Abstract
Abstract. In this study, published Holocene ostracod data for Osaka, Hiroshima, and Iyo-nada Bays within the Seto Inland Sea (SIS), Japan, are re-examined. Five major faunal changes are recognized at different stratigraphic horizons (Horizons 1–5) in the Holocene sedimentary record. Horizons 1 and 5 (faunal changes) were most likely triggered by the respective openings of the Bisan-seto (and resulting formation of the SIS) and Akashi Straits. Three other major faunal changes, at Horizons 2–4, may also be linked to the opening of straits and resulting formation of bays, although the exact nature of these events is less certain. This re-examination indicates that the opening of straits extensively affected the benthic inner-bay community during the Holocene transgression in the SIS. Further high-resolution studies are needed for confirmation of faunal changes triggered by opening of straits.
Highlights
The impact of opening and closing of coastal straits on organisms has been recognized over relatively long time-scales (e.g. Isthmus of Panama, Collins et al, 1996; Bering Strait, Marincovich & Gladenkov, 1999) and includes ostracod speciation (Cronin & Ikeya, 1990)
There is no clear relationship between the present-day distribution of the five dominant ostracod species considered in this study and water depth
A variation in distribution from Spinileberis quadriaculeata to Bicornucythere bisanensis, corresponding to Horizons 1–4, is not recognized in modern ostracod data of Japan. This is despite the existence of abundant ostracod data for Japanese inner-bay areas (Ishizaki, 1968 – Uranouchi Bay; Ishizaki, 1969 – Lakes Nakaumi and Shinji; Ishizaki, 1971 – Aomori Bay; Ikeya & Hanai, 1982 – Lake Hamana; Frydl, 1982 – Tateyama Bay; Bodergat & Ikeya, 1988 – Ise and Mikawa Bays; Takayasu et al, 1990 – Lakes Nakaumi and Shinji; Ikeya & Itoh, 1991 – Sendai Bay; Ikeya et al, 1992 – Otsuchi Bay; Iwasaki, 1992 – Ariake-kai Bay; Itoh, 1998 – Lake Hamana; Tanaka et al, 1998 – Miho Bay to Lake Shinji; Yamane, 1998 – Hiuchi-nada Bay; Yasuhara & Irizuki, 2001 – Osaka Bay; Irizuki et al, 2003 – Lake Nakaumi; Irizuki et al, 2006 – Urauchi Bay)
Summary
The impact of opening and closing of coastal straits on organisms has been recognized over relatively long time-scales (e.g. Isthmus of Panama, Collins et al, 1996; Bering Strait, Marincovich & Gladenkov, 1999) and includes ostracod speciation (Cronin & Ikeya, 1990). The impact of opening of straits due to Holocene sea-level rise on organisms is not well known (Irizuki et al, 2001; Yasuhara et al, 2004). Irizuki et al (2001) and Yasuhara et al (2004) described a major ostracod faunal change in Osaka Bay at the eastern end of the Seto Inland Sea (SIS), Japan, which was probably triggered by the opening of the strait. Two factors have enabled the reconstruction of Holocene high-resolution records of inner-bay ostracods: the recent development of Accelerator Mass Spectrometry (AMS) radiocarbon dating, and the relatively high sedimentation rate within inner-bay areas. The resultant high-resolution ostracod record assembled in this study is tested to reveal whether the opening of straits and resulting formation of bays had a major impact on the inner-bay ostracod benthos
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