RESULTS

The results of opal analysis at Site 1143 indicate the following variabilities (Fig. F2):

  1. 12.3-7.9 Ma: opal content and MAR were relatively high and obviously variable (average opal = ~2.9 wt%; average opal MAR = 0.24 g/cm2/k.y.).
  2. 7.90-5.8 Ma: opal content and MAR dropped sharply (average opal = 2.0 wt%; average opal MAR = 0.16 g/cm2/k.y.).
  3. 5.8-4.4 Ma: opal content and MAR were stable (average opal = 1.2 wt%; average opal MAR = 0.06 g/cm2/k.y.).
  4. 4.4 -3.2 Ma: opal content and MAR were at their lowest (average opal = 0.63 wt%; average opal MAR = 0.02 g/cm2/k.y.).

Both opal content and MAR increased from ~3.2 Ma and then showed a rapid increase at ~0.7 Ma (Table T3). The increase from low values (4.4-3.2 Ma) could be important in understanding the response of the SCS to large-scale Northern Hemisphere glaciation.

Changes in opal content and MAR directly reflect changes in sea-surface productivity: higher opal MAR indicates higher surface productivity, and lower opal MAR indicates lower surface productivity. Thus, the variations of surface productivity for the past 12.3 m.y. in the southern SCS can be reconstructed in terms of opal content and MAR data at Site 1143. The surface productivity was enhanced and varied during the period from 12.3 to 7.9 Ma, reduced sharply during the period from 7.9 to 5.8 Ma, and was relatively stable during the period from 5.8 to 4.4 Ma. The lowest levels during the period occurred from 4.4 to 3.2 Ma and then increased from ~3.2 Ma and showed a rapid increase at ~0.7 Ma.

The extremely high biogenic silica accumulation rates were first found in the central equatorial Pacific from the late Miocene to the early Pliocene (Leinen, 1979). This was confirmed and designated as the "Biogenic Bloom Event" during DSDP Leg 85 (Theyer et al., 1985). According to a study during ODP Leg 138, biogenic accumulation rates evidently increased in the eastern equatorial Pacific from the late Miocene to the early Pliocene, with high peaks of carbonate MAR between 6.5 and 3.5 Ma. High opal MAR was contemporaneous with high carbonate MAR, indicating a biogenic bloom (Farrell et al., 1995). Dickens and others documented that the Biogenic Bloom Event occurred commonly in the Indian-Pacific Ocean from the late Miocene to the early Pliocene, when the surface productivity increased conspicuously in the upwelling areas of the Indian and Pacific Oceans. Based on data from 12 ODP sites, they concluded that the age of the Biogenic Bloom Event was 9.0-3.5 Ma (Dickens and Owen, 1996, 1999; Dickens and Barron, 1997). The Biogenic Bloom Event was also reported at the Ontong Java Plateau during ODP Leg 130. Both high biogenic sedimentation and accumulation rates occurred during the period from 9.4 to 4.1 Ma, indicating high biogenic productivity (Berger et al., 1993).

As shown in Figure F2, high biogenic productivity is clearly indicated by high values in opal, carbonate, and total MAR as well as opal content from 12.3 to 5.8 Ma at Site 1143, which corresponds to the late Miocene-early Pliocene Biogenic Bloom Event reported from the eastern equatorial Pacific, Indian Ocean, and Ontong Java Plateau. Thus, the conclusion can be drawn that the late Miocene Biogenic Bloom Event occurred in the southern SCS.

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