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doi:10.2204/iodp.proc.310.201.2009 ResultsIn the following, a detailed inventory of the ichnotaxa is given. Traces which require further investigation are left in open nomenclature (e.g., “Dendrinid-form”). Spectrum of macrobioerosionThree ichnotaxa of macrobioeroders are present in corals, encrusting coralline algae, and in microbialites in the entire post-LGM reef. The sponge borings of the ichnogenus Entobia Bronn, 1837 (Fig. F3), are common to abundant in all samples. The producer of Entobia is the boring sponge Cliona and other Hadromerida. Additionally, domiciles of polychaetes have been identified. Caulostrepsis isp. is produced by the spionid worm Polydora, and the ichnospecies Maeandropolydora isp. is produced by the same or other polychaete worms. Spectrum of microbioerosionIn typical Tahitian reef substrates of Expedition 310 (corals, encrusting coralline algae, and microbial crusts), a total of nine different traces of microborers were identified. Two main groups can be distinguished: traces of phototrophic euendoliths (cyanobacteria and chlorophytes) and traces of heterotrophic euendoliths (bacteria and fungi). The microbioerosion patterns produced by phototrophic organisms are dominated by cyanobacteria. These include Fascichnus dactylus (trace maker, e.g., Hyella caespitosa), Eurygonum nodosum (trace maker Mastigocoleus testarum), and Scolecia filosa (trace maker Plectonema terebrans). Additionally, ichnotaxa of chlorophytes were identified, including Rhopalia catenata (trace maker Phaeophila sp.) and Ichnoreticulina elegans (trace maker Ostreobium quekettii). The microborings produced by fungi are Saccomorpha clava and Saccomorpha cf. clava (both trace maker Dodgella priscus) and Orthogonum fusiferum (trace maker Ostracoblabe implexa). Ichnotaxa produced by unknown heterotrophic organisms identified during this study are Scolecia serrata, Scolecia cf. serrata, and Orthogonum lineare. Previously unknown traces produced by unknown organisms are the Dendrinid-form and the “worm-form.” Three samples were from aphotic habitats or more accurately referred to as “cryptophotic” habitats (cryptic and/or shaded habitats characterized by low light conditions within the otherwise euphotic reef; e.g., dead underside of corals, shaded crevices, etc.) (Heindel et al., in press), as is reflected by the identified heterotrophic microendoliths: Sample 310-M0025A-9R-1, 22–29 cm, is from a downward surface of a coral, and Samples 310-M0025B-10R-1, 62–69 cm, and 310-M0007B-11R-1, 54–60 cm, are from small cavities in coral framework encrusted by microbialite. Traces of phototrophic euendolithsTraces produced by cyanobacteria
Ichnotaxon Fascichnus dactylus (Radtke, 1991)
Description: The trace was found in radiating bundles or larger carpets of up to 100 µm long galleries 2–4 µm in diameter. The relief of the galleries is smooth to slightly rough. Their distal ends are slightly thickened. Bifurcations were only rarely observed.
Ichnotaxon Eurygonum nodosum Schmidt, 1992
Description: The gallery diameter of this ichnospecies varies from 6 to 10 µm and the traces are characterized by lateral swellings developed along the individual galleries in irregular intervals (heterocysts). These swellings are globular (7–15 µm in diameter). The repetitive bifurcations alternate from unilateral to bilateral mode with angles between 45° and 90°.
Ichnotaxon Scolecia filosa Radtke, 1991
Description: S. filosa is characterized by “spaghetti-like” curved, thin (almost constantly 2 long individual galleries forming large networks that are commonly found collapsed to the cast surface. They only rarely bifurcate. Traces produced by chlorophytes
Ichnotaxon Rhopalia catenata Radtke, 1991
Description: The type of R. catenata in the present substrates is characterized by 5–7 µm thick galleries. Different pronounced swellings (5–20 µm in diameter) occur in irregular intervals along the galleries and are connected to the substrate surface by short rhizoidal appendixes (2 µm). The swellings appear from oval (egg-shaped) to nodular/spheroidal, whereas the latter morphology is mostly more pronounced than the oval form. The bifurcations of R. catenata are dichotomous (angles between 45° and 60°).
