6.10: The Sediment Historical Record

Sediments accumulate continuously by deposition of new layers of particles on top of previously deposited layers. Therefore, individual layers that make up the sediment below the sediment surface were deposited progressively earlier as depth below the sediment surface increases. The type of particles deposited at a specific location and time depends on many factors, including, the proximity to land, whether rivers or glaciers flowed into the adjacent coastal ocean, the type and amount of dust in the atmosphere, and the composition and production rates of organisms in the water column. These factors do not remain constant over geological time because they are affected by plate tectonic movements, climate changes, and changes in volcanic activity, all of which may be related to each other in complex ways. Such changes, singly or in combination, can move coastlines, change seafloor depth, raise mountains, alter rivers, create and melt glaciers, change atmospheric dust composition by creating or reducing deserts or by altering the amounts of volcanic material injected into the atmosphere, and change the temperature, salinity, and nutrient distributions in the oceans. Each of these changes affects the type of sediment that accumulates. Hence, buried sediments may be very different from those currently accumulating.

Buried sediments provide information about the conditions at the time they were buried. Ocean sediments, therefore, can provide a history of plate tectonics and climate changes in that location covering about 170 million years, the approximate age of the oldest remaining oceanic crust in the major oceans. Older fragments of oceanic crust are known to exist, but it is not yet known whether they support undisturbed sediment layers. To read the history of ocean sediment layers, we must be able to determine the date at which a particular layer of sediment was deposited. Then, we must examine the composition of the sediment particles to reconstruct the sedimentation conditions at the time the layer was formed.

Stratigraphy is the study of the Earth’s history through investigation of the sediment layers beneath the ocean floor (and on land, where sediments have been compressed and converted into sedimentary rock). Stratigraphic studies of ocean sediments usually involve the examination of sediment cores (Chap. 3), which are sampled every 1 or 2 cm (Fig. 6-20). Each 1–2 cm section represents the accumulation during a period of thousands or tens of thousands of years, depending on the sedimentation rate.

Figure 6-20. Sediment cores, such as this deep-sea drilling core, are first cut down the middle and then marked every 1 to 2 cm, each 1–2 cm section representing the accumulation of sediments during a period of hundreds or thousands of years in the past.

The stratigraphic record can be complicated to read. Gaps may appear in the record if previously deposited sediment was eroded because of a temporary increase in current speed or a decrease in sea level relative to the continent. Furthermore, adjacent layers may have had substantially different sedimentation rates. Biological activity also may have altered the historical record. Bottom-dwelling organisms rework, or mix, the upper few centimeters or tens of centimeters of sediment as they consume the organic matter, feed on other sediment dwellers, or build burrows. Their churning of the sediments is called bioturbation. Despite these limitations, the stratigraphic record provides vital information about the Earth’s history, including previous climate changes. Understanding climate changes will enable us to better understand the factors that control changes in climate, such as those expected to result from the enhancement of the greenhouse effect.

Sediment Age Dating

The age (date of deposition) of the sediment layers is essential for stratigraphic, or sediment layering, studies. While we can date the sediments relative to one another (younger sediments are on top), determining their numerical age is often difficult. Methods for determining sediment ages include radioactive dating (CC7), magnetic dating, and biological dating.

Radioactive dating of sediments is difficult because often the assumptions on which the technique is based, including minimal weathering of the grains, are not met. Therefore, researchers commonly use several different methods to be certain of a measured age, including dating of sediments based on their magnetic properties. To explain how magnetic dating is done, we need to understand first that the magnetic poles appear to have been close to the Earth’s north and south poles of rotation for billions of years, even though they wander slightly. However, the Earth’s magnetic field completely reverses periodically for reasons we do not yet fully understand. During a reversal, the north magnetic pole is located close to the geographic South Pole and the south magnetic pole close to the geographic North Pole. The sequence and ages of these reversals are well documented from studies of terrestrial rocks. In sediments, particles that contain iron or other magnetic materials tend to align with the magnetic field during or soon after sediment deposition. As a result, the magnetic orientation of sediment varies with depth below the sediment surface, changing at each depth in the sediment column that corresponds to a magnetic reversal. Measuring the magnetic properties of the sediment layers and correlating the depth of layers where changes in magnetic field occur with the known dates of magnetic pole reversals, therefore allows us to date the sediments.

