Data report: refinement of calcareous nannofossil biostratigraphy from the late Oligocene to the Pleistocene, IODP Expedition 363 Hole U1490A

An Oligocene to recent sedimentary sequence was recovered in Hole U1490A in the northern portion of the Eauripik Rise (western Pacific Ocean) during International Ocean Discovery Program Expedition 363. High-resolution sampling and moderate to good calcareous nannofossil preservation allowed us to adjust the depths for nannofossil events reported shipboard. This study identified 22 zonal boundary markers and 30 secondary calcareous nannofossil events. Because of better preservation of the calcareous nannofossils in the Pliocene–Pleistocene units, more calcareous nannofossil events were observed in that interval than in the older units. In the Miocene units, the discoasters and helicosphaerids (which are important zonal boundary markers) are poorly preserved (fragmented and recrystallized), making it difficult to identify nannofossil zones. In lieu of zonal boundary markers, secondary calcareous nannofossil events were used to refine the biostratigraphy in the Miocene interval. Introduction International Ocean Discovery Program (IODP) Site U1490 is located in the northern portion of the Eauripik Rise, which is a north–south trending tectonically stable elevated ridge separating the East and West Caroline Basins. The site is situated at 05°48.95ʹN, 142°39.27ʹE at a water depth of 2341 m (Figure F1). Three holes were drilled at Site U1490: Holes U1490A, U1490B, and U1490C. Hole U1490A recovered ~380 m of upper Oligocene to recent sediments. Compared to the sediments recovered from other sites cored during Expedition 363, Site U1490 has a relatively low sedimentation rate. The sedimentation rates varied through the recovered intervals and ranged from 0.9 cm/ky in the early to middle Miocene to 5 cm/ky in the late Oligocene, whereas the average sedimentation rate for the whole core was ~1.5 cm/ky. This rate is lower than the rates at nearby sites, including Sites U1488 (~2.5 cm/ky) and U1489 (~2 cm/ky). The sediments from Site U1490 contain calcareous microfossils (nannofossils and foraminifers), siliceous microfossils (radiolarians and diatoms), sponge spicules, clay minerals, and ash. The hole was divided into three subunits (IA, IB, and IC) during shipboard description based on the lithology and composition (i.e., kind of biogenic material, type of clay minerals, and the presence or absence of volcanic ash) (Rosenthal et al., 2018b). A generalized lithologic log for Site U1490 is shown in Figures F2 and F3. Subunit IA is ~185 m thick and is composed of early Miocene to recent calcareous nannofossil and foraminifer ooze and low amounts of clay minerals and volcanic ash (Figure F2). Subunit IB is ~78 m thick and ranges in age from late early to early late Miocene (Figure F3). Compared to the previous unit, a significant increase in clay minerals and biosiliceous material was observed in Subunit IB. Subunit IC is a 124 m thick early Oligocene to middle Miocene succession. The siliceous particles that dominate this subunit have distinct dark gray to black indurated layers and nodules and chert fragments (Figure F3) (Rosenthal et al., 2018a). Shipboard calcareous nannofossil biostratigraphic study of core catcher samples (~10 m interval), supplemented in some parts by samples collected from selected sections, recognized 30 calcareous nannofossil events; 15 of these events were zonal boundary markers, and the other 15 were supplementary events. The Pleistocene interval was constrained by 7 calcareous nannofossil events, the Pliocene was constrained by 4 events, the Miocene was constrained by 17 events, and the Oligocene was constrained by 2 events. The top common occurrence of Cyclicargolithus abisectus, which has an estimated age of ~24 Ma (Rio et al., 1990; Backman et al., 2012), was found near the base of the hole (Rosenthal et al., 2018a). This study refines the shipboard biostratigraphy of Hole U1490A by examining sediments collected at a higher resolution sampling interval (~1.5 m) of one sample per section. Y.I.L. Doyongan and A.G.S. Fernando Data report: refinement of calcareous nannofossil biostratigraphy IODP Proceedings 2 Volume 363 Figure F1. Location of Site U1490.


