4:00 PM - 4:15 PM
[MIS14-28] Analysis of carbon isotope record at the Cretaceous/Paleogene boundary using an ocean biogeochemical model
Keywords:Cretaceous/Paleogene boundary, Ocean carbon cycle, Biological productivity, Carbon isotope
At the Cretaceous-Paleogene (K/Pg) boundary (66 Ma), an asteroid impact at the Yucatán Peninsula (Mexico) is thought to have triggered a severe mass extinction. The impact event and its environmental and biological consequences left a global geochemical imprint in oceanic carbon isotope records, as measured in planktonic and benthic foraminifera and bulk (nannoplankton) calcite. These data indicate that surface values rapidly decreased immediately after the K/Pg boundary and the collapse of the oceanic bottom-to-surface gradient for several hundred thousand years, followed by a gradual recovery of the vertical gradient (e.g., Zachos et al., 1989).
The collapse of the gradient at the K/Pg boundary has been traditionally explained by a severe drop in oceanic primary and/or export productivity. Representative hypotheses include the global cessation of primary productivity (the "Strangelove Ocean" hypothesis) (Hsü and McKenzie, 1982), the cessation of primary productivity in the open ocean excluding coastal regions (Kump, 1991), and an increase in organic matter decomposition rate in the open ocean, leading to a decrease in export productivity (the "Living Ocean" hypothesis) (e.g., D’Hondt et al., 1998). However, quantitative and comprehensive evaluations of these hypotheses remain insufficient. In addition, the changes in marine biogeochemical cycles immediately after the K/Pg boundary and through the recovery process are not well understood.
In this study, we focused on variations in marine carbon isotope records and developed a one-dimensional vertical ocean carbon cycle model to investigate changes in export productivity and particulate organic matter (POM) decomposition rate after the K/Pg boundary. We assumed that POM remineralization profile depends on export productivity and decomposition rate in this model and calculate these parameters.
First, we prepared scenarios based on several hypotheses and investigated the patterns of carbon isotope variations using forward calculations. The results show that the collapse of the vertical carbon isotope gradient after the K/Pg boundary could be explained by either a decrease in export productivity in the open ocean, an increase in POM decomposition rate or a decrease in POM sinking velocity, provided that primary productivity and carbon burial continued in coastal areas. However, the increased decomposition rate or the decreased sinking velocity scenario would require that POM decomposition rate during diagenesis is lower than previous K/Pg value. The carbon isotope records of intermediate water, particularly at depths of several hundred meters, are considered crucial for distinguishing between these scenarios because of the significant differences of carbon isotope gradient at intermediate depth.
Next, we reviewed geological records and used inversion analysis. In this calculation, we prepared the time-series data of carbon isotopes in surface and deep waters as boundary conditions and imposed the various conditions at intermediate depth to reconstruct the time-series variations of export productivity and the POM decomposition rate. The results indicated that export productivity and organic carbon burial rate in coastal regions rapidly recovered after K/Pg boundary, while export productivity significantly declined in the open ocean for several hundred thousand years. This supports the discussion of Kump (1991). The simultaneous recovery of the vertical carbon isotope gradient and export productivity in the open ocean suggests at least the gradual recovery of lower trophic levels.
The collapse of the gradient at the K/Pg boundary has been traditionally explained by a severe drop in oceanic primary and/or export productivity. Representative hypotheses include the global cessation of primary productivity (the "Strangelove Ocean" hypothesis) (Hsü and McKenzie, 1982), the cessation of primary productivity in the open ocean excluding coastal regions (Kump, 1991), and an increase in organic matter decomposition rate in the open ocean, leading to a decrease in export productivity (the "Living Ocean" hypothesis) (e.g., D’Hondt et al., 1998). However, quantitative and comprehensive evaluations of these hypotheses remain insufficient. In addition, the changes in marine biogeochemical cycles immediately after the K/Pg boundary and through the recovery process are not well understood.
In this study, we focused on variations in marine carbon isotope records and developed a one-dimensional vertical ocean carbon cycle model to investigate changes in export productivity and particulate organic matter (POM) decomposition rate after the K/Pg boundary. We assumed that POM remineralization profile depends on export productivity and decomposition rate in this model and calculate these parameters.
First, we prepared scenarios based on several hypotheses and investigated the patterns of carbon isotope variations using forward calculations. The results show that the collapse of the vertical carbon isotope gradient after the K/Pg boundary could be explained by either a decrease in export productivity in the open ocean, an increase in POM decomposition rate or a decrease in POM sinking velocity, provided that primary productivity and carbon burial continued in coastal areas. However, the increased decomposition rate or the decreased sinking velocity scenario would require that POM decomposition rate during diagenesis is lower than previous K/Pg value. The carbon isotope records of intermediate water, particularly at depths of several hundred meters, are considered crucial for distinguishing between these scenarios because of the significant differences of carbon isotope gradient at intermediate depth.
Next, we reviewed geological records and used inversion analysis. In this calculation, we prepared the time-series data of carbon isotopes in surface and deep waters as boundary conditions and imposed the various conditions at intermediate depth to reconstruct the time-series variations of export productivity and the POM decomposition rate. The results indicated that export productivity and organic carbon burial rate in coastal regions rapidly recovered after K/Pg boundary, while export productivity significantly declined in the open ocean for several hundred thousand years. This supports the discussion of Kump (1991). The simultaneous recovery of the vertical carbon isotope gradient and export productivity in the open ocean suggests at least the gradual recovery of lower trophic levels.