4. Late Cenozoic Ice Age

4.1 Causes of Glaciation

The greenhouse conditions that began following the end of the Late Paleozoic Ice Age and peaked during the Eocene Epoch of the Early Cenozoic Era lasted until 34 million years ago, when the planet went back into an ice age at the start of the Oligocene Epoch. This ice age has persisted through the present day (Figure 3B.4.1). This began with the growth of ice sheets on Antarctica around 34 Ma followed by growth of Northern Hemisphere ice sheets around 3 Ma. Since the onset of the current ice age, the extent of continental ice sheets has waxed and waned through glacial and interglacial cycles. In the current interglacial cycle, only the Antarctic and Greenland ice sheets are still present.

Graph showing global surface temperatures from 66 Ma to present day. Temperatures were about 25 degrees Celsius (C) through the Paleocene from 66 to 56 Ma, rising to a high around 28-29 C in the Early Eocene through 50 Ma. Then temperatures steadily dropped from 50 Ma through 35 Ma where they stayed at a steady temperature of around 20 C until dropping again starting around 15 Ma through to present day where temperatures are about 12 C. Blue bars showing ice sheet growth show Antarctic ice sheet beginning around 34 Ma and continuing to present day and the Northern Hemisphere ice sheets beginning around 3 Ma and continuing to the present day. Green arrows for plate tectonic events have India collision with Asia beginning 50 Ma, Antarctic Circumpolar Current beginning 40 Ma, and the Isthmus of Panama beginning 3-5 Ma.
Figure 3B.4.1 Global surface temperature trends over the Cenozoic Era. Growth of ice sheets over this time shown in dark blue bars. Major plate tectonic initiated events shown in green arrows. Red curve is the 500,000-year running average for the data. Source: Lindsay Iredale (2024). CC BY-4.0 Modified after Steven Earle (2015). CC BY-4.0. Found here. Using global surface temperature data from Hansen et al. (2013). CC BY-3.0. Found here.

The cooling of the planet from the warm climate of the Early Eocene is attributed to a series of plate tectonic events. The collision of the Indian Plate with the Eurasian Plate began the process of Himalayan mountain building around 50 Ma. The rapid uplift of these mountains and formation of the high landscape of the Himalayan-Tibetan Plateau increased rates of weathering and erosion pulling CO2 out of the atmosphere and initiating long term cooling.

Around 40 Ma, plate motions widened the narrow gap between South America and Antarctica opening the Drake Passage. Prior to this, the arm of land connecting South America and Antarctica allowed warm ocean currents to carry heat from the equator to Antarctica, but when the gap opened up, a new cold-water current, the Antarctic Circumpolar Current, formed (Figure 3B.4.2). This current flows in a continuous loop around Antarctica and blocks warm water from reaching Antarctica. In the already cooling climate, the isolation of Antarctica from warm water currents cooled the southern polar region further, allowing ice sheets to grow by about 34 Ma, marking the onset of the Late Cenozoic Ice Age.

Diagrams illustrating how ocean currents changed when the Drake Passage opened. Left: 40 million years ago, South America is connected to Antarctica with a narrow area of land, this allows the warm water current that passes down the western side of the Atlantic to bring warm water from the equator down to Antarctica. Right: The Drake Passage opened around 40 million years ago (the narrow area of land connecting South America to Antarctica split apart to create the Drake Passage which is the narrow ocean area between these two continents). With this open, cold water could circulate all the way around Antarctica, which blocked the warm water current that flows down from the equator along the western side of the Atlantic Ocean from reaching Antarctica.
Figure 3B.4.2 The opening of the Drake Passage changed how ocean currents moved heat around the planet. Left: Prior to 40 Ma Antarctica and South America were connected allowing a warm water current to bring heat to Antarctica. Right: After 40 Ma, the Drake Passage opened between South America and Antarctica and the Antarctic Circumpolar Current began (cold water current) looping Antarctica and blocking the warm water current. Source: Adapted from Karla Panchuk (2018) CC BY-NC-SA 4.0. Found here. (cropped and reorganized position of images) Map: Modified after Woods Hole Oceanographic Institution.

