Showing papers on "Foraminifera published in 1989"

Miocene deepwater oceanography

Abstract: A global synthesis of Miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that Miocene thermohaline circulation evolved through three regimes corresponding approximately to early, middle, and late Miocene times. There is evidence for major qualitative differences between the circulation of the modern ocean and the Miocene ocean prior to 11 Ma. The 13C/12C ratios of the benthic foraminifera Cibicidoides are interpreted in terms of water mass aging, i.e., the progressive depletion of dissolved O2 and lowering of δ13C values as the result of oxidation of organic matter as water flows further from its sources at the surface of the oceans. Both isotopic and faunal data indicate that the early Miocene regime, from 22 to 15 Ma, was the most different from today's. During that interval intermediate and deep waters of both the Atlantic and the Pacific oceans aged in a northward direction, and the intermediate waters of the Indian, the South Atlantic and the South Pacific oceans were consistently the youngest in the global ocean. We speculate that early Miocene global thermohaline circulation may have been strongly influenced by the influx of warm saline water, Tethyan Indian Saline Water, from the Tethys into the northern Indian Ocean. The isotopic and faunal data suggest that flow from the Tethyan region into the Indian Ocean diminished or terminated at about 14 Ma. Isotopic and faunal data give no evidence for North Atlantic Deep Water (NADW) formation prior to about 14.5 Ma (with the exception of a brief episode in the early Miocene). From 14.5 to 11 Ma NADW formation was weak, and circumpolar and Antarctic water flooded the deep South Atlantic and South Pacific as the Antarctic ice cap grew. From about 10 Ma to the end of the Miocene, thermohaline circulation resembled the modern circulation in many ways. In latest Miocene time (6 to 5 Ma) circulation patterns were very similar to today's except that NADW formation was greatly diminished. The distribution pattern of siliceous oozes in Miocene sediments is consistent with our proposed reconstruction of thermohaline circulation. Major changes which occurred in circulation during the middle Miocene were probably related to the closing of the Tethys and may have contributed to rapid middle Miocene growth of the Antarctic ice cap. Appendices 1, 4, 6, and 7 are available withentire article on microfiche. Order fromAmerican Geophysical Union, 2000 FloridaAvenue, N.W., Washington, DC 20009.Document 88P-002; $5.00. Payment mustaccompany order.

Down-core distribution of live and dead deep-water benthic foraminifera in box cores from the Weddell Sea and the California continental borderland

Abstract: Five short cores sub-sampled from box cores from three sites in the eastern Weddell Sea off Antarctica and in the eastern Pacific off southern California, covering a range in water depth from 500 to 2000 m, were analysed for the down-core distribution of live (stained with Rose Bengal) and dead benthic foraminifera. In the California continental borderland, Planulina ariminensis, Rosalina columbiensis and Trochammina spp. live attached to agglutinated polychaetes tubes that rise above the sediment-water interface. Bolivina spissa lives exclusively in or on the uppermost sediment. Stained specimens of Chilostomella ovoidea are found down to 6 cm within the sediment and specimens of Globobulimina pacifica down to a maximum of 8 cm. δ13C values of live G. pacifica decrease with increasing depth from the sediment surface down to 7 cm core depth, indicating that this infaunal species utilizes 13C-depleted carbon from pore waters. In the dead, predominantly calcareous benthic forminiferal assemblage, selective dissolution of small delicate tests in the upper sediment column causes a continuous variation in species proportions. In the eastern Weddell Sea, the calcareous Bulimina aculeata lives in a carbonate corrosive environment exclusively in or on the uppermost sediment. The arenaceous Cribrostomoides subglobosum, Recurvoides contorus and some Reophax species are frequently found within the top 4 cm of the sediment, whereas stained specimens of Haplophragmoides bradyi, Glomospira charoides and Cribrostomoides wiesneri occur in maximum abundance below the uppermost 1.5 cm. Species proportions in the dead, predominantly arenaceous, benthic foraminiferal assemblage change in three distinct steps. The first change is caused by calcite dissolution at the sediment-water interface, the second coincides with the lower boundary of intense bioturbation, and the third results from the geochemical shift from oxidizing to reducing conditions below a compacted ash layer.

