ALBERT

Description: Methane contributes substantially to global warming as the second most important anthropogenic greenhouse gas. Radiocarbon (14C) measurements of atmospheric methane can be used as a source apportionment tool, as they allow distinction between thermogenic and biogenic methane sources. However, these measurements remain scarce due to labor-intensive methods required. A new setup for the preparation of atmospheric methane samples for radiocarbon analysis is presented. The system combines a methane preconcentration line with a preparative gas chromatography technique to isolate pure methane samples for a compound-specific radiocarbon analysis. In order to minimize sample preparation time, we designed a simplified preconcentration line for the extraction of methane from 50 L atmospheric air, which corresponds to 50 µg C as required for a reliable 14C analysis of methane-derived CO2 gas measurement with accelerator mass spectrometry (AMS). The system guarantees a quantitative extraction of methane from atmospheric air samples for 14C analysis, with a good repeatability and a low processing blank. The setup was originally designed for the measurement of samples with low methane concentrations, but it can also be adapted to apportion sources from environmental compartments with high methane levels such as freshwaters or wetlands.

Description: Alpine glaciers are valuable archives for the reconstruction of human impact on the environment. Besides dating purposes, measurement of radiocarbon (14C) content provides a powerful tool for long-term source apportionment studies on the carbonaceous aerosols incorporated in ice cores. In this work, we present an extraction system for 14C analyses of dissolved organic carbon (DOC) in ice cores. The setup can process ice samples of up to 350 g mass and offers ultra-clean working conditions for all extraction steps. A photo-oxidation method is applied by means of external UV irradiation of the sample. For an irradiation time of 30 min with catalyzation by addition of Fe2+ and H2O2, we achieve an efficiency of 96 ± 6% on average. Inert gas working conditions and stringent decontamination procedures enable a low overall blank of 1.9 ± 1.6 μg C with a F14C value of 0.68 ± 0.13. This makes it possible to analyze the DOC in ice samples with a carbon content of as low as 25 μg C kg−1 ice. For a first validation, the new method was applied to ice core samples from the Swiss Alps. The average DOC concentration and F14C values for the Fiescherhorn ice core samples show good agreement with previously reported data for the investigated period of 1925–1936 AD.

Description: Neodymium (Nd) isotopes extracted from authigenic sediment phases are increasingly used as a proxy for past variations in water mass provenance. To better constrain the controls of water mass provenance and nonconservative effects on the archived Nd isotope signal, we present a new depth transect of Nd isotope reconstructions from the Blake Bahama Outer Ridge along the North American continental margin covering the past 30 ka. We investigated five sediment cores that lie directly within the main flow path of the Deep Western Boundary Current, a major advection route of North Atlantic Deep Water. We found offsets between core tops and seawater Nd isotopic compositions that are observed elsewhere in the Northwest Atlantic. A possible explanation for this is the earlier suggested redistribution of sediment by nepheloid layers at intermediate as well as abyssal depths, transporting material downslope and along the continental margin. These processes potentially contributed to Nd isotope excursions recorded in Northwest Atlantic sediment cores during the Bølling‐Allerød and early Holocene. An Atlantic‐wide comparison of Nd isotope records shows that the early Holocene excursions had an additional contribution from conservative advection of unradiogenic dissolved Nd. Nevertheless, the trends of the Nd isotope records are in general agreement with previous reconstructions of water mass provenance from the entire Atlantic and also reveal millennial‐scale changes during the last deglaciation in temporal high resolution, which have rarely been reported before. Further, the new records confirm that during cold periods the Northwest Atlantic was bathed by an increased contribution of southern sourced water.

