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ENVIRONMENTAL QUALITY AND PRESERVATION— Tiny Windows to Environmental Change: the Fascinating World of Microfossils

Though small in size, (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 groups used in environmental provide signifi cant scientifi c information studies is . 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 . They the . 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 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 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 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, , 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 . 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- 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

Neogene

i d middle i

b Micropaleontology is the study of 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 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 and Deep-sea biostratigraphy is the study of Paleogene middle the movement of tree lines with chang- and pelagic forams and nan-

Eocene ing climate through time. 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 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 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. 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. The paleotemperature inhabit pristine waters, marginally pol- St. Croix in the U.S. Virgin Islands curve correlates with other data on luted waters, or survive in heavily are sandwiched between pelagic climate change, such as glacial and polluted areas. Some assemblages are layers that contain late Miocene foram interglacial intervals and corresponding known to be tolerant of variable and species. If there are deep-sea marine data on sea-level changes. Planktic extreme conditions such as high or low sediments onshore in a tectonically forams can develop abnormal shells salinity. In the fossil record, such assem- stable area, age of the sediments as as a response to environmental stress. blages could refl ect a restricted shallow determined by the microfossils will Deformed shells of species living in the nearshore environment that was peri- denote a time when sea level was higher water column on the shelf off Newport odically impacted by high infl ux of than today. Biostratigraphy of sediments Beach in Southern California have been freshwater runoff. Some species contain from multiple cores in an area also linked to presence of offshore sewage living when alive, as do species of enables correlation of the strata within outfalls. Many abnormal shells of fossil- the stony or reef-building corals. Stress the cores from which a cross section of ized species developed toward the end symptoms in these species and in corals the subsurface structure of the land can of the Miocene in waters over what is and coral reef communities are similar. be developed. now St. Croix, probably in response to Both types of organisms require clear increasing Antarctic glaciation and asso- ambient oceanic conditions. Both are Planktic Forams: Indicators of ciated cooling of the world’s oceans. now being affected in coral reef areas Paleoclimates and Paleoenvironments The youngest pelagic forams on the worldwide and are considered bioindica- Living planktic forams deposit in island, early Pliocene species, indicate tors of changing conditions. Like plank- their shells different concentrations of the island emerged from the sea shortly tic forams, benthic forams also produce and oxygen isotopes and other after ~5 Ma (Fig. 2). abnormal shells under stress. Pollution minerals dissolved in seawater depend- is a type of stress and often includes ing on water temperature and latitude. Benthic Forams: Indicators of Stressed presence of heavy metals. Heavy metals Rapid changes in carbon-isotope concen- Environments and Pollution can be incorporated into benthic foram trations in fossil planktic forams have Living benthic forams are especially shells as they grow. The type and been linked to sudden releases of frozen useful in detecting environmental per- concentration of metals in normal and methane from the ocean fl oor, events turbations. They have relatively short abnormal parts of the shell can be that are thought to have caused periods life cycles and specifi c habitats; thus, detected through x-ray analysis. The of rapid global warming in the past. they tend to respond quickly to changes USGS in cooperation with the National Though methane oxidizes to carbon in their environments. They are diverse, Park Service is currently conducting dioxide, a greenhouse gas, atmospheric abundant, easily sampled, and provide a investigations of the effects of pollutants methane has 20 times the heat-trapping cost-effective “bioindicator” that occurs on benthic forams in Biscayne Bay, power of carbon dioxide. Variations in in statistically signifi cant numbers. Florida. The bay is the marine com-

