https://doi.org/10.1130/G48591.1

Manuscript received 6 April 2020 Revised manuscript received 5 November 2020 Manuscript accepted 7 January 2021

© 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 4 March 2021

Magnetostratigraphy of U-Pb–dated boreholes in Svalbard, Norway, implies that magnetochron M0r (a proposed Barremian-Aptian boundary marker) begins at 121.2 ± 0.4 Ma Yang Zhang1*, James G. Ogg2,1†, Daniel Minguez3, Mark W. Hounslow4, Snorre Olaussen5, Felix M. Gradstein6 and Selen Esmeray-Senlet3 1Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907-2051, USA 2State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu, Sichuan 610059, China 3Chevron Energy Technology Company, 1500 Louisiana Street, Houston, Texas 77002, USA 4Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YB, UK 5Department of Arctic Geology, University Centre in Svalbard, PO Box 156, 9171 Longyearbyen, Norway 6Geology Museum, University of Oslo, N-0318 Oslo, Norway

ABSTRACT 121.5 Ma age suggested in earlier scales (e.g., The age of the beginning of magnetic polarity Chron M0r, a proposed marker for the Channell et al., 1995). A younger age was partly base of the Aptian Stage, is disputed due to a divergence of published radioisotopic dates and supported by 40Ar/39Ar dates of 122.0 ± 0.5 Ma ambiguities in stratigraphic correlation of sections. Our magnetostratigraphy of core DH1 on basalt flows yielding reversed polarity in from Svalbard, Norway, calibrates a bentonite bed, dated by U-Pb methods to 123.1 ± 0.3 Ma, northeastern China that were interpreted as be- to the uppermost part of magnetozone M1r, which is ∼1.9 m.y. before the beginning of Chron longing to magnetozone M0r (He et al., 2008), M0r. This is the first direct calibration of any high-precision radioisotopic date to a polarity although biostratigraphic constraints and bound- chron of the M sequence. The interpolated age of 121.2 ± 0.4 Ma for the beginning of Chron ing magnetozones were lacking. M0r is younger by ∼5 m.y. than its estimated age used in the Geologic Time Scale 2012, However, U-Pb dates published after which had been extrapolated from radioisotopic dates on oceanic basalts and from Aptian 2012, which used an isotope dilution–thermal cyclostratigraphy. The adjusted age model implies a commensurate faster average global ionization mass spectrometry (ID-TIMS) method oceanic spreading rate of ∼12% during the Aptian–Santonian interval. Future radioisotopic ­coupled with new techniques for processing of dating and high-resolution cyclostratigraphy are needed to investigate where to expand the zircons and EARTHTIME standards (http:// mid-Jurassic to earliest Cretaceous interval by the required ∼4 m.y. www​.earthtimetestsite.com/working-groups/u- pb-isotope-dilution/), seem to suggest that nearly INTRODUCTION In particular, the age model for the Ap- all of the 40Ar/39Ar dates derived from ODP basalt Rates of plate tectonic motions, biologic tian Stage in the Geologic Time Scale 2012 sites and most of the pre-2012 U-Pb dating of evolution, geochemical excursions, and other (GTS2012; Gradstein et al., 2012) had used ca. volcanic ashes were systematically too old by processes in Earth’s history depend on an ac- 126 Ma for the beginning of Chron M0r, the an average of ∼4 m.y. (Fig. 1B). For example, curate geologic time scale. The high-resolution proposed magnetozone marker for the base of a bentonite bed in the uppermost Hauterivian time scale for the Late Jurassic through Early the Aptian Stage (Erba et al., 1996). That age of Argentina that had yielded a U-Pb date of Cretaceous is compiled mainly from the cor- estimate was based on (1) the consistency of 132.5 ± 1.3 Ma was re-dated by the ID-TIMS relation of biostratigraphy to the M sequence 40Ar/39Ar and U-Pb radioisotopic dating of oce- method at 129.09 ± 0.16 Ma (Aguirre-Urreta of magnetic polarity chrons, and the durations anic basalts and volcanic ash beds from Ocean et al., 2015). of many of those biozones and polarity chrons Drilling Program (ODP) sites, from the Great Critical to this study is a bentonite layer in the have been derived from cyclostratigraphy on Valley Group in California (USA), and from Helvetiafjellet Formation in Svalbard, Norway, reference sections (e.g., Channell et al., 1995; Argentina (Fig. 1A); and (2) an assumed dura- dated using ID-TIMS U-Pb at 123.1 ± 0.3 Ma Sprovieri et al., 2006; Malinverno et al., 2012). tion of 13 m.y. for the Aptian Stage according to (Corfu et al., 2013; Midtkandal et al., 2016). cyclostratigraphic interpretation of the Piobbico Based on regional palynology and a negative 13 *Current address: Faculty of Geosciences, Uni- core of central Italy (Huang et al., 2010) relative carbon-isotopic (δ Corg, org—organic carbon) versity of Bremen, Klagenfurter Street 2-4, 28359 to a U-Pb date of 113.1 ± 0.3 Ma near the Ap- excursion in the overlying Carolinefjellet For- Bremen, Germany tian-Albian boundary (Selby et al., 2009). This mation (Midtkandal et al., 2016), which was †E-mail: [email protected] 126 Ma age was significantly older than the ca. proposed to be equivalent to the δ13C excursion