Another important dating technique is based on the fossil species found within the sediments. Many of the marine species whose remains make up the biogenous fraction of older sediments have become extinct, have been replaced by new species, or have evolved into substantially different forms. Consequently, the ages of sediments at different locations can often be matched by comparing the species compositions of organisms found in their biogenous fractions. If the sediment at a specific depth at one sampling site contains the fossil remains of all the same species that the sediment at a different depth at another sampling site contains, the sediments are likely to be the same age. Thus, we can date sediments in relation to each other and determine relative sedimentation rates.

Diagenesis

Interpreting the stratigraphic record requires an understanding of diagenesis, the physical and chemical changes that occur within sediment after it is buried. In some cases, during the millions of years they lie buried, minerals are altered from one form to another as weathering processes that began on land or in suspended sediment continue. However, the more important diagenetic changes occur in the pore water (interstitial water) that is trapped between the sediment grains.

Chemicals that dissolve in pore waters during diagenesis include silica and calcium carbonate, which continue to dissolve from buried siliceous and calcareous material. Also, during diagenesis, other substances are released by continuing oxidation of sediment organic matter and minerals, and these too may be dissolved in the pore water. Chemicals dissolved from the sediment particles during diagenesis migrate upward by diffusion toward (and in some cases into) the overlying water. As sediments accumulate, their weight compacts the underlying sediment. In the process, pore waters are slowly squeezed out and migrate upward through the sediment.

As organic matter decomposes in the sediment, dissolved oxygen in the pore water is consumed, and oxidation continues with oxygen from nitrates and then sulfates (Chap. 12). The sulfates are reduced to sulfide, which dramatically changes the solubility of many elements. Iron, manganese, and several other metals form sulfides that are much more soluble than their hydrated oxides, which form in oxygenated water. In sediment with enough organic matter to produce sulfides, iron and manganese are dissolved in pore waters that migrate upward. The metal sulfides migrate with the pore water until it approaches the sediment surface, where it encounters dissolved oxygen diffusing down into the sediment or introduced by bioturbation. The iron and manganese then are oxidized and redeposited on sediment grains. As sediment accumulates, these elements may be dissolved and moved upward continuously, accumulating in a layer near the sediment surface where sulfide and oxygen mix.

In contrast to iron and manganese, the sulfides of many elements (e.g., copper, zinc, and silver) are less soluble than their hydrated oxides. These metals are not dissolved in pore waters that contain sulfide, so they do not migrate up toward the oxygenated surface sediments and overlying seawater. On the contrary, these elements can diffuse with oxygenated seawater downward into the sediment, where they can be converted to sulfides and buried with the accumulating sediment. The complex migrations of elements within pore waters and the associated changes in the nature of the sediment particles must be understood if we are to deduce the nature of the sediment particles at the time of their deposition and thus read the stratigraphic record.

Diagenetic processes are also important to life. For example, the nutrients nitrate, phosphate, and silicate are transported to the sediments in detritus that falls to the sediment surface. The nutrients are released to the pore water as organic matter is oxidized and they then diffuse back into the water column, where they can be reused by living organisms.

Tectonic History in the Sediments 

Much of the history of tectonic processes can be revealed by studies of the changes in sediment characteristics with depth below the sediment surface. For example, consider the sediment layers that have accumulated at locations between the oceanic ridges and continents (Fig. 6-11). Remember that the seafloor is progressively older with increased distance from the oceanic ridge (Chap. 4). However, the older seafloor was once at the oceanic ridge and sank isostatically (CC2) as it cooled, so it becomes progressively deeper. Remember also that all sediments are mixtures (Fig. 6-16). As a result, sediment deposits can reveal the history of plate tectonic movements. For example, the age of a buried calcareous sediment layer can reveal the times during which the seafloor was at depths shallower than the CCD.

In reality, reading the sediment history is complicated because important factors, such as the CCD level, have undoubtedly varied over the millennia. In addition, the locations at which certain types of organisms are abundant may have changed. Currently, siliceous diatoms abound near the poles, and siliceous radiolaria dominate near the equator. However, the distributions of these organisms might have been very different at times in history when the climate and paleogeography (Fig. 4-9) was different from the present.

