Exceptionally well-preserved radiolarian fossils within Miocene trace fossils from the Amatsu Formation, Chiba, Japan
Abstract
This study focuses on the siliceous microfossil preservation potential of trace fossils. To understand this potential, we studied the trace fossil Tasselia ordamensis from the Middle to Late Miocene Amatsu Formation (Miura Group), which is exposed in the Boso Peninsula, Chiba, Japan. We examined the state of preservation of radiolarian fossils within the infill of trace fossils and from the surrounding host siltstones. The following conclusions were drawn from this study. Exceptionally well-preserved radiolarian fossils with fine and delicate structures were recognized within the Tasselia ordamensis compared to those from siltstones. Furthermore, stratigraphically important radiolarian species were identified only in trace fossil samples. Thin-walled collodarian fossils, which are generally more difficult to preserve in consolidated deposits compared to spumellarian and nassellarian fossils, were found only within trace fossils. The collodarian fossils found in this study remained intact with fragile spine tips which have rarely been reported in previous studies. Finally, it is suggested here that because sediments, including microfossils, were transported into the burrows (now carbonate concretions) in a geologically extremely short period of time and were protected from burial compaction and destructive diagenesis, this led to the siliceous microfossils being exceptionally well preserved. This should be useful in elucidating the morphological diversity of radiolarians that were previously unnoticed.
Keywords
Microfossils such as radiolarians, diatoms, foraminifers, calcareous nannofossils and conodonts are well known as very important and powerful tools for geology, stratigraphy and structural geology as they can help correlate and determine the age of strata. In particular, extracting well-preserved microfossils from significantly deformed sedimentary rocks, such as accretionary prisms and their cover deposits, is essential for reconstructing their stratigraphy and revealing geological structures. However, the preservation conditions of these microfossils deteriorate because of burial compaction and subsequent diagenesis after deposition (e.g. Littke et al. 1991; Kameda et al. 2012; Tsuji et al. 2013; Suto et al. 2016). As microfossil preservation deteriorates, surface morphology of skeletons, fragile parts such as an apical horn, spines, feet, and spongy columns (which are essential for species identification) might be lost, making age determination and strata correlation difficult.
To extract well-preserved microfossils, many geologists and palaeontologists have utilized various concretions within sedimentary rocks. For instance, many well-preserved siliceous microfossils, such as radiolarian and diatom fossils, have been reported from carbonate and phosphate concretions (e.g. Bramlette 1946; Foreman 1959; Yao 1972, 1979; Pessagno & Blome 1980; Blome 1984; Harwood 1988; Aita & Bragin 1999; Hori et al. 2003, 2015; Bragin 2011; Shimada et al. 2022). Blome & Albert (1985) also suggested that the delicate and fragile portions of the siliceous microfossil skeletons are easily broken by compaction because they are brittle. However, in some carbonate concretions, because carbonate minerals fill the original pore space between siliceous microfossil skeletons and deposit particles prior to the beginning of compaction of the host deposits, the skeletons tend to be better preserved than in the host rocks. In addition, detailed studies have revealed that the growth rates of carbonate concretions in response to decaying organic matter are at least three to four orders of magnitude faster than those previously estimated (Yoshida et al. 2015, 2018). The early formation of carbonate concretions is considered to be a significant phenomenon that increases the microfossil preservation potential, preventing the effects of burial compaction and subsequent diagenesis.
Therefore, concretions are an ideal repository of past biomaterials, as the microfossil preservation conditions are kept intact or better than those outside of them. Recently, Kikukawa et al. (2024) focused on the Oligocene trace fossils which form concretions and studied the potential for extracting numerous well-preserved microfossils. Trace fossils are records of biological activities such as feeding, excretion, dwelling, grazing, resting, and so on, preserved in sedimentary rocks. Among them, the activities of some trace makers, such as deposit-feeding and excretion, cause the active transportation of sediments, including biogenic debris such as microfossil skeletons, from the seafloor surface to sub-seafloor environments. In addition, when dwelling burrows open on the seafloor surface, these sediments are passively transported to the sub-seafloor. These two types of biological sediment infill are called ‘active fill’ and ‘passive fill’ (Bromley 1990).
Many occurrences of siliceous and calcareous microfossils from various kinds of trace fossils observed in shallow to deep marine environments as a result of these sediment transportations have been reported (e.g. Miller & Vokes 1998; Löwenmark & Werner 2001; Leuschner et al. 2002; Löwenmark & Grootes 2004; Yanagisawa et al. 2003; Hirasawa et al. 2010; Olivero & López Cabrera 2010; Rodríguez-Tovar et al. 2010; Kotake et al. 2013; Alegret et al. 2015; Izumi 2015). Furthermore, it is well known that the sediment transportations from a seafloor surface to the interior of the trace fossil can help prevent loss of records of sediments from sediment erosion processes (Alegret et al. 2015; Wetzel 2015). Therefore geological and stratigraphical data which are usually lost can be preserved in trace fossils. In addition, some trace fossils are known to form carbonate, phosphate, and siliceous concretions or cementations (e.g. Hallam 1960; Weimer & Hoyt 1964; Bromley 1967; Radwański 1977; Carvalho et al. 2007; Olivero & López Cabrera 2010; Kotake et al. 2013). In other words, trace fossils increase the preservation of microfossil-bearing sediments which accumulate within them and protect the microfossil skeletons from the deformation and dissolution effects of burial compaction and diagenesis.
