Low 10Be Research Articles

10Be imaging of sediment accretion and subduction along the northeast Japan and Costa Rica convergent margins

Research Article| January 01, 2002 10Be imaging of sediment accretion and subduction along the northeast Japan and Costa Rica convergent margins J. Morris; J. Morris 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA Search for other works by this author on: GSW Google Scholar R. Valentine; R. Valentine 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA Search for other works by this author on: GSW Google Scholar T. Harrison T. Harrison 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (1): 59–62. https://doi.org/10.1130/0091-7613(2002)030<0059:BIOSAA>2.0.CO;2 Article history received: 17 May 2001 rev-recd: 26 Sep 2001 accepted: 01 Oct 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. Morris, R. Valentine, T. Harrison; 10Be imaging of sediment accretion and subduction along the northeast Japan and Costa Rica convergent margins. Geology 2002;; 30 (1): 59–62. doi: https://doi.org/10.1130/0091-7613(2002)030<0059:BIOSAA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity (the seismogenic zone) and the subduction recycling of volatiles and continentally derived materials to the Earth's mantle. Identifying and quantifying sediment accretion, essential for a complete mass balance across the margin, can be difficult. Seismic images do not define the processes by which a prism was built, and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy. This contribution reports the first use of cosmogenic 10Be depth profiles to define the origin and structural evolution of forearc sedimentary prisms. Biostratigraphy and 10Be model ages generally are in good agreement for sediments drilled at Deep Sea Drilling Project Site 434 in the Japan forearc, and support an origin by imbricate thrusting for the upper section. Forearc sediments from Ocean Drilling Program Site 1040 in Costa Rica lack good fossil or paleomagnetic age control above the decollement. Low and homogeneous 10Be concentrations show that the prism sediments are older than 3–4 Ma, and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments. Low 10Be in Costa Rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Last‐Glacial to Post‐Glacial 10Be fluctuations in a sediment core from the Academician Ridge, Lake Baikal

10Be concentrations, sediment composition and sediment accumulation rates were determined in a sediment core from the Academician Ridge of Lake Baikal. The 10Be concentrations vary from 5.07 × 108 atoms/g in the last glacial section to 11.25 × 108 atoms/g in the section representing the post‐glacial climate stage. 10Be fluxes, on the other hand, increase toward the last‐glacial stage coinciding with an increase of the sediment accumulation rate. These facts imply that climate‐related sedimentation is related to both flux and concentration of the nuclide in the sediment core. During the last‐glacial stage, arid conditions in the lake's watershed caused a marked increase in flux of clay particles with low 10Be content to the lake, resulting in an increased flux but lowered concentration of 10Be in the sediments. It is suggested that the 10Be record of the bottom sediments of Lake Baikal can be used as a proxy indicator of climate change in central Asia.

Estimation of slip rates in the southern Tien Shan using cosmic ray exposure dates of abandoned alluvial fans

Cosmic ray exposure dates (based on in situ–produced 10 Be), in combination with measured heights of fault scarps that cut three abandoned fans, imply a slip rate of less than ∼2 mm/yr on the bounding thrust fault along a segment of the southern flank of the Tien Shan of central Asia. This rate, somewhat lower than those for the segment farther west and for the northern margin of the belt, implies that the distribution of shortening across the Tien Shan must change markedly along the belt. The date of abandonment of the smallest of the three fans is consistent with fan formation since the last glacial maximum, but the other two fans appear to have been formed and abandoned prior to that time. These and other results suggest that advances and retreats of alpine glaciers may not be synchronized with continental ice sheets and that not all prominent geomorphological features in arid Asian regions have formed since the last glacial maximum. In this study we have assessed the relationship between cosmogenic nuclide content of individual clasts and the time since their deposition on a surface. Cosmogenic nuclide accumulation represents the integrated total surface exposure of rocks in source regions during transport and since deposition. Postdepositional processes (erosion, burial, bioturbation, and cryoturbation), all of which decrease the quantity of cosmogenic nuclides produced in a rock in a given time and hence the apparent exposure age, also cause significant scatter among apparent ages of samples from the surfaces. Very low 10 Be concentrations in material from active streambeds and at depth below one of the fan surfaces indicate that exposure prior to deposition onto fan surfaces was minimal (