Short Paper: A standardized model for Tethyan Tertiary carbonate ramps

Abstract: Low latitude carbonates in massive and clinoform bedded sheets, termed ramps, occur throughout the Tertiary Tethyan realm, and are characterized by recurring biofacies usually including large foraminifera, rhodolithic algae, coralgal patch-reef and gastropod dominated sequences. All can be interpreted broadly within a standardized Tertiary carbonate ramp model. Biota can be used to define precise palaeobathymetric zones which have a predictive utility in palaeoenvironmental modelling, especially where data are limited. The carbonate ramp model of earlier authors, has been elaborated by Read (1982) and ramp formation occurs mostly in passive margin and extensional tectonic settings. Ramps are characterized by slope gradients of less than 1 degree and, once established, are conservative to change. Much use has been made of the ramp model for Palaeozoic and Mesozoic lithofacies modelling. In contrast, published work on Cenozoic ramps is limited, however, Tethyan ramps are particularly widespread and extend tens to hundreds of kilometres along strike and occupy tens of kilometres of slope. Many studies have dealt with Cenozoic ramp associations and sequences (e.g. Pedley 1983; Reiss & Hottinger 1984; Purser 1973) and have demonstrated the diversity of biotal and sediment detail, but without ascribing them to a ramp model. Biota were quick to adapt to the Tertiary environments left vacant after the K–T boundary extinction events. New genera of Foraminifera, Mollusca, Bryozoa and Echinoder-mata rapidly replaced the extinct forms in bursts of innovative opportunism. Codiacean algae, Rhodophyta and marine angiosperms were equally adaptive colonizers of the carbonate platforms. Novel biofacies associations were derived from

Barium content of benthic foraminifera controlled by bottom-water composition

Abstract: THE carbon isotope ratio (δ13;C) and cadmium content (Cd/Ca) of benthic foraminifera shells have been used to reconstruct deep-water circulation patterns of the glacial oceans1–7. These tracers co-vary with phosphorus in the modern ocean because they are nearly quantitatively regenerated from sinking biological debris in the upper water column. Hence they can be used to reconstruct the distribution of labile nutrients in glacial water masses. Independent constraints on glacial deep-ocean circulation patterns could be provided by a tracer of the distribution of silica and alkalinity, the deeply regenerated constituents of planktonic hard parts. Barium shares key aspects of its behaviour with these refractory nutrients because it is removed from solution in surface waters and incorporated into sinking particles which slowly dissolve deep in the water column and in the sediments8. The fractionation of Ba between deep-water masses of the major ocean basins is largely controlled by thermohaline circulation patterns, so Ba conforms to different boundary conditions from Cd andδ 13C. As Ba substitutes into trigonal carbonates9, it is a potential palaeoceano-graphic tracer if the Ba content of foraminifera shells reflects ambient dissolved Ba concentrations. Here we present data from Recent core-top benthic foraminifera which indicate that the Ba content of some recent calcitic benthic foraminifera does co-vary with bottom-water Ba.

Ocean temperatures and isotopic compositions through time

Abstract: Fossil assemblages can give quantitative estimates of palaeotemperatures, by comparison with modern biota, only in the recent geological past. Oxygen isotopic palaeotemperatures on calcareous or phosphatic fossils are potentially available for the whole Phanerozoic. Their reliability is limited by physiological effects (generally believed minor), preservation (for which criteria are available), and by uncertainty in the isotopic composition of ancient seawater. The latter is greatly affected by glaciation. In the Cenozoic, the relative contribution of ice-volume change and temperature change in producing isotopic variations can largely be resolved by analysing planktonic and benthic foraminifera in deep-sea cores. For earlier times only continental shelf deposits are available. In the Mesozoic, reasonable assumptions about ocean isotopic composition lead to palaeotemperature estimates that suggest generally higher temperatures than at present, particularly for mid- to high latitudes. This agrees with estimates based on biotic distributions. Late Palaeozoic glaciation is reflected in variable isotopic compositions in high palaeolatitude areas. In the earlier Palaeozoic, well-preserved fossils indicate either oceans enriched in 16O compared to today's or generally higher temperatures; controversy continues about the relative importance of the two effects.

