Tilo von Dobeneck

Sahel megadroughts triggered by glacial slowdowns of Atlantic meridional overturning

[1] The influence of the large-scale ocean circulation on Sahel rainfall is elusive because of the shortness of the observational record. We reconstructed the history of eolian and fluvial sedimentation on the continental slope off Senegal during the past 57,000 years. Our data show that abrupt onsets of arid conditions in the West African Sahel were linked to cold North Atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with Heinrich Stadials. Climate modeling suggests that this drying is induced by a southward shift of the West African monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric African Easterly Jet.

The Mid-Pleistocene climate transition as documented in the deep South Atlantic Ocean: initiation, interim state and terminal event

Abstract 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. Climate proxy records not exclusively linked to global ice volume are necessary to advance understanding of the MPT. Here we present a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic essentially reflecting dissolution driven variations in carbonate accumulation controlled by changes in deep water circulation. In addition to characteristics known from δ18O records, the data sets reveal 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 540–530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.

Isothermal magnetization of samples with stable Preisach function: A survey of hysteresis, remanence, and rock magnetic parameters

Isothermal magnetization curves, like hysteresis loops, initial curves, back field curves, acquisition curves and demagnetizing curves of isothermal remanent magnetization, are commonly used for rock magnetic purposes. In this study we investigate the relations among these curves and other useful magnetization curves (saturation initial curve and induced and remanent hysteretic magnetization curves) in order to compare coercivity and domain state parameters which can be derived from them. Most natural samples, especially sediments, are weakly magnetic and possess relatively stable Preisach functions. Their magnetization states can therefore be described by classical Preisach theory. This approach verifies well-known rules and establishes some formerly unreported relations between isothermal magnetization curves and parameters. It is possible to point out sets of mathematically independent isothermal magnetization curves and to state theoretical interrelations between dependent curves by simply inspecting Preisach diagrams. Furthermore, we define six elemental isothermal magnetization curves from a general partition scheme of the Preisach diagram. They can be easily obtained from common measurements and generate all above mentioned curves. The experimental applicability of our results is demonstrated for three (single-domain, pseudo-single-domain, multidomain) marine sediment samples. A physical rationale of the elemental curves reveals favorable properties for the investigation of interaction and domain state. As a spin-off from the general results, a new hysteresis-based procedure for the measurement of Hcr is presented. We also propose an apparently more robust hysteresis-derived domain state parameter and a generalized version of the well-known R parameter. All presented methods can be applied without actually measuring Preisach functions.

Magnetic characterization of Holocene sedimentation in the South Atlantic

Surface sediment samples representative for the tropical and subtropical South Atlantic (15°N to 40°S) were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant Holocene climatic and oceanographic controls. Individual parameters reveal distinct, yet frequently overlapping, regional sedimentation characteristics. A probabilistic (“fuzzy c-means”) cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine-particle magnetite grain size and coercivity spectra. The resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes, authigenic biogenic magnetite accumulation in high-productivity areas, transport by ocean current systems, and effects of bottom water velocity on depositional regimes. Distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the South Atlantic reflect prominent contrasts of both oceanic and continental influences.

Late Quaternary sedimentation and early diagenesis in the equatorial Atlantic Ocean: patterns, trends and processes deduced from rock magnetic and geochemical records

This is an interdisciplinary and synoptic study of Equatorial Atlantic sediment formation in the Late Quaternary aimed at untangling the interlaced signatures of terrigenous and biogenous deposition and early diagenesis. It is based on a stratigraphic network of 16 gravity core records arranged along one meridional and three zonal transects (4°N, 0° and 4°S) crossing the Amazon and Sahara plumes as well as the Equatorial Divergence high productivity region. All newly introduced sediment sequences are collectively dated by their coherent CaCO3 content profiles and two available δ18O age models. To infer proxy records indicative of individual fluxes and processes, we analyze environmental magnetic parameters describing magnetite concentration, magnetic grain sizes and magnetic mineralogy along with CaCO3, Corg, Fe, Mn, Ba and color data. Diagenetically affected layers are identified by a newly introduced Fe/κ index. Reach and climatic variability of the major regional sedimentation systems is delimited from lithological patterns and glacial/interglacial accumulation rate averages. The most prominent regional trends are the N-S decrease in terrigenous accumulation and the Equatorial Divergence high in glacial Corg accumulation, which decays much faster south- than northwards. Glacial enrichments in Corg and proportional depletions in CaCO3, content appear to reflect sedimentary carbonate diagenesis more than lysoclinal oscillations and dominate temporal lithology changes. Suboxic iron mineral reduction is low at Ceara Rise and Sierra Leone Rise, but more intense on both flanks of the Mid-Atlantic ridge, where it occurs within organic rich layers deposited during oxygen isotope stages 6, 10 and 12, in particular at the terminations. To the equator, these zones reflect a full precessional rhythm with individual diagenesis peaks merging into broader magnetite-depleted zones. Rock magnetic and geochemical data show, that the depths of the Fe3+/Fe2+ redox boundary in the Equatorial Atlantic are not indicative of average productivity and were frequently shifted in the past. They are now located just above the topmost preserved productivity pulse. At 4°N, this organically enriched layer coincides with glacial stage 6, at 0° with glacial stage 2. Subsequent oxic and suboxic degradation of organic material entails stratigraphically coincident carbonate and magnetite losses opening new analytical perspectives.

