Topics: Changes in climate conditions, ocean circulation and the sedimentary environments usually modify the composition and grain size distribution of the sediments that we find on the sea floor. Sediments transported by rivers, wind and currents can have different colours, grain sizes and a different composition in terms of magnetic minerals. With rock magnetic and physical property measurements and combined cyclo- and magnetostratigraphic dating, we aim to reconstruct the sources and depositional history with regard to climatic and oceanic conditions. Post-depositional modifications of the magnetic mineralogy are further characteristic for changing redox conditions in the ocean and sediment column, associated with ocean chemistry. We integrate our investigations into the global context in order to identify the factors and variations in the Earth system responsible for changes in terrestrial and oceanic environmental conditions.
Conceptual model of stagnant glacial deep ventilation of the NW Pacific evidenced by cyclic magnetite dissolution
Petromagnetism and provenance of Heinrich Event layers at the SE Grand Banks Slope, Newfoundland Scientists: Shettima Bukar Projects: ArcTrain
Summary: Periodic mass discharges of icebergs from the Laurentide ice-sheet into the North Atlantic Ocean during the last glacial period deposited abundant ice-rafted detritus (IRD) accumulated in sequences of typically six major Heinrich Event layers, each with some tens of cm thickness, at all eastern slopes of the Grand Banks submarine platform of Newfoundland. Compositionally, it is well established that these IRD layers consist of varied rock contents emanating from distinct, but not yet clearly defined bedrock provinces of the Canadian Shield. The most prominently reported constituent is detrital dolomite but the entire lithological range of the IRD is much broader. Rock magnetic records, e.g. magnetic susceptibility logs of SE Grand Banks cores, therefore depict complex and partly repeating internal substructures across the Heinrich Event layers owing to distinct successions in IRD lithology over the course of every mass calving event. We investigated IRD sieve fractions (1mm â 4cm) of the entire glacial section (550â1054 cm) of SE Grand Banks slope gravity core GeoB 18530-1, sampled in 2.3 cm steps. Therefrom, we identified and classified distinct IRD rock types as well as monocrystalline rock-forming mineral particles, for which we established so far 24 well-defined lithological categories of sedimentary, igneous and metamorphic origin. This initial identification of IRD lithology was performed based on all available visual criteria including texture (crystallinity, grain-size), color and translucency (mineralogy), hardness and surface structures (e.g., cleavage) using a binocular microscope. This rock type classification is now being substantiated by polarized light microscopy of exemplary thin sections created from larger IRD clasts.
Representative specimens of 24 idetified IRD rock and mineral classes
Plio-Pleistocene climate of and oceanography off East Africa (completed) Scientists: Janna Just Projects: IODP Exp 361 (Southern African Climates), DFG SFB 806
Summary: No paleoclimatic record exceeding 1.5 Ma is available for SE Africa and the SW Indian Ocean, although this region has integral importance for the evolution and environmental adaption of Early Hominins that started in the Pliocene. To fill this gap sediment cores obtained in course of IODP Expedition 361 are used to investigate the paleoclimatic and paleoceanographic conditions during the Pliocene Warm Period, and into the Pleistocene with the dominant glacial-interglacial variability that was initiated at the Mid-Pleistocene Transition. In particular changes in the magnetic mineral concentration and mineral assemblage together with other sedimentological properties of the sediment cores provide information on continental weathering conditions, and transport mechanisms of terrigenous material to the coring sites as well as post-depositional processes. In this regard, the investigation will provide information on continental climate, varying surface and deep water current strength and chemistry. The comparison of marine and continental environmental conditions enable to disentangle teleconnections between these two environments and potential influence on the evolution of Early Hominins in East Africa.
Summary: The carbonate-free abyss of the NW Pacific defies most paleoceanographic proxy methods and remains a âblank spotâ in ocean history. Paleo- and rock magnetic, geochemical, and sedimentological methods were combined to date and analyze seven NW Pacific sediment cores from water depths of 5100-5700 m collected on the SO202-INOPEX expedition in 2009. A striking features of these records are nearly magnetite-free zones corresponding to glacial MIS 22, 12, 10, 8, 6, and 2. Within interglacial sections and glacial stages MIS 20, 18, 16, and 14, magnetite of detrital, volcanic, and bacterial origin is well preserved. Such alternating successions of magnetite preservation and depletion are known from sapropel-marl cycles deposited under periodically changing redox conditions. The only conceivable mechanism to cause such abrupt oxygeneation change in the open Pacific is a modified glacial bottom water circulation. During all major glaciations since MIS 12, stagnant, oxygen-depleted Antarctic Bottom Water sourced bottom water seems to have crept into the abyssal NW Pacific basin, thereby changing redox conditions in the sediment and trapping and preserving dissolved and particulate organic matter that reacted with magnetite. At deglaciation, the downward progressing oxidation front apparently remineralized and released these sedimentary carbon reservoirs without replenishing the magnetite losses.
