Nummulites: small wonders of nature and distant memory of a subtropical northwestern Europe – part 2

Robert Speijer is professor in geology and paleontology at KU Leuven (Belgium). With his research group he studies the interaction between climate change and the biosphere.

During the Eocene Epoch, between 56 and 34 million years ago, nummulites (see part 1) thrived in the warm shallow seas, bordering the former Tethys Ocean. At that time, this ancient ocean was gradually closing due to the northward movements of the African and Indian tectonic plates. The silent remnants of this ocean and its bordering shallow seas, can now be found in rocks of the mountain chains from the Pyrenees in the west to the Himalayas in the east, but also all over North Africa, where broad shallow seas covered present-day land (see deeptimemaps.com/global-series/ for paleogeographic reconstructions of the Eocene and other time intervals).

In many parts of the warm shallow seas of the Eocene zillions of nummulite shells have formed thick layers of sediment, often up to tens of meters thick. After burial under younger sediments, these nummulites accumulations became solidified into hard limestone rocks, sometimes almost exclusively consisting of nummulites. One of the greatest examples of the rock-forming capacity of nummulites can be observed at the pyramids of Giza (Gizeh) near Cairo in Egypt: many huge building blocks of the pyramids consist of such nummulitic limestones. Perhaps not surprisingly, the aptly named Nummulites gizehensis is the most abundant species in these rocks (see objekte.nhm-wien.ac.at/objekt/th1577/ob1583 for an example)

Further north, such as in Belgium, nummulites were less prolific and not larger than about 1,5 cm in diameter. Nevertheless, they form ancient relicts from a time that the North Sea was a permanent warm shallow sea. Nowadays, nummulites and other photosymbiont-bearing larger foraminifera live in shallow and warm (sub)tropical seas, such as the Bahama Platform, the Persian Gulf and coastal waters around the numerous Indonesian islands. The North Sea is currently not a suitable habitat for them, but during the Eocene that was quite a different story: the presence of nummulites in Eocene sands and clays of Belgium, northern France and southern England indicates that for a period of almost 20 million years the North Sea provided also good living conditions for certain nummulites species. Research by KU Leuven PhD student Lise Martens on the chemical composition of the carbonate of nummulites demonstrates that the average temperature of early Eocene sea water in the southern part of the North Sea Basin (i.e. present day Belgium) ranged from 20 ^OC to almost 30 ^OC. This is a stunning 10 to 20 ^OC warmer than the present day average North Sea water temperature. This may sound incredible, but when we consider the global climatic conditions of that time, it is not really surprising. The early Eocene period represents the most recent geological time interval which is characterized by so-called hothouse conditions. During hothouse conditions there were no significant icecaps at the poles and globally averaged surface temperatures were up to ~15 ^OC higher than during the current (interglacial) icehouse conditions. Even at very high-latitudes, enhanced temperatures enabled many tropical to subtropical organisms to live far beyond their current geographic range limits. For instance, Ellesmere Island, which is part of the Arctic territory of present-day Canada (80^O North!) is famous for its fossil remains of alligators, giant tortoises and palm trees. These are all organisms that we nowadays only encounter in the tropics and subtropics. Ellesmere Island has not significantly moved north since the Eocene, indicating that even polar regions must have been very warm and essentially ice free throughout the year. These early Eocene globally warm conditions are generally linked to high atmospheric carbon dioxide levels, two to three times higher than today.

