Figures F3 and F4 are age-depth models that depict the primary planktonic foraminiferal marker species listed in Table T3 (Site 1143; southern South China Sea) and Table T4 (Site 1146; northern South China Sea), respectively. The plot for Site 1143 (Fig. F3) shows three distinct intervals in the upper Miocene having sediment accumulation rates of 25 m/m.y. (5.5-6.4 Ma), 99 m/m.y. (6.4-8.6 Ma), and 8 m/m.y. (8.6-10.5 Ma). The lowermost leg of this plot (8.6-10.5 Ma; 8 m/m.y.) may contain a diastem in the sedimentary column (at ~455 mcd) based on the concentration of both foraminiferal and nannofossil datums. Although Site 1143 exhibits dramatic changes in sedimentation rates through the studied interval, the entire section corresponds to a single lithologic unit (Subunit IB; Wang, Prell, Blum, et al., 2000). In addition, shipboard studies of carbonate and total organic carbon show no appreciable change in trend through the Miocene.
The age-depth model for Site 1146 (Fig. F4) includes the same three upper Miocene intervals as Site 1143, though the differences in sedimentation rates between these intervals are more subtle. The observed rates are 21 m/m.y. (5.5-6.4 Ma), 32 m/m.y. (6.4-8.6 Ma), and 20 m/m.y. (8.6-10.5 Ma). At both sites, the interval from 6.4 to 8.6 Ma has the highest sedimentation rates of the upper Miocene. As at Site 1143, the interval of Zone 14 to Zone N19 (uppermost middle Miocene to basal Pliocene) at Site 1146 lies entirely within a single lithologic unit (Unit II). Total organic carbon shows no appreciable change in trend for this interval, though carbonate levels decrease from ~50-60 wt% to ~35-40 wt% below ~420 mcd (Zone N16; at about Section 184-1146A-42X-CC) (Wang, Prell, Blum, et al., 2000).
Because the drilled interval at Site 1146 extends deeper than at Site 1143, three additional sedimentary intervals have been identified in the middle and lower Miocene. These intervals and their sedimentation rates are 21 m/m.y. (10.5-15.1 Ma), 58 m/m.y. (15.1-16.4 Ma), and 8 m/m.y. (16.4-18.8 Ma). At Site 1146 the base of Zone N9 (FO of Orbulina suturalis at 15.1 Ma; between Sections 184-1146A-55X-2 and 54X-CC) corresponds to a dramatic change in sedimentation rate (58 to 21 m/m.y.) and the boundary between lithologic Units II and III (553.02 mcd), as described by the shipboard sedimentologists (Wang, Prell, Blum, et al., 2000). This lithologic boundary is based on a color change in the sediment from brownish gray with an increasing number of green intervals (base of Unit II) to a distinct greenish gray (Unit III). However, shipboard analyses of carbonate and total organic carbon show no significant changes across this boundary. A summary of the sedimentation history for both sites, with respect to age and core depth, is presented in Figure F5.
The large increase in mass accumulation rates (MARs) from 16.4 to 15.1 Ma at Site 1146 is mostly due to an increase in the noncarbonate fraction (Figs. F6, F7). This increase could be the result of lowered sea level or greater fluvial input. From 15.1 to 8.58 Ma, carbonate MARs increase steadily, whereas noncarbonate MARs decrease. A second large increase in MARs is observed in the interval from 8.58 to 6.4 Ma. At Site 1146 this increase is largely in the carbonate fraction, whereas at Site 1143 both the carbonate and noncarbonate fractions increase sharply. The observed increases in MARs during the late Miocene may be attributable to the onset of the East Asian monsoon. At 6.4 Ma, both sites show a marked decrease in MARs to approximately pre-8.58-Ma values.