Topics: Large spatial variations in magnetic and heavy mineral content are typical for sublittoral sediment facies. These patterns reflect modern sediment dynamics driven by waves, tides and currents as well as relict structures of past sea-level change, tectonics or human impact. With electro- and enviromagnetics we can map these structures to reveal sediment facies, transport pathways and fractionation mechanisms.
Heinrich stadials as reddish dust layers in GeoB sediment cores off NW Africa (Senegal)
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 â€˜dustyâ€™ 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. Further, the pedogenic magnetic mineral assemblage bears a high potential for reconstructing environmental conditions prevailing in the source areas of aeolian 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 Brazilâ€“Malvinas 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. This shift enabled the transport of Antarctic diatoms and more strongly magnetic Argentinean shelf sands to the Rio Grande Cone, while sediment contributions from the La Plata River became less important. During the Late Holocene, the modern El NiÃ±o Southern Oscillation set in and the SAMS and the austral tradewinds intensified, causing a southward shift of the STSF to its modern position. This reinforced a significant deposition of La Plata River silts at the Rio Grande Cone. These higher magnetic silts with intermediate Fe contents mirror the modern more humid terrestrial climatic conditions over SE South America.
Sedimentological, geochemical and rock-magnetic parameters show sediment source changes
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 540â€“530 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.