Introduction
International Ocean Discovery Program (IODP) Site U1490 is located in the northern portion of the Eauripik Rise, which is a north-south trending tectonically stable elevated ridge separating the East and West Caroline Basins. The site is situated at 05°48.95ʹN, 142°39.27ʹE at a water depth of 2341 m ( Figure F1). Three holes were drilled at Site U1490: Holes U1490A, U1490B, and U1490C. Hole U1490A recovered ~380 m of upper Oligocene to recent sediments. Compared to the sediments recovered from other sites cored during Expedition 363, Site U1490 has a relatively low sedimentation rate. The sedimentation rates varied through the recovered intervals and ranged from 0.9 cm/ky in the early to middle Miocene to 5 cm/ky in the late Oligocene, whereas the average sedimentation rate for the whole core was ~1.5 cm/ky. This rate is lower than the rates at nearby sites, including Sites U1488 (~2.5 cm/ky) and U1489 (~2 cm/ky). The sediments from Site U1490 contain calcareous microfossils (nannofossils and foraminifers), siliceous microfossils (radiolarians and diatoms), sponge spicules, clay minerals, and ash. The hole was divided into three subunits (IA, IB, and IC) during shipboard description based on the lithology and composition (i.e., kind of biogenic material, type of clay minerals, and the presence or absence of volcanic ash) (Rosenthal et al., 2018b).
A generalized lithologic log for Site U1490 is shown in Figures  F2 and F3. Subunit IA is ~185 m thick and is composed of early Miocene to recent calcareous nannofossil and foraminifer ooze and low amounts of clay minerals and volcanic ash ( Figure F2). Subunit IB is ~78 m thick and ranges in age from late early to early late Miocene ( Figure F3). Compared to the previous unit, a significant increase in clay minerals and biosiliceous material was observed in Subunit IB. Subunit IC is a 124 m thick early Oligocene to middle Miocene succession. The siliceous particles that dominate this subunit have distinct dark gray to black indurated layers and nodules and chert fragments ( Figure F3) (Rosenthal et al., 2018a).
Shipboard calcareous nannofossil biostratigraphic study of core catcher samples (~10 m interval), supplemented in some parts by samples collected from selected sections, recognized 30 calcareous nannofossil events; 15 of these events were zonal boundary markers, and the other 15 were supplementary events. The Pleistocene interval was constrained by 7 calcareous nannofossil events, the Pliocene was constrained by 4 events, the Miocene was constrained by 17 events, and the Oligocene was constrained by 2 events. The top common occurrence of Cyclicargolithus abisectus, which has an estimated age of ~24 Ma (Rio et al., 1990;Backman et al., 2012), was found near the base of the hole (Rosenthal et al., 2018a). This study refines the shipboard biostratigraphy of Hole U1490A by examining sediments collected at a higher resolution sampling interval (~1.5 m) of one sample per section.
IODP Proceedings 2 V o l u m e 3 6 3 Figure F1. Location of Site U1490.  Figure F2. Composite lithologic log of Subunit IA, Holes U1490A-U1490C. Age equivalents, nannofossil zonations, and calcareous nannofossil events (zonal boundary markers and secondary calcareous nannofossil events) observed in this study are also shown. In Hole U1490A, Subunit IA consists of 20 cores extending from late Miocene to recent (from Rosenthal et al., 2018c). T = top, B = base, Bc = base common occurrence, Bpa = base paracme.

Methodology
The samples were prepared using the standard smear slide preparation technique of Bown and Young (1998). The calcareous nannofossils were examined under an Olympus BX51 polarizing microscope at 1000× magnification in plane-transmitted and crosspolarized light with the occasional use of a quartz wedge compensator. At least 300 fields of view were observed per sample. Calcareous nannofossil taxa were identified to species level if possible using the descriptions provided by the holotypes in Perch-Nielsen (1985) and Aubry (1984aAubry ( , 1984bAubry ( , 1984c and supplemented by the illustrations in Bown (1998) and the guide to the biodiversity and taxonomy Nannotax3 online database (Young et al., 2017). Relative abundances of calcareous nannofossil taxa and degree of preservation were estimated following the same criteria used during Expedition 363 (Table T1) (Rosenthal et al., 2018c). Index markers, as well as calcareous nannofossil assemblages, were used to establish the biostratigraphic zonations based on the NP/NN nannofossil zonation scheme of Martini (1971), the CP/CN nannofossil zonation scheme of Okada and Bukry (1980), and the references compiled by Rosenthal et al. (2018c). All calcareous nannofossil species identified in the present study are listed in the Appendix. Digital images of calcareous nannofossils were taken using a QImaging Go-21 camera attached to the polarizing microscope and the Image ProPlus imaging software.
A total of 198 samples were observed in this study (see results in NANNOS in Supplementary material). The samples examined were chosen using the calcareous nannofossil shipboard data as reference ( Figure F4). The shipboard biostratigraphic data identified calcareous nannofossil events based on core catcher samples and a few samples collected from selected sections. The present study examined samples between these intervals to further refine the depth of the nannofossil events and identify additional nannofossil events not observed during the expedition. In the absence of zonal boundary markers, secondary calcareous nannofossil events were used to supplement the biostratigraphic data. Several calcareous nannofossil events were considered: • Bc = base common occurrence. • Bpa = base paracme. Figure F3. Composite lithologic log of Subunits IB and IC, Holes U1490A-U1490C. Age equivalents, nannofossil zonations, and calcareous nannofossil events (zonal boundary markers and secondary calcareous nannofossil events) observed in this study are also shown. In Hole U1490A, Subunit IB and IC consist of 23 cores extending from late Oligocene to late Miocene (from Rosenthal et al., 2018c). T = top, Tc = top common occurrence, B = base.

