MICROFOSSILS AND THE OCEAN HISTORY.

Forams from deep-sea. Credit: Miriam Katz, Rensselaer Polytechnic Institute. (Originally published by Micropress.)

Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments.

The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contains clues about the temperatures in which they lived.

Following this pioneering work, Schott working on sediments of the METEOR Expedition (1925-1927) introduced quantitative counting of species within the fossil assemblages on the sea floor and realized that surface water temperature changed as the climate fluctuated between glacial and interglacial conditions.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

In 1955, Emiliani, who was then a student of Harold Urey at the University of Chicago,  published a paper entitled “Pleistocene temperatures” where introduced isotope stratigraphy to paleoceanography. He used the density of a heavy oxygen isotope in planktonic foraminifera from deep sea cores to outline oxygen isotope stages for the Quaternary, believing these would reflect surface temperature changes and the ice volume changes.  He concluded that the last glacial cycled had ended about 16,000 years ago, and found that temperature increased steadily between that time and about 6000 years ago. Many of Emiliani’s findings are still valid today, however in 1970 several improvements to Emiliani’s work were made, such as a revision of the temperature scale.

Oxygen isotope records have also been obtained from well-preserved microfossil materials in the Late Cretaceous  when bottom waters appear to have been much warmer than at present.

This concepts of paleotemperature reconstruction, as first developed for planktic foraminifera, apply to other groups of microfossils. Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antartic Oceans, especially where calcareous fossils are less abundant. Diatom assemblage are also used in reconstructions of paleoproductivity.

Climatic modes and sea-level fluctuations indicated by calcareous nannofossils of the Oligocene deposits from the Romanian Carpathians. (Melinte, 2004)

The calcareous nannoplankton represents good proxy for the sea-level fluctuations. The group exhibit  a clear latitudinal distribution pattern, for instance, the presence of mixed nannofloral assemblages (taxa of low-middle latitudes together with high ones) are indicative of the sea-level rise,  while endemic assemblages characterize periods of low sea-level.

By studying cores from those ocean sediments, its possible determine the ages of the rocks, the ocean environment and some atmospheric conditions using the information  provided by the microfossils present in that core, as well as stable isotope analysis and magnetic stratigraphy.

Each layer of the core recorded the geological history of the ocean basins, changing climates, evolving biota and the events that could altered the course of Earth history.

References:

Armstrong, Howard A. and Martin D. Brasier.  Microfossils.  Blackwell Publishing, 2005.

Berger, W. H., Sea level in the late Quaternary: patterns of variation and implications, Int J Earth Sci (Geol Rundsch) (2008) 97:1143–1150

Pollen analysis and the science of climate change.

Example of fossil pollen grains from Eocene of Huadian , Jilin, China.

Pollen grains are the carriers of the male gametes or their progenitor cell, in higher plants. They also are important tools for paleoclimatic reconstruction.  They reflects the ecology of their parent plants and their habitats and provide a continuous record of their evolutionary history.

Like spores, pollen grains possess a wall highly resistant to microbial attack. This wall comprises two layers,  the outer, highly resistant exine mostly composed by sporopollenin, a biopolymer considered “the most resistant organic material known”, and the inner intine that surrounds the cytoplasm. The morphology of pollen grains is diverse. Gymnosperms pollen often is saccate (grains with two or three air sacs attached to the central body), while Angiosperm pollen shows more variation and covers a multitude of combinations of features: they could be  in groups of four (tetrads),  in pairs (dyads),  or single (monads). The individual grains can be inaperturate, or have one or more pores, or slit-like apertures or colpi (monocolpate, tricolpate).

Arabis pollen has three colpi and prominent surface structure. From Wikimedia Commons

Pollen grains  could enter into the fossil record by falling directly into swamps or lakes, or being washed into them or into the rivers and seas. The ones which are not buried in reducing sediments will tend to become oxidized and be destroyed.

