How and why do we collect sediment cores in Yellowstone Lake?

Release Date:

In August 2021, YVO scientists collected sediment cores from the floor of Yellowstone Lake. Analysis of the sediment composition, as well as the fluids contained within the sediment, can provide new information about hydrothermal activity occurring out of view beneath the lake water.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Pat Shanks and Lisa Morgan, Scientists Emeriti with the U.S. Geological Survey, and Rob Harris, Professor in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University.

Bathymetric map of Yellowstone Lake

Bathymetric map of Yellowstone Lake showing hydrothermal features in the north part of the lake, including Elliott's Crater, Mary Bay, and Deep Hole.  Colors correspond to lake depth, with cooler colors indicating greater depths.

(Public domain.)

Although hidden from view of the many visitors to Yellowstone, the floor of Yellowstone Lake has many hydrothermal features.  In fact, it is the third-largest thermal basin in the Park!  Scientists study the lake floor using a variety of sophisticated tools, including sonar imaging of the lake bottom, magnetic surveys, heat-flow surveys using thermal probes, and direct sampling of rocks, sediment, and biota using remotely operated vehicles (ROVs) and sediment coring devices.  These tools provide a better understanding of the geologic, geochemical, and geophysical processes operating beneath the floor of Yellowstone Lake.  

In August of 2021, a group of geoscientists ventured out onto the lake on the R/V Annie, a specially built, 40-ft-long boat designed for lake research, to collect relatively short (<1 m / 3 ft) sediment cores from carefully selected target areas.  The goal was to improve knowledge of lake-bottom hydrothermal hot springs, hydrothermal alteration of sediments, hydrothermal explosion craters and deposits, and structures (faults, fractures, fissures) that cut the lake floor.

Sediments contained within the short cores represent hundreds of years of lake history, where the oldest sediment is at the bottom of the core and the youngest at the top.  The mineralogy and geochemistry of the collected sediments will be studied to better understand variations brought about by interactions with hot fluids or hydrothermal explosion ejecta.  Pore water contained within the sediments also was extracted to determine whether the fluids have compositions representing normal lake water, or rather are hydrothermal fluids resulting from high-temperature reactions with sediment or rocks deep beneath the lake floor.

Gravity coring device after sampling Yellowstone Lake sediment

Gravity coring device on the rear deck of the R/V Annie after coring the floor of Yellowstone Lake, with dark mud coating the outside of the corer.  The 100-lb. green coring head is at the top, and the bottom of the barrel has a tapered stainless steel core cutter.  Mud-filled plastic core-liners are removed from the core barrel and replaced with a new core liner for the next core.  Thermal outriggers are the two fin-like appendages on the core barrel that hold recording thermistors for heat flow measurements.

(Credit: Pat Shanks, USGS. Public domain.)

The coring targets included the deepest part of the 13,000-year-old Mary Bay and 8,000-year-old Elliott’s Crater hydrothermal explosion craters in the northern part of Yellowstone Lake.  The greatest depths in these large craters are actually in smaller craters that were formed after the main crater-forming explosions.  Both Mary Bay and Elliott’s Crater continue to have active hydrothermal vents today.  The team also targeted:

  • a vent area on the North Basin Hydrothermal Dome to sample stiff, hydrothermally altered mud that caps the dome
  • the extensional fissures west of Stevenson Island, which are suspected of once being a hydrothermal area but have low heat flow values today
  • the Bridge Bay spire area that includes an area of inactive silica-rich spires rising ≥8 m (25 ft) above the lake floor from depressions interpreted as former hydrothermal craters 

Sediment cores were collected using a gravity corer connected to the boat by a Kevlar line on a winch.  The coring device was lowered to near the lake bottom and then allowed to free-fall ~5–10 m (15–30 ft) into the lake mud.  This study collected thermal data by fitting the coring barrel with outriggers that carried temperature-measuring devices.  These thermal measurements provide information on present-day heat flow in the subsurface that can reflect ongoing hydrothermal fluid flow.  During the work, the team collected nine cores that varied in length from 14 to 59 cm (5.5 to 23.2 in). 