Results
A total of 52 calcareous nannofossil events were identified in this study: 22 were zonal boundary markers, and 30 were secondary calcareous nannofossil events. Generally, the calcareous nannofossils observed were well preserved, especially in the Pliocene-Pleistocene interval. However, in the Miocene sediments, the larger calcareous nannofossils such as the discoasters and ceratoliths were noticeably recrystallized and much more fragmented. As demonstrated by the thickening arm widths and narrowing arm spaces of the discoasters, recrystallization was much more evident in the Oligocene sediments. Therefore, it was difficult to identify the specimens to species level. The results were divided into four sections: Pleistocene, Pliocene, Miocene, and Oligocene. In each section, the zonal boundary markers were given emphasis, whereas the secondary events were briefly mentioned to supplement the establishment of the zonations. The reliability of selected index markers is described in this study based on published literature and recent zonation scheme references.

Pleistocene
The Pleistocene interval was constrained by 12 calcareous nannofossil events: 5 zonal boundary markers and 7 secondary calcareous nannofossil events ( Table T2). Most of the calcareous nannofossils observed in the Pleistocene interval were placoliths, helicoliths, and discoasters with good preservation and minimal traces of recrystallization or etching. This interval was characterized by the high diversity of species observed, likely due to the good preservation. Gephyrocapsids were prominent in the Pleistocene interval. Following Raffi et al. (1993), appearance and extinction events based on their size variations were used as secondary calcareous nannofossil events (small = <4 μm, medium = 4-5.5 μm, and large = >5.5 μm). All of the nannofossil zones in the NN zonation scheme of Martini (1971) and CN zonation scheme of Okada and Bukry (1980) were clearly identified in this interval.
The . This event was also near the Pliocene/Pleistocene boundary and was used to approximate the position of the boundary at ~34 mbsf.
Among the zonal boundary markers used in this study, biohorizon base E. huxleyi, biohorizon top P. lacunosa, and biohorizon top D. brouweri were reported to have high reliability with clear appearance/extinction events that are globally synchronous (Raffi et al., 2006). Thierstein et al. (1977) and Gartner (1977) first reported the high reliability of biohorizon base E. huxleyi and biohorizon top P. lacunosa. These datums were retained in the recent zonation scheme of Backman et al. (2012). Biohorizon top D. pentaradiatus and biohorizon top D. surculus were given a lower reliability rank by Raffi et al. (2006). In Backman et al. (2012), biohorizon top D. pentaradiatus was used as a zonal boundary marker but biohorizon top D. surculus was used as a secondary calcareous nannofossil event.

