Benthic foraminiferal response to changes in mining pattern: a case study from the Zuari estuary,Goa, India

Being sensitive to environmental changes, foraminifera have been extensively used to monitor pollution level in the marine environment, including the effect of mining in coastal areas. In the Goa state of India, the rejects from opencast mining on land largely find their way to the estuaries, as washout during monsoon. Additionally, the Mormugao Port at the mouth of the Zuari estuary is the hub of activities due to the transport of ore from hinterland areas by barges and its subsequent loading for export. On the directive of the Supreme Court of India, all the mining-related activities abruptly stopped throughout India, including that in Goa in 2012, and got reinstated in 2015. Therefore, it provided a fit case to test the effectiveness of benthic foraminifera as an indicator of environmental impact due to mining activities. A total of ten surface sediment samples from five locations in Zuari estuary were collected from a depth range of 4.5–8.5 m in the years of 2013 and 2016 and were analyzed for both the living (stained) and dead benthic foraminifera. The year 2013 represents a time interval immediately after the closure of extensive mining activity, and the sampling during 2016 represents minimal mining. The living benthic foraminiferal abundance was higher (19–54/g sediment) during 2013 and decreased substantially during 2016 (3–22/g sediment), suggesting an adverse effect of activities associated with mine closure on benthic foraminifera. Additionally, the relative abundance of Ammonia was also significantly low during the year 2016. The temporal variation in dead foraminifera was, however, different than that of the living foraminifera. The differential response was attributed to the terrigenous dilution as a result of change in sedimentation rate. Therefore, we conclude that living foraminifera correctly incorporate the changes in mining pattern and may be used as an effective tool to monitor the impact of mining. We further suggest that the potential counter effect of terrigenous dilution on total and living benthic foraminiferal population should be considered while interpreting temporal variations in foraminiferal abundance in marginal marine settings.     

Late Quaternary sedimentation on the Southern Sohm Abyssal Plain

Six closely spaced sediment cores taken below the carbonate compensation depth penetrated fine silty muds and entered sandy sediment at 10–12 m below the seafloor. Foraminiferal assemblages and δ⁸O analyses on planktonic foraminifera indicated that the surface muds down to 2 m are Holocene and derived from local promontories above the CCD. Below these sediments are about 6 m of clays deposited during the late Wisconsin. These are unfossiliferous and have a possible northern source suggested by the higher chlorite content. Sandy sediments below 9 m in the cores contain well preserved benthic foraminifera from the Scotian Shelf. Glacial δ¹⁸O values on planktonic tests indicate the sandy sediments are most likely of latest Wisconsin age. Thus during the recent interglacial, the sand fraction of the southern Sohm Abyssal Plain sediments is mostly locally derived, but during glacial periods the sediments have a distant northern source containing quartz sand that was initially deposited on the Scotian Shelf 1,500 km to the north.     

"Biostratigraphy and Paleoecology of Paleocene/Eocene (P/E) interval of some geological sections in Central Egypt"

The Paleocene/Eocene boundary intervals were studied in three outcrops along the Nile Valley: Gabal Taramsa, Gabal Qreiya, and Gabal Nag El Quda in Qena and Esna regions. The planktonic and benthic foraminifera have been examined. The qualitative study of planktonic foraminifera distinguishes eight planktonic biozones from (P4 and P5) Paleocene age to (E1, E2, E3, E4, E5, and E6) Early Eocene age. The analysis of quantitative distribution patterns of benthic foraminifera allows the reconstruction of the paleoenvironmental settings in the studied area. The disappearance or scarce appearance of deeper-water benthic foraminifera (Angulogavelinella avnimelechi and Gavelinella rubiginosus) and increasing dominance of shallow-marine taxa (Buliminides, Loxostomoides applinae) indicate deposition in shallow water environments. The benthic foraminiferal assemblages which dominated by Loxostomoides applinae, Buliminids, and Lenticulina indicate Dysoxic conditions and maximum food levels. The species of mid-way type fauna dominate the assemblages of the studied area; the species of Velasco-type fauna are very rare.     

