A few photos from the field

We’ve all been hard at work collecting data for the past two weeks.  Rain-less weather at last!  Here are few photos from today’s field activities:

Steep karst towers and sunflower fields.  It’s pretty incredible to hike up into these high valleys and see fields everywhere.  We often come across farmers coming to and from their fields, carrying their harvest, or leading their livestock – they like to chat and give us cucumbers!

About to descend into endless corn fields.  The corn is taller than all us – it makes it hard to see any rock exposures.

Large oncoids – microbial laminated carbonate grains – that are indicative of a lagoon environment.

My field assistants, Xiao Fu and Jiao Jiao, ready to collect a sample and record data.

Sunny days make for very productive sampling

We’ve had some dry weather, and after putting in some long days of hard work, we’ve finished measuring and sampling our first section!  A lot of our time at work has been spent following a very picturesque stream, but it also can be tricky to maneuver our way around.

Our team is hard at work collecting hand samples of the outcrop at regular intervals, which will be used for thin sections and geochemical analyses.  Thin sections are very thin slices of rock mounted onto glass slides, and are often used to study the mineral and fossil components using a petrographic microscope.  My main focus for sampling, however, is to collect rocks for geochemical studies.  The goal is to unravel the environmental conditions, such as ocean acidification and anoxia, using isotopes and other similar geochemical proxies that have been preserved in the carbonate rock.  Because carbonate minerals, such as calcite (CaCO₃), are precipitated directly from seawater, these rocks should record the ocean chemistry from the time that they were deposited.  Isotopes of Ca, C, and O from calcite, and other elements that have been incorporated as impurities, may be used as proxies for different paleoceanographic conditions.

In addition, we have been collecting larger samples (of around 5 pounds) for conodont analyses.  Conodonts (below) are tooth-like elements that belonged to an extinct eel-like vertebrate animal.  These conodont elements are found in many carbonate rocks from the Late Cambrian (around 500 million years ago) to the Late Triassic (around 200 million years ago) and are very useful in creating a relative time scale.  Based on their evolutionary pattern, the conodont elements appear at different time periods and this sequence can be used to “date” carbonate rocks and to correlate between rocks from different locations.  The conodonts below are primarily Hindeodus parvus, a conodont whose appearance defines the start of the Triassic period.

(Haas, J. et al., 2007: http://www.sciencedirect.com/science/article/pii/S0921818106001676)

In addition to collecting these rocks samples, we are using a handheld gamma ray spectrometer to measure the radioactivity level of the outcrop.  This tool is often used in geophysical and petroleum exploration studies because the level of radiation is generally related to the level of siliclastic sediment in the rock.  In other words, it is an indicator of how much non-carbonate clays are present.  Creating a record of the radiation throughout the section may be useful in correlating the rocks between different locations.

Below is the team hard at work: Brian and Jake (both wisely in the shade) are measuring and collecting rock samples, and Xiao Fu is working with the gamma ray spectrometer.

The highest temperature so far has been 39.5°C – scorching and humid – and after sweating and carrying all the rocks back to Xiliang, we get to enjoy a cold beer with a view of town and the karst towers further in the carbonate platform.

 

Sampling in the Rain

It’s still raining daily, which is not typical for the summer months at the GBG.  Every day, we hike in from a small town, Xiliang, along a dirt road, and turn off onto a dirt path before reaching the section that we are measuring.  In total, this is about a 30 minute one-way hike up and down rutted and muddy roads, occasionally walking along rice paddies and hoping not to slip and fall into the flooded fields.  As a pleasant addition, water buffalo, cow, and horse scat can often be found along the roads as well, making for a very stinky and mucky trek at the start and end of every field day.  After the hike, the traction on my boots isn’t ideal for walking along the stream-eroded rocks, but when the rain clears, we are still treated to a spectacular view.  Here is a photo of some karst hills and stream with carbonate bedrock.

During a break in the rain, Brian (a fellow graduate student at Stanford) and a Chinese student, Xiao Fu, are hard at work measuring section and sampling with a gamma ray spectrometer.

My notes station, sheltered a bit from the rain.  The umbrella was a great find (and only $5!).

