5 ocean sediments

An Atlantic stingray emerges from its hiding place in the calcareous sediments between the coral reefs of the Florida Keys.

72

5 Ocean Sediments

cha16543_ch05_072-089.indd 72 11/15/06 2:24:09 PM

Chapter 5 Ocean Sediments 73

The Ocean’s Memory

Any solid fragment of inorganic or organic material may be termed sediment. Familiar ocean sediments include those you find along the coast: rocks and cobbles at the beach, fragments of seashells, or sand and mud at the bottom of a bay. What

you may not appreciate about those sediments is the story they tell to a trained observer. Sediments reveal the processes that form beaches, shape coastlines, carve submarine can-yons, and control the abundance of marine organisms, to name a few. Sediments also tell a story of Earth’s past, what it was like on land and in the sea thousands and millions of years ago. In fact, sediments reveal a history of the ocean over a wide range of temporal and spatial scales (see inside front cover).

Over geologic time, the sinking and settling of sediments—both constant and epi-sodic—can build thick deposits. If undisturbed, the deepest layers will represent the oldest sediments. Those on the surface will be the youngest. Recognition of this process provides geologists with an important tool for interpreting the history of the Earth. It is formalized in the principle of superposition, which states that in a deposit of undisturbed sedimentary rock layers, the oldest rocks are at the base of the deposit and the youngest rocks are at the top (Figure 5-1). Knowing this, geological oceanographers can determine the relative ages of sediment layers even if their true age is unknown. They can also infer the relative ages for deposits far away from each other as long as the deposits have at least one sediment layer in common.

The study of Earth’s history in the ocean’s sedimentary record is called paleocean-ography. As a chronicle of ocean and climatic conditions, marine sediments serve as the ocean’s memory. In that sense, they offer one of the rarest of insights: a chance to see what conditions were like on Earth before humans. To study ancient oceans, paleoceanogra-phers typically obtain sediment cores, long, narrow-diameter cylinders of sediments, some of which may be hundreds of feet long. Sediment cores dating back to 145 million years ago have been extracted from the ocean floor. The information they provide about Earth’s past enables oceanographers and climate scientists to infer patterns of change in Earth’s climate. Although the story remains incomplete, at least some evidence exists that human activities are altering our planet as never before.

Questions to Consider 1. Why are seafloor sediments called the “memory” of the ocean, and

how is that memory being “read” to obtain longer and more accurate records of the world ocean’s past?

2. What processes cause sediments to form, and once formed, how do sediments arrive at the seafloor?

3. How does climate affect the origins, transport, and deposition of sedi-ments in the world ocean?

4. What role do biological processes play in the origins, transport, and deposition of sediments?

5. How do oceanographic studies of sinking particles and seafloor sedi-ments help scientists to better understand the possible consequences of global warming?

The Ocean’s Memory

The Foundations of Paleoceanography

Classification of Marine Sediments

Sedimentary Processes

Spotlight 5.1: Dawn of the Anthropocene?

Spotlight 5.2: Carbon Flux to the Deep Sea

Global Distribution of Seafloor Sediments

cha16543_ch05_072-089.indd 73 11/15/06 2:24:10 PM

Time intervals over which species existed

Tim

e

Older

Younger

Change inenvironment

The Foundations of Paleoceanography

The first oceanographic expedition to study the global dis-tribution of sediments in the world ocean was the Challenger Expedition (1872–1876). Attached to the sounding line was a metal tube surrounded by a donut of weights (called a Baille rod) that would drive the tube into the seafloor and capture a small sample of sediment. From these modest beginnings, a catalog of seafloor sediment types and their descriptions was begun. Expeditions following the Challenger Expedition obtained cores of 1–2 m (3–6 feet) lengths, which revealed variations in the abundance of the skeletal remains of fora-minifera, microscopic marine organisms found throughout the world ocean (Figure 5-2). Analyzing sediment samples from the German Meteor Expedition (1925–1927), Wolfgang Schott (1905–1989) in 1935 reasoned that the absence of foraminifera skeletons in the lower layers of the core was caused by cold sea-water temperatures during the Ice Age. In effect, the presence or abundance of foraminifera skeletons could be used to trace changes in seawater temperature and, by inference, Earth’s climate. Schott is credited with establishing the first links between the abundance of marine organisms and climate.

The major advance in paleoceanography came when Cesare Emiliani (1922–1995) demonstrated that measurements of 18O, a stable isotope of oxygen in the shells of foraminifera, could be used as a proxy of seawater temperature. The abundance of

74 www.mhhe.com/chamberlin1e

■ FIGURE 5-1 The principle of superposition and the types of fos-sils present in sediment layers are tools that paleoceanographers use to interpret the ocean’s history. Changes in fossil species over time can reflect changes in their environment. Chemical characteristics of their skeletons also hold clues to ocean conditions at the time they formed.

18O relative to 16O, the normal form of oxygen (see Chapter 2), varies with temperature, providing a convenient indicator of the seawater temperature in which the foraminfera lived. Plotted as delta 18O, or d 18O (the difference in the ratio of 18O/16O relative to a standard), Emiliani found that colder waters tend to have higher values (+3.5‰), that is, they are 18O-enriched, while warmer waters have lower values (0 to −2‰), that is, they are 18O-depleted. Measuring the 18O of foraminifera shells from different sediment layers revealed variations in seawater temperature. Emiliani’s work was the first to demonstrate that ocean conditions were not constant through geologic time. The trend in his three-point graph agrees with modern plots based on hundreds of data points and reveals that the world ocean has cooled by about 12° C (about 21° F) over the past 100 million years. Because of his work, Emiliani is considered by many to be the “father” of paleoceanography.

The biggest boost to an understanding of the history of the oceans came with the invention of a new type of cor-ing device, the Kullenberg piston corer. This instrument enabled oceanographers on the Swedish Deep-Sea Expedition (1947–1948) to obtain 10–20 m (33–66 feet) sediment cores representing 1–2 million years of deposition. Application of traditional piston core technology culminated during the Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) project (1971–1982), organized by the National Science Foundation to study and understand changes in Earth’s climate during the past 700,000 years. CLIMAP demonstrated that ice ages result from millennial-scale deviations in Earth’s orbit around the Sun, a pattern known as the Milankovitch

■ FIGURE 5-2 Single-celled protists, like the multi-chambered fora-minifera, live planktonically or on the seafloor within a specific range of ocean temperatures. Because their shells are naturally preserved in sediments, their occurrence and abundance provide a record of ocean temperatures in the past, as first realized by Wolfgang Schott. Chemical isotopes, like oxygen-18, also provide a record of the tem-perature conditions when the shell was formed.

cha16543_ch05_072-089.indd 74 11/15/06 2:24:13 PM

100,000 years

More eccentric

Less eccentric

Less obliqueMore oblique

Pointedaway

Pointedtowards

41,000 years

Normal to plane oforbit around sun

Northern Hemisphere tiltedaway from Sun when Earth

is farthest away from the Sun.

