Presentation Information
[T16-P-24]Reef formation and dolomitization on Kitadaito Jima: Insight from a marginal drill core
*Goitse MOSEKIEMANG1, Momo YOSHIBE1, Hideko TAKAYANAGI1,2, Shigeyuki WAKAKI3, Kazuyuki YAMAMOTO4, Yasufumi IRYU1,2 (1. Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Japan, 2. Advanced Institute for Marine Ecosystem Change (WPI-AIMEC), Tohoku University, Sendai, Japan, Sakura, Japan, 3. National Museum of Japanese History, 4. Technical Headquarters, INPEX Corporation, Tokyo, Japan)
Keywords:
KitaDaito Jima,Dolomite,Reef formation
Carbonate platforms are excellent archives of sea-level, ocean chemistry, and tectonic histories, with isolated atolls serving as key subsurface analogs. Kitadaito Jima (NW Pacific) is an uplifted remnant atoll whose past morphology is well preserved. While previous work focused on the lagoonal interior and surface outcrops, this study investigates a new 130m long drill core from the eastern reef margin to resolve the coastal lithostratigraphy, depositional history, and dolomitization through sedimentological and geochemical analyses (δ13C, δ18O, 87Sr/86Sr).
The drill core is divided into four lithostratigraphic units (A–D, top to bottom). Unit A (5.6–4.6 Ma) comprises coral framestone and bioclastic packstone; Unit B (∼7 Ma) consists of coral rudstone; Unit C (∼10 Ma) is dominated by bioclastic grainstone and coral rudstone/floatstone; and Unit D (13.2–11.7 Ma) features coral floatstone with Halimeda-rich rudstone. These units reflect shifting reef accretion dynamics: the basal Unit D shows a steady "keep-up" growth matching gradual subsidence, whereas Units C, B, and A record rapid, episodic upward growth. Continuous rapid growth at ∼10 Ma argues against a globally synchronous Late Miocene "carbonate crash", indicating that reported carbonate declines reflect regional rather than global forcings.
Dolomitization occurs throughout the drill core but is more pervasive in Units A–C than in Unit D. Comparison with the lagoonal drill core highlights spatial heterogeneity in dolomitization. Seawater-driven dolomitization is supported by a consistent 2.8‰ δ18O offset between coexisting dolomite and calcite. Similar dolomite δ18O values across both drill cores imply repeated dolomitization by the same fluid type at comparable temperatures island-wide. However, marginal drill core dolomite and calcite exhibit 1–2‰ heavier δ13C values than the lagoonal interior, reflecting higher marginal productivity and greater meteoric diagenetic overprinting in the lagoon.
The drill core is divided into four lithostratigraphic units (A–D, top to bottom). Unit A (5.6–4.6 Ma) comprises coral framestone and bioclastic packstone; Unit B (∼7 Ma) consists of coral rudstone; Unit C (∼10 Ma) is dominated by bioclastic grainstone and coral rudstone/floatstone; and Unit D (13.2–11.7 Ma) features coral floatstone with Halimeda-rich rudstone. These units reflect shifting reef accretion dynamics: the basal Unit D shows a steady "keep-up" growth matching gradual subsidence, whereas Units C, B, and A record rapid, episodic upward growth. Continuous rapid growth at ∼10 Ma argues against a globally synchronous Late Miocene "carbonate crash", indicating that reported carbonate declines reflect regional rather than global forcings.
Dolomitization occurs throughout the drill core but is more pervasive in Units A–C than in Unit D. Comparison with the lagoonal drill core highlights spatial heterogeneity in dolomitization. Seawater-driven dolomitization is supported by a consistent 2.8‰ δ18O offset between coexisting dolomite and calcite. Similar dolomite δ18O values across both drill cores imply repeated dolomitization by the same fluid type at comparable temperatures island-wide. However, marginal drill core dolomite and calcite exhibit 1–2‰ heavier δ13C values than the lagoonal interior, reflecting higher marginal productivity and greater meteoric diagenetic overprinting in the lagoon.
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