Relationships between primary productivity and bottom‐water oxygenation off northwest Africa during the last deglaciation

The upwelling region off northwest Africa is one of the most productive regions in the world ocean. This study details the response of surface‐ and deep‐water environments off Mauritania, northwest Africa, to the rapid climate events of the last deglaciation, especially the Bølling–Allerød (15.5–13.5 ka BP) and Younger Dryas (13.5–11.5 ka BP). A high accumulation rate gravity core GeoB7926‐2, recovered at ～20° N, 18° W, was analysed for the grain size distribution of the terrigenous sediment fraction, the organic carbon content, diatom and benthic foraminifera communities. Humid conditions were observed during the Bølling–Allerød with a high contribution of fluvial sediment input. During the Younger Dryas intensified trade winds caused a larger sediment input of aeolian dust from the Sahara and more intense upwelling with higher primary productivity, as indicated by high diatom concentrations. The abrupt and large increase of organic matter caused low oxygen conditions at the sea floor, reflected by the poor benthic foraminiferal fauna and the dominance of the low‐oxygen‐tolerant foraminiferal species Bulimina exilis. This is surprising since low‐oxygen conditions have not been recorded during modern times at the sea floor in this region, despite present‐day intensive upwelling and high primary productivity. After the Younger Dryas, more humid conditions returned, diatom abundance decreased and B. exilis was replaced by typical deep‐sea species as found in the region today, indicating the return of more oxygenated conditions at the sea floor. Copyright © 2011 John Wiley & Sons, Ltd.     

A method for determination of the isotopic composition of authigenic uranium in Baikal bottom sediments

A simple, reliable, and high-performance method has been proposed for direct determination of the isotopic composition of authigenic uranium in silica lacustrine sediments. The method is based on studying the kinetics of the selective extraction of authigenic uranium from sediments with weak solutions of ammonium hydrocarbonate followed by the ICP-MS analysis of the nuclides. To estimate the contamination of authigenic uranium by terrigenous one, the contents of ²³²Th and some other clastogenic elements in the extracts were measured simultaneously. The selectivity of extraction of authigenic uranium from the sediments treated with a 1% NH₄HCO₃ solution appeared to be no worse than 99%. The method was used to analyze the isotopic composition of authigenic uranium at several key horizons of a core dated before. The measurements directly prove that the ²³⁴U/²³⁸U values in Baikal water varied depending on climate, which contradicts the previous statements. The measured ²³⁴U/²³⁸U ratios in paleo-Baikal water match the values reconstructed from isotopic data for total uranium in the sediments on the supposition that the U/Th ratio is constant in the terrigenous part of the sediment. Direct experimental determination of total and authigenic nuclides in sediments enhances the potentiality of the method for absolute ²³⁴U-²³⁰Th dating of carbonate-barren lacustrine sediments, including those from Lake Baikal, within the intervals corresponding to the periods of glaciation, where the sediments contain a large fraction of terrigenous component. Given the fractions of terrigenous and authigenic uranium are accurately determined, we have an opportunity to study the variability of the sources of terrigenous matter and to refine the previous model for reconstructing the climate humidity in East Siberia.     

晚上新世以来南海北部深部水团演化

对南海北部大洋钻探184航次1146站晚上新世以来底栖有孔虫属种组合的Q型因子分析, 发现底栖有孔虫组合以2.1Ma, 1.5Ma和0.7Ma为界, 分为Stilostomella-Globocassidulina subglobosa-Nodogenerina, Bulimina alazanensis, Uvigerina perigrina和Melonis barleeanus-Globobulimina affinis-Bulimina aculeata4个组合.结合底层水溶解氧含量和浮游、底栖有孔虫碳同位素分析, 认为底栖有孔虫组合的变化是南海底层水影响所致, 以及南海北部表层和底层海水营养盐含量变化的共同结果.      

