Anthony C. Gary

Taphonomic process and species microhabitats in the living to fossil assemblage transition of deeper water benthic foraminifera

A detailed analysis was made of living and death assemblage abundances of benthic foraminiferal species in the top 10 cm of three high quality box cores from the Gulf of Mexico (water depth=1170, 1050, and 1020 m). These data, combined with sedimentological studies and analysis of the biotic activity in the sediments (using Pb-210 profiles), were used to examine the processes controlling the transformation of living to fossil assemblages. These results have implications for the paleoceanic interpretation of benthic foraminiferal assemblage and isotopic data

Shape and isotopic differences between conspecific foraminiferal morphotypes and resolution of paleoceanographic events

The principal axiom of isotope paleoceanography is that the isotopic behavior of the foraminiferal species being used to provide proxy paleoenvironmental information is consistent throughout the geographic and stratigraphic range of the taxon. We present evidence that populations of Uvigerina peregrina, Globorotalia truncatulinoides, Neogloboquadrina pachyderma, and species of the Sphaeroidinellopsis lineage are composed of complex mixtures of morphological types (morphotypes). Many of the conspecific morphotypes exhibit a different response with respect to their δ18O and δ13C compositions. Morphotypes are defined by a combination of image analysis, extended cabfac/extended qmodel vector analysis and isotopic analysis. In many cases, morphotypes of specimens of the same size and from the same sample exhibit significant oxygen and/or carbon isotopic differences. This “morphotype effect” may be an important contributor to intraspecific isotope variability among foraminiferal species and to the observed magnitude of isotopic changes associated with paleocenographic events. Such morphotypic-isotopic differences may have significant consequences for the interpretation of isotope paleoceanographic records.

Time/depth distribution of late Quaternary Uvigerina peregrina, North American continental margin; morphological and paleoceanographic implications

Uvigerina peregrina from the continental slope of the eastern United States exhibits morphological and distributional evidence for distinct populations at water depths from about 700 to 5000 m since 25, 000 years before present (YBP). During the glacial maximum (18, 000 YBP) this species occupied water depths from 2400 to 4350 meters. Beginning 16, 000 YBP, U. peregrina diverged into two populations. The main population rangedfrom 3000 to 4350 m water depth, contracted during late glacial and early Holocene time, and finally vanished from a depth of 3600 m at 8000 YBP. An isolated portion of the glacial age population survived between 2100 and 2300 m and connected the late glacial and late Holocene populations of U. peregrina. The modern population of U. peregrina first appeared about 4000 YBP and is confined to water depths of 900 to 3000 m with its acme between 1600 and 2500 meters. Fourier shape analysis shows that the modern and glacial age populations of U. peregrina are morphologically distinct. The observed morphological and distributional differences indicate that the modern and glacial-age specimens represent discrete populations rather than an upslope migration of a single population. Water masses of the western North Atlantic have significant control on the spatial and temporal distribution of this species. Paleobathymetric and paleoceanographic reconstructions using U. peregrina should consider the population affinity, especially in the western North Atlantic.

UNMIXING FORAMINIFERAL ASSEMBLAGES: POLYTOPIC VECTOR ANALYSIS APPLIED TO YAKATAGA FORMATION SEQUENCES IN THE OFFSHORE GULF OF ALASKA

Abstract The Yakataga Formation contains mudstones, sandstones, and diamictites that represent fluctuating marine and glaciomarine deposition in the Gulf of Alaska since the latest Miocene. These rocks contain distinctive benthic foraminiferal assemblages that provide insight into the depositional, structural, and climatic history of the northeastern Pacific Ocean. Qualitative analyses of benthic foraminiferal distribution patterns have been used previously to define eight stratigraphic sequences in the offshore Yakataga Formation. Results of multivariate quantitative methods are presented here to provide additional insight into the nature of foraminiferal faunas and to demonstrate the potential for using quantitative methods in sequence biostratigraphy. Cluster and polytopic vector analyses of 158 samples from the offshore Yakataga Formation indicate distinct mixtures of foraminiferal faunas within the samples. Polytopic vector analysis suggests that samples contain mixtures of seven end members representing the neritic through bathyal environments: EM1: Elphidium excavatum clavatum biofacies (inner neritic); EM2: Epistominella pacifica biofacies (upper bathyal and middle bathyal); EM3: sharp-margined Cassidulina spp. biofacies (outer neritic); EM4: round-margined Cassidulina spp. biofacies (outer neritic); EM5: Cassidulina californica biofacies (outer neritic); EM6: Uvigerina ex gr. peregrina biofacies (upper to middle bathyal); and EM7: Haplophragmoides spp. biofacies (outer neritic through upper bathyal). Identification of several outer neritic biofacies indicates heterogeneity within this environment, probably due to differences in substrates and nutrient levels. Mixing of biofacies is primarily due to downslope transport of faunas across the shelf and slope and may also be the result of constriction of environmental zones due to advances of the tidewater glacier margins across the shelf.

