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Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats , erosion history, lava flows, meteorite impacts, rock slides, fault scarps , cave development, and other geological events. It is most useful for rocks which have been exposed for between 10 years and 30, years [ citation needed ]. The most common of these dating techniques is Cosmogenic radionuclide dating [ citation needed ]. Earth is constantly bombarded with primary cosmic rays , high energy charged particles - mostly protons and alpha particles. These particles interact with atoms in atmospheric gases, producing a cascade of secondary particles that may in turn interact and reduce their energies in many reactions as they pass through the atmosphere.

Korschinek; A. Bergmaier; T. Faestermann; U. Gerstmann Chmeleff; F. Kossert; D. Jakob Earth and Planetary Science Letters. Categories : Isotopes of beryllium. Hidden categories: Isotope content page. Namespaces Article Talk. Views Read Edit View history. By using this site, you agree to the Terms of Use and Privacy Policy. Decay energy MeV. Isotopes of beryllium Complete table of nuclides. Lighter: Beryllium Beryllium is an isotope of beryllium.

Heavier: Beryllium Decay chain of beryllium Although each meteorite had different concentrations of these radionuclides, they were present in a constant ratio, as expected, with the notable exception in some samples of 36 Cl, the most short-lived nuclide. By measuring the ratio of a short-lived to a long-lived radionuclide, the terrestrial age of the meteorite can be easily calculated from the deficit in the activity of the short-lived species, assuming that prior to landing all cosmogenic nuclides were at secular equilibrium.

In Situ-Produced Cosmogenic Nuclides and Quantification of Geological Processes

The arguments above can be expressed in very simple equations governed by production and decay. These equations can be modified for more complex exposure histories.

Continuing studies of cosmogenic nuclides in meteorites and lunar rocks have revealed increasingly complex exposure histories over the timescales of radioactive decay. Nishiizumi et al. Later, Nishiizumi et al. They used these five radionuclides plus cosmogenic noble gases to solve for five unknowns in several lunar meteorites' irradiation histories: preejection exposure depth on the moon, depth in the meteoritic body in space, exposure time on the moon t mexposure time in space t sand terrestrial age since the time of fall t a.

Each cosmogenic nuclide's activity is determined by equation 9 :. By solving equation 9 simultaneously for all radionuclides, and minimizing the misfit between measurements and the model, all five of the unknowns can be constrained. As with any inverse problem, it is important to realize that the number of measurements must meet or exceed the number of unknowns, and that the unknowns can only be resolved over timescales during which the cosmogenic nuclides behave differently.

Nevertheless, the use of multiple cosmogenic nuclides with different radioactive mean-lives and production rates can reveal complex histories of exposure and burial.

Similar methods using chi-square minimization to resolve complex histories are beginning to be used in the terrestrial field today e. The summary above is by no means a comprehensive review, but rather a glimpse into the development of multiple-nuclide methods.

The point to be made is that the theoretical and mathematical framework for using multiple cosmogenic nuclides has existed since the very beginning of the field. The development of cosmogenic nuclide methods in terrestrial research has closely followed that of its meteoritic predecessor, and in many cases merely elaborates upon these early-established methods. The following section reviews the development of multiple-nuclide measurements in the terrestrial field.

There are far fewer applications in this arena, partly because terrestrial cosmogenic nuclides are more difficult to measure, but also because many Earth surface processes occur quickly with respect to the radioactive decay of the cosmogenic nuclides. In these cases, the cosmogenic nuclides behave nearly identically to each other, and little new information can be gleaned from analysis of multiple species.

It is only either by analyzing short-lived radionuclides such as 14 C or by studying processes that occur over million-year timescales that the strength of multiple-nuclide approaches may be realized. Although the measurement of cosmogenic nuclides in meteorites was in full swing by the s, terrestrial samples were far more difficult to measure by the decay counting methods then in use. Only two early attempts to measure cosmogenic nuclides were successful.

The first of these was by Davis and Schaefferwho detected 36 Cl in rock. They were able to measure the accumulation of this nuclide in an unglaciated surface, but had difficulty with glaciated surfaces that had been exposed for less time.

