Geochemical and rock magnetic investigation of sediment cores of the western South Atlantic@en

Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (k) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS- is generated by the process of AOM. The released HS- reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm/kyr during glacial times to 5 cm/kyr in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ~9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.为研究由甲烷厌氧氧化（anaerobic oxidation of methane, AOM）驱动的铁矿物成岩蚀变过程，本次研究对阿根廷与乌拉圭外大陆边缘3个站位的沉积物开展了地球化学与岩石磁学调查。西阿根廷海盆具备高度动态的沉积条件，是开展非稳态过程研究的理想沉积环境。矿物学与全固相数据表明，研究区沉积物以陆源物质为主，且铁矿物含量较高。该类沉积的典型特征为磁化率（k）出现显著低值。孔隙水数据显示，这些磁化率低值与当前硫酸盐-甲烷界面（sulfate/methane transition, SMT）的深度一致，而该界面正是由AOM过程产生硫氢根离子（HS⁻）的区域。所释放的HS⁻与大量铁（羟基）氧化物发生反应，生成硫化铁沉淀，同时伴随磁化率近乎完全丧失。地球化学数据的模拟结果显示，该区域的磁记录深受更新世/全新世过渡期发生的平均沉积速率（sedimentation rate, SR）剧变的影响。我们推测，此次冰期-间冰期过渡期内平均沉积速率的大幅降低，导致硫酸盐-甲烷界面固定于某一特定深度。这种停滞状态显然强化了特定沉积层段内铁（羟基）氧化物的成岩溶解作用。该假设通过数值模拟得到进一步验证：模拟中将冰期的平均沉积速率从100 cm/kyr调整至全新世的5 cm/kyr，并将下方甲烷通量设为恒定值。若要重现观测到的地球化学与磁学特征，硫酸盐-甲烷界面需保持固定位置约9000年，这一计算值与更新世/全新世过渡期的时间高度吻合。模拟结果还显示，稳态条件下恒定的高平均沉积速率会导致孔隙水硫酸盐呈现凹形剖面。