Barbara Seth

Mesoproterozoic U–Pb and Pb–Pb ages of granulites in NW Namibia: reconstructing a complete orogenic cycle

Abstract The chronology of the crustal evolution of the granulite facies fault-bounded Epembe Unit south of the anorthositic Kunene Intrusive Complex (KIC) in NW Namibia is presented. U–Pb zircon ages by ion microprobe (SHRIMP II) from a heterogeneous set of granulite facies rocks from the Epembe Unit reveal Mesoproterozoic ages between 1520 and 1510xa0Ma for prograde zircon growth. Cathodoluminescence (CL) imaging clearly shows metamorphic growth of zircon grains and zircon rims around igneous cores. Zircon core analyses obtained by SHRIMP yield ages between 1635 and 1810xa0Ma. These ages are comparable to those of the Epupa basement and indicate derivation of the inherited zircon cores from Palaeoproterozoic protoliths. Pb–Pb stepwise leaching (PbSL) TIMS ages on peak-metamorphic garnet and retrograde sapphirine reflect granulite facies conditions at ca. 1490 to 1447xa0Ma. Our combined age data from the granulite facies Epembe Unit reveal a complete early Mesoproterozoic orogenic cycle between ca. 1640 and 1450xa0Ma. The cycle begins with erosion of the Palaeoproterozoic Epupa basement and involves sedimentation, burial down to the crust–mantle boundary leading to high-grade metamorphism and rapid exhumation. An orogenic cycle of this age matches in time with events dated in the Pinwarian (Grenville Province, Canada), in the Amazonian craton (Brazil) and in the Gothian (Baltic Shield, Sweden) but has not been reported in southwestern Africa before.

Accurate measurements of Th–U isotope ratios for carbonate geochronology using MC-ICP-MS

We present an advanced method to produce accurate and reproducible U and Th isotope data for carbonate geochronology using very small sample sizes. We analysed speleothem, marine calcite and coral samples by MC-ICP-MS and compared the data and its precision to U and Th data obtained by TIMS using the same samples. MC-ICP-MS needs between 0.05 and 0.34 ng of Th compared with 0.3–1.4 ng Th for TIMS analyses to obtain comparable precision. No separation of Th and U is necessary because of U and Th recoveries of circa 100% and 75–100%, respectively. Th and U analyses are carried out for a range from masses 228 to 238 in two separate runs, with mass 230Th on the Daly detector for measuring Th isotope ratios and 234U on the Daly detector for measuring U isotope ratios. Silicate rock standard “Table Mountain Latite” n(TML) n235U/238U ratios are used to correct for instrumental mass bias. For calibrating the gain of the Daly and Faraday detectors (Daly–Faraday gain), a 1 ppt Aldrich U solution spiked with 233U–236U double spike is measured with mass 235U in the Daly detector. We demonstrate the accuracy and precision of our analytical data obtained for standard solution UCSC-Th-A, the TML and eleven spiked TML samples over the past 10 months. Literature data obtained by both TIMS and MC-ICP-MS lie well within the error of our UCSC-Th-A 230Th/232Th value of 0.00000585 ± 5 (2 sd, n n= 29). Our (234U/238U) and (230Th/238U) activity ratios for the TML compare well with results of other MC-ICP-MS studies. Our external precision of 230Th/232Th and 234U/238U ratios for the unspiked TML is slightly worse than other MC-ICP-MS studies, but reflects either significantly smaller sample sizes or much shorter time for measurement.

Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records

Significance Polar ice is a unique archive of past atmosphere. Here, we present methane stable isotope records (used as source fingerprint) for the current and two past interglacials and their preceding glacial maxima. Our data are used to constrain global emissions of methane. Tropical wetlands and floodplains seem to be the dominant sources of atmospheric methane changes, steered by past variations in sea level, monsoon intensity, temperature, and the water table. In contrast, geologic emissions of methane are stable over a wide range of climatic conditions. The long-term shift seen in both isotopes for the last 25,000 y compared with older intervals is likely connected to changes in the terrestrial biosphere and fire regimes as a consequence of megafauna extinction. Atmospheric methane (CH4) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ13CH4 and δD(CH4)] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH4) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.

Bipolar carbon and hydrogen isotope constraints of the Holocene methane budget

Atmospheric methane concentration shows a wellknown decrease over the first half of the Holocene following the Northern Hemisphere summer insolation before it started to increase again to preindustrial values. There is a debate about what caused this change in the methane concentration evolution, in particular, whether an early anthropogenic influence or natural emissions led to the reversal of the atmospheric CH4 concentration evolution. Here, we present new methane concentration and stable hydrogen and carbon isotope data measured on ice core samples from both Greenland and Antarctica over the Holocene. With the help of a two-box model and the full suite of CH4 parameters, the new data allow us to quantify the total methane emissions in the Northern Hemisphere and Southern Hemisphere separately as well as their stable isotopic signatures, while interpretation of isotopic records of only one hemisphere may lead to erroneous conclusions. For the first half of the Holocene our results indicate an asynchronous decrease in Northern Hemisphere and Southern Hemisphere CH4 emissions by more than 30 Tg CH4 yr−1 in total, accompanied by a drop in the northern carbon isotopic source signature of about −3 ‰. This cannot be explained by a change in the source mix alone but requires shifts in the isotopic signature of the sources themselves caused by changes in the precursor material for the methane production. In the second half of the Holocene, global CH4 emissions increased by about 30 Tg CH4 yr−1, while preindustrial isotopic emission signatures remained more or less constant. However, our results show that this early increase in methane emissions took place in the Southern Hemisphere, while Northern Hemisphere emissions started to increase only about 2000 years ago. Accordingly, natural emissions in the southern tropics appear to be the main cause of the CH4 increase starting 5000 years before present, not supporting an early anthropogenic influence on the global methane budget by East Asian land use changes.