Thus, trace fossils should have enormous potential for increasing microfossil preservation. In particular, the trace fossil Tasselia ordamensis is known to contain well-preserved siliceous microfossils (Kikukawa et al. 2024) and should be the ideal target sample to investigate the potential throughout geologic time. T. ordamensis is interpreted as a deposit and detritus-feeding trace of a polychaete worm (Olivero & López Cabrera 2010). Kikukawa et al. (2024) suggested that Oligocene Tasselia ordamensis is mainly composed of the trace makers’ burrows and excreted materials. Furthermore, it was considered that carbonate concretions are formed by the chemical reaction of cations such as Ca2+, Mg2+, and/or Mn2+ in seawater with CO32- derived from organic matter contained in the mucus of trace makers and excreted materials. As a result, it was suggested that siliceous microfossils, such as radiolarian and diatom fossils, were better preserved in trace fossils than in mudstones outside of them (Kikukawa et al. 2024). However, except for this example from the Oligocene, there are no detailed studies comparing the differences in microfossil preservation inside and outside trace fossils. Therefore, further case studies are required to understand whether trace fossils have the potential to maintain the preservation conditions of microfossils through geological time.
In this study, we focused on the Middle to Late Miocene succession of the Amatsu Formation of the Miura Group, distributed on the Boso Peninsula, to understand the microfossil preservation potential of trace fossils. This formation is ideal for exploring any taphonomic relationship between trace fossils and microfossils because the occurrences of poorly to moderately preserved Miocene siliceous microfossils, such as radiolarian and diatom fossils, have been reported from mudstones and siltstones (Haga & Kotake 1996; Sawada et al. 2009). Many trace fossils, including Tasselia ordamensis, occur in this formation. The purpose of this study was to investigate the effect of the trace fossil Tasselia ordamensis on the state of microfossil preservation and to compare radiolarian fossils preserved within trace fossils with the surrounding host rocks. We also discussed the stratigraphical importance of radiolarian fossils derived from the trace fossils.
Geological setting
The Miura Group and the Amatsu Formation
The Boso Peninsula is located north of the plate boundary between the Philippine Sea Plate and North American/Eurasian Plate, and the strata and their structures affected by plate subduction can be observed on the Peninsula. The geology of the Boso Peninsula is mainly divided into Eocene to Lower Miocene accretionary complexes (Mineoka and Hota groups), Middle Miocene to Middle Pleistocene trench slope basin deposits (Miura, Chikura and Toyofusa groups), and Middle Miocene to Upper Pleistocene forearc basin deposits (Miura, Kazusa and Shimosa groups) (Figs 1, 2). The Miura Group in the Boso Peninsula is subdivided into forearc basin deposits in the northern part and trench slope basin deposits in the southern part which are intervened by the Mineoka Group that represent a trench slope break (e.g. Takahashi et al. 2016; Yamamoto et al. 2017; Shibata et al. 2021). This study focused on the Miura Group, composed of forearc deposits which are subdivided into five stratigraphical units (Mitsunashi et al. 1979; Nakajima et al. 1981; Kawakami & Shishikura 2006; Takahashi et al. 2016; Fig. 2). These formations consist mainly of sandstones, and mudstones with frequent interbedded characteristic tuff layers (Nakajima et al. 1981; Kawakami & Shishikura 2006; Takahashi 2008). Some of these tuffs have been recognized as key regional markers from east to west of the Peninsula and have been used to correlate strata and understand geological structures (e.g. Tokuhashi 1976, 1979; Nakajima et al. 1981; Ishihara & Tokuhashi 2001; Kawakami & Shishikura 2006).
The Amatsu Formation, comprising the middle part of the Miura Group, is considered to be of late Middle Miocene to earliest Early Pliocene age and consists mainly of siltstones and claystones with frequent coarse to fine-grained tuff beds such as basaltic scoria, crystal, and vitric tuffs (Nakajima et al. 1981; Takahashi 2008; Sawada et al. 2009). In the Amatsu Formation, the Am1 to Am101 key tuff markers (101 markers in total) were defined and recognized from bottom to top and correlated from west to east of the Boso Peninsula (Nakajima et al. 1981; Natural History Museum and Institute, Chiba 1997, 1998, 2002; Nakajima & Watanabe 2005). In particular, the section between Am29 and Am40 is dominated by many tuff layers, so it is very distinct and named as the Kominato Tuff Member (Nakajima et al. 1981; Figs 1, 2).
Although the preservation conditions vary from poor to good depending on the formation, the occurrences of various kinds of calcareous and siliceous microfossils have been reported from the Miura Group in the Boso Peninsula (e.g. Oda 1977; Haga & Kotake 1996; Watanabe & Takahashi 1997; Motoyama & Takahashi 1997; Mita & Takahashi 1998; Kameo et al. 2002). In terms of radiolarian fossils, since the assemblages from the Boso include warm-water (low latitude) taxa and cold-water (middle to high latitude) taxa, this region was influenced by both warm and cool water currents and the biozones of both low and middle-high latitude schemes have been applied to the group (Sawada et al. 2009; Motoyama et al. 2017). Furthermore, the K-Ar and fission track ages of some key tuff markers have been investigated and reported (e.g. Kasuya 1987; Takahashi et al. 1999; Tokuhashi et al. 2000). As a result of these studies, the integrated biostratigraphy has been established for the entire Miura Group (e.g. Motoyama & Takahashi 1997; Takahashi 2008; Sawada et al. 2009). Therefore, the group represents an important on-land section where a continuous stratigraphical sequence exists from the earliest Middle Miocene to the early Late Pliocene in the western Pacific in mid-latitudes.
