RESULTS

XRD measurements indicate that Idmidronea spp. and Nevianipora sp. have calcite skeletons with 1.5-10.0 and 0.9-6.4 mol% MgCO₃, respectively (Table T1). These bryozoans fall into the low magnesian calcite (LMC) (mol% MgCO₃) and the intermediate magnesian calcite (IMC) (4-12 mol% MgCO₃) mineralogical groupings of Bone and James (1993). SEM observations, however, indicate that the bryozoan skeletons have been subject to variable marine diagenesis below the flushed zones (Fig. F6). Therefore, the above values do not totally represent the original skeletal mineralogy, possibly with lower magnesian contents. Adeonellopsis spp. have an aragonite skeleton accompanied by significant amounts (up to 59%) of IMC (Table T1). The volume of calcite varies from stratigraphic horizon to horizon and from sample to sample even within the same horizon.

Bryozoan carbon and oxygen isotopic profiles for Holes 1129C, 1131A, and 1132B are shown in Figures F2, F3, and F4, respectively. In these depth profiles, we used the average value of the same groups collected from the same depth. The groups include the LMC and IMC mineralogical groups of Idmidronea spp. and Nevianipora sp. and aragonite and aragonite + calcite groups of Adeonellopsis spp. It can be considered that the LMC groups of Idmidronea and Nevianipora and aragonitic Adeonellopsis may have less marine cements and retain more original isotopic signals than the rests. A sharp negative shift of ¹⁸O values is discernible at ~40 mbsf in Hole 1129C, above which the profiles are characterized by relatively lighter values and larger amplitudes (up to 1.3). Idmidronea spp. and Nevianipora sp. show the lightest oxygen isotopic values of ~0.2 and 0.3 at the top of Hole 1129C, which consists of a thin Holocene sediment veneer. In contrast, the isotopic values are heavier, and the profiles have smaller amplitudes (<1.0) at 40 mbsf downhole. This negative shift is not clearly recognized in Holes 1131A or 1132B.