NOAA/WDS Paleoclimatology - Pichevin et al. 2009 Eastern Equatorial Pacific 35KYr Paleonutrient Data

The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere. Primary productivity over large areas of the EEP is limited by silicic acid and iron availability, and because of this constraint the organic carbon export to the deep ocean is unable to compensate for the outgassing of carbon dioxide that occurs through upwelling of deep waters. It has been suggested that the delivery of dust-borne iron to the glacial ocean could have increased primary productivity and enhanced deep-sea carbon export in this region, lowering atmospheric carbon dioxide concentrations during glacial periods. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments, but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP results from a decrease in the silicon to carbon uptake ratio of diatoms under conditions of increased iron availability from enhanced dust input. Consequently, our study supports the idea of an invigorated biological pump in this region during the last glacial period that could have contributed to glacial carbon dioxide drawdown. Additionally, using evidence from silicon and nitrogen isotope changes, we infer that, in contrast to the modern situation, the biological productivity in this region is not constrained by the availability of iron, silicon and nitrogen during the glacial period. We hypothesize that an invigorated biological carbon dioxide pump constrained perhaps only by phosphorus limitation was a more common occurrence in low-latitude areas of the glacial ocean.

现代东赤道太平洋（Eastern Equatorial Pacific, EEP）是向大气释放碳的大型海洋源区。东赤道太平洋大片海域的初级生产力受硅酸与铁有效性的限制，受此约束，向深海输出的有机碳无法补偿因深层水上升流引发的二氧化碳脱气过程。已有研究提出，冰期海洋中粉尘输送的铁输入可能提升了该区域的初级生产力，并强化了深海碳输出，进而降低冰期时段的大气二氧化碳浓度。东赤道太平洋的这一作用得到了其下伏冰期沉积物中更高有机碳埋藏率的佐证，但较低的蛋白石堆积率却对该区域作为冰期显著二氧化碳固存海洋区的重要性提出了质疑。本文报道了一套全新的硅同位素记录，该记录表明，冰期东赤道太平洋蛋白石堆积率的反常下降，源于粉尘输入增强导致铁有效性提升后，硅藻的硅碳摄取比降低。据此，本研究支持末次冰期期间该区域生物泵（biological pump）活性增强的观点，这一过程或对冰期二氧化碳固存起到了积极贡献。此外，结合硅与氮同位素变化的证据，我们推断：与现代状况相反，冰期时段该区域的生物生产力并未受铁、硅与氮有效性的限制。我们提出假说：仅受磷限制的强化型生物碳泵，在冰期海洋的低纬度区域或许更为普遍。