Stratigraphic Tools: Basic Sequence Stratigraphy

Conformable Ordovician limestone layers near Carthage, Tennessee
Conformable Ordovician limestone layers near Carthage, Tennessee

KEY CONCEPTS

After reading this chapter, students should be able to:

  • Define a sequence in the context of stratigraphy.
  • Describe basic sequence hierarchy and architecture.
  • Describe a basic understanding of accommodation space and the variables that affect that space.

An Introduction to Sequence Stratigraphy

A sequence is a pattern of observable phenomena that occur in a predictable order. In sequence stratigraphy, these are layers of sediment deposited over time in a predictable manner due to the effects of environmental changes that affect tectonics, sediment supply, and sea level.

Over time, environments change due to a wide variety of long-term and short-term events within the Earth system. Earth system events can be extrinsic, coming from the space environment, or intrinsic, resulting from an event within one of the Earth’s spheres. Examples of an intrinsic event might be a large flood-basalt eruption. Such a geosphere event will directly impact the hydrosphere, atmosphere, and biosphere. An example of an extrinsic event would be the gradual changes in Earth’s orbital or tilt parameters (Milankovitch Cycles). These affect the amount of incident solar radiation at a number of levels, which impacts all of the Earth’s spheres.

A representation of five "spheres" that are a part of the Earth system. From top around clockwise, the atmosphere, biosphere, geosphere, cryosphere, and hydrosphere. Events that occur within the system have direct and indirect effects that are transferred through interactions between systems. Such interactions lead to changes in sedimentary deposition that is recorded in basins.
A representation of five “spheres” that are a part of the Earth system. From top around clockwise, the atmosphere, biosphere, geosphere, cryosphere, and hydrosphere. Events that occur within the system have direct and indirect effects that are transferred through interactions between systems. Such interactions lead to changes in sedimentary deposition that is recorded in basins.

Events like these that occurred in Earth’s deep past are often hard to understand as a whole, let alone when attempting to examine their genesis, progress, or ultimate result. Geoscience historians care about these events because they have direct and indirect effects on the Earth’s individual spheres. And, understanding how systems like the hydrosphere, geosphere, and atmosphere have interacted in the past provides a great deal of insight into similar processes occurring today. If we can understand how Earth system events lead to change, we can create predictive models for what that change looks like in the rock record. In deep time, those records are recorded in sedimentary basins. Sequence stratigraphy is a field that provides important tools for understanding the course and cause of these events in Earth’s past. And, we can apply these lessons to understanding them today.

The ultimate goal of sequence stratigraphy is to understand the Earth’s past as recorded in the stratigraphic history of a basin. Sometimes, the impetus for this work comes through the search for natural resources, like coal and petroleum. Sometimes, the impetus is pure research and curiosity. We can then use the stratigraphic tools like lithostratigraphy and biostratigraphy to measure and analyze stacks of rock layers. Then, correlating units across a region, it is possible to reconstruct this history. At this point, it becomes possible to quantify changes in water depth, sedimentation rates, rates of tectonic change, and rates of global (eustatic) sea level change. Over time, all of these factors will produce packages of sediment that are related to one another and bounded on the top and bottom by unconformities, or sequence boundaries, that can be correlated over distances. These related and predictable packages of rock are referred to as sequences. Sequences of strata are genetically-related packages of rock bounded by unconformities. Within sequences, Walther’s Principle of facies migration apply. Across these sequence boundaries, it does not.

Sediment needs its space: Accommodation, Sea Level, and Sediment supply

Basins are not static features. Basins are influenced by the physical processes around them as they record the history of the surrounding land and of their own past. They also come in a variety of shapes and sizes. Some basins, like the Atlantic or Pacific Ocean basins today, are massive. As sediments erode,transport downhill via wind and streams, and accumulate, they fill in the “accommodation space” that is available in that basin. Like a bowl of cereal, a basin only holds so much sediment, and fills in over time. Sedimentary systems strive toward an equilibrium between between sediment infill and available accommodation space, but various Earth events will upset this balance.

