Research

Research Interests

My research field is paleomagnetism, geomagnetism and environmental magnetism. Research activity in recent years can be subdivided into four categories:

   1. The application of magnetic polarity stratigraphy to the generation of geologic time scales.  Magnetic polarity stratigraphy (the record of geomagnetic polarity reversals in sediments and sedimentary rocks) is a preferred means of global stratigraphic correlation. The method can be used to correlate biostratigraphies, cyclostratigraphies, isotope stratigraphies and radiometric age determinations.


   2. Environmental magnetism and geomagnetic paleointensity: the use of rock magnetic characteristics as sedimentological tools in recent sediments (e.g. magnetic granulometry) and the use of secular variation and paleointensities for high-resolution correlation of recent (last few Myr) sediments. Modern studies of past climate require millennial-scale correlation of climate-proxy records which usually cannot be provided by the stable isotopes, biostratigraphy or radiometric ages. Variations in the intensity of the geomagnetic dipole field, when recorded by sediments, appear to provide the desired means of global correlation.


   3. Geomagnetism: high-resolution records of geomagnetic field behavior from deep-sea sediment drifts have revolutionized our knowledge of the geomagnetic field during the last ~2 million years. The records are now being used to constrain computer-generated models of the geodynamo.  As a result, we have a clearer picture of the spatial and temporal characteristics of geomagnetic (secular) variation, and the morphology of polarity transition fields and magnetic excursions.  Excursions occur at paleointensity minima, constitute pairs of polarity reversals when optimally recorded, and have durations of a few kyr or less.  About seven excursions are well documented and age-calibrated in the Brunhes Chron, and about 10 have been recorded in the Matuyama Chron.


   4. The application of paleomagnetism to paleogeographies in mountain belts, notably in the Alpine-Mediterranean area. The objective has been to reconstruct the pre-deformational relative position of rock bodies (on scales from individual thrust sheets to continents). Field-work has been carried out in Italy, Austria, Switzerland, Spain, Slovakia and Turkey in the last 15 years.

 

Research for graduate students:

(1) Stratigraphy in the North Atlantic

The North Atlantic Ocean is one of the most climatically sensitive regions on Earth because the ocean-atmosphere-cryosphere system is prone to mode jumps that are triggered by changes in freshwater delivery to source areas of deepwater formation. During the last glaciation, these abrupt jumps in climate state are manifest by Dansgaard-Oeschger (D-O) cycles and Heinrich Events in ice and marine sediment cores, respectively. Abrupt climate variability in the North Atlantic has been well documented for the last glacial cycle, but extension of the record beyond this period has been limited by the availability of appropriately located cores with suitable sedimentation rates and stratigraphic continuity. In 2004, IODP Expedition 303 recovered high sedimentation-rate sequences at several sites from the Ruddiman ice-rafted detritus (IRD) belt of the North Atlantic. Preliminary stable isotopic, elemental (scanning XRF), core logging, and paleomagnetic data demonstrate that these sections have the potential for documenting millennial-scale climate variability for much of the Quaternary.  In this project, we use oxygen isotope stratigraphy and geomagnetic paleointensity to correlate the records among three sites located along a proximal-to-distal transect from the mouth of the Labrador Sea to the central North Atlantic, including Sites U1302/03 (Orphan Knoll), U1304 (Gardar Drift), and U1308 (re-drill of DSDP Site 609).  A central theme of this project is the combined use of oxygen isotope and paleointensity data to provide improved stratigraphic correlation across the North Atlantic, and in so doing provide reference templates linking oxygen isotopes, paleointensity and Heinrich-type detrital layers, as important indicators of abrupt climate change in the region. The improved stratigraphies will be used to document: 1) the occurrence of detrital carbonate (Heinrich) and other IRD events beyond the last glacial cycle; 2) the amplitude and frequency of millennial-scale climate variability as orbital and glacial boundary conditions changed during the Pleistocene; and 3) the relationship between millennial-scale variability and deep-water circulation over the last several million years. In addition, the data will be used to explore pressing issues related to paleointensity records such as: 1) sources of lithologic contamination, 2) sources of orbital power in paleointensity records, and 3) the scale at which paleointensity records can be correlated globally. 

