Greater Yellowstone Area Eocene to Recent Hydrothermal Springs

The Gravelly Range spring deposits depicted in this photo are late Eocene (probably 34-36 million years in age).

Geologic field work is always fun, but especially so when it turns up something unexpected. Working on Eocene to Recent geology and vertebrate paleontology in the Gravelly Range, southwestern Montana promised to be enthralling because the volcanics, sedimentary units, and vertebrate fossils are at elevations of about 9,000 feet. But to come across extensive, unmapped calcareous spring deposits of probable Eocene age is topping off research efforts.

At this point, I’ll just say that our field team is still at work on the Tertiary spring deposits. We’ve found numerous leaf impressions including those of ginkgo, palm, metasequoia, Fagopsis (extinct member of Beech family), and alder – just to name a few. We’ve shown the plant assemblage collected to date to several paleobotanists, and, at least for age, their take is that the assemblage is probably latest Eocene in age, and bears many similarities to Florissant, Colorado fossil plant assemblages.

Palm frond impression from Gravelly Range spring deposit.
Ginkgo leaf impression from a Gravelly Range spring deposit.
Alnus cone from a Gravelly Range Spring deposit.

The spring deposits in the Gravelly Range are extensive, covering an area roughly 2 miles in length with deposits up to 120 feet in thickness. The springs are best characterized as travertine, although the spring systems’ edges contain clastic fluvial units and both the springs’ edges and pools have features such as plant impressions, root systems, and small travertine balls.

Gravelly Range Eocene spring deposit. Field backpacks in lower left corner for scale.

Because the Gravelly Range is so close to Yellowstone National Park, it is extremely interesting to compare its Eocene spring deposits to hydrothermal units at both the currently active Mammoth Hot Springs (which probably began its activity about 7,700 years ago), and to the fossil travertine found just north of Gardiner, Montana, that formed about 19.500 to 38,700 years ago (Fouke and Murphy, 2016: The Art of Yellowstone Science: Mammoth Hot Springs as a Window on the Universe).

The Gardiner travertine is fairly well exposed because it has been extensively quarried for several decades. Of interest for comparison are numerous plant impressions that occur within microterracettes. Fouke and Murphy (2016) suggest that these may be impressions of sage brush. A photo of the quarried wall with the plant impressions is shown below.

Plant impressions in Gardiner travertine. These impressions may be from sage brush. The travertine in this quarry face is estimated at about 30,000 years in age.

Other features in the Gardiner travertine, now partly covered by graffiti, include a quarry wall that shows terracettes and microterracettes that are outlined by darker lines within the travertine. These features are probably indicative of a proximal slope facies.

Gardiner travertine with its slope facies depicted well in smooth quarry face. The dark, irregular lines delineate terracettes and microterracettes.

Jumping forward in time to the extensive spring deposits of Mammoth Hot Springs (just within the northeast park boundary of Yellowstone National Park), is mind boggling. As in any comparison with rocks as old as Eocene to active deposition, one realizes how much detail is lost over time. But it is still worthwhile to try to compare spring features, so I’ll show a few photos of the Mammoth Hot Springs that may match up with various features of the fossil springs.

Branch and plant fragments in the process of becoming calcified at Mammoth Hot Springs – main terrace.
Calcified plant debris – Mammoth main terrace.
Terracettes – Mammoth main terrace, proximal slope facies.
Trees engulfed by prograding spring activity – Mammoth main terrace.
Travertine balls in small pond – Mammoth main terrace.

Suffice it to say, that the upcoming field season should be a good one, with more work to be done on the Gravelly Range spring deposits. And – it’s always fun to get a trip in to Yellowstone!









Ocean Acidification and Climate Change

A news item caught my interest recently – a National Public Radio (NPR) news segment of 11/23/2012 on whether shellfish can adapt to increasingly acidic oceans (NPR shellfish link). Because UN Climate Talks opened in Doha, Qatar today, I thought it would be an appropriate time to talk about ocean acidification trends. As noted by Lauren Sommer during the NPR broadcast, “Scientists say oceans are becoming more acidic as they absorb the carbon dioxide added to the air through the burning of fossil fuels. That can be bad news for oysters, mussels and others animals that are key to the seafood industry and to the marine food web. Scientists are using the unique ocean conditions off the California coast to monitor developments”.

The first comment posted on the NPR web site page of the shellfish story tried to debunk ocean acidification. However, as noted in a second comment, data from several studies clearly show that ocean pH is decreasing. In fact, a study cited by both commentors is the Wootton,, 2008 study (Wootton study link) of ocean pH which is based upon a multi-year data set. The authors of this article give a summary statement of “…our results indicate that pH decline is proceeding at a more rapid rate than previously predicted in some areas, and that this decline has ecological consequences for near shore benthic ecosystems”.

Being a geologist, I like to look at the present oceanic decline in pH in a geologic context. One of the most interesting articles in this area that I’ve found is a 2012 paper by Honisch et. al. that was published in Science (Honisch study link). The authors of this paper, 21 ocean/climate geoscientists, examined the geologic record (these authors confined their study to only approximately the past 300 million years due to the presence of pelagic calcifiers similar to those living today that make the deep-sea carbonate buffer of the modern Earth system) for events that could be associated with ocean acidification, such as mass extinctions and evolutionary turnovers among marine calcifiers. They recognized eight geological events that could be similar to what is occurring today, but state, “…Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry—a consequence of the unprecedented rapidity of CO2 release currently taking place”.

One of the better analog events identified by the Honisch study is the Paleocene-Eocene Thermal Maximum (PETM) which began about 56 million years ago. During the PETM, global temperature rose about 5 degrees Celsius – a temperature elevation that occurred within just a few thousand years. The increased temperature probably resulted from massive additions of heat-trapping greenhouse gases into the atmosphere sourced initially by volcanic eruptions and later by destabilized methane hydrate deposits and other related events such as wide-spread forest fires and thawed permafrost. It took about 200,000 years for the earth’s systems to counteract this elevated temperature. Consequences of this climate change are wide-ranging and include occurrences such as the largest extinction among deep-sea benthic foraminifers of the past 75 million years, poleward shift of many animals and plants, redistribution of mammals over high-latitude land bridges, and organism adaptation such as smaller body size. However, to put the PETM’s massive greenhouse gas injection into the atmosphere into a context for the present day, Lee Kump, in a 2011 Scientific American article on the PETM (Kump paper link), suggests that the PETM greenhouse gas release “… was only 10 percent of the rate at which heat-trapping greenhouse gases are building up in the atmosphere today”.

As the 21 geoscience authors in the 2012 Honish study summarized, “…the current rate of (mainly fossil fuel) CO2 release stands out as capable of driving a combination and magnitude of ocean geochemical changes potentially unparalleled in at least the last ~300 My of Earth history, raising the possibility that we are entering an unknown territory of marine ecosystem change”.

Let’s hope that there is huge progress made at the UN Climate Change talks.