In order to calculate the accommodation space within a given current or past basin, the following equation is used:

T + E = S + W

Where:

T = Rate of tectonic subsidence, or sinking, in the basin

E = Rate of eustatic, or global, sea level change, relative to a reference ellipsoid

S = Rate of sedimentation/accumulation in the basin

W = Rate of change in water depth within basin

This equation represents a simple balance. If the left side of the equation is positive, then there is increasing accommodation space in the basin and, if negative, decreasing space. On the right, if the number is positive then there is increasing sedimentation and negative if there is not. Thus, if the numbers on both sides of the equation are both positive, then both space and sedimentation (S) are increasing. If the left is negative and the right side positive, then we can predict that the basin will be infilling over time until no more sedimentation is possible. While it seems simple enough, this equation is very difficult to actually use in practice and better serves as a model for thinking about and considering stratigraphic packages of sedimentary rocks (Holland, 2018).

Absolute sea level (ASL), also known as eustatic sea level, is an attempt at calculating a global average. It is the level that, under the force of gravity and if all waters were perfectly calm. Today, when we think of sea level rise due to global warming, this is the kind of change to which we refer. If so much ice melts in Antarctica or Greenland, sea level globally will rise to some amount (10cm to multiple meters, perhaps) over the next century. Due to global warming, the ocean waters are undergoing thermal expansion, causing the rises we see along our shorelines today. The rate of this change (E) can be difficult to measure. Absolute sea level is measured against a reference ellipsoid, an imaginary sphere within the Earth that can be used as a datum. From this, a geoid can be produced for sea level, which is the level at which it would be under the force of gravity and the Earth’s rotation alone, taking out tidal and other short-term variations. Changes in ASL happen slowly, but occur in reaction to global events, such as changes in climate.

Cross-section of the Earth showing important sequence stratigraphic definitions (from Holland, 2018).
Cross-section of the Earth showing important sequence stratigraphic definitions (from Holland, 2018).

Tectonic changes include variables like the uplift of land, isostatic rebound or the sinking of land, subsidence. Plate tectonic changes are the primary driver of all of the changes in these variables. Recall that oceanic lithosphere is primarily made up of basalt and that continental lithosphere is primarily made up of granite. Lithospheric plates are like ice cubes. These different rock types have different bulk densities, basalt being much more dense. This causes oceanic lithosphere to sit lower on the Asthenosphere than continental lithosphere. At subduction zones, new mountain ranges are thrust up as lithosphere buckles and thickens. This concentrates mass and leads to the subsidence of the land as foreland basins are created through downward flexure. These kinds of processes tend to deepen basins.

Other situations, such as ice melt on land, tend to shallow basins. During ice ages, times when the continents bear the burden of massive ice sheets, the extra mass of ice weighs down the plates. When the ice melts, local basins will rise as continents isostatically adjust their elevation. The rate of subsidence or uplift (T) is very important to know when calculating the accommodation space of a basin. For the purposes of accommodation space, the rate of subsidence and the rate of eustatic sea level change are the biggest and longest term factors. If eustatic sea level drops at the same time that subsidence increases, no new sedimentation space is created. If eustatic sea level drops at a slower rate than subsidence, new space is made, and available for deposition.

Seas are dynamic. It is a bit misleading to refer to “sea level”, in the sense that there is no absolute level at which the water rests, when measured against the floor of a basin. Measured relative sea level is an average depth, relative to this basin floor in a particular region. Relative sea level (RSL) is the water depth (W) of the basin. It is affected by many factors that can cause it to rise or fall. Some of these factors include increases in water mass, thermal expansion of water with a rise in temperature, changes in ocean currents or seawater salinity, and isostatic rebound adjustments of continents. When looking at NOAA tidal gage data today, it is the case that the eastern shore of Virginia is experiencing rising seas while the Gulf coast of Alaska is experiencing a net drop in sea level.

