Carbon cycle responses to changes in weathering and the long-term fate
of stable carbon isotopes
Abstract
The causes of the variations in CO2 of the past million
years remain poorly understood. Imbalances between the input of elements
from rock weathering and their removal from the
atmosphere-ocean-biosphere system to the lithosphere likely contributed
to reconstructed changes. We employ the Bern3D Earth system model of
intermediate complexity to investigate carbon-climate responses to
step-changes in the weathering input of phosphorus, alkalinity, carbon,
and carbon isotope ratio (δ13C) in simulations
extending up to 600,000 years.
CO2 and climate approach a new equilibrium within a few
ten thousand years, whereas the equilibration lasts several hundred
thousand years for δ13C. These timescales represent a
challenge for the initialization of sediment-enabled models and
unintended drifts may be larger than forced signals in simulations of
the last glacial-interglacial cycle. Changes in dissolved
CO2 change isotopic fractionation during marine
photosynthesis and δ13C of organic matter. This
mechanism and changes in the organic matter export cause distinct
spatio-temporal perturbations in δ13C of dissolved
inorganic carbon.
A cost-efficient emulator is built with the Bern3D responses and applied
in contrasting literature-based weathering histories for the past
800,000 years. Differences between scenarios for carbonate rock
weathering reach around a third of the glacial-interglacial
CO2 amplitude, 0.05 ‰ for δ13C, and
exceed reconstructed variations in marine carbonate ion. Plausible input
from the decomposition of organic matter on shelves causes variations of
up to 10 ppm in CO2 , 4 mmol m−3 in
CO2−3, and 0.09‰ in
δ13C. Our results demonstrate that weathering-burial
imbalances are important for past climate variations.