Could corals help study the variability of past Indian monsoons?

Featured image: A coral colony from Maldives, Indian Ocean. Picture credit: Андрей Корман from Pixabay (Public domain)

Paper: Potential of reef building corals to study the past Indian monsoon rainfall variability

Author: Supriyo Chakraborty

Paleooceanographers have often used reef-building corals to study oceanic processes like the El Niño and Southern Oscillation, ocean circulation patterns, air–sea gas exchange, and the Indian Ocean dipole (a.k.a Indian Niño), among others. Yet how exactly do corals provide clues about the physical and chemical conditions of their environments? The answer lies in their skeletons. 

Like some other marine invertebrates, corals extract calcium carbonate from seawater and use it to build their solid skeletal structures. During this process, regulated by the environmental temperatures, they incorporate different ratios of stable isotopes of oxygen, namely, oxygen-18 (18O) and oxygen-16 (16O). Isotopes are variants of a chemical element with similar chemical behaviour but differing physical properties. Oxygen isotopic composition is a universally accepted standard for studying past climates – the same concept that underlies the dating of ice cores, fossils, tree rings, and marine sediments.

In addition to the oxygen isotopes, coral skeletons incorporate trace metals like Strontium, Magnesium, and Uranium along with Calcium. The proportions of each of these trace metals with respect to calcium can provide estimates of past sea surface temperatures and sea salinity.

The Indian monsoon circulation is closely linked to sea surface temperatures and salinity – and is negatively affected by an increase in either. If corals record both parameters, could they provide long-term insights into the seasonal variations of rainfall? A recent study published in Current Science explores the potential of corals in the Indian Ocean basin to study monsoon variability across the Indian subcontinent.

Unlike the Pacific and Atlantic ocean basins, the Indian Ocean is rather landlocked – with the Indian subcontinent to its north. Thereby, its seasonal reversal of circulation is dependent on the differential heating of land and sea. A strong circulation causes cold water from the ocean’s depths to rise to the surface (a process known as upwelling), and leads to a cooling of 2°C-4°C in the Arabian Sea – the northern portion of the Indian Ocean. Free-floating foraminifera – single-celled organisms that are neither plants nor animals, are greatly affected by upwelling, and hence, can be used to study both oceanic and atmospheric processes. Similarly, the researcher was interested in if and how well firmly-attached corals could serve as proxies for studying the link between oceanic processes and monsoon circulation. 

He compared over 12 research papers on coral cores that tried to establish an empirical relation between oxygen isotope values of corals in the Indian Ocean, the sea surface temperature, and past rainfall data from 50-100 years (gathered traditionally, via instruments). The papers span locations across the Indian Ocean: the Gulf of Kutch and the Lakshadweep Archipelago (Arabian Sea), the Andaman and Nicobar Islands (Bay of Bengal), the Chagos Island (Indian Ocean), Mahe (Seychelles), Rasdhoo Atoll in the central Maldives, Marbat (Oman coast), and Ras Umm Sidd and the Dahlak Archipelago in the northern and southern Red Sea, respectively. He then compared the findings from these papers with similar studies on Pacific Ocean corals.

In conclusion, he found that most corals in the Indian Ocean are less sensitive to ocean-atmospheric processes and rainfall than those in Pacific colonies. This provides indirect evidence that the equatorial Pacific plays a greater role in modulating the Indian monsoon than the equatorial Indian Ocean. An exception to this was the coral colony near Lakshadweep. The author proposes a synchronised analysis of coral isotopic data from Lakshadweep and the equatorial Pacific Ocean belt to obtain a long-term record of the onset and withdrawal patterns of monsoon and further clues to the seasonal variability of the Indian monsoon.

Could corals help study the variability of past Indian monsoons? by Devayani Khare is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

What do deep-sea sediment cores tell us about past fish populations?

Black background with fish teeth of different heights and widths

Featured Image: Ichthyoliths (microfossil fish teeth) from deep-sea sediment cores displaying the variety of tooth morphology. Photo courtesy of Elizabeth Sibert, lead author of the paper.

Paper: No state change in pelagic fish production and biodiversity during the Eocene–Oligocene transition

Authors: Elizabeth C. Sibert, Michelle E. Zill, Ella T Frigyik, Richard D. Norris

The seafloor at the bottom of the ocean records what is happening in the water above. Sediments capture silica from diatoms and phytoplankton, carbon from zooplankton poop and detrital marine snow, and teeth after dead fish sink. This last piece of evidence is particularly important: fossilized fish teeth or icthyoliths can help estimate past fish abundance and can show shifts in fish species or biodiversity in the ocean over time.