They are filtered by differential dispersal in the air and in the water. For instance, large miospores, pollen grains and megaspores will tend to settle out in rivers, estuaries, deltas or shallow shelf areas, whereas small miospores and pollen grains may settle out in outer shelf and oceanic conditions. The differences in pollen productivity and dispersion rates pose a significant problem for palaeoclimatic reconstruction because the relative abundances of pollen grains in a deposit cannot be directly interpreted in terms of species abundance in the study area. Another difficult is that spores and pollen may suffer several cycles of reworking and redeposition, leading to some confusion in the fossil record.

Scanning electron microscope image of different types of pollen grains. Image from Wikimedia Commons

Gunnar Erdtman, a Swedish botanist, published  in 1921,  his thesis about pollen as a tool for study the Quaternary vegetation and climate change. He was the first to suggest this application for fossil pollen.

Pollen analysis involves the quantitative examination of spores and pollen at successive horizons through a core, specially in lake, marsh or delta sediments, especially in Quaternary sediments where the parent plants are well known. This provide information on regional changes in vegetation through time, and it’s also a valuable tool for archaeologists because gives clues about man’s early environment and his effect upon it.

In a recent study, applied to the Crisis in the Late Bronce Age (LBA) in Cyprus and Syria, the pollen record reveals the presence of plants adapted to drier weather, which indicates a decrease in rainfall. Researchers suggest that this drought lasted about three hundred years causing crop failures, dearth and famine, and forcing regional human migrations at the end of the LBA in the Eastern Mediterranean and southwest Asia.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Traverse, A. (1988), Paleopalynology. Unwin Hyman

Kaniewski D, Van Campo E, Guiot J, Le Burel S, Otto T, et al. (2013), Environmental Roots of the Late Bronze Age Crisis. PLoS ONE 8(8): e71004.doi:10.1371/journal.pone.0071004

Mysterious fossils: Chitinozoans

Scanning electron micrograph of Sphaerochitina sp.(Late Silurian of Sweden). From Wikimedia Commons

Chitinozoans are marine organic-walled microfossils,  bottle-shaped, consisting of  hollow organic vesicles of uncertain affinity.  The vesicle ranges from 30 to 1500 μm, but most are 150–300 μm long. The outer wall of the vesicle may be smooth, striate, tuberculate, folded into hollow  spines or extended into a tubular sleeve. The group was named by Eisenack in 1931 who was the first to noted that the material of the wall was composed by a pseudochitinous material.

Chitinozoans can be found as single forms or joined together in chains. They first appeared in the Early Ordovician and became an abundant and diverse group, until the Silurian, and they extinct in the Early Carboniferous. They were exclusively marine and lived in a wide range of shelf environments.

Because of their rapid evolution and wide diversity of forms,  are useful  for local and global stratigraphical correlations.

Lagenochitina sp. (right) Conochitina sp. (left). From UCL.

Systematic position of the taxon still remains problematic.  The pseudochitin wall suggests animal affinities, but whether they are metazoan or protistan is still uncertain. In 1963, Kozlowski  proposed they were the eggs of annelid worms. The co-occurrence of chitinozoa with scolecodonts is a strong argument to his proposal.

Prosomatifera, longitudinal section.

The Chitinozoans are classified based on the structure of their opening and the overall shape of flask and neck. The Order Operculatifera contains one family, the Desmochitinidae, characterized by an operculum, reduced oral tubes and a relatively small subspherical vesicle. The Order Prosomatifera contains two families the Conochitinidae and the Lagenochitinidae, distinguished by the relationship between the chamber and the neck.

Armoricochitina nigerica, from late Ordovician Hirnantian glaciation.

Most  of chitinozoan genera appear to be cosmopolitan, others show latitudinal provinciality. A reconstruction of the paleobiogeographic distribution of chitinozoan before and after the Hirnantian glaciation (∼440 Ma) showed a pattern that revealed the position of ancient climate belts. This patterns look “very modern” and suggests that ancient carbon dioxide levels could not have been as high as previously thought, but were more modest, about five times current levels.