Stratigraphical position and lithofacies of the Tasselia-bearing section
This study investigated a section of the Amatsu Formation lower than the Am29 key tuff marker, which outcrops in the coastal region of the Amatsu-Kominato area (Figs 1, 3). According to Motoyama & Takahashi (1997) and Sawada et al. (2009), the section between the Am29 and Am31 key tuff markers corresponds to the boundary between low-latitude radiolarian zones RN6 and RN5. Thus, the stratigraphical position in this study was considered to be the upper part of RN5 (Fig. 2C). The Am29 key tuff marker in this area was recognized as a succession of tuff layers and siltstone beds with approximately 1.5 m thick, consisting of tuff layers from bottom to top: four layers of 1–5 cm thick, coarse to very coarse-grained scoria and pumice tuffs; 10 cm thick, fine to very fine-grained crystal and vitric tuff layer; 10 cm thick, fine to very fine-grained crystal tuff layer; 20 cm thick, very fine-grained crystal and vitric tuff layer; and 15 cm thick, medium-grained crystal tuff layer (Fig. 3).
The studied section is mainly composed of alternating beds of tuffaceous siltstone and volcaniclastic sandstone, dominated by tuffaceous siltstone (Fig. 3). The tuffaceous siltstone beds have a massive texture with no primary sedimentary structures, implying that they were intensely bioturbated. Scattered scoria particles are occasionally present in tuffaceous sandy siltstone beds. The thickness of the tuffaceous siltstone beds ranges from 3 to 50 cm. Trace fossils, such as Chondrites isp., Nereites missouriensis, Palaeophycus isp., Phycosiphon incertum, Planolites isp., Tasselia ordamensis, Thalassinoides suevicus, and Zoophycos isp., are abundant in the tuffaceous siltstone. Some of them are preserved in concretions; and in particular, in concretions bearing Tasselia ordamensis could be easily identified in this section because of their cylindrical shape (Fig. 4E-F).
The volcaniclastic sandstone beds mainly consist of very coarse to fine-grained scoria particles and generally show normal grading. Their thicknesses ranges from 1 to 20 cm, and each bed has a sheet-like geometry. The bottom surface of the beds is sharp. The boundary between the volcaniclastic sandstone and the overlying tuffaceous siltstone beds is obscure because of the gradual change in grain size by the fining upward of the sandstones or reworking by bioturbation. Current ripple cross-lamination and parallel lamination are occasionally observed in the volcaniclastic sandstone beds (Fig. 3). The trace fossils Chondrites isp., Scolicia prisca, Thalassinoides suevicus, and Zoophycos isp., are observed on top of the volcaniclastic sandstone beds
The sedimentological characteristics suggest that the study section was deposited in the basin plain or slope in the marginal part of the forearc basin (Arnott 2010). The fine-grained, bioturbation structures of the tuffaceous siltstone beds imply that the deposition resulted from hemipelagic advection and suspension cascading (Stow & Smillie 2020). Occurrence of Zoophycos isp. indicates that the water depth of the depositional area was over 1,000 m (Olivero 2003; Löwenmark 2012). The sedimentary structures observed in the volcaniclastic sandstone beds, such as current ripple cross-lamination, parallel lamination, and normal grading, strongly suggest that they were affected by unidirectional flow during their deposition (Stow & Smillie 2020). In addition, the combination of sedimentary structures can be interpreted as the Bouma Ta and Tb divisions (Bouma 1962), which represent the features of turbidites. Thick-bedded tuffaceous siltstones indicate a physically stable condition in the depositional area which is temporally sufficient for benthic animals to disturb the deposits. In contrast, the intercalation of sheet-like volcaniclastic sandstone beds originating from sediment-gravity-flows suggests that physical disturbances occasionally affected the depositional area. However, it is considered that these sediment-gravity-flows did not form any specific landforms, such as frontal splays, because no significant patterns, such as upward thickening or thinning trends, cannot be recognized. Based on these considerations, the study section is expected to be deposited on a slope in the marginal part of the forearc basin or basin plain.
Material and methods
The trace fossil Tasselia ordamensis and surrounding host tuffaceous siltstone samples were soaked in acid to extract microfossils and compare their states of preservation. The examined samples from the infill of the trace fossils and siltstone samples were also subjected to XRD analysis to determine their mineral composition. The characteristics of these samples and detailed treatment methods are described below.
