Update on climate and sea-level change during the Cenozoic

The Cenozoic Era was a period of fundamental change in the outer part of the Earth system. It culminated in the greatest climatic cooling since the Permian Period, during which upright apes emerged between 6 to 10 Ma ago.  The most decisive part of hominin evolution – the appearance of our own genus Homo – took place in the last 2.5 Ma that saw icecaps plastered over both polar regions and repeated pulses of major climate upheaval that dramatically affected all parts of the continents. Whereas the Mesozoic was dominated by reptiles, most famously the dinosaurs, the Cenozoic is rightly known as the age of mammals and of birds. The flowering plants, especially grasses, also transformed terrestrial ecosystems. The background to what has become ‘our time’ is not only climate change, but massive shifts in sea level and the outlines of the continents. For more than two decades many palaeoclimatologists have focused on the Cenozoic, gathering data using a variety of rapidly advancing technologies from a growing number of sites, in sediments from the continents and the ocean floor. One problem has been correlating all this global data precisely, coming as it does from many incomplete sedimentary sequences dotted around the planet. A great deal of basic information has come from the petroleum industry, which, of course, has continually eyed sedimentary rocks as the source of hydrocarbons through the 20th century. It was seismic reflection surveying that first gave clues to global ups and downs of sea level from onlaps and offlaps of strata that are visible on seismic sections, amplified by sequence stratigraphy. Six geoscientists from Rutgers University in New Jersey, USA have blended oil-industry archives with academic research to produce the first fully calibrated, comprehensive record of the Cenozoic (Miller, K.G. et al. 2020. Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records. Science Advances, v. 6, article eaaz1346; DOI: 10.1126/sciadv.aaz1346).

Latest palaeoclimate data for the Cenozoic Era. A oxygen-isotope data from benthic foraminifera (pale blue = polar icecaps, green = ice-free, pink – hothouse); B estimated mean sea-surface temperature from the calcium/magnesium ratio in Pacific Ocean cores; C variation in global mean sea level estimated from A and corrected for changes in the density of seawater due to water temperature (B); D atmospheric CO2 variations estimated using various proxies – see top right box. Click on the image to show a full-size version in a new browser tab. (Credit: Miller et al. 2020; Fig. 1)

Fluctuations in the proportion of 18O (δ18O) in the tests of foraminifera that lived in deep water are the key to global changes in sea level  and point to the influence of glacial ice accumulating on land (see Cooling sets in: Stepping Stones, Chapter 17). This is because glaciers are made from water that has evaporated from the oceans. When this happens, water that incorporates the lighter 16O isotope evaporates more easily and becomes enriched in atmospheric water vapour. When this water falls as snow that accumulates on land to form ice, the oceans are slightly enriched in the heavier 18O: δ18O incorporated into the shelly material of organism dwelling in the deep ocean increases at levels of a few parts per thousand. Conversely, their δ18O decreases when huge ice caps melt (A on the figure). The oxygen isotope records from fossils in ocean floor sediments give a far more precise impression of fluctuating sea level than do seismic sections and sequence stratigraphy of sedimentary rocks that interest the oil industry. But it is an ‘impression’, because other factors affect sea level.

Not only the global volume of ocean water is involved: the volume of the ocean basins changes too. This can occur because of changes in the rate of sea-floor spreading: when that is fast the hot new oceanic lithosphere is less dense and so buoys-up part of the ocean floor to drive sea level upwards. Slow spreading does the converse, as more lithosphere cools and sinks slightly. Another factor is the changing rate of marine sedimentation of material eroded from the continents. That fills ocean basins to some extent, again displacing the water upwards. When sediments are compacted as they become more deeply buried that has an effect too, to increase basin volume and result in sea-level fall. Oil industry geoscientists have attempted to allow for these long-term, slow mechanisms, to give a more accurate sea-level record.

