A 50-million-year tectonic pause stabilized the climate, allowing trees to flourish. A new study reveals that Earth's climate experienced chaotic and calm periods during the Late Paleozoic era, 360 to 250 million years ago.
During this prolonged period of tectonic inactivity, orbital rhythms stabilized temperature and rainfall, and organic carbon accumulated in rocks that later formed coal. The research, led by Zhijun Jin, an academician and geoscientist at Peking University, delves into the intricate relationship between plate motions and climate over deep time.
The study divided the interval into three distinct phases. Two active periods, from 360 to 330 million years ago and again from 280 to 250 million years ago, were separated by a calmer middle phase between 330 and 280 million years ago.
During the active periods, volcanic carbon dioxide levels rose, leading to increased climate variability. In contrast, the quiet middle phase witnessed a decline in carbon dioxide, stable ice formation, and a synchronization of seasonal patterns with orbital rhythms.
Jin emphasized that each phase exhibited its own climate signature, influenced by tectonic activity and carbon dioxide fluctuations.
The quiet phase held significance as orbital cycles altered sunlight distribution across latitudes. This phase resulted in shorter and more consistent sea level cycles, while active intervals stretched and blurred their timing. Evidence suggests that orbital pacing can influence organic carbon burial on shorter time scales, explaining why calm conditions transform subtle astronomical rhythms into durable rock records.
The middle phase also favored the growth of widespread forests and wetlands near the equator, enhancing organic carbon burial and long-term storage of dead biomass in sediments.
The study's focus on the tightness of short cycles during each phase revealed steady climate pacing during quiet intervals and unstable conditions during broader spreads. Researchers also evaluated the clarity of orbital pacing alignment in each interval, indicating stronger influence when tectonic forcing was low.
The tectonic climate theory was tested using plate reconstructions, geochemical markers, and paired climate and carbon models. The orbital solution precisely tracked insolation over 250 million years. A widely used Paleozoic sea level curve provided a benchmark to assess changes in short-period cycles over time.
The study also examined subduction and ridge length over time, revealing longer ridges and faster recycling as indicators of stronger volcanic outgassing. Model runs with 400 and 800 parts per million carbon dioxide demonstrated a clear pattern: higher carbon dioxide levels led to more significant month-to-month temperature and rainfall fluctuations.
The quiet middle phase resulted in the formation of coal and organic-rich shales across multiple basins, with warm and humid tropics between 0 and 40 degrees latitude being prime zones for organic carbon burial. Under calm tectonics, astronomical forcing could guide ice growth and sea level with regular beats, facilitating the long-term storage of carbon.
In contrast, active tectonics disrupted habitats and muddled sedimentary signals due to frequent carbon dioxide pulses and shifting shorelines. The authors argue that low variability supports ecosystem productivity and organic matter burial, while high variability shortens growing seasons and depletes soil and shelf nutrients.
While deep history doesn't set policy, it clarifies climate physics. Rising carbon dioxide intensifies natural climate swings, making them more sensitive to external influences. This sensitivity allowed orbital changes to impact the system during active tectonic periods, while quiet intervals gave the cosmos a dominant role.
The study concludes that energy balance controls natural swings, and carbon plays a significant role in this balance. Buried carbon is not permanently lost; it can resurface through volcanic activity as tectonic plates shift.
