Palaeogeography, Palaeoclimatology, Palaeoecology xxx (xxxx) xxx–xxx

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Cool eastern rim of the North Pacific during Late time: A seep- carbonate paleothermometry from the Nanaimo Group, British Columbia, Canada

⁎ Robert G. Jenkinsa, Takashi Hasegawaa, , James W. Haggartb, Akiko S. Gotoa, Yuya Iwasec, Chiharu Nakasec a Department of Earth Sciences, Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan b Geological Survey of Canada, 1500-605 Robson Street, Vancouver, British Columbia V6B 5J3, Canada c Department of Earth Sciences, Division of Natural System, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920- 1192, Japan

ARTICLE INFO ABSTRACT

Keywords: A crater-shaped carbonate body newly recognized in the uppermost Northumberland Formation (Nanaimo Methane seep Group) of the Canadian Pacific coast, is identified as a remnant of methane seepage during latest time in the northeastern Pacific region. A two meter-long elliptical mound with central concavity is found on a Paleotemperature Cretaceous bedding plane observed at Hornby Island, British Columbia, exposed on an expansive intertidal Oxygen isotope platform. Microstructures characteristic of carbonate associated with methane seepage are readily observed AOM within the rock body and indicate it was formed near the sediment-water interface. Carbon isotope values of the carbonate range between −45.5 and +4.9‰, where the lowermost values characterize anaerobic oxidation of biogenic methane as the primary process that controlled the carbonate precipitation. Oxygen isotope values also exhibit a wide range of values, from −9.8 to +0.6‰. Areas preserving features of earliest precipitation (i.e., radiaxial bladed calcite and micrite with clotted fabric) on cross-sections exhibit the highest δ18O values and indicate a paleotemperature 8.0 ± 0.7 °C for seawater at the seafloor. Development of cool water on uppermost bathyal or outer shelf depths in the northeastern North Pacific is thus implied during the Campanian- Maastrichtian transition. This paleotemperature determination shows a significant variance with those obtained from similar paleo-water depths of the northwestern North Pacific.

1. Introduction reported stable isotope values from one and two Cretaceous sequences along the North American Pacific coast and interpreted the Hydrocarbon or methane seep-carbonate bodies have been re- observed δ18O signals as seriously compromised by post-depositional cognized in forearc basin settings where upward seepage from deeper diagenetic processes, including recrystallization. These authors postu- sediments meets with sea water at the seafloor. Some seep-carbonates lated this was resultant from authigenic clay mineral precipitation, have been reported from Cretaceous sequences of the Pacific coast of resulting in further 18O-depleted δ18O values relative to the original North America and were summarized by Campbell (2006). Among four ones. In contrast, δ13C values partly retained the original signal and Cretaceous slope basin sites from California mentioned by Campbell, allowed Campbell et al. (2002) to understand its formative processes. the Guenoc Ranch-Romero Creek site in the upper Campanian Moreno These workers also reviewed Cenozoic and modern seep-carbonates Formation of central California (Elder and Miller, 1993) is the only one from various settings and found that these younger deposits preserved from the Upper Cretaceous; however, no geochemical data have yet the original oxygen and carbon isotope signatures recorded under their been reported from that site. environments of precipitation. Therefore, it is possible to obtain the Carbon and oxygen isotope compositions (δ13C and δ18O) are useful original oxygen and carbon isotopic signatures from Cretaceous de- to characterize seep-carbonates and provide important insight on their posits, provided the preservation of sampled materials is exceptionally origin, namely the carbon source, formation processes, and the bio- good. geochemistry processes occurring at the seafloor. Campbell et al. (2002) In this study, we describe a crater-shaped seep-carbonate body,

⁎ Corresponding author. E-mail address: jh7ujr@staff.kanazawa-u.ac.jp (T. Hasegawa). http://dx.doi.org/10.1016/j.palaeo.2017.09.027 Received 6 June 2017; Received in revised form 21 September 2017; Accepted 25 September 2017 0031-0182/ © 2017 Published by Elsevier B.V.

