Using Non-Marine Ostracodes to Reconstruct Quaternary Paleoenvironments

LaDorna Jo Pfaff, November 17, 2004, Emporia University, Kansas

Table of Contents

Abstract

Introduction

Taxonomy

History of scientific uses of ostracodes

Using ostracodes to reconstruct Quaternary paleoclimates

Changing lake depths

Water energy regimes

Salinity

Position and duration of air masses

Age dating methods

Geochemical analysis

Biomonitors

References

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Abstract

Ostracodes are crustaceans whose remains are commonly preserved in aquatic environments. Different species are sensitive to changes in water salinity, temperature, solutes, and dissolved oxygen. They are increasingly being used to reconstruct Quaternary paleoenvironments

Ostracode morphology

Introduction

Ostracodes are small bivalve crustaceans that live in most types of aquatic environments, including damp leaf litter, mosses, and wet fen soils. Typically the adults are 0.5 - 2.0 mm long. Their low-magnesium calcite valves (shells) are often preserved in Quaternary sediments. Some species of ostracodes can live in slightly acid environments, however, their calcite shells are not preserved in this environment. (Benson 2003)

In life a bivalve carapace totally encloses the ostracode's body. The adult has 5 - 8 appendages; ostracodes either crawl or swim. Most are free living, but some are commensal on other aquatic animals. Reproduction is mostly sexual, but a few are assumed parthenogenic, since in these species no males have ever been found. The ostracodes go through 4 - 9 juvenile molt stages (instars) before becoming an adult. The Ostracoda class is very diverse.

Types of ostracodes from fresh water lakes to oceanic abyssal plains

Note: The term Ostracoda is the formal taxonomic name for the class, but two common spellings are encountered: "ostracode" is usually used in North America and France, and "ostracod" is used in other areas of Europe, and in Australia. (Holmes et al 2002)

Taxonomy

There are a wide variety of crustacean classifications. This taxonomic tree follows the form in the paragraph below.

    Arthropoda - jointed limbs, exoskeleton

    Crustacea - mostly aquatic shrimp-like invertebrates

    Maxillopoda (ostracodes, copepods, barnacles)

    Class: Ostracoda

    Subclass: Podocopa

    Order: Platycopida

    Order: Palaeocopida

    Order: Podocopida

    Subclass: Myodocopa

The Ostacoda class is divided into two subclasses: Podocopa and Myodocopa. Most ostracodes found in Quaternary sediments are likely to be of the Podocopa subclass. The molt stages are dramatically different between the two subclasses.

Podocopa has 9 instars: 8 juvenile, and 1 adult. Unlike many other aquatic invertebrates, Podocopa lacks swimming larval stages for dispersal. In Podocopa eggs are deposited singly or in clusters and are carried by currents or by fish (they remain viable after passing thru the fish). The Podocopa subclass has 3 orders: Platycopida are benthic, filter feeders found mainly in marine environments; Palaeocopida, once diverse, are represented in the Quaternary only by a rare ostracode in shallow marine, high-energy conditions off New Zealand; and Podocopida are the most diverse and widespread ostracodes today. The Podocopida class is found in marine, brackish and freshwater environments.

The Myodocopa subclass consists of 4 - 7 juvenile instars and a single adult. Ostracodes in the Myodocopa subclass have brood care within the parent ostracode shell.

History of scientific uses of ostracodes

Ostracode fossils (along with foraminifer fossils) were used by the oil and gas companies to locate petroleum reservoirs.

In the 1970s the scanning electron microscope (SEM) was developed. It enabled better measurements on ostracode morphology. The deep-sea ostracode species, Bythoceratina scaberrima, was the first biological specimen seen with the new SEM at Cambridge, United Kingdom.

The development of the hydraulic piston corer (HPC) allowed controlled continuous sections from the ocean floor. Abundant and well-preserved deep-sea ostracodes were found in the continuous stratigraphic succession of the hydraulic piston cores. This ocean drilling set the precedent for later land drilling.