Relative sea level (RSL) trends for two sites on opposite sides of the North American craton. Kiptopeke, Virginia, is experiencing a relative rise in sea level of 3.97mm/yr where Kodiak, Alaska (southern coast) is experiencing a drop in relative sea level of -9.92mm/yr. While global (eustatic) sea levels are rising, local records will vary. Forecast trends for both sites, according to NOAA, indicate positive sea level trends into the next century. Note: The Kodiak, Alaska data has a large gap around the time of the 1964 Good Friday earthquake. Prior to the quake, the sea level trend was flat. After the quake, the sea level trend is negative. This negative trend is temporary, however, and may be a tectonic remnant of this event.
Relative sea level (RSL) trends for two sites on opposite sides of the North American craton. Kiptopeke, Virginia, is experiencing a relative rise in sea level of 3.97mm/yr where Kodiak, Alaska (southern coast) is experiencing a drop in relative sea level of -9.92mm/yr. While global (eustatic) sea levels are rising, local records will vary. Forecast trends for both sites, according to NOAA, indicate positive sea level trends into the next century. Note: The Kodiak, Alaska data has a large gap around the time of the 1964 Good Friday earthquake. Prior to the quake, the sea level trend was flat. After the quake, the sea level trend is negative. This negative trend is temporary, however, and may be a tectonic remnant of this event.
Glacial Isostatic Adjustment as equivalent H2O thickeness variation rate. In other words, gravitational anomalies (changes) as a result of tectonic rebound from the last glacial advance of the Pleistocene. Note the high levels of adjustment over areas of northern North America, Europe, and also Antarctica. Gravitation, as measured by NASA's GRACE mission, is balanced against the gradual recovery of ocean basins as they slowly spring upward, causing relative sea level around these noted areas to drop. Gravitational anomalies are due to mass loss, which can be mantle material moving to accommodate areas of the crust that are rebounding. Rates of mass change have been estimated to be as high as 104 gigatons under Greenland (Wu et al., 2010).
Glacial Isostatic Adjustment as equivalent H2O thickeness variation rate. In other words, gravitational anomalies (changes) as a result of tectonic rebound from the last glacial advance of the Pleistocene. Note the high levels of adjustment over areas of northern North America, Europe, and also Antarctica. Gravitation, as measured by NASA’s GRACE mission, is balanced against the gradual recovery of ocean basins as they slowly spring upward, causing relative sea level around these noted areas to drop. Gravitational anomalies are due to mass loss, which can be mantle material moving to accommodate areas of the crust that are rebounding. Rates of mass change have been estimated to be as high as 104 gigatons under Greenland (Wu et al., 2010). Image: NASA

The graph of tidal gauge data for Kodiak, Alaska, illustrates nicely the effect tectonics can have on relative sea level. Prior to the great 1964 “Good Friday” quake, sea level was stable. After the quake, not only did sea level start out much higher than the original datum prior to 1964, but it has been dropping ever since. This indicates that the land surface is undergoing uplift due to tectonic changes since the earthquake. Long term trends (not shown) according to NOAA do see sea level eventually rising here also, but overcoming tectonic effects takes time. Another important measure of relative sea level can be analyzed using satellite gravity measurements. In areas of the Earth’s lithosphere where there is more mass (thicker lithosphere), gravity is higher than normal, and vice versa. As in the GRACE data in the image above, areas subject to uplift now due to the retreat of glaciers since the last ice age are experiencing intense changes in gravity, indicating increasing mass in those areas, most likely due to mantle material moving into place beneath uplifting lithosphere. such tectonic change is important to consider also throughout Earth’s history, as a component of both relative and global changes in sea level. It is not always the water that changes, but sometimes it is the land.

When water levels are rising and a basin deepens, we call this change a transgression. When water levels are falling and the basin gets more shallow, we call it a regression. Whether it happens due to rising or lowering water or rising or lowering land, the effect is the same.

Sedimentation rates (S) vary a great deal. As already seen, they are most closely linked with the rate of change in water level (W) in the equation above. How do sedimentation rates change? There are a wide variety of factors. As mountains rise, sedimentation rates will also rise, as the land moves upward into the atmosphere and is exposed to ever more chemical and physical weathering. Likewise, the eventual erosion of such mountains will lead to drop offs in sedimentation into the basin. Sedimentation is also affected by the climate and latitude. Humid environments will have more runoff. This will lead to higher sedimentation rates. The type of geologic setting also matters. Siliciclastic environments source their sediment from outside the basin and so in these situations sediment supply is a much more significant factor in basin analysis. In carbonate settings, the sediment is produced within the basin by organisms precipitating CaCO3 directly from seawater.

