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The Fascinating World of Microfossils

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The Fascinating World of Microfossils ENVIRONMENTAL QUALITY AND PRESERVATION— Tiny Windows to Environmental Change: the Fascinating World of Microfossils Though small in size, calcareous (calcium record, a record of natural processes and carbonate) microfossils reveal volumes change. about geologic time, events, processes, and past climate. The U.S. Geological One of the most common micro- Survey (USGS) has the capability to fossil groups used in environmental provide signifi cant scientifi c information studies is foraminifera. Forams are pro- based on microfossils that can be used tozoans with external chambered shells. to help resolve issues raised in many There are two types, planktic and ben- other disciplines. A source for pioneering thic (Plate 1A, B). Both are distributed investigations and new discoveries little globally. Planktic (short for planktonic) recognized outside the fi eld, microfossils forams are pelagic organisms that fl oat and the stories they tell lie preserved in as plankton in the open oceans. They the geologic record. live at various depths and construct their shells from elements dissolved in Introduction seawater. Species vary depending upon water depth, temperature, and latitude. Microfossils are everywhere (Fig. An abundance of fossil pelagic species 1). Generally the size of small sand in deep-sea sediment indicates the over- grains, they are the fossilized remains lying water was warm and nutrient-rich. of marine or terrestrial organisms. There Planktic species are relatively short- are many kinds of microfossils ranging lived (a few million to tens of millions from terrestrial pollen spores to con- of years old) and are used to determine odonts (toothlike fossil elements in form precise geologic age of the sediment or but not in function, produced by marine rock in which they are found, called worms) to shells of calcareous marine host rock. Planktic forams reveal more organisms such as mollusks, pelecypods, than benthic forams about geologic time, ostracods, and foraminifera (forams). processes, past environments and cli- Nannofossils are extremely small cal- mates. Benthic forams are bottom dwell- careous marine microfossils not visible ers. They live on or within the top to the naked eye and include radiolar- layers of sediment from the shoreline ians and diatoms. Marine microfossils to abyssal depths. Most also build their are important tools used to determine shells from elements dissolved in seawa- various aspects of the geologic and cli- ter, but some species use sand grains or matic record. In the modern world, some other materials from their surroundings living faunas that become microfossils to construct their shells. Benthic forams upon death are also used to measure are extremely long ranging (hundreds of changing marine conditions, including millions of years old) and are seldom pollution. Pollution is generally con- used for precise age dating. Instead, sidered a result of human activities, assemblages consisting of specifi c spe- but other types of change such as vari- able salinity (salt content) or tempera- ture of the water are more diffi cult to Figure 1. (top) Open-water fl oating plankton consists of organisms of all sizes. (center) Extruded-protoplasm defi ne as strictly natural or manmade. To bubbles around long thin spines on a planktic foram help ascertain causes of these types of entrap other types of plankton for food. (bottom) Reef- change, we must examine the geologic building branching coral Acropora palmata and certain species of bottom-dwelling forams require similar con- ditions for survival. U.S. Department of the Interior USGS Open-File Report 02–192 U S Geological Survey May 2002 cies are used to indicate the general set- local site as well as possible distant ting in which they lived (i.e., estuary, sources and the types of processes that Cenozoic Age (Ma) inner shelf, outer shelf, slope, and transported them. Regardless of spe- Holocene 0.01 abyss), which implies a general water cialty, given only the microfossils and Pleistocene 1.9 late depth. In the geologic record of a tec- little other solid evidence, a micropa- Pliocene 3.4 early tonically stable area where there are leonologist is a forensic geologist who 5.3 no mountain-building processes, general applies scientifi c knowledge and logical host rock water depth has implications for past reasoning to interpret and reconstruct late position of sea level. events of the past. 11.5 e t The Field of Micropaleontology Biostratigraphy Neogene i d middle i b Micropaleontology is the study of Stratigraphy is the study of sedi- r u t Miocene 15.2 microfossils. A micropaleontologist usu- mentary or rock layers (strata) in the ally specializes in a single type of geologic (stratigraphic) record (Fig. 2). microfossil but is able to recognize Layered strata are often observed in road early organisms from other groups. A forami- cuts. A technique called seismic survey- niferal micropaleontologist uses forams ing is used to map subsurface strata. 