A Meromictic Treasure in Petroglyph Park

 by Nina Munteanu

 Looking for ancient treasure, I drove north from Peterborough to Petroglyph Park in the Great Lakes-St. Lawrence Lowlands Forest Region, a sought-after destination for its impressive ancient petroglyphs (rock carvings). Holes in the rock were considered entrances to the spirit world, situated directly beneath the surface (spirits prefer to live near water).

When I reached the park, I discovered that the glyphs were off-limits because of COVID. Disappointed, I looked to salvage my trip by hiking the 2 km loop trail to McGinnis Lake. The walk from the west day use parking lot took me through dense pine forest. Giant pines thrust high above me like columns of a sacred cathedral. Their deep green canopies swayed and creaked in the breeze as they strained toward the heavens in a low baritone hush. I passed pink granite outcrops in the soft limestone and found myself on a rocky promontory that overlooked the over 12 m deep lake. The 4.4 ha lake’s water was a deep blue-green jade colour rimmed by shallows of lighter green that graded to a cream colour. Rocks, logs and large shore debris hung precariously over the steep sides of the lake below me, covered in creamy marl.

On the opposite side of the jade-coloured lake—accessed by the east day use parking lot, the marl-covered shallows extended further out, creating a stunning visual colourscape that shifted from deep blue-green to yellow and cream.

The information sign on the promontory describes McGinnis Lake as a rare meromictic lake.

What a treasure! I’d studied meromictic lakes at university as a limnology student; I’d never actually seen one before. Until now. Meromictic lakes aren’t just rare; they are fascinating in the study of lake formation, type and function.

Many lakes in the northern temperate area of Canada are dimictic: they mix completely (holomixis) twice a year, once in the spring and once in the fall. In shallow lakes, warmed by the sun and mixed by the wind, wind-driven currents keep the water mixed all year round. In deeper lakes, the currents can’t compete with the active summer warming of the upper water mass and density differences develop between upper and lower waters. The lake stratifies into an upper epilimnion and lower hypolimnion, separated by a metalimnion or thermocline barrier. In the fall, with cooling, the density barrier breaks down and the wind-driven currents penetrate into the lower layers to thoroughly mix the lake to the bottom sediments (holomixis) in what’s called vernal turnover.

Unlike dimictic lakes, meromictic lakes experience incomplete vertical mixing of only the upper water mass during the circulation period (called meromixis). The upper water mass (mixolimnion) mixes twice yearly like a dimictic lake; however, below this upper mixing layer lies a salinity barrier known as a chemocline (where dissolved oxygen decreases markedly with depth) and beneath it lies the anoxic water mass known as the monimolimnion, which experiences a fairly constant temperature and higher salinity. The higher dissolved salt at the bottom—and greater associated water density—prevents wind-driven mixing of this bottom quiescent layer and accumulates hydrogen sulfide and methane.

Lake morphology—particularly the relationship of depth and surface area—contributes largely to whether a lake is meromictic and capable of preserving undisturbed laminated sediments. A meromictic lake may develop if it contains a deep hole in a shallow basin or is sheltered from the prevailing wind by tall vegetation or other barriers—like McGinnis Lake, which rests in a steep-sided limestone basin, sheltered from the winds by a dense pine forest. McGinnis Lake may have formed through karst erosion; it may also simply occupy a deep glacial trough. Because of the barrier and lack of mixing, any exchange of dissolved materials from the lower quiescent layer into the mixing layer occurs very slowly through eddy diffusion across the chemocline. This makes Lake McGinnis’s monimolimnion a nutrient sink and why it is, like most meromictic lakes, unproductive (oligotrophic).

This drawing shows how the lake is shallow at its edges and grows deeper in two places.

In summer, when McGinnis Lake is stratified, the top 6 m layer of McGinnis Lake reaches 20-22˚C and its middle 6-12 m layer is typically 7-12˚C. However, below the chemocline, the anoxic monimolimnion (below 12 m), stays a constant 5-6˚C year-round, and is a pinky-brown colour. Few organisms live in the oxygen-depleted monimolinion. An exception are the cyanobacteria (Cyanophyceae), autotrophic bacteria that can survive on hydrogen sulfide at the lake bottom and are responsible for lime depositing in lakes.

