Nancy Healy-Williams

Fourier shape analysis of Globorotalia truncatulinoides from late Quaternary sediments in the southern Indian Ocean

Abstract Fourier shape analysis has been used to determine the morphological variation within the planktonic foraminifera, Globorotalia truncatulinoides from the southern Indian Ocean. Measured specimens are from 20 Recent core top samples distributed from 22°S (central gyre water mass) to 47°S (subantarctic water mass). Fourier analysis determined that the two most significant components of shape within this species are elongateness and conicalness which are characterized by harmonics 2 and 3, respectively. Mean harmonic 2 values are significantly correlated with the percentage of sinistral coiled individuals in each sample ( r = +0.91). Mean values for harmonic 3 have a correlation coefficient with water temperature and salinity at 200 m of r = +0.85 and +0.88. Specimens become less elongate and more conical in a traverse from south to north. Fourier shape analysis of planktonic foraminifera was then shown to be useful in investigations of late Quaternary paleoceanographic problems. Four piston cores were examined from the southern Indian Ocean. Total faunal and oxygen isotope analyses provided the stratigraphic framework. The results indicated that the central gyre water mass remained relatively stable during glacial stages. The region of the Subtropical Convergence was strongly affected, however, by the northward migration of the Polar Front.

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.

Dissolution and water-mass patterns in the Southeast Indian Ocean, Part I: Evidence from Recent to late Holocene foraminiferal assemblages

A quantitative study of the planktonic foraminiferal fauna and various related parameters in more than 100 surface sediment samples from the Southeast Indian Ocean has identified the major relationships which can be utilized to reconstruct past changes in water-mass boundaries and degree of CaCO 3 dissolution. The samples represent Recent to late Holocene sedimentation in water depths from Ntogloboquadrina pachyderma with increasing latitude. First-order dissolution effects related to increased water depth produce (a) increases in test fragmentation and benthic foraminiferal abundance and (b) decreases in coarse fraction >63µm and planktonic foraminiferal abundance. A schematic model of dissolution and productivity changes in high latitudes of the Southeast Indian Ocean shows (1) that the percentage of N. pachyderma is the least susceptible to dissolution effects, (2) faunal dissolution effects are most evident in regions of high diversity gradients such as the Polar Front and Subtropical Convergence, and (3) that the numbers of benthic foraminifera and of foraminiferal fragments provide the best indicators of spatial and temporal variations in dissolution intensity in the Southern Ocean.

Dissolution and water-mass patterns in the Southeast Indian Ocean, Part II: The Pleistocene record from Brunhes to Matuyama age sediments

Calcium carbonate dissolution patterns were determined for portions of the past 2.5 m.y. in 9 Eltanin piston cores distributed along the Southeast Indian Ridge in the Southeast Indian Ocean. Oxygen isotope and foraminiferal faunal records were used to determine the relationship between paleoclimate water-mass changes and the dissolution intervals. CaCO 3 dissolution was generally more intense during glacial periods, but dissolution pulses of short duration also occur during several interglacial periods, particularly on the shallower portions of the Southeast Indian Ridge. The glacially related increases in dissolution are in-phase with northern migrations of the Polar Front. During the past 250,000 yr, the Polar Front has migrated to 45°S latitude in the region south of Australia and as far north as 40°S in the mid-southern Indian Ocean during the same time period. From the mid-Brunhes chron to the present, an over-all increase in CaCO 3 content indicates a gradual deepening of the carbonate lysocline. Unlike the Brunhes chron, the carbonate fluctuations in the Matuyama age cores are due mainly to productivity variations as a function of water-mass migrations, and are of lower magnitude than are the Brunhes fluctuations. The magnitude of the oxygen isotopic fluctuations in the Matuyama chron is also smaller than it was during the Brunhes chron, suggesting less climatic variability.

Morphological changes in living foraminifera and the thermal structure of the water column, western North Atlantic

Changes in the morphological characteristics of living planktonic foraminiferal species, Globorotalia inflata and Globorotalia hirsuta, reflect the thermal structure of the upper water column in the western North Atlantic. Described using Fourier series analysis, morphologic variation in these species proves useful in extracting paleoceanographic information from fossil foraminiferal assemblages. For example, a well defined thermocline is accompanied by a significant decrease in elongation and quadrateness in G. inflata tests. Changes in the test shape of G. hirsuta are minimal in an area where the thermocline is poorly developed and water temperatures change gradually over a large water depth range (20over 800 meters). The observed shape changes in G. inflata with depth indicate that this globorotalid species adds an additional chamber in the final whorl with depth, dependent on the nature of the thermal stratification. This indication is corroborated by the fact that approximately 25% of the G. inflata specimens from below the thermocline have thin transparent fourth chambers in the final whorl compared to only 2% in waters above the thermocline. The results of this study further suggest that morphological changes observed in fossil populations of globorotalid species are to some degree the result of shape variation in the water column due to the nature of the thermal structure. The shape characteristics of fossil globorotalid foraminifera may possibly be used to determine the past hydrographic conditions of a region.

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.

Stable isotope gradients in modern benthic foraminifera of the Vema Channel, South Atlantic

Abstract Oxygen and carbon isotopic analyses of benthic foraminifera from core top sediment samples were made to determine the extent to which the benthic foraminiferal species, Planulina wuellerstorfi, accurately records the isotopic signatures of the water masses in the Vema Channel—Rio Grande Rise region. The δ18O of P. wuellerstorfi to parallel the present day vertical δ18O gradient within the channel. The δ 13C of P. wuellerstorfi appears to accurately reflect the present δ 13 C gradient of total dissolved CO2 in the channel and thereby provides a potential monitor of past changes in thermohaline circulation processes.

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 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------------