Showing papers on "Foraminifera published in 1987"

North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature

Abstract: During a surface cooling event 10,000 to 12,000 years ago, higher Cd/Ca and lower 13C/12C ratios are observed in benthic foraminifera shells from rapidly accumulating western North Atlantic sediments. Data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial North Atlantic Deep Water flux. It is proposed that cold high-latitude sea surface temperatures enhance intermediate-water formation at the expense of deep-water formation.

Paleoecology of Mid-Cretaceous planktonic foraminifera; a comparison of open ocean and epicontinental sea assemblages

Abstract: Although our knowledge of living planktonic foraminifera is limited, useful paleoecologic information can be obtained from ancient assemblages. During the mid-Cretaceous, as today, the simple, inflated morphotypes inhabited the nearsurface waters while the flatter, keeled forms probably occupied deeper habitats. Three faunal groups are proposed, an Epicontinental Sea Fauna (ESF) characterized by species of Gubkinella, Guembelitria and Heterohelix, an open marine Shallow Water Fauna (SWF) composed primarily of species of Hedbergella and Globigerinelloides, among others, and finally an open marine Deep Water Fauna (DWF) represented by species of Planomalina, Rotalipora and Praeglobotruncana (keeled taxa). Open ocean assemblages of DSDP Sites 545 and 547 are numerically dominated by species of the SWF in the >63-,um size fraction while the ESF comprises about 5% of the assemblages, and the biostratigraphically important DWF generally comprises less than 2%. These assemblages demonstrate remarkable stratigraphic consistency. In contrast to open ocean assemblages, epicontinental sea assemblages of the Vocontian Basin are characterized by loss of the DWF with decreasing water depth and an increase in the proportion of the ESF. The ESF:SWF ratio also becomes increasingly variable with decreasing water depth. Upwelling also has pronounced effects on the composition of planktonic assemblages as suggested by an increase in the proportion of the ESF, interpreted to include opportunistic species.

Abyssal circulation and benthic foraminiferal changes near the Paleocene/Eocene boundary

Abstract: A major change in Cenozoic deep-sea benthic foraminifera occurred in the Atlantic, Indian, and Pacific oceans near the Paleocene/Eocene boundary. Benthic foraminiferal abundance changes began at about 61.5 Ma at Pacific Deep Sea Drilling Project (DSDP) Site 577. A major extinction event followed at 58–57 Ma (between Zones P6a and P6b), and a series of first appearances continued until circa 55.5 Ma (Zone P6c). These faunal changes occurred during a 6°C warming of Pacific bottom water and may indicate that the primary cause was changing temperature. Other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions. Comparison of benthic and planktonic δ13C records requires no change in the ratio of oceanic phosphorous to carbon during the late Paleocene to early Eocene, which weakens the case for (but does not disprove) a change in surface ocean productivity at this time. Interbasinal comparisons of benthic foraminiferal δ13C records document that water with high δ13C values filled the Cape Basin during the late Paleocene and possibly the early Eocene (circa 61–57 Ma), but apparently did not extend into the western basins of the Atlantic. This pattern suggests a supply of Antarctic source water for the Cape Basin and possible tectonic isolation of the western Atlantic basins during at least part of the late Paleocene. Carbon isotope comparisons show that bottom water supply to the Cape Basin was reduced in the early Eocene. Eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the Paleocene. These late Paleocene climatic changes (bottom water warming and decreased wind intensity) correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism, which may have contributed to climatic warming through increased supply of CO2.

Antarctic marine temperatures: Late Campanian through Early Paleocene

Abstract: An oxygen isotope paleotemperature study of benthic foraminifera from the upper Campanian through lower Paleocene section at Seymour Island, near the coast of Antarctica, was done in conjunction with careful scanning electron microscope observations to monitor the state of foraminiferal preservation. The isotopic compositions of those samples which were deemed to be well preserved were the basis of the conclusion that Campanian through Paleocene high-latitude shelf waters near Antarctica had temperatures between approximately 4.5° and 10.5°C. Temperatures during early and middle Maastrichtian time were slightly higher than those of either the late Campanian or late Maastrichtian. The inference of temperate climatic conditions in Antarctica is in agreement with paleoclimatic conclusions drawn on the species composition of the planktonic foraminiferal assemblages and the character of the planktonic foraminifera, as well as with published conclusions based upon paleobotanical evidence. The Maastrichtian and Campanian water temperatures at Seymour Island are similar to those inferred from published isotope ratios of Pacific Late Cretaceous benthic foraminifera from low latitudes, indicating that Campanian through early Paleocene deep ocean waters, at least in the Pacific and perhaps elsewhere as well, could have formed by sinking of surface waters in polar regions.

