How Well Do We Understand the Geological Carbon Cycle?

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How Well Do We Understand the Geological Carbon Cycle?

The foundation of our understanding of the geological carbon cycle, and how this acts as Earth's "thermostat," was articulated in a seminal paper in nineteen eighty-one. They suggested that silicate weathering on the continents acts as a stabilizing feedback on the carbon cycle such that increased atmospheric carbon dioxide leads to increased weathering rates and hence increased removal of carbon dioxide from the atmosphere. This "textbook model" is at the core of most models of long-term biogeochemical cycles. We summarize evidence that there are many other processes in the geological carbon cycle that may be equally or more important than those in the Walker model. We argue there is a need to move beyond the textbook model in both teaching and research.

Keywords: carbon cycle; feedback; weathering; geological non-uniform Earth system sensitivity

INTRODUCTION AND MOTIVATION FOR THIS ISSUE

"We are so prone to attribute a phenomenon to a single cause, that, when we find an agency present, we are liable to rest satisfied therewith, and fail to recognize that it is but one factor, and perchance a minor factor, in the accomplishment of the total result." Chamberlain eighteen ninety

Carbon dioxide is degassed from the solid Earth into the surface environment (used here to mean the ocean, atmosphere, and biosphere) due to various geodynamic (e.g., magmatism, metamorphism) and surface (e.g., weathering of carbon-rich rocks) processes. As a greenhouse gas, if this carbon dioxide remained in the surface environment, temperatures would continuously increase and Earth would eventually become uninhabitable, like Venus. Fortunately, processes operate to remove carbon from the surface environment, preventing a runaway greenhouse. The return flux of carbon from the surface environment into the solid Earth comes from a variety of biotic and abiotic processes, such as the deposition of organic carbon-rich sediments and the net removal of carbon into carbonate minerals. This cycle of carbon transfer from rocks to the surface environment and back again constitutes the geological carbon cycle. Operating on one hundred five to one hundred six year timescales, the controls on the geological carbon cycle are generally thought to act as a planetary thermostat, both on Earth and potentially on habitable exoplanets. Because the

ELEMENTS, VOL. twenty-two, PP. twelve-seventeen

Calcite, the largest reservoir of carbon in the crust, in thin section.

residence time of carbon in the surface environment is short (less than one million years), differences between the rate of carbon addition to, and removal from, the surface environment over geological timescales could lead to much larger changes in atmospheric carbon dioxide than have occurred. Preventing these large changes in carbon dioxide and temperature requires a tight policing of the carbon cycle by natural processes that link solid Earth and surface processes, and that are a fundamental part of what makes Earth a habitable planet. Better understanding the tempo and pathways of geological carbon cycling is of increasing importance as human-induced perturbations dramatically modify Earth's natural carbon cycle and climate. This is especially true as the need for carbon capture and storage mechanisms, many of which mimic natural processes, become more urgent. However, much of how most scientists think about, and teach, this topic is based on a four-decade-old conceptualization that rapidly became the "textbook model." In this issue, we outline the reasons for moving beyond this "textbook model," both in research and education.

THE TEXTBOOK MODEL

BEYOND THE TEXTBOOK MODEL

Multiple Links and Feedbacks Regulate the Geological Carbon Cycle

The Moderating Role of Ocean Chemistry

A net negative feedback is still required

Testing carbon cycle models using Earth history

LOOKING FORWARD

REFERENCES ELEMENTS

ACKNOWLEDGMENTS

Overview

The document critiques the traditional understanding of the geological carbon cycle articulated in 1981 by Walker et al., emphasizing that many processes are overlooked in this model. It calls for a reevaluation of teaching and research approaches to incorporate these additional factors.

Key Points

1The geological carbon cycle acts as Earth's thermostat through various feedback mechanisms
2The textbook model, established by Walker et al., oversimplifies the complexities of carbon cycling
3Evidence suggests other processes in the geological carbon cycle may be equally or more significant
4A broader understanding is necessary to adapt to human-induced changes in carbon dynamics
5Reevaluating historical models can improve education and research in Earth sciences

Details

Authors: Laurence A. Coogan, Kimberly V. Lau, Jeremy K.C. Rugenstein
Category: Environmental Studies

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