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In Situ Magnetic Identification of Giant, Needle-Shaped Magnetofossils in Paleocene–Eocene Thermal Maximum Sediments

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In Situ Magnetic Identification of Giant, Needle-Shaped Magnetofossils in Paleocene–Eocene Thermal Maximum Sediments In situ magnetic identification of giant, needle-shaped magnetofossils in Paleocene–Eocene Thermal Maximum sediments Courtney L. Wagnera,1, Ramon Eglib, Ioan Lascuc, Peter C. Lipperta,d, Kenneth J. T. Livie, and Helen B. Searsf aDepartment of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112; bDivision of Data, Methods and Models, Central Institute of Meteorology and Geodynamics, 1190 Vienna, Austria; cDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560; dGlobal Change and Sustainability Center, University of Utah, Salt Lake City, UT 84112; eMaterials Characterization and Processing Center, Department of Materials Sciences and Engineering, Johns Hopkins University, Baltimore, MD 21218; and fDepartment of Geology, Colby College, Waterville, ME 04901 Edited by Lisa Tauxe, University of California San Diego, La Jolla, CA, and approved November 24, 2020 (received for review August 27, 2020) Near-shore marine sediments deposited during the Paleocene– These magnetofossils are interpreted to be the predominant Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant source of the PETM magnetic enhancement of these cores (7, conventional and giant magnetofossils. We find that giant, 11–14), although alternative sources have been suggested (15–18). needle-shaped magnetofossils from Wilson Lake produce distinct Giant magnetofossils have so far only been identified in sediments magnetic signatures in low-noise, high-resolution first-order rever- from the PETM and the Middle Eocene Climatic Optimum, sal curve (FORC) measurements. These magnetic measurements on leading to the interpretation that they are unique to hyperthermal bulk sediment samples identify the presence of giant, needle- events(6,7,11–14). For example, Chang et al. (6) suggest that shaped magnetofossils. Our results are supported by micromag- giant magnetofossils are linked to oceanic deoxygenation during netic simulations of giant needle morphologies measured from the PETM. transmission electron micrographs of magnetic extracts from Wil- Previous studies, which used micromagnetic simulations, son Lake sediments. These simulations underscore the single- electron holography, or both, suggest that giant magnetofossils domain characteristics and the large magnetic coercivity associ- have distinct magnetic properties (6, 11, 14, 18, 19). These in- ated with the extreme crystal elongation of giant needles. Giant EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES terpretations are limited, however, by assumptions regarding magnetofossils have so far only been identified in sediments de- posited during global hyperthermal events and therefore may crystal arrangement, spacing, and magnetic domain structure, serve as magnetic biomarkers of environmental disturbances. and they lack independent confirmation of defining character- Our results show that FORC measurements are a nondestructive istics. Additionally, some of these methods have been applied to method for identifying giant magnetofossil assemblages in bulk only a few giant magnetofossil morphologies. sediments, which will help test their ecology and significance with Here we show independent, physical evidence of the magnetic respect to environmental change. signature of giant magnetofossils in situ (i.e., not in extracts) using low-noise, high-resolution first-order reversal curves magnetofossils | magnetotactic bacteria | first-order reversal curves | (FORCs). FORC routines measure the response of all magnetic micromagnetic modeling | biogenic needles particles, including giant magnetofossils, within a bulk sediment he Paleocene–Eocene Thermal Maximum (PETM; ∼56 Ma) Significance Tis a geologically rapid global warming event with many characteristics that make it an analog for the Anthropocene (1, Giant magnetofossils are the preserved remains of iron- 2). These characteristics include rapid warming of the sea surface biomineralizing organisms that have so far been identified and atmosphere, increased seasonality of precipitation and only in sediments deposited during ancient greenhouse cli- temperature, and biological extinctions (1). The PETM is iden- mates. Giant magnetofossils have no modern analog, but their tified globally by a −3‰ carbon isotope excursion (CIE) in bulk association with abrupt global warming events links them to marine carbonate and is characterized by three stratigraphic in- environmental disturbances. Thus, giant magnetofossils may tervals: 1) a preonset excursion, 2) the main CIE, and 3) a re- encode information about nutrient availability and water covery toward baseline δ13C