HR: 1330h
AN: PP12A-0224    [PDF]
TI: The Use Of Multiple Proxies From Subtropical Shallow-Water Carbonates In The Study Of Quaternary Environments: Core Analyses Of The Ryukyu Group, Southwest Japan
AU: * Sakai, S
EM: saburo@soc.shimane-u.ac.jp
AF: Institute for Frontier Research on Earth Evolution, Japan Marine Science & Technology Center, 2-15 natsushima-cho, Yokosuka, 237-0061 Japan
AU: Sagawa, N
EM: na-sa@bj8.so-net.ne.jp
AF: Tohoku University, 2-14-27 sakuragaoka, aoba-ku, Sendai, 981-0961 Japan
AB: Quaternary shallow-water carbonates, which are often characterized by high sedimentation rates, can provide high-resolution records of sea-level fluctuation, climatic change, and oceanographic evolution for tropical and subtropical shallow waters. However, most previous studies using Quaternary shallow-water carbonates have focused on facies distribution or sedimentological data because, in these commonly cemented sediments, (1) the original geochemical signals are normally altered by post-depositional diagenesis and (2) faunal censusing is difficult, although a few studies have been reported excellent biotic proxies such as coral assemblages, benthic foraminifer, calcareous algae, and bryozoans. Here we present and discuss biotic (coral and benthic foraminifers), geochemical (d18O, d13C, Mg/Ca, and Alkenone SST), and sedimentologic proxies of environmental change in Quaternary shallow-water carbonates using cores from the Ryukyu Group (southwestern Japan). Dating: Nannofossil and planktic foraminiferal biostratigraphy, oxygen isotope stratigraphy, and paleomagnetic stratigraphy were used to establish core chronology. Multiple dating methods are used to maintain inter-core chronology with other workers on the Ryukyu Group. Biotic proxies: In the case of the sedimentary facies shallower than 50 m, coral assemblages can be used to track fifth- and sixth- order sea-level change. If paleo-depth was deeper than 50 m, benthic foraminifer assemblages and morphologies (especially thickness / diameter ratio of the large foraminifer Amphistegina spp.) may be used to reconstruct the Quaternary sea-level fluctuations. Stratigraphic and sedimentological proxies: The Ryukyu Group is composed of repeating lithologic units. A complete succession of units consists of the basal shallow-water limestone overlain by deep-water deposits that grade upward into shallow-water sediments. We assume each packet is formed in response to a single sea-level change, and from this have constructed a relative sea-level curve for the last 1.3 million years. Geochemical proxies: Although Quaternary shallow-water carbonates have typically undergone significant alteration by post-depositional meteoric diagenesis, we demonstrate here that original d18O and d13C signals can be preserved in carefully cleaned low-Mg calcitic planktic foraminifers, even when the isotopic values of the carbonate host rock have been altered by meteoric fluids and subaerial exposure. We also discuss the preservation of original Mg/Ca ratios in planktic foraminifers and alkenone SST calculations in the diagenetically altered Ryukyu Group.
DE: 1040 Isotopic composition/chemistry
DE: 4556 Sea level variations
DE: 4825 Geochemistry
DE: 9355 Pacific Ocean
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting