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  • 1
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    Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years (2018)
    Nature Research
    In:  Nature Communications, 9 (4702).
    Details
    Publication Date: 2021-03-19
    Description: The orbital-scale timing of South Asian monsoon (SAM) precipitation is poorly understood. Here we present new SST and seawater δ18O (δ18Osw) records from the Bay of Bengal, the core convective region of the South Asian monsoon, over the past 1 million years. Our records reveal that SAM precipitation peaked in the precession band ~9 kyrs after Northern Hemisphere summer insolation maxima, in phase with records of SAM winds in the Arabian Sea and eastern Indian Ocean. Precession-band variance, however, accounts for ~30% of the total variance of SAM precipitation while it was either absent or dominant in records of the East Asian monsoon (EAM). This and the observation that SAM precipitation was phase locked with obliquity minima and was sensitive to Southern Hemisphere warming provides clear evidence that SAM and EAM precipitation responded differently to orbital forcing and highlights the importance of internal processes forcing monsoon variability.
    Type: Article , PeerReviewed
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    DOI: 10.1038/s41467-018-07076-2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    H2S events in the Peruvian oxygen minimum zone facilitate enhanced dissolved Fe concentrations (2018)
    Nature Research
    In:  Scientific Reports, 8 . Art.Nr. 12642.
    Details
    Publication Date: 2021-04-23
    Description: Dissolved iron (DFe) concentrations in oxygen minimum zones (OMZs) of Eastern Boundary Upwelling Systems are enhanced as a result of high supply rates from anoxic sediments. However, pronounced variations in DFe concentrations in anoxic coastal waters of the Peruvian OMZ indicate that there are factors in addition to dissolved oxygen concentrations (O2) that control Fe cycling. Our study demonstrates that sediment-derived reduced Fe (Fe(II)) forms the main DFe fraction in the anoxic/euxinic water column off Peru, which is responsible for DFe accumulations of up to 200 nmol L-1. Lowest DFe values were observed in anoxic shelf waters in the presence of nitrate and nitrite. This reflects oxidation of sediment-sourced Fe(II) associated with nitrate/nitrite reduction and subsequent removal as particulate Fe(III) oxyhydroxides. Unexpectedly, the highest DFe levels were observed in waters with elevated concentrations of hydrogen sulfide (up to 4 µmol L-1) and correspondingly depleted nitrate/nitrite concentrations (〈0.18 µmol L-1). Under these conditions, Fe removal was reduced through stabilization of Fe(II) as aqueous iron sulfide (FeSaqu) which comprises complexes (e.g., FeSH+) and clusters (e.g., Fe2S2|4H2O). Sulfidic events on the Peruvian shelf consequently enhance Fe availability, and may increase in frequency in future due to projected expansion and intensification of OMZs.
    Type: Article , PeerReviewed
    Format: text
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    DOI: 10.1038/s41598-018-30580-w
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Major intensification of Atlantic overturning circulation at the onset of Paleogene greenhouse warmth (2018)
    Nature Research
    In:  Nature Communications, 9 (4954).
    Details
    Publication Date: 2021-02-08
    Description: During the Late Cretaceous and early Cenozoic the Earth experienced prolonged climatic cooling most likely caused by decreasing volcanic activity and atmospheric CO2 levels. However, the causes and mechanisms of subsequent major global warming culminating in the late Paleocene to Eocene greenhouse climate remain enigmatic. We present deep and intermediate water Nd-isotope records from the North and South Atlantic to decipher the control of the opening Atlantic Ocean on ocean circulation and its linkages to the evolution of global climate. The marked convergence of Nd-isotope signatures 59 million years ago indicates a major intensification of deep-water exchange between the North and South Atlantic, which coincided with the turning point of deep-water temperatures towards early Paleogene warming. We propose that this intensification of Atlantic overturning circulation in concert with increased atmospheric CO2 from continental rifting marked a climatic tipping point contributing to a more efficient distribution of heat over the planet.
