Jan Kramers

Molybdenum isotope records as a potential new proxy for paleoceanography

New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition (‘mean ocean water 98 Mo/ 95 Mo’ (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1^3 Myr time resolution as indicated from thick sections of Fe^Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of 33.1 and 32.9x). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe^Mn crust surface layers range from 32.7 to 32.9x and 32.7 to 33.1x, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (30.7 to 31.6x relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (32.0 to 32.3x). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO 23 4 and (minor) Mo(OH)6 species in solution, of which the latter is preferentially scavenged. @ 2003 Elsevier Science B.V. All rights reserved.

Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust

A combined solution for the two major terrestrial lead paradoxes has been sought. These are the ‘future’ paradox (upper crustal and upper mantle Pb isotope compositions plot in the future field in 207pb/204Pb vs. 206Pb/204Pb space) and the Th/U mantle paradox (The Th/U ratio of the upper mantle is ca. 2.6, whereas Pb isotopes indicate a value of 3.8). Constraints considered other than UThPb data include siderophile element concentrations of the mantle (which restrict core formation scenarios), W isotopes and the oldest lunar ages (bracketing the accretion and core formation time scale to 60–100 Ma), and noble gas systematics (requiring a two-layered mantle structure dating back to just after accretion). Further, Nd isotopes allow a test of the validity of crustal growth models used. The transport balance model used includes a continental crust divided into four parts: upper (high U/Pb) and lower (low U/Pb), as well as older and younger. The latter division is generated by erosion removing proportionally more younger than older crust. After 2 Ga ago erosion transfers U to the ocean floor in preference to Pb and Th, as a consequence of U solubility in an oxidizing environment. Within the constraints imposed on the model, the future paradox cannot be solved by postulating a delayed core formation. An additional low U/Pb reservoir required for this can be found in the continental crust. Solving the future paradox requires that particularly the older lower crust reservoir is conservative, which limits the amount of continental crust that can have,been recycled into the. mantle over Earths history. On the other hand, a solution of the mantle Th/U paradox requires a considerable amount of continent recycling, particularly in the last 1–2 Ga. A restricted family of crustal history scenarios allows a solution to both paradoxes. These are characterized by < I0% of the present amount of continental crust existing just after Earth accretion, rapid crustal growth, with relatively insignificant recycling into the mantle, during the Archaean, and increasing continent recycling in the Proterozoic, reaching ca. 60% of the rate of continent formation today. Scenarios in which delamination of lower crust accounts for over 5% of continent recycling do not provide solutions. The result portrays a non-steady-state Earth in which the net mass of continental crust is at present still growing at 2 × 1015 g/a, and the U content of the upper mantle is increasing.

Geochemistry of the peat bog at Etang de la Gruère, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 14C yr BP

A 650 cm core from a Swiss bog represents 12,370 14C years of peat accumulation and provides the first complete record of atmospheric Pb deposition for the entire Holocene. Excess, non-atmospheric Sr in the peat was calculated by normalizing Sr/Sc to crustal abundance; this was used to differentiate between the ombrogenic section of the bog in which inorganic solids are supplied exclusively by atmospheric deposition and the minerogenic zone where mineral-water interactions contribute metals to the peat. While sediment dissolution contributes significantly to the Sr inventory of the minerogenic section of the peat profile, there is no measurable effect of this process on the Pb burden. Isotopic analyses (204Pb, 206Pb, 207Pb, 208Pb) show that effectively all of the Pb in the peat profile was supplied exclusively from the atmosphere. To separate natural and anthropogenic Pb, Sc was selected over Ti, Y, Zr, Hf and REE as a conservative, reference element which is supplied by soil dust aerosols derived from rock weathering. Enrichment factors (EF) were calculated using the Pb/Sc ratio in the peat samples, normalized to the “natural, background” Pb/Sc which is found in peats dating from 8030 to 5320 14C yr BP. The results show that anthropogenic source have dominated the supply of atmospheric Pb to the peat core continuously since 3000 14C yr BP. The aerosols supplied to the bog can be divided into 3 classes: a) Pre-Anthropogenic (older than 3000 14C yr BP with Pb EF 1.194); b) Pre-Industrial (dating from 3000–240 14C yr BP, with Pb EF ≥ 2 but 20 and 206Pb/207Pb < 1.179). Elevated soil dust fluxes are observed at 5320, 8230 and 10,590 14C yr BP; the latter corresponds to the Younger Dryas. Aluminum, Zr, Hf, and REE/Sc ratios also are elevated at the same depths, suggesting differences in particle size, wind strength, or source regions. Pre-Anthropogenic aerosols deposited since 8230 14C yr BP reveal Pb/Sc which is significantly higher, and 206Pb/207Pb which is less radiogenic, than during the early Holocene. While the trend toward increasing Pb/Sc could be due to chemical weathering and soil development, this could not explain the shift in Pb isotopic composition. The changes which took place at 8230 14C BP, therefore, may be related to a large scale climatic reorganization which, at present, is poorly understood.