The work will be carried out in close collaboration with D.A. Hodell (University of Cambridge, UK). As part of the project, we plan student exchange between Cambridge and Gainesville with a graduate student from each institution spending time at the other institution during the course of the project. The sequences recovered during IODP Expedition 303 are important reference sections for the paleoclimate community. As the Greenland ice cores have revolutionized our understanding of climate change during the last glacial cycle, we expect the EPICA and other Antarctic ice cores to revolutionize our knowledge of climate change during the last ~1 Myrs.  The challenge for the paleoceanographic community is to generate marine records with resolution and chronostratigraphic control comparable to that of the ice-cores. The integrated stratigraphies developed from this project will serve the broader community of scientists interested in studying climate change on long timescales in the North Atlantic and elsewhere.  The stratigraphies will be shared with scientists conducting parallel studies on sediments from Expedition 303, and will be archived at the Paleoclimate World Data Center for future access. Undergraduate and graduate students will be directly and significantly involved in this research project that deals with diverse stratigraphic tools and proxies for climate change, providing an excellent interdisciplinary educational experience for those involved.

 

(2) The Earth’s magnetic field of the last few million years

Over the last 40 years, magnetic polarity stratigraphy has revolutionized stratigraphic correlation through the establishment of global timelines at polarity reversals. The proposed work addresses the potential of relative paleointensity (RPI) stratigraphy and magnetic excursions (which occupy minima in RPI records) for providing global timelines within polarity chrons. Quaternary sedimentary sequences with mean sedimentation rates in the 5-20 cm/kyr range, collected during IODP Expedition 303/306 in 2004-2005, will be the focus of this investigation. Channell et al. (2009) adapted the “Match” protocol of Lisiecki and Lisiecki (2002) to optimally correlate oxygen isotope and RPI records from 13 deep-sea coring sites for the last 1.5 Myr using one of the Expedition 303/306 sites (Site U1308) as the basis for the age model. The tandem correlation of two ostensibly global, but independent, signals resulted in RPI and oxygen isotope stacks, and provides a template for correlation of RPI (and isotope) records. Here we plan to build on this work by analyzing the older part of the record at Site U1308 and other IODP Expedition 303/306 sites (U1302-3, U1304, U1306) with the view to incorporating their RPI records into the stack, and investigating their records of polarity transitions and excursions, and their environmental signatures of Heinrich-type detrital events. The elevated mean sedimentation rates, combined with the high fidelity of the magnetic records, appear to have captured not only high quality RPI records but also high-fidelity records of excursions and polarity transitions.  Heinrich-type detrital layers, an important signature of North Atlantic Quaternary environmental change, cannot be unequivocally correlated from site to site using oxygen isotope data alone, providing a useful test for combined isotope/RPI correlations.  Magnetic hysteresis data show differences among detrital layers that may be useful as provenance indicators. Orbital periods (at ~100 and ~41 kyr) are embedded in many RPI records and there is continuing debate regarding the origin of this orbital power; it may represent lithologic contamination of the RPI records or an inherent characteristic of the geomagnetic field. We plan to build on our understanding of the origin of the orbital power by development of “depth-derived” age models (see Huybers, 2007) for isotope and RPI events (e.g. Terminations, RPI minima etc.) by using compaction-compensated sedimentation models between geomagnetic reversals, and also between reversals and excursions. The resulting depth-derived age models, constrained by both RPI and oxygen isotopes, would improve the resolution of depth-derived age models acquired using the oxygen isotope records alone. Wavelet analyses and coherence analyses, that incorporate uncertainty estimates generated by Monte Carlo simulations, will be used to compare normalized remanence records with their normalizers, and with other lithologic/climatic signals and magnetic parameters.  We also plan to build on software designed to facilitate the analysis of u-channel paleomagnetic data (Xuan and Channell, 2009), and investigate normalization procedures that minimize the lithological influence on RPI records.

