SS: GP21B
LO: MC:Hall D
DA: Tuesday
HR: 0800h
SN: Paleogeomagnetism From Marine and Continental Drilling II Posters
PR: J Stoner, College of Oceanic and Atmospheric Sciences, Oregon State UIniversity; G Acton, University of California, Davis
MN: 2008 Fall Meeting

HR: 16:00h
AN: GP14A-01 INVITED
TI: A Tour of Paleomagnetic Results From Ocean Drilling
AU: * Acton, G
EM: acton@geology.ucdavis.edu
AF: University of California, Davis, Department of Geology, One Shields Avenue, Davis, CA 95616, United States
AB: After over a decade of working on the paleomagnetism of ocean drill cores, I have had the opportunity to collaborate with many other magnetists as we have tried to extract the paleomagnetic and rock magnetic information stored in the sediments and rocks cored from the ocean basins. In many instances, the cores have acted as high-fidelity magnetic recorders, providing detailed chronstratigraphies and exceptionally detailed records of geomagnetic field behavior, including the occurrence of multiple Brunhes-age excursions and complex changes in the direction and intensity of the field across geomagnetic reversals. They have also retained records of the long-term paleomagnetic direction at numerous sites around the globe, which have been used to constrain plate and hotspot motions. And, although one needs only to tow a magnetometer across the seafloor to map the marine magnetic lineations and date the ocean floor, it is the drill cores that allow us to examine the origin of the marine magnetic anomaly signal and to learn more about magmatic, tectonic, and alteration processes at mid-ocean ridges. In other instances, the paleomagnetic record has been less robust than expected owing to a myriad of natural processes, which provide clues about the environment, microbial activity, tectonic deformation, and fluid flow, or owing to drilling and handling artifacts, which are very important to recognize if the paleomagnetic record is to be interpreted accurately. I will take you through a tour of some of these results and suggest that the paleomagnetic journey has an important path to follow, with a focus on coring high and ultra-high resolution geomagnetic records. The records would become part of a constellation of paleomagnetic stations. Comparable to the modern magnetic observatories that provide data for modeling the present-day geomagnetic field, these stations will provide both the paleomagnetic directions and paleointensities necessary to construct spherical harmonic models of the geomagnetic field back in time.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1521 Paleointensity
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 3036 Ocean drilling
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 13:55h
AN: GP33A-02 INVITED
TI: Cryptochrons and Short Subchrons in the Marine Sedimentary Record
AU: * Evans, H F
EM: helen@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States
AU: Acton, G D
EM: acton@geology.ucdavis.edu
AF: Department of Geology, University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
AB: Cryptochrons are short geomagnetic events (less than about 30 k.y. in duration) that are caused by large changes in the direction and/or intensity of the geomagnetic field. As such, they may equate to paleointensity fluctuations, geomagnetic excursions, or full polarity reversals that have durations less than that of a subchron. Their existence was first recognized by Cande and Kent (1992) by the "tiny wiggles" that they caused in marine magnetic anomaly profiles. Since then, our ability to document and study these and other forms of short-term geomagnetic field variability has been greatly enhanced by the recovery of long, continuous sedimentary sections from the World's oceans. The Ocean Drilling Program (ODP) and its successor, the Integrated Ocean Drilling Program (IODP), have played a crucial role in this effort, not only in acquiring the necessary cores but also in supporting the scientific community as they have developed new sampling strategies, stratigraphic analysis tools, and instrumentation. In particular, coring multiple holes at a site has allowed the construction of composite sections that yield stratigraphically complete records. Continuous, high-resolution measurements, made possible by automated-track systems, has provided the necessary quantity and quality of data to resolve the age, duration, and geomagnetic characteristics of several of the cryptochrons that had previously only be observed in marine magnetic anomaly profiles. We will discuss the geomagnetic changes associated with eight cryptochrons that occur in the Miocene and Plio- Pleistocene. We will also examine intervals in the Paleocene, Eocene, and Oligocene where cryptochrons were expected based on marine magnetic anomaly profiles but where marine sedimentary sections fail to record any notable changes in direction. This may indicate that the events are too short-lived to be recorded in the lower sedimentation rate records from these older time periods or that the cryptochrons are due only to fluctuations in the intensity of the field and not directional changes.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1530 Rapid time variations
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 14:40h
AN: GP43C-07
TI: Vibrating Sample Magnetometer (VSM) Paleointensity Determinations
AU: Petronotis, K
EM: petronotis@iodp.tamu.edu
AF: Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station, TX 77845, United States
AU: * Acton, G
EM: acton@geology.ucdavis.edu
AF: University of California - Davis, Department of Geology, One Shields Avenue, Davis, CA 95616, United States
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: University of Hawaii, Hawaii Institute of Geophysics and Planetology, 1680 East-West Road, POST 602, Honolulu, HI 96822, United States
AB: Determining the absolute intensity of Earth's magnetic field from rocks has proven to be fraught with pitfalls that paleomagnetists have sought to overcome with carefully crafted but often tediously time-consuming measurement protocols. Without at least some form of protocol, little confidence is given to a paleointensity determination, and even with rigorous protocols, doubt can remain as to the validity of the determination. Most protocols involve heating, cooling, and then measuring the remanence of a sample in and out of an applied magnetic field and then repeating this process at progressively higher temperatures. Completion of an experiment takes considerable time for a reasonable number of temperature steps and requires a large amount of manual sample manipulation. We have investigated whether it is possible to improve the speed while increasing the number of thermal steps by about an order of magnitude by using a vibrating sample magnetometer (VSM). In our test, we use a MicroMag 3900 VSM produced by Princeton Measurements Corporation. This VSM includes a furnace capable of heating samples to 800°C and an automated rotating head that allows the magnetic moment of a sample to be measured in multiple orientations within a plane. Although this instrument was not designed for paleointensity determinations, it has several features that facilitate them. The instrument is sensitive enough to measure basalt samples that weigh only a few milligrams to a few tens of milligrams, measurements can be made while the sample is being heated, the sample does not need to be manually manipulated once the experiment starts, heating or cooling of the small samples can be accomplished quite rapidly (from about a minute to a few minutes), and the sample is bathed in helium gas while being heated, which helps reduce alteration. So far, we have obtained highly accurate and precise VSM paleointensity determinations on basalt samples in experiments that were completed in only a few hours. We will discuss some of the advantages and disadvantages of the VSM paleointensity method and show results obtained for the 1960 Hawaii basalt flow and for oceanic basalt samples from ODP Leg 200 (Site 1224) and IODP Expedition 206 (Site 1256).
