Double scavenging processes explain the vertical distribution of rare earth elements in the oceans: Importance of surface plankton as a primary scavenger and carbonate/oxide as a secondary scavenger

抄録

To examine the hypothesis that vertical distribution of rare earth elements (REEs) in the oceans are controlled by surface plankton, a self consistent algorithm was developed, in which reported vertical profiles of REEs and nutrients (Si or P) are given variables with the REE composition of surface plankton being one of the solutions. In the calculations the composition of surface plankton (primary scavenging) was tuned such that the sum of two relative standard deviations (RSDs), i.e., the RSD of partitioning pattern of secondary scavenging at sampling depths of all REEs and the RSD of discrepancy of the reproduced vertical profiles from those observed at all depths, is minimized, where the partitioning pattern of scavenging is obtained by averaging the obtained partitioning pattern of scavenging at all depths. In the Pacific Ocean the calculation was successful in both the Si- and P-based calculations; whereas in the Indian and Atlantic Oceans the calculation was much more successful in the P-based calculation than the Si-based calculation. This implies that diatom frustules and organic components in surface plankton comprise primary scavengers: the former mainly in the Pacific Ocean and the latter possibly in all three oceans examined. In the successful calculations the results were consistent: the calculated composition of surface plankton is typically enriched with heavier (H) REEs against that of shale; the obtained partitioning patterns of the scavenging consistently showed maxima around lighter (L) + middle (M) REEs and slight increases toward Lu. In the North Pacific Ocean, if Si is regarded as a representative nutrient, the composition of surface plankton thus calculated is almost identical to the reported composition of diatom frustules. Carbonate/oxide and organic matter are considered to be the main secondary scavengers of L+M and H REEs, respectively. The residence times of REEs can be calculated relative to the turnover rate of nutrient. Those of M+HREEs are typically in the range of 1000–2500 years in the three oceans; the residence times of LREEs in the Pacific Ocean (1000–2500 years) are longer than those of the other two oceans (<1000 years).