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  • 2015-2019  (177)
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  • 1
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    Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature estimations for G. bulloides and N. pachyderma left (2016)
    Wiley
    Details
    Publication Date: 2016-03-05
    Description: Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking. Here we present species-specific calibrations of Mg/Ca vs. calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca-temperature calibration for mid and high latitudes of both hemispheres between 2 - 18°C, where Mg/Ca = 1.006 ± 0.032 * e 0.065 ± 0.003*Tiso (R 2 = 0.82). G. bulloides is found to calcify deeper in the Southern Ocean (∼ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between ∼ -1 and 9°C, of the form Mg/Ca = 0.580 ± 0.016 * e 0.084 ± 0.006*Tiso (R 2 = 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1σ propagated error in Mg/Ca-derived temperatures is 1.1°C for G. bulloides and 0.9°C for N. pachyderma left for the presented datasets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Brine formation recorded by stable isotopes of Recent benthic foraminifera in Storfjorden, Svalbard: palaeoceanographical implications (2016)
    Wiley
    In: Boreas
    Details
    Publication Date: 2016-03-10
    Description: We discuss water oxygen isotopes (δ 18 O w ) and carbon isotopes of dissolved inorganic carbon (δ 13 C DIC ) of brine-enriched shelf water (BSW) from Storfjorden (southern Svalbard) in comparison to Recent benthic foraminiferal δ 18 O c and δ 13 C c calcified in the same water. We determined relatively high δ 18 O w values of 0.15±0.03‰ VSMOW in BSW below sill depth at temperatures below −1.8 °C, and high δ 18 O c values of 3.90±0.18‰ VPDB. Such high BSW δ 18 O w cannot significantly deplete 18 O w contents of Arctic Ocean deep water; furthermore, such high δ 18 O c cannot be distinguished from δ 18 O c values of 3.82±0.12‰, calcified in warmer Arctic and Nordic seas intermediate and deeper waters. Today, in Storfjorden low benthic δ 13 C c and high δ 18 O c reflect the low δ 13 C DIC and relatively high δ 18 O w of BSW. High benthic δ 18 O c is in contrast to expected low δ 18 O c as brine rejection is widely thought to predominantly take place in surface water diluted by meteoric water with very low δ 18 O w . Low epibenthic δ 13 C c values of 0.50±0.12‰ partly reflect low δ 13 C DIC caused by enhanced uptake of atmospheric low δ 13 C CO 2 decreased by anthropogenic activities. An adjustment for preindustrial higher values would increase δ 13 C c by about 0.6‰. Therefore, in Storfjorden brine formed before the industrial era would be characterized by both high δ 13 C c as well as high δ 18 O c values of benthic foraminiferal calcite. Our data may cast doubt on scenarios that explain negative excursions in benthic foraminiferal stable isotope records from the Atlantic Ocean during cold stadials in the last glacial period by enhanced brine formation in Nordic seas analogously to modern processes in Storfjorden.
    Print ISSN: 0300-9483
    Electronic ISSN: 1502-3885
    Topics: Geography , Geosciences
    Published by Wiley
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  • 3
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    The seasonal water temperature cycle in the Arctic Dicksonfjord (Svalbard) during the Holocene Climate Optimum derived from subfossil Arctica islandica shells (2015)
    Sage
    In: Holocene
    Details
    Publication Date: 2015-07-03
    Description: Future climate change will have significant effects on ecosystems worldwide and on polar regions in particular. Hence, palaeo-environmental studies focussing on the last warmer-than-today phase (i.e. the early Holocene) in higher latitudes are of particular importance to understand climate development and its potential impact in polar systems. Molluscan bivalve shells constitute suitable bio-archives for high-resolution palaeo-environmental reconstructions. Here, we present a first reconstruction of early Holocene seasonal water temperature cycle in an Arctic fjord based on stable oxygen isotope ( 18 O shell ) profiles in shells of Arctica islandica (Bivalvia) from raised beach deposits in Dicksonfjorden, Svalbard, dated at 9954–9782 cal. yr BP. Reconstructed maximum and minimum bottom water temperatures for the assumed shell growth period between April and August of 15.2°C and 2.8°C imply a seasonality of about 12.4°C for the early Holocene. In comparison to modern temperatures, this indicates that average temperature declined by 6°C and seasonality narrowed by 50%. This first palaeo-environmental description of a fjord setting during the Holocene Climate Optimum at Spitsbergen exceeds most previous global estimates (+1–3°C) but confirms studies indicating an amplified effect (+4–6°C) at high northern latitudes.