Northern Hemisphere tiltedtoward the Sun when Earth

is farthest away from the Sun.

23,000 years

Eccentricity cycle

Obliquity cycle

Precession cycle

del O-18 ‰

More iceLess ice

2 31-1 0-2900

-3

700

600

500

400

300

200

100

0

Tho

usan

ds o

f yea

rs a

go


ment studies was the discovery of Heinrich events, abrupt climate change in the form of rapid cooling within decades to centuries. Six Heinrich events have occurred within the past 75,000 years. Scientists are concerned that human activities may initiate another Heinrich event or other types of abrupt climate change, unleashing a scenario like that depicted in the popular movie Day After Tomorrow, albeit at a much reduced pace and scale.

By the 1960s, it became apparent that drilling of the sea-floor was the only means to obtain a sufficient length of core for studying past ocean conditions beyond 2 million years. In July 1968, the newly built Glomar Challenger set sail on an era of deep-sea drilling that continues to this day. The first drill-ing program, the Deep-Sea Drilling Project (DPSP), operated from 1968 to 1983. However, in 1983, the project obtained a new and larger vessel, the JOIDES Resolution (Figure 5-5) and became the Ocean Drilling Program (ODP). The accomplish-ments of the DPSP and ODP contributed significantly to our understanding of plate tectonics, climate change, seafloor processes, and hydrothermal vents, among other research top-ics. Results compiled from these programs fill more than 100 volumes! In October 2003, the ODP transferred its scien-tific objectives to the International Ocean Drilling Program (IODP), a cooperative effort between a nonprofit consortium and academic institutions. Initial phases of the project will be conducted aboard the JOIDES Resolution, until a new Japanese vessel, the Chikyu, commences operations in 2007. The Chikyu is part of a multivessel fleet intended to expand drilling capa-bilities into shallow-water and ice-covered environments.

■ FIGURE 5-3 The 100,000-, 41,000-, and 23,000-year patterns of the Milankovitch cycle. These variations slightly alter the amount of solar radiation that reaches Earth. The result is up to 10-degree C changes in Earth’s temperature, sufficient to trigger Ice Ages when other climate factors favor glaciation. Discovery of the influence of the Milankovitch cycle on Earth’s temperatures came from studies of microscopic shells in marine sediments and other paleotemperature indicators. See Fig. 5-4.

■ FIGURE 5-4 Studies of oxygen isotopes in foraminifera shells found in sediment cores reveal a sawtooth pattern that matches the patterns of the Milankovitch cycle. Peak values represent periods of glaciation.

cycle (Figures 5-3 and 5-4). Recognition of the Milankovitch cycle and its effect on climate represents an important mile-stone in scientific understanding of temporal scales of vari-ability in Earth processes. Another important result of sedi-

cha16543_ch05_072-089.indd 75 11/15/06 2:24:17 PM

Continental environments Marine environmentsVolcanism

GlaciersWind and eolian sediments

Marine sediments

Submarine canyonturbidity currents

Mixed terrestrialand marinesediments

Sinking

DustBiological

activity

Cosmic dust

Evaporites

LakeSand

Terrestrialsediments

Parentrock


Classification of Marine Sediments

Marine sediments originate from a variety of sources, includ-ing continental and oceanic crust, volcanoes, microbes, plants and animals, chemical processes, and outer space (Figure 5-6). However, identifying the source of a particular deposit of marine sediments often proves difficult. Sediments may be altered from their original condition by any of a number of physical, chemical, and biological transformations that take place after the sediment is formed. Sedimentologists employ a number of different classi-fication and analysis techniques to characterize sediments. Visual analysis of the texture and composition of a sediment sample, or descriptive classification, is often the first step in differentiating sediments. Size classification, based on visual, mechanical, or laser-based sizing of sediments, aids in understanding physical and chemical changes in sediments that occur during transport and deposition. Genetic classification includes a more complete description of the physical, chemical, and biological properties of sediments. Like forensic scientists working a crime scene, ocean-ographers search for any clues to piece together an understanding of the history of sediments from the time they were formed to the time they were deposited.

Descriptive ClassificationsDescriptive classifications prove highly useful for work at sea and for describing the patterns of sediment distribution on the seafloor. Oceanographers employ a visual classification system ■ FIGURE 5-5 Drilling operations aboard the Joides Resolution.

■ FIGURE 5-6 A few of the sources of sediments in continental margins and in ocean basins. A number of processes may produce sediments from a number of different sources, including rock, organisms, and cosmis debris.

cha16543_ch05_072-089.indd 76 11/15/06 2:24:24 PM

■ FIGURE 5-8 Examples of major types of granular and chemical sediments. Top: Southern California beach sand (lithogenous granular sediments). (b) Marine shells on Sanibel Island, Florida (biogenous granular sediments). (c) Salt crystals or halites (chemical sediments).

called logging to create a written description of sediments obtained while at sea (Figure 5-7). Occasionally, microscopic observations and other analyses will accompany logging, but in general, the idea is to quickly develop a written description that can be used to guide more detailed analyses on shore at a later time.

In the logging system adopted by the Ocean Drilling Program, sediments are separated into two broad categories: granular, resulting from the fragmentation of inorganic or organic parent materials; and chemical, forming directly from dissolved compounds in seawater (Figure 5-8). Granular sedi-ments include familiar examples such as mud, silt, and sand, and unfamiliar ones, such as the microscopic shells of marine organisms. Granular sediments may be formed by mechanical processes (weathering and erosion), biological processes (secre-tion of shell or mineralized cell wall), or volcanic processes (above- or below-water ejection of ash and particles). Granular sediments may also be categorized as biogenous, originating from biological processes, or lithogenous, originating from rocks. Chemical sediments have a variety of pseudonyms and may be referred to collectively as hydrogenous (coming from water) or authigenic (forming in place). Chemical sedi-ments include the fragments of limestone and limestone-like rocks (dolomite, chert, chalk, etc.), which come from the compacted, buried, and mineralized remains of marine organ-isms whose cell walls are composed of calcium carbonate and other chemical substances. They also include evaporites, such as those found beneath the seafloor in the Gulf of Mexico (see Chapter 4). As well, metalliferous compounds produced by hydrothermal vents and manganese nodules are included among chemical sediments.

