G. P. Lohmann

The environmental and climatic distribution of dinoflagellate cysts in modern marine sediments from regions in the North and South Atlantic Oceans and adjacent seas

Abstract Quantitative treatments of 168 dinoflagellate cyst assemblages from modern marine sediments were used to decipher the salient environmental and climatic features of the distribution of common living cyst-based taxa. Surface sediment samples were studied by routine palynological methods from estuarine, continental shelf, slope and rise zones, and abyssal plains between latitudes 62°N and 27°S within fourteen geographic regions of the North and South Atlantic Oceans, the Caribbean and Mediterranean Seas and from one region in the southeastern Pacific Ocean near Peru, to meet this general objective. The results were expressed as percentages of species present and statistically analyzed by multivariate Q -mode factor analysis, cluster analysis and by calculation of a diversity index, using pre-existing computerized routines. Inshore to offshore and latitudinal (climatic) variations in distribution were identified and they involved individual taxa, associations of species, species diversity and specimen densities (cysts per gram of sediment). They were related empirically to changing surface water environments above and this suggested that biologic-ecologic phenomena, which involve species-water type relationships for different taxa, combined with hydrodynamic (current) systems, are the most important factors that control cyst distribution in the bottom thanatocoenosis. However, geologic factors such as recycling of older specimens and outcroppings of relict Pleistocene-Early Holocene sediments exert their influence too. An ecologic classification for extant cyst-based species is proposed in the format of classical “plankton elements”. This format is dictated by the existence of dual trends (environmental and climatic) in cyst distribution. It is suggested that genesis of ecologic species-groups of this nature during evolution can be interpreted by or “predicted” from stability-predictability concepts as developed in contemporary ecologic work, if surface water masses are considered to represent unique hydrographic climates with innate stability and predictability characteristics. This concept of “hydroclimatic stability” is used to identify fossilizable living dinoflagellates as being environmentally adapted to unstable conditions around continental margins and comparable shallow-water environments around oceanic islands. In addition there is a tendency for the more stenotopic extant cyst-based taxa to be adaptively specialized towards more stable sectors of nearshore and offshore plankton environments which develop at the terminal and subterminal ends of temperature and salinity regimes or profiles. One taxonomic change is introduced: Planinosphaeridium choanum (Reid 1974) nov. comb. is proposed from the basionym Ataxiodinium choanum Reid 1974.

A model for variation in the chemistry of planktonic foraminifera due to secondary calcification and selective dissolution

A quantitative model for describing the within-population variation in planktonic foraminifer shell chemistry that results from secondary calcification and selective dissolution is presented. The objective is to construct a basis for inferring the chemistries of different shell components and estimating the extent of shell dissolution. Variation is modeled as mixtures of two kinds of shell calcite, a primary calcite that forms the chambers and a secondary calcite that forms crust. Bulk shell chemistries are intermediate between the chemistries of these calcites and lie on mixing lines between them. For two component systems, mass balance relationships can be reformulated as a linear regression and solved for the chemistries of the primary and secondary calcites and for the uncertainties associated with these estimates. To apply this model, one needs measurements of bulk shell chemistries and estimates of the relative proportions of secondary and primary calcites. For most planktonic foraminifer species the proportion of secondary calcite can be estimated from differences in the relationship between shell size and mass before and after crusting. Preliminary results are consistent with previous work showing that secondary calcites are added at depth. However, even deep-dwelling species appear to grow most of their primary shell in surface waters and some surface-dwelling species add secondary calcite in the deep ocean. In contrast to the models simple description of secondary calcification, the variation in chemistry from selective dissolution is more complicated because undissolved shells are themselves mixtures of primary and secondary calcites and therefore present a wide range of initial shell compositions. Nevertheless, the model allows both the compositions of different components to be inferred and the amount of dissolution to be estimated. Preliminary results indicate that dissolution of planktonic foraminifera is apparent nearly 2 km above the foraminifer lysocline and even apparently well-preserved shells may be over 50% dissolved.

Incorporation and preservation of Mg in Globigerinoides sacculifer: implications for reconstructing the temperature and 18O/16O of seawater

Using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution, we examined the processes which affect the relationship between foraminiferal Mg/Ca and δ18O. We found that Globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their Mg content. On the seafloor, foraminiferal Mg/Ca is substantially altered by dissolution with the degree of alteration increasing with water depth. Selective dissolution of the chamber calcite, formed in surface waters, shifts the shells bulk Mg/Ca and δ18O toward the chemistries of the secondary crust acquired in colder thermocline waters. The magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved. In spite of this shift the initial relationship between Mg/Ca and δ18O, determined by their temperature dependence, is maintained. We conclude that paired measurements of δ18O and Mg/Ca can be used for reconstructing δ18Owater, though care must be taken to determine where in the water column the reconstruction applies.