Yet there is another important factor: the density and thus volume of ocean water changes with temperature. The warmer it is the greater the volume of ocean water and the higher is sea level. This is where academic work comes in handy. Two common elements that are dissolved in ocean water are magnesium and calcium. They also occur in the carbonate tests of the same deep-water forams that are used for oxygen-isotope measurements. It turns out that the warmer the water is the more magnesium enters the foram tests, and vice versa: their Mg/Ca ratio is a reliable proxy for mean ocean temperature and can be measured easily, centimetre-by-centimetre through cores. Kenneth Miller and colleagues have used this with the oxygen isotope proxy for land-ice volume to correct the sea-level record.

The Cenozoic ocean temperature record (B on the figure) is, in itself, interesting. It reveals far more large fluctuations than previously thought, especially in the Palaeocene and early Eocene. Yet, overall, the trend is one of steady cooling compared with the sudden shifts in δ18O that mark the onset of the Antarctic ice cap at the Eocene-Oligocene boundary around 34 Ma ago, and the apparent, temporary emergence from ‘ice-house conditions in the Middle Miocene. Also, sea level corrected for ocean temperature effects (C on the figure) suggests that for much of the Cenozoic sea level was lower than expected; i.e. it rarely exceeded 60 m above the current level, which is that expected when no substantial mass of  land ice exists.

The other important compilation made by Miller et al. is that of the CO2 content of the atmosphere estimated using six different proxies. It is a lot more fuzzy than the oceanic records because the proxies are not precise. Nevertheless, it is interesting. The current, partly anthropogenic level of around 400 parts per million (ppm) is not unique. In fact from 55 to 23 Ma it was consistently above this ‘Anthropocene’ level, peaking at twice that level at the end of the Eocene. That’s odd, because it doesn’t tally with the oxygen isotopes that indicate the onset of large scale Antarctic glaciation shortly afterwards. In fact most of the climatic highlights shown by A on the figure are not reflected in the Cenozoic history of the most influential greenhouse gas. In the short term of glacial-interglacial cycles during the late Pleistocene, atmospheric CO2 levels are very closely related to fluctuations of land-ice volume. In the 65 Ma of the Cenozoic such a link is hard to argue for. There are more puzzles than revelations in this otherwise major addition to palaeoclimatology.

An Early Archaean Waterworld

In Earth-logs you may have come across the uses of oxygen isotopes, mainly in connection with their variations in the fossils of marine organisms and in ice cores. The relative proportion of the ‘heavy’ 18O isotope to the ‘light’ 16O, expressed by δ18O, is a measure of the degree of fractionation between these isotopes under different temperature conditions when water evaporates. What happens is that H216O, in which the lighter isotope is bound up, slightly more easily evaporates thus enriching the remaining liquid water in H218O. As a result the greater the temperature of surface water and the more of evaporates, the higher is its δ18O value. Shells that benthonic (surface-dwelling) organism secrete are made mainly of the mineral calcite (CaCO3). Their formation involves extracting dissolved calcium ions and CO2 plus an extra oxygen from the water itself, as calcite’s formula suggests. So plankton shells fossilised  in ocean-floor sediments carry the δ18O and thus a temperature signal of surface water at the place and time in which they lived. Yet this signal is contaminated with another signal: that of the amount of water evaporated from the ocean surface (with lowered  δ18O) that has ended up falling as snow and then becoming trapped in continental ice sheets. The two can be separated using the δ18O found in shells of bottom-dwelling (benthonic) organisms, because deep ocean water maintains a similar low temperature at all time (about 2°C). Benthonic δ18O is the main guide to the changing volume of continental ice throughout the last 30 million year or so. This ingenious approach, developed about 50 years ago, has become the key to understanding past climate changes as reflected in records of ice volume and ocean surface temperature. Yet these two factors are not the only ones at work on marine oxygen isotopes.

