GP21B-01
Late Holocene Sediment Study From Santa María del Oro Crater Lake, Nayarit, México, Using Environmental Magnetism
The lake is located near the Pacific coast of Mexico, at the western end of the Trans Mexican Volcanic Belt. It is a deep lake (ca. 65 m) with steep sides and only a small bay (Agua Caliente) has shallower water (ca. 12 m). Four parallel cores between 4 and 9 m long were recovered in March 2002 from this shallower area. Sediments are characterized by alternated laminations (few millimeters to 2 cm) of sand, brown silt, green silt, reddish silt, ochre silt, and peat. The 14-C dated sequence spans the last ca. 2,600 yrs. Given this age, it is possible that each set of laminations represent annual sedimentation cycles. The record is a potential high- resolution archive of environmental and climatic variability for western Mexico for late Holocene. Magnetic measurements of susceptibility along the cores show a high variability in the concentration of magnetic mineralogy. Different magnetic and non-magnetic properties show two sets of facies in relation to its magnetic mineralogy; one group composed by sand, brown silt, green silt and peat has the magnetite and Ti-magnetite as the principal magnetic phase; the second group, composed by reddish and ochre silt, has a low Ti magnetite component and siderite, as the principal paramagnetic component. The effects of climatic variations such as the drought occurred in the archeological Classic period (100 - 900 dC), the Medieval Warm Period (950 - 1350 dC), the Little Ice Age (1400 - 1800 dC), and the droughts over the last 700 years, documented in sites along central Mexico, are recognized in the magnetic mineralogy of Santa Maria del Oro.
GP21B-02
Magnetic Pattern of Luvisol Sequences From Mexico and Russia: An Alternative Analysis of Soil Studies
Recently, soil has been consolidated like a worthy and complementary source of information to environmental and paleo-climatic reconstruction research. The study of magnetic mineralogy present in soil horizons play an important role to understand a lot of complex process which to allow obtain information about environment condition present during genesis and evolution of soil. The main aim of this study is to compare soils with different parent material, as well as to obtain a magnetic sign from soils profile which could be used to determinate the responsible of pedogenic process of their magnetic behavior of each horizon, and their possible relationship with environmental condition. Study carried out of two complete profiles of buried interglacial Luvisols, one formed in loess in Russia (Alexandrovsky quarry, AQ) and the other in volcaniclastics in Mexico (Barranca Tlalpan, BT). In the AQ, the magnetic susceptibility is enhanced in the paleosol compared to parent material. In the BT sequence, susceptibility enhancement is absent in the soil profile. Increase of fine-grained magnetic components in the soil is attributed to neoformed minerals. However, this process cannot compensate for the loss of lithogenic magnetic minerals in any of the genetic horizons, and the resulting trend is susceptibility depletion in the whole soil profile. The pedogenic environment of eluvial horizons in both Luvisols is destructive to all magnetic components, both primary and secondary. Higher concentrations of antiferromagnetic components (hematite and goethite) found in E horizons are related to redoximorphic processes.
GP21B-03
Deposition, Diagenetic, or Bacterial Origin of Magnetite in Sediment Drifts From the Western Antarctic Peninsula
A 607 meter sediment core from the western Antarctic Peninsula was collected during Ocean Drilling Program (ODP) Leg 178 at Site 1096. An abrupt shift, first recognized during the construction of the geomagnetic reversal stratigraphy, is observed in the magnetic mineral assemblage in core 1096A at 18 meters below seafloor (mbsf). This magnetic shift led us to question the fidelity of the geomagnetic reversal record. We evaluated the magnetic mineral assemblage using rock magnetic and electron microscopy techniques. Magnetic susceptibility showed variable concentration of magnetic material with depth, but not an abrupt change in concentration at 18 mbsf. Hysteresis data reveal a grain size shift at 18 mbsf. Magnetic particle size measurements show that with increasing depth the magnetic grain size increases. One possible explanation for the grain size shift is the presence of bacterial magnetite above 18 mbsf. However, TEM analysis did not conclusively prove the presence of magnetosomes. TEM images show grains in the 20-60 nm size range, but these grains were not in chains and they do not have the typical morphologies associated with bacterial magnetite. It is possible that these ultra fine grains are authigenic. Dissolution of these ultra fine magnetic particles could cause the grain size shift, with the smaller grains dissolving below18 mbsf. Temperature dependent FORC analyses are in progress to further characterize the particle size distribution above and below 18 mbsf. Curie temperature analyses indicate that magnetite is present above and below 18 mbsf. A magnetic iron sulfide with a Curie temperature of 350° C is present above 18 mbsf. X-ray microanalysis results agree with Curie temperature analyses, and show the presence of both Ti-poor iron oxides and iron sulfides in the magnetic extract. The presence of magnetic iron sulfides above 18 mbsf suggests iron-sulfur diagenesis is occurring with pyrrhotite preserved above 18 mbsf, and non-magnetic pyrite present below 18 mbsf. This could explain why the bottom of the core has a weak-amplitude natural remnant magnetization and a noisy geomagnetic reversal record.