Sample treatment was extremely laborious, and the results had high uncertainty, so the method languished. A second measurement of a cosmogenic nuclide was to by Hampel et al. A combination of difficult chemistry and counting efforts rewarded by imprecise results effectively prohibited terrestrial cosmogenic nuclide geochronology. With the development of accelerator mass spectrometry AMS in the s, the measurement of terrestrial cosmogenic nuclides suddenly became far more tractable.

Yiou et al. In a brief but influential paper, Lal and Arnold then suggested that the nuclide pair 26 Al and 10 Be would be particularly suitable for measurement in the mineral quartz. It is notable that Lal and Arnold emphasized the simultaneous measurement of these two radionuclides to take advantage of their radioactive decay.

The first two data-intensive papers on terrestrial cosmogenic 26 Al and 10 Be used these nuclides' radioactive decay to date buried sediments. Klein et al. This glass is an impact melt scattered across a large area of desert. The glass must have been repeatedly buried and reexposed by passing sand dunes and during transport by flowing water. Although Klein et al. The uppermost line of the triangle is defined by the condition of constant accumulation.

Although their assumption of saturation was later revealed to be incorrect, the method of constraining erosion rates and half-lives from the exposure-burial triangle can still be useful today, as I will show in a later section on half-life determination.

The exposure-burial triangle was modified by Nishiizumi et al. Recall that Whipple and Fireman pointed out that cosmic-ray exposure ages could be equally well interpreted in terms of erosion rates. This same issue remains for terrestrial exposure; the erosion rate of a rock can be calculated using equation 1. The ratio of two radionuclides can be calculated from this equation, assuming that they have similar production rate profiles.

This is a valid assumption for 26 Al and 10 Be, whose production profiles are nearly identical Brown et al. For example, 36 Cl may have a different depth dependence than 26 Al and 10 Be due to its production from thermal-energy neutron capture on 35 Cl. The curves are shown in Figure 1on logarithmic axes, together with lines showing radioactive decay and burial isochrons.

I will refer to it as the exposure-burial diagram following the usage of Klein et al. View large Download slide Figure 1.

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Exposure-burial diagrams illustrating the evolution of 26 Al and 10 Be in quartz. The top left diagram shows conditions of steady erosion or constant exposure followed by burial.

A sample that has experienced constant exposure will plot on the uppermost line. A sample that has experienced long-term steady-state erosion will plot on the lower line, at a position indicated by the erosion rates shown. Samples at the surface that were exposed and then eroded will plot between these curves, in the steady-state erosion island Lal, Million-year decay isochrons are shown.

The bottom left diagram illustrates the evolution of a sample whose production rate decreases, but does not stop. Each growth curve on the diagram begins at a node corresponding to the uppermost diagram, and evolves over time to approach the secular equilibrium endpoint. SLHL-sea level, high latitude; P-production rate.

Figure 1. The exposure-burial diagram has played an important role in multiple-nuclide methods; it will be presented in several configurations later in this paper. Although the fundamentals of the diagram have not varied since its inception, some details have varied among users.

For example, Klein et al. Lal later expressed both axes logarithmically. One modification to the exposure-burial diagram that is useful when comparing samples from multiple sites is to normalize the abscissa by the local production rates, so that the 10 Be concentrations are scaled to sea level, high latitude SLHL. This can be done in two ways: Ivy-Ochs et al.

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Because it is more intuitive to maintain the 10 Be concentration in terms of atoms per gram, and because cosmogenic nuclide practitioners are familiar with scaling production rates to SLHL, I will use the dimensionless normalization factor in this paper.

The exposure-burial diagram has several important characteristics. First, if production rates are constant through time, then all rocks with continuous exposure should plot along the exposure line.

Their position on the exposure line depends on the exposure time. Rocks that were suddenly exposed, and subsequently began to erode but have not yet eroded through many penetration lengths, plot within the steady-state erosion island.

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The position within the steady-state erosion island can in principle be used to determine both how long a rock has been exposed and how fast it is eroding Nishiizumi et al. In practice, it is often difficult to distinguish erosion from exposure with the 26 Al- 10 Be pair because measurement uncertainties may be comparable to the difference between the exposure and erosion lines.

