Pieter Vermeesch

Loess plateau storage of northeastern Tibetan plateau-derived Yellow River sediment

Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the rivers upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO2 and erosion records.

CosmoCalc: An Excel add‐in for cosmogenic nuclide calculations

As dating methods using Terrestrial Cosmogenic Nuclides (TCN) become more popular, the need arises for a general-purpose and easy-to-use data reduction software. The CosmoCalc Excel add-in calculates TCN production rate scaling factors (using Lal, Stone, Dunai, and Desilets methods); topographic, snow, and self-shielding factors; and exposure ages, erosion rates, and burial ages and visualizes the results on banana-style plots. It uses an internally consistent TCN production equation that is based on the quadruple exponential approach of Granger and Smith (2000). CosmoCalc was designed to be as user-friendly as possible. Although the user interface is extremely simple, the program is also very flexible, and nearly all default parameter values can be changed. To facilitate the comparison of different scaling factors, a set of converter tools is provided, allowing the user to easily convert cut-off rigidities to magnetic inclinations, elevations to atmospheric depths, and so forth. Because it is important to use a consistent set of scaling factors for the sample measurements and the production rate calibration sites, CosmoCalc defines the production rates implicitly, as a function of the original TCN concentrations of the calibration site. The program is best suited for Be-10, Al-26, He-3, and Ne-21 calculations, although basic functionality for Cl-36 and C-14 is also provided. CosmoCalc can be downloaded along with a set of test data from http:// cosmocalc. googlepages. com.

Remotely sensed dune celerity and sand flux measurements of the world's fastest barchans (Bodélé, Chad)

Quantifying sand flux with field measurements is an expensive and time-consuming process. We here present an alternative approach using the COSI-Corr software package for Earth surface deformation detection. Using pairs of ASTER satellite images, we detected dune migration in the Bodele depression of northern Chad over time intervals of one month to 6.5 years. The displacement map can be used to automatically distinguish dunes from interdunes, which is a crucial step towards calculating sand flux. We interpolated a surface between the interdune areas and subtracted it from a digital elevation model, thus obtaining dune heights and volumes. Multiplying height with celerity yields a pixel-by-pixel estimate of the sand flux. We applied this method to large diatomite dunes in the Bodele, confirming that these are some of the worlds fastest moving barchans. Plotting dune height against inverse celerity reveals sand flux at the dune crest of > 200 m(3)/m/yr. Average dune sand flux values for the eastern and western Bodele are 76 and 99 m3/m/yr, respectively. The contribution of the dunes to the total area-averaged sand flux is 24-29 m(3)/m/yr, which is similar to 10% of the saltation flux determined by previously published field measurements. Citation: Vermeesch, P., and N. Drake ( 2008), Remotely sensed dune celerity and sand flux measurements of the worlds fastest barchans ( Bodele, Chad), Geophys. Res. Lett., 35, L24404, doi: 10.1029/2008GL035921.

Provenance of Passive-Margin Sand (Southern Africa)

This study investigates the petrographic, mineralogical, geochronological, and geochemical signatures of river sands across southern Africa. We single out the several factors that control sand generation, including weathering and recycling, and monitor the compositional changes caused by chemical and physical processes during fluvial transport from cratonic sources to passive-margin sinks. Passive-margin sands have two first-cycle sources. Quartz and feldspars with amphibole, epidote, garnet, staurolite, and kyanite are derived from crystalline basements exposed at the core of ancient orogens and cratonic blocks (dissected continental block provenance). Volcanic rock fragments, plagioclase, and clinopyroxene are derived from flood basalts erupted during the initial phases of rifting (volcanic rift provenance). First-cycle detritus mixes invariably with quartzose detritus recycled from ancient sedimentary successions (undissected continental block provenance) or recent siliciclastic deposits (e.g., Kalahari dune sands; recycled clastic provenance). U-Pb ages of detrital zircons mirror the orogenic events that affected southern Africa since the Archean. Damara (0.5–0.6 Ga) and Namaqua (1 Ga) age peaks are prominent throughout Namibia, from the Orange mouth to the Namib and Skeleton Coast Ergs, and also characterize Kalahari dunes and sands of the Congo, Okavango, and Zambezi Rivers. Instead, sharp old peaks at 2.1 Ga and 2.6 Ga characterize Limpopo and Olifants sands, matching the age of the Bushveld intrusion and the final assembly of the Zimbabwe and Kaapvaal Cratons, respectively; discordant ages indicate Pb loss during the Pan-African event. Chemical indices confirm that weathering is minor throughout the tropical belt from South Africa and Zimbabwe to Namibia and coastal Angola but major for quartzose sands of the Congo, Okavango, and upper Zambezi Rivers, largely produced in humid subequatorial regions. Recycling of quartzose sediments is extensive in all of these catchments. From Congo to Mozambique, along the >5000-km Atlantic and Indian Ocean rifted margins, polycyclic detritus reaches commonly 50% and locally up to 100%, in line with the estimated incidence of recycling worldwide. Quantitative information provided by provenance studies of modern sands helps us to better understand the relationships between sediment composition and plate-tectonic setting and to upgrade the overly simplified and often misleading current provenance models. This is a necessary step if we want to decipher the stratigraphic record of ancient passive margins and reconstruct their paleotectonic and paleoclimatic history with greater accuracy.

