8.7. Results and Discussion
8.7.1. CaCO 3 and biogenic silica as a proxy for lysocline fluctuation
fluctuations in percentage, with high concentrations occurring in the glacial stages and significantly lower values during the interglacials (Farrell and Prell, 1989). It has been reasoned that during the glacial intervals the trade winds would be more intense than during the nonglacial intervals and would prompt the equatorial current system generating higher upwelling and hence greater productivity. The variation in the percentage of CaCO3 in the Pacific carbonates sediments is been said to be due to the corrosivity of the bottom water (Thompson and Saito, 1974) while Valencia, 1977; Adelseck and Anderson, 1978 both said that productivity cannot be ruled out especially when productivity is above the lysocline. The use of opal as a paleoceanographic tool along the equatorial Pacific transects is known to reflect a distribution of surface primary productivity of the equatorial Pacific surface waters. Murray et al 2012, depicted links between iron and opal deposition in the Pleistocene equatorial Pacific, explaining that horizons of high Fe and opal accumulation rate indicates high surface productivity. In contrast, Rea et al 1991 indicated that opal is not a good indicator of past productivity during the late Pleistocene of the central equatorial Pacific but represents the transition from low to high sea surface biological productivity. Thus, the question now rises whether or not CaCO3 and biogenic silica could be used as a proxy for surface productivity in this study.
The average CaCO3% during the early Pleistocene was 88.9% having the lowest percentage of 83.1% during the Jaramillo while that of the middle Pleistocene had an average of 85.3%. Average biogenic silica percentage during the early Pleistocene was 9.4% having the highest percentage of 14.8% during
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the Jaramillo and an average of 7.7% during mid- Pleistocene. CaCO3 % and biogenic silica % stratigraphy of PC 932 showed carbonate- siliceous cycles (Fig.
8-2a, Fig. 8-2b) with the early Pleistocene showing no significant carbonate- biogenic silica fluctuation except Jaramillo- Matuyama boundary, unlike that of the middle Pleistocene. In accordance to (Farrell and Prell, 1989), a higher percentage of CaCO3s said to be associated with ncreased biological productivity at the sea surface during the glacial, based on this it could be said that the high percentage of the CaCO3 during the early Pleistocene is associated with increased biological productivity. Although increased biological productivity is paramount in determining carbonate accumulation in the central equatorial Pacific (Rea et al., 1991), the contradicting results of similar pattern flux of calcium carbonate and organic carbon mass accumulation rate (MAR) from his study (Fig. 8-3d, Fig. 8-3e) with this study (Fig. 8-3b, Fig. 8-3c), indicates that the combination of both proxies is not a better indicator of past productivity for this study because of the contradictions of both proxies. As illustrated in (Fig. 8-4), we assume that dissolution phenomena could be a predominant factor governing the CaCO3 accumulation in early-middle Pleistocene. The CaCO3 percentage appears to be controlled by the corrosiveness of deep water to carbonates. This is partly due to the effect of pressure on the solubility of carbon dioxide (CO2). Since more CO2 is held in solution at higher pressure, the addition of more CO2 in deep ocean water by the respiration of organisms’ results to more corrosive activities at the deep ocean leading dissolution of calcareous shells. At a shallow depth of lysocline, foraminiferal and coccolith tend to be well preserved in the bottom sediments, while at depths approaching the calcium carbonate compensation depth (CCD) below the lysocline, preservation declines and more foraminiferal and coccolith increases in dissolution. Due to this fact, we speculate that during the early Pleistocene the high CaCO3 MAR and Corg MAR was as a consequence of well-preserved calcareous shells by increased sedimentation rate (Fig. 8-3b, Fig.
8-3c), emerging from a shallow depth of the lysocline between the bottom sediment. Reports from Balsam,1983; Croley,1983; Rea and Leinen, 1985 addressed the change in depth of the lysocline and CCD over a geological scale. Records from such studies have indicated that the lysocline and the CCD deepened and remained constant during the Brunches Chron (Farrell and Prell, 1989). The aspect of the change of depth of lysocline is also said to affect the change in biogenic silica chemistry in the water column. Generally, the
solubility of silica decreases with a decrease in temperature, however, the increase in solubility with pressure results in the progressive dissolution of skeletal remains of diatoms and radiolarians. In addition to our assumption of the dissolution of CaCO3% during the mid-Pleistocene, the CaCO3% is strongly in phase with the 100kyr band, and it has been said that dissolution is being responsible for the CaCO3 variability during this cycle (Murry et al., 2000).
Fig. 8-2. Abundance of biogenic materials in PC 932. (a) Biogenic silica percentage (b) Calcium carbonate percentage (c) Organic carbon percentage.
Shown in (Fig. 8-3a, Fig. 8-3b), the records opal MAR and CaCO3 MAR appears to have a similar profile during the early Pleistocene. A regression of opal MAR to Corg MAR and CaCO3 MAR to Corg MAR shows no correlation (R2=0.02 and 0.08 respectively). Also, regression of opal MAR and CaCO3 MAR showed no correlation (R2=0.01). Phytoplankton and zooplankton are both known to secrete skeletal calcite and opal (Lyle et al., 1988), with the lack of correlation of both proxies (CaCO3 and opal) with Corg MAR and opal MAR to and CaCO3 MAR we assume that we have at least different plankton communities during the early Pleistocene, one rich in opal secreting and one more dominate by calcite-secreting. Since biogenic silica is undersaturated through out the world’s ocean, we presume that he high opal MAR during the early Pleistocene indicates deposition of biogenic silica in a saturated (CO3) environment. Our presumption is in agreement with the correlation of sedimentation rate and planktonic
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foraminiferal dissolution cycles for RC11-210 (Thompson and Saito, 1974) which showed intense dissolution at horizons where linear sedimentation rate and carbonate amounts are low (Fig. 8-5a, Fig. 8-5b). In summary, the biogenic CaCO3 and silica accumulation in the PC932 are dependent on the post depositional dissolution of CaCO3 and silica. Indicating little or no dissolution of CaCO3 and silica during the early Pleistocene resulting to the high CaCO3 MAR and opal MAR recorded at a shallow depth of lysocline between the bottom sediment.
8.7.2. Stable carbon isotope (δ13C)andoxygenisotope(δ18O)offoraminifera