The complex origin of ooids in the Middle Jurassic Carmel Formation of southwestern Utah: Anna Cooke’s Senior Independent Study thesis

Editor’s Note: Independent Study (IS) at The College of Wooster is a three-course series required of every student before graduation. Earth Sciences students typically begin in the second semester of their junior years with project identification, literature review, and a thesis essentially setting out the hypotheses and parameters of the work. Most students do fieldwork or lab work to collect data, and then spend their senior years finishing extensive Senior I.S. theses. This year we have the COVID-19 pandemic to deal with in the spring, so our students have not had a chance to publicly present their hard work and scientific ideas. Some, then, will be writing blog posts like this. The text and images here are from Anna Cooke (’20) who is a member of Team Utah 2019. The picture above shows Anna and fellow team member Evan Shadbolt (’20) on the top of Angel’s Landing in Zion National Park. (Photo by Nick Wiesenberg.) Now Anna takes over —

The Carmel Formation (shown above) formed in a shallow inland sea during the Middle Jurassic and is located in parts of Utah and Arizona. It can be broken into four distinct members, one of which, the Co-op Creek Limestone Member, contains ooid shoals. The ooids in these shoals are calcitic with radial crystals and sparry cement. Several noteworthy features are found in the Carmel ooids, such as delamination, pressure solution, and microborings created by the cyanobacteria: Hyella sp. and/or Solentia sp. Foraminifera are sometimes incorporated into ooids as their nuclei. Seventeen of 21 Carmel thin sections contain foraminiferans inside or outside of ooids. Of these 17, 16 thin sections (94%) show more foraminiferans inside ooids than outside, meaning that ooids can act as taphonomic engineers, preserving what might otherwise not be preserved in the rock record. These foraminiferans likely belong to genera Turrispirulina and/or Ammodiscus. Eolian quartz silt is common in the Carmel shoals. The hypothesis of this study is that a pulse of quartz silt provided nuclei for the formation of the shoals and extinction of the shoals occurred when another pulse smothered it. This is partially supported by point counts, used to determine the percentage of each individual component of these limestones, and nuclei counts, used to determine the percentage of each type of nucleus found in these ooids. The locality that supports this hypothesis most strongly is C/W 142 EMR, which shows three distinct pulses of quartz accompanied by an inverse effect on the percentage of quartz nuclei. Locality C/W 757 DV is also of note, displaying a large amount of quartz early in the shoal’s life, decreasing over time. The percentage of ooids in the shoal shows the inverse. However, other shoals show no such pattern; one method of formation cannot be attributed to all of the Carmel Formation’s shoals, and even those geographically close show marked differences.

Cross-bedded ooid shoal deposit in the Carmel Formation.

Ooids in unit C/W-758A.

I have nothing but positive things to say about my I.S. experience at Wooster. Over the last three semesters, I have had the privilege of researching the Carmel, a formation in southwestern Utah that several other students have done research in. My focus was on ooids: tiny spherical grains composed of calcium carbonate which form in specific marine environments. I have learned so much about these amazing little grains, though at times they made me want to tear my hair out (I personally marked, counted, and recorded the nuclei of over 17,000 ooids)! Though I.S. is a process that comes with a certain amount of stress and frustration, it was also a rich and rewarding experience for me. I learned so much about geology, as well as fieldwork methods and research, writing, and presentation skills. My favorite part of this experience was the field work, which I conducted the spring semester of my junior year with the help of the rest of Team Utah 2019. I am so grateful to everyone who has helped me along in this process, especially my wonderful advisor Dr. Wilson! Independent Study is something I will no doubt remember fondly for the rest of my life!

