by Claudia Schröder-Adams
The Arctic summer field season is a short one. A team of dedicated students from Carleton University’s Earth Sciences Department have accompanied me into the field for the last four years, to spend a few short weeks unravelling the ancient climate history as imprinted in the rocks of the High Arctic. With just two to three weeks available to us, we came prepared to work quickly looking for evidence of climate change at the top of the world through an examination of marine fossils.
We witnessed extensive glacier melt during our summer field season in 2011 at the head of Glacier Fiord, southern Axel Heiberg Island, noting that fresh rock outcrops had become exposed. These exposed rocks tell a story of an ancient Arctic region that was icefree during summers with much warmer average temperatures than today. Extensive sedimentary sections along the glacier’s erosional path date back to a geological time period called the Cretaceous (145 to 65 million years ago).
Earth climate history can be divided into greenhouse and icehouse conditions depending on the absence or presence of continental glaciers. By using this qualifier our present climate is in icehouse conditions and the Cretaceous Period was in greenhouse conditions.
The Cretaceous was a unique time during earth history where our planet was shaped by extensive formation of mid-oceanic ridges, intense volcanism and high levels of atmospheric carbon dioxide (CO2) and other greenhouse gases that resulted in temperature maxima. At times these processes created a nearly ice-free globe with high sea levels flooding large continental segments. These marine sedimentary records with their fossil content tell the story of a much different earth than the one we know today. It is that marine record that my team is here to examine.
Polar regions are most sensitive to climate change. Ice-free poles will trigger changes in marine water column structure by delivering large amounts of freshwater to oceans due to glacier melting. This lowers salinity in surface waters causing watermass stratification to which marine organisms have to respond. For example, organisms that are stenohaline, which means adapted to a narrow salinity range, are more likely to become extinct, as salinities will inevitably change. Increased CO2 uptake in marine waters can ultimately lead to ocean acidification that in turn will be detrimental to marine organisms that secrete calcium carbonate to build their shells. This will open up niches for organisms with siliceous tests such as diatoms, which are microscopic algae made out of biogenic silica, or radiolaria, which are microscopic zooplankton. As ice covers melt earlier in the spring or retreat completely, summer primary productivity explodes, sending large amounts of organic matter to the seafloor for burial. As stratified oceans inhibit the vertical transfer of oxygen from the surface to the seafloor, benthic environments suffer from the lack of oxygen and biological communities living on the seafloor collapse. These environmental changes are predictions for a future Arctic Ocean if the current trend to warmer temperatures continues.
Can the Cretaceous rocks in the Arctic Polar Region serve as an analogue to better predict future environmental changes? We believe they can, although let’s not forget that all Cretaceous climate drivers were unaffected by humans because our species was not around yet. Two summer field seasons have been devoted to studying the Cretaceous record on beautiful Axel Heiberg Island and desolate Ellef Ringnes Island and have shed light on an Arctic region where trees once grew and oceans were ice-free during summer months. These sedimentary records are part of the Sverdrup Basin, a sedimentary basin that holds the answer to more than 250 million years of Arctic paleoenvironmental changes that are preserved in the rock record. During Cretaceous time this basin had its deepest parts where Ellef Ringnes and Amund Ringnes islands are now situated, whereas the Axel Heiberg study locality reflects a shallower marine shelf setting of that ancient sea.
Ellef Ringnes Island has relatively flat topography and large areas are covered by indistinct black mud giving the uninhabited island a forbidding appearance. Those muddy sections were, of course, our target since they reflect the upper Cretaceous seafloor of the ancient Sverdrup Basin. After finding our camp spot, tents were set up, radio antenna assembled and our kitchen dome tent became our home when the weather turned nasty. During a routine radio check-up we were told that we had to ration our water because of a helicopter breakdown at the main camp of the Geological Survey of Canada. As this was well beyond walking distance, we resorted to gathering snow from the tops of our tents to melt as valuable drinking water since local creeks were salty due to contamination from salt domes on the island.
As we started piecing our sedimentary sections together, often stuck knee-deep in mud, we doubted how such a miserable environment, where barely a bird was sighted, could deliver any good science. Now two years after our journey and with the dedicated research of my two graduate students Julie Andrews and Adam Pugh we know we had chosen the right place. A multidisciplinary approach of studying several fossil groups augmented with geochemical analyses unravelled a complex paleoenvironmental history of the Late Cretaceous ocean that might serve as an ancient analogue to a future Arctic Ocean.
With great anticipation, the following year we travelled to Axel Heiberg Island where we had hoped to find the entire Cretaceous rock record. Polar Continental Shelf flew us in a Twin Otter to Sherwood Head, our landing spot at the mouth of Glacier Fiord. A helicopter delivered us to the head of the fiord allowing a bird’s eye view of a spectacular glaciated landscape under sunny skies. The added bonus of 24 hours of daylight made for long work days and occasionally provided opportunities to observe the habits of a herd of musk oxen that kept their distance. I am sure they claimed back their green pastures as soon as we left. Our camp was set up at the foot of one of the glaciers that was retreating before our very eyes. The melt provided the most delicious drinking water I have ever sampled.
The fresh rock outcrops in the vicinity of the glaciers are the sort that geologists cherish for field investigations. We went to work with immense enthusiasm. After measuring a section of nearly three kilometres in length we had enough sample material for years of study. Careful field observations already revealed an interesting Cretaceous Arctic landscape that was shaped by large sea-level changes and varying climate. The lower Cretaceous is dominated by sandstones that reflect an ancient large delta where coal beds testify to rich vegetation at the time. Large chunks of petrified wood attest to trees in the area, which are completely unknown in the modern icehouse Arctic. The lower Cretaceous section contains multiple beds of so-called glendonites, a pseudomorph of the mineral ikaite. These tweaked our interest because ikaite is a calcium carbonate mineral that forms under near freezing temperatures on muddy seafloors and therefore becomes a paleoclimate indicator. Sure enough, as we moved towards the upper Cretaceous sections and geological time that globally reflect a temperature maximum, these crystals disappeared telling us that the Late Cretaceous global warming affected the Arctic as well.
Fossils are a testament to ancient ecosystems and their animal and plant inhabitants. Microscopic life is a great indicator for paleoenvironmental reconstructions because they are usually abundant and widely spread in ocean basins allowing for stratigraphic correlations between different sites. As we then compare time-equivalent fossil and sedimentary records from Ellef Ringnes and Axel Heiberg islands, we are able to correlate times of high surface productivity and times when no oxygen remained on the seafloor preventing animals living on the seafloor to thrive. As predicted for a future warmer Arctic Ocean, the warm upper Cretaceous deep Polar ocean as exposed on Ellef Ringnes Island was barren of calcium carbonate but rich in siliceous planktic organisms in the form of diatoms and radiolaria. In contrast to the stratified deep offshore regions, the shelf regions of that ancient basin, as documented in Axel Heiberg Island sections, remained oxygenated due to storms mixing the water column and allowing benthic life to prevail.
Our two field seasons have delivered a wealth of information about ancient climate conditions of the Arctic and biotic responses to a much warmer Arctic Ocean. While we cannot forget the contribution humans have made to the warming trend of our current climate, the geological rock record tells a story of constant and inevitable climate change with periods of warmer and colder climates than today.
As with most scientific studies, our work has left us with more questions to ask and more answers to find. We hope to return in the summer of 2014 for another exciting field season in the High Arctic looking for more stories the rocks can unveil.
Claudia J. Schröder-Adams is a Professor with the Department of Earth Sciences at Carleton University in Ottawa, Ontario.