 

 

Rain Delay

After about half an hour measuring and sampling in a stream bed in light rain, it began to shower with thunder and lightning.  Despite waiting two hours for it to let up, we were out of luck today.  Back to the hotel to catch up on other work.  Unfortunately, the forecast for the next 10 days shows a high probability of rain, but hopefully it sticks to small drizzles and not large storms!

View from the hotel

Essential field equipment

Left to right, top to bottom: belt with hammer holster and Brunton case; large rock hammer; clipboard with graphic log sheets; waterproof field book; Brunton compass; HCl acid bottle; Sharpies and hand lens; and sample bag.

First day out!

Today was our first day out in the field.  We did some reconnaissance to check out the rocks and to see where to collect my samples.  First, we hiked in (the trail runs somewhere behind this house).

Along the way we see a lot of rice paddies, corn fields, and sunflower beds.

There is a lot of vegetation to deal with – sometimes it is challenging to find good outcrop.

Below is a contact between two very different rock formations.  The one on the left below the ferns is a fossiliferous limestone, while the one to the right is a very fine-grained carbonate, which weather more easily.  The fossiliferous limestone is older than the fine-grained carbonate.  This represents a relatively rapid shift in depositional environment (also known as ‘facies’) from a shallow water facies, which supports a lot of carbonate-producing biological activity, to a deep water facies.  The deep water environment cannot support a lot of life, and therefore carbonate fossils, which are common on shallow shelf environments, are rare and the rock is made up of very fine grained material that has fallen down slowly in the water column.  In addition to the fine silt and clay that has settled into this deep area, siliceous microscopic organisms known as radiolarians are deposited into a hard rock known as chert.  Radiolarians are an example of a pelagic organism, meaning that they live in open water at the top of the water column.  While not very visible in this photo, chert bands are common in this deep water rock formation.

Note that at this time, carbonate pelagic organisms that are common in modern oceans, such as pelagic forams and coccoliths had not yet evolved.

Later in the day, we explored another rock formation that consists of carbonate breccias alternating with thinly bedded limestones.  Below, the breccia can be seen in the large gray rock in the middle of the photo, dotted with lighter gray pieces of rock.  A breccia is a type of sedimentary rock that consists of large clasts (or grains) that have been deposited through high energy processes such as a rock fall or a large flood.  The clasts are angular, meaning that they traveled quickly and didn’t have time to become rounded through abrasion with other grains, as you may observe in river pebbles.  Because this is a carbonate breccia, it means that the grains are pieces of carbonate rocks that have been broken apart and transported (in this case, most likely via a rock fall), and have been cemented together with smaller carbonate mud grains.  (Mud, in geology, refers to a very small grain size.)

(For scale, the thin beds to the right are centimeter-scale.)

Finally, to end the day, we hiked along this stream bed to look at the rocks that have been exposed by the erosion of the water (which isn’t flowing much today).

T-minus 12 hours … heading to China!

I leave tomorrow for China, and I am all packed and ready to go!  This past quarter has been extremely busy with classes, lab work, and field work preparation, but I can finally look forward to the trip.

Before I leave, however, I should explain why I’m heading to China.  My research currently focuses on studying a very important time period in Earth history, the end-Permian extinction.  As the largest mass extinction ever recorded, when up to 90% of marine species went extinct, there is a lot of scientific research focused on the causes and consequences of this event.  (I hope to go into more detail in future posts.)  I am studying the recovery of life from this extinction from a geochemistry perspective by using chemical tracers left behind from ancient oceans.   While I have started some lab work with the aim to elucidate the marine environmental conditions during the Triassic, the geologic period following the extinction, I will need my own samples for future work.

The GBG (Great Bank of Guizhou), where I’m heading, is located in Guizhou province, south China.  It is an exceptional example of a carbonate platform and where a lot of work has already by done by my adviser, Jon Payne, and others in the Payne paleobiology group (as well as many other notable geologists).  I will go into more detail about the GBG later, but for now, the goal of my trip is to collect samples spanning the late Permian to the middle Triassic in order to better understand what conditions may be influencing the recovery of life.