Climate History in the Sediments 

The past 170 million years of the Earth’s climate history are preserved in ocean sediments, primarily in biogenous particles. Each marine species that contributes calcareous and siliceous material to ocean sediments has an optimal set of conditions for growth. Although nutrient concentrations, light intensity, and other factors are important, temperature is crucial for most species. Hence, fossils in sediments can be used to determine the geographic variations in ocean water temperatures.

Because different species of foraminifera thrive at different temperatures, the species composition of foraminiferal fossils in buried oozes can be used to study past climate. However, because some species have evolved or become extinct, the temperature tolerances of fossil species often are assumed to be the same as those of modern species. This assumption may not be correct.

Past climate information also can be obtained from measurements of the ratio of oxygen isotope concentrations in calcareous sediments. The basis for this method is somewhat complicated. Oxygen has three isotopes: oxygen-16, oxygen-17, and oxygen-18. Almost all ocean water molecules consist of either an oxygen-16 atom combined with two hydrogen atoms, or an oxygen-18 atom combined with two hydrogen atoms (the concentration of oxygen-17 is very small). Oxygen-16 is lighter than oxygen-18. Consequently, water containing oxygen-16 is lighter than water containing oxygen-18. The lighter water molecules evaporate more easily than the heavier molecules. Hence, when seawater evaporates, the water vapor is enriched in oxygen-16. When the Earth’s climate cools, more water is precipitated and stored in the expanding glaciers and polar ice sheets. Because this water was evaporated from the oceans, oxygen-16 is transferred preferentially to the ice, the ratio of oxygen-16 to oxygen-18 in ocean water decreases, and sea level falls. Organisms that live in seawater incorporate the oxygen-16 and oxygen-18 of seawater into their calcium carbonate body parts in a ratio that is determined partly by the ratio of these two isotopes in the seawater. Therefore, the ratio of oxygen-16 to oxygen-18 in calcareous sediment can be used to deduce climatic characteristics and sea level at the time of deposition. The ratio is low when sea level is lowered by evaporation and more water is stored on land as ice and snow and high when the climate is warm and sea level is high.

In the same way that water molecules containing oxygen-16 and oxygen-18 evaporate at slightly different rates, molecules containing these oxygen isotopes react at slightly different rates in the chemical processes that produce the calcium carbonate of calcareous organisms. The ratio of oxygen-16 to oxygen-18 in the calcareous parts differs between species, but for a single species, it depends on the ratio in the seawater and the seawater temperature. At any given time, the oxygen isotope ratio of ocean surface water is relatively uniform throughout the oceans. Hence, calcareous remains of the same species deposited in different parts of the ocean at the same time have different ratios of oxygen-16 to oxygen-18 because the water temperatures were different where the particles were deposited. A low ratio of oxygen-16 to oxygen-18 indicates colder water.

Studies of sediments aimed at revealing the Earth’s climate history have become more important because of growing concerns about human enhancement of the greenhouse effect. An understanding of past climates can help us understand future climate change due to both natural causes and human activities. Stratigraphic studies are becoming ever more sophisticated and are now beginning to reveal details of ancient ocean current systems and ocean chemistry, including carbon dioxide concentrations. Such information continually improves our understanding of the complicated feedback mechanisms between the atmosphere and oceans (Chap. 7, CC9).

History of Ocean Acidification and Deoxygenation in the Sediments

As we have seen in the previous section, ocean sediments can be used to study the variation of temperatures in Earth’s past. Sediments also provide a historical record of changes in ocean water chemistry. For example, past changes in the acidity of the oceans and in the extent of oxygenation of the oceans can be revealed by studies of sediments and the fossils found in sediments. The techniques used to extract acidity and oxygenation information from sediments are complex and beyond the scope of this text. However, the results of such studies provide important information on the likely effects of anthropogenic emissions on ocean ecosystems. This information suggests that acidification and deoxygenation may have severe adverse effects comparable to, and perhaps greater than, the adverse effects of climate change. 