Timing and climatic impact of Greenland interstadials recorded in stalagmites from northern Turkey.

A 50 kyr-long exceptionally well-dated and highly resolved stalagmite oxygen (δ 18O) and carbon (δ 13C) isotope record from Sofular Cave in northwestern Turkey helps to further improve the dating of Greenland Interstadials (GI) 1, and 3–12. Timing of most GI in the Sofular record is consistent within ±10 to 300 years with the “iconic” Hulu Cave record. Larger divergences (>500 years) between Sofular and Hulu are only observed for GI 4 and 7. The Sofular record differs from the most recent NGRIP chronology by up to several centuries, whereas age offsets do not increase systematically with depth. The Sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern Mediterranean to GI, whereas a phase lag of ∼100 years between climate and full ecosystem response is evident. Finally, results of spectral analyses of the Sofular isotope records do not support a 1,470-year pacing of GI.

Two thousand years of atmospheric arsenic, antimony, and lead deposition recorded in an ombrotrophic peat bog profile, Jura Mountains, Switzerland

A peat core from a Swiss bog reveals significant enrichments of As, Sb and Pb extending back to Roman times, indicating that the anthropogenic fluxes of these metals have exceeded the natural fluxes for more than 2000 years. The isotopic composition of Pb provides no evidence of vertical downward Pb migration, suggesting that the bog has faithfully preserved the historical record of atmospheric Pb deposition. Age dating using210Pb (verified independently using pollen markers) provides the chronology of changing metal concentrations during the past 150 years. Present day enrichment factors (relative to the metal/Sc ratios of typical crustal rocks) are of the order of 20 times (As), 70 (Sb), and 130 (Pb). Given the potential toxicity of As and Sb, these new findings suggest that the environmental significance of these, and perhaps other less common trace elements, deserve more attention.

Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase dating of metamorphic minerals

Abstract Dating of relic metamorphic assemblages can provide important information about the timing and character (metamorphic grade and/or PT-evolution) of early high grade episodes in polymetamorphic provinces. Using Pb stepwise leaching of metamorphic silicates, we have dated multiple granulite facies metamorphic episodes in the Central Zone (CZ) of the Limpopo Belt. Ages of 2.52 Ga were obtained from sillimanite and cogenetic garnet and ages of about 2.01 Ga from titanite, garnet and clinopyroxene. Together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data, these results lead us to a reinterpretation of the tectono-metamorphic history of the CZ. Three distinct high grade events at about 3.2−3.1 Ga, 2.65−2.52 Ga and 2.0−0.05 Ga are recognized. Each of these is suggested to correspond to a tectonic episode of distinct character: (a) for the 3.2 Ga event magmatic activity can mainly be identified (best represented, for example, by the Sand River Gneisses or the Messina Layered Intrusion). The field relationships concerning the tectonometamorphic history of this Early-Archean event are largely erased by at least two high grade metamorphic overprints. (b) Late-Archean (∼2.6-2.52 Ga) low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism. The anticlockwise P-T evolution of these granulites probably reflects deep crustal processes, associated with magmatic underplating (or in-plating), contemporaneous with vertical crustal growth of the Zimbabwe craton around 2.6 Ga. (c) During the Proterozoic event (∼2.05-1.95 Ga) tectonic thickening was caused by the collision of the Kaapvaal and Zimbabwe cratons. The CZ was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise P-T evolution. The structural, metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny.

Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary.

Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the ‘Cambrian explosion’. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.

The δ44Ca‐temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures

We report direct δ44Ca-temperature calibration on cultured and fossil calcite foraminifera, showing that Ca isotopes are potentially a new proxy for past sea surface temperatures (SST). Samples have been analyzed using a 43Ca-48Ca double spike and thermal ionization mass spectrometry (TIMS). In order to avoid species-dependent isotope fractionation we focused our investigations on a single foraminifera species (Globigerinoides sacculifer), which is known to inhabit shallow euphotic waters in tropical and subtropical oceans. Ca isotope measurements were performed on cultured G. sacculifer that grew in seawater kept at temperatures of 19.5°, 26.5°, and 29.5°C. A δ44Ca change of 0.24 ± 0.02 per 1°C is defined by the weighted linear regression through reproduced δ44Ca data of the three temperatures (95% confidence level). Application of this new method to fossil G. sacculifer of an Equatorial East Atlantic sediment core (GeoB1112; 5°46.7′S, 10°45.0′W, 3125 m) indicates that the δ44Ca difference between marine isotope stage 1 (MIS-1) and MIS-2 is 0.71 ± 0.24. According to the current δ44Ca-temperature calibration this value corresponds to a temperature difference between MIS-1 and MIS-2 of ∼3.0 ± 1.0°C.

Geological Setting and Age of Australopithecus sediba from Southern Africa

From Australopithecus to Homo Our genus Homo is thought to have evolved a little more than 2 million years ago from the earlier hominid Australopithecus. But there are few fossils that provide detailed information on this transition. Berger et al. (p. 195; see the cover) now describe two partial skeletons, including most of the skull, pelvis, and ankle, of a new species of Australopithecus that are informative. The skeletons were found in a cave in South Africa encased in sediments dated by Dirks et al. (p. 205) to about 1.8 to 1.9 million years ago. The fossils share many derived features with the earliest Homo species, including in its pelvis and smaller teeth, and imply that the transition to Homo was in stages. A new species of Australopithecus, about 1.9 million years old, shows many derived features with Homo, helping to reveal its evolution. We describe the geological, geochronological, geomorphological, and faunal context of the Malapa site and the fossils of Australopithecus sediba. The hominins occur with a macrofauna assemblage that existed in Africa between 2.36 and 1.50 million years ago (Ma). The fossils are encased in water-laid, clastic sediments that were deposited along the lower parts of what is now a deeply eroded cave system, immediately above a flowstone layer with a U-Pb date of 2.026 ± 0.021 Ma. The flowstone has a reversed paleomagnetic signature and the overlying hominin-bearing sediments are of normal polarity, indicating deposition during the 1.95- to 1.78-Ma Olduvai Subchron. The two hominin specimens were buried together in a single debris flow that lithified soon after deposition in a phreatic environment inaccessible to scavengers.

Geochronology of the Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa

Abstract In this paper monazite U–Pb and zircon evaporation dates, stepleaching Pb/Pb results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in the Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaapvaal Craton. The Southern Marginal Zone of the Limpopo Belt is considered to be a lower crustal equivalent of the northern Kaapvaal Craton. Granulite exhumation is associated with southward thrusting along the Hout River Shear Zone which is a set of thrust and strike slip shear zones. Zircon ages for the Matok Intrusive Complex which was emplaced within the zone during this thrusting (charno-enderbites: 2671±4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhumation of the Southern Marginal Zone within only ∼7 Ma. We have obtained a U/Pb date of 2691±7 Ma for monazite from the Bandelierkop Quarry in the Southern Marginal Zone, interpreted as the age of high grade metamorphism. A single zircon evaporation Pb/Pb date of 2643±1 Ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation. Anatexis of metapelitic xenoliths within the Matok Intrusive Complex was dated at 2663±4 Ma by U/Pb on monazite, indistinguishable from existing zircon ages for this complex. Pb/Pb step leaching dates obtained on synkinematically grown garnet (2691±20 Ma), staurolite (2712±37 Ma) and kyanite (2672±51 Ma) from the Khavagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneous with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations, probably reflecting repeated or continuous tectonic activity of the Hout River Shear Zone up to about 2600 Ma.