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  • Articles  (49)
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  • 1
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    Antarctic icebergs reorganize ocean circulation during Pleistocene glacials (2021)
    Nature Publishing Group
    In:  EPIC3Nature, Nature Publishing Group, 589(7841), pp. 236-241, ISSN: 0028-0836
    Details
    Publication Date: 2021-02-03
    Description: The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50°E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 2
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    A Plio-Pleistocene (c. 0–4 Ma) cyclostratigraphy for IODP Site U1478 (Mozambique Channel, SW Indian Ocean): Exploring an offshore record of paleoclimate and ecosystem variability in SE Africa (2021)
    In:  EPIC3Newsletters on Stratigraphy, 54(2), pp. 159-181, ISSN: 0078-0421
    Details
    Publication Date: 2021-05-07
    Description: The paleoclimate and ecosystem variability in Africa during the Plio/Pleistocene has received considerable attention due to its potential links to hominid evolution. However, the reconstruction of this variability hinges critically upon highly temporally resolved proxy data from continuous, well-dated sediment archives. In light of these requirements we use a new XRF core-scanning record from International Ocean Discovery Program (IODP) Site U1478 off the Limpopo River mouth (Mozambique Channel, SW Indian Ocean) spanning the past c. 4 Ma to identify the climate variability in SE Africa. Our results show that the elemental distribution in the Site U1478 cores is mainly controlled by the rate of terrigenous input and – to a lesser extent – by bottom-current transport and post-depositional processes such as propagation of paleoredox boundaries and diagenesis across some intervals. The log(Ti/Ca) ratio, which is used as a tracer of terrigenous sediment input, shows quasi-cyclical variability across the entire record that closely matches the periods of orbital parameters. However, the cyclical behaviour of the log(Ti/Ca) signal varies through time, with the uppermost 106 m of the sequence (0–1.07 Ma) displaying a mix of precession and obliquity signals, the intervals 106–223 m (1.07–2.80 Ma) and 240–257 m (3.68–4.05 Ma) being dominated by precession, and the interval 223–240 m (2.80–3.68 Ma) being controlled by eccentricity. To refine the available chronology for Site U1478, which is based on shipboard biostratigraphic and paleomagnetic data, we have tuned the log(Ti/Ca) record to the LR04 benthic oxygen isotope record, summer insolation at 25° S, and orbital eccentricity depending on the dominant cyclicities in the XRF dataset across individual time intervals. The resulting chronology enables us to evaluate the XRF data as well as the previously available shipboard sedimentological and geochemical datasets within a regional and global climatic context. This allows the connection of a c. 7-m-thick contourite deposit and a prominent paleoredox boundary to hydroclimate and ocean-circulation changes during the early Pleistocene and across the Mid-Pleistocene Transition, respectively. Moreover, a decoupling of the log(Ti/Ca) and the log(Ti/K) records, with the latter indicating the degree of sediment weathering, from 3.2 to 2.8 Ma points to an increased delivery of highly weathered sediments to Site U1478. We attribute this to temporarily wetter and warmer conditions in the catchment of the Limpopo River and/or a change in the sediment source, perhaps associated with the tectonically driven enlargement of the Zambezi River catchment during the late Pliocene/early Pleistocene.