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New 23 million year record of atmospheric carbon dioxide highlights current human influence on the atmosphere

Paper: A 23 m.y. record of low atmospheric CO2

Featured image: Modern vascular land plants (Raphanus sativus), growing in a carbon dioxide experiment (Figure 1A from Jahren et al., 2008)

Authors: Ying Cui, Brian A. Schubert, A. Hope Jahren

Carbon dioxide is a greenhouse gas, trapping warmth within the Earth’s atmosphere. Sixty years of measurements on Hawaii’s Mauna Loa summit have shown rising amounts of carbon dioxide in our atmosphere. In addition, the carbon dioxide levels in our modern atmosphere are  significantly higher than  those we have seen on Earth over the last 800,000 years, according to measurements on bubbles of ancient air trapped in  Antarctic ice. When combined with measurements of global temperatures, these direct measurements are irrefutable evidence for rapid modern climate change. However, understanding our current position relative to Earth’s climate farther back in time is trickier, since scientists have to estimate atmospheric composition indirectly (through a “proxy”). A new study tackles this problem with a new method of estimating past carbon dioxide, showing  that modern carbon dioxide levels have been unprecedented since at least 7 million years ago.

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Got an apatite for minerals? Of quartz you do!

Minerals, those naturally occurring, inorganic materials with well-defined chemical compositions and crystal structures have long influenced human culture and fascinated (geo)scientists. Some of the earliest descriptions of minerals and their uses date back to Ancient Egypt, recorded on papyri, as well as on stelae (blocks of stone or wood), and ostraca (clay tablets or pottery shards). Minerals and their uses have been intertwined with human history for thousands of years from the gemstone bracelets of the Egyptians and their belief that color was a strong reflection of personality (color symbolism, e.g., the use of gold for crowns on pharaohs and its association with the sun), to the Greeks and their wide use of gemstones in necklaces, and bracelets. 

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Did a change in phosphorus cycling lead to the diversification of macroscopic life?

Featured image: The earliest examples of life on Earth are microbial buildups known as stromatolites, like these 1.8 Ga old examples from Great Slave Lake, Canada. What changed on our planet for organisms to evolve from microbes to macroscopic lifeforms?

Paper: Ediacaran reorganization of the marine phosphorus cycle
Authors: Laakso, T.A., Sperling, E.A., Johnston, D.T., and Knoll, A.H.

This is a guest post by Akshay Mehra and Danielle Santiago Ramos. Contact us to submit a guest post of your own!

The history of life on Earth—as recorded in the rock record—stretches back to more than 3.5 billion years ago (Ga). The earliest fossilized remains of living organisms appear in the form of stromatolites, which are laminated constructions built in part (or completely) by microbes. While there have been some tantalizing hints that living organisms were mobile by 2.1 Ga (Albani et al., 2019) and multicellular by 1.6 Ga (Bengston et al. 2017), what is definitively known is that by ~750 million years ago (Ma), complex microscopic lifeforms were widespread on our planet. As time progressed, life became macroscopic. Then, during the Cambrian Era (beginning 539 Ma), most modern phyla (i.e. a grouping of organisms based on body plans) appeared in a flurry of diversification so drastic that it has been nicknamed “the Cambrian explosion.” Scientists are still trying to understand what combination of physical and biological processes may have driven the Cambrian explosion.

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Warmer climate could mean corals thrive in the southern Great Barrier Reef

Featured image: Jeremy Bishop on Pexels

Paper: Re-evaluating mid-Holocene reef “turn-off” on the inshore Southern Great Barrier Reef
Authors: Leonard, N.D., Lepore, M.L., Zhao, J.X., Rodriguez-Ramirez, A., Butler, I.R., Clark, T.R., Roff, G., McCook, L., Nguyen, A.D., Feng, Y. and Pandolfi, J.M.

A new study has reconstructed the complex growth history of coral communities in the Keppel Islands, southern Great Barrier Reef, revealing that the area might provide a safe-haven for coral under climate change.

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Marsquakes give scientists an InSight to Mars

Featured image: An artist’s concept of NASA’s InSight lander on Mars with a cutaway of the surface below. Credit: IPGP/Nicolas Sarter.

Paper: Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data

Authors: Philippe Lognonné et al.,

Scientists are able to ‘see’ the internal structure of the Earth based on seismic waves recorded during Earthquakes. Earthquakes send seismic waves out in all directions with two main types: (1) surface waves are the major culprits of Earthquake damage as they remain on the surface; (2) faster body waves can travel down within Earth’s interior. The body waves are the fastest seismic waves, consisting of the first (primary; P-wave) and second (secondary, S-wave) waves to arrive at a location away from the epicentre of an Earthquake.

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