    Type: Article , PeerReviewed
    Format: text
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    DOI: 10.1038/s41467-018-07457-7
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Two-step closure of the Miocene Indian Ocean Gateway to the Mediterranean (2019)
    Nature Research
    In:  Scientific Reports, 9 (8842).
    Details
    Publication Date: 2022-01-31
    Description: The Tethys Ocean was compartmentalized into the Mediterranean Sea and Indian Ocean during the early Miocene, yet the exact nature and timing of this disconnection are not well understood. Here we present two new neodymium isotope records from isolated carbonate platforms on both sides of the closing seaway, Malta (outcrop sampling) and the Maldives (IODP Site U1468), to constrain the evolution of past water mass exchange between the present day Mediterranean Sea and Indian Ocean via the Mesopotamian Seaway. Combining these data with box modeling results indicates that water mass exchange was reduced by similar to 90% in a first step at ca. 20 Ma. The terminal closure of the seaway then coincided with the sea level drop caused by the onset of permanent glaciation of Antarctica at ca. 13.8 Ma. The termination of meridional water mass exchange through the Tethyan Seaway resulted in a global reorganization of currents, paved the way to the development of upwelling in the Arabian Sea and possibly led to a strengthening of South Asian Monsoon.
    Type: Article , PeerReviewed
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    DOI: 10.1038/s41598-019-45308-7
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Late Eocene onset of the Proto-Antarctic Circumpolar Current (2019)
    Nature Research
    In:  Scientific Reports, 9 (Article number: 10125).
    Details
    Publication Date: 2022-01-31
    Description: The formation of the Antarctic Circumpolar Current (ACC) is critical for the evolution of the global climate, but the timing of its onset is not well constrained. Here, we present new seismic evidence of widespread Late Eocene to Oligocene marine diagenetic chert in sedimentary drift deposits east of New Zealand indicating prolonged periods of blooms of siliceous microorganisms starting ~36 million years ago (Ma). These major blooms reflect the initiation of the arrival and upwelling of northern-sourced, nutrient-rich deep equatorial Pacific waters at the high latitudes of the South Pacific. We show that this change in circulation was linked to the initiation of a proto-ACC, which occurred ~6 Ma earlier than the currently estimated onset of the ACC at 30 Ma. We propose that the associated increased primary productivity and carbon burial facilitated atmospheric carbon dioxide reduction contributing to the expansion of Antarctic Ice Sheet at the Eocene-Oligocene Transition.
    Type: Article , PeerReviewed
    Format: text
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    DOI: 10.1038/s41598-019-46253-1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Miocene restriction of the Pacific-North Atlantic throughflow strengthened Atlantic overturning circulation (2019)
    Nature Research
    In:  Nature Communications, 10 (Article number: 4025 ).
    Details
    Publication Date: 2022-01-31
    Description: Export of warm and salty waters from the Caribbean to the North Atlantic is an essential component of the Atlantic Meridional Overturning Circulation (AMOC). However, there was also an active AMOC during the Miocene, despite evidence for an open Central American Seaway (CAS) that would have allowed low-salinity Pacific waters to enter the Caribbean. To address this apparent contradiction and to constrain the timing of CAS closure we present the first continuous Nd isotope record of intermediate waters in the Florida Strait over the past 12.5 million years. Our results indicate that there was no direct intermediate water mass export from the Caribbean to the Florida Strait between 11.5 and 9.5 Ma, at the same time as a strengthened AMOC. After 9 Ma a strong AMOC was maintained due to a major step in CAS closure and the consequent cessation of low-salinity Pacific waters entering the Caribbean.
    Type: Article , PeerReviewed
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    DOI: 10.1038/s41467-019-12034-7
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Radiogenic isotopes: Tracers of past ocean circulation and erosional input (2002)
    AGU
    In:  Reviews of Geophysics, 40 (1).