The sequences recovered on Expedition 303/306 are destined to become important reference sections for Quaternary climate change and geomagnetic field behavior.  Integrated paleomagnetic/oxygen isotope stratigraphies will serve the broader community of scientists interested in climate and geomagnetic change on 103-106 yr timescales in the North Atlantic and elsewhere, and will address a conundrum that affects the study of abrupt climate change: the lack of marine stratigraphic tools with an appropriate resolution. Evidence for a direct geomagnetic-climate linkage has been muted although unequivocal evidence is lacking. Improved knowledge of the paleointensity record is a key to understanding possible linkages. Undergraduate and graduate students will be directly and significantly involved in this research project through mentoring and teaching providing an excellent interdisciplinary educational experience for those involved.

(3) Paleomagnetism in the Arctic Ocean

Magnetic stratigraphy has become a lynch-pin of paleoceanographic studies through its ability to provide global correlation and age control.  For reasons that are not yet understood, magnetic stratigraphies from the Arctic Ocean and Norwegian-Greenland Sea have been problematic.  Difficulties in magnetostratigraphic interpretation have been exacerbated by lack of corroborating isotopic or biostratigraphic data from the region. The presence of zones of apparently reverse magnetization, denoted by negative inclinations, in Brunhes-aged sediments from the Arctic and Norwegian-Greenland Sea has led to the conclusion that the Arctic is fertile ground for the detection of Brunhes-aged magnetic excursions.  Excursional zones are particularly thick, compared to excursional intervals from outside this region, and this has led to the conclusion that excursion duration is somehow enhanced at high latitudes, or that sedimentation rates amplified during excursional intervals, presumably implying a geomagnetic-climatic link in which excursions are associated with particular climatic/paleoceanographic conditions.

With the aim of solving this dilemma, we are studying Arctic cores from the 2005 Healy-Oden Trans-Arctic Expedition (HOTRAX) cruise. The results indicate that these cores are characterized by negative component inclinations within the uppermost few meters of the core.  Thermal demagnetization of composite IRM, and susceptibility-temperature curves, indicate the presence of maghemite and magnetite.  Hysteresis parameters, and AF demagnetization of NRM, imply the absence of high coercivity minerals such as hematite or goethite.  Our working hypothesis is that the negative inclinations in these sediments are a result of a partial self-reversal in maghemite, where the maghemite forms as alteration coatings from magnetite.  Ionic reordering of the ferrimagnet during maghemitization may have led to partial self-reversal as alteration proceeds. We plan to study nine HOTRAX cores in order to establish the stratigraphic control on natural remanent magnetization (NRM) component directions from core to core. SEM studies, and X-ray diffraction of magnetic separates, will augment the magnetic measurements.

The societal importance of the Arctic Oceans has increased in the last few years as evidenced by the political claims of sovereignty, and the dramatic demise of sea-ice cover that has opened this region to the possibility of increased commercial traffic during the next few decades.  It is crucial to global climate studies to understand the climate evolution of the Arctic, and yet age models for sediment cores collected in the Arctic remain difficult to interpret due to lack of isotopic and biostratigraphic data (lack of carbonate) and the uncertainties in magnetostratigraphic interpretations (outlined above). Undergraduate and graduate students will be directly and significantly involved in a research project that deals with diverse stratigraphic tools, providing an excellent interdisciplinary educational experience for those involved.

(4) Paleoenvironmental Change, and a Tuned Geologic Timescale from Pacific Eocene–Pleistocene Sediments, IODP Expeditions 320-321.