Artistic impression of the Early Archaean Earth dominated by oceans (Credit: Sci-news.com)

When rainwater flows across the land, clays in the soil formed by weathering of crystalline rocks preferentially extract 18O and thus leave their own δ18O mark in ocean water. This has little, if any, effect on the use of δ18O to track past climate change, simply because the extent of the continents hasn’t changed much over the last 2 billion years or so. Likewise, the geological record over that period clearly indicates that rain, wet soil and water flowing across the land have all continued somewhere or other, irrespective of climate. However, one of the thorny issues in Earth science concerns changes of the area of continents in the very long term. They are suspected but difficult to tie down. Benjamin Johnson of the University of Colorado and Boswell Wing of Iowa State University, USA, have closely examined oxygen isotopes in 3.24 billion-year old rocks from a relic of Palaeoarchaean ocean crust from the Pilbara district of Western Australia that shows pervasive evidence of alteration by hot circulating ocean water (Johnson, B.W. & Wing, B.A. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, v. 13, p. 243-248; DOI: 10.1038/s41561-020-0538-9). Interestingly, apart from the composition of the lavas, the altered rocks look just the same as much more recent examples of such ophiolites.

The study used many samples taken from the base to the top of the ophiolite along some 20 traverses across its outcrop. Overall the isotopic analyses suggested that the circulating water responsible for the hydrothermal alteration 3.2 Ga ago was much more enriched in 18O than is modern ocean water. The authors’ favoured explanation is that much less continental crust was exposed above sea level during the Palaeoarchaean Era than in later times and so far less clay was around on land. That does not necessarily imply that less continental crust existed at that time compared with the Archaean during the following 700 Ma , merely that the continental ‘freeboard’ was so low that only a few islands emerged above the waves. By the end of the Archaean 2.5 Ga ago the authors estimate that oceanic δ18O had decreased to approximately modern levels. This they attribute to a steady increase in weathering of the emerging continental landmasses and the extraction of 18O into new, clay-rich soils as the continents emerged above sea level. How this scenario of a ‘drowned’ world developed is not discussed. One possibility is that the average depth of the oceans then was considerably less than it was in later times: i.e. sea level stood higher because the volume available to contain ocean water was less. One possible explanation for that and the subsequent change in oxygen isotopes might be a transition during the later Archaean Eon into modern-style plate tectonics. The resulting steep subduction forms deep trench systems able to ‘hold’ more water. Prior to that faster production of oceanic crust resulted in what are now the ocean abyssal plains being buoyed up by warmer young crust that extended beneath them. Today they average around 4000 m deep, thanks to the increased density of cooled crust, and account for a large proportion of the volume of modern ocean basins.

Glacial cycles and sea-floor spreading

The London Review of Books recently published a lengthy review (Godfrey-Smith, P. 2015. The Ant and the Steam Engine. London Review of Books, v. 37, 19 February 2015 issue, p. 18-20) of the latest contribution to Earth System Science by James Lovelock, the man who almost singlehandedly created that popular paradigm through his Gaia concept of a self-regulating Earth (Lovelock, J. A Rough Ride to the Future. Allen Lane: London; ISBN 978 0 241 00476 0). Coincidentally, on 5 February 2015 Science published online a startling account of the inner-outer-inner synergism of Earth processes and climate (Crowley, J.W. et al. 2015. Glacial cycles drive variations in the production of oceanic crust. Science doi:10.1126/science.1261508). In fact serendipity struck twice: the following day a similar online article appeared in a leading geophysics journal (Tolstoy, M. 2015. Mid-ocean ridge eruptions as a climate valve. Geophysical Research Letters, doi:10.1002/2014GL063015)

Both articles centred on the most common topographic features on the ocean floor, abyssal hills. These linear features trend parallel to seafloor spreading centres and the magnetic stripes, which chart the progressive additions to oceanic lithosphere at constructive margins. Abyssal hills are most common around intermediate- and fast-spreading ridges and have been widely regarded as fault-tilt blocks resulting from extensional forces where cooling of the lithosphere causes it to sag towards the abyssal plains. However, some have suggested a possible link with variations in magma production beneath ridge axes as pressure due to seawater depth varied with rising and falling sea level through repeated glacial cycles. Mantle melting beneath ridges results from depressurization of rising asthenosphere: so-called ‘adiabatic’ melting. Pressure changes equivalent to sea-level fluctuations of around 100-130 m should theoretically have an effect on magma productivity, falls resulting in additional volumes of lava erupted on the ocean floor and thus bathymetric highs.

