Eelco J. Rohling 2017. - The Oceans. A deep history. Princeton: Princeton University Press, 262 pp.

Despite a few significant flaws, this is a fascinating, compelling and sobering summary of what we think we know about the history of the oceans and climate, and the perspective that gives to an understanding of climate change due to humans in the past ~200 years.

Eelco Rohling is a “paleoceanographer” [a daft but apparently nearly universal misspelling of what should surely be “palaeo-oceanographer”, or at least “paleo-oceanographer”. Who says or writes “paleclimatolagist”, for example?]. His goal is to convey what he and his colleagues have learned about the history of the oceans and climate through geological time to a readership “beyond the narrow circle of specialists” (p. 2). Unfortunately I suspect he will fail, since the book may be a bit too detailed and have too many graphs for a general audience. A more likely audience would include scientists from other fields, especially related fields. Like a typical reader of other books by the publisher (Princeton University Press). It isn’t a hard book to read, however, and I learned a lot, and when I forget it again I’ll return to this book and to these notes.

It is a pity that many of the interesting/important/debatable statements are not explicitly supported by references. The best one can do is attempt to guess which of the several dozen primary literature citations for each chapter might be applicable to each statement. Or to read each paper.

For example: “we have good evidence of their [oceans] existence by about four billion years ago” (p. 15)

“the Sun was only about 70% as strong as it is today” [about four billion years ago] (p. 19)

“Earth is not significantly gaining of losing mass or size” (p. 24)

Chapter 1, Origins, presents a very readable and accurate summary of plate tectonics as understood by the vast majority of geologists (even though most of them have done no research pertinent to that subject). It is therefore wrong, as the handful of living adherents of expanding Earth theory will rush to exclaim. This leads to such circular arguments as “plate tectonics is needed to maintain the heat-flow motion in Earth’s outer core” (p. 19; textbooks state the converse, that heat-flow is needed to maintain plate motion, but you can’t have it both ways). The maps of continental reconstructions over geological time on page 29 would be great if the diameter of the Earth before 20 Ma was not expanded by inserting a fictitious Panthalassic Ocean (for which there is no evidence) and Tethys (for which at least some marine fossil evidence exists). Even on the facing page, Rohling emphasises the anomaly by stating that the oldest Pacific crust is at most 200 Ma [indeed, the majority of it is of course far younger, less than 65 Ma]. Thus, here is a news bulletin to team plate tectonics: the simplest explanation, gang, is not to invent unnecessary circumference but to listen to the evidence, which is shouting out that the Earth was smaller in the past. Of course Rohling can’t be blamed for this; he isn’t an Earth scientist. To those prepared to entertain other hypotheses of Earth history, mentally substituting “expansion” for “subduction” will make this otherwise wonderful book more logical and more accurate.

Elsewhere, the Origins chapter does entertain alternative hypotheses. For example in the matter of the origin of the oceans. My understanding was that there was no mystery, and that the oceans were understood to have originated from outgassing from the mantle (vulcanism) condensing as water. But apparently there is still debate on the subject, and an alternative explanation is that water is cometary in origin (even though the hydrogen isotope ratio of ice in comets is different from that of water on Earth).

The last couple of pages of the Origins chapter (pp. 54-55) has a very nice summary of the three types of oceans characterised by paleoceanographers [sic!]:

“Strangelove oceans” - 4 Ba-542 Ma and also post-mass extinction events. Times when abiotic processes dominate carbon precipitation in shallow oceans. “Neritic oceans” - 542-252 Ma. Biological carbonate production occurs now in mostly shallow oceans. “Cretan oceans” - 252 Ma-present. In these times, planktonic organisms with calcareous skeletons become important agents of carbon precipitation. In the following chapter, Controls on change, the interrelated influences of these phases, and geological factors, on atmospheric CO^2^ levels through geological time is lucidly summarised with, thankfully, some of the many uncertainties admitted.

Lots of other great explanations to return to here. For example:

Solar variability and sunspots (pp. 67-69): luminosity of the sun was about 70% of the modern value at the earliest time in Earth history; and, sunspots are darker (=cooler) but faculae occur at the same times and they are brighter and hotter, so a sunspot maximum is also a solar output maximum, but only by about 0.1%.

A great summary of CO₂ variation over geological time (vulcanism is responsible for only about a few % of CO₂ rise due to humans (pp. 69-79).

Sponges may have had a role increasing oxygenation in the early oceans because they were probably the most abundant filter feeders and could have promoted photosynthesis by making the water clearer (p. 98; relevant refs may be Lenton et al., 2014?; Li et al., 2008; Malouf et al., 2010?).