Pliocene
The Pliocene interval was constrained by 7 calcareous nannofossil events, 4 of which were zonal boundary markers and 3 of which were secondary calcareous nannofossil events (Table T2). Similar to the Pleistocene interval, most of the calcareous nannofossils identified were placoliths, helicoliths, and discoasters, with the addition of ceratoliths and sphenoliths. Calcareous nannofossils were generally moderately preserved in this interval, although most discoasters and ceratoliths appear to be overgrown/affected by recrystallization. Despite the lower degree of preservation in the Pliocene interval than in the Pleistocene, nannofossil diversity was still high because of the presence of more Discoaster species. All of the nannofossil zones in Martini (1971) and Okada and Bukry (1980)  Biohorizon base C. cristatus was the event nearest the Miocene/Pliocene boundary, which is located at ~90 mbsf. Among the zonal boundary markers, biohorizon top R. pseudoumbilicus (>7 μm) is considered a highly reliable marker (Raffi et al., 2006) because of its wide distribution, synchroneity across basins, and well-documented extinction (Backman and Shackleton, 1983;Rio et al., 1990). Biohorizon top D. tamalis was given a low reliability rank by Raffi et al. (2006) because of its low abundance and diachroneity across basins. Biohorizon base C. cristatus was given a very low reliability rank (Raffi et al., 2006) but still was included in the zonation scheme by Backman et al. (2012) as a secondary calcareous nannofossil event. The low rank was due to the low abundance and sparse distribution of the species (Raffi and Flores, 1995;Backman and Raffi, 1997). Biohorizon top A. primus was not assessed in Raffi et al. (2006) but was included in the zonation scheme by Backman et al. (2012) as a secondary calcareous nannofossil event. The distribution of A. primus has been noted to be sporadic and rare in several basins, which made it a less reliable marker (Backman and Shackleton, 1983;Rio et al., 1990).
Among the Miocene zonal boundary markers, biohorizon base C. armatus, biohorizon top D. quinqueramus, and biohorizon base Amaurolithus spp. were considered reliable markers by Raffi et al. (2006) and were also used as zonal boundary markers in the recent zonation scheme of Backman et al. (2012). Biohorizon base D. berggrenii was also used as a zonal boundary marker in Backman et al. (2012) but was ranked low in terms of reliability (Raffi et al., 2006). There was difficulty in determining biohorizon base D. berggrenii because of its evolution from D. bellus. The difference is the distinct central knob in D. berggrenii, which can be difficult to observe in overgrown specimens (Backman and Raffi, 1997). Biohorizon top and base D. hamatus were also used as zonal boundary markers in Backman et al. (2012) but were given a low reliability rank (Raffi et al., 2006). This was due to its low abundance, sporadic occurrence (Backman and Raffi, 1997), and diachroneity between low-and mid-latitude sediments . Similar to the previously mentioned zonal boundary markers, biohorizon base C. coalitus and biohorizon top S. heteromorphus were also given a low reliability rank in Raffi et al. (2006). Despite the low rank, both were used as zonal boundary markers in the zonation scheme of Backman et al. (2012). The low reliability rank of biohorizon base C. coalitus resulted from the presence of transitional morphotypes from Discoaster micros believed to be its ancestor (Raffi et al., 1998), which made recognition of the event difficult. It was also reported by  that C. coalitus could be influenced by ecological conditions resulting in inconsistency in its presence across different basins, which was also postulated for biohorizon top S. heteromorphus (Olafsson, 1989;Backman and Raffi, 1997).
Biohorizon top common D. deflandrei group was not assessed in terms of reliability in Raffi et al. (2006) but was used in Backman et al. (2012) as a secondary calcareous nannofossil event. Biohorizon base S. heteromorphus and biohorizon top S. belemnos were used in the zonation scheme of Backman et al. (2012) as a zonal boundary marker and as a secondary calcareous nannofossil marker, respectively. The use of biohorizon base common S. heteromorphus was suggested by several studies (Olafsson, 1989;Fornaciari and Rio, 1996;Raffi et al., 2016) because the increase in abundance of the species was much easier to document than its initial appearance in the fossil record. Rather than biohorizon top T. carinatus, biohorizon top common T. carinatus was used as a zonal boundary marker in Backman et al. (2012). T. carinatus was reported to have a sporadic occurrence near its biohorizon top, which made it difficult to determine the exact extinction event, hence its low reliability (Fornaciari and Rio, 1996;Backman et al., 2012).

Oligocene
The upper Oligocene units recovered from Hole U1490A were constrained by biohorizon top and base Sphenolithus delphix (23.06-23.38 Ma) between Samples 363-U1490A- 39X-4, 148-149 cm, and 39X-5, 98-99 cm (343.88-344.88 mbsf ), and between Samples 40X-CC and 41X-1, 98-99 cm (353.26-358.38 mbsf), respectively (Table T3). Similar to the Miocene interval, the calcareous nannofossils observed were mostly placoliths, discoasters, and sphenoliths. The placoliths and sphenoliths showed good preservation, whereas the discoasters were highly overgrown and recrystallized. Identification of discoasters to species level in the Oligocene interval was difficult. Biohorizon top common C. abisectus (~24 Ma) was observed between Samples 43X-2, 116-117 cm, and 43X-3, 54-55 cm (379.36-379.94 mbsf ), near the bottom of the hole and could therefore be used to approximate the age of the oldest sedi- Age-depth model Figure F5 shows the age-depth model for Hole U1490A based on all the calcareous nannofossil events identified between 0.29 Ma (biohorizon base E. huxleyi) and 24 Ma (biohorizon top common C. abisectus). The sedimentation rates were highest in the Oligocene to early early Miocene interval at 3.67 cm/ky, decreasing to 1.58 cm/ky in the early to middle middle Miocene interval from 334 to 224 mbsf. A continued decrease in the sedimentation rate to 0.94 cm/ky occurred from the middle middle to early late Miocene from 224 to 172 mbsf, and then an increase to 2.15 cm/ky occurred from 172 to 25 mbsf in the late Miocene to early Pleistocene interval. For the rest of the Pleistocene, above 25 mbsf, the sedimentation rate was 1.11 cm/ky. The middle middle to early late Miocene interval had the lowest sedimentation rate in Hole U1490A and was also the interval where several events were either reversed or observed occurring in the same sample interval. Figure F5. Age-depth model using all calcareous nannofossil biohorizons identified in this study, Hole U1490A. See Figures F2 and F3 for lithology key. See Tables T2 and T3 for calcareous nannofossil events corresponding to numbers.