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  • 1
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    Climate oscillations reflected within the microbiome of Arabian Sea sediments (2017)
    Nature Publishing Group
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    Publikationsdatum: 2022-05-25
    Beschreibung: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 7 (2017): 6040, doi:10.1038/s41598-017-05590-9.
    Beschreibung: Selection of microorganisms in marine sediment is shaped by energy-yielding electron acceptors for respiration that are depleted in vertical succession. However, some taxa have been reported to reflect past depositional conditions suggesting they have experienced weak selection after burial. In sediments underlying the Arabian Sea oxygen minimum zone (OMZ), we performed the first metagenomic profiling of sedimentary DNA at centennial-scale resolution in the context of a multi-proxy paleoclimate reconstruction. While vertical distributions of sulfate reducing bacteria and methanogens indicate energy-based selection typical of anoxic marine sediments, 5–15% of taxa per sample exhibit depth-independent stratigraphies indicative of paleoenvironmental selection over relatively short geological timescales. Despite being vertically separated, indicator taxa deposited under OMZ conditions were more similar to one another than those deposited in bioturbated intervals under intervening higher oxygen. The genomic potential for denitrification also correlated with palaeo-OMZ proxies, independent of sediment depth and available nitrate and nitrite. However, metagenomes revealed mixed acid and Entner-Dourdoroff fermentation pathways encoded by many of the same denitrifier groups. Fermentation thus may explain the subsistence of these facultatively anaerobic microbes whose stratigraphy follows changing paleoceanographic conditions. At least for certain taxa, our analysis provides evidence of their paleoenvironmental selection over the last glacial-interglacial cycle.
    Beschreibung: his work was primarily supported by NSF MGG Grant #1357017 to MJLC, VG, and LG, and the KAUST-WHOI Special Academic Partnership Program OCRF-SP-WHOI-2013 (grants 7000000463 to XI and 7000000464 to MJLC). Additional financial support was provided via a C-DEBI grant #OCE-0939564 to WDO.
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  • 2
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    Alongshore sediment bypassing as a control on river mouth morphodynamics (2016)
    John Wiley & Sons
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    Publikationsdatum: 2022-05-25
    Beschreibung: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 121 (2016): 664–683, doi:10.1002/2015JF003780.
    Beschreibung: River mouths, shoreline locations where fluvial and coastal sediments are partitioned via erosion, trapping, and redistribution, are responsible for the ultimate sedimentary architecture of deltas and, because of their dynamic nature, also pose great management and engineering challenges. To investigate the interaction between fluvial and littoral processes at wave-dominated river mouths, we modeled their morphologic evolution using the coupled hydrodynamic and morphodynamic model Delft3D-SWAN. Model experiments replicate alongshore migration of river mouths, river mouth spit development, and eventual spit breaching, suggesting that these are emergent phenomena that can develop even under constant fluvial and wave conditions. Furthermore, we find that sediment bypassing of a river mouth develops though feedbacks between waves and river mouth morphology, resulting in either continuous bypassing pathways or episodic bar bypassing pathways. Model results demonstrate that waves refracting into the river mouth bar create a zone of low alongshore sediment transport updrift of the river mouth, which reduces sediment bypassing. Sediment bypassing, in turn, controls the river mouth migration rate and the size of the river mouth spit. As a result, an intermediate amount of river discharge maximizes river mouth migration. The fraction of alongshore sediment bypassing can be predicted from the balance between the jet and the wave momentum flux. Quantitative comparisons show a match between our modeled predictions of river mouth bypassing and migration rates observed in natural settings.
    Beschreibung: NSF Grant Number: EAR-0952146
    Beschreibung: 2016-10-21
    Schlagwort(e): River mouth ; Bypassing ; Alongshore sediment transport ; Delft3D ; River mouth migration ; River mouth spit
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  • 3
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    U‐PB detrital zircon geochronology of the Lower Danube and Its tributaries : implications for the geology of the Carpathians (2018)
    John Wiley & Sons
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    Publikationsdatum: 2022-05-25
    Beschreibung: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 19 (2018): 3208-3223, doi:10.1029/2018GC007659.