The best summary I’ve found on the carbonate compensation depth (CCD - transition from depths where sediments contain carbonate to those where carbonate is absent because deep sea waters are more acid and colder due) starting on p. 104 and changes over time due to weathering of carbonate rocks, buffering of atmospheric CO₂ and comparisons of the Atlantic and Pacific Oceans. I didn’t know that the CCD his thought to have increased by 1.6 km over the past 55 Ma (p. 117). The system is a buffer, because when oceans become more acidic carbonate in the sediments dissolves and corrects. Just a lot more slowly.

Sea levels are said to be about 200 m higher than present values during the mid-Cretaceous but only about 80 m of this can be achieved by melting all the polar ice. This is no mystery to proponents of a smaller palaeo-Earth (pp. 131-132; also p. 167).

A compelling description of anoxic events in the Mediterranean (p. 145 ff.) and correlation with astronomical phenomena (precession). Precession also alternately magnifies then minimises seasonal difference every ~11.5K years.

The theory and method of inferring sea-level change over time is masterfully summarised on pages 168-174, although I would have liked to read a short description of the methods by which palaeo-oceanographers correct for uplift or subsidence since the deposition of a [coral] sample (p. 169).

Maybe my biggest lesson was that causes of the most recent ice ages is a mystery. Ice ages older than 1.2 Ma occurred about every 41K years and are reasonably well understood as they correlate with orbital variations. However ice ages since 1.2 Ma occur about every 100K years and their causes are still not known (p. 176 and elsewhere). Given the present predicament of humanity this is a hugely significant knowledge gap.

I didn’t know about the “seesaw” of temperatures between north and south. North Atlantic deepwater formation is a heat pump that ensures when high northern latitudes receive warm, high salinity tropical waters, then the south cools (p. 190).

All this provides a convincing and compelling context for evaluating changes in climate, CO₂ and interrelated phenomena due to humans. This is the material covered in the final chapter Future oceans and climate. For example, the current amount and rate of CO₂ increase due to human activity is at least an order of magnitude greater than has occurred anytime since the end-Permian mass extinction (pp. 191-195). And: ocean acidification has already taken up about twice the amount of carbon as has been added to the atmosphere as CO₂ (p. 201) . And: higher CO₂ convincingly correlates with higher temperatures throughout Earth history (p. 203). It is no help to human time-scales, but tectonic events are also significant; continents are more relective than oceans so changing ration of continent to ocean will change absorption of solar radiation. It has been suggested that this has had a lot to do with deep sea cooling since the warm mid-Mesozoic (p. 203).

A few refs to follow up:

Brinkhuis, H., Schouten, S., Collinson, M.E., Sluijs, A., Damsté, J.S.S., Dickens, G.R., Huber, M., Cronin, T.M., Onodera, J., Takahashi, K., Bujak, J.P., Stein, R., van der Burgh, J., Eldrett, J.S., Harding, I.C., Lotter, A.F., Sangiorgi, F., Cittert, H. van K., de Leeuw, J.W., Matthiessen, J., Backman, J., Moran, K. & the Expedition 302 Scientists (2006) Episodic fresh surface waters in the Eocene Arctic Ocean. Nature 441, 606.

Butterfield, N.J. (2006) Hooking some stem-group “worms”: fossil lophotrochozoans in the Burgess Shale. BioEssays 28, 1161–1166.

Canfield, D.E., Poulton, S.W. & Narbonne, G.M. (2007) Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life. Science 315, 92.

Caron, J.-B., Scheltema, A., Schander, C. & Rudkin, D. (2007) Reply to Butterfield on stem-group “worms”: fossil lophotrochozoans in the Burgess Shale. BioEssays 29, 200–202.

Hönisch, B., Ridgwell, A., Schmidt, D.N., Thomas, E., Gibbs, S.J., Sluijs, A., Zeebe, R., Kump, L., Martindale, R.C., Greene, S.E., Kiessling, W., Ries, J., Zachos, J.C., Royer, D.L., Barker, S., Marchitto, T.M., Moyer, R., Pelejero, C., Ziveri, P., Foster, G.L. & Williams, B. (2012) The Geological Record of Ocean Acidification. Science 335, 1058. Huber, B.T., Norris, R.D. & MacLeod, K.G. (2002) Deep-sea paleotemperature record of extreme warmth during the Cretaceous. Geology 30, 123–126.

Purvis, A., Jones, K.E. & Mace, G.M. (2000) Extinction. BioEssays 22, 1123–1133.

Shen, B., Dong, L., Xiao, S. & Kowalewski, M. (2008) The Avalon Explosion: Evolution of Ediacara Morphospace. Science 319, 81.

Whaley, J. (2007) The Azolla Story: Climate Change and Arctic Hydrocarbons. Geo ExPro 4, 66–72.