Across the seafloor of the equatorial Pacific Ocean, scientists have discovered unusual pits. Similar depressions have been found around the world, caused by fluids escaping from sediments or salt dissolving in rocks below the seabed. However, during 40 years of drilling into the tropical Pacific Ocean’s seafloor, no one has found pressurized fluids to explain how these pits formed. Another mechanism for the creation of seafloor depressions is the flow of hot, hydrothermal fluids circulating in the upper crustal rocks. Read More
The highly fractured crust is formed at the ocean ridge spreading centers. It slowly moves away from these centers and is blanketed by the calcareous and silicious shells of tiny planktonic plants and animals falling from near-surface waters. This sediment blanket effectively seals the waters circulating in the upper crust.
To investigate the cause of the pits in the Pacific Ocean, Dr. Ted Moore and an international team of co-workers used imaging techniques on over 60,000 square kilometers of tropical Pacific Ocean seafloor. The team discovered that most of the pits occur over high points of the underlying crust, known as “basement highs”. The pits are between 50 and 100 meters deep and often several kilometers wide.
Seismic images revealed that the pits are associated with disrupted sediment reflections over basement highs. This disruption is evidence of faulting in the sediments between the pits and basement highs.
The greater compaction of the thicker surrounding sediments results in tensional stress in the thinner sediments capping the basement highs. This tension produces cracks in the sediments that might allow waters circulating in the crust to escape.
Dr. Moore and his colleagues suggest that depressions were created by hydrothermal fluids discharging through cracks within the sediment atop the basement highs. But what evidence is there for hydrothermal activity?
The effects of hydrothermal circulation of waters can be seen when looking at the pore waters in drill core sediments taken from the Pacific Ocean seabed. The concentration of calcium and strontium ions tend to increase with depth. However, this trend is reversed in sediments directly overlying basement rock, becoming similar to pore waters in near-surface sediments.
In addition, siliceous shells in sediments overlying basement rock have been dissolved, and carbonate shells show signs of carbonate precipitation on their outer surfaces. These changes are consistent with calcium carbonate precipitating and siliceous shells dissolving in warm waters.
If hydrothermal fluids flow through basement rocks and sediment, how does this create pits on the seafloor? According to Dr. Moore’s team, as the hydrothermal water moves up through the sediment cracks, it cools to the near-freezing temperatures of the deep ocean. With cooling, the waters become very corrosive to carbonate and dissolve carbonate shells. Thus, a seafloor depression is formed by the fracturing of the sediment and dissolution of carbonate in the sediments.
The ascending cooler hydrothermal waters are much less corrosive to siliceous shells. The venting waters carry up the tiny shells of ancient siliceous plankton and spread them out on the surrounding seafloor. These older shells are found in near-surface sediments throughout the tropical Pacific. The full explanation for their presence has eluded scientists until now.
After decades of studies, the researchers have shown that hydrothermal flow and expulsion through overlying sediment are likely causes of pit formation in the tropical Pacific Ocean. Documenting the hydrothermal circulation of waters in older ocean crust is important for understanding the impact of these fluids on the chemistry and ecology of the deep ocean.
