ALBERT

Description: Magnetic susceptibility (MS) is a powerful tool, which is being applied increasingly on sedimentary rocks to constrain stratigraphic correlations, or as a palaeo-environmental or palaeo-climatic tool. The origin of the magnetic minerals responsible for the variations in MS can be linked to various phenomena such as detrital inputs, pedogenesis, bacterial precipitation or diagenesis. Therefore, it is critical to improve our knowledge of the origin of the MS signal in order to apply it for correlations or as a proxy. Here, we present a synthesis of the techniques that can be applied to get a better understanding of the origin of the MS signal, through comparison with other palaeo-environmental proxies, through magnetic measurements or through dissolution and direct observation of the extracted minerals. We also propose an overview of the different techniques applied in order to use MS as a correlation tool, and we show various examples of successful applications of MS as a recorder of change in past sea-level and climate. We also present the main results and activities of the IGCP-580 project ‘Application of magnetic susceptibility as a palaeo-climatic proxy on Palaeozoic sedimentary rocks and characterization of the magnetic signal’.

Description: In the Eifel area (western Rheinisches Schiefergebirge), a shallow- to deep-subtidal sequence of mixed carbonates and siltstones around the Kačák Event Interval close to the Eifelian–Givetian stage boundary was studied. An overall transgressive trend is inferred by the microfacies evolution. The stratigraphic variations of magnetic susceptibility in carbonates and in shale intervals show an overall decreasing evolution towards the top, which fits well with the transgressive trend. In addition, carbon and oxygen isotopes, and major, trace and rare earth element (REE) analysis have been used to get a better understanding of palaeoenvironmental variations in a shallow-water realm in the late Eifelian ( kockelianus and ensensis conodont biozones): for example, the 13 C excursion and Ce anomaly are interpreted to be the local representation of the beginning of the Kačák Event Interval, which is also consistent with the stratigraphy and microfacies analyses.

Description: The Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility ( in ) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between in and the facies; the average in values were higher for the distal facies than for the proximal facies. The in profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average in values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between in and facies indicates at least a partly preserved primary in signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the in , we performed hysteresis measurements on selected samples. It appears that the in signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).

Description: Limestones at the Puech de la Suque Global Boundary Stratotype Section and Point (GSSP) of the Givetian–Frasnian boundary show a drastic change towards much higher magnetic susceptibility values in the Givetian rocks. Different rock magnetic parameters indicate that ferromagnetic minerals are the main controlling factor. The ferromagnetic fraction is composed of low- (magnetite-type) and high-coercivity (hematite and goethite) phases. Confirmed by the spectral reflectance, high coercivity minerals are fluctuating along the section with a higher abundance in the basal Frasnian. These phases may be of secondary origin and produced during burial stage. The magnetite-type phase contains two different grain-size populations. The identified that Stable Single-domain/Superparamagnetic (SSD/SP) particles are of diagenetic origin and their amount decreases slightly upwards. A second group of magnetite grains correspond to coarse-grained particles identified using the squareness v. coercive force plot. It is tentatively suggested here that these particles present throughout the section are of primary origin. On the contrary, the paramagnetic minerals underwent a clear diagenetic overprinting and may represent secondary minerals. The presence of a primary ferromagnetic carrier allowed the use of spectral analyses, leading to the detection of spectral peaks at 1.25 and 4 cycles/m, which can be interpreted as the result of 405-kyr and 100-kyr eccentricity forcing, respectively.