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)."流星号"（R/V Meteor）科考船第16-2航次获取的亚马孙深海扇沉积物岩芯，详细揭示了沉积有机碳的现代空间分布、有机质（organic matter, OM）的稳定有机碳同位素组成，以及沉积过程的演化特征。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米的钻孔沉积记录展开分析，以此探究亚马孙扇的时间尺度沉积变化。本研究结果为亚马孙扇中下段在海平面变化时期，沉积过程与有机碳源的快速转变提供了全新证据。为估算亚马孙扇末次冰期沉积物中储存的陆源有机碳储量，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模型。本研究结果显示，亚马孙深海扇的有机碳堆积过程受冰川-海面升降振荡的调控。间冰期海平面高位域的沉积物以海源有机质为主；而冰期海平面低位域时期，陆源有机碳可通过亚马孙峡谷跨陆架搬运，并直接沉积于亚马孙深海扇之上。亚马孙扇冰期沉积物在2万年的时间尺度内，共储存了约73×10¹⁵克陆源有机碳，年均堆积量达3.7×10¹²克/年；假设古径流量恒定，该值相当于河流有机碳输出量的7%~12%。这一储量与现今亚马孙陆架沉积的陆源有机碳总量相当——现今亚马孙陆架年均堆积陆源有机碳3.1×10¹²克/年，占现代河流有机碳输出量的6%~10%。