UR: http://paleomag.ucdavis.edu/paleoint-test.html
DE: 1521 Paleointensity
DE: 1594 Instruments and techniques
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3036 Ocean drilling
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 17:50h
AN: GP14A-10
TI: Magnetic Properties of Fast Spreading Ocean Crust Drilled by (I)ODP Hole 1256D Into the Gabbro Zone Revealed by Fuzzy c-Means Cluster Analysis
AU: * Dekkers, M J
EM: dekkers@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, Budapestlaan 17, Utrecht, 3584 CD, Netherlands
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, University of Hawaii at Manoa, 1680 East West Rd Honolulu, Honolulu, HI 96822, United States
AU: Krasa, D
EM: david.krasa@ed.ac.uk
AF: School of GeoSciences, University of Edinburgh, West Main Road, Edinburgh, EH9 3JW, United Kingdom
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: Department of Geology, University of California, One Shields Ave., Davis, CA 95616, United States
AB: Despite its importance for plate tectonics the nature of ocean crust is relatively poorly constrained primarily related to the extreme difficulty of direct sampling. Here we report on a magnetic mineral analysis of the entire oceanic crust down to in the gabbro zone at Site 1256 (~15Ma) that was drilled during three (I)ODP Legs (206, 309 and 312) in a super-fast spreading zone in the Pacific Ocean on the Cocos Plate. Mineral magnetically, sheet flows and massive basalts are characterized by low Mrs/Ms ratios of ~0.1 with sheet flows tending to be slightly higher. Sheeted dikes are typified by Mrs/Ms ratios between 0.1 and 0.2 and the underlying gabbros have < 0.1. Sheeted dikes have highest Bc values of ~15 mT, gabbros are 5-10 mT, very similar to sheet flows and massive basalts. Bcr/Bc ratios increase with depth in the sheet flows and massive basalts from ~1.5 to ~2.0. Sheeted dykes are remarkable constant at ~2.0 while gabbros range 2.0-4.0 for the coercivity ratio. Curie temperatures appeared to be very distinctive, gradually increasing with depth from 200-300°C in the top of the sheet flows and massive basalts to 500°C at the base of the extrusive rock package. Sheeted dikes and gabbros have Curie temperatures of 580°C. Fuzzy c-means cluster analysis (333 samples with 5 variables: Mrs/Ms, Bcr/Bc, Bc, low-field susceptibility, and Curie temperature) enables a distinction between gabbros, sheeted dikes and upper crustal rocks on the basis of their combined magnetic properties that is very consistent with lithological observation. Individual magnetic parameters were less discriminative. On the non-linear mapping (NLM) plot sheeted dikes and gabbros on the one hand and sheet flows and massive basalts on the other, follow different trends. Quenched glass samples appear to have a very distinct position on this plot. Most clusters show signs of thermochemical overprinting leading to impure TRMs so they are less suited for paleointensity analysis. The most pristine samples have fairly low Mrs/Ms ratios of ~0.1-0.15 and Bcr/Bc ratios of ~1.6-2.0.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
DE: 3036 Ocean drilling
DE: 5109 Magnetic and electrical properties (0925)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 17:30h
AN: GP14A-08
TI: Rock Magnetic Characterization Through an Intact Sequence of Oceanic Crust, IODP Hole 1256D
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, University of Hawaii at Manoa, 1680 East West Road, Honolulu, HI 96822, United States
AU: Krasa, D
EM: david.krasa@ed.ac.uk
AF: School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JW, United Kingdom
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: Department of Geology, University of California, Davis, Davis, CA 95616, United States
AU: Rodriguez Durand, S
EM: durand@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Geochemistry Bldg., Palisades, NY 10964, United States
AB: One goal of drilling a complete oceanic crust section is to determine the source of marine magnetic anomalies. For crust generated by fast seafloor spreading, is the signal dominated by the upper extrusive layer, do the sheeted dikes play a role, what role do the gabbros play relative to slow spreading centers, and what is the timing of acquisition of the magnetization? To address these questions, we are conducting a comprehensive set of rock magnetic and paleomagnetic measurements that extend through the intervals drilled on Leg 206 and Expeditions 309 and 312. Recent drilling in the Eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate (i.e. up to 200mm/year full rate) on the Cocos-Pacific plate boundary between 19 and 12 million years ago. Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Continuous downhole variations in magnetic grain size, coercivity, mass-normalized susceptibility, Curie temperatures, and composition have been mapped. Compositionally, we have found that the iron oxides vary from being titanium-rich (TM60) to titanium-poor magnetite as determined semi-quantitatively from Curie temperature analyses. Magnetic grain sizes vary from few Single Domain (SD), to the majority of them being Pseudo Single Domain (PSD) and some on the Multi Domain (MD) area of the Day diagram. The low-Ti magnetite or stoichiometric magnetite is present mainly in the lowest part of the section and is associated with higher Curie temperatures (550°C to near 580°C) and higher coercivities than in the extrusive basalts. Skeletal titanomagnetites with varying degrees of alteration is the most common magnetic mineral throughout the section and is often bordered by large iron sulfide grains. Last but not least, absolute paleointensity experiments have been determined on several samples, although the success rate is low as has been found in other studies of oceanic basalts.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 0800h
AN: C21B-0542
TI: Dating Antarctic climatic and tectonic events of the Neogene – Age models from the ANDRILL 1B and 2A cores
AU: Wilson, G
EM: gary.wilson@otago.ac.nz
AF: Geology Department, University of Otago, PO Box 56, Dunedin, 9001, New Zealand
AU: * Levy, R
EM: rlevy2@unlnotes.unl.edu
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: Geology Department, University of California-Davis, One Shields Ave, Davis, CA 95616, United States
