Pleistocene Calcareous Nannofossil Biostratigraphy and Gephyrocapsid Occurrence in Site U1431D, IODP 349, South China Sea

geosciences

Article Pleistocene Calcareous Nannofossil Biostratigraphy and Gephyrocapsid Occurrence in Site U1431D, IODP 349, South China Sea

, Jose Dominick S. Guballa * † and Alyssa M. Peleo-Alampay Nannoworks Laboratory, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 1101, Philippines; [email protected] * Correspondence: [email protected]; Tel.: +1-28-9707-5905 Current address: Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, † ON M5S 3B1, Canada. Received: 25 August 2020; Accepted: 23 September 2020; Published: 28 September 2020

Abstract: We reinvestigated the Pleistocene calcareous nannofossil biostratigraphy of Site U1431D (International Ocean Discovery Program (IODP) Expedition 349) in the South China Sea (SCS). Twelve calcareous nannofossil Pleistocene datums are identified in the site. The analysis confirms that the last occurrence (LO) of Calcidiscus macintyrei is below the first occurrence (FO) of large Gephyrocapsa spp. (>5.5 µm). The FO of medium Gephyrocapsa spp. (4–5.5 µm) is also identified in the samples through morphometric measurements, which was unreported in shipboard results. Magnetobiochronologic calibrations of the numerical ages of LO of Pseudoemiliania lacunosa and FO of Emiliania huxleyi are underestimated and need reassessment. Other potential markers such as a morphological turnover of circular to elliptical variants of Pseudoemiliania lacunosa and a small Gephyrocapsa acme almost synchronous with the FO of Emiliania huxleyi may offer biostratigraphic significance in the SCS. The morphologic changes in Gephyrocapsa coccoliths are also examined for the first time in Site U1431D. Placolith length and bridge angle changes are comparable with other ocean basins, suggesting that morphologic changes are most likely evolutionary novelties rather than being caused by local climate anomalies.

Keywords: calcareous nannofossils; South China Sea; Pleistocene; International Ocean Discovery Program; Site U1431; Gephyrocapsa

1. Introduction The Pleistocene is characterized by worldwide sea level lows and highs due to increased amplitudes of glacial and interglacial episodes, respectively [1–3]. Ice volume ﬂuctuations are well represented in deep sea sedimentary sequences by variations in proxies such as sediment properties, fossil assemblages, and/or isotope concentrations. The basis for proxy correlations is a precise and robust chronology constructed from natural cycles of sediment physical properties, radiometric ages, magnetic reversals, and/or biostratigraphic data. An accurate age model is important because major climatic shifts in the Pleistocene occurred within a few hundred thousand years, as exempliﬁed by glacial–interglacial amplitude variations in δ18O records [2]. Furthermore, several periods in the Pleistocene are thought to be analogs to the modern-day climate, such as Marine Isotope Stage (MIS) 11 [4,5] and may contribute to a further understanding of past and future climate change dynamics. These climatic events can be put into geologic context through relative dating by using calcareous nannofossils. Calcareous nannofossils are mostly made up of remains of marine, unicellular phytoplankton known as coccolithophores. Coccolithophores produce a robust, calcium carbonate exoskeleton called the coccosphere. The coccosphere disintegrates into its coccoliths (individual calcite plates)

Geosciences 2020, 10, 388; doi:10.3390/geosciences10100388 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 388 2 of 26 upon death and is incorporated in fecal pellets and/or marine snow, thus dominating the carbonate fraction of deep-sea pelagic sediments. Calcareous nannofossils are utilized in sediment core studies because of their widespread geographic occurrence and stratigraphic significance. They underwent extinction and speciation events (datums) since the Triassic period, defining brief spans of geologic time [6]. The calibration of nannofossil datums through astronomical tuning and cyclostratigraphy has been substantial in creating a robust time scale for the Pleistocene [7–9]. Star-shaped discoasters are mainly used in subdividing the Early Pleistocene [7,10–14]. Gephyrocapsids are also major components of Pleistocene calcareous nannofossil assemblages [15–22]. Gephyrocapsids have been extensively studied because of complex morphological changes, which have been utilized to subdivide the Pleistocene [7,21–24]. However, semi-enclosed marginal seas may have different oceanographic parameters, resulting to a different calcareous nannofossil assemblage. It is therefore important to study the calcareous nannofossil biostratigraphy of marginal seas to verify if standard nannofossil zonations apply to these areas. The South China Sea (SCS) is the largest marginal sea in the western Pacific area and has been subject to multiple paleoceanographic [25–30] and sedimentological studies [31–34]. The SCS is characterized by better carbonate preservation compared to other western Pacific marginal basins, providing a valuable sediment archive for East Asian Monsoon System (EAMS) variations since the Miocene [35]. Calcareous nannoplankton in surface sediments [36–39] have also been studied in the northern, western, and eastern SCS. Despite increasing scientific attention on the SCS, calcareous nannofossil biostratigraphic studies of Pleistocene deep sea sediments are limited. Most works are concentrated on the northern flanks of the SCS to acquire a record of the spreading history of the SCS [40–42]. International Ocean Discovery Program (IODP) Expedition 349 aimed to establish the ages of cessation of different spreading centers in the SCS Basin [43,44]. Site U1431D was the most successful in providing a continuous archive from the Miocene to Holocene, recovering around 400 m of sediment. Since Ocean Drilling Program (ODP) Leg 184 [42], IODP 349 is one of the very few research expeditions to obtain records since the start of ocean floor spreading of the SCS basin. The acquired cores, therefore, represent a valuable archive in which age constraints for different spreading events in the sub-basins can be studied. Furthermore, the thick sediment recovery provides a continuous sedimentary record of paleoceanographic changes in the SCS. Shipboard data have been able to estimate age dates of different sections of the whole core through biostratigraphic analysis using different microfossils [43]. Calcareous nannofossils have been useful in shipboard analysis to determine sedimentation rates and estimate the cessation of the sub-basin spreading. However, further refining of datum placements is needed to achieve a better time constraint for paleoceanographic changes. This study reinvestigates the calcareous nannofossil biostratigraphy of the Pleistocene sediments acquired from IODP 349. Temporal size and bridge angle changes in the genus Gephyrocapsa are discussed in detail to assess the reliability of these bioevents in the SCS. The results of the research contribute to the assessment of nannofossil datums and their importance in Pleistocene correlation and chronology, especially in the SCS.

2. Materials and Methods

2.1. Present Day Circulation of the South China Sea Surface water circulation in the SCS is currently inﬂuenced by the EAMS [30,45,46]. The EAMS is characterized by having a stronger winter and a weaker summer season. The winter season (December–February) is dominated by the northeast monsoon, causing a basin-wide cyclonic gyre (Figure1). The winter monsoon is mainly driven by cold winds from Siberia, which runs southward along the coast of East Asia. Mountains near the East Asian coast force the winter monsoon to produce a southwest traveling jet over the SCS. The summer season (June–August) is characterized by a weaker southwest monsoon forming an anticyclonic gyre concentrated at the south and a cyclonic cell at the Geosciences 2020, 10, 388 3 of 26 north (Figure1). The ocean jet separates near Vietnam: one branch travels northeast crossing the Geosciences 2020, 10, x FOR PEER REVIEW 3 of 26 central basin while the other continues on the northern path. Both branches of the current exit the theSCS current through exit the the Bashi SCS Strait.through The the ocean Bashi dynamics Strait. The of theocean SCS dynamics produce of localized the SCS annual produce upwelling localized in annualVietnam upwelling and in northwest in Vietnam Luzon, and in although northwest upwelling Luzon, although during winterupwelling is generally during winter stronger is generally based on strongerchlorophyll based concentrations on chlorophyl [30l concentrations,45,46]. [30,45,46].

FigureFigure 1. 1. SiteSite U1431 U1431 (15°22.5371 (15◦22.5371′ N,0 N, 117°00.0022 117◦00.0022′ E)0 E) enclosed enclosed by by the the box box is isth thee core core used used in in this this study. study. OtherOther dots dots correspond correspond to to drill drill sites sites from from Ocean Ocean Drilling Drilling Program Program (ODP) (ODP) Expedition Expedition 184. 184. The The black black dotteddotted line line represents represents the the winter winter surface surface circulation circulation of of the the South South China China Sea Sea (SCS). (SCS). The The white white solid solid line line indicatesindicates summer summer surface surface circul circulationation (modified (modiﬁed from from [46]). [46]).

2.2.2.2. Site Site U1431D U1431D TheThe sediment sediment cores cores were were obtained obtained from from the the central central SCS SCS by by IODP IODP Expedition Expedition 349 349 from from 1 to 1 to6 February6 February 2014, 2014, at Site at Site U1431 U1431 (Figure (Figure 1). 1Site). Site U1431 U1431 is located is located near near a relict a relict spreading spreading ridge ridge where where the youngestthe youngest magnetic magnetic anomalies anomalies in the in SCS the are SCS located are located [43]. [The43]. primary The primary objective objective of the of expedition the expedition was towas determine to determine the cessation the cessation of seafloor of seafloor spreading spreading at the East at theSub-basin East Sub-basin of the SCS. of The the SCS.site also The provides site also anprovides almost ancontinuous almost continuous sediment sedimentrecord, providin record,g providing the opportunity the opportunity to study to a studycontinuous, a continuous, high- resolutionhigh-resolution paleoceanographic paleoceanographic evolution evolution of the SCS. of the Core SCS. U1431D Core U1431Dwas recovered was recovered from a mean from seafloor a mean depthseafloor of depth4240.5 ofm 4240.5below m sea below level sea (mbsl). level (mbsl).About 402.11 About m 402.11 of core m ofwas core recovered was recovered in Site in U1431D Site U1431D in a 617-min a 617-m interval interval (65.2% (65.2% core recovery). core recovery). The uppermos The uppermostt 135 m 135of core m of material core material (from (fromsection section 1H to 15H) 1H to was15H) sampled was sampled every every20 cm 20for cm the for first the two first sections two sections (1H and (1H 2H), and and 2H), every and every 150 cm 150 for cm the for rest the of rest the of sections.the sections. A total A of total 168 of samples 168 samples were analyzed were analyzed for this for study this study(Figure (Figure 2). 2). ElevenEleven (11) (11) lithostratigraphic lithostratigraphic units units were were defined defined by by the the shipboard shipboard team team based based on on visual visual core core descriptions,descriptions, thin thin section section analysis, analysis, biogenic biogenic materials, materials, and and physical physical properties. properties. The The first first two two units units werewere investigated investigated in in this this study, study, which which spans spans the the late late Pliocene Pliocene (section (section 15H) 15H) to to the the latest latest Pleistocene Pleistocene (section(section 1H; 1H; Figure Figure 22).). UnitUnit I I (0–101.16 (0–101.16 m m below below sea sea floor floor (mbsf)) (mbsf)) is characterizedis characterized by aby sequence a sequence of clay, of clay,silty silty clay, clay, and and clayey clayey silt thatsilt that has has substantially substantially more more silt silt than than Unit Unit II. II. The The silt silt layers layers are are composed composed of ofquartz, quartz, feldspars feldspars (plagioclases (plagioclases and and K-feldspars), K-feldspar ands), and minor minor amounts amounts of amphiboles, of amphiboles, micas, micas, and heavy and heavyminerals. minerals. The layers The layers with more with siltmore are silt greenish are greenish gray interbeds, gray interbeds, usually usually with graded, with graded, fining upward fining upward sequences interpreted as turbidites [43]. Nannofossil oozes occur locally and often show a fining upward character, also identified as turbidite deposits. Unit I contains multiple dark gray to brown volcanic ash layers, 0.5- to 5.0-cm thick. Geosciences 2020, 10, 388 4 of 26 sequences interpreted as turbidites [43]. Nannofossil oozes occur locally and often show a fining upward character, also identified as turbidite deposits. Unit I contains multiple dark gray to brown volcanic ash layers, 0.5- to 5.0-cm thick. Geosciences 2020, 10, x FOR PEER REVIEW 4 of 26