Carbonate settings are very good at keeping up with sea level rise and this leads to the remarkable diversity in depositional settings. Today, carbonate platforms are more rare, existing in the Caribbean, Persian Gulf, and portions of the Pacific basin. In the past, particularly in the early to middle Paleozoic Era, such environments were much more common. The Great American Carbonate Bank, represented by the thousands of feet of variously fossiliferous Sauk Sequence limestones throughout the Mid-Atlantic region and the Cincinnati, Ohio area, are excellent evidence of this past. When it comes to sediment supply, we can also describe how relative sea level behaves with respect to sedimentation rates. When sediment supplies are increasing more rapidly than water levels, it is said that the coastal land is prograding. When sediment supplies are decreasing, it is said that the coastal land is retrograding. Thus, the rate of sedimentation (S) is critical to grasp when trying to discern whether sedimentation or water level is controlling changes in accommodation space. If progradation of a coastline occurs and sea level regresses, then accommodation space decreases. If coastlines are in retrograde and sea levels are transgress, then accommodation space in the basin will increase.

Sequence Stratigraphic: A Model for Discerning Basinal and Environmental Change

In the stratigraphic record of a basin, there can be packages of beds that repeat in a cyclic fashion. Sometimes, these are bounded by clear unconformities and mark major changes in tectonic regimes. In other situations, they are a remnant of changes in sea level. An entire sub-field of stratigraphy, sequence stratigraphy, is dedicated to the study of such cyclic phenomena. It will only be treated very lightly here. These cycles, however, can occur on a variety of scales. Sometimes, they represent tens of millions of years and sometimes only days. Their causes are open to interpretation, depending upon the scale of time involved, and the evidence contained in the record. One thing that is very clear is the important story these cyclic beds tell about the nature of repetitive phenomenon in nature. Climates change, continents move, and sea levels rise and fall. Such cycles have been occurring during the entirety of Earth’s history. However, stratigraphy can give us important insights into how one cycle differs from another or, in our modern situation, the sediments we leave today will tell the story of human impact on the environment. Packages of cyclic sediment being deposited today represent different inputs created by our activities.

Seismic data collected in the search for new petroleum reserves in the 1960s and 1970s led to the identification of packages of genetically-related strata. These strata, bound by unconformity surfaces, came in various thicknesses. Later on, the field of sequence stratigraphy was developed that sought to make sense of these packages of strata. The longest of these cycles, called 1st Order Megasequences, have periods of several hundred million years. The shortest of these, what are referred to as 5th order or higher, could have a cyclicity of just days, a remarkable degree of resolution for the geologic record. This stratigraphic revolution has led not only to new successes in economic geology, but arguably more important understandings of how natural cyclicity works on a wide variety of scales, from global to a very specific location.

Sequence Hierarchy

Stratigraphic sequences are superimposed on one another. This is because environmental phenomena that create cycles of various orders are also happening concurrently. One of the jobs of a sequence stratigrapher is to separate these storylines. Nested within the time span of a 1st order megasequence, a number of 2nd order supersequences will occur. Within a supersequence, a number of smaller sequences occur, and so on. Each of these different packages of genetically-related strata are bounded by unconformities, created by erosional flooding surfaces (transgressions), that can be traced over distances. Strata are genetically-related if all facies within the sequence were deposited in lateral continuity with one another, following Walther’s Principle. Flooding surfaces are sharp contacts that separate underlying shallower-water facies from overlying deeper water facies. Sequences of any order can be related across time and studied for what they have to teach about the Earth’s past, a region’s past, and a particular location’s past.

The causes of sequences of various hierarchical levels is subject to debate. 1st Order Megasequences are known to reflect global phenomena, such as the formation and breakup of supercontinents. The ancient supercontinents of Rodinia and Pangaea would be examples of sequence boundaries. An unconformity often called “The Great Unconformity” is a good example of a bounding surface that marks the lower boundary of the megasequence that would eventually lead to the formation of Pangaea, starting with the flooded cratons that make up today’s continents. 2nd order supersequences are thought to be more regional in scope and again driven by tectonics, but this time within an ocean basin. During the Paleozoic Era, a global megasequence developed, within which four 2nd Order Supersequences would progress (The Sauk, Tippecanoe, Kaskaskia, and Absaroka). The Sauk Sequence represents the earliest of these, bounded by the Great Unconformity and the Knox Unconformity. The Tippecanoe Sequence came next, bounded by the Knox Unconformity on the bottom and the Wallbridge Unconformity on top. Each of these supersequences carry with them signals for shorter, 3rd order sequences. Within those, 4th order, and within those, 5th order. By the level of a 5th order sequence, the data is entirely within a single stratigraphic formation. These shortest sequences, 4th and 5th Order, can be driven by a number of factors, but Milankovitch orbital variations are certainly likely drivers.