Biogenous sediments originate from organisms that secrete mineral skeletons. In general, they may be subdivided into calcare-

■ FIGURE 5-7 Scientists visually examine a sediment core during a leg of the Ocean Drilling Program.

a.

b.

c.

cha16543_ch05_072-089.indd 77 11/15/06 2:24:27 PM

Volcanism

Seafloor spreading

Subduction,accretion,melting

Weatheringof parent rock

WindAshTransportby water

Riverineparticles

Sinking,aggregation,fragmentation

Decomposition,diagenesis, lithification

Coprophagy,scavenging,dissolution Hydrothermal

EolianparticlesMarine biogenic

particles

10.0100

5004096 250 10050 2010 5 2 1 0.5 0.2 0.05 0.01 0.005 0.001

inch

Boulders

Cobbles

Gravel

Sand

Silt Clay

mm

1.0 0.1 0.01 0.001 0.0001


Size ClassificationsAnother way to classify sediments is by the average size of sediment particles, called grains. The most frequently used size classification system is the Udden-Wentworth scale. Using this system, sediments are classified according to their diameter (Figure 5-9), often determined using a series of sieves with dif-ferent-sized pores. The logarithmic scale of grain sizes ranges over six orders of magnitude. Clay and silt represent the smallest diameter sediments, while cobble and boulder make up the larg-est grain size classification.

Sedimentary ProcessesCollectively, processes that involve the production, transport, and deposition of sediments are called sedimentary processes, or simply sedimentation (Figure 5-10). Sediments are produced through the erosion of rocks, mostly continental, and through the activities of organisms, especially microorganisms that form skeletons or shells. The transport of sediments occurs by winds, rivers, ocean currents, ice flows, glaciers, and a number of other geologic processes. Deposition involves a number of physical, chemical, and biological processes that trap sediments and allow them to accumulate. Ultimately, sediments may be transformed through metamorphosis or melted during subduction, although some evidence exists of a net gain in sediment accumulations

■ FIGURE 5-10 Idealized diagram representing major sedimentary processes. Note that physical, chemical, and biological processes affect the production of sediments and their ultimate fate on the seafloor.

ous or siliceous and, if known, named after the type of organism that formed the sediment. For example, calcareous sediments originating from foraminifera may be called foraminifera oozes. Diatoms, single-celled, photosynthetic microbes whose cell walls are constructed of silica, produce diatom oozes. The particular modifier chosen is usually at the discretion of the person clas-sifying the sediment and often arises from a desire to emphasize the type of study performed. Thus, sediments rich in silica may be called siliceous sediments by a geologist studying chemical processes, while a biologist may call these sediments diatom oozes to emphasize their origins. These biogeneous granular sediments make up a significant fraction of seafloor sediments. Foraminifera oozes constitute about 50% of the deep-sea floor, while diatom oozes comprise about 15%.

Two types of rare sediments are occasionally encountered during logging of sediments using microscopic analysis. These sediments have origins in extraterrestrial processes. Tektites were once thought to be the fragments of meteorites that disintegrated in Earth’s atmosphere. We now know, however, that tektites originate from the ejection of melted fragments of terrestrial rocks during meteorite impacts. These glassy frag-ments appear in a dazzling array of shapes and colors, and may settle directly into the ocean or be transported by wind or water. Tektites are considered to be a special class of granular lithog-enous sediments that have undergone what is known as shock metamorphism. Interplanetary dust particles (also known as cosmic dust) represent a type of cosmogenous sediment. These small particles, captured by Earth’s gravitational field, originate from asteroids, active comets, and other planetary bodies in the solar system. The tiny fragments, less than 3–35 microns (mm) (0.000118–0.001378 inches) in diameter, avoid vaporization on entering Earth’s atmosphere. Interplanetary dust particles are enriched in 3He, an isotope of helium, making them easy to distinguish from terrestrial sediments. Interplanetary dust has received increasing attention in recent decades because it provides information on the “dustiness” of space with possible implications for millennial-scale climate changes.

■ FIGURE 5-9 The Udden-Wentworth size classification of sediments.

cha16543_ch05_072-089.indd 78 11/15/06 2:24:32 PM

Ice and snow(43,400)

Groundwater

infiltration

Subsurface(15,300)

Terrestrialatmosphere

(4.5)

Marineatmosphere

(11)

Oceans(1,400,000)

Land(59,000)

Evapotranspiration(71)

Mixed layer (50,000) Thermocline (460,000)

Abyssal (890,000)

Evaporation(434)

Condensation

Precipitation(107)

Winds(36)

Surface runoff(36)

Snowmelt runoffRain (398)

Solar driven

Groundwater

discharge


fragmentation of rocks due to rock slides, debris flows, and other processes, such as earthquakes. Chemical weathering arises due to oxidation of minerals or elements within minerals, dissolution by natural acids, and dissolution of rocks in water. Biological weathering stems from the activities of organisms that fracture, dissolve, or chemically alter rocks. Weathering varies according to

■ FIGURE 5-11 The hydrologic cycle, includ-ing major reservoirs and rates of exchange. Movements of water across and within Earth’s crust dominate the origination, transport, alteration, and deposition of marine sedi-ments. Bold numbers in parentheses indicate size of reservoirs in 1015 kg. Other numbers in parentheses indicate rates of exchange in 1015yr -1. Source: Based on data from Leeder (1999) after Chahine (1992).

through geologic time. In a global sense, the origination, trans-port, deposition, and destruction of sediments describes what is known as the sediment cycle. Let’s explore various aspects of the sediment cycle with the goal of understanding how and where sediments move about on our planet.

The Hydrologic CycleA natural starting point for the sediment cycle is the hydrologic cycle, the solar- and gravity-driven exchange of water between various reservoirs (Figure 5-11). The hydrologic cycle plays a central role in the formation, transport, and deposition of sedi-ments. Precipitation of water over landmasses leads to dissolu-tion and fragmentation of rocks through weathering. Surface runoff, rivers, and groundwater flow transport sediments and dissolved substances into the ocean. Glaciers, ice, and snow form sediments through mechanical weathering and transport sediments and dissolved substances into the ocean through their movements and when they melt. Ultimately, sediments are trans-ported to continental basins, continental margins, or the deep-ocean floor (i.e., oceanic crust), where they are deposited.