FigureFigure 2. SchematicSchematic diagram diagram of core of coresections sections (1H to (1H15H) toexamined 15H) examined in this study. in It this is generally study. Itcomposed is generally composedof clays/claystone of clays /claystonewith interbeds with interbedsof minor ofsand/san minordstone, sand/sandstone, silt/siltstone, silt /clayeysiltstone, silt, clayey silty clay, silt, siltyand clay, foraminiferal oozes. Age dates are determined from shipboard biostratigraphic data. White areas in the and foraminiferal oozes. Age dates are determined from shipboard biostratigraphic data. White areas “Core Recovery” column approximates coring gaps. Horizontal lines in the “Samples” column in the “Core Recovery” column approximates coring gaps. Horizontal lines in the “Samples” column approximate positions of the samples utilized for this study Abbreviations listed: CSF = core depth below approximate positions of the samples utilized for this study Abbreviations listed: CSF = core depth seafloor, H = advanced piston corer, Plio = Pliocene. Figure modified from [43]. below seafloor, H = advanced piston corer, Plio = Pliocene. Figure modified from [43]. Unit II (101.16–267.82 mbsf) is divided into two sub-units. Only samples from Sub-unit IIA Unit II (101.16–267.82 mbsf) is divided into two sub-units. Only samples from Sub-unit IIA (101.16–194.95 mbsf) are analyzed for calcareous nannofossils. This sub-unit is composed of clay, (101.16–194.95 mbsf) are analyzed for calcareous nannofossils. This sub-unit is composed of clay, nannofossil-rich clay, and silty clay. The greenish gray clays are thickly bedded and are usually nannofossil-rich clay, and silty clay. The greenish gray clays are thickly bedded and are usually interbedded with greenish gray medium- to thickly-bedded nannofossil-rich clays and dark greenish interbedded with greenish gray medium- to thickly-bedded nannofossil-rich clays and dark greenish gray thin- to medium-bedded silty clay layers. The clayey silt layers often show sharp, planar bases, graywhich thin- grade to medium-beddedinto clay and are interpreted silty clay layers.as turbidite The deposits clayey silt [43]. layers Moreover, often showfine sand sharp, foraminiferal planar bases, whichoozes gradewith sharp, into clay erosive and bases, are interpreted fining upward as turbidite towards deposits nannofossil-rich [43]. Moreover, layers, are fine also sand interpreted foraminiferal oozesas turbidites. with sharp, Bioturbation erosive bases, from the fining Nereites upward ichnofacies towards are nannofossil-rich darker colored clay layers, within are alsolighter interpreted colored as turbidites.carbonate-rich Bioturbation sediments. from Thin thevolcanicNereites ashichnofacies layers 0.5- to are 2.0-cm darker thick colored are also clay identified within but lighter are much colored carbonate-richless compared sediments.to Unit I and Thin are volcanicrestricted ash at the layers upper 0.5- portions to 2.0-cm of thickthe sub-unit are also [43]. identified but are much less compared to Unit I and are restricted at the upper portions of the sub-unit [43]. 2.3. Calcareous Nannofossil Analysis 2.3. Calcareous Nannofossil Analysis Smear slide replicates from unprocessed sediments were prepared following the standard procedureSmear of slide [47]. replicatesAt least 300 from random unprocessed fields of view sediments (FOVs) were were scann prepareded using following an Olympus the standardBX51 procedurephase contrast of [47 polarizing]. At least microscope 300 random under fields 1000× of view magnification. (FOVs) were Additional scanned using fields an of Olympusview were BX51 analyzed for samples with important calcareous nannofossil taxa. A sample was considered barren of calcareous nannofossils if there were none present after 300 FOVs. Relative abundances of individual calcareous nannofossils identified per slide were estimated using IODP standards [43]: D = dominant (>50%, or 100 specimens per FOV) A = abundant (10–50%, or 10–100 specimens per FOV) Geosciences 2020, 10, 388 5 of 26 phase contrast polarizing microscope under 1000 magnification. Additional fields of view were × analyzed for samples with important calcareous nannofossil taxa. A sample was considered barren of calcareous nannofossils if there were none present after 300 FOVs. Relative abundances of individual calcareous nannofossils identified per slide were estimated using IODP standards [43]: GeosciencesD = dominant2020, 10, x FOR (> 50%,PEER REVIEW or 100 specimens per FOV) 5 of 26 A = abundant (10–50%, or 10–100 specimens per FOV) C = common (1–10%, or 1–10 specimens per FOV) C = common (1–10%, or 1–10 specimens per FOV) F = few (0.1–1%, or 1 specimen per 1–10 FOV) F = few (0.1–1%, or 1 specimen per 1–10 FOV) R = rare (<0.1%, or <1 specimen per 10 FOV) R = rare (<0.1%, or <1 specimen per 10 FOV) Holotypes in [48] and [49,50], supplemented by nannofossil illustrations in [6], were used for Holotypes in [48–50], supplemented by nannofossil illustrations in [6], were used for identification. identification. Taxonomic concepts are listed in Appendix A. The calcareous nannofossil distribution Taxonomic concepts are listed in AppendixA. The calcareous nannofossil distribution is shown in the is shown in the Supplementary Materials. Supplementary Materials. Nannoliths from Discoaster species are usually reworked specimens in younger sediments and Nannoliths from Discoaster species are usually reworked specimens in younger sediments and occur both as fragmented and complete fossils within their age ranges. Thus, the last occurrences (LO) occur both as fragmented and complete fossils within their age ranges. Thus, the last occurrences (LO) of discoasters are difficult to recognize because of reworking. Complete, incomplete, and arm fragments of discoasters are difficult to recognize because of reworking. Complete, incomplete, and arm fragments of 100 Discoaster specimens were counted from 135 mbsf to 91 mbsf to semi-quantitatively determine of 100 Discoaster specimens were counted from 135 mbsf to 91 mbsf to semi-quantitatively determine their relative abundances. In this study, specimens that have ≤60% of their original form intact are their relative abundances. In this study, specimens that have 60% of their original form intact are considered incomplete specimens. One or two arms of discoasters≤ without a central area are considered considered incomplete specimens. One or two arms of discoasters without a central area are considered arm fragments. The LO of the Discoaster is assigned to the midpoint depth between the sample arm fragments. The LO of the Discoaster is assigned to the midpoint depth between the sample containing >1% of the total count that are complete specimens and the level where it has disappeared. containing >1% of the total count that are complete specimens and the level where it has disappeared. The informal grouping “Discoaster spp.” is used for ambiguous stellate nannofossils. The Base Acme The informal grouping “Discoaster spp.” is used for ambiguous stellate nannofossils. The Base Acme (Ba) of D. triradiatus was identified using the definition of [51]. Based on the counts, barren intervals (Ba) of D. triradiatus was identified using the definition of [51]. Based on the counts, barren intervals were noted at 133.2–130.2 mbsf, 127.5 mbsf, 124.9 mbsf, 121.9–118.9 mbsf, and 99.9–98.39 mbsf. were noted at 133.2–130.2 mbsf, 127.5 mbsf, 124.9 mbsf, 121.9–118.9 mbsf, and 99.9–98.39 mbsf. Morphological analysis of gephyrocapsids was done using smear slides prepared from Morphological analysis of gephyrocapsids was done using smear slides prepared from unprocessed unprocessed sediments. The Gephyrocapsa coccoliths were photographed and measured using Image- sediments. The Gephyrocapsa coccoliths were photographed and measured using Image-Pro Plus Version Pro Plus Version 7.0 mounted on an Olympus BX51 polarizing microscope under 1000× 7.0 mounted on an Olympus BX51 polarizing microscope under 1000 magnification. The Gephyrocapsa magnification. The Gephyrocapsa coccoliths were measured based× on the parameters set by [52] coccoliths were measured based on the parameters set by [52] (Figure3), mainly the coccolith length (Figure 3), mainly the coccolith length and width, coccolith central area length and width, and bridge and width, coccolith central area length and width, and bridge angle. At least 40 specimens per angle. At least 40 specimens per sample were analyzed. In samples with few nannofossils, 200 FOVs sample were analyzed. In samples with few nannofossils, 200 FOVs were scanned and all Gephyrocapsa were scanned and all Gephyrocapsa placoliths seen were measured. The study adopted the biometric placoliths seen were measured. The study adopted the biometric subdivision of [21] which is mainly subdivision of [21] which is mainly based on the maximum length of the coccolith. Forms <4 μm are based on the maximum length of the coccolith. Forms <4 µm are small Gephyrocapsa spp.; medium small Gephyrocapsa spp.; medium Gephyrocapsa spp. are those with placolith lengths from ≥4 to <5.5 Gephyrocapsa spp. are those with placolith lengths from 4 to <5.5 µm, and large Gephyrocapsa spp. are μm, and large Gephyrocapsa spp. are ≥5.5 μm in length.≥ All measurements are provided in the 5.5 µm in length. All measurements are provided in the Supplementary Materials. ≥Supplementary Materials.

Figure 3. Measurements done on digitized photos of Gephyrocapsa placoliths under phase contrast Figure 3. Measurements done on digitized photos of Gephyrocapsa placoliths under phase contrast research microscope at 1000 magniﬁcation. research microscope at 1000×× magnification.

3. Results Twelve (12) calcareous nannofossil datums were detected for the Pleistocene section of Site U1431D. The preservation of calcareous nannofossils ranged from very poor to pristine specimens. Overgrowth and fragmentation of calcareous nannofossils (especially among discoasters) are evident. ForaminiferalGeosciences 2020, tests, 10, x FOR sponge PEER REVIEW spicules, radiolarian tests, and diatom frustules were observed in6 theof 26 ﬁrst 3 m of the core. Samples barren of nannofossils were dominated by detrital materials such as clay minerals,in the first biotite, 3 m of plagioclase, the core. Samples quartz, barren and muscovite. of nannofossils Some were barren dominated intervals by coincide detrital withmaterials volcanic suchash layersas clay [43 minerals,]. biotite, plagioclase, quartz, and muscovite. Some barren intervals coincide with volcanicCalcareous ash layers nannofossil [43]. abundances were generally low throughout the core, but were high in portionsCalcareous of sections nannofossil 1H and abundances 2H (0 to 12.2 were mbsf). generally Recrystallization low throughout of the nannofossil core, but specimenswere high in was uncommonportions of throughout sections 1H the and core. 2H Samples(0 to 12.2 with mbsf). partially Recrystallization to heavily dissolved of nannofossil placoliths specimens coincide was with decreasesuncommon in calcareousthroughout nannofossilthe core. Samples relative with abundances. partially to heavily Figure dissolved4 illustrates placoliths important coincide calcareous with decreases in calcareous nannofossil relative abundances. Figure 4 illustrates important calcareous nannofossil species used as biostratigraphic markers for Site U1431D. Table1 summarizes all calcareous nannofossil species used as biostratigraphic markers for Site U1431D. Table 1 summarizes all calcareous nannofossil biostratigraphic events documented in Site U1431D. The calcareous nannofossil distribution nannofossil biostratigraphic events documented in Site U1431D. The calcareous nannofossil distribution for all samples analyzed in Site U1431D is provided in the Supplementary Materials. for all samples analyzed in Site U1431D is provided in the Supplementary Materials.

Figure 4. Photomicrographs of calcareous nannofossil marker taxa used in subdividing the Figure 4. Photomicrographs of calcareous nannofossil marker taxa used in subdividing the uppermost uppermost Pliocene and Pleistocene. All specimens are photographed under 1000× magnification. Pliocene and Pleistocene. All specimens are photographed under 1000 magniﬁcation. Abbreviations Abbreviations listed: XPL = cross-polarized light image. PC = phase contrast× image. (A) Emiliania listed: XPL = cross-polarized light image. PC = phase contrast image. (A) Emiliania huxleyi, XPL. huxleyi, XPL. Sample 2H-1W, 60–61 cm. (B) Pseudoemiliania lacunosa, XPL. Sample 2H-2W, 80–82 cm. Sample 2H-1W, 60–61 cm. (B) Pseudoemiliania lacunosa, XPL. Sample 2H-2W, 80–82 cm. (C) Gephyrocapsa (C) Gephyrocapsa sp. 3, XPL. Sample 4H-4W, 20–21 cm. (D) Gephyrocapsa spp. >4 μm, XPL. Sample 7H- sp. 3, XPL. Sample 4H-4W, 20–21 cm. (D) Gephyrocapsa spp. >4 µm, XPL. Sample 7H-4A, 20–21 cm. 4A, 20–21 cm. (E) Gephyrocapsa spp. >5.5 μm, XPL. Sample 8H-1W, 50 cm. (F) Calcidiscus macintyrei, (E) Gephyrocapsa spp. >5.5 µm, XPL. Sample 8H-1W, 50 cm. (F) Calcidiscus macintyrei, XPL. Sample XPL. Sample 10H-6W, 20–21 cm. (G) Discoaster brouweri, PC. Sample 12H-6W, 20–21 cm. (H) Discoaster 10H-6W,triradiatus 20–21, PC. cm. Sample (G) Discoaster13H-7W, 20–21 brouweri cm., PC.(I) Discoaster Sample 12H-6W, surculus, 20–21 PC. Sample cm. (H 15H-3W,) Discoaster 20–21 triradiatus cm. , PC.White Sample scale 13H-7W, bar at the 20–21 lower cm. right (I) ofDiscoaster each photomicrograph surculus, PC. Sampleis 5 μm 15H-3W,in length. 20–21 cm. White scale bar at the lower right of each photomicrograph is 5 µm in length.