AU: Naish, T
EM: Tim.Naish@vuw.ac.nz
AF: Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington, 6140, New Zealand
AU: Harwood, D
EM: dharwood1@unl.edu
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AU: Florindo, F
EM: florindo@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, I- 00143, Italy
AU: Powell, R
EM: ross@geol.niu.edu
AF: Department of Geology and Environmental Geosciences, Northern Illinois University, 312 Davis Hall, Normal Road, DeKalb, IL 60115-2854, United States
AU: MIS Science Team, A
EM: andrill@andrill.org
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AU: SMS Science Team, A
EM: andrill@andrill.org
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AB: A network of seismic surveys in McMurdo Sound provides an image of the tectonic, glacial, and sea-level controls on accumulation of Cenozoic strata in the West Antarctic Rift. Integration with previously recovered drill cores revealed a younger Neogene succession, which was the target of drilling from floating ice platforms under the ANDRILL program in the austral summers of 2006 and 2007. Continuously recovered core from the ANDRILL 1B and 2A drillholes provides key paleoenvironmental data regarding climatic variation and ice volume fluctuation of the Antarctic Ice Sheets through the Neogene. Chronostratigraphic data available from the drillcores includes diatom and nannofossil biostratigraphy, magnetic polarity stratigraphy, 40Ar/39Ar ages on numerous ashes from the McMurdo Volcanic Complex, and strontium dates on carbonate material. Integration and optimization of the currently available data provides a robust age model for the Plio- Pleistocene (upper 600m) of the AND-1B drillcore and the middle and lower Miocene (224-1139m) interval of the AND-2A drillcore. The Plio-Pleistocene record is punctuated by multiple hiatuses that correlate with climate (ice sheet and sea level fluctuations) events and account for approximately half of the time spanned by the record. Despite these hiatuses, the distribution of chronostratigraphic data still enable the identification of orbital influence in remaining strata. The lower 600m of the AND-1B extends well down into the Miocene as constrained by a 40Ar/39Ar age of approximately 13.8 Ma towards the base of the core. The upper 300m interval of the AND-2A core contains punctuated middle Miocene to Pliocene succession with hiatuses accounting for more than two thirds of the time spanned by the record. The ~800-m thick lower Miocene interval of the AND-2B core is nearly continuous and enables new constraints on regional tectonic and climatic events of the West Antarctic Rift sedimentary succession.
DE: 0726 Ice sheets
DE: 0728 Ice shelves
DE: 1621 Cryospheric change (0776)
DE: 9310 Antarctica (4207)
DE: 9605 Neogene
SC: Cryosphere [C]
MN: 2008 Fall Meeting

HR: 17:40h
AN: GP14A-09
TI: Magnetic Mineralogy of a Complete Oceanic Crustal Section (IODP hole 1256D)
AU: * Krasa, D
EM: david.krasa@ed.ac.uk
AF: University of Edinburgh, School of GeoSciences King's Buildings, Edinburgh, EH9 3JW, United Kingdom
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: University of Hawaii at Manoa, SOEST-HIGP 1680 East West Rd, Honolulu, HI 96822, United States
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: University of California at Davis, Department of Geology One Shields Ave, Davis, CA 95616, United States
AU: Rodriguez Durand, S
EM: durand@ldeo.columbia.edu
AF: Lamont-Doherty-Earth Observatory, Columbia University 61 Rt. 9W P.O. Box 1000, Palisades, NY 10964, United States
AB: Oceanic crust is the carrier of the marine magnetic anomalies and is therefore a valuable archive of geomagnetic information. Hole 1256D, which was drilled during ODP leg 206 and IODP expeditions 309 and 312, was the first to retrieve a core comprising of an entire sequence of oceanic crust down to the gabbro. This provides a unique opportunity to study the carriers of the marine magnetic anomalies. We used reflected light microscopy, scanning electron microscopy and energy dispersive x-ray analysis in addition to rock magnetic measurements to study the grain size, morphology, composition, and alteration state of the magnetic minerals. This comprehensive data set allows us not only to understand the magnetic stability of the minerals but also the mode and timing of remanence acquisition. The extrusive layer contains dendritic, low-temperature (LT) oxidized titanomagnetites (TMs) typical for mid- ocean ridge basalts (MORBs). The initial composition of these is close to previously reported values for MORB TMs with an ulvöspinel content of about 60%. The degree of LT oxidation remains fairly constant across the whole extrusive part of the section with an oxidation parameter z=0.6. Therefore, the increase in Curie temperature from 200C at the top to about 500C at the bottom of the extrusives cannot be accounted for by LT oxidation alone. Instead, we favor a model involving submicron inversion of LT oxidized TMs to an intergrowth of TMs and nonmagnetic phases, where the Ti-content of the TM phase is continuously decreasing with depth due to higher inversion temperatures. In the underlying sheeted dykes and gabbros, TMs precipitated as the primary opaque phase, as well. Due to the slower cooling rate, these particles are in most cases oxy-exsolved and form lamellar intergrowths of Ti- poor TMs and ilmenite. After emplacement, these minerals were altered to a much higher degree than the extrusive lavas. Secondary minerals frequently replace the original TMs, and the lower part of the sheeted dykes witnesses the onset of hydrothermal alteration. In the gabbroic part of the section, grain sizes of TMs reach values of up to several mm. These findings lead to the conclusion that the different parts of the section had acquired their remanent magnetisation by different mechanisms: The extrusive part carries a TRM, the intensity of which was later influenced by LT oxidation and inversion. The sheeted dyke part is likely to carry a CRM acquired during hydrothermal alteration, and the underlying gabbro acquired a TCRM at a time significantly after emplacement due to slow cooling at this depth.