Summary: Cold conditions in the North Atlantic during Heinrich Stadials correspond to phases of great aridity in NW Africa. These changing climate conditions are recorded in marine sediment cores by variations of aeolian and fluvial sediment accumulation and accordingly changes in geochemical composition and grain-size. In our sediment core studies off NW Africa, we address how these environmental conditions are expressed in the grain-size, concentration and mineralogy of magnetic minerals. Heinrich Stadials are characterized by high concentration of hematite and by a coarse magnetic grain size. The thickness of the âdustyâ layers decreases from East to West, indicating the gravitational settling of coarse-grained dust particles close to the continent. Mass budgets of aeolian sediment accumulation rates suggest that deposition of dust was enhanced by one to two orders of magnitude with respect to the Late Holocene. Furthermore, the pedogenic magnetic mineral assemblage bears a high potential for reconstructing environmental conditions prevailing in the source areas of eolian and fluvial sediments.
Summary: Surface currents and sediment distribution of the SE South American upper continental margin are under the influence of the South American Monsoon System (SAMS) and the Southern Westerly Wind Belt (SWWB). Both climatic systems determine the meridional position of the Subtropical Shelf Front (STSF) and probably also of the BrazilâMalvinas Confluence (BMC). We reconstruct the changing impact of the SAMS and the SWWB on sediment composition at the upper Rio Grande Cone off southern Brazil during the last 14 cal kyr BP combining sedimentological, geochemical, micropaleontological and rock magnetic proxies of marine sediment core GeoB 6211-2. Sharp reciprocal changes in ferri- and paramagnetic mineral content and prominent grain-size shifts give strong clues to systematic source changes and transport modes of these mostly terrigenous sediments. Our interpretations support the assumption that the SAMS over SE South America was weaker than today during most of the Late Glacial and entire Early Holocene, while the SWWB was contracted to more southern latitudes, resembling modern austral summer-like conditions. In consequence, the STSF and the BMC were driven to more southern positions than today`s, favoring the deposition of Fe-rich but weakly magnetic La Plata River silts at the Rio Grande Cone. During the Mid Holocene, the northern boundary of the SWWB migrated northward, while the STSF reached its northernmost position of the last 14 cal kyr BP and the BMC most likely arrived at its modern position.
Summary: The Mid-Pleistocene transition (MPT) of the global climate system, initiated by a shift towards much larger northern hemisphere ice shields at around 920 ka and ending with predominance of 100 kyr ice age cyclicity since about 640 ka, is one of the fundamental enigmas in Quaternary climate evolution. Based on a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic we could demonstrate dissolution driven variations in carbonate accumulation controlled by changes in MPT deep water circulation. In addition to characteristics known from δ18O records, these data sets revealed three remarkable features intimately related to the MPT: (1) an all-Pleistocene minimum of carbonate accumulation in the South Atlantic at 920 ka, (2) a MPT interim state of reduced carbonate deposition, indicating that the MPT period may have been a discrete state of the Pleistocene deep water circulation and climate system and (3) a terminal MPT event at around 540â530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.
Summary: In a number of original studies on submarine river fans and continental slopes in high productivity zones, we investigated the progressive chemical reduction and dissolution of ferrimagnetic oxides of the redox-sensitive element iron in suboxic and sulfidic sedimentary environments. Fine-grained magnetite is generally more strongly affected than hematite, while inclusions in silicate matrix are well protected. Titanium substituted Fe oxides are considerably more resistant due to their lower Fe3+ content. Titanomagnetite-hemoilmenite intergrowths form skeleton structures due to the preferential dissolution of titanomagnetite. In the course of early diagenesis, new magnetic minerals such as greigite, goethite or biogenic magnetite prcipitate at geochemical boundaries, which we could substantiate in great detail by rock magnetic diagnostics. Diagenetic overprinting of paleo- and enviromagnetic records is a common problem in organic-rich layers, but often provides us with valid clues on sediment redox zonation and history. We were the first to quantitatively simulate non-steady state magnetite dissolution with geochemical transport and reaction models and giving estimates for the age of sedimentation events.