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Quarternary and Tertiary |
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Whole-core (WC) logging of low-field magnetic susceptibility (MS) is an extremely simple, rapid and non-destructive method of acquiring a quantifiable, lithologically related signal which is ideally suited to the task of correlating between deep-sea sediment sequences recovered from related holes at ODP sites. This is particularly useful for reconstructing uninterrupted composite sequences by splicing together stratigraphically continuous subsections of offset cores from adjacent holes. Continuity of sequence is established by pattern-matching the WCMS profiles of each hole, which are based on measurements made at intervals of only a few centimetres. It is essential that interhole correlation is effected with the highest possible lithostratigraphic resolution if composite sequences are to be used, for example, in studies of Milankovitch-band cyclicity in palaeoceanographic records, involving spectral analysis of down-core data.
In addition to identifying coring irregularities such as unrecorded gaps in the recovered sequence between cores, or repetition of sequence due to repenetration of the corer, WCMS profiling can be used to identify turbidite horizons and also intervals contaminated by ferrous metal artifacts from drilling (e.g. pipe rust). The presence of contaminants often indicates that the sequence contains horizons of slumped, or mechanically reworked sediment washed-in from uphole. Thus identified, turbidites and contaminated intervals can be avoided during sampling, and eliminated from stratigraphic reconstructions or accumulation rate estimates. The only category of deep-sea sediment lithology which exhibits WCMS values similar to those of contaminated intervals, is that of basic volcanic ash-rich horizons. Accordingly, this makes WCMS logging a potentially valuable tool in studies of marine tephrostratigraphy.
Inter-regional correlation between the WCMS profiles for holes drilled at different ODP sites is also possible in many instances, especially when lithological variations at each site are controlled by large scale palaeoceanographic, or global (i.e. orbitally forced) palaeoclimatic changes. In such circumstances, WCMS may also be used as a proxy palaeoclimatic indicator, duly assisting more conventional microfossil and isotopic methods of climato-stratigraphical zonation of the sequences recovered.
