Saturday, September 30, 2023

Iron Pisolites From Western Ghats

A reader sent me this photograph of a pebble he had collected from a stream bed near Belgaum, Karnataka.

 Photo credit: Gopisundar

These look to me like iron rich pisolites. These spheroidal grains form by the accretion of iron, manganese, and aluminum hydroxides around a nucleus. The core may be an aggregate of soil particles, a rock fragment, or even wood debris. 

You will notice that the core is quite massive and structureless but in a few grains a crude concentric layering is seen at the margins. The pisolites are bound together into a firm mass by mixture of clay and iron aluminum hydroxide. Pisolites form during prolonged episodes of chemical weathering of  rocks like basalt or shale or iron aluminum rich metamorphic rocks. They are present in thick laterite and bauxite profiles. 

The picture below is a representative example of pisolite from a location in Brazil. It shows the occurrence of pisolite layers in a weathered soil profile, along with a hand sample and a cross section under high magnification.


Source: K Marques 2022: Geochronology (preprint).

Here is a map of the landscape just south of Belgaum that I am describing in this post. 


 Source: Amanda Jean 2020: Journal of Geological Society

It shows the distribution of three distinct horizons of chemically weathered soil, named as the S1, S2 and S3 surfaces. Each of these horizons consist of tens of meters of laterite or bauxite and manganese rich ore. There has been some recent success in dating these weathered layers using the mineral crytomelane, a potassium rich manganese oxide. Three distinct weathering periods are documented. As India broke away from Gondwanaland it eventually drifted northwards into tropical climatic belts. Throughout the Eocene to Miocene, long phases of hot humid climate resulted in intense chemical alteration of the Western Ghat landscape. 

The oldest soil, surface S1 in the map, formed between 53-44 million years ago. Surface S2 formed later in the Oligocene-Miocene between 39-22 million years ago. And surface S3 developed in the mid Miocene, between 14-10 million years ago. 

The graphic below tells a story of landscape evolution recorded in the formation of these three weathering profiles. 


 Source: Amanda Jean 2020: Journal of Geological Society

Episodic dissection of a plateau through the Cenozoic kept stripping away rock layers, and younger bauxite and manganese rich soils formed at lower altitudes on freshly exposed rock and debris flows. The youngest surface S3 has developed on a pediment. These are layers of debris eroded from surrounding hills that accumulate in low lying areas. Surface S3 indicates a very active phase of weathering and erosion of the surrounding mountains ranges that took place between 22 and 14 million years ago. As a once contiguous plateau was fragmented, older surfaces S1 and S2 remain preserved on isolated mesas and table lands.

The pisolite my friend sent me could have broken off from one of these surfaces. Its hard to say which one. The overall light color suggests that it is aluminum rich, and may have been sourced from the bauxitic S1 or S2. But this is just a guess. 

Pebbles from a stream can hold many secrets. Don't just chuck them away :)

Monday, September 11, 2023

Agglutinated Foraminifera From The Deep Sea

No matter how many David Attenborough specials you may watch, nature always throws more surprises at you.

Foraminifera are protists that build a skeleton or a test made up of calcium carbonate. The calcium carbonate is precipitated out of sea water. There is a sub-group of foraminifera which construct a shell not by capturing the calcium carbonate via chemical precipitation, but by assembling sedimentary grains and then cementing them together like a brick and mortar structure. This group of forams are known as agglutinated foraminifers. 

All this is well known. Many different species of forams use a variety of grains as bricks. They are mostly different shell fragments, but even mineral grains like ilmenite (iron titanium oxide) , rutile (titanium dioxide), or garnet are used. 

Now there is a new report of a deep sea living agglutinated foraminifera which constructs a tube made up of  planktonic foraminifera shell fragments of a single species

Read that again. A benthic (bottom living) foram shell made up of bits of another foram which lived floating in the upper water column!  

These shells are selected to the exclusion of all other types of available sedimentary grains. The specimens were recovered in a core drilled off shore northwest Australia by the International Ocean Discovery Program. The paper by Paul N. Pearson and IODP 363 Shipboard Scientific Party is published in the Journal of Micropalaeontology.

Here is the entire abstract. It is mind boggling. 

Agglutinated foraminifera are marine protists that show apparently complex behaviour in constructing their shells, involving selecting suitable sedimentary grains from their environment, manipulating them in three dimensions, and cementing them precisely into position. Here we illustrate a striking and previously undescribed example of complex organisation in fragments of a tube-like foraminifer (questionably assigned to Rhabdammina) from 1466 m water depth on the northwest Australian margin. The tube is constructed from well-cemented siliciclastic grains which form a matrix into which hundreds of planktonic foraminifer shells are regularly spaced in apparently helical bands. These shells are of a single species, Turborotalita clarkei, which has been selected to the exclusion of all other bioclasts. The majority of shells are set horizontally in the matrix with the umbilical side upward. This mode of construction, as is the case with other agglutinated tests, seems to require either an extraordinarily selective trial-and-error process at the site of cementation or an active sensory and decision-making system within the cell.


The photographs from the paper shows the tube of the agglutinated foraminifera made up of planktonic foraminifera shells of a single species.

Charles Darwin knew of agglutinated foraminifera from reports he had read and was astonished....“almost the most wonderful fact I ever heard of. One cannot believe that they have mental power enough to do so, and how any structure or kind of viscidity can lead to this result passes all understanding”.  This was a letter he wrote to W.B Carpenter who had described them in 1873.

I'll leave you to ponder upon this most exquisite of natural wonders. The paper is open access: A deep-sea agglutinated foraminifer tube constructed with planktonic foraminifer shells of a single species.