Continental WeatheringThe hydrologic cycle plays a major role in continental weather-ing, the dissolution, fracturing, or chemical alteration of rocks through physical, chemical, or biological processes (Figure 5-12). Physical/mechanical weathering refers to the breaking apart or

■ FIGURE 5-12 Weathering breaks down solid rock into fragments that become sediments.

cha16543_ch05_072-089.indd 79 11/15/06 2:24:39 PM

River flow/ocean currentsSettling

Rolling

Sliding

Silt and claysuspended by turbulence

Siltand clay

Suspendedload

Bed loadSand

Gravel

Dissolved load

Sand movedby saltation

Riverbed or seafloor

Attachment pointfor floats

Sample bottleswith preservative

Funnel

Seafloor

Watersurface

Floats

Sedimenttrap

Release

Weight

Honeycombbaffle

Programmablemotorcontroller

Motor for movingnew samplebottle into place


the chemical makeup of the rock (some rocks dissolve more easily than others), climate (temperature, precipitation, humidity), pres-ence of soil (inhibiting rock erosion or promoting plant growth), and length of exposure of the rock at the surface. Weathering plays an important role in the cycling of elements and the salinity of seawater, subjects we explore in Chapter 6.

Sediment TransportSediments move when they interact with moving fluids, including air and water. Whether a given sediment grain moves as a result of fluid flow depends on a complex number of variables (Figure 5-13). For our purposes, grain size serves as a useful indicator. Grain size determines the threshold velocity of a particle, defined as the speed of fluid flow (air or water) that causes the sediment to move. In general, three types of movement occur: 1) rolling, the tumbling of a grain along a surface; 2) saltation, the hop-ping of a grain along a surface; and 3) suspension, the floating of a grain within the fluid. Small-sized grains, such as dust, are easier to move and suspend than large-sized grains, such as cobble. Similarly, more energetic flows (e.g., fast-moving winds and cur-rents) will move larger grains, whereas less energetic flows (e.g., slow-moving winds and currents) will move only small-grained sediments. By examining the proportions of different sizes—the grain size distribution—oceanographers can learn something about how the sediments were transported and the conditions under which they were deposited. Samples that exhibit a narrow range of sizes (i.e., when the grains are nearly all the same size) are said to be well-sorted, whereas samples with a wide range of sizes are poorly sorted. Sorting occurs because grains of different sizes and density behave differently under a given flow; some particles are carried away, while others are left behind.

Sediment SinkingGrain size is also an important factor in the sinking or sedi-mentation of particles. Large grains may settle quickly, but silts and clays may not be deposited for days or months. This means that fine particles may remain suspended in the water and carried much farther from their site of origin than large particles. The rate of sinking of fine particles (less than 0.125 mm or 0.005 inches) is described mathematically by Stoke’s law.

■ FIGURE 5-13 The movement of sediment grains under the influence of flowing water in a river or on the seafloor. Sediment grains may roll, slide, jump (saltation), or become sus-pended. An understanding of the movements of sediment grains under a given set of conditions is important for interpreting sediment deposits and their history.

■ FIGURE 5-14 Sediment traps deployed at fixed depths in the ocean act like giant funnels to collect sinking particles in sample bottles at regular time intervals. By moving a new sample bottle into place after a period of time (days to weeks) the rates of particle sinking can be determined and compared. At the end of an experiment (weeks to months), an acoustic signal from the ship releases the trap from its weight and it floats to the surface where it can be retrieved. Sediment traps have proven highly useful for understanding processes that affect the flux of carbon and other elements to the deep sea.

Oceanographers often estimate the sinking rates of small parti-cles and marine microorganisms using Stoke’s law. Alternatively, direct measurements of particle sinking rates may be obtained (with certain restrictions) using an instrument called a sediment trap (Figure 5-14; see also Spotlight 5.2). Determination of

cha16543_ch05_072-089.indd 80 11/15/06 2:24:48 PM

Dawn of the Anthropocene?At the start of the twenty-first century, it has

become clear that human activities affect many facets of the earth system. So pervasive is the human influ-ence on geological, physical, chemical, and biological processes that many scientists believe that our planet has entered a new geologic age, the Anthropocene. The Anthropocene marks an era when human activi-ties rival natural processes in their scale and scope of influence on Earth’s environment. The beginning of the Anthropocene may be traced to the first signs of increased carbon dioxide in Antarctic ice cores dating back to the eighteenth century. Some scientists place its origins as far back as 100,000 years ago, when human civilizations began to alter the landscape and change the course of rivers. Whether the Anthropocene as a true geologic age will resonate with the earth science com-munity remains to be determined. Yet already several major studies provide clear evidence for the dominance of human activities in many earth systems.

One study, published in 2005, used models and observations to compare prehuman and modern dis-charges of sediments from 4462 river basins around the

world. Globally, rivers discharge some 40,000 km3 (9,596 cubic miles) of water to the world ocean annually. However, water flow and sediment flux to these rivers have been altered by land-use practices and the construction of dams. More than 45,000 dams (above 15 m or about 49 feet high) now control river flows.

5.1

Reservoirs associated with these dams have a capacity to store at least 15% of the global annual discharge. At the same time, human activities have accelerated land erosion and increased the rate of sediment transport by rivers. While a greater amount of sediment runoff from land has been estimated for modern times (an increase of nearly 2 billion tons annually over prehuman times), trapping of sediments behind reservoirs has reduced the global discharge of sediments to the world ocean by about 1.5 billion tons annually. Significant regional differences in sediment discharge are also evident. Africa and Asia discharge a reduced volume of sediments, while much of India transports larger volumes of sediment to the world ocean.

The overall effects of reduced sediment discharge to the world ocean are difficult to ascertain. On a local level, reduced sediment transport by the damming of rivers has caused severe beach erosion along many parts of the US coastline. Perhaps the most significant outcome of human alteration of river basins has been increases in the amount of dissolved and toxic substances discharged to the world ocean. Further research aimed at better quantifying the regional and seasonal transports of sediments and identification of issues related to increased or reduced loads will allow us to determine with greater certainty the consequences of human activities on sediments in the world ocean.

Dawn of the Anthropocene?

FIGURE 5a Though heralded as a triumph of technological achieve-ment, the construction of dams, such as the Hoover Dam shown here, has significant and unforeseen consequences for the earth systems on which human survival depends.

cha16543_ch05_072-089.indd 81 11/15/06 2:24:54 PM

CO2, N2

Fixation ofC and N by

phytoplankton

Grazing

Benthic wormCarbondeposition

Seafloor

Sloppy feedingEgestion

EgestionIngestion

Consuming

Zooplankton

Passivesinking

Respiration

Excretion

Respiration

Excretion

Base ofeuphotic zone

Aggregateformation

Break up

Bioturbation

Decomposition

Two-wayverticalmigration

Physicalprocesses

Lysocline

Photic zone

Foraminifer (calcite shell)

Carbonate compensation depth

Dead organisms sink Calcite andsilica oozes

Silica oozes only

Diatom (silica shell)


particle sinking rates is especially useful for estimating carbon flux to the seafloor, an important component of the global car-bon cycle and climate change.