Please cite this article as: Jenkins, R.G., Palaeogeography, Palaeoclimatology, Palaeoecology (2017), http://dx.doi.org/10.1016/j.palaeo.2017.09.027 R.G. Jenkins et al. Palaeogeography, Palaeoclimatology, Palaeoecology xxx (xxxx) xxx–xxx including its geometry, interior textures, and stable isotope geochem- intervals of magnetic reversals reported from the Northumberland istry, from the Upper Cretaceous Northumberland Formation of the Formation on Hornby Island. The lower interval of magnetic reversal Nanaimo Group of British Columbia, Canada. This study is the first to was identified by Raub et al. (1998) in the middle part of the North- recognize such seep-carbonate bodies along the Pacific coast of Canada umberland Formation section; based on the characteristic signal pair of and provides the opportunity to compare the paleotemperature of the this reversal, and its associated macrofossil data, Ward et al. (2012) bottom water from the North American Pacific coast with that of assigned this reversal to Chron 32r. The higher reversal horizon was comparable deposits of the northwestern Pacific margin (Japan). reported by Enkin et al. (2001; their sample HOR20) from the western The late Campanian age of strata of the Northumberland Formation part of the tidal flat discussed above. Enkin et al. (2001) assigned this on Hornby Island is well constrained based on macrofossil stratigraphy horizon to C32n.1r and Ward et al. (2012) accepted this, noting that the and magnetostratigraphy (Ward et al., 2012), and the strata at this associated biostratigraphic data indicate a late Campanian age. locality are well known for their excellently preserved ammonite The carbonate body we studied and report on herein is found at the with unaltered aragonite shells (Usher, 1952; Ludvigsen and Beard, western end of the intertidal platform on northwestern Hornby Island 1997; Zakharov et al., 2013; McLachlan and Haggart, in press). Low (49°32.931′N, 124°41.914′W; UTM coordinates: 10 U, 377136E, levels of organic maturity of the strata (Enkin et al., 2001) indicate their 5489858 N) (Fig. 1). This location is almost exactly the same as the relatively shallow burial depth. geographic position of Enkin et al.'s (2001) reported reversed sample The Campanian-Maastrichtian transition is a period of global HOR20, within the resolution of mapping (UTM coordinates given in greenhouse cooling (Barrera and Savin, 1999; Huber et al., 2002; Hague Enkin et al., 2001 appear to be inaccurate and may be related to NAD et al., 2012) and possible greenhouse glaciation (Miller et al., 2005), datum issues). As the strata in this area of the intertidal platform are which is still a matter of debate (Huber et al., 2002; Miller et al., 2005). nearly horizontal, the studied carbonate body can be considered to Hague et al. (2012) offered direct evidence of deep or intermediate essentially correspond with the level of the sample HOR20 and the water formation in northern high latitudes of the paleo-Pacific Ocean magnetic reversal C32n.1r. Ogg and Hinnov (2012) suggest that based on neodymium (Nd) isotopes from Maastrichtian deposits at ODP C32n.1r is of earliest Maastrichtian age (~71.8 Ma), but the biostrati- Site 883. Hague et al. (2012) furthermore used geophysical simulations graphic data from Hornby Island (Haggart in Mustard et al., 2003; to infer that bipolar production of deep water and possible bi-cellular Haggart et al., 2009; Ward et al., 2012) indicate these strata are of latest deep water circulation lasted at Site 883 in the Pacific Ocean basin at Campanian age. It is also possible that the reversed locality C32n.1r least from until Paleocene. Unfortunately, direct evi- may actually be a fault-repeated exposure of the Chron 32r interval dence of paleothermometry cannot be obtained from this site as benthic found to the south. carbonate-forming organisms are not preserved there due to dissolu- tion. Thus, the uppermost Northumberland Formation deposit analyzed 3. Material and methods in this study provides a crucial record of sea-bottom paleotemperature and for paleoceanographic discussion, from the viewpoint of its age and The crater carbonate mound is located at the edge of the tidal flat water depth. and exposed above the sea only during the lowest tide, thus, the time available for studying and sampling at the