Following the start of Antarctic glaciation, temperatures leveled off, remaining around 18-20 °C throughout the Oligocene and Early Miocene (Figure 3B.4.1). During this time, Antarctic ice sheets waned but did not melt away altogether and the Northern Hemisphere remained ice free. Ongoing plate tectonics forced another change to climate by creating the Isthmus of Panama. This is the narrow strip of land that separates the Caribbean Sea from the Pacific Ocean in the region of the present-day country of Panama. It was created through volcanism associated with eastward subduction under the Caribbean Plate. Prior to the Isthmus of Panama, the Central American Seaway allowed movement of warm tropical water between the Pacific and Caribbean through this area. The closure of this seaway 3-5 million years ago forced ocean currents into a new path intensifying the warm Gulf Stream current, the shallow current that carries heat and moisture north along the coast of eastern North America before heading across the North Atlantic to Northern Europe (Figure 3B.4.3). Cooling and evaporation of this water in the North Atlantic strengthened thermohaline circulation and introduced extra moisture to the Arctic region. This influx of moisture provided the necessary water to feed ice sheet growth in the Northern Hemisphere beginning about 3 million years ago in the Late Pliocene.

Two diagrams showing how ocean currents changed before and after the Isthmus of Panama was created as described in the figure caption.
Figure 3B.4.3 Formation of the Isthmus of Panama changed ocean currents. Left: Before the Isthmus formed there was a single tropical ocean between North and South America that had relatively uniform conditions of temperature and nutrients. Right: The formation of the isthmus created two different oceans. The Caribbean became warmer and saltier which created a strong Gulf Stream current taking heat and moisture into the North Atlantic. Source: Smithsonian Tropical Research Institute (n.d.) CC BY-NC. Found here.

The Pleistocene Epoch, the time period from 2.58 Ma to about 12,000 years ago, is characterized by intense glaciation across the Northern Hemisphere, with average global temperatures being 4 to 10 °C (about 8-20 F) cooler than present day. This time period is known as the Pleistocene Glaciation, the Quaternary Glaciation, or simply the Ice Age. It is the time of woolly mammoths and saber tooth tigers popularized in movies.

As the most recent time period of extensive glaciation, this cold period also has the best-preserved record of temperature fluctuations. Through the study of ice cores and seafloor sediments a clear pattern of glacial-interglacial cycles has been identified throughout this time frame (Figure 3B.4.4). The high to low temperature variations correspond to contraction and expansion of ice sheets, respectively, with major fluctuations occurring on a 100,000-year cycle. This time interval is a result of Milankovitch Cycle orbital changes. Changing solar irradiance from the Milankovitch Cycles drove the glacial-interglacial cycles in the cooler climate brought on by the earlier plate tectonic events.

Temperature record for the past 5 million years shows global temperatures were around 13 degrees Celsius from 5 to 4 Ma then slowly decreased to an average around 8 degrees Celsius over the past 1 million years. Through this time there are distinct highs and lows that are cyclical and occurring about every 100,000 years.
Figure 3B.4.4 Temperature record for the past 5 million years based on Oxygen isotope data from seafloor sediments. Note the high to low cycling of temperatures on a 100,000-year interval. Source: Steven Earle (2015), CC BY 4.0. Found here. Using data from Lisiecki and Raymo (2005). Access the data.

4.2 Effects of the Pleistocene Glaciation

Glaciation reached a peak about 20,000 years ago during the Last Glacial Maximum (LGM). At this time ice sheets were at their largest, covering sizable portions of North America and Northern Europe (Figure 3B.4.5). The water to grow ice sheets was supplied from the oceans, so as ice sheets grew sea level dropped. Sea level was lower by 125 m (over 400 ft) during LGM, exposing land areas that flooded once the ice sheets receded again. On Figure 3B.4.5 the present-day coastline is outlined in white for comparison with ocean levels during the LGM.

Glaciers during the Last Glacial Maximum covered all of the arctic, Greenland, Canada, and extended into the northern US and northern Europe. Lower ocean levels at this time exposed more land globally. In the US this is most prevalent along the east coast and in the Gulf of Mexico.
Figure 3B.4.5 Map of glacial ice coverage during the Last Glacial Maximum. Source: NOAA (2021). Public Domain. Found here. Based on data from the University of Zurich Applied Sciences, provided by Science on a Sphere.
One of the effects of having lower sea level was that land bridges were created where before there was water. An important one for North America was the Bering Land Bridge connecting Siberia with Alaska. This allowed land animals to travel between Asia and North America. The Bering Land Bridge also allowed humans to migrate into North America. As climate warmed at the end of the LGM, the ice sheets melted and sea level rose flooding the land bridge. This area is the present day shallow sea called the Bering Strait (Figure 3B.4.6).
short gif showing the Bering Land Bridge, present at 21,000 years ago, slowly flooding over to become the Bering Strait (current ocean) as glaciers melted and sea level rose.
Figure 3B.4.6 The Bering Land Bridge was present between Siberia and Alaska but as glaciers melted and sea level rose, the land bridge was flooded and replaced with the present day Bering Strait. Source: NOAA (1999). Public Domain. Found here.