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Coryphodon and the Paleocene-Eocene Thermal Maximum: Emily Randall’s Senior Independent Study thesis

Editor’s Note: Independent Study (IS) at The College of Wooster is a three-course series required of every student before graduation. Earth Sciences students typically begin in the second semester of their junior years with project identification, literature review, and a thesis essentially setting out the hypotheses and parameters of the work. Most students do fieldwork or labwork to collect data, and then spend their senior years finishing extensive Senior I.S. theses. This year we have the COVID-19 pandemic to deal with in the spring, so our students have not had a chance to publicly present their hard work and scientific ideas. Some, then, will be writing blog posts like this. The text and images below are from Emily Randall (’20) who participated in a Keck Geology Consortium project last summer. The picture above shows Emily on the right in Wyoming (with Isaac and Mike) collecting Coryphodon teeth. And now Emily takes over —

Abstract

Preliminary data point toward a new hypothesis in which Coryphodon lived in wetter habitats before the Paleocene-Eocene Thermal Maximum (PETM), but was able to adapt to drier habitats in order to survive post-PETM. Early Paleogene nonmarine strata are extensively exposed in the Bighorn Basin of northwestern Wyoming. The Fort Union and Willwood Formations represent alluvial deposition within a Laramide Basin formed from the Paleocene through early Eocene. Therefore, the basin is an ideal place to study the local effects of the PETM, a rapid global warming event that occurred about 55.5 million years ago at the Paleocene–Eocene boundary. During this event, an initial decrease in rainfall was followed by wet and dry cycles with increased temperature and decreased precipitation. Some flora and fauna went extinct, but many others exhibited dwarfing during this interval. The response of the large mammal Coryphodon to the PETM is poorly understood, but is of special interest due to its inferred semiaquatic nature.

We collected 14 stratigraphic sections from 5 mammalian biozones within the Bighorn Basin, each centered around depositional units containing Coryphodon. The depositional environments of these units were evaluated by describing the grain size; matrix and mottling colors; mottling percent; abundance and type of nodules; shrink-swell features such as slickensides and clay cutans; and other interesting attributes such as organic matter, invertebrate fossils, sedimentary features, and mottling color or percentage stratigraphic changes. The depositional environments include ponds, swamps, fluvial deposits, soils with evidence of wet and dry cycles, and dry soils.

 

Reflection

Completing my independent study was an extremely rewarding process and I am so happy I was able to have this experience. I was lucky enough to be part of a larger Keck Geology Consortium project, which allowed the team to tackle many more research questions than just one student project ever could. We spent about a month in the Bighorn Basin in northwestern Wyoming collecting data over the summer before I began working on my independent study on campus. It was amazing to be able to gain so much field experience and get to work with such a great team! Back on campus, I was able to focus on data analysis and teaching myself Adobe Illustrator in order to create stratigraphic columns. And then, of course, there was a lot of writing, reading, thinking, and analysis to do to complete my independent study. In the end, I am very proud of how my stratigraphic columns and independent study turned out!

Stratigraphic columns from Clarkforkian (Cf) 2 and 3 mammalian biozones (Pre-PETM).

Some of the Keck Wyoming team collecting Coryphodon fossils. From top to bottom left and then top to bottom right, Michael, Richard, Grant, Simone, Danika, Isaac, and Emily.

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Jurassic bivalves in a shallow epicontinental seaway: Evan Shadbolt’s Senior Independent Study thesis

Editor’s Note: Independent Study (IS) at The College of Wooster is a three-course series required of every student before graduation. Earth Sciences students typically begin in the second semester of their junior years with project identification, literature review, and a thesis essentially setting out the hypotheses and parameters of the work. Most students do fieldwork or labwork to collect data, and then spend their senior years finishing extensive Senior I.S. theses. This year we have the COVID-19 pandemic to deal with in the spring, so our students have not had a chance to publicly present their hard work and scientific ideas. Some, then, will be writing blog posts like this. The text and images below are from Evan Shadbolt (’20) who worked with me on Team Utah 2019. The picture above shows Anna Cooke (’20), Evan Shadbolt (’20) and me at an outcrop of the Carmel Formation (Middle Jurassic) near Gunlock, Utah, in March 2019. And now Evan takes over —

The Jurassic bivalves Plagiostoma ziona (right) and Camptonectes stygius (left).

The Carmel Formation of the Middle Jurassic has many mysteries. One of these enigmas is its bivalves. The formation contains the famous oyster balls called ostreoliths. Despite bivalves making up 80 percent of the fossils found in the Carmel Formation, it is not understood how the bivalves lived in this community. The formation is located in southwestern and central Utah. It was deposited when an epicontinental seaway covered most of Utah. The paleoclimate of Utah was hot and dry, which meant that the environment was evaporite heavy. This also meant that the seawater at the southernmost extent of the seaway in Utah was hypersaline. The bivalves lived in normal marine conditions, but there was little biological diversity. During the Jurassic, there was a calcite sea, and aragonite shells were dissolved away.

