Abstract
Abstract. The late Paleocene is characterized by warm and stable climatic conditions that served as the background climate for the Paleocene–Eocene Thermal Maximum (PETM, ~55 million years ago). With respect to feedback processes in the carbon cycle, the ocean biogeochemical background state is of major importance for projecting the climatic response to a carbon perturbation related to the PETM. Therefore, we use the Hamburg Ocean Carbon Cycle model (HAMOCC), embedded in the ocean general circulation model of the Max Planck Institute for Meteorology, MPIOM, to constrain the ocean biogeochemistry of the late Paleocene. We focus on the evaluation of modeled spatial and vertical distributions of the ocean carbon cycle parameters in a long-term warm steady-state ocean, based on a 560 ppm CO2 atmosphere. Model results are discussed in the context of available proxy data and simulations of pre-industrial conditions. Our results illustrate that ocean biogeochemistry is shaped by the warm and sluggish ocean state of the late Paleocene. Primary production is slightly reduced in comparison to the present day; it is intensified along the Equator, especially in the Atlantic. This enhances remineralization of organic matter, resulting in strong oxygen minimum zones and CaCO3 dissolution in intermediate waters. We show that an equilibrium CO2 exchange without increasing total alkalinity concentrations above today's values is achieved. However, consistent with the higher atmospheric CO2, the surface ocean pH and the saturation state with respect to CaCO3 are lower than today. Our results indicate that, under such conditions, the surface ocean carbonate chemistry is expected to be more sensitive to a carbon perturbation (i.e., the PETM) due to lower CO32− concentration, whereas the deep ocean calcite sediments would be less vulnerable to dissolution due to the vertically stratified ocean.
Highlights
The late Paleocene has received interest because of its role as the background climate for the Paleocene–Eocene Thermal Maximum (PETM), which could have been a possible analog for present-day greenhouse warming and ocean acidification (e.g., Zachos et al, 2005; Ridgwell and Zeebe, 2005; Zeebe and Zachos, 2013)
Using the biogeochemistry model Hamburg Ocean Carbon Cycle model (HAMOCC) coupled with the Max Planck Institute ocean model (MPIOM) ocean general circulation model, we establish a steady-state ocean biogeochemistry simulation with late Paleocene boundary conditions
We present spatial and vertical tracer distributions within a warmer climate and display how the late Paleocene ocean physical state influences the biogeochemistry
Summary
The late Paleocene has received interest because of its role as the background climate for the Paleocene–Eocene Thermal Maximum (PETM), which could have been a possible analog for present-day greenhouse warming and ocean acidification (e.g., Zachos et al, 2005; Ridgwell and Zeebe, 2005; Zeebe and Zachos, 2013). The late Paleocene ocean biogeochemistry has been addressed exclusively with earth system models of intermediate complexity (EMIC) or box models (e.g., Panchuk et al, 2008; Zeebe et al, 2009; Ridgwell and Schmidt, 2010; Winguth et al, 2012) These modeling studies cover the whole PETM, with the major objective of constraining the absolute amount of the carbon perturbation. The depth of the pre-PETM CCD is still under discussion for wider geographical areas of the late Paleocene oceans (Zeebe and Zachos, 2013) This leads to quite different estimates of total carbon mass and carbon injection speed into the climate system necessary for obtaining the observed sedimentary CaCO3 dissolution (Dunkley Jones et al, 2010).
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