    Beschreibung: We performed a detrital zircon (DZ) U‐Pb geochronologic survey of the lower parts of the Danube River approaching its Danube delta, Black Sea sink, and a few large tributaries (Tisza, Jiu, Olt, and Siret) originating in the nearby Carpathian Mountains. Samples are modern sediments. DZ age spectra reflect the geology and specifically the crustal age formation of the source area, which in this case is primarily the Romanian Carpathians and their foreland with contributions from the Balkan Mountains to the south of Danube and the East European Craton. The zircon cargo of these rivers suggests a source area that formed during the latest Proterozoic and mostly into the Cambrian and Ordovician as island arcs and back‐arc basins in a Peri‐Gondwanan subduction setting (~600–440 Ma). The Inner Carpathian units are dominated by a U‐Pb DZ peak in the Ordovician (460–470 Ma) and little inheritance from the nearby continental masses, whereas the Outer Carpathian units and the foreland have two main peaks, one Ediacaran (570–610 Ma) and one in the earliest Permian (290–300 Ma), corresponding to granitic rocks known regionally. A prominent igneous Variscan peak (320–350 Ma) in the Danube's and tributaries DZ zircon record is difficult to explain and points out to either an extra Carpathian source or major unknown gaps in our understanding of Carpathian geology. Younger peaks corresponding to arc magmatism during the Alpine period make up as much as about 10% of the DZ archive, consistent with the magnitude and surface exposure of Mesozoic and Cenozoic arcs.
    Beschreibung: Unitatea Executiva pentru Finantarea Invatamantului Superior, a Cercetarii, Dezvoltarii si Inovarii (UEFISCDI); Romanian Executive Agency for Higher Education, Research, Development and Innovation Funding Grant Number: PN‐III‐P4‐ID‐PCE‐2016‐0127; U.S. National Science Foundation. Grant Number: EAR 1725002
    Beschreibung: 2019-03-14
    Schlagwort(e): Danube ; Carpathians ; Detrital zircon ; U‐Pb geochronology ; Continental crust
    Repository-Name: Woods Hole Open Access Server
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  • 4
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    Climatically driven changes in the supply of terrigenous sediment to the East China Sea (2018)
    John Wiley & Sons
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    Publikationsdatum: 2022-05-25
    Beschreibung: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 19 (2018): 2463-2477, doi:10.1029/2017GC007339.
    Beschreibung: We examine the paleoceanographic record over the last ∼400 kyr derived from major, trace, and rare earth elements in bulk sediment from two sites in the East China Sea drilled during Integrated Ocean Drilling Program Expedition 346. We use multivariate statistical partitioning techniques (Q‐mode factor analysis, multiple linear regression) to identify and quantify five crustal source components (Upper Continental Crust (UCC), Luochuan Loess, Xiashu Loess, Southern Japanese Islands, Kyushu Volcanics), and model their mass accumulation rates (MARs). UCC (35–79% of terrigenous contribution) and Luochuan Loess (16–55% contribution) are the most abundant end‐members through time, while Xiashu Loess, Southern Japanese Islands, and Kyushu Volcanics (1–22% contribution) are the lowest in abundance when present. Cycles in UCC and Luochuan Loess MARs may indicate continental and loess‐like material transported by major rivers into the Okinawa Trough. Increases in sea level and grain size proxy (e.g., SiO2/Al2O3) are coincident with increased flux of Southern Japanese Islands, indicating localized sediment supply from Japan. Increases in total terrigenous MAR precede minimum relative sea levels by several thousand years and may indicate remobilization of continental shelf material. Changes in the relative contribution of these end‐members are decoupled from total MAR, indicating compositional changes in the sediment are distinct from accumulation rate changes but may be linked to variations in sea level, riverine and eolian fluxes, and shelf‐bypass processes over glacial‐interglacials, complicating accurate monsoon reconstructions from fluvial dominated sediment.
    Beschreibung: U.S. National Science Foundation Grant Numbers: NSF‐EAR1434175, NSF‐EAR1433665, NSF‐EAR1434138
    Schlagwort(e): East China Sea ; Bulk sediment ; Provenance ; Multivariate statistics ; East Asian Monsoon ; Loess
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  • 5
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    Decrease in coccolithophore calcification and CO2 since the middle Miocene (2016)
    Nature Publishing Group
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    Publikationsdatum: 2022-05-25
    Beschreibung: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 7 (2016): 10284, doi:10.1038/ncomms10284.
