Michael Staubwasser

Iron isotopes in the early marine diagenetic iron cycle

Measurements of Fe oxyhydroxides [reactive Fe(III)] in two shallow sediment profiles from reducing and oxic environments on the Arabian Sea margin demonstrate Fe isotope fractionation during early marine diagenesis. Reactive Fe(III) has δ 56 Fe values between −0.77‰ and −0.19‰. Values are lowest at the top of the sediment profile and considerably lower than bulk sediment Fe (δ 56 Fe ≈ 0‰). Preferential reduction and dissolution of light Fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive Fe(III) residual. Initially, the isotopic composition of reactive Fe(III) evolves down-core according to the fractionation factor typical of microbial dissimilatory Fe reduction. Deeper in the profile, δ 56 Fe values remain unchanged despite further Fe reduction. Here, another process with a different fractionation factor becomes dominant, probably reduction by dissolved sulfide. The δ 56 Fe of the residual reactive Fe(III) suggests that ∼25% of the initially present reactive Fe(III) was reduced by microbial Fe reduction. When Fe is diagenetically recycled between reducing sediments at depth and an oxic top layer, the process, depending on recycling efficiency, may result in the accumulation of light Fe in the top layer while the complementary heavier residual is buried. Fe diffusing from the seafloor back into the ocean should reflect the low-δ 56 Fe diagenetic source of dissolved Fe.

South Asian monsoon climate change and radiocarbon in the Arabian Sea during early and middle Holocene

The 1 4 C ages of planktonic foraminifers Globigerinoides sacculifer bracketing the Younger Dryas in a δ 1 8 O record of Globigerinoides ruber from a laminated sediment core on the Pakistani continental margin suggest thatsurface reservoir ages in the Arabian Sea were in excess of 1000 years during the deglaciation. A least squares error fit of a detailed 1 4 C chronology to the (atmospheric) tree ring record gave variable early Holocene reservoir ages between 780 and 1120 years, well above the prebomb value of 640 years. Mid-Holocene reservoir ages are less well constrained but were probably closer to the prebomb value. The method used to fit individual core sections to the tree ring record was designed to require only a rough a priori estimate of the time spans, which in the core presented here were taken from each sections range of 1 4 C ages. A significant 220-year quasi-oscillation was present in the δ 1 8 O record during the early Holocene but not thereafter. This frequency and amplitude pattern resembles an early Holocene 207- to 227-year oscillation previously observed in the atmospheric 1 4 C record, which is generally interpreted as reflecting solar irradiance variability. An early Holocene climate event at 8150-8400 calendar years B.P. observed elsewhere within the Asian-East African monsoon system was again found in our record, suggesting a reduction in precipitation over NW India and Pakistan.

Palaeoenvironmental changes in the arid and sub arid belt (Sahara-Sahel-Arabian Peninsula) from 150 kyr to present

The PEP III Arid to Subarid Belt includes the largest hot desert in the world, the Sahara- Arabian desert and the Sahel zone. The region of interest extends south of the Atlas Mountains and south and east of the Mediterranean Sea to approximately 10 °N and shows a broadly zonal pattern with a varying seasonal distribution of precipitation. In the north (ca. 20–23 °N), rainfall results from the southward displacement of the midlatitude westerlies during winter whereas the south is governed by seasonal northward migration of the Intertropical Convergence Zone (ITCZ). Contraction and expansion phases of these presently semi-arid to hyper-arid desert areas result from significant changes in local precipitation. Palaeoenvironmental records from Northern Africa (north of 10 °N) and the surrounding seas document long-term changes in the magnitude and extent of the African monsoon in response to orbitally-forced changes in insolation. However, marine records as well as terrestrial palaeohydrological indicators (e.g., lakes, speleothems, rivers, pollen and charcoal) show that there have been changes in the hydrological cycle superimposed on the long-term waxing and waning of the monsoon which cannot be explained exclusively by changes in insolation. These fluctuations in space, time and magnitude were on a regional to continental scale. Here, we review available data on near-surface palaeohydrological indicators and vegetational changes in arid North Africa and the Arabian Peninsula as well as changes in the intensity of the South Asian Monsoon identified from marine sediments of the Arabian Sea. A comparison of regional environmental changes can clarify relations between the environment and changes in the Earth’s climate system. Each data-set is initially presented independently because they represent heteregeneous records from different regions and time periods and thereby emphasise their potential to provide evidence of continental chronostratigraphic palaeoenvironmental changes. Data-sets of lake status and vegetational change are complementary as they strongly reflect hydrological variation. Deep-sea sediments from the Arabian Sea were used to generate continuous records of oceanic upwelling, continental humidity, and dust and river discharge, that are closely related to palaeoenvironmental changes on the surrounding continents.After presenting the individual data-sets we compare the palaeoclimatic reconstructions derived from the different types of evidence.