Pleistocene oceanographie changes indicated by deep sea benthic foraminifera in the northern Indian Ocean

An attempt has been made to understand the Pleistocene bottom water history in response to the paleoclimatic changes in the northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among the 150 benthic foraminifera recorded a few species show dominance with changing percent frequencies during most of the sequence. The dominant benthic foraminiferal assemblages suggest that most of the Pleistocene bottom waters at site 219 and Early Pleistocene bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears to be closely associated with a water mass intermediate between North Indian Deep Water (NIDW) and Antarctic Bottom Water (AABW). Uvigerina proboscidea is the most dominant benthic foraminiferal species present during the Pleistocene at both the sites. A marked increase in the relative abundance ofU. proboscidea along with less diverse and equitable fauna during Early Pleistocene suggests a relative cooling, an intensified oceanic circulation and upwelling of nutrient rich bottom waters resulting in high surface productivity. At the same time, low sediment accumulation rate during Early Pleistocene reveals increased winnowing of the sediments possibly due to more corrosive and cold bottom waters. The Late Pleistocene in general, is marked by relatively warm and stable bottom waters as reflected by low abundance ofU. proboscidea and more diverse and equitable benthic fauna. The lower depth range for the occurrence ofBulimina aculeate in the Indian Ocean is around 2300 m, similar to that of many other areas.B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative abundance ofStilostomella lepidula occurs close to the Early/Late Pleistocene boundary.     

Benthic-pelagic coupling in the Greenland-Norwegian Sea and its effect on the geological record

The sedimentation pattern of organic material in the Greenland-Norwegian Sea is reflected in the surface sediments, although less than 0.5% of the organic matter is buried in the sediment. Maximum fluxes and benthic responses are observed during June and/or August/September, following the pattern of export production in the pelagial zone. The annual remineralization rate on the Vøring Plateau is 3.0 g C m^–2 a ^–1 Freshly settled phytodetritus, as detected by chlorophyll measurements, is rapidly mixed into the sediment and decomposed. It stimulates the activity of benthic organisms, especially foraminifera. The mixing coefficient for this material is D _b=0.2 cm² d^–1, which is two to three orders of magnitude higher than that estimated from radiotracer methods. The effect on the geological record, however, is likely to be small. Chlorophyll-containing particles are at first very evenly distributed on the seafloor. After partial decomposition and resuspension, a secondary redistribution of particles occurs which can result in the formation of a high accumulation area, with an up to 80-fold increase in the sedimentation rate by lateral advection. This is mainly due to physical processes, because biodeposition mediated by benthic animals increases sedimentation by only a factor of two or three.     

Benthonic and planktonic foraminifera in relation to the Early Holocene stagnation in the Ionian Basin, Central Mediterranean

The distribution patterns of benthonic and planktonic foraminifera in cores from the Ionian Basin, central Mediterranean, were investigated in relation to the deposition of sapropel S-1. The sapropel is a dark organic-rich sediment deposited under anoxic conditions during the last marine stagnation in the Early Holocene. The major divisions between both benthonic and planktonic faunas correspond to changes in the core lithology and coincide with the transitions between pre-sapropel, sapropel/'oxidized layer' and post-sapropel sediments. The faunal evidence shows that the oxidized layer belongs to the sapropel sequence. The planktonic faunas have the same species composition as in the sapropel sediment and the high density of planktonic species continues into the oxidized layer. The oxidized layer is devoid of a benthonic fauna or contains a 'sapropel-associated' fauna composed of infaunal species with an affinity to high supply of organic material material and low oxygen. High depletion in the heavy oxygen isotope in the oxidized layer substantiates the faunal evidence. A peak in abundance of the planktonic species Globorotalia inflata at the top of the oxidized layer marks the time when turnover of the water masses ended the stagnation phase and sapropel sedimentation in the Ionian Basin at about 6000 BP. The distribution of the benthonic and planktonic foraminifera shows that the sapropel in the central part of the Ionian Basin was originally almost twice as thick as it is today. When oxygen returned to the deep sediments the top of the sapropel was oxidized to 4–7 cm below its original surface. Only the lower part of the sapropel is preserved. The remainder is now a red laminated layer, the 'oxidized layer'.     