500 m.y. of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel)

Following the Neoproterozoic Pan-African orogeny, the Arabian-Nubian Shield of North Africa and Arabia was eroded and then covered by Cambrian sandstones that record the onset of platform sedimentation. We applied K-feldspar 40 Ar/ 39 Ar, zircon and apatite fi ssion-track, and apatite (U-Th)/He thermochronology to detritus from Cambrian sandstones of southern Israel deposited at ca. 500 Ma. U-Pb detrital zircon ages from these sandstones predate deposition and record the earlier Neoproterozoic crustal evolution of the Pan-African orogens. 40 Ar/ 39 Ar ages from 50 single grains of K-feldspar yield a Cambrian mean of ca. 535 Ma. The 40 Ar/ 39 Ar age spectrum of a multi-grain K-feldspar aliquot displays diffusion behavior compatible with >560 Ma cooling later affected by a heating event. Assuming that the high-temperature domains of the K-feldspars have not been affected by subsequent (hydro)thermal events, and taking previously published K-Ar and Rb-Sr ages from other parts of the East African Orogen at face value, these ages apparently record Pan-African thermal resetting below a thick volcano-sedimentary pile similar to the Saramuj conglomerate in Jordan and/or the Hammamat in Egypt. Detrital zircon fi ssion-track (ZFT) ages cluster around 380 Ma, consistent with previous ZFT results from Neoproterozoic basement and sediments of the region, revealing that the Cambrian platform sequence experienced a middle Devonian thermal event and low-grade metamorphism. Regional correlation indicates that during Devonian time the sedimentary cover atop the Cambrian in Israel was never in excess of 2.5 km, requiring an abnormally steep geothermal gradient to explain the complete ZFT annealing. A basal Carboniferous unconformity can be traced from Syria to southern Saudi Arabia, suggesting that the observed Devonian ZFT ages represent a regional tectonothermal event. Similar Devonian ZFT ages were reported from Arabian-Nubian Shield basement outcrops in the Eastern Desert, 500 km south of Eilat. The detrital apatites we studied all have extremely rounded cores suggestive of a distant provenance, but some grains also feature distinct euhedral, U-rich apatite overgrowth rims. Authigenic apatite may have grown during the late Devonian thermal event we dated by ZFT, coinciding with existing Rb-Sr ages from authigenic clays in the same deposits and leading to the conclusion that the Devonian event was probably hydrothermal. Like the ZFT ages, the detrital apatite fi ssion-track (AFT) ages were also completely reset after deposition. Sixty single-grain detrital AFT ages group at ca. 270 Ma with signifi cant dispersion. Inverse modeling of the AFT data indicate extended and/or repeated residence in the AFT partial annealing zone, in turn suggesting an episodic burial-erosion history during the Mesozoic caused by low-amplitude vertical motions. Seven detrital apatite (U-Th)/ He ages scatter between 33 and 77 Ma, possibly resulting from extreme compositional zonation associated with the authigenic U-rich overgrowths. The ca. 70 Ma (U-Th)/ He ages are more likely to be accurate, setting 1‐2 km as an upper limit (depending on the geothermal gradient) on the post-Cretaceous exhumation of the Cambrian sandstone and showing no evidence for substantial denudation related to Tertiary rifting of the Red Sea. Regional Geological Setting

Bathymetry and Sedimentary Environments of Lake Issyk-Kul, Kyrgyz Republic (Central Asia): A Large, High-Altitude, Tectonic Lake

Lake Issyk-Kul, located in the northern Tien Shan of the Kyrgyz Republic, in the heart of Central Asia, is one of the deepest and largest lakes in the world. Although Kyrgyz and Russian scientists have studied the lake quite intensively since the 1850’s, not much of the scientific literature has found its way to the international community. It is only recently that Lake Issyk-Kul has also started to attract international attention, and this is essentially thanks to the fact that the area occupies a potentially interesting location for paleoclimate research as well as for geodynamic studies.