DE: 1039 Alteration and weathering processes (3617)
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3036 Ocean drilling
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 0800h
AN: C21B-0519
TI: Paleomagnetism of the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica
AU: * Acton, G
EM: acton@geology.ucdavis.edu
AF: University of California - Davis, Department of Geology, One Shields Avenue, Davis, CA 95616, United States
AU: Florindo, F
EM: florindo@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, I- 00143, Italy
AU: Jovane, L
EM: jovane@geology.ucdavis.edu
AF: University of California - Davis, Department of Geology, One Shields Avenue, Davis, CA 95616, United States
AU: Ohneiser, C
EM: christian.ohneiser@stonebow.otago.ac.nz
AF: University of Otago, Department of Geology, PO Box 56, Dunedin, 9010, New Zealand
AU: Sagnotti, L
EM: sagnotti@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, I- 00143, Italy
AU: Strada, E
EM: strada@ingv.it
AF: Università di Siena, Dipartimento di Scienze della Terra, Via Laterina 8, Siena, I- 53100, Italy
AU: Strada, E
EM: strada@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, I- 00143, Italy
AU: Verosub, K L
EM: verosub@geology.ucdavis.edu
AF: University of California - Davis, Department of Geology, One Shields Avenue, Davis, CA 95616, United States
AU: Wilson, G S
EM: gary.wilson@otago.ac.nz
AF: University of Otago, Department of Geology, PO Box 56, Dunedin, 9010, New Zealand
AU: Science Team, A
EM: www.andrill.org/projects/sms/team.html
AF: ANDRILL, University of Nebraska-Lincoln 125C Bessey Hall, Lincoln, NE 68588, United States
AB: Paleomagnetic data from Site AND-2A (77°45.488'S, 165°16.605'E, ~383.57 m water depth) provide a rare view of geomagnetic field behavior at high latitude and are the basis for a magnetostratigraphic study, which aims to date climatic change in the Ross Sea since the early Miocene. A total of 813 mini-core samples were collected that span from the top of the section down to the base at 1138.54 mbsf. In addition, 11 U-channel samples were collected from a fine-grained, laminated interval at 1016-1040 mbsf to examine short-term geomagnetic field variability over a period of 100-200 k.y. in the early Miocene. AF demagnetization was generally able to resolve a characteristic remanent magnetization (ChRM) as well or better than thermal demagnetization. Above Lithostratigraphic Unit (LSU) 8 (436.18 mbsf), where lithologies are generally more coarse grained than lower in the section, resolving a ChRM is difficult and recent overprints or a drilling overprint are a concern. Within LSU 8 and below, most samples have a ChRM that can be resolved. The ChRM is most likely an original depositional magnetization throughout most of this lower section. Based on 40Ar/39Ar dates and diatom datums, the magnetozones identified from the base of the hole up to ~266 mbsf are consistent with spanning from either Chron C6n (18.748-19.772 Ma) or C6An.1n (20.040-20.213 Ma) up through Chron C5Br (15.160-15.974 Ma). The spacing of polarity reversals below 266 mbsf and their correlation with the GPTS indicates that this part of the stratigraphic section was deposited between 15 and 20 Ma at a mean sedimentation rate of about 18 cm/k.y. Analysis of the inclinations from the mini-cores and U-channels allow us to assess the fidelity of the geomagnetic record and to estimate geomagnetic field properties over the tangent cylinder (the cylinder coaxial with Earth's rotation axis and tangent to the inner core/outer), where paleomagnetic data are rare and geomagnetic field variability may provide clues about the nature of fluid flow in the outer core.