In mid-March 2019, I went with a College of Wooster group to southwestern Utah. There we collected bivalves from the Carmel Formation and identified them. Then we researched them and constructed a systematic paleontological overview of the known bivalves. We have possibly identified ten different types of bivalves, and three distinct communities in the Co-op Creek Limestone Member of the Carmel Formation. The communities were the Plagiostoma community, Camptonectes community, and the Liostrea Community. Each of these communities was dominated by a unique bivalve. The Liostrea community was associated with hardgrounds, while the Camptonectes and Plagiostoma communities lived in the same type of environment. We also hypothesize that the area was frequently hit by storms, which caused damage to these communities. The communities were possibly ephemeral, but the bivalves themselves could be considered opportunists. The communities in the Carmel Formation were also small and patchy throughout the area. The bivalve genera that appeared in the Carmel Formation were common in other Jurassic bivalve communities around the world.

My IS experience was fun and unique. Getting to travel to Utah and collect fossils with Team Utah 2019 was a rewarding experience. We spent a week there exploring the Utah environment. Luckily, I was able to collect my fossils over the spring break of my Junior year, so I could start my research early. I felt I was well prepared to start my IS, thanks to the help of the Team Utah 2018 and my advisor, Dr. Wilson. The IS writing experience was not as stressful as I thought it would be. The deadlines were all reasonable and even if I felt I did not do enough work that week, Dr. Wilson was always fine with the amount of work. I feel that the Earth Sciences department at The College of Wooster properly prepare you for writing your IS.

A reconstruction of the bivalve community sampled at Water Tank.

A reconstruction of the bivalve community sampled at Eagle Mountain Ranch.

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Wooster’s Team Utah 2020 Fieldwork

This is the index page for Wooster’s Team Utah 2020 expedition (March 9-13, 2020). The team members above are, from the left, Will Santella (’21), Juda Culp (’21), Nick Wiesenberg (geological technician), and Dr. Shelley Judge (structural geologist and tectonicist). Plus me, of course, Wooster’s sedimentologist and paleontologist. The Pine Valley Mountains are in the background.

This stratigraphic column from the National Park Service details the stratigraphy of southwestern Utah. Our expedition was to continue long-term Wooster explorations of the Carmel Formation (Middle Jurassic) near the top (marked with the red dot). We are preceded by several teams in the 1990s and most recently by Team Utah 2018 and Team Utah 2019. I am a most fortunate professor and geologist to work with such fine people in such a beautiful, stimulating place.

Here are the links to our daily field posts —

March 10: Field geology in a time of plague
March 11: On a Jurassic tidal flat
March 12: Final day in the field (alas)

I hope you enjoy these descriptions and images.

ADDENDUM on March 19, 2020 — Boxes of samples safely arrived in Wooater!

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Wooster’s Team Utah 2020: Final Day in the Field (Alas)

Hurricane, Utah — Last night we made the sad decision to leave for home as soon as possible because of the COVID-19 pandemic. The College has mandated no more in-person teaching, and we don’t want our flight plans to be complicated by cancellations and other mass-transit issues. This is thus our last day in the field.

We started at our treasured oyster-ball locality in Manganese Wash just north of the Gunlock Reservoir (C/W-157; field code MW). This was a key site for Team Utah 2018, but we could not access it last year because the bridge over the Santa Clara River had washed out. The bridge is back so over it we went. This is now Juda’s second site for trace fossils in the upper part of the Co-op Creek Limestone Member of the Carmel Formation. As you can see in the image above with Dr. Judge, there is more brush and weathering at this location than at Eagle Mountain Ranch. This made the trace fossils less crisp in their preservation.

This diffuse set of traces is new to us. It seems to be a deposit-feeding swirl.

Herringbone cross-stratification in this location as well. The paleoenvironment is still shallow and normal marine.