On the formation of laminated sediments on the continental margin off Pakistan: the effects of sediment provenance and sediment redistribution

Abstract The sedimentary processes and sediment sources contributing to the formation of laminated sediments along the upper slope off Pakistan are unravelled using inorganic bulk sediment geochemistry of 43 surface cores from the Pakistani continental margin and additional geochemical and Pb and Nd-isotope data for different types of layers. An important process everywhere along the margin is redeposition of fluvial-derived detritus from the shelf onto the slope. This process is of considerably higher intensity along the Makran margin than on the Indus margin. Trace element enrichment related to early diagenesis or surface productivity, which is commonly detectable in bulk sediment composition, is swamped by the high clastic supply in the Makran region, but may be observed in the Indus region. Four types of layers are found in the laminated sediment cores from the upper slope. They reflect different mechanisms of deposition and different sediment sources. An alternating pattern of olive-grey and black layers results from downslope redeposition of fluvial material over most of the year, to which organic matter from sea surface production is added during the late summer monsoon season. Distinctive white to grey coloured layers along the Makran slope originate from large scale expulsion of sediments from the Makran accretionary wedge through mud volcanoes on the shelf, subsequent erosion by waves, and downslope redeposition. These layers may dominate the sedimentary record within the Makran accretionary wedge, but are absent on the Indus margin. Occasional red coloured turbidites, which probably represent larger floods on the Indus plain, contribute to this mixture of varying sedimentary processes and sediment sources along the Pakistani continental slope.

Ba, Ra, Th, and U in marine mollusc shells and the potential of 226Ra/Ba dating of Holocene marine carbonate shells

The geochemistry of Ba, Ra, Th, and U and the potential of using 226 Ra/Ba ratios as an alternative dating method are explored in modern and Holocene marine mollusc shells. Five modern shells of the Antarctic scallop Adamussium colbecki collected from the present day beach and six radiocarbon dated specimens from Holocene beach terraces of the Ross Sea region (Antarctic) between 700 and 6100 calibrated yr BP old have been analysed by mass spectrometry. In clean shells 226 Ra concentrations and 226 Ra/Ba ratios show a clear decrease with increasing age, suggesting the possibility of 226 Ra dating. Limiting factors for such dating are Ba and 226 Ra present in surface contaminants, and ingrowth of 226 Ra from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using 232 Th. Sub-samples from the same shell with different proportions of contamination form a mixing line in a 226 Ra/Ba- 232 Th/Ba graph, and the 226 Ra/Ba of the pure shell can be derived from the intercept on the 226 Ra/Ba axis. Contaminant corrected 226 Ra/Ba ratios of late-Holocene 14 C-dated samples fall close to that expected from simple 226 Ra excess decay from seawater 226 Ra/Ba values. 226 Ra ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of post mortem U uptake into the shell makes a correction for 226 Ra ingrowth from secondary U difficult to achieve. In the A. colbecki shells, 226 Ra ingrowth from such secondary U becomes significant only when ages exceed 2500 yr. In younger shells, 226 Ra/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between 226 Ra and Ba in the ocean can be confirmed for that time scale, the 226 Ra/Ba chronometer may enable the reconstruction of variability in sea surface 14 C reservoir ages from mollusc shells and allow its use as a paleoceanographic

Simultaneous analysis of (17) O/(16) O, (18) O/(16) O and (2) H/(1) H of gypsum hydration water by cavity ring-down laser spectroscopy.