UR: http://www.andrill.org
DE: 1165 Sedimentary geochronology
DE: 1520 Magnetostratigraphy
DE: 1522 Paleomagnetic secular variation
DE: 1527 Paleomagnetism applied to geologic processes
DE: 3036 Ocean drilling
SC: Cryosphere [C]
MN: 2008 Fall Meeting

HR: 0800h
AN: C21B-0536
TI: Provenance Investigations Using Magnetic Susceptibility of Pebble- to Cobble-Sized Clasts in the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica
AU: * Strada, E
EM: strada@ingv.it
AF: Dipartimento di Scienze della Terra, Universita' di Siena, Via Laterina, 8, Siena, 53100, Italy
AU: * Strada, E
EM: strada@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma, 00143, Italy
AU: Florindo, F
EM: florindo@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma, 00143, Italy
AU: Sandroni, S
EM: sandroni@unisi.it
AF: Dipartimento di Scienze della Terra, Universita' di Siena, Via Laterina, 8, Siena, 53100, Italy
AU: Talarico, F
EM: talarico@unisi.it
AF: Dipartimento di Scienze della Terra, Universita' di Siena, Via Laterina, 8, Siena, 53100, Italy
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: Geology Department, University of California-Davis, One Shields Ave., Davis, CA 95616, United States
AU: Jovane, L
EM: jovane@geology.ucdavis.edu
AF: Geology Department, University of California-Davis, One Shields Ave., Davis, CA 95616, United States
AU: Ohneiser, C
EM: christian.ohneiser@stonebow.otago.ac.nz
AF: Department of Geology, University of Otago, Geology Building, Leith St., Dunedin, 9016, New Zealand
AU: Sagnotti, L
EM: sagnotti@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma, 00143, Italy
AU: Verosub, K L
EM: verosub@geology.ucdavis.edu
AF: Geology Department, University of California-Davis, One Shields Ave., Davis, CA 95616, United States
AU: Wilson, G S
EM: gary.wilson@otago.ac.nz
AF: Department of Geology, University of Otago, Geology Building, Leith St., Dunedin, 9016, New Zealand
AU: Science Team, A
EM: sms@andrill.org
AF: http://www.andrill.org/projects/sms/team.html, ANDRILL Science Management Office, 126 Bessey Hall University of Nebraska Lincoln, Lincoln, NE 68588-0341, United States
AB: Magnetic susceptibilities of pebble- to cobble-sized clasts recovered in the ANDRILL SMS core (site AND-2A: 77° 45.488'S, 165° 16.605'E) (McMurdo Sound, Ross Sea) were measured on ice with a Bartington MS-2B susceptibility meter. Measurements were made on thin section billets or on clasts themselves when they were of suitable size for the instrument. The variability of the magnetic susceptibility is related both to variations in the primary magnetic mineral content of the source rocks as well as to secondary magnetic mineral formation/dissolution prior to and during the diagenetic process. Volcanic clasts, the dominant clast type thoughout the core, display the highest susceptibility values, but there is extreme heterogeneity of values within the same compositional type (i.e. felsic, intermediate, mafic). Given this, the susceptibilities of volcanic clasts in the AND-2A core are poorly suited for provenance studies. In contrast, the basement clasts (consisting of a variety of metasedimentary and intrusive rocks) can play an important role in defining ice provenance and dynamics. The textures and mineralogical compositions of intrusive and metamorphic rocks indicate the region between the Ferrar Glacier and the Mulock Glacier as the most likely provenance region. In order to better understand this result, we chose to undertake a magnetic petrology study on the most magnetic (Low-Field mass susceptibility, χ > 90*10-8 m3/kg) basement clasts sampled on ice. A comparison of the magnetic susceptibilities of our AND-2A clasts with samples collected from the outcrops of Southern Victoria Land (SVL) indicate that there is a good correlation between the highly magnetic metamorphic rocks of the region south of Ferrar Glacier and the most magnetic basement clasts in the core. In particular, the petrographic and magnetic features of metasedimentary clasts closely match both metasandstones from Baronick Glacier (Skelton Glacier area) and gneisses and schists from Hobbs Peak (Blue Glacier area), corroborating the inferences of the petrography-based provenance studies. These preliminary results point out the importance of mineral magnetic measurements in providing additional constrains and a useful data set for on-going provenance investigations of Cenozoic sediments recovered by recent ANDRILL drillholes in the McMurdo Sound area.
DE: 1519 Magnetic mineralogy and petrology
DE: 1621 Cryospheric change (0776)
DE: 3337 Global climate models (1626, 4928)
DE: 3344 Paleoclimatology (0473, 4900)
SC: Cryosphere [C]
MN: 2008 Fall Meeting

HR: 1340h
AN: B13B-0450
TI: Environmental Magnetism of Cores from Peat Deposits of the Sacramento-San Joaquin Delta, California
AU: * Verosub, K L
EM: verosub@geology.ucdavis.edu
AF: Geology Department, University of California - Davis One Shields Avenue, Davis, CA 95616, United States
AB: We have conducted an environmental magnetic study of cores from 12 sites in the Sacramento-San Joaquin Delta of California. Eight of the sites are on tracts and islands that were created in the late 19th and 20th centuries when marshes were leveed and drained to create agricultural land. The other four sites are march islands that have never been drained. Most of the cores penetrate underlying clay deposits that predate the formation of the peat. Over 50 meters of core was studied from the 12 sites. Measurements were made of the acquisition and demagnetization of anhysteretic remanence and isothermal remanence, two laboratory- induced magnetizations that provide information about the concentration and mineralogy of the magnetic fraction. In all of the cores, the peat-rich intervals have fairly low magnetic intensities, indicating significant dilution of the clastic material by the organic material of the peat. Some variations in intensity can be discerned, which appear to be due to changes in climate and/or landscape. Material from the top of the cores from the farmed islands has much higher magnetic intensities, indicative of the concentration of clastic material that occurs as organic material is lost by oxidation and/or wind erosion. The most interesting results come from the bases of the cores where the transition from non-organic clays and silts to organic-rich peat is marked by a 100-fold decrease in the concentration of magnetic material. Detailed analysis of this transition indicates that it was usually very abrupt although in several cases the sudden decrease in concentration is followed by a short interval of moderate concentration before the final, lowest concentrations are attained. This intermediate state provides insights into the nature of the transition from an estuarine to a paludal environment.