Abundance ofBulimina exilis andMelonis barleeanum: Relationship to the quality of marine organic matter

Abstract: Studies on diatoms showed that locations of Mauritanian coastal upwellings during the last glacial and deglaciation epochs were More westerly than present-day sites, i.e., off the continental shelf, rather than nearshore. Benthic foraminifera revealed a stratigraphic coverage from the present to the last glacial maximum (0 to 18000 yrs BP). Factor analysis produced four factor assemblages that account for 95.4 percent of the variance. Two factors are dominated by eitherBulimina exilis orMelonis barleeanum. The quantitative stratigraphic distribution of these two deep-sea species is related to the quality of marine organic matter derived from surface upwellings.B. exilis develops only when organic matter reaching the bottom is relatively unchanged;M. barleeanum prefers organic matter in a more altered form.

Carbon isotope composition of tropical surface water during the past 22,000 years

Abstract: Carbon isotope values of many species of planktonic foraminifera are size-dependant. Values of δ13C of individual size fractions are therefore an integration of low δ13C values of calcite precipitated in the juvenile growth stage and high values precipitated in later stages. In general, the selection of a particular size fraction for downcore studies has been subjective. Analysis of different size fractions of planktonic foraminifera, however, yields different time series in downcore studies. By measuring the δ13C of many size fractions, the δ13C precipitated throughout the growth of the foraminifera can be calculated. Time series of the δ13C of calcite precipitated in the late growth stages of planktonic foraminifera from two nearby cores from the Caribbean Sea result in a better correlation than time series of constant size fractions. These δ13C records indicate similar δ13C values during the last glaciation and the late Holocene and a broad deglacial minimum approximately synchronous with a minimum in the δ13C records of benthic foraminifera from intermediate-depth Atlantic cores. The deglacial minimum may be a reflection of the isotopic composition of the underlying intermediate water (which may be affected by circulation changes or changes in preformed δ13C values) and/or of the flux of water upwelled into the surface layer. The small (−0.2 to 0.0‰) glacial-interglacial δ13C difference in tropical surface water is also believed to be affected by the chemistry of the underlying source water.

Extended Cretaceous/Tertiary boundary extinctions and delayed population change in planktonic foraminifera from Brazos River, Texas

Abstract: High-resolution planktonic foraminiferal analysis of three Brazos River sections indicates a nearly continuous Cretaceous/Tertiary [K/T] boundary sedimentary record second only to the world's most complete record at El Kef, Tunisia. Species extinctions occur over an extended period of time and with two major extinction episodes. The first extinction episode with 46% of the species extinct occurs at and just below [10–15 cm] a short hiatus at the base of a sandy shell hash and clay-sand unit which was interpreted by Bourgeois et al. [1988] to represent a tsunami bed generated by the K/T boundary bolide impact. The top of this tsunami bed is about 17–20 cm below the K/T boundary as defined by the first appearance of Tertiary planktonic foraminifera. The second extinction phase with 45% of the species extinct occurs 25 cm above the K/T boundary [Zone P0/P1a boundary]. Of the remaining seven surviving Cretaceous species, six gradually disappear during planktonic foraminiferal Subzones P1a and basal P1b. No species extinctions or major faunal assemblage changes are directly associated with the K/T boundary. Iridium distribution is ambiguous, with one peak in the upper part of the tsunami bed and a second peak at the micropaleontologically defined K/T boundary. Relative abundances of dominant species are stable through the Late Maastrichtian, and only minor abundance changes coincide with the first extinction episode or the K/T boundary. The first major faunal change in the dominant species group coincides with the second extinction episode and leads to decline and eventual extinction of this group in Subzone P1a. Species disappearing at the two extinction episodes [46% and 45%] constitute only a small percentage [8% and 5%] of the individuals of the total planktonic foraminiferal population. This suggests that weakened species with low numbers of individuals and sensitive to relatively minor environmental changes were primarily affected by these extinction episodes. Magnetostratigraphy indicates that the first extinction phase began about 310,000 years before the K/T boundary, and the second extinction phase occurred 50,000 years after the K/T boundary. This stepped pattern of species extinctions suggests a progressively stressed ecosystem in continental shelf settings which may be related to an observed sea level regression and global cooling. The hypothesis of a global catastrophic mass extinction at the K/T boundary caused by a large extraterrestrial impact is not supported by the Brazos River planktonic foraminiferal data.