CITATION: Zhang, Y., et al., 2021, Magnetostratigraphy of U-Pb–dated boreholes in Svalbard, Norway, implies that magnetochron M0r (a proposed Barremian- Aptian boundary marker) begins at 121.2 ± 0.4 Ma: Geology, v. 49, p. 733–737, https://doi.org/10.1130/G48591.1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/6/733/5323341/g48591.1.pdf by guest on 02 October 2021 periments including thermal demagnetization ABFigure 1. (A) Radioiso- topic dates (in Ma) used of orthogonal isothermal remanent magnetiza- in the Geologic Time Scale tion (IRM) and rotational remanent magneti- 2012 (GTS2012; Gradstein zation (RRM). Additional details are discussed et al., 2012). (B) Selected radioisotopic dates (in Ma) in the Supplemental Material. The characteris- published since 2012 that tic remanent magnetization (ChRM) for each imply systematic offset to sample was computed by a three-dimensional younger ages by ∼4 m.y. “least-squares fitting” technique (Kirschvink, Dates in both columns are positioned according to 1980) using the public software PaleoMagX their constraints from bio- (Jones, 2002), and a subset was analyzed us- stratigraphic assignment ing LINEFIND (Kent et al., 1983) utilizing the to geologic stages or from variance of each measurement. Quality ratings magnetic polarity (central on each ChRM and polarity interpretation were columns). All 40Ar/39Ar dates are recalibrated to assigned [N (or R)—confident; NP (RP)—val- the Fish Canyon Tuff mon- id; NPP (RPP)—probable; N? (R?)—possible; itor standard of 28.20 Ma or INT—uncertain; where N refers to normal (Kuiper et al., 2008). Two polarity, and R to reversed polarity] based on dates (in green) from MIT Guyot (Pacific Ocean) the stability of the magnetic vectors during the include alternate assign- progressive demagnetization (Tables S2 and S3 ment to polarity chrons in the Supplemental Material). Examples of the in the right column. The quality ratings are illustrated in Figure S3. 123.1 ± 0.3 Ma date (in red) is from the bentonite layer in the Helvetiafjellet RESULTS AND DISCUSSION Formation (Svalbard). See The coercivity and IRM results (Fig. S1) in- Table S1 (see footnote 1) dicate mixed magnetic mineralogy in the sam- for geochronology locali- ples, with magnetite being the main phase of ties and references for [1] to [13]. primary remanent magnetization carriers with some minor contributions from detrital pyrrho- tite or maghemite. segment “C3” accompanying the onset of Oce- the Helvetiafjellet Formation is on a subaerial The magnetostratigraphy of the Helvetiafjel- anic Anoxic Event 1a (OAE1a) of the midle– unconformity surface of early Barremian age let Formation in core DH1 yielded three pairs early Aptian (e.g., Menegatti et al., 1998; Erba (Fig. 2B; Śliwińska et al., 2020). A 20-cm- of reversed- and normal-polarity magnetozones et al., 1999), the bentonite was interpreted as thick bentonite layer occurs in boreholes DH3 (Hv0 to Hv2) based on the higher-quality-rated being of midle–late Barremian age. A postu- and DH7 (Corfu et al., 2013; Midtkandal et al., samples (Fig. 2). Reversed-polarity magne- lated triggering cause of that global negative 2016) and projects to just below a fluvial sand- tozone Hv0r in the uppermost Helvetiafjel- δ13C excursion is the eruption of the immense stone unit at about level 180 m in borehole DH1 let Formation occurs just below the negative 1 13 Ontong Java Plateau large igneous province, (see Section S1 in the Supplemental Material ). δ Corg excursion documented by Midtkandal which rapidly released isotopically light carbon The bentonites are considered to be a product of et al. (2016; Fig. 2). This relationship is appar- into the ocean-atmosphere system (e.g., Erba the regional High Arctic large igneous province ently very similar to the stratigraphic position 13 et al., 2015). (e.g., Polteau et al., 2016). The transgressive of magnetozone M0r below the OAE1a δ Ccarb We collected magnetostratigraphic data base of the overlying mudstone-rich Caroline- (carb—carbonate) excursion within the Aptian from a core (DH1; Fig. 2) penetrating the Hel- fjellet Formation is a reworked deposit mark- reference sections in northern Italy (e.g., the vetiafjellet Formation to accurately place the ing the flooding of the Helvetiafjellet Formation Cismon core; Erba et al.,1999, 2015; Fig. 2). U-Pb–dated 123.1 Ma bentonite into the global coastal plain (Midtkandal et al., 2016; Grundvåg Guided by recent constraints from palynol- framework of the Barremian through earliest et al., 2019). Details are discussed in the Supple- ogy and dinocysts (Śliwińska et al., 2020), the Aptian magnetic polarity time scale, thereby mental Material. base of the Helvetiafjellet Formation appears helping to resolve the disputed age model for Paleomagnetic minicores were drill-pressed to be no older than middle–early Barremian. parts of the Early Cretaceous. at ∼1 m spacing from levels 130 m to 215 m Therefore, the magnetozones Hv1 and Hv2 of core DH1, spanning the entire Helvetiafjel- of paired normal and reversed polarity under- MATERIALS AND METHODS let and the lowermost part of the Carolinefjellet lying Hv0r are correlated to magnetochrons The cores investigated in this study were Formations (Fig. 2). The paleomagnetic direc- M1n, M1r, M3n, and uppermost M3r, respec- retrieved from a series of research wells (wells tions of the 110 minicores were obtained using tively (blue guidelines in Fig. 2). This implies DH1, DH3, and DH7; Fig. 2D) drilled in relation a composite scheme of thermal demagnetiza- that the bentonite dated at 123.1 ± 0.3 Ma by to a carbon-sequestration project (Longyearbyen tion to 200–300 °C (as dictated by lithology) Corfu et al. (2013) occurs in the uppermost part