Interactions of Arctic and Atlantic water-masses and associated environmental changes during the last millennium, Hornsund (SW Svalbard)

The fjords of southwestern Spitsbergen (European Arctic) are a climatically sensitive area neighbouring the mixing zone of warm northward-flowing Atlantic water-masses and cold Arctic Water. Owing to reasonably high accumulation rates, these settings are especially suitable for providing high-resolution sedimentary records of regional hydrological and environmental changes. A sediment core spanning the last millennium was retrieved from the outer Hornsund fjord basin, ¹⁴C dated and analysed for sediment grain size, ice-rafted debris (IRD), the distribution of benthic foraminifera and their oxygen and carbon stable isotope composition. The record of sub-centennial resolution reveals three distinctive periods: the Medieval Warm Period, the Little Ice Age (∼AD 1600–1900) and 20th-century warming. The marine record obtained is well correlated with regional high-resolution ice-core records as well as with atmospheric palaeotemperature reconstructions and sea-ice data. The colder periods stay in phase with the greater influence of less saline, cold Arctic Water indicated by subtle changes in benthic foraminifera assemblages and the δ¹⁸O signal, which is dominated by changes in salinity. The IRD record clearly indicates that tidewater glaciers were present in SW Spitsbergen throughout the last millennium, and most actively from the late 16th century until the end of the 19th century.     

Laminated sediments in the Bay of La Paz, Gulf of California: a depositional cycle regulated by pluvial flux

Abstract Core BAP96‐CP, sampled from the deepest part of the Bay of La Paz, Gulf of California, has been analysed sedimentologically taking into account regional climate and oceanography. Laminated sediments at the bottom of the bay are essentially not bioturbated by benthic fauna. A subanoxic condition (O₂ < 0·2 mL L^?1) inhibits the proliferation of benthic fauna. Within the bay, the relative abundances of terrigenous and biogenic inputs change periodically. The terrigenous input is greater than the biogenic input and apparently experiences larger fluctuations. The terrigenous input dominates in dark laminae, whereas the biogenic input mostly occurs in light laminae. Thus, it is assumed that, down the core, the alternation of dark and light laminae represents cycles in the extent of dilution of the biogenic input by terrigenous input. The terrigenous input into the Bay of La Paz is mostly regulated by pluvial runoff. Thus, its temporal fluctuation follows the periods shown by the regional pluvial regime, particularly the 11·2 year period. This is equal to the frequency of sunspot cycles.     

Palaeoceanographic development in Storfjorden,Svalbard, during the deglaciation and Holocene: evidence from benthic foraminiferal records

Brines can have a profound influence on the relative abundance of calcareous and agglutinated foraminiferal faunas. Here we investigated the distribution of benthic foraminiferal species in four cores from a brine‐enriched environment in Storfjorden, Svalbard. Stratigraphically, the cores comprise the last 15 000 years. The purpose of the study was to reconstruct changes in the palaeoecology and palaeoceanography of Storfjorden in relation to past climate changes, and to identify potential indicator species for brine‐affected environments. The benthic foraminifera in Storfjorden all have widespread occurrences in the Arctic realm. Calcareous species dominated Storfjorden during the deglaciation and early Holocene until c. 8200 a BP. However, agglutinated species increased in abundance whenever conditions became colder with more sea ice and stronger brine formation, such as during the Older Dryas, the Intra‐Allerød Cold Period and the Younger Dryas. Following a moderately cold period with numerous agglutinated foraminifera from c. 8200–4000 a BP, conditions became more changeable from c. 4000 a BP with repeated shifts between warmer periods dominated by calcareous species and colder periods dominated by agglutinated species. The warmer periods show a stronger influence of Atlantic Water, with reduced brine formation and less corrosive conditions at the sea bottom. Conversely, the colder periods show a stronger influence of Arctic water, with higher brine production and more corrosive bottom water. The distribution patterns of the calcareous species are basically the same whether calculated relative to the total fauna (including agglutinated specimens) or relative to calcareous specimens alone. Moreover, the patterns are similar to the patterns found elsewhere along western Svalbard in areas without the influence of brines. No particular species appear to be specifically linked to brine formation. However, the most persistent agglutinated species R. scorpiurus and A. glomerata are also the species most tolerant of the acidic bottom water that normally is associated with brine formation.     

