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doi:10.2204/iodp.proc.310.201.2009

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 310, drowned Pleistocene–Holocene barrier reef terraces seaward of the modern fringing reefs of Tahiti were drilled in order to recover the deglacial reef sequence. The drilling strategy aimed at recovering cores along transects perpendicular to the strike direction of the reefs (Camoin et al., 2007). More than 600 m of core material was recovered (Camoin et al., 2007).

Tahiti is located at 17°50′S, 149°20′W in the central Pacific Ocean (French Polynesia, Society archipelago). A total of 19 samples from 14 holes from the three areas (Tiarei, Maraa, and Faaa) were studied. For the exact positions of the sites and samples studied see Tables T1 and T2. Terrestrial input of material eroded from the volcanic island is strongest in the Tiarei area, which is located close to the discharge of the largest drainage system of Tahiti, the mouth of the Papenoo River. The samples studied here are from sites in water depths up to 117 m, whereas samples from sites located on ridges are in water depths of 56–81 m (Table T2; Fig. F1). Eleven samples were studied from the Tiarei area to the north, seven samples from Maraa to the south, and one sample from Faaa west of the island of Tahiti (Fig. F2). This study focuses on the post-Last Glacial Maximum (LGM) interval of the Expedition 310 cores. The typical repetitive pattern of the Tahitian post-LGM reef sequence consists of corals encrusted by coralline algae and subsequently by microbial crusts (Camoin et al., 2007). According to U/Th age dates, the post-LGM interval is from 16,000 to 8,000 y before present (BP) (Camoin et al., 2007).

Approach: bioerosion in marine carbonate substrates

Bioerosion describes the erosion of marine hard substrates by living organisms via a number of mechanisms. Bioeroders are grazing organisms like gastropods, chitons, echinoids, and so on; sponges, bryozoans, worms, and so on (macroborers, trace diameter >100 µm); and bacteria, algae, and fungi (microborers, trace diameter <100 µm) (e.g., Golubic et al., 1975; Warme, 1975). Mechanisms of bioerosion include biotic boring, rasping, and scraping. The traces left by those bioerosive activities are classified as ichnotaxa and are highly sensitive paleoenvironmental indicators (Bromley, 2004; Glaub and Vogel, 2004). Microbioeroders are considered reliable paleoenvironmental indicators (temperature, light availability, and trophic conditions) because they are evolutionary conservative organisms (Vogel and Glaub, 2004).

For standardized interpretation of photic conditions, typical communities of microbioerosion traces have been defined (index ichnocoenoses) (Glaub, 1994, 1999; Vogel et al., 1995, 1999; Glaub et al., 2002; Vogel and Marincovich, 2004). Borings of the different photic zones show typical preferred orientations. In the shallower euphotic zones the penetration activities tend to be vertically oriented (e.g., Fascichnus isp. produced by the cyanobacterium Hyella spp.), whereas the traces found in deeper euphotic and dysphotic zones tend to be horizontally oriented (e.g., Ichnoreticulina elegans produced by the chlorophyte Ostreobium quekettii).

Three photic zones have been defined: the euphotic zone (>1% surface illumination), dysphotic zone (0.01%–1% surface illumination), and aphotic zone (<0.01% surface illumination) (Liebau, 1984; Glaub, 1994). The euphotic zone is divided into shallow euphotic Zones I, II, and III and the deep euphotic zone (Liebau, 1984; Glaub, 1994). Index ichnocoenoses describe most of the different photic zones. No index ichnocoenosis has been defined yet for shallow euphotic Zone I, but it is well known that this zone is typically dominated by cyanobacteria with sheath pigmentation. The Fascichnus acinosus/​Fascichnus dactylus ichnocoenosis is the typical vertically orientated trace community of shallow euphotic Zone II and is produced by cyanobacteria. The change from vertical to horizontal orientation of the borings starts with the index ichnocoenosis of shallow euphotic Zone III: F. dactylus/​“Palaeoconchocelis starmachii.” In shallow euphotic Zone III, microbioerosion is still dominated by traces of cyanobacteria but the traces now are horizontally orientated. Additionally, borings of eukaryotes are frequently encountered. The index ichnocoenoses of the deep euphotic zone is “P. starmachii”/​I. elegans. Entirely, that zone shows the maximal trace diversity and is dominated by horizontally oriented bioerosion patterns of eukaryotes, mainly produced by rhodophytes and chlorophytes. The abundance of heterotrophs increases. The dysphotic zone is controlled by heterotrophs and the traces I. elegans (chlorophytes) and/or Scolecia filosa (cyanobacterium), whose producers can cope with very low illumination rates. The index ichnocoenosis of the dysphotic zone is not yet defined. Under aphotic conditions, bioerosion is limited to heterotrophic organisms. The index ichnocoenosis is composed of Saccomorpha clava/​Orthogonum lineare (Glaub, 1994, 1999; Vogel and Marincovich, 2004; Wisshak, 2006).

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