DE: 0442 Estuarine and nearshore processes (4235)
DE: 0486 Soils/pedology (1865)
DE: 0497 Wetlands (1890)
DE: 1512 Environmental magnetism
SC: Biogeosciences [B]
MN: 2008 Fall Meeting

HR: 1340h
AN: B13B-0449
TI: Can Phytolith Concentrations Indicate That Wind Erodes Drained Peatlands?
AU: * Lunning, N G
EM: lunning@geology.ucdavis.edu
AF: Department of Geology, University of California, Davis, One Shields Ave, Davis, CA 95616, United States
AU: Verosub, K L
EM: verosub@geology.ucdavis.edu
AF: Department of Geology, University of California, Davis, One Shields Ave, Davis, CA 95616, United States
AU: Singer, M J
EM: mjsinger@ucdavis.edu
AF: Department of Land, Air and Water Resources, University of California, Davis, One Shields Ave, Davis, CA 95616, United States
AB: Surface elevations of organic soils in peatlands in the Sacramento-San Joaquin Delta, California, have dropped by as much as fifteen meters since they were drained for agricultural use in the late 19^th and early 20^th centuries. In this and analogous areas, peat loss is commonly attributed to microbial oxidation of the organic material; while wind erosion is generally considered to be a minor factor. However, in the soil science literature it is widely accepted that organic soils are highly susceptible to wind erosion. The goal of this study is to investigate the competing roles of wind erosion and oxidation in the loss of peat in the Sacramento-San Joaquin Delta by using natural phytoliths found in these peat soils. Phytoliths are biogenic opal; unlike organic matter they are not removed by oxidation. Because of their size, mostly between 2-50 microns, phytoliths are very susceptible to aeolian removal and hence are potentially a good proxy for wind erosion. In this study we sampled peat from drained farmlands and nearby undrained wetlands of the Sacramento-San Joaquin Delta. Samples were treated using conventional phytolith extraction methods. Phytolith concentrations were determined by adding a known quantity (lycopodium spore tablet) of tracer to the phytolith extract (coarse silt fraction >20 microns) and counting the tracer along with the phytoliths. Compared to the phytolith concentrations in undrained peat, the phytolith concentrations should substantially increase in the drained soils as the organic matter oxidizes, unless wind is also eroding the peat. Preliminary results demonstrate an increase in phytolith concentrations for the portion of soil profile (top 45 cm) that has been mixed by agricultural equipment of the drained peat (mean=3.6x10⁴ phytoliths/cm³) compared to the undrained wetland peat (mean=2.3x10⁴ phytoliths/cm³.) However, this higher phytolith concentration in the drained peat is well below the expected phytolith concentration (3.4x10⁵ phytoliths/cm³) if there had been no removal of phytoliths, suggesting that wind erosion is a contributor to the degradation of peat drained for farming.
DE: 0402 Agricultural systems
DE: 0471 Oxidation/reduction reactions (4851)
DE: 0486 Soils/pedology (1865)
DE: 0497 Wetlands (1890)
SC: Biogeosciences [B]
MN: 2008 Fall Meeting

HR: 0800h
AN: GP31C-0809
TI: Environmental Magnetism of Late Quaternary Sediments From the Southern Okinawa Trough: Paleoceanographic Implications and History of the Kuroshio Current
AU: * Richter, C
EM: richter@louisiana.edu
AF: Department of Geology, University of Louisiana at Lafayette, P.O. Box 44530, Lafayette, LA 70504, United States
AU: Venuti, A
EM: venuti@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Roma, 00143, Italy
AU: Verosub, K L
EM: verosub@geology.ucdavis.edu
AF: Department of Geology, University of California at Davis, One Shields Avenue, Davis, CA 95616, United States
AU: Wei, K
EM: weiky@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei, 106, Taiwan
AB: The objective of this study is to advance our understanding of the late Cenozoic environmental and paleoceanographic history of the Southern Okinawa Trough and the Kuroshio Current. Our investigation is based on the environmental magnetic record of a suite of u-channel samples that span the upper 140 m (0.42 m.y.) of ODP Site 1202. This site was drilled in the southernmost Okinawa Trough off the northeast coast of Taiwan and provides, with sedimentation rates of up to 9 m/k.y., one of the highest-resolution records of environmental and paleoceanographic change ever recovered. Variations in the concentration of fine-grained magnetite (anhysteretic remanent magnetization, ARM), magnetic grain size (ARM/k), magnetic hardness (ARM(30)/ARM(0)), and overall mineralogy (susceptibility, k) reflect fluctuations in climate, sea level, source material, and depositional environment. Eleven AMS C-14 dates of planktonic foraminifers and scaphopod samples combined with oxygen isotope stratigraphy (Wei et al., 2005) provide a relatively low- resolution, but well-defined age model. Based on the magnetic characteristics we distinguished five distinctive intervals with boundaries at 11.3, 14.3, 18.6, and 27.4 ka. From the beginning of the record at about 40 ka to 27.4 ka (MIS-3) all magnetic parameters remain remarkably constant indicating very little variation in sediment source and processes. Sea level during this time interval fluctuated at around -80 m. At 27.4 ka a significant decrease in magnetic grain size is accompanied by an increase in the amount of magnetic material. This change coincides with a drop in sea level below -80 m in the buildup to the sea level lowstand of the Last Glacial Maximum (LGM). Sediment source during this time interval was predominantly the East China Sea (Wei, 2006; Diekmann et al., in press). MIS-2 is characterized by constantly increasing magnetic grain size and a decreasing amount of magnetic material. At 14.3 ka after melt water pulse (MWP) 1A, which is expressed as a short spike in all magnetic parameters, the magnetic signature changes significantly, shows very little variability and remains constant until MWP-1B at 11.3 ka. An effect of the Younger Dryas is possibly preserved in the magnetic susceptibility signal. Associated with a significant sea level rise at the onset of the Holocene, a change in sediment source to Taiwan (Wei, 2006; Diekmann et al., in press) and the onset of the Kuroshio Current as sea level rose above -50 m, magnetic parameters shift and remain relatively constant throughout the Holocene. This allowed for the extraction of a high-resolution relative paleointensity record from 0-9.4 ka. MWP-1C is associated with a short and sudden increase in magnetic grain size.
