Evaluation of Oxygen / Carbon Isotopes and Weights of Primary and Secondary Calcite in Planktonic Foraminifer Tests

Since Lohmann (1995), it is generally believed that the weight of particular planktonic foraminifer test with a similar size is identical when it is formed and it increases with sinking in the water column due to formation of secondary calcite. Finally, the calcite of the foraminifer test is affected by dissolution and loose its weight in the deep sea sediments. In order to extract the information of both surface and subsurface waters from one sample, oxygen / carbon isotopes and weight of individual test of G. sacculifer collected from plankton tow (18° 58'N 137° 01'E 0-500 m) and piston core (Site KPR4: 26° 52'N 135° 29'E 2704 m) samples in subtropical wetern North Pacific during KH04-02 cruise, and relationship among them were examined in detail. To compare the weight and isotope values for tests with different sizes, we refer to a size-weight relationship of living foraminifers (Takahashi, 1984). The simple relationship between length and weight with the power close to 3 suggests that G. sacculifer tests with different sizes can be treated as similarity shape. Therefore, we express the weight of individual test as "normalized weight" (= real weight / weight expected from length). Distribution of "normalized weight" of living G. sacculifer is trivially centered at 1 and deviated symmetrically. On the other hand, normalized weight of dead tests collected from both plankton tow and sediments are all 6-25% heavier than living one. These facts strongly suggest that secondary calcite was added in the water column to the primary calcite tests formed when they were living. Oxygen isotope variation measured from G. inflata of KPR4 piston core samples shows 23 cm bsf is the boundary of MIS 1 and 2. We selected 3 samples from this core and measured oxygen and carbon isotopes for 20-23 individual tests of G. sacculifer in each sample. Oxygen isotope value averaged for each sample is concordant to the variation of G. inflata. However, deviation (dynamic range) of oxygen isotopes for individual tests very large and up to 3.7‰. Each individual foraminifer formed its test in a certain season and water depth. If G. sacculifer formed its test in a surface water (0-100 m), δ18O is expected to be within the range from -3.3 to -0.5‰ considering the temperature and salinity of water column at the location where KPR4 piston core was recovered. However, realized δ18O show larger values up to +0.2‰. Calcite with such a heavy oxygen isotope can be formed in the water deeper than 400 m, which suggests that some secondary calcite is added to foraminifer test in water column deeper than 400 m. As expected from Lohmann's model, foraminifer tests with heavier weight show larger oxygen and smaller carbon isotopes because secondary calcite formed in subsurface water should reflect lower temperature and higher nutrient, respectively. Ideally, oxygen and carbon isotope values at 1 / "normalized weight" =1 and 0 reflect the isotope values for primary and secondary calcite, respectively. Relationship between oxygen / carbon isotopes and weight of individual tests could enable us to reconstruct δ18O and δ13C of both surface and subsurface water as well as carbonate ion concentration of subsurface water, which could strongly constrain the carbonate budget of Pacific in the past.