Dessication Cracks In Mars Lake Bed

This Comment and Reply published in the August 2018 issue of Geology is worth reading.

Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale Crater: COMMENT Brian R. Pratt

Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale crater: REPLY N. Stein; J.P. Grotzinger; J. Schieber; N. Mangold; B. Hallet; D.Y. Sumner; C. Fedo

The argument concerns the origin of polygonal shaped ridges found on the surface of mudstones deposited in a Martian lake. These millimeter to centimeter high ridges made of sand were interpreted as having formed by sand filling in dessication cracks that form on the surface of a drying lake bed. The alternate view argued is that the sand was injected into cracks formed during seismic events taking place on early Mars.

New about Mars is usually dominated by grand questions about evidence for extraterrestrial life. But scientists inch towards answering such bigger themes by working the nitty gritty. In this case, grit filling up cracks in a mudstone. Such a debate may seem arcane, but understanding these details matter. They form small but essential building blocks of a knowledge base, incrementally adding to the larger picture.

Mineralogy Of The Earliest Animal Shells

Carbonate sedimentology and evolution. Two of my favorite subjects converge in this interesting study published in the September 2018 issue of Geology.

Calcium isotope evidence that the earliest metazoan biomineralizers formed aragonite shells-
Sara B. Pruss; Clara L. Blättler; Francis A. Macdonald; John A. Higgins

Ediacaran Cloudina and Namacalathus are among the earliest shell-forming organisms. The debated carbonate phase of their skeletons, high-magnesium calcite or aragonite, has been linked to seawater chemistry and pCO2, yet independent constraints on the original mineralogy are lacking. We present a new method to distinguish primary skeletal mineralogy using δ44/40Ca values and trace element compositions of the skeletons and associated cements. Ca isotopes are useful because they are relatively insensitive to diagenetic alteration during burial, and they vary with the primary mineralogical phase of carbonate. We applied this method to microdrilled carbonate and cements associated with both Namacalathus and Cloudina skeletons from the Ediacaran Omkyk Member of the Nama Group in southern Namibia. These data demonstrate that both organisms originally produced aragonitic skeletons, which later underwent diagenetic conversion to calcite. We suggest that calcium isotopes can be used to further constrain unknown skeletal mineralogies through time and to reassess the relationship between seawater chemistry and the mineralogy of biocalcifiers.

Different animals groups acquired the ability to precipitate hard protective shells made up of calcium carbonate at different times between the latest Neoproterozoic (~ 580-541 mya) to early Paleozoic (541 - ~ 450 mya).  What is the larger picture of the evolution of the biomineralization in different animal groups and the mineralogy of the skeleton? Through geologic time the chemistry of sea water has oscillated from that favoring the precipitation of aragonite and high magnesium calcite to that favoring the precipitation of low magnesium calcite. From Late Neoproterozoic to Middle Cambrian sea water chemistry favored the precipitation of aragonite and high Mg Calcite. Animal groups which evolved skeletonization during this time largely adopted aragonite to build their skeletons. Animals which first evolved biomineralization in the Late Cambrian to Ordovician times, during the time of calcite seas, adopted calcite skeletons.

In most animal groups, skeletal mineralogy was conserved even when sea water chemistry changed later in history. There have been only rare instances of animals changing the mineral phase used to build its skeleton. I wrote about one such instance in the Mesozoic when some groups of molluscs switched from  aragonite to calcite. The most prominent example is from the Hippuritoidea (rudist) bivalves. The switch to calcite shells seems to have triggered an evolutionary diversification beginning in the Late Jurassic. By Late Cretaceous times rudist bivalves were so abundant that they displaced corals (which built their skeletons with aragonite) as the chief reef builders in the shallow shelf environments.

Fascinating topic!

Geology Photo Contest

The Centre for Education and Research in Geosciences, a geology outreach group based in Pune, is organizing a photo contest. Entry is open to all. The selected entries will be exhibited during their annual Geology Week event to be held in Pune from October 8 to October 12. Last date for entry in September 21 2018.

Brochure.

.. and go to this webpage for details regarding the photo contest: Geology Photo Contest

Please share widely!

Before The Himalaya: Story Of A Late Cretaceous Subduction Zone

My article on the geology of the upper catchment of the Brahmaputra River in southern Tibet has been published in The Wire Sciences. This region is technically called a suture zone. It contains the remnants of the Tethys Ocean floor made up of basaltic oceanic crust and overlying deep sea sediments. These rocks were deformed and uplifted during the India Asia collision.

I focus on a paper on the Jiachala Formation by Hanpu Fu and colleagues published in a recent issue of Science China Earth Sciences. They use a technique known as detrital zircon geochronology to resolve a long standing problem about the age of that sedimentary deposit. In my article I explain how this technique works. I also elaborate on the broader story these deposits tell us about plate tectonics and the beginnings of the Himalaya.

An excerpt:

In the Cretaceous Period, the Indian plate, which had been moving northwards since the breakup of Gondwanaland, was approaching the Asian continent. The southern edge of the Asian continent was lighter continental crust, whereas the leading part of the Indian plate was denser oceanic crust. As a result, in the zone where the the two plates converged, the denser Indian plate slid below the Asian plate, forming a subduction zone.

As the Indian lithosphere sunk deeper into the mantle, it heated up and released water trapped in sediments and hydrated oceanic crust. This water penetrated the overlying Asian plate, lowering the melting point of its rocks and triggering magma generation. This buoyant magma rose through the Asian continental crust. Some of it reached the surface, resulting in extensive volcanism. The rest solidified in the subsurface, forming giant bodies of granite known as batholiths.

Such terrains have the grandiose name of magmatic arcs. The town of Leh and the surrounding settlements in the Indian region of Ladakh are situated partly on a magmatic arc.

Two sedimentary basins since developed south of this arc. Immediately adjacent to the magmatic arc was the forearc basin. A deeper depression, known as the trench, formed further away on the Indian oceanic lithosphere, at the junction where the Indian plate had slid under the Asian plate. Both were receiving sediments derived from the erosion of the Asian continent.

During subduction, slices of the Indian plate were scraped off and thrust to the surface. Such fault-bounded piles of sediment and oceanic crust are called accretionary wedges, and they, along with a chain of oceanic volcanoes  that formed to the west in the region between Ladakh and Kohistan, would have been the first island ranges formed in the Tethys Ocean between India and Asia.

 Read the complete article here.

Geoscience Education Woes In India

Dilip Saha of the Indian Statistical Institute, Kolkata,  has written an editorial in Current Science on the many problems with geology education in India.

He identifies a lack of attention to field work and the quality of teachers as the two major weaknesses that need correction.

I agree with many of the points he has made. I had a very poor quality field training experience during my Master's education at University of Pune (now Savitribai Phule Pune University). That was somewhat compensated for by some very good classroom teaching. Across State Universities and local colleges, the quality of teaching suffers not just because subject experts are not up to the task, but because many departments are understaffed and don't have subject experts.  Often, just two or three faculty end up teaching all the subjects.

I will also add that besides the obvious improvements in field courses, teacher quality and pedagogy, a module on research ethics is desperately needed. This is not a geology specific issue. Plagiarism is a big problem in Indian academia. I occasionally mentor students from local colleges. I have found out, to my dismay, that copying and pasting material from a research paper in to one's thesis seems to be commonplace. Students don't even realize that they are crossing serious ethical lines.

Open Access.