3 Plate 1. (A) Examples of planktic foraminifera. Shell shapes in many planktic species might be said to resemble popped popcorn. Shells of some species are essentially spherical. Others have elongate chambers and some have a single thick spine on each chamber. Other shells are conical, or fl at like discs and have a peripheral keel. Shell shape, thickness, and other morphologic characteristics vary according to the water depth range in which the species primarily lives. In planktic species, shell morphology is linked to fl otation. Plate 1. (B) Examples of benthic foraminifera. Shell shapes include fl at, angular, tubular, branching, elongate, oval, and round. Some benthic species also have a single thick spine on each chamber. Depending on orientation, some species look like fans, tri-cornered hats, or single, double, or triple-scoop ice cream cones. Some species are encrusting and attach themselves to hard objects on the sea fl oor. Some burrow beneath the sediment surface. Some that live on grass blades are ‘mobile’ to the degree that they move up the blades toward sunlight. In benthic species, shell morphology is linked to seafl oor setting and depth below sea level. ponent of Biscayne National Park. ball consistency and species composi- far more information about the bank Abnormal benthic species found off- tion also show that subsea reworking margin and the regional and global shore may indicate that pollutants are and mixing of fi ne-grained sediments record than was ever imagined at the reaching offshore waters. Benthic were occurring at the mudball source beginning of the study. forams and geochemical analyses are at about 15.2 Ma. The turbidite-mudball complementary tools used for monitor- package further documents submarine Besides host-rock faunas, the lime- ing water quality in estuarine or coral processes of large-scale sediment trans- stone contained abundant specimens of reef settings. port and admixing of dissimilar sedi- imported planktic forams that ranged mentary bodies. Until the mudball was from ~66.5 to 15.2 Ma (Fig. 2). The Signifi cance of Reworked found, the oldest pelagic sediments iden- 82 species identifi ed were also uniquely Microfossils: What Can They Reveal? tifi ed on the island belonged to the early deposited in the host rock. Paleocene, Miocene (~23.0 Ma, Fig. 2). To this Eocene, Oligocene and early Miocene The geologic record contains the only evi- day, the mudball forams remain the only species were present in the bottoms of dence anywhere for determining cause record known of earlier pelagic sedi- the cores. The middle parts of the cores and effect of natural change. The micro- mentation in the area of St. Croix. The contained Eocene, Oligocene and early fossil part of the record can contain the reworked microfossils had expanded the Miocene faunas. Only Oligocene and potential and capacity to provide far more timeframe and depositional record of early Miocene species were found near information than may at fi rst be evident on island-history knowledge by nearly 35 the core tops. In other words, the oldest the surface of a study. million years. displaced species were progressively and completely eliminated in successively St. Croix, U.S. Virgin Islands Western Margin of the Great Bahama younger intervals of host rock. Such an The elongate island of St. Croix Bank, Bahamas orderly distribution of reworked forams consists of volcanic rock at both ends in two long drill cores is unique to the geologic record in the separated by that was depos- from the western margin of the Great Bahamas. Because of low-lying topogra- ited as layers of in an Bahama Bank contain a particularly rich phy on the bank and very young host ancient seaway prior to tectonic uplift. and enlightening record. The cores were rock relative to ages of the reworked Depositional strata are revealed in road obtained to verify ages of the host rock, specimens, the displaced forams could cuts throughout the seaway. In 1980, inferred from seismic data to represent a not have been derived from elsewhere a specifi c hillside outcrop behind a pri- complex depositional setting. The plank- on the bank. This discovery led to a new vate residence was free of vegetation tic microfossils revealed the magnitude direction of research, to determine their and revealed a coarse-grained limestone of complexity. The setting itself posed a source. layer encased in massive chalky deep- challenge for the foram/nannofossil part water limestone (Fig. 3). The coarse- of the study: the cores were taken in The new direction required applica- grained layer, called a turbidite, had shallow-water sediments in a shallow- tion of ancient regional geographic con- been deposited by a submarine debris margin setting that would normally pre- straints, speed and direction of ancient fl ow. The turbidite contained a mudball clude presence of deep-water faunas. currents, paleoclimatic data, and sea- inclusion consisting of homogenous But they were there, in abundance, level changes. The investigation required clay. When analyzed for microfossils, especially in thin, condensed layers of correlation of the original (source) and the mudball yielded 39 species of strictly pelagic sediments. Presence of secondary (bank) depositional records reworked planktic forams (Fig. 2); the pelagic microfossils and sediment of the reworked species with regional 23 species had not previously been layers demonstrated several facts. Water and global proxy data from studies by described from the island. The turbidite depth at the platform edge was once others. The reworked species represent contained younger middle Miocene spe- deep enough that planktic faunas lived pelagic deposition over 51 million years cies. Though the mudball clay and the there, off the bank at their source. Their abundance indi- older fossils were the same ages, the was intermittent, and the top of the cates that the sediments in which they mudball entity was middle Miocene platform was once farther inland and were originally deposited were easily because it was a part of the turbidite. smaller than it is today. Critical fi rst eroded (i.e., not rock) millions of years The source of the mudball could not and last occurrences of marker species later. Their unique redistribution in the be determined, but the mudball dem- (those that indicate age) of both types cores implies tectonic forces at the onstrates the existence relative to the of microfossils were out of stratigraphic source that produced a certain type submerged seaway fl oor of a steep order because of mixing by shallow- of stratigraphically ordered submarine submarine slope, probably of tectonic water debris shed from the platform. structure, then much later erosion of that origin, that was composed of clay and The host rock was eventually deter- structure by marine bottom currents over was undergoing pelagic mined to range from ~7.5 to 1.9 Ma a period of ~6 million years (duration of between ~57.8 and 15.2 Ma. The mud- (Fig. 2), but the planktic forams yielded accumulation in the host rock). Although

