【氏名】ハンブレ・マーク・アンドレ

【専攻・学年】地学専攻 博士課程後期３年

【指導教員名】井龍 康文

【研究グループ名】地球進化史研究グループ

【Travel purposes】

1)Poster presentation entitled “Variations of Pleistocene coral community structure in response to relative sea-level changes in the Ryukyu Islands” at the SEAL’AIX symposium “Sea level changes: records, processes, modelling” held in Giens, France (Monday 25th to Friday 29thof September).

2)Lecture given at the University of Liège, Belgium, entitled “Quaternary development of coral reefs in the Ryukyu Islands” (Monday 10th of October).

1) The SEAL’AIX symposium (Monday 25th to Friday 29thof September) was subdivided into 4 successive sessions including both oral and poster presentations. Each session covered a specific time span. The first session, held on Monday, was about sea-level changes during Holocene and the Last Deglaciation. The second session took place on Tuesday and dealt with the last glacial cycle and the Pleistocene. Presentations scheduled on Wednesday targeted fluctuations of sea level during the last 33 Ma. (icehouse world). The last two days were dedicated to sequences dating as far back as 250 Ma.(greenhouse world). The chronological organisation of the sessions, from historical records to Palaeozoic times, provided a perfect mean for highlighting the different techniques used to reconstruct sea-level changes back in time, the loss of accuracy affecting sea-level reconstructions based on older sedimentary records, and the different factors causing sea-level fluctuations at various time scales.

Understanding how sea-level has fluctuated through time is of prime importance for my research as sea-level changes have had a great impact on the temporal successions of coral associations observed in the fossil record, as well as their preservation potential. I was particularly interested in the first two sessions, which focused on the Quaternary. I report hereafter a few examples of information that have particularly excited my curiosity. Dr. Cazenave (LEGOS GRGS/CNES) pointed out the striking increase of the contribution of thermal expansion in the rise of sea level during the last 50 years. Dr. Statteger (Univ. of Kiel) presented a sea-level curve for the Holocene based on beachrock and tidal lagoonal sediments from NE Brazil characterized by a highstand followed by progressive sea-level drop linked to differences in redistribution of water masses in equatorial regions (“equatorial ocean siphoning”). Dr. Saito (Geological Survey of Japan) also evoked this phenomenon. Dr. Gischler (Johann Wolfgang Goethe Universität) discussed differences between Holocene sea-level curves obtained in different reef areas of the Caribbean and Western Atlantic based on calibrated 14C dates of A. palmata and mangrove peat. His talk was particularly relevant for my research as it was partly related to the problem of preservation of coral community structure in the fossil record. Dr. Lericolais (IFREMER) presented the results of the European Union-funded ASSEMBLAGE Project aiming to unravel the sea-level history of the Black Sea since the LGM. His presentation clearly highlighted the influence of climate change on the isolated Black Sea basin through the melting of the Fennoscandian ice cap resulting in S-L rise and the subsequent dry/cool weather of the Young Dryas evidenced by characteristic geomorphological features left on the sea floor during the regression. The final rapid S-L rise prevented the pre-existing coastal features from being eroded. Dr. Fairbanks (Lamont-Doherty Earth Observatory) provided interesting new data on the timing of MWP-1A and 1B, the Younger Dryas stillstand and the last glacial lowstand in Barbados. It is interesting to note that some participants have questioned the existence of MWP-1B. They claimed that one might draw a line showing a more progressive trend (without any slope break) and still fall within the error bar of each dot.

Given the main subject of my research, i.e. Pleistocene coral communities, two posters have particularly caught my attention. The first was presented by Dr. Sugihara (FukuokaUniversity) and showed differences in coral community structure between Ishigaki-jima and Tahiti. His results demonstrate that coral-based bathymetric reconstructions of quaternary reef deposits are valid only if the coral zonation model is established in the same region as the fossil material studied. Regional variations of coral community structure arise from differences in the cover of dominant taxa and differences in the depth distribution of coral zones. The author of the second poster was Phd student Kate Bromfield (University of Queensland). She presented the preliminary results of her Phd thesis on the relationship between sea-level change and species turnover in Indo-Pacific corals over the Miocene-Pliocene boundary. We discussed on the various styles of responses of coral communities to external environmental forcing, mainly relative sea-level changes, and their preservation potential in the fossil record. Our respective experience of the field is very similar and sharing observations of fossil coral associations from different places in the Pacific and of different ages was very enriching.

2) The lecture that I gave at the University of Liège on Monday, the 10th of October, was mainly addressed to 3rd and 4th grade students in geology, and organized by the laboratory of sedimentology headed by Pr. Frédéric Boulvain. I had the pleasure to see that not only geologists were interested in my subject but also members of the department of biology as some of them came to attend my presentation. My purpose was to present a general overview of the history of coral reef growth in the Ryukyu Islands during the Quaternary. I first introduced our current knowledge of present-day sedimentary systems in the Ryukyu islands. Then I highlighted the main mechanisms driving the reef growth during the Holocene and finally summarized the evolution of coral reef complexes during the Pleistocene with a special focus on fossil coral associations and their preservation in the fossil record. The discussion following my talk led to interesting remarks on coral taxonomy, the influence of turbidity on coral morphology, the comparison between quaternary reef drowning and Paleozoic carbonate platform drowning and the potential impact of climate/S-L changes on the depth distribution of rhodoliths.

SEAL’AIX meeting 2006 SEAL’AIX meeting 2006

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東北大学21世紀COEプログラム「先端地球科学技術による地球の未来像創出」