While Juda, Dr. Judge and I worked in the upper Co-op Creek, Will and Nick climbed up a ridge and then down towards the Gunlock Reservoir to visit the lower Co-op Creek and its stromatolites. They again measured, described and collected the unit.

And that was it for our fieldwork! We shipped three heavy boxes of samples back to our Wooster lab. We met our field goals, despite the truncated schedule.

To celebrate, we had another round of Veyo pies and then visited Snow Canyon State Park north of St. George. The Jurassic Navajo Sandstone is weathered in three dimensions here, enabling us to scramble about on its “petrified dunes”. Such a beautiful mix of orange white and black rocks with the green plants and blue skies.

Needless to say, Juda and Will liked the place.

The Jurassic dunes here have deeply eroded foresets at sometimes surprisingly steep angles.

Team Utah 2020! Plus Nick, who took this image. Such a fine crew in skills and enthusiasm.

(Links to the First Day, Second Day, and Third Day.)

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Wooster’s Team Utah 2020: On a Jurassic Tidal Flat

Hurricane, Utah — Our second day was devoted to measuring, describing and sampling Will’s stromatolite-bearing rocks in the lower half of the Co-op Creek Limestone Member of the Carmel Formation. This locality is only a couple of hundred meters west of Juda’s study location yesterday. The rocks are very different: lime mudstones with beautiful markers for their tidal flat origins. We worked in a deep wadi and thus had cliff sections with some bedding plane exposures. Above the team is describing the top of a depositional cycle. (I don’t know why Nick is giving me the side-eye!)

These are bedding-plane exposures of the top of a laterally-linked hemispheroids stromatolite unit.

Just above the previous stromatolites are these desiccation cracks. The tiny pockmarks may be raindrop imprints. The mudcracked units are thick enough in some places to make unusual sedimentary columnar bedding.

These are casts of evaporative gypsum or anhydrite nodules.

An intraclastic limestone grading into a breccia was one of our marker horizons. These rocks are often referred to as “evaporative breccias” because they are associated with the dissolution of evaporite mineral layers and collapse of the mudstones above.

These are delicious columnar stromatolites that made mounds on the sediment surface. The stromatolites are like thick fingers reaching upwards.

This close view shows the packing of the stromatolites. It is almost hexagonal.

An even closer view shows that the stromatolites were burrowed while still relatively soft. Were the trace-makers feeding on the decaying cyanobacterial mats inside? The interstitial sediment in the burrows and between the columns appears to be dolomitized.

Can’t have tidal sediments without herringbone cross-stratification, can we? These structures indicate bidirectional currents, likely from storms or tides.

Lunch in the shade! We had much more sun than yesterday.

Another successful day of field geology. We celebrated at the Veyo pie shop, now a Wooster Utah tradition.

(Links to the First Day, Second Day, and Third Day.)

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Wooster’s Team Utah 2020: Field Geology in a Time of Plague

Hurricane, Utah — This is Team Utah 2020 at Gunlock Reservoir in the far southwestern corner of beautiful Utah. Starting on the left is Juda Culp (’21), Will Santella (’21), Dr. Shelley Judge (our ace structural geologist and tectonicist), and Nick Wiesenberg (our invaluable geological technician). The dipping exposure in the background is the Carmel Formation, a Middle Jurassic (about 170 million-year-old) unit with wonderfully diverse sedimentary rocks and fossils. It is why we are here.

The Carmel has been one of my favorite formations since the early 1990s. I’ve been bringing students and colleagues to study it for many years, the most recent being Team Utah 2019 and Team Utah 2018. This unit has enough variability and mystery for a dozen future teams.

We are again pursuing the Independent Study projects of Wooster students with this field trip. Juda is studying the Carmel trace fossils in a paleoenvironmental context, and Will is examining a series of stromatolites preserved in the lower part of the Carmel.

As you will see, the students were very successful with their fieldwork, but we had to go back to Ohio early because of the COVID-19 pandemic producing travel and health complications. We left Wooster on Monday, March 9, into a risky but predictable world. By Thursday, March 12, it was clear we needed to get back home. We had three days of fieldwork. Juda and Will adapted immediately to the geology and the gorgeous landscapes, so they were disappointed to leave. We accomplished all our measuring and sampling goals, though.

