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  • 2020-2024  (30)
  • 2021  (30)
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  • 1
    Online Resource
    Online Resource
    Stable barium isotopes : biogeochemical cycling and palaeoceanographic applications (2021)
    Kiel : Universitätsbibliothek Kiel
    Details Availability
    Keywords: Hochschulschrift ; Bariumisotop ; Biogeochemie ; Paläoozeanographie
    Type of Medium: Online Resource
    Pages: 1 Online-Ressource (v,147 Seiten) , Illustrationen
    URL: https://nbn-resolving.org/urn:nbn:de:gbv:8:3-2021-00445-5
    URL: https://d-nb.info/1236897250/34
    URL: https://macau.uni-kiel.de/receive/macau_mods_00001491
    URN: urn:nbn:de:gbv:8:3-2021-00445-5
    DDC: 551.462
    Language: English
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  • 2
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    The impact of postdepositional alteration on iron- and molybdenum-based redox proxies (2021)
    GSA (Geological Society of America)
    Details
    Publication Date: 2024-02-07
    Description: Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Moauth) in sedimentary rocks are geochemical proxies that are widely used to reconstruct past marine redox states, which have been calibrated in modern marine settings covering oxic to euxinic conditions. However, syn- and postdepositional processes can result in alterations and ambiguities of proxy-derived redox signals that can challenge the validity of paleoreconstructions. We present new data from modern organic-rich sediments of two oxygen minimum zone settings in the Gulf of California and the Peruvian margin. The results show that Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S produced deeper in the sediment (e.g., by sulfate reduction coupled to anaerobic methane oxidation) leaves the initially deposited Mo concentrations and δ98Mo signatures unaltered, which supports the robustness of Mo-based redox proxies. In contrast, the Fe speciation data reveal continued pyritization due to constant exposure of Fe minerals to H2S. Importantly, both Fe bound to oxides and carbonates (highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates) undergo pyritization during early diagenesis. This process generates Fe-based proxy signatures that falsely imply ferruginous or euxinic conditions.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling (2021)
    Frontiers
    Details
    Publication Date: 2024-02-07
    Description: The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (δ 30 Si dSi ) and biogenic silica (δ 30 Si bSi ) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More δ 30 Si bSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ 30 Si bSi data (together with δ 30 Si dSi ) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The δ 30 Si dSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ 30 Si bSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ 30 Si) ranging from −0.4‰ to −1.0‰. In contrast, δ 30 Si dSi and δ 30 Si bSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ 30 Si dSi . A strong relationship between δ 30 Si bSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ 30 Si bSi . A comparison of δ 30 Si bSi samples from the water column and from underlying core-top sediments (δ 30 Si bSi_ sed. ) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ 30 Si bSi_ sed. = +0.9‰ to +1.7‰), although we observed small differences in δ 30 Si bSi depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using δ 30 Si bSi data as a paleo proxy for the reconstruction of dSi utilization in the region.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    An experimental investigation of the acquisition of Nd by authigenic phases of marine sediments (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: The neodymium isotope composition (ɛNd) of authigenic phases in marine sediment is widely used to reconstruct the origin and mixing of water masses of overlying seawater through time. However, at some locations in the modern ocean, the ɛNd of authigenic phases in surface sediment is not consistent with that of local seawater, raising concerns about its current interpretation as a paleotracer of water masses. To further investigate this question, we conducted a laboratory-based incubation experiment with a Mn-oxide phase placed at the sediment–water interface of multicores to assess the extent to which the authigenic phase records seawater ɛNd. Multicores were collected from the Strait of Georgia (SoG), which is a relatively deep coastal waterway with high sedimentation rates, oxygenated surface sediments, and active