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.

本研究针对东赤道太平洋与中赤道太平洋的3支岩芯，生成了约30万年的生物成因蛋白石（biogenic opal）、方解石（calcite）与有机碳（Corg）记录，并对这些记录开展比对分析，以探究整个区域是否存在共同的生物沉积周期。研究发现，有机碳（Corg）在全区范围内呈现共同的脉冲式沉积特征，而蛋白石的变化则具有更强的局域性；方解石的变化呈现区域特征，但其记录受叠加的溶解作用与生产力过程共同调控。规模最大的有机碳峰值出现在18 ka，其有机碳含量可达全新世水平的2倍以上，其余主要有机碳峰值分别出现在150 ka以及可能的280 ka时段。我们将其中一支岩芯V19-28的有机碳记录，与基于邻近岩芯V19-30的δ13C数据构建的深水氧气记录模型进行比对，以检验有机碳记录主要受控于有机质降解过程，还是源自真光层（euphotic zone）的有机质沉降通量。研究发现二者记录并无相关性，这表明有机碳记录主要反映了区域生产力水平。通过对位于溶跃面（lysocline）及以上的岩芯V19-28的蛋白石、方解石与有机碳记录进行比对，我们发现方解石沉积通量与蛋白石沉积通量的升高，均对应着有机碳的高堆积速率。但同时我们也发现，方解石与蛋白石的记录并无相关性，即蛋白石沉积高值时段未必伴随方解石的快速堆积。我们据此提出假说：该站位上方水体中至少存在两类占主导的浮游生物群落，一类以分泌蛋白石的浮游生物为主，另一类则以产方解石的生物占优。其中，以分泌蛋白石的浮游生物为主的群落，在10~15 ka与35~60 ka时段占据主导地位。