Geosciences 2020, 10, 388 7 of 26

Table 1. Summary of calcareous nannofossil biostratigraphic events of [43] and this study for International Ocean Discovery Program (IODP) Expedition 349, Site U1431D. Age discrepancies from [43] are due to the incorporation of Equatorial Pacific ages (compiled by [9]) when available. Abbreviations listed: FO—first occurrence, LO—last occurrence, Ba—base acme, reemG—reentrance Geosciences 2020, 10, x FOR PEER REVIEW 7 of 26 event of medium Gephyrocapsa, tlG—top (last occurrence) of large Gephyrocapsa, blG—base (first

occurrence)Table of 1. largeSummaryGephyrocapsa of calcareous, bmG—base nannofossil of biostratigra medium Gephyrocapsaphic events of. [43] and this study for International Ocean Discovery Program (IODP) Expedition 349, Site U1431D. Age discrepancies from IODP Expedition 349 Shipboard Results [43] This Study [43] are due to the incorporation of Equatorial Pacific ages (compiled by [9]) when available. Age (Ma)Abbreviations Depth (mbsf) listed: FO—first Nannofossil occurrence, Event LO—lastAge occurrence, (Ma) Ba—base Depth (mbsf) acme, reemG—reentrance Nannofossil Event 0.29event of 3.15 medium Gephyrocapsa FO Emiliania, tlG—top huxleyi (last occurrence)0.29 of large 3.905Gephyrocapsa, blG—base FO Emiliania (first huxleyi 0.44occurrence) 12.15 of large LOGephyrocapsaPseudoemiliania, bmG—base lacunosa of medium0.44 Gephyrocapsa. 5.42 LO Pseudoemiliania lacunosa 0.61 32.08 LO Gephyrocapsa sp. 3 0.61 25.925 LO Gephyrocapsa sp. 3 IODP Expedition 349 Shipboard Results [43] This Study 1.02 39.67 FO Gephyrocapsa sp. 3 1.026 57.605 FO Gephyrocapsa sp. 3 Age (Ma) Depth (mbsf) Nannofossil Event Age (Ma) Depth (mbsf) Nannofossil Event 1.04 50.90 reemG 1.034 58.24 reemG 0.29 3.15 FO Emiliania huxleyi 0.29 3.905 FO Emiliania huxleyi 1.24 60.70 tlG 1.062 60.60 tlG 0.44 12.15 LO Pseudoemiliania lacunosa 0.44 5.42 LO Pseudoemiliania lacunosa Calcidiscus macintyrei 1.600.61 89.0132.08 LO LO Gephyrocapsa sp. 3 1.541 0.61 25.925 78.985 LO Gephyrocapsa blG sp. 3 1.621.02 96.1039.67 FO Gephyrocapsa blG sp. 3 1.738 1.026 57.605 85.705 FOLO GephyrocapsaCalcidiscus sp. macintyrei3 1.04 50.90 reemG 1.789 1.034 58.24 87.635 reemG bmG 1.931.24 107.5760.70 LO Discoaster tlG brouweri 1.905 1.062 60.60 94.15 LO Discoaster tlG brouweri 2.221.60 116.8589.01 Ba DiscoasterLO Calcidiscus triradiatus macintyrei 2.3371.541 78.985 115.905 Ba Discoaster blG triradiatus 2.491.62 135.2096.10 LO Discoaster blG surculus 2.491 1.738 85.705 127.855 LO Calcidiscus LO Discoaster macintyrei surculus 1.789 87.635 bmG 1.93 107.57 LO Discoaster brouweri 1.905 94.15 LO Discoaster brouweri 3.1. Calcareous2.22 Nannofossil116.85 BiostratigraphyBa Discoaster triradiatus 2.337 115.905 Ba Discoaster triradiatus 2.49 135.20 LO Discoaster surculus 2.491 127.855 LO Discoaster surculus 3.1.1. LO of Discoaster surculus 3.1. Calcareous Nannofossil Biostratigraphy The LO of D. surculus was previously assigned to Sample 15H-CC (135.20 mbsf) in the IODP 349 biostratigraphic3.1.1. LO of results Discoaster [43 surculus]. Rare to few specimens are observed from Sample 15H-CC to Sample 15H-3W untilThe they LO of occur D. surculus sporadically was previously as reworked assigned to specimens Sample 15H-CC throughout (135.20 mbsf) the in younger the IODP portions 349 of the sedimentbiostratigraphic record (Figureresults [43].5A). Rare There to few is a specimens barren interval are observed of nannofossils from Sample from 15H-CC 133.2 to toSample 130.2 mbsf, but D. surculus15H-3W reappeareduntil they occur as well-preservedsporadically as reworked specimens specimens above throughout this interval the (Sample younger 15H-3W).portions of It again the sediment record (Figure 5A). There is a barren interval of nannofossils from 133.2 to 130.2 mbsf, reappearsbut atD. 112.4surculus mbsf, reappeared but is as regarded well-preserved as reworked specimens in above this samplethis interval because (Sample not 15H-3W). only is It it higher than thatagain of thereappears bottom at 112.4 acme mbsf, of D. but triradiatus is regarded, a as younger reworked datum in this sample (Figure because5A), but not itsonly occurrence is it higher is also anomalouslythan that located of the abovebottom samplesacme of D. with triradiatus consistent, a younger absences datum of (FigureD. surculus 5A), but. Thus, its occurrence the LO ofis alsoD.surculus is placedanomalously between Sampleslocated above 15H-3W, samples 20 towith 21 consistent cm and 15H-2W, absences 20of toD. 21surculus cm (127.85. Thus, mbsf).the LO of D. surculus is placed between Samples 15H-3W, 20 to 21 cm and 15H-2W, 20 to 21 cm (127.85 mbsf).

Figure 5. Relative percentages of (A) Discoaster surculus, (B) D. pentaradiatus, (C) D. triradiatus/D. Figure 5. Relative percentages of (A) Discoaster surculus,(B) D. pentaradiatus,(C) D. triradiatus/D. brouweri + D. triradiatus, and (D) D. brouweri with depth. Highlighted portions approximate the brouweriposition+ D. triradiatus of the datum, and from (D biostr) D. brouweriatigraphicwith data. depth. Solid lines Highlighted denote complete portions specimens approximate while broken the position of the datumlines signify from incomplete biostratigraphic specimens. data. Abbreviati Solidons lines listed: denote LO—last complete occurrence, specimens Ba—base while acme. broken lines signify incomplete specimens. Abbreviations listed: LO—last occurrence, Ba—base acme. Geosciences 2020, 10, 388 8 of 26

3.1.2. LO of Discoaster pentaradiatus The occurrence of D. pentaradiatus is very sporadic as they occur as complete and fragmented specimens throughout the deeper sections of the core (Sample 11H-CC to Sample 15H-CC; from 96 to 135 mbsf; Figure5B). The LO of D. pentaradiatus may lie between 120 and 125 mbsf based on the sharp decrease in abundance between these depth intervals and the relative placement of the LO of D. surculus and Ba D. triradiatus. However, there is a pervasive occurrence of fragmented specimens above and below this depth. Similar problems regarding the LO of D. pentaradiatus is also reported in ODP 149 in the Iberia Margin (Holes 897C and 898A, [53]) and Deep Sea Drilling Project (DSDP) Site 662 of the eastern North Atlantic [10]. Based on discoaster counts for Site U1431D, the LO of this species is unclear and therefore not utilized in this study.

3.1.3. Base Acme (BA) of Discoaster triradiatus Relative percentages based on counts of D. triradiatus against D. brouweri have been used in determining the base acme (>20%) of the former [51]. Counts at Sample 13H-7W (116.41 mbsf) show the ﬁrst increased abundance of well-preserved specimens (Figure5C). Because D. triradiatus was not observed below Sample 13H-CC (116.9 mbsf), the sudden increase of well-preserved specimens in Sample 13H-7W is considered the base acme event. Above 110 mbsf (Sample 13H-1W), D. triradiatus also increases in abundance greater than that of Sample 13H-7W. However, the paucity of well-preserved species and the abundance decrease of D. triradiatus from Sample 13H-6W to Sample 13H-2W (115–110 mbsf) make extending the acme event to younger sediments questionable (Figure5C). Thus, this datum is placed at 115.905 mbsf, between Sample 13H-7W, 20–21 cm (116.41 mbsf) and Sample 13H-6W, 20–21 cm (114.91 mbsf).

3.1.4. LO of Discoaster brouweri A steady decrease in complete Discoaster brouweri and increase in fragmented specimens are noted from the initial placement of the datum (Sample 12H-CC; 107.57 mbsf) until its complete absence in Sample 12H-2W, 20–21 cm (99.9 mbsf; Figure5D). Only one complete specimen is found in 12H-3W, 20–21 cm (101.4 mbsf), probably due to decreased nannofossil abundance. Samples 12H-2W, 20–21 cm (99.9 mbsf) and 12H-1W, 19–20 cm (98.39 mbsf) are nearly barren of calcareous nannofossils. However, two consecutive samples above Sample 12H-1W (Sample 11H-5-CC, 140–145 cm; 96.10 mbsf and 11H-5W, 20–21 cm; 94.9 mbsf) still contain at least 2–3% D. brouweri. The species is absent in Sample 11H-4W, 20–21 cm (93.4 mbsf), and in four consecutive younger samples (6 m above Sample 11H-4W). Therefore, the LO of D. brouweri is reassigned between Sample 11H-5W, 20–21 cm (94.9 mbsf) and Sample 11H-4W, 20–21 cm (94.15 mbsf) from Sample 12H-CC (107.57 mbsf; [43]; Figure5D).

3.1.5. First Occurrence (FO) of Medium Gephyrocapsa spp. (bmG) The base of medium Gephyrocapsa (bmG) lies between Samples 10H-7W, 22 to 23 cm and 10H-8W, 20 to 21 cm (87.635 mbsf), which was previously unreported in shipboard results [43]. Placolith length measurements have been useful in determining the nannofossil datum. The gephyrocapsid specimens observed generally have a bridge oriented nearer the long axis of the coccolith (15◦–40◦) and a relatively open central area. These gephyrocapsids are most likely equivalent to Gephyrocapsa oceanica, G. margereli, and G. muellerae.

3.1.6. LO of Calcidiscus macintyrei The species C. macintyrei generally occurs sporadically (only 1 or 2 specimens) in Sample U1431D-15H-CC, 10–16 cm to 7H-6W, 20 to 21 cm (135.33 to 57.9 mbsf), except in Samples 14H-CC (125.48 mbsf) and 10H-6W, 20 to 21 cm (85.8 mbsf) where they occur as few specimens. Recognition of C. macintyrei above Sample 10H-CC is diﬃcult because siliciclastic materials dominate the samples and nannofossils are reworked to younger portions of the core. Coccolith preservation is moderate to good Geosciences 2020, 10, 388 9 of 26 in Samples 10H-CC to 10H-6W, while very few, poorly preserved and broken specimens of C. macintyrei are found above the interval and occur sporadically in younger sediment samples. Furthermore, coccoliths ~9 µm but similar to the morphology of C. macintyrei are noted in some samples. In this study, the last occurrence is placed between Samples 10H-6W, 20 to 21 cm and 10H-5W, 20 to 21 cm (85.705 mbsf).

3.1.7. FO of Large Gephyrocapsa spp. (blG) The base (first occurrence) of large Gephyrocapsa (blG) had been documented at Sample U1431D 11H-CC (98.15 mbsf) based on IODP shipboard biostratigraphy (Li et al., 2014). However, this study did not observe large Gephyrocapsa at any sample within sections 11H and 10H (from 96.10 to 79.40 mbsf). It is detected much shallower (at Samples 9H-CC (78.91 mbsf) and above) and occurs above the LO of C. macintyrei rather than below it. Few to common large Gephyrocapsa specimens occur at Sample 9H-CC (78.91 mbsf) and they appeared abruptly in the sediment record. The first occurrence of large Gephyrocapsa morphotypes has also been documented above the LO of C. macintyrei in the Shatsky Rise in the northwest Pacific Ocean [54] and in ODP Leg 111 [55] and ODP Leg 138 [56] in the Equatorial Pacific. This pattern is also seen in the Caribbean Sea from ODP Leg 165 [57] and in the South Atlantic at ODP Sites 925 and 926 [58]. Thus, the blG is placed between Samples 9H-CC and 9H-7W, 36 cm (78.985 mbsf), above the LO of C. macintyrei. The distinct first appearance of large gephyrocapsids suggests that they are reliable markers in the SCS.