Palaeoenvironmental interpretation of Holocene coastal sequences in the southern bay of Biscay

The main sedimentary features of the northern coast of the Iberian Peninsula during the Holocene transgression are characterized by the formation of estuaries, the deposition of sand bars and sand beaches, and the accumulation of aeolian dunes. These coastal deposits are very favourable for identifying Quaternary sea-level changes as they contain great volumes of well preserved sediments including marine, brackish and freshwater beds. The micropalaeontological analysis (benthic foraminifera) of diverse littoral sequences has allowed different microfaunal assemblages to be recognised and their corresponding depositional environments determined. Two recurrent phases of sediment build-up as the sea level rose have been distinguished. They have been interpreted as the consequence of two different marine advances in this region: one dated at around 8 000 years BP and the second around 2 500 years BP.     

Tracing seafloor methane emissions with benthic foraminifera in the Baiyun Sag of the northern South China Sea

Changes in the concentrations of atmospheric greenhouse gases are an important part of the global climate forcing. The hypothesis that benthic foraminifera are useful proxies of local methane emission from the seafloor has been verified on sediment cores by numerous studies. The calcium carbonate (CaCO₃) content and the high-resolution carbon and oxygen isotope composition of the benthic foraminifera from the core 08CF7, from the northeastern Shenhu gas hydrate drilling area in the Baiyun Sag of the northern South China Sea were analyzed, and the benthic foraminifera’s evidence for methane release from gas hydrate decomposition are presented here for the first time. Two rapid obvious carbon isotope negative excursions were observed in the oxygen isotope stage boundaries 5d/5c and 6/5e (penultimate deglaciation, about 130 ka) of the cold-to-warm climatic transition period. The largest negative value of δ¹³C is about ?2.95 ‰, and the whole change of carbon and oxygen isotope is strikingly similar and is in consonance with the atmospheric methane concentration recorded by the Vostok ice core and the carbon isotopic record from Lake Baikal. Combining these results with the analysis of the geological conditions of the study area and the fact that gas hydrate exists in the surrounding area, it can be concluded that the carbon isotope negative excursions of the benthic foraminifera in the northern South China Sea are associated with methane release from gas hydrate decomposition due to deglacial climate warming. By recording the episodes of massive gas hydrate decomposition closely linked with the northern hemisphere temperatures during major warming periods, the new δ¹³C record from the Baiyun Sag provides further evidence for the potential impact of gas hydrate reservoir on rapid deglacial rises of atmospheric methane levels.     

Reappraisal of early Paleogene CCD curves: foraminiferal assemblages and stable carbon isotopes across the carbonate facies of Perth Abyssal Plain