Early Cenozoic calcareous nannoplankton biogeography of the Atlantic Ocean

Biogeographic patterns of Early Cenozoic calcareous nannoplankton assemblages are delineated for the North and South Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Nannoplankton assemblages are defined byQ-mode Varimax Factor and Oblique Factor Analyses of census data on 44 taxa from 113 deep-sea and land-based samples. Examination of their latitudinal distribution through time allows recognition of those assemblages which can be used as environmental indicators. Comparison of the distributions of contemporaneous nannoplankton assemblages with the distribution of the appropriate environmental indicator assemblage permits their classification as either low-, mid-, or high-latitude nannoflora. Early Paleocene is characterized by a high-latitude Thoracosphaerid-Markalius astroporus Assemblage and a mid- to low-latitude Braarudosphaerid Assemblage. Eight Middle Paleocene-Early Eocene nannoplankton assemblages are identified and grouped according to their relative environmental distribution: (1) Low-latitude nannoflora: theToweius craticulus-Coccolithus pelagicus Assemblage, the Discoaster-Cyclococcolithus formosus Assemblage, theToweius craticulus-Ericsonia subpertusa-Discoaster Assemblage, and the Fasciculith-Discoaster Assemblage. (2) Mid-latitude nannoflora: theEricsonia subpertusa Assemblage and theCoccolithus pelagicus Assemblage. (3) High-latitude nannoflora: thePrinsius martinii Assemblage and theP. bisulcus Assemblage. These groupings are indicated by comparison of the distribution of our Paleocene-Early Eocene environmental indicator, the high-latitudePrinsius martinii Assemblage, with the distributions of contemporaneous assemblages. Seven Eocene nannoplankton assemblages are identified: (1) Low-latitude nannoflora: the Reticulofenestrid Assemblage and theCyclococcolithus formosus-Sphenolith Assemblage. (2) Mid-latitude nannoflora: the Discoaster Assemblage, theReticulofenestra umbilica-R. bisecta-Coccolithus pelagicus Assemblage, and theCribrocentrum reticulatum Assemblage. (3) High-latitude nannoflora: theToweius craticulus-Coccolithus pelagicus Assemblage, and theC. pelagicus-Cyclococcolithus formosus-C. aff.gammation Assemblage. Our Eocene environmental indicator is the low-latitude Reticulofenestrid Assemblage. Five Oligocene nannoplankton assemblages are identified: (1) Low-latitude nannoflora: the Sphenolith-Discoaster Assemblage. (2) Mid-latitude nannoflora: theCyclococcolithus neogammation Assemblage and theDictyococcites hesslandii Assemblage. (3) High-latitude nannoflora: the Reticulofenestrid-R. bisecta Assemblage and theCoccolithus pelagicus Assemblages. Our Oligocene environmental indicator is the low-latitude Sphenolith-Discoaster Assemblage. If it is assumed (1) that the latitudinal differentiation of calcareous nannoplankton assemblages we observe in the Early Cenozoic is related to a latitudinal temperature gradient, and (2) that the ecological preferences of these assemblages remain stable through time, then the latitudinal nannofloral migrations we recognize delineate paleotemperature changes: The maximum equatorward migration (cooling) of high- and mid-latitude nannofloras in the Paleocene occurs at about 58 m.y. B.P. This is followed by their poleward migration, the disappearance of the high-latitude nannoflora, and the appearance of a new low-latitude nannoflora. A major poleward migration (warming) occurs at about 49 m.y. B.P. The Middle Eocene is characterized by the return of high-latitude nannofloras into mid-latitudes, with the maximum equatorward migration (cooling) occurring at 48-43 m.y. B.P. Low-latitude nannoflora again gradually invade high latitudes through the Late Eocene, indicating a second major Eocene warming by at least 38 m.y. B.P. There is a well-defined migration of high-latitude nannoflora into the mid-latitudes during the Middle Oligocene, with the maximum indicated cooling between 32 and 27 m.y. B.P.; an earlier, though minor, Oligocene cooling may have occurred at about 36 m.y. B.P. During the Late Oligocene, low-latitude nannoflora migrate into high latitudes, indicating a warming by 26 m.y. B.P. These inferred paleotemperature changes are similar to those delineated by some workers on the basis of terrestrial flora. Nannoflora characterized by the cool-water coccolithCoccolithus pelagicus predominate at the equator throughout most of the Paleocene, are confined mostly to high latitudes in the Eocene, and generally remain above mid-latitudes during the Oligocene. Although this migration could indicate that the Paleocene was the coolest epoch in the Early Cenozoic, and the Oligocene the warmest, we suggest instead that this shift ofC. pelagicus to higher latitudes is a result of evolution in its ecology. The implications of paleobiogeography to high-latitude biostratigraphy are discussed and theacme horizon as a time-stratigraphic concept is informally introduced. The following new taxa are described:Fasciculithus rotundus, Neochiastozygus imbriei andThoracosphaera atlantica. The following taxa are recombined:Cyclococcolithus protoannulus, Cyclolithella bramlettei Dictyococcites hesslandii, andNannotetrina alata.