3.1.8. LO of Large Gephyrocapsa spp. (tlG) The LO of large Gephyrocapsa (>5.5 µm) was reported in Sample U1431D 8H-1W, 50 cm (60.70 mbsf; [43]). Very good coccolith preservation is observed in this sample. Samples above and below this interval (8H-1W, 40 to 41 cm, 8H-1W, 20 to 21 cm and 8H-2W, 19 to 20 cm, respectively) are barren of nannofossils while Sample 7H-CC has no large Gephyrocapsa. Thus, the LO lies between Samples 8H-1W, 50 cm and 8H-1W, 40 to 41 cm (60.65 mbsf), and is consistent with shipboard results [43].

3.1.9. Reentrance Event of Medium Gephyrocapsa spp. (reemG) Measurements of Gephyrocapsa coccoliths recognized the reentrance event of medium Gephyrocapsa (reemG) in Sample 7H-6W, 20 to 21 cm (57.90 mbsf). The placoliths are characterized by generally high bridge angle values (60◦–70◦) but are not exactly parallel to the short axis. The reemG is reassigned between Samples 7H-CC, 24 to 29 cm and 7H-6W, 20 to 21 cm (58.24 mbsf) from Sample 6H-CC reported in shipboard results (50.90 mbsf; [43]). In Site U1431D, the reemG event does not exactly correspond to the ﬁrst occurrence (FO) of the omega-type Gephyrocapsa (Gephyrocapsa sp. 3). This trend is also observed in ODP 198, Site 1209B, in the Shatsky Rise [59] and in ODP 161 in the western Mediterranean [60].

3.1.10. FO and LO of Gephyrocapsa sp. 3 The FO of Gephyrocapsa sp. 3 was initially observed at Sample U1431D 5H-6W, 47 cm (39.67 mbsf; [43]). The occurrence of Gephyrocapsa sp. 3 in the sediment core shows a cyclical appearance and disappearance of nannofossils. Detrital material seen in smear slides probably diluted the nannofossil content. These barren layers are punctuated by samples with well-preserved, abundant calcareous nannofossils. The reﬁnement of nannofossil biostratigraphy extended the FO further downcore to Sample 7H-5W, 60 to 61 cm (57.30 mbsf). The FO of Gephyrocapsa sp. 3 is then placed between Samples 7H-5W, 60 to 61 cm and 7H-6W, 20 to 21 cm (57.60 mbsf). This datum is 60 cm above the reemG event in Site U1431D. The LO of this nannofossil was initially placed at Sample U1431D 4H-CC, 29 to 34 cm (32.08 mbsf; [43]). The results of this study show that the LO is found between Samples 4H-3W, 20–21 cm, and 4H-3W, 120 to 121 cm (25.925 mbsf). Geosciences 2020, 10, 388 10 of 26

3.1.11. LO of Pseudoemiliania lacunosa Shipboard results from IODP Site U1431D placed the LO of P. lacunosa at Sample 2H-CC, 0 to 4 cm (12.15 mbsf; [43]). In this study, the datum is found at younger portions of the core—between SamplesGeosciences 2H-2W,2020, 10, x80 FOR to PEER 82 cm REVIEW and 2H-2W, 60 to 62 cm (5.42 mbsf). Circular morphotypes are10 of found 26 more abundantly in the deeper sections of the core, while elliptical types are more common in younger sediments.abundantly Only in the the deeper elliptical sections forms of Pseudoemilianiathe core, while areelliptical documented types are in themore last common two samples in younger (Samples 2H-2W,sediments. 100 Only to 102 the cm elliptical and 2H-2W, forms 80of Pseudoemiliania to 82 cm; 5.7 and are 5.5documented mbsf, respectively). in the last two samples (Samples 2H-2W, 100 to 102 cm and 2H-2W, 80 to 82 cm; 5.7 and 5.5 mbsf, respectively). 3.1.12. FO of Emiliania huxleyi 3.1.12. FO of Emiliania huxleyi The FO of E. huxleyi was previously reported at Sample 1H-CC, 7 to 14 cm (3.22 mbsf; [43]). The datumThe FO has of been E. huxleyi reassigned was previously between Samples reported 2H-1W, at Sample 60 to 1H-CC, 61 cm and 7 to 2H-1W, 14 cm 80(3.22 to 81mbsf; cm (3.905[43]). The mbsf). Nannofossilsdatum has been above reassigned and below between the event Samples show a cyclical2H-1W, trend 60 to of 61 barren cm and and 2H-1W, well-preserved 80 to 81 assemblages,cm (3.905 whichmbsf). made Nannofossils the recognition above and of this below bioevent the event diﬃcult. show Dissolution a cyclical trend in some of ofbarren the nannofossils, and well-preserved especially amongassemblages, the placoliths, which made is also the observed. recognition of this bioevent difficult. Dissolution in some of the nannofossils, especially among the placoliths, is also observed. 3.2. Morphologic Changes in Gephyrocapsids in Site U1431D 3.2. Morphologic Changes in Gephyrocapsids in Site U1431D Small Gephyrocapsa is the only morphotype present from 135 to 87.94 mbsf (~2.58 to 1.79 Ma). It is characterizedSmall Gephyrocapsa by a large centralis the only area morphotype and has varying present bridge fromangles, 135 to 87.94 although mbsf low (~2.58 average to 1.79 values Ma). It are recordedis characterized in the interval.by a large The central overall area trend and has shows varying an increasing bridge angles, small althoughGephyrocapsa low averagecoccolith values length throughoutare recorded the in the core. interval. Medium The Gephyrocapsaoverall trend showsﬁrst appeared an increasing at 87.635 small mbsfGephyrocapsa (~1.79 Ma) coccolith and doeslength not throughout the core. Medium Gephyrocapsa first appeared at 87.635 mbsf (~1.79 Ma) and does not show any long-term changes in length. The increase in the length of medium forms corresponds to show any long-term changes in length. The increase in the length of medium forms corresponds to an increase in the overall length of small morphotypes throughout most of the interval (Figure6). an increase in the overall length of small morphotypes throughout most of the interval (Figure 6). Large Gephyrocapsa occurs consistently only from 78.91 to 60.70 mbsf. Exceptionally large forms Large Gephyrocapsa occurs consistently only from 78.91 to 60.70 mbsf. Exceptionally large forms (between 7.5 and 8.0 µm) are documented at around 0.8 Ma. No observable long-term trends in size (between 7.5 and 8.0 μm) are documented at around 0.8 Ma. No observable long-term trends in size changes are noted for the large morphotypes. changes are noted for the large morphotypes.

FigureFigure 6.6. SizeSize measurements of of GephyrocapsaGephyrocapsa fromfrom Site Site U1431D U1431D correlated correlated with with the the LR04 LR04 Stack Stack of [2]. of [ 2]. DotsDots in in the the leftmost leftmost line line graph graph are are sample sample points. points. Gray Gray bars bars highlight highlight onsets onsets of relative of relative abundance abundance peaks inpeaks medium in mediumGephyrocapsa Gephyrocapsaforms. forms. Blank intervalsBlank intervals in the relativein the rela abundancetive abundance graph indicategraph indicate barren barren samples. Thesamples. age of The each age sample of each was sample calculated was calculated through linearthrough interpolation linear interpolation of data points of data following points following the results ofthe nannofossil results of nannofossil biostratigraphy. biostratigraphy. The dashed The arrows dashed in thearrows relative in the abundance relative abundance graph and graph LR04 and Stack showLR04 trendsStack inshow increasing trends amplitude in increasing ﬂuctuations. amplitud Abbreviationse fluctuations. listed: Abbreviations LO—last occurrence, listed: LO—last FO—ﬁrst occurrence,occurrence, reemG—reentranceFO—first occurrence, event reem ofG—reentrance medium-sized eventGephyrocapsa of medium-sized, tlG—top Gephyrocapsa of large Gephyrocapsa, tlG—top , blG—baseof large Gephyrocapsa of large Gephyrocapsa, blG—base, of bmG—base large Gephyrocapsa of medium, bmG—baseGephyrocapsa of medium. Gephyrocapsa.

Bridge angle changes across the Gephyrocapsa morphotypes range from 30° to 75° (Figure 6). A general decrease in bridge angle is documented from 2.58 to 1.79 Ma (small Gephyrocapsa) while an increase in bridge angle from 1.79 to 0.9 Ma is evident in the record (small and medium Gephyrocapsa). The highest values for bridge angles for small and medium forms are noted from 1.04 to 0.9 Ma and correlate with the first occurrence of Gephyrocapsa sp. 3. The bridge angle rapidly decreases in value Geosciences 2020, 10, 388 11 of 26

Bridge angle changes across the Gephyrocapsa morphotypes range from 30◦ to 75◦ (Figure6). A general decrease in bridge angle is documented from 2.58 to 1.79 Ma (small Gephyrocapsa) while an increase in bridge angle from 1.79 to 0.9 Ma is evident in the record (small and medium Gephyrocapsa). The highest values for bridge angles for small and medium forms are noted from 1.04 to 0.9 Ma and correlate with the first occurrence of Gephyrocapsa sp. 3. The bridge angle rapidly decreases in value from 0.9 to 0.8 Ma. An overall increase in bridge angle is seen from 0.8 Ma to present, with distinct lows between 0.5 and 0.45 Ma and at 0.2 Ma. In general, the bridge angle changes seem to respond similarly, regardless of the size fraction, and there is no apparent relation between the bridge angle with respect to average placolith length (Figure6). Relative abundances of small, medium, and large Gephyrocapsa morphotypes show oscillations in relative abundances of small and medium forms at around 1.79 and 0.65–0.6 Ma (Figure6). Medium Gephyrocapsa comprise 20-30% of the total gephyrocapsid assemblage in this interval. From 0.65 to 0.6 Ma, oscillations of the shifting dominance of small and medium forms became more evident. When medium Gephyrocapsa is dominant, large morphotypes also appear in the record in few numbers, except for a spike in large Gephyrocapsa at around 0.2 Ma. The increasing amplitude fluctuation of medium Gephyrocapsa roughly coincides with the increasing glacial–interglacial variations trending towards younger sections (Figure6). However, it is not clear if the driving mechanism of the changes in relative abundance fluctuations is correlated to glacial or interglacial stages, as both increasing and decreasing trends are evident in both conditions.

4. Discussion

4.1. Magnetobiochronologic Calibration of Nannofossil Datums in Site U1431D Nannofossil events identiﬁed in Site U1431D are tied with magnetostratigraphic data from shipboard results to estimate a numerical age for the calcareous nannofossil datums. Magnetostratigraphic reversals are calibrated against [61], compiled in [9], while the calculation of the numerical ages using magnetostratigraphy is explained in the Supplementary Materials. Low- to mid-latitude Deep Sea Drilling Project (DSDP) and ODP sites are utilized to compare the nannofossil ages of datums observed in Site U1431D. The locations of these DSDP and ODP Sites are listed in Table2 and are shown in Figure7. The magnetobiochronologic calibrations are mostly in agreement with the compilation of [9], except for some datums (Table1). These inconsistencies are focused on the discussion below.

Table 2. Location of Deep Sea Drilling Project (DSDP), ODP Sites, and onshore references used for comparison.

Hole/Name Latitude Longitude Water Depth (m) Reference(s)

DSDP 552 56◦02.560 N 23◦13.880 W 2301 [13,62] DSDP 607 41◦00.070 N 32◦57.440 W 3427 [21,22,63] ODP 964 36◦15.620 N 17◦45.0250 E 3650 [64] Boso Peninsula 35◦ N 140◦ E-[65] ODP 1146 19◦27.400 N 116◦16.370 E 2092 [42] ODP 1148 18◦50.170 N 116◦33.930 E 3294 [42,66] ODP 769B 8◦47.120 N 121◦17.680 E 3644 [67] ODP 768B 8◦00.050 N 121◦13.190 E 4385 [67] ODP 925B 4◦12.250 N 43◦29.350 W 3041 [58] ODP 677 1◦12.030 N 83◦44.160 W 3461 [21,22] ODP 848 2◦59.60 S 110◦28.80 W 3855 [68] ODP 710 4◦18.70 S 60◦58.80 E 3824 [13,69] DSDP 593 40◦30.470 S 167◦40.470 E 1068 [70,71]

The LO of Calcidiscus macintyrei in Paciﬁc, Atlantic, Indian, and Mediterranean sections are correlated to Chron C1r.3r; however, the placement of the datum within the chron varies by location Geosciences 2020, 10, 388 12 of 26

(Figure8). Excluding Site 1146, the Indian, Atlantic, and Mediterranean records have younger age estimates than the Paciﬁc and the SCS. In Site U1431D, correlation with shipboard magnetostratigraphy suggests that the event falls just above the top of the Olduvai Chron (C2n; 1.778–1.945 Ma, [9]), but still within the lowermost portions of Chron C1r.3r, while previous works document the LO Geosciences 2020, 10, x FOR PEER REVIEW 12 of 26 of C. macintyrei at a slightly higher magnetostratigraphic position (lower C1r.3r but not directly on top of Olduvai—[7,9,22,58,64,70–72]). Discrepancies in the ages of the extinction event (0.2 Ma) may not directly on top of Olduvai—[7,9,22,58,64,70–72]). Discrepancies in the ages of the extinction event suggest diachroneity. (0.2 Ma) may suggest diachroneity.