North Pacific deep water formation during the latest glaciation

Abstract: For at least 20 years palaeoceanographers have speculated about the existence of a source of young (nutrient-depleted) deep water in the North Pacific (NPDW) during glaciations. Proof of its existence has eluded researchers because the present deep North Pacific is very corrosive to calcium carbonate. Thus, it has been difficult to obtain a long time series of oxygen isotope data on benthic foraminifera for dating the sediment, and carbon isotope data for use as a proxy for deep water nutrient content. I report here on a stable isotope record from a western subarctic Pacific core taken at 3 km water depth. The carbon isotope ratios in the benthic foraminifera Cibicidoides show no evidence of nutrient-depleted NPDW from glacial to Holocene time.

Seasonal and interannual changes in planktonic foraminiferal production in the North Pacific

Abstract: It is well established that there are distinct seasonal cycles in the production of plankton, that vary both geographically and in terms of phytoplankton-zooplankton coupling1 . Only recently have attempts been made to measure seasonal variability in the flux of various biogenic components from the ocean surface to the sea-floor2–4 and to relate this flux to local hydrographic conditions5. For example, a simple inverse relationship has been found between sediment flux to the seafloor and sea-surface temperature in the Sargasso Sea5. A three-year sediment trapping project in the subpolar North Pacific has allowed us to document both seasonal and interannual variability in the flux of planktonic foraminifera from the sea surface. We propose a model, based on the temperature preferences of planktonic foraminiferal species, the thermal structure of the upper water column and food availability, to explain the observed changes over time in foraminiferal production and flux.

Equatorial Pacific deep-sea benthic foraminifera: Faunal changes before the middle Miocene polar cooling

Abstract: Data on the composition of benthic foraminiferal faunas at Deep Sea Drilling Project Site 575 in the eastern equatorial Pacific Ocean were combined with benthic and planktonic carbon- and oxygen-isotope records and CaCO 3 data. Changes in the composition of the benthic foraminiferal faunas at Site 575 predated the middle Miocene period of growth of the Antarctic ice cap and cooling of the deep ocean waters by about 2 m.y., and thus were not caused by this cooling (as has been proposed). The benthic faunal changes may have been caused by increased variability in corrosivity of the bottom waters, possibly resulting from enhanced productivity in the surface waters.

Cd/Ca in late Miocene benthic foraminifera and changes in the global organic carbon budget

Abstract: The late Miocene carbon shift (∼6.2 Myr)—a 0.5–1.0‰, δ13C decrease in benthic and planktonic foraminifera—has been ascribed to changes in global inventory, deep-ocean circulation, and/or productivity1–16. Cadmium, δ13C, and nutrients in the ocean are linked; comparison of δ13C and Cd/Ca yields circulation and chemical inventory information not available from either alone17,18. We determined Cd/Ca ratios in late Miocene benthic foraminifera from DSDP Site 289. Results include: (1) late Miocene Pacific Cd/Ca values fall between those of late Quaternary Atlantic and Pacific benthic foraminifera; (2) there are no systematic Cd/Ca offsets between Cibicidoides kullenbergi, Cibicidoides wuellerstorfi and Uvigerina spp.; and (3) there is a very slight Cd/Ca change coincident with δ13C. Cd/Ca, slightly higher in younger, isotopically lighter samples, exhibits a smaller increase than predicted if circulation were the primary cause of the carbon shift. The carbon shift may have been due to a long-term shift in the steady-state carbon isotope input or to a change in the sedimentation of organic carbon relative to calcium carbonate.