23.0 along with other types of data from Seismic records or profi les show vertical a sample site (for example, type of slices through the ground that reveal the late x host rock or sediment, geographic set- different layers. Biostratigraphy is the k ting, and seismic-survey data) to deter- differentiation of rock layers based on mine various attributes of the sample distribution of the fossils they contain. 30.0 or group of samples. The attributes are The identifying fossils within each bed Oligocene then placed into the context of local geo- are to be found only in that particular early logic history—what happened at the site portion of time when the bed was depos- and when did it happen. Once ‘what’ ited (Winchester, 2001). Stratigraphic 36.6 and ‘when’ are resolved, the data can order represents the order through time late often be extrapolated into a larger pic- in which the beds accumulated. Whether ture—the regional or global record. For working with microfossils or dinosaurs, 41.3 example, pollen spores from sediment biostratigraphy is the primary method cores are used to trace the presence for determining ages of geologic events. ages of reworked species in mudball, St. Croi and location of terrestrial plants and Deep-sea biostratigraphy is the study of Paleogene middle the movement of tree lines with chang- sediments and pelagic forams and nan- Eocene ing climate through time. Conodonts nofossils deposited in a deep-water envi- become darker with age and thermal ronment. Identifi cation of forams and ages of reworked species from Cuba, Great Bahama Ban pressure through time and are used in nannofossils is also a key component 52.0 the oil and gas industry as indicators in oil and gas exploration. Rock frag- early of conditions favorable for hydrocarbon ments containing microfossils are recov- formation. When conodonts recovered ered from various intervals during drill- 57.8 from a certain subsurface depth within ing of a well. Fossils are identifi ed and an exploratory well have attained a spe- ages determined, and the depth interval late cifi c dark color, they verify that organics of the sample is compared to seismic at that depth have reached the matura- profi les and other types of data obtained 63.6 Paleocene tion stage to form oil and gas. Assuming in the area. Drilling continues until the early that foram specimens are found in place, fossils indicate that rock layers of an 66.5 Mesozoic i.e., where they lived in the case of age known to contain hydrocarbons in 66.5 benthic species or where they fell to the region are reached. Whether the new the ocean fl oor after death in the case well will contain oil or gas depends on late of planktic species, the species are the structure of the hydrocarbon-bearing Cretaceous used to develop part of the local geo- rocks and whether there is an imperme- logic record. Where fl oods, rivers, debris able layer or structural trap above them fl ows, and other processes have that would confi ne the oil or gas. Figure 2. Geologic time scale showing Cenozoic epochs. Stratigraphic order occurs when sediments in a rock imported microfossils from somewhere section progressively range from oldest at the bottom to else, the displaced or reworked faunas Deep-sea biostratigraphy is also youngest at the top, such as is observed in a core of are used to tell something about the used in settings that are no longer in undisturbed deep-sea sediments. Ma = million years. 2 deep water, such as at a shallow car- bonate-platform margin to ascertain how and when the platform built seaward and the duration of expansion. The Great Bahama Bank in the Bahamas is an example of a carbonate platform. During high stands of sea level, carbonate platforms generally build seaward by shedding of shallow-water debris pro- duced on the bank top. The process is called margin progradation. In tec- tonically active zones, biostratigraphy is used to tell the age of pelagic sediments now above sea level. Knowing the age of uplifted pelagic sediments can help determine the timing of processes, such as when tectonics occurred in the area or when volcanoes erupted. For instance, Figure 3. Excavated site of mudball (arrow) and turbidite (dashed lines) encased within fi ne-grained pelagic lime- stone on St. Croix. Turbidite layer is ~0.5 m thick. Visible dimensions of mudball are 15 x 25 cm. Rock hammer for uplift took place after deposition of scale. m = meters. cm = centimeters. Cretaceous, Paleogene, and early oxygen-isotope concentrations in fossil Benthic foram assemblages are becom- Neogene (Fig. 2) pelagic forams and species have also established ocean ing increasingly important as we learn marls that now form mountains in temperatures and temperature variations more about the particular species that northern Cuba. Volcanic-ash beds on through time.
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