Temporal variation of diatom benthic propagules in a monsoon-influenced tropical estuary

Temporal variations in the diatom benthic propagule (DBP) community and their role in the phytoplankton community in a monsoon-affected tropical estuary, Zuari estuary, Goa (India) are presented. The DBP from the sediments was enumerated using an extinction dilution method (most probable number method), which allows estimation of resting stages through examination of germinated vegetative cells in culture. The DBP community was dominated by planktonic species belonging to the genera Skeletonema, Fragilariopsis, Thalassiosira, and Chaetoceros. Benthic propagules (BPs) of Skeletonema costatum and Fragilariopsis sp. were dominant throughout the year. Between these two species, only S. costatum showed a linear relationship between the BP and planktonic cells, indicating that this species is particularly important in coupling of pelagic and benthic ecosystems. During the onset and restart of monsoon after an intermittent break, water column was stratified, with a low-salinity layer arising from riverine discharge and precipitation at the surface and relatively cold, saline, low-oxygen waters at the bottom. Stratification favored blooming of S. costatum and Fragilariopsis sp. in nutrient-rich surface and bottom waters, respectively. The decline in these blooms ensuing nitrate depletion and salinity change resulted in an increased abundance of BP. Chaetoceros bloom was observed during the monsoon break as well as during non-monsoon period and on both the occasions the decline in bloom was coupled with freshwater discharge. During the non-monsoon season, Thalassiosira blooms were encountered subsequent to high nitrate inputs. These findings suggest that in such shallow tropical regions, physical processes during monsoon (freshwater discharge) and non-monsoon seasons (currents, waves and tides) cause resuspension of diatom BP. Since light is not a limiting factor for germination in such regions, the blooming of resuspended BP depends on nutrient availability.     

南海大鹏湾海洋卡盾藻赤潮发生的环境背景

根据１９９１年２－３月，大鹏湾盐田水域的理化、水文、气象、海况的动态变化和大鹏湾底泥中发现海洋卡盾藻孢囊的事实，从生态学的角度分析并提出了１９９１年３月２０－２１日发生在大鹏湾盐田水域的海洋卡盾藻赤潮形成过程和机理。分析结果认为，此次赤潮应从３月７－１０日的水温急升而引发的孢囊一齐萌芽开始，此后在东南风引起的表层风海流作用下，营养细胞逐渐在湾西北部的盐田水域聚集，而１６－１９日水 营养盐和金属元素     

New biostratigraphic data for the Chikkim Formation (Cretaceous, Tethyan Himalaya, India)

The Chikkim Formation as exposed in the Tethyan Himalaya (India) has been studied at its type locality, using planktonic foraminifera for a detailed biostratigraphic elaboration. Divided into two members, the Lower and Upper Chikkim members, this formation ranges in age from Albian to early Maastrichtian(?), and reaches a maximum thickness of 150 m. Examination of thin sections has yielded 34 species of foraminifera in five genus-level assemblages. The Lower Chikkim Member is about 55 m thick; its basal portion is of Albian age based on the presence of Biticinella breggiensis and Planomalina buxtorfi. At 26 m above the base, Whiteinella archaeocretacaea documents OAE 2 (Oceanic Anoxic Event 2), and thus the Cenomanian/Turonian boundary in this section. The carbonate sequence is capped by a Santonian-age hardground with iron oxide crusts and bioturbation. Macrofossils, including belemnites (at the base) and irregular echinoids (upper part), are present. The basal carbonaceous marls of the Upper Chikkim Member yield both large (benthic) rotaliid as well as planktonic foraminifera (Globotruncanita elevata, Gl. stuartiformis, Gl. stuarti, Gansserina gansseri and others), indicating a Campanian age. The co-occurrence of Gl. elevata and G. gansseri in a single thin section results either from condensation or reworking in the basal part of the Upper Chikkim Member. Late Cretaceous index foraminifera such as Gl. elevata document deposition within the Tethyan Realm. The original thickness of the Upper Chikkim Member is uncertain, but would have been around 100 m; the unit appears markedly reduced through weathering at a height of about 5000 m above sea level. Equivalent sediments are exposed in the Zanskar area to the northwest, and in Nepal and Tibet. Cretaceous Oceanic Red Beds (CORBs) are probably missing due to the proximality of these pelagic settings.     