Tasselia ordamensis and siltstone samples
The Tasselia ordamensis was sampled approximately 3.5 m below the Am29 key tuff marker (sample no. AMTH-Tasselia; Figs 3, 4F, 5), and three siltstone samples were collected from the lower, same, and upper parts of the trace fossil-bearing horizon (sample no. AMTH-L, -M, and -U; Fig. 3). First, the Tasselia ordamensis sample was cut vertically through its centre to observe its internal structure, which was mainly composed of three distinct parts from the inside to the outside: a lined tube, an inner burrow fill, and an outer burrow fill (Fig. 5; Olivero & López Cabrera 2010). In the sample shown in Figure 5, the central axis (consisting of a lined tube and an inner burrow fill) was slightly inclined behind the surface of the cross-section. Then, the sample was subdivided into slabs several centimetres in width and 1 cm thick, and treated with acid as described below to visualize and confirm the preservation condition of microfossils on the surface of the slabs. The counterpart of the cut Tasselia ordamensis (AMTH-Tasselia) shown in Figure 5 and the three siltstone samples (AMTH-L, -M, and -U) were also soaked in acid, as described below, to extract radiolarian fossils for taxonomic examination and comparative investigation of the state of preservation.
Acid treatment and observation method
The slabs of Tasselia ordamensis were soaked in a mixed solution of 10% hydrochloric acid and 5% hydrofluoric acid for 3 h to dissolve the surface of the slabs, in order to investigate the state of preservation of the siliceous microfossils within them. Digital images of the surface of the slab containing the internal structures (inside the white dashed square in Fig. 5) of Tasselia ordamensis were captured using a scanning electron microscope (JEOL JSM-6500 at National Museum of Nature and Science, Tsukuba, Japan).
The counterpart of the cut Tasselia ordamensis shown in Figure 5 was soaked in a mixed solution of 10% hydrochloric acid and 5% hydrofluoric acid for 11 h to extract radiolarian fossils for taxonomic examination and comparative investigation of the state of preservation. Deposit residues were separated from the Tasselia ordamensis sample during acid treatment were collected at the bottom of the acid baths. These residues were washed gently over a 45 µm mesh sieve, and radiolarian fossils were randomly picked. One hundred specimens were mounted and observed under a scanning electron microscope (JEOL JSM-6460 at Chiba University, Japan) to identify the species. The fine structures of the skeleton surfaces were investigated using scanning electron microscopy. In addition, as described in more detail below, because some radiolarian fossils belonging to Collodaria, which have more fragile skeletons and are more difficult to preserve in fossil records than Spumellaria and Nassellaria were identified from this sample, we selected only collodarian fossils and observed under a field emission-scanning electron microscope (Hitachi S-4800 at Shimane University).
Three bulk siltstone samples, stratigraphically adjacent to the Tasselia ordamensis sample (AMTH-L, -M, and -U; Fig. 3), were crushed to a size range of several centimetres and soaked in a 5% hydrofluoric acid solution for 7–12 h. After that, the residues that had fallen off the rock pieces were washed gently over a 45 µm mesh sieve, and radiolarian fossils were randomly picked. One hundred specimens were mounted and observed under a scanning electron microscope (JEOL JSM-6460 at Chiba University, Japan) to identify the species. The fine structures of the skeleton surfaces were investigated using scanning electron microscopy.
Spumellaria, Nassellaria and Collodaria specimens were housed in a collection at the Natural History Museum and Institute, Chiba, Japan.
XRD analysis
Samples for XRD analysis were obtained from three bulk siltstone samples (sample no. AMTH-L, -M, and -U), and three specific locations of the Tasselia ordamensis sample (lined tube, inner burrow fill, and outer burrow fill of sample no. AMTH-Tasselia; Fig. 6a) to determine and compare the mineral compositions of the deposits within T. ordamensis and siltstones. The mineralogy of the samples was identified by X-ray powder diffraction (XRD) using a RIGAKU Ultima IV X-ray diffractometer (Chiba University, Japan). This diffractometer had a Cu Kα radiation source and was operated at 40 kV and 20 mA. The scans were recorded between 3° and 60°.
Results
Radiolarian fossil assemblages and correlation of their preservation with each sample
The entire surface of the Tasselia ordamensis slab and selected locations after acid treatment are shown in Figure 7. Observation of the slab revealed that a large quantity of well-preserved diatoms and radiolarians belonging to Spumellaria, Nassellaria and Collodaria are preserved within Tasselia ordamensis. In particular, the radiolarian fossils remained intact with delicate fine spines and by-spines (Fig. 7B, D). Furthermore, a single cortical shell of collodarian fossils which are more fragile than that of spumellarians and nassellarians are preserved in the slab of Tasselia ordamensis (Fig. 7E). In addition, the diatom frustules were complete, without any separation or breakage (Fig. 7C). These well-preserved microfossils and other deposited particles such as lithic fragments derived from tuff beds or tuffaceous siltstones are cemented by much finer carbonate minerals.