GP21B-04
Contributions of Sediment Provenance and Sediment Diagenesis to Western Antarctic Peninsula Magnetic Proxy Records
We examine the role of sediment provenance, iron-sulfur diagenesis, and iron-silica diagenesis in shaping environmental magnetic proxy records in Antarctic biosiliceous sediments. A 20-m sediment core from the Gerlache Drift, western Antarctic Peninsula, contains an ultra-high resolution record of environmental change spanning the last 4000 years. Magnetic susceptibility and several remanence parameters appear to record regularly repeating patterns in biogenic and terrigenous sedimentation, with a major shift at the Middle Holocene to Late Holocene transition. This shift is manifested as an abrupt downcore decrease in magnetic susceptibility, concomitant with an increase in biogenic silica. However, this record does not show the typical pattern associated with iron-sulfur diagenesis, i.e., selective dissolution of fine particles and coarsening of the residual assemblage. We observe multidomain magnetite above the transition, and PSD-magnetite plus hematite below the transition, which suggests a change in sediment provenance. However, we observe a change in clay mineralogy downcore that is consistent with predictions for iron-silica diagenesis. In order to investigate the role of changing sediment sources vs. post-depositional diagenesis, we have examined bulk sediment geochemistry, total organic carbon and total sulfur content to identify possible diagenetic fronts, and bulk mineralogic assemblages and lithic clast composition to examine sediment sources. In addition, we have examined the lithologic and geochemical characteristics of surface sediment samples and diamict samples from a North- South suite of western AP fjords and inner shelf basins, which we use to fingerprint potential sediment source regions. These tools may be able to distinguish between sediment supplied from local Antarctic Peninsula geologic terranes, Bransfield Basin volcanics entrained in the nearshore countercurrent, or West Antarctic sediment supplied by the Antarctic Circumpolar Current.
GP21B-05
Multi-proxy analyses of kasten cores and surface samples from the Joinville-d'Urville Trough, Northeastern Antarctic Peninsula: Results and correlation with the Western Antarctic Peninsula
Two sediment cores, LMG04-04 KC16 and KC3, an expanded and a condensed section, respectively, were recovered from a sediment drift within the Joinville-d'Urville Trough on the northeastern Antarctic Peninsula (AP), in approximately 750 m water depth. Both cores show a two-part magnetic susceptibility profile: the upper section shows regularly spaced highs and lows, followed by a large (>50%) amplitude drop. The susceptibility profiles of these two cores are similar to those found on the western side of the AP. The purpose of this study is to determine whether the same processes drive magnetic susceptibility on both sides of the AP. Major element geochemical analyses show that susceptibility is positively correlated with Ca, Al, Mn, Mg, and Ti, and is inversely correlated with Fe/Ti. Paleoproductivity, as indicated by Si/Al and Ba/Al ratios, has no consistent relationship with susceptibility. These parameters alternately show direct, inverse, or no correlation. P/Al ratios were uniform downcore. The lack of consistent correlation between Si/Al and susceptibility may indicate that siliceous phytoplankton productivity is not a main driving factor, although there is a weak positive correlation between % benthic diatoms and susceptibility peaks. Susceptibility peaks and troughs in KC16 and KC3 were sampled for low-temperature magnetic susceptibility. All samples exhibit a magnetic order/disorder transition at ~117 K, which we interpret as the magnetite Verwey transition. The Verwey transition peak is less prominent below the susceptibility drop in KC3. Particle size analyses of KC16 and KC3 show a bimodal distribution of grain sizes. Medium to fine silt-sized particles dominate in both cores, as well as in surface samples collected across the sediment drift. Both cores contain ~20-54% very fine sand above the susceptibility drop, but percentages in the fine sand region and larger are complicated by the presence of macroalgal detritus. Unlike the western AP susceptibility profiles, hysteresis measurements from KC16 show no relationship between magnetic grain size and susceptibility. We speculate that paramagnetic detrital minerals may be an important driver of the susceptibility profile on the east side, where biogenic silica is not a significant component of the sediment. XRD and SEM analyses are in progress in order to study the bulk mineralogic assemblage and investigate the source of the inverse relationship between susceptibility and Fe/Ti.