Cenozoic bathyal and abyssal calcareous benthic foraminiferal zonation

Abstract: Deep-sea (bathyal-abyssal) benthic foraminifera provide a potential supplement to planktonic biostratigraphy. Benthic foraminifera are more resistant to dissolution and are often found in upper bathyal, lower abyssal, and higher latitude sediments which contain impoverished planktonic faunas. The biostratigraphic utility of benthic foraminifera was previously limited by poor understanding of their taxonomy and paleobathymetric distributions. During the past ten years, study of deep-sea benthic foraminifera from Deep Sea Drilling Project (DSDP) and industry wells has provided an improved taxonomic base and empirical observations of age-depth distributions. Building on this data base, we establish 14 bathyal and 12 abyssal zones for the Cenozoic deep sea. Biostratigraphic resolution is higher in the abyssal realm during the Paleogene (9 abyssal versus 6 bathyal zones) and higher in the bathyal realm during the Neogene (3 abyssal versus 8 bathyal zones). During periods of accelerated taxonomic turnover (e.g., in the middle Miocene bathyal zone), deep-sea benthic foraminifera yield fairly refined zonations; in contrast, slow taxonomic turnover occurred during the Paleocene in the bathyal and abyssal realms and in the late Neogene in the abyssal realm, yielding few biostratigraphic subdivisions.

Biostratigraphy and late Cenozoic paleoceanography of the Arctic Ocean: Foraminiferal, lithostratigraphic, and isotopic evidence

Abstract: Detailed studies of benthonic foraminifera, stable isotopes, and lithofacies in cores from the southeastern Alpha Ridge, central Arctic Ocean, reveal some new aspects of Arctic Ocean paleoceanography. High ratios of benthonic to planktonic foraminifera are found in most of the Quaternary sediment units, and ratios of 1:1 appear to characterize the Arctic deep-water sediments. Benthonic foraminifera in the carbonate mud unit M show a succession of calcareous species reflecting increased influx of Norwegian Sea bottom water to the Arctic Ocean during the past 0.4 m.y. Foraminiferal and lithological data indicate less-uniform sedimentation during a warmer interval from 0.4 to 0.6 Ma, when most of the silty lutite unit L was deposited at the CESAR site. Lower Pleistocene units J to I contain less limestone and more dolomite, and they contain a uniform faunal assemblage with low numbers of calcareous foraminifera. Upper Pliocene units H to AB contain rare limestone and relatively large amounts of do-lomite and quartz sand. Middle to upper Pliocene units AB to A3 are marked by abundant sand-sized ferromanganese-coated particles, which in many cases have a silt nucleus; hence, much of the coarse sand in these units does not indicate increased ice rafting. The Pliocene sediments mostly contain a low-diversity assemblage of agglutinated foraminifera, but a mixed calcareous/arenaceous fauna occurs in a short interval above the Matuyama-Gauss boundary (2.4 Ma). Stable-isotopic curves occur within sequences which broadly correspond to stages 1-9 of the global record; below stage 9, the record is discontinuous. Strong vertical mixing apparently prevailed during most of the Pliocene and early Pleistocene, then decreased during the past 0.4 m.y. owing to damping by a perennial ice cover. Isotopic and foraminiferal data, however, suggest that an interval of perennial sea ice also occurred during the late Pliocene at the time of the earliest glacial event recorded in the North Atlantic.

