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Carbonate deposition, climate stability, and Neoproterozoic ice ages

Andy J. Ridgwell, Martin J. Kennedy, and Ken Caldeira · Science 302, 859-862 · 2003

Key finding. The evolutionary success of planktic calcifiers during the Phanerozoic stabilized the climate system by introducing saturation-dependent preservation of carbonate in sea-floor sediments as a new buffering mechanism — before which buffering depended on shallow-water carbonate deposition alone, leaving the Precambrian carbon cycle far more sensitive to the loss of shallow-water environments.

Four schematic cross-sections through an ocean margin, from shelf to abyssal plain. Panels A and B show the modern system before and after a fall in sea level, with carbonate produced by plankton in the open ocean and buried both on the shelves and in the deep sea, and a marked lysocline separating carbonate-rich from carbonate-poor sediments. Panels C and D show possible Neoproterozoic states, in which carbonate is buried only in shallow shelf environments because open-ocean calcifying plankton had not yet evolved.
Before calcifying plankton evolved, carbonate could only be buried in shallow water (C and D), so a loss of shelf area removed the buffer entirely. In the modern system (A and B) deep-sea burial responds to saturation state and stabilises ocean chemistry regardless of sea level. Figure 1 from Ridgwell, Kennedy, and Caldeira (2003), Science 302, 859-862. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

The Neoproterozoic, before about 540 million years ago, saw ice ages of near-global extent lasting millions of years, together with distinctive cap carbonate deposits. Nothing comparable has happened since.

This paper asked what changed. Specifically, whether the evolutionary rise of shell-building plankton altered how the carbon cycle buffers itself, and whether that shift can account for the difference between the Precambrian world and the one that followed.

What did we find?

Before planktic calcifiers evolved, ocean carbonate-ion concentration was buffered mainly by adjustments in shallow-water carbonate deposition balancing the input of weathering products from land.

Once calcifying plankton became abundant, carbonate began raining onto the deep sea floor, where the fraction preserved depends on saturation state. That dependence is a feedback, and it operates over the whole ocean rather than only in shallow shelf seas.

The older mechanism was fragile because it was geographically confined. A loss of shallow-water environments removed the buffer, leaving the carbon cycle exposed.

Neoproterozoic ice ages of near-global extent and multi-million-year duration, and the cap carbonates that follow them, become intelligible in those terms — a weakly buffered carbon cycle combined with the feedback between CO2, climate, and ice-sheet growth.

Why does it matter?

The paper makes a strong claim about the relationship between life and planetary stability. Marine plankton are not merely passengers of the climate system; their evolution changed how strongly that system resists perturbation.

It also explains an absence, which is harder than explaining an event. Any account of the Neoproterozoic ice ages must also explain why nothing similar has occurred in the half-billion years since, and a change in buffering capacity does both at once.

Citation

Andy J. Ridgwell, Martin J. Kennedy, and Ken Caldeira (2003). Carbonate deposition, climate stability, and Neoproterozoic ice ages. Science 302, 859-862.

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