Figure 7.7. LocationLocation of of DSDP DSDP/ODP/IODP/ODP/IODP Sites Sites used inused this in study this for study comparison for comparison of Pleistocene of nannofossilPleistocene nannofossildatums. Site datums. IODP 1431 Site isIODP examined 1431 is in examined the present in study.the present study.

Although thethe taxonomic taxonomic definition definition of C.of macintyreiC. macintyreiis simple, is simple, several several inconsistencies inconsistencies and diff eringand differingspecies concepts species concepts exist in the exist literature. in the literature. In Sites In 768 Sites and 768 769 and in the769 Suluin the Sea, Sulu [67 Sea,] cited [67] [cited73] for [73] their for theiradopted adopted age for ageC. tropicusfor C. tropicusto subdivide to subdivide NN19 of NN19 [74]. However,of [74]. Howeve there isr, no there written is no account written of accountC. tropicus of C.in tropicus [73]. The inC. [73]. tropicus The C.of tropicus [67] may of be[67] equivalent may be equivalent to the C. to macintyrei the C. macintyreiof [73]. This of [73]. equivalence This equivalence may be mayunlikely be unlikely because because large age large discrepancies age discrepancies exist in the exist literature in the literature between thebetween two species the two (1.48 species Ma—[ (1.4867]; 1.57Ma—[67]; Ma—[ 1.5773]) andMa—[73]) thus the and di ffthuserences the maydifferences be an e maffecty ofbe varyingan effect species of varying concepts. species The concepts. stratigraphic The stratigraphicinconsistency inconsistency for the datum fo isr the also datum reported is also in the reported Japan Seain the (1.33 Japan Ma; Sea [75 ]).(1.33 The Ma; LO [75]). in the The Japan LO in Sea the is Japanyounger Sea by is 0.22younger Ma compared by 0.22 Ma to compared the study ofto [the51] st andudy by of 0.27 [51] Ma and compared by 0.27 Ma to [compared7]. Size variations to [7]. Size in C.variations leptoporus in C.(5–8 leptoporusµm, sometimes (5–8 μm, reachingsometimes up reaching to 10 µm) up may to 10 have μm) causedmay have confusion caused whetherconfusion the whether larger theend larger of the end species of the belonged species belonged to C. macintyrei to C. macintyreior a larger or a variant larger variant of C. leptoporus of C. leptoporus(e.g., [ 75(e.g.,]). [75]). Studies concerning the LO of Gephyrocapsa sp. 3 are limited only to the Mediterranean. The limited occurrence and distribution of studies of the LO of Gephyrocapsa sp. 3 is the reason [7] did not include the event for global biostratigraphic correlation. The datum age (0.58 Ma) was first identified by [76] and calibrated through magnetostratigraphy. More recent calibrations in the Mediterranean correlated the event to MIS 15 (0.570–0.577 Ma; [64,77]). In the SCS, there is very little documentation on the distribution of this nannofossil downcore [43]. The LO of Gephyrocapsa sp. 3 is placed within the middle portions of Chron C1n (Brunhes Chron; 0–0.780 Ma) in Site U1431D, suggesting an age of 0.44 Ma for the datum (Figure9). Although events using Gephyrocapsa are used in some studies in the SCS [41,42], this study presents one of the very few evidence for the occurrence and distribution of Gephyrocapsa sp. 3 in SCS Pleistocene sediments. Its clear appearance downcore implies its significance as a calcareous nannofossil marker in the SCS. It must be noted with caution that Gephyrocapsa sp. 3 is still present in the open ocean today [77]; thus, careful analysis of the bridge angles of Gephyrocapsa downcore may be necessary to determine the LO of this species.

Figure 8. Comparison of stratigraphic ranges of the LO of Calcidiscus macintyrei from IODP/ODP/DSDP Sites. Magnetostratigraphic records from IODP Site U1431D, ODP Sites 1148, 848, 710 and DSDP Site 593 are calibrated to the astronomically tuned Geomagnetic Polarity Time Scale (GPTS) of [61] as compiled by [9]. Oxygen isotope data is used to determine the ages in ODP Sites 1146, 925, 964 and DSDP Site 607. These datums are calibrated to the benthic foraminiferal δ18O time scale of [2]. Horizontal bars represent the recalculated datum ages for each site. Med.—Mediterranean. Black—normal; white— reversed polarity. Geosciences 2020, 10, x FOR PEER REVIEW 12 of 26

not directly on top of Olduvai—[7,9,22,58,64,70–72]). Discrepancies in the ages of the extinction event (0.2 Ma) may suggest diachroneity.

Figure 7. Location of DSDP/ODP/IODP Sites used in this study for comparison of Pleistocene nannofossil datums. Site IODP 1431 is examined in the present study.

Although the taxonomic definition of C. macintyrei is simple, several inconsistencies and differing species concepts exist in the literature. In Sites 768 and 769 in the Sulu Sea, [67] cited [73] for their adopted age for C. tropicus to subdivide NN19 of [74]. However, there is no written account of C. tropicus in [73]. The C. tropicus of [67] may be equivalent to the C. macintyrei of [73]. This equivalence may be unlikely because large age discrepancies exist in the literature between the two species (1.48 Ma—[67]; 1.57 Ma—[73]) and thus the differences may be an effect of varying species concepts. The stratigraphic inconsistency for the datum is also reported in the Japan Sea (1.33 Ma; [75]). The LO in the Japan Sea is younger by 0.22 Ma compared to the study of [51] and by 0.27 Ma compared to [7]. Size Geosciences 2020, 10, 388 13 of 26 variations in C. leptoporus (5–8 μm, sometimes reaching up to 10 μm) may have caused confusion whether the larger end of the species belonged to C. macintyrei or a larger variant of C. leptoporus (e.g., [75]).

Geosciences 2020, 10, x FOR PEER REVIEW 13 of 26

Studies concerning the LO of Gephyrocapsa sp. 3 are limited only to the Mediterranean. The limited occurrence and distribution of studies of the LO of Gephyrocapsa sp. 3 is the reason [7] did not include the event for global biostratigraphic correlation. The datum age (0.58 Ma) was first identified by [76] and calibrated through magnetostratigraphy. More recent calibrations in the Mediterranean correlated the event to MIS 15 (0.570–0.577 Ma; [64,77]). In the SCS, there is very little documentation on the FiguredistributionFigure 8. Comparison8. Comparison of this nannofo of of stratigraphic stratigraphicssil downcore ranges ranges [43]. ofof The thethe LOLO of of GephyrocapsaCalcidiscus macintyrei macintyrei sp. 3 is placedfromfrom IODP/ODP/DSDP IODP within/ODP the /middleDSDP Sites.portionsSites. Magnetostratigraphic Magnetostratigraphic of Chron C1n (Brunhes records records Chron; from from IODP0–0.780 IODP Site SiMa)te U1431D, U1431D, in Site ODP U1431D, ODP Sites Sites 1148,suggesting 1148, 848, 848, 710 an710 andage and DSDPof DSDP 0.44 Site Ma Site 593 for arethe593 calibrated datum are calibrated (Figure to the 9). astronomically to Although the astronomically events tuned using Geomagnetic tuned Gephyrocapsa Geomagnetic Polarity are Polarityused Time in Scale someTime (GPTS) studiesScale of(GPTS) in [61 the] as ofSCS compiled[61] [41,42], as this study presents one of the very few evidence for the occurrence and distribution of Gephyrocapsa sp. bycompiled [9]. Oxygen by [9]. isotope Oxygen data isotope is used data to is determine used to determine the ages the in ODPages in Sites ODP 1146, Sites 925,1146, 964 925, and 964 DSDPand 3 in SCS Pleistocene sediments. Its clear appearance downcore implies18 its significance as a calcareous SiteDSDP 607. TheseSite 607. datums These datums are calibrated are calibrated to the to benthic the benthic foraminiferal foraminiferalδ18 δOO time time scale scale of [2]. [2]. Horizontal Horizontal nannofossil marker in the SCS. It must be noted with caution that Gephyrocapsa sp. 3 is still present in barsbars represent represent the the recalculatedrecalculated datum datum ages ages for foreach each site. site.Med.—Mediterranean. Med.—Mediterranean. Black—normal; Black—normal; white— the open ocean today [77]; thus, careful analysis of the bridge angles of Gephyrocapsa downcore may be white—reversedreversed polarity. polarity. necessary to determine the LO of this species.

FigureFigure 9. Comparison 9. Comparison of stratigraphic of stratigraphic ranges ranges of the of FO the and FO LO and of GephyrocapsaLO of Gephyrocapsasp. 3 from sp. IODP3 from/ODP / DSDPIODP/ODP/DSDP Sites. Magnetostratigraphic Sites. Magnetostratigraphic records from IODPrecords Site from U1431D IODP Site and U1431D ODP Site and 848ODP are Site calibrated 848 are to the astronomicallycalibrated to the tuned astronomically Geomagnetic tuned Polarity Geomagnetic Time ScalePolarity (GPTS) Time ofScale [61 ](GPTS) as compiled of [61] as by compiled [9]. Oxygen isotopeby data[9]. Oxygen is used isotope to determine data is used the to ages determine in ODP the Site ages 964 in ODP and Site DSDP 964 Siteand 607.DSDP These Site 607. datums These are 18 calibrateddatums to theare benthiccalibrated foraminiferal to the benthicδ foraminiferal18O time scale δ ofO time [2]. Horizontalscale of [2]. Hori barszontal represent bars represent the recalculated the datumrecalculated ages for each datum site. ages Upward for each facing site. horizontal Upward fa barscing representhorizontalFO bars datums, represent while FO downwarddatums, while facing downward facing bars signify LO datums. Med.—Mediterranean. Black—normal; white—reversed bars signify LO datums. Med.—Mediterranean. Black—normal; white—reversed polarity. polarity.

Correlation with magnetostratigraphy places the LO of P. lacunosa within the upper portions of Chron C1n (Figure 10), which corresponds to an age of 0.092 Ma in Site U1431D. There is a considerable age discrepancy in the LO of P. lacunosa in Site U1431D compared to the literature. The difference ranges from 0.37 Ma between Sites U1431D and Sites 1146 (SCS; [42]), 925 (Atlantic; [58]), and 964 (Mediterranean; [77]) to as much as 0.55 Ma compared to the southwest Pacific [74,75]. The results of this study differ significantly compared from previous works which suggest the LO of P.Geosciences lacunosa 2020to be, 10 correlated, x FOR PEER to REVIEW MIS 12 [17,21,22]. The upper portions of the core are defined mainly14 of by 26 interbedded to interlaminated clayey silt to silty clay which are interpreted as turbidite sequences [43]. interbedded to interlaminated clayey silt to silty clay which are interpreted as turbidite sequences Turbidity currents may have caused the resuspension of coccoliths of P. lacunosa to younger sediments [43]. Turbidity currents may have caused the resuspension of coccoliths of P. lacunosa to younger of the core. Uncertainties in magnetostratigraphic calibration may also be a factor in the age discrepancy. sediments of the core. Uncertainties in magnetostratigraphic calibration may also be a factor in the Because the age calculated from magnetostratigraphy is too young compared to other sites, the standard age discrepancy. Because the age calculated from magnetostratigraphy is too young compared to age of 0.44 Ma [9] is adopted for the LO of P. lacunosa (Figure 10, gray datum). other sites, the standard age of 0.44 Ma [9] is adopted for the LO of P. lacunosa (Figure 10, gray datum).