Elemental U and Th variations in island arc rocks: implications for U-series isotopes

Isotope and trace element data from well characterised arc suites are used to identify contributions from fluids and subducted sediments in arc rocks. U-series isotopes are then used to argue that the fluid and sediment components are characterised by different transfer times through the mantle wedge. U, but not Th, is readily mobilised in the fluid component. Th behaves as a high field strength element (HFSE), and the negative array between Th/Ce and Nd isotopes indicates that significant amounts of the Th and by implication other HFSE, in arc rocks are derived from sediments in the subducted slab. These elements may therefore only be regarded as conservative in the sense that they are not mobilised in the fluid component. As the fluid component contains U, but little if any Th, the resultant ( 238U/ 230Th) disequilibria may be used to estimate transfer times for the fluid component, and these are typically 30–120 ka. In contrast, rocks with a greater contribution from subducted sediments (high Th/Ce and low 143Nd/ 144Nd) tend to have ( 238U/ 230Th) ∼ 1, and low 10Be, suggesting transfer times of several million years. Overall, the Th, LREE and Ta contents of many arc rocks are dominated by the sediment component which is probably partial melts of sediments in the subducted slab. It is estimated that ∼ 30% of the Th in subducted sediments is returned to the crust in arc magmas. The fractionation of U/Th and Sm/Nd requires that average continental crust was generated in the presence of residual garnet, presumably by processes early in Earth history that were different from those at recent plate margins.

10Be records of sediment cores from high northern latitudes: Implications for environmental and climatic changes

The 10Be records of four sediment cores forming a transect from the Norwegian Sea at 70°N (core 23059) via the Fram Strait (core 23235) to the Arctic Ocean at 86°N (cores 1533 and 1524) were measured at a high depth resolution. Although the material in all the cores was controlled by different sedimentological regimes, the 10Be records of these cores were superimposed by glacial/interglacial changes in the sedimentary environment. Core sections with high 10Be concentrations ( > 1 · 10 9 at/g) are related to interglacial stages and core sections with low 10Be concentrations ( < 0.5 · 10 9 at/g) are related to glacial stages. Climatic transitions (e.g., Termination II, 5/6) are marked by drastic changes in the 10Be concentrations of up to one order of magnitude. The average 10Be concentrations for each climatic stage show an inverse relationship to their corresponding sedimentation rates, indicating that the 10Be records are the result of dilution with more or less terrigenous ice-rafted material. However, there are strong changes in the 10Be fluxes (e.g., Termination II) into the sediments which may also account for the observed oscillations. Most likely, both processes affected the 10Be records equally, amplifying the contrast between lower (glacials) and higher (interglacials) 10Be concentrations. The sharp contrast of high and low 10Be concentrations at climatic stage boundaries are an independent proxy for climatic and sedimentary change in the Nordic Seas and can be applied for stratigraphic dating ( 10Be stratigraphy) of sediment cores from the northern North Atlantic and the Arctic Ocean.