    Details
    Publication Date: 2020-06-03
    Description: The radiogenic isotope composition of dissolved trace metals in the ocean represents a set of relatively new and not yet fully exploited tracers with a large potential for oceanographic and paleoceanographic research on timescales from the present back to at least 60 Ma. The main topic of this review are those trace metals with oceanic residence times on the order of or shorter than the global mixing time of the ocean (Nd, Pb, Hf, and, in addition, Be). Their isotopic composition in the ocean has varied as a function of changes in paleocirculation, source provenances, style and intensity of weathering on the continents, as well as orogenic processes. The relative importance of these processes for each trace metal is evaluated, which is a prerequisite for reliable interpretation of their time series in terms of changes in paleocirculation or weathering inputs. This analysis of processes includes a discussion of the long-term isotopic evolution of Sr and Os, which are well mixed in the ocean and have thus not been influenced by circulation changes. The radiogenic isotope evolution of those trace metals with intermediate oceanic residence times can be used as paleoceanographic proxies to reconstruct paleocirculation and weathering inputs into the ocean. This is demonstrated by studies from different ocean basins, mainly carried out on ferromanganese crusts, which show that radiogenic trace metal isotopes provide important new insights and can complement results obtained by other well-established paleoceanographic tracers such as carbon isotopes.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2000RG000094
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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
    Format: text
    Format: other
    Format: other
    Format: other
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Overlooked riverine contributions of dissolved neodymium and hafnium to the Amazon estuary and oceans (2023)
    Nature Research
    Details
    Publication Date: 2024-02-08
    Description: The Amazon River accounts for 20% of global freshwater runoff and supplies vital trace metals to the Atlantic Ocean. Suspended particles within its plume are thought to partially dissolve, constituting a large potential source of metals, which is, however, not well constrained. Here we used combined neodymium (Nd) and hafnium (Hf) isotopes to disprove the release of Nd and Hf from particles as the cause of the observed dissolved concentration increases and isotopic variability across the plume. Instead, the changes reflect admixture of nearby Pará River freshwater with exceptionally high dissolved Nd and Hf concentrations contributing 45–100% of the riverine fraction to the southern and outer estuary. This result led us to develop an empirical relationship between riverine Nd concentration and pH to revise the global dissolved riverine Nd flux, which accordingly is at least three times higher than commonly used estimates. Future work should focus on contributions of low-pH rivers to global metal fluxes.
    Type: Article , PeerReviewed
    Format: text
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    Format: other
    Format: other
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Circumpolar mapping of permafrost temperature and thaw depth in the ESA Permafrost CCI project (2018)
    AGU
    In:  EPIC3AGU Fall Meeting 2018, Washington, D.C., USA, 2018-12-10-2018-12-14Washington, D.C., USA, AGU
    Details
    Publication Date: 2019-12-24
    Description: Permafrost is an Essential Climate Variable (ECV) within the Global Climate Observing System (GCOS), which is characterized by subsurface temperatures and the depth of the seasonal thaw layer. Complementing ground-based monitoring networks, the Permafrost CCI project funded by the European Space Agency (ESA) 2018-2021 will establish Earth Observation (EO) based products for the permafrost ECV spanning the last two decades. Since ground temperature and thaw depth cannot be directly observed from space-borne sensors, we will ingest a variety of satellite and reanalysis data in a ground thermal model, which allows to quantitatively characterize the changing permafrost systems in Arctic and High-Mountain areas. As recently demonstrated for the Lena River Delta in Northern Siberia, the algorithm uses remotely sensed data sets of Land Surface Temperature (LST), Snow Water Equivalent (SWE) and landcover to drive the transient permafrost model CryoGrid 2, which yields ground temperature at various depths, in addition to thaw depth. For the circumpolar CCI product, we aim for a spatial resolution between 10 and 1km, but ensemble runs will be performed for each pixel to represent the subgrid variability of snow and land cover. The performance of the transient algorithm crucially depends on the correct representation of ground properties, in particular ice and organic contents. Therefore, the project will compile a new subsurface stratigraphy product which also holds great potential for improving Earth System Model results in permafrost environments. We report on simulation runs for various permafrost regions and characterize the accuracy and ability to reproduce trends against ground-based data. Finally, we evaluate the feasibility of future “permafrost reanalysis” products, exploiting the information content of various satellite products to deliver the best possible estimate for the permafrost thermal state over a range of spatial scales.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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