Biological SedimentationA number of in situ (in place) biological processes affect the transport and deposition of sediments. An important aspect of biological sedimentation are those activities of organisms that

increase the rate at which carbon (as biogenous sediments) sinks into the ocean, a process called the biological pump (Figure 5-15). In general, the biological pump is affected by three principal factors: 1) those factors that affect the produc-tivity of sediment-producing organisms; 2) those factors that alter the rate of sinking of biogenous (and other) sediments; and 3) those factors that affect the solubility of biogenous (and other) sediments in seawater. The productivity of biogenous sediments is intimately connected with the physical, chemical, and biological processes in the water column that promote and inhibit the growth of planktonic organisms. Regions of high productivity produce the greatest abundance of biogeneous sediments, whereas regions of low productivity produce fewer biogenous deposits. Factors that affect the sinking rate involve processes that alter the size distribution of particles and, thus, the rate at which they sink. Activities that aggregate particles will cause them to sink more quickly, while activities that frag-ment particles will cause them to sink more slowly. Solubility factors determine which types of biogeneous sediments may dissolve before their deposition on the seafloor. We will explore factors affecting biological productivity and sedimentation in greater detail in Chapters 12, 13, and 14.

Calcium Carbonate Compensation DepthBelow a certain depth in the ocean, sediments composed of calcium carbonate (mainly, the shells and skeletons of marine organisms) begin to dissolve rapidly. This depth, called the lysocline, is the point at which seawater becomes under-saturated in dissolved calcium carbonate (Figure 5-16). That means that solids composed of calcium carbonate begin to go into solution. The factors that determine the solubility of calcium carbonate in seawater are complex and not fully under-stood, but in general, lower temperatures and higher pressures increase its solubility. Thus, warm surface waters are more eas-ily saturated, while cold deeper waters remain undersaturated. As a result, carbonate sediments that sink beneath the lysocline begin to dissolve.

■ FIGURE 5-15 Conceptual model of the biological “pump.” The transport of carbon from the upper ocean to the seafloor is not simple! Nonetheless, a small but significant percentage of carbon is delivered to the seafloor via the biological pump. Many of these processes are discussed in detail in Chapters 12, 13, and 14.

■ FIGURE 5-16 The solubility of calcium car-bonate in seawater affects the distribution of calcium-containing (calcite) sediment on the sea-floor. The surface waters of the world ocean are generally saturated in calcium carbonate and calcite sediments. The surface waters of the world ocean are generally saturated in calcium cabonate and and calcite sediments may accu-mulate. Below the lysocline, seawater is under-saturated in calcium carbonate, and the shells of calcium-carbonate-secreting organisms dissolve.

cha16543_ch05_072-089.indd 82 11/15/06 2:24:58 PM

Particles settlingon seafloor

Intense mixing

Less intense mixing

Permanent sedimentary record

Lysocline

High productivity High productivityLow productivity

90°S 90°N0° 30°N 60°N60°S 30°S

Net accumulation

Netdissolution

Calcium carbonate compensation depth


Whether a carbonate sediment makes it to the seafloor and survives intact once deposited depends both on the rate of pro-duction of the sediment and the depth of the lysocline. High rates of production are generally accompanied by high rates of sinking that allow carbonate shells to accumulate on the seafloor before they are dissolved. Alternatively, a shallow lysocline in deep water increases the likelihood that carbonate sediments will dissolve before they reach the bottom. Oceanographers define the depth at which the rates of supply and dissolution of carbonate sediments are equal as the calcium carbonate com-pensation depth (CCD), that is, the depth at which there is no net accumulation of carbonate sediments (Figure 5-17). If a given region of the seafloor is above the CCD, carbonate sedi-ments may accumulate; below the CCD, carbonate sediments do not accumulate. Note the important distinction between the lysocline and the CCD: in regions of low productivity, their depths may be equal, but in regions of high productivity, the CCD will be at a greater depth than the lysocline. Regional dif-ferences in the depth of the lysocline may also be important. The depth of the lysocline in the Pacific is shallower (about 3.5 km or 2.2 miles) than the Atlantic (~5 km or 3.5 miles). Note that regions of high productivity promote a deeper calcium carbonate compensation depth.

Deposition and Fate of SedimentsWhen sediments reach the seafloor, a number of processes may occur before their burial as sedimentary layers or rocks. Immediately upon settling, sediments may be consumed or processed by organisms living on the seafloor. A host of benthic or bottom-dwelling animals extract meager supplies of energy and nutrients from biogenous sediments. Sea cucumbers and sea urchins plow the surface of seafloor sediments in search of nutri-tion. Deposit-feeding worms burrow through seafloor sediments much like earthworms in soil. The reworking and processing of sediments by organisms is called bioturbation (Figure 5-18). Bioturbation disturbs the layering and chemistry of sediments because of the activities of organisms. The degree of bioturba-

tion and the depth to which it occurs depend on the kinds of species present and their abundance.

Another important process on the seafloor is diagen-esis, the physical, chemical, or biological transformation of sediments or sedimentary rocks that occurs once they are deposited (in any environment). Microbial degradation, in particular, alters the chemical composition of sediments soon after they are deposited. The chemical environment of interstitial waters—waters in the spaces between sediment grains—determines the elements that may be released or retained by sediments. Digenesis is a complex process that varies considerably over the broad expanse of the seafloor from the continental shelves to the deep basins.

Eventually, sediments become buried and preserved. The pressure of overlying sediments may cause compaction and cementation of sediment grains. Eventually, sediment grains crystallize into sedimentary rocks, a process called lithifica-tion. Most sedimentary rocks result from the consolidation of sediment grains that were once part of solid rocks themselves. Sandstones and shales are good examples of sedimentary rocks formed in this manner.

Once deposited, sediments and sedimentary rocks form successive layers whose depth and properties reveal the history of their formation, transport, and deposition. In some cases, tectonic processes may uplift and expose these sediments to the atmosphere. Alternatively, sediments and sedimentary rocks may be carried by plate motions to subduction zones, where they may be scraped off the oceanic crust into accre-tionary wedges or subducted and melted. In many cases, the composition of subducted sediments may contribute to the chemical composition of magmas generated by subduction. The silica-rich magmas along the western United States result, in part, from subduction of silica-rich sediments on the seafloor of the Pacific.

■ FIGURE 5-17 A generalized schematic of the calcium carbonate compensation depth. Note that in high productivity regions the CCD may be below the lysocline.

