Organic carbon ranges from 0.1 to 0.3 wt.% with the mean of 0.2 wt.%. The Corg/N atomic ratio of organic matter varies from 5.0 to 8.9 with the mean ratio of 7.3. MARoc varies from 0.7 to 2.1 mg cm-2 kyr-1 and the average of 1.4 mg cm-2 kyr-1. The MARoc profile exhibits a general trend of higher values during times of glaciation6)(Fig. 3b).
(3) NGC36 (Ontong Java Plateau)
Organic carbon makes up 0.1 to 0.2 wt.% in this core with the average value of 0.10 wt.%. The Corg/N atomic ratio of organic matter varies from 3.9 to 8.0 with the mean ratio of 5.5. The MARoc ranges between 0.3 to 2.5 mg cm-2 kyr-1. The mean value of MARoc is 0.7 mg cm-2 kyr-1. The profile shows no distinct fluctuation of MARoc through this core except the surface sediments (Fig. 3b).
(4) NGC38 (North Fiji Basin)
Organic carbon forms 0.1 to 0.2 wt.% of the sediments. The average value is 0.1 wt.%. The Corg/N atomic ratio of organic matter varies from 3.5 to 12.6 with the mean ratio of 6.2. Calculated MARoc varies 0.3 to 1.3 mg cm-2 kyr-1. The average value of MARoc is 0.6 mg cm-2 kyr-1. The MARoc record show relatively higher values at the early Stage 5 and the surface sediment although no remarkable changes are found in the rest of the core7)(Fig. 3b).
Fig. 3
Time series of mass accumulation rate of organic carbon (MARoc) records for (a) C4402 (solid square), (b) NGC34 (dot), NGC36 (cross) and N0C38 (open square).
4. DISCUSSION
4-1. Sedimentation of organic matter
The MARoc profile demonstrates fluctuations with time as well as from place to place. We need to determine whether the fluctuation was due to changes of primary productivity, the preservation degree, or supply rates of organic matter from another source. The organic carbon to total nitrogen atomic ratio (Corg/N) is considered to be a useful indicator of the origin of organic matter in marine sediments8). The average Corg/N atomic ratios of the sediments of C4402, NGC34, NGC36 and NGC38 are 8.3, 7.3, 5.5 and 6.2, respectively. These results suggest that the organic carbon in the sediments are mainly of marine origin.
Recent evidence suggests that bottom oxygen does not play a primary role in promoting organic matter preservation9). By eliminating an additional organic matter source and variations in preservation we are led to conclude that changes in productivity are mainly responsible for differences in organic carbon accumulation in the western equatorial Pacific.
4-2. MARoc and global climatic change
We conducted a spectral analysis of MARoc records from the four cores. We choose the oxygen isotope record of each core for the second variable which is regarded as a well-established climate indicator. Results are shown in a total amplitude density and a coherent amplitude density. The difference between the total amplitude and the coherent amplitude reflect a combination of influences, including responses to orbital forcing that are not linear in the Milankovitch cycle10).
C4402 shows high total amplitude at the three interest frequencies (23 kyr,41 kyr and 100 kyr) and beside the high coherent amplitude at each orbital periods (Fig.4). These results suggest that the fluctuation of MARoc reflects climatic changes in the west Caroline Basin. The fluctuation of MARoc in NGC34 sediment also show high total amplitude at the three interest frequencies but the coherent amplitude doesn't show liner response at 100 kyr period. The results suggest that MARoc of the Eauripic Rise wealdy response to the climatic oscillation. Other two regions show no relation between the MARoc records and the global climatic changes.
5.CONCLUSIONS
1 .The mean Corg/N atomic ratio of the four sediments is 6.8, which indicates that marine organic
