We assessed the performance of a transfer function model for sea-level studies using salt-marsh foraminifera from two estuaries of northern Portugal. An independent data set of 12 samples and 13 sub-fossil samples from a core were used to evaluate if reconstructions and errors derived from current models are adequate. Initial transfer function models provided very strong results as indicated by cross-validation (component 2; r2 = 0.80-0.82; RMSEP ranged from 10.7 to 12.3 cm) and improved its performance by ca. 10% when sample size reached ca. 50. Results derived using an independent test data set indicate that cross-validation is a very effective approach and produces conservative errors when compared to observed errors. We additionally explored the possible effect of transforming the concentration data into percent in the error estimations by comparing the results obtained based on the use of both concentration and compositional data. Results indicate that this type of transformation does not affect the performance of the transfer function. Results derived from a reconstruction of sub-fossil samples from a core indicate that high-resolution sea-level reconstructions are possible, but show that depositional environments have to be selected carefully in order to minimize the impact of possible taphonomical loss. 相似文献
An integrated study of the ammonites, inoceramid bivalves, planktonic foraminifera, calcareous nannofossils, geochemistry, stable carbon isotopes, and cyclostratigraphy is provided for the upper Middle to upper Upper Albian sucession exposed in the Col de Palluel section east of Rosans in Hautes-Alpes, France. The Albian-Cenomanian boundary interval described by Gale et al. at Mont Risou is re-examined, a total thickness of 370 m of the Marnes Bleues Formation. Zonal schemes based on ammonites, inoceramid bivalves, planktonic foraminifera, and calcareous nannofossils are integrated with the stable carbon isotope curve and key lithostratigraphic markers to provide a sequence of more than 70 events in the uppermost Middle Albian to basal Cenomanian interval. Time series analysis of the Al2O3 content of the 500 m Albian sequence present in the Col de Palluel and Risou sections reveals the presence of the 20 kyr precession, 40 kyr tilt, 100 kyr short eccentricity, and 406 kyr long eccentricity cycles. Correlation using planktonic foraminiferan and nannofossil data provide a link between the Col de Palluel and Risou sections and the Italian sequence at Gubbio, and in the Piobbico core. This provides a basis for the extension of the orbital time scale of Grippo et al. to the sequence. It reveals a major break in the Col de Palluel succession at the top of the distinctive marker bed known as the Petite Vérole that may represent as much as 2 Ma. It also provides a basis for the estimation of the length of the Albian Stage at 4.12 Ma, 0.8 Ma for the early Albian, 2.84 Ma for the Middle Albian, and 3.68 Ma for the late Albian substages. 相似文献
Samples were collected continuously and systematically from Beds 2 to 30 of the Meishan D section, and nearly 4,000 foraminiferal specimens were recognized. In total, 72 species (partially including undetermined species and conformis species) in 34 genera of foraminifers were identified. Twelve foraminiferal community zones were established from Beds 2 to 27 based on the abundance, dominance, diversity, and equitability of the foraminiferal fauna and a functional morphological analysis of the dominant and characteristic species. The habitat type indicated by each community zone and the sedimentary features were used to determine the paleo‐water depths and the relative changes in sea level. We generated curves of both the habitat type and the hydrodynamic conditions, which together indicate the relative changes in the paleo‐water depth and the substrate features in the studied area during the Changhsingian. This study discusses the possible relationship between the foraminiferal community zones and the sea‐level changes based on a statistical analysis of the foraminiferal community zones. 相似文献
We reconstructed the paleoenvironmental history of surface and deep water over the last 130 kyr from oxygen and carbon isotope ratios of planktonic and benthic foraminifera in two cores (MD179-3312 and MD179-3304) from the Joetsu Basin, eastern margin of the Japan Sea. Our data showed that paleoceanographic changes such as influx of surface currents and vertical circulation were associated with global glacial–interglacial sea level change. Surface water conditions were influenced by the influx of Tsushima Current, East China Sea coastal or off-shore waters through the Tsushima Strait during interglacial or interstadial stages, and strongly affected by freshwater input during the glacial maximum. During interglacial maximums such as Marine Isotope Stages 1 and 5e, development of well-oxygenated bottom water was indicated. A density-stratified ocean with weak ventilation was inferred from the isotopic records of benthic foraminifera during the Last Glacial Maximum. Local negative excursions in carbon isotopes during deglacial or interglacial periods may suggest the dissolution of gas hydrates or methane seep activities. 相似文献
A high-resolution study of a marsh sedimentary sequence from the Minho estuary provides a new palaeoenvironmental reconstruction from NW Iberian based on geological proxies supported by historical and instrumental climatic records. A low-salinity tidal flat, dominated by Trochamminita salsa, Haplophragmoides spp. and Cribrostomoides spp., prevailed from AD 140–1360 (Roman Warm Period, Dark Ages, Medieval Climatic Anomaly). This sheltered environment was affected by high hydrodynamic episodes, marked by the increase in silt/clay ratio, decrease of organic matter, and poor and weakly preserved foraminiferal assemblages, suggesting enhanced river runoff. The establishment of low marsh began at AD 1380. This low-salinity environment, marked by colder and wet conditions, persisted from AD 1410–1770 (Little Ice Age), when foraminiferal density increased significantly. Haplophragmoides manilaensis and Trochamminita salsa mark the transition from low to high marsh at AD 1730. Since AD 1780 the abundances of salt marsh species (Jadammina macrescens, Trochammina inflata) increased, accompanied by a decrease in foraminiferal density, reflecting climate instability, when droughts alternate with severe floods. SW Europe marsh foraminifera respond to the hydrological balance, controlled by climatic variability modes (e.g., NAO) and solar activity, thus contributing to the understanding of NE Atlantic climate dynamics. 相似文献
The Early Cretaceous Fahliyan Formation (middle part of the Khami Group), is one of the important reservoir rocks in the Zagros Fold-Thrust Belt. The Zagros Fold-Thrust Belt is located on the boundary between the Arabian and Eurasian lithospheric plates and formed from collision between Eurasia and advancing Arabia during the Cenozoic. In this study area, the Fahliyan Formation with a thickness of 325 m, consists of carbonate rocks (limestone and dolomite). This formation overlies the Late Jurassic Surmeh Formation unconformably and underlies the Early Cretaceous Gadvan Formation conformably at Gadvan Anticline. The formation was investigated by a detailed petrographic analysis to clarify the depositional facies, sedimentary environments and diagenetic features in the Gadvan Anticline. Petrographic studies led to recognition of the 12 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open marine in mid-ramp environments. The absence of turbidite deposits, reefal facies, and gradual facies changes show that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. The diagenetic settings favored productioning a variety of features which include cements from early to late marine cements, micritization, dolomitization, compaction features, dissolution fabric, and pores. The diagenetic sequence can be roughly divided into three stages: (1) eugenic stage: marine diagenetic environment, (2) mesogenic stage: burial environment, and (3) telogenic stage: meteoric diagenetic environment. 相似文献
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. 相似文献