A second important characteristic of the exposure-burial diagram is that all samples must plot below and to the left of the exposure line, and to the left of radioactive decay from secular equilibrium. This is the original exposure-burial triangle of Klein et al. All other space on this diagram is forbidden. Samples that plot below the exposure or erosion line have experienced at least one episode of burial, in which production rates decrease. Radioactive decay lines may take two forms, depending on the depth of burial.

Lal,his Fig. For deeply buried samples, the radioactive decay lines on the exposure-burial diagram can be traced back to the exposure or erosion line to determine the inheritance, or the amount of cosmogenic nuclides present prior to burial.

A sample that experiences multiple episodes of exposure and burial may thus evolve along a zigzag path on the exposure-burial diagram, moving parallel to the burial and reexposure curves Fig. View large Download slide Figure 2. Exposure-burial diagram showing two different pathways leading to the same final point.

The shortest path, shown as a solid black line, represents the minimum time required, while the complex path, shown as a dashed line, represents one of many possible scenarios involving repeated exposure and burial. Figure 2. It is important to realize that the position of a sample on the exposure-burial diagram alone cannot uniquely resolve the sample's exposure and burial history. There are many paths that a sample can take, involving repeated episodes of exposure, erosion, shallow burial, or deep burial, that may yield identical 26 Al and 10 Be concentrations.

Only a minimum age can be established with absolute confidence. It is only through the judicious choice of a sampling site, and the addition of geologic or stratigraphic information, that a sample's exposure and burial history can be reconstructed. In a list that presaged the next two decades of terrestrial cosmogenic nuclide research, Lalproposed an array of geologic applications for cosmogenic nuclides.

Many of these applications implicitly involve the use of multiple nuclides, in particular the 26 Al- 10 Be pair in quartz, but also stable nuclides and shorter-lived species such as 36 Cl, 14 C, and 39 Ar.

Several of these proposed applications can be classified into two groups that are discussed here in more detail: 1 constraining complex burial histories of glacial sediments and sand dunes, and 2 dating single-stage burial histories of deeply and rapidly buried sediments, such as found in caves, river terraces, and potentially marine or lake deposits.

The fate of sediments on the surface is often complicated, with repeated episodes of burial and exposure as the sediment is transported down hillslopes and rivers, and into basins. It is often impossible to say with certainty how much time any individual grain has spent buried in a sedimentary deposit versus exposed while in transport.

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The fate of sand grains in deserts or in longshore transport is equally complex, with individual grains being recycled many times to the surface. Likewise, rocks exposed at the surface may be buried and exhumed many times, either by sediments or by glacial ice. In these situations, the use of multiple cosmogenic nuclides such as 26 Al and 10 Be can provide useful information, such as whether sediment or rock has been buried, and a minimum estimate of the burial and exposure times.

Although these are not discrete dates, they can nevertheless be critical for geologic and geomorphic interpretation.

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Case 1: Libyan Desert Glass. The first use of the 26 Al- 10 Be pair in Libyan Desert Glass, discussed above, was just such a case of complex exposure and burial Klein et al. It is likely that each clast had experienced different histories as they were repeatedly buried and exhumed by sand dunes, and transported by flowing water. In other words, the shortest path to a particular point on the exposure-burial diagram is taken by a single episode of exposure followed by a single episode of burial Fig.

The total exposure and burial history of the clast must exceed the sum of these two ages. The Libyan Desert Glass was found to be at least several million years old, consistent with its fission-track age of 28 Ma.

Case 2: Littoral Sediments. Sediment on shorelines or marine terraces may have complicated histories of burial and exposure during longshore transport or due to eustasy. A similar situation was observed by Boaretto et al. The sand was derived from dunes along the shore, which were in turn likely derived from longshore transport from the mouth of the Nile River.

Case 3: Pediments. Pediments are a common desert land-form where bedrock is beveled to a nearly flat surface at the base of mountains. They are thought to form by weathering and erosion as a thin layer of gravel is transported over the surface by sheetwash. Bierman and Caffee measured 26 Al and 10 Be in gravelly pavements on piedmonts, and found that individual quartz clasts have greatly varying exposure and burial histories.