CO2 Lab, http://co2-ccs.unis.no/) and are stored followed by alternating field (AF) treatments. of magnetozone M1r. in the University Centre in Svalbard (UNIS; This scheme was guided by rock magnetic ex- Cyclostratigraphy of Italian sections yields Longyearbyen, Svalbard). In these wells, the durations for magnetochrons M1r and M1n of Helvetiafjellet Formation is an ∼75-m-thick 0.20 m.y. and 1.85 m.y., respectively (Sprovieri paralic succession sandwiched between open- 1Supplemental Material. Additional details on et al., 2006). Therefore, the placement of the marine shelfal facies of the lower Valanginian paleomagnetic analyses of core DH1, supplemental bentonite bed (123.1 Ma) within magnetozone Figures S1–S3, and Tables S1–S5. Please visit to lowermost Barremian Rurikfjellet Forma- https://doi.org/10.1130/GEOL.S.14047025 to access M1r is 1.9 m.y. (±0.1 m.y.) prior to the onset tion and the Aptian to Albian Carolinefjellet the supplemental material, and contact editing@ of magnetozone M0r, implying that Chron M0r Formation (Grundvåg et al., 2019). The base of geosociety.org with any questions. begins at 121.2 Ma. This interpolated age has an

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/6/733/5323341/g48591.1.pdf by guest on 02 October 2021 A B