macrofauna, separating the mainland coast of British Columbia and Vancouver Island. Manganese oxide-coated XAD resin beads were placed at the sediment surface and the cores were incubated for 6 months in a tank filled with SoG seawater spiked with Nd. While the ɛNd of the Mn-oxide coated resin (−4.0) was similar to that of SoG seawater used for the incubation (−3.7), the Nd/Nd of the Mn-oxide phase measured after the incubation indicates that, under our experimental conditions, a minimum of 83% of the Nd associated with the Mn-oxide phase is not sourced from seawater, but from pore water. The Nd/Nd ratio of the Mn-oxide resin is necessary to determine the predominant source of Nd to the resin because the ɛNd of SoG pore water (−3.9) is within analytical error of seawater (−3.7). Using field data and constraints from the Nd mass balance during the incubation, we conclude that the similarity of ɛNd in pore water and seawater in the SoG is fortuitous and not a result of a top-down or bottom-up control. Although the setting of our sediments is not directly comparable to open ocean locations, this study raises concerns about the use of ɛNd in paleocirculation studies, and points to the necessity of elucidating the factors controlling local lithogenic dissolution in pore waters as a prerequisite for the correct interpretation of ɛNd in the authigenic phases of marine sediments.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Stable Barium Isotope Dynamics During Estuarine Mixing (2021)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: Stable barium isotopes are a potential proxy for riverine inputs into the ocean that reflect monsoon variability and climate change. However, dissolved Ba isotope (δ138BaDBa) geochemistry in river estuaries, a dynamic land to ocean transition zone, has rarely been systematically examined to date. Here, we show that significant Ba isotope fractionation occurs at near-zero salinities in the Yangtze and Pearl River Estuary, whereas conservative mixing dominates δ138BaDBa distributions beyond low salinities, which are well predicted by an ion exchange model. Elevated δ138BaDBa in the river endmember results from preferential removal of light Ba isotopes by adsorption to fluvial particles. Subsequently, δ138BaDBa rapidly drops to minimum signatures at increased salinities indicating particle desorption of isotopically light Ba. Nevertheless, the apparently conservative δ138BaDBa-salinity relationship beyond the low-salinity minimum in both estuaries provides a modern calibration for using Ba isotopes as a proxy for paleosalinity and river water inputs into the ocean.
    Type: Article , PeerReviewed
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    Format: other
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Persistent Provenance of South Asian Monsoon‐Induced Silicate Weathering Over the Past 27 Million Years (2021)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: The development of the South Asian monsoon (SAM) and Himalaya‐Tibetan Plateau uplift were closely intertwined with some studies suggesting that uplift initiated the monsoon whereas others link tectonics with monsoon‐controlled exhumation. Silicate weathering controls atmospheric CO2 on geological timescales resulting in a large potential for monsoon strength and the Himalayan orogeny to influence global climate but detailed records of SAM‐induced weathering on million year (Myr) timescales are lacking. Here, we present radiogenic Sr, Nd, and Pb isotope compositions of clay minerals produced by silicate weathering and transported to the central Bay of Bengal. The radiogenic isotope data exhibit a relatively small range and demonstrate a remarkably consistent mixture of sources dominated by Himalayan rocks and the Indo‐Burman ranges, which consist of sediments derived from the Himalayas. This suggests that the spatial pattern of regional weathering, which today is highest in the regions of strongest monsoon rains, has persisted in a similar form for the last 27 Myrs. A pronounced increase in primary clay mineral abundance (from 9% to 22%) coincident with global cooling 13.9 Myrs ago points to a shift in the weathering regime given that the clay provenance did not change dramatically. Relatively weaker chemical weathering intensity during the mid and late Miocene cooling suggests increased aridity and changes in the large scale atmospheric circulation in the SAM domain. The establishment of the dry winter monsoon season during the mid and late Miocene may have caused this shift in the weathering regime and can reconcile much of the contrasting evidence for SAM initiation.