Using the same scheme as Klein et al. Case 4: Soil Cover. The 26 Al- 10 Be pair can also be used to detect burial by accumulated soil.

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Albrecht et al. The authors suggested that a much thicker soil once covered the tuff, thus burying the surface. The soil was later stripped from much of the landscape, reexposing the original surface. In a very different environment, Braucher et al.

These data help to resolve discrepant views of whether stone lines represent a former surface now buried or if they developed by in situ differentiation. All of these examples share similar characteristics. The samples, whether individual clasts or bulk sediment, experienced repeated episodes of burial and exposure in a complex history of erosion, transport, and sedimentation.

It is impossible to distinguish a detailed history from two radionuclides alone, although a minimum age for the grains can be inferred from the most direct exposure-burial path on the exposure-burial diagram Fig. An important lesson to be learned from these measurements is that sediment samples must be interpreted with caution, particularly when their maturity suggests a long history of weathering and transport.

TERRESTRIAL COSMOGENIC NUCLIDES

Even deeply buried sediments may have a history of earlier burial from their source area that is difficult to detect with cosmogenic nuclides alone, and impossible to detect with a single nuclide such as 10 Be. Sediments and rocks can be buried not only by other sediments, but also by snow and ice. Such is the case beneath cold-based glaciers. Under this scenario, the rocks themselves are not eroded because the ice is frozen to its bed and does not slide, but they are shielded from secondary cosmic rays by the glacier itself.

After deglaciation, there may be little or no geomorphic evidence that the ice was ever there. Two different efforts have detected burial of rocks beneath cold-based glaciers using 26 Al and 10 Be. Case 1: Laurentide Ice Sheet. Bierman et al. The northern site on Baffin Island was characterized by a high degree of weathering on upland surfaces.

Previous interpretations of this area had suggested that the area remained ice-free during the last glaciation. Moreover, erratics on this surface showed young exposure ages consistent with deposition by ice during the Last Glacial Maximum. They concluded that the area must have been buried at least once beneath a thick cover of ice. Following the procedure of Klein et al.

The fact that this total near-surface time exceeds the glacial-interglacial period suggests that the bedrock surface could have been buried multiple times. They also considered the condition of partial shielding under a thin cover, so that production rates are lowered but not halted. Under the partial shielding scenario, the samples could be exposed for a long period of time.

If the cover were suddenly and recently removed, then the samples would lie under the steady-erosion island as drawn for the present-day production rates.

Gosse and Phillips considered a similar scenario in which glacial plucking rapidly removed an overburden in a rock with a very long exposure history, causing the sample to shift to the left on the exposure-burial diagram and thus fall beneath the steady-erosion island. Case 2: Fennoscandian Ice Sheet. A different research group followed a similar approach for the Fennoscandian Ice Sheet.

Stroeven et al. Previous work had suggested that this area had been repeatedly buried beneath cold-based ice, and that the tors were part of a relict preglacial landscape. These authors found that 26 Al and 10 Be plotted consistently beneath the steady-erosion island, demonstrating conclusively that this landscape had been preserved for at least several glacial-interglacial cycles.

They carried the analysis one step further by using a specific model for the timing of glaciation over the past 2. The modeled history slightly overestimated the final 10 Be concentration but could be reconciled with a subdued production rate due to thin cover, or perhaps due to surface erosion during interglacials. These applications of 26 Al and 10 Be to bedrock beneath the Fennoscandian Ice Sheet represent a further advance in methodology, in that the exposure-burial histories are expressly modeled.

Although a detailed history is still not attainable from the data alone, the regularity of glacial-interglacial cycles lends itself to constructing a credible model of periodic exposure and burial.

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This leads to more information than can be gleaned from more poorly constrained or stochastic processes such as sand dune migration or longshore transport. However, it is still difficult to reconstruct an exact exposure and burial history, because many different scenarios could lead to the same 26 Al and 10 Be concentrations. In the next section, I will discuss a different sort of application, in which the 26 Al- 10 Be pair is used to date discrete events that are clearly evident from the sedimentary or geologic record.