Description: The neodymium isotope proxy has become a valuable tool for the reconstruction of past ocean water mass provenance and mixing. For its accurate application, knowledge about the origin and preservation of Nd in sedimentary archives is crucial. Recently, concerns have emerged regarding the applicability of neodymium isotopes as a conservative palaeo water mass tracer, given potential Nd fluxes from sediments into bottom waters (Abbott et al., 2015a) and inferred relabelling of ocean waters by settling detrital material (Roberts and Piotrowski, 2015). Consequently, a decoupling of water mass provenance and proxy variations may arise. We investigate the mobility of Nd around extreme detrital sedimentation events such as glacial ice rafting pulses and turbidite deposition in the Northeast Atlantic. The constructed records from sediment leachates span extreme Nd isotope variations including volcanic (εNd ∼ 0) and Laurentian (εNd ∼ −27) sources. We find that Nd was released into pore waters from reactive detritus inside some detrital layers during early diagenesis, thereby overprinting any archived bottom water Nd signature and precluding the reconstruction of past water mass provenance during the affected time intervals. However, we do not observe any definite indication of diffusive vertical migration of Nd into adjacent layers. Furthermore, bottom water Nd isotope signatures were not modified to a measurable degree by any potential benthic flux of Nd during the deposition of these detrital sediment layers. Consequently, the Nd isotope composition of the pelagic glacial Northeast Atlantic water masses were resilient to such episodic large detrital fluxes. Apart from extreme local sedimentation events, we confirm the presence of detritally overprinted deep waters north of 47°N during the peak glacial from comparison of Northeast Atlantic depth transects. We furthermore suggest that the sensitivity of deep waters to this overprinting effect increased during periods of reduced Atlantic Meridional Overturning Circulation and elevated ice rafting. Overall, our study demonstrates that a thorough evaluation of the proportion of Nd originating from physical water mass advection versus in situ chemical inputs is crucial for the reliable application of Nd isotopes as a water mass tracer.

Description: Neodymium (Nd) isotopes extracted from authigenic sediment phases are increasingly used as a proxy for past variations in water mass provenance. To better constrain the controls of water mass provenance and nonconservative effects on the archived Nd isotope signal, we present a new depth transect of Nd isotope reconstructions from the Blake Bahama Outer Ridge along the North American continental margin covering the past 30 ka. We investigated five sediment cores that lie directly within the main flow path of the Deep Western Boundary Current, a major advection route of North Atlantic Deep Water. We found offsets between core tops and seawater Nd isotopic compositions that are observed elsewhere in the Northwest Atlantic. A possible explanation for this is the earlier suggested redistribution of sediment by nepheloid layers at intermediate as well as abyssal depths, transporting material downslope and along the continental margin. These processes potentially contributed to Nd isotope excursions recorded in Northwest Atlantic sediment cores during the Bølling-Allerød and early Holocene. An Atlantic-wide comparison of Nd isotope records shows that the early Holocene excursions had an additional contribution from conservative advection of unradiogenic dissolved Nd. Nevertheless, the trends of the Nd isotope records are in general agreement with previous reconstructions of water mass provenance from the entire Atlantic and also reveal millennial-scale changes during the last deglaciation in temporal high resolution, which have rarely been reported before. Further, the new records confirm that during cold periods the Northwest Atlantic was bathed by an increased contribution of southern sourced water.

Description: Neodymium (Nd) isotopes extracted from authigenic sediment phases are increasingly used as a proxy for past variations in water mass provenance. To better constrain the controls of water mass provenance and nonconservative effects on the archived Nd isotope signal, we present a new depth transect of Nd isotope reconstructions from the Blake Bahama Outer Ridge along the North American continental margin covering the past 30 ka. We investigated five sediment cores that lie directly within the main flow path of the Deep Western Boundary Current, a major advection route of North Atlantic Deep Water. We found offsets between core tops and seawater Nd isotopic compositions that are observed elsewhere in the Northwest Atlantic. A possible explanation for this is the earlier suggested redistribution of sediment by nepheloid layers at intermediate as well as abyssal depths, transporting material downslope and along the continental margin. These processes potentially contributed to Nd isotope excursions recorded in Northwest Atlantic sediment cores during the Bölling-Alleröd and early Holocene. An Atlantic-wide comparison of Nd isotope records shows that the early Holocene excursions had an additional contribution from conservative advection of unradiogenic dissolved Nd. Nevertheless, the trends of the Nd isotope records are in general agreement with previous reconstructions of water mass provenance from the entire Atlantic and also reveal millennial-scale changes during the last deglaciation in temporal high resolution, which have rarely been reported before. Further, the new records confirm that during cold periods the Northwest Atlantic was bathed by an increased contribution of southern sourced water.