    Type: Article , PeerReviewed
    Format: other
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Silicon Isotope Signatures of Radiolaria Reveal Taxon-Specific Differences in Isotope Fractionation (2021)
    Frontiers
    Details
    Publication Date: 2024-02-07
    Description: The global silicon (Si) cycle plays a critical role in regulating the biological pump and the carbon cycle in the oceans. A promising tool to reconstruct past dissolved silicic acid (DSi) concentrations is the silicon isotope signature of radiolaria (δ 30 Si rad ), siliceous zooplankton that dwells at subsurface and intermediate water depths. However, to date, only a few studies on sediment δ 30 Si rad records are available. To investigate its applicability as a paleo proxy, we compare the δ 30 Si rad of different radiolarian taxa and mixed radiolarian samples from surface sediments off Peru to the DSi distribution and its δ 30 Si signatures (δ 30 Si DSi ) along the coast between the equator and 15°S. Three different radiolarian taxa were selected according to their specific habitat depths of 0–50 m ( Acrosphaera murrayana ), 50–100 m ( Dictyocoryne profunda/truncatum ), and 200–400 m ( Stylochlamydium venustum ). Additionally, samples containing a mix of species from the bulk assemblage covering habitat depths of 0 to 400 m have been analyzed for comparison. We find distinct δ 30 Si rad mean values of +0.70 ± 0.17‰ ( Acro ; 2 SD), +1.61 ± 0.20 ‰ ( Dictyo ), +1.19 ± 0.31 ‰ ( Stylo ) and +1.04 ± 0.19 ‰ (mixed radiolaria). The δ 30 Si values of all individual taxa and the mixed radiolarian samples indicate a significant ( p 〈 0.05) inverse relationship with DSi concentrations of their corresponding habitat depths. However, only δ 30 Si of A. murrayana are correlated to DSi concentrations under normally prevailing upwelling conditions. The δ 30 Si of Dictyocoryne sp., Stylochlamydium sp., and mixed radiolaria are significantly correlated to the lower DSi concentrations either associated with nutrient depletion or shallower habitat depths. Furthermore, we calculated the apparent Si isotope fractionation between radiolaria and DSi (Δ 30 Si ∼ 30 ε = δ 30 Si rad − δ 30 Si DSi ) and obtained values of −1.18 ± 0.17 ‰ ( Acro ), −0.05 ± 0.25 ‰ ( Dictyo ), −0.34 ± 0.27 ‰ ( Stylo ), and −0.62 ± 0.26 ‰ (mixed radiolaria). The significant differences in Δ 30 Si between the order of Nassellaria ( A. murrayana ) and Spumellaria ( Dictyocoryne sp. and Stylochlamydium sp.) may be explained by order-specific Si isotope fractionation during DSi uptake, similar to species-specific fractionation observed for diatoms. Overall, our study provides information on the taxon-specific fractionation factor between radiolaria and seawater and highlights the importance of taxonomic identification and separation to interpret down-core records.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Drivers of river reactivation in North Africa during the last glacial cycle (2021)
    Nature Research
    Details
    Publication Date: 2024-02-07
    Description: North African greening phases, during which large rivers ran through the Sahara Desert, occurred repeatedly during the Quaternary and are regarded as key periods for the development of past human populations. However, the timing and mechanisms responsible for the reactivation of the presently dormant fluvial systems remain highly uncertain. Here we present hydroclimate changes over the past 160,000 years, reconstructed from analyses of the provenance of terrestrial sediments in a marine sediment record from the Gulf of Sirte (offshore Libya). By combining high-resolution proxy data with transient Earth system model simulations, we are able to identify the various drivers that led to the observed shifts in hydroclimate and landscapes. We show that river runoff occurred during warm interglacial phases of Marine Isotope Stages 1 and 5 due to precession-forced enhancements in the summer and autumn rainfall over the entire watershed, which fed presently dry river systems and intermittent coastal streams. In contrast, shorter-lasting and less-intense humid events during glacial Marine Isotope Stages 3 and 4 were related to autumn and winter precipitation over the Libyan coastal regions driven by Mediterranean storms. Our results reveal large shifts in hydroclimate environments during the last glacial cycle, which probably exerted a strong evolutionary and structural control on past human populations, potentially pacing their dispersal across northern Africa.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Deoxygenation dynamics on the western Nile deep-sea fanduring sapropel S1 from seasonal to millennial timescales (2021)
