Browsing by Author "Hodell, David A."
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- Centennial-millennial scale ocean-climate variability in the northeastern atlantic across the last three terminationsPublication . Singh, Harshit; Singh, Arun Deo; Tripathi, Ravi; Singh, Pradyumna; Verma, Komal; Voelker, Antje; Hodell, David A.Changes in Earth's orbital parameters pace the Pleistocene glacial-interglacial cycles, although considerable ambiguity still remains over the interaction of the internal climatic variables, such as ice-sheet instability and ocean circulation that allow transitions into and out of an interglacial. Here, we analyse high-resolution planktic foraminiferal proxies including sea-surface temperature (SST) based on an Artificial Neural Network (ANN) across the last three terminations (TI, TII and TIII) and the subsequent interglacials (Holocene, Marine Isotope Stages (MIS) 5e and 7e) from IODP Site U1385, SW Iberian Margin. To demarcate the stadials and interstadials, we combined the faunal and SST records with existing data at Site U1385 including log (Ca/Ti) and benthic and planktic 818O. The composite records reveal details of the last three terminations in terms of abrupt climatic events occurring during these terminations. Termination I included three well-known climatic events: Heinrich stadial (HS)1, Bolling/Allerod (B/A) complex & Younger Dryas (YD). Termination II was interrupted by only HS11. Termination III included HS8.2 and HS8.1 which show more resemblance to HS2 (a stadial prior to TI) and HS1, suggesting the YD is a unique feature of the last deglaciation. Additionally, TI and TII reveal similar du-rations (-6 kyr) with rates of SST change (-1.5 degrees C/kyr to-2.1 degrees C/kyr), whereas TIII represents a longer process (-10 kyr) with a relatively slow rate of SST change (-0.8 degrees C/kyr). The anatomy of stadials (HS1, HS2, HS11 & HS8.1) reveals a complex history ('W' shaped anatomy) with two or three cold phases sandwiching (a) brief warm event(s). The European ice-sheet melting possibly initiated the stadial cooling at the Iberian Margin fol-lowed by the mid-latitude summertime warming and the intermediate-depth water mass warming that probably induced the Laurentide ice-sheet melting resulting in the complex stadial pattern. Our records further reveal a major reorganization of the surface current system, oceanographic fronts and productivity conditions across these terminations. We also document broad similarities in the climatic evolution of Holocene, MIS 5e and 7e interglacials in terms of SST, surface productivity and current system. The long-term interglacial trends were superimposed by multiple brief cold events interrupting the Holocene (-11.3, 9.9, 8.2, 7.1, 5.5, 2.5 ka), MIS 5e (C28, C27, C27', C27a, C27b, C26, C26', C25), and MIS 7e (-238, 234, 231, 230 ka) interglacials. Integration of our records with benthic foraminiferal 813C records from the Iberian Margin and central North Atlantic suggest fluctuations in the deep water convection process (which in turn were influenced by the conditions in subpolar gyre) possibly resulted in the brief cold events interrupting the interglacials at the Iberian Margin.
- Modern relationships between microscopic charcoal in marine sediments and fire regimes on adjacent landmasses to refine the interpretation of marine paleofire records: an Iberian case studyPublication . Genet, Marion; Daniau, Anne-Laure; Mouillot, Florent; Hanquiez, Vincent; Schmidt, Sabine; David, Valérie; Georget, Muriel; Abrantes, Fatima; Anschutz, Pierre; Bassinot, Franck; Bonnin, Jérome; Dennielou, Bernard; Eynaud, Frédérique; Hodell, David A.; Mulder, Thierry; Naughton, Filipa; Rossignol, Linda; Tzedakis, Polychronis; Sánchez-Goñi, Maria FernandaMarine microcharcoal records provide invaluable information to understand changes in biomass burning and its drivers over multiple glacial and interglacial cycles and to evaluate fire models under warmer climates than today. However, quantitative reconstructions of burnt area, fire intensity and frequency from these records need calibration studies of the current fire-microcharcoal relationship. Here, we present the analysis of microcharcoal concentration and morphology in 102 core-top sediment samples collected in the Iberian margin and the Gulf of Cadiz. We show that microcharcoal concentrations are influenced by the water depth or the distance from the river mouth. At regional scale, the mean microcharcoal concentrations and microcharcoal elongation (length to width ratio) show a marked latitudinal variation in their distribution, primarily controlled by the type of burnt vegetation in the adjacent continent. High microcharcoal concentrations in marine sediments represent rare, large and intense fires in open Mediterranean woodlands. Based on these results, the increasing trend of microcharcoal concentrations recorded since 8 ka in the well-known marine sedimentary core MD95-2042 off the Iberian margin indicates the occurrence of large and infrequent fires of high intensity due to the progressive degradation of the Mediterranean forest and the expansion of shrublands.
- Reconstruction of deep-water undercurrent variability from the outer Labrador Sea during the past 550,000 yearsPublication . Kaboth-Bahr, Stefanie; Bahr, André; Blaser, Patrick; Voelker, Antje; Lippold, Jörg; Gutjahr, Marcus; Hodell, David A.; Channell, James E.T.; Vernal, Anne de; Hillaire-Marcel, ClaudeWe present a comprehensive multi-proxy analysis spanning 550,000 years from the outer Labrador Sea region at the Integrated Ocean Drilling Program (IODP) Sites U1302/1303. We combine new benthic foraminiferal stable oxygen (delta 18O) and carbon (delta 13C) isotope records, with sediment elemental composition and authigenic neodymium isotope measurements, to provide insights into deep-water mass sourcing and changes of the Deep Western Boundary Current (DWBC), which exports North Atlantic Deep Water (NADW) into the wider North Atlantic as part of the lower limb of the Atlantic Meridional Overturning Circulation. We find that a prominent DWBC likely remained a persistent feature within the Labrador Sea region throughout the past 550 kyr. However, glacial peaks of marine isotope stage (MIS) 14 to MIS 2 were consistently characterized by a weaker or shallower DWBC, while all interglacial periods of MIS 13a to MIS 1, with the exception of MIS 7e, were marked by enhanced DWBC. Additionally, the dominant deep-water masses feeding into the DWBC during these glacial-interglacial periods varied from regional (K-rich sediment, unradiogenic epsilon Nd) to more distal sources from the Nordic Seas (Ti-rich sediment, radiogenic epsilon Nd). Yet, these changes in deep-water provenance did not consistently correlate with DWBC strength, suggesting that additional factors may have played a significant role in shaping the DWBC strength or core depth throughout the geological past.