Relevance of specimen size in distribution studies of deep-sea benthic foraminifera

Abstract: There is no agreement among investigators of abyssal benthic foraminifera on the sieve-size of samples best suited for distribution studies; 63, 125, and 250 Fxm + sizes are commonly used. A faunal analysis of these fractions in two sets of deep-sea samples, one from the Caribbean, the other from the equatorial North Atlantic, demonstrates that a signifcant amount of information, including that on species diversity and dominance, is lost when the coarser sieves are used. The reported abundances of species in known water-mass associations of benthic foraminifera are not independent of specimen size. In an Antarctic Bottom Water assemblage from the Equatorial North Atlantic, a 250 ,um threshold size obliterates the dominance of Epistominella eidgua and inflates the proportion of Cibicides wuellerstorfi. Eponides turgidus, another dominant member of this assemblage, is almost absent in both the 125 and 250 ,um + fractions. The same is true for Nuttallides decorata, the dominant benthic foraminifer in the Caribbean Bottom Water in the southeastern Venezuela Basin. Eponides polius, common in the Venezuela Basin assemblage, is absent in the coarsest fraction. We suggest that researchers choosing to work with coarser sieves in the context of their particular problems indicate what the proportions of common species would be if the 63 ,um sieve is used. This test could be made with just a few samples.

Toward A High-Resolution Chronology for Latest Miocene Paleoceanographic Events

Abstract: It is now widely accepted that expansion of continental glaciers is one manifestation of profound paleoenvironmental change in latest Miocene time. The oxygen isotopic record in deep-sea cores is inferred to document brief pulses in glaciation, based on covariance of δ18O in benthic and planktonic foraminifera, which probably lowered latest Miocene sea level by a minimum of 60 m. Additional evidence of glacial activity in the oxygen isotope record is obscured by the small signal amplitude. Before further details of paleoenvironmental change can be investigated it is essential to have a high-resolution stratigraphy and chronology. This has not been achieved previously because most studies of the latest Miocene have had a sampling interval greater than the duration of events they seek to resolve. A high-resolution stable isotope record from the north Atlantic (Deep Sea Drilling Project (DSDP) 552) is correlated with a second record from the southwest Pacific (DSDP 588). Each core has magnetostratigraphy, but there are two possible interpretations of magnetic results in the Atlantic core. By assuming that brief events of 18O enrichment and 13C depletion in each core are synchronous it is possible to assign the most likely age model to the Atlantic core. Results of this exercise indicate that two δ18O maxima occurred at 4.8 and 5.2 Ma. If these events reflect continental glaciation, and if sea level was lowered enough to contribute to isolation and desiccation of the Mediterranean, then the Miocene/Pliocene boundary must be close to 4.8 Ma in age because Messinian deposits are latest Miocene in age. This interpretation is consistent with biostratigraphy at DSDP 552; magnetostratigraphy, biostratigraphy, and isotope stratigraphy at a site commonly referred to because of its proximity to the Mediterranean (DSDP 397, northwest African continental margin); and recent work on land in the Mediterranean region.

Cenozoic deep-water agglutinated foraminifera in the North Atlantic

Abstract: 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…

Danian faunal succession: Planktonic foraminiferal response to a changing marine environment

Abstract: Planktonic foraminiferal evolution following the Cretaceous/Tertiary boundary extinction event is marked by a series of events in which one assemblage is rapidly replaced by another. This pattern is characteristic of a highly changing pelagic environment. The observed changes in the planktonic foraminifera may be related to long-term changes in oceanic productivity that persisted for about the first 2 m.y. of the Danian.

Patterns of sediment composition of Jamaican fringing reef facies

Abstract: Recent carbonate sediments from Jamaican north coast fringing reefs were collected along three parallel traverses in the vicinity of Discovery Bay. Each traverse extended from near shore across the back reef, reef crest, and fore reef to a depth of 75 m. Relative abundances of the biotic constituents vary between sites, reflecting general patterns of reef community composition. The sediment is dominated by highly comminuted coral fragments (27.1% to 63.1%), plates of the calcareous green alga Halimedu (0.4% to 38.7%), coralline algae (4.7% to 16.2%) and the encrusting foraminiferan Homotremu rubrum (0.7% to 9,5%), with lesser amounts of other taxonomic groups (non-encrusting foraminifera 1.3--5.5%; molluscs 1.4-7.0%; echinoderms 0.9-5.0%). Coral fragments, coralline algae and particles of Homotrema rubrum dominate the sediments of the shallow portions of the fore reef (5-15 m), whereas plates of Hulimedu are most abundant in sediments from the back reef and deeper portions of the fore reef (2 24 m), Q-mode cluster analysis, using sediment constituent data, resulted in the delineation of four reef biofacies over the depth range of this study (1-75 m).