T. von Dobeneck

Geomagnetic Events and Relative Paleointensity Records — Clues to High-Resolution Paleomagnetic Chronostratigraphies of Late Quaternary Marine Sediments?

Magnetochronostratigraphies of marine sediment series are generally determined by correlating the polarity pattern derived from their natural remanent magnetization (NRM) to a geomagnetic polarity time scale (GPTS) inferred from dated marine magnetic anomaly lineations. With regard to the Quaternary, this conventional method can only provide a poor resolution as even the latest available GPTS comprises but a few reversals for this time interval. Numerous recent paleomagnetic studies have revealed supplementary prominent NRM features in marine sedimentary deposits, namely (1) recurrent abrupt changes in stable remanent directions documenting a succession of several geomagnetic events within the Brunhes Chron and (2) conspicuous variations in NRM intensity normalized to the concentration of magnetic minerals (the so-called relative paleointensity) which were attributed to pronounced fluctuations in Earth’s paleofield strength. Detailed records of both phenomena are presented here for late Quaternary sediments recovered on the Ceara Rise in the western equatorial Atlantic. Based on an oxygen isotope age model, that was further refined by orbital tuning and sub-Milankovitch correlations of continuous magnetic susceptibility logs, the polarity and normalized intensity time series indicate repeated full reversals of the Earth’s magnetic field configuration during the last about 380 kyr associated with distinct changes in relative paleointensity. The data sets are discussed as primary proxies to unravel the geomagnetic field history and reviewed in their perspectives and deficiencies to develop a high-resolution magnetostratigraphic framework for the late Quaternary. They also substantially contribute to the basic understanding of NRM acquisition processes in marine sediments summarized in a model outlining their temporal and spatial variability as function of sediment lithology.

Using Rock Magnetic Proxy Records for Orbital Tuning and Extended Time Series Analyses into the Super- and Sub-Milankovitch Bands

High-resolution rock magnetic proxy records of marine sediments, in particular magnetic susceptibility logs, delineate variations of sediment lithology and mirror climatic and oceanographic changes of different duration. Most commonly, Milankovitch cyclicity resulting from orbital forcing of carbonate dissolution and terrigenous sedimentation prevails. Extracted by bandpass filtering, these signal components can serve for multiple core correlation, cyclostratigraphic analyses and orbital tuning. Phase relations between astronomical obliquity and precession cycles and their equivalents in rock magnetic records depend on regional sedimentological settings. Two case studies are developed to demonstrate specific aims, strategies, strengths and restrictions of rock magnetic time series analyses and their extension into the super- and sub-Milankovitch bands. In the first example, twelve Pleistocene susceptibility records spanning the oligotrophic subtropical South Atlantic (SUSAS stack) were successfully tuned to obliquity and precession despite their low average sedimentation rates < 1 cm/kyr. Multiple bandpass filtering and evolutionary spectral analysis reveal two major base line shifts at around 0.95 and 0.6 Ma and various superstructures (e.g., amplitude modulations) of orbital proxy response. Compared to analogous analyses of an adapted astronomical target curve, converse residue patterns in the 41 kyr and 100 kyr bands indicate that Pleistocene ice age cycles were mainly triggered by obliquity before 1.25 Ma and from 1.05 to 0.7 Ma, while eccentricity modulation of precession predominated between 1.25 and 1.05 Ma and during the last 0.6 Ma. In the second example, eight susceptibility records from the western equatorial Atlantic Ceara Rise (CEARIS stack) were tuned to a lagged precession index signal. Subsequently, a high resolution core correlation scheme was established on basis of their coherent high-frequency (< 15 kyr) signal patterns. Basic harmonics and intermodulation frequencies of obliquity, and, predominately, precession resulting from nonlinear proxy response were detected by sub-Milankovitch spectral and bispectral analyses. Twin susceptibility peaks corresponding to a tropical double precession cycle appear even in the unfiltered records. Millennial signal variations (< 7 kyr) seem to coincide with Bond- and Dansgaard-Oeschger cycles.