Sequence OrderDurationTitleAverage thickness of strataRise/Fall rate (cm/1000yr)Areal extent of deposition (Sq. mi.)
First50-100 million yearsSloss Megasequence1000+<1Global
Second5-50 million yearsSupersequence500-5000+1-3Regional
Third0.5-5 million yearsSequence500-15001-10500-50,000
Fourth0.1-0.5 millionParasequence Set20-80040-50020-2,000
Fifth0.01-0.1 million yearsParasequence10-200600-70020-2,000

Systems Tracts

A sequence can be divided up into four phases, sandwiched on top and bottom by sequence boundaries. In sequence stratigraphy, these phases are referred to as systems tracts. These systems tracts are directly tied to behavior of relative water level within the basin. The placement of a particular set of strata within a specific systems tract is done using chronostratigraphic and lithostratigraphic tools. The systems tract phases of a sea level cycle include, Falling Stage Systems Tract and Sequence Boundary Formation (FST), Lowstand systems Tract (LST), Transgressive Systems Tract (TST), Highstand Systems Tract (HST), and back to Falling Stage Systems Tract and Sequence Boundary Formation (FST). Systems tracts are made up of all sediments and material that accumulate during one sea level cycle and, as much as possible, are bounded by chronostratigraphic markers, or global stratotype sections and points (GSSP).

A sequence begins with the lowstand systems tract. At this point in the sea level cycle, the main process is sedimentation. The thickest strata deposited during this tract will be located in the center of the basin. Deeper water deposits, such as turbidites, are often common at this phase of the cycle and progradation of the coastline is the norm. Sedimentation rates are high. Accommodation space decreases. Sea level is at its lowest ebb in the basin. This phase is bounded at its bottom by the sequence boundary, always an erosional unconformity following the prior falling stage systems tract.

Once sea level begins to rise, the transgressive systems tract begins. As sea levels rise, a depositional pattern of strata displaying a retrograding (receding) shoreline and rising sea level appear. Sedimentation rates during this phase are greatest around the basin margin and lower in the center of the basin. Eventually, the basin center will see a significant drop off in sedimentation and coastal areas will show evidence of tidal influence. Such evidence might include stromatolite growth or the successive deposits of mud atop evaporative salt casts. In either situation, this evidence will largely be microstratigraphic.

Once sea level reaches its peak for the cycle, the highstand systems tract has been reached. This is the peak of the cycle. At this point, sediment supply is much greater and a progradational pattern will begin to emerge once more, as accommodation space decreases on the back front side of the sea level curve. Coastal deposits during such times will be dominated by waves and fluvial inputs and will tend to be thin, but with great areal extent. Once this highstand systems tract gives way to sea level fall, erosion will begin again during the regression of sea level and a new sequence boundary will form during the falling stage systems tract. The end of a sea level cycle is then marked by the onset once again of the lowstand systems tract.

Ultimately, there are two very important observations to be made about the sea level cycle and the progression through these systems tracts. Within one sea level cycle, there may be many, much smaller sea level cycles occurring, forming their own unique depositional systems tracts, sequence boundaries, and leaving important evidence of environmental changes in the stratigraphic record. Second, the depositional portion of a systems tract always begins with lowstand and ends with the highstand. The formation of sequence boundaries, which are unconformities, is by definition non-depositional or erosional. As such, in sequence stratigraphy, all sediment deposited in a basin is deposited during an overall sea level transgression. Within a sequence, there may be brief periods of sedimentation interruption, called diastems, but otherwise there are no substantial periods of erosion or nondeposition.

