Carbon geochemistry of sediments from the Amazon deep sea fan

Sediment cores from the Amazon deep sea fan recovered during R/V Meteor cruise 16-2 show in detail the modern areal distribution of sedimentary organic carbon, stable organic carbon isotopes of the organic matter (OM), as well as variations in the depositional processes. In addition, we studied up to 300 m long drilled sediment records recovered during ODP Leg 155 which allow evaluation of temporal variations on the Amazon fan. Our results reveal new evidence for a very rapid change of fan depositional processes and organic carbon source at times of sea-level change over the middle and lower Amazon fan. To estimate the amount of terrestrial organic carbon stored in sediments from the last glacial in the Amazon fan we used stable organic carbon isotopes of the OM (delta13Corg), organic carbon content (Corg), and age models based on oxygen isotopes, faunal data, and magnetic excursions. Following our results, the organic carbon accumulation on the Amazon deep sea fan is controlled by glacio-eustatic sea-level oscillations. Interglacial sea-level high stand sediments are dominated by marine OM whereas during glacial sea-level low stands terrestrial organic carbon is transported beyond the continental shelf through the Amazon canyon and deposited directly onto the Amazon deep sea fan. Glacial sediments of the Amazon fan stored approximately 73*10**15 g terrestrial Corg in 20,000 years or 3.7*10**12 g terrestrial Corg/yr (equivalent to 7-12% of the riverine organic carbon discharge; assuming constant paleo discharge), which is about the same amount of terrestrial organic carbon as deposited on the Amazon shelf today (3.1*10**12 g terrestrial Corg/yr or 6-10% of the modern riverine organic carbon discharge).

本研究依托科研考察船"流星号"（R/V Meteor）第16-2航次获取的亚马逊深海扇（Amazon deep sea fan）沉积物岩芯，详细揭示了沉积有机碳的现代空间分布特征、有机质（organic matter, OM）的有机碳稳定同位素组成，以及沉积作用过程的变化规律。此外，本研究还对大洋钻探计划（Ocean Drilling Program, ODP）第155航次获取的长达300米的钻探沉积物记录开展分析，以此评估亚马逊深海扇的时间尺度沉积变化。研究结果为中低纬度亚马逊深海扇在海平面变化时期，扇体沉积作用过程与有机碳源的快速突变提供了新的直接证据。为估算亚马逊深海扇末次冰期沉积物中赋存的陆生有机碳储量，本研究采用了有机质的有机碳稳定同位素（δ¹³Corg）、有机碳含量（Corg），以及基于氧同位素、生物化石数据与地磁漂移事件建立的年代学模型。研究结果显示，亚马逊深海扇的有机碳堆积量受冰盖-全球海平面升降旋回（glacio-eustatic sea-level oscillations）调控：间冰期海平面高位期的沉积物以海相有机质为主；而冰期海平面低位期时，陆生有机碳可通过亚马逊峡谷跨越大陆架，直接沉积于亚马逊深海扇。亚马逊深海扇的冰期沉积物在20000年期间累计存储了约73×10¹⁵克陆生有机碳，年均堆积量达3.7×10¹²克/年（假设古河流输出通量恒定，该值相当于亚马逊河流有机碳入海输出量的7%-12%），这一储量与现今亚马逊陆架上沉积的陆生有机碳总量大致相当——现今亚马逊陆架陆生有机碳年均堆积量为3.1×10¹²克/年，占现代河流有机碳入海输出量的6%-10%。