    Repository Name: EPIC Alfred Wegener Institut
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  • 3
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    Surface and deep-water variability on the southern Agulhas Plateau: Interhemispheric links over the past 2 Ma (2019)
    In:  EPIC3AGU Fall Meeting 2019, San Francisco, U.S.A., 2019-12-09-2019-12-13
    Details
    Publication Date: 2019-12-22
    Description: The Southern Ocean is involved in setting the state of global climate through its role in redistributing heat and salt through the world ocean and its control on atmospheric CO2. Utilising sediment core sites on the southern Agulhas Plateau (AP) in the southwest Indian Ocean, we present new records of ice-rafted debris mass accumulation rate (IRDMAR), intermediate and benthic oxygen and carbon isotope, sortable silt mean grain size and bulk sediment chemistry (XRF) spanning the past 2 Ma. The AP is situated at the southern extent of the Indian-Atlantic Ocean Gateway (I-AOG); the upper water column is dominated by Indian Ocean waters not leaked into the South Atlantic and instead flowing eastward as the Agulhas Return Current. South of the AP, the relatively cold and fresh waters of the Sub-Antarctic Zone (SAZ) meet their northern limit and steep meridional property gradients occur. The AP region is therefore highly sensitive to variations in both the Sub-Antarctic Zone (SAZ) to the south and the Agulhas Current System to the north. IODP Site U1475 (41°25.61’S; 25°15.64’E, 2669 m water depth), was recovered from a contourite drift deposit on the southern AP, situated close to the modern-day subtropical front. Together with complementary data from sediment core MD02-2588 from the same location, our results indicate that during glacial periods there was a persistent influence of a well-ventilated water mass within the I-AOG with a carbon isotope signature similar to present-day Northern Component Water (NCW). The records of chemical ventilation and near-bottom flow vigour closely reflect changes in the advection of NCW and meridional variability in the location of the Antarctic Circumpolar Current and its associated fronts, as recorded by IRDMAR. We suggest that equatorward expansions of the circum-Antarctic frontal system, occurring relatively early in the glacial sequence, are central in triggering this glacial overturning circulation, hence modulating global climate. On orbital timescales, the SAZ represents a window through which external forcing may be translated into the global climate system; likely relevant for the enigmatic Mid-Pleistocene Transition.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 4
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    Latitudinal Migrations of the Subtropical Front at the Agulhas Plateau Through the Mid‐Pleistocene Transition (2021)
    In:  EPIC3Paleoceanography and Paleoclimatology, 36(7), ISSN: 2572-4517
    Details
    Publication Date: 2021-09-20
    Description: The meridional variability of the Subtropical Front (STF) in the Southern Hemisphere, linked to expansions or contractions of the Southern Ocean, may have played an important role in global ocean circulation by moderating the magnitude of water exchange at the Indian-Atlantic Ocean Gateway, so called Agulhas Leakage. Here we present new biomarker records of upper water column temperature ( and ) and primary productivity (chlorins and alkenones) from marine sediments at IODP Site U1475 on the Agulhas Plateau, near the STF and within the Agulhas retroflection pathway. We use these multiproxy time-series records from 1.4 to 0.3 Ma to examine implied changes in the upper oceanographic conditions at the mid-Pleistocene transition (MPT, ca. 1.2?0.8 Ma). Our reconstructions, combined with prior evidence of migrations of the STF over the last 350 ka, suggest that in the Southwestern Indian Ocean the STF may have been further south from the Agulhas Plateau during the mid-Pleistocene Interim State (MPIS, MIS 23?12) and reached its northernmost position during MIS 34?24 and MIS 10. Comparison to a Globorotalia menardii-derived Agulhas Leakage reconstruction from the Cape Basin suggests that only the most extreme northward migrations of the STF are associated with reduced Agulhas Leakage. During the MPIS, STF migrations do not appear to control Agulhas Leakage variability, we suggest previously modeled shifting westerly winds may be responsible for the patterns observed. A detachment between STF migrations and Agulhas Leakage, in addition to invoking shifting westerly winds may also help explain changes in CO2 ventilation seen during the MPIS.
    Repository Name: EPIC Alfred Wegener Institut
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  • 5
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    Deep North Atlantic last glacial maximum salinity reconstruction (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-10-19
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 36(7), (2021): e2020PA004088, https://doi.org/10.1029/2020PA004088.
    Description: We reconstruct deep water-mass salinities and spatial distributions in the western North Atlantic during the Last Glacial Maximum (LGM, 19–26 ka), a period when atmospheric CO2 was significantly lower than it is today. A reversal in the LGM Atlantic meridional bottom water salinity gradient has been hypothesized for several LGM water-mass reconstructions. Such a reversal has the potential to influence climate, ocean circulation, and atmospheric CO2 by increasing the thermal energy and carbon storage capacity of the deep ocean. To test this hypothesis, we reconstructed LGM bottom water salinity based on sedimentary porewater chloride profiles in a north-south transect of piston cores collected from the deep western North Atlantic. LGM bottom water salinity in the deep western North Atlantic determined by the density-based method is 3.41–3.99 ± 0.15% higher than modern values at these sites. This increase is consistent with: (a) the 3.6% global average salinity change expected from eustatic sea level rise, (b) a northward expansion of southern sourced deep water, (c) shoaling of northern sourced deep water, and (d) a reversal of the Atlantic's north-south deep water salinity gradient during the LGM.
    Description: This work was supported by the US National Science Foundation (grant numbers 1433150 and 1537485).
    Description: 2021-10-24
    Keywords: Carbon cycle ; Climate change ; Deep water ; Glaciation ; Meridional overturning circulation ; Paleosalinity ; Porewater
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Slope currents and contourites in an eastern boundary current regime: California Continental Borderland (2007)
    In:  Geological Society Special Publication 276: 155-169.
