Sr isotope variations in Oligocene–Miocene and modern biogenic carbonate formations of Koko Guyot (Emperor Seamount Chain, Pacific Ocean)

The Hawaiian–Emperor Seamount Chain, a major topographic feature of the Pacific Ocean floor, is composed of seamounts capped with fossil coral reef deposits that originally formed close to sea level but are now covered by hundreds of meters of water owing to prolonged subsidence. These fossil reef deposits are important archives of paleoenvironmental change and yield information on the subsidence history of the seamounts. We studied the Sr isotope compositions of Oligocene–Miocene coral reef limestone from Koko Guyot in the southern Emperor Seamount Chain to assess the dynamics of the subsidence. The ages of the studied samples containing coral fragments established by Sr isotope stratigraphy vary from 26.3 to 20.1 Ma. In contrast, the youngest samples (15.3 Ma), which were deposited in water depths of >120 m, are barren of corals and are composed exclusively of bryozoans and coralline algae. The subsidence rate of the Koko Guyot volcanic structure was not constant over time. Integration of our new data with the results of previous studies reveals that the subsidence rate was 0.046 ± 0.005 mm/yr during the first 25–30 Myr (from 49–44 to 20 Ma). During this period, Koko Guyot was in a bathymetric interval favorable for coral reef development, and its subsidence was compensated by rapid vertical growth of the reef. Subsequently, the subsidence rate decreased to an average value of 0.019 ± 0.003 mm/yr from 20 to 15 Ma. The decrease in the rate of bottom subsidence coincided with unfavorable environmental conditions for coral reef development, leading to the disappearance of corals. The average subsidence rate has been 0.015 ± 0.002 mm/yr since 15 Ma, comparable to the present-day subsidence rate. We also analyzed the stable Sr isotope ratios (δ88/86Sr) of warm-water coral samples formed at 25–20 Ma (0.32‰ ± 0.1‰), as well as carbonate of large benthic foraminifera, coralline algae, and other non-coral species for the period 20–15 Ma (0.10‰ ± 0.09‰). We suggest that the large difference in carbonate δ88/86Sr between 25–20 and 20–15 Ma corresponds to a difference in the fractionation factor caused by environmental and benthic community change.