FigureFigure 10. 10.Comparison Comparison of stratigraphic of stratigraphic ranges of ranges LO of Pseudoemiliania of LO of lacunosaPseudoemilianiafrom IODP lacunosa/ODP/DSDP from Sites.IODP/ODP/DSDP Magnetostratigraphic Sites. Magnetostr records fromatigraphic IODP Siterecords U1431D, from ODPIODP Sites Site 1148,U1431D, 769B, ODP 768B, Sites 848, 1148, 710 769B, and DSDP768B, Site848, 593 710 are and calibrated DSDP Site to 593 the are astronomically calibrated to tuned the astronomically Geomagnetic Polaritytuned Geomagnetic Time Scale (GPTS)Polarity ofTime [61] asScale compiled (GPTS) by of [9 [61]]. Oxygen as compiled isotope by data [9]. is Oxygen used to determineisotope data the is ages used in ODPto determine Sites 1146, the 925 ages and in 964.ODP These Sites datums1146, 925 are and calibrated 964. These to thedatums benthic are foraminiferalcalibrated to theδ18 Obenthic time scale foraminiferal of [2]. Horizontal δ18O time bars scale representof [2]. Horizontal the recalculated bars represent datum ages the recalculated for each site. datum Black datumages for in each IODP site. Site Black U1431D datum represents in IODP age Site acquiredU1431D fromrepresents magnetostratigraphy. age acquired from Gray magnetostr datum in IODPatigraphy. Site U1431D Gray datum represents in IODP placement Site U1431D of the datumrepresents from thisplacement study usingof the the datu datumm from age reportedthis study in using [9]. Med.—Mediterranean. the datum age reported in [9]. Med.— Mediterranean. The age of the FO of E. huxleyi in Site U1431D also has a large discrepancy. Correlation to Site U1431DThe magnetostratigraphy age of the FO of E. huxleyi showed in that Site the U1431D FO of E.also huxleyi has acorresponds large discrepancy. to the upperCorrelation portions to Site of ChronU1431D C1n magnetostratigraphy at 0.066 Ma (Figure 11 showed, black datum).that the TheFO of oldest E. huxleyi age estimates corresponds are found to the in upper ODP portions Sites 1146 of andChron 1148 C1n followed at 0.066 by Ma ODP (Figure Sites 768 11, and black 769 datum). in the Sulu The Seaoldest (Figure age 11estimates). Younger are ages found are in noted ODP from Sites the1146 open and ocean 1148 (Pacificfollowed and by Atlantic)ODP Sites and 768 in and the 769 Mediterranean in the Sulu Sea Sea; (Figure however, 11). Younger the age acquired ages are fromnoted Sitefrom U1431D the open is much ocean younger (Pacific and than Atlantic) all other and sites. in Thethe Mediterranean uncertainty ranges Sea; fromhowever, 0.305 the Ma age compared acquired tofrom Site 1148Site U1431D [42] to 0.188 is much Ma comparedyounger than to Site all 925other [58 ].sites. Bioturbation The uncertainty and terrigenous ranges from input 0.305 to the Ma basincompared may haveto Site aff 1148ected [42] the to signal 0.188 ofMa the compared FO. Bioturbation to Site 925 causes [58]. Bioturbation vertical sediment and terrigenous mixing which input mayto the reposition basin may a datum have affected to a slightly the signal deeper of depth. the FO However,. Bioturbation shipboard causes results vertical reveal sediment only mixing slight bioturbationwhich may (10–30%)reposition in a thedatum interval to a slightly where the deeper FO of depth.E. huxleyi However,is documented shipboard [ 43results] and reveal may have only hadslight minor bioturbation effect in the (10–30%) position in of the the interval datum. where Terrigenous the FO input of E. mayhuxleyi be ais problem documented especially [43] and for lessmay robusthave specieshad minor like effectE. huxleyi in the. Previous position reports of the suggestdatum. theTerrigenous sporadic occurrenceinput may of beE. a huxleyi problemnear especially its first appearancefor less robust [15,17 species,78]. Carbonate like E. huxleyi dilution. Previous through reports terrigenous suggest input the mightsporadic further occurrence obscure of the E. datum. huxleyi Shipboardnear its first data appearance suggest that [15,17,78]. carbonate Carbonate percentages dilution in Pleistocene through sediments terrigenous of Site input U1431D might are further very lowobscure (1–10%; the [ 43datum.]). This Shipboard interval is data also suggest defined that by an ca abundancerbonate percentages of plagioclase, in Pleistocene mica, and sediments amphibole of Site U1431D are very low (1–10%; [43]). This interval is also defined by an abundance of plagioclase, mica, and amphibole seen in smear slides, suggesting increased detrital input. Magnetostratigraphic calibration may have also contributed to the age discrepancy recorded in IODP Site U1431D. Thus, the age of 0.29 Ma [9] is adopted for the datum (Figure 11, gray datum). Geosciences 2020, 10, 388 15 of 26 seen in smear slides, suggesting increased detrital input. Magnetostratigraphic calibration may have also contributed to the age discrepancy recorded in IODP Site U1431D. Thus, the age of 0.29 Ma [9] is adopted for the datum (Figure 11, gray datum). Geosciences 2020, 10, x FOR PEER REVIEW 15 of 26

FigureFigure 11. ComparisonComparison of of stratigraphic stratigraphic ranges ranges of FO of FOof Emiliania of Emiliania huxleyi huxleyi fromfrom IODP/ODP/DSDP IODP/ODP/DSDP Sites. Sites.Magnetostratigraphic Magnetostratigraphic records records from fromIODP IODP Site U1431D, Site U1431D, ODP ODPSites Sites1148, 1148, 769B, 769B, 768B, 768B, and andDSDP DSDP Site Site593 593are arecalibrated calibrated to the to theastronomically astronomically tuned tuned Geomag Geomagneticnetic Polarity Polarity Time Time Scale Scale (GPTS) (GPTS) of of[61] [61 as] ascompiled compiled by by[9]. [ 9Oxygen]. Oxygen isotope isotope data data is used is used to determine to determine the theages ages in OD inP ODP Sites Sites 1146, 1146, 925 925and and964. 18 964.These These datums datums are arecalibrated calibrated to tothe the benthic benthic foraminiferal foraminiferal δ18δOO time time scale scale of of [2]. [2]. Horizontal barsbars representrepresent thethe recalculatedrecalculated datumdatum agesages forfor eacheach site.site. BlackBlack datumdatum inin IODPIODP SiteSite U1431DU1431D representsrepresents ageage acquiredacquired fromfrom magnetostratigraphy.magnetostratigraphy. GrayGray datumdatum inin IODPIODP SiteSite U1431DU1431D representsrepresents placementplacement ofof thethe datumdatum fromfrom thisthis studystudy usingusing thethe datumdatum ageage reportedreported inin [9[9].]. Med.—Mediterranean.Med.—Mediterranean. 4.2. The Importance and Complexity of Gephyrocapsa Morphometry in Site U1431D 4.2. The Importance and Complexity of Gephyrocapsa Morphometry in Site U1431D The overall time-transgressive size increase correlates well with those of previous The overall time-transgressive size increase correlates well with those of previous works [19,21– works [19,21–24,59,79]. The work of [19] complements the data gathered; however, no large Gephyrocapsa 24,59,79]. The work of [19] complements the data gathered; however, no large Gephyrocapsa were were observed in the lower Pleistocene sections of the core studied. Only small Gephyrocapsa spp. observed in the lower Pleistocene sections of the core studied. Only small Gephyrocapsa spp. placoliths placoliths are documented from Sample 15H-CC (~135.3 m; latest Pliocene) to 10H-8W, 20–21 cm are documented from Sample 15H-CC (~135.3 m; latest Pliocene) to 10H-8W, 20–21 cm (~87.94 m; early (~87.94 m; early Pleistocene). The Pliocene–Pleistocene was defined at the base of the Calabrian Pleistocene). The Pliocene–Pleistocene was defined at the base of the Calabrian (~1.8 Ma) by [19] and (~1.8 Ma) by [19] and may account for the disparity. Nevertheless, the size increase is documented across may account for the disparity. Nevertheless, the size increase is documented across ocean basins; thus, ocean basins; thus, the morphologic change in Gephyrocapsa most probably points to an evolutionary the morphologic change in Gephyrocapsa most probably points to an evolutionary trend. The time- trend. The time-transgressive size increase is applicable for biostratigraphic use and may explain the transgressive size increase is applicable for biostratigraphic use and may explain the comparable record comparable record of Site U1431D sediments to other ocean basins. Global climate changes longer of Site U1431D sediments to other ocean basins. Global climate changes longer than glacial–interglacial than glacial–interglacial cycles, such as ocean cooling brought on by a steady increase in ice cover, cycles, such as ocean cooling brought on by a steady increase in ice cover, may have affected the long- may have affected the long-term changes in the size of Gephyrocapsa coccoliths [79]. It is also noted that term changes in the size of Gephyrocapsa coccoliths [79]. It is also noted that the changes in the changes in morphological parameters of the gephyrocapsids are not in one-to-one correspondence morphological parameters of the gephyrocapsids are not in one-to-one correspondence with glacial– with glacial–interglacial variations. Higher resolution sampling, correlatable to suborbital variations in interglacial variations. Higher resolution sampling, correlatable to suborbital variations in δ18O, is δ18O, is needed to further explain the response of Gephyrocapsa during these time scales. needed to further explain the response of Gephyrocapsa during these time scales. Specimens of Gephyrocapsa coccoliths analyzed by [80] from tropical Atlantic waters generally Specimens of Gephyrocapsa coccoliths analyzed by [80] from tropical Atlantic waters generally have high bridge angles (parallel to the short axis of the coccolith). This observation is supported have high bridge angles (parallel to the short axis of the coccolith). This observation is supported by by [52], noting further that the bridge angle may be correlated with geographic distribution—the [52], noting further that the bridge angle may be correlated with geographic distribution—the position of the bridge approaches the long axis towards high latitudes, in contrast to a mean bridge position of the bridge approaches the long axis towards high latitudes, in contrast to a mean bridge angle of 60◦ within 15 degrees north and south of the Equator. If temperature has a direct effect on angle of 60° within 15 degrees north and south of the Equator. If temperature has a direct effect on bridge angle orientation, then there should be a glacial–interglacial trend with respect to bridge angle, bridge angle orientation, then there should be a glacial–interglacial trend with respect to bridge angle, with relatively colder periods shifting to lower bridge angle values. However, in Site U1431D there is with relatively colder periods shifting to lower bridge angle values. However, in Site U1431D there is very little correlation between the average bridge angle values of all size fractions measured and the δ18O record (LR04 Stack of [2]; Figure 6). Most probably, environmental factors other than temperature may have had an influence on the development of the bridge. The presence of medium-sized forms with high bridge angle values at around 1.0– 0.6 Ma is documented in this study and is comparable to previous works [7,19,21,23,24,59,81–83]. This Geosciences 2020, 10, 388 16 of 26 very little correlation between the average bridge angle values of all size fractions measured and the δ18O record (LR04 Stack of [2]; Figure6). Most probably, environmental factors other than temperature may have had an influence on the development of the bridge. The presence of medium-sized forms with high bridge angle values at around 1.0–0.6 Ma is documented in this study and is comparable to previous works [7,19,21,23,24,59,81–83]. This Gephyrocapsa morphotype is most probably equivalent to Gephyrocapsa sp. 3 of [19], Gephyrocapsa oceanicaGeosciencesvertical 2020 of, 10 [, 23x FOR], and PEERGephyrocapsa REVIEW sp. C of [24]. Small forms of Gephyrocapsa with a bridge16 of 26 angle parallel to the short axis are also documented mostly from 1.0 to 0.9 Ma near the “small Gephyrocapsa acme”Gephyrocapsa [23,24]. These morphotype studies is are most correlatable probably equivalent and span to di ffGephyrocapsaerent ocean sp. basins, 3 of [19], each Gephyrocapsa with their own localoceanica climate. vertical Thus, of the [23], changes and Gephyrocapsa in the morphology sp. C of [24]. of Gephyrocapsa Small forms ofin Gephyrocapsa the SCS may with not a be bridge factors of angle parallel to the short axis are also documented mostly from 1.0 to 0.9 Ma near the “small regional climate perturbations. These results show the importance of both bridge angle and size for Gephyrocapsa acme” [23,24]. These studies are correlatable and span different ocean basins, each with Gephyrocapsa the determinationtheir own local ofclimate. certain Thus, the changesmorphotypes in the morphology for Pleistocene of Gephyrocapsa biostratigraphy. in the SCS may not be factors of regional climate perturbations. These results show the importance of both bridge angle and 4.3. Age–Depth Model for Site U1431D size for the determination of certain Gephyrocapsa morphotypes for Pleistocene biostratigraphy. An age–depth model is constructed based on magnetostratigraphic and nannofossil datums identified4.3. Age–Depth in Site Model U1431D. for Site The U1431D datum ages are calculated based on linear interpolation between magnetostratigraphicAn age–depth chronsmodel is (Supplementary constructed based Materials). on magnetostratigraphic The ages for the and FO nannofossil of E. huxleyi datumsand LO of P. lacunosaidentifiedare in adopted Site U1431D. from [The9] because datum ages thecalculated are calculated site-specific based on ageslinear are interpolation too young between compared to publishedmagnetostratigraphic figures. The LOchrons of Gephyrocapsa (Supplementarysp. Materials). 3 is also The taken ages from for the [9] FO because of E. huxleyi the only and calibrationLO of doneP. for lacunosa this datum are adopted is from from [76 [9]] and because [64] the in thecalculated Mediterranean. site-specific Thus, ages are the too age–depth young compared model to for Site published figures. The LO of Gephyrocapsa sp. 3 is also taken from [9] because the only calibration U1431D is a combination of ages compiled by [9] and site-specific numerical ages. done for this datum is from [76] and [64] in the Mediterranean. Thus, the age–depth model for Site The age–depth curve generated from the nannofossil datums using calcareous nannofossils U1431D is a combination of ages compiled by [9] and site-specific numerical ages. shows consistencyThe age–depth with curve other generated microfossil from groups the nanno (Figurefossil 12 ).datums From 135using mbsf calcareous to ~80 mbsf,nannofossils planktonic foraminiferashows consistency and calcareous with other nannofossil microfossil data groups generally (Figure agree12). From with 135 each mbsf other to ~80 (Figure mbsf, planktonic12), with slight divergenceforaminifera at the and 100–135 calcareous mbsf interval.nannofossil Above data generally 90 mbsf, planktonicagree with each foraminiferal other (Figure datums 12), with imply slight younger agesdivergence than the calcareous at the 100–135 nannofossils. mbsf interval. Further Above analysis 90 mbsf, of planktonic planktonic foraminiferal foraminiferal datums biostratigraphy imply is neededyounger to verifyages than the agethe discrepanciescalcareous nannofossils. between theFurther two microfossilanalysis of groups.planktonic The foraminiferal age model from radiolariansbiostratigraphy for the is uppermostneeded to verify layers the (0–10 age disc mbsf)repancies agrees between with thethe two nannofossils, microfossil reflectinggroups. The a slow sedimentationage model ratefrom withinradiolarians the interval. for the uppermost layers (0–10 mbsf) agrees with the nannofossils, reflecting a slow sedimentation rate within the interval.