Integrated Ocean Drilling Program (IODP) Expeditions 320 and 321 were part of a single drilling program that was designed to core thick sedimentary sections along a Pacific Equatorial Age Transect (PEAT). Each of the eight sites (Sites U1331–U1338) was selected to recover a portion of the interval spanning the Eocene through the Pleistocene. Multiple holes were cored at each site ensuring complete stratigraphic coverage and resulting in the recovery of over 6 km of core.  The shipboard magnetic records from these sites indicate that the sediments have recorded the paleomagnetic field with high fidelity, and the shipboard data indicate the potential of these sediments for studies of geomagnetic field variability, paleoenvironmental change, chronostratigraphy, and Pacific plate kinematics and geodynamics.  The proposed study will (1) refine the shipboard magnetostratigraphies by improving the placement of polarity reversal boundaries and resolving brief polarity subchrons and excursions; (2) generate long continuous relative geomagnetic intensity (RPI) and directional paleosecular variation (PSV) records; (3) construct high-resolution environmental magnetic records; (4) tune the geologic timescale over much of the Cenozoic by combining astronomical variations in physical and magnetic properties with the magnetostratigraphy and biostratigraphy constraints; and (5) improve the Pacific Plate apparent polar wander path (APWP) by integrating paleolatitudes estimated from PEAT sites with other Pacific paleomagnetic data. The magnetostratigraphies to be generated are primary chronostratigraphic constraints for the PEAT sites and will be fundamental in efforts to extend the astronomically-tuned timescale from the Neogene back into the Paleogene. Such a timescale provides the foundation for dating and correlating geologic events, determining rates of change, and for establishing the role of astronomical climate forcing. The RPI and PSV records will give valuable new insights into both long-term and short-term geomagnetic field behavior over a time interval that is currently sparsely sampled globally. The proposed study will extend dated RPI records, which are now largely restricted to the Pliocene-Quaternary, back into the Eocene. Besides being invaluable for a number of geomagnetic investigations, including fundamental modeling of the geomagnetic field through time, a new astronomically-tuned (stacked) RPI record will eventually serve as a basis for global correlations that augment more traditional correlation methods such as oxygen isotopes. The high-resolution rock magnetic records, some of which will be capable of defining millennial-scale lithologic variability and detrital layer stratigraphy, will be integrated with physical properties, geochemistry, and core color reflectance data to develop paleoenvironmental proxies. These have the potential to constrain eolian fluxes and provide insights into paleoceanographic change, like the opening and closing of ocean gateways that may have been responsible for some of the largest paleoclimatic changes in Earth’s history. A revised Pacific APWP will improve constraints on the motion of Pacific hotspots relative to Earth’s spin axis, which will in turn provide new constraints on mantle geodynamics and plate motions.

(5) Possible future Integrated Ocean Drilling Program (IODP) drilling in the Antarctic (IODP proposal 732)

There is intense interest in the response of the Antarctic Peninsula and West Antarctica to global warming because recent observations suggest this region is undergoing rapid changes including warming, ice-shelf disintegration and ice-sheet thinning and retreat.  These changes can be considered in a longer geologic perspective by studying ice-cores and marine sediment cores to decode the paleoceanographic and climatic changes, as well as evaluate the past history and stability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS).  We have proposed a series of drilling sites on sediment drifts (contourites) located on the continental rise west of the Antarctic Peninsula and West Antarctica. The proposed sites contain continuous sections with high sedimentation rates that can be dated using relative (geomagnetic) paleointensity and, at shallow-water sites, oxygen isotope stratigraphy.  Six proposed sites target expanded Pliocene-Quaternary sequences, with two sites targeting the pre-Pliocene record at locations characterized by thinned younger sediment cover.

Previous cores collected in the region, including those recovered during ODP Leg 178 (1998), have indicated that these drift deposits carry a rich high-resolution archive of Antarctic margin paleoceanography and APIS and WAIS history .  The potential of existing ODP cores is compromised by two factors: (1) incomplete composite sections and (2) lack of precise chronological control.  Imprecise chronological control, due in large part to lack of foraminiferal carbonate for isotopic analyses, has stymied paleoceanographic interpretations of sediment cores from high southerly latitudes.  The chronological problem can now be partially offset by using relative paleointensity records, and by placing sites at water depths less than 2800 m where enhanced carbonate preservation allows stable isotope analyses.

There are few targets in the circum-Antarctic region that rival the potential offered by the sediment drifts off West Antarctica.  The recovery of these sediment cores will elucidate the evolution of sedimentary processes along the margin, and the integration of these data with polar ice cores will contribute significantly our understanding of the role of the WAIS and APIS and the adjacent Southern Ocean in global atmospheric and oceanographic processes.