Ichnotaxon Ichnoreticulina elegans (Radtke, 1991)
Description: The morphology of I. elegans is highly complex (Radtke and Golubic, 2005). The typical zigzag pattern is the most obvious feature of I. elegans. From a parallel and close to the substrate surface extending straight or winding main first-order gallery (4–5 µm in diameter), second- and third-order galleries develop mainly at right angles and form large and dense zigzag-shaped networks (2–5 µm). Thin and straight “exploratory” filaments (1–2 µm) extend from the same colony. In some cases little appendixes (≤1 µm) emerge from individual first-order galleries (e.g., in Maraa Sample 310-M0007B-26R-1, 77–92 cm, and Tiarei Sample 310-M0023A-2R-1, 40–47 cm). Some thick galleries (as thick as 5 µm) show archlike branches which connect I. elegans with the substrate surface (cf., Radtke and Golubic, 2005). The individual arches span a distance of 10–20 µm). Traces of heterotrophic euendolithsTraces produced by fungi
Ichnotaxon Saccomorpha clava Radtke, 1991
Description: Sphere-, pear- and club-shaped sacks (up to 30 µm long) connected to the surface by a narrow neck, usually lacking a collar, were identified as S. clava. The individual sacks are interlinked by one or several thin filaments originating from the main sack or at the base of the necks. Four morphotypes exist in the analyzed reef substrates: (1) scattered straight or (2) curved sacks (both 10–15 µm in diameter), (3) clusters of mainly sphere- and club-shaped sacks (10–15 µm in diameter), and (4) large individual branched sacks (20–30 µm in diameter).
Ichnotaxon Saccomorpha cf. clava
Description: Generally, the morphology is comparable to Morphotypes 2 and 3 of S. clava (see above), but S. cf. clava is longer and slightly thicker (30–60 µm long and 10–20 µm in diameter). Figure F9E shows one specimen with several noticeably long and thin filaments originating mainly from the base of the neck.
Ichnotaxon Orthogonum fusiferum Radtke, 1991
Description: Thin straight to slightly winding galleries (~2 µm in diameter) with typical swellings (3–5 µm in diameter) along the galleries or at the mostly perpendicular bifurcations. Traces produced by unknown heterotrophs
Ichnotaxon Scolecia serrata Radtke, 1991
Description: This trace is the thinnest of all the observed ichnotaxa (~1 µm in diameter) and shows a characteristic serrate microsculpture. S. serrata forms dense often interconnected networks in very narrow windings parallel to the substrate surface.
Ichnotaxon Scolecia cf. serrata
Description: This unusual appearance of S. serrata shows the typical serrate microsculpture with rare bifurcations, but the galleries of S. cf. serrata are slightly thicker (~2 µm in diameter) and they often form spherical aggregates resembling “bags of wool.” In the latter, narrow windings of galleries seem to twine around spherelike traces such as Saccomorpha sphaerula, the emerging hyphae of which are clearly visible. Nevertheless, the association of spherical buildings of larger diameter filaments with radiating very thin filaments is unique and quite distinct from the usual prostrate and space-filling habit of S. serrata. Therefore, this trace might refer to a new ichnotaxon. However, this type of trace (association of two traces?) was only found in one current sample. There is no “fossil affirmation” (older than the last deglacial). Future studies are required to introduce a new ichnotaxon.
Ichnotaxon Orthogonum lineare Glaub, 1994
Description: O. lineare has a perpendicular bifurcation pattern with 8–10 µm thick galleries without swellings. Casually, short spiny protrusions (apophysis; Fig. F13C, F13D) protrude from the main galleries. Traces of unknown affinity
Ichnotaxon Dendrinid-form
Description: A central gallery bifurcates in a dendriniform pattern often at right angles and forms a dense network parallel to the substrate surface. The microborings have developed a zigzag pattern otherwise typical for I. elegans but are much larger in diameter (10–15 µm). The surface of the individual galleries appears rough.
