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- Data report: IODP Site U1387: the revised splice between Sections U1387B-18X-3 and U1387C-8R-3 (>171.6 mcd)Publication . Voelker, Antje; Jimenez-Espejo, F. J.; Bahr, A.; Rebotim, Andreia; Cavaleiro, Catarina; Salgueiro, Emilia; Röhl, U.; Stow, Dorrik A. V.; Hernández-Molina, F. J.; Alvarez Zarikian, Carlos A.; Expedition 339 ScientistsThe Expedition 339 shipboard splice of Integrated Ocean Drilling Program (IODP) Site U1387 deeper than ~155 meters composite depth (mcd) is based on a composite of the magnetic susceptibility and natural gamma radiation data. When generating high-resolution paleoceanographic reconstructions for the Mid-Pleistocene Transition and early Pleistocene sections of Site U1387, it quickly became obvious that proxy data misfits existed at several splice transitions. Thus, a revised splice was generated for Site U1387 below Core 339-U1387B-18X based on X-ray fluorescence– derived element records (e.g., ln[Fe/Ca]) and the stable isotope records obtained for planktonic and benthic foraminifers. Corrections were needed at most of the splice transitions below Core 339-U1387A-19X, with adjustments ranging from a few centimeters to several meters. In addition, Core 339-U1387A-33X and sections of Core 36X were integrated into the revised splice to replace Core 339-U1387C-2R and sections of Core 5R, respectively. The replacement of Core 339-U1387C-2R with Core 339-U1387A33X is an option for the intended paleoceanographic research and not essential for lower resolution studies. The splice tie point table, therefore, also includes an option for a splice that retains Core 339-U1387C-2R. The extensive revision of the shipboard splice reveals that making a splice for sediment sequences rich in contourite layers and coring disturbances (biscuiting in the extended core barrel cores) can be tricky and that data misfits at splice transitions are not necessarily a data problem but could indicate a splice problem.
- Mediterranean outflow conditions during the early to middle Pleistocene linked to precession forcingPublication . Voelker, Antje; Salgueiro, Emilia; Henning, KuhnertThe Mediterranean Outflow Water (MOW) is an important intermediate depth water mass in the North Atlantic. Here we reconstruct changes in the MOW during the early Pleistocene interval from 630 to 1760 ky, encompassing the Mid-Pleistocene Transition (MPT), which marks an important change in orbital climate forcing. Neither MOW ventilation nor MOW flow strength reveal changes related to the MPT, but incorporate variations related to precession (insolation) forcing. The MPT related change to higher glacial benthic δ O values occurs earlier at Site U1387 than in most deep-sea records, i.e. with MIS 26 instead of the MIS 24 to MIS 22 interval
- Repeated near-collapse of the Pliocene sea surface temperature gradient in the North AtlanticPublication . Naafs, B. David A.; Voelker, Antje; Karas, Cyrus; Andersen, Nils; J. Sierro, FranciscoSea surface temperature (SST) is used to infer past changes in the state of the climate system. Here we use a combination of newly generated and published organic paleothermometer records, together with novel high‐resolution benthic foraminiferal δ18O stratigraphy, from four sites in the midlatitude North Atlantic (41–58°N) to reconstruct the long‐term evolution of the latitudinal SST gradient during the Pliocene and early Pleistocene (4.0 to 2.4 Myr), the last time atmospheric CO2 reached concentrations above 400 ppmv. We demonstrate that the latitudinal SST gradient in the North Atlantic nearly collapsed twice during this period. We conclude that the latitudinal SST gradient in the midlatitude North Atlantic has two end‐members: a maximum as existing at present and a minimum that existed during certain periods of the (late) Pliocene. Our results suggest that the 400‐ppmv Pliocene world was more dynamic than currently thought.
- Data report: diatom and silicoflagellate records of marine isotope Stages 25-27 at IODP Site U1387, Faro DriftPublication . Ventura, C.; Abrantes, Fatima; Loureiro, I.; Voelker, Antje; Stow, Dorrik A. V.; Hernandez-Molina, F. Javier; Alvarez Zarikian, Carlos A.; Expedition 339 ScientistsDuring Integrated Ocean Drilling Program Expedition 339, the shipboard micropaleontological studies of Site U1387 core catcher samples revealed the preservation of diatoms and radiolarians in specific depths from early Pleistocene age (900–1000 ka). To evaluate the ecological significance of those diatoms, we analyzed 98 samples from the intervals of 244 to 274 m along the corrected splice, corresponding to marine isotope Stage (MIS) 25 to MIS 29, for the abundance of diatoms and silicoflagellates. In 6 samples, the composition of the diatom assemblage was determined as well. Although most samples were barren of siliceous microfossils, the downcore record revealed two intervals, 249–252 corrected meters composite depth (cmcd) and 263–265 cmcd, where diatoms and silicoflagellates reach their maximum values. These maxima occurred from the MIS 26/25 transition to interglacial MIS 25, and again during early MIS 27. The diatom assemblage includes 27 identified taxa with Chaetoceros (Hyalochaete) resting spores being dominant and Thalassionema nitzschioides and Paralia sulcata significant. The Chaetoceros spores clearly indicate strong influence of seasonal upwelling and associated high primary productivity. Special to the diatom record is, however, the occurrence of the large-diameter (>125 µm) centric diatoms Coscinodiscus asteromphalus, Coscinodiscus apiculatus, and Coscinodiscus cf. gigas that imply incursions of low-nutrient, open-ocean water into the southern Portuguese coast during MIS 25.
