150 documents found in 212ms
# 141
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: Higher abundance of greigite during glacial intervals also coincides with small increases of the S content (Fig. 11B). Greigite levels in glacial sediments cannot be correlated between cores (Fig. 12), which suggests that greigite concentrations are driven by local processes. We suggest that faecal pellets could be a suitable microenvironment for sulphate reduction. And while greigite could potentially act as proxy for faecal pellets in glacial sediments, unfortunately, we cannot rely on this possible indicator since the greigite is very sensitive to onshore alterations after sampling (Snowball and Thompson, 1990).
# 142
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: Increased presence of greigite (high SIRM/κLF) coincides with maximum sulphur contents observed at the beginning of interglacial stages (Fig. 11A). At similar levels in another sediment core of Lake Baikal, Watanabe et al. (2004) observed pyrite mineralization. They attributed these pyrite-rich levels to mineralization at sediment/water interface under anoxic bottom water conditions. However, we prefer to interpret the greigite as a result of magnetite transformation when sulphate reduction occurs in the interglacial sediments. Peak sulphur contents would therefore be due to sulphur mineralization within the sediment and would not result from an enrichment of the sediment in sulphur at the sediment/water interface.
# 143
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: In interglacial sediments, the magnetic assemblages depict changes in the sedimentation rate, which are traced using the ratio of magnetite over hematite (S-ratio). At the beginning of interglacials, the sedimentation rate is constant with an assemblage magnetite+greigite (high S-ratio), and at the end of some interglacials, the sedimentation rate decreases with a predominance of hematite (low S-ratio).
# 144
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: In selected intervals, we measured titanium and iron contents in parallel to rock magnetic parameters (Fig. 9). Titanium content is a good reflection of detrital input since minerals containing titanium are not very sensitive to dissolution. Iron, however, is rather mobile and involved in the redox history of highly porous sediments: the spike of iron observed on top of the sedimentary column (Fig. 9A) marks the redox front. We observed a strong similarity between the titanium and HIRM curves: the detrital input decreases from the late glacial to the Holocene. In ancient sediments, HIRM and titanium display similar variations with high values in glacials and low values in interglacials (Fig. 9B).
# 145
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: No significant HIRM change is observed at the transition between oxidising and reducing conditions in the sediment (Fig. 9A). This implies that HIRM is not affected by redox conditions and further confirms that the “hard” magnetic mineral content is the best tracer of detrital input (Peck et al., 1994). On the other hand, the S-ratio seems to be related to the redox conditions in the sediment (see Section 7.2). The ARM has also to be considered with caution as it is mainly influenced by the ferrimagnetic contribution, which is itself influenced by post depositional processes. This is seen in Fig. 9 where ARM variations are partly influenced by S-ratio variations.
# 146
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: The day plot (Fig. 6B; Day et al., 1977) indicates that samples with S-ratios >0.95 plot rather in the single-domain (SD) to pseudosingle-domain (PSD) range. Samples with S-ratios between 0.9 and 0.95 instead, plot rather in the PSD to the multidomain (MD) range. Low S-ratio samples are not greatly dispersed and have ratios of Bcr/Bc and Mrs/Ms of 3.5 and 0.1, respectively.
# 147
Demory, Francois • Oberhänsli, Hedi • Nowaczyk, Norbert • Gottschalk, Matthias • Wirth, Richard • (et. al.)
Abstract: Downcore variations of rock magnetic parameters and simplified lithological description for the sedimentary sequence VER 98-1-14. Here, MIS are denoted by numbers in the lithological column. The black squares filled intervals mark occurrences of greigite characterised by a high magnetic susceptibility (κLF) in parts, with a low coercive mineral dominating the magnetic signal (S-ratio close to 1), a high SIRM/κLF, a strong loss of ARM intensity between the demagnetisations steps 50 and 65 mT and finally a deviation of the inclination of ARM. The dark grey intervals mark occurrences of magnetite dissolution, with a low S-ratio resulting from relative higher hematite content in the ferromagnetic components. The assignment of greigite and dissolved magnetite is based on subsequent interpretation. Magnetic susceptibility (κLF) vs. S-ratio for the sedimentary sequence VER 98-1-14 showing S-ratio gathered around 0.95 in glacial sediments and scattered from 0.7 to 1 in interglac
# 148
Demory, Francois • Nowaczyk, Norbert • Witt, Annette • Oberhänsli, Hedi
Abstract: Paleointensity versus age of all the sedimentary sequences of the present study, of the synthetic curve resulting from its compilation from other curves, and of the reference curve from ODP Site 984 (Channell, 1999). For the compilation, data have been averaged using a sliding window of 2 ka (the variance is marked by the grey shadow). Dashed lines show some of the correlations. The grey lines show the location of the low paleointensities related to geomagnetic excursions. Note that the lowest paleointensities in the time span of Blake are at c. 129 ka. (see Fig.11)
# 149
Demory, Francois • Nowaczyk, Norbert • Witt, Annette • Oberhänsli, Hedi
Abstract: We established a mastercurve “Baikal 200” of relative paleointensity, which represents a new synthetic paleomagnetic archive for Central Eurasia. The synthetic record is composed of mean values of the six records with respect to a sliding time window of 2 ka. This compilation has been restricted to the last 200 ka in order to maintain a representative population of points (between 2 and 68). However, we present this synthetic record together with individual records and the reference paleointensity curve (Fig. 11) for the following reasons: – Each relative paleointensity record has a different resolution (e.g., sedimentation rates in CON 01-605-3 are five times higher than in VER 98-1-14). During stack procedure, smoothing of the data had the effect of lowering the resolution of the paleomagnetic information. – This stack does not provide more information on timing of the geodynamo changes since the records are tuned to ODP Site 984.
# 150
Demory, Francois • Nowaczyk, Norbert • Witt, Annette • Oberhänsli, Hedi
Abstract: In order to characterise Lake Baikal sedimentary responses to global climatic changes that may be recorded in marine sediments, we compared our paleomagnetically dated climate-proxy record from Lake Baikal with benthic and plankontic δ18O curves of ODP Site 983, a site close to ODP Site 984. The neighbouring site was chosen for comparison because although the quality of the ODP Site 984 paleomagnetic record is high, its δ18O records are of lower quality than those of ODP Site 983. Synchronous paleomagnetic variations observed in ODP Sites 983 and 984 sediments (Fig. 10) show that the premise of our age model based on paleomagnetic correlation is identical, if the reference curve used for correlation is from ODP Site 983. We can, therefore, compare climatic records from ODP site 983 and Lake Baikal. The climatic proxy used for Lake Baikal sediment is the HIRM record since it displays the detrital input variations (Peck et al., 1994).
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