Now how does information on the early Eocene hothouse translate to the current debate on global warming? Some may argue that curbing global warming through reducing greenhouse gasses is a pointless effort because in the past climate has always varied, without man being the culprit. Although this may perhaps sound credible, this is actually a non-argument. Even the fastest global warming events of the geological past, like the Paleocene-Eocene thermal maximum (PETM) or the transitions from Quaternary glacials to interglacials, which were characterized by global warming of about 5 degrees, occurred over a time interval of several thousands of years. This equals, at most, a few tenths of a degree of global temperature rise per 100 years. Current global warming and future projections range from 1 to 5 degrees for the coming 100 years, so we are currently dealing with a rate of temperature change that is about ten times larger than the most rapid natural global warming phases of the geological past. Most ecosystems and many biological species will probably not be resilient enough to cope with this rate of change. To put this into the perspective of the PETM, one of the main and most rapid warming events of the last 60 million years: this global warming phase led to major disruptions of marine and non-marine ecosystems (more information can be found here: en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Thermal_Maximum)

Yet, even this natural phenomenon progressed with a speed of only about one tenth of the current rate of warming. Moreover, the PETM occurred in a world that was already very warm and without major polar ice caps. This means that ecosystems were relatively well pre-adapted to globally hot conditions; this is in stark contrast with modern ecosystems of which many are adapted to moderate to cold conditions; conditions that will disappear in a rapidly warming world.

Without significantly reducing current greenhouse emissions, the world of the late 21^st century and beyond will function increasingly similar to the warm world of the geological past. This will have many repercussions, amongst others warming of the poles, especially in summer, a decrease in sea ice and glaciers, an increasing rate of sea-level rise, changing ocean circulation, limited deep-sea ventilation, surface ocean acidification, etc. For people living at high latitudes a warmer world may seem comfortable: mild winters and warmer summers? There will also be opportunistic organisms that profit from rapid global warming. Yet, the vast majority of organisms on land and in the sea, ranging from microbes to top predators form intricate ecosystem networks that partly evolved under the cool conditions that have been prevailing since the last few millions of years. These ecosystems will not be able to adapt quickly enough, and this will lead to the decline and ultimately even extinction of numerous organisms. Also for the other end of the temperature spectrum, in the tropics, current processes and the geological and fossil record provides no good news. For instance, prior to the PETM, tropical seas bordering the Tethys Ocean were characterized by reef ecosystems, somewhat similar to those of the modern world: these reefs were composed of many different organisms, including colonies of stony corals and various kinds of algae with a calcareous skeleton (and many other organisms like fish that are rarely preserved as a fossils). After the PETM and during the Eocene hothouse, these organisms almost disappeared or became very rare, giving way to much less biodiverse shallow platforms. As can be judged from the information above, these Eocene platforms often became dominated by nummulites and related fossil groups. In contrast to what might be expected, it turns out that there was hardly any refuge for coral reef ecosystems further away from the hot equatorial region, where sea water temperatures were milder. Unfortunately, this is again not good news for these highly diverse modern marine ecosystems, which are already now frequented by overheated conditions (marine heatwaves during El Nino events), leading to bleaching of the coral reefs, which basically means a complete collapse of the local ecosystems (see coralreefwatch.noaa.gov/satellite/analyses_guidance/global_coral_bleaching_2014-17_status.php for more information)

As yet, many reefs have been able to recover from these bleaching events, but if bleaching events are going to occur too frequently in a warming world, reef ecosystems will not have sufficient time to recover and may eventually almost disappear.  This not only means a loss of coral species, but also a loss of all organisms that use the coral reefs as a nursery room (for numerous tropical fish and crustaceans) or as foraging area for predator fish. These diverse reef ecosystems also provide numerous known and yet unknown ecosystem services to mankind, like coastal protection during storms. Such ecosystem services will also vanish in a rapidly warming world and as such have negative consequences for human society.

Will there also be winners in this process? Undoubtedly yes. Even after the greatest mass extinctions of the geological past (such as the one at the Cretaceous/Tertiary boundary), during which up to 95% of all species died out, there have always been temporary winners: opportunists that could thrive because of a lack of competitors or predators. Under continuing global warming in this century, perhaps some nummulites or closely related organisms (other larger benthic foraminifera) may thrive again, as they were during the hothouse of the Eocene.

Robert P. Speijer – Department of Earth & Environmental Sciences, KU Leuven, Belgium