In these cases, because there is geologic and stratigraphic information that supports a simple exposure and burial history, the burial histories can be dated with much higher confidence. Although the cases discussed so far emphasize the stochastic nature of sediment transport and burial, it is also common for sediment to experience a simple history of a single exposure followed by burial.

For example, sediment can be laid at the bottom of a thick deposit, perhaps on an alluvial terrace that accumulates over hundreds or thousands of years. Sediment can be buried beneath volcanic ash or lava that is laid down in days.

Or stream sediment can be carried into a cave, where it is instantly shielded by up to hundreds of meters of rock as it passes beneath the surface. In these cases, cosmogenic nuclide production practically ceases, and the sedimentation event can be dated with confidence by the relative radioactive decay of multiple nuclides. The methodology of this type of burial dating has been recently reviewed by Granger and Muzikar ; I will not review the methodology here, but will instead focus on several geologic examples.

Case 1: New River, Virginia. The first use of the 26 Al- 10 Be pair to date sediment burial events was by Granger et al. Quartz-bearing sediments in caves are almost always derived from outside of the cave itself, which is generally formed in a soluble rock such as limestone.

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Prior to deposition, the quartz was eroded from its parent rock and transported over the ground surface, accumulating cosmogenic nuclides. Once washed into the cave, it is instantly shielded by tens to hundreds of meters of solid rock, so cosmogenic nuclide production practically stops. This simple, single-stage history of exposure followed by burial allows the burial age to be readily calculated using equation 13 for both 26 Al and 10 Be, using the simplifying assumption that sediment begins under a condition of steady-state erosion equation For different initial conditions, see Granger and Muzikar, Granger et al.

These quartz pebbles were derived from a metamorphic rock more than 75 km upstream; thus the only source of quartz pebbles in this landscape is from the New River itself. Where the New River flows through limestone, caves discharge water as springs at river level. Some of the New River's bed load inevitably spills into the cave openings, particularly if the caves are large.

Divers in the river can observe this happening today. As the river cuts down over time, the caves and the sediments contained within them are left abandoned above the river. The water finds a new path to the river underground, dissolving a new cave, and the ancient alluvium can remain virtually undisturbed in the cave for millions of years.

In many ways the caves are similar to terraces. The caves form at river level, just as terraces do. When the river level is stable for a long time, the caves enlarge, in the same way that terraces widen. When the river incises, the horizontal cave passages are abandoned; sometimes caves form narrow sinuous canyons that match the river's incision rate, or just as often the groundwater chooses a new flow path and abandons the old cave entirely.

If the river aggrades, then the caves can fill with sediment, just as terraces are capped with alluvium. However, caves offer an advantage over terraces, in that erosion and weathering is negligible underground.

The cave passage, because it is formed in bedrock, leaves a faithful record of river level that is not lowered or degraded over time. Although caves are sometimes more difficult to find than terraces, the record of river incision and aggra-dation contained within them may offer much higher resolution than from terraces alone. Moreover, caves offer an additional advantage, in that their sediments can be readily dated with cos-mogenic nuclides.

In a survey of 55 caves along the New River, Granger et al. One nagging problem remains unresolved with the New River study. When Granger et al. It is possible that this inherited burial age reflects the travel time of quartz down the New River, as it was deposited in point bars and later reworked. If this is the case, then the cave sediments should haveyr subtracted from their age. A similar problem of prior burial, discussed above, was found for the beach sediments studied by Boaretto et al.

However, Granger et al. A dam now prevents quartz pebbles from the mountain source area from reaching the limestone in the Valley and Ridge. Moreover, entire cities are built on old river terraces, and erosion due to construction has introduced old gravels into the river. It is not clear that the modern river sediments are a suitable analog for those from the past.

A better analog in this case would be sediments deposited only - yr ago, prior to major land use but recent enough to be little affected by cosmogenic nuclide production and decay. Case 2: Mammoth Cave, Kentucky. The Mammoth Cave system is the largest, and arguably the most studied, cave system in the world Granger et al.