    Print ISSN: 0959-6836
    Electronic ISSN: 1477-0911
    Topics: Geography , Geosciences
    Published by Sage
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  • 4
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    Deciphering the Variability in Mg/Ca and Stable Oxygen Isotopes of Individual Foraminifera (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Foraminifera are commonly used in paleoclimate reconstructions as they occur throughout the world's oceans and are often abundantly preserved in the sediments. Traditionally, foraminifera‐based proxies like δ18O and Mg/Ca are analyzed on pooled specimens of a single species. Analysis of single specimens of foraminifera allows reconstructing climate variability on timescales related to El Niño–Southern Oscillation or seasonality. However, quantitative calibrations between the statistics of individual foraminifera analyses (IFA) and climate variability are still missing. We performed Mg/Ca and δ18O measurements on single specimens from core top sediments from different settings to better understand the signal recorded by individual foraminifera. We used three species of planktic foraminifera (Globigerinoides ruber (s.s.), T. sacculifer, and N. dutertrei) from the Indo‐Pacific Warm Pool and one species (G. ruber (pink)) from the Gulf of Mexico. Mean values for the different species of Mg/Ca versus calculated δ18O temperatures agree with published calibration equations. IFA statistics (both mean and standard deviation) of Mg/Ca and δ18O between the different sites show a strong relationship indicating that both proxies are influenced by a common factor, most likely temperature variations during calcification. This strongly supports the use of IFA to reconstruct climate variability. However, our combined IFA data for the different species only show a weak relationship to seasonal and interannual temperature changes, especially when seasonal variability increases at a location. This suggests that the season and depth habitat of the foraminifera strongly affect IFA variability, such that ecology needs to be considered when reconstructing past climate variability.
    Print ISSN: 0883-8305
    Electronic ISSN: 2572-4525
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Variability in mid-depth ventilation of the western Atlantic Ocean during the last deglaciation (2017)
    Wiley
    Details
    Publication Date: 2017-08-29
    Description: Negative stable carbon isotopic excursions have been observed throughout most of the mid-depth (~1000–3000m) Atlantic Ocean during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Although there is agreement that these mid-depth excursions were in some way associated with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), there is still no consensus on the precise mechanism(s). Here, we present benthic stable carbon and oxygen isotopic (δ 13 C and δ 18 O) records from five cores from the western equatorial Atlantic (WEA). Together with published benthic isotopic records from nearby cores, we produced a WEA depth transect (~800–2500m). We compare HS1 and YD data from this transect with data from previously published North- and South Atlantic cores and demonstrate that the largest negative δ 13 C excursions occurred in the WEA during these times. Moreover, our benthic δ 18 O records require the presence of two water masses flowing from the Southern Ocean, bisected by a Northern Component Water (NCW). Given that δ 18 O is a conservative water mass tracer, we suggest that δ 13 C was decoupled from water mass composition and does not correspond to simple alternations between northern and southern sourced waters. Instead, δ 13 C behaved non-conservatively during HS1 and the YD. Consistently with our new 231 Pa/ 230 Th record from the WEA transect, that allowed the reconstruction of AMOC strength, we hypothesize that the negative δ 13 C excursions reflect an increase in the residence time of NCW in response to a weakened AMOC, allowing for a marked accumulation of 13 C-depleted respired carbon at the mid-depth WEA.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Lapsyn controls branch extension and positioning of astrocyte-like glia in the Drosophila optic lobe (2017)
    Springer Nature
    Details
    Publication Date: 2017-08-22
    Description: Lapsyn controls branch extension and positioning of astrocyte-like glia in the Drosophila optic lobe Nature Communications, Published online: 22 August 2017; doi:10.1038/s41467-017-00384-z How glial cells, such as astrocytes, acquire their characteristic morphology during development is poorly understood. Here the authors describe the morphogenesis of astrocyte-like glia in the Drosophila optic lobe, and through a RNAi screen, they identify a transmembrane LRR protein–Lapsyn–that plays a critical role in this process.