■ FIGURE 5-18 Bioturbation affects rates of exchange of materials and gases between the seafloor and the overlying water column. They also affect the rate at which carbon and other elements are perma-nently buried. These processes may be important to biogeochemical cycles and climate change.

cha16543_ch05_072-089.indd 83 11/15/06 2:25:01 PM

Carbon Flux to the Deep SeaTo improve our understanding of global climate and especially the role of the world ocean in climate change, oceanographers want to know how much carbon is deliv-ered to the seafloor and under what circumstances. Ideally, such studies will help to establish a carbon budget for the world ocean that will allow scientists to assess its role as a source or sink of carbon dioxide, an important green-house gas. To answer such questions, oceanographers employ an ocean-going instrument called a sediment trap (Figure 5b). In use since the late 1970s, sediment traps have been deployed to catch the rain of ocean sedi-ments, or sediment flux (the amount or mass of material that falls through a horizontal area per unit time, i.e., mg m-2 day-1). The sediments found in these traps consist of a variety of sinking sediments, but scientists are most interested in sediments containing carbon, also known as particulate organic carbon (POC). Modern sediment traps are designed with multiple containers attached to a wheel that sequentially rotates beneath the trap funnel on a pre-timed schedule. Particles that fall into the fun-nel sink into these containers which hold preservatives to prevent their decay. The preserved samples are later weighed and carefully analyzed to identify the chemical

and isotopic composition of the particles and the species of organism that produced them (if possible).

Sediment traps have been deployed from moorings in prac-tically all ocean basins. In some cases, instruments attached to the sediment trap or nearby moorings collect meteorological, physical, biological, and chemical data that allow oceanographers to establish causal effects for the changes in carbon delivery to sediments. The Ocean Flux Program (OFP), established in 1978, represents one of the longest sediment trap time-series studies ever conducted. The program was initiated in the Sargasso Sea off the coast of Bermuda by Werner Deuser of the Woods Hole Oceanographic Institution. Since 2000, the OFP has been led by Maureen Conte of the Bermuda Biological Station for Research. The decades-long record of the OFP illustrates the importance of long-term and continuous oceanographic observations. During this time, oceanographers have observed unexpected variability related to seasonal cycles, interannual climate phenomena, passing ocean eddies, and extreme storm or hurricane events.

One example of the importance of these studies occurred when the OFP sediment traps captured an extreme sediment flux event

5.2

during Hurricane Fabian, which passed over the OFP mooring in 2003. The amount of sediment collected by a sediment trap deployed at 1500 m (4921 feet) was greater for a single two-week period than the total amount of sediments collected at that depth since 1978. What caused this record-setting pulse of sediment into the traps? One hypothesis was that the hurricane mixed the ocean and stimu-lated the growth and sinking of phytoplankton to depth. Another hypothesis was that the sediment arrived from a source some distance away from the site. As it turned out, the chemical composition of the posthurricane sediments was largely calcium carbonate instead of phytoplankton carbon. In addition, a color satellite image showed a large plume of material originating from the Bermuda archipelago and drifting very near the OFP mooring. Thus, a plume of sedi-ment appears the most likely explanation. These results demonstrate the importance of keeping a close and constant eye on the ocean. Episodic events like these have already altered the way oceanogra-phers think about carbon delivery to the ocean. The steady rain of particles from above has now become a storm!

Carbon Flux to the Deep Sea

FIGURE 5b Deployment of a sediment trap from the stern of the Weatherbird II off Bermuda.

cha16543_ch05_072-089.indd 84 11/15/06 2:25:04 PM

ice-rafted carbonate siliceous red clay siliceous/red clay terrigenous


Global Distribution of Seafloor Sediments

The distribution of sediments on the seafloor represents the inte-gration of a number of geological, physical, chemical, and biologi-cal processes. By examining the types of sediments on the seafloor in the major basins, we can draw a few generalizations about the processes that brought them there. According to Table 5.1, car-bonate sediments occupy a greater percentage of the seafloor in the Atlantic basin (67.5%) versus the Pacific basin (36%). It has been hypothesized that lower overall rates of microorganism pro-ductivity (on a per area basis) may be responsible. A contributing factor is the shallower lysocline in the Pacific.

Although maps of sediment distributions are generally depict-ed as broad swaths of single types (defined on the basis of the sedi-ment type whose concentration exceeds 30%), in fact, a mixture of types are present at most locations. The global patterns of surficial sediments may be explained by the processes that dominate in a given location (Figure 5-19). For example, the “ring” of siliceous

sediments surrounding Antarctica comes from diatoms associated with ice-edge processes. Siliceous sediments along the equatorial Pacific have been attributed to radiolarians, a protist with a silica skeleton that appears to thrive in the upwelling of cold waters in this region. Terrigenous deposits dominate continental margins, especially at the mouths of major rivers.

The thickness of sediments typically corresponds to the age of the seafloor (Figure 5-20). Because all seafloor originates at mid-ocean ridges, it only gradually acquires a blanket of sediments. Newly formed seafloor will be practically devoid of sediments, while the seafloor farthest from the spreading center will have the thickest sediments. Sediments accumulate slowly in most of the ocean basins, except near large rivers and where biological pro-cesses are active. Plate tectonics plays a role in the distribution of sediments by altering the speed at which the seafloor spreads. In fast-spreading ridges, such as the East Pacific Rise, sediment lay-ers may be shallow over a broad stretch of the basin. In the North Atlantic, where seafloor spreading is slow, the sediment layers increase in thickness at shorter distances from the ridge. Tectonic processes also trap sediments in submarine trenches. Some of the thickest sediment layers in the ocean can be found in the subma-rine trenches along Alaska and Chile.

5.1 Proportion of sediment types covering the deep-sea floor.

Sediments Atlantic Basin (%) Pacific Basin (%) Indian Basin (%) World Ocean (%)

Calcareous oozes 65.1 36.2 54.3 47.1

Pteropod oozes 2.4 0.1 — 0.6

Diatom oozes 6.7 10.1 19.9 11.6

Radiolarian oozes — 4.6 0.5 2.6

Red clays 25.8 49.1 25.3 38.1

Relative size of ocean 23.0 53.4 23.6 100.0

Source: Schulz and Zabel, 2000; after Berger 1976.