Be isotope and B-Be investigations of the historic eruptions of Mt. Vesuvius

Cosmogenic and radioactive 10Be, stable 9Be and B concentrations have been determined for four alkaline lavas from the recent cycle of historical activity at Mt. Vesuvius, in the Central Campania Province (Italy). The goal of this study was to use the Be isotope and Be-B systematics of these lavas in a manner analogous to that used in regions unequivocally related to active subduction, in hopes of being able to document a subduction origin for the Vesuvian lavas. Four lavas measured to date have low 10Be concentrations as well as low B/Be ratios. While the low 10Be concentrations could reflect subduction and incorporation of old sediments and/or only contributions from the basaltic part of the subducting plate, the combination of low 10Be and low B/Be ratios must be interpreted as the absence of a subduction signature in these lavas. Unfortunately, the absence of a subduction signature in the lavas cannot be used unequivocally to argue against recent subduction in the Central Campania region. Subduction of an unusually young, hot slab which has lost its B through prograde metamorphic reactions at shallow levels could explain the absence of a subduction signature, as could extensive crustal contamination. In addition, recent studies in Java show that alkaline lavas in this region of active subduction never show a subduction signature, even when erupting in close proximity to calc-alkaline and tholeiitic lavas which do; by analogy, the Vesuvian lavas studied may have been generated in a part of the sub-arc mantle which either does not experience or does not preserve the chemical signature of subduction modification. The present data set does not allow us to make definitive statements about the role of subduction in the origin of the Central Campania lavas.

10Be chronometry of bedrock-to-soil conversion rates

We report concentrations of cosmogenic 10Be ( t 1/2 = 1.5 × 10 6 yrs) in soil excavated from a soil-mantled hillslope in Black Diamond Mines Regional Park, Contra Costa County, California. The most striking features of the data are: (1) the similarity in the downward decreasing trends of 10Be concentrations in two soil profiles collected 75 m apart, (2) the coincidence in each soil profile of the soil/bedrock interface (as defined by visual inspection of soil pits) and the level at which 10Be concentrations attain very low values ( ∼4 × 10 6 atoms/g), and (3) the extremely low 10Be concentrations in the underlying regolith (0.5 × 10 6 atoms/gram). The inventory of 10Be in these soils is low, equivalent to about 6000 yrs of 10Be accumulation in a soil initially containing no 10Be. On the basis of these measurements, and with the aid of simple models of soil ( 10Be) motions on the hillslope, we conclude that 10Be loss from the surface is dominated by its removal in soil by creep. We calculate local rates of bedrock-to-soil conversion of between 0.15 and 0.27 km/10 6 yrs. Comparing these with uplift rates determined for coastal regions of California indicates that soil creep alone is capable of removing soil from the local geomorphic system at a rate equivalent to the rate of uplift of much of the coast.

10Be study of rapid erosion in Taiwan

Cosmogenic 10Be was measured using accelerator mass spectrometry in soils and sediments to study the erosion of Taiwan, which has the highest denudation rate in the world. The river sediments in Taiwan have very low 10Be concentrations, around 5 million atoms per gram, about 1 45 the world wide average. This is the direct consequence of its high sediment yield of more than 1000 mg/cm 2/yr, 70 times the world average. Combining these values we found that, for Taiwan as a whole, the 10Be output to the sea only slightly exceeds the input from the rain, a situation typical of many areas around the world. Therefore, even in this example of extremely rapid erosion, 10Be seems to remain a useful indicator for erosion status. The total 10Be inventory found in a soil profile from a geologically stable area is at least 18% of the maximum possible inventory. The minimum age thus estimated for the soil is 0.11 Ma, in reasonable agreement with estimates from other means. The maximum erosion rate for this area thus estimated is at most 0.14 mg/cm 2/yr, four orders of magnitude slower than the average value observed for Taiwan. We also demonstrate that the shelf sediments around Taiwan have received the admixing of a 10Be rich component from the ocean and would give a poor estimate for material eroded from Taiwan.