Oceanographic significance of Pacific late miocene calcareous nannoplankton

Abstract An analysis of the variability in the composition and distribution of Pacific Late Miocene calcareous nannoplankton about their average biogeography shows that there are primarily two environmental factors causing that variability, climate and dissolution. Climate produces a latitudinal, biogeographic differentiation of the Late Miocene nannoflora, while selective dissolution superimposes a bathymetric differentiation of the nannoflora on that due to climate. Together, these two factors produce three distinct Late Miocene nannofloral assemblages, a high-latitude, temperate assemblage characterized by Reticulofenestra pseudoumbilica and Coccolithus pelagicus , and two tropical assemblages, their differences in composition depending on water depth and surface-water productivity: (1) in shallower water and beneath areas of higher organic production and sedimentation of calcite there is an undissolved assemblage characterized by sphenoliths, small elliptical placoliths and Coccolithus pataecus ; (2) in deeper water and areas of lower productivity there is a dissolved assemblage dominated by discoasters. Selective dissolution produces most of the apparent biogeographic variation in Pacific Late Miocene nannoplankton compositions, the variation in compositions observed between the seventeen sites studied. Dissolution preferentially removes the more soluble constituents of the tropical nannoflora so that increasing dissolution tends to give tropical nannoflora a cooler, more temperate aspect. At the same time, selective dissolution shifts the composition of the warmer, tropical component towards its more resistant taxa. Nannoplankton records show a period of greatly decreased calcite dissolution in deep tropical and temperate South Pacific sites between about 8 and 10 m.y. ago. This decrease is strongly correlated with a temporary increase in the 13 C composition of Pacific deep waters. Calcite dissolution increased during this same period in the deep North Pacific. Nannoplankton records of Late Miocene climate in the tropics are distinctly different from those at higher, south temperate latitudes. Tropical records show a sharp warming in the earliest Late Miocene after a generally cool late Middle Miocene. This was followed by a temporary cooling, nearly to Middle Miocene levels, about 7 m.y. ago. Toward the end of the Late Miocene, the tropical Pacific warmed again and remained warm into the Pliocene. Warming of temperate climates occurred much later. Not until latest Miocene did the southern the Pliocene. Warming of temperate climates occurred much later. Not until latest Miocene did the southern temperate latitudes warm appreciably. Southern subpolar climate cooled continuously through the Late Miocene. We attribute the resulting increases in the latitudinal climatic contrast across the southern Pacific Ocean to the development and migration of a strong subtropical convergence. On the basis of the nannoplankton oceanographic records we postulate that beginning about 10.5 m.y. ago Pacific surface circulation became primarily zonal and the production of deep and bottom waters in the Southern Ocean increased sharply. This produced a northward decrease in calcite preservation, an increase in benthic 13 C, and a strong climatic gradient across southern latitudes. The period of most vigorous deep Pacific circulation ended 7 m.y. ago in response, we speculate, to the reduced ocean salinities during the Messinian.

Increasing seasonal upwelling in the subtropical South Atlantic over the past 700,000 yrs: Evidence from deep-living planktonic foraminifera

Abstract The distribution and character of deep-living planktonic foraminifera (such asGloborotalia truncatulinoides andG. hirsuta) are particularly sensitive to the structure of the upper ocean because of the wide range of water depths over which their life cycles extend. Like other planktonic foraminifera, the deep-living species begin growing in shallow surface waters, but unlike the other species they continue growing while sinking into much deeper water. Their reproduction is synchronized with the seasonal deep-mixing and upwelling of nutrient-rich waters that precedes spring bloom. Where deep-mixing is curtailed the abundances of the deeper-dwelling species (G. hirsuta), the deeper-dwelling varieties (e.g., left-coilingG. truncatulinoides), and the larger (presumably faster-sinking) shells are reduced (leading to an apparent acceleration in shell development). These changes are observed today in transects from the seasonally deep-mixed subtropical gyres to the seasonally-stable surface waters equatorward of the subtropical convergences. Past changes in the abundance and character of these deep-living planktonic foraminifera have been measured in a Late Pleistocene record from the subtropical western South Atlantic Ocean. Over the past 700 kyrs, abundances of these species have increased,G. truncatulinoides has changed from the predominantly shallower-dwelling right-coiling to the deeper-dwelling left-coiling variety, and the accelerated shell development characteristic of shallow mixing has shifted toward the rates normally associated with deep mixing. All this indicates that the structure of the upper ocean in the subtropical South Atlantic before 700 kyrs ago was more like that nearer the equator today where mixing is limited to the upper 100–200 m. Since that time the scale of mixing has gradually increased to more than 400 m. This implies a gradual increase through the Late Pleistocene in the seasonal upwelling of nutrients in the subtropical South Atlantic and, consequently, in the intensity of the seasonal maximum in organic productivity.