4 proto-Cuba GBB

K

P E O K K M upper limb P P E E O O

A M B M C (A) Erosion (dashed lines), faulting, and subsidence of upper limb of overturned anticline expose stratigraphically reversed beds in lower limb. (B) Erosion of lower limb with slump block failure. (C) Uplift and emergence removes oldest sediments from erosional window. Note Cretaceous slump block (from K layer) in Eocene sediments. Diagram is based on actual stratification in uplifted mountain located ~500 m from the shore near Bahia Nuevitas in northeast Cuba and ~400 km from the western margin of the Great Baham Bank (GBB).

Miocene Miocene Miocene

Oligocene Oligocene Oligocene

Eocene Eocene Eocene

Paleocene Paleocene Paleocene Miocene Miocene Miocene D Oligocene E Oligocene F Oligocene

(D-F) Erosion of stratigraphically ordered thrust fault hanging wall with progressive burial of older strata by pelagic deposition or slumping.

M M M M

O M O O M O

O E E O E E M E P P E P P O P P M M E G O H O I

(G) Erosion of stratigraphically ordered faulted horst blocks. (H) Subsidence of fault blocks below zone of active erosion or below graben surface or burial by pelagic deposition or slumping removes oldest sediments from erosional window. (I) Uplift of horst-and-graben system places horst blocks in less erosive shallow-marine setting.

Figure 4. Simplifi ed structural models and generalized events characteristic of Cuban geology. The events could have produced the richly fossiliferous submarine source of pelagic foraminifera that were imported to the western margin of the Bahama Bank. Letters indicate epochs (stratigraphic ages) as shown in Figure 2. K = Cretaceous. km = kilometers. m = meters.

Florida is built of pelagic limestone of along that coast are evidence. The only at the bank margin represents systematic the same ages as the reworked species, type of structure that could have been removal of layers containing the oldest the direction and velocity of the Gulf the source is a submarine wall with species from the erosional window at Stream, which was well established by erodible strata either in stratigraphic or the source, either by burial (submarine the time of secondary deposition, elimi- reverse stratigraphic order. deposition) or by emergence of an nates Florida as the source. The only eroded overturned anticline above sea area in the Caribbean that meets all cri- There are three possible models, all level. Processes that occurred at the teria for type and site of the source common tectonic structures. An eroded source include pelagic deposition, fault- is located somewhere along the tectoni- overturned anticline can exhibit layers in ing, changing sea level, subsidence, cally unstable northern coast of Cuba, reversed stratigraphic order (Fig. 4A-C). erosion, and uplift and gradual emer- some 400 to 700 km away from the Stratigraphically ordered layers can be gence of Cuba. Lastly, using forami- western bank margin. The Cretaceous, found in a thrust fault hanging wall or niferal settling rates, size range of the Paleogene, and early Neogene pelagic a horst-and-graben system (Fig. 4D-I). reworked shells, and velocity, turbulence forams now found in uplifted mountains Systematic redistribution of the forams and direction of currents in channels

