Climate Analysis Using Planktonic Foraminifera

CLIMATE ANALYSIS USING PLANKTONIC FORAMINIFERA

INTRODUCTION

Foraminifera (forams for short) are single-celled protists with shells. Their shells are also referred to as tests because in some forms the protoplasm covers the exterior of the shell. Depending on the species, the shell may be made of organic compounds, sand grains and other particles cemented together, or crystalline calcite.

The small size of most foraminifera may make them difficult to see, but it makes them much more useful than larger fossils for applications such as petroleum exploration, because there can be thousands of specimens in the small chips of rock collected when drilling a well. In addition, many species of foraminifera are geologically short-lived, and others are only found in specific environments, so a paleontologist can examine the specimens in a sample and determine the geologic age and environment when the rock formed. As a result, since the 1920's the oil industry has been a major employer of paleontologists who specialize in these microscopic fossils. It is unusual to drill an oil well without a paleontologist onsite to determine when the desired oil-bearing rock layer has been reached.

Most of the estimated 4,000 living species of forams live in the world's oceans. Of these, 40 species are planktonic, that is they float in the water. The remaining species live on the bottom of the ocean, on shells, rock and seaweeds or in the sand and mud of the bottom. In places, foraminifera are so abundant that the sediment on the bottom is mostly made up of their shells. For example, the pink sands of Bermuda get their color from the shells of a foraminiferan called Homotrema rubrum which has pink to red-colored shells. Far from land in the deep sea, where little material comes from erosion of the land, the bottom sediment is made up mainly of shells of planktonic organisms, especially foraminifera.

Foraminifera are found in all marine environments, from the intertidal to the deepest ocean trenches, and from the tropics to the poles, but species of foraminifera can be very particular about the environment where they live. Some are abundant only in the deep ocean, others are found only in brackish estuaries or salt marshes along the shore, and most live at certain depths and water temperatures in between.

Because different species of foraminifera are found in different environments, paleontologists can use their fossils to determine past environments. If a sample of fossil foraminifera contains many living species, the present-day distribution of those species can be used to infer the environment there when the fossils were alive. Even when samples contain all or mostly extinct species, data such as species diversity, the relative numbers of planktonic and benthic species (planktic:benthic ratio), and the ratios of different shell types are used to infer past environments.

In addition to using species distributions (whether directly or through diversity and other ratios) to study past environments, the chemistry of the shell can tell us about the chemistry of the water in which it grew. Most importantly, the ratio of stable oxygen isotopes depends on the water temperature, because warmer water tends to evaporate off more of the lighter isotopes. Studies of stable oxygen isotopes in planktonic and benthic foram shells from hundreds of deep-sea cores worldwide have been used to map past water temperatures. These data help us understand how climate has changed in the past and thus how it may change in the future.

CLIMATE ANALYSIS USING PLANKTONIC FORAMINIFERA

Instructions: Follow the instructions given below and those of your teacher to complete this activity.

1) You have been given a series of samples containing planktonic foraminifera representing time from the present to 160,000 years ago. After your analysis of the material, you decide to look at the climatic signal from these samples. You are aware that a particular species of foraminifera, Neogloboquadrina pachyderma, is an excellent recorder of water temperature through geologic time. When the earth experiences periods of relatively cold temperatures, ocean waters are cooler and Neogloboquadrina pachyderma forms its test (shell) such that it coils to the left. Alternatively, during periods of relatively warm temperatures when ocean waters are warmer, Neogloboquadrina pachyderma constructs its test with a coiling direction to the right. Therefore, you have separated out the specimens of Neogloboquadrina pachyderma from your samples. For each sample you have counted how many of this species coil to the left and how many coil to the right. Your data is recorded in Table 1. Your next step is to calculate the percentage data for each sample. You must calculate percentage data so that your information is not biased by the total amount of foraminifera present in the sample. Complete the worksheet in Table 1 by calculating the total number of Neogloboquadrina pachyderma for each sample and the percentages of right- and left-coiling forms of the species. The first sample is done for you on the worksheet.

2) Next, use graph paper to plot your results. On the vertical axis you should plot the age of the samples with "0" at the top and "-160,000" at the bottom. On the horizontal axis, plot the percentage of right-coiling Neogloboquadrina pachyderma with "0%" on the left side and "100%" on the right side. You should now have a graph representing the climatic signal derived from the coiling ratios of Neogloboquadrina pachyderma.

3) Analyze your graph and make an interpretation of the climatic history on our planet during the last 160,000 years. Describe this history below and provide evidence for this interpretation.

Table 1. Neogloboquadrina pachyderma coiling rations worksheet