Biogeochemistry of the North Atlantic during oceanic anoxic event 2: role of changes in ocean circulation and phosphorus input

[摘要] The geological record provides evidence for the periodic occurrence of watercolumn anoxia and formation of organic-rich deposits in the North AtlanticOcean during the mid-Cretaceous (hereafter called the proto-North Atlantic). Bothchanges in primary productivity and oceanic circulation likely played a rolein the development of the low-oxygen conditions. Several studies suggest thatan increased input of phosphorus from land initiated oceanic anoxic events(OAEs). Other proposed mechanisms invoke a vigorous upwelling system and anocean circulation pattern that acted as a trap for nutrients from the PacificOcean.

Here, we use a detailed biogeochemical box model for the proto-North Atlanticto analyse under what conditions anoxia could have developed during OAE2 (94 Ma).The model explicitly describes the coupled water, carbon, oxygen andphosphorus cycles for the deep basin and continental shelves. In oursimulations, we assume the vigorous water circulation from a recent regionalocean model study. Our model results for pre-OAE2 and OAE2 conditions arecompared to sediment records of organic carbon and proxies for photic zoneeuxinia and bottom water redox conditions (e.g. isorenieratane,carbon/phosphorus ratios). Our results show that a strongly elevated input ofphosphorus from rivers and the Pacific Ocean relative to pre-OAE2 conditionsis a requirement for the widespread development of low oxygen in theproto-North Atlantic during OAE2. Moreover, anoxia in the proto-NorthAtlantic is shown to be greatly influenced by the oxygen concentration ofPacific bottom waters. In our model, primary productivity increasedsignificantly upon the transition from pre-OAE2 to OAE2 conditions. Our modelcaptures the regional trends in anoxia as deduced from observations, witheuxinia spreading to the northern and eastern shelves but with the mostintense euxinia occurring along the southern coast. However, anoxia in thecentral deep basin is difficult to achieve in the model. This suggests thatthe ocean circulation used in the model may be too vigorous and/or thatanoxia in the proto-North Atlantic was less widespread than previouslythought.

[学科分类] 地球化学与岩石