    Details
    Publication Date: 2007-10-08
    Description: Analysis of piston cores from lower slopes of central and outer basins of the California Continental Borderland shows the presence of structures ranging from starved silt ripples and lenses to erosion surfaces and truncated burrows to cross-bedded units that testify to reworking of the sediments by bottom currents. Adjacent basin-floor piston cores do not reveal these structures, but exhibit the usual bioturbated hemipelagic mud and turbidites. Slope sediments generally contain more silt than the adjacent basin-floor clay silts. The slope grain-size distributions are multimodal as a result of mixing of hemipelagic mud and thin (millimetre scale) silt layers as a consequence of the sampling intervals at centimetre scale. Carbonate and organic carbon contents tend to be low during the glacial periods but variations from this pattern occur that are probably related to the shift in the upwelling associated with the California Counter Current between inter-glacial and glacial conditions. The evidence for reworking is most abundant from late in Marine Isotope Stage 5 (MIS 5) to early Stage 4 (MIS 4), and it decreases into the Holocene. No structures are observed in the Holocene sections of the slope cores. The temporal distribution of the reworking structures can be explained either by changes in the degree of bioturbation (bottom-water oxygenation), which would work to erase the structures, or variation in the intensity of lower slope circulation through time. The observed pattern is in rough agreement with documented changes in bottom-water oxygenation conditions for these basins. However, the occurrence of reworking is also dominantly within glacial intervals, where lower sea levels produce stronger circulation within the basins as a result of exposure of banks and resulting restriction of cross-sections of the deeper flow pathways. These observations add to the increasing evidence that sediment transport by bottom currents is not restricted to the intensively studied western boundary current drift deposits. The action of bottom currents on eastern boundary slopes can introduce subtle effects on what are commonly assumed to be continuous records.
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  • 7
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    Fresh Water Inflow and Oyster Productivity in Apalachicola Bay, FL (USA) (2011)
    Springer
    Details
    Publication Date: 2011-02-25
    Print ISSN: 1559-2723
    Electronic ISSN: 1559-2731
    Topics: Geography
    Published by Springer
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  • 8
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    Diagenetic aluminum uptake into diatom frustules and the preservation of diatom-bound organic nitrogen (2013)
    Elsevier
    Details
    Publication Date: 2013-09-01
    Print ISSN: 0304-4203
    Electronic ISSN: 1872-7581
    Topics: Geosciences
    Published by Elsevier
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  • 9
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    Structure and Recognition of Graphs with No 6-wheel Subdivision (2008)
    Springer
    Details
    Publication Date: 2008-01-15
    Print ISSN: 0178-4617
    Electronic ISSN: 1432-0541
    Topics: Computer Science , Mathematics
    Published by Springer
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  • 10
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    Rutile (U‐Th)/He Thermochronology: Temperature sensitivity and radiation damage effects (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Rutile (TiO2) is a common U‐Th‐bearing accessory mineral in metamorphic and, to a lesser extent, igneous rocks. Its development as a (U‐Th)/He thermochronometer would diversify the lithologies dateable with the (U‐Th)/He technique. We report (U‐Th)/He dates for rutile with crystallization ages that vary from Cretaceous to Proterozoic from 8 samples with independent constraints on their thermal history to empirically calibrate the temperature sensitivity of the rutile (U‐Th)/He (RHe) system. The results document an analytical challenge for the Proterozoic rutile, where degassing at laser powers typical for zircon volatilized the parent isotopes while lower laser powers failed to consistently and completely extract all 4He from the crystals. In contrast, the 6 Phanerozoic samples were easily degassed without U‐Th loss. RHe dates for 5 of these samples are fairly reproducible (6‐22% dispersion) or yield positive RHe date‐eU correlations. RHe dates are older than or overlap with apatite (U‐Th)/He dates available for four of these samples and are younger than zircon (U‐Th)/He dates available for two of these samples. This implies that the He closure temperature (Tc) for these Phanerozoic rutile is between apatite (~70 °C) and lower‐damage zircon (~200°C). 4He diffusion experiment data are consistent with anisotropic He diffusion, suggest differing retentivities for the analyzed rutile, and yield Tc estimates of ~155‐159°C for diffusion parallel to the c‐axis (at a 10°C/Ma cooling rate). These results, the degassing patterns for Phanerozoic versus Proterozoic rutile, and the positive RHe date‐eU correlations in two samples imply that radiation damage increases rutile He retentivity.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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