Palaeogeography, Palaeoclimatology, Palaeoecology · The burrow is produced by selective sand-grain feeding ... are able to reject unwanted mineral grains ... a nearly horizontal

Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 224–229

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Large Macaronichnus in modern shoreface sediments: Identification of the producer,the mode of formation, and paleoenvironmental implications

Koji Seike a,⁎, Shin-ichi Yanagishima a, Masakazu Nara b, Takenori Sasaki c

a Coastal and Estuarine Sediment Dynamics Group, Port and Airport Research Institute, 3-1-1 Nagase, Yokosuka, Kanagawa 239-0826, Japanb Department of Natural Sciences, Kochi University, 2-5-1 Akebono-cho, Kochi 780-8520, Japanc The University Museum, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

⁎ Corresponding author at: 3-1-1 Nagase, YokosukaTel.: +81 46 844 5045; fax: +81 46 846 9812.

E-mail address: [email protected] (K. Seike).

0031-0182/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.palaeo.2011.08.023

a b s t r a c t

Article history:Received 27 March 2011Received in revised form 24 August 2011Accepted 30 August 2011Available online 16 September 2011

Keywords:Trace fossilIchnologyBioturbationIndicatorBeachWorm

The trace fossilMacaronichnus segregatis is an intrastratal trail occurring in shallow marine deposits. Previousstudies have shown that this trace fossil can be used as a powerful indicator of ancient sea level, shorelineorientation, beach morphodynamics, and paleoceanographic conditions. Here we describe another type ofMacaronichnus, an incipient large Macaronichnus isp., which is 5–12 mm in diameter, from upper shorefacedeposits of a modern sandy beach in western Japan. The burrow is produced by selective sand-grain feedingof the opheliid polychaete Travisia japonica, although the detailed mechanism remains unclear. Based on thedistribution of T. japonica and other Travisia species, large Macaronichnus isp. seems to be more widely dis-tributed across tidal flats, the upper–lower shoreface, the continental shelf, and possibly even the deep-seafloor. The distribution of the large burrow is quite different to that of the standardM. segregatis. It is thereforenecessary to distinguish between the sizes of the burrows in order to use the ichnogenus Macaronichnus as apaleoenvironmental indicator.

, Kanagawa 239-0826, Japan.

rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Trace fossils not only provide information on the autecology of ancientanimals but also on the paleoenvironment in which the trace-producinganimals lived (e.g., Seilacher, 1967, 2007); however, improving our un-derstanding of trace fossils (i.e., their origin and paleoenvironmental use-fulness) requires analysis of their modern analogs (e.g., Bromley, 1996;Gingras et al., 2008).

The trace fossil Macaronichnus segregatis Clifton and Thompson,1978 (Fig. 1A) is an intrastratal trail occurring in post-Permian fine- tomedium-grained sandy marine deposits (e.g., Clifton and Thompson,1978; Nara, 1994; Bromley, 1996; Pemberton et al., 2001; Gingras etal., 2002; Uchman and Krenmayr, 2004; Bromley et al., 2009; Quirozet al., 2010). This trace fossil comprises a non-branching cylindricalshape, 3–5 mm in diameter, which preferentially runs parallel to thebedding plane (Fig. 1A). It is characterized bymineralogical segregationbetween the tube core and a surrounding rim. The former consists oflight-colored felsic sand grains, whereas the latter consists of dark-colored mafic sand grains. Studies of its modern counterpart haverevealed that the burrow is created during the feeding process of theopheliid polychaetes Ophelia limacina Rathke, 1843 (Clifton andThompson, 1978) and several species of the genus Euzonus Grube,

1866 (Pemberton et al., 2001; Gingras et al., 2002; Nara and Seike,2004; Seike, 2007, 2008; Dafoe et al., 2008a, b; Seike, 2009).

On the basis of studies of modern burrows, the trace fossil M. segre-gatis is known to be a powerful indicator of ancient sea level, shorelineorientation, and beach morphodynamics (e.g., Seike, 2007, 2009, andreferences therein), although shoreface specimens are also recognized(Bromley et al., 2009). In addition, Quiroz et al. (2010) reported thatthe occurrence of the trace fossil in tropical coastal deposits providespaleoceanographic information, such as upwelling conditions.

In addition to the previously describedM. segregatis (Fig. 1A), pre-vious studies have reported a larger Macaronichnus isp. (Fig. 1B) – aburrow 5–15 mm in diameter – in nearshore deposits at several local-ities and of variable ages, including Cretaceous upper shoreface–lower foreshore deposits of a low-energy beach in the USA (Curran,1985), Pliocene inner shelf deposits of Spain (Aguirre et al., 2010),and Pleistocene upper-shoreface deposits in Japan (Masuda andYokokawa, 1988; Nara, 1998). Although Savrda and Uddin (2005) de-scribed the trace fossils from the Cretaceous tidal inlet deposits of theUSA as “large Macaronichnus”, the trace fossil should be identified asanother ichnogenus Bichordites Plaziat and Mahmoudi 1988 owingto its shape and internal structure.

The features of the large burrows (e.g., cylindrical shape and min-eral segregation) are very similar to those of standard M. segregatis(here called small burrows) in all but size. Although size of trace fossilis not valid as ichnotaxobases (Bertling et al., 2006), recognizing thedifference between the distributions of the small and large burrowis essential if they are to be used in reconstructing detailed

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paleoenvironments; however, no previous studies have reported thelarge burrow's modern counterpart; thus, its producer and anypaleoenvironmental implications have remained uncertain to date.

Although the mineralogical particle segregation is the main crite-rion used to identify the ichnogenus Macaronichnus, the mechanismof segregation remains uncertain. On the basis of neoichnological ob-servations on the smaller burrows, there are two hypotheses for theirformation. Clifton and Thompson (1978) proposed that the producersare able to reject unwanted mineral grains individually, and therebyconstruct a mineralogically grain-segregated burrow structure. Con-versely, Dafoe et al. (2008b) attributed the segregation to preferentialmass feeding of the producer in felsic-rich sand microenvironments.It has proved difficult to test these hypotheses because the burrowand its producer are too small to enable quantitative analysis of thegrain segregation. Observation of the larger Macaronichnus isp.would enable an understanding of the nature of the segregation, be-cause the burrow is large enough for a quantitative analysis of miner-al composition.

The purpose of the present study is to identify the distribution ofthe large Macaronichnus isp. structure as well as its producer in amodern sandy beach, and to discuss its usefulness in paleoenviron-mental studies. Here we describe the modern analog of the largeMacaronichnus isp. and its producer, Travisia japonica Fujiwara, 1933(Polychaeta; Opheliidae) from upper shoreface deposits in a sandybeach in western Japan. We also compared the mineral compositionof the gut contents of the worm with the surrounding host sediment

2 cm

2 cm

B

A

Fig. 1. Two size variants of the trace fossil Macaronichnus. Vertical section. (A) Macar-onichnus segregatis (small burrow) from foreshore deposits of the upper PleistoceneKatori Formation, Central Japan. (B) Large Macaronichnus from upper shoreface de-posits of the middle Pleistocene Yabu Formation. Note that the two burrow types aresimilar in terms of mineral segregation.

to better understand the cause of the mineral particle segregation inthe Macaronichnus.

2. Study sites

Fieldwork was carried out on Nijigahama Beach, Hikari, Yamagu-chi Prefecture, western Japan (Fig. 2). This arcuate beach is 2.5 kmlong and faces the Seto Inland Sea. The average tidal range at springtide is 2.58 m; the high, mean, and low water levels based on theTokuyama datum level (DL: Tokyo Peil −1.845 m) are 3.09 m,1.80 m, and 0.51 m, respectively (Yanagishima et al., 2003). On thebasis of wave data measured with an ultrasonic wave gauge in thisarea for 10 min/h during 1995–1999, the mean significant waveheight and period are 0.3 m and 3.2 s, respectively. The beach coast-line has not migrated measurably in any systematic way from its po-sition surveyed in 1962, although temporary sand erosion duringstorms and typhoons, and sand accretion under fair-weather condi-tions, has occurred (Yanagishima et al., 2000). The beach consists ofthree depositional environments (Fig. 3): a nearly horizontal back-shore at an elevation of around 5 m, a seaward sloping foreshore at

400 m

N

Study Site

Hikari Station

Route 2

Shimada River

Seto Inland Sea

San’yo Main Line

Nijigahama Beach

35°

500 km

J

Fig. 2. Locality map (top) showing Nijigahama Beach, Hikari, Yamaguchi Prefecture,western Japan. The stippled area indicates sandy beach. The bottom image shows theemerged shoreface bar at low water level during a spring tide.

