Calcium carbonate, sedimentation and mass accumulation rates at DSDP Site 85-572@en

At Site 572, located at 1°N, 114° W (3903 m water depth), we recovered a continuous hydraulic piston cored section of upper Miocene to upper Pleistocene pelagic sediments. The sediment is composed of biogenic carbonate and silica with nonbiogenic material as a minor component. Detailed analysis of the calcium carbonate content shows that the degree of variability in carbonate deposition apparently changed markedly between the late Miocene and Pliocene at this equatorial Pacific site. During this interval carbonate mass accumulation rates decreased from 2.6 to 0.8 g/cm**2 per 10**3 yr. If we assume that variations in CaCO3 content reflect changes in the degree of dissolution, then the detailed carbonate analysis would suggest that the degree of variability in carbonate deposition decreases by a factor of 5 as the dominant wavelength of variations increases significantly. However, if the variability in carbonate concentration is described in terms of changes in mean mass accumulation, calculations then suggest that relatively small changes in noncarbonate rates may be important in controlling the observed carbonate records. In addition, the analysis suggests that the degree of variability observed in pelagic carbonate data may in part reflect total accumulation rates. Intervals with high sedimentation rates show lower amplitude variations in concentration than intervals with lower sedimentation rates for the same degree of change in the carbonate accumulation rate.本研究在位于北纬1°、西经114°，水深3903米的572站位，获取了一套连续的液压活塞岩心柱，其沉积序列涵盖中新世晚期至更新世晚期的远洋沉积物。该沉积物由生源碳酸盐与二氧化硅组成，非生源物质仅占次要组分。对碳酸钙（CaCO3）含量的详细分析显示，该赤道太平洋站位的碳酸盐沉积波动程度，在中新世晚期至上新世期间发生了显著变化。此时段内，碳酸盐质量堆积速率从2.6克/(平方厘米·千年)降至0.8克/(平方厘米·千年)。若假设碳酸钙含量变化反映了溶蚀程度的改变，则详细的碳酸盐分析结果表明，随着波动主导波长显著增大，碳酸盐沉积的波动程度会降低至原来的1/5。但若以平均质量堆积速率的变化来描述碳酸盐浓度的波动，则计算结果显示，非碳酸盐组分堆积速率的小幅变化，可能是调控观测到的碳酸盐沉积记录的关键因素。此外，本分析表明，远洋碳酸盐数据中观测到的波动程度，在一定程度上可反映总堆积速率。当碳酸盐堆积速率的变化幅度相同时，高沉积速率层位的浓度波动幅度低于低沉积速率层位。