 

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Vandenbroucke, T.R.A., Armstrong, H.A., Williams, M., Paris, F., Zalasiewicz, J.A., Sabbe, K., Nolvak, J., Challands, T.J., Verniers, J. & Servais, T. 2010. Polar front shift and atmospheric CO2 during the glacial maximum of the Early Paleozoic Icehouse. PNAS doi/10.1073/pnas.1003220107.

AN INTRODUCTION TO FORAMINIFERA

The Foraminiferida are an important group of single celled protozoa.  There are an estimated 8,000 species living in the world’s oceans today, among  the sea floor and the marine plankton and comprise over 55% of Arctic biomass and over 90% of deep sea biomass.

The first ocurrence of forams is from the Early Cambrian and the range extend to the present day. A recent study suggest that the rise of the forams coincides with the demise of stromatolites. Those earliest forms had organic test walls or  simple agglutinated tubes and were benthics.

The Order Foraminiferida (informally foraminifera) possesses a shell (test) of different composition, and granuloreticulose pseudopodia (extensions of ectoplasm with grains o tiny particles of various composition).

Benthic foraminiferid in cross-section.

At the cellular level, the cytoplasm is differentiated into an outer layer of clear ectoplasm and an inner layer of darker endoplasm. The ectoplasm forms a mobile film around the test with numerous, granuloreticulose pseudopodia whose form is ever changing. The endoplasm often contains diatoms and dinoflagellates as symbionts.

Living species of foraminifera present various types of reproductive strategies, with alternation of sexual and asexual generation. The size range is from about 100 micrometers to almost 20 centimeters long. They also use a great variety of feeding mechanisms, as evidenced by the great variety of test morphologies that exhibit.

The test consists of one or multiple chambers interconnected by an opening, the foramen. The composition and structure of the test wall is very important for the classification of the group. There are three basic types of wall composition: organic, agglutinated and secreted calcium carbonate.

Allogromia laticollaris

The suborder Allogromiina comprises all the organic-walled forms, composed  by a proteinaceous mucopolysaccharide.

Textularia agglutinans

The suborder Textulariina encompasses forms with agglutinated tests composed of randomly accumulated grains or grains selected by specific gravity, shape or size.

Quinqueloculina seminulum.

Secreted test foraminifera are subdivided into three major groups. First, the suborder Fusulinina, with microgranular tests.  Second, the suborder Miliolina with porcelaneous test.

Globigerina bulloides.

In third place, the hyaline test  may be of calcite (which encompasses the suborders Spirillinina, Globigerinina, Rotaliina), or aragonite (Involutinina, Robertinina).

The external surface of the test may bear spines, keels, rugae, granules or a reticulate sculpture.

The morphology of foraminifera tests varies enormously, but in terms of classification two features are important: chamber arrangement and aperture style. The tests of many primitive foraminifera are unilocular, although test form varies greatly. Unilocular tests may be globose, tubular, branched, radiated or irregular. And in the case of the chambers of multilocular forms they could be globular, tubular, compressed lunate and wedge-shaped.

Two types of chamber arrangement: single chambered and uniserial.

In the Lower Palaeozoic, the tests were mainly agglutinated. By the late Devonian, septate periodic growth evolved and Foraminifera with hard test became more common. In the early Carboniferous first appeared the miliolids, followed in the Mesozoic by the appearance and radiation of the rotalinids and the textularinids. Also planktic forms appeared in the Mid Jurassic in the strata of the northern margin of Tethys and epicontinental basins of Europe. During the Palaeocene appeared the planktic globigerinids and globorotalids. The diversity of planktic forms has also generally declined since the end of the Cretaceous with brief increases during the warm climatic periods of the Eocene and Miocene.

In 1835, Dujardin recognised foraminifera as protozoa and shortly afterwards d’Orbigny produced the first classification. Early data on deep-sea benthic foraminifera (and on other deep-sea groups) were collected on the 1872-1876 Challenger Expedition and E. Hackel also included forams in his master work “Kunstformen der Natur”.

