In a fascinating glimpse into Earth's ancient past, a massive volcanic eruption has been linked to a catastrophic event that turned the oceans deadly, resulting in a mass extinction of tiny plankton species. This event, which occurred approximately 113 million years ago, left an indelible mark on the fossil record and provides a sobering reminder of the profound impact of geological forces on marine life.
The Plankton's Plight
The story begins with the planktonic foraminifera, single-celled organisms that drift through the upper ocean, building intricate shells from calcium carbonate. These microscopic creatures, no wider than a grain of sand, play a disproportionately large role in the carbon cycle, contributing up to half of the annual calcium carbonate production in the oceans.
However, during the transition from the Aptian to the Albian age, something catastrophic occurred. Large plankton species with thick, robust shells, which were once common, suddenly disappeared from the rock record. In their place, smaller species with thin, fragile shells emerged.
A Tale of Two Oceans
What makes this event particularly intriguing is the contrast between the upper and lower oceans. While the plankton near the surface experienced a catastrophic die-off, their relatives living on the seafloor remained relatively unscathed. This disparity can be attributed to the unique nature of the volcanic eruption that triggered this event.
The eruption, which occurred on the Kerguelen Plateau, a vast volcanic province in the southern Indian Ocean, released carbon dioxide into the atmosphere rather than directly into the ocean. This meant that the surface waters became acidic first, with the deep ocean experiencing the effects much later. This sequence of events is similar to what we observe with modern ocean acidification, where the surface waters are affected first, leaving the deep ocean relatively protected, at least initially.
The Role of Alkalinity
One key factor in the survival of the seafloor plankton is alkalinity. Building their shells requires plankton to draw alkalinity from the seawater, which helps neutralize acid. When the surface plankton reduced their shell-building activities, more alkalinity remained in the water, eventually circulating downward and providing additional buffering capacity for the deep ocean.
However, the seafloor was not entirely spared. Near the boundary, a larger proportion of bottom-dwelling foraminifera were species that constructed their shells by gluing sediment grains together rather than using calcite. This suggests that even in the deep ocean, the effects of the volcanic eruption were felt, albeit to a lesser extent.
Implications for Today
This ancient event holds important lessons for our modern world, where ocean chemistry is once again undergoing significant changes. The surface ocean has already crossed a critical threshold, with acidity rising by approximately 30% in the past 200 years. As we continue to study this ancient event, we gain a deeper understanding of the potential consequences of our actions on the delicate balance of our oceans.
While there are still gaps in our knowledge, such as the exact amount of carbon dioxide in the atmosphere at the time of the eruption and the precise dating of the Kerguelen eruptions, ongoing research is shedding light on these mysteries. The team led by Jonathan Chen is already conducting further calcium measurements at other extinction intervals, including the event that led to the demise of the dinosaurs.
As we delve deeper into the past, we uncover not only the stories of ancient life but also a cautionary tale for our own time. The oceans, it seems, have a remarkable capacity for resilience, but only if we heed the lessons of the past and take action to protect them.
