[B-PT04] Biomineralization and the Geochemistry of Proxies

[BPT04-09] Inter-specific variations in trace element concentrations of modern brachiopod shells collected from Otsuchi Bay, Japan

*西尾 拓哉¹、高柳 栄子¹、浅海 竜司¹、新城 竜一²、山本 鋼志³、井龍 康文¹ (1.東北大学大学院理学研究科地学専攻、2.琉球大学大学院理工学研究科物質地球科学科地学系、3.名古屋大学大学院環境学研究科地球環境科学専攻)

キーワード：現生腕足動物、微量金属元素濃度、古環境指標、大槌湾

Carbon- (δ¹³C) and oxygen-(δ¹⁸O) isotope composition and trace element to calcium ratios (Me/Ca; e.g., Mg/Ca and Sr/Ca) in biogenic carbonates have been used as a powerful tool for reconstructing paleoceanographic and paleoenvironmental conditions throughout the Phanerozoic. Rhynchonelliformea brachiopods, hereafter called “brachiopod”, are regarded as one of the excellent archives for the conditions because of (1) their extensive fossil records from the early Cambrian onwards, (2) their dense, low-magnesium calcite shells less susceptible to diagenetic alteration than aragonitic precipitates of marine calicifiers, and (3) the occurrence of modern analoguesin the oceans.For these reasons, δ¹³C and δ¹⁸O values of fossil brachiopod shells have been served as a major data source in many studies on paleoenvironments, especially those in the Paleozoic and Mesozoic. In contrast, Me/Ca in brachiopod shells has not been used as paleoenvironmental indicators because the relationship between shell chemistry and seawater composition and physical properties (e.g., temperature, salinity) are not well understood. This is at least partly due to difficulties in obtaining time-series of Me/Ca from modern brachiopod shells to compare directly with those in seawater chemistry (pH and carbonate system as well as Me/Ca) and the oceanographic properties at their growth sites.
We investigated inter-specific variations in Mg/Ca, Sr/Ca, Mn/Ca, and Fe/Ca in the shells of four modern brachiopod species collected from two different water depths in Otsuchi Bay, Japan, and established a new technique for converting distance domain Me/Ca profiles of the ontogenetic-series samples to the Me/Ca time-series. Mg/Ca of Terebratulina crosseiis higher than that of the other species (Laqueus blanfordi,Laqueus rubellus, and Terebratalia coreanica). Significant correlations are found between Mg/Ca and δ¹⁸O values of some samples. If the correlations are statiscally significant, regression equations established for Mg/Ca and seawater temperatures for brachiopod shells have steeper slopes and higher intercepts than those for foraminiferal tests. Sr/Ca profiles are likely different in their patterns among species even though they live at the same water depth. Mn/Ca and Fe/Ca of the all species are verylow or below detection limits.