Spumellarian and nassellarian fossils, including 35 species belonging to 28 genera, were identified in the residue of the Tasselia ordamensis sample (sample no. AMTH-Tasselia; Fig. 8, Table 1). These radiolarians include Anthocyrtidium ehrenbergi Stöhr, Cycladophora cosma Lombari & Lazarus, Cyrtocapsella japonica (Nakaseko), Diartus petterssoni (Riedel & Sanfilippo), Dictyopodium charybdeum allium (Lazarus), Eucyrtidium cienkowskii Haeckel, Hexacontium sp., Larcopyle polyacantha (Campbell & Clark), Lithopera bacca Ehrenberg, L. thornburgi Sanfilippo & Riedel, Lophospyris pentagona pentagona (Ehrenberg), Stichocorys wolffii Haeckel, Tholospyris ? infericosta Goll (Fig. 8, Table 1). Delicate structures such as an apical horn, spines, basal feet, spongy columns, and other parts of the skeleton of each specimen were well preserved, similar to the results of the slab surface (Fig. 8). A rare, complete specimen of as T. ? infericosta (Fig. 8, Table 1) a late form of the species (Goll 1985) of the Middle to Late Miocene age, is possibly and seldom occurs in on-land sections because of its complicated and fragile structure. The occurrences of the species has been reported only in various piston cores and several DSDP sites in the deep sea (Goll 1969, 1972). Furthermore, several collodarian fossils belonging to the family Collosphaeridae were identified from this sample (sample no. AMTH-Tasselia) are shown in Figure 9. The cortical shell and spines of each specimen remained intact well-preserved.
In contrast, fewer species and genera from the three siltstone samples stratigraphically adjacent to Tasselia ordamensis sample were identified as follows: 15 radiolarian species belonging to 13 genera (sample no. AMTH-L), 12 species belonging to eight genera (sample no. AMTH-M) and eleven species belonging to ten genera (sample no. AMTH-U) (Table 1, Fig. 8). Although these samples include such radiolarians as Cyrtocapsella japonica (Nakaseko), C. sp, Diartus sp., Didymocyrtis cf. laticonus (Riedel), Eucyrtidium calvertense Martin, E. cienkowskii Haeckel, Hexacontium sp., Larcopyle sp., Lophospyris sp. and Stichocorys wolffii Haeckel, many delicate structures such as an apical horn, spines, spongy columns and other parts of the skeleton of each specimen are not well enough preserved (Fig. 8). No collodarian fossils were found in any of the three siltstone samples.
Radiolarian species | AMTH-Tasselia | Siltstone | ||
|---|---|---|---|---|
AMTH-L | AMTH-M | AMTH-U | ||
Acanthodesmia sp. | ○ | |||
Albatrossidium sp. | ○ | |||
Anthocyrtidium ehrenbergi (Stöhr) | ○ | |||
Anthocyrtidium ehrenbergi pliocenica (Sequenza) | ○ | |||
Anthocyrtidium cf. ehrenbergi Stöhr | ○ | |||
Anthocyrtidium sp. | ○ | |||
Botryostrobus auritus/australis (Ehrenberg) group | ○ | |||
Calocycletta sp. | ○ | |||
Carpocanium sp. | ○ | |||
Ceratocyrtis sp. | ○ | |||
Ceratocyrtis ? sp. | ○ | |||
Cornutella profunda Ehrenberg | ○ | |||
Cycladophora cabrilloensis (Campbell & Clark) | ○ | |||
Cycladophora cosma Lombari & Lazarus | ○ | |||
Cycladophora sp. | ○ | |||
Cyrtocapsella japonica (Nakaseko) | ○ | ○ | ||
Cyrtocapsella cf. japonica (Nakaseko) | ○ | |||
Cyrtocapsella sp. | ○ | ○ | ||
Dictyopodium charybdeum allium (Lazarus) | ○ | |||
Eucecryphalus sp. | ○ | |||
Eucyrtidium calvertense Martin | ○ | |||
Eucyrtidium cienkowskii Haeckel | ○ | ○ | ||
Eucyrtidium yatsuoense Nakaseko | ○ | |||
Eucyrtidium sp. | ○ | ○ | ||
Lithopera thornburgi Sanfilippo & Riedel | ○ | |||
Lithopera bacca Ehrenberg | ○ | |||
Lophophaena sp. | ○ | |||
Lophospyris laventaensis (Campbell & Clark) | ○ | |||
Lophospyris pentagona pentagona (Ehrenberg) | ○ | |||
Lophospyris sp. | ○ | |||
Pseudodictyophimus lectairi Caulet | ○ | |||
Pterocorys clausus (Popofsky) | ○ | |||
Siphocampe arachnea (Ehrenberg) group | ○ | |||
Siphocampe sp. | ○ | |||
Stichocorys peregrina (Riedel) | ○ | |||
Stichocorys wolffii Haeckel | ○ | ○ | ||
Stichocorys cf. wolffii Haeckel | ○ | |||
Stichocorys sp. | ○ | ○ | ○ | |
Theocorythium sp. A | ○ | |||
Theocorythium sp. B | ○ | |||
Tholospyris ? infericosta Goll (late form) | ○ | |||
Amphisphaera sp. | ○ | |||
Cladococcus sp. | ○ | |||
Diartus petterssoni (Riedel & Sanfilippo) | ○ | |||
Diartus sp. | ○ | ○ | ||
Didymocyrtis cf. laticonus (Riedel) | ○ | |||
Didymocyrtis sp. | ○ | ○ | ○ | ○ |
Didymocyrtis ? sp. | ○ | ○ | ||
Druppatractus sp. | ○ | |||
Dictyocoryne profunda Ehrenberg | ○ | |||
Dictyocoryne cf. marylanddicum (Martin) | ○ | |||
Dictyocoryne sp. | ○ | |||
Flustrella sp. | ○ | |||
Hexacontium sp. | ○ | ○ | ||
Larcopyle polyacantha (Campbell & Clark) | ○ | |||
Larcopyle sp. | ○ | ○ | ||
Larcopyle ? sp. | ○ | |||
Perichlamydium scutaeforme Campbell & Clark | ○ | |||
Perichlamydium sp. | ○ | |||
Spongodiscus sp. | ○ | ○ | ||
Further detailed observations were conducted to compare the differences in the preservation conditions of the very fine structures of spumellarian and nassellarian skeletons from the trace fossil sample with those from the siltstone samples. The results are shown in Figures 10 and 11. Specimens belonging to the same genera with several micro- to nano-scale structures, such as fine spines, by-spines, sponge columns, costae and irregular surfaces, were selected for comparison within each group. In terms of spumellarians, fine structures such as the sponge columns of the genus Diartus, the by-spines of the genus Hexacontium and the thorned cortical shell with fine spines of the genus Larcopyle were well preserved within the trace fossil sample, whereas these structures of the specimens in siltstone samples were broken and/or dissolved (Fig. 10). Concerning nassellarians, fine structures such as the spines of the genus Lophospyris, the irregular surface of cephalis of the genus Cyrtocapsella, and costae on the skeleton of the genus Anthocyrtidium were well preserved within the trace fossil sample, whereas their structures in siltstone samples were broken and/or dissolved, similar to the spumellarian trends (Fig. 11).