Ichnotaxon worm-form
Description: This previously unknown microboring shows a complex morphology with widening galleries, from which a lateral prolongation of the main gallery emerges close to their rounded ends, and the pattern is repeated in reduced dimension. The basal galleries reach diameters of up to 40 µm, whereas the more distal galleries are only few micrometers in diameter. Relative abundances and paleobathymetric significanceThe mean abundances of all traces in the coral skeletons are higher than those in microbialites and coralline algal crusts. On average, S. filosa, I. elegans, and S. clava are common, whereas F. dactylus, E. nodosum, R. catenata, S. cf. clava, O. fusiferum, S. serrata, S. cf. serrata, O. lineare, and Dendrinid-form are rare. In coralline algal crusts, S. filosa, I. elegans, S. serrata, and O. lineare are rarely encountered, whereas all other traces are absent. In microbialites, S. filosa, I. elegans, S. clava, S. cf. clava, S. serrata, O. lineare, and worm-form are found in low mean abundance, whereas F. dactylus, E. nodosum, R. catenata, O. fusiferum, S. cf. serrata, and Dendrinid-form are absent. In total, I. elegans is the most frequent ichnotaxon with a presence in 79% of the analyzed samples, followed by S. serrata (68%), S. filosa and S. clava (58% each), O. lineare (42%), E. nodosum (21%), and F. dactylus and S. cf. clava (16% each). R. catenata, O. fusiferum, and worm-form show presences in 11% each. The least frequent traces are S. cf. serrata and Dendrinid-form (5% each). Microbioerosion at the base of deglacial successionMicrobioerosion patterns in corals at the base of the deglacial succession are dominated by light-dependent cyanobacterial traces. F. dactylus, E. nodosum, and S. filosa are common at Faaa, whereas the sole heterotrophic trace O. lineare is rare. In the Maraa area, the cyanobacterial traces occur in varying abundances: F. dactylus from rare to common and E. nodosum and S. filosa from common to abundant. The chlorophytal traces R. catenata and I. elegans are abundant. At Maraa, the only heterotrophic trace observed in corals is S. serrata with common abundance. In the Tiarei area, E. nodosum and S. filosa are common, whereas the sole chlorophytal trace I. elegans is rare. The fungal S. clava is common, whereas S. cf. clava and the traces of unknown heterotrophic producers S. serrata and O. lineare are rare (Table T3). The trace associations in coralline algal crusts at the Maraa sites are dominated by common I. elegans and S. serrata, whereas S. filosa is very rare. At Tiarei sites, S. filosa, I. elegans, S. serrata, and O. lineare are all rare. At Faaa, only the bioerosion patterns that were found inside coral skeletons could be identified with confidence (Table T3). The trace associations in microbialites at the base of the deglacial succession at Maraa sites are dominated by the common ichnotaxon I. elegans, whereas S. filosa and S. serrata are rare. At Tiarei, the microbioerosion inventory is dominated by the common fungal trace S. clava, whereas S. filosa, I. elegans, S. serrata, O. lineare, and worm-form are rare (Table T3). The key ichnotaxon of shallow euphotic Zones II and III, F. dactylus, was found exclusively in the sample from Faaa and in two samples from Maraa. The horizontally oriented traces E. nodosum and R. catenata are known to be common in shallow euphotic Zone III and the deeper euphotic zone (Vogel and Marincovich, 2004) and are considered indicative of these zones. In analyzed samples, F. dactylus, E. nodosum, and R. catenata are the ichnotaxa with the shallowest photic indication and were found exclusively inside coral skeletons and at the base of the deglacial reef succession. Traces produced by microborers that cope with very low illumination rates (dysphotic conditions), S. filosa and I. elegans, were found in all three substrates alongside traces of heterotrophic bioerosion agents such as S. serrata and O. lineare (cryptophotic conditions). The fungal trace S. clava was not identified in coralline algae, whereas the other fungal trace O. fusiferum was not found at all at the base of the succession. In summary, at the base of the deglacial succession differential microbioerosion patterns were observed in the various substrates with microbioerosion in corals indicating shallow euphotic