Sedimentation rates and geochemistry of three cores from eastern tropical Pacific

We have generated approx. 300 Kyr records of biogenic opal, calcite, and organic carbon (Corg) for three cores in the eastern and central equatorial Pacific Ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region. We find that Corg has been deposited in common pulses throughout the area, while opal has a much more local pattern of variation. Calcite varies regionally, but the record is shaped by superimposed dissolution and productivity processes. The most intense Corg peak occurs at 18 ka and can have greater than 2 times the Holocene Corg content. Other major Corg peaks occur 150 ka and perhaps at 280 ka. We have compared the Corg record in one of the cores, V19-28, to a model deepwater oxygen record developed from d13C data in the nearby V19-30 to test whether the Corg record has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone. We found no coherence between the two records, implying that the Corg record is primarily a measure of productivity. By comparing the opal, calcite, and Corg records in V19-28, a core which is at or above the lysocline, we found that both increased calcite and opal deposition matches high Corg accumulation. We also found, however, that the calcite and opal records were uncorrelated, so that episodes of high opal deposition do not necessarily accumulate calcite rapidly. We hypothesize that at least two different plankton communities have been dominant in the waters above this site, one rich in opal-secreting plankton and one more dominated by calcite producers. The opal-rich plankton community was dominant during the intervals 10-15 ka and 35-60 ka.

我们针对赤道东太平洋和中太平洋的三个岩芯，生成了约300千年（Kyr）的生物成因蛋白石（biogenic opal）、方解石（calcite）及有机碳（Corg）记录，并通过对比这些记录，探究该区域是否存在统一的生物沉积作用（biogenic sedimentation）时期。研究发现，有机碳（Corg）在整个区域内呈统一脉冲式沉积，而蛋白石的变化模式则具有更强的局域性。方解石的变化具有区域性，但其记录受溶解作用与生产力过程的叠加影响所塑造。有机碳（Corg）含量的最显著峰值出现在18千年（ka），其含量可达到全新世（Holocene）有机碳含量的两倍以上。其他主要的有机碳（Corg）峰值出现在150千年（ka），可能还存在于280千年（ka）。我们将其中一个岩芯（V19-28）的有机碳（Corg）记录，与基于邻近岩芯V19-30的δ¹³C数据构建的深水氧气模型记录进行对比，以验证有机碳（Corg）记录主要是由降解作用还是真光层（euphotic zone）有机物质沉降所主导。结果显示两者记录之间无相关性，这表明有机碳（Corg）记录主要反映了生产力水平。通过对比位于溶跃面（lysocline）或其上方的岩芯V19-28中的蛋白石、方解石及有机碳（Corg）记录，我们发现方解石和蛋白石沉积量的增加均与有机碳（Corg）的高积累量相匹配。然而，我们也发现方解石与蛋白石记录之间无相关性，因此高蛋白石沉积期并不一定伴随方解石的快速积累。我们推测，该区域上方水体中至少存在两种不同的优势浮游生物群落（plankton communities）：一种以分泌蛋白石的浮游生物（opal-secreting plankton）为主，另一种则以方解石生产者（calcite producers）为主。富含蛋白石的浮游生物群落在10-15千年（ka）及35-60千年（ka）期间占据主导地位。