Ostracode assemblages demonstrated sudden change in ocean systems: by recognizing marine ostracodes in Tethys Sea sediments, these fossils help demonstrate the fact that the Tethys Sea was an ocean before the end of the Miocene. Finding deep-sea ostracodes in mountains regions (such as the Alps) helped bring about major revisions in tectonic history. (Benson 2002)

Today ostracodes are playing an important role in Quaternary climate change research. Ecological and biogeographical databases are being compiled from many researchers world wide. Using Geographic Information Systems (GIS) the data can be portrayed spatially. Examples of on-going ostracode databases are the North American Non-Marine Ostracode Database (NANODe) and the Arctic Ostracode Database. Even though marine species are important for defining ocean temperature, and salinity, and dissolved oxygen (Cronin 2002), this report focuses mainly on non-marine species.

Using ostracodes to reconstruct Quaternary paleoclimates

To use ostracodes for Quaternary research, researchers must first have a detailed knowledge of the biology and ecology of present-day ostracodes - their morphology, taxonomy, life cycle, factors in secreting their valves, reproduction, etc. There is much needed work to be done in this field.

Ostracodes do not sense climate impacts like terrestrial plants and animals; however, they respond to the climatic impacts of their aquatic environment. Climate has large effects on aquatic environments, especially with respect to moisture balance and vegetation zones. In temperate climates, ostracodes respond mainly to water chemistry and water depth, and less to temperature. In this way ostracodes are different from pollen successions. Pollen, representing the regional vegetation, responds mainly to temperature, and secondarily to water moisture. It is important to correlate ostracode and pollen stratigraphy within regional settings.

When looking at ostracode valves from aquatic settings, two questions to ask are 1) does the ostracode assemblage relate to the water characteristics, and 2) are these aquatic characteristics sensitive to climate. There are non-climate factors that effect ostracodes, as well, such as basin shoaling, change in water chemistry, abrupt changes in groundwater, and basin overflows. Basin overflows shortened water residence time in areas were precipitation exceeds evaporation. (Curry 2003)

Changing lake depths

Freshwater ostracode species and abundance correlates well with changing lake depth. During dry periods, where the water is shallow, light reaches the bottom of the lake, and there is much aquatic vegetation. In this scenario one finds many swimming ostracode varieties. During wet periods, the deepest part of the lake is dark, vegetation-free, and muddy. Burrowing benthic species will be found in the dark lake bottom. If a lake dries out completely, there will be almost no ostracodes found. (Curry 2003)

Water energy regimes

The different proportions of various ostracode molt stages indicate different water energy regimes. In lakes, the discarded ostracode shells (valves) are deposited on the lake floor. It there is a complete range from young to adult valves, it indicates absence of strong currents. In a beach environments, different sizes are sorted; one would find the large size adult valves, while the smaller ones would be washed away. If there is a ostracode assemblage that has no adults, one would surmise that a condition changed (anoxia, salinity, or temperature change) that prevented the ostracodes from reaching adulthood. (De Deckker, 2002)

Salinity

The biodiversity of ostracodes within lakes, ponds, wetlands and streams is very high. It is composed of different species living along gradients defined by differences in temperature, water energy, salinity, nutrients, etc. Salinity is a major control on ostracode distribution. Although, within the same salinity environments, different ionic composition of the water can determine the kind of ostracodes that live there. (A. Smith and Horne 2002)

Hammer(1986)constructed salinity scales for ostracodes found in saline lakes. Australocypris rectangularis and Platycypris baueri can live in salinities up to 200 0/00. Most species of ostracodes that tolerate high salinities can exist in a wide range of environments from fresh to saline. If a sediment layer has either of these ostracodes, and lacks other ostracodes, it would indicate high salinities. Some ostracodes can live in only a narrow range of salinity, such as Candona rawsoni. It can live in freshwater and up to 30 0/00. Finding Candona rawsoni in sediment layers would bracket salinity measurements to this range. It is important to correlate probable salinities from ostracode data with electrical conductivity logs.