Mg/Ca-paleothermometry in the western Mediterranean Sea on planktonic foraminifer species Globigerina bulloides: Constraints and implications

We generated a high-resolution SST_Mg/Ca record for the surface-dwelling planktonic foraminifera Globigerina bulloides from the core MD99-2346 collected in the Gulf of Lion, and compared it to that obtained using modern analogue techniques applied to fossil foraminiferal assemblages (SST_MAT). The two temperature records display similar patterns during the last 28,000 years but the SST_Mg/Ca estimates are several degrees warmer (∼ +4 °C) than SST_MAT. The temperature shift between SST_Mg/Ca and SST_MAT remained relatively constant over time. This seems to exclude a bias on the Mg/Ca record associated with salinity or secondary Mg-rich calcite encrustation on the foraminiferal tests during early diagenesis. Therefore, anomalously high Mg/Ca suggests either: (1) the empirical equation for G. bulloides of Elderfield and Ganssen (2000) is incorrect; or (2) there is a specific Mediterranean genotypes of G. bulloides for which a specific Mg/Ca-temperature calibration is needed.     

Late Cretaceous calcareous nannofossil and planktonic foraminiferal bioevents of the shallow-marine carbonate platform in the Mitla Pass, west central Sinai, Egypt

The first detailed biostratigraphic analyses of the Coniacian-middle Campanian shallow-marine carbonate successions exposed in the Mitla Pass, west central Sinai, Egypt have revealed the stratigraphic distribution of diverse calcareous nannofossil and planktonic foraminiferal species. Thirty-six calcareous nannofossils and thirty-two planktonic foraminifera are identified, indicating a Coniacian to middle Campanian age and four Tethyan planktonic foraminiferal and five calcareous nannofossil zones. A comparison of these bioevents from different palaeolatitudes shows considerable variation in age.Three sequence boundaries coincident with the Turonian/Coniacian, Coniacian/Santonian and Santonian/Campanian stage boundaries are recognized. A fourth sequence boundary is marked by a major upper Campanian to early Ypresian (early Eocene) unconformity. These sequence boundaries are primarily related to regional tectonism associated with the Syrian Arc Fold System and secondarily to eustatic sea-level fluctuations.     

Dinoflagellate biostratigraphy at the Campanian-Maastrichtian boundary in Zumaia,northern Spain

Dinoflagellate cyst assemblages from a well-exposed uppermost Cretaceous section at Zumaia (northern Spain) provide a basis for comparison with previous biostratigraphic and magnetostratigraphic studies on the problematic location of the Campanian-Maastrichtian boundary in the section. The position of the last occurrence of Corradinisphaeridium horridum and first common occurrence of Alterbidinium acutulum, correspond well with the bioevents defining the Campanian-Maastrichtian boundary in the Global boundary Stratotype Section and Point of Tercis les Bains (130 km to the North). Together with other age-diagnostic dinoflagellate cyst bioevents, we suggest that the boundary should be placed between 239.75 and 224.75 m below the Cretaceous-Palaeogene boundary, about 46 m lower than an interpretation based on the first occurrence of the planktonic foraminifer Pseudoguembelina palpebra and the last occurrence of the nannofossil Broinsonia parca subsp. constricta. A conspicuous acme of the dinoflagellate cyst Thalassiphora cf. delicata is encountered around the lower-upper Maastrichtian boundary (calibrated by foraminiferal, calcareous nannoplankton and magnetic polarity data), which may prove to be a useful correlatable event.     

青海湖介壳与无机碳酸盐氧同位素组成差异的环境意义

青海湖Q14B沉积物柱芯560~415 cm(约14.0~10.5 ka B.P.)段,介壳1δ8O高于无机碳酸盐1δ8O可能反映了此时青海湖表层水温高于底层水温;介壳1δ8O与无机碳酸盐1δ8O之间的较小差值可能揭示了此时青海湖水位很浅,气候干冷;介壳1δ8O变幅大于无机碳酸盐1δ8O变幅,则可能源于此时青海湖水位大幅度波动导致的底层水温变幅超过表层,以及无机碳酸盐1δ8O测自全碳酸盐所致。在利用湖泊碳酸盐1δ8O进行气候及环境变化研究时,有必要分别测试不同种属介形虫及不同无机碳酸盐矿物的同位素值。