Evidence of Mass Extinctions 

Earth has experienced at least five major mass extinctions of species. All but one of these occurred farther in the past than the record provided by current ocean sediments. These earlier extinctions can be studied by records left in sedimentary rocks that were formed from sediment deposited when each event occurred. Sedimentary rocks are more difficult to study than sediments since they are more likely to have been altered by chemical and physical processes that occurred after they were formed and uplifted by tectonic processes. However, each of the mass extinctions has been investigated and it has been learned that, in at least two of these events, the extinction was the result of climate warming due to rising atmospheric carbon dioxide levels, and associated ocean acidification and deoxygenation. The atmospheric carbon dioxide rise in most or all extinctions was likely due to increased volcanic activity. Volcanic activity can introduce large amounts of dust and gas to the atmosphere, altering global climate and causing extinctions. For example, the 1883 eruption of Krakatau, a relatively small eruption in comparison with some that have occurred in the past, is known to have affected the world climate for a decade. Large increases in volcanic activity are known to have occurred periodically in Earth’s history

The rate of increase of carbon dioxide in the atmosphere during Earth’s mass extinctions is much slower than what Earth is now experiencing due to anthropogenic releases, but past extinctions were not instantaneous events and instead took place over at least thousands of years. Volcanic activity can release large quantities of sulfur that have a similar but different effect on the atmosphere and oceans than carbon dioxide. Despite these differences, a better understanding of past extinctions can provide us with a better understanding of the relative effects of anthropogenic climate change, ocean acidification, deoxygenation in the future and of whether or not anthropogenic inputs may lead to another mass extinction.

Investigating a Mass Extinction 

About 65 million years ago, the last of the dinosaurs became extinct. Fossil and sediment records show that, at the same time, about 70% of all species then on Earth, including more than half of all species of marine animals, became extinct. Although other extinctions have been found at different times in the sedimentary rock record, the dinosaur extinction was among the most dramatic. 

For a number of years, there was a major debate about what caused this extinction. It was known that a major increase in volcanic activity occurred at about the time of the extinction event. However, dating of events such as the eruption and extinction is difficult and until recently the volcanic event, called the Deccan Traps, flood basalt eruption in China was thought to have occurred about 300,000 years before the extinction event. Instead, evidence supported the hypothesis that a meteorite impact near Chicxulub, Mexico could have been responsible for the extinction. The studies leading to this meteorite impact conclusion provide a good illustration of how past events can be deduced from sediments.

The area that is now the Yucatán Peninsula of Mexico was underwater 65 million years ago at a depth of about 500 m. Magnetic surveys in this area reveal a circular feature (Fig. 6-21), 180 km in diameter, that may be the buried remains of the impact crater of a huge meteorite. Studies of sedimentary deposits from many areas surrounding this circular feature, now called Chicxulub, show an unusual series of layers that correspond to deposition 65 million years ago.

Figure 6-21. The location of the Chicxulub crater, which is thought to be the impact site of a massive meteorite that may have been responsible for climate changes that, in turn, led to the extinction of the dinosaurs. The crater is at least 180 km in diameter, but it may be twice that size.

Sediments below the strange layers are fine-grained biogenous oozes of the type normally deposited at a depth of approximately 500 m. The unusual layers immediately above these normal sediments have characteristics that could be explained by the sequence of events that followed this massive meteorite impact. The deepest and oldest strange layer consists primarily of rounded, coarse grains several millimeters in diameter. These grains were not rounded by water weathering. They are glassy tektites, which form when rocks are melted by meteorite impacts, ejected into the atmosphere where they resolidify, and fall back to the Earth. This layer also contains high concentrations of iridium, an element rare on earth but more common in meteorites. An iridium-rich layer is found in 65-million-year-old sediments from all parts of the oceans but decreasing in concentration with distance from Chicxulub, providing evidence that a major meteorite hit spread dust through the entire Earth’s atmosphere at this time and that the impact took place near Chicxulub.

The next higher layer consists of coarse-grained sediments that contain fossilized wood and pinecone fragments. These materials are not normally found in marine sediments. It is hypothesized that this layer was created when the meteorite impact generated a huge tsunami (Chap. 9) that smashed onto land, tore rocks and trees loose, and carried them back to sea as it receded. The tsunami may have been so large that it sloshed back and forth across the Gulf of Mexico and around the world for at least several days. It must have been many times higher than the tsunamis that hit Indonesia in 2004 and Japan in 2011, perhaps several hundred meters or more high.