Brilliant Jade Colour
The intense jade colour of marl-based McGinnis Lake is partially explained by the presence of calcium carbonate (CaCO3) in the lake from marl—calcium carbonate and clay. The dominant carbonate mineral in most marls is calcite, along with other carbonate minerals such as aragonite, dolomite and siderite. Marl formation and settling is encouraged by bacteria, phytoplankton, and periphyton (attached algae) as temperatures increase in summer. The calcium carbonate—which is present in the limestone bedrock surrounding the lake—acts like a flocculent to clear the lake of the coloured, dissolved substances; as the brown hue is removed, blue and green light can penetrate into the deepest parts of the lake. The most brilliant jades can be seen when the microscopic algae thrive and when the suspended marl increases in volume in mid to late summer.

Presence of marl is also why the water-sunken trees and debris and the entire shoreline are covered in a milky cream-coloured floc—likely a combination of marl deposit and periphyton (attached encrusting and filamentous algae) that help deposit the marl. Examples may include stalked diatoms (Gomphonema) and blue-green alga Oedogonium. The periphyton secrete glycocalyx (fibrous meshwork of carbohydrates) and other mucilage secretions that coat the sediment particles and adsorb organics and other nutrients for their use. This is why the lake’s shallow shores are a dramatic cream-yellow colour and grade to a brilliant green then deep blue-green of deeper overlying waters. Marl are tiny white coloured crystals and as the water warms in the day, so does the volume of crystals in the water. As the summer progresses, the clear deep blue of McGinnis Lake may transform into lighter milkier turquoise with suspended calcium carbonate crystals.

Undisturbed Sediments & Varves
Because a meromictic lake’s bottom water layer doesn’t mix and is permanently anoxic (without oxygen), no burrowing benthic organisms are present to destroy the sediment layers (varves) laid down over time—mostly organics that don’t decay. Because of this, these varves provide an undisturbed history of biological succession and climate change of the last 10,000 years.

Here's a cutaway image of varves - layers of sediment

Undisturbed annual sediment laminations can provide accurate chronology, just like tree rings, over thousands of years, dating back to the late Pleistocene and Holocene 10,000 to 12,000 years ago. This is because sediment accumulation—just like tree growth—often follows a seasonal pattern. Annual accumulations of sediment may consist of a simple two-component couplet (summer vs. winter sedimentation). In summer increased photosynthesis causes settling of CaCO3; while in winter, when the lake is ice-covered, fine organic material and clay settles to the bottom.

This is a layer of diatoms in a varve

Varve couplets (summer-winter layers of a year) typically consist of a dark layer of organic sludge with algal filaments, iron sulfides, and clay that grades upward into a lacy network of diatom frustules and organic matter; this would be overlain by a light layer of diatom frustules and calcite that turns into pure calcite at the top. In summer, calcium carbonate and diatoms (algae with silica shells) accumulate on the bottom; in winter more fine organic matter and clay settle.

These shapes are tiny diatoms!

On my way home, I considered my fortune: I’d found a real ancient treasure after all, something
I hadn’t expected to see.

References:
Anderson, Roger Y., Walter E. Dean, J. Platt Bradbury, and David Love. 1985. “Meromictic Lakes and Varved Lake Sediments in North America. USGS Bulletin 1607.

Burkholder, JoAnn M. 1996. “Interactions of Benthic Algae with Their substrata. B. The Edaphic Habit: Epipsammic and Epipelic Algae among Sands and Other Sediments. Algal Ecology: Freshwater Benthic Ecosystems, R. Jan Stevenson et al., editors. Academic Press. 753pp.

Cheek, Michael Ross. 1979. “Paleo-indicators of Meromixis.” M.Sc. Thesis, Brock University, St. Catherines, Ontario. 129pp.

Stewart, K.M., G.E. Likens. 2009. “Meromictic Lakes.” In: Encyclopedia of Inland Waters, G. E. Likens, editor-in-chief. Academic Press. 2250pp.