    Electronic ISSN: 2041-1723
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 7
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    Alternating influence of northern versus southern-sourced water masses on the equatorial Pacific sub-thermocline during the past 240 ka (2017)
    Wiley
    Details
    Publication Date: 2017-10-29
    Description: The Eastern Equatorial Pacific (EEP) is a key area to understand past oceanic processes that control atmospheric CO 2 concentrations. Many studies argue for higher nutrient concentrations by enhanced nutrient transfer via Southern Ocean Intermediate Water (SOIW) to the low-latitude Pacific during glacials. Recent studies, however, argue against SOIW as the primary nutrient source, at least during early Marine Isotope Stage 2 (MIS 2), as proxy-data indicate that nutrients are better utilized in the Southern Ocean under glacial conditions. New results from the subarctic Pacific suggest that enhanced convection of nutrient-rich Glacial North Pacific Intermediate Water (GNPIW) contributes to changes in nutrient concentrations in equatorial sub-thermocline water masses during MIS 2. However, the interplay between SOIW versus GNPIW and its influence on the nutrient distribution in the EEP spanning more than one glacial cycle are still not understood. We present a carbon isotope (δ 13 C) record of sub-thermocline waters derived from deep-dwelling planktonic foraminifera Globorotaloides hexagonus in the EEP, which is compared with published δ 13 C records around the Pacific. Results indicate enhanced influence of GNPIW during MIS 6 and MIS 2 compared to today with largest contributions of northern-sourced intermediate waters during glacial maxima. These observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past EEP nutrient concentrations. A switch from increased GNPIW (decreased SOIW) to diminished GNPIW (enhanced SOIW) influence on equatorial sub-thermocline waters is recognized during glacial terminations and marks changes to modern-like conditions in nutrient concentrations and biological productivity in the EEP.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
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  • 8
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    Calibration of the Carbon Isotope Composition (δ13C) of Benthic Foraminifera (2017)
    Wiley
    Details
    Publication Date: 2017-05-13
    Description: The carbon isotope composition (δ 13 C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump and the global carbon cycle, and is reflected by the δ 13 C of foraminifera tests. Here, more than 1,700 δ 13 C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ 13 C Cibnat ) are compiled and compared with newly updated estimates of the natural (pre-industrial) water-column δ 13 C of dissolved inorganic carbon (δ 13 C DICnat ) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples we find high correlation between δ 13 C Cibnat and δ 13 C DICnat , confirming earlier work. Regression analyses indicate significant carbonate ion (-2.6 ± 0.4)×10 -3 ‰/(μmol kg -1 ) [CO 3 2- ] and pressure (-4.9 ± 1.7)×10 -3 ‰ m -1 (depth) effects, which we use to propose a new global calibration for predicting δ 13 C DICnat from δ 13 C Cibnat . This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ 13 C DICnat = δ 13 C Cibnat ). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25 ‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4 ‰) and for species other than Cibicides wuellerstorfi . Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ 13 C record.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
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  • 9
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    Carbon isotope offsets between benthic foraminifer species of the genus Cibicides (Cibicidoides) in the glacial sub-Antarctic Atlantic (2016)
    Wiley
    Details
    Publication Date: 2016-11-25
    Description: Epibenthic foraminifer δ 13 C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ 13 C (δ 13 C DIC ), which are used to infer global ocean circulation patterns. Epibenthic δ 13 C, however, may also reflect the influence of 13 C-depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here, we compare the δ 13 C of two benthic foraminifer species, Cibicides kullenbergi and Cibicides wuellerstorfi , and their morphotypes, in three sub-Antarctic Atlantic sediment cores over several glacial-interglacial transitions. These species are commonly assumed to be epibenthic, living above or directly below the sediment-water interface. While this might be consistent with the small δ 13 C offset that we observe between these species during late Pleistocene interglacial periods (Δδ 13 C = -0.19 ±0.31 ‰, N =63), it is more difficult to reconcile with the significant δ 13 C offset that is found between these species during glacial periods (Δδ 13 C =-0.76 ±0.44‰, N =44). We test possible scenarios by analysing Uvigerina spp. δ 13 C and benthic foraminifer abundances: 1) C. kullenbergi δ 13 C is biased to light values, either due to microhabitat shifts or phytodetritus effects; and 2) C. wuellerstorfi δ 13 C is biased to heavy values, relative to long-term average conditions, for instance by recording the sporadic occurrence of less depleted deep water δ 13 C DIC . Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer δ 13 C in the sub-Antarctic Atlantic may reflect several factors rather than being a sole function of bottom water δ 13 C DIC , which directly bear on the interpretation of extremely light South Atlantic δ 13 C values at the last glacial maximum.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
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  • 10
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    Evidence for enhanced convection of North Pacific Intermediate Water to the low-latitude Pacific under glacial conditions (2016)
    Wiley
    Details
    Publication Date: 2016-12-27
    Description: We provide high-resolution foraminiferal stable carbon isotope (δ 13 C) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extra-tropical and tropical North Pacific during the past 60 kyr. We measured the δ 13 C composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201-2-101KL; 58°52.52’N, 170°41.45’E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the δ 13 C of sub-thermocline water masses in the EEP is derived from the deep-dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31’N, 82°27.76’W; 2921 m water depth). Apparent similarities in the long-term evolution of δ 13 C between GNPIW, intermediate waters in the eastern tropical North Pacific and sub-thermocline water masses in the EEP suggest the expansion of relatively 13 C-depleted, nutrient-enriched, and northern-sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface-waters of the glacial EEP. Our findings highlight potential links between North Pacific mid-depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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