Now the good parts! The images in the following posts were taken by Shelley, Nick and me.

Today we worked on Juda’s project at the productive Eagle Mountain Ranch locality (C/W-142 EMR). Thank you again to the Smith family for giving us access to their land. The thick conglomerate at the top of the section is the Middle Cretaceous Iron Springs Formation. It rests unconformably on the Middle Jurassic Co-op Creek Limestone Member of the Carmel Formation. We spent all our time in the Co-op Creek Limestone Member, which is informally divided into an upper unit (buff-colored; Juda’s rocks) and lower unit (light gray; Will’s rocks). Our prime targets are the loose slabs eroded from meter-thick oolitic limestones. They often have fantastic trace fossils.

Above is a typical slab collected by Juda for its trace fossils. These are burrow-fillings on the bottom of the bed, formally preserved as convex hyporelief.

Every day starts with a field briefing and exchange of initial observations.

Juda hard at work on the steep slope. The skies are cloudy, with temperatures pleasantly in the 50s. Behind Juda’s head are the light-colored rocks Will is studying.

Will collecting trace fossils for Juda. The slabs are weathered just right to show the fossils in crisp relief.

Team Utah 2020 celebrating a successful first field day.

There was just enough time left in the day to visit the St. George Dinosaur Discovery Site.

This museum is always cool, but it was especially relevant today because it is all about trace fossils! We visit every year we’re in town. Dinosaur trackways are the primary subject — most of them in place.

The students were fascinated, especially since they could now consider themselves ichnologists (trace fossil experts).

After our museum visit we had a delicious barbecue dinner and then went back home to our Hurricane lodgings with our samples and observations.

(Links to the First Day, Second Day, and Third Day.)

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Petroleum Experts Ltd. Donates MOVE Suite to Wooster Once Again!!

Wooster, Ohio — The Department of Earth Sciences is pleased to announce that Petroleum Experts Ltd. recently donated ten licenses of their MOVE suite software package to be used for educational and training purposes.  This is the second year in which Petroleum Experts Ltd. has generously donated 10 licenses of the MOVE suite to the College.  The MOVE suite currently has a market value of $2.54 million (US$), and it is accessed through a hardware based software protection device (a bitlock by Petex’s Network Licensing Manager, HARDLOCK).

The MOVE suite is the global industry standard for structural modelling, and its software modules include 2D/3D kinematic modelling, geomechanical modelling, sediment modelling, fracture modelling, fault analysis, and stress analysis, to name a few.  When using the MOVE suite, Wooster faculty and students will be able to interpret data, build cross-sections, and kinematically and dynamically analyze structural histories.  More information about Petroleum Experts Ltd. and the MOVE suite can be found at http://www.petex.com/products/move-suite/.

Petroleum Experts Ltd. is based in Edinburgh, Scotland, with a satellite office in Houston, Texas.  The Department of Earth Sciences is appreciative for the diligent team effort at Petroleum Experts Ltd. that worked to make this current year’s donation possible.  We are also grateful for the conscientious work of numerous colleagues at the College (Vince DiScipio, Ellen Falduto, Lisa Perfetti), especially those in Technology Services who install and upkeep the MOVE suite.

During the upcoming calendar year, our faculty and students will benefit enormously from using the integrative MOVE suite; those students enrolled in the following courses will have access to class modules that require MOVE modelling capabilities: ESCI 340 (Structural Geology), ESCI 345 (Tectonics and Basin Analysis), and ESCI 401/451/452 (Independent Study).

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New Paper: Synoptic Climatology of Rain-on-Snow Events in Alaska

Back in 2018, some internal sophomore research funding through the College of Wooster allowed me to hire Anna Cooke (’20) to begin an investigation into rain-on-snow events in Alaska. Rain-on-snow is exactly what it sounds like: rain falling on top of snow.  We see this fairly often in Ohio in winter, and our biggest problem with rain-on-snow is usually that all of that rain melts the snow packs. This makes flooding more likely because it’s not just the rain falling that raises river levels — it’s also the melting snow.

