The Cambrian Period, spanning approximately 538.8 to 485.4 million years ago, is defined by the rapid appearance and diversification of complex, multi-celled life, known as the Cambrian Explosion. Understanding the prevailing climate is fundamental to explaining the environmental conditions that allowed this biological revolution to occur. The stable, warm, and chemically unique environment of the Cambrian set the stage for the evolution of nearly all modern animal body plans.
Global Thermal Conditions
The Cambrian world was characterized by sustained global warmth, contrasting sharply with the severe “Snowball Earth” glaciations that immediately preceded it in the Neoproterozoic Era. Evidence suggests that for much of the period, there were no major continental ice sheets, as continents were not situated at the poles where ice could accumulate. This lack of glaciation indicates a stable, greenhouse climate persisted across the planet.
Average global temperatures during the Cambrian were significantly higher than they are today, averaging around 22 °C compared to the modern average of about 14 °C. Tropical sea surface temperatures were particularly elevated, ranging from 28 °C to as high as 38 °C. While the overall trend was warm and stable, some evidence suggests episodic cold periods in the late Cambrian, indicating the climate was not entirely uniform.
Atmospheric Composition and Greenhouse Gases
The warmth of the Cambrian climate was driven by an atmosphere rich in greenhouse gases, primarily carbon dioxide (\(text{CO}_2\)). Estimates suggest \(text{CO}_2\) concentrations were as high as 4,000 parts per million (ppm) near the start of the period, a dramatic contrast to the modern level of approximately 420 ppm. This high concentration was maintained by intense geological activity, particularly outgassing from volcanism associated with the movement of continental plates.
Oxygen (\(text{O}_2\)) levels continued the long-term rise that began in the Precambrian due to early life’s photosynthetic activity. Atmospheric oxygen was still lower than today, likely ranging between 3% and 14% compared to the modern 21%. The combination of high temperatures and lower \(text{O}_2\) resulted in less dissolved oxygen in the oceans, leading to widespread anoxia, or oxygen-depleted zones, in the deeper marine basins. The lack of a robust ozone layer also meant that ultraviolet radiation reached the surface, confining most complex life to the protective depths of the oceans.
Continental Drift and Shallow Seas
The physical arrangement of the continents was a major control on the Cambrian climate, particularly for the marine environment where life flourished. The supercontinent Pannotia had begun to break apart, and the massive continent of Gondwana was forming, situated largely in the Southern Hemisphere. This tectonic activity, characterized by the rifting of continents, led to a significant increase in global sea levels.
The rising seas flooded the low-lying continental interiors, creating vast, shallow, and sunlit marine environments known as epeiric or epicontinental seas. These shallow waters were generally warm, well-mixed, and oxygenated. These extensive, stable habitats buffered temperature extremes and provided favorable conditions for the rapid proliferation and spread of marine invertebrates.
Climate’s Influence on Biological Diversification
The unique climate and geological factors of the Cambrian created an environmental window responsible for the sudden burst of life. The stable, warm temperatures and the extensive, shallow seas provided an increase in habitable space with high energy input from the sun. The warm, oxygenated waters of the continental shelves offered a more favorable environment than the deeper, anoxic ocean basins, allowing organisms to develop complex metabolisms.
High atmospheric \(text{CO}_2\) drove the high concentration of dissolved calcium carbonate in the warm oceans. This chemical availability was a prerequisite for the development of hard parts, such as shells and skeletons, which defines the fossil record of the Cambrian Explosion. The combination of environmental stability, abundant resources, and rising oxygen provided the necessary conditions for the evolution of new body plans, enabling the transition to diverse, mineralized organisms.