Geochemistry of terrigenous material in deep-sea sediments of the equatorial Pacific

Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs vs. Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from sediment along a cross-equatorial transect in the central equatorial Pacific at 140°W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash, Papua New Guinea, and ocean island basalt) varies through time, latitude, and climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no connection between a particular Fe source or pathway (eolian vs. Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene "Biogenic Bloom". This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, may be responsible for past changes in total export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes. Additionally, perhaps a change in Fe source or flux is related to a change in a particular component of the total productivity (e.g., the production of organic matter, calcium carbonate, or biogenic opal).

现代赤道太平洋为高营养盐低叶绿素区域，其当前的生物生产力受微量营养元素铁（Fe）限制。为验证过去铁是否曾为生物生产力的限制因子，并识别可能驱动铁施肥作用的铁输入潜在路径（即风成输入的风尘输送 vs 赤道潜流（Equatorial Undercurrent）供给的铁），我们对太平洋中部赤道海域140°W跨赤道断面的沉积物，以及东赤道太平洋大洋钻探计划（Ocean Drilling Program）850站位的沉积物，通过化学方法分离其中的陆源物质。我们采用一系列化学与同位素判别方法，结合多元统计技术，量化了各潜在含铁陆源的贡献占比。研究发现，陆源物质（即亚洲黄土、南美火山灰、巴布亚新几内亚物质与洋岛玄武岩）的分布随时间、纬度与气候条件发生变化。无论采用何种方法确定堆积速率，任一铁源的通量与气候均无关联。此外，在末次冰盛期、更新世冰期事件与中新世生物勃发事件（"Biogenic Bloom"）期间，特定铁源或输入路径（风成路径 vs 潜流路径）与总生物生产力均无关联。这表明，过去开放海域总输出生产力的变化可能由其他过程主导，例如大洋间营养盐库的重组，而非仅由风尘或赤道潜流带来的铁供给所决定。此外，铁源或铁通量的变化可能与总生产力的特定组分变化相关，例如有机质、碳酸钙或生物成因蛋白石的生产。