Zone II to deep euphotic conditions, whereas microbioeroders in coralline algae and microbialites imply dysphotic conditions (except from microhabitats with prevailing cryptophotic conditions) (Table T3). Along the deglacial succession, the changing bioerosion patterns are outlined in the following. In contrast to the base of the succession, the main photic indication of the trace patterns found in all three substrates (corals, coralline algae, and microbialites) from the middle to the top ranges of the post-LGM is dysphotic. Traces of microbioeroders typical for euphotic conditions are completely absent in the younger part of the succession (Table T4, T5; Fig. F16). Microbioerosion in the middle ranges of deglacial successionMicrobioerosion patterns in corals in the middle ranges of the deglacial succession are composed of traces produced by low-light specialists and by traces of microbioeroders penetrating in the dark. On average, all identified ichnotaxa are common at Maraa: S. filosa, I. elegans, S. clava, S. serrata, and O. lineare. At Tiarei sites, the most frequent traces are I. elegans (abundant), S. filosa (common), and S. clava (common), whereas S. serrata and O. lineare are rare to common and O. fusiferum is rare (Table T4). In coralline algal crusts, the trace associations at Maraa and Tiarei sites are composed of S. filosa, I. elegans, S. serrata, and O. lineare. All observed traces are rare (Table T4). The trace associations in microbial crusts at Maraa and Tiarei consist of S. filosa, I. elegans, S. clava, S. cf. clava, S. serrata, O. lineare, and worm-form. At Maraa, I. elegans, S. serrata, and O. lineare show common abundances, whereas all other ichnotaxa are rare. At Tiarei, I. elegans and S. clava are also represented commonly in microbialites, whereas other ichnotaxa are rare (Table T4). Microbioerosion at the top of deglacial successionThe top of the deglacial succession is mainly characterized by microborings with dysphotic indication (cf. the middle ranges; Table T5, Fig. F16). In the youngest part of the deglacial reef sequence, the mean diversity of traces is strongly reduced when compared to the base and slightly reduced when compared to the middle ranges. The trace associations in corals are dominated at Maraa by the abundant I. elegans and S. serrata, whereas O. lineare and S. cf. serrata are common and S. clava is rare. At Tiarei, I. elegans and S. clava are abundant, whereas S. filosa is common and O. fusiferum, S. serrata, and Dendrinid-form are rare (Table T5). Coralline algal crusts at Maraa show no signs of microbioerosion. At Tiarei sites, all identified ichnotaxa (S. filosa, I. elegans, S. serrata, and O. lineare) are rare (Table T5). In microbial crusts at the top of the succession of Maraa, I. elegans is common, whereas S. serrata, S. clava, and O. lineare are rare. Microbialites at Tiarei from the top of the succession are composed of common I. elegans and S. clava as well as rare S. filosa, S. serrata, and O. lineare (Table T5). The traces S. filosa and I. elegans produced by low-light specialists (cyanobacterium P. terebrans, chlorophyte O. quekettii) are known to thrive down to the dysphotic zone (Vogel and Marincovich, 2004). Both ichnotaxa were found along with traces of heterotrophic origin (see below) throughout the entire Tahitian deglacial reef sequence and in all three framework elements. This indicates dysphotic conditions for the deposition of the middle and upper part of the deglacial reef sequence (Table T4, T5). The producer of S. serrata is a heterotrophic organism and was observed in all substrates, apparently independent of photic conditions. O. lineare was also identified in the entire Tahitian reef succession and occurs mainly inside Entobia cavities (Fig. F13). S. clava and O. fusiferum produced by fungi are present in coral and microbialite samples independent of sampling positions. However, the abundances of all heterotrophic traces are most pronounced in cryptophotic microhabitats: dead underside of corals, deep inside the coral porosity, and in Entobia cavities (Sample 310-M0007B-11R-1, 54–60 cm, from the upper part of the reef and Samples 310-M0025A-9R-1, 22–29 cm, and 310-M0025B-10R-1, 62–69 cm, from the base of the deglacial reef; Fig. F16). |
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