Mineralogical composition of infill of Tasselia ordamensis and siltstones
The XRD profiles of all the samples (lined tube, inner burrow fill, and outer burrow fill) from the Tasselia ordamensis trace fossil (sample no. AMTH-Tasselia) were characterized by several minerals, such as dolomite (CaMg(CO3)2), calcite (CaCO3), quartz (SiO2), and anorthite (CaAl2Si2O8) (Fig. 6(b)). In contrast, the profiles of all the siltstone samples (sample no. AMTH-L, -M, and -U) were characterized by quartz (SiO2) and anorthite (CaAl2Si2O8) (Fig. 6(b)). Carbonate minerals, such as dolomite and calcite were notably absent in the siltstone samples.
Discussion
The results of the detailed examination and comparison of the state of preservation of radiolarian skeletons from trace fossil Tasselia ordamensis and surrounding host siltstones revealed that exceptionally well-preserved spumellarian and nassellarian fossils occurred within the infill of the trace fossil, and the fine delicate structures of the skeletons, which were rather damaged in the siltstone samples, remained intact. Furthermore, collodarian fossils were preserved only within the trace fossil sample.
The order Collodaria is a relatively new group that appeared during the Cenozoic (Nakamura et al. 2020), and its abundance in the modern ocean is generally higher than that of the other polycystine orders, such as Spumellaria and Nassellaria (Suzuki & Not 2015; Biard et al. 2016; Sogawa et al. 2022). Spumellaria and Nassellaria have multi-layered robust skeletons. On the other hand, many species of Collodaria do not have skeletons. The collodarian family, Collosphaeridae, has skeletons, but its shells are single-layered and very thin (Suzuki & Not 2015; Nakamura et al. 2018), which would easily be broken in deposits. Because of these fragile skeletal structures, the microfossils of Collodaria are rarely reported compared to that of Spumellaria and Nassellaria, especially in on-land section. Thus, Collodaria are difficult to be preserved as fossils, and their species diversity and abundance during the Cenozoic era have not been sufficiently clarified.
Most fossil records of Miocene Collodaria have been reported from core samples obtained by deep-sea drilling. In particular, the skeletons of some species of genera such as Acrosphaera, Collosphaera, Otosphaera, Siphonosphaera, Solenosphaera and Trisolenia belonging to the family Collosphaeridae have been reported from the eastern to western equatorial Pacific, the eastern to western north Pacific and the south Pacific Ocean (e.g. Riedel & Sanfilippo 1971; Bjørklund & Goll 1979; Reynolds 1980; Caulet et al. 1985; Kamikuri et al. 2004, 2009; Kamikuri 2010, 2017, 2019a, 2019b; Sandoval et al. 2017; Sandoval 2018). Furthermore, these collodarian fossils, belonging to the family Collosphaeridae, are also found in core samples from the Atlantic, the Indian, and the Antarctic oceans (e.g. Petrushevskaya & Kozlova 1972; Sanfillipo & Riedel 1974; Lazarus 1990, 1992; Kamikuri 2022). Thus, most of the well-preserved collodarian fossils are generally found from deep-sea core samples and not so abundant in on-land samples. However, the samples examined in this study contain well-preserved collodarian fossils with a characteristic combination of a cortical shell and spines (Fig. 9) which have never been reported in previous records. Notably, these fossils remained intact with fragile spine tips which are usually damaged by burial compaction. Their preservation conditions seem to be comparable to that of deep-sea core samples. Therefore, Tasselia ordamensis should be suitable samples to elucidate the previously unnoticed morphological diversity of radiolarians as well as the core samples, since the radiolarians of excellent preservation were successfully found within the trace fossil.
