Magnetic Stratigraphy and Characterization of Sediments from the Northeast Margin of Australia and Their Relationship to Environmental Change during the Quaternary

Abstract

This study is a synthesis of paleomagnetic and mineral magnetic results for Sites 819 through 823 of Ocean Drilling Program (ODP) Leg 133, which lie on a transect from the outer edge of the Great Barrier Reef (GBR) down the continental slope to the bottom of the Queensland Trough. Because of viscous remagnetization and pervasive overprinting, few reversal boundaries can be identified in these extremely high-resolution Quaternary sequences. Some of the magnetic instability, and the differences in the quality of the paleomagnetic signal among sites, can be explained in terms of the dissolution of primary iron oxides in the high near-surface geochemical gradients.Well-defined changes in magnetic properties, notably susceptibility, reflect responses to glacio-eustatic sea-level fluctuations and changes in slope sedimentation processes resulting from formation of the GBR. Susceptibility can be used to correlate between adjacent holes at a given site to an accuracy of about 20 cm. Among-site correlation of susceptibility is also possible for certain parts of the sequences and permits (tentative) extension of the reversal chronology. The reversal boundaries that can be identified are generally compatible with the calcareous nannofossil biostratigraphy and demonstrate a high level of biostratigraphic consistency among sites. A revised chronology based on an optimum match with the susceptibility stratigraphy is presented.Throughout most of the sequences there is a strong inverse correlation both between magnetic susceptibility and calcium carbonate content, and between susceptibility and δ 18 O. In the upper, post-GBR, sections a more complicated type of magnetic response occurs during glacial maxima and subsequent transgressions, resulting in a positive correlation between susceptibility and 6 O.Prior to and during formation of the outer-reef barrier, the sediments have relatively uniform magnetic properties showing multidomain behavior and displaying cyclic variations in susceptibility related to sea-level change. The susceptibility oscillations are controlled more by carbonate dilution than by variation in terrigenous influx. Establishment of the outer reef between 1.01 and 0.76 Ma restricted the supply of sediment to the slope, causing a four-fold reduction in sedimentation rates and a transition from prograding to aggrading seismic geometries (see other chapters in this volume). The Brunhes/Matuyama boundary and the end of the transition period mark a change to lower and more subdued susceptibility oscillations with higher carbonate contents. The major change in magnetic properties comes at about 0.4 Ma in the aggrading sequence, which contains prominent sharp susceptibility peaks associated with glacial cycles, with distinctive single-domain magnetite and mixed single-domain/superparamagnetic characteristics. Bacterial magnetite has been found in the sediments, particularly where there are high susceptibility peaks, but its importance has not yet been assessed. A possible explanation for the characteristic ...

Diagenetic sensitivity of paleoenvironmental proxies: A rock magnetic study of Australian continental margin sediments

“…The downhole changes in magnetic properties were interpreted in the context of glacioeustatic sea level fluctuations [i.e., Barton et al , 1993a, 1993b; Peerdeman and Davies , 1993]. The sharp susceptibility maxima correlate with the start of marine transgressions, following lowstands in sea level (high δ 18 O, glacial maxima).…”

“…Volumetrically significant amounts of the highcoercivity phase were found throughout most of the sequence; however, the surface zone and peaks 1 and 2 each showed predominance of magnetite. [14] The downhole changes in magnetic properties were interpreted in the context of glacioeustatic sea level fluctuations [i.e., Barton et al, 1993aBarton et al, , 1993b. The sharp susceptibility maxima correlate with the start of marine transgressions, following lowstands in sea level (high d 18 O, glacial maxima).…”

“…not have been explained by carbonate dilution only; a change in the terrigenous fraction assemblage thus was proposed. As the prevailing southeasterly wind pattern at the location of Site 820 precludes a large influx of wind-borne material from the continental interior [Thiede, 1979;Hesse, 1994b], the following complicated scenario for variations in the type of magnetic particles that reached the slope was proposed by Barton et al [1993b]. [16] At low sea level, fine-grained terrigenous sediments were trapped in river estuaries and deltas on the exposed shelf.…”

“…The variations in the susceptibility and NRM relate to fluctuation of sea level. Two prominent peaks at about 7 (peak 1) and 32 (peak 2) meters below surface (mbsf) correspond to marine transgressions [ Barton et al , 1993a, 1993b]; the sharp susceptibility maxima occur at the start of the transgressions. The susceptibility peaks are superimposed on a weakly magnetic background representative of the sea level highstand sedimentation.…”

“…However, the difference in the NRM intensity between the background and high‐magnetization zones could not have been explained by carbonate dilution only; a change in the terrigenous fraction assemblage thus was proposed. As the prevailing southeasterly wind pattern at the location of Site 820 precludes a large influx of wind‐borne material from the continental interior [ Thiede , 1979; Hesse , 1994b], the following complicated scenario for variations in the type of magnetic particles that reached the slope was proposed by Barton et al [1993b].…”