- Planktonic foraminifera evidence for surface-water conditions off Southern Portugal during Mid-Pleistocene marine isotope stages 20 to 24 (790 – 940 ky)Publication . Mega, Aline; Salgueiro, Emilia; Voelker, AntjeCharacterized by a drastic change in the deep thermohaline circulation, the Mid Pleistocene Transition (MPT) was a global climatic event resulted in more intense and longer lasting glacial periods and cooler sea -surface temperatures. The present study's objective is characterized surface -water variations during the MPT interval at the mid -latitude southern Portuguese margin using planktonic foraminifera faunal data from Site U1387. Stratigraphy records reveals dominantly tropical and subtropical species interrupted by incursions of subpolar waters when the North Atlantic's subarctic front moved to southernmost position during the MPT.
- Data report: IODP Expedition 339 Site U1391: an improved splice and preliminary age model on the basis of XRF dataPublication . Abrantes, Fatima; Santos, C.; Ventura, C.; Voelker, Antje; Röhl, U.; Stow, Dorrik A. V.; Hernandez-Molina, F. Javier; Alvarez Zarikian, Carlos A.; Expedition 339 ScientistsWith the objective to reconstruct past primary production on the southwest Portuguese margin at orbital and millennial timescales through the Pleistocene, we chose to compare two sites recovered during Expedition 339 at similar latitude but different distances to the coast (Sites U1385 and U1391). Site U1385 has a well-established chronology, but for Site U1391 no isotope data are yet available. In order to develop a correct chronology without isotope data, we performed XRF analysis of cores from Holes U1391A and U1391B between 76.51 and 205.77 mbsf. With these data, in particular using log(Ca/Ti) and log(Ca/Fe), it was possible to enhance the Site U1391 splice and to establish an age model based on the correlation to the well-defined and robust Site U1385 age model.
- Data report: early Pleistocene calcareous nannofossils, IODP Expedition 339, Site U1387Publication . Balestra, B.; Voelker, Antje; Flores, J.A.; Stow, D. A. V.; Hernandez-Molina, F. Javier; Alvarez Zarikian, Carlos A.; Expedition 339 ScientistsWe present a revision and refinement of semiquantitative analyses of calcareous nannofossil assemblages in early Pleistocene samples from Holes U1387A and U1387C recovered toward the eastern end of the Faro Drift (36°48.3210N, 7°43.1321W) during Integrated Ocean Drilling Program Expedition 339, Mediterranean Outflow (November 2011–January 2012). The record is characterized by intervals very rich in calcareous nannofossils that are in general moderately to well preserved. On the other hand, the record contains an interval directly above the youngest dolomite layer in Section 339-U1387C-19R-4 (~0.7 m) where no coccoliths were preserved. The new stratigraphic constraints of events such as the lowest occurrence (LO) of large Gephyrocapsa, the highest occurrence (HO) of Calcidiscus macintyrei (1.66 Ma), the LO of medium-sized Gephyrocapsa group, and the HO of Discoaster brouweri (1.95 Ma) allow better interpretation of the isotope stratigraphy applied to this interval.
- Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes (MIS 9-14) and their implications for the thermohaline circulationPublication . Voelker, Antje; Rodrigues, T.; Billups, K.; Oppo, D.; McManus, J.; Stein, R.; Hefter, J.; Grimalt, J.O.Stable isotope and ice-rafted debris records from three core sites in the mid-latitude North Atlantic (IODP Site U1313, MD01-2446, MD03-2699) are combined with records of ODP Sites 1056/1058 and 980 to reconstruct hydrographic conditions during the middle Pleistocene spanning Marine Isotope Stages (MIS) 9-14 (300-540 ka). Core MD03-2699 is the first high-resolution mid-Brunhes record from the North Atlantic's eastern boundary upwelling system covering the complete MIS 11c interval and MIS 13. The array of sites reflect western and eastern basin boundary current as well as north to south transect sampling of subpolar and transitional water masses and allow the reconstruction of transport pathways in the upper limb of the North Atlantic's circulation. Hydrographic conditions in the surface and deep ocean during peak interglacial MIS 9 and 11 were similar among all the sites with relative stable conditions and confirm prolonged warmth during MIS 11c also for the mid-latitudes. Sea surface temperature (SST) reconstructions further reveal that in the mid-latitude North Atlantic MIS 11c is associated with two plateaus, the younger one of which is slightly warmer. Enhanced subsurface northward heat transport in the eastern boundary current system, especially during early MIS 11c, is denoted by the presence of tropical planktic foraminifer species and raises the question how strongly it impacted the Portuguese upwelling system. Deep water ventilation at the onset of MIS 11c significantly preceded surface water ventilation. Although MIS 13 was generally colder and more variable than the younger interglacials the surface water circulation scheme was the same. The greatest differences between the sites existed during the glacial inceptions and glacials. Then a north - south trending hydrographic front separated the nearshore and offshore waters off Portugal. While offshore waters originated from the North Atlantic Current as indicated by the similarities between the records of IODP Site U1313, ODP Site 980 and MD01-2446, nearshore waters as recorded in core MD03-2699 derived from the Azores Current and thus the subtropical gyre. Except for MIS 12, Azores Current influence seems to be related to eastern boundary system dynamics and not to changes in the Atlantic overturning circulation.