The internal structures (lined tube, inner burrow fill, and outer burrow fill) of Tasselia ordamensis containing exceptionally well-preserved radiolarian fossils are filled with carbonate minerals such as dolomite and calcite and form carbonate concretions, whereas these carbonate minerals are not contained in the siltstone samples which occur poorly preserved radiolarians. Generally, the origins of carbonate concretions formed on the deep seafloor can be explained in two possible ways: firstly, the chemical reaction of cations such as Ca2+ and/or Mg2+ in seawater with CO32- derived from the activities of sulfur-reducing bacteria and anaerobic methane-oxidizing archaea in methane seepage areas (e.g. Takeuchi et al. 2001; Orphan et al. 2004; Ueda et al. 2005); and second, the decomposition of organic matter derived from dead organisms (e.g. Yoshida et al. 2015; Yoshida et al. 2018). Kikukawa et al. (2024) suggested that the trace fossils such as T. ordamensis which are mainly formed by trace makers’ excretion activities contain their excreted material presumably including both bodily fluids of the animal (such as mucus) as well as organic waste matter, which could be a potential source of biogenic CO32- which would promote the precipitation of carbonate minerals. According to Yoshida et al. (2015, 2018, 2020), the growth rate of carbonate concretions that form in response to decaying organic matter is very fast, and depending on their size, the formation periods are extremely short (several months to several years) on a geological timescale. Therefore, it is suggested that, as a result of the rapid precipitation of carbonate minerals derived from biogenic CO32-, deposits containing well-preserved microfossils stored inside the trace fossils were sealed by carbonate minerals and protected from burial compaction and diagenesis. In particular, carbonate concretions associated with Tasselia ordamensis could serve as a repository for keeping microfossils intact within a certain time and space.
As mentioned earlier, methane seepage areas are known as places where carbonate concretions are observed because they are formed as a result of the chemical reaction of cations such as Ca2+ and/or Mg2+ in seawater with CO32- derived from activities of bacteria and archaea around the areas (e.g. Takeuchi et al. 2001; Orphan et al. 2004; Ueda et al. 2005). In general, the chemosynthetic assemblages are found around areas of methane seepage (e.g. Takeuchi et al. 2001; Ueda et al. 2005; Jenkins et al. 2007). In the Miura Group, chemosynthetic molluscan fossil assemblages have been reported from the Kinone Formation (a stratigraphically lower unit than the Amatsu Formation) (Kase et al. 2007) and the Am40 key tuff marker horizon (stratigraphically upper than the Am29 horizon in the Amatsu Formation) with carbonate concretions (Isaji & Kato 2017). This fact should suggest that a methane seepage might have occurred in the study area. Therefore, it might be possible that the trace fossil concretions in the study area are affected not only by CO32- derived from the organic waste matter of trace makers but also by that from methane seepage areas. As numerous carbonate sediment precipitations have been reported on the deep seafloor surface around recent methane seepage areas (e.g. Ritger et al. 1987; Peckmann et al. 1999; Takeuchi et al. 2001), the growth rates of carbonate concretions are fast enough on geological timescales to form before burial. Thus, even if the trace fossil concretions in the study area were formed by the chemical reaction of cations such as Ca2+ and/or Mg2+ in seawater with CO32- derived from the methane seepage areas and/or organic waste matter of trace makers, it is suggested that the deposits, including exceptionally well-preserved siliceous microfossils within the trace fossils, were possibly sealed by the carbonate minerals in a geologically extremely short period and protected from burial compaction and diagenesis.
Radiolarian skeletons and diatom frustules mainly made of biogenic amorphous silica (Opal-A: SiO2·nH2O) with minor elements (e.g. Coradin & Lopez 2003; Nakamura et al. 2018). It has been suggested that biogenic silica is chemically unstable because of the migration of water molecules (H2O) and hydroxide ions (OH-) in seawater (e.g. Kamatani & Takeda 2007; Chiba 2014). Moreover, because seawater is unsaturated with respect to biogenic silica, not all the fragile and fine structures of siliceous skeletons can be preserved because of their dissolution at the sediment-water interface (e.g. Bernstein et al. 1990; Takahashi 1991; Lange et al. 1997; Moore et al. 2012; Chiba 2014). Therefore, the rapid deposition and transportation of sediments, including siliceous skeletons, from the seafloor surface into the infill of trace fossils and isolation from seawater could be important processes for the preservation of fine and fragile siliceous microfossils. Furthermore, as burial compaction and diagenesis progress, opal-A, which comprises siliceous skeletons such as radiolarians and diatoms, is dissolved and re-precipitated to convert to opal-CT and then quartz, which deteriorates the state of preservation of the skeletons (e.g. Isaacs 1981; Tada & Iijima 1983; Kameda et al. 2012; Tsuji et al. 2013). It is suggested that the conversions due to these processes are mainly affected by increases in palaeothermal temperatures, and the transitions from opal-A to opal-CT start at around 50 ℃, and from opal-CT to quartz at around 70 ℃ (e.g. Tada & Iijima 1983; Mero et al. 1992; Behl 2011; Kameda et al. 2012).
