Major Faunal Changes Research Articles

Macrofaunal palaeoecology and stratigraphy in late quaternary shelf sediments off Northern Norway

In a Late Pleistocene/Holocene glaciomarine/marine sequence, a minimum of 115 taxa, macrofossils and trace fossils, are recognized on the basis of 690 samples from gravity cores from Andfjorden (nine) and Malangsdjupet (one). The biostratigraphy is proposed on the basis of reconstructions of autochthonous macrofaunal assemblages. A total of seven zones are defined, five in the Pleistocene and two in the Holocene, the ages of which are estimated by radiocarbon datings. In ascending order the zones are: Mixed Assemblage-zone, Barren Interzone, Arctinula greenlandica Assemblage-zone (before 14,000 yr B.P.), Yoldiella intermedia Assemblage-zone (14,000-13,000 yr B.P.), Bathyarca glacialis Assemblage-zozne (13,000-yr. B.P.), Bathyarca glacialis-Kelliella miliaris Interval-zone (10,000-ca. 7800 yr B.P.) and Kelliella miliaris Assemblage-zone (ca. 7800-present). The Bathyarca glacialis Zone is divided into three faunules: I (13,000-12,000 yr B.P.); II (12,200-11,100 yr B.P.) and III (11,100-10,000 yr B.P.). The palaeoecology of the faunal succession is elucidated by means of the sample-frequency method. Two major environmental events have influenced the faunal history. The first was the onset of the deglaciation of the thoughs, the second the intrusion of the Norwegian Current, bringing warm saline Atlantic water into the troughs at 10,000 yr B.P. The Pleistocene fossils demonstrate an Arctic macrofaunal succession from a low-order stage to a high-order equilibrium stage that is in principle identical to that of a modern faunal development in fjords following a disturbance causing anoxia. An estimated polychaete/mollusc ratio together with changes in sediment influx, depositional rates and grain size of substrate, indicate the presence of different energy regimes, where the inner and outer reaches of Andfjorden are compared. About 10,000 yr B.P. a major faunal change took place. The previous Arctic fauna was replaced by a Boreal fauna which developed into the modern fauna. The most important factor causing the replacement, and renewed faunal succession, were probably the rise in temperature and salinity as well as an increase in nutrient supply, brought about by the intrusion of the Norwegian Current. A comparison with shallower shelf areas indicates a somewhat different faunal composition in the Pleistocene brought about by different depositional environments. However, a similar major faunal change took place at 10,000 yr B.P. and there are fewer discrepancies among the Holocene faunas. Regional comparison indicates that some of the phases in the development of the fauna probably are rather local while other have a more regional nature.

Biochronology and paleoclimatic implications of Middle Eocene to Oligocene planktic foraminiferal faunas

Planktic foraminiferal assemblages have been analyzed quantitatively in six DSDP sites in the Atlantic (Site 363), Pacific (Sites 292, 77B, 277), and Indian Ocean (Sites 219, 253) in order to determine the nature of the faunal turnover during Middle Eocene to Oligocene time. Biostratigraphic ranges of taxa and abundance distributions of dominant species are presented and illustrate striking similarities in faunal assemblages of low latitude regions in the Atlantic, Pacific and Indian oceans. A high resolution biochronology, based on dominant faunal characteristics and 55 datum events, permits correlation between all three oceans with a high degree of precision. Population studies provide a view of the global impact of the paleoclimatic and paleoceanographic changes occurring during Middle Eocene to Oligocene time. Planktic foraminiferal assemblage changes indicate a general cooling trend between Middle Eocene to Oligocene time, consistent with previously published oxygen isotope data. Major faunal changes, indicating cooling episodes, occur, however, at discrete intervals: in the Middle Eocene 44-43 Ma (P13), the Middle/Late Eocene boundary 41-40 Ma ( P14 P15 ), the Late Eocene 39-38 Ma ( P15 P16 ), the Eocene/Oligocene boundary 37-36 Ma (P18), and the Late Oligocene 31-29 Ma ( P20 P21 ). With the exception of the E 0 boundary, faunal changes occur abruptly during short stratigraphic intervals, and are characterized by major species extinctions and first appearances. The Eocene/Oligocene boundary cooling is marked primarily by increasing abundances of cool water species. This suggests that the E 0 boundary cooling, which marks a major event in the oxygen isotope record affected planktic faunas less than during other cooling episodes. Planktic foraminiferal faunas indicate that the E 0 boundary event is part of a continued cooling trend which began during the Middle Eocene. Two hiatus intervals are recognized in low and high latitude sections at the Middle/Late Eocene boundary and in the Late Eocene ( P15 P16 ). These hiatuses suggest that vigorous bottom water circulation began developing in the Middle Eocene, consistent with the onset of the faunal cooling trend, and well before the development of the psychrosphere at the E 0 boundary.