English: A close-up showing mid-ocean ridge to...
Formation of mid-ocean ridge topography, including abyssal hills that parallel the ridge axis. (credit: Wikipedia)

A test of this hypothesis would be see how the elevation of the sea floor adjacent to spreading axes changes with the age of the underlying crust. John Crowley and colleagues from Oxford and Harvard Universities and the Korea Polar Research Institute analysed new bathymetry across the Australian-Antarctic Ridge, whereas Maya Tolstoy of Columbia University performed similar work across the Southern East Pacific Rise. In both studies frequency analysis of changes in bathymetry through time, as calibrated by local magnetic stripes, showed significant peaks at roughly 23, 41 and 100 ka in the first study and at 100 ka in the second. These correspond to the well known Milankovitch periods due to precession, changing axial tilt and orbital eccentricity: persuasive support for a glacial control over mid-ocean ridge magmatism.

Enlarged by 100% & sharpened file with IrfanView.
Periodicities of astronomical forcing and global climate over the last million years (credit: Wikipedia)

An interesting corollary of the observations may be that pulses in sea-floor eruption rates emit additional carbon dioxide, which eventually percolates through the ocean to add to its atmospheric concentration, which would result in climatic warming. The maximum effect would correspond to glacial maxima when sea level reached its lowest, the reduction in pressure stimulating the greatest magmatism. One of the puzzling features of glacial cycles over the last million years, when the 100 ka eccentricity signal dominates, is the marked asymmetry of the sea-level record; slowly declining to a glacial maximum and then a rapid rise due to warming and melting as the Earth changed to interglacial conditions. Atmospheric CO2 concentrations recorded by bubbles in polar ice cores show a close correlation with sea-level change indicated by oxygen isotope data from oceanic sediments. So it is possible that build-up of polar ice caps in a roundabout way eventually reverse cooling once they reach their greatest thickness and extents, by modulating ocean-ridge volcanism and thereby the greenhouse effect.

Not-so-light, but essential reading

In its 125th year the Geological Society of America is publishing invited reviews of central geoscience topics in its Bulletin. They seem potentially useful for both undergraduate students and researchers as accounts of the ‘state-of-the-art’ and compendia of references. The latest focuses on major controls on past sea-level changes by processes that operate in the solid Earth (Conrad, C.P. 2013. The solid Earth’s influence on sea level. Geological Society of America Bulletin, v. 125, p. 1027-1052), a retrospective look at how geoscientists have understood large igneous provinces (Bryan, S. E. & Ferrari, L. 2013. Large igneous provinces and silicic large igneous provinces: Progress in our understanding over the last 25 years. Geological Society of America Bulletin, v. 125, p. 1053-1078) and the perennial topic of how granites form and end up in intrusions (Brown, M. 2013. Granite: From genesis to emplacement Geological Society of America Bulletin, v. 125, p. 1079-1113).

Sea level change

Conrad covers sea-level changes on the short- (1 to 100 years), medium- (1 to 100 ka) and long term (1 to 100 Ma). The first two mainly result from local deformation of different kinds associated with glacial loading and unloading. These result in changes in the land surface, the sea surface nearby and on thousand year to 100 ka timescales to ups and downs of the sea-bed. Global sea-level changes due to melting of continental glaciers at the present day amount to about half the estimated 2 to 3 mm of rise each year. But increasingly sensitive measures show it is more complex as the rapid shifts of mass involved in melting ice also result in effects on the solid Earth. At present solid mass is being transferred polewards, but at rates that differ in Northern and Southern hemispheres and which are changing with anthropogenic influences on glacial melting. Viscous movement of the solid Earth is so slow that effects from previous glacial-interglacial episodes continue today. As a result rapid elastic movements are tending to produce relative sea-level falls in polar regions of up to 20 mm per year with rising sea level focusing on areas between 30°N and 30°S. The influence of the slower viscous mass transfer has an opposite sense: sea-level rise at high latitudes. Understanding the short- and medium-term controls is vital in predicting issues arising in the near future from natural and anthropogenic change.