5 between Cuba and the bank, remarkably short transit times were calculated for Additional Reading: the imported specimens to reach the bank. Depending upon precise distance Ausich, W.I., 2000, in the ivory tower: Geotimes, October, p. 18-21. between the source on Cuba and the Cockey, E., Hallock, P., and Lidz, B., 1996, Decadal-scale changes in benthic foraminiferal western bank margin, the reworked fos- assemblages off Key Largo, Florida: Coral Reefs, v. 15, p. 237-248. sils could have made the trip in as many as 27 to 48 days or as few as 5 to 8 Farley, M.B., and Armentrout, J.M., 2000, Fossils in the oil patch: Geotimes, October, p. days. 14-17. Lidz, B.H., 1982, Biostratigraphy and paleoenvironment of Miocene-Pliocene hemipelagic The western margin of the Great limestone: Kingshill Seaway, St. Croix, U.S. Virgin Islands: Journal of Foraminiferal Bahama Bank has long been believed Research, v. 12, no. 3, p. 205-233. to have built seaward solely through pro- gradation. Yet through pelagic foramin- Lidz, B.H., 1984, Oldest (early Tertiary) subsurface carbonate rocks of St. Croix, USVI, revealed in a turbidite-mudball: Journal of Foraminiferal Research, v. 14, no. 3, p. ifera, a new dimension and new deposi- 213-227. tional component are identifi ed. The link between the bank and Cuba implies that Lidz, B.H., 1984, Neogene sea-level change and emergence, St. Croix, Virgin Islands: the bank margin has advanced seaward Evidence from basinal carbonate accumulations: Geological Society of America Bulletin, primarily but not solely through pro- v. 95, p. 1268-1279. gradation. Sediments from Cuba had to Lidz, B.H., and McNeill, D.F., 1998, New allocyclic dimensions in a prograding carbonate have been rafted along with the micro- bank: Evidence for eustatic, tectonic, and paleoceanographic control (Late Neogene, fossils and therefore have helped build Bahamas): Journal of Sedimentary Research, v. 68, no. 2, p. 269-282. the margin. If it had not been for the microfossils, we would not have known Martin, R., 2000, The environmental stories of microfossils: A new research path for that the young (~7.5-1.9 Ma) host rock micropaleontology: Geotimes, January, p. 19-21. represents an approximately 6-million- Scott, D.B., and Lipps, J.H., 1995, eds., Environmental applications of foraminiferal studies: year-long time capsule. Through the Journal of Foraminiferal Research, v. 25, no. 3, p. 189-286. window opened by the reworked spe- cies, we have a glimpse into, and a Winchester, S., 2001, The Map That Changed the World: William Smith and the Birth of better understanding of, regional geo- Modern Geology: HarperCollins Publishers Inc., New York, 330 p. logic processes and events and the global climatic record spanning nearly For more information, This report is preliminary and has not been 66.5 million years of time. please contact: reviewed for conformity with U.S. Geological Survey editorial standards or with the North Summary Barbara H. Lidz American Stratigraphic Code. Reference therein U. S. Geological Survey to any specifi c commercial product, process, or The application of research involv- 600 4th Street South service by trade name, trademark, manufacturer, St. Petersburg, FL 33701 ing living benthic forams is an increas- or otherwise does not necessarily constitute phone: 727-803-8747 x3031 or imply its endorsement, recommendation, or ingly important part of assessing chang- fax: 727-803-2030 favoring by the United States Government or ing conditions in the nearshore marine [email protected] any agency thereof. environment. The problem lies in deter- mining whether the changes are natural, manmade, or both. The geologic record contributed to understanding past pro- forams of the Great Bahama Bank contains the only evidence anywhere cesses and climatic events that produced are evidence of paleoceanographic, tec- for determining cause and effect of nat- fl uctuations in global sea level. The tonic, and sea-level control on regional ural change. Earth-science studies that more we know about past climate and Cenozoic history. The quantity and incorporate microfossils have a very the causes that led to changes, the better diversity of the imported fossil faunas high potential for leading to pioneering we can predict what may trigger climate indicate that the global deep-sea record investigations and new discoveries. The and sea-level change in the future. may be far more prevalent in uncommon study of microfossils can touch on many settings or regions where it has not important questions and can benefi t The planktic foraminiferal record been sought than is presently realized. many other disciplines. Pelagic forams has unveiled remarkable evidence for Microfossils provide fascinating infor- in particular are excellent and precise heretofore-unknown sedimentary com- mation in ways that no other part of the tools for a wide range of studies. ponents and depositional complexity in geologic record can. Different types of microfossil data have a large carbonate bank. The reworked

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