    Copernicus Publications (EGU)
    Details
    Publication Date: 2024-02-07
    Description: Ocean deoxygenation is a rising threat to marine ecosystems and food resources under present climate warming conditions. Organic-rich sapropel layers deposited in the Mediterranean Sea provide a natural laboratory to study the processes that have controlled changes in seawater oxygen levels in the recent geological past. Our study is based on three sediment cores spanning the last 10 000 years and located on a bathymetric transect offshore from the western distributaries of the Nile delta. These cores are partly to continuously laminated in the sections recording sapropel S1, which is indicative of bottom-water anoxia above the western Nile deep-sea fan. We used a combination of microfacies analyses and inorganic and organic geochemical measurements to reconstruct changes in oxygenation conditions at seasonal to millennial timescales. Millimetre-thick laminations are composed of detrital, biogenic and chemogenic sublayers reflecting seasonal successions of sedimentation. Dark layers reflect the deposition of summer floods and two types of light layers correspond to autumn plankton blooms and authigenic carbonates formed in the water column during spring–early summer, respectively. The isotopic signature of the authigenic carbonates suggests permanent anoxic to euxinic bottom waters resulting in high levels of anaerobic remineralization of organic matter and highlights their potential to reconstruct seawater chemistry at times when benthic fauna was absent. Ratios of major elements combined with biomarkers of terrestrial and marine organic matter and redox-sensitive compounds allow changes in terrigenous input, primary productivity and past deoxygenation dynamics on millennial timescales to be tracked. Rapid fluctuations of oxygenation conditions in the upper 700 m water depth occurred above the Nile deep-sea fan between 10 and 6.5 ka BP, while deeper cores recorded more stable anoxic conditions. Synchronous changes in terrigenous input, primary productivity and past oxygenation dynamics after 6.5 ka BP show that runoff-driven eutrophication played a central role in rapid oxygenation changes in the south-eastern Levantine Basin. These findings are further supported by other regional records and reveal time-transgressive changes in oxygenation state driven by rapid changes in primary productivity during a period of long-term deep-water stagnation.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Provenance and weathering of clays delivered to the Bay of Bengal during the middle Miocene: Linkages to tectonics and monsoonal climate (2021)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: Tectonics and regional monsoon strength control weathering and erosion regimes of the watersheds feeding into the Bay of Bengal, which are important contributors to global climate evolution via carbon cycle feedbacks. The detailed mechanisms controlling the input of terrigenous clay to the Bay of Bengal on tectonic to orbital timescales are, however, not yet well understood. We produced orbital‐scale resolution geochemical records for International Ocean Discovery Program Site U1443 (southern Bay of Bengal) across five key climatic intervals of the middle to late Miocene (15.8 – 9.5 Ma). Our new radiogenic Sr, Nd, and Pb isotope time series of clays transported to the Ninetyeast Ridge suggest that the individual contributions from different erosional sources overall remained remarkably consistent during the Miocene despite major tectonic reorganizations in the Himalayas. On orbital timescales, however, high‐resolution data from the five investigated intervals show marked fluctuations of all three isotope systems. Interestingly, the variability was much higher within the Miocene Climatic Optimum (around 16‐15 Ma) and across the major global cooling (~13.9‐13.8 Ma) until ~13.5 Ma, than during younger time intervals. This change is attributed to a major restriction on the supply of High Himalayan erosion products due to migration of the peak precipitation area towards the frontal domains of the Himalayas and the Indo‐Burman Ranges. The transient excursions of the radiogenic isotope signals on orbital timescales most likely reflect climatically driven shifts in monsoon strength.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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