0

0

2

4

3

1

20 40 60 80

July 7 2009

Seaward distance (m)

Ele

vatio

n (m

)

Mean tide level

Large Macaronichnus

Foreshore ShorefaceBack-shore

Fig. 3. Beach profile at the study site, showing the distribution of large Macaronichnuson Nijigahama Beach. The burrows occur exclusively in the shoreface bar deposits.

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an elevation of 0.5–5 m, and a shoreface showing decimeter-scale barand trough topographic undulations below 0.5 m elevation. Sedi-ments of the beach are mainly siliciclastic sand. The median grainsize (d50) of backshore, foreshore, and shoreface deposits is 0.7, 2.0(data from Hosogai et al., 1997), and 0.27 mm, respectively.

3. The modern large Macaronichnus and its producer

Resin peels were prepared from vertical sections of sedimentobtained from the bar area of the shoreface, using a specially designedbox corer (24 cm wide×60 cm deep×10 cm thick) to observe physi-cal and biogenic sedimentary structures. Resin peel samples enable usto clearly observe sedimentary structures, because they emphasizethe structural differences in the sands (see Nara and Seike, 2004;Seike, 2009). In addition, this procedure yields excellent undisturbedsediment samples, not only from the intertidal region, but also fromsubtidal water-saturated sediments (see Greenwood et al., 1984;Davidson-Arnott and Van Heyningen, 2003). Producers of the bur-rows were collected using a sieve (2-mm diameter mesh). Observa-tions on the whole foreshore and uppermost shoreface werepossible during spring ebb tides. For this reason, fieldwork was car-ried out during 6–10 July 2009, when spring tides occurred.

Modern burrows comparable to the large Macaronichnus isp.(Fig. 4A–C) are clearly visible in the trough cross-laminated sedi-ments on the resin peel taken from the shoreface bar at an elevationof 0.1–0.4 m (Fig. 3), and occurred at a depth of 5–20 cm below thesediment surface (Fig. 4A). Individual burrows are circular shape incross section, and are ≤12 mm wide and ≤30 mm long on the resinpeel surface (Fig. 4B). The core of the burrow shows a much higherproportion of felsic grains than does the host sediment. Burrow man-tles are distinctly fringed with a high concentration of mafic mica-ceous sands which are darker than both the felsic burrow-fillmaterial and the surrounding matrix (Fig. 4C).

The opheliid polychaete T. japonica (Fig. 4D) was also found inthe upper shoreface bar sediments where the burrows occur(Fig. 3). Collected worms were ≤8 mm in diameter and ≤80 mm inlength. This species is the only soft-bodied, large worm (≥5 mm indiameter) in the bar area, although other burrowing hard-bodiedinvertebrates (e.g., callianassid crustaceans) and their burrows werealso seen.

As mentioned above, the opheliid polychaete O. limacina and Euzo-nus spp. are known to be infaunal deposit feeders, and produce thesmall (standard) M. segregatis. The large opheliid polychaete T. japon-ica is also an infaunal deposit feeder, similar to Euzonus. In the present

study, T. japonica was recovered from the same area and level as thelarge Macaronichnus isp., and the diameter of the worm correspondsto that of the burrow. No large worms other than T. japonica were re-covered from the bar deposits. These findings strongly indicate thatT. japonica is responsible for producing the larger Macaronichnus isp.

