Diagenesis of magnetic minerals in Recent haemipelagic sediments

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 ...

Section: Geochemical Gradients Sulfate Reduction and Dissolutionmentioning

confidence: 99%

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

Barton¹,

Omarzai²,

Alexer³

et al. 1993

“…Reductive dissolution of iron oxides in the organic-rich upper layers also can profoundly reduce the magnetic content and also change the magnetic grain size (Karlin and Levi, 1983). Diagenesis and authigenic changes also can produce additional post-depositional magnetic minerals (Henshaw and Merrill, 1980).…”

Section: Mineral Magnetismmentioning

confidence: 99%

“…We interpret the magnetic concentration peaks at depths of 21.7, 24.6, 67.2, and 76.2 mbsf as horizons that have escaped dissolution. As described by Karlin and Levi (1983), dissolution changes the grain size of the magnetic minerals as well as the concentration. Such grain size changes clearly are seen in the downcore log of the S and ARWX ratios (see Fig.…”

Section: Mineral Magnetismmentioning

confidence: 99%

Late Quaternary Paleoenvironmental Change on the Northeast Australian Margin as Evidenced in Oxygen Isotope Stratigraphy, Mineral Magnetism, and Sedimentology

Alexander¹,

Kroon²,

Thompson³

1993

To investigate late Quaternary paleoclimatic and paleoceanographic change in the sedimentary record, preserved on the Australian Continental Margin during the late Quaternary, core material was collected from Ocean Drilling Program, Leg 133, Site 819. An expanded sequence of late Quaternary, rhythmically bedded, predominantly hemipelagic sediments were recovered from Hole 819A. The foraminiferal δ 18 θ record preserved at Hole 819A suggests that the late Quaternary section is incomplete. Both benthic and planktonic δ 18 θ stratigraphies can be traced tentatively downcore to stage 9 at about 32.5 mbsf, where a major hiatus occurs. At this level, a slump detachment surface has been identified (Shipboard Scientific Party, 1991). This slump has removed marine oxygen isotope stages 10 to 13. Below 32.5 mbsf, continuous correlation can be achieved in the planktonic δ 18 θ curve, with existing deep-sea foraminiferal oxygen isotope stratigraphies from stage 14 through stage 28.The major hiatus at 32.5 mbsf marks the position of a significant change in the character of the sedimentation at Site 819. Sediments below 32.5 mbsf, relative to those above 32.5 mbsf, are characterized by less variation in mean particle size; lower percentages of carbonate content in the coarse fraction (>63 µm); a stronger relationship between the percentage of fine fraction and magnetic mineral concentration, and lower foraminiferal abundances. Above the hiatus, large fluctuations in mean particle size occurred, which have been interpreted to be the result of high foraminiferal abundances. Early highstands show high terrigenous influx in the fine fraction above the hiatus. This is the opposite of the general idea of high terrigenous influx during lowstands of sea level on siliciclastic dominated continental margins. We are far from understanding the origin of this material and further investigation will be required (see also Glenn et al., this volume).All our records, except the planktonic foraminiferal oxygen isotope record, indicate that the major hiatus marks the position of a significant change in the environment at Site 819. The planktonic foraminiferal δ 18 θ record suggests that environmental change occurred prior to the formation of the hiatus (i.e., near the Brunhes/Matuyama [B/M] boundary). The interval between the B/M boundary and the hiatus represents a transitional period between two different patterns of ocean circulation.Throughout most of the lower part of the sequence, Site 819 was at a shallow-water depth and local Oceanographic conditions were dominated by sluggish Subtropical Central Water (SCW) flow. However, near the B/M boundary, ocean circulation patterns intensified, reflecting a worldwide change in paleoenvironment. Enhanced ocean circulation patterns were possibly aided by tectonic subsidence. During this period Site 819 became progressively more under the influence of Antarctic Intermediate Water (AAIW), than SCW.In the upper part of the sequence at Hole 819 A, we see a continuation of the pattern of Oceanog...

“…Reductive and oxidative diagenesis affects the redox stability of iron-bearing minerals, and the resulting changes in magnetic properties can decouple any potential relationship between susceptibility and terrigenous percent (Kent and Lowrie, 1974; Froelich et al, 1979;Henshaw and Merrill, 1980;Karlin and Levi, 1983;Canfield and Berner, 1987). An additional factor that limits the use of susceptibility as a conservative indicator of terrigenous percent is the possibility of multiple terrigenous sediment sources.…”

Section: Magnetic Susceptibilitymentioning

confidence: 99%

A Rock-Magnetic Record of Monsoonal Dust Deposition to the Arabian Sea: Evidence for a Shift in the Mode of Deposition at 2.4 Ma

Menocal¹,

Bloemendal²,

King³

1991

A detailed record of eolian deposition from the Asian monsoon has been determined from measurements of magnetic susceptibility at Sites 721 and 722 located on an elevated ridge in the western Arabian Sea. Susceptibility was measured at 5-cm intervals (1 k.y.) on whole-core sections (15,000 measurements) and these data were used to construct complete composite records extending to 3.2 Ma. The composite records at Sites 721 and 722 are virtually identical, suggesting that both are complete and without local hiatuses. Bulk X-ray diffraction analyses indicate that (eolian) quartz and dolomite abundances are strongly covariant with the susceptibility data {r = 0.83, 0.70; « = 172). An almost perfect correlation (r = 0.98; n = 94) between susceptibility and terrigenous percent (determined by sequential carbonate, opal, and organic carbon extraction) indicates that susceptibility is a rapid and sensitive tracer of terrigenous fraction variations. Furthermore, terrigenous and biogenic flux calculations indicate that the observed susceptibility variations are indeed the result of terrigenous accumulation rate variations and not dilution by variations in biogenic input.In the frequency-domain, variance is concentrated at orbital periodicities for the entire 3.2 Ma sequence; however, there is a shift in the dominant periodicity at ca. 2.4 Ma. From 3.2 to 2.4 Ma, the susceptibility data vary almost purely at 23-19 k.y. periodicities and coherency with the precessional insolation forcing is high (0.89). Strong and coherent variance at the 23-19 k.y. band is maintained after 2.4 Ma but there is a dramatic increase in variance at the 41 k.y. periodicity corresponding to orbital obliquity. The timing of this shift coincides with the initiation of Northern Hemisphere glaciation and suggests a linkage between high-and low-latitude climate processes. General circulation model experiments and paleoclimatological evidence from northeast Africa suggest that the increase in 41 k.y. power after 2.4 Ma may be reflecting periodic increases in monsoon dust source area aridity due to the coeval expansion of the Eurasian and/or North American ice sheets, which varied predominantly at this periodicity.