Cenozoic deep-water agglutinated foraminifera in the North Atlantic
Kaminski, Michael A.
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Cenozoic (predominantly Paleogene) "flysch-type" agglutinated foraminiferal assemblages and their modern analogs in the North Atlantic and adjacent areas have been studied to provide an overview of their spatial and temporal distribution and utility for paleoenvironmental analysis. Over 200 species of agglutinated foraminifera have been recognized in Paleogene sediments from North Atlantic and Tethyan basins. This unified taxonomic data base enables the first general synthesis of biostratigraphic, paleobiogeographic and paleobathymetric patterns in flysch-type agglutinated assemblages from upper Cretaceous to Neogene sediments in the North Atlantic. The majority of taxa are cosmopolitan, but latitudinal, temporal and depth-related trends in diversity and species composition are observed among flysch-type assemblages. Modern deep-sea agglutinated foraminiferal faunas provide an analog to fossil flysch-type assemblages and serve as models for paleoecologic studies. Core-top samples from the Panama Basin, Gulf of Mexico and Nova Scotian continental rise were examined in order to determine the habitats of modern species of agglutinated foraminifera. The ecology of modern taxa provides constraints on the paleoenvironmental significance of fossil agglutinated assemblages in the North Atlantic, and their utility for paleoceanography. Towards this end, spade core samples from a 3912 m deep station in the Panama Basin were studied to determine abundance and microhabitat partitioning among living agglutinated foraminiferal populations and the preservation of dead assemblages. The genera Dendrophrya, Cribrostomoides and Ammodiscus have epifaunal habitats and the genus Reophax is predominantly infaunal. Species of Reophax are probably responsible for fine reticulate burrows observed in xradiographs. An experiment using recolonization trays in the Panama Basin was designed to identify opportunistic species of benthic foraminifera, and to assess the rate at which a population can colonize an abiotic substrate. The most successful colonizer at this site is Reophax, while Dendrophrya displays the lowest capability for dispersal. After nine months the abundance of living individuals in sediment trays was one-tenth to one-third that of background abundance, but the faunal diversity did not differ greatly from control samples. Recolonization by benthic foraminifera is more rapid than among macrofaunal invertebrates. Modern agglutinated assemblages from the Louisiana continental slope were examined to determine changes in species composition associated with hydrocarOrganic- bon seeps. rich substrates are characterized by a decrease in astrorhizids and an increase in trochamminids and textulariids. Highly organicenriched substrates with chemosynthetic macrofauna are dominated by Trochammina glabra and Glomospira charoides. The biostratigraphy of fossil agglutinated foraminifera in the North Atlantic is based on detailed analysis of 670 samples from 14 wells and one outcrop section, and examination of additional picked faunal slides from industry wells. Local biostratigraphic schemes are established for Trinidad, Northern Spain, the Labrador Sea, Baffin Bay, and the Norwegian-Greenland Sea. These schemes are compared with existing biostratigraphic frameworks from the Labrador Margin, the North Sea, and the Polish Carpathians. A number of species show utility for biostratigraphy in the North Atlantic. Lineages which contain stratigraphically useful species include the Haplophragmoides cf. glabra - Reticulophragmium group, Hormosina, and Karreriella. Significant faunal turnovers are observed at the Paleocene/Eocene, Ypresian/ Lutetian and Eocene/Oligocene boundaries. A reduction in diversity occurs at the Paleocene/Eocene boundary in all bathyal sections studied, and agglutinated forminifera disappear entirely from abyssal low-latitude DSDP sites. In the Gibraltar Arch, the Labrador Sea and the Norwegian-Greenland Sea, the Ypresian/Lutetian boundary is characterized by a Glomospira-facies. This is attributed to a rise in the lysocline associated with increased paleoproductivity and the NP14 sealevel lowstand. The Eocene/Oligocene boundary is delimited by another major turnover and the last occurrence of a number of important taxa. At Site 647, where recovery across the Eocene/Oligocene boundary was continuous, the change from an Eocene agglutinated assemblage to a predominantly calcareous assemblage in the early Oligocene took place gradually, over a period of about 4 m.y. The rate of change of the faunal turnover accelerated near the boundary. This faunal turnover is attributed to changes in the preservation of agglutinated foraminifera, since delicate species disappeared first. Increasingly poorer preservation of agglutinated foraminifera in the late Eocene to earliest Oligocene reflected the first appearance of cool, nutrient-poor deep water in the southern Labrador Sea. The approximately coeval disappearance of agglutinated assemblages along the Labrador Margin was caused by a regional trend from slope to shelf environments, accentuated by the "mid"-Oligocene sealevel lowstand. Paleobiogeographic patterns in flysch-type foraminifera were examined in the Paleogene of the North Atlantic. In the early Paleogene, general decrease in diversity is observed from low to high latitudes and from the continental slope to the deep ocean basins. The diversity of these microfossils declines in most studied sections throughout the Paleogene. The last common occurrence (LCO) of flysch-type foraminifera in the North Atlantic exhibits a pattern of diachrony with latitude and depth. Extinctions occurred first at abyssal depths and at low latitudes. Agglutinated assemblages disappeared from the northern Atlantic region in the early Oligocene. However, the deep Norwegian-Greenland Sea served as a refuge for many species, and agglutinated assemblages persisted there until the early Pliocene. The LCO of flysch-type foraminifera may have been related to the transition from a warm, sluggish deep sea environment to a cooler, more oxygenated, thermohaline-driven deep circulation pattern caused by bipolar cooling. The paleobathymetry of Paleogene agglutinated assemblages in the North Atlantic differs from Cretaceous patterns. Shallow-water assemblages of Paleogene age contain robust astrorhizids, loftusiids and coarse lituolids, whereas deep assemblages possess delicate tubular forms, ammodiscids, and smooth lituolids. At low latitudes, upper bathyal assemblages contain abundant calcareous ataxophragmiids. Paleocene paleobathymetric patterns in the North Atlantic compare well with patterns observed in the Carpathian troughs. The utility of agglutinated foraminifera in paleoceanography is illustrated by a study of the paleocommunity structure of fossil assemblages in ODP Hole 646B on the Eirik Ridge (Labrador Sea). The synecology of benthic foraminifera in Hole 646B places constraints on the history of Denmark Straits Overflow Water over that site. Below seismic horizon "R3", a Miocene assemblage contains smooth agglutinated species with abundant Nuttalides umbonifera, indicating corrosive bottom water and tranquil conditions. A coarse agglutinated assemblage with "NADW-type" calcareous benthics is observed above the seismic horizon. This faunal turnover at horizon "R3" reflects the onset (or renewal) of significant Denmark Straits overflow at -7.5 Ma. Agglutinated species disappear between reflector "R2", and the base of the sediment drift, indicating a change in deep-water properties associated with the re-opening of the Mediterranean. The onset of drift sedimentation at the Eirik Ridge is dated at -4.5 Ma. Drift formation ceased at -2.5 Ma, concomitant with the appearance of ice-rafted sediments.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1987
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