13:45 〜 15:15
[ACG41-P01] Reconstruction of late Middle Pleistocene reef growth history and environmental changes in the Ryukyu Islands
キーワード：Minatogawa Formation, Ryukyu group, Quaternary reefs, Okinawa Island, Corals, Depositional environment
The Middle to late Quaternary (< 500 ka) is a critical period to study rapid sea-level and climate changes and their impact on ice sheet stability. However, there is still considerable uncertainty regarding the timing and amplitude of sea-level change during the Last Interglacial (Marine Isotope Stage 5; 80–130 ka), and data on sea-level history becomes incomplete as we look further back in time. This lack of data hinders fully understanding long-term sea-level and climate changes and validating models that predict future trends. Quaternary fossil coral reefs are treasure troves of information on past sea-level and climate changes and are very helpful in filling this knowledge gap. Recent sea-level reconstructions based on fossil coralgal assemblages and associated reef organisms have shed light on sea-level history since the Last Interglacial (Yokoyama et al., 2018; Webster et al., 2022). The Ryukyu Islands, whose largest island is Okinawa, are extensively covered with Pleistocene reef limestones and offer an excellent opportunity to study Quaternary sea-level and climate history and its impact on coral reef ecosystems.
In this study, we focus on the late Middle to late Pleistocene Minatogawa Formation exposed in southern Okinawa Island at elevations of < 50 m and resting unconformably on the Lower to Middle Pleistocene Naha Formation. The age of the Minatogawa Formation is still unclear but is thought to be < 400 ka based on the calcareous nannofossil biochronology of the Naha Formation (Iryu et al., 2006). We report the preliminary results of a drilling campaign conducted in 2021, which collected five cores along a one km-long transect perpendicular to the shore. The drilled length ranges from 10 to 27 m. The main objectives of this study are (1) to understand the 3D facies architecture of the Minatogawa Formation and reconstruct relative sea-level history, (2) to determine the age of the Minatogawa Formation, and (3) to investigate the response of reef organisms to millennial-scale sea-level and environmental changes.
Results of our preliminary examination of the cores show that the thickness of the Minatogawa Formation decreases inland and ranges from 0 to 23.7 m. At the most distal site, only the Naha Formation was recovered. The drilling reached the Naha Formation at all sites, which consisted mainly of bioclastic packstone. It reached a massive siltstone of the Shimajiri Formation beneath a ~3 m-thick section of the Naha Formation at the most proximal site. The boreholes were capped with 1–3 m-thick reddish modern soil at the four sites. Modern soil was pervasive in dissolution cavities within the Pleistocene limestones. The Minatogawa Formation consisted of two main lithologies: coral limestone with varying proportions of in situ corals and well-sorted grainstone rich in large benthic foraminifers. The fossil coral fauna is dominated by laminar and massive forms, mostly belonging to Porites and Merulinidae, with noticeably few branching Acropora. There is a clear lateral variation in taxonomic and morphological compositions of corals, with an increase in the proportion of laminar and branching corals inland, which could reflect a lower wave energy or higher turbidity. Several paleosol layers were identified, attesting to the influence of relative sea-level change: two within the Minatogawa Formation and one at the boundary between the Minatogawa Formation and the Naha Formation. Further work is needed to clarify the timing of the deposition of the Minatogawa Formation.
References cited
Imai et al. (2013). Island Arc 22, 522–537.
Iryu et al. (2006). Island Arc 15, 393–406.
Webster et al. (2022). International Association of Sedimentology Special Publication 49, 215–273.
Yokoyama et al. (2018). Nature 559, 603–607.
In this study, we focus on the late Middle to late Pleistocene Minatogawa Formation exposed in southern Okinawa Island at elevations of < 50 m and resting unconformably on the Lower to Middle Pleistocene Naha Formation. The age of the Minatogawa Formation is still unclear but is thought to be < 400 ka based on the calcareous nannofossil biochronology of the Naha Formation (Iryu et al., 2006). We report the preliminary results of a drilling campaign conducted in 2021, which collected five cores along a one km-long transect perpendicular to the shore. The drilled length ranges from 10 to 27 m. The main objectives of this study are (1) to understand the 3D facies architecture of the Minatogawa Formation and reconstruct relative sea-level history, (2) to determine the age of the Minatogawa Formation, and (3) to investigate the response of reef organisms to millennial-scale sea-level and environmental changes.
Results of our preliminary examination of the cores show that the thickness of the Minatogawa Formation decreases inland and ranges from 0 to 23.7 m. At the most distal site, only the Naha Formation was recovered. The drilling reached the Naha Formation at all sites, which consisted mainly of bioclastic packstone. It reached a massive siltstone of the Shimajiri Formation beneath a ~3 m-thick section of the Naha Formation at the most proximal site. The boreholes were capped with 1–3 m-thick reddish modern soil at the four sites. Modern soil was pervasive in dissolution cavities within the Pleistocene limestones. The Minatogawa Formation consisted of two main lithologies: coral limestone with varying proportions of in situ corals and well-sorted grainstone rich in large benthic foraminifers. The fossil coral fauna is dominated by laminar and massive forms, mostly belonging to Porites and Merulinidae, with noticeably few branching Acropora. There is a clear lateral variation in taxonomic and morphological compositions of corals, with an increase in the proportion of laminar and branching corals inland, which could reflect a lower wave energy or higher turbidity. Several paleosol layers were identified, attesting to the influence of relative sea-level change: two within the Minatogawa Formation and one at the boundary between the Minatogawa Formation and the Naha Formation. Further work is needed to clarify the timing of the deposition of the Minatogawa Formation.
References cited
Imai et al. (2013). Island Arc 22, 522–537.
Iryu et al. (2006). Island Arc 15, 393–406.
Webster et al. (2022). International Association of Sedimentology Special Publication 49, 215–273.
Yokoyama et al. (2018). Nature 559, 603–607.