Archive for August, 2019

Two Records in Arctic Melt This Summer

August 11th, 2019

There is perhaps a bit too much media hype about July 2019 being the warmest month on record. If you go to the source — the European Copernicus Climate Service article,  the official statement is that “July 2019 was on a par with, and possibly marginally higher than, that of July 2016”.  This is important to note because NASA and NOAA have yet to provide their own summaries for July, and because of uncertainties in the surface temperature record and varying techniques for measuring global temperature among groups, it’s possible July 2016 will remain the record based on other scientific agencies. However, it’s still been a hot summer overall, with June 2019 very clearly the hottest June ever.

This warmth has been felt in the Arctic, as well. Surface melt on the Greenland Ice Sheet has been particularly high this summer, and it recently experienced its highest daily melt area ever recorded, based on data from the National Snow and Ice Data Center. Over 60% of the ice sheet was melting at the surface. In the world of sea ice, it is unlikely that we break the all-time record from 2012 for the annual daily minimum sea ice extent, as that coincided with the strongest storm ever recorded in the Arctic (which helped break up and melt the ice pack).  However, there has never been less sea ice in the Arctic Ocean in July than in 2019.

 

Last fieldwork of the summer: A local section with a mystery limestone

August 7th, 2019

It was a delightful August day in northeastern Ohio with pleasant temperatures and thunderstorms that obligingly went around us. Nick Wiesenberg, our geology technician, and I were invited by Dr. Nigel Brush (Ashland University) and friends to examine an outcrop of the Cuyahoga Formation (Lower Carboniferous) near Jeromesville, about a 25 minute drive west of Wooster. Nigel has been investigating a strange limestone in a thick shale sequence that doesn’t seem to fit into our known stratigraphy for the region. I’m always up for a new limestone to look at! As a bonus, the shales here have excellent fossils, especially in concretionary beds. The outcrop, with Nick for scale, is about ten meters high. It is on land owned by the Bricker family, who are geology enthusiasts with beautiful rock and fossil collections.

The shale cliff is steep and has been cut rapidly by the stream (Quaker Springs Run). assisted by occasional dredging of the channel for gravel. The limestone in question is about a meter over our heads here. That’s me, Nigel Brush, and Jeff Dilyard. Photo by Nick.

The shale beds contain siderite concretionary beds loaded with fossils. The most common are spiriferid and spiriferinid brachiopods preserved as internal and external molds and occasionally their original calcite. The taxonomy of these brachiopods is well known, but their paleoecology is still to be determined. (Future Independent Study projects!)

Here is a piece of that limestone eroded from the cliff into the stream. You can immediately see how different it is from the surrounding muddy shales. It is cross-bedded, coarse-grained, and loaded with shale intraclasts (the gray chips), which are mud flakes that were incorporated into the rock while it was being deposited. It looks very much like a storm deposit, possibly associated with a sudden drop in sea level (and rise again).

We took a piece of the above limestone and cut it with a rock saw to begin assessing its composition and fabric. The round grains you see are almost entirely crinoid fragments, mostly stem and arm fragments. The gray chips are the shale intraclasts. Clearly this rock has a story to tell! We will now make acetate peels of this limestone for microscopic study.

One primary question we must address is just where this unit fits in the local stratigraphy. We know it is the Cuyahoga Formation, but which member? It could be the Wooster Shale, Armstrong, or Meadville Shale. One clue would be the conodonts present in the rock. Conodonts are phosphatic dental elements of long-extinct swimming animals. They have considerable biostratigraphic value and so can be used to place units in a stratigraphic column. Since they are phosphatic, they can be collected from limestones by dissolving the carbonate matrix with formic acid and sifting through the remaining insoluble residue. I did this for my dissertation many years ago, and most recently Dean Thomas (’17) dissolved Ordovician limestones for these little critters. I started the process this evening in the paleo lab. Above you see two basins which contain limestone fragments in a 10% solution of formic acid. They are bubbling furiously as the carbonate dissolves. In a few days I’ll sieve the residues and see if we have any conodonts. You’ll be the first to know.

Thank you, Nigel Brush, for the invitation and all your expert fieldwork. It was a fun day as the summer wanes and our first classes of the fall rush towards us.

 

Shamrock Glacier, Neacola Mountains, Alaska

August 7th, 2019

While in the Neacola Mountains of Alaska last month, we flew over Shamrock Glacier. This first image is from the head of the glacier, where crevasses have been filled in with snow during the accumulation season.

Farther down the north-flowing glacier, we see the merging of the east and west branches and a fine example of a medial moraine.  However, also note on the far lower-right the recently exposed rock. This part of the glacier is shrinking in size, becoming a narrower ice stream.

The toe of Shamrock Glacier is just plain beautiful, ending at a small lake that is dammed from two larger lakes by a ring of moraines. Estimates in a blog post by Mauri Pelto are that the glacier extended all the way out to that moraine as recently as 1950.

In fact, back in 2015, Mauri showed a Landsat satellite image showing the retreat of Shamrock Glacier away from its moraine from 1987 to 2014.  Updating this with the most recent July 2019 imagery, you can see the continued retreat of Shamrock Glacier just in the past four years.  A big section on the left has calved off, and the glacier has slipped off a rise on the right.

Finally, taking a view from the ground, we can better see how the glacier has not only retreated back, but also thinned and narrowed over the past few years. (The 2015 line is based on a photo from Jerry Pillarelli It’s still a pretty glacier, and the sound of calving icebergs while eating lunch  is always welcome.  However, it won’t be long before it retreats upslope sufficiently to no longer calve off into the lake.