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2008 Fall Meeting

HR: 0800h
AN: C21B-0522
TI: Over-Sea-Ice Multi-Component VSP at the AND-2A Drill Hole: ANDRILL Southern McMurdo Sound Project
AU: * Patterson, T
EM: tbpatterson@mtech.edu
AF: Geophysical Engineering Department, Montana Tech, 1300 W Park St, Butte, MT 59701, United States
AU: Speece, M
EM: mspeece@mtech.edu
AF: Geophysical Engineering Department, Montana Tech, 1300 W Park St, Butte, MT 59701, United States
AU: Henrys, S
EM: s.henrys@gns.cri.nz
AF: GNS Science, 1 Fairway Drive, Lower Hut, 5042, New Zealand
AU: Wonik, T
EM: wonik@gga-hannover.de
AF: GGA-Institute, Stillweg 2, Hannover, D-30655, Germany
AU: Dunbar, G
EM: gavin.dunbar@vuw.ac.nz
AF: Antarctic Research Centre, Victoria University of Wellington, Wellington, 6005, New Zealand
AU: Levy, R
EM: rlevy2@unl.edu
AF: Department of Geosciences and ANDRILL SMO, University of Nebraska-Lincoln, Lincoln, NE 68588, United States
AU: Harwood, D
EM: dharwood@unl.edu
AF: Department of Geosciences and ANDRILL SMO, University of Nebraska-Lincoln, Lincoln, NE 68588, United States
AU: the SMS Science Team
AF: http://andrill.org/projects/sms/team.html
AB: During the austral summer, 2007, the ANtarctic geological DRILLing Program (ANDRILL) drilled, cored and logged the AND-2A drill hole as part of the Southern McMurdo Sound (SMS) Project in the western Ross Sea, Antarctica. As part of the logging program, a single near-offset over-sea-ice Vertical Seismic Profile (VSP) was collected in the AND-2A drill hole. The VSP was collected using a Generator-Injector (GI) air-gun source that was suspended by a cable through a hole in the sea-ice. The GI air gun was selected to minimize the bubble-pulse effects common to explosive sources placed in the water column and because of the poor coupling of sea-ice surface seismic sources. Inconsistent injection-bubble timing caused source wavelet stretching and bubble oscillation. Two-pass deconvolution with short and long prediction lags helped shorten the wavelets and minimize oscillation. A single three-component geophone provided high-quality downgoing P- and S-wave data from which subsurface velocities were determined. The VSP P-wave time-depth curve closely agrees with the time-depth curve derived from whole-core velocity measurements. Downgoing P- wave events were recorded with strong water-bottom multiples occurring throughout the section. A P-wave corridor stack is in good agreement with the surface seismic data at the SMS site. Major seismic reflectors in the corridor stack correspond to important stratigraphic boundaries that are observed in the core. Downgoing and upgoing mode-converted PS-waves were also recorded on both radial and transverse components. Mode-conversion and S-wave birefringence indicate significant anisotropy beginning in the near sea floor (approximately 20 mbsf) and continuing to depth in the drill hole. The S-wave radial component travels as the slow shear wave and the transverse as the fast shear wave. This is in agreement with the overall fracture orientation expected from the regional stress regime. Two component rotation analysis generated an average fracture azimuth of N 45o W (+/- 10o). A 2C coarse-layer stripping method could better constrain fracture orientation over select intervals within the section.
DE: 3025 Marine seismics (0935, 7294)
SC: Cryosphere [C]
MN: 2008 Fall Meeting

HR: 0800h
AN: C21B-0534
TI: Transient Establishment of a Seasonal Sea-Ice Regime in the Late Pliocene, McMurdo Embayment, Ross Sea, Antarctica
AU: * Riesselman, C R
EM: criessel@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States
AU: Dunbar, R B
EM: dunbar@stanford.edu
AF: Department of Environmental Earth System Science, Stanford University, Stanford, CA 94305, United States
AU: Harwood, D M
EM: dharwood1@unl.edu
AF: Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588, United States
AU: Olney, M P
EM: cyclingolney@yahoo.co.uk
AF: Department of Geology, University of South Florida, Tampa, FL 33620, United States
AU: Scherer, R P
EM: reed@geol.niu.edu
AF: Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, United States
AU: Sjunneskog, C M
EM: charlottems60@yahoo.com
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Tuzzi, E
EM: evatuzzi@libero.it
AF: Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588, United States
AU: Winter, D M
EM: dwinter1@juno.com
AF: Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588, United States
AU: SMS science team, A
EM: info@andrill.org
AF: http://andrill.org/projects/sms/team.html, ANDRILL Science Management Office, University of Nebraska – Lincoln, Lincoln, NE 68588, United States
