ALBERT

Description: In 2016 the International Ocean Discovery Program (IODP) Expedition 361 (“SAFARI”) recovered complete high-resolution Plio-/Pleistocene sediment sections at six drilling locations on the southeast African margin and at the oceanic connection between the Indian and South Atlantic Oceans. Site U1475 is located on the southern flank of the Agulhas Plateau, proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. The site was drilled into a sediment drift in 2669 m water depth and comprises a complete carbonate rich (74 – 85%) stratigraphic section of the last ~7 Ma. The contourite deposits hold detailed information on past changes in the bottom water flow history in the Indian-Atlantic ocean gateway. Here we present results from the integration of physical properties, seismic reflection data, and major element records. The whole spliced sediment record (292 meters) of Site U1475 was measured using an X-ray fluorescence (XRF) core scanner to derive multi-centennial resolution records of major element intensities. Based on these measurements it is possible to derive biogenic (e.g. %CaCO3) and siliciclastic (e.g. TiO2, K2O) mineral phases. Elemental log-ratios, such as Ca/Ti and K/Fe, reflect variations in biogenic (CaCO3) vs. terrigenous supply and variability of the terrigenous provenance, respectively. While long-term changes in physical properties and elemental ratios can be linked to the seismic reflection patterns associated with deep water circulation changes, short-term cyclicities reflect Plio-Pleistocene climate variations at Milanlovitch-frequencies. Evolutionary spectra show that the orbital control on sediment composition was variable over time. During the last 4 Ma energy is concentrated at the 41ka band of obliquity and at lower frequencies. In contrast, the orbital precession cycle (19-23ka) is very prominent in a peculiar high sedimentation rate interval in the early Pliocene (~4 to 5 Ma) that is bounded by seismic reflectors and characterized by the development of sediment waves.

Description: IODP Expedition 361 drilled six sites on the southeast African margin and in the Indian-Atlantic ocean gateway, southwest Indian Ocean, from 30 January to 31 March 2016. The sites, situated in the Mozambique Channel, at locations directly influenced by discharge from the Zambezi and Limpopo River catchments, the Natal Valley, the Agulhas Plateau, and the Cape Basin were targeted to reconstruct the history of the Greater Agulhas Current System over the past ~ 5 Ma. The main objectives of the expedition were to establish the sensitivity of the Agulhas Current to climatic changes during the Plio-Pleistocene, to determine the dynamics of the Indian-Atlantic gateway circulation during this time, to examine the connection of the Agulhas leakage and AMOC, to address the influence of the Agulhas Current on African terrestrial climates and potential links to human evolution. Additionally, the Expedition set out to fulfill the needs of the APL, consisting of high-resolution interstitial water samples that will constrain the temperature and salinity profiles of the ocean during the Last Glacial Maximum. Here we highlight some of the expedition successes and show how it has made major strides toward fulfilling each of these objectives. The recovered sequences allowed complete spliced stratigraphic sections to be generated spanning the interval of 0 to between ~0.13 and 7 Ma. These sediments provide an exceptional opportunity to generate decadal to millennial-scale climatic records that will resolve key paleoceanographic questions from a region poorly represented in the database of scientific drills sites.

Description: Evidence from the joint interpretation of proxy data as well as geodynamical and biogeochemical modeling results point to complex interactions between sea level drawdown, volcanic degassing, and atmospheric CO2 that hampered the climate system’s decent into the last ice age. Ice core data shows that atmospheric CO2 dropped abruptly into glacial Marine Isotope Stage (MIS) 4 at ~71 ka, while Antarctic temperatures display a more gradual decline between ~85 ka to ~71 ka across the MIS 5/4 transition. Based on 2D and 3D geodynamical simulations, we show that a ~60-100 m sea level drop associated with the MIS 5/4 transition led to a significant increase in magma and possibly CO2 flux at mid-ocean ridges (MOR) and oceanic hotspot volcanoes. The MOR signal is assessed with 2D thermomechanical models that account for mantle melting and resolve the flux of incompatible carbon dioxide. These models have been run at different spreading rates and integrated with the global distribution of opening rates to compute global variations in magma and CO2 flux across the MIS 5/4 transition. 3D plume models have been used to quantify the impact of a dropping sea level on oceanic hotspot melting and CO2 release. Here a wide range of simulations with differing plume fluxes, lithospheric thicknesses as well as speeds, and plume excess temperatures have been integrated with data from ~40 hotspots in order to compute a global signal. Biogeochemical carbon cycle modeling shows that the predicted increase in volcanic emissions is likely to have raised atmospheric CO2 by up to 15 ppmv, sufficient to explain the bulk of the decoupling between temperature and atmospheric CO2 during the global change to pronounced glacial conditions across the MIS 5/4 transition.