Model of a typical sequence stratigraphic progression. Relative sea level will transgress or regress based upon actual sea level rise or the progradation or retrogradation of the shoreline with changes in sediment supply. Beginning at highstand, where a flooding surface is formed, sea level enters the falling stage systems tract. This relative regression leaves an erosional sequence boundary behind. As lowstand approaches, relative sea level is at its lowest. The trasgressive systems tract follows the lowstand and leads to the bulk of deposition as increased sediment supply begins a relative rise in sea level, eventually leading to a new highstand.
Model of a typical sequence stratigraphic progression. Relative sea level will transgress or regress based upon actual sea level rise or the progradation or retrogradation of the shoreline with changes in sediment supply. Beginning at highstand, where a flooding surface is formed, sea level enters the falling stage systems tract. This relative regression leaves an erosional sequence boundary behind. As lowstand approaches, relative sea level is at its lowest. The trasgressive systems tract follows the lowstand and leads to the bulk of deposition as increased sediment supply begins a relative rise in sea level, eventually leading to a new highstand.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sequence stratigraphy systems tracts. From sequence boundary to sequence boundary (Falling Stage Systems Tracts), a sequence repeats. This model provides a lens through which packages of rock strata, deposited along the margins of a basin, can be interpreted.
Sequence stratigraphy systems tracts. From sequence boundary to sequence boundary (Falling Stage Systems Tracts), a sequence repeats. This model provides a lens through which packages of rock strata, deposited along the margins of a basin, can be interpreted.

The Parasequence

The 5th Order Sequence, the parasequence, provides an accessible pattern for understanding all of this. These sequences represent some of the shortest depositional periods, the smallest areal extent, and the most rapid change in sea level, providing a nice laboratory for exploring how these systems tracts work. Two examples of parasequences, or parasequence sets (as in the second figure below) are provided. In both cases, parasequences represent a single episode of shoreline progradation, or seaward movement of the shoreline. This creates a typical shallowing-upward succession of strata typical of a more rapid filling of accommodation space than may have been created at the time. The first represents a set of facies that progressively become coarser in grain size as deposition occurs, which is typical in a deltaic environment along a coast. The sequence boundary at the bottom is an unconformity that represents the end of the falling stage systems tract from the prior sea level cycle. The fine-grained muds that begin the sequence represent the deeper waters of the offshore facies residing in deeper waters. As the sequence develops, a typically shallowing-upward prograding progression will develop. This is a predictable pattern for this kind of environment. Sea level is still transgressing, but sedimentation rates are very high.

A Model Parasequence. Muds at the base (gray) represent the sea level highstand systems tract. As the parasequence deposits over time (bottom to top), sea level is rising as sediments are prograding seaward. Accommodation space in the basin is being filled faster than space is created. The sequence is typically shallowing-upward, or coarsening-upward.
A Model Parasequence. Muds at the base (gray) represent the sea level highstand systems tract. As the parasequence deposits over time (bottom to top), sea level is rising as sediments are prograding seaward. Accommodation space in the basin is being filled faster than space is created. The sequence is typically shallowing-upward, or coarsening-upward.
Characteristics of two upward-fining parasequences formed in a tidal flat to subtidal environment. (Van Wagoner et al., 1995).
Characteristics of two upward-fining parasequences formed in a tidal flat to subtidal environment. (Van Wagoner et al., 1995).

In the field, tracing out sequence boundaries is challenging. Given the spotty nature of the rock record and the variation that occurs as beds pinch out or new beds emerge over distances, it is impossible to ever have all of the data necessary to reconstruct ancient environments or the entire history of a basin. These sequence stratigraphic tools have provided valuable help in identifying important deposits of economic importance and for better understanding how a wide variety of environmental processes work. Such insights are important as we strive to understand the processes we are experiencing today along our shorelines and beyond.

A deeper exploration of sequence stratigraphy is beyond the scope of this work. Introducing the concept of sequences in this chapter serves as a way to tie together all of the various lines of discussion surrounding stratigraphy. The use of basic stratigraphic principles to understand how beds are correlated across distances is foundational and descriptive. Sequence stratigraphy provides a set of models that allow for deeper interpretations of basin history. They also provide opportunities for further research. Testable questions abound through the application of these models of change within basins.

References:

Holland, Steven (2018). An Online Guide to Sequence Stratigraphy. https://strata.uga.edu/sequence/index.html.

Wilson, J. Tuzo (1966). Die the Atlantic close and then re-open?. Nature, 211, 676-681.

Wu, X. P., M. B. Heflin, H. Schotman, B. L. A. Vermeersen, D. A. Dong, R. S. Gross, E. R. Ivins, A. Moore, and  S. E. Owen (2010), Simultaneous estimation of global present-day water transport and glacial isostatic adjustment, Nature Geoscience, 3(9), 642-646, doi: http://dx.doi.org/10.1038/NGEO938.