Bulk carbonate content, planktic and benthic foraminiferal assemblages, stable isotope compositions of bulk carbonate and Nuttallides truempyi (benthic foraminifera), and non-carbonate mineralogy were examined across ∼30 m of carbonate-rich Paleogene sediment at Deep Sea Drilling Project (DSDP) Site 259, on Perth Abyssal Plain off Western Australia. Carbonate content, mostly reflecting nannofossil abundance, ranges from 3 to 80% and generally exceeds 50% between 35 and 57 mbsf. A clay-rich horizon with a carbonate content of about 37% occurs between 55.17 and 55.37 mbsf. The carbonate-rich interval spans planktic foraminiferal zones P4c to P6b (∼57–52 Ma), with the clay-rich horizon near the base of our Zone P5 (upper)—P6b. Throughout the studied interval, benthic species dominate foraminiferal assemblages, with scarce planktic foraminifera usually of poor preservation and limited species diversity. A prominent Benthic Foraminiferal Extinction Event (BFEE) occurs across the clay-rich horizon, with an influx of large Acarinina immediately above. The δ¹³C records of bulk carbonate and N. truempyi exhibit trends similar to those observed in upper Paleocene–lower Eocene (∼57–52 Ma) sediment from other locations. Two successive decreases in bulk carbonate and N. truempyi δ¹³C of 0.5 and 1.0‰ characterize the interval at and immediately above the BFEE. Despite major changes in carbonate content, foraminiferal assemblages and carbon isotopes, the mineralogy of the non-carbonate fraction consistently comprises expanding clay, heulandite (zeolite), quartz, feldspar (sodic or calcic), minor mica, and pyrolusite (MnO₂). The uniformity of this mineral assemblage suggests that Site 259 received similar non-carbonate sediment before, during and after pelagic carbonate deposition. The carbonate plug at Site 259 probably represents a drop in the CCD from ∼57 to 52–51 Ma, as also recognized at other locations.     

Cenomanian-Coniacian Sea-level Change and Dissolved Oxygen Fluctuations in Tethys-Himalaya: Evidences from Benthic Foraminifera of Gamba, Tibet

Benthic foraminifera, preserved in the Late Cretaceous organic carbon-rich sediments of Gamba, southern Tibet, provide high-resolution proxies for sea-level changes and dissolved oxygen fluctuations of southeastern Tethys. The fossils were statistically analyzed and divided into three faunas of "Cenomanian fauna", "Turonian fauna", and "Coniacian fauna". A middle neritic-upper bathal environment (50-250m) was estimated considering the ratios of planktonic and epifaunal benthic foraminifera (P/(P+E)), the morphological analysis according to the studies of recent foraminifera and the abundant distributions of depth-related species such as Alabamina creta, Laevidentalina sp., Praebulimina spp., Pleurostomella cf. naranjoensis, Pyrulina sp., Quinqueloculina spp., Haplophragmoides spp., etc. The result shows an almost parallel trend with the global transgressive and regressive cycles, but the former fluctuates more frequently at upper Cenomanian, which probably indicates tectonic instability of the continental margin. According to the benthic foraminiferal richness (BFN), Shannon-Weiner diversity (H(s)), as well as benthic foraminiferal oxygen index (BFOI), five periods of oxygen depleted conditions (dysoxic-anoxic) have been recognized. They correspond to the OAE2, the lower Turonian, the upper Turonian, the Turonian-Caniacian boundary event and the probably OAE3. In addition, the oxygen fluctuations in Gamba might be controlled directly by sea-level changes, while the paleoproductivity and oxygen conditions interacted with each other under oxygen deficiency environments.     

Diagenetic disturbances of marine sedimentary records from methane‐influenced environments in the Fram Strait as indications of variation in seep intensity during the last 35 000 years