Eocene-Oligocene: A Time of Transition: ABSTRACT

The Eocene-Oligocene is a time of transition from a warm early Tertiary world with low vertical and latitudinal thermal gradients to the Neogene world with steep vertical oceanic gradients and high latitudinal gradients between equator and poles. The transition between these two regimes occurred primarily between the middle Eocene and middle Oligocene and can be observed in faunal and floral assemblage changes, associated paleotemperature changes, periodic current intensification as implied by increased carbonate dissolution and hiatuses, eustatic sea level changes, and the curious association of microtektites and iridium anomalies with several of these intervals. Population studies of planktonic foraminifers in 14 DSDP sites in the Atlantic, Pacific, and Indian Oceans indicate a general cooling trend between the middle Eocene and Oligocene. Major faunal changes indicating cooling episodes occur, however, at discrete intervals: middle Eocene 44 to 43 Ma (P13); middle/late Eocene boundary 41 to 40 Ma (P14/P15); late Eocene 39 to 38 Ma (P15/P16); Eocene/Oligocene boundary 37 to 36 Ma (P18); and late Oligocene 31 to 29 Ma (P20/P21). Each cooling episode resulted in the extinction of warmer water species and evolution and dominance of cooler water species. This trend is associated with the development of a steep vertical thermal gradient and resulting stratification in the upper water masses (0 to 300 m, 1,000 ft) in the latest Eocene. 18The presence of microtektites and iridium anomaly in latest Eocene sediments has resulted in the scenario of catastrophic extinctions due to a bolide impact. The present study reveals multiple microtektite occurrences at 43 Ma, 40 Ma, 38 Ma, 34 Ma, and 30 Ma. Moreover, these microtektite occurrences coincide with intervals of increased carbonate dissolution and/or hiatuses. This suggests that microtektites are concentrated as a result of carbonate dissolution and selective winnowing of sediments at these intervals. Consequently, a concentration of microtektites in deep-sea sediments may not always imply a bolide impact, nor is there any evidence of catastrophic extinctions during Eocene-Oligocene time. End_of_Article - Last_Page 494------------

Deep-sea benthonic foraminiferal faunal turnover near the Eocene/Oligocene boundary

