Percorrer por autor "Kuhnert, Henning"
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- Early Pleistocene (1.94–1.46 Ma) records for the upper mediterranean outflow Water branch reveal low and high latitude climate influencesPublication . Voelker, Antje; Ducassou, Emanuelle; Balestra, Barbara; Flores, Jose Abel; Acton, Gary D.; Richter, Carl; Xuan, Chuang; Lofi, Johanna; Alberto, Ana; Kuhnert, Henning; Zarikian, Carlos A. AlvarezThe Mediterranean Outflow Water (MOW), modified by paleoceanographic conditions and tectonic processes, played a significant role in the formation of sediments drifts along the Iberian Margin. Using sediment samples from IODP Hole U1387C, we explore the Early Pleistocene history of the upper MOW core above the central Faro Drift in the Gulf of Cadiz. The time series of benthic foraminifer stable isotope and grain size related data have a rigorous stratigraphic framework consisting of nannofossil biostratigraphy and paleomagnetic and delta 18O stratigraphy. The paleoenvironmental records are supplemented by natural gamma ray downhole logging data. Above the hiatus associated with the youngest dolostone, sandy to muddy contourite sedimentation started at 1.946 Ma, i.e., within Marine Isotope Stage (MIS) 74, at IODP Site U1387, slightly younger than at IODP Site U1389. Formation of contourite layers, reflected in the sortable silt and sand percentage records, strongly reacted to precession forcing, including semi- and quarter-precession cycles. The majority of the contourite beds developed during stadial (colder) climate periods, like previous observations from the Early to Late Pleistocene. Formation of contourite layers within MIS 53, MIS 55 and MIS 65, however, appear to be linked to the prevailing atmospheric conditions over North Africa. Periods of poor ventilation in the upper MOW were linked to insolation maxima and reduced ventilation in the Mediterranean Sea. Here, MIS 51 presents a peculiar case as poor ventilation reached from the surface to the lower North Atlantic Deep Water range, reflecting unique interglacial conditions that merit future exploration.
- The early–middle pleistocene transition in the gulf of Cadiz (NE Atlantic) – an interplay between subtropical gyre and extremely cold surface watersPublication . Mega, Aline; Rodrigues, Teresa; Salgueiro, Emilia; Padilha, Mária; Kuhnert, Henning; Voelker, AntjeBesides the shift in dominant orbital cyclicity depicted in paleoclimate proxy records, the Mid-Pleistocene Transition or Early-Middle Pleistocene Transition (EMPT) was linked to a change in the deep thermohaline circulation. Those changes contributed to more intense and longer-lasting glacial periods and cooler sea surface temperatures (SSTs) during glacials. Within the Atlantic Ocean, the Iberian Margin is considered a key location to study climatic variations influenced by both high- and low-latitude processes. In this study we focus on IODP Site U1387 on the southern Portuguese margin to reconstruct surface water circulation changes and related plankton foraminifera ecosystems during the interval of Marine Isotope Stage (MIS) 28 to MIS 18 (1006-750 ka). Our planktonic foraminifera assemblages and SST reconstructions (foraminifera assemblages and U-37(K ') alkenone index) demonstrate warm, relative stable SST conditions during much of the interval due to persistent influence of subtropical gyre waters as indicated by the tropical-subtropical and Azores Current-related foraminifera species and the periods with dominant sinistral coiling direction of the species Globorotalia truncatulinoides. Maximum interglacial SSTs were up to 2 degrees C warmer than at present in both summer and winter, with the exception of interglacial MIS 23 with SSTs similar to 1.5 degrees C colder than in the other interglacials. Subsequent to the respective glacial inception, the relatively warm conditions were periodically interrupted by millennial-scale extreme cold events when polar species Neogloboquadrina pachyderma became abundant (> 30 %), and the SSTs, reconstructed from the foraminifera assemblage data, dropped below 10 degrees C in summer and 5 degrees C in winter, although some of those values might be overestimated. The most pronounced event, considering the amplitude of cooling and duration, occurred between 870 and 864 ka, marking the terminal stadial event of the MIS 22-MIS 21 transition (Termination X). Extreme cold events, always associated with the incursion of subpolar waters into the Gulf of Cadiz, mark all the terminal stadial events from Terminations XII to IX and the millennial-scale variability during the transitions to full glacial conditions, although the duration of the cooling varied greatly. The extreme cooling was only possible through migration of the subarctic front into the lower mid-latitudes as a consequence of cooling and freshening in the higher latitudes and the associated extreme reduction in the Atlantic Meridional Overturning Circulation. The amplitude of cooling, duration, and frequency of subpolar water incursions during MIS 24 to MIS 22 stands out, providing further evidence for the "900 ka event" being a key feature of the EMPT.