C

D

Figure 2. Magnetostratigraphy of Svalbard, Norway, borehole DH1 (drilled by Longyearbyen CO2 Lab, http://co2-ccs.unis.no/) (B) and its cor- relation (blue lines) to the Geologic Time Scale 2012 (GTS2012) magnetic polarity scale (Gradstein et al., 2012) (A) in which the base of the Aptian Stage (set as base of Chron M0r) was placed at 126.3 Ma (H.—Hauterivian). Circles in the magnetostratigraphy represent the quality ratings (N, NP, NPP, etc.; see the Materials and Methods section) of magnetic polarity of each sample. Biostratigraphic constraints include palynomorphs of interpreted Barremian and Hauterivian ages and dinoflagellate cyst of Aptian age (Vesperopsis longicornis) (LO—last occur- rence). Carbon-isotope curve from core DH1 (modified from Midtkandal et al., 2016) is compared to the Cismon core of Italy (C), which shows 13 δ Ccarb (carb—carbonate) data and “C” segments according to Menegatti et al. (1998), Oceanic Anoxic Event 1a (OAE1a; shaded in gray), and magnetostratigraphy (VPDB—Vienna Peedee belemnite; Nanno.—Nannofossil zones). Level of the U-Pb date (in red; Corfu et al., 2013) in the DH1 core is projected from two nearby bentonite-bearing cores (DH3 and DH7) with well locations shown in the location map (D). VGP—virtual geomagnetic pole; lat.—latitude; ChRM—characteristic remanent magnetization; inc.—inclination.

uncertainty of ∼0.4 m.y. from its relative place- ammonite zones and microfossil datums of the 2019, 4 July 2019, Milan, Italy) are (1) the base ment within magnetozone Hv1r (±0.1 m.y.) Tethyan and Subboreal regions have calibrations of magnetozone M0r, (2) the onset of negative and the total uncertainty on the U-Pb date relative to Chron M0r and to carbon-isotope δ13C excursion “C3” or the beginning of OAE1a, (±0.3 m.y., which includes external uncertain- trends, and some ammonite zones have dura- or (3) the base of ammonite zone Deshayesites ties). This derived 121.2 Ma age for the onset tions derived directly from cyclostratigraphy oglanlensis or another biological datum. The of Chron M0r is significantly younger than the (e.g., Frau et al., 2018; Frau, 2020; Luber et al., indicated 121.2 Ma age in the DH1 core (Fig. 2) ca. 126 Ma age used in GTS2012 and younger 2019; Gale et al., 2020; Martinez et al., 2020). for the base of Chron M0r thus enables age es- than a recent range estimate of between 123.8 The interpolated 121.2 Ma age for the base of timates for other markers (Fig. 3). and 121.8 Ma determined by reevaluation of the Chron M0r enables the assignment of an age constraints from published radioisotopic dates model for this integrated bio-magneto-isotopic IMPLICATIONS AND CONCLUSIONS (Olierook et al., 2019). stratigraphic scale (Fig. 3). According to the estimated 121.2 Ma age The duration of Chron M0r is estimated as Three of the markers currently under discus- for the beginning of Chron M0r, the Cretaceous 0.5 m.y. (e.g., Huang et al., 2010), whereas the sion for assigning the base of the Aptian Stage Normal Superchron spanning the Aptian to the onset of OAE1a occurred 0.3 m.y. after the end (i.e., meeting minutes of the International Sub- beginning of Chron C33r (beginning of the of Chron M0r (e.g., Malinverno et al., 2010). commission on Cretaceous Stratigraphy, Third Campanian Stage) becomes 5 m.y. shorter than Several latest Barremian through earliest ­Aptian International Congress on Stratigraphy, STRATI its estimated span in GTS2012. This revised age

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/6/733/5323341/g48591.1.pdf by guest on 02 October 2021 Figure 3. Revised age model for the latest Bar- remian through earliest Aptian with selected zones of Subboreal and Tethyan ammonites (modified nomenclature, durations, and calibra- tions from Reboulet et al., 2018; Martinez et al., 2020; C. Frau, 2020), planktonic foraminifers (Huber and Petrizzo, 2014; Coccioni, 2019), and calcareous nannoplankton (Bottini et al., 2015). Schematic 13 δ Ccarb (carb—carbonate) curve (Cramer and Jarvis, 2020) and duration of the organic-rich shale event Oceanic Anoxic Event 1a (OAE1a; also known as the Selli or Goguel event) are scaled to ammonite zones accord- ing to the compilation by Frau (2020). Correlation constraints adopted here are from Gale et al. (2020) and include (1) base of Deshayesites deshayesi Zone coeval with base of δ13C segment C6; (2) base of D. forbesi coeval with base of OAE1a (or base δ13C segment C3); (3) base of D. oglanlensis Zone lying just below top of Chron M0r; and (4) base of Toxancyloceras van- denheckii Zone at base of Chron M1r (see also Table S5 [see footnote 1]). Astronomically tuned durations of Tethyan Bar- remian ammonite zones and of D. oglanlensis are from Martinez et al. (2020). Three candidate markers for the base of the Aptian Stage in this age model are (1) base of OAE1a (or C3; marked by red dashed line) at 120.4 Ma, (2) base of D. oglanlensis Zone at 120.8 Ma (blue dashed line), and (3) base of Chron M0r at 121.2 Ma. The age model has an external uncertainty of ∼0.4 m.y., and placements of zonal boundaries and events relative to each other and durations of zones have an estimated uncertainty of ∼0.1–0.2 m.y. A modified version of this compilation using a slightly different age model is incorporated in the Geologic Time Scale 2020 (Gale et al., 2020).

