TY - JOUR
T1 - The Effect of Differential Weathering on The Magnetic Properties of Paleosols
T2 - A Case Study of Magnetic Enhancement vs. Magnetic Depletion in the Pleistocene Blackwater Draw Formation, Texas
AU - Stine, J.
AU - Geissman, J. W.
AU - Sweet, D. E.
AU - Baird, H.
N1 - Funding Information:
Research funding for JS includes: the Geological Society of America for Graduate Research Awards No. 11267-16 and No.12277-18, the Dr. Oscar Wilhelm Memorial Student Research Fund at the University of Texas at Dallas, and the Visiting Fellowship Awarded by the Institute for Rock Magnetism (IRM) at the University of Minnesota in October, 2018. The visiting fellowship awarded by IRM allowed for Travel and use of their rock-magnetic instrumentation. HB student research grant from the Southwest Section of the American Association of Petroleum Geology. Dr. JG Faculty Start-up support provided by the University of Texas at Dallas Dr. DS Faculty Start-up support provided by Texas Tech University.
Funding Information:
JS thanks the Geological Society of America for Graduate Research Awards no.11267-16 and no.12277-18. JS also acknowledges support awarded through the Dr. Oscar Wilhelm Memorial Student Research Fund at The University of Texas at Dallas (UTD). JS also would like to thank, Leah Thompson and Dr. Ignacio Pujano at UTD for all their help in preparing specimens and interpreting data from the Scanning Electron Microscope. JS would also like to thank the Institute for Rock Magnetism (IRM) at the University of Minnesota for awarding him a visiting fellowship in October, 2018. The visiting fellowship allowed JS to utilize their rock-magnetic instrumentation. The authors would also like to thank Joshua Feinberg (IRM), Dario Bilardello (IRM), and Bruce Moskowitz (IRM) for providing valuable insight on the interpretation of the low-temperature data gathered at the IRM. The IRM is a US National Multi-user Facility supported through the Instrumentation and Facilities Program of the National Science Foundation, Earth Sciences Division, and by funding from the University of Minnesota. HB appreciates the student research grant provided by the Southwest Section of the American Association of Petroleum Geology. JG and DS acknowledge faculty start-up support from UTD and Texas Tech University, respectively. We would also like to thank our three reviewers for their many helpful suggestions.
Funding Information:
JS thanks the Geological Society of America for Graduate Research Awards no.11267-16 and no.12277-18. JS also acknowledges support awarded through the Dr. Oscar Wilhelm Memorial Student Research Fund at The University of Texas at Dallas (UTD). JS also would like to thank, Leah Thompson and Dr. Ignacio Pujano at UTD for all their help in preparing specimens and interpreting data from the Scanning Electron Microscope. JS would also like to thank the Institute for Rock Magnetism (IRM) at the University of Minnesota for awarding him a visiting fellowship in October, 2018. The visiting fellowship allowed JS to utilize their rock-magnetic instrumentation. The authors would also like to thank Joshua Feinberg (IRM), Dario Bilardello (IRM), and Bruce Moskowitz (IRM) for providing valuable insight on the interpretation of the low-temperature data gathered at the IRM. The IRM is a US National Multi-user Facility supported through the Instrumentation and Facilities Program of the National Science Foundation, Earth Sciences Division, and by funding from the University of Minnesota. HB appreciates the student research grant provided by the Southwest Section of the American Association of Petroleum Geology. JG and DS acknowledge faculty start-up support from UTD and Texas Tech University,
Publisher Copyright:
© Copyright © 2021 Stine, Geissman, Sweet and Baird.
PY - 2021/6/17
Y1 - 2021/6/17
N2 - The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.
AB - The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.
KW - blackwater draw formation
KW - continental weathering
KW - environmental magnetism
KW - eolian strata
KW - magnetic mineralogy
KW - mid-pleistocene transition
KW - rock magnetism
KW - soil magnetism
UR - http://www.scopus.com/inward/record.url?scp=85109019275&partnerID=8YFLogxK
U2 - 10.3389/feart.2021.601401
DO - 10.3389/feart.2021.601401
M3 - Article
AN - SCOPUS:85109019275
SN - 2296-6463
VL - 9
JO - Frontiers in Earth Science
JF - Frontiers in Earth Science
M1 - 601401
ER -