Wooster undergraduate researchers will share findings at international conference

August 2nd, 2019

Wooster, OH – Team Geochemistry finished a highly productive summer research season with two conference abstracts that we submitted to the 2019 Fall Meeting of the American Geophysical Union. AGU will be an excellent opportunity for the team to hone their presentation skills, network with potential employers and graduate school advisors, and explore the wide range of disciplines in the geosciences. Read about our work in the abstracts:

Ok and Bræðravirki offer us opportunities to explore past climates and interactions between volcanoes and glaciers, helping us predict what might be in store for Iceland and other ice-covered volcanoes across the world. (Photo Credit: Hannah Grachen)

A Geochemical Study of Bræðravirki Ridge (Western Volcanic Zone, Iceland) Reveals Regional Glaciovolcanic Variations and Complex Tindar Construction 

By Hannah Grachen, Simon Crawford-Muscat, Billy Irving, Meagen Pollock, Benjamin R. Edwards, Shelley A. Judge, Layali Banna, Kendra Devereux, Marisa Schaefer

Summary: As global ice recedes in response to climate change, volcanoes that erupted under the ice are becoming more exposed. We studied one such subglacial volcano in Iceland called Bræðravirki Ridge, and found that Bræðravirki is chemically different from other nearby subglacial volcanoes. We also found that there were several packages of volcanic ash and lava that were erupted at different times. These findings show that subglacial eruptions are complicated, and we might be able to use these types of observations to understand more about the history of volcanoes and glacial ice in an area.

Although tuyas in the Western Volcanic Zone (WVZ) of Iceland have been studied in detail, little work has been done on the numerous smaller tindars there. This study compares Bræðravirki Ridge, a 3-km long tindar on the southeastern flank of Ok shield volcano in the WVZ, to regional tuyas and creates a model for ridge formation based on combined mapping and geochemical analyses. Bræðravirki is dominated by palagonitized lapilli tuff with scattered intrusions, rare exposures of intact pillow lavas, and multiple tuff/lapilli tuff units. Whole rock samples were measured for major and trace elements by XRF and ICP-MS. Mineral compositions were measured by SEM-EDS. Major elements show that Bræðravirki is relatively enriched in SiO2 and depleted in CaO, FeO*, and MgO compared to regional data. Within the ridge, vitric tuff/lapilli tuff units are more evolved (MgO 6.29-6.66 wt.%) than other units (MgO 6.94-7.72 wt.%). Preliminary Rhyolite-MELTS (v. 1.0.1) models are consistent with the two compositional groups being genetically related by 3 kb fractional crystallization of a common parent magma. The MELTS models generate plagioclase (An61 to An70) and two pyroxene (low- and high-Ca) compositions that are observed in the SEM-EDS mineral data. Incompatible trace elements show limited variation between the two compositional groups (Nb/Y 0.37-0.41), suggesting stable melting conditions. We propose a two-stage eruptive model that begins with an explosive phase, followed by a second explosive-effusive phase that forms intrusions and pillows. The second phase is hypothesized to be initiated by a recharge event, emplacing the higher-MgO units. This study demonstrates that small tindars can be constructed through multiple eruptive events that shift in eruptive style and that glaciovolcanic edifices in the same region can have significant compositional differences, possibly providing insights into the understanding of their timing, magmatic history, and paleo-ice conditions.

Microscope photo of one of the granitic rock samples that was classified using the new and old naming systems.

New Igneous Classification System Produces Consistent Rock Names and Illuminates Modal Data

By Hannah Grachen, Anna Cooke, Charley Hankla, Ethan Killian, Cody Park, Layali Banna, Kendra Devereux, Meagen Pollock

Summary: Rock names are useful for understanding what a rock is made of, how it formed, and how it might be useful. For rocks that form from molten magma, the current naming system doesn’t communicate all of the relevant information. Earlier this year, scientists proposed a controversial new naming system to make rock names more meaningful. We tested the new naming system and found that it worked well at the microscopic scale but was more confusing when using the naked eye. We found that the new rock names contained more useful information than the old rock names, and we think that the new system could become the future of rock classification.

In the February 2019 issue of GSA Today, Glazner et al. proposed a new classification system for igneous rocks, citing shortcomings within the standard International Union of Geological Sciences (IUGS) classification system, in use since 1974; chiefly its inability to convey modal data and its arbitrary division of interrelated rocks into disparate types. Their proposed system recognizes rock classifications on ternary diagrams to be “fuzzy” rather than distinct, therefore better representing the continuous nature of rock sequences and lending their system the term “fuzzy classification scheme”. They suggest a system of limited root names combined with modal percentage numbers and significant accessory minerals to provide more informative and straightforward rock names. To test this claim, we have named a suite of six thin sections and hand samples of granitic rocks from Songo Pluton, North Jay Quarry, Mount Waldo, and Deer Isle, using both the IUGS and “fuzzy” classification schemes. We also obtained modes on thin sections through quantitative image analysis using Adobe Illustrator and ImageJ, this being an important step in our process due to the difficulty of differentiating alkali feldspar and plagioclase in our samples. We determined that while the comments on Glazner et al.’s paper are critical, in the tests we performed, some of those criticisms, such as the non-replicability of the proposed system due to interpretive bias, are insubstantial. In thin section, the fuzzy system naming led to more consistent naming results than in hand sample observations, while the opposite was true in hand sample. One of the things we would suggest after conducting our tests is to include boundaries to distinguish the names in a better way. Overall, the proposed system’s ease of data communication is an improvement on the IUGS system that warrants more attention and modification.