Rationale The recent development of cavity ring‐down laser spectroscopy (CRDS) instruments capable of measuring 17O‐excess in water has created new opportunities for studying the hydrologic cycle. Here we apply this new method to studying the triple oxygen (17O/16O, 18O/16O) and hydrogen (2H/1H) isotope ratios of gypsum hydration water (GHW), which can provide information about the conditions under which the mineral formed and subsequent post‐depositional interaction with other fluids. Methods We developed a semi‐automated procedure for extracting GHW by slowly heating the sample to 400°C in vacuo and cryogenically trapping the evolved water. The isotopic composition (δ17O, δ18O and δ2H values) of the GHW is subsequently measured by CRDS. The extraction apparatus allows the dehydration of five samples and one standard simultaneously, thereby increasing the long‐term precision and sample throughput compared with previous methods. The apparatus is also useful for distilling brines prior to isotopic analysis. A direct comparison is made between results of 17O‐excess in GHW obtained by CRDS and fluorination followed by isotope ratio mass spectrometry (IRMS) of O2. Results The long‐term analytical precision of our method of extraction and isotopic analysis of GHW by CRDS is ±0.07‰ for δ17O values, ±0.13‰ for δ18O values and ±0.49‰ for δ2H values (all ±1SD), and ±1.1‰ and ±8 per meg for the deuterium‐excess and 17O‐excess, respectively. Accurate measurement of the 17O‐excess values of GHW, of both synthetic and natural samples, requires the use of a micro‐combustion module (MCM). This accessory removes contaminants (VOCs, H2S, etc.) from the water vapour stream that interfere with the wavelengths used for spectroscopic measurement of water isotopologues. CRDS/MCM and IRMS methods yield similar isotopic results for the analysis of both synthetic and natural gypsum samples within analytical error of the two methods. Conclusions We demonstrate that precise and simultaneous isotopic measurements of δ17O, δ18O and δ2H values, and the derived deuterium‐excess and 17O‐excess, can be obtained from GHW and brines using a new extraction apparatus and subsequent measurement by CRDS. This method provides new opportunities for the application of water isotope tracers in hydrologic and paleoclimatologic research.

Triple oxygen isotope signatures in evaporated water bodies from the Sistan Oasis, Iran

Natural samples from water bodies in the arid and semiarid environment of the Sistan Oasis, Iran, demonstrate a systematic evolution of 17O-excess and δ18O as a result of nonequilibrium fractionation during extreme evaporation. Residual water samples exhibit a significant and systematic decrease of 17O-excess with progressive evaporation loss, reaching values of −160 per meg over a 35‰ range of δ18O. Waters from heavily evaporated volume-limited natural bodies with limited or no recharge fall on the theoretically predicted isotopic evolution curve in agreement with ambient relative humidity of 30 to 35%. Recharged water bodies appear to follow a different trend. These new results demonstrate the potential of 17O-excess for the estimation of evaporation loss and ambient conditions in an arid environment.

Effect of increased glacier melt on diagenetic Fe cycling in marine sediments at King George Island (Antarctica)

The supply of trace metals to the surface ocean via dust deposition is important for primary productivity and the global biogeochemical cycle of many elements. Here we utilise the systematic variation of the chemical properties of yttrium and the rare earth elements (YREE) to investigate trace metal release from dust in the equatorial Atlantic Ocean. We present YREE data for the dissolved (<0.45 μm) and suspended particulate matter (SPM) collected from the mixed layer during Polarstern cruise ANT-XXIII/1 in Oct-Nov 2005. Saharan dust can be traced with the Al content of the SPM revealing a broad maximum extending from 15° to 3°N. The PAAS normalised YREE patterns of the dust dominated SPM are relatively flat with a broad peak centred around Eu and Gd. This dust dominated SPM is also characterised by lower Y/Ho and Er/Nd ratios than the particulate material from outside the high Al zone. The dissolved YREE distributions show normal seawater patterns with the relative enrichment of heavy REE over light REE. The samples with dust dominated SPM are enriched in the light and middle REE by a factor of approximately 2 compared to the other samples and a Sargasso Sea surface water. The dissolved Y/Ho and Er/Nd ratios obtained from the dust dominated SPM zone are also low compared to the samples outside the zone but display a fractionation between the SPM and the dissolved phase. This comparison indicates a consistent incongruent dissolution of the dust associated YREE which are probably mainly hosted by oxide coatings on the particles.In order to analyse differences in concentration, speciation and total mobility of arsenic two different locations were studied near the Helgoland Mud Area, North Sea. The first location is characterised by natural sedimentation, the second by deposited sediments dredged from the port of Hamburg. Porewater as well as sediment profiles were analysed with respect to arsenic compounds (As (III) and total As) and major redox species as total and reactive manganese and iron. The sediment samples were handled under inert atmosphere before and during extraction by water, phosphate, hydrochloric acid and aqua regia. Total element contents in porewater and leachable extracts of sediment fractions were analysed. The results show a strong redox coupling of arsenic with manganese and iron. Oxidized arsenic seems to adsorb to manganese- and iron-oxyhydroxides in surface sediments. In contrast to the solid samples, the pore water data shows a release of As (III) into porewater when manganese- and ironoxyhydroxides are reduced in the upper part of the cores. Also a remobilisation of As (V) occurs. Downward diffusing arsenic can be fixed by carbonate below the zone of manganese and iron reduction. In the anoxic parts of the sediments As (III) and As (V) are released and could be fixed at authigenic iron sulphide or arsenic sulphides formation. A sulfidic precipitation of arsenic in iron-dominated systems is limited by the occurrence of HS-. Total solid-phase contents in leachable extracts of sediment fractions of the natural area show significant higher arsenic concentrations than the core of the anthropogenic dumping area. This is due to the higher fines content of the Helgoland mud area. Higher total porewater contents of iron and arsenic in the core of the anthropogenic dumping area thus due to higher turnover rates of organic matter by iron reduction. Higher concentrations of arsenic may be due to a higher availability of iron in the dumped sediments.