Mammoth Cave has developed in a ridge of nearly horizontally bedded limestones capped by a protective cover of sandstone and quartz-pebble conglomerate.

The cave formed by carrying water from a karst plain to discharge as springs on the Green River. Sinking streams carry abundant quartz gravel into passages containing underground streams many kilometers long. There, the small caves were primarily repositories of river sediment that washed into the caves through happenstance. At Mammoth Cave, the quartz sediments were carried by the cave-forming waters themselves.

The Mammoth Cave system is an example of a multilevel cave. Each level corresponds to a time when the Green River was stable, and the water table remained at the same elevation for a long period of time.

The cave morphology shows that these long episodes of river stability were punctuated by rapid incision, when the Green River cut to a lower elevation. Sometimes, river aggradation would fill the cave passages with thick sediment packages. This is an ideal setting for a geologic application of burial dating, because the timing of river incision is thought to be linked to climate history Miotke and Palmer, They were able to identify seven major events in the cave's development, and to link most of these with the expansion of the Laurentide Ice Sheet and associated drainage rearrangements within the Ohio River system.

Major river incision events were associated with the earliest glaciations at ca. Importantly, sediment from outside the cave had a burial age of zero, as expected. In contrast to the quartz pebbles in the New River, these sediments came from a local source and thus had little opportunity for long-term deposition and remobilization. The data from this study are shown in Figure 3 and fall clearly along a radioactive decay line, indicating that the sediment entering the cave maintained a uniform inheritance over millions of years.

Similar results were obtained by Anthony and Grangerwho dated sediments in large caves along the Cumberland River, Tennessee. These large caves are associated with periods of river stability and were abandoned in sequence as the Cumberland River incised in response to climate change and drainage rearrangement. View large Download slide Figure 3.

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Data from Mammoth Cave, Kentucky, show burial ages ranging from 0 to 4 Ma. The data spread parallel to the radioactive decay lines, indicating that local erosion rates remained nearly constant over this time. Modified from Granger et al. Figure 3. The Mammoth Cave study made several advances in the development of the burial dating methodology. First, it demonstrated that burial ages are reproducible to within analytical uncertainty, helping to confirm that a simple sediment burial scenario is appropriate.

Sediment was not remobilized from one cave level to another, and the sediment entered the cave with little or no history of prior burial. Second, it extended the burial dating record to beyond 4 Ma.

At these ages, the remaining concentrations of 26 Al and 10 Be are small, and they are particularly susceptible to postburial production of cosmogenic nuclides.

Third, it demonstrated that 26 Al and 10 Be can be used to determine a record of paleoerosion rates. Erosion rates in the Mammoth Cave area had remained slow and unvaried for millions of years.

Despite these apparent successes, it must be emphasized that cave sediments should be collected and interpreted cautiously; the sediment source area should be identified, and the possibility of sediment remobilization in the cave should be minimized.

Case 3: Sierra Nevada, California. Other cave sediment studies soon followed. The most notable of these is that of Stock et al.

Although the Sierra Nevada are a batholith, remnants of marble country rock are scattered through the range.

Stock et al. These caves could be used to reconstruct the river incision history in much the same way as the work of Granger et al. They posited that rapid incision was a transient response to tectonic uplift, and that the incision later slowed as rivers were choked with sediment due to glaciation in the headwaters. Case 4: Sterkfontein, South Africa. All of the above cave-dating studies used caves to infer long-term river incision rates.

This is a natural application of caves in geomorphology, because cave formation is linked to the regional water table and is controlled by river incision and aggradation. However, caves are often interesting for other reasons as well.

For example, caves often contain fossils, including those of our own ancestors. As suggested by Lal and Arnol the burial dating method is applicable over a timescale that spans hominin evolution, and is thus ideal for dating hominin-bearing caves.

The caves at Sterk-fontein, South Africa, offer a good example of this application. Sterkfontein is the source of more hominin fossils than any other site in the world. It is the source of most Australopithecus africanus fossils ever found and thus holds an important place in paleoanthropology. The only previous dating of the cave sediments, however, was by paleomagnetic stratigraphy of intercalated flowstone Partridge et al.



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