4. Origin of grain segregation

We attempted to identify the cause of the grain segregation in theburrows, by comparing the sands ingested by its producer (T. japoni-ca) with sands of the host sediment. We collected T. japonica individ-uals, and later dissected the soft body of the worms, therebyretrieving the ingested sand from 5 specimens. We also collected 5samples of the host sediments. Each of the samples contained1000–1500 sand grains: the numbers of felsic and mafic sand grainswere counted for each sample. Statistical comparison was made ofthe proportions of felsic grains between the sands ingested and thehost sediments, using the Mann–Whitney U-test.

The proportion of felsic grains from the host sediment (Fig. 5A)ranges from 82% to 87% (mean: 85.6%) whereas that in the ingestedsand (Fig. 5B) ranges from 96% to 98% (mean: 97.3%), indicating sig-nificant compositional differences between the host sediments andthe sand ingested by the worm (Pb0.01). The ingested sand is alsoslightly finer and better sorted than that of the host sediments(Fig. 5B).

The above result strongly indicates that the worm selectively in-gests felsic sand grains and rejects mafic grains, as previous studieshave suggested. In addition, the core of the burrow contains a muchhigher concentration of felsic grains than does the surrounding sedi-ment. This finding means that the worm ingests felsic grains by grainselection rather than by mass feeding. The larger burrow rim alsocontains a much higher concentration of mafic grains than does thesurrounding sediments. This result supports the hypothesis originallyproposed by Clifton and Thompson (1978). Because the morphologi-cal characteristics and the feeding strategies of the producers are verysimilar in all but size for the large and small burrows, the smaller(standard) M. segregatis may also be produced by the same selectivesegregation mechanism; however, as in previous studies, the precisetrace-producing mechanism – the way in which the mineralogicalgrain segregation actually occurs – remains unclear.

5. Paleoenvironmental significance

The type ichnospecies for the ichnogenusMacaronichnus,M. segre-gatis, is considered to be a strong indicator of the foreshore environ-ment of high-energy beaches; however, we have revealed that themodern larger Macaronichnus isp. occurs within upper shoreface de-posits on a sandy beach. The environmental range of the larger bur-row is therefore different from the smaller burrow, which occursdominantly at the foreshore level.

Travisia japonica (the producer of the larger burrow) is distributedacross coasts of the North Pacific and inhabits not only the uppershoreface, as shown in the present study, but also tidal flat and conti-nental shelf sea-floor environments (Uchida, 1992). For example,Nishi et al. (2001) reported T. japonica at a depth of 40–45 m belowsea level on a sandy coast off the Izu Peninsula, Central Japan. Hintonet al. (1992) described T. japonica from 40.2 m below the water sur-face, offshore from the mouth of the Columbia River, USA. These find-ings indicate that the distribution of the large Macaronichnus isp.ranges from upper shoreface or tidal flat, to lower shoreface and con-tinental shelf zones. This is consistent with the paleoenvironmentalsettings of the large Macaronichnus isp. in the geological record (Cur-ran, 1985; Masuda and Yokokawa, 1988; Nara, 1998; Aguirre et al.,2010).

The worms Travisia have a cosmopolitan distribution. For exam-ple, Elías et al. (2003) described several species of Travisia from the

2 mm

2 cm

1 cm

1 cm

BB

CC

DD

AA

Fig. 4.Modern largeMacaronichnus and its producer. (A) A resin peel sample showing the largeMacaronichnus in trough cross-laminated sediments, vertical section. This sample ishoused at the University Museum, the University of Tokyo, Japan (UMUT-RR30909). (B) and (C) Higher-magnification views of the burrows in (A). Note that the burrow core con-tains a larger proportion of felsic sand grains (light-colored) than any other areas, and the mantle of the burrow is composed of dark-colored mafic sand grains. (D) Opheliid poly-chaete Travisia japonica, producer of the large Macaronichnus.

227K. Seike et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 224–229

southwestern Atlantic Ocean. The worms are also known to inhabitthe Antarctic Sea (Maciolek and Blake, 2006). It is unclear whetherall the worms Travisia produce the Macaronichnus isp. structure;however, the cosmopolitan distribution of the worms implies thatthe larger Macaronichnus isp. structure might be seen in nearshoredeposits worldwide, because the members of the genus are of suffi-cient size to produce the large burrow.

