Stable oxygen and carbon isotopic measurements were performed using approximately 40 individuals (approximately 1 mg in weight) of a shallow-dwelling planktonic foraminifera, Globigerinoides sacculifer (250 > 425 μ m), for the NGC34, NGC36, and NGC38. Approximately 10 individuals (approximately 1 mg in weight) of a deep-dwelling planktonic foraminifera Globorotalia tumida (> 250μm) were analyzed for the C4402 core because of their continuous occurrenece through this sediment section. All measurements were performed using Finnigan Mat delta E mass spectrometer at the University of the Ryukyus. The standard deviation for the σ 18 O was 0.036‰.
(2)Organic carbon measurement
Organic carbon was determined with a Yanako MT-S CHN analyzer after removing carbonate by acidification. At least 6 concentrates were measured to obtain a linear calibration (r2>0.99). Total nitrogen measurements way also performed on dried and ground bulk sediment samples using the CHN analyzer. The relative standard deviation for duplicate analyses of carbonate-rich samples with organic carbon content of approximately 0.16 wt.% is better than 7%. For measurements of dry bulk density, four cubic centimeter of sediments were taken and dried at 50℃.
3.RESULTS
3-1 .Stratigraphy
The chronostratigraphy of the four sedimentary cores are based on the stable oxygen isotopic records. Correlation with the SPECMAP stack3)and the isotopic events taxonomy4)shows that the records in the sediments appear to extend back to Stage 9 (about 300ka)(C4402), Stage 8 (about 260ka) (NGC34), Stage 15 (about 570ka) (NGC36) and Stage 11 (400ka)(NGC38), respectively (Fig.2). Average linear sedimentation rate of each core is 3.6cm kyr-1 for C4402, 1.0cm kyr-1 for NGC34, and 0.9 cm kyr-1 for NGC36 and NGC38, respectively.
Fig. 2
Oxygen isotope stratigraphies of the four cores. All oxygen isotope data are plotted relative to the PDB standard. The profiles of NGC34 (dot), NGC36 (cross) and NGC38 (open square) are based on analyses of planktonic foraminifera Globigerinoides sacculifer. The profiles of C440'2 (solid square) are based on analyses of Globolotaliatumida.
3-2.Organic carbon mass accumulation rate:
MARoc (mg cm-2kyr-1) are calculated using the thefollowing equation.
MARoc =10 x OC x LSR x DBD
where OC is organic carbon content (wt.%), LSR is linear sedimentation rate (cm kyr-1) and DBD is dry-bulk density (g cm-3). The fluctuations of MARoc in the four cores are
shown in Fig.3a, b.
(1) C4402 (West Caroline Basin)
Organic carbon forms 0.20 to 2.02 wt.% of the sediments. The average value is 0.7 wt.%. The Corg/N atomic ratio of organic matter varies from 4.7 to 11.6 with the mean ratio of 8.3. Calculated MARoc varies from 2 to 34 mg cm-2kyr-1. The average value of MARoc is 11 mg cm-2kyr-1. Two large MARoc maxima are the middle Stage 6 to the early Stage 5 and Stage 4 to Stage 2. Relatively high values are observed during Stage 8 and in Stages 7/6 boundary (Fig. 3a).