Figure 12. Comparison of age–depth models using different microfossil groups for Site U1431D. Figure 12. Comparison of age–depth models using diﬀerent microfossil groups for Site U1431D. Symbols represent specific microfossil groups found in the site. The nannofossil age model Symbols represent speciﬁc microfossil groups found in the site. The nannofossil age model incorporates incorporates published ages for FO of E. huxleyi, LO of P. lacunosa, and LO of Gephyrocapsa sp. 3 found published ages for FO of E. huxleyi, LO of P. lacunosa, and LO of Gephyrocapsa sp. 3 found in [9] and in [9] and magnetostratigraphic ages acquired from this study for the older datums. The planktonic magnetostratigraphicforaminifera and radiolarian ages acquired curves from use the this ages study from for [9]. the older datums. The planktonic foraminifera and radiolarian curves use the ages from [9]. The sedimentation rate in Site U1431D may be divided into three distinct segments based on nannofossil datums (Figure 12). From 135 to 60.6 mbsf, the estimated sedimentation rate is ~46.7 m/Ma, representing the time frame between 2.49 to 1.06 Ma. Sedimentation rates almost doubled from 60.6 to 5.42 mbsf, estimated at about 85.2 m/Ma. This period of high sedimentation characterizes the interval between 1.06 and 0.44 Ma. A very slow sedimentation rate of ~12.7 m/Ma occurred after 0.44 Ma. Geosciences 2020, 10, 388 17 of 26

The sedimentation rate in Site U1431D may be divided into three distinct segments based on nannofossil datums (Figure 12). From 135 to 60.6 mbsf, the estimated sedimentation rate is ~46.7 m/Ma, representing the time frame between 2.49 to 1.06 Ma. Sedimentation rates almost doubled from 60.6 to 5.42 mbsf, estimated at about 85.2 m/Ma. This period of high sedimentation characterizes the interval between 1.06 and 0.44 Ma. A very slow sedimentation rate of ~12.7 m/Ma occurred after 0.44 Ma. Geosciences 2020, 10, x FOR PEER REVIEW 17 of 26 4.4. Other Possible Nannofossil Events 4.4. Other Possible Nannofossil Events There are other possible nannofossil events found in the core which may need further reexamination. There are other possible nannofossil events found in the core which may need further These events are recorded as shifts in the abundance values of nannofossil species, or a cross-over reexamination. These events are recorded as shifts in the abundance values of nannofossil species, or of the abundance of variants within a species. These shifts may offer biostratigraphic significance if a cross-over of the abundance of variants within a species. These shifts may offer biostratigraphic assessed further and documented across other basins. significance if assessed further and documented across other basins. The genus Pseudoemiliania had been differentiated based on the number of distal slits [23] and the The genus Pseudoemiliania had been differentiated based on the number of distal slits [23] and degree of ellipticity [84]; these morphotypes are not routinely used in biostratigraphy. However, large, the degree of ellipticity [84]; these morphotypes are not routinely used in biostratigraphy. However, circular forms only appeared in the late Pleistocene and the changes in their morphology may be related large, circular forms only appeared in the late Pleistocene and the changes in their morphology may to temperature fluctuations [48,85]. Absolute abundance counts from Site U1431D show a shift in be related to temperature fluctuations [48,85]. Absolute abundance counts from Site U1431D show a abundance between Pseudoemiliania circular and elliptical variants between 120 to 130 mbsf (Figure 13). shift in abundance between Pseudoemiliania circular and elliptical variants between 120 to 130 mbsf Circular variants of P. lacunosa are dominant from 135 to 130 mbsf and become replaced by elliptical (Figure 13). Circular variants of P. lacunosa are dominant from 135 to 130 mbsf and become replaced types. This observation is different from the work of [84] wherein the circular forms of Pseudoemiliania by elliptical types. This observation is different from the work of [84] wherein the circular forms of dominate the younger portions of DSDP Leg 4 cores. It is possible that ecological preferences and the Pseudoemiliania dominate the younger portions of DSDP Leg 4 cores. It is possible that ecological marginal basin setting of the SCS may have affected the distribution of Pseudoemiliania compared to the preferences and the marginal basin setting of the SCS may have affected the distribution of open-ocean setting of DSDP Leg 4 [84]. Pseudoemiliania compared to the open-ocean setting of DSDP Leg 4 [84].

Figure 13. Absolute abundance of the calcareous nannofossil Pseudoemiliania lacunosa from 0 to 135 Figure 13. Absolute abundance of the calcareous nannofossil Pseudoemiliania lacunosa from 0 to 135 mbsf. mbsf. Broken line shows abundance of circular variants. Solid line shows abundance of elliptical Broken line shows abundance of circular variants. Solid line shows abundance of elliptical morphotypes. morphotypes. The gray highlighted area estimates the transition of circular-dominated to elliptical- The gray highlighted area estimates the transition of circular-dominated to elliptical-dominated dominated Pseudoemiliania in the core. Pseudoemiliania in the core.

TheThe first first and and last last occu occurrencesrrences of of the the small small GephyrocapsaGephyrocapsa acmeacme (1.24 (1.24 to to 1.02 1.02 Ma, Ma, respectively respectively [9]), [9]), waswas not detected inin SiteSite U1431D. U1431D. Possible Possible carbonate carbonate dilution dilution through through terrigenous terrigenous input input may may have have been beenthe cause the cause of the of absence the absence of the event. of the However, event. Howe Zonever, NN21 Zone [74 ]NN21 consists [74] of severalconsists other of severalGephyrocapsa other Gephyrocapsaacme intervals, acme one intervals, of which one may of be which recorded may in be the recorded SCS. Nannofossil in the SCS. biostratigraphy Nannofossil biostratigraphy from the Kings fromTrough the atKings the northeasternTrough at the Atlanticnortheastern Ocean Atlantic show Ocean several show acme several intervals acme corresponding intervals corresponding to different tospecies different of smallspeciesGephyrocapsa of small Gephyrocapsa[85]. They [85]. have They documented have documented the dominance the dominance of G. aperta of G.near aperta the near FO theof E.FO huxleyi of E.. huxleyi In Sample. In Sample 2H-1W, 2H-1W, 60–61 cm 60–61 (3.80 cm mbsf), (3.80 an mbsf), increase an inincrease small Gephyrocapsain small Gephyrocapsaplacoliths placolithsis observed is observed (Figure 14 (Figure). These 14). small These placoliths small placoliths have a large have central a large area central and area a bridge and a near bridge the near long the long axis, in agreement with the taxonomic description of G. aperta. Interestingly, the first occurrence of E. huxleyi is assigned to this sample and correlates well with observations in Kings Trough. A minor small Gephyrocapsa acme event occurs between Zones NN20 and NN21 [86] and was also documented by [24], most likely corresponding to the earlier portions of their Assemblage “B”. The reliability of these secondary events can be assessed through correlation with oxygen isotope stratigraphy or cyclostratigraphy using the physical properties of sediments in the SCS. These minor events have been noted in some studies [24,84–86]. Details of these events in the SCS can be studied further through higher resolution sampling in the intervals where they are noted. Additional Geosciences 2020, 10, 388 18 of 26 axis, in agreement with the taxonomic description of G. aperta. Interestingly, the first occurrence of E. huxleyi is assigned to this sample and correlates well with observations in Kings Trough. A minor small Gephyrocapsa acme event occurs between Zones NN20 and NN21 [86] and was also documented by [24], most likely corresponding to the earlier portions of their Assemblage “B”. The reliability of these secondary events can be assessed through correlation with oxygen isotope stratigraphy or cyclostratigraphy using the physical properties of sediments in the SCS. These minor events have been noted in some studies [24,84–86]. Details of these events in the SCS can be studied Geosciences 2020, 10, x FOR PEER REVIEW 18 of 26 further through higher resolution sampling in the intervals where they are noted. Additional data from otherdata from ocean other basins ocean are neededbasins are to verify needed whether to veri thesefy whether nannofossil these eventsnannofossil are recorded events are on arecorded regional on or a globalregional scale. or a global scale.

Figure 14. AbsoluteAbsolute abundance of small Gephyrocapsa morphotypes fromfrom 00 to 8 mbsf. Gray highlighted area estimatesestimates anan acmeacme intervalinterval ofof thethe nannofossil.nannofossil. The secondary event coincides with the ﬁrstfirst occurrence of Emiliania huxleyi,, whichwhich alsoalso lieslies withinwithin thethe graygray area.area.