Although the analysis is just beginning, the cores already are providing important insights.  Two hot cores with temperatures up to 91°C (196°F) were collected from the Mary Bay explosion crater.  In contrast, two cores from Elliott’s Crater were cold, but the cores may have interesting chemical signatures in the pore fluids or sediments.  Three additional cores were collected from the area west of Stevenson Island and should provide a good basis for determining if the region was formerly a hydrothermally active area.  Two cores from the Bridge Bay spire area will provide evidence for its hydrothermal history.  Preliminary results indicate pH values of pore fluids from 6.0–7.5, slightly acid to slightly alkaline (neutral pH is 7.0).  Some of the pH values are outside the normal range for lake water, suggesting that the samples may be alkaline-chloride hydrothermal fluids, while others may be influenced by acidic vapor-dominated fluids.

During the winter of 2021–2022, YVO scientists will complete a full analysis of the sediment composition from the sampled cores, as well as the fluids those cores contain.  Analysis of heat flow data also will provide new understanding of thermal activity and thermal history of these important sites.  The lessons learned should provide new insights into the dynamic hydrothermal system hidden from view on the floor of Yellowstone Lake!

Extracting pore water from Yellowstone Lake sediment cores

Pore waters from Yellowstone Lake sediment cores collected in August 2021 are extracted through filtration devices into plastic syringes.  Note that the second core from the left appears light in color because the plastic core liner was etched by very hot 91°C (196°F) fluids.

(Credit: Pat Shanks, USGS. Public domain.)

Related Content

Filter Total Items: 7
Date published: June 7, 2021

Henry Wood Elliott and the first map of Yellowstone Lake

Henry Wood Elliott was a dedicated conservationist and explorer who, in 1871, helped create the first bathymetric map of Yellowstone Lake. Unlike many of his contemporaries, however, he declined to leave his name on any feature in Yellowstone. Geologists now honor Elliott’s legacy by referring to a very large explosion crater beneath Yellowstone Lake as Elliott’s Crater.

Date published: January 18, 2021

Where do acid-sulfate hot springs come from and why are they important?

Yellowstone hosts thousands of thermal features which have diverse chemistries and origins. The most iconic features, like Old Faithful, have neutral to alkaline pH. Some Yellowstone features, however, can be acidic enough to break down the very rock that hosts them!

Date published: December 28, 2020

Hydrothermal explosions hidden beneath Yellowstone Lake’s serene waters

Although Yellowstone Lake itself may seem calm, the floor of the lake is littered with hydrothermal explosion craters.  Detailed studies are beginning to reveal the details of these explosions, like the one that formed Elliott’s Crater about 8000 years ago.

Date published: August 17, 2020

Why do most geyser- and sinter-producing hot springs have alkaline (basic) pH?

It’s a common misconception that all geysers and hot springs in Yellowstone are acidic.  Some are, but the water that comes out of many of Yellowstone’s most iconic features, like Old Faithful and Grand Prismatic Spring, is actually basic.  But why

Date published: July 22, 2019

Alterations to go! Hydrothermal alteration in Yellowstone

What is hydrothermal alteration, and why is it important? Most visitors to Yellowstone National Park are only vaguely aware of hydrothermal (hot water) alteration (chemical and mineral reactions with hot water).

Date published: December 3, 2018

The Hydrothermal System in Yellowstone Lake

When you think of a lake bed, what comes to mind? Squishy bottom with some grasses, rocks, and sunken logs?

Date published: April 2, 2018

Exploring the depths of Yellowstone Lake

Yellowstone Lake is huge. It is the largest high-altitude (above 2130 m, or 7000 ft) freshwater lake in North America, covering about 341 square kilometers (about 130 square miles). That's about 100 times the size of New York City's Central Park!