The evolution of 17 O-excess in surface water of the arid environment during recharge and evaporation

This study demonstrates the potential of triple O-isotopes to quantify evaporation with recharge on a salt lake from the Atacama Desert, Chile. An evaporative gradient was found in shallow ponds along a subsurface flow-path from a groundwater source. Total dissolved solids (TDS) increased by 177 g/l along with an increase in δ18O by 16.2‰ and in δD by 65‰. 17O-excess decreased by 79 per meg, d-excess by 55‰. Relative humidity (h), evaporation over inflow (E/I), the isotopic composition of vapor (*RV) and of inflowing water (*RWI) determine the isotope distribution in 17O-excess over δ18O along a well-defined evaporation curve as the classic Craig-Gordon model predicts. A complementary on-site simple (pan) evaporation experiment over a change in TDS, δ18O, and 17O-excess by 392 g/l, 25.0‰, and −130 per meg, respectively, was used to determine the effects of sluggish brine evaporation and of wind turbulence. These effects translate to uncertainty in E/I rather than h. The local composition of *RV relative to *RWI pre-determines the general ability to resolve changes in h. The triple O-isotope system is useful for quantitative hydrological balancing of lakes and for paleo-humidity reconstruction, particularly if complemented by D/H analysis.

Impact of climate change on the transition of Neanderthals to modern humans in Europe

Significance A causality between millennial-scale climate cycles and the replacement of Neanderthals by modern humans in Europe has tentatively been suggested. However, that replacement was diachronous and occurred over several such cycles. A poorly constrained continental paleoclimate framework has hindered identification of any inherent causality. Speleothems from the Carpathians reveal that, between 44,000 and 40,000 years ago, a sequence of stadials with severely cold and arid conditions caused successive regional Neanderthal depopulation intervals across Europe and facilitated staggered repopulation by modern humans. Repetitive depopulation–repopulation cycles may have facilitated multiple genetic turnover in Europe between 44,000 and 34,000 years ago. Two speleothem stable isotope records from East-Central Europe demonstrate that Greenland Stadial 12 (GS12) and GS10—at 44.3–43.3 and 40.8–40.2 ka—were prominent intervals of cold and arid conditions. GS12, GS11, and GS10 are coeval with a regional pattern of culturally (near-)sterile layers within Europe’s diachronous archeologic transition from Neanderthals to modern human Aurignacian. Sterile layers coeval with GS12 precede the Aurignacian throughout the middle and upper Danube region. In some records from the northern Iberian Peninsula, such layers are coeval with GS11 and separate the Châtelperronian from the Aurignacian. Sterile layers preceding the Aurignacian in the remaining Châtelperronian domain are coeval with GS10 and the previously reported 40.0- to 40.8-ka cal BP [calendar years before present (1950)] time range of Neanderthals’ disappearance from most of Europe. This suggests that ecologic stress during stadial expansion of steppe landscape caused a diachronous pattern of depopulation of Neanderthals, which facilitated repopulation by modern humans who appear to have been better adapted to this environment. Consecutive depopulation–repopulation cycles during severe stadials of the middle pleniglacial may principally explain the repeated replacement of Europe’s population and its genetic composition.