■ FIGURE 5-19 Global distribution of surficial sediments in the world ocean. Equatorial upwelling in the Pacific and ice-edge processes in the Antarctic contribute to the productivity of diatoms and, hence, the accumulation of siliceous sediments. Carbonate sediments are generally con-fined to shallower regions of the world ocean. Terrigenous sediments dominate near the mouths of major rivers.

cha16543_ch05_072-089.indd 85 11/15/06 2:25:09 PM

Thickness (kilometers)

0 0.5 1 5 10 20

-60˚

60˚

70˚

80˚

-50˚

50˚

-40˚

40˚

-30˚

30˚

-20˚

20˚

-10˚

10˚

0˚

-70˚180˚ -150˚ -120˚ -90˚ -60˚ -30˚ 0˚ 30˚ 60˚ 90˚ 120˚ 150˚ 180˚

The distribution of sediments in the oceansis controlled by five primary factors:1) Age of the underlying crust2) Tectonic history of the ocean crust3) Structural trends in basement4) Nature and location of sediment sources, and 5) The nature of the sedimentary processesdelivering sediments to depocenters.

The data values are in meters and represent the depth to acoustic basement. It should be noted that acoustic basement may not actually represent the base of the sediments. These data are intended to provide a minimum value for the thickness of the sediment in a particular geographic region.

A digital total sediment thickness database for the world’s oceans and marginal seas is being compiled by the National Geophysical Data Center (NGDC), Marine Geology & Geophysics Division. The data are gridded with a spacing of 5 arc-minutes by 5 arc-minutes. Sediment thickness data were compiled from three principle sources: previously published isopach maps; ocean drilling results, both ODP and DSDP; and seismic reflection profiles archived at NGDC as well as seismic data and isopach maps available as part of the IOC’s Geological/Geophysical Atlas of the Pacific (GAPA) project.


■ FIGURE 5-20 Thickness of sediments on continental margins and ocean basins. The thick-est sediment layers occur near the mouths of major rivers or in submarine trenches where sediments are scraped off the subducting plate. The thinnest sediment layers occur on oce-anic ridges. Moderately thick layers occur in the Southern Ocean and the equatorial Pacific.

1. What are the oldest marine sediments ever recovered? In December 1989, ODP scientists drilled Hole 801C

and recovered Jurassic-age rocks and sediments, about 170–165 million years old, from the Pigafetta Basin in the western Pacific, near the Mariana Islands. Sediments of nearly identical age had been found previously from the Deep Sea Drilling Project Hole 534A, located on the Blake-Bahama basin in the central Atlantic. These sedi-ments and the rocks on which they rest are among the oldest oceanic crust found in the world ocean.

2. What are the oldest sedimentary rocks ever found? Before about 1998, scientists thought that the Isua supra-

crustal belt in Greenland represented the oldest sedimentary rocks, about 3.8 billion years old. Reanalysis and reinter-pretation of these rocks indicate that they are metamorphic in origin and not sedimentary. This finding was significant because claims were made for chemical signs of life in these rocks. Their metamorphic origin excludes the possibility that life ever existed in these rocks. The next candidates are

sedimentary rocks in Western Australia and South Africa, dating to about 3.5 billion years ago. Some scientists ques-tion their origins as well, so the question of the oldest sedi-mentary rocks on Earth remains open to interpretation.

3. If the Earth is getting cooler, why are we worried about global warming?

While it is true that our natural climate has been transi-tioning from “hothouse” to “icehouse” conditions over the past 5 million years, considerable evidence exists that human activities have reversed that trend and created unprecedented warming in the past 100 years. Some scientists argue that if our climate had not been cooling, then global warming would be even more severe than it is now. Still other scientists suggest that human activities might counteract the effects of cool-ing. A great deal of uncertainty remains concerning the long-term effects of anthropogenic releases of carbon dioxide, which is why studies of marine sediments and other climate research studies are so important.

cha16543_ch05_072-089.indd 86 11/15/06 2:25:11 PM


Anthropocene, 81authigenic, 77biogenous, 77biological pump, 82biological weathering, 79bioturbation, 83calcium carbonate compensa-

tion depth, 83chemical, 77chemical weathering, 79continental weathering, 79cosmogenous sediment, 78delta 18O, 74

descriptive classification, 76diagenesis, 83diatoms, 78foraminifera, 74genetic classification, 76grain size distribution, 80grains, 78granular, 77Heinrich events, 75hydrogenous, 77hydrologic cycle, 79interplanetary dust particles, 78Kullenberg piston corer, 74

lithification, 83lithogenous, 77logging, 77lysocline, 82Milankovitch cycle, 74paleoceanography, 73physical/mechanical weather-

ing, 79poorly sorted, 80principle of superposition, 73proxy, 74radiolarians, 85rolling, 80

saltation, 80sediment, 73sediment cores, 73sediment cycle, 79sediment trap, 80sedimentation, 78size classification, 76Stoke’s law, 80suspension, 80tektites, 78threshold velocity, 80Udden-Wentworth scale, 78well-sorted, 80

Sediments can provide insights into past ocean and atmo-spheric climates, a field of study known as paleoceanography.

Sediments may be classified according to their origins, their properties, or their size.

Sedimentary processes include the origin, transport, deposi-tion, and lithification of sediments.

Biological processes play an important role in the formation, transport, and deposition of sediments.

The global distribution of sediments on the seafloor depends on the factors and processes that form, transport, and deposit sediments at a particular location.

Visit www.mhhe.com/chamberlin1e for access to chapter quizzing, key term flash cards, video clips, interactive activi-

1. Visit the Ocean Drilling Program website, www. oceandrill-ing.org/, and explore one of the preliminary reports under Publications. Prepare a brief summary of the drilling expe-dition, including drill sites and major findings. Discuss the importance of the study in terms of improving our under-standing of climate change, plate tectonics, or the nature of marine sediments.

2. Find a sample of sand, mud, clay, or some other type of sedi-ment. Examine it using a magnifying glass and describe its characteristics. Classify your sediment as terrigenous, biog-enous, or chemical. Postulate on its origins, using your obser-vations as evidence. Write a story or scientific essay describing a possible path for your sediment from the location where you found it to the bottom of the sea.

3. Compare and contrast the weathering of rock and the trans-port of sediments under dry and wet conditions. Perform

an identical comparison with opposite conditions during weathering and transport (i.e., dry weathering-wet transport and wet weathering-dry transport). Discuss the clues a pale-oceanographer might find millions of years later that would suggest what kind of climate produced the sediment and what kind of climate transported it to the site where it was found.

4. Based on what you have learned about the role of biological processes on sedimentation in the ocean, discuss the ways in which biological processes might affect sediment processes on land. Provide a few examples based on your own outdoor experiences. (Hint: what do tree roots do to sidewalks?)