Travisia has even been described from the deep-sea floor. For ex-ample, Nishi et al. (2001) described Travisia spp. at a depth of 210–218 m below sea level on a sandy coast off the Izu Peninsula, CentralJapan, and Maciolek and Blake (2006) reported Travisia from the

Antarctic Sea at a depth of 20–6010 m below sea level. Hence, it isalso possible that the large Macaronichnus isp. may occur in a deep-sea environment, although the occurrence of the tracemaker doesnot directly imply the presence of the trace fossil in different deposi-tional settings (muddy sand and/or muddy sea floors).

Although M. segregatis has been considered a strong indicator ofthe foreshore (intertidal) environment, descriptions of the environ-mental range of this trace fossil remain confusing (Bromley et al.,2009): the small burrows are known to occur in both foreshore andshoreface deposits. This occurs because a few species of Euzonus –

the producer of the small burrows – also inhabit subtidal (shoreface)

1 mm

1 mm

80

75

85

90

95

Fel

sic

grai

ns (

%)

B

C

A

100

Host sediments(n=5)

Ingested sand(n=5)

Fig. 5.Mineral composition of the sediments. (A) Surrounding host sediments contain-ing abundant light-colored felsic sand grains and some dark-colored mafic sand grains.(B) Sands ingested by Travisia japonica, which are mostly composed of light-coloredfelsic grains. (C) Graph showing the difference in mineral composition between sur-rounding host sediments and ingested sand. Note that the proportion of felsic grainsof the latter is significantly higher than that of the former (Pb0.01). Error bars are±SD.

228 K. Seike et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 224–229

settings (Misaka and Sato, 2003). In addition, juveniles of the wormsTravisia in shoreface environments are possibly small-burrow pro-ducers. However, the small M. segregatis in foreshore deposits andthose near the foreshore/shoreface boundary have a unique feature:they occur densely packed in association with a high population den-sity of the producer Euzonus (McConnaughey and Fox, 1949; Seike,2008). In contrast, such a high population density of opheliidworms has never been reported from shoreface settings. Hence, themode of occurrence of densely packed M. segregatis is indicative of aforeshore environment.

In contrast, Travisia has never been reported from the foreshoreenvironment of a high-energy sandy beach, where the smallerM. seg-regatis burrows are abundant. This indicates that no large Macaro-nichnus isp. occurs in such an environment. Foreshore opheliids aresmaller than those in shoreface settings, possibly because of the geo-physical properties of intertidal sediments: in intertidal areas

(cyclically submerged and exposed areas), the dynamics of suction(negative pore-water pressure relative to atmospheric air pressure),combined with tide-induced fluctuations in groundwater level, resultin reduced sediment porosity. Thus, the suction dynamics result inharder sediment compared with environments in which suctiondoes not develop, such as the subtidal zone (Sassa and Watabe,2007). Such stiff sediments may be unsuitable for burrowing bylarge polychaete worms, because the optimal sediment stiffness forburrowing depends on the body size of the worm (Sassa et al.,2009). This may explain the environmental segregation of burrowsof different sizes.

The ichnogenus Macaronichnus has been considered a powerfultool in reconstructing nearshore paleoenvironments; however, itis essential to distinguish between the burrows with regard tosize. We conclude that the small-densely packed M. segregatis is in-dicative of the foreshore environment (i.e., sea level; see Seike,2009). The larger Macaronichnus isp. represents a wider range of de-positional environments and bathymetry, including tidal flats, theupper–lower shoreface, the continental shelf, and possibly the deep-sea floor.

Acknowledgments

KS is indebted to K. Tanabe (University of Tokyo) for his supervi-sion throughout this study. We thank M. Sato (Kagoshima University)for identification of the polychaete and valuable discussions, theYamaguchi Prefectural Government for providing wave data of thestudy area, and Y. Seike for assistance in the field. This work was fi-nancially supported by JSPS Fellowships awarded to KS (20-7991and 22-10734). Thanks are also due to two anonymous reviewersand Editor F. Surlyk, who improved manuscript.

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