Quantitative paleobathymetry using oxygen isotopes and shape changes in benthic foraminifera

Accurate estimates of paleodepth are of critical importance to oil exploration in determining environment of deposition and geologic history. Models based on the test shape and the /sup 18/O//sup 16/O ratio in benthic foraminifera from the northwestern Gulf of Mexico indicate that a resolution of +/- 75 ft can be achieved in paleobathymetric reconstructions. The proportion of /sup 18/O and /sup 16/O incorporated into the tests of benthic foraminifera varies with bottom water temperature in a predictable manner. This depth/temperature relationship is the result of the temperature dependence of oxygen isotopic fractionation between sea water and calcium carbonate, and it allows the tests of benthic foraminifera to be used as indicators of paleotemperature. Since subbottom water temperatures on the outer shelf and slope decrease systematically with increasing water depth, these paleotemperatures can be used to reconstruct paleobathymetric trends. Paleobathymetric interpretations can also be independently inferred from Fourier shape analysis of benthic foraminiferal species. Combining the oxygen isotope and shape relationships relative to water depth increases the resolution of paleobathymetric reconstructions and provides an independent check on interpretations based on faunal assemblages and sedimentological data. These paleodepth models should allow extinct taxa to be used for paleobathymetric reconstructions as well.

Relationship Between Test Morphology and Bathymetry in Recent Bolivina albatrossi Cushman, Northwestern Gulf of Mexico: ABSTRACT

An automated video digitizer and closed-form Fourier series analysis were used in this study to quantify benthic foraminiferal test morphology. This approach allows morphology to be described rapidly, objectively, and accurately and to be compared statistically to important environmental variables. Canonical discriminant analysis reduced the variable dimensions and revealed specific shape components relatable to bathymetry in Holocene specimens of Bolivina albatrossi Cushman from the northwestern Gulf of Mexico. Populations of B. albatrossi exhibit reduced test triangularity and increased surface sculpture relief with increasing water depth. Margin lobateness decreases with increasing depth. A depth classification algorithm using Fourier harmonic amplitudes divides the ba hyal zone into four subzones. With further development this approach could allow paleoenvironmental reconstructions to be automated and quantified. End_of_Article - Last_Page 1422------------

Relationship of Morphologic Variation and Environment in Recent Bolivina (Foraminiferida) from Northwestern Gulf of Mexico: ABSTRACT

This study investigates intraspecific morphological variation trends of recent benthic foraminifera in relation to depth and environmental factors (temperature and salinity). Twelve samples were obtained from traverses trending south-southeast from offshore Galveston, Texas, to the Sigsbee Deep. Samples are cuts of short cores (30-40 cm, 12-16 in.) from 33-3,431 m (108-11,257 ft) in depth. Fifty specimens from the total population (all growth stages) and 30 specimens of a specific growth stage were randomly selected for each of 4 foraminifera species, Bolivina albatrossi, B. lowmani, B. subspinescens, and B. ordinaria, from each sample site. Test outline, as an indicator of overall shape, was quantified by an automated video digitizer using closed-form Fourier series anal ses. Significant variations in shape outline components were tested for their relationships to bathymetry and environmental factors using analysis of variance and multiple discriminant analysis for both total populations and the specific growth stage. Morphologic trends relatable to both depth and environmental variables are recognizable in the 2 data sets. Such trends may be observable in the fossil record, thus indicating relative depths and suggesting possible absolute depth and values of environmental variables. This morphologic approach may be utilized even though different species are incorporated in analyses and different absolute depths involved. End_of_Article - Last_Page 479------------