Above the two older extraordinary layers is a series of layers of progressively finer-grained sand mixed with other mineral particles that are not normally found in marine sediments. These progressively finer-grained layers may consist mostly of beach sands and eroded soil particles suspended and transported to the deep water by waves generated by the meteorite impact. As wave energy decreased after the initial meteorite impact and tsunami, progressively smaller particles would have been deposited in the same way that turbidites are. Much of the finer-grained material overlying the graded sequence may have come from the fallout of dust ejected into the atmosphere during the impact. 

Above these finer-grained layers, the sediments finally grade into normal biogenous oozes, similar to those that underlie the unusual layers, but with fewer and different species. For example, below the anomalous layers, foraminifera species were large, abundant, and diverse, but above they were almost absent, smaller and of many fewer species.

There are many possible variations of the details of the events that deposited the anomalous sediment layers 65 million years ago. For example, one suggestion is that the tsunami may not have been caused directly by the impact of the meteorite on the ocean. Instead, the impact may have caused a magnitude-11 earthquake (about a million times more powerful than the 1989 Loma Prieta earthquake near San Francisco), which could have sent giant turbidity currents down the continental slope and thus generated massive tsunamis. 

Additional details of the Chicxulub impact have emerged as studies of the impact continue. For example, it has been estimated that the object that hit the Earth was about 10 km in diameter, that the impact created a crater more than 180 km in diameter, and that it must have caused strong earthquakes, and most likely a massive fireball. The meteorite impacted carbonate and sulfur-rich rocks and it has been estimated that 100–150 gigatons of sulfur were released to the atmosphere. This would have caused devastating acid rains, while the dust blown into Earth’s upper atmosphere would have blocked enough sunlight to substantially cool Earth’s climate for decades or longer. This amount of dust would also drastically reduce the sunlight available for photosynthesis. Photosynthetic organisms are the base of both terrestrial and marine food chains, so the loss of available food would have caused many species populations to crash to, or close to, extinction even if they were not adversely affected by the initial firestorm, earthquakes, and tsunami or the subsequent acid rain.

As we can see, the odd sediment layers in the Gulf of Mexico have begun to reveal a detailed picture of what must have been one of the largest events in Earth’s history. This evidence appeared to support a hypothesis that the mass extinctions were caused by the meteorite impact rather than by a major increase in volcanic activity. However, as often happens in science, the simple answer is not always correct. More recent studies of fossils in sediments from before and after the impact, and better dating of sediment layers have now revealed the impact is likely not the single cause of the mass extinction. This more recent data now suggests that a series of massive volcanic eruptions took place over a time period that straddles the extinction, with at least four large pulses of volcanic activity, one of which preceded the extinction. Also, more detailed dating of the disappearance of species in the fossil record showed that a number of species became extinct before the impact, while a comparable number of species became extinct after the impact. 

It now appears that this mass extinction, that included an end to the dinosaurs and led to the rise of mammals, was caused by a double disaster. First, there was an episode of increased volcanic activity on the planet, causing rapid climate warming and almost certainly acidification and deoxygenation (as scientific evidence supports likely occurred during other extinctions). Second, during the period of increased volcanic activity, there was the massive Chixulub meteorite impact. 

The sediments related to this mass extinction will undoubtedly be studied extensively, and their meaning debated for many years, and this double disaster scenario may change again, but there are two important lessons to learn. First, environmental events do not always have a simple single cause. They are most often a combination of several processes interacting with each other. Second, our tendency to think of natural events on human time scales can be misleading. A mass extinction that takes 100,000 years or longer to take place is a very abrupt change on the timescale that controls Earth’s climate and life. We think of the anthropogenic impacts on Earth as having taken place over a long period of centuries, but in geological time, humans have caused changes that are far faster than anything seen on Earth before, or at least since the Earth’s formation and early history. 

Evidence of Other Impacts

Researchers believe that there may have been a number of other large impacts similar to the Chicxulub event at different times in the past, and they are searching for evidence of the impact craters. A number of locations have been identified as possible impact sites, including an area off the northwestern coast of Australia, where there may have been an impact about 250 million years ago, and an area at the mouth of the Chesapeake Bay, where an impact may have left an 85-km-wide crater about 35 million years ago. Decades of careful research will undoubtedly be needed to demonstrate that meteorite impacts did or did not occur in these areas and others. If confirmed, additional research will be needed to investigate and identify the effects that those impacts may have had on the biosphere.