5. Conclusions Twelve (12)(12) calcareouscalcareous nannofossilnannofossil datumsdatums are documenteddocumented in PleistocenePleistocene sediments of Site U1431D. This study is one of thethe firstfirst attemptsattempts toto describedescribe thethe distributiondistribution ofof GephyrocapsaGephyrocapsa sp.sp. 3 in SCS sediments and is suggested to be a well-definedwell-defined marker for the subdivision of the Pleistocene in the marginal sea.sea. The FO of Gephyrocapsa sp. 3 is near the reentrance event of medium Gephyrocapsa (reemG),(reemG), whichwhich is is also also observed observed in in several several studies studies [7,21 [7,21–23,60,81,83].–23,60,81,83]. Nannofossil Nannofossil analysis analysis suggests suggests that thethat FO the of FO large of largeGephyrocapsa Gephyrocapsa(blG) (blG) is found is found above above the LO the of LOCalcidiscus of Calcidiscus macintyrei macintyrei, which, which differs differs from shipboardfrom shipboard results results [43]. The [43]. LO The of DiscoasterLO of Discoasterspecies species are determined are determined using a semi-quantitativeusing a semi-quantitative method tomethod reduce to the reduce effect the of reworking;effect of reworking; however, thehowever, LO of D.the pentaradiatus LO of D. pentaradiatusis very sporadic is very and sporadic therefore and is nottherefore used asis anot marker used as for athe marker Pleistocene for the inPleistocen Site U1431D.e in Site The U1431D. numerical The ages nume ofrical nannofossil ages of datumsnannofossil are generateddatums are through generated correlation through correlation with magnetostratigraphy. with magnetostratigraphy. The ages The of theages FO of oftheE. FO huxleyi of E. huxleyiand LO and of P.LO lacunosa of P. lacunosaare underestimated are underestimated by 0.22 by and 0.22 0.35 andMa, 0.35respectively, Ma, respectively, while while bmG bmG and LOandof LOC. of macintyrei C. macintyreiare slightlyare slightly overestimated overestimated by by 0.12 0.12 and and 0.15 0.15 Ma, Ma, respectively. respectively. The The results results of of the the magnetobiochronological magnetobiochronological calibration suggests further analys analysisis using other stratigraphies (e.g., oxygen isotope stratigraphy, cyclostratigraphy). Morphometric analysis of Gephyrocapsa coccoliths fromfrom the late Pliocene to Pleistocene sediments of Site Site U1431D U1431D allowed allowed for for the the recognition recognition and and refinement refinement of bioevents of bioevents of the ofgephyrocapsids. the gephyrocapsids. These Theseevents events are useful are useful in updating in updating the nannofossil the nannofossil biostratigraphy biostratigraphy of the of site. the site.The Theevents events documented documented are arethe theFO FOof medium of medium GephyrocapsaGephyrocapsa morphotypesmorphotypes (87.635 (87.635 mbsf), mbsf), the the FO FO and and LO LO of of large large Gephyrocapsa (78.985(78.985 andand 60.65 60.65 mbsf, mbsf, respectively), respectively), the reemGthe reem eventG (58.24event mbsf),(58.24 andmbsf), the FOand and the LO FO of Gephyrocapsaand LO of sp.Gephyrocapsa 3 (57.605 sp. and 3 25.925(57.605 mbsf,and 25.925 respectively). mbsf, respectively). The trends The of placolith trends of length placolith and length bridge and angles bridge of Gephyrocapsaangles of Gephyrocapsain Site U1431D in Site are U1431D correlatable are correlatable with previous with studiesprevious on studies other oceanon other basins. ocean The basins. data The data suggest that the change in the size of the gephyrocapsids points to an evolutionary trend rather than being caused by local or regional climate anomalies.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1.

Author Contributions: Conceptualization, J.D.S.G. and A.M.P.-A.; methodology, J.D.S.G. and A.M.P.-A.; validation, J.D.S.G. and A.M.P.-A.; formal analysis, J.D.S.G.; investigation, J.D.S.G. and A.M.P.-A.; resources, A.M.P.-A.; data curation, J.D.S.G.; writing—original draft preparation, J.D.S.G.; writing—review and editing, J.D.S.G. and A.M.P.-A.; visualization, J.D.S.G.; supervision, A.M.P.-A.; project administration, A.M.P.-A.; funding acquisition, J.D.S.G and A.M.P.-A. All authors have read and agreed to the published version of the manuscript. Geosciences 2020, 10, 388 19 of 26 suggest that the change in the size of the gephyrocapsids points to an evolutionary trend rather than being caused by local or regional climate anomalies.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3263/10/10/388/s1. Author Contributions: Conceptualization, J.D.S.G. and A.M.P.-A.;methodology, J.D.S.G. and A.M.P.-A.;validation, J.D.S.G. and A.M.P.-A.; formal analysis, J.D.S.G.; investigation, J.D.S.G. and A.M.P.-A.; resources, A.M.P.-A.; data curation, J.D.S.G.; writing—original draft preparation, J.D.S.G.; writing—review and editing, J.D.S.G. and A.M.P.-A.; visualization, J.D.S.G.; supervision, A.M.P.-A.; project administration, A.M.P.-A.; funding acquisition, J.D.S.G and A.M.P.-A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the University of the Philippines Diliman, Office of the Vice Chancellor for Research and Development (OVCRD), grant number 171706 TSNE, and the University of the Philippines National Institute of Geological Sciences (UP NIGS) Research Grant for J.D.S.G. Acknowledgments: The authors are thankful to the two anonymous reviewers whose reviews improved this paper. This research used samples and data provided by the International Ocean Discovery Program (IODP). The authors are grateful to the scientists and crew of IODP 349, UP NIGS and the Nannoworks Laboratory. J.D.S.G. gratefully acknowledges student assistants from the Institute for their invaluable help with sample processing. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Appendix A. Taxonomic Remarks Calcidiscus leptoporus (Murray and Blackman, 1989) Loeblich and Tappan, 1978 Circular to subcircular proximal shield, birefringent. Distal shield non-birefringent with curved sutures. Shields easily separated. Closed central area. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Calcidiscus macintyrei (Bukry and Bramlette, 1969a) Loeblich and Tappan, 1978 Circular proximal shield, birefringent. Distal shield non-birefringent with curved sutures, >10 µm with central opening.

Ceratolithus cristatus Kamptner, 1950 Strongly birefringent, c-axis lies in plane of horseshoe, perpendicular to length. Variable in size and degree of ornamentation. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Ceratolithus telesmus Norris, 1965 Strongly birefringent, c-axis lies in plane of horseshoe, perpendicular to length. Long, sinuous arms. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Coccolithus pelagicus (Wallich, 1877) Schiller, 1930 Elliptical placolith; central area open or with simple bar on the proximal surface. REMARKS: Specimens of Coccolithus pelagicus with simple bar are only noted from section 10H and below (79.2 mbsf and below). The disappearance of C. pelagicus with bar may be a result of the migration of the species to higher latitudes within the Pleistocene. However, this event was not detected in the samples.

Cyclicargolithus ﬂoridanus (Roth and Hay in Hay et al., 1967) Bukry, 1971a [Coccolithus] Coccoliths circular to sub-circular with small central area, <11 µm in width. REMARKS: Occurs as reworked specimens.

Discoaster asymmetricus Gartner, 1969c Asymmetric ﬁve-rayed variety of D. brouweri. REMARKS: Occurs as reworked specimens. Geosciences 2020, 10, 388 20 of 26

Discoaster bellus Bukry and Percival, 1971 Five-rayed discoasters without any central knob or central area development. Straight rays that taper slightly and terminate in points. All rays lie on the same plane. REMARKS: Occurs as reworked specimens.

Discoaster berggrenii Bukry, 1971b Symmetric; ﬁve-rayed; rays curved; without bifurcations; well-developed central boss; central area 1-2x free ray length; adjacent rays separated at central-area. REMARKS: Occurs as reworked specimens.

Discoaster blackstockae Bukry, 1973 four-rayed form with inter-ray angles of 60◦ and 120◦. Considered a variant of D. brouweri. REMARKS: Distribution within the core is very limited and does not parallel that of D. brouweri.

Discoaster brouweri Tan, 1927b emend. Bramlette and Riedel, 1954 Ray tips non-bifurcate but with marked proximal extensions giving it a distinctive concavo-convex form; six-rayed asterolith species.

Discoaster calcaris Gartner, 1967 Six-rayed asterolith, bifurcating asymmetrically at tip. Longer branch of bifurcation curved sharply proximally but extending only slightly laterally beyond tip of ray. REMARKS: Occurs as reworked specimens.

Discoaster kugleri Martini and Bramette, 1963 Central-area wide and ﬂat; free rays short with notched ends rather than true bifurcations. REMARKS: Occurs as reworked specimens.

Discoaster pentaradiatus Tan, 1927b Five-rayed acute bifurcations; strongly concavo-convex and birefringent (each ray is a separate unit, with c-axes inclined slightly from vertical toward radial). REMARKS: The distribution of D. pentaradiatus is sporadic in Site U1431D sediments, thus the LO of the species was not used for biostratigraphy.

Discoaster quinqueramus Gartner, 1969c Five-rayed with concavo-convex rays. Central area with prominent distal sutural ridges and large proximal boss. REMARKS: Occurs as reworked specimens.

Discoaster surculus Martini and Bramlette, 1963 Six-rayed and rarely with fewer rays with a stellate knob in the central area. From the knob, small ridges extend to the margin between the rays on one side of the asterolith and along the arms on the side. The slender rays are slightly enlarged at the end, and show a trifurcation, with the central spine extending beyond the arms.

Discoaster tamalis Kamptner, 1967 Concavo-convex rays. Calcite body consists of four arms arranged to form an orthogonal cross. There is no boss in the central area. REMARKS: Occurs as reworked specimens.

Discoaster triradiatus Tan, 1927b Symmetric three-rayed variety of D. brouweri. Geosciences 2020, 10, 388 21 of 26

Discoaster variabilis Martini and Bramlette, 1963 Central-area with proximal and distal ridges but weak bosses. Bifurcations well developed. REMARKS: Occurs as reworked specimens.

Emiliania huxleyi (Lohmann, 1902) Hay and Mohler in Hay et al., 1967 No bridge, slits between distal shield elements.

Florisphaera profunda Okada and Honjo, 1973 Coccoliths formed of single angular 5-sided calcite units. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Gephyrocapsa Kamptner, 1943 Like Reticulofenestra but with bridge formed from inner tube. REMARKS: Size diﬀerences of coccoliths are used to separate the genus into diﬀerent morphotypes for biostratigraphic purposes. The size criteria assigned by [21] is adopted in this study.

Gephyrocapsa sp. 3 Rio, 1982 (synonym: G. parallela Hay and Beaudry, 1973) Large (4.0–6.4 µm) circular to sub-circular placolith, open central-area spanned by a bridge aligned 60◦–90◦ to the long axis. REMARKS: In this study, only those with a bridge aligned at 85◦–90◦ to the long axis are considered as Gephyrocapsa sp. 3.

Helicosphaera carteri (Wallich, 1877) Kamptner, 1954 Medium to large size; ﬂange ends in wing; two pores in central-area. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Helicosphaera carteri var. wallichi (Lohmann, 1902) Theodoridis, 1984 [H. wallichi] Like H. carteri but central-area with inclined pores. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Helicosphaera inversa (Gartner, 1977 ex Gartner, 1980) Theodoridis, 1984 Like H. carteri but with large pores separated by prominent conjunct sub-horizontal bar usually with inverse direction. REMARKS: Only one specimen of H. inversa is seen in Site U1431D samples; thus, it is not utilized for biostratigraphy.

Helicosphaera sellii (Bukry and Bramlette, 1969a) Jafar and Martini, 1975 [Helicopontosphaera] Like H. carteri but pores large. REMARKS: Distribution is sporadic in Site U1431D samples; thus, it is not utilized for biostratigraphy.

Neosphaera coccolithomorpha Lecal-Schaluder, 1950 Circular, ring-shaped coccoliths with open central-area. Single shield and tube, no distal shield. Superﬁcially similar in light microscope to Umbilicosphaera rotula but shield paler in phase contrast and distinctive pseudoextinction cross seen in cross-polars at high focus. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Pseudoemiliania lacunosa (Kamptner 1963) Gartner, 1969 [Ellipsoplacolithus] Circular to broadly elliptical with variable number of slits in distal shield. Abundant in late Pliocene but well-developed, large circular forms occur only in the Pleistocene. REMARKS: [84] observed that circular forms became much abundant during the late Pleistocene. However, circular forms are more abundant in the early Pleistocene in samples from Site U1431D. Geosciences 2020, 10, 388 22 of 26

Reticulofenestra minuta Roth, 1970 <3 µm which probably includes several diﬀerent species. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Reticulofenestra haqii Backman, 1978 3–5 µm which intergrades with R. pseudoumbilicus. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Reticulofenestra pseudoumbilicus (Gartner, 1967b) Gartner, 1969c [Coccolithus] >5 µm but for biostratigraphy it is better to separate the >7 µm specimens. REMARKS: Occurs as reworked specimens.

Rhabdosphaera clavigera Murray and Blackman, 1898 Robust, club-shaped, 7–10 µm long. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Scyphosphaera Lohmann, 1902 Vase-like coccoliths, generally large. REMARKS: Species is of little biostratigraphic value.

Sphenolithus abies Deﬂandre in Deﬂandre and Fert, 1954 Similar to S. moriformis but more elevated and with cuspate outline. REMARKS: Occurs as reworked specimens.

Sphenolithus moriformis (Brönnimann and Stradner, 1960) Bramlette and Wilcoxon, 1967a (Nannoturbella) Generalized form, no spine, upper and lower parts of similar size. REMARKS: Occurs as reworked specimens.

Syracosphaera histrica Kamptner, 1941 Oblong or irregularly elliptical with bright rim and central-area. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Umbellosphaera tenuis (Kamptner, 1937) Paasche in Markali and Paasche, 1955 [Coccolithus] Trumpet-shaped coccoliths, with ridges on the distal surface REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

Umbilicosphaera sibogae var. sibogae (Weber-van Bosse, 1901) Gaarder, 1970 [Coccosphaera] Central opening wide, proximal shield monocyclic, wider than the distal shield. REMARKS: Species is of little biostratigraphic value for the Pleistocene sediments of Site U1431D.

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