than the pore waters, the major factor causing this difference is believed to be the short residence time of pore waters in the coarse-grained sediments, which is the result of the high-energy hydrodynamic environments in which they reside.     

Composition of Pliocene to Quaternary mixed terrigenous and calcareous sandy beds in contourite drift deposits at ODP Site 1119 off New Zealand: Insights into sandy drift development and drift petroleum reservoir characterization

ODP (Ocean Drilling Program) Site 1119 was drilled in drift successions deposited by northward flowing currents on the Canterbury slope, South Island, New Zealand. Pliocene (∼3.9 Ma) to Pleistocene drift accumulation was concomitant with tectonic uplift and sediment supply from the Alpine Fault plate boundary to the west, glacial development and eustatic sealevel change that created cyclicity in sandy input. Sand detrital and biogenic modes of very fine and fine sand fractions from 24 unconsolidated core samples (44 thin sections) were determined. Fine sand fractions contain an average of 52% bioclastic debris, with bioclasts making up 23% of the very fine sand fraction. Foraminifers are the dominant bioclasts, with pelagic more common than benthic varieties. The average proportions of monocrystalline quartz, plagioclase and potassium feldspar are similar in both size fractions (average QmKP of ∼25:12:63). In contrast, the very fine sand fractions have, on average, higher lithic (QFL%L of 16 vs. 3%), mica (Totalgrain%M of 10 vs. 6%), and dense mineral (Totalgrain%D of 8 vs. 5%) content as compared with the fine sand fractions. Younger clinoform drape (Unit I) and older mounded drift (Unit II) seismic facies showed no distinct detrital compositional differences in their sand fractions, though bioclast content ranges higher in the fine sand fraction of Unit II. Albite feldspar, metamorphic rock fragment, and chlorite components in the sand are consistent with a schist provenance. Other components such as biotite, plagioclase, and K-feldspar require a contribution from metasedimentary and volcanic rocks. Thus there is some along-slope mixing during sediment transport by south-to-north flowing currents. Cemented microporous sandstone at the base of the section provides some insight into contourite burial diagenesis. This work provides a better understanding of the compositional variability of sand components of drift successions which are potential hydrocarbon reservoirs elsewhere.     

网纹藤壶壳板矿化结构及其碳酸钙含量测定

采用X射线衍射技术,初步研究了网纹藤壶(Balanus reticulates Utinomi)壳板矿化结构特征,并采用钙黄绿素法、钙-羧酸指示剂法和铬黑T指示剂三种方法测量了来自不同海域藤壶壳板的碳酸钙含量。实验发现,网纹藤壶壳板中的矿化组织主要由方解石构成,碳酸钙微晶体排列有一定的取向,较为复杂。海口和陵水两地藤壶壳板中碳酸钙含量没有显著差异(P<0.05),最大值为93.87%,最小值90.77%。尽管三种碳酸钙含量测量法的结果没有显著性差异(P<0.05),但进一步综合分析发现钙黄绿素法测定结果最为稳定,且简单易操作,可作为首选方法。     

沉积物埋藏深度和碳酸钙含量对南海沉积物干密度的影响

在古海洋学研究中,质量堆积速率是反映地质历史时期某种物质沉积通量变化趋势的重要指标。计算质量堆积速率的一个必要参数是沉积物的干密度（DBD）。但是,很多钻孔沉积物的干密度数据往往因采样间隔稀疏而分辨率太低,甚至没有实测数据,为后续研究带来一定限制。前人研究中曾经发现深海沉积物的DBD可以用沉积物中的CaCO3含量进行估算,但这一方法在南海是否可行尚缺乏研究。笔者根据前人经验,对南海ODP184,IODP 349、367、368共4个航次17个站位的DBD、CaCO3含量与样品埋藏深度数据进行了分析,发现沉积物埋藏深度是影响DBD的重要因素,DBD和CaCO3含量仅在南海南部的ODP1143站位表现出良好的关系。而在南海北部及中部可能由于受到不同程度深层流活动的影响,导致沉积环境极不稳定,非碳酸钙组分来源复杂多变,故DBD与CaCO3%的相关性差。在南海南部,以沉积物埋藏深度和CaCO3%为变量,对ODP1143站位用二元多项式拟合出一经验公式,该公式适用于南海南部海域与1143站位有相似沉积环境的沉积物样品干密度的估算。     

Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean

The taxonomic composition and types of particles comprising the downward particle flux were examined during the mesoscale artificial iron fertilisation experiment LOHAFEX. The experiment was conducted in low-silicate waters of the Atlantic Sector of the Southern Ocean during austral summer (January–March 2009), and induced a bloom dominated by small flagellates. Downward particle flux was low throughout the experiment, and not enhanced by addition of iron; neutrally buoyant sediment traps contained mostly faecal pellets and faecal material apparently reprocessed by mesozooplankton. TEP fluxes were low, ≤5 mg GX eq. m⁻² d⁻¹, and a few phytodetrital aggregates were found in the sediment traps. Only a few per cent of the POC flux was found in the traps consisting of intact protist plankton, although remains of taxa with hard body parts (diatoms, tintinnids, thecate dinoflagellates and foraminifera) were numerous, far more so than intact specimens of these taxa. Nevertheless, many small flagellates and coccoid cells, belonging to the pico- and nanoplankton, were found in the traps, and these small, soft-bodied cells probably contributed the majority of downward POC flux via mesozooplankton grazing and faecal pellet export. TEP likely played an important role by aggregating these small cells, and making them more readily available to mesozooplankton grazers.     

Characteristics and distribution patterns of reef complexes on the carbonate platform margin in deep water areas: the western South China Sea

As a potential oil and gas reservoir, reef complexes have been a research focus from petroleum geologists for a long time. There are favorable conditions for the development of reef complexes in the South China Sea; however, their internal structures, evolution and distribution are still poorly understood. Based on 2D and 3D seismic data, the internal structures and evolution patterns of the reef complexes on the carbonate platform margin in the deep water areas over the western South China Sea were studied in detail. The result shows that two types of reef complexes, i.e., fault controlling platform margin reef complexes and ramp reef complexes have been developed in the study area. The reef complexes have independent or continuous mound or lenticular seismic reflections, with three internal structures (i.e., aggrading, prograding and retrograding structures). There are different growth rates during the evolution of the reef complexes, resulting in the formation of catch-up reefs, keep-up reefs and quick step reefs. The study also reveals that different platform margin reef complexes have different internal structures and distributions, because of the different platform types. These results may be applied to the exploration and prediction of carbonate platform margin reef complexes in other areas that are similar to the study area.     

Clay mineral assemblages,siliciclastic input and paleoproductivity at ODP Site 1085 off Southwest Africa: A late Miocene–early Pliocene history of Orange river discharges and Benguela current activity,and their relation to global sea level change

A late Miocene to early Pliocene sequence drilled on the continental slope of southwest Africa off the Orange river mouth (ODP Site 1085) has been investigated. Clay mineral assemblages, coarse siliciclastics and benthic foraminifer accumulation rates (BFAR) unravel a step by step evolution of marine and continental environments closely related to sea level variations, ocean circulation and global climate: (1) smectite is a typical tracer of the Orange river load, whereas illite is mostly transported by the Benguela current (like chlorite) and winds, and kaolinite is derived from low latitudes by the poleward undercurrent and the North Atlantic Deep Water (NADW); (2) increased erosion and influence of the Orange river after 9.6 Ma is linked to a sea level drop at a time of Antarctic ice-growth. This has been followed by an increased seasonality of precipitation and high productivity, but low oxygen content and associated dissolution of carbonates; (3) increased productivity and dissolution of carbonates, and coeval increase of continental aridity after 8.9 Ma express a further development of the Benguela current and upwelling; (4) better preservation of carbonates and increased contribution of terrigenous material from northern sources at 6.9 Ma are related to increased circulation of NADW after an early stage of northern hemisphere glaciation; (5) increased erosion and contribution from the Orange river and westward shift of the area of higher productivity from 5.9–5.8 Ma to 5.3–5.2 Ma are related to a significant fall of sea level, and encompass the time of the entire Mediterranean salinity crisis; (6) short-term variations of the smectite/illite ratio (S/I) and BFAR suggest a major control of productivity by wind and current activities (and related upwelling), but may express brief variations of sea level in specific intervals before 8.9 Ma and during the late Messinian especially.     

Late Quaternary variations in calcium carbonate preservation of deep-sea sediments in the northern Cape Basin: results from a multiproxy approach

In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today’s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDX′) (Volbers and Henrich, submitted). The BDX′ is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6.

The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDX′. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages.

Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDX′. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate dissolution.