In the depths of the Bay of Bengal, scientists have discovered a 50,000-year-old sediment — a giant magnetofossil and one of the youngest to be found yet.
What are magentofossils?
Magnetofossils are the fossilised remains of magnetic particles created by magnetotactic bacteria, also known as magnetobacteria, and found preserved within the geological records.
Magnetotactic bacteria are mostly prokaryotic organisms that arrange themselves along the earth’s magnetic field. These unique creatures were first described fairly recently, in 1963, by Salvatore Bellini, an Italian doctor and then again in 1975 by Richard Blakemore of the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts.
These organisms were believed to follow the magnetic field to reach places that had optimal oxygen concentration. Using an electron microscope, Blakemore found the bacteria contained “novel structured particles, rich in iron” in small sacs that essentially worked as a compass.
These magnetotactic bacteria create tiny crystals made of the iron-rich minerals magnetite or greigite, inside their cells The crystals help them navigate the changing oxygen levels and saturated sediment in the water body they reside in. The crystals within the cells are arranged in a chain configuration in a process called magnetotaxis.
However, giant magnetofossils are much rarer than conventional magnetofossils and are most likely produced by eukaryotes.
The fossils left behind by the crystal-creating bacteria help scientists glean conditions that prevailed millions of years ago and which contributed to “the sediment magnetic signal”.
What makes the Bay of Bengal sediment special?
In previous studies on magnetofossils often ascertained their origins to be hyperthermal vents, comet impacts, changes in oceanic ventilation, weathering or the presence of oxygen-poor regions.
Sediments deposited at the core site originate predominantly from the Godavari, Krishna, and Penner Rivers, highlighted on the map. | Photo Credit: Kadam, N et al/Nature
In fact, the origin of giant magnetofossils has remained a mystery. Most giant magnetofossils have been found in sediments dating to two geological time periods — Paleocen-Eocene Thermal Maximum (roughly 56,000 million years ago) and Middle Eocene Climatic Optimum (about 40 million years ago) — both of which were known for a rise in global temperature. This suggested that the magnetofossils formed only during periods of extreme warming.
In an email to The Hindu, Dr Firoz Kadar Badesab, one of the lead of authors of the study and a senior scientist at the National Institute of Oceanography, Goa, said, “However, our present study challenges this assumption by discovering giant magnetofossils in a geologically recent sediments from the Bay of Bengal, making them a modern analogue.”
In the new study, published in the journal Nature in February, Dr Firoz and his team found that the sediment from the Bay of Bengal to be from the late Quaternary period, or about 50,000 years ago, making it the youngest giant magnetofossil to have been found found yet.
Shedding light on the samples collected, Dr Firoz said, “The sediments examined in the present study cover only the last 42.7 thousand years and not been formed during the Paleocene-Eocene Thermal Maximum or the Middle Eocene Climatic Optimum.”
What did the study find?
In the study, scientists at the CSIR-National Institute of Oceanography, Goa, combined magnetic analyses and electron microscopy to study the sediment sample.
The three-metre-long sediment core from the southwestern Bay of Bengal consisted mainly of “pale green silty clays,” they wrote in their paper. They also reported finding abundant benthic and planktic foraminifera — single-celled organisms with shells found near the sea bed and free-floating in water.
High-resolution transmission electron microscopy revealed the fossil to be in the shape of needles, spindles, bullets, and spearheads. The microscopy also confirmed the presence of ‘conventional’ magnetofossils along with giant ones.
At a depth of around 1,000-1,500 m, the Bay of Bengal has a distinctively low oxygen (suboxic) concentration.
Earlier, studies of sediments suggested that nearly 29,000 to 11,700 years ago, during the last Glacial Maximum-Halocene period, the northeast and southwest monsoon strengthened and resulted in significant weathering and sedimentation.
Analysis of the sediment sample confirmed fluctuations in the monsoon took place as the scientists found particles of magnetic minerals from the two distinct geological periods.
The scientists also suggested the rivers Godavari, Mahanadi, Ganga-Brahmaputra, Cauvery, and Penner, which empty into the Bay of Bengal, played a crucial role in the formation of the magnetofossils.
“Our study suggests that a warming event is not required for the growth of giant magnetofossil-producing organisms. Rather, an optimum balance of iron and organic carbon with suboxic conditions in water is required,” he said. “Therefore, giant magnetofossils in these sediments can encode information about nutrient availability, oxygen conditions and water stratification in ancient aquatic environments.”
The nutrient-rich sediment carried in by these rivers provided a sufficient supply of reactive iron, which combined with the available organic carbon in the suboxic conditions of the Bay of Bengal to create a favourable environment for the growth of magnetotatic bacteria.
The freshwater discharge from these rivers along with the other oceanographic processes, like eddy formation, rendered the oxygen content in these waters that isn’t usually found in other low-oxygen zones.
Highlighting the significance of the find, Dr Firoz said, “Iron-biomineralising organisms are sensitive to changes in iron, organic carbon, and oxygen concentrations in the water/sediment. Hence, the preserved remains of the magnetofossils provide information about the past environmental conditions on geological timescale.”
“Our study, for the first time, reports the occurrence of giant magnetofossils in the late Quaternary Bay of Bengal sediments including the last glacial period and suggests that the sustained suboxic conditions in the Bay of Bengal during the late Quaternary supported the proliferation of giant magnetofossils,” he added.