Description: The Pliocene epoch represents a discrete interval which reversed a long-term trend of late Neogene cooling and is also the most recent geological interval in which global temperatures were several degrees warmer than today. It is therefore often considered as the best analogue for a future anthropogenic greenhouse world. However, there is growing evidence that the Pliocene was not a stable period but can rather be subdivided in several distinct climate phases. Our understanding of Pliocene climate variability in the Southern Hemisphere, and especially in the Atlantic-Indian ocean gateway, is limited by scarce marine records and poor age control on existing terrestrial climate archives. At five drilling locations IODP Exp. 361 recovered high resolution complete late Miocene to Pleistocene sections (Hall et al., 2017). Our research proposal is based on the Sites U1474 (Natal Valley), U1475 (Agulhas Plateau), and U1479 (Cape Basin) forming a latitudinal transect. The main focus is on the interplay between northern and southern sourced deep water masses in the Atlantic-/Indian Ocean gateway during the Pliocene and combines chemical, physical property and seismic methods. Our research is driven by three working hypotheses: Seismic stratigraphies for the last 6 Ma and sediment drift growth in the Atlantic-Indian gateway are mainly controlled by bottom water flow changes Using the sediment archives and physical property records from IODP Exp. 361 we aim to construct and compare detailed seismic stratigraphies for the Agulhas Plateau, the Natal valley and the Cape basin for the last 6 Ma. At all Exp. 361 sites P-wave velocity and density records enable detailed correlations of drilling results and site survey data through the calculation of synthetic seismograms. Our working hypothesis implies that seismic reflection patterns and sediment accumulation during the Pliocene are closely linked to deep water circulation changes associated with climate Pliocene phases. Furthermore four distinct high latitude Pliocene glaciation events have been identified. We speculate that these phases and events have led to deep water circulation changes in Agulhas region, have altered the sediment physical properties and thus may be recognized as reflectors in the seismic profiles. How did the sediment input of terrigenous vs. biogenic sediment components in the gateway change during these events? Are these changes driven by dilution, dissolution, or productivity? We strive to answer these questions by interpreting edited and in-situ corrected physical core scanning records in combination major element variabilty derived from post cruise XRF-scanning. Trajectories and intensities of deep water masses in the Agulhas region during the Pliocene were influenced by Antarctic ice volume rather then by the closure of the Central American Seaway. The Exp. 361 drill sites offer the possibility to inter-correlate different flow speed proxies and to derive a detailed picture of flow changes during the Pliocene. By comparing core-measurements of sortable silt (S̅S̅), physical properties and XRF-core scanning data with seismic features we will tie the major flow speed changes to our seismic grid covering the Agulhas Plateau such that changing current intensities and pathways can be mapped together. Here we hypothesize that these changes are mainly driven by climate (Antarctic ice volume). How have the sedimentation patterns changed under the growing influence of North Atlantic Deep Water (NADW) during the Pliocene? What were the main changes associated with the instability of Antarctic ice sheets and was the production of Antartic Bottom Water (AABW) reduced or enhanced during these intervals? Was there also a potential influence of tectonic processes on the flow changes in the Agulhas region? Especially the closure of the Centarl American Seaway (CAS) in various phases between ~14 and ~2.7 Ma is thought to have had a profound impact on climate. Changes in physical and chemical sediment properties in the Agulhas region are largely controlled by earth’s orbital variations and allow a significant improvement of age models by cyclostratigraphy. Another primary objective of our research is the detection and characterization of orbital and sub-orbital cycles in the Agulhas sedimentary environment in relation to paleoceanographic changes. The presence of orbital cycles in ocean sediments has widely been used to derive high resolution age models in Cenozoic sediments. Typically orbital chronologies are based on benthic oxygen isotope records (δ18O) that are correlated to astronomical forcing functions (“orbital tuning”). However, the generation of such records at high resolution over long time intervals is time consuming and will likely not be completed for the Exp. 361 sites over the next years. In the absence of δ18O records cyclic changes in high resolution measurements of physical (e.g. density, colour reflectance, magnetic susceptibility) and chemical (major elements from XRF core scanning) parameters have been successfully used for orbital tuning. At the Exp. 361 Sites very regular cyclic amplitude changes are evident in the Pliocene sections, but up to now have not been further investigated. Which orbital frequency do these cycles represent and how do the dominent frequencies change over time? What is the potential of the observed cycles for stratigraphic purposes? We will analyse those cyclicities in the depth and time domain and strive to generate orbitally tuned time series of sediment provenance.