AB: The marine sediment drillcores collected by the Antarctic Geological Drilling (ANDRILL) Program from sites beneath the McMurdo Ice Shelf (AND-1B) and in Southern McMurdo Sound (AND-2A) together represent the longest, most complete record to date of Neogene climatic evolution proximal to the Antarctic continent. Multiple extensive, well-preserved diatomite units distributed throughout the upper six hundred meters of the McMurdo Ice Shelf (MIS) core provide a unique perspective on the transition from the mid-Pliocene climatic optimum into modern cold-polar conditions. While the diatom assemblages preserved within these diatomites record a variety of paleoenvironmental conditions, the modern, fully-developed sea-ice community that characterizes the modern Ross Sea environment is absent from the record. Here, we focus on two discrete diatomite units deposited between ~3.3 and 3.0 million years ago. The base of the lower unit, at 292 mbsf, is marked by the radiation of a Fragilariopsis species complex that is previously undescribed from Antarctic sediments. This complex dominates the diatom assemblage throughout the late Pliocene, comprising up to 55 % of the community at 288 meters. By contrast, modern sea-ice marker species are poorly represented in most mid- to late Pliocene sediments at AND-1B. However a transient sea-ice event is recorded in the upper portion of the diatomite unit that spans 252-258 mbsf. The assemblage at the base of this diatomite is similar in character to the diatomite units above and below, however the proportion of F. curta, a taxon with well-established sea-ice affinity, increases gradually, reaching a transient but significant maximum at 253.11 mbsf. The mid-Pliocene climatic optimum is not recovered from the shallower Southern McMurdo Sound (SMS) site, which is comprised predominantly of diamictite and mudstone. However a diverse Pliocene assemblage between 44.5 and 48.3 mbsf in AND-2A appears equivalent to the AND-1B diatomite 164-180 mbsf. Preserved in five pulses incorporated into sandy, clast-poor diamictite, this assemblage includes numerous age diagnostic species but lacks a significant contribution from the sea-ice assemblage that dominates modern Ross Sea coastal environments. The 1138-meter AND-2B drillcore includes only two meters of true diatomite (from the Miocene), reflecting both the continental proximity and shallow paleodepth of the site. In this context, the episodic nature of Pliocene diatom preservation in AND-2B suggests that sedimentation in Southern McMurdo Sound was paced by an unusual oscillating environmental or depositional influence around 2.5 million years ago. The nearshore Southern McMurdo Sound section also provides an opportunity to place the dynamic McMurdo Ice Shelf record within a regional context.
DE: 0750 Sea ice (4540)
DE: 4944 Micropaleontology (0459, 3030)
DE: 9310 Antarctica (4207)
DE: 9605 Neogene
SC: Cryosphere [C]
MN: 2008 Fall Meeting

HR: 0800h
AN: C21B-0542
TI: Dating Antarctic climatic and tectonic events of the Neogene – Age models from the ANDRILL 1B and 2A cores
AU: Wilson, G
EM: gary.wilson@otago.ac.nz
AF: Geology Department, University of Otago, PO Box 56, Dunedin, 9001, New Zealand
AU: * Levy, R
EM: rlevy2@unlnotes.unl.edu
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: Geology Department, University of California-Davis, One Shields Ave, Davis, CA 95616, United States
AU: Naish, T
EM: Tim.Naish@vuw.ac.nz
AF: Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington, 6140, New Zealand
AU: Harwood, D
EM: dharwood1@unl.edu
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AU: Florindo, F
EM: florindo@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Rome, I- 00143, Italy
AU: Powell, R
EM: ross@geol.niu.edu
AF: Department of Geology and Environmental Geosciences, Northern Illinois University, 312 Davis Hall, Normal Road, DeKalb, IL 60115-2854, United States
AU: MIS Science Team, A
EM: andrill@andrill.org
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AU: SMS Science Team, A
EM: andrill@andrill.org
AF: ANDRILL Science Management Office, University of Nebraska-Lincoln, 126 Bessey Hall, Lincoln, NE 68588-0340, United States
AB: A network of seismic surveys in McMurdo Sound provides an image of the tectonic, glacial, and sea-level controls on accumulation of Cenozoic strata in the West Antarctic Rift. Integration with previously recovered drill cores revealed a younger Neogene succession, which was the target of drilling from floating ice platforms under the ANDRILL program in the austral summers of 2006 and 2007. Continuously recovered core from the ANDRILL 1B and 2A drillholes provides key paleoenvironmental data regarding climatic variation and ice volume fluctuation of the Antarctic Ice Sheets through the Neogene. Chronostratigraphic data available from the drillcores includes diatom and nannofossil biostratigraphy, magnetic polarity stratigraphy, 40Ar/39Ar ages on numerous ashes from the McMurdo Volcanic Complex, and strontium dates on carbonate material. Integration and optimization of the currently available data provides a robust age model for the Plio- Pleistocene (upper 600m) of the AND-1B drillcore and the middle and lower Miocene (224-1139m) interval of the AND-2A drillcore. The Plio-Pleistocene record is punctuated by multiple hiatuses that correlate with climate (ice sheet and sea level fluctuations) events and account for approximately half of the time spanned by the record. Despite these hiatuses, the distribution of chronostratigraphic data still enable the identification of orbital influence in remaining strata. The lower 600m of the AND-1B extends well down into the Miocene as constrained by a 40Ar/39Ar age of approximately 13.8 Ma towards the base of the core. The upper 300m interval of the AND-2A core contains punctuated middle Miocene to Pliocene succession with hiatuses accounting for more than two thirds of the time spanned by the record. The ~800-m thick lower Miocene interval of the AND-2B core is nearly continuous and enables new constraints on regional tectonic and climatic events of the West Antarctic Rift sedimentary succession.
DE: 0726 Ice sheets
DE: 0728 Ice shelves
DE: 1621 Cryospheric change (0776)
DE: 9310 Antarctica (4207)
DE: 9605 Neogene
SC: Cryosphere [C]
MN: 2008 Fall Meeting