5. How is the past or present a key to the future where climate is concerned? Write an essay that discusses the role of pale-oceanography or modern-day sediment-related research in helping us to better understand the causes and consequences of increased atmospheric greenhouse gases.

ties, and more. Further enhance your knowledge with web links to chapter-related material!

cha16543_ch05_072-089.indd 87 11/15/06 2:25:13 PM


Exploration Activity 5-1

Question for InquiryHow do scientists use records of the past prior to the existence of humans to understand the impacts of humans on natural processes in the present?

SummaryIn Spotlight 5.2, we introduced the Anthropocene, a newly proposed geologic time period that recognizes human influ-ence on Earth systems. Like our Exploration Activity for Chapter One, we examine here in greater detail the impacts of humans on natural systems. As before, our goal is to illuminate the scientific evidence for these impacts. However, we also wish to provoke your thinking about the role and limitations of studies of past climates in the interpretation of present-day climate change.

Learning ObjectivesBy the end of this activity, you should be able to:

Compile evidence of human activities and impacts since the mid-1800s

Explain the limitations of evidence for supporting claims of human impacts

Propose a set of observations or experiments that would strengthen the case for or against human impacts on natural systems

Evaluate scientific attempts to determine impacts on natu-ral systems

Part I: Sharpen Your Analytical SkillsThe quotation below comes from the Preface to Man and Nature (re-published as The Earth As Modified by Human Action) written by George Marsh in 1864. Carefully review these paragraphs and make notes on the key points. Identify sections that pertain to geological, physical, chemical, and bio-logical processes. Record your thoughts and opinions, too. . . . In the rudest stages of life, man depends upon spontaneous ani-mal and vegetable growth for food and clothing, and his consump-tion of such products consequently diminishes the numerical abun-dance of the species which serve his uses. At more advanced periods, he protects and propagates certain esculent vegetables and certain fowls and quadrupeds, and, at the same time, wars upon rival organisms which prey upon these objects of his care or obstruct the

increase of their numbers. Hence the action of man upon the organic world tends to derange its original balances, and while it reduces the numbers of some species, or even extirpates them altogether, it multiplies other forms of animal and vegetable life. The extension of agricultural and pastoral industry involves an enlargement of the sphere of man’s domain, by encroachment upon the forests which once covered the greater part of the earth’s surface otherwise adapted to his occupation. The felling of the woods has been attended with momentous consequences to the drainage of the soil, to the external configuration of its surface, and probably, also, to local climate; and the importance of human life as a transforming power is, perhaps, more clearly demonstrable in the influence man has thus exerted upon super-ficial geography than in any other result of his material effort. Lands won from the woods must be both drained and irrigated; river-banks and maritime coasts must be secured by means of artifi-cial bulwarks against inundation by inland and by ocean floods; and the needs of commerce require the improvement of natural and the construction of artificial channels of navigation. Thus man is com-pelled to extend over the unstable waters the empire he had already founded upon the solid land. The upheaval of the bed of seas and the movements of water and of wind expose vast deposits of sand, which occupy space required for the convenience of man, and often, by the drifting of their particles, overwhelm the fields of human industry with inva-sions as disastrous as the incursions of the ocean. On the other hand, on many coasts, sand-hills both protect the shores from erosion by the waves and currents, and shelter valuable grounds from blasting sea-winds. Man, therefore, must sometimes resist, sometimes promote, the formation and growth of dunes, and subject the barren and fly-ing sands to the same obedience to his will to which he has reduced other forms of terrestrial surface. Besides these old and comparatively familiar methods of mate-rial improvement, modern ambition aspires to yet grander achieve-ments in the conquest of physical nature, and projects are meditated which quite eclipse the boldest enterprises hitherto undertaken for the modification of geographical surface. . . .

In teams or individually, complete the following. Refer to sec-tions in your textbook and other sources when necessary.

1. For each paragraph above, list and briefly describe in a table (see the example) of the types of mid-1800 human activities on the next page. For example, agricultural activity in the mid-1800s involved plows that were pulled by animals, and so on. Consult historical sources to expand and refine your list and descriptions.

Exploration Activity 5-1 Exploring the Anthropocene Through Critical Thinking

cha16543_ch05_072-089.indd 88 11/15/06 2:25:14 PM


Part II. Gathering Evidence of Human ImpactsSpotlight 5.2 offers one example of the kinds of scientific stud-ies that are being conducted to quantify the impacts of humans on natural systems. Yet determination of human impacts can be understood at least qualitatively by a number of other different means. Complete the following:

1. Brainstorm and list the various ways that humans recorded their activities in the 18th century and in modern times. (Hint: how do you keep “memories” of important events for future enjoyment and reference.)

2. Use any available resources to document at least three 21st century modifications to natural systems that have been or could be attributed to human activities. (A google search on glacier retreat will bring up an obvious example of the effects of global warming on glaciers. We’re confident you can think of many others.)

3. Write a short comparison of your 18th century and 21st century evidence. Describe as objectively as possible the differences between the two centuries (as revealed by your evidence). Can you conclusively state that humans are the cause or do other possible causes exist? What information is missing and how might you gather additional or alterna-tive evidence to better support human-caused change?

Part III. If You Could Turn Back Time . . .Late one night while surfing the Internet, you accidentally stumble upon a blueprint for a time machine, which you quick-ly build. Unfortunately, just as you are about to get away from it all, your oceanography professor pulls the plug. “Bring me sci-entific evidence of natural processes before the time of humans and I’ll give you an A in the class,” your professor offers. You agree and are soon on your way. To pass the time as the centu-ries roll backwards, you begin to think about where you will go and what you will do when you get there. You prepare a written proposal that includes the following:

The geologic time periods you would visit and the ratio-nale for visiting them

The evidence you would obtain and your rationale for obtaining it

The ways in which your evidence would complement modern-day evidence

Use the Rubric for Analzying and Presenting Scientific Information to self-evaluate your proposal. (See Exploration Activity 1-1.) Discuss it with your instructor and classmates and revise it as needed.

Analogous 19th century activity 21st century activity 19th century impact 21st century impact

Farming with plows Farming with tractors Conversion of natural Release of carbon dioxide lands to agricultural and expansion of ones. agricultural land use

Fertilization with manure Fertilization Possible local impact Eutrophication and with chemicals. on water quality other water quality impacts.

Add your own . . .

Examples of 19th and 21st century human activities and impacts.

2. For each item you entered in the table, list and briefly describe its modern analogue. For example, farming in the 21st cen-tury involves combustion-driven farm machinery.

3. Complete the table by listing the impacts of 19th and 21st century practices on geological, physical, chemical, and bio-logical processes.

cha16543_ch05_072-089.indd 89 11/15/06 2:25:14 PM

5 ocean sediments

Documents