Position and duration of air masses

Some non-marine ostracode species found worldwide appear to have distributions that closely correlate with the position and duration of air masses. It seems that, in other words, some species are limited to air mass areas: Cyclocyris ampla is found in cold, fresh water lakes in the northern part of North America, and its distribution is limited by the summer extent of the Arctic air mass; Candona rawsoni and Limnocythere ceriotuberosa are found in the western part of North America, and appear to mark the extent of the dry Pacific Air mass.

Air mass configuration and duration change through time. Cytherissa lacustris is found in the fossil record of the lakes in the Great Basin; today C. lacustris lives in the range of the polar circulation cells. The presence of C. lacustris in the fossil record indicates that past air masses (influenced by precipitation and temperature) have been quite different from today. In this case, past conditions in climate can be reconstructed from the distribution of ostracode species. (A. Smith and Horne 2002)

Age dating methods

Age dating methods on cores with ostracodes. Isolecine amino-acid racemization dating is done on ostracode shells.(De Deckker 2002) Carbon dating is done on bone or shell fragments found in the sedimentary layers. Uranium fission track dating can be done if there are volcanic tephra layers in the sediment core.

Even though ostracode species have existed since the Ordovician Period, during the Quaternary there has not been much evolutionary radiation of ostracodes. Therefore scientists are unable to date sediment layers, with high temporal resolution, from identifying the time range in which that ostracode species lived. (Horne 2003)

Geochemical analysis

The ostracode shells are also used for geochemical analysis.

    1) Ostracode chemistry can reveal trace elements of Mg and Sr uptake. These are related to water temperature and water chemistry in both lakes and oceans.

    2) Oxygen and carbon isotope analysis is done on ostracode shells in lacustrine settings. They provide a carbonate shell in an environment where other carbonate fossils are largely absent.

    3) In temperate deep lakes benthic ostracode shell chemistry is related to the isotopic water composition.

    4) In marginal marine settings, ostracode shell chemistry is related to salinities.

    5) In the deep ocean, Mg in the benthic ostracode shells is related to ocean bottom water temperatures (Holmes and Chivas 2002)

    6) Ostracode shell chemistry can be used for potential indicators of past heavy metal pollution. For instance, the ostracodes in Owens Lake, California, show a high percentage or uranium from natural Uranium sources in the sediments. (G. Smith 1993)

    7) Ostracode shells can be used in isoleucene dating and paleotemperaterue studies by amino acid racemization.

    8)In shallow open lakes, the shell chemistry is more complex; however, if the ionic composition of the lake remains constant, Mg values in the shells may relate to water temperature and water ionic composition.

    9) Ostracode shells are being used in DNA evolutionary genetic studies De Deckker (2002) cautions that quality assurance testing needs to be provided in publications using ostracode shell chemistry. The ostracodes should be well-preserved, and from assemblages where both juveniles and adults exist.

Biomonitors

Living ostracodes are beginning to be used as biomonitors in wetlands, streams and springs. These non-marine aquatic environments can be assessed by studying ostracode species living there. Unfortunately, non-marine ostracodes tend to be without ornamentation or other easily identifiable characteristics. It is a hurdle to identify different species. L. Smith is focusing on the three common species. Cavernocypris wardi is found in springs. This species is strongly regulated by temperature. Fabaeformiscandona rawsoni is found in wetlands, and Physocypria globula is found in streams. These latter ostracodes are most strongly influenced by salinity and alkalinity composition. Variations in the water quality show different assemblages of ostracodes. (A. Smith et al 2003)

De Deckker(2002) notes that finding Candona caudate in lake settings can indicate that there is anoxia from eutrophication.

Recommendations for future Quaternary ostracode work: Researchers need more detailed knowledge abput living ostracode ecology from along biological gradients. Global Positioning Systems (GPS) transects and bathymetric data are needed to correlate information in the ostracode databases. Ostracode studies should be carried out in conjunction with pollen and foraminifer research, physical measurements on site, such as electrical conductivity logs and geomorphology analysis. Accurate dating is essential.

References

This web page was made for Earth Science 767, Quaternary Geology, Emporia State University, Emporia, Kansas, Professor: James S. Aber, Ph.D.

Link to course homepage

http://academic.emporia.edu/aberjame/ice/icehome.html