Comparison of two sea level reconstructions du...
Comparison of two sea level reconstructions during the last 500 Ma. (credit: Wikipedia)

 Most geologists are concerned in practice with explanations for major sea-level changes in the distant past, which have a great deal to do with changes in the volumes of the ocean basins. If the global sea-floor rises on average water is displaced onto former land to produce transgressions, and subsidence of the sea floor draws water down from the land. Conrad gives a detailed account of what has been going on since the start of the Cretaceous Period, based on the rate of sea-floor spreading, marine volcanism and sedimentation, changes in the area of the ocean basins and the effects of thermally-induced uplift and subsidence of the continents, showing how each contribution acted cumulatively to give the vast transgressions and regressions that affected the late Phanerozoic. On the even longer timescale of opening and closing of oceans and the building and disintegration of supercontinents the entire mantle becomes involved in controls on sea level and a significant amount of water is chemically exchanged with the mantle.

Large igneous provinces

Cathedral Peak, 3004m above sea level in the K...

The Web of Science database marks the first appearance in print of “large igneous province” in 1993, so here is a topic that is indeed new, although the single-most important attribute of LIPs, ‘flood basalt’ pops up three decades earlier and the term ‘trap’ that describes their stepped topography is more than a century old. Bryan and Ferrari are therefore charting progress in an exciting new field, yet one that no human – or hominin for that matter – has ever witnessed in action. One develops, on average, every 20 Ma and since they are of geologically short duration long periods pass with little sign of one of the worst things that our planet can do to the biosphere. In the last quarter century it has emerged that they blurt out the products of energy and matter transported as rising plumes from the depths of the mantle; they, but not all, have played roles in mass extinctions; unsuspected reserves of precious metals occur in them; they play some role in the formation of sedimentary basins and maturation of petroleum and it seems other planets have them – a recipe for attention in the early 21st century. Whatever, Bryan and Ferrari provide a mine of geological entertainment.

 

Granites

In comparison, granites have always been part of the geologist’s canon, a perennial source of controversy and celebrated by major works every decade, or so it seems, with twenty thousand ‘hits’ on Web of Science since 1900 (WoS only goes back that far). Since the resolution of the plutonist-neptunist wrangling over granite’s origin one topic that has been returned to again and again is how and where did the melting to form granitic magma take place? If indeed granites did form by melting and not as a result of ‘granitisation. Lions of the science worried at these issues up to the mid  20th century: Bowen, Tuttle, Read, Buddington, Barth and many others are largely forgotten actors, except for the credit in such works as that of Michael Brown. Experimental melting under changing pressure and temperature, partial pressures of water, CO2 and oxygen still go on, using different parent rocks. One long-considered possibility has more or less disappeared: fractional crystallisation from more mafic magma might apply to other silicic plutonic rocks helpfully described as ‘granitic’ or called ‘granitoids’, but granite  (sensu stricto) has a specific geochemical and mineralogical niche to which Brown largely adheres. For a while in the last 40 years classification got somewhat out of hand, moving from a mineralogical base to one oriented geochemically: what Brown refers to as the period of ‘Alphabet Granites’ with I-, S- A- and other-type granites. Evidence for the dominance of partial melting of pre-existing continental crust has won-out, and branched into the style, conditions and heat-source of melting.

English: Kit-Mikayi, a rock formation near Kis...
Typical granite tor near Kisumu, Kenya (credit: Wikipedia)

All agree that magmas of granitic composition are extremely sticky. The chemical underpinnings for that and basalt magma’s relatively high fluidity were established by physical chemist Bernhardt Patrick John O’Mara Bockris (1923-2013) but barely referred to, even by Michael Brown. Yet that high viscosity has always posed a problem for the coalescence of small percentages of melt into the vast blobs of low density liquid able to rise from the deep crust to the upper crust. Here are four revealing pages and ten more on how substantial granite bodies are able to ascend, signs that the puzzle is steadily being resolved. Partial melting implies changes in the ability of the continental crust to deform when stressed, and this is one of the topics on which Brown closes his discussion, ending, of course, on a ‘work in progress’ note that has been there since the days of Hutton and Playfair.