Planktonic foraminifera: Differential dissolution and the Quaternary stable isotope Record in the west equatorial Pacific

Abstract: Differential dissolution affects the isotopic composition of different species of planktonic foraminifera in different ways. In the two species studied here in cores from Ontong Java Plateau, the less resistant species, Globigerinoides sacculifer, is more readily affected at a shallower depth than the more resistant species, Pulleniatina obliquiloculata (2.9 versus 3.4 km), but shows a smaller and less predictable response to partial dissolution (0.2 to 0.3 per mil versus 0.6 to 0.7 per mil). Comparison of isotopic values from the last glacial period with those from the late Holocene indicates that the apparent dissolution effect is considerably reduced during the last glacial, presumably due to reduced dissolution intensity during glacial time. A change in the level of the lysocline of about 400 m is suggested. In the published isotope records from Pacific cores V28-238 and V28-239, the dissolution-generated difference in δ18O (noted previously by Shackleton and Opdyke [1976]) is seen to describe a mid-Brunhes dissolution maximum, between 300 and 500 thousand years ago. This mid-Brunhes dissolution excursion is well known from the Pacific and the Indian Ocean.

Stable isotope stratigraphy of latest Miocene sequences in northwest Morocco: the Bou Regreg section

Abstract: Oxygen and carbon isotopic ratios of foraminifers were measured from outcrop and drill core sequences from the Bou Regreg Section, northwest Morocco. This composite section was located at the western end of the Rifian Corridor during the late Miocene and thus potentially contains a record of water exchange between the Atlantic and Mediterranean during the late Tortonian and Messinian stages. Here we correlate isotopic and sedimentologic events in the Bou Regreg Section with Mediterranean and deep-sea sequences during the time leading up to and including the deposition of the Messinian evaporites. The late Miocene chron 6 carbon shift was identified in two Moroccan sequences, providing a valuable 6.3-Ma datum level. In both sections, the carbon shift coincides with the first occurrence of Globorotalia conomiozea and the Tortonian/Messinian boundary. Near this boundary, a major faunal turnover occurred in ostracod, planktonic foraminiferal, and nannofossil assemblages that indicates a cooling of surface and deep water in the Rifian Corridor. At 6.1 Ma, just above the Tortonian/Messinian boundary, mean ∂18O values of benthic foraminifers increased by 0.4-0.5‰ suggesting decreased temperature and/or increased continental ice volume. The faunal and isotopic changes are interpreted as reflecting a reversal in the direction of deep water flow through the Rifian Corridor that occurred between 6.3 and 6.1 Ma. At this time, cold, nutrient-rich waters filled the Mediterranean basins from intermediate depths of the Atlantic, and the production of Mediterranean Outflow Water ceased. The upwelling of nutrient-rich Atlantic intermediate water stimulated productivity in the Mediterranean, which led to the deposition of organic-rich strata such as those found in the Tripoli Formation. At ∼5.5 Ma, coincident with a coiling shift in Neogloboquadrina acostaensis, the variability of the benthic ∂18O signal increased markedly, and strong color variations appeared in the sediments (alternating between red and blue marls). These isotopic and sedimentologic changes at Bou Regreg represented markedly fluctuating oceanographic conditions in the Rifian Corridor between 5.5 and 4.8 Ma. We speculate that these cycles were related to sea level variations that controlled the periodic influx of marine water into the Mediterranean during the time of evaporite deposition. The onset of these cycles occurred at 5.5 Ma and is correlated to a eustatic fall in sea level at the base of the Caliza Tosca Formation in Carmona, Spain, and to the base of the lowermost evaporite unit in the Mediterranean (e.g., Calcare di Base in Sicily). In this proposed scenario, the duration of the Messinian salinity crisis was 700,000 years and lasted between 5.5 and 4.8 Ma.