    Beschreibung: Marine algae are instrumental in carbon cycling and atmospheric carbon dioxide (CO2) regulation. One group, coccolithophores, uses carbon to photosynthesize and to calcify, covering their cells with chalk platelets (coccoliths). How ocean acidification influences coccolithophore calcification is strongly debated, and the effects of carbonate chemistry changes in the geological past are poorly understood. This paper relates degree of coccolith calcification to cellular calcification, and presents the first records of size-normalized coccolith thickness spanning the last 14 Myr from tropical oceans. Degree of calcification was highest in the low-pH, high-CO2 Miocene ocean, but decreased significantly between 6 and 4 Myr ago. Based on this and concurrent trends in a new alkenone εp record, we propose that decreasing CO2 partly drove the observed trend via reduced cellular bicarbonate allocation to calcification. This trend reversed in the late Pleistocene despite low CO2, suggesting an additional regulator of calcification such as alkalinity.
    Beschreibung: Funding for this research was provided by the European Research Council under grant UE-09-ERC-2009-STG-240222-PACE (HMS), the Principado de Asturias under award FC-13-COF13-044 (HMS) and a French ANR infrastructure project EMBRC-France (IP).
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  • 6
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    Tracing the Vedic Saraswati River in the Great Rann of Kachchh (2017)
    Nature Publishing Group
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    Publikationsdatum: 2022-05-25
    Beschreibung: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 7 (2017): 5476, doi:10.1038/s41598-017-05745-8.
    Beschreibung: The lost Saraswati River mentioned in the ancient Indian tradition is postulated to have flown independently of the Indus River into the Arabian Sea, perhaps along courses of now defunct rivers such as Ghaggar, Hakra and Nara. The persistence of such a river during the Harappan Bronze Age and the Iron Age Vedic period is strongly debated. We drilled in the Great Rann of Kachchh (Kutch), an infilled gulf of the Arabian Sea, which must have received input from the Saraswati, if active. Nd and Sr isotopic measurements suggest that a distinct source may have been present before 10 ka. Later in Holocene, under a drying climate, sediments from the Thar Desert probably choked the signature of an independent Saraswati-like river. Alternatively, without excluding a Saraswati-like secondary source, the Indus and the Thar were the dominant sources throughout the post-glacial history of the GRK. Indus-derived sediment accelerated the infilling of GRK after ~6 ka when the Indus delta started to grow. Until its complete infilling few centuries ago, freshwater input from the Indus, and perhaps from the Ghaggar-Hakra-Nara, probably sustained a productive marine environment as well as navigability toward old coastal Harappan and historic towns in the region.
    Beschreibung: The drilling effort and subsequent study of the cores was funded by Department of Science and Technology (DST), Government of India sponsored research project to DMM (Project No. SR/S4/ES-21/Kachchh Window/P1) under the science of Shallow Subsurface Programme (SSS). N. Khonde gratefully acknowledges Indo-US Post-doctoral Fellowship sponsored by SERB-IUSSTF for research work at Woods Hole Oceanographic Institution.
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  • 7
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    Southern Hemisphere forcing of South Asian monsoon precipitation over the past ~1 million years (2018)
    Nature Publishing Group
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    Publikationsdatum: 2022-05-25
    Beschreibung: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 9 (2018): 4702, doi:10.1038/s41467-018-07076-2.
    Beschreibung: 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.
    Beschreibung: This study was partly funded by the German Science Foundation, DFG, IODP Priority Program (grant HA 5751/3-1).
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  • 8
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    South Asian monsoon history over the past 60 kyr recorded by radiogenic isotopes and clay mineral assemblages in the Andaman Sea (2015)
    John Wiley & Sons
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    Publikationsdatum: 2022-05-26
    Beschreibung: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 505–521, doi:10.1002/2014GC005586.
    Beschreibung: The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial-interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay-size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval εNd values changed little, generally oscillating between −7.3 and −5.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo-Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite + chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite-poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short-term northern Atlantic climate variability on millennial time scales.
    Beschreibung: Part of this work was funded by German Science Foundation (DFG), grant HA5751/3-1.
    Beschreibung: 2015-08-24
    Schlagwort(e): Marine sediments ; Clay minerals ; Radiogenic isotopes ; Andaman Sea ; South Asian monsoon
    Repository-Name: Woods Hole Open Access Server
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