- Environmental changes at the seafloor of the Faro drift (Gulf of Cadiz) during the transition from the early to the middle pleistocenePublication . Silveira Molina, Giulia; Schmiedl, Gerhard; Jiménez-Espejo, Francisco; Kuhnert, Henning; Anica Claro Rodrigues, Teresa Isabel; Voelker, AntjeThis study explores the ecology of the benthic foraminifera fauna and reconstructs bottom water oxygenation, organic matter fluxes, and Mediterranean Outflow Water (MOW) dynamics in the Gulf of Cadiz during the Early to Middle Pleistocene Transition (EMPT) interval of Marine Isotope Stage (MIS) 28 to MIS 19 (1014–761 ka) using high-resolution multiproxy data from IODP Site U1387. Along with benthic foraminifera assemblages, we integrate stable isotopes (δ 18O and δ 13C), organic carbon, alkenone concentrations, and geochemical and sedimentological proxies (Zr/Al ratio, grain size) to identify environmental drivers across glacial– interglacial cycles and millennial-scale events. Furthermore, the absolute abundance of Planulina ariminensis is applied as a proxy for bottom current strength. Principal component analysis confirms assemblage responses to variations in organic matter quality and oxygenation. Periods of intensified MOW during stadial climate stages correspond to enhanced bottom water ventilation, reflected in higher abundances of epifaunal and porcelaneous taxa, higher diversity, and increased dissolved oxygen, with the exception of the late MIS 22. Intervals of reduced ventilation (e.g. interglacial MIS 27, MIS 25e, MIS 21g, MIS 19c) coincide with higher total alkenone concentrations, potentially contributing to low oxygen conditions and increased proportions of infaunal taxa. Our results reveal that bottom water dynamics at Site U1387 were controlled by local oceanographic processes (e.g. coastal upwelling, river discharge, water column stratification) rather than by global ice volume changes only. These findings highlight the importance of understanding regional oceanographic variations during the EMPT and emphasize the value of combining food supply, oxygenation, and bottom current proxies to interpret benthic foraminifera ecological changes.
- A southern Portuguese Margin Perspective of Marine Isotope Stage 47—An interglacial in the 41 kyr WorldPublication . Voelker, Antje; Rodrigues, Teresa; Trotta, Samanta; Marino, Maria; Kuhnert, HenningIn order to better understand interglacial climate variability within the 41 kyr world, we produced high-resolution climate records for interglacial Marine Isotope Stage (MIS) 47 (1424-1452 ka) at IODP Site U1387 (36 degrees 48 ' N, 7 degrees 43 ' W) on the southern Portuguese margin. Using benthic and planktonic foraminifera stable isotope records, U-37(k') sea-surface temperature (SST), and plankton assemblage data we investigated Mediterranean Outflow Water (MOW) and surface water conditions. The MOW-level records indicate a poorly ventilated and sluggish bottom current during the MIS 48/MIS 47 transition in association with the insolation maximum, whereas a well-ventilated MOW formed a contourite layer during the second insolation maximum. The benthic delta O-18 record shows a fairly abrupt change during the deglaciation of MIS 48, while the surface waters experienced a terminal stadial event that was associated with initial cooling and freshening followed by stepwise warming until interglacial SST was reached at 1450 ka. Interglacial conditions with SST of 24 degrees C or higher persisted until 1427 ka, although warm SST prevailed into MIS 46. The persistent and prolonged warmth is attributed to a northward expansion of the subtropical gyre during MIS 47 as reflected by the dominance of subtropical-tropical planktonic foraminifera species and the presence of warm water coccolithophores taxa.