Foraminifera have been widely utilised for biostratigraphy. They also have a wide  environmental range and changes in the composition of foraminiferal assemblages could be used to track changes in the circulation of water masses and in sea-water depth. They are particularly important in studies of Mesozoic to Quaternary climate history because isotopes within their CaCO3 tests record changes in temperature and ocean chemistry.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Gooday, Andrew J., Rothe, Nina and Pearce, Richard B. (2013) New and poorly known benthic foraminifera (Protista, Rhizaria) inhabiting the shells of planktonic foraminifera on the bathyal Mid-Atlantic Ridge,  Marine Biology Research, 9, (5-6), 447-461 (doi:10.1080/17451000.2012.750365).

Bernhard JM, Edgcomb VP, Visscher PT, McIntyre-Wressnig A, Summons RE, Bouxsein ML, Louis L, Jeglinski M., Insights into foraminiferal influences on  of microbialites at Highborne Cay, Bahamas, PNAS June 11, 2013 vol. 110 no. 24 9830-9834.

An introduction to diatoms.

Diatoms are almost ubiquitous. They live in aquatic environments, soils, ice, attached to trees or anywhere with humidity and their remains accumulates forming diatomite, a type of soft sedimentary rock.
They are the dominant marine primary producers in the oceans and play a key role in the carbon cycle and in the removal of biogenic silica from surface waters.

Diatoms are unicellular algae with golden-brown photosynthetic pigments. They have a siliceous skeleton known as frustule that comprise two valves, one overlaps the other like the two pieces of a petri dish. The frustule is ornamented with pores, processes, spines, hyaline areas and other features The size range is between 1 to 2000 μm in length.

Diagrammatic sections of a frustule. From UCL.

They belong to the Division Chrysophyta, Class Bacillariophyceae and are divided in two Orders: The Centrales or Biddulphiales and the Pennales or Bacillariales. The Pennales or pennate diatoms have frustules that are elliptical or rectangular in valve view, with sculpture that is bilaterally symmetrical about a central line while the Centrales are characterized by frustules which are circular, triangular or quadrate in valve view and rectangular or ovate in girdle view.

Diatoms could be single or could form colonies. The cell has two or more golden-brown photosynthetic chloroplasts, a central vacuole, a large central diploid nucleus. Diatoms also store oils rather than starch and lacks of flagella or pseudopodia.

The first record of diatom frustules are centric forms from the Early Jurassic although very few remains are known before the Late Cretaceous, they were moderately affected by the massive extinction at the end of the Cretaceous.

Crossophialus gyroscolus and C. glabrus from the Upper Cretaceous of Antarctica. Image from the Ocean Drilling Program.

Centric diatoms had a major radiation in the Paleocene. Pennate diatoms also appeared during the Paleocene. The provincialism among diatoms increased in the latest Miocene and during the Pleistocene, diatom assemblages start to closely resemble modern ones.

Azpeitia tabularis (Miocene to Recent) and Fragilariopsis ritscheri (Pliocene to Recent). From UCL

During the 19th century, Ehrenberg, Grunow, Schmid and others studied and made great illustrations of diatoms, but was E. Hackel who created the most beautiful and artistic representations of diatoms in his work “Kunstformen der Natur”.

Living diatoms are very sensitive to parameters like salinity, oxigenation and other physical and chemical conditions, so they provide a valuable tool for studies of modern water quality and for the reconstruction of past environments, particularly for evidence of climatic cooling and changing sedimentation rates in the Arctic and Antarctic oceans. Also the evolutionary history of diatoms has been punctuated by several floristic turnovers which makes diatoms great tools for biostratigraphic correlations.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Barron, J.A. (2003). Appearance and extinction of planktonic diatoms during the past 18 m.y. in the Pacific and Southern oceans. “Diatom Research” 18, 203-224