Eocene-Oligocene deep-sea benthonic foraminifera in D.S.D.P. Site 277 in the southwest Pacific have been analyzed to determine the benthonic foraminiferal response to the development of the psychrosphere near the Eocene/Oligocene boundary. Biostratigraphic ranges of 41 taxa show that 23 taxa are found throughout the Late Eocene to Early Oligocene sequence, while 18 taxa exhibit first or last occurrences. Comparison of the faunal changes in Site 277 with a benthonic foraminiferal oxygen isotope record shows that the development of the psychrosphere did not have a profound effect upon the benthonic foraminifera, and the overall faunal change preceding and subsequent to the bottom-water circulation event occurred gradually. The inferred water-mass event affected the relative abundance of one species, Epistominella umbonifera. The lack of major faunal changes at the Eocene/Oligocene boundary in Site 277 probably reflects either wide environmental tolerances of the benthonic foraminifera, or a bottom-water temperature change less than 3°C.Examination of previously published benthonic foraminiferal biostratigraphic data from D.S.D.P. Sites 167, 171, 357, 360, 363, and 400A, and deep-sea ostracode data from D.S.D.P. Leg 3 show faunal changes occurred during discrete intervals in the Middle Eocene-Early Oligocene. The faunal patterns from these data and from Site 277 show that the Eocene/Oligocene cooling event did not cause rapid, catastrophic changes of the benthonic faunas of the open ocean, although significant faunal changes are associated with the water mass event in Sites 167, 171 and 400A.The benthonic faunal changes in Middle Eocene-Early Oligocene time are consistent with the gradual decrease of inferred bottom-water temperatures, based on previously published oxygen isotopic data. The δ 18O Eocene/Oligocene enrichment of 0.76‰ is a major event in the Southern Ocean oxygen isotopic record, but is considerably less in magnitude than the 1.75-2.00‰ change that occurred gradually from mid-Early Eocene to the Eocene/Oligocene boundary. The benthonic foraminiferal and isotopic data indicate that bottom-water circulation may have developed during the Middle Eocene to Early Oligocene interval, with the 3°C bottom-water cooling near the Eocene/Oligocene boundary representing part of this development.

Late Miocene—early Pliocene planktonic foraminiferal biostratigraphy and paleoceanography of low-latitude marine sequences

Plate reorganization and development of polar glaciation are closely associated with the changing climatic conditions of the Cenozoic. The planktonic foraminiferal fauna in three low-latitude DSDP sites (224, Ninety East Ridge; 317B, Manihiki Plateau; 366A, Sierra Leone Rise) has been examined to determine how these regions responded to the late Miocene climatic cooling that has been previously observed in high-latitude regions. It has been proposed that an expansion of Antarctic glaciation and a resultant eustatic regression are associated with this cooling, with the latter being at least partially responsible for the Messinian “salinity crisis” in the Mediterranean. Only one of the sites, 366A, shows any significant faunal change during the late Miocene and early Pliocene (approximately 8-3 Ma). During the late Miocene, there is a decrease in the abundance of species considered to be tropical-subtropical, suggesting the incursion of a cool water mass into the Sierra Leone Rise region during this time. The lack of any major faunal changes at Sites 214 and 317B indicates that the water masses of the low-latitude regions of the Indo-Pacific were relatively unaffected by this cooling in the late Miocene. The three sites also show no evidence for a change in the level of the CCD between the late Miocene and early Pliocene; however, this is most likely due to their equatorial position and shallow water depth. Higher-latitude sites from both the Atlantic and the Pacific reveal a definite shoaling of the CCD during the late Miocene.

Middle to late Miocene planktonic foraminiferal datum levels and paleoceanography of the North and Southeastern Pacific Ocean

Quantitative middle—late Miocene analysis of planktonic foraminifera of the North Pacific DSDP Sites 310, 173, 296, and 292 and the Southeast Pacific Site 319 reveal strikingly similar abundance distributions and foraminiferal datum levels permitting correlations between all sites. The following first (FA) and last (LA) appearances appear to be isochronous in tropical, subtropical, and temperate faunas: Globigerina nepenthes FA, G. druryi FA, Globorotalia mayeri LA, G. siakensis LA, Globoquadrina dehiscens LA, Globigerinoides kennetti FA and LA, and G. obliquus extremus FA. Isotope analysis is available for Site 310 and indicates a depletion in carbon at about 6.2 Ma following the extinction of G. kennetti and the first appearance of G. obliquus extremus. An increase in dissolution of calcium carbonate is apparent in the interval preceding the carbon shift at Site 310 and two further dissolution intervals are present lower in the section within paleomagnetic Epochs 7–8 (N16) and 12 (N14). Similar dissolution intervals have been observed in these same biostratigraphic intervals at DSDP Sites 296, 292, and 319. These dissolution intervals are often associated with a short hiatus and are marked by major faunal changes apparently due to climatic cooling.

PLEISTOCENE HISTORY OF THE BRITISH VERTEBRATE FAUNA

PLEISTOCENE HISTORY OF THE BRITISH VERTEBRATE FAUNA