outcrop is limited. Firmly 2. Geologic setting and age attached modern oysters, barnacles, and algae cover almost the entire outside surface of the rock body. The crater carbonate mound has an The carbonate body is within the uppermost Northumberland elliptical or ovate shape with 2.4 m and 1.6 m major and minor axes, Formation (Katnick and Mustard, 2001, 2003) of the Nanaimo Group respectively, a 45 cm height above the bedding plane, and an associated exposed near Collishaw Point on the west coast of Hornby Island, east ~80 cm central concavity. The concavity may be ascribed to a lesser of Vancouver Island, British Columbia, Canada (Fig. 1). The North- degree of cementation at the center of the body, although this was not umberland Formation is exposed here predominantly on an intertidal confirmed as it is totally filled with shells of modern oysters and bar- platform that extends over 1 km2, and is associated with minor faulting nacles, and algae. and folding to the east of the sample locality. As the Upper Cretaceous Samples MNP-103A, B, and C were collected from inside or on the strata on the west coast of Hornby Island are flat to very gently periphery of the concavity, whereas samples MNP-103D and E were northeastward-dipping, the horizon that the carbonate body is found in collected from outside of the concavity (Fig. 2A, B). A fist-sized block of can be traced laterally over a distance of > 100 m. carbonate from each sample site was cut perpendicular to the hor- Northumberland Formation strata on Hornby Island have been in- izontal plane and polished with a series of carborundum powders. The terpreted previously as being of deep-water origin, based on the pre- polished slabs were observed by both naked-eye and binocular micro- sence of sedimentological structures, primarily slump facies, suggesting scope. A thin-section of selected areas of MNP-103C was made and a depth of ~500 m, or upper slope depths (Katnick and Mustard, 2001). described under plane- and cross-polarized light (Fig. 3C–F). Two to However, this view has been challenged by Haggart (in Mustard et al., five subsamples were taken from each polished slab for isotopic study. 2003), who suggested that the rich and diverse benthic and nektic A dental grinder was used to extract an aliquot of powder from each molluscan assemblage present within the Northumberland For- subsample position, as shown in Fig. 3A, B. Powdered samples were mation on Hornby Island suggests an appreciably shallower marine dried overnight, then weighed (ca. 0.3 mg each) with a Sartorius MC5 setting, probably mid- to outer shelf depths, ca. ~100–300 m. McGugan microbalance and processed. Carbon dioxide was produced by reaction (1979) also suggested an outer neritic environment for the North- with 100% phosphoric acid in helium atmosphere at 70 °C through umberland Formation on a regional level, based on benthic for- GasBenchII (ThermoFisher), and the gas was introduced and analyzed aminiferal paleontology, and this interpretation has been corroborated by a Delta V Advantage (ThermoFisher) mass spectrometer. Carbon and by Cameron (1988) for strata on Hornby Island. Furthermore, we note oxygen isotopic compositions were expressed in the conventional δ that slump deposits are not restricted to slope settings, but can be found notation relative to the VPDB standard. The reference NBS19 (δ18O: in shallow marine stratigraphic successions with steep topography. −2.20‰; δ13C: +1.95‰) was analyzed twice daily and used for in- Based on the available evidence, we suggest a paleodepth of 100–300 m ternational calibration. Laboratory standard carbonates were analyzed (i.e., mid- to outer shelf) is most plausible for the Northumberland more often to check the stability of the machine. Analytical errors of the Formation on Hornby Island. isotopic measurement were < 0.1‰ for both oxygen and carbon iso- Ward et al. (2012) discussed and summarized the chronostrati- topes during the analyses of all samples studied, based on repeated graphy of the Nanaimo Group succession on Hornby Island, based on analyses of these standard materials. Carbonate content is roughly es- macrofossil biostratigraphy and magnetostratigraphy. There are two timated from comparison of the peak area between NBS19 and samples

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