Testing models of past ocean chemistry using foraminifera 15N/14N

Abstract: Previous models have attempted to explain glacial to interglacial changes in atmospheric pCO2 by invoking changes in the ocean's nutrient concentration or regional changes in nutrient utilization. Nitrogen is limiting to primary production and has a residence time in the ocean compatible with glacial to interglacial variations. But up to now, there has been no geological indicator for glacial to interglacial changes in the ocean's nitrogen cycle. We propose that 15N/14N ratios for the organic matrix of preserved foraminifera yield an interpretable δ15N record. We have developed models to study the relationship between plausible changes in the ocean's nitrogen cycle and resulting changes in the 15N/14N ratio of dissolved NO3−. Our model results demonstrate that 15N/14N ratios exhibit excursions during periods in which the oceans are accumulating or losing nitrogen regardless of the means by which these changes occur. Foraminifera 15N/14N results for a Pacific and an Atlantic core provide evidence against wide spread glacial anoxia, a consequence of many models of ocean chemistry. We suggest that increasing shelf-sediment denitrification upon deglaciation reduced nitrogen concentrations in the modern ocean. Ambiguities between the two foraminifera 15N/14N records indicate that local effects associated with changes in hydrography or ecology need to be studied further.

Atlantic paleogene biserial heterohelicid foraminifera and oxygen minima

Abstract: Through most of the Paleogene the planktonic foraminifera called biserial heterohelicids record the most depleted carbon isotope ratios among planktonic foraminiferal species. They are interpreted to occupy an oxygen minimum habitat. Stronger interplanktonic carbon isotope contrasts and more enriched near surface carbon isotope values correspond to higher abundances of biserial heterohelicids. This group appears to have preferred latitudes and times when the oxygen minimum was well developed. Biserial heterohelicid generic and specific diversity are directly related to the interplanktonic carbon isotope contrast. Minimal specific diversity characterizes the Oligocene when interplanktonic and surface to bottom carbon isotope contrasts were consistently low. Diversity and abundance decreases were coeval with increased oceanic ventilation.

Latest Quaternary and Holocene paleoceanography of the eastern Baffin Island continental shelf, Canada: benthic foraminiferal evidence

Abstract: Foraminiferal zones, radiocarbon ages on shells, and corrected ages on pretreated organic sediment from four cores from the eastern Baffin Island continental shelf suggest a three-stage deglacial to postglacial history (Late Wisconsin to Holocene). The earliest sediments in the cores contain foraminiferal species (Elphidium excavatum, Cassidulina reniforme, Islandiella helenae) indicative of distal glaciomarine environments that lasted at least several thousand years. An oceanographic change about 8500 years ago is indicated by a Melonis zaandamae zone in northern and central shelf cores collected from 200–800 m water depth. The presence of M. zaandamae in the north and its absence in the south suggest warmer and more saline postglacial water in northern Baffin Bay, whose influence became diluted with cooler coastal meltwater as the current flowed south along the Baffin Island Shelf. Sediments after 6000 years ago on the northern and central shelf are dominated by agglutinated foraminifera, suggesting dis...

Adriatic deep water formation during the Holocene: Implication for the reoxygenation of the deep eastern Mediterranean Sea

Abstract: Under modern conditions, the reoxygenation of eastern Mediterranean deep water is the result of the formation of dense bottom water in the southern Adriatic Sea. Oxygen isotopic records of planktonic foraminfera used as a paleosalinity tracer show that Po River and/or the alpine glaciers meltwater are not directly responsible of the inhibition of the deep water formation in the Adriatic, during sapropel (S1) formation at 9,000 years B.P.. Sedimentological evidence indicates that fully oxygenated conditions in the deep Adriatic occurred only by 1,650±100 years B.P., a long time after the total reoxygenation of the deep eastern Mediterranean. Organic carbon content and isotopic ratio indicate that persistence of a reduced sediment phase until 1,650 years B.P., does not result from increased primary productivity or large input of terrestrial organic matter.

Sensitivity of climate and atmospheric CO2 to deep-ocean and shallow-ocean carbonate burial

Abstract: A model of the carbonate-silicate geochemical cycle is presented that distinguishes carbonate masses produced by shallow-ocean and deep-ocean carbonate burial and shows that reasonable increases in deep-ocean burial could produce substantial warmings over a few hundred million years. The model includes exchanges between crust and mantle; transients from burial shifts are found to be sensitive to the fraction of nondegassed carbonates subducted into the mantle. Without the habitation of the open ocean by plankton such as foraminifera and coccolithophores, today's climate would be substantially colder.