The palaeothermal structure and consolidation trends of the Miura Group in the Boso Peninsula were investigated and the maximum palaeotemperatures calculated from vitrinite reflectance were 71 to 98 ℃ in the western part of the peninsula (Kamiya et al. 2017), 71 to 128 ℃ in the central part and 76 to 91 ℃ in the eastern part (Kamiya et al. 2020), respectively. Thus, the poorly preserved radiolarian fossils from the siltstone samples in this study might have been affected by burial compaction and diagenesis that converted opal-A of their skeletons to opal-CT and quartz, which resulted in the deterioration of their state of preservation. However, exceptionally well-preserved radiolarian fossils from the trace fossil concretion were tightly sealed by fine carbonate minerals. Therefore, even if the burial compaction and diagenesis processes progressed, the skeletons might seldom have been in contact with the formation water to dissolve and re-precipitate to convert opal-A to opal-CT and quartz.
According to a previous study which investigated the radiolarian biostratigraphy of the Miura Group, the first occurrence (FO) horizon of D. petterssoni in the Amatsu Formation was assigned to a stratigraphical section between the Am29 and Am31 key tuff markers (Sawada et al. 2009). In contrast, the trace fossil Tasselia ordamensis examined in this study was collected from the approximately 3.5 m lower horizon of the Am29 key tuff marker, and the sample contained D. petterssoni (Figs 3, 8, 12). Therefore, the gap between the first occurrence horizon of D. petterssoni of the previous study and the horizon of our samples in the Amatsu Formation is at least about 20 m (Sawada et al. 2009). This suggests that the first occurrence (FO) horizon of D. petterssoni in this region was lower than that defined in the previous study. D. petterssoni is a stratigraphically important species because its first occurrence datum (FOD) defines the base of the low-latitude radiolarian zonation RN6. The ages of the base of the RN6 Zone in the biostratigraphy constructed in the Pacific regions are generally assigned to approximately 12 Ma, with slight regional differences (e.g. Sanfilippo & Nigrini 1998; Kamikuri et al. 2009; Kamikuri 2017). On the other hand, the fission track (FT) age of the Am19 key tuff marker which corresponds to the lower horizon than the base of RN6 Zone of the Amatsu Formation in the previous study (Sawada et al. 2009) is assigned to 11.7±0.3 Ma (Tokuhashi et al. 2000; Fig. 12). This may indicate that the base of RN6 corresponds to a lower horizon in the formation than that in the previous study (Fig. 12).
The sampling horizon in this study (approximately 3.5 m lower than Am29) which was previously considered to be in the upper part of RN5, should be assigned to the lower part of RN6 (Fig 12). According to Sawada et al. (2009), mudstone samples were used to investigate the radiolarian biostratigraphy of the lower horizons of the Am29 key tuff marker. Sawada et al. (2009) also suggested that the preservation conditions of radiolarian fossils in the samples were moderate (no dissolution, but mechanically broken) to poor (important structures for identifying the species of many individuals are missing due to dissolution and/or mechanically breakage). Fine and delicate structures such as spongy columns attached to the both sides of the cortical shell are one of the important parts to identify the D. petterssoni. Radiolarian species belonging to the genus Diartus with such features were not found in the siltstone samples examined in the present study. Therefore, it might be suggested that the D. petterssoni could not be identified from the samples examined by Sawada et al. (2009) because the individuals were damaged and the delicate structures for species identification were not preserved. Here, it can be seen that, even if only poorly preserved microfossils were found in the strata, trace fossil concretions such as Tasselia ordamensis could preserve more stratigraphical information to be considered.
Conclusions
To understand the microfossil preservation potential of trace fossils, Tasselia ordamensis from the Amatsu Formation around the Middle/Late Miocene border was chosen as the target sample for a comparative investigation of the state of preservation between the inside and outside of the trace fossil. The results of this study revealed the following findings:
1.
Exceptionally well-preserved radiolarian fossils with fine and delicate structures occur within Tasselia ordamensis compared to those from the surrounding host deposits. Furthermore, a stratigraphically important radiolarian species which define the base of the radiolarian zone was identified only in the trace fossil sample.
2.
It should be emphasized that excellently well-preserved collodarian fossils which are generally difficult to preserve in consolidated deposits compared to spumellarians and nassellarians found in the trace fossil, remained intact with the fragile tip parts of spines. Because species belonging to the Collodaria with such long spines have rarely been reported, this group may have been overlooked.
3.
It is suggested that because deposits, including microfossils, were transported within the infill of the trace fossils which formed carbonate concretions in an extremely short geological period and were protected from burial compaction and diagenesis, the siliceous microfossils were exceptionally well preserved. This should be useful in elucidating the morphological diversity of radiolarians that were previously unnoticed.
Acknowledgments
Special thanks are due to Prof. Nobuhiro Kotake who gave us helpful suggestions on Tasselia ordamensis trace fossil bearing section in the Miura Group. We are grateful to Prof. Hidekazu Yoshida and Dr Yusuke Muramiya for their valuable suggestions on carbonate concretion formation process. Dr Megumi Saito and Dr Hideki Ishida instructed us in the SEM operations. We thank Dr Hisayoshi Kato for his information regarding chemosynbiotic molluscan assemblage from the Miura Group. Dr Nana Kamiya provided useful suggestions about palaeothermal structure and consolidation trends of the Miura Group. This paper benefited from constructive reviews by Prof. Alfred Uchman and an anonymous referee. We thank the editor Prof. Peter Doyle for important comments. We would like to thank Editage (www.editage.jp) for English language editing. This work was financially supported by JSPS KAKENHI Grant Number JP23K19075 (A. Kikukawa). All these contributions are gratefully acknowledged.
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