The effect of seeping of methane on marine sediment records has been studied in four gravity cores from Vestnesa Ridge, Svalbard margin. The area shows acoustic signs in the form of flares indicating active methane gas seepage. For a better understanding of the timing and variability of the flux of methane in the past and the effects on potential proxies, a detailed study of the diagenetic processes that may affect the composition and structure of both sediments and foraminiferal shells is needed. Here we discuss deep‐sea records from methane‐influenced environments in three cores from an active and very heterogeneous seep‐area (pockmark) and one core from outside the pockmark for background. The results include the distribution and stable isotopes of authigenic carbonates and of benthic and planktonic foraminifera, magnetic susceptibility, AMS‐¹⁴C dates, sedimentary data and biostratigraphy. Extremely low δ¹³C values recorded in both benthic and planktonic foraminifera during the Bølling‐Allerød interstadials indicate possible increased methane flux beginning at late Heinrich event H1. The recorded low values are mainly a result of diagenetic overprint by methane‐derived authigenic carbonates. The δ¹⁸O signals of authigenic carbonates are close to those of foraminiferal calcite and thus the δ¹⁸O records remain a valid stratigraphical tool in methane seep sites, except in the case of severely encrusted samples. In addition, the records from the active pockmark show nearly constant values of low magnetic susceptibility in contrast to higher and more variable magnetic susceptibility values from the control station and other published records from normal sediments west of Svalbard. This phenomenon is probably caused by dissolution of magnetic minerals in the reducing environmental conditions of methane seep sediments, associated with anaerobic oxidation of methane and formation of paramagnetic minerals (pyrite). This process enables magnetic susceptibility to be used as a common diagnostic tool for identifying methane‐related palaeo‐reductive environments.     

台湾峡谷HD133和HD77柱状样的沉积构成和发育背景

分别对南海东北部台湾峡谷内水深3 280 m的HD133和峡谷外水深3 378 m的HD77重力活塞柱状样进行了沉积物粒度、古生物和碳酸钙含量分析,利用AMS14C同位素测年和沉积速率初步认定是属于MIS3a以来的沉积。按沉积物粒度和碳酸钙含量可将两支柱状样划分为3套沉积层段:上部层段1和下部层段3均以粉砂质黏土为主,夹薄层粉砂,深水底栖有孔虫含量高,碳酸钙低于10%,代表受重力流作用较弱的正常深海沉积;中部层段2发育一套以中-细粒为主的厚砂层,含大量浅水底栖有孔虫,碳酸钙含量可高达60%,AMS14C测年出现倒置现象,表明主要为浅水重力流沉积。柱状样的沉积构成响应同期海平面变化,特别表现在深水砂层沉积的两大控制因素:在时间上,低海平面时期大量浅水和陆源碎屑物质直接输送到陆坡之下的深水区,形成富砂的层段2;在空间上,峡谷水道是重力流的物质输送通道,地形优势使得重力流携带物优先在水道中发生沉积,造成HD133柱的含砂量明显高于HD77柱状样。     

Oxygen level,primary productivity,and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): Benthic foraminiferal variations in the carbonate microfacies

Cenomanian/Turonian boundary (upper Sarvak Formation) benthic foraminiferal assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ¹³C positive excursion (peak “a”), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (“Heterohelix shift”) in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ¹³C peaks “b” and “c”) coincides with harbinger of Neo-Tethys closure in the Arabian Plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera.     

A Paleoecological History of the Late Precolonial and Postcolonial Mesohaline Chesapeake Bay Food Web

Geochemical (total nitrogen, total organic carbon, total phosphorus, total sulfur, and carbon and nitrogen stable isotopes) and selected biotic (diatom, foraminifera, polychaete) indicators preserved in two estuarine sediment cores from the mesohaline Chesapeake Bay provide a history of alterations in the food web associated with land-use change. One core from the mouth of the Chester River (CR) (collected in 2000) represents a 1,000-year record. The second core (collected in 1999), from the Chesapeake Bay’s main stem opposite the Choptank River (MD), represents a 500-year record. As European settlers converted a primarily forested landscape to agriculture, sedimentation rates increased, water clarity decreased, salinity decreased in some areas, and the estuarine food web changed into a predominantly planktonic system. Representatives of the benthic macrofaunal community (foraminifera and the polychaetes Nereis spp.) were affected by local changes before there were widespread landscape alterations. Nitrogen stable isotope records indicated that land-use changes affected nitrogen cycling beginning in the early 1700s. Extreme changes were evident in the mid-nineteenth century following widespread deforestation and since the mid-twentieth century reflecting heightened eutrophication as development increased in the Chesapeake Bay watershed. Results also demonstrate how paleoecological records vary due to the degree of terrestrial inputs of freshwater runoff and nutrients at core locations within the Chesapeake Bay.