Planktonic foraminiferal distribution record productivity cycles: evidence from the Aptian‐Albian Piobbico core (central Italy)

Abstract: The Aptian-Albian ‘Scisti a Fucoidi’ varicoloured pelagic sediments in central Italy, show a ‘couplet’ alternation of carbonate-rich/carbonate-poor layers, which are interpreted as the sedimentary expression of precession (frequency 19–23 kyr). Carbonate content, chromatic variation, and planktonic foraminiferal abundance were analysed at a 1-cm spacing for a 10-m interval of the Piobbico core, specifically drilled through this formation. Spectral analysis of these parameters shows a prominent signal equated to the c. 100 kyr cycle of orbital eccentricity at a sedimentation rate of 5 mm kyr−1. The coherency of the spectral response of each parameter suggests that a single mechanism controlled the whole sedimentary record. Detailed study of planktonic foraminiferal distribution of the same section at 1-mm scale resolves the Milankovitch frequencies of 41 kyr and 18 to 23 kyr, equated with the obliquity and precessional cycles. But foraminiferal abundance is not in phase with carbonate content, which was largely controlled by calcareous nannofossils, but peaked at intermediate carbonate values. The proposed model for explaining the discrepancy at the precessional level is that foraminifera thrived at intermediate values of the precession index, when the environment was only moderately fertile but stable, while during highs of the precession index, mixing of the water column increased fertility and caused calcareous nannofossil blooms and restriction of planktonic foraminifera to few and tolerant species. The resulting bimodality of foraminiferal abundance per precessional cycle appears to be recorded in the spectrum by peaks at the 11 and 14 kyr levels. Cross correlation of foraminiferal abundances with the calcium carbonate curve over 1–2 Myr intervals produces discrepant results (apparent phase lags) which we attribute to differences in the response to the fundamental eccentricity cycles.

New constraints on early Tertiary palaeoproductivity from carbon isotopes in foraminifera

Abstract: ONE of the most pronounced features of marine sections at the Cretaceous/Tertiary (K/T) boundary is the negative excursion in biogenic carbonate δ13C in the early Tertiary carbon isotope record1–9. The coincidence of this excursion with the elimination of many marine plankton species supports predictions that large-scale reduction of primary productivity in the oceans would disrupt the biological carbon pump, resulting in a diminished surf ace-to-deep δ13C gradient10 (Δδ13C). Carbonate accumulation rates and other geochemical indices support the idea that the oceans were less productive in the early Palaeocene9. Here we present carbon isotope analyses of individual species of benthic and planktonic foraminifera spanning the K/T boundary in OOP Hole 690C (Weddell Sea, Antarctica) which demonstrate that primary productivity following the K/T extinctions may not have diminished to the extent or for the duration indicated by previous isotope studies. The magnitude of the δ13C gradient change is dependent on the species analysed, reflecting differing depths of calcification, seasonal contrasts and/or disequilibrium effects. The lack of any change in benthic δ13C at the K/T boundary indicates that the Antarctic may not have been a significant source of deep water during the early Palaeocene, contrary to previous suggestion11–13.

Reasons for Spatial Microdistributions of Foraminifers in an Intertidal Pool (Northern Adriatic Sea)

Abstract: . The spatial microdistribution of foraminifers was tested by the method of quadratic samplings on 2 sample grids in an intertidal pool of the northernmost Adriatic sea. Abundant species of foraminifers exhibit patchy distributions throughout; the distribution-patterns of some species correspond nearly completely. Using statistical methods (regression- and correlation-analyses) correlations to abiotic and biotic factors (water depth, exposure rate, seagrass, benthonic algae) were ascertained. The significant correlations of the foraminiferal frequencies to blue-green algae and/or diatoms enable an interpretation of these connexions as food dependences. Several foraminiferal species seem to have specific diets.