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  • Nature Research  (8)
  • ASLO (Association for the Sciences of Limnology and Oceanography)  (2)
  • 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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    Format: text
    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
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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
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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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    Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation (2015)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 60 (5). pp. 1619-1633.
    Details
    Publication Date: 2018-06-20
    Description: We present the first set of dissolved silicon isotope data in seawater (delta Si-30(Si(OH)4)) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface delta Si-30(Si(OH)4) signatures of +2.1 parts per thousand corresponding to the highest Si(OH)(4) concentrations (similar to 30.0 mu mol L-1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface delta Si-30(Si(OH)4) increased rapidly to +3.2 parts per thousand while Si(OH)(4) became depleted, indicating increasing biological utilization of the Si(OH)(4) originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of delta Si-30(Si(OH)4) and an increase of Si(OH)(4) with depth on the East China Sea mid-shelf and slope. In winter (December 2009-January 2010), however, the delta Si-30(Si(OH)4) was nearly constant at +1.9 parts per thousand throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in delta Si-30(Si(OH)4) were detected in the surface waters beyond the Changjiang Diluted Water-influenced region on the East China Sea shelf, where the winter values were similar to 1.0 parts per thousand lower than those in summer, despite the same primary Si(OH)(4) supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)(4) in summer than in winter.
    Type: Article , PeerReviewed
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    DOI: 10.1002/lno.10124
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Silicon and nitrogen cycling in the upwelling area off Peru: A dual isotope approach (2016)
    ASLO (Association for the Sciences of Limnology and Oceanography)
    In:  Limnology and Oceanography, 61 (5). pp. 1661-1676.
    Details
    Publication Date: 2019-09-23
    Description: We present a comparison of the dissolved stable isotope composition of silicate (δ30Si(OH)4) and nitrate (δ15 inline image) to investigate the biogeochemical processes controlling nutrient cycling in the upwelling area off Peru, where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Besides strong upwelling of nutrient rich waters mainly favoring diatom growth, an anticyclonic eddy influenced the study area. We observe a tight coupling between the silicon (Si) and nitrogen (N) cycles in the study area. Waters on the shelf showed high Si(OH)4 concentrations accompanied by diminished inline image concentration as a consequence of intense remineralization, high Si fluxes from the shelf sediments, and N-loss processes such as anammox/denitrification within the OMZ. Correspondingly, the surface waters show low δ30Si(OH)4 values (+2‰) due to low Si utilization but relatively high δ15 inline image (+13‰) values due to upwelling of waters influenced by N-loss processes. In contrast, as a consequence of the deepening of the thermocline in the eddy center, a pronounced Si(OH)4 depletion led to the highest δ30Si(OH)4 values (+3.7‰) accompanied by high δ15 inline image values (+16‰). In the eddy center, high inline image: Si(OH)4 ratios favored the growth of non-siliceous organisms (Synechococcus). Our data show that upwelling processes and the presence of eddies play important roles controlling the nutrient cycles and therefore also exert a major influence on the phytoplankton communities in the Peruvian Upwelling. Our findings also show that the combined approach of δ30Si(OH)4 and δ15 inline image can improve our understanding of paleo records as it can help to disentangle utilization and N-loss processes.
    Type: Article , PeerReviewed
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    DOI: 10.1002/lno.10324
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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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
  • 7
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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
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    DOI: 10.1038/s41598-019-46253-1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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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
  • 9
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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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    Format: other
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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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
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    Format: other
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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