Terrigenous Flux in the Rio Grande Rise Area during the Past 1500 ka: Evidence of Deepwater Advection or Rapid Response to Continental Rainfall Patterns?

Surface sediment samples and three gravity cores from the eastern terrace of the Vema Channel, the western flank of the Rio Grande Rise, and the Brazilian continental slope were investigated for physical properties, grain size, and clay mineral composition. Discharge of the Rio Doce is responsible for kaolinite enrichments on the slope south of 20° and at intermediate depths of the Rio Grande Rise. The long-distance advection of kaolinite with North Atlantic Deep Water from lower latitudes is of minor importance as evidenced by low kaolinite/chlorite ratios on the Mid-Atlantic Ridge. Cyclic variations of kaolinite/chlorite ratios in all our cores, with maxima in interglacials, are attributed to low- and high-latitude forcing of paleoclimate on the Brazilian mainland and the related discharge of the Rio Doce. A long-term trend toward more arid and “glacial” conditions from 1500 ka to present is superimposed on the glacial-interglacial cyclicity.

Cyclic magnetite dissolution in Pleistocene sediments of the abyssal northwest Pacific Ocean: Evidence for glacial oxygen depletion and carbon trapping

The carbonate-free abyss of the North Pacific defies most paleoceanographic proxy methods and hence remains a “blank spot” in ocean and climate history. Paleomagnetic and rock magnetic, geochemical, and sedimentological methods were combined to date and analyze seven middle to late Pleistocene northwest Pacific sediment cores from water depths of 5100 to 5700 m. Besides largely coherent tephra layers, the most striking features of these records are nearly magnetite-free zones corresponding to glacial marine isotope stages (MISs) 22, 12, 10, 8, 6, and 2. Magnetite depletion is correlated with organic carbon and quartz content and anticorrelated with biogenic barite and opal content. Within interglacial sections and mid-Pleistocene transition glacial stages MIS 20, 18, 16, and 14, magnetite fractions of detrital, volcanic, and bacterial origin are all well preserved. Such alternating successions of magnetic iron mineral preservation and depletion are known from sapropel-marl cycles, which accumulated under periodically changing bottom water oxygen and redox conditions. In the open central northwest Pacific Ocean, the only conceivable mechanism to cause such abrupt change is a modified glacial bottom water circulation. During all major glaciations since MIS 12, oxygen-depleted Antarctic Bottom Water (AABW)-sourced bottom water seems to have crept into the abyssal northwest Pacific below ~5000 m depth, thereby changing redox conditions in the sediment, trapping and preserving dissolved and particulate organic matter and, in consequence, reducing and dissolving both, biogenic and detrital magnetite. At deglaciation, a downward progressing oxidation front apparently remineralized and released these sedimentary carbon reservoirs without replenishing the magnetite losses.

The Termination of the Olduvai Subchron at Lingtai, Chinese Loess Plateau: Geomagnetic Field Behavior or Complex Remanence Acquisition?

We present a detailed investigation of the geomagnetic polarity transition that terminated the Olduvai subchron as recorded by loess/paleosol sediments at Lingtai in the central Chinese Loess Plateau (CLP). The polarity transition occurs within loess layer L25, where mineral magnetic parameters show considerable variations and sedimentation rate changes occur. The magnetic record obtained after thermal cleaning exhibits more than twenty apparent polarity flips, most of which occur within a stratigraphic distance corresponding to no more than ∼15,000 years. We argue that these results do not represent the actual behavior of the geomagnetic field. Instead, we propose that the combined effect of detrital and pedogenic remanences—which almost always co-exist on the central CLP—are responsible. These cause significant, lithologically-controlled, delays in the acquisition of the total remanence. In effect, the sediments act as a filter that generates noisy magnetic output from possibly simple input. We conclude that loess/paleosol sediments from the central CLP are poor candidates for tracking short-term geomagnetic field behavior such as polarity transitions, geomagnetic excursions and paleosecular variation.

Reconstruction of Magnetic Susceptibility and Electric Conductivity from Marine Small-loop EMI Data

Electric conductivity and magnetic susceptibility are standard physical parameters in environmental geophysics. Both can be obtained from electromagnetic induction (EMI) methods. In comparison to the extensive use on land, near surface applications of EMI methods in coastal and shelf regions are relatively novel. Recognizing the power of electromagnetic data to provide information, useful for many different fields in marine and coastal geoscience ranging from geotechnics to sediment dynamics, we developed the electromagnetic benthic profiler NERIDIS III at the MARUM – Center for Marine Environmental Sciences. This bottom-towed sled is equipped with a commercial, controlled-source EMI sensor modified for underwater applications. We use an inversion algorithm employing the in-phase and quadrature components of five operation frequencies to reconstruct the lateral distributions of electric conductivity and magnetic susceptibility of shallow marine deposits. Vertical distribution of conductivity is also recovered through 1-D inversion. Both parameter offers complementary information as electric conductivity is primarily considered as a measure of porosity (grain size, sorting), while magnetic susceptibility is used as a proxy for fine-grained terrigenous or iron mineral content and anthropogenic metallic contaminants.