[B-PT03] バイオミネラリゼーションと古環境プロキシー

[BPT03-01] CCD 以上の深さに存在する石灰質有孔虫群集維持のメカニズムに関する考察

*北里 洋¹ (1.国立大学法人東京海洋大学/ Danish Center for Hadal Research)

キーワード：CCD、超深海、緻密な殻

There exist calcite compensation depth in the oceans. Calcite is saturated at sea surface and undersaturated according to the ocean depths. At regular ocean floor, calcareous tests of foraminifers totally dissolve below 3500 or 4000m deep. It calls Calcite Compensation Depth (CCD). However, there are some exceptions. We count several cases where we can find calcium carbonate tests even under CCD. I found three cases. 1) Calcareous foraminiferal shells are sometimes remained at eutrophic oceans where calcareous foraminifera productions are so high that sediments contain a lot of calcareous foraminiferal tests. This is the most common case. We call it as snow line under the sea. 2) Existence of large limestone blocks below CCD. At Palau Trench, there exist big limestone blocks at 6500m deep. Sediments covered on the surface of the limestone blocks, we found delicate calcareous foraminifers such as Globobulimina and Chilostomella spp. Due to high concentration of carbonates, calcareous tests survive even at 6500m deep. 3) Some foraminifera are covered with chitinous films on its calcareous test. The film prevents calcite dissolution below the CCD.
I recently found the fourth case. During the Pressure Drop Expedition on 2022, I found fresh calcareous foraminiferal communities at 7500m deep of the Japan Trench axial zone. There are neither eutrophic environments nor limestone blocks, nor chitinous skins. Approximately 10 % of benthic foraminiferal populations are calcareous species. They are Alabaminella, Pullenia and Fissulina species. These species consists of very dense calcite crystals as calcareous test. These physically tight and robust crystals may prevent dissolution of calcareous tests from the dissolution. If this explanation can be reasonable, it is new adaptation mechanism for benthic foraminifera that flourish at hadal depths.