The Magnetic View on the Marine Paleoenvironment: Parameters, Techniques and Potentials of Rock Magnetic Studies as a Key to Paleoclimatic and Paleoceanographic Changes

The eminent potential of Environmental Magnetism analytical techniques to delineate depositional regimes and climatic changes in the marine realm is reviewed and illustrated with results of three individual studies of sediment series from the South Atlantic Ocean. Rock magnetic properties related to the mineralogy, concentration, domain state and hence grain-size of the magnetic mineral assemblage are explained on grounds of physical principles and discussed as proxy parameters for terrigenous particle fluxes, bioproductivity and diagenetic redox conditions. With cluster analysis of rock magnetic parameters determined for a large collection of surface samples, the regional characteristics of recent depositional environments in the equatorial South Atlantic are established. Notably, the different input mechanisms of terrigenous material via fluvial transport by the Amazon and Congo Rivers at the African and South American continental margins are as clearly identified as the eolian transport from the Sahara and Sahel Zone into the central and eastern equatorial South Atlantic. Based on a detailed susceptibility log and measurements of various laboratory remanences, high-coercive hematite components and different magnetite grain-size fractions could quantitatively be discriminated in a late Quaternary sediment sequence from the central equatorial Atlantic. The data sets allow to assess variations in eolian influx from the Saharan dust plume and several redox events during the last 400 kyr can be recognized. While biogenic magnetite is generally of minor importance in pelagic deposits, it may completely dominate the sediment magnetic properties in high productive areas. An intense primary biologic productivity in surface waters of the Benguela upwelling center supplies a high flux of organic matter to the sea floor at the continental slope off Namibia and causes reducing conditions in the sediment column. Resulting strong diagenetic effects on the biomagnetic mineral component are traced in detail by high-resolution rock magnetic analyses and transmission electron micrographs.

Integrated Rock Magnetic and Geochemical Quantification of Redoxomorphic Iron Mineral Diagenesis in Late Quaternary Sediments from the Equatorial Atlantic

Rock magnetic and geochemical data logged by fast, non-destructive X-ray fluorescence and susceptibility half core scanning techniques have been combined to create high-resolution records of redoxomorphic iron mineral diagenesis in suboxic marine sediments. The great potential of this approach and advantage to standard single sample methods is demonstrated on two Late Quaternary sequences from the central Equatorial Atlantic (GeoB 2908–7 and 4317–2). Reducti ve dissolution of ferric minerals, most prominently magnetite (Fe3O4) and hematite (Fe2O43), induced by organic carbon degradation is shown to represent a gradual, mineral- and grain-size selective process. Proportionality of Fe, Ti and magnetite concentrations in the unaltered sections lead us to define proxy parameters for magnetite depletion (Fe/κnd) below and precipitation (κnd/Ti) above the modem and numerous fossil redox boundaries, while iron relocation was detected on basis of the Fe/Ti ratio. By calibrating all three ratios internally, we reconstruct and quantify primary deposition and secondary change of both, magnetite and total Fe profiles. Fine-scaled Corg variations (0.1 to 0.6%) and susceptibility losses (up to 200 · 10−6 SI) show high signal resemblance and appear to be equivalent signatures of cyclic productivity pulses in the study area. Some minor suboxic events are still expressed in the rock magnetic proxy signal, but are not accompanied by residual Corg enrichments.

Late Quaternary Terrigenous Sedimentation in the Western Equatorial Atlantic South American versus African Provenance Discriminated by Magnetic Mineral Analysis

Magnetic mineral accumulation at the Ceara Rise has been studied with the aim to discriminate and reconstruct fluvial South American and eolian African terrigenous fluxes to the late Quaternary western Equatorial Atlantic. Seven sediment series recovered along two bathymetric transects were investigated with standard environmental magnetic techniques. Climatically controlled fluctuations in continental detrital discharge and marine biogenic carbonate fluxes strongly modulate the susceptibility records. Their coherent precessional and higher-frequent signal components could be used to establish a high-resolution age framework for these sediments. According to a partial susceptibility analysis, on average 79 % of the susceptibility signal originates from magnetite of different grain size, 13 % from hematite and 8 % from paramagnetic matrix compounds. In terms of absolute concentrations this implies that hematite is almost twenty times more abundant than magnetite, because of its orders of magnitude lower intrinsic susceptibility. The longitudinal gradients of their respective accumulation rates document a delivery from two major sources characterized by largely different magnetite to hematite ratios (about 1:12 versus 1:50). A mixing model of this scenario provided detailed insight into the past variability of the separate magnetic mineral fluxes and their most probable provenance. Overall about 56 % of hematite and 84 % of magnetite were transported in the Amazon fluvial load. Their accumulation is closely related to sea level changes, reaching highest (lowest) rates, when most South American shelf areas fell dry (were flooded) before and after Termination I and II. Hematite and magnetite of African provenance, 44 and 16 %, respectively, follow a distinctly different accumulation pattern with prominent maxima during cold intervals of glacial periods. By statistically linking these trace minerals to total lithogenic fluxes, we find that during the last 200 kyr, on average 79 % of total terrigenous material in the Ceara Rise area originates from South American sources in the Amazon River catchment, while African dust sources contributed 21 %.