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  • 2020-2024  (32)
  • 2010-2014  (19)
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  • 1
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    The timing and properties of deep water exchange between the Nordic Seas and open North Atlantic during the last 20,000 years (2012)
    In:  [Invited talk] In: AGU Fall Meeting 2012, 06.12.2012, San Francisco, USA .
    Details
    Publication Date: 2020-11-04
    Description: PP43C-06 Deepwater formation in the North Atlantic is an essential component of the overturning circulation of the Atlantic Ocean, which helps regulate global climate. Paleoceanographic studies have revealed that deep water formation in the North Atlantic was altered during the last ice age as compared with today; although the circulation of the North Atlantic during the subsequent deglaciation remains poorly constrained. In order to understand the mechanisms by which deglaciation proceeded, it is crucial that we further investigate the timing and nature of North Atlantic circulation changes. Much of the uncertainty surrounding the circulation of the deglacial Northeast Atlantic centres on the cause of mid-depth low benthic δ18O recorded during Heinrich Stadial 1, and episodic incursions of an extremely 14C-depleted (i.e., poorly ventilated) water mass in the Northeast Atlantic. Existing proxy data are insufficient to unambiguously resolve the cause of these ‘anomalies’ and models typically fail to reconstruct such variability. To investigate the cause of these events, we have reconstructed deep water temperature, δ18O, δ13C and Δ14C at sites in the Northeast Atlantic and deep Norwegian Seas over the past 20ka. Using our multi-proxy datasets from north and south of the Greenland-Scotland ridge we robustly constrain the timing and properties of water mass exchange across the Iceland-Scotland Ridge. We deconvolve the temperature and seawater δ18O signals contained within the benthic δ18O records and examine the source and subsequent spreading of highly 14C-depleted water.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 2
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    Direct astronomical influence on abrupt climate variability (2021)
    Nature Research
    Details
    Publication Date: 2024-02-07
    Description: Changes in the magnitude of millennial-scale climate variability (MCV) during the Late Pleistocene occur as a function of changing background climate state over tens of thousands of years, an indirect consequence of slowly varying incoming solar radiation associated with changes in Earth’s orbit. However, whether astronomical forcing can stimulate MCV directly (without a change in the background state) remains to be determined. Here we use a comprehensive fully coupled climate model to demonstrate that orbitally driven insolation changes alone can give rise to spontaneous millennial-scale climate oscillations under intermediate glacial conditions. Our results demonstrate that an abrupt transition from warm interstadial to cold stadial conditions can be triggered directly by a precession-controlled increase in low-latitude boreal summer insolation and/or an obliquity-controlled decrease in high-latitude mean annual insolation, by modulating North Atlantic low-latitude hydroclimate and/or high-latitude sea ice–ocean–atmosphere interactions, respectively. Furthermore, contrasting insolation effects over the tropical versus subpolar North Atlantic, exerted by obliquity or precession, result in an oscillatory climate regime, even within an otherwise stable climate. With additional sensitivity experiments under different glacial–interglacial climate backgrounds, we synthesize a coherent theoretical framework for climate stability, elaborating the direct and indirect (dual) control by Earth’s orbital cycles on millennial-scale climate variability during the Pleistocene.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    A salty deep ocean as a prerequisite for glacial termination (2021)
    Nature Research
    Details
    Publication Date: 2024-02-07
    Description: Abstract Deglacial transitions of the middle to late Pleistocene (terminations) are linked to gradual changes in insolation accompanied by abrupt shifts in ocean circulation. However, the reason these deglacial abrupt events are so special compared with their sub-glacial-maximum analogues, in particular with respect to the exaggerated warming observed across Antarctica, remains unclear. Here we show that an increase in the relative importance of salt versus temperature stratification in the glacial deep South Atlantic decreases the potential cooling effect of waters that may be upwelled in response to abrupt perturbations in ocean circulation, as compared with sub-glacial-maximum conditions. Using a comprehensive coupled atmosphere–ocean general circulation model, we then demonstrate that an increase in deep-ocean salinity stratification stabilizes relatively warm waters in the glacial deep ocean, which amplifies the high southern latitude surface ocean temperature response to an abrupt weakening of the Atlantic meridional overturning circulation during deglaciation. The mechanism can produce a doubling in the net rate of warming across Antarctica on a multicentennial timescale when starting from full glacial conditions (as compared with interglacial or subglacial conditions) and therefore helps to explain the large magnitude and rapidity of glacial terminations during the late Quaternary.