model indicates a commensurate ∼12% increase of Oxfordian through Barremian biozones, the Aptian Stage can be resolved by applying in average global oceanic spreading rates during substages, and magnetic polarity zones (e.g., ID-TIMS methods with EARTHTIME standards the Aptian–Santonian interval. A similar conclu- Martinez et al., 2020; and reviews in Gradstein to re-date the California volcanic ash layers; by sion, but of slightly lesser magnitude (∼6%), et al. [2012], and in Gale et al. [2020]) imply acquiring and verifying cyclostratigraphy of the was reached by Olierook et al. (2019) from their that many of these must retain approximately Kimmeridgian through Aptian from outcrops, reevaluation of published radioisotopic dates. the same durations as compiled in GTS2012. ODP cores and other boreholes; and by acquiring The implied ∼5 m.y. shortening of the dura- We thus suggest that a significant portion of the additional radioisotopic dates from successions tion of the Aptian Stage relative to its span in required expansion of the age model would be that have precise age frameworks established GTS2012 does not necessarily imply that the for the relatively brief Middle Jurassic stages, from bio-, magneto- and cyclostratigraphy. duration of the underlying Barremian Stage which were constrained by the 40Ar/39Ar date becomes longer. Instead, it appears that the of 168.7 ± 1.7 Ma on ODP Site 801C on Pa- ACKNOWLEDGMENTS array of new U-Pb dates (Fig. 1) requires that cific magnetic anomaly M42n.4r (Koppers et al., We thank editor James Schmitt, Elisabetta Erba, an expansion by ∼4 m.y. should be distributed 2003; Tominaga et al., 2008). Helmut Weissert, Andrew Gale, Sten-Andreas across portions of the age model for much of In the future, the distribution of the suggested Grundvåg, Camille Frau, Kenneth Kodama, and an anonymous reviewer for their constructive suggestions the Middle Jurassic through Barremian (i.e., a ∼4 m.y. expansion and upward shifting of the on this paper. This study was inspired by numerous time period of ∼50 m.y.). Numerous consis- age models for the Middle Jurassic through Bar- discussions with Beatriz Aguirre-Urreta, Víctor A.

tent cyclostratigraphy studies of the durations remian and of the commensurate shortening of Ramos, and Elizabeth Johnson. We thank UNIS CO2

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/6/733/5323341/g48591.1.pdf by guest on 02 October 2021 LAB (University Centre Svalbard, Longyearbyen CO2 Cerbara (central Italy): An alternative explanation Paleoceanography, v. 25, PA2203, https://doi​ Lab) for access to core material and logistics, and of the bio-, litho- and chemostratigraphic mark- .org/10.1029/2009PA001769. Julian Janocha for his careful drill pressing; Anita ers: Newsletters on Stratigraphy, v. 51, p. 311– Malinverno, A., Hildebrandt, J., Tominaga, M., de Chiara and Vassil Karloukovski for their help in 326, https://doi.org/10.1127/nos/2017/0422. and Channell, J.E.T., 2012, M-sequence geo- the Lancaster Environment Centre (UK) lab work; Gale, A.S., Mutterlose, J., and Batenburg, S., 2020, magnetic polarity time scale (MHTC12) that and sponsorship from the LoCrA (Lower Cretaceous The Cretaceous Period, in Gradstein, F.M., et al., steadies global spreading rates and incorporates basin studies in the Arctic) consortium managed by eds., Geologic Time Scale 2020: Boston, Elsevier, astrochronology constraints: Journal of Geo- the University of Stavanger (Stavanger, Norway) and p. 1023–1086. https://doi.org/10.1016/B978-0- physical Research, v. 117, B06104, https://doi​ the UNIS. 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