Worm burrows and seaweed fossils paint a picture of past climate

Between 56 and 50 million years ago, a range of crustaceans and marine worms burrowed into shallow sediments, during episodes of ancient global warming.

The Gulf of Kutch, on the Arabian Sea on the west coast of India — a mosaic of desert, outcrops and salt flats is now revealing those ancient burrows¹, called ichnofossils, providing scientists a snapshot of climatic conditions from that era.

Trace fossils, a series of cylindrical burrows connected by vertical shafts, show signs of prehistoric biological activity, giving clues into how marine organisms coped with extreme environments.

Ichnofossils such as tracks and resting impressions of animals, belong to specific ichnogenus and ichnospecies. “But it is tricky to match the traces to the trace makers,” according to Mohuli Das, at the Indian Institute of Technology, in Mumbai.

Das studies the trace fossils of the Naredi basin, which evolved over three global warming events, the Eocene Thermal Maximum 2 and 3 and the Early Eocene Climatic Optimum, characterised by rapid rise in global temperatures, atmospheric carbon dioxide and oceanic temperatures over relatively short geological timescales.

Das shows these organisms were trying to adapt to the warming stresses. Their body size, number and diversity, which reduced in response to the warming, rebounded when they reached Early Eocene Climatic Optimum.

They needed time to adjust to the stresses. “The rate of warming today is about 10 times faster than the warming in the past, which was rapid enough to take the benthic dwellers to the brink,” Das says.

In the southern Indian state of Kerala, small outcrops around the historic port town of Kollam harbour signs of ancient seagrass meadows dating back to more recent prehistory — 16 to 20 million years ago or the Miocene. This epoch was 5-8°C warmer and had slightly higher carbon dioxide levels than today.

Pumice-like rock fossils of the segmented green seaweed Halimeda, single-celled organisms Pseudotaberina malabarica, and seagrass-chomping small sea snails, Smaragdia, all hint² at the existence of meadows which were significant carbon stocks that evolved over 100 million years ago but waned globally in the past decades due to climate change and anthropogenic stress.

While P. malabrica has since disappeared, it has close relatives thriving today. Halimeda still drapes modern-day oceans. They have been ecologically successful for a 100 million years, including in the Miocene, when many key components of the Earth as we know it today, such coral reefs had already developed, says marine micropaleontologist, Suman Sarkar, at Birbal Sahni Institute of Palaeosciences.

“If we look at the modern-day distribution of the fossils’ nearest living relatives, it can tell us the most suitable overlapping range where the marine species could grow in today's conditions and also in the future,” Sarkar adds.

In the Lakshadweep archipelago, just 1-2 metres above sea level in the Arabian Sea, A. A. Fousiya, of the Indian Institute of Technology, in Kanpur is prying open giant clams³ Tridacna maxima, the largest bivalve species in the ocean,

Like tree rings, these clams can live for 100 years, creating growth bands in their carbonate shells, capturing snapshots of ocean environment changes as they grow. Because they are fixed, they are accurate climate proxies for specific locations. Fousiya’s imaging and isotopic studies on an 11-year-old, 18 cm-long clam shell taken from the island’s reef showed a slowing growth rate in 2010 corresponding to a massive coral bleaching event triggered by El Nino. But, after a few years, the clam’s growth rate recovered, suggesting a degree of resilience.

With more samples, she plans to use Tridacna shells to map past regional weather extremes such as tropical cyclones and cold surges. “Consistent data from giant clams and old reef-building organisms can fill data gaps in instrumental climate records which cause variability in climate projections, especially for remote regions such as Lakshadweep.”

Lakshadweep corals have also pinpointed divergence between proxy data and model simulations.

A reconstruction⁴ of Arabian Sea pH across 23 years using a massive Porites lutea coral reveals that contrary to a long-term declining trend in pH shown by models, the ocean acidification level didn’t go up, even with a rise in atmospheric carbon dioxide.

This means changes caused by El Niño-Southern Oscillation affect how acidic the Arabian Sea surface water is from year to year, says isotope geochemist, Sambuddha Misra, at the Indian Institute of Science, Bengaluru. The pH reconstruction, the first of its kind for the Arabian Sea, exploited the ability of corals to incorporate elemental boron as borate ions into their skeletons in a way that is influenced by the surrounding seawater pH.

“We need more in-situ ecosystem monitoring and regional proxy-based reconstruction to understand how different parts of the globe will react to climate change, not only in terms of warming but also ocean acidification,” Misra says.