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Ice sheet decline and rising atmospheric CO2 control AMOC sensitivity to deglacial meltwater discharge (2022)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Highlights: • Climate model sensitivity experiments are performed using state-of-the-art ice sheet and freshwater reconstructions • Declining Northern Hemisphere ice sheets increase the sensitivity of the AMOC to North Atlantic meltwater discharge • Deglacial rise in atmospheric CO2 concentration decreases the sensitivity of the AMOC to North Atlantic meltwater discharge • Both effects provide a complementary perspective to existing explanations for abrupt AMOC transitions Abstract: The last deglaciation was characterized by a sequence of abrupt climate events thought to be linked to rapid changes in Atlantic meridional overturning circulation (AMOC). The sequence includes a weakening of the AMOC after the Last Glacial Maximum (LGM) during Heinrich Stadial 1 (HS1), which ends with an abrupt AMOC amplification at the transition to the Bølling/Allerød (B/A). This transition occurs despite persistent deglacial meltwater fluxes that counteract vigorous North Atlantic deep-water formation. Using the Earth system model COSMOS with a range of deglacial boundary conditions and reconstructed deglacial meltwater fluxes, we show that deglacial CO2 rise and ice sheet decline modulate the sensitivity of the AMOC to these fluxes. While declining ice sheets increase the sensitivity, increasing atmospheric CO2 levels tend to counteract this effect. Therefore, the occurrence of a weaker HS1 AMOC and an abrupt AMOC increase in the presence of meltwater, might be explained by these effects, as an alternative to or in combination with changes in the magnitude or routing of meltwater discharge.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    A Systematic Role for Extreme Ocean‐Atmosphere Oscillations in the Development of Glacial Conditions Since the Mid Pleistocene Transition (2023)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: We introduce a new hypothesis concerning the role of internal climate dynamics in the non-linear transitions from interglacial to glacial (IG-G) state since the Mid Pleistocene Transition (MPT). These transitions encompass large and abrupt changes in atmospheric CO2, ice volume, and temperature that we suggest involve critical interactions between insolation and high amplitude oscillations in ocean/atmosphere circulation patterns. Specifically, we highlight the large amplitude of millennial-scale climate oscillations across the transition from Marine Isotope Stage (MIS) 5 to 4, which we argue led to amplified cooling of the deep ocean and we demonstrate that analogous episodes of extreme cooling systematically preceded glacial periods of the last 800 kyr. We suggest that such cooling necessitates a reconfiguration of the deep ocean to avoid a density paradox between northern and southern-sourced deep waters (SSW), which could be accomplished by increasing the relative volume and or salinity of SSW, thus providing the necessary storage capacity for the subsequent (delayed) and relatively abrupt drawdown of CO2. We therefore explain the transient decoupling of Antarctic temperature from CO2 across MIS 5/4 as a direct consequence of millennial activity at that time. We further show that similar climatic decoupling typically occurred during times of low obliquity and was a ubiquitous feature of IG-G transitions over the past 800 kyr, producing the appearance of bimodality in records of CO2, benthic δ18O and others. Finally we argue that the apparent lack of bimodality in the pre-MPT record of benthic δ18O implies that the dynamics associated with IG-G transitions changed across the MPT
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Millennial scale feedbacks determine the shape and rapidity of glacial termination (2021)
    Nature Research
    Details
    Publication Date: 2024-02-07
    Description: Within the Late Pleistocene, terminations describe the major transitions marking the end of glacial cycles. While it is established that abrupt shifts in the ocean/atmosphere system are a ubiquitous component of deglaciation, significant uncertainties remain concerning their specific role and the likelihood that terminations may be interrupted by large-amplitude abrupt oscillations. In this perspective we address these uncertainties in the light of recent developments in the understanding of glacial terminations as the ultimate interaction between millennial and orbital timescale variability. Innovations in numerical climate simulation and new geologic records allow us to highlight new avenues of research and identify key remaining uncertainties such as sea-level variability.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions (2011)