Figure 1: Alaskan caribou (photo credit: Alex Crawford)

But in Arctic environments, rain-on-snow can have a more insidious impact. If, instead of melting the snowpack, the rain trickles through the snow and cools down to the point of freezing, it will form lenses of ice at the base of the snow. That ice impedes that ability of animals like caribou (Figure 1) to access lichens on the ground below.  Digging through snow is easy, but breaking through ice is energy-intensify at best and impossible at worse. Several examples of rain-on-snow followed by freezing and mass die-offs have been recorded in AlaskaCanada, Svalbard, Russia, and most recently Finland.

Figure 2: Counts per year for various precipitation events in Alaska from the MERRA-2 atmospheric reanalysis (1980-2018).

In a new paper just published in final form at Monthly Weather Review, we built on the work that Anna began as a sophomore researcher by applying a storm detection and tracking algorithm that I developed for my PhD and connecting rain-on-snow events at particular locations to the storms that generated them. This allowed us to answer questions that had never been addressed before, especially: Is there anything special about storms that produce rain-on-snow?  Are they more intense than other storms?  Are they generated in different ways? Do they take distinct paths? Answering such questions can better help us understand and predict these events, which sometimes have dire consequences for caribou/reindeer, musk oxen, and the people who rely on them. Dr. Karen Alley here at the College of Wooster and Dr. Mark Serreze at the University of Colorado Boulder helped out in the planning, interpretation, and writing of this project.

Figure 3: Distribution of storm tracks leading to (left) rain-on-snow and (center) only snow-on-snow in winter months at Bethel, Alaska. The right-hand plot shows where rain-on-snow-producing storms are more common (in red, e.g., the Bering Sea) and where snow-on-snow-producing storms are more common (in blue, e.g., the Gulf of Alaska). A statistical simulation for the region outlined in purple showed that two observed samples of storm tracks have more distinct spatial patterns than 1000 out of 1000 (i.e., 100%) of randomly assigned samples.

The most important finding of our research is this: For most of Alaska (the south, southwest, and interior regions) storms that generate rain-on-snow generally take different paths than other storms, tracking much more often into the Bering Sea than the Gulf of Alaska (Figure 3). Such a storm track places Alaska in what’s called the “warm sector” of a storm, where winds are blowing from the south, bringing more warm, moist air poleward. We also found that storms tend to track into the Bering Sea instead of the Gulf of Alaska when “atmospheric blocking” takes place (Figure 4). We see atmospheric blocking in the lower 48, too: whenever a storm stalls out, not moving, and there’s a big blue H (i.e.,  a high-pressure center) sitting to the east of the storm, that’s an indication of atmospheric blocking. (They’re sometimes called “blocking highs” for that reason.) Why is that important? Well, there’s been a lot of talk about whether atmospheric blocking will become more common in a warming world.  That’s still uncertain, it seems, but it’s something atmospheric scientists will continue to monitor.

Figure 4: Detrended sea-level pressure anomalies (colored shading) and upper-level atmospheric height anomalies (contours) for (right) rain-on-snow events and (center) only snow-on-snow events and (right) their difference. The darker purple in the left-hand plot indicates that rain-on-snow events are accompanied by strong atmospheric blocking. This blocking forces storms north into the Bering Sea and (eventually) the Arctic Ocean. The lighter purple in the center plot shows weaker high pressure and no tendency for blocking. In these situations, storms can plow on through toward the Gulf of Alaska.

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New paper: “Chemical composition of carbonate hardground cements as reconstructive tools for Phanerozoic pore fluids”

My friend Paul Taylor and I are junior authors on a paper that has just appeared in the journal Geochemistry, Geophysics, Geosystems (“G-Cubed”) as an in press accepted manuscript. We’ll be the first to admit that it is a bit outside our comfort zone in geology, but our contributions, along with those of the other authors, are a good example of interdisciplinary team work. We were led by Dr. Andrea Erhardt at the University of Kentucky. This project took about four years from first draft to published article. Here is the abstract —