A 45‐year time series of dune mobility indicating constant windiness over the central Sahara

Although evidence is mounting that links global warming to changes in atmospheric dynamics over the Atlantic realm, similar studies over the African continent are lacking. And even if such models would exist, it would be difficult to verify their validity due to the paucity of meteorological observations and anemometers in the central Sahara. A pragmatic way around this problem is to monitor barchan dune velocity as a proxy for the windiness of desert areas. Dune migration rates are a measure of the amount of work done by the wind which does not require field measurements but can be observed from space instead. This paper presents a novel application of the remote sensing tool COSI-Corr for the construction of time series of dune mobility from sequences of optical satellite imagery. The technique has been applied to the Bodele Depression in northern Chad, to demonstrate that dune migration rates in the central Sahara have been remarkably constant for nearly half a century, leading us to conclude that wind velocities have not changed more than 0.2% per year over that period. It is therefore unlikely that the frequency and intensity of dust storms originating from this ‘hot spot’ has significantly changed over the past decades either.

An in situ technique for (U–Th–Sm)/He and U–Pb double dating

We report on a new laser-based technique for rapid, quantitative and automated in situ double dating (U–Pb and (U–Th–Sm)/He) of minerals, for applications in geochronology, thermochronology and geochemistry. In situ laser microanalysis offers several advantages over conventional bulk crystal methods in terms of spatial resolution, productivity, and safety. This new approach/methodology utilizes an interoperable and integrated suite of analytical instruments including a 193 nm ArF excimer laser system, quadrupole ICP-MS, quadrupole helium mass spectrometry system and swappable flow-through and ultra-high vacuum analytical chambers. We describe the analytical protocols for zircon analysis including grain mounting in Teflon, parameters for parent and daughter isotopic measurement, and standard development, and provide a freeware application for determining (U–Th–Sm)/He ‘pairwise’ ages from analytical data. The in situ double dating method described is applied to the Ellendale lamproite pipe and country rocks, Western Australia and successfully replicates conventional U–Pb and (U–Th–Sm)/He age variations determined previously by conventional techniques.

Lies, Damned Lies, and Statistics (in Geology)

According to Karl Poppers epistemology of critical rationalism, scientists should formulate falsifiable hypotheses rather than produce ad hoc answers to empirical observations. In other words, we should predict and test rather than merely explain [Popper, 1959]. Sometimes, statistical tests such as chi-square, t, or Kolmogorov-Smirnov are used to make deductions more “objective.” Such tests are used in a wide range of geological subdisciplines [see Reimann and Filzmoser, 2000; Anderson and Johnson, 1999; Lorup et al., 1998; Sircombe and Hazelton, 2004].

Multimethod detrital thermochronology of the Great Valley Group near New Idria, California

The simultaneous use of several thermochronological methods on replicate sedimentary rock samples can reveal their pre- and postdepositional history. Single grain U/Pb dating of zircon, zircon and apatite fi ssion track dating, and vitrinite refl ectance measurements were performed on Cretaceous through Miocene sedimentary rocks of the Great Valley Group and the Temblor Formation near Coalinga and New Idria, California. The data show that the Sierra Nevada was exhumed and cooled at ~0.5–1 km/m.y. or ~20 °C/m.y. during the Cretaceous. After deposition in the Great Valley forearc basin, Sierra Nevada erosional products were buried at great depth under low thermal gradients. At ca. 12–14 Ma, northward progression of the Mendocino triple junction triggered folding on the eastern fl anks of the California Coast Ranges and rapid exhumation of the New Idria serpentinite diapir. This middle Miocene event caused the deposition of spectacular deposits of sedimentary serpentinite (Big Blue Formation). The rapid rise of the hot serpentinite body created a thermal pulse that may have provided the enigmatic heat source for oil fi elds in the shallow Vallecitos syncline, a few kilometers north of New Idria.