    Royal Society of Chemistry (RSC)
    Details
    Publication Date: 2011-05-21
    Description: Synergistic ESR and ESI-MS analyses reveal modification of TEMPO derivatives at the 1- and 4-positions upon exposure to hydroxyl radicals. David L. Marshall, Meganne L. Christian, Ganna Gryn'ova, Michelle L. Coote, Philip J. Barker, Stephen J. Blanksby ( from Org. Biomol. Chem.) David L. Marshall, Org. Biomol. Chem., 2011, DOI: 10.1039/C1OB05037K To cite this article before page numbers are assigned, use the DOI form of citation above. The content of this RSS Feed (c) The Royal Society of Chemistry
    Print ISSN: 1477-0520
    Electronic ISSN: 1477-0539
    Topics: Chemistry and Pharmacology
    Published by Royal Society of Chemistry (RSC)
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    PAPER CURRENT
  • 8
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    Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions (2011)
    Royal Society of Chemistry (RSC)
    Details
    Publication Date: 2011-04-13
    Description: Employing 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as models for the active intermediate of hindered amine light stabilisers (HALS) has brought about new mechanistic insights through study of their reactions with... David Lachlan Marshall, Meganne Christian, Ganna Gryn'ova, Michelle L Coote, Philip J Barker, Stephen Blanksby ( from Org. Biomol. Chem.) David Lachlan Marshall, Org. Biomol. Chem., 2011, DOI: 10.1039/C1OB05037K To cite this article before page numbers are assigned, use the DOI form of citation above. The content of this RSS Feed (c) The Royal Society of Chemistry
    Print ISSN: 1477-0520
    Electronic ISSN: 1477-0539
    Topics: Chemistry and Pharmacology
    Published by Royal Society of Chemistry (RSC)
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    PAPER CURRENT
  • 9
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    Biological response to millennial variability of dust and nutrient supply in the Subantarctic South Atlantic Ocean (2014)
    Royal Society Publishing
    In:  EPIC3Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society Publishing, 372(2019), pp. 20130054-20130054, ISSN: 1364-503X
    Details
    Publication Date: 2018-08-10
    Description: Fluxes of lithogenic material and fluxes of three palaeo-productivity proxies (organic carbon, biogenic opal and alkenones) over the past 100 000 years were determined using the 230Th-normalization method in three sediment cores from the Subantarctic South Atlantic Ocean. Features in the lithogenic flux record of each core correspond to similar features in the record of dust deposition in the EPICA Dome C ice core. Biogenic fluxes correlate with lithogenic fluxes in each sediment core. Our preferred interpretation is that South American dust, most probably from Patagonia, constitutes a major source of lithogenic material in Subantarctic South Atlantic sediments, and that past biological productivity in this region responded to variability in the supply of dust, probably due to biologically available iron carried by the dust. Greater nutrient supply as well as greater nutrient utilization (stimulated by dust) contributed to Subantarctic productivity during cold periods, in contrast to the region south of the Antarctic Polar Front (APF), where reduced nutrient supply during cold periods was the principal factor limiting productivity. The anti-phased patterns of productivity on opposite sides of the APF point to shifts in the physical supply of nutrients and to dust as cofactors regulating productivity in the Southern Ocean.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , peerRev
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  • 10
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    Evidence of silicic acid leakage to the tropical Atlantic via Antarctic Intermediate Water during Marine Isotope Stage 4 (2013)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 28 (2013): 307–318, doi:10.1002/palo.20030.
    Description: Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silicon (silicic acid) from the deep Southern Ocean (SO) to the low-latitude surface ocean and therefore have an important control on low-latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate-producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pCO2. This mechanism is known as the “silicic acid leakage hypothesis” (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage 4 (~60–70 ka). This leakage was approximately coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO2 drawdown during full glacial development.
    Description: The work is part of a wider project on the MIS 5/4 transition, supervised by S. B. and supported by NERC (UK) grant NE/F002734/1. K.R.H. is funded by National Science Foundation grant MCG-1029986. T.v.d.F. acknowledges funding from the European Commission (IRG 230828).
    Description: 2013-12-27
    Keywords: Silica leakage ; Diatom ; Carbon dioxide ; SAMW ; AAIW
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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