This study uses the chemical composition of early carbonate cement precipitates in carbonate hardgrounds to understand the geochemical signature of near-surface carbonate mineral precipitation. As carbonate hardgrounds lithify at or near the sediment-water interface, they acquire cements that may be minimally evolved from paleo-seawater. While hardgrounds can be subaerially exposed during sea-level regression, geochemical changes from interactions with meteoric water can leave a distinct geochemical signature. Using a suite of chemical measurements, we explore the potential of carbonate hardground cements as paleoenvironmental proxies. Trace metal and isotopic ratios, including rare earth elements, Mg/Ca, manganese and strontium concentrations, d18O, d13C, and 87Sr/86Sr, were analyzed in the carbonate cements from 17 Phanerozoic carbonate hardgrounds. Of these samples, only our sample from the modern oceans has measurements consistent with primary precipitation from seawater; all other samples precipitated from chemically evolved seawater or were influenced by meteoric water, even if only minimally changed. While the more recent Cenozoic samples had seawater 87Sr/86Sr, the Mesozoic samples, in contrast, did not preserve seawater 87Sr/86Sr, even though the Mg/Ca, d18O, and d13C values were consistent with precipitation from seawater. Finally, the Paleozoic samples preserved expected seawater 87Sr/86Sr, though REE and d18O suggest primary precipitation was from evolved seawater. Additionally, we place our results in the context of open vs. closed system precipitation using transects of the Mg/Ca ratios across individual cements. Overall, we stress that one proxy provides only a partial record of fluid composition, but multiple measurements allow a potential understanding of the seawater geochemical signal. [Sorry that I couldn’t figure out how to include superscripts and Greek letters!]

Fortunately this journal requires a Plain Language Summary —

All potential archives for reconstructing ancient seawater chemistry have complicating factors, be it biological modification or secondary alteration. This study investigates a promising alternative, carbonate hardground cements. As carbonate hardgrounds form relatively quickly and in equilibrium with seawater, if a sample has remained unaltered it should retain the primary seawater chemistry. We evaluate 17 samples from across the Phanerozoic, compiling trace element concentrations and isotopic ratios to determine if a sample has undergone significant diagenesis. Overall, no ancient sample satisfies all criteria, but the suite of measurements allows for an evaluation framework for future samples.

Hardgrounds are synsedimentarily-cemented seafloor. In other words, sediments that have essentially lithified into rock on the seafloor. The top image is of an echinoderm-encrusted Ordovician carbonate hardground from the Kanosh Formation of west-central Utah, which was included in this study. I’ve loved hardgrounds for decades now, learning much from my friend, the master of hardgrounds, Tim Palmer of the University of Wales, Aberystwyth. You can see in the first sentence of the discussion in this paper the primary role Paul Taylor and I played: “Our samples were selected based on evidence of early lithification at the sediment/water interface through the presence of marine boring and encrusting organisms.” That early lithification is with calcite cement generated from seawater in some form, thus the possibility that these hardgrounds are archiving ancient seawater composition. Seawater composition, of course, tells us much about marine paleoenvironments.

Figure 4 caption: “Mg/Ca ratios, strontium concentrations, and Mn/Sr ratios for samples showing examples of A) closed and B) open system precipitation behavior. Samples from potential closed system environments show an increase in Mg/Ca ratios along the growth axis, while samples from open systems show uniform Mg/Ca ratios. Strontium concentrations and Mn/Sr ratios can be indicators of diagenetic alteration, with thresholds of less than 300ppm for strontium and Mn/Sr ratios greater than 2 consistent with carbonate recrystallization under chemically evolved pore waters. The red lines indicate the trace of the LA-ICPMS.”

One aspect of this project I appreciate very much: The results are fuzzier than we expected. No single geochemical proxy shows a full record of the composition of the original cementing fluids. It is the combination of proxies that gives us the best clues, which is an incremental move towards better understanding of ancient seawater geochemistry. It is nice to see such data, observations and ideas published without a tight evidentiary ribbon around it all. Science in progress!

Reference:

Erhardt, A.M., Alexandra V. Turchyn, A.V., Dickson, J.A.D., Sadekov, A.Y., Taylor, P.D., Wilson, M.A. and Schrag, D.P. 2020. Chemical composition of carbonate hardground cements as reconstructive tools for Phanerozoic pore fluids. Geochemistry, Geophysics, Geosystems (in press; accepted manuscript online; https://doi.org/10.1029/2019GC008448).

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