Description: The Deep Western Boundary Current (DWBC) inflow to the SW Pacific is one of the largest, transporting ~40% of the total input of deep water to the world’s oceans. Here we use a sedimentary record from the giant piston core MD97-2114 collected on the northern flank of the Chatham Rise located at 1935 m water depth, east of New Zealand, to investigate DWBC variability during the Pleistocene epoch when the period of glacial cycles changed progressively from a 41 kyr to 100 kyr rhythm. Magnetic grain-size may be directly related to orbitally forced fluctuations in the strength of the upper circumpolar deep water (UCDW) through its interaction with terrigenous sediments supplied from the south and west. The long-term trends in magnetic properties are characterized by two main perturbations centered at 870 ka (Marine Isotope Stage, MIS 22) 450 ka (MIS 12), which is broadly consistent with the inferred perturbation during the mid-Pleistocene climate transition based on sedimentological paleocurrent reconstruction from Ocean Drilling Program Site 1123 located at 3290 m water depth in the main core of the DWBC flow on the North Chatham Drift. This similarity suggests that both the upper and middle CDW are modulated by similar processes and fluctuations of Antarctic Bottom Water production could be directly responsible for this deep Pacific Ocean inflow variability over the past 1.2 Ma.

Description: Published

Description: 107-118

Description: 2.2. Laboratorio di paleomagnetismo

Description: JCR Journal

Description: reserved

Description: Agulhas Current warm water transports and their leakage into the Atlantic form a key link in the global thermohaline circulation (THC) as they feed the warm upper limb of the northward THC in the South Atlantic. The interocean exchange off southern Africa is potentially involved in the initiation, maintenance and stability of North Atlantic THC and thus, it bears importance within the Plio-Pleistocene climate development. On regional scales it is involved with setting past environmental conditions in continental southern Africa with plausible significance for the evolution of mammals including hominins. While the southwest African margin has been the target of scientific ocean drilling several times in the past, only a single drill site has presently been occupied within the direct reaches of the warm water Agulhas Current along the southeast African margin. With this IODP drilling initiative we target 9 sites to retrieve multiple APC-XCB cores spanning the main routing of the Agulhas head waters, the Agulhas corridor linking the Indian and Atlantic Oceans, and the retroflected waters returning into the Indian Ocean. One additional RCB hole is planned to drill 〉200 m deep into the Agulhas Plateau basement.The SAFARI drilling proposal combines four overarching themes: (i) Variability of warm water transports along the southeast African margin during the late Neogene and its linking with regional to hemisphere-wide climate changes; (ii) the role of gateway circulation around South Africa and ensuing Indian-Atlantic Ocean water exchanges in changing Atlantic THC modes during key-periods of the Plio-Pleistocene climatic evolution; (iii) place southern African terrestrial climates and the evolution of mammals including hominins into a framework of long-term changes of ocean circulation and marine heat budgets in the SW Indian; (iv) origin and development of the Agulhas plateau as a continental fragment vs. a large igneous province of oceanic origin.Specific palaoceanographic objectives, on Plio-Pleistocene timescales, of the SAFARI drill sites are: variations of Agulhas Current warm-water transports during orbital and faster climate changes; role of such variations during key-periods of global change: mid-Pliocene onset of northern hemisphere glaciation and mid-Pleistocene revolution; influence of upstream forcing i.e., monsoonal, Indonesian Throughflow and Red Sea outflow, on SE African warm water transports in the 41-kyr and 100-kyr worlds; advective salinity feedback between Agulhas Leakage and Atlantic MOC mode changes at these times; contribution of Southern Ocean deep and bottom water masses to global ocean THC circulation prior to and after the onset of northern hemisphere glaciation; linking between ocean circulation changes in the SW Indian, southern African terrestrial climates and mammal / hominin evolution; extending the southern hemisphere paleoclimatic record beyond EPICA.Specific tectonic objectives of the SAFARI drill sites are: basement petrology and geochemical composition and hence, the origin of the Agulhas Plateau; age of the basement and timing of lava flows relative to the separation of Africa from Antarctica; age and composition of Agulhas Plateau lava flows within the framework of worldwide LIP occurrence and excessive volcanism; emplacement of volcanics on the southern Agulhas Plateau in the light of a possible volcanic period; age and composition of the sediments on the Agulhas Plateau as an indication for the earliest occurrence of modern-type current systems.