levels (1). The main CIE interval is stratification in ancient aquatic environments. Identification of further subdivided into the CIE onset and CIE core, which giant magnetofossils has previously required destructive ex- correspond to the first ∼6 to 10 kyr and 100 to 200 kyr of the traction techniques. We show that giant, needle-shaped mag- PETM (1, 3, 4). The Wilson Lake A (WL-A) core from Wilson netofossils have distinct magnetic signatures. Our results Lake, NJ, contains a continental shelf section of the PETM provide a nondestructive method for identifying giant mag- within the Marlboro Clay and, within the Marlboro Clay, an netofossil assemblages in bulk sediments, which will help test expanded and nearly complete record of the CIE onset and CIE their significance with respect to environmental change. core (1, 5). Several near-shore and offshore cores complement Author contributions: C.L.W. designed research; C.L.W., R.E., I.L., P.C.L., K.J.T.L., and H.B.S. the WL-A record and enable a broader understanding of how performed research; C.L.W., R.E., I.L., P.C.L., and K.J.T.L. analyzed data; C.L.W., P.C.L., and Paleogene coastal ecosystems responded to the rapid onset K.J.T.L. provided TEM interpretations; R.E. provided FORC interpretations; I.L. provided of global hyperthermal conditions (5–8). The New Jersey con- micromagnetic interpretations; and C.L.W., R.E., I.L., and P.C.L. wrote the paper. tinental shelf experienced an overall rapid influx of clay, min- The authors declare no competing interest. eralization of iron oxides, dinoflagellate blooms, and benthic This article is a PNAS Direct Submission. foraminifera species turnover coincident with the CIE onset Published under the PNAS license. (5,9,10). 1To whom correspondence may be addressed. Email: [email protected]. Abundant conventional and giant magnetofossils, the fossil re- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ mains of magnetotactic bacteria and other iron-biomineralizing doi:10.1073/pnas.2018169118/-/DCSupplemental. microorganisms, were identified in several New Jersey cores. Published February 1, 2021. PNAS 2021 Vol. 118 No. 6 e2018169118 https://doi.org/10.1073/pnas.2018169118 | 1of7 Downloaded by guest on September 25, 2021 sample. We also present micromagnetic simulations of giant also been proposed (15, 16, 18, 20). One of the reasons for the needle-shaped crystals whose morphologies were characterized ambiguity in interpreting the origin of magnetic nanoparticles in with transmission electron microscopy (TEM) of magnetic ex- the Marlboro Clay is the selective extraction of magnetofossils tracts. Our results show that giant, needle-shaped magnetofossils with large magnetic moments, in contrast to abiotic iron oxides; produce a high-coercivity component distinct from conventional this selective extraction might explain their dominance in TEM magnetofossils that is identified using a specific FORC mea- observations (18, 20). Here we provide a detailed characteriza- surement protocol. We argue that these are definitive magnetic tion of the magnetofossil signature in a bulk sediment sample by signatures of giant magnetofossils, which further supports the comparing low-noise, high-resolution FORC measurements on interpretation that the magnetic enhancement of the WL-A core specimen WL35950b, from the CIE onset interval at WL-A, to has a biogenic origin. The link between giant magnetofossils, that of BAL13, a sediment sample from Lake Baldeggersee in hyperthermal events, and oceanic deoxygenation makes the Switzerland (Fig. 1), where the magnetic signature of conven- magnetic signature of giant needles a powerful tool for identifying tional magnetofossils was isolated for the first time (21). The giant magnetofossil assemblages and, by extension, testing their magnetic signature of conventional magnetofossils consists of ecological significance in the context of global change events in the two narrow coercivity components associated with single-domain geologic record. particles, referred to as biogenic soft and biogenic hard (21, 22). These magnetic characteristics have been confirmed by analysis Results of several freshwater and marine magnetofossil-rich sediments Low-Noise, High-Resolution FORCs. Previous studies suggest that (23–26) and are therefore considered representative of conven- the magnetic enhancement of the WL-A core is due to abundant tional magnetofossils. The single-domain character and nar- magnetofossils (12, 13) found in magnetic extracts. Other sour- rowness of these biogenic coercivity components reflect the ces of single-domain magnetite particles, such as debris from a tightly genetically controlled biomineralization process in pro- comet or pyrogenic magnetite produced during wildfires, have ducing highly uniform magnetic structures. AB CD F E Fig. 1. Low-noise, high-resolution FORC measurements
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