Beryllium 10 in hydrothermal vent deposits from the East Pacific Ridges: Role of sediments in the hydrothermal processes

Beryllium 10 concentrations were determined for 31 handpicked hydrothermal sulfides, six oxyhydroxides, seven basalts, and five sediments collected from the hydrothermally active areas of the East Pacific ridges. The samples include specimens from the East Pacific Rise (EPR) at 21°N and 13°N, the Galapagos Rift, the Guaymas Basin, and the Gorda and the Juan de Fuca ridges. Additional samples from massive sulfides associated with the Oman ophiolites were studied. In all samples, we obtained values ranging from 0.04 × 106 atoms/g to 125 × 106 atoms/g, with the lowest values being very close to our blank (0.015 × 106 atoms/g). The data show systematic variations with sample location and type. The 10Be concentrations measured for the mid‐ocean basalts are of the order of 0.3 × 10 atoms/g and reach 3800 × 106 atoms/g for the pelagic deep‐sea sediments collected near the EPR 21°N. Based on their 10Be concentrations, we can clearly distinguish two categories of sulfides: sulfides containing low 10Be concentration (&lt;106 atoms/g) sitting directly on the mid‐ocean basalt (EPR or Juan de Fuca), and sulfides with high 10Be concentration (&gt;106 atoms/g) located atop of a thick pile of young sediments (Guaymas Basin or Gorda Ridge). The oxy‐hydroxides, collected on some of the Juan de Fuca chimneys, are always very rich in 10Be with concentrations reaching 125 × 106 atoms/g. We propose three mechanisms to explain these data: (1) mechanical entrapment of sediments in the hydrothermal chimneys as observed in the Gorda Ridge sulfides, (2) leaching of 10Be from the ocean crust sediments, transport by hydrothermal fluids, and coprecipitation with heavy metals as hypothesized to explain the Be concentrations observed in the Guaymas Basin, and (3) passive adsorption of 10Be from seawater, preferred hypothesis oxy‐hydroxides of the areas with no sediment cover.

10Be in a deep-sea core: implications regarding 10Be production changes over the past 420 ka

The influx of 10Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ 18O stages covering the last 423 ka. It is shown that the marine deposition of 10Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing 10Be to the bottom sediments. In core CH72-02, the deposition rates of 10Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 10 8 atoms cm −2 ka −1. However, after correction for changes in lithology, the data show that the production rate of 10Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low 10Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.

Pb, Sr and 10Be isotopic studies of volcanic rocks from the Northern Mariana Islands. Implications for magma genesis and crustal recycling in the Western Pacific

We report Sr and Pb isotope analyses for an extensive suite of volcanic rocks from the N. Mariana arc together with Sr and Pb isotope analyses of sediments from the nearby Mariana and Nauru basins. In addition ten of the most recent volcanic samples were analysed for 10Be. The Sr isotope compositions cluster tightly around 87Sr 86Sr = 0.7035 being slightly but significantly higher than the Pacific ocean floor basalts on either side of the arc and agreeing well with previous data. In contrast, the large number of new Pb isotopic data presented significantly extends the observed range of Pb isotope compositions for volcanic rocks from the Mariana arc to more radiogenic compositions. The concentrations of 10Be were very low (< 0.5 × 10 6 atom g −1). These new data require either that the Pb and Sr isotopic compositions of the Mariana sub-arc mantle be substantially different from those of the mantle source of ocean floor basalts on either side of the arc, or that the enrichment in radiogenic Pb and Sr relative to the values observed in Pacific ocean floor basalts be related to the subduction process. We prefer the latter hypothesis in which radiogenic Sr and Pb in ocean floor sediments are added to M.O.R.B. type mantle either by direct assimilation of the sediments in partial melts or, more probably, by transfer in a fluid phase into the zone of magma production. The low 10Be concentrations observed suggest the removal of at least the top few metres of sediment during subduction.

10Be in Arctic Ocean sediments

The 10Be deposition rate in two Arctic Ocean cores, ∼80°N, was measured to be ≤0.6 atoms cm −2 sec −1, much less than the 10Be deposition rate reported for other oceanic regions. Possible causes for the low 10Be deposition in the Arctic are: (1) latitudinal variation in the 10Be delivery rate from the stratosphere; (2) the presence of an ice cover on the Arctic, which may inhibit the transfer of atmospherically supplied 10Be from sea surface to sediments; and (3) locally low scavenging efficiency of sedimentary particles for 10Be in the Arctic.