At the end of the Pleistocene Glaciation, the climate was warming and entering the current interglacial cycle in the Late Cenozoic Ice Age. As the ice sheets retreated, they left massive lakes of glacial meltwater on the landscape. Lakes formed in this way are termed proglacial lakes. A significant one of these in North America was Lake Agassiz, which covered most of Manitoba and western Ontario in Canada and covered a major portion of northern Minnesota with a long arm that extended along the Minnesota-North Dakota border in the present-day location of the Red River Valley (Figure 3B.4.7, left). At its maximum, it was the largest freshwater lake to have ever existed on the planet, with a volume greater than all of the Great Lakes combined. Lake Agassiz existed because this area of land drains north into Hudson Bay, but the slow retreat of the ice sheet kept the outlet to Hudson Bay blocked (Figure 3B.4.7, right). The meltwater had nowhere to go other than to pool into a massive lake on the edge of a retreating ice sheet. As the edge of the ice sheet slowly retreated north, the location of Lake Agassiz shifted north as well until finally an outlet to Hudson Bay was established and the lake water drained into the Bay. Present day remnants of Lake Agassiz include Red Lake, Rainy Lake, and Lake of the Woods in Minnesota, Lake Manitoba, Lake Winnipeg, and Lake Winnipegosis in Manitoba.

left: map showing the full extent of Lake Agassiz. It covered almost all of Manitoba, most of western Ontario, and northern Minnesota with a long arm that extended down along the North Dakota/Minnesota border.Right: Map of Lake Agassiz 13,000 years ago. At this time the edge of the ice sheet was not far north of the US/Canada border so only a small part of southern Manitoba and western Ontario was covered in water. Northern Minnesota and the arm of the lake that extends south along the Minnesota/North Dakota border were covered in water.
Figure 3B.4.7 Left: Map showing maximum extent of Lake Agassiz. Right: Extent of Lake Agassiz 13,000 years before present. Source: Left: “Maximum extent and major features of Lake Agassiz” by Manitoba Historical Maps (1983). CC BY-2.0. Found here. Right: “Lockhart Phase of Lake Agassiz, c. 13,000 YBP”. Teller and Leverington, 2004 (U.S. Geological Survey). Public Domain. CC BY-SA 3.0. Found here.

Minnesota Connection: Flooding in the Red River Valley

Flooding along the Red River Valley, the area situated on the North Dakota-Minnesota border and extending north into Manitoba, is a yearly occurrence. The propensity for flooding in this region is a direct result of the Pleistocene Glaciation.

Glacial ice is dirty, carrying lots of clay, silt, sand, and gravel. As the ice flows over the landscape, it essentially behaves like a giant piece of sandpaper, scraping the landscape flat. Lake Agassiz pooled over this already flat landscape contributing a layer of lake floor sediments rich in clay and silt. As the lake emptied off of the landscape, the Red River was left to drain this area. The Red River flows north along the Minnesota-North Dakota border through what was the southern arm of Lake Agassiz; a landscape that is both very flat and very rich in clay sediments. Almost every year during the spring, snow meltwater raises the river levels and the Red River Valley floods. With a flat landscape and clay rich sediment, the flood waters do not have anywhere to go. The flatness means water can spread quite far and then pool on the landscape as there is no slope to encourage it to flow back into the river channel (Figure 3B.4.8). In addition, the clay rich soils have low permeability making it hard for floodwater to infiltrate into groundwater.

Left satellite image shows flood water extending across most of the area around the Red River. Right: Water is at least a foot deep across the entire area, showing hay bales stacked up on a farmer's field.
Figure 3B.4.8 Images of Red River Flooding. Left: Satellite image showing flood extent in 2010 flood. Right: Flooded farm field in the Red River Valley in Manitoba. Source: Left: “Red River Flooding in North Dakota” NASA Goddard Space Flight Center (2010). CC BY 2.0. Found here. Right: “Red River Valley Overland Flooding” Jordan Morningstar (2009). CC BY 2.0. Found here.

